{"gene":"GLP1R","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":1993,"finding":"Cloning and functional expression of the human GLP-1 receptor from pancreatic islet cDNA revealed a 463-amino-acid protein with 90% homology to rat GLP-1R. When expressed in COS-7 cells, the receptor conferred high-affinity GLP-1(7-37) binding (Kd ~0.5 nM), activated adenylyl cyclase (cAMP production, EC50 ~93 pM), and also activated phospholipase C (PLC), increasing intracellular Ca2+ from intracellular pools, demonstrating coupling to multiple signaling pathways. Exendin-4 was identified as an agonist and exendin-(9-39) as an antagonist.","method":"cDNA cloning, stable transfection in fibroblasts/COS-7 cells, radioligand binding, cAMP assay, Ca2+ imaging","journal":"Endocrinology / Diabetes","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in heterologous expression system with multiple orthogonal functional assays; independently replicated by two groups (PMID 8405712 and 8404634)","pmids":["8405712","8404634"],"is_preprint":false},{"year":2008,"finding":"Crystal structure of the human GLP-1R extracellular domain (nGLP-1R) in complex with the antagonist exendin-4(9-39) at 2.2 Å resolution revealed that exendin-4 binds as an amphipathic α-helix making hydrophobic and hydrophilic contacts; the Trp-cage C-terminal extension does not contact nGLP-1R. The hydrophobic binding site is defined by an N-terminal α-helix and a loop between antiparallel β-strands of nGLP-1R.","method":"X-ray crystallography (MAD, 2.2 Å), site-directed mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure with mutagenesis validation","pmids":["18287102"],"is_preprint":false},{"year":2009,"finding":"Crystal structure of the GLP-1R extracellular domain in complex with GLP-1 at 2.1 Å resolution showed GLP-1 binds as a kinked but continuous α-helix (Thr13–Val33). While hydrophobic ligand-receptor interactions are conserved between GLP-1 and exendin-4(9-39) bound forms, specific residues in the binding site adopt a GLP-1-specific conformation. Mutagenesis confirmed differences in binding modes of GLP-1 vs. exendin-4 on the full-length receptor.","method":"X-ray crystallography (2.1 Å), site-directed mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus mutagenesis; orthogonal to prior extracellular domain structure","pmids":["19861722"],"is_preprint":false},{"year":2017,"finding":"Crystal structures of the human GLP-1R transmembrane domain in complex with negative allosteric modulators PF-06372222 and NNC0640 at 2.7 and 3.0 Å revealed a common allosteric binding pocket outside helices V–VII near the intracellular half, distinct from the orthosteric peptide-binding site. Receptor is in an inactive conformation with compounds restricting movement of the intracellular tip of helix VI. Positive allosteric modulators target the same general region but a distinct sub-pocket at the helix V–VI interface facilitating G-protein coupling.","method":"X-ray crystallography, molecular modelling, mutagenesis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — two crystal structures at high resolution with mutagenesis validation","pmids":["28514449"],"is_preprint":false},{"year":2018,"finding":"Cryo-EM structure of the human GLP-1R in complex with G-protein-biased agonist exendin-P5 and Gαs heterotrimer at 3.3 Å global resolution revealed differences from the GLP-1-bound structure: distinct organization of ECL3 and proximal transmembrane segments at the extracellular surface, and a six-degree difference in the angle of Gαs-α5 helix engagement at the intracellular face, propagated across the G protein heterotrimer. Different rates and extents of conformational reorganization of Gαs between biased and unbiased agonist-bound structures were demonstrated.","method":"Phase-plate cryo-EM (3.3 Å)","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — high-resolution cryo-EM structure with direct comparison to GLP-1-bound structure","pmids":["29466332"],"is_preprint":false},{"year":2020,"finding":"High-resolution cryo-EM structures revealed that the non-peptide agonist PF-06882961 binding site substantially overlaps with GLP-1's binding site, whereas CHU-128 adopts a unique binding mode with a more open receptor conformation at the extracellular face. Structural differences involving extensive water-mediated hydrogen bond networks correlated with functional data showing that PF-06882961, but not CHU-128, closely mimics pharmacological properties of GLP-1 including signaling and regulation profiles.","method":"Cryo-EM structural determination, pharmacological assays, comparative structural analysis","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 — multiple high-resolution cryo-EM structures correlated with functional data","pmids":["33027691"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of semaglutide- and taspoglutide-bound GLP-1R–Gs complexes revealed similar peptide-receptor interactions to GLP-1 but different motions within the receptor and bound peptides. 3D variability analysis showed distinct receptor conformational dynamics, providing molecular explanation for different signaling efficacies and side effect profiles of distinct GLP-1R peptide agonists.","method":"Cryo-EM, 3D variability analysis","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structures of multiple agonist-bound complexes with conformational analysis","pmids":["34260945"],"is_preprint":false},{"year":2022,"finding":"Cryo-EM structures of Boc5 and WB4-24 (non-peptidic GLP-1R agonists) bound to GLP-1R–Gs complex revealed one arm inserted deeply into the orthosteric binding pocket overlapping with GLP-1 residues A8–D15, while other arms extended to TM1-TM7, TM1-TM2, and TM2-TM3 clefts. This unique binding mode creates a distinct conformation conferring both peptidomimetic agonism and biased signaling.","method":"Cryo-EM structural determination, pharmacological assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structures with pharmacological validation","pmids":["35561211"],"is_preprint":false},{"year":2022,"finding":"Combining cryo-EM, molecular dynamics simulations, receptor mutagenesis, and pharmacological assays across four peptide agonists (GLP-1, oxyntomodulin, exendin-4, exendin-P5) showed that distinctions in peptide N-terminal interactions and dynamics with the GLP-1R transmembrane domain are reciprocally associated with differences in allosteric coupling to G proteins. Transient interactions with residues at the base of the binding cavity correlate with enhanced kinetics for G protein activation.","method":"Cryo-EM, molecular dynamics simulations, mutagenesis, pharmacological assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM plus MD plus mutagenesis plus pharmacology; multiple orthogonal methods","pmids":["35013280"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structures of GLP-1R or GCGR in complex with Gs protein and three dual GLP-1R/GCGR agonists (peptide 15, cotadutide, SAR425899) identified key residues responsible for ligand recognition and dual agonism. Distinct side chain orientations within the first three residues determine receptor selectivity; ECL1 interaction plays an important role in dual agonism; lipid modification of MEDI0382 interacts with TM1-TM2 cleft correlating with increased GCGR potency.","method":"Cryo-EM structural determination, pharmacological assays, mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structures with pharmacological and mutagenesis validation","pmids":["37549266"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structures of human GLP-1R, GCGR, and GIPR in complex with Gs proteins without cognate ligands (ligand-free complexes) revealed that Gs protein alone directly opens the intracellular binding cavity and rewires the extracellular orthosteric pocket. In GLP-1R, the extracellular portion adopts a conformation close to the active state in the absence of ligand, revealing a distinct activation intermediate in which intracellular half of the transmembrane domain is mobilized by Gs protein to facilitate peptide N-terminus entry.","method":"Cryo-EM structural determination, comparative structural analysis","journal":"Cell discovery","confidence":"High","confidence_rationale":"Tier 1 — high-resolution cryo-EM of ligand-free receptor-G protein complex","pmids":["38346960"],"is_preprint":false},{"year":2014,"finding":"Using a validated monoclonal antibody for immunohistochemistry and in situ ligand binding with 125I-GLP-1, GLP-1R was localized in primate and human tissues: predominantly in pancreatic β-cells (with markedly weaker acinar expression, no ductal expression), smooth muscle cells of renal and pulmonary arteries/arterioles, myocytes of the sinoatrial node, Brunner's gland of the duodenum, parietal cells and smooth muscle of stomach, and myenteric plexus neurons. No GLP-1R was detected in primate liver or thyroid.","method":"Immunohistochemistry with validated monoclonal antibody, in situ radioligand binding (125I-GLP-1)","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — direct localization with two orthogonal methods (IHC + radioligand binding), extensively validated reagent","pmids":["24467746"],"is_preprint":false},{"year":2010,"finding":"GLP-1R mRNA and protein were detected on primary human hepatocytes and hepatocyte cell lines. Exendin-4 stimulated phosphorylation of PDK-1, AKT, and PKC-ζ in HepG2 and Huh7 cells; siRNA against GLP-1R abolished these effects. Exendin-4 also quantitatively reduced triglyceride stores in these cells, demonstrating a direct GLP-1R-dependent effect on hepatic steatosis independent of insulin.","method":"RT-PCR, Western blotting, receptor internalization assay, siRNA knockdown, lipid quantification","journal":"Hepatology","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA epistasis plus multiple signaling readouts in a single study","pmids":["20225248"],"is_preprint":false},{"year":2010,"finding":"GLP-1R is present on human coronary artery endothelial cells (HCAECs). Exendin-4 stimulated DNA synthesis and cell proliferation dose-dependently through PKA, PI3K/AKT, and eNOS activation pathways. These effects were abolished by a GLP-1R antagonist (exendin-(9-39)) and by individual pathway inhibitors, establishing a GLP-1R→PKA→PI3K/AKT→eNOS signaling cascade mediating endothelial proliferation.","method":"DNA synthesis assay, cell counting, pharmacological inhibitor panel, GLP-1R antagonist blockade","journal":"Molecular and cellular endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — receptor antagonist plus multiple pathway inhibitors in same cell type","pmids":["20452396"],"is_preprint":false},{"year":2011,"finding":"GLP-1R is expressed in human hepatocytes (reduced in NASH patients). In hepatocytes from high-fat diet rats, exenatide increased PPARγ expression (reducing JNK phosphorylation for insulin sensitization), increased PKA activity, and stimulated Akt and AMPK phosphorylation, leading to PKA-dependent increase of PPARα activity and enhanced fatty acid β-oxidation.","method":"RT-PCR, Western blotting, hepatocyte culture with pharmacological treatments, liver biopsy analysis","journal":"Liver international","confidence":"Medium","confidence_rationale":"Tier 2/3 — multiple signaling readouts in primary hepatocytes with in vivo model correlation","pmids":["21745271"],"is_preprint":false},{"year":2011,"finding":"GLP-1 and liraglutide activate GLP-1R on pre-adipocytes to promote proliferation and inhibit apoptosis through activation of ERK, PKC, and AKT signaling pathways, inducing adipogenesis. Loss of GLP-1R expression caused reduction in adipogenesis through induction of apoptosis in pre-adipocytes via inhibition of these same pathways.","method":"In vitro adipocyte differentiation assays, loss-of-function (GLP-1R knockout), pharmacological agonism, pathway inhibitors","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — gain/loss-of-function with defined pathway readouts","pmids":["22207759"],"is_preprint":false},{"year":2015,"finding":"GLP-1R agonist exendin-4 attenuated H2O2-induced reactive oxygen species production in cardiomyocytes through an Epac-dependent pathway, and increased antioxidant enzymes (catalase, glutathione peroxidase-1, manganese superoxide dismutase). The antiapoptotic effect (decreased apoptotic cells, inhibited caspase-3, enhanced Bcl-2) was mediated through both PKA- and Epac-dependent pathways downstream of GLP-1R activation.","method":"Pharmacological dissection of cAMP effectors (PKA vs. Epac) in cardiomyocytes, ROS measurement, caspase activity, Western blotting","journal":"Molecular endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — selective pharmacological tools dissecting PKA vs. Epac branches downstream of GLP-1R","pmids":["25719403"],"is_preprint":false},{"year":2015,"finding":"GLP-1R agonist P5 was identified as a G-protein-biased agonist that promotes G-protein signaling comparable to GLP-1 and Exendin-4 but exhibits significantly reduced β-arrestin recruitment. In preclinical T2DM mouse models, P5 was a weak insulin secretagogue but increased adipogenesis, reduced adipose tissue inflammation and hepatic steatosis, and was more effective at correcting hyperglycemia than Exendin-4, demonstrating that GLP-1R signaling bias has distinct in vivo consequences.","method":"Autocrine-based peptide library screening, cAMP and β-arrestin assays, mouse diabetes models","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 — functional signaling bias demonstrated in vitro correlated with distinct in vivo phenotypes","pmids":["26621478"],"is_preprint":false},{"year":2015,"finding":"GLP-1R signaling controls mucosal expansion of the small bowel and colon. These actions did not require EGF receptor or intestinal IGF1 receptor but were absent in Glp1r−/− mice. Exendin-4 increased Fgf7 expression in intestinal tissue and failed to increase intestinal growth in Fgf7-deficient mice, placing Fgf7 downstream of GLP-1R signaling in mediating intestinal proliferation.","method":"Conditional knockout mice, genetic epistasis (Glp1r−/−, Fgf7−/−, EGF/IGF1R knockouts), pharmacological GLP-1R agonist treatment","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic epistasis experiments with defined mechanistic pathway","pmids":["25738454"],"is_preprint":false},{"year":2019,"finding":"Systematic investigation using Wnt1-Cre2 and Phox2b-Cre conditional Glp1r knockout mice showed that widespread neural Glp1r loss had no effect on basal food intake or gastric emptying but preserved the glucoregulatory actions of GLP-1R agonists. In contrast, selective Glp1r loss in Phox2b+ neurons impaired glucose homeostasis, gastric emptying, and attenuated weight loss from sustained GLP-1R agonism, identifying Phox2b+ neural cells as required for physiological GLP-1R-mediated gut-brain axis regulation.","method":"Conditional Cre-lox knockout mice, GLP-1R agonist treatment, metabolic phenotyping","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — multiple conditional knockout lines with specific phenotypic readouts","pmids":["31189118"],"is_preprint":false},{"year":2021,"finding":"In vivo fiber photometry and patch-clamp electrophysiology showed that liraglutide and semaglutide directly activate hypothalamic POMC neurons, requiring GLP-1Rs in POMC neurons and a downstream TRPC5-subunit mixed cation channel. They also indirectly upregulate excitatory input to POMC neurons from glutamatergic cells requiring TRPC5. GLP-1RAs inhibit NPY/AgRP neurons indirectly through activation of K-ATP and TRPC5 channels in GABAergic neurons. These temporal effects depend on metabolic (fed/fasted) state.","method":"Neuron-specific transgenic mice, in vivo fiber photometry, patch-clamp electrophysiology, pharmacological channel blockers","journal":"Molecular metabolism","confidence":"High","confidence_rationale":"Tier 2 — in vivo imaging plus electrophysiology plus genetic requirement established in same study","pmids":["34626854"],"is_preprint":false},{"year":2022,"finding":"The gut IEL GLP-1R is not required for enteroendocrine L cell GLP-1 secretion or glucose homeostasis, but is essential for the full effects of GLP-1RAs on gut microbiota composition. The anti-inflammatory actions of GLP-1RAs require the gut IEL GLP-1R to selectively restrain T cell-induced (but not LPS-induced) inflammation, mediated by suppression of IEL effector functions through dampening of proximal TCR signaling in a protein-kinase-A-dependent manner.","method":"IEL-specific conditional Glp1r knockout mice, microbiota analysis, T cell activation assays, PKA inhibitor experiments","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 — conditional knockout with mechanistic PKA pathway identification","pmids":["36027914"],"is_preprint":false},{"year":2022,"finding":"GLP-1 regulates skeletal muscle remodeling to enhance exercise endurance via GLP-1R signaling-mediated phosphorylation of AMPK. AAV-mediated GLP-1 overexpression in skeletal muscle enhanced endurance and promoted glycogen synthesis, glucose uptake, type I fiber proportion, and mitochondrial biogenesis. In vitro, AMPK knockdown reversed the effects of GLP-1R activation on glucose uptake, type I fiber formation, and mitochondrial respiration.","method":"AAV-mediated GLP-1 overexpression, siRNA AMPK knockdown, in vitro exendin-4 treatment, metabolic/functional assays","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 — genetic gain/loss-of-function establishing GLP-1R→AMPK pathway","pmids":["35636559"],"is_preprint":false},{"year":2022,"finding":"GLP-1R agonists activate 15 signaling pathways in 4 cellular compartments. Using biosensors, time-lapse microscopy, and phosphoproteomics, modifications to GLP-1R agonists were shown to greatly influence compound efficacy, potency, and safety in a pathway- and compartment-selective manner. Unique signaling signatures at the level of receptor conformation, functional selectivity, and location bias were associated with functionally distinct cellular outcomes and adverse clinical events.","method":"Biosensor panel (15 pathways/4 compartments), comparative structural analysis, time-lapse microscopy, phosphoproteomics","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 — multi-compartment biosensors plus structural and proteomic analyses across multiple agonists","pmids":["37813859"],"is_preprint":false},{"year":2022,"finding":"Ligand dissociation kinetics (Koff), but not association kinetics (Kon), of GLP-1R peptide agonists were positively correlated with onset of receptor-G protein coupling/conformational change, onset of cAMP production, and duration of cAMP signaling. This established that peptide off-rate is a key determinant of the kinetics of GLP-1R–G protein coupling and downstream signaling.","method":"Kinetic binding assays, BRET-based receptor-G protein conformational biosensors, cAMP time-course measurements across multiple GLP-1R agonists","journal":"Biochemical pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — systematic kinetic analysis across multiple agonists with multiple orthogonal biosensors","pmids":["35300966"],"is_preprint":false},{"year":2023,"finding":"In adult β cell-specific β-arrestin 2 knockout mice, acute GLP-1R agonist (exendin-4, semaglutide, tirzepatide) responses were sex-dimorphically impaired, but improved 6 hours post-injection. β-arrestin 2 KO impaired acute cAMP increases (attributed to enhanced β-arrestin 1 and phosphodiesterase 4 activities) but reduced desensitization, co-occurring with impaired GLP-1R recycling and lysosomal targeting, increased trans-Golgi network signaling, and reduced GLP-1R ubiquitination. Biased agonist exendin-phe1 did not show this phenotype.","method":"Adult β cell-specific conditional knockout mice, islet cAMP measurements, GLP-1R trafficking/ubiquitination assays, in vivo glucose tolerance","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 2 — conditional KO plus multiple orthogonal mechanistic readouts (cAMP, trafficking, ubiquitination) in vivo and in vitro","pmids":["37134170"],"is_preprint":false},{"year":2023,"finding":"Functional profiling of 60 GLP1R variants across four signaling pathways revealed unexpected diversity including defective cell surface expression, complete or pathway-specific gain/loss-of-function. Defective insulin secretion of GLP1R loss-of-function variants was rescued by allosteric GLP1R ligands or high concentrations of exendin-4/semaglutide. Impaired GLP1R cell surface expression (a distinct variant category) was associated with poor glucose control and increased adiposity in UK Biobank analysis.","method":"Functional signaling profiling of 60 variants (4 pathways), INS-1 cell insulin secretion rescue experiments, UK Biobank genetic association","journal":"Nature metabolism","confidence":"High","confidence_rationale":"Tier 2 — systematic functional profiling of multiple variants with mechanistic follow-up rescue experiments","pmids":["37709961"],"is_preprint":false},{"year":2021,"finding":"GRK2 negatively regulates GLP-1R-mediated insulin secretion. GRK2 hemizygous mice showed enhanced early-phase (but not late-phase) insulin release upon oral glucose, GLP-1R agonists, or feeding, correlated with an increased readily releasable pool of insulin granules. Using nanoBRET in β-cells, GLP-1R stimulation promoted GRK2 association with GLP-1R, and GRK2 kinase activity was required for subsequent β-arrestin recruitment.","method":"GRK2+/− mice, isolated islets, nanoBRET in β-cell lines, patch-clamp/capacitance measurements of readily releasable pool","journal":"BMC biology","confidence":"High","confidence_rationale":"Tier 2 — genetic model plus direct receptor-GRK2 interaction assay plus mechanistic signaling readout","pmids":["33658023"],"is_preprint":false},{"year":2021,"finding":"GLP-1R shows increased cell surface levels, internalization, degradation, and endosomal vs. plasma membrane signaling activity compared to GIPR in pancreatic β-cells. GIPR is instead associated with increased plasma membrane recycling, reduced desensitization, and enhanced downstream signal amplification. These differences in spatiotemporal signaling profiles underlie distinct pharmacological responses to each incretin receptor.","method":"Comparative surface expression, trafficking, and multi-compartment signaling analysis between GLP-1R and GIPR in pancreatic β-cells","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — direct comparative functional and trafficking analysis in relevant cell type","pmids":["36774542"],"is_preprint":false},{"year":2020,"finding":"Exendin-4 (GLP-1R agonist) restored airway mucus homeostasis via a GLP1R→PKA→PPARγ-dependent signaling cascade that activated phosphatases PTEN and PTP1B, which in turn inhibited key kinases within both EGFR and STAT6 signaling cascades to restore FOXA2 expression and attenuate mucin production in COPD and CF airway cells.","method":"Pharmacological dissection, signaling pathway analysis, siRNA, human airway cell culture, mouse lung model","journal":"Mucosal immunology","confidence":"Medium","confidence_rationale":"Tier 2 — defined GLP1R→PKA→PPARγ→phosphatase pathway with multiple inhibitor/knockdown experiments","pmids":["32034274"],"is_preprint":false},{"year":2020,"finding":"GABA requires GLP-1R to inhibit TxNIP in pancreatic β-cells; this function was attenuated in GLP-1R−/− islets. In GLP-1R−/− mice, GABA failed to alleviate STZ-induced diabetic syndrome or increase β-cell mass. GABA treatment increased cellular cAMP and β-catenin S675 phosphorylation in WT but not GLP-1R−/− islets, identifying TxNIP as a common downstream target of both GABA and GLP-1 acting through GLP-1R-cAMP-β-catenin signaling.","method":"GLP-1R knockout mice, isolated islets, INS-1 cells, siRNA knockdown of TxNIP, STZ challenge model","journal":"The Journal of endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic knockout epistasis establishing GLP-1R requirement for GABA-mediated β-cell effects","pmids":["32544878"],"is_preprint":false},{"year":2024,"finding":"In vivo two-photon imaging of hindbrain GLP1R neurons showed that area postrema (AP) GLP1R neurons are broadly responsive to both nutritive and aversive stimuli, whereas NTS GLP1R neurons are biased toward nutritive stimuli. Selective activation of NTS GLP1R neurons triggered satiety without aversion, while AP GLP1R neurons triggered aversion with food intake reduction. GLP1R agonists reduce food intake even when the aversion pathway is inhibited, demonstrating dissociable neural circuits for satiety vs. aversion.","method":"In vivo two-photon calcium imaging, chemogenetic activation/inhibition of subregion-specific GLP1R populations, anatomical projection tracing","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — in vivo imaging plus selective circuit manipulation with specific behavioral readouts","pmids":["38987598"],"is_preprint":false},{"year":2023,"finding":"GABAergic LepRb neurons expressing Glp1r (LepRbGlp1r neurons) are required for food intake suppression by both leptin and GLP1R agonists. Ablating Lepr from LepRbGlp1r cells caused hyperphagic obesity. Restoration of Glp1r expression in LepRb neurons on an otherwise Glp1r-null background enabled food intake suppression by liraglutide, demonstrating that GLP1R expression in this specific hypothalamic population is sufficient for the anorectic effect.","method":"Cell-specific knockout/reactivation mouse models, single-nucleus RNA-Seq, chemogenetic and GLP1R agonist challenges","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — cell-specific loss- and gain-of-function with defined behavioral phenotype","pmids":["37581939"],"is_preprint":false},{"year":2024,"finding":"GLP-1R-positive neurons in the lateral septum (LS) mediate anorectic and weight-lowering effects of liraglutide. Chemogenetic activation of LS GLP1R neurons suppressed feeding; targeted knockdown of GLP-1R in LS (but not hypothalamus) attenuated liraglutide's anorectic and weight-lowering effects. Synaptic inactivation of LS GLP1R neurons diminished liraglutide-triggered anorexia, establishing LS GLP1R neurons as a critical circuit node.","method":"Chemogenetics (DREADD), targeted shRNA knockdown, synaptic inactivation, in vivo feeding assays in Glp1r-Cre mice","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — multiple loss/gain-of-function approaches targeting specific brain nucleus","pmids":["39225090"],"is_preprint":false},{"year":2024,"finding":"GLP-1R activation in gut IELs modulates microbiota composition and restrains T cell inflammation via PKA-dependent suppression of TCR signaling (established previously, 2022); separately, dorsolateral septal (dLS) GLP-1R neurons project GABAergically to the lateral hypothalamic area (LHA) and regulate food intake. Chemogenetic inhibition of dLSGLP-1R neurons or the dLSGLP-1R→LHA pathway increased food intake; optogenetic terminal stimulation in LHA rapidly suppressed feeding. Exendin-4 enhanced dLSGLP-1R→LHA GABA release.","method":"Channelrhodopsin-assisted circuit mapping, chemogenetics, electrophysiology, optogenetics, GABA release measurement in Glp1r-ires-Cre mice","journal":"Molecular metabolism","confidence":"High","confidence_rationale":"Tier 2 — multiple circuit dissection methods establishing GABAergic projection mechanism","pmids":["38763494"],"is_preprint":false},{"year":2025,"finding":"GLP-1RAs alleviate AD-related phenotypes by activating CaMKK2-AMPK signaling, which reduces BACE1-mediated cleavage of APP and Aβ generation. GLP-1RAs also increased AMPK activity in microglia, inhibiting neuroinflammation and promoting Aβ phagocytosis. GLP-1 plasma levels were reduced in AD model mice and negatively correlated with Aβ load.","method":"AD transgenic mouse models, GLP-1RA treatment, CaMKK2-AMPK pathway analysis, BACE1 activity assay, microglial phagocytosis assay","journal":"Nature aging","confidence":"Medium","confidence_rationale":"Tier 2 — defined mechanistic pathway in multiple models with relevant pharmacological and genetic tools","pmids":["40394225"],"is_preprint":false},{"year":2021,"finding":"The MafA-target gene PPP1R1A is expressed in pancreatic β-cells and its silencing in INS-1 cells impaired GLP-1-mediated GSIS amplification, PKA-target protein phosphorylation, and mitochondrial coupling efficiency. PPP1R1A mRNA levels positively correlated with GLP-1-mediated GSIS amplification in human islets and were reduced in type 2 diabetic islets, placing PPP1R1A as a required downstream effector of GLP1R-PKA signaling in β-cells.","method":"siRNA knockdown in INS-1 cells, human islet correlational analysis, PKA substrate phosphorylation, mitochondrial coupling assay","journal":"Metabolism: clinical and experimental","confidence":"Medium","confidence_rationale":"Tier 2 — loss-of-function with defined PKA-mediated signaling readout","pmids":["33631146"],"is_preprint":false},{"year":2025,"finding":"VTA dopamine (VTADA) neuron responsiveness was suppressed during food consumption by semaglutide (a GLP-1R agonist). Mice recovered palatable food appetite and VTADA neuron activity during repeated semaglutide treatment, which was reversed by consumption-triggered VTADA neuron inhibition. A neural pathway from peri-locus ceruleus to VTA controls hedonic eating, and GLP-1R agonism opposes VTADA neuron activation to reduce palatable food intake.","method":"Photometry-calibrated optogenetics, in vivo calcium imaging, chemogenetics, semaglutide treatment","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — direct neural circuit imaging plus optogenetic manipulation establishing GLP-1R/VTADA interaction","pmids":["40146831"],"is_preprint":false},{"year":2024,"finding":"GLP-1R activation attenuates pulmonary fibrosis by inhibiting the interaction between NLRP3 inflammasome and PFKFB3-driven glycolysis in lung fibroblasts, reducing lactate production. This prevents lactate-mediated histone H3K9 lactylation at profibrotic gene promoters (demonstrated by ChIP-qPCR) and reduces EMT-related fibrotic gene expression. GLP-1R activation also represses p300-mediated histone lactylation in exogenous lactate-treated fibroblasts.","method":"In vivo silica fibrosis model, in vitro TGF-β1+IL-1β fibroblast activation, ChIP-qPCR for histone lactylation, glycolysis/mitochondrial respiration assays, RNA-seq","journal":"Journal of translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP-qPCR plus metabolic assays establishing GLP-1R→NLRP3/PFKFB3→lactate→histone lactylation mechanism","pmids":["39434134"],"is_preprint":false},{"year":2024,"finding":"Dual activation of GCGR and GLP1R reduces intestinal fibrosis; downregulation of GLP1R led to lactate accumulation and histone H3K9 lactylation driving EMT-mediated fibrosis. Dual agonist peptide 1907B reduced H3K9 lactylation in epithelial cells and ameliorated intestinal fibrosis in vivo, identifying lactate-mediated epigenetic regulation as downstream of GLP1R signaling.","method":"Crohn's disease patient tissue analysis, mouse chronic colitis model, dual agonist treatment, H3K9 lactylation measurement, GLP1R/GCGR knockdown","journal":"Acta pharmaceutica Sinica. B","confidence":"Medium","confidence_rationale":"Tier 2 — receptor knockdown epistasis plus epigenetic mechanistic readout","pmids":["40041889"],"is_preprint":false},{"year":2023,"finding":"GLP1R inhibits endometrial carcinoma progression by activating the cAMP/PKA signaling pathway. GLP1R overexpression increased cAMP content and p-PKA, promoting apoptosis and inhibiting proliferation, migration, and invasion. These effects were blunted by PKA knockdown (siRNA), establishing a direct GLP1R→cAMP→PKA axis mediating tumor suppression.","method":"GLP1R overexpression vector transfection, siRNA-PKA knockdown, cAMP ELISA, Western blotting, cell functional assays, xenograft tumor model","journal":"Journal of clinical laboratory analysis","confidence":"Medium","confidence_rationale":"Tier 2 — gain/loss of function with defined pathway mechanism in vitro and in vivo","pmids":["35989517"],"is_preprint":false},{"year":2024,"finding":"GLP-1R activation (liraglutide) ameliorated oxidized LDL-induced endothelial dysfunction via GLP-1R-dependent downregulation of LOX-1, which reduced NOX4/NF-κB-mediated oxidative stress and ICAM-1/VCAM-1 expression. The protective effects were abrogated by GLP-1R knockdown or LOX-1 overexpression, establishing a GLP-1R→LOX-1 suppression axis in endothelial protection.","method":"LDLR-KO mouse model, HUVECs with GLP-1R knockdown/LOX-1 overexpression, vascular reactivity measurements, ROS assays, Western blotting","journal":"Redox report","confidence":"Medium","confidence_rationale":"Tier 2 — receptor knockdown epistasis plus LOX-1 overexpression to establish mechanism","pmids":["37278349"],"is_preprint":false},{"year":2024,"finding":"GLP-1R signaling is required for maintenance of colonic physiology during energy deprivation; GF GLP-1R KO mice showed 25% mortality associated with enlarged ceca, increased cecal water content, increased colonic apical ion transporter expression, reduced goblet cells, and loss of colonic epithelial integrity. Colonocytes were energy-deprived with increased ER stress, mitochondrial fragmentation, and loss of stemness. Restoring energy (Western diet or microbiota colonization) normalized gut phenotypes and prevented lethality.","method":"GF GLP-1R KO mice, histology, organoid stemness assays, metabolic rescue experiments (diet/microbiota colonization)","journal":"Molecular metabolism","confidence":"Medium","confidence_rationale":"Tier 2 — receptor KO with defined phenotype rescued by multiple interventions","pmids":["38521185"],"is_preprint":false}],"current_model":"GLP1R is a class B1 GPCR that, upon binding GLP-1 or agonists at its extracellular N-terminal domain and transmembrane core, undergoes conformational changes that couple to Gαs (raising cAMP) and PLC (raising intracellular Ca2+), with subsequent PKA, Epac, AKT, AMPK, and ERK/PKC effectors mediating tissue-specific outcomes including β-cell insulin secretion (requiring PPP1R1A/PKA), intestinal growth (via Fgf7), hepatic lipid oxidation (via PPARα/PPARγ), endothelial proliferation (via PKA/PI3K/AKT/eNOS), adipogenesis, cardioprotection (via Epac), gut IEL T cell suppression (via PKA/TCR dampening), and central appetite regulation (via NTS GLP1R neurons driving satiety and AP GLP1R neurons driving aversion); receptor desensitization is regulated by GRK2-mediated β-arrestin 2 recruitment, receptor internalization, lysosomal targeting, and ubiquitination, with ligand off-rate kinetics governing G protein coupling kinetics, and allosteric modulators engaging a distinct transmembrane pocket outside helices V–VII."},"narrative":{"teleology":[{"year":1993,"claim":"Cloning of human GLP1R and heterologous reconstitution established that a single receptor couples to both adenylyl cyclase (cAMP) and PLC (Ca²⁺), resolving whether GLP-1 effects required one or multiple receptors.","evidence":"cDNA cloning from pancreatic islets, stable expression in COS-7/fibroblasts, radioligand binding, cAMP and Ca²⁺ assays","pmids":["8405712","8404634"],"confidence":"High","gaps":["Downstream effectors beyond cAMP and Ca²⁺ not yet mapped","Tissue-specific signaling outputs unknown","Receptor desensitization mechanism not addressed"]},{"year":2008,"claim":"Crystal structures of the GLP1R extracellular domain with exendin-4(9-39) and then GLP-1 defined the peptide-binding interface and showed ligand-specific conformational differences, answering how peptide agonists versus antagonists engage the N-terminal domain.","evidence":"X-ray crystallography at 2.1–2.2 Å resolution plus site-directed mutagenesis","pmids":["18287102","19861722"],"confidence":"High","gaps":["No transmembrane domain structure yet available","Mechanism of signal transduction across the membrane unresolved","Contribution of ECL loops to full-length binding unknown"]},{"year":2014,"claim":"Systematic immunohistochemistry and radioligand binding in primate/human tissues resolved longstanding controversies about GLP1R tissue distribution, confirming expression in β-cells, vascular smooth muscle, sinoatrial node myocytes, and gut but not in liver or thyroid C cells.","evidence":"Validated monoclonal antibody IHC plus ¹²⁵I-GLP-1 in situ binding on human and primate tissues","pmids":["24467746"],"confidence":"High","gaps":["Neural distribution not systematically mapped in this study","Conflicting evidence for hepatocyte GLP-1R from cell-line studies","Cell-type resolution within pancreatic islets limited"]},{"year":2015,"claim":"Genetic epistasis experiments identified Fgf7 as the obligate downstream mediator of GLP-1R-driven intestinal growth, and pharmacological dissection in cardiomyocytes distinguished PKA- versus Epac-dependent branches for cardioprotection, resolving which cAMP effectors mediate specific tissue outcomes.","evidence":"Glp1r⁻/⁻ and Fgf7⁻/⁻ mice for intestinal growth; selective PKA/Epac activators in cardiomyocytes for anti-apoptotic readouts","pmids":["25738454","25719403"],"confidence":"High","gaps":["Fgf7-producing cell type not identified","Whether Epac arm operates in β-cells independently of PKA not settled","Mechanism linking GLP-1R to Fgf7 transcription unknown"]},{"year":2015,"claim":"Identification of exendin-P5 as a G-protein-biased agonist with reduced β-arrestin recruitment but enhanced metabolic efficacy in diabetic mice demonstrated that GLP-1R signaling bias translates into distinct in vivo therapeutic profiles.","evidence":"Autocrine peptide library screen, cAMP versus β-arrestin assays, T2DM mouse models","pmids":["26621478"],"confidence":"Medium","gaps":["Structural basis for bias not yet determined at this time","Long-term safety of biased agonism unknown","Relative contribution of β-arrestin scaffolding versus desensitization unclear"]},{"year":2017,"claim":"Crystal structures of the GLP1R transmembrane domain with negative allosteric modulators revealed an allosteric pocket outside helices V–VII, establishing that receptor activation can be modulated from a site distinct from the orthosteric peptide-binding cavity.","evidence":"X-ray crystallography (2.7–3.0 Å) of TMD with PF-06372222 and NNC0640, mutagenesis","pmids":["28514449"],"confidence":"High","gaps":["How positive allosteric modulators engage the same region to facilitate G-protein coupling not fully resolved","No full-length inactive-state structure available","Dynamics of transition from inactive to active conformation unknown"]},{"year":2018,"claim":"Cryo-EM of GLP-1R–Gs complexes with biased versus unbiased agonists showed that agonist-specific conformational differences at the extracellular face propagate through the transmembrane domain to alter the Gαs engagement angle, providing a structural explanation for signaling bias.","evidence":"Phase-plate cryo-EM at 3.3 Å comparing exendin-P5 and GLP-1 bound complexes","pmids":["29466332"],"confidence":"High","gaps":["β-arrestin-bound receptor structure absent","Dynamics of conformational propagation not captured in static structures","Limited to Gs-coupled state"]},{"year":2020,"claim":"Cryo-EM structures of non-peptide agonist-bound GLP-1R–Gs complexes showed that small-molecule agonists can closely mimic peptide binding or adopt entirely different binding modes with distinct receptor conformations, establishing molecular rules for non-peptide agonist design.","evidence":"Cryo-EM structures of PF-06882961 and CHU-128 bound complexes with pharmacological profiling","pmids":["33027691"],"confidence":"High","gaps":["Clinical translation of structural insights to oral drug design not demonstrated","Conformational dynamics of non-peptide binding not explored by MD"]},{"year":2021,"claim":"GRK2 was identified as the kinase responsible for GLP-1R phosphorylation that enables β-arrestin recruitment and desensitization, with GRK2 hemizygosity enhancing early-phase insulin secretion by increasing the readily releasable granule pool.","evidence":"GRK2⁺/⁻ mice, nanoBRET for GLP-1R–GRK2 interaction, islet capacitance measurements","pmids":["33658023"],"confidence":"High","gaps":["Specific GRK2 phosphorylation sites on GLP-1R not mapped","Role of other GRKs not excluded","Whether GRK2 inhibition improves GLP-1RA efficacy in vivo not tested"]},{"year":2021,"claim":"Electrophysiology and in vivo photometry in hypothalamic circuits showed GLP-1RAs directly activate POMC neurons via TRPC5 channels and indirectly inhibit AgRP neurons, establishing the ion channel effectors and circuit logic underlying central appetite suppression.","evidence":"Neuron-specific transgenics, fiber photometry, patch-clamp, pharmacological channel blockers","pmids":["34626854"],"confidence":"High","gaps":["Whether peripheral versus central GLP-1R pools are differentially engaged by long-acting agonists unclear","Role of TRPC5 in human hypothalamic GLP-1R signaling unconfirmed","Interaction with other nutrient-sensing circuits not mapped"]},{"year":2022,"claim":"Comprehensive biosensor and phosphoproteomic analysis across 15 signaling pathways in 4 cellular compartments revealed that GLP-1R agonist modifications produce pathway- and compartment-selective signaling signatures, and that ligand off-rate governs G-protein coupling kinetics and cAMP duration.","evidence":"Multi-pathway biosensor panel, time-lapse microscopy, phosphoproteomics, kinetic BRET across multiple agonists","pmids":["37813859","35300966"],"confidence":"High","gaps":["How compartment-specific signals are decoded into cellular outcomes remains unclear","In vivo relevance of compartment bias not tested","Contribution of receptor recycling versus degradation to sustained signaling not quantified"]},{"year":2022,"claim":"GLP-1R on gut intraepithelial lymphocytes was shown to suppress T-cell-mediated inflammation via PKA-dependent dampening of proximal TCR signaling and to shape microbiota composition, identifying an immune-regulatory function independent of glucose homeostasis.","evidence":"IEL-specific conditional Glp1r knockout mice, T-cell activation assays, microbiota sequencing, PKA inhibitor experiments","pmids":["36027914"],"confidence":"High","gaps":["Which specific microbiota taxa are regulated and through what mechanism not defined","Relevance of IEL GLP-1R to human IBD not established","Downstream PKA substrates in IELs not identified"]},{"year":2023,"claim":"β-arrestin 2 conditional knockout in adult β-cells revealed that β-arrestin 2 is required for acute cAMP production yet simultaneously promotes desensitization, lysosomal targeting, and ubiquitination of GLP-1R, resolving the paradoxical dual role of β-arrestin 2 in GLP-1R signaling.","evidence":"Adult β-cell-specific β-arrestin 2 KO mice, islet cAMP measurements, receptor trafficking and ubiquitination assays","pmids":["37134170"],"confidence":"High","gaps":["Mechanism by which β-arrestin 2 loss upregulates β-arrestin 1 and PDE4 not elucidated","Sex dimorphism in β-arrestin effects not mechanistically explained","Whether trans-Golgi signaling is therapeutically exploitable unknown"]},{"year":2023,"claim":"Systematic functional profiling of 60 GLP1R coding variants established that impaired cell-surface expression is a genetically distinct mechanism from signaling loss-of-function, with the former associated with poor glycemic control in humans and rescuable by allosteric ligands.","evidence":"Multi-pathway signaling profiling of 60 variants, INS-1 cell insulin secretion rescue, UK Biobank association","pmids":["37709961"],"confidence":"High","gaps":["Whether allosteric rescue translates to improved clinical outcomes untested","Structural basis for expression-defective variants not determined","Gain-of-function variant physiology largely unexplored"]},{"year":2024,"claim":"In vivo imaging and circuit manipulation in the hindbrain demonstrated that GLP1R neurons in the NTS and area postrema drive dissociable satiety and aversion responses respectively, while lateral septum GLP1R neurons suppress feeding via GABAergic projections to the lateral hypothalamus, mapping the circuit architecture for GLP-1RA-induced appetite control.","evidence":"Two-photon calcium imaging, chemogenetic/optogenetic manipulation, projection tracing in Glp1r-Cre mice","pmids":["38987598","39225090","38763494"],"confidence":"High","gaps":["How peripheral GLP-1RA access reaches specific brain nuclei not resolved","Human neuroanatomical equivalence of these circuits not confirmed","Interaction between hindbrain and forebrain GLP1R populations during chronic treatment unclear"]},{"year":2024,"claim":"Ligand-free cryo-EM structures showed that Gs protein alone partially activates GLP-1R by opening the intracellular cavity and rewiring the extracellular pocket, revealing a pre-engagement activation intermediate that facilitates peptide binding.","evidence":"Cryo-EM of ligand-free GLP-1R–Gs complex, comparative structural analysis with GCGR and GIPR","pmids":["38346960"],"confidence":"High","gaps":["Whether this intermediate is populated under physiological ligand concentrations unknown","Kinetic lifetime of the ligand-free complex not measured","Implications for constitutive activity not explored"]},{"year":null,"claim":"Key unresolved questions include the structural basis of β-arrestin-bound GLP-1R, how compartment-specific signaling is decoded into distinct cellular outcomes in vivo, how peripheral versus central GLP-1R pools are differentially engaged by long-acting agonists, and whether allosteric rescue of expression-defective variants can improve clinical outcomes.","evidence":"","pmids":[],"confidence":"High","gaps":["No β-arrestin-bound GLP-1R structure","Compartment-specific signaling decoded in vivo not demonstrated","Mechanism by which GLP-1RAs cross the blood–brain barrier to engage specific neuronal populations unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,3,4,5,6,7,8,24]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,26]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,11,25,26,28]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[25,28]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,3,4,8,16,23,24,25,27]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[12,14,22]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[21]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[20,31,32,33,34,37]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[26,35,40]}],"complexes":["GLP-1R–Gαs–Gβγ heterotrimer"],"partners":["GNA S","GRK2","ARRB2","PPP1R1A","TRPC5","FGF7"],"other_free_text":[]},"mechanistic_narrative":"GLP1R is a class B1 G-protein-coupled receptor that transduces signals from GLP-1 and related peptide or non-peptide agonists through Gαs-coupled cAMP/PKA and Epac pathways, as well as PLC-mediated Ca²⁺ mobilization, ERK/PKC, PI3K/AKT, and AMPK cascades to control insulin secretion, appetite, intestinal growth, lipid metabolism, and inflammation [PMID:8405712, PMID:25719403, PMID:25738454, PMID:36027914]. Structural studies of the extracellular domain, transmembrane core, and full-length receptor–Gs complexes have defined a two-step peptide-binding mechanism—N-terminal domain capture followed by transmembrane insertion—and revealed that agonist-specific contact dynamics at the peptide N-terminus allosterically tune G-protein coupling kinetics, providing a molecular basis for biased signaling [PMID:18287102, PMID:29466332, PMID:35013280, PMID:28514449]. Receptor desensitization is governed by GRK2-dependent β-arrestin 2 recruitment that controls internalization, lysosomal targeting, and ubiquitination; loss of β-arrestin 2 impairs acute cAMP generation yet reduces desensitization, and ligand off-rate is a principal determinant of G-protein coupling onset and cAMP duration [PMID:33658023, PMID:37134170, PMID:35300966]. In the CNS, GLP1R delineates dissociable neural circuits—NTS neurons driving satiety, area postrema neurons driving aversion, lateral septum neurons suppressing feeding via GABAergic projections to the lateral hypothalamus, and LepRb⁺ hypothalamic neurons required for both leptin- and GLP-1RA-mediated anorexia [PMID:38987598, PMID:39225090, PMID:37581939]."},"prefetch_data":{"uniprot":{"accession":"P43220","full_name":"Glucagon-like peptide 1 receptor","aliases":[],"length_aa":463,"mass_kda":53.0,"function":"G-protein coupled receptor for glucagon-like peptide 1 (GLP-1) (PubMed:19861722, PubMed:26308095, PubMed:27196125, PubMed:28514449, PubMed:7517895, PubMed:8216285, PubMed:8405712). Ligand binding triggers activation of a signaling cascade that leads to the activation of adenylyl cyclase and increased intracellular cAMP levels (PubMed:19861722, PubMed:26308095, PubMed:27196125, PubMed:28514449, PubMed:7517895, PubMed:8216285, PubMed:8405712). Plays a role in regulating insulin secretion in response to GLP-1 (By similarity)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P43220/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GLP1R","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GLP1R","total_profiled":1310},"omim":[{"mim_id":"603659","title":"GLUCAGON-LIKE PEPTIDE 2 RECEPTOR; GLP2R","url":"https://www.omim.org/entry/603659"},{"mim_id":"602228","title":"TRANSCRIPTION FACTOR 7-LIKE 2; TCF7L2","url":"https://www.omim.org/entry/602228"},{"mim_id":"147559","title":"INTEGRIN, BETA-7; ITGB7","url":"https://www.omim.org/entry/147559"},{"mim_id":"138032","title":"GLUCAGON-LIKE PEPTIDE 1 RECEPTOR; GLP1R","url":"https://www.omim.org/entry/138032"},{"mim_id":"138030","title":"GLUCAGON; GCG","url":"https://www.omim.org/entry/138030"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nuclear bodies","reliability":"Approved"},{"location":"Plasma membrane","reliability":"Approved"},{"location":"Golgi apparatus","reliability":"Additional"},{"location":"Centrosome","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"heart muscle","ntpm":2.6},{"tissue":"pancreas","ntpm":8.6}],"url":"https://www.proteinatlas.org/search/GLP1R"},"hgnc":{"alias_symbol":["GLP-1R"],"prev_symbol":[]},"alphafold":{"accession":"P43220","domains":[{"cath_id":"4.10.1240.10","chopping":"33-123","consensus_level":"high","plddt":91.3262,"start":33,"end":123},{"cath_id":"1.20.1070.10","chopping":"136-416","consensus_level":"high","plddt":87.7164,"start":136,"end":416}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P43220","model_url":"https://alphafold.ebi.ac.uk/files/AF-P43220-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P43220-F1-predicted_aligned_error_v6.png","plddt_mean":81.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GLP1R","jax_strain_url":"https://www.jax.org/strain/search?query=GLP1R"},"sequence":{"accession":"P43220","fasta_url":"https://rest.uniprot.org/uniprotkb/P43220.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P43220/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P43220"}},"corpus_meta":[{"pmid":"30567927","id":"PMC_30567927","title":"Anti-obesity 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the GLP-1 binding site, while CHU-128 adopts a unique binding mode with a more open extracellular receptor conformation; structural differences involving water-mediated hydrogen bond networks explain how PF 06882961 but not CHU-128 mimics GLP-1 pharmacology.\",\n      \"method\": \"High-resolution cryo-EM structural determination combined with pharmacological assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures with functional validation across multiple compounds\",\n      \"pmids\": [\"33027691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM and molecular dynamics simulations of GLP-1R bound to four peptide agonists (GLP-1, oxyntomodulin, exendin-4, exendin-P5) reveal that distinctions in peptide N-terminal interactions and dynamics within the transmembrane domain are reciprocally associated with differences in allosteric coupling to G proteins; transient interactions at the base of the binding cavity correlate with enhanced G protein activation kinetics.\",\n      \"method\": \"Cryo-EM, molecular dynamics simulations, receptor mutagenesis, and pharmacological assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM plus mutagenesis and MD simulations across multiple agonists\",\n      \"pmids\": [\"35013280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of semaglutide- and taspoglutide-bound GLP-1R-Gs complexes reveal similar peptide-receptor interactions to GLP-1 but distinct motions within the receptor and bound peptides, providing molecular determinants of differential GLP-1R peptide engagement.\",\n      \"method\": \"Cryo-EM and 3D variability analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM structures with comparative structural analysis\",\n      \"pmids\": [\"34260945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM structures of Boc5 and its analog WB4-24 in complex with human GLP-1R and Gs protein show these non-peptidic agonists bind to the extracellular domain, ECL2, and TM helices 1, 2, 3, and 7, with one arm inserted into the orthosteric binding pocket overlapping with GLP-1 residues A8-D15, creating a distinct conformation that confers both peptidomimetic agonism and biased signaling.\",\n      \"method\": \"Cryo-EM structural determination with functional pharmacological assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures with functional validation and mutagenesis context\",\n      \"pmids\": [\"35561211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of GLP-1R and GCGR in complex with Gs protein and dual agonists (peptide 15, MEDI0382/cotadutide, SAR425899) identify key residues for dual agonism; distinct N-terminal side chain orientations determine receptor selectivity, and ECL1 conformational changes are important for dual agonism at GLP-1R versus GCGR.\",\n      \"method\": \"High-resolution cryo-EM structural determination with pharmacological data\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM with multiple compounds and supported by pharmacological data\",\n      \"pmids\": [\"37549266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structures of ligand-free GLP-1R, GCGR, and GIPR in complex with Gs proteins show that Gs protein alone directly opens the intracellular binding cavity and rewires the extracellular orthosteric pocket; the extracellular portion of ligand-free GLP-1R adopts a conformation close to the active state, revealing that intracellular half of the transmembrane domain is mobilized by Gs, which rearranges the extracellular half to facilitate peptide N-terminus entry.\",\n      \"method\": \"Cryo-EM structural determination of ligand-free receptor-G protein complexes\",\n      \"journal\": \"Cell discovery\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures revealing novel ligand-independent activation mechanism\",\n      \"pmids\": [\"38346960\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Selective reduction of Glp1r expression within Phox2b-Cre neurons impaired glucose homeostasis and gastric emptying and attenuated weight loss achieved with sustained GLP-1R agonism, while widespread loss of neural Glp1r in Wnt1-Cre2 mice preserved glucoregulatory actions of GLP-1R agonists, identifying Phox2b+ neuronal GLP-1R as mediators of physiological gut-brain axis regulation.\",\n      \"method\": \"Conditional knockout mice (Cre-lox), glucose tolerance tests, gastric emptying measurements\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean cell-type-specific KO with defined metabolic phenotypes\",\n      \"pmids\": [\"31189118\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In vivo two-photon imaging showed most hindbrain GLP1R neurons are tuned to either nutritive or aversive stimuli but not both; area postrema (AP) GLP1R neurons are broadly responsive while NTS GLP1R neurons are biased towards nutritive stimuli; separate activation of NTS GLP1R neurons triggers satiety without aversion while AP GLP1R neurons trigger aversion with food intake reduction; NTS and AP GLP1R neurons project to different downstream brain regions.\",\n      \"method\": \"In vivo two-photon imaging, optogenetic/chemogenetic manipulation, circuit tracing\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (imaging, optogenetics, chemogenetics, anatomy) in single study\",\n      \"pmids\": [\"38987598\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Gut intraepithelial lymphocyte (IEL) GLP-1R is not required for GLP-1 secretion or glucose homeostasis but is essential for the full effects of GLP-1R agonists on gut microbiota; gut IEL GLP-1R is required for anti-inflammatory actions of GLP-1R agonists to restrain T cell-induced (but not LPS-induced) inflammation through suppression of IEL effector functions via dampening of proximal T cell receptor signaling in a protein-kinase-A-dependent manner.\",\n      \"method\": \"Conditional knockout mice, immune cell functional assays, PKA pathway analysis\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type specific KO with multiple orthogonal mechanistic readouts\",\n      \"pmids\": [\"36027914\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GLP-1R-positive neurons in the lateral septum (LSGLP-1R) are robustly activated by liraglutide; chemogenetic activation suppresses feeding; targeted knockdown of GLP-1R in the LS (but not hypothalamus) substantially attenuates liraglutide's ability to inhibit feeding and lower body weight; LSGLP-1R neuron activity decreases during naturalistic feeding.\",\n      \"method\": \"Chemogenetics (DREADD), viral knockdown, in vivo fiber photometry, pharmacology\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods identifying specific neuronal population required for drug efficacy\",\n      \"pmids\": [\"39225090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GABAergic hypothalamic LepRb neurons co-expressing Glp1r (LepRbGlp1r neurons) control food intake suppression by leptin and GLP-1R agonists; ablating Lepr from LepRbGlp1r cells caused hyperphagic obesity; restoration of Glp1r expression in LepRbGlp1r neurons in Glp1r-null mice enabled food intake suppression by liraglutide.\",\n      \"method\": \"Single-nucleus RNA-seq, conditional KO mice, Cre-dependent Glp1r rescue, pharmacology\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic rescue and KO with defined cellular and behavioral phenotypes\",\n      \"pmids\": [\"37581939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Liraglutide and semaglutide directly activate POMC neurons and increase excitatory tone in a time-dependent manner requiring GLP-1Rs in POMC neurons and downstream TRPC5 channel subunits; GLP-1R agonists indirectly inhibit NPY/AgRP neurons through activation of K-ATP and TRPC5 channels in GABAergic neurons; temporal activation pattern depends on nutritional state.\",\n      \"method\": \"Neuron-specific transgenic mice, patch-clamp electrophysiology, in vivo fiber photometry\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — electrophysiology with transgenic models and photometry, mechanistic pathway identified\",\n      \"pmids\": [\"34626854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"β-arrestin 2 knockout in adult β-cells produced divergent acute vs. prolonged GLP-1R agonist responses: impaired acute cAMP increases attributed to enhanced β-arrestin 1 and phosphodiesterase 4 activities; reduced desensitization co-occurred with impaired GLP-1R recycling and lysosomal targeting, increased trans-Golgi network signaling, and reduced GLP-1R ubiquitination; GRK2 promotes β-arrestin 2 recruitment to GLP-1R.\",\n      \"method\": \"Adult β-cell-specific conditional KO, in vivo glucose tolerance, islet cAMP assays, nanoBRET, trafficking assays\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with multiple mechanistic readouts and orthogonal methods\",\n      \"pmids\": [\"37134170\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GLP-1R shows increased internalization, degradation, and endosomal vs. plasma membrane signaling activity compared to GIPR in pancreatic beta cells; GIPR has increased plasma membrane recycling, reduced desensitization, and enhanced downstream signal amplification; these differences underlie distinct pharmacological responses of the two incretin receptors.\",\n      \"method\": \"Direct comparison of surface expression, trafficking, and spatiotemporal signaling in pancreatic beta cells\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct comparative assays in beta cells, single lab study\",\n      \"pmids\": [\"36774542\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GRK2 promotes β-arrestin 2 recruitment to GLP-1R following stimulation (demonstrated by nanoBRET); GRK2 protein and kinase activity are required for β-arrestin recruitment; GRK2 hemizygous mice show enhanced early phase insulin release in response to GLP-1R agonists due to increased size or priming efficacy of the readily releasable pool of insulin granules.\",\n      \"method\": \"GRK2+/- mice, isolated islets, nanoBRET assay in β-cell lines, insulin secretion assays\",\n      \"journal\": \"BMC biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo KO + nanoBRET + islet assays providing mechanistic pathway\",\n      \"pmids\": [\"33658023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Functional profiling of 60 GLP1R variants across four signaling pathways revealed unexpected diversity of phenotypes including defective cell surface expression, and pathway-specific gain or loss of function; defective insulin secretion from GLP1R LoF variants is rescued by allosteric GLP1R ligands or high concentrations of exendin-4/semaglutide; genetic association in 200,000 UK Biobank participants showed impaired GLP1R cell surface expression contributes to poor glucose control and increased adiposity.\",\n      \"method\": \"Functional profiling of variants in signaling assays, INS-1 cell rescue experiments, population genetics\",\n      \"journal\": \"Nature metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — systematic functional variant analysis with multiple pathways and large population validation\",\n      \"pmids\": [\"37709961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"GLP-1R agonist peptide dissociation kinetics (Koff), but not association kinetics (Kon), are positively correlated with the onset of receptor-G protein coupling/conformational change, onset of cAMP production, and duration of cAMP signaling, establishing that ligand-receptor unbinding kinetics drive downstream signaling dynamics.\",\n      \"method\": \"Kinetic binding assays, BRET-based G protein engagement assays, cAMP production assays\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple pharmacological assays correlating binding kinetics to signaling, single lab\",\n      \"pmids\": [\"35300966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GLP-1R signaling in dorsolateral septum (dLS) GABAergic neurons projecting to the lateral hypothalamic area (LHA) regulates feeding behavior; dLSGLP-1R→LHA projections are GABAergic; chemogenetic inhibition of this pathway promotes food intake; GLP-1R agonist exendin-4 enhances dLSGLP-1R→LHA GABA release.\",\n      \"method\": \"Chemogenetics, channelrhodopsin-assisted circuit mapping, optogenetics, electrophysiology\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal circuit-mapping methods identifying GABAergic projection mechanism\",\n      \"pmids\": [\"38763494\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"VTA dopamine neuron responsiveness was suppressed during food consumption by semaglutide (GLP-1R agonist), demonstrating that GLP-1R activation opposes the hedonic eating pathway driven by VTADA neurons; VTADA neuron-triggered palatability responses sustain hedonic eating and oppose GLP-1R-mediated appetite reduction.\",\n      \"method\": \"Photometry-calibrated optogenetics, in vivo neural activity imaging, pharmacology\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — photometry-calibrated optogenetics with mechanistic circuit identification\",\n      \"pmids\": [\"40146831\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Exendin-4 increases Fgf7 expression in intestinal tissue and fails to increase intestinal growth in mice lacking Fgf7, placing Fgf7 downstream of GLP-1R signaling as a required mediator of GLP-1R-stimulated mucosal intestinal growth.\",\n      \"method\": \"Glp1r knockout mice, Fgf7 knockout mice, genetic epistasis with exendin-4 treatment\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with two independent KO models identifying Fgf7 as required downstream mediator\",\n      \"pmids\": [\"25738454\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GLP-1R signaling activates ERK, PKC, and AKT pathways to promote pre-adipocyte proliferation and inhibit apoptosis; loss of GLP-1R reduces adipogenesis through induction of apoptosis in pre-adipocytes via inhibition of these pathways.\",\n      \"method\": \"In vitro and in vivo adipogenesis assays with GLP-1R loss-of-function\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — pathway identification with KO phenotype, limited mechanistic depth\",\n      \"pmids\": [\"22207759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"A GLP-1R G-protein-biased agonist (P5) promotes G-protein signaling comparable to GLP-1 but with significantly reduced β-arrestin response, demonstrating that β-arrestin pathway engagement is separable from G-protein-mediated effects and that G-protein-biased activation produces distinct metabolic outcomes including increased adipogenesis and reduced hepatic steatosis.\",\n      \"method\": \"High-throughput autocrine-based peptide library screening, cAMP/β-arrestin assays, diabetic mouse models\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple signaling assays plus in vivo models, single lab\",\n      \"pmids\": [\"26621478\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Stimulation of GLP-1R with exendin-4 attenuates H2O2-induced ROS production and increases antioxidant enzyme synthesis in cardiomyocytes via an Epac-dependent mechanism; antiapoptotic effects of GLP-1R activation are mediated through both PKA- and Epac-dependent pathways.\",\n      \"method\": \"Pharmacological inhibitors of Epac and PKA in cardiomyocyte cell lines with functional assays\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — pharmacological dissection of signaling pathways in cell lines, single lab\",\n      \"pmids\": [\"25719403\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Exendin-4 (GLP-1R agonist) restores FOXA2 expression and mucus homeostasis by activating GLP1R-PKA-PPARγ-dependent phosphatases PTEN and PTP1B, which inhibit key kinases within both EGFR-AKT/ERK1/2 and STAT6 signaling cascades.\",\n      \"method\": \"Cell-based assays with pharmacological inhibitors and agonists, in vitro and mouse lung models\",\n      \"journal\": \"Mucosal immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — pathway dissection in cell lines and in vivo with specific mechanistic readouts\",\n      \"pmids\": [\"32034274\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"GLP1R mRNA is expressed in a subset of GABAergic neurons in the anterior lateral bed nucleus of the stria terminalis (alBST); knockdown of GLP1R mRNA in alBST reduces anxiety-like behavior, light-enhanced acoustic startle, and stress-induced hypophagia, but prolongs stress-induced elevation of plasma corticosterone.\",\n      \"method\": \"In situ hybridization, AAV-shRNA knockdown, behavioral assays, plasma hormone measurements\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — targeted knockdown with defined behavioral phenotypes and cell-type identification\",\n      \"pmids\": [\"30683681\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cannabinoid 1 receptor (CB1) is a negative modulator of GLP-1R-mediated insulin secretion; cannabinoids reduce GLP-1R-mediated intracellular cAMP accumulation and subsequent insulin secretion in mouse insulinoma cells and human islets; CB1 blockade enhances GLP-1R-mediated insulin secretion.\",\n      \"method\": \"CB1 knockout mice, mouse insulinoma cells, human islets, cAMP and insulin secretion assays\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mice plus human islet validation identifying regulatory interaction\",\n      \"pmids\": [\"26724516\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"GABA shares a common downstream target (TxNIP inhibition) with GLP-1 in pancreatic β-cells; GABA's ability to inhibit TxNIP is attenuated in GLP-1R-/- islets; GABA's protective effects in STZ-challenged mice (increased β-cell mass) are absent in GLP-1R-/- mice; GABA treatment increases cellular cAMP and β-catenin S675 phosphorylation in WT but not GLP-1R-/- islets, identifying GLP-1R-cAMP-β-catenin as a mediator of GABA's effects.\",\n      \"method\": \"GLP-1R knockout mice, STZ challenge model, INS-1 cell line knockdown, signaling assays\",\n      \"journal\": \"The Journal of endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mice plus cell line epistasis identifying shared signaling pathway\",\n      \"pmids\": [\"32544878\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PPP1R1A (a MafA target gene) is required for GLP-1R-mediated amplification of glucose-stimulated insulin secretion in β-cells; PPP1R1A silencing in INS-1 β-cells impairs GSIS amplification and PKA-target protein phosphorylation, placing PPP1R1A downstream of GLP-1R/PKA signaling.\",\n      \"method\": \"siRNA knockdown in INS-1 cells, PKA substrate phosphorylation assays, human islet correlational data\",\n      \"journal\": \"Metabolism: clinical and experimental\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — knockdown with defined signaling readouts, human islet correlation\",\n      \"pmids\": [\"33631146\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R activation inhibits pulmonary fibrosis by disrupting the interaction between NLRP3 inflammasome and PFKFB3-driven glycolysis in lung fibroblasts, preventing lactate production and subsequent lactate-mediated histone lactylation at promoters of pro-fibrotic genes; GLP-1R activation also represses p300-mediated histone lactylation and protects mitochondria from increased oxidative phosphorylation.\",\n      \"method\": \"Mouse silica-induced fibrosis model, primary lung fibroblasts, ChIP-qPCR, RNA-seq, metabolic assays\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in vitro and in vivo, novel epigenetic mechanism identified\",\n      \"pmids\": [\"39434134\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"A GIPR-Ab/GLP-1 peptide-antibody conjugate requires both CNS GIPR and CNS GLP-1R for maximal weight loss; the conjugate is detected in circumventricular organs and activates c-FOS in downstream neural substrates involved in appetite regulation, demonstrating direct CNS action.\",\n      \"method\": \"CNS-specific conditional KO mice, IHC for c-FOS, tissue distribution, gene expression profiling\",\n      \"journal\": \"Nature metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO genetic evidence with imaging of receptor engagement in brain\",\n      \"pmids\": [\"40301582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"GLP-1 regulates skeletal muscle remodeling and exercise endurance via GLP-1R signaling-mediated phosphorylation of AMPK; knockdown of AMPK reverses the effects of GLP-1R activation on glucose uptake, type I fiber formation, and mitochondrial respiration in vitro.\",\n      \"method\": \"AAV-mediated GLP-1 overexpression in skeletal muscle, in vitro siRNA knockdown, metabolic assays\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — in vivo overexpression plus in vitro mechanistic knockdown, single lab\",\n      \"pmids\": [\"35636559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R activation with semaglutide increases CaMKK2-AMPK signaling, reducing BACE1-mediated cleavage of amyloid precursor protein (APP) and Aβ generation; GLP-1RAs also increase AMPK activity in microglia, inhibiting neuroinflammation and promoting Aβ phagocytosis in AD model mice.\",\n      \"method\": \"AD transgenic mouse models, signaling assays, BACE1 cleavage assays, microglial functional assays\",\n      \"journal\": \"Nature aging\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple mechanistic readouts in transgenic models, single lab\",\n      \"pmids\": [\"40394225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Genetic variation in Glp1r expression correlates with allelic imbalance in stomach and with differences in gastric emptying rate; congenic mice with CAST Glp1r allele show accelerated gastric emptying and impaired response to the GLP-1R antagonist exendin-(9-39), establishing that Glp1r expression level influences gastric emptying.\",\n      \"method\": \"Congenic mouse strains, gene expression profiling, allelic imbalance analysis, gastric emptying measurements\",\n      \"journal\": \"American journal of physiology. Regulatory, integrative and comparative physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic model with functional pharmacological validation, single lab\",\n      \"pmids\": [\"18077508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R signaling in the colon is required for maintenance of colonic physiology under energy deprivation; GLP-1R KO germ-free mice showed enlarged ceca, reduced goblet cell number, loss of colonic epithelial integrity, colonocyte energy deprivation with increased ER stress and mitochondrial fragmentation, and loss of stemness; restoring colonic energy (Western diet or microbiota colonization) normalized phenotypes.\",\n      \"method\": \"GLP-1R KO mice rederived as germ-free, histology, intestinal organoids, rescue experiments\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with germ-free model and rescue experiments, multiple readouts\",\n      \"pmids\": [\"38521185\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"GLP-1R agonist supramammillary nucleus (SuM) microinjection causes potent anxiety-like behavior in rats; reduction of GLP-1R expression in SuM by AAV-shRNA causes anxiolytic response (only in females); GLP-1R mRNA is expressed throughout the SuM, identifying SuM GLP-1R as sufficient for anxiogenesis in both sexes but necessary only in females.\",\n      \"method\": \"Intra-SuM microinjection of agonist, AAV-shRNA knockdown, behavioral assays (open field, elevated plus maze)\",\n      \"journal\": \"Psychoneuroendocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — site-specific pharmacology plus targeted knockdown with behavioral phenotypes\",\n      \"pmids\": [\"32563174\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"LepRbGlp1r neurons in the DMH of nonhuman primates exist (establishing translational relevance); reactivation of Glp1r expression in Lepr neurons on Glp1r-null background (Glp1rLeprRe mice) was sufficient to permit food intake suppression and body weight loss by a GLP-1R/LepR dual agonist; ablating Lepr in Glp1r-expressing neurons abrogated dual agonist's suppression of food intake.\",\n      \"method\": \"Conditional KO and Cre-dependent rescue mice, NHP anatomical analysis, pharmacological challenge\",\n      \"journal\": \"Science translational medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic rescue and KO with defined phenotypes, cross-species validation\",\n      \"pmids\": [\"39630884\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GLP-1R activation with liraglutide protects cardiomyocytes from necroptosis through GLP-1R/PI3K/Akt pathway; both GLP-1R inhibitor exendin(9-39) and PI3K inhibitor LY294002 abrogated liraglutide's protective effects in H9c2 cells, and liraglutide reduced necroptosis markers (RIPK3, p-RIPK3, p-MLKL) in myocardial I/R injury models.\",\n      \"method\": \"H9c2 cell H/R model, rat MI/RI model, pharmacological inhibitors, western blot for necroptosis markers\",\n      \"journal\": \"Cardiovascular toxicology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — pharmacological epistasis in cell and animal models, single lab\",\n      \"pmids\": [\"36934206\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GLP-1R is a class B1 GPCR that couples primarily to Gs/cAMP/PKA and Epac signaling upon binding GLP-1 or agonist ligands; structural studies define overlapping and distinct binding modes for peptide and non-peptide agonists within the transmembrane and extracellular domains, with ligand N-terminal interactions and dissociation kinetics determining allosteric coupling to G proteins and signaling efficacy; receptor desensitization is mediated by GRK2-dependent β-arrestin 2 recruitment, which controls recycling, lysosomal targeting, and ubiquitination; in the brain, discrete GLP-1R neuronal populations (NTS, AP, lateral septum, LepRb/Glp1r hypothalamic neurons, Phox2b+ neurons) mediate separable physiological effects including satiety, aversion, and glucose homeostasis via defined circuit projections; peripheral GLP-1R signaling regulates insulin secretion, adipogenesis, intestinal growth (via Fgf7), colonic epithelial integrity, and immune modulation (gut IEL GLP-1R controls T cell-mediated inflammation via PKA-dependent suppression of TCR signaling).\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1993,\n      \"finding\": \"Cloning and functional expression of the human GLP-1 receptor from pancreatic islet cDNA revealed a 463-amino-acid protein with 90% homology to rat GLP-1R. When expressed in COS-7 cells, the receptor conferred high-affinity GLP-1(7-37) binding (Kd ~0.5 nM), activated adenylyl cyclase (cAMP production, EC50 ~93 pM), and also activated phospholipase C (PLC), increasing intracellular Ca2+ from intracellular pools, demonstrating coupling to multiple signaling pathways. Exendin-4 was identified as an agonist and exendin-(9-39) as an antagonist.\",\n      \"method\": \"cDNA cloning, stable transfection in fibroblasts/COS-7 cells, radioligand binding, cAMP assay, Ca2+ imaging\",\n      \"journal\": \"Endocrinology / Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in heterologous expression system with multiple orthogonal functional assays; independently replicated by two groups (PMID 8405712 and 8404634)\",\n      \"pmids\": [\"8405712\", \"8404634\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Crystal structure of the human GLP-1R extracellular domain (nGLP-1R) in complex with the antagonist exendin-4(9-39) at 2.2 Å resolution revealed that exendin-4 binds as an amphipathic α-helix making hydrophobic and hydrophilic contacts; the Trp-cage C-terminal extension does not contact nGLP-1R. The hydrophobic binding site is defined by an N-terminal α-helix and a loop between antiparallel β-strands of nGLP-1R.\",\n      \"method\": \"X-ray crystallography (MAD, 2.2 Å), site-directed mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure with mutagenesis validation\",\n      \"pmids\": [\"18287102\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structure of the GLP-1R extracellular domain in complex with GLP-1 at 2.1 Å resolution showed GLP-1 binds as a kinked but continuous α-helix (Thr13–Val33). While hydrophobic ligand-receptor interactions are conserved between GLP-1 and exendin-4(9-39) bound forms, specific residues in the binding site adopt a GLP-1-specific conformation. Mutagenesis confirmed differences in binding modes of GLP-1 vs. exendin-4 on the full-length receptor.\",\n      \"method\": \"X-ray crystallography (2.1 Å), site-directed mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus mutagenesis; orthogonal to prior extracellular domain structure\",\n      \"pmids\": [\"19861722\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Crystal structures of the human GLP-1R transmembrane domain in complex with negative allosteric modulators PF-06372222 and NNC0640 at 2.7 and 3.0 Å revealed a common allosteric binding pocket outside helices V–VII near the intracellular half, distinct from the orthosteric peptide-binding site. Receptor is in an inactive conformation with compounds restricting movement of the intracellular tip of helix VI. Positive allosteric modulators target the same general region but a distinct sub-pocket at the helix V–VI interface facilitating G-protein coupling.\",\n      \"method\": \"X-ray crystallography, molecular modelling, mutagenesis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — two crystal structures at high resolution with mutagenesis validation\",\n      \"pmids\": [\"28514449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Cryo-EM structure of the human GLP-1R in complex with G-protein-biased agonist exendin-P5 and Gαs heterotrimer at 3.3 Å global resolution revealed differences from the GLP-1-bound structure: distinct organization of ECL3 and proximal transmembrane segments at the extracellular surface, and a six-degree difference in the angle of Gαs-α5 helix engagement at the intracellular face, propagated across the G protein heterotrimer. Different rates and extents of conformational reorganization of Gαs between biased and unbiased agonist-bound structures were demonstrated.\",\n      \"method\": \"Phase-plate cryo-EM (3.3 Å)\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM structure with direct comparison to GLP-1-bound structure\",\n      \"pmids\": [\"29466332\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"High-resolution cryo-EM structures revealed that the non-peptide agonist PF-06882961 binding site substantially overlaps with GLP-1's binding site, whereas CHU-128 adopts a unique binding mode with a more open receptor conformation at the extracellular face. Structural differences involving extensive water-mediated hydrogen bond networks correlated with functional data showing that PF-06882961, but not CHU-128, closely mimics pharmacological properties of GLP-1 including signaling and regulation profiles.\",\n      \"method\": \"Cryo-EM structural determination, pharmacological assays, comparative structural analysis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple high-resolution cryo-EM structures correlated with functional data\",\n      \"pmids\": [\"33027691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of semaglutide- and taspoglutide-bound GLP-1R–Gs complexes revealed similar peptide-receptor interactions to GLP-1 but different motions within the receptor and bound peptides. 3D variability analysis showed distinct receptor conformational dynamics, providing molecular explanation for different signaling efficacies and side effect profiles of distinct GLP-1R peptide agonists.\",\n      \"method\": \"Cryo-EM, 3D variability analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures of multiple agonist-bound complexes with conformational analysis\",\n      \"pmids\": [\"34260945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM structures of Boc5 and WB4-24 (non-peptidic GLP-1R agonists) bound to GLP-1R–Gs complex revealed one arm inserted deeply into the orthosteric binding pocket overlapping with GLP-1 residues A8–D15, while other arms extended to TM1-TM7, TM1-TM2, and TM2-TM3 clefts. This unique binding mode creates a distinct conformation conferring both peptidomimetic agonism and biased signaling.\",\n      \"method\": \"Cryo-EM structural determination, pharmacological assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures with pharmacological validation\",\n      \"pmids\": [\"35561211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Combining cryo-EM, molecular dynamics simulations, receptor mutagenesis, and pharmacological assays across four peptide agonists (GLP-1, oxyntomodulin, exendin-4, exendin-P5) showed that distinctions in peptide N-terminal interactions and dynamics with the GLP-1R transmembrane domain are reciprocally associated with differences in allosteric coupling to G proteins. Transient interactions with residues at the base of the binding cavity correlate with enhanced kinetics for G protein activation.\",\n      \"method\": \"Cryo-EM, molecular dynamics simulations, mutagenesis, pharmacological assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM plus MD plus mutagenesis plus pharmacology; multiple orthogonal methods\",\n      \"pmids\": [\"35013280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of GLP-1R or GCGR in complex with Gs protein and three dual GLP-1R/GCGR agonists (peptide 15, cotadutide, SAR425899) identified key residues responsible for ligand recognition and dual agonism. Distinct side chain orientations within the first three residues determine receptor selectivity; ECL1 interaction plays an important role in dual agonism; lipid modification of MEDI0382 interacts with TM1-TM2 cleft correlating with increased GCGR potency.\",\n      \"method\": \"Cryo-EM structural determination, pharmacological assays, mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures with pharmacological and mutagenesis validation\",\n      \"pmids\": [\"37549266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structures of human GLP-1R, GCGR, and GIPR in complex with Gs proteins without cognate ligands (ligand-free complexes) revealed that Gs protein alone directly opens the intracellular binding cavity and rewires the extracellular orthosteric pocket. In GLP-1R, the extracellular portion adopts a conformation close to the active state in the absence of ligand, revealing a distinct activation intermediate in which intracellular half of the transmembrane domain is mobilized by Gs protein to facilitate peptide N-terminus entry.\",\n      \"method\": \"Cryo-EM structural determination, comparative structural analysis\",\n      \"journal\": \"Cell discovery\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM of ligand-free receptor-G protein complex\",\n      \"pmids\": [\"38346960\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Using a validated monoclonal antibody for immunohistochemistry and in situ ligand binding with 125I-GLP-1, GLP-1R was localized in primate and human tissues: predominantly in pancreatic β-cells (with markedly weaker acinar expression, no ductal expression), smooth muscle cells of renal and pulmonary arteries/arterioles, myocytes of the sinoatrial node, Brunner's gland of the duodenum, parietal cells and smooth muscle of stomach, and myenteric plexus neurons. No GLP-1R was detected in primate liver or thyroid.\",\n      \"method\": \"Immunohistochemistry with validated monoclonal antibody, in situ radioligand binding (125I-GLP-1)\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct localization with two orthogonal methods (IHC + radioligand binding), extensively validated reagent\",\n      \"pmids\": [\"24467746\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"GLP-1R mRNA and protein were detected on primary human hepatocytes and hepatocyte cell lines. Exendin-4 stimulated phosphorylation of PDK-1, AKT, and PKC-ζ in HepG2 and Huh7 cells; siRNA against GLP-1R abolished these effects. Exendin-4 also quantitatively reduced triglyceride stores in these cells, demonstrating a direct GLP-1R-dependent effect on hepatic steatosis independent of insulin.\",\n      \"method\": \"RT-PCR, Western blotting, receptor internalization assay, siRNA knockdown, lipid quantification\",\n      \"journal\": \"Hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA epistasis plus multiple signaling readouts in a single study\",\n      \"pmids\": [\"20225248\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"GLP-1R is present on human coronary artery endothelial cells (HCAECs). Exendin-4 stimulated DNA synthesis and cell proliferation dose-dependently through PKA, PI3K/AKT, and eNOS activation pathways. These effects were abolished by a GLP-1R antagonist (exendin-(9-39)) and by individual pathway inhibitors, establishing a GLP-1R→PKA→PI3K/AKT→eNOS signaling cascade mediating endothelial proliferation.\",\n      \"method\": \"DNA synthesis assay, cell counting, pharmacological inhibitor panel, GLP-1R antagonist blockade\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor antagonist plus multiple pathway inhibitors in same cell type\",\n      \"pmids\": [\"20452396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"GLP-1R is expressed in human hepatocytes (reduced in NASH patients). In hepatocytes from high-fat diet rats, exenatide increased PPARγ expression (reducing JNK phosphorylation for insulin sensitization), increased PKA activity, and stimulated Akt and AMPK phosphorylation, leading to PKA-dependent increase of PPARα activity and enhanced fatty acid β-oxidation.\",\n      \"method\": \"RT-PCR, Western blotting, hepatocyte culture with pharmacological treatments, liver biopsy analysis\",\n      \"journal\": \"Liver international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — multiple signaling readouts in primary hepatocytes with in vivo model correlation\",\n      \"pmids\": [\"21745271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"GLP-1 and liraglutide activate GLP-1R on pre-adipocytes to promote proliferation and inhibit apoptosis through activation of ERK, PKC, and AKT signaling pathways, inducing adipogenesis. Loss of GLP-1R expression caused reduction in adipogenesis through induction of apoptosis in pre-adipocytes via inhibition of these same pathways.\",\n      \"method\": \"In vitro adipocyte differentiation assays, loss-of-function (GLP-1R knockout), pharmacological agonism, pathway inhibitors\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — gain/loss-of-function with defined pathway readouts\",\n      \"pmids\": [\"22207759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GLP-1R agonist exendin-4 attenuated H2O2-induced reactive oxygen species production in cardiomyocytes through an Epac-dependent pathway, and increased antioxidant enzymes (catalase, glutathione peroxidase-1, manganese superoxide dismutase). The antiapoptotic effect (decreased apoptotic cells, inhibited caspase-3, enhanced Bcl-2) was mediated through both PKA- and Epac-dependent pathways downstream of GLP-1R activation.\",\n      \"method\": \"Pharmacological dissection of cAMP effectors (PKA vs. Epac) in cardiomyocytes, ROS measurement, caspase activity, Western blotting\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — selective pharmacological tools dissecting PKA vs. Epac branches downstream of GLP-1R\",\n      \"pmids\": [\"25719403\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GLP-1R agonist P5 was identified as a G-protein-biased agonist that promotes G-protein signaling comparable to GLP-1 and Exendin-4 but exhibits significantly reduced β-arrestin recruitment. In preclinical T2DM mouse models, P5 was a weak insulin secretagogue but increased adipogenesis, reduced adipose tissue inflammation and hepatic steatosis, and was more effective at correcting hyperglycemia than Exendin-4, demonstrating that GLP-1R signaling bias has distinct in vivo consequences.\",\n      \"method\": \"Autocrine-based peptide library screening, cAMP and β-arrestin assays, mouse diabetes models\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional signaling bias demonstrated in vitro correlated with distinct in vivo phenotypes\",\n      \"pmids\": [\"26621478\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GLP-1R signaling controls mucosal expansion of the small bowel and colon. These actions did not require EGF receptor or intestinal IGF1 receptor but were absent in Glp1r−/− mice. Exendin-4 increased Fgf7 expression in intestinal tissue and failed to increase intestinal growth in Fgf7-deficient mice, placing Fgf7 downstream of GLP-1R signaling in mediating intestinal proliferation.\",\n      \"method\": \"Conditional knockout mice, genetic epistasis (Glp1r−/−, Fgf7−/−, EGF/IGF1R knockouts), pharmacological GLP-1R agonist treatment\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic epistasis experiments with defined mechanistic pathway\",\n      \"pmids\": [\"25738454\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Systematic investigation using Wnt1-Cre2 and Phox2b-Cre conditional Glp1r knockout mice showed that widespread neural Glp1r loss had no effect on basal food intake or gastric emptying but preserved the glucoregulatory actions of GLP-1R agonists. In contrast, selective Glp1r loss in Phox2b+ neurons impaired glucose homeostasis, gastric emptying, and attenuated weight loss from sustained GLP-1R agonism, identifying Phox2b+ neural cells as required for physiological GLP-1R-mediated gut-brain axis regulation.\",\n      \"method\": \"Conditional Cre-lox knockout mice, GLP-1R agonist treatment, metabolic phenotyping\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple conditional knockout lines with specific phenotypic readouts\",\n      \"pmids\": [\"31189118\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"In vivo fiber photometry and patch-clamp electrophysiology showed that liraglutide and semaglutide directly activate hypothalamic POMC neurons, requiring GLP-1Rs in POMC neurons and a downstream TRPC5-subunit mixed cation channel. They also indirectly upregulate excitatory input to POMC neurons from glutamatergic cells requiring TRPC5. GLP-1RAs inhibit NPY/AgRP neurons indirectly through activation of K-ATP and TRPC5 channels in GABAergic neurons. These temporal effects depend on metabolic (fed/fasted) state.\",\n      \"method\": \"Neuron-specific transgenic mice, in vivo fiber photometry, patch-clamp electrophysiology, pharmacological channel blockers\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo imaging plus electrophysiology plus genetic requirement established in same study\",\n      \"pmids\": [\"34626854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The gut IEL GLP-1R is not required for enteroendocrine L cell GLP-1 secretion or glucose homeostasis, but is essential for the full effects of GLP-1RAs on gut microbiota composition. The anti-inflammatory actions of GLP-1RAs require the gut IEL GLP-1R to selectively restrain T cell-induced (but not LPS-induced) inflammation, mediated by suppression of IEL effector functions through dampening of proximal TCR signaling in a protein-kinase-A-dependent manner.\",\n      \"method\": \"IEL-specific conditional Glp1r knockout mice, microbiota analysis, T cell activation assays, PKA inhibitor experiments\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional knockout with mechanistic PKA pathway identification\",\n      \"pmids\": [\"36027914\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"GLP-1 regulates skeletal muscle remodeling to enhance exercise endurance via GLP-1R signaling-mediated phosphorylation of AMPK. AAV-mediated GLP-1 overexpression in skeletal muscle enhanced endurance and promoted glycogen synthesis, glucose uptake, type I fiber proportion, and mitochondrial biogenesis. In vitro, AMPK knockdown reversed the effects of GLP-1R activation on glucose uptake, type I fiber formation, and mitochondrial respiration.\",\n      \"method\": \"AAV-mediated GLP-1 overexpression, siRNA AMPK knockdown, in vitro exendin-4 treatment, metabolic/functional assays\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic gain/loss-of-function establishing GLP-1R→AMPK pathway\",\n      \"pmids\": [\"35636559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"GLP-1R agonists activate 15 signaling pathways in 4 cellular compartments. Using biosensors, time-lapse microscopy, and phosphoproteomics, modifications to GLP-1R agonists were shown to greatly influence compound efficacy, potency, and safety in a pathway- and compartment-selective manner. Unique signaling signatures at the level of receptor conformation, functional selectivity, and location bias were associated with functionally distinct cellular outcomes and adverse clinical events.\",\n      \"method\": \"Biosensor panel (15 pathways/4 compartments), comparative structural analysis, time-lapse microscopy, phosphoproteomics\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multi-compartment biosensors plus structural and proteomic analyses across multiple agonists\",\n      \"pmids\": [\"37813859\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Ligand dissociation kinetics (Koff), but not association kinetics (Kon), of GLP-1R peptide agonists were positively correlated with onset of receptor-G protein coupling/conformational change, onset of cAMP production, and duration of cAMP signaling. This established that peptide off-rate is a key determinant of the kinetics of GLP-1R–G protein coupling and downstream signaling.\",\n      \"method\": \"Kinetic binding assays, BRET-based receptor-G protein conformational biosensors, cAMP time-course measurements across multiple GLP-1R agonists\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic kinetic analysis across multiple agonists with multiple orthogonal biosensors\",\n      \"pmids\": [\"35300966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In adult β cell-specific β-arrestin 2 knockout mice, acute GLP-1R agonist (exendin-4, semaglutide, tirzepatide) responses were sex-dimorphically impaired, but improved 6 hours post-injection. β-arrestin 2 KO impaired acute cAMP increases (attributed to enhanced β-arrestin 1 and phosphodiesterase 4 activities) but reduced desensitization, co-occurring with impaired GLP-1R recycling and lysosomal targeting, increased trans-Golgi network signaling, and reduced GLP-1R ubiquitination. Biased agonist exendin-phe1 did not show this phenotype.\",\n      \"method\": \"Adult β cell-specific conditional knockout mice, islet cAMP measurements, GLP-1R trafficking/ubiquitination assays, in vivo glucose tolerance\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO plus multiple orthogonal mechanistic readouts (cAMP, trafficking, ubiquitination) in vivo and in vitro\",\n      \"pmids\": [\"37134170\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Functional profiling of 60 GLP1R variants across four signaling pathways revealed unexpected diversity including defective cell surface expression, complete or pathway-specific gain/loss-of-function. Defective insulin secretion of GLP1R loss-of-function variants was rescued by allosteric GLP1R ligands or high concentrations of exendin-4/semaglutide. Impaired GLP1R cell surface expression (a distinct variant category) was associated with poor glucose control and increased adiposity in UK Biobank analysis.\",\n      \"method\": \"Functional signaling profiling of 60 variants (4 pathways), INS-1 cell insulin secretion rescue experiments, UK Biobank genetic association\",\n      \"journal\": \"Nature metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — systematic functional profiling of multiple variants with mechanistic follow-up rescue experiments\",\n      \"pmids\": [\"37709961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GRK2 negatively regulates GLP-1R-mediated insulin secretion. GRK2 hemizygous mice showed enhanced early-phase (but not late-phase) insulin release upon oral glucose, GLP-1R agonists, or feeding, correlated with an increased readily releasable pool of insulin granules. Using nanoBRET in β-cells, GLP-1R stimulation promoted GRK2 association with GLP-1R, and GRK2 kinase activity was required for subsequent β-arrestin recruitment.\",\n      \"method\": \"GRK2+/− mice, isolated islets, nanoBRET in β-cell lines, patch-clamp/capacitance measurements of readily releasable pool\",\n      \"journal\": \"BMC biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic model plus direct receptor-GRK2 interaction assay plus mechanistic signaling readout\",\n      \"pmids\": [\"33658023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GLP-1R shows increased cell surface levels, internalization, degradation, and endosomal vs. plasma membrane signaling activity compared to GIPR in pancreatic β-cells. GIPR is instead associated with increased plasma membrane recycling, reduced desensitization, and enhanced downstream signal amplification. These differences in spatiotemporal signaling profiles underlie distinct pharmacological responses to each incretin receptor.\",\n      \"method\": \"Comparative surface expression, trafficking, and multi-compartment signaling analysis between GLP-1R and GIPR in pancreatic β-cells\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct comparative functional and trafficking analysis in relevant cell type\",\n      \"pmids\": [\"36774542\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Exendin-4 (GLP-1R agonist) restored airway mucus homeostasis via a GLP1R→PKA→PPARγ-dependent signaling cascade that activated phosphatases PTEN and PTP1B, which in turn inhibited key kinases within both EGFR and STAT6 signaling cascades to restore FOXA2 expression and attenuate mucin production in COPD and CF airway cells.\",\n      \"method\": \"Pharmacological dissection, signaling pathway analysis, siRNA, human airway cell culture, mouse lung model\",\n      \"journal\": \"Mucosal immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined GLP1R→PKA→PPARγ→phosphatase pathway with multiple inhibitor/knockdown experiments\",\n      \"pmids\": [\"32034274\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"GABA requires GLP-1R to inhibit TxNIP in pancreatic β-cells; this function was attenuated in GLP-1R−/− islets. In GLP-1R−/− mice, GABA failed to alleviate STZ-induced diabetic syndrome or increase β-cell mass. GABA treatment increased cellular cAMP and β-catenin S675 phosphorylation in WT but not GLP-1R−/− islets, identifying TxNIP as a common downstream target of both GABA and GLP-1 acting through GLP-1R-cAMP-β-catenin signaling.\",\n      \"method\": \"GLP-1R knockout mice, isolated islets, INS-1 cells, siRNA knockdown of TxNIP, STZ challenge model\",\n      \"journal\": \"The Journal of endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout epistasis establishing GLP-1R requirement for GABA-mediated β-cell effects\",\n      \"pmids\": [\"32544878\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In vivo two-photon imaging of hindbrain GLP1R neurons showed that area postrema (AP) GLP1R neurons are broadly responsive to both nutritive and aversive stimuli, whereas NTS GLP1R neurons are biased toward nutritive stimuli. Selective activation of NTS GLP1R neurons triggered satiety without aversion, while AP GLP1R neurons triggered aversion with food intake reduction. GLP1R agonists reduce food intake even when the aversion pathway is inhibited, demonstrating dissociable neural circuits for satiety vs. aversion.\",\n      \"method\": \"In vivo two-photon calcium imaging, chemogenetic activation/inhibition of subregion-specific GLP1R populations, anatomical projection tracing\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo imaging plus selective circuit manipulation with specific behavioral readouts\",\n      \"pmids\": [\"38987598\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GABAergic LepRb neurons expressing Glp1r (LepRbGlp1r neurons) are required for food intake suppression by both leptin and GLP1R agonists. Ablating Lepr from LepRbGlp1r cells caused hyperphagic obesity. Restoration of Glp1r expression in LepRb neurons on an otherwise Glp1r-null background enabled food intake suppression by liraglutide, demonstrating that GLP1R expression in this specific hypothalamic population is sufficient for the anorectic effect.\",\n      \"method\": \"Cell-specific knockout/reactivation mouse models, single-nucleus RNA-Seq, chemogenetic and GLP1R agonist challenges\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-specific loss- and gain-of-function with defined behavioral phenotype\",\n      \"pmids\": [\"37581939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R-positive neurons in the lateral septum (LS) mediate anorectic and weight-lowering effects of liraglutide. Chemogenetic activation of LS GLP1R neurons suppressed feeding; targeted knockdown of GLP-1R in LS (but not hypothalamus) attenuated liraglutide's anorectic and weight-lowering effects. Synaptic inactivation of LS GLP1R neurons diminished liraglutide-triggered anorexia, establishing LS GLP1R neurons as a critical circuit node.\",\n      \"method\": \"Chemogenetics (DREADD), targeted shRNA knockdown, synaptic inactivation, in vivo feeding assays in Glp1r-Cre mice\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple loss/gain-of-function approaches targeting specific brain nucleus\",\n      \"pmids\": [\"39225090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R activation in gut IELs modulates microbiota composition and restrains T cell inflammation via PKA-dependent suppression of TCR signaling (established previously, 2022); separately, dorsolateral septal (dLS) GLP-1R neurons project GABAergically to the lateral hypothalamic area (LHA) and regulate food intake. Chemogenetic inhibition of dLSGLP-1R neurons or the dLSGLP-1R→LHA pathway increased food intake; optogenetic terminal stimulation in LHA rapidly suppressed feeding. Exendin-4 enhanced dLSGLP-1R→LHA GABA release.\",\n      \"method\": \"Channelrhodopsin-assisted circuit mapping, chemogenetics, electrophysiology, optogenetics, GABA release measurement in Glp1r-ires-Cre mice\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple circuit dissection methods establishing GABAergic projection mechanism\",\n      \"pmids\": [\"38763494\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GLP-1RAs alleviate AD-related phenotypes by activating CaMKK2-AMPK signaling, which reduces BACE1-mediated cleavage of APP and Aβ generation. GLP-1RAs also increased AMPK activity in microglia, inhibiting neuroinflammation and promoting Aβ phagocytosis. GLP-1 plasma levels were reduced in AD model mice and negatively correlated with Aβ load.\",\n      \"method\": \"AD transgenic mouse models, GLP-1RA treatment, CaMKK2-AMPK pathway analysis, BACE1 activity assay, microglial phagocytosis assay\",\n      \"journal\": \"Nature aging\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined mechanistic pathway in multiple models with relevant pharmacological and genetic tools\",\n      \"pmids\": [\"40394225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The MafA-target gene PPP1R1A is expressed in pancreatic β-cells and its silencing in INS-1 cells impaired GLP-1-mediated GSIS amplification, PKA-target protein phosphorylation, and mitochondrial coupling efficiency. PPP1R1A mRNA levels positively correlated with GLP-1-mediated GSIS amplification in human islets and were reduced in type 2 diabetic islets, placing PPP1R1A as a required downstream effector of GLP1R-PKA signaling in β-cells.\",\n      \"method\": \"siRNA knockdown in INS-1 cells, human islet correlational analysis, PKA substrate phosphorylation, mitochondrial coupling assay\",\n      \"journal\": \"Metabolism: clinical and experimental\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined PKA-mediated signaling readout\",\n      \"pmids\": [\"33631146\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"VTA dopamine (VTADA) neuron responsiveness was suppressed during food consumption by semaglutide (a GLP-1R agonist). Mice recovered palatable food appetite and VTADA neuron activity during repeated semaglutide treatment, which was reversed by consumption-triggered VTADA neuron inhibition. A neural pathway from peri-locus ceruleus to VTA controls hedonic eating, and GLP-1R agonism opposes VTADA neuron activation to reduce palatable food intake.\",\n      \"method\": \"Photometry-calibrated optogenetics, in vivo calcium imaging, chemogenetics, semaglutide treatment\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct neural circuit imaging plus optogenetic manipulation establishing GLP-1R/VTADA interaction\",\n      \"pmids\": [\"40146831\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R activation attenuates pulmonary fibrosis by inhibiting the interaction between NLRP3 inflammasome and PFKFB3-driven glycolysis in lung fibroblasts, reducing lactate production. This prevents lactate-mediated histone H3K9 lactylation at profibrotic gene promoters (demonstrated by ChIP-qPCR) and reduces EMT-related fibrotic gene expression. GLP-1R activation also represses p300-mediated histone lactylation in exogenous lactate-treated fibroblasts.\",\n      \"method\": \"In vivo silica fibrosis model, in vitro TGF-β1+IL-1β fibroblast activation, ChIP-qPCR for histone lactylation, glycolysis/mitochondrial respiration assays, RNA-seq\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP-qPCR plus metabolic assays establishing GLP-1R→NLRP3/PFKFB3→lactate→histone lactylation mechanism\",\n      \"pmids\": [\"39434134\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Dual activation of GCGR and GLP1R reduces intestinal fibrosis; downregulation of GLP1R led to lactate accumulation and histone H3K9 lactylation driving EMT-mediated fibrosis. Dual agonist peptide 1907B reduced H3K9 lactylation in epithelial cells and ameliorated intestinal fibrosis in vivo, identifying lactate-mediated epigenetic regulation as downstream of GLP1R signaling.\",\n      \"method\": \"Crohn's disease patient tissue analysis, mouse chronic colitis model, dual agonist treatment, H3K9 lactylation measurement, GLP1R/GCGR knockdown\",\n      \"journal\": \"Acta pharmaceutica Sinica. B\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor knockdown epistasis plus epigenetic mechanistic readout\",\n      \"pmids\": [\"40041889\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GLP1R inhibits endometrial carcinoma progression by activating the cAMP/PKA signaling pathway. GLP1R overexpression increased cAMP content and p-PKA, promoting apoptosis and inhibiting proliferation, migration, and invasion. These effects were blunted by PKA knockdown (siRNA), establishing a direct GLP1R→cAMP→PKA axis mediating tumor suppression.\",\n      \"method\": \"GLP1R overexpression vector transfection, siRNA-PKA knockdown, cAMP ELISA, Western blotting, cell functional assays, xenograft tumor model\",\n      \"journal\": \"Journal of clinical laboratory analysis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — gain/loss of function with defined pathway mechanism in vitro and in vivo\",\n      \"pmids\": [\"35989517\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R activation (liraglutide) ameliorated oxidized LDL-induced endothelial dysfunction via GLP-1R-dependent downregulation of LOX-1, which reduced NOX4/NF-κB-mediated oxidative stress and ICAM-1/VCAM-1 expression. The protective effects were abrogated by GLP-1R knockdown or LOX-1 overexpression, establishing a GLP-1R→LOX-1 suppression axis in endothelial protection.\",\n      \"method\": \"LDLR-KO mouse model, HUVECs with GLP-1R knockdown/LOX-1 overexpression, vascular reactivity measurements, ROS assays, Western blotting\",\n      \"journal\": \"Redox report\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor knockdown epistasis plus LOX-1 overexpression to establish mechanism\",\n      \"pmids\": [\"37278349\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1R signaling is required for maintenance of colonic physiology during energy deprivation; GF GLP-1R KO mice showed 25% mortality associated with enlarged ceca, increased cecal water content, increased colonic apical ion transporter expression, reduced goblet cells, and loss of colonic epithelial integrity. Colonocytes were energy-deprived with increased ER stress, mitochondrial fragmentation, and loss of stemness. Restoring energy (Western diet or microbiota colonization) normalized gut phenotypes and prevented lethality.\",\n      \"method\": \"GF GLP-1R KO mice, histology, organoid stemness assays, metabolic rescue experiments (diet/microbiota colonization)\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor KO with defined phenotype rescued by multiple interventions\",\n      \"pmids\": [\"38521185\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GLP1R is a class B1 GPCR that, upon binding GLP-1 or agonists at its extracellular N-terminal domain and transmembrane core, undergoes conformational changes that couple to Gαs (raising cAMP) and PLC (raising intracellular Ca2+), with subsequent PKA, Epac, AKT, AMPK, and ERK/PKC effectors mediating tissue-specific outcomes including β-cell insulin secretion (requiring PPP1R1A/PKA), intestinal growth (via Fgf7), hepatic lipid oxidation (via PPARα/PPARγ), endothelial proliferation (via PKA/PI3K/AKT/eNOS), adipogenesis, cardioprotection (via Epac), gut IEL T cell suppression (via PKA/TCR dampening), and central appetite regulation (via NTS GLP1R neurons driving satiety and AP GLP1R neurons driving aversion); receptor desensitization is regulated by GRK2-mediated β-arrestin 2 recruitment, receptor internalization, lysosomal targeting, and ubiquitination, with ligand off-rate kinetics governing G protein coupling kinetics, and allosteric modulators engaging a distinct transmembrane pocket outside helices V–VII.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GLP1R is a class B1 G-protein-coupled receptor that transduces incretin and satiety signals by coupling to Gs to activate cAMP/PKA and Epac pathways, with ligand dissociation kinetics governing the onset and duration of G-protein engagement and downstream cAMP production [PMID:35300966, PMID:35013280]. Cryo-EM structures with peptide agonists (GLP-1, semaglutide, exendin-4, oxyntomodulin), non-peptide agonists (PF 06882961, Boc5), and dual GLP-1R/GCGR agonists reveal that diverse ligands engage overlapping orthosteric sites within the transmembrane domain and extracellular domain, with distinct N-terminal interactions and ECL1 conformational changes dictating receptor selectivity and biased signaling; Gs protein alone can open the intracellular cavity and rewire the extracellular pocket even in the absence of ligand [PMID:33027691, PMID:35561211, PMID:37549266, PMID:38346960]. Receptor desensitization proceeds through GRK2-dependent β-arrestin 2 recruitment, which controls internalization, recycling, lysosomal targeting, and ubiquitination, and loss of β-arrestin 2 in β-cells shifts signaling to sustained trans-Golgi network cAMP production [PMID:37134170, PMID:33658023]. In the CNS, discrete GLP-1R-expressing neuronal populations—NTS neurons mediating satiety, AP neurons mediating aversion, lateral septum neurons required for GLP-1R agonist-induced anorexia, and hypothalamic LepRb/Glp1r neurons integrating leptin and GLP-1 signaling—project to distinct downstream regions and are differentially required for appetite suppression, glucose homeostasis, and body weight control [PMID:38987598, PMID:39225090, PMID:37581939, PMID:31189118].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Establishing that natural variation in Glp1r expression levels directly controls a peripheral physiological output—gastric emptying—linked genetic dosage of the receptor to functional consequence.\",\n      \"evidence\": \"Congenic mouse strains with CAST Glp1r allele showed allelic imbalance in stomach expression and accelerated gastric emptying impervious to GLP-1R antagonist\",\n      \"pmids\": [\"18077508\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which expression level alters gastric motility not defined\", \"Single congenic background limits generalizability\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Downstream effector pathways of GLP-1R were mapped in multiple peripheral tissues: Fgf7 was identified as a required mediator of intestinal mucosal growth, Epac and PKA were shown to be parallel anti-apoptotic arms in cardiomyocytes, and ERK/PKC/AKT were placed downstream for adipogenesis.\",\n      \"evidence\": \"Genetic epistasis using Fgf7-KO mice with exendin-4; pharmacological inhibition of Epac/PKA in cardiomyocyte lines; GLP-1R loss-of-function in adipocyte assays\",\n      \"pmids\": [\"25738454\", \"25719403\", \"22207759\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct substrates linking GLP-1R to Fgf7 transcription not identified\", \"Epac-dependent and PKA-dependent effector branches not fully resolved in vivo\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstration that G-protein versus β-arrestin signaling arms of GLP-1R are pharmacologically separable established the concept of biased agonism at this receptor, with a G-protein-biased peptide (P5) producing distinct metabolic outcomes.\",\n      \"evidence\": \"High-throughput peptide library screening yielding P5, cAMP and β-arrestin assays, diabetic mouse metabolic studies\",\n      \"pmids\": [\"26621478\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo confirmation of biased signaling contributions to specific metabolic endpoints limited\", \"β-arrestin-independent desensitization mechanisms not excluded\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identifying Phox2b+ neurons as a specific CNS population through which GLP-1R mediates physiological glucose homeostasis and gastric emptying distinguished gut-brain axis GLP-1R from broader neural GLP-1R pools.\",\n      \"evidence\": \"Phox2b-Cre and Wnt1-Cre2 conditional Glp1r knockout mice with glucose tolerance and gastric emptying assays\",\n      \"pmids\": [\"31189118\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream circuit targets of Phox2b+ GLP-1R neurons not mapped\", \"Whether Phox2b+ neurons mediate pharmacological GLP-1RA weight loss independently of other populations unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Cryo-EM structures of GLP-1R with non-peptide agonists revealed that small molecules can substantially overlap the GLP-1 binding site or adopt unique conformations, explaining differential pharmacological profiles and opening structure-based drug design.\",\n      \"evidence\": \"Cryo-EM of PF 06882961 and CHU-128 bound to GLP-1R–Gs complexes with pharmacological validation\",\n      \"pmids\": [\"33027691\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structures captured in active Gs-bound state only; inactive-state non-peptide binding unknown\", \"Water-mediated hydrogen bond network contributions to signaling bias not functionally validated\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"GRK2 was established as the kinase required for β-arrestin 2 recruitment to GLP-1R, with GRK2 haploinsufficiency enhancing insulin secretion by increasing the readily releasable granule pool—directly linking desensitization machinery to secretory output.\",\n      \"evidence\": \"GRK2+/− mice, nanoBRET in β-cell lines, isolated islet insulin secretion assays\",\n      \"pmids\": [\"33658023\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"GRK2 phosphorylation sites on GLP-1R C-tail not mapped\", \"Contribution of other GRKs not fully excluded\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Multiple cryo-EM structures of peptide agonists (GLP-1, exendin-4, oxyntomodulin, exendin-P5) revealed that N-terminal peptide dynamics within the TMD base allosterically control G-protein activation kinetics, and that ligand dissociation kinetics (Koff) govern the onset and duration of cAMP signaling.\",\n      \"evidence\": \"Cryo-EM with MD simulations and mutagenesis across four peptides; kinetic binding and BRET-based G-protein coupling assays\",\n      \"pmids\": [\"35013280\", \"35300966\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How dissociation kinetics map onto biased signaling (G-protein vs. arrestin) not resolved\", \"In vivo relevance of kinetic differences for therapeutic efficacy untested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"GLP-1R on gut intraepithelial lymphocytes was shown to suppress T-cell-mediated inflammation through PKA-dependent dampening of proximal TCR signaling, establishing an immune-modulatory function independent of metabolic roles.\",\n      \"evidence\": \"Conditional GLP-1R knockout in IELs, immune cell functional assays, PKA pathway analysis\",\n      \"pmids\": [\"36027914\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific PKA substrates mediating TCR suppression not identified\", \"Whether IEL GLP-1R contributes to anti-inflammatory effects of clinical GLP-1RAs in humans unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"β-Arrestin 2 was shown to have dual roles at GLP-1R in β-cells: it mediates acute cAMP signaling (through restraining PDE4/β-arrestin 1), controls receptor recycling and lysosomal targeting, and regulates GLP-1R ubiquitination—resolving how desensitization and sustained signaling are balanced.\",\n      \"evidence\": \"Adult β-cell-specific β-arrestin 2 conditional KO with in vivo glucose tolerance, cAMP nanoBRET, and trafficking assays\",\n      \"pmids\": [\"37134170\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase responsible for GLP-1R ubiquitination not identified\", \"Trans-Golgi network signaling compartment contribution to insulin secretion kinetics not quantified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Hypothalamic LepRb/Glp1r co-expressing neurons were identified as a convergence node for leptin and GLP-1R agonist action on food intake; genetic rescue of Glp1r in these neurons alone was sufficient for liraglutide efficacy, and cross-species validation in nonhuman primates confirmed translational relevance.\",\n      \"evidence\": \"snRNA-seq, conditional KO, Cre-dependent Glp1r rescue in Glp1r-null mice, NHP anatomical studies\",\n      \"pmids\": [\"37581939\", \"39630884\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effectors within LepRb/Glp1r neurons mediating appetite suppression not mapped\", \"Whether these neurons are required for weight loss with all GLP-1RA classes unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Systematic functional profiling of 60 human GLP1R variants revealed that impaired cell surface expression is a dominant loss-of-function mechanism, validated by UK Biobank association with poor glycemic control and increased adiposity, providing genotype-phenotype architecture for this receptor.\",\n      \"evidence\": \"Multi-pathway signaling assays for 60 variants, INS-1 insulin secretion rescue, population genetics in ~200,000 individuals\",\n      \"pmids\": [\"37709961\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether variant-specific signaling bias predicts differential drug response clinically untested\", \"Structural basis for variant-specific surface expression defects not determined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"In vivo two-photon imaging resolved the long-standing question of whether hindbrain GLP-1R neurons encode satiety and aversion through shared or separate populations: NTS GLP-1R neurons drive satiety without aversion while AP GLP-1R neurons drive aversion, with distinct downstream projections.\",\n      \"evidence\": \"In vivo two-photon calcium imaging, optogenetic and chemogenetic activation of NTS versus AP GLP-1R neurons, circuit tracing\",\n      \"pmids\": [\"38987598\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular markers distinguishing satiety vs. aversion NTS subpopulations not identified\", \"Whether pharmacological GLP-1RAs differentially engage AP vs. NTS populations at therapeutic doses unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Ligand-free cryo-EM structures revealed that Gs protein alone can open the intracellular TM cavity and remodel the extracellular orthosteric pocket, establishing a mechanism for constitutive or ligand-independent receptor activation.\",\n      \"evidence\": \"Cryo-EM of ligand-free GLP-1R–Gs, GCGR–Gs, and GIPR–Gs complexes\",\n      \"pmids\": [\"38346960\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of ligand-free Gs coupling in vivo not demonstrated\", \"Whether constitutive activity contributes to basal insulin secretion untested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"GLP-1R agonist semaglutide was shown to suppress VTA dopamine neuron responsiveness during food consumption, demonstrating that the hedonic eating circuit is a direct target opposing GLP-1R-mediated appetite reduction.\",\n      \"evidence\": \"Photometry-calibrated optogenetics and in vivo neural activity imaging in mice treated with semaglutide\",\n      \"pmids\": [\"40146831\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether GLP-1R is expressed on VTA DA neurons or acts indirectly via afferent inputs not resolved\", \"Contribution of VTA suppression vs. hypothalamic/hindbrain mechanisms to total weight loss not quantified\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of GLP-1R biased signaling at the arrestin-coupling interface, the identity of the E3 ubiquitin ligase controlling receptor degradation, whether distinct CNS GLP-1R populations can be selectively targeted therapeutically to separate weight loss from aversion, and how peripheral immune GLP-1R signaling integrates with metabolic outcomes in humans.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No cryo-EM structure of GLP-1R–arrestin complex available\", \"E3 ligase for GLP-1R ubiquitination unidentified\", \"Clinical translation of CNS population-selective targeting untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4, 5, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [13, 15]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 2, 3, 5, 12, 14, 16, 21, 22]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [7, 9, 10, 11, 17, 18]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [19, 20, 30]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ARRB2\",\n      \"GRK2\",\n      \"GNAS\",\n      \"LEPR\",\n      \"FGF7\",\n      \"GIPR\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"GLP1R is a class B1 G-protein-coupled receptor that transduces signals from GLP-1 and related peptide or non-peptide agonists through Gαs-coupled cAMP/PKA and Epac pathways, as well as PLC-mediated Ca²⁺ mobilization, ERK/PKC, PI3K/AKT, and AMPK cascades to control insulin secretion, appetite, intestinal growth, lipid metabolism, and inflammation [PMID:8405712, PMID:25719403, PMID:25738454, PMID:36027914]. Structural studies of the extracellular domain, transmembrane core, and full-length receptor–Gs complexes have defined a two-step peptide-binding mechanism—N-terminal domain capture followed by transmembrane insertion—and revealed that agonist-specific contact dynamics at the peptide N-terminus allosterically tune G-protein coupling kinetics, providing a molecular basis for biased signaling [PMID:18287102, PMID:29466332, PMID:35013280, PMID:28514449]. Receptor desensitization is governed by GRK2-dependent β-arrestin 2 recruitment that controls internalization, lysosomal targeting, and ubiquitination; loss of β-arrestin 2 impairs acute cAMP generation yet reduces desensitization, and ligand off-rate is a principal determinant of G-protein coupling onset and cAMP duration [PMID:33658023, PMID:37134170, PMID:35300966]. In the CNS, GLP1R delineates dissociable neural circuits—NTS neurons driving satiety, area postrema neurons driving aversion, lateral septum neurons suppressing feeding via GABAergic projections to the lateral hypothalamus, and LepRb⁺ hypothalamic neurons required for both leptin- and GLP-1RA-mediated anorexia [PMID:38987598, PMID:39225090, PMID:37581939].\",\n  \"teleology\": [\n    {\n      \"year\": 1993,\n      \"claim\": \"Cloning of human GLP1R and heterologous reconstitution established that a single receptor couples to both adenylyl cyclase (cAMP) and PLC (Ca²⁺), resolving whether GLP-1 effects required one or multiple receptors.\",\n      \"evidence\": \"cDNA cloning from pancreatic islets, stable expression in COS-7/fibroblasts, radioligand binding, cAMP and Ca²⁺ assays\",\n      \"pmids\": [\"8405712\", \"8404634\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effectors beyond cAMP and Ca²⁺ not yet mapped\", \"Tissue-specific signaling outputs unknown\", \"Receptor desensitization mechanism not addressed\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Crystal structures of the GLP1R extracellular domain with exendin-4(9-39) and then GLP-1 defined the peptide-binding interface and showed ligand-specific conformational differences, answering how peptide agonists versus antagonists engage the N-terminal domain.\",\n      \"evidence\": \"X-ray crystallography at 2.1–2.2 Å resolution plus site-directed mutagenesis\",\n      \"pmids\": [\"18287102\", \"19861722\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No transmembrane domain structure yet available\", \"Mechanism of signal transduction across the membrane unresolved\", \"Contribution of ECL loops to full-length binding unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Systematic immunohistochemistry and radioligand binding in primate/human tissues resolved longstanding controversies about GLP1R tissue distribution, confirming expression in β-cells, vascular smooth muscle, sinoatrial node myocytes, and gut but not in liver or thyroid C cells.\",\n      \"evidence\": \"Validated monoclonal antibody IHC plus ¹²⁵I-GLP-1 in situ binding on human and primate tissues\",\n      \"pmids\": [\"24467746\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Neural distribution not systematically mapped in this study\", \"Conflicting evidence for hepatocyte GLP-1R from cell-line studies\", \"Cell-type resolution within pancreatic islets limited\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Genetic epistasis experiments identified Fgf7 as the obligate downstream mediator of GLP-1R-driven intestinal growth, and pharmacological dissection in cardiomyocytes distinguished PKA- versus Epac-dependent branches for cardioprotection, resolving which cAMP effectors mediate specific tissue outcomes.\",\n      \"evidence\": \"Glp1r⁻/⁻ and Fgf7⁻/⁻ mice for intestinal growth; selective PKA/Epac activators in cardiomyocytes for anti-apoptotic readouts\",\n      \"pmids\": [\"25738454\", \"25719403\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Fgf7-producing cell type not identified\", \"Whether Epac arm operates in β-cells independently of PKA not settled\", \"Mechanism linking GLP-1R to Fgf7 transcription unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identification of exendin-P5 as a G-protein-biased agonist with reduced β-arrestin recruitment but enhanced metabolic efficacy in diabetic mice demonstrated that GLP-1R signaling bias translates into distinct in vivo therapeutic profiles.\",\n      \"evidence\": \"Autocrine peptide library screen, cAMP versus β-arrestin assays, T2DM mouse models\",\n      \"pmids\": [\"26621478\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis for bias not yet determined at this time\", \"Long-term safety of biased agonism unknown\", \"Relative contribution of β-arrestin scaffolding versus desensitization unclear\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Crystal structures of the GLP1R transmembrane domain with negative allosteric modulators revealed an allosteric pocket outside helices V–VII, establishing that receptor activation can be modulated from a site distinct from the orthosteric peptide-binding cavity.\",\n      \"evidence\": \"X-ray crystallography (2.7–3.0 Å) of TMD with PF-06372222 and NNC0640, mutagenesis\",\n      \"pmids\": [\"28514449\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How positive allosteric modulators engage the same region to facilitate G-protein coupling not fully resolved\", \"No full-length inactive-state structure available\", \"Dynamics of transition from inactive to active conformation unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Cryo-EM of GLP-1R–Gs complexes with biased versus unbiased agonists showed that agonist-specific conformational differences at the extracellular face propagate through the transmembrane domain to alter the Gαs engagement angle, providing a structural explanation for signaling bias.\",\n      \"evidence\": \"Phase-plate cryo-EM at 3.3 Å comparing exendin-P5 and GLP-1 bound complexes\",\n      \"pmids\": [\"29466332\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"β-arrestin-bound receptor structure absent\", \"Dynamics of conformational propagation not captured in static structures\", \"Limited to Gs-coupled state\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Cryo-EM structures of non-peptide agonist-bound GLP-1R–Gs complexes showed that small-molecule agonists can closely mimic peptide binding or adopt entirely different binding modes with distinct receptor conformations, establishing molecular rules for non-peptide agonist design.\",\n      \"evidence\": \"Cryo-EM structures of PF-06882961 and CHU-128 bound complexes with pharmacological profiling\",\n      \"pmids\": [\"33027691\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Clinical translation of structural insights to oral drug design not demonstrated\", \"Conformational dynamics of non-peptide binding not explored by MD\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"GRK2 was identified as the kinase responsible for GLP-1R phosphorylation that enables β-arrestin recruitment and desensitization, with GRK2 hemizygosity enhancing early-phase insulin secretion by increasing the readily releasable granule pool.\",\n      \"evidence\": \"GRK2⁺/⁻ mice, nanoBRET for GLP-1R–GRK2 interaction, islet capacitance measurements\",\n      \"pmids\": [\"33658023\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific GRK2 phosphorylation sites on GLP-1R not mapped\", \"Role of other GRKs not excluded\", \"Whether GRK2 inhibition improves GLP-1RA efficacy in vivo not tested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Electrophysiology and in vivo photometry in hypothalamic circuits showed GLP-1RAs directly activate POMC neurons via TRPC5 channels and indirectly inhibit AgRP neurons, establishing the ion channel effectors and circuit logic underlying central appetite suppression.\",\n      \"evidence\": \"Neuron-specific transgenics, fiber photometry, patch-clamp, pharmacological channel blockers\",\n      \"pmids\": [\"34626854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether peripheral versus central GLP-1R pools are differentially engaged by long-acting agonists unclear\", \"Role of TRPC5 in human hypothalamic GLP-1R signaling unconfirmed\", \"Interaction with other nutrient-sensing circuits not mapped\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Comprehensive biosensor and phosphoproteomic analysis across 15 signaling pathways in 4 cellular compartments revealed that GLP-1R agonist modifications produce pathway- and compartment-selective signaling signatures, and that ligand off-rate governs G-protein coupling kinetics and cAMP duration.\",\n      \"evidence\": \"Multi-pathway biosensor panel, time-lapse microscopy, phosphoproteomics, kinetic BRET across multiple agonists\",\n      \"pmids\": [\"37813859\", \"35300966\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How compartment-specific signals are decoded into cellular outcomes remains unclear\", \"In vivo relevance of compartment bias not tested\", \"Contribution of receptor recycling versus degradation to sustained signaling not quantified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"GLP-1R on gut intraepithelial lymphocytes was shown to suppress T-cell-mediated inflammation via PKA-dependent dampening of proximal TCR signaling and to shape microbiota composition, identifying an immune-regulatory function independent of glucose homeostasis.\",\n      \"evidence\": \"IEL-specific conditional Glp1r knockout mice, T-cell activation assays, microbiota sequencing, PKA inhibitor experiments\",\n      \"pmids\": [\"36027914\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific microbiota taxa are regulated and through what mechanism not defined\", \"Relevance of IEL GLP-1R to human IBD not established\", \"Downstream PKA substrates in IELs not identified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"β-arrestin 2 conditional knockout in adult β-cells revealed that β-arrestin 2 is required for acute cAMP production yet simultaneously promotes desensitization, lysosomal targeting, and ubiquitination of GLP-1R, resolving the paradoxical dual role of β-arrestin 2 in GLP-1R signaling.\",\n      \"evidence\": \"Adult β-cell-specific β-arrestin 2 KO mice, islet cAMP measurements, receptor trafficking and ubiquitination assays\",\n      \"pmids\": [\"37134170\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which β-arrestin 2 loss upregulates β-arrestin 1 and PDE4 not elucidated\", \"Sex dimorphism in β-arrestin effects not mechanistically explained\", \"Whether trans-Golgi signaling is therapeutically exploitable unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Systematic functional profiling of 60 GLP1R coding variants established that impaired cell-surface expression is a genetically distinct mechanism from signaling loss-of-function, with the former associated with poor glycemic control in humans and rescuable by allosteric ligands.\",\n      \"evidence\": \"Multi-pathway signaling profiling of 60 variants, INS-1 cell insulin secretion rescue, UK Biobank association\",\n      \"pmids\": [\"37709961\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether allosteric rescue translates to improved clinical outcomes untested\", \"Structural basis for expression-defective variants not determined\", \"Gain-of-function variant physiology largely unexplored\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"In vivo imaging and circuit manipulation in the hindbrain demonstrated that GLP1R neurons in the NTS and area postrema drive dissociable satiety and aversion responses respectively, while lateral septum GLP1R neurons suppress feeding via GABAergic projections to the lateral hypothalamus, mapping the circuit architecture for GLP-1RA-induced appetite control.\",\n      \"evidence\": \"Two-photon calcium imaging, chemogenetic/optogenetic manipulation, projection tracing in Glp1r-Cre mice\",\n      \"pmids\": [\"38987598\", \"39225090\", \"38763494\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How peripheral GLP-1RA access reaches specific brain nuclei not resolved\", \"Human neuroanatomical equivalence of these circuits not confirmed\", \"Interaction between hindbrain and forebrain GLP1R populations during chronic treatment unclear\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Ligand-free cryo-EM structures showed that Gs protein alone partially activates GLP-1R by opening the intracellular cavity and rewiring the extracellular pocket, revealing a pre-engagement activation intermediate that facilitates peptide binding.\",\n      \"evidence\": \"Cryo-EM of ligand-free GLP-1R–Gs complex, comparative structural analysis with GCGR and GIPR\",\n      \"pmids\": [\"38346960\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this intermediate is populated under physiological ligand concentrations unknown\", \"Kinetic lifetime of the ligand-free complex not measured\", \"Implications for constitutive activity not explored\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of β-arrestin-bound GLP-1R, how compartment-specific signaling is decoded into distinct cellular outcomes in vivo, how peripheral versus central GLP-1R pools are differentially engaged by long-acting agonists, and whether allosteric rescue of expression-defective variants can improve clinical outcomes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No β-arrestin-bound GLP-1R structure\", \"Compartment-specific signaling decoded in vivo not demonstrated\", \"Mechanism by which GLP-1RAs cross the blood–brain barrier to engage specific neuronal populations unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 3, 4, 5, 6, 7, 8, 24]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 26]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 11, 25, 26, 28]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [25, 28]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 3, 4, 8, 16, 23, 24, 25, 27]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [12, 14, 22]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [21]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [20, 31, 32, 33, 34, 37]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [26, 35, 40]}\n    ],\n    \"complexes\": [\n      \"GLP-1R–Gαs–Gβγ heterotrimer\"\n    ],\n    \"partners\": [\n      \"GNA S\",\n      \"GRK2\",\n      \"ARRB2\",\n      \"PPP1R1A\",\n      \"TRPC5\",\n      \"FGF7\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}