{"gene":"GCG","run_date":"2026-04-28T18:06:52","timeline":{"discoveries":[{"year":1983,"finding":"The human preproglucagon gene (GCG) encodes multiple peptides through exon duplication and divergence, establishing that glucagon, GLP-1, and GLP-2 are all products of a single preproglucagon precursor gene.","method":"cDNA cloning and sequence analysis of the human preproglucagon gene","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — foundational molecular cloning, foundational discovery replicated across species","pmids":["6877358"],"is_preprint":false},{"year":1993,"finding":"The two bioactive forms of GLP-1 (7-36 amide and 7-37) are equipotent in stimulating insulin secretion, suppressing glucagon, and lowering plasma glucose in healthy humans, and share similar metabolic clearance rates (~12–15 pmol/kg·min) and plasma half-lives (~5–6 min), indicating C-terminal amidation does not significantly alter GLP-1 biological activity or metabolism.","method":"Intravenous infusion of GLP-1 7-36 amide vs. GLP-1 7-37 in healthy human subjects with measurement of insulin, C-peptide, glucagon, glucose, and metabolic clearance rates","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 1 — direct in vivo pharmacokinetic and pharmacodynamic comparison with multiple orthogonal endpoints","pmids":["8482423"],"is_preprint":false},{"year":1996,"finding":"Central GLP-1 (ICV injection) potently inhibits feeding in fasted rats and activates c-fos expression in the paraventricular nucleus (PVN) of the hypothalamus and central nucleus of the amygdala; the specific GLP-1 receptor antagonist exendin(9-39) blocks these effects and augments feeding in satiated animals, establishing central GLP-1 as a physiological mediator of satiety acting through hypothalamic circuits.","method":"Intracerebroventricular (ICV) injection of GLP-1 and exendin(9-39) in rats; food intake measurement; c-fos immunohistochemistry","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — receptor antagonist epistasis plus functional readout (food intake) plus neuronal activation marker; highly cited foundational study","pmids":["8538742"],"is_preprint":false},{"year":1996,"finding":"GLP-1 and PACAP depolarize pancreatic beta cells and stimulate insulin secretion through at least two mechanisms: closure of ATP-sensitive K+ channels and activation of nonselective cation channels (NSCCs) carrying predominantly Na+-dependent current; NSCCs are dually regulated by intracellular cAMP and Ca2+, and activation of GLP-1 signaling raises [Ca2+]i and promotes exocytosis of insulin-containing granules.","method":"Electrophysiology (patch-clamp), intracellular Ca2+ imaging, and pharmacological manipulation in pancreatic beta cells","journal":"Annals of the New York Academy of Sciences","confidence":"High","confidence_rationale":"Tier 1 — direct electrophysiological recordings with pharmacological dissection of ion channel mechanisms","pmids":["8993395"],"is_preprint":false},{"year":1999,"finding":"GLP-2 (a proglucagon-derived peptide) exerts its intestinotrophic actions through a specific G protein-coupled receptor (GLP-2R) expressed in the gut; GLP-2R activation by GLP-2 but not GLP-1 increases cAMP (EC50 = 0.58 nM) and shows saturable high-affinity radioligand binding (Kd = 0.57 nM), demonstrating ligand specificity among proglucagon-derived peptides.","method":"cDNA cloning of rat and human GLP-2R; cAMP assays; radioligand binding; in vivo intestinotrophic activity of GLP-2 analogs","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — receptor cloning with functional reconstitution, binding assays, and in vivo validation","pmids":["9990065"],"is_preprint":false},{"year":2003,"finding":"Glucagon is the principal counterregulatory hormone that increases hepatic glucose output by simultaneously promoting glycogenolysis and gluconeogenesis while inhibiting glycogenesis and glycolysis, acting through the glucagon receptor; diabetic hyperglucagonemia contributes to pathological hyperglycemia.","method":"Review integrating in vivo and in vitro mechanistic studies of glucagon receptor signaling and hepatic glucose metabolism","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 — synthesis of multiple mechanistic studies with well-established in vivo and in vitro experimental support","pmids":["12626323"],"is_preprint":false},{"year":2003,"finding":"GLP-1 inhibits apoptosis in freshly isolated human islets by downregulating active caspase-3 and upregulating bcl-2 at both mRNA and protein levels, while enhancing intracellular insulin content and glucose-dependent insulin secretion; this establishes a direct antiapoptotic mechanism of GLP-1 in human beta cells.","method":"Culture of human islets with GLP-1 (10 nM); measurement of apoptotic nuclei, caspase-3 and bcl-2 mRNA and protein; insulin content and secretion assays","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (histology, RT-PCR, Western blot, secretion assay) in human tissue","pmids":["12960095"],"is_preprint":false},{"year":2003,"finding":"GLP-1 stimulates pancreatic beta-cell proliferation, induces islet neogenesis, promotes differentiation toward a beta-cell phenotype from exocrine or progenitor cells, and exerts antiapoptotic actions preserving beta-cell mass in vivo; GLP-2 analogously stimulates gastrointestinal mucosal cell proliferation and exerts antiapoptotic actions on gut epithelium.","method":"Review integrating in vivo genetic models, cell culture experiments, and proglucagon-derived peptide biology","journal":"Molecular endocrinology (Baltimore, Md.)","confidence":"High","confidence_rationale":"Tier 2 — multiple experimental systems cited; well-established mechanistic framework","pmids":["12554744"],"is_preprint":false},{"year":2003,"finding":"Leptin directly stimulates GLP-1 secretion from intestinal L cells via the long form of the leptin receptor (Ob-Rb), inducing STAT3 phosphorylation; leptin resistance in diet-induced obese mice is associated with reduced basal GLP-1 and diminished GLP-1 response to oral glucose, linking leptin signaling to GLP-1 secretory regulation.","method":"GLP-1 secretion assays in fetal rat intestinal cells, GLUTag and NCI-H716 L-cell lines, and in vivo in rats and ob/ob mice; Western blot for STAT3 phosphorylation; high-fat diet mouse model","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 2 — multiple cell models plus in vivo validation with receptor expression and downstream signaling confirmed","pmids":["12540594"],"is_preprint":false},{"year":2003,"finding":"GLP-1 rapidly degrades in plasma with a half-life of ~1–2 min primarily through DPP-IV cleavage; kidneys are important for final elimination of the metabolite GLP-1(9-36 amide), as patients with chronic renal insufficiency show significantly longer half-lives for the metabolite (5.3 vs. 3.3 min) but not for intact GLP-1.","method":"Intravenous infusion of GLP-1 in patients with chronic renal insufficiency vs. healthy controls; specific immunoassays for intact and total GLP-1; pharmacokinetic modeling","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 1 — controlled intravenous pharmacokinetic study with specific immunoassays distinguishing intact vs. metabolite forms","pmids":["14988249"],"is_preprint":false},{"year":2003,"finding":"Oxyntomodulin (a proglucagon-derived peptide) suppresses appetite and reduces food intake in humans when infused intravenously, and significantly suppresses circulating ghrelin levels (by ~44% of postprandial decrease), identifying oxyntomodulin as a physiological appetite-regulating signal from the GCG locus.","method":"Randomized, double-blind, placebo-controlled crossover study with IV OXM infusion in healthy subjects; ad libitum food intake measurement; plasma ghrelin, appetite VAS scores","journal":"The Journal of clinical endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 — double-blind crossover RCT with multiple physiological endpoints","pmids":["14557443"],"is_preprint":false},{"year":2005,"finding":"GLP-1 directly protects the heart against ischemia/reperfusion injury via GLP-1 receptor-dependent activation of cAMP, PI3-kinase, and p42/44 MAPK prosurvival pathways, leading to phosphorylation of the proapoptotic peptide BAD; protection is abolished by the GLP-1 receptor antagonist exendin(9-39), Rp-cAMP, LY294002 (PI3K inhibitor), or UO126 (MEK inhibitor).","method":"Isolated perfused rat heart and whole animal ischemia/reperfusion models; infarct size measurement; pharmacological inhibition with receptor antagonist and kinase inhibitors; Western blot for phospho-BAD","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 1 — reconstituted organ model with multiple orthogonal pharmacological antagonists and downstream signaling readout","pmids":["15616022"],"is_preprint":false},{"year":2005,"finding":"GLP-1 receptor activation in pancreatic beta cells promotes nuclear exclusion of Foxo1 through PKB (Akt)-mediated phosphorylation, downstream of IRS-2 and PI3K signaling; this upregulates PDX-1, a master regulator of beta-cell growth and differentiation, providing a mechanism for GLP-1-induced expansion of beta-cell mass.","method":"Mechanistic review integrating cell-based studies of GLP-1R → IRS-2 → PI3K → PKB → Foxo1 phosphorylation → PDX-1 upregulation in pancreatic beta cells","journal":"Science's STKE : signal transduction knowledge environment","confidence":"Medium","confidence_rationale":"Tier 2 — synthesis of multiple cell-based mechanistic experiments; single review perspective","pmids":["15671479"],"is_preprint":false},{"year":2006,"finding":"GLP-2 (a proglucagon-derived peptide) stimulates glucagon secretion in humans both in the fasting state and postprandially, enhances intestinal lipid absorption (elevated postprandial triglycerides and free fatty acids), and inhibits pentagastrin-stimulated gastric acid secretion by ~15%, without affecting gastric emptying.","method":"Randomized crossover studies in 15 healthy male volunteers each; IV infusion of GLP-2 vs. placebo; plasma glucagon, lipid, and gastric acid measurements; 13C-octanoate breath test for gastric emptying","journal":"Gastroenterology","confidence":"High","confidence_rationale":"Tier 2 — controlled crossover human studies with multiple physiological endpoints","pmids":["16401467"],"is_preprint":false},{"year":2006,"finding":"Insulin-degrading enzyme (IDE) degrades glucagon (as well as insulin, amyloid-β, and amylin) by encapsulating it within an enclosed catalytic chamber formed by IDE-N and IDE-C domains; substrate access requires repositioning of these domains, and the substrate undergoes conformational changes to form beta-sheets with IDE for degradation.","method":"Crystal structures of human IDE in complex with substrates including glucagon; in vitro activity assays; mutagenesis disrupting IDE-N/IDE-C contacts increases catalytic activity 40-fold","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus mutagenesis plus in vitro reconstitution","pmids":["17051221"],"is_preprint":false},{"year":2007,"finding":"Intestinal L cells express sweet taste receptors and the taste G protein α-gustducin; glucose-stimulated GLP-1 secretion is defective in α-gustducin null mice and in isolated intestinal tissue from these mice; GLP-1 release from NCI-H716 cells is promoted by sugars and the noncaloric sweetener sucralose and blocked by the sweet receptor antagonist lactisole or siRNA for α-gustducin, establishing that L cells 'taste' luminal glucose through the same machinery as lingual taste cells to trigger GLP-1 secretion.","method":"α-Gustducin knockout mice; glucose tolerance tests; isolated small bowel GLP-1 secretion assays; NCI-H716 cell line; siRNA knockdown of α-gustducin; pharmacological antagonism with lactisole","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout plus in vitro siRNA knockdown plus pharmacological antagonism across multiple systems","pmids":["17724330"],"is_preprint":false},{"year":2007,"finding":"GLP-1 binds to its class B GPCR through a two-domain mechanism: the N-terminal extracellular domain (ECD) provides most binding energy for the C-terminal region of GLP-1, while the core transmembrane domain binds the N-terminal region of GLP-1 to transmit the activation signal; in contrast, exendin-4 can bind the isolated ECD with full-receptor affinity, indicating mechanistic differences between GLP-1 and exendin-4 receptor engagement.","method":"Peptide binding studies using full-length and truncated GLP-1 receptor variants and truncated ligands; radioligand competition assays","journal":"Biochemical Society transactions","confidence":"Medium","confidence_rationale":"Tier 2 — systematic binding studies with receptor and ligand truncations; mechanistic model supported by multiple constructs","pmids":["17635131"],"is_preprint":false},{"year":2007,"finding":"Crystal structure of the GIP receptor extracellular domain (ECD) bound to GIP(1-42) reveals that the incretin hormone binds in an alpha-helical conformation within a surface groove of the ECD through predominantly hydrophobic interactions; the N-terminal residues of GIP remain free, consistent with a two-domain binding model conserved across class B GPCRs including GLP-1R.","method":"X-ray crystallography of human GIP receptor ECD in complex with GIP(1-42); thermodynamic binding analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with thermodynamic validation; provides structural framework for incretin-receptor binding","pmids":["17715056"],"is_preprint":false},{"year":2008,"finding":"β-Arrestin-1 physically associates with the GLP-1 receptor (demonstrated by co-immunoprecipitation in INS-1 beta cells) and mediates GLP-1 signaling to insulin secretion; β-arrestin-1 knockdown reduces cAMP production, ERK and CREB activation, IRS-2 expression, and insulin secretion in response to GLP-1, without affecting GLP-1R surface expression or ligand-induced internalization.","method":"Co-immunoprecipitation in INS-1 cells; siRNA knockdown of β-arrestin-1; measurement of cAMP, ERK/CREB phosphorylation, IRS-2 expression, and insulin secretion","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP plus siRNA knockdown with multiple downstream signaling readouts","pmids":["18445652"],"is_preprint":false},{"year":2011,"finding":"IL-6 stimulates GLP-1 secretion from both intestinal L cells and pancreatic alpha cells; in alpha cells, IL-6 increases GLP-1 production by upregulating proglucagon (GCG) gene expression and prohormone convertase 1/3 expression, establishing a mechanism by which exercise-elevated IL-6 enhances GLP-1 availability to improve insulin secretion.","method":"IL-6 administration in mice and in vitro in L-cell and alpha-cell lines; measurement of GLP-1 secretion, proglucagon mRNA, and prohormone convertase 1/3 expression; IL-6 neutralization experiments; exercise models","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 — multiple cell types, in vivo and in vitro, with gene expression and secretion endpoints, and loss-of-function (IL-6 neutralization)","pmids":["22037645"],"is_preprint":false},{"year":2012,"finding":"A major lineage of enteroendocrine cells co-expresses proglucagon (GLP-1), CCK, GIP, PYY, neurotensin, and secretin but not somatostatin throughout the intestine; cell ablation driven by the proglucagon promoter (diphtheria toxin receptor model) markedly reduces not only GLP-1 cells but also PYY, neurotensin, GIP, CCK, and secretin cells, demonstrating that GCG-expressing L cells represent a broad shared lineage of enteroendocrine cells.","method":"Transgenic mice with CCK-eGFP reporter; FACS purification and quantitative PCR; LC-MS proteomics; immunohistochemistry; proglucagon-promoter-driven diphtheria toxin receptor cell ablation; single-cell qPCR","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including genetic cell ablation, proteomics, and single-cell analysis","pmids":["23064014"],"is_preprint":false},{"year":2016,"finding":"NTS astrocytes express functional GLP-1 receptors; systemic or central administration of fluorophore-labeled exendin-4 localizes within NTS astrocytes and neurons in a GLP-1R-dependent manner (blocked by exendin(9-39)); GLP-1R agonists activate NTS astrocytes (live-cell calcium signaling) and increase cAMP in astrocytes in vitro; pharmacological inhibition of NTS astrocytes attenuates the anorectic and body-weight-suppressive effects of intra-NTS GLP-1R activation.","method":"Fluorophore-labeled GLP-1R agonist localization in vivo; ex vivo and in vitro calcium signaling; cAMP assays in astrocytes; immunohistochemistry; pharmacological inhibition of NTS astrocytes with behavioral readout","journal":"The Journal of neuroscience : the official journal of the Society for Neuroscience","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods establishing astrocytic GLP-1R localization and functional consequence on food intake","pmids":["27013681"],"is_preprint":false},{"year":2018,"finding":"GLP-1 receptor agonists that retain GLP-1R at the plasma membrane (reduced internalization and slower agonist dissociation rates, with reduced β-arrestin recruitment) produce greater long-term insulin release compared to FDA-approved GLP-1 mimetics; this enhanced insulin secretion is dependent on reduced β-arrestin recruitment, establishing a mechanism by which GLP-1R trafficking determines insulinotropic efficacy.","method":"Series of biased GLP-1R agonists tested for GLP-1R internalization, recycling, β-arrestin recruitment, and insulin secretion in primary islets; glycemic studies in mice; comparison to panel of FDA-approved GLP-1 mimetics","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — structure-activity series with multiple signaling and functional endpoints including in vivo glycemic testing","pmids":["29686402"],"is_preprint":false},{"year":2020,"finding":"Crystal structure of the full-length human GLP-1R in peptide-free inactive state at 3.2 Å reveals a unique closed conformation of the extracellular domain (ECD); disulfide cross-linking validates the physiological relevance of this closed conformation; EM and MD simulations show large ECD conformational dynamics necessary for GLP-1 binding, providing a mechanistic framework for receptor activation.","method":"X-ray crystallography (3.2 Å); disulfide cross-linking; electron microscopy; molecular dynamics simulation","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with multiple orthogonal validation methods","pmids":["32152292"],"is_preprint":false},{"year":2020,"finding":"A non-peptide agonist (TT-OAD2) binds the GLP-1 receptor at an unpredicted extracellular pocket distinct from the peptide-binding site, causing reorganization of ECL3 and TM helices 6 and 7 independently of direct interaction with the deep transmembrane pocket; TT-OAD2 shows biased agonism with distinct G-protein activation kinetics compared to peptide agonists, and protrudes beyond the receptor core to interact with lipid/detergent, explaining its distinct activation kinetics.","method":"Cryo-EM structure of GLP-1R bound to TT-OAD2; signaling assays (cAMP, β-arrestin); binding competition studies","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure plus pharmacological characterization; reveals novel binding site","pmids":["31915381"],"is_preprint":false},{"year":2020,"finding":"A positive allosteric modulator (LSN3160440) of GLP-1R acts as a 'molecular glue' that cooperatively binds both the receptor and orthosteric ligand GLP-1(9-36), stabilizing the active-state conformation; cryo-EM structure reveals the modulator binds at an interface between TM1 and TM2 high in the helical bundle, allowing access to the peptide ligand; LSN3160440 enhances insulin secretion in a glucose-, ligand-, and GLP-1R-dependent manner.","method":"Cryo-EM structure of GLP-1R bound to LSN3160440 + GLP-1 + Gs; insulin secretion assays; pharmacological characterization with multiple ligands; probe-dependence analysis","journal":"Nature chemical biology","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure plus functional insulin secretion assay and pharmacological characterization","pmids":["32690941"],"is_preprint":false},{"year":2020,"finding":"Tirzepatide shows biased agonism at the GLP-1 receptor, favoring cAMP generation over β-arrestin recruitment, and drives less GLP-1R internalization compared to native GLP-1; β-arrestin1 limits the insulin response to GLP-1 but not GIP or tirzepatide, and tirzepatide engages the GIP receptor more than the GLP-1R at clinically efficacious doses; these signaling properties together enhance insulin secretion.","method":"Receptor occupancy calculations; cAMP and β-arrestin recruitment signaling assays; GLP-1R internalization assays; insulin secretion in primary islets from β-arrestin1 knockout and wild-type mice","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 — multiple signaling assays with genetic validation (β-arrestin1 KO islets)","pmids":["32730231"],"is_preprint":false},{"year":2021,"finding":"PPGNTS neurons (preproglucagon-expressing neurons in the nucleus tractus solitarii) encode satiation through vagal signaling of gastrointestinal distension and receive vagal input predominantly from oxytocin-receptor-expressing vagal neurons rather than GLP-1 receptor-expressing vagal neurons; PPGNTS neurons are not necessary for eating suppression by GLP-1 receptor agonists, and concurrent PPGNTS neuron activation suppresses eating more potently than semaglutide alone, demonstrating that central and peripheral GLP-1 systems suppress eating via independent gut-brain circuits.","method":"Optogenetic manipulation of PPGNTS neurons; calcium imaging; vagal circuit mapping; genetic deletion of PPGNTS neurons; comparison with semaglutide treatment in mice","journal":"Nature metabolism","confidence":"High","confidence_rationale":"Tier 2 — optogenetics plus calcium imaging plus genetic circuit dissection with behavioral readout","pmids":["33589843"],"is_preprint":false},{"year":2022,"finding":"Intestinal Gpr17 (an orphan GPCR) is co-expressed in GLP-1-expressing enteroendocrine cells and functions as an inhibitory regulator of GLP-1 secretion; Gpr17 knockout or acute intestinal epithelial ablation improves glucose tolerance and GSIS; Gpr17 agonism in vitro reduces voltage-gated calcium currents and decreases cAMP production, the two critical factors regulating GLP-1 secretion; Gpr17 null intestinal organoids show increased GLP-1 (but not GIP) secretion in response to glucose or lipid.","method":"Inducible intestinal-specific Gpr17 knockout mice; intestinal organoids; electrophysiology (voltage-gated Ca2+ currents); cAMP assays; GSIS and glucose tolerance tests","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — genetic KO plus organoid model plus electrophysiology and cAMP mechanistic studies","pmids":["34986353"],"is_preprint":false},{"year":2024,"finding":"GLP-1 receptor neurons in the dorsomedial hypothalamus (DMH) encode preingestive satiation; optogenetic activation of DMHGLP-1R neurons causes satiation; calcium imaging shows these neurons are activated specifically during eating behavior; GLP-1RA administration selectively increases DMHGLP-1R neuron activity, and these neurons interact with NPY/AgRP neurons of the arcuate nucleus to regulate food intake.","method":"Optogenetic manipulation and calcium imaging of DMHGLP-1R neurons in mice; GLP-1RA administration; human and mouse brain sample analysis; food intake measurements","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 2 — optogenetics plus calcium imaging plus pharmacological GLP-1RA treatment with defined circuit mapping","pmids":["38935778"],"is_preprint":false},{"year":2024,"finding":"GLP-1 receptor activation in the olfactory bulb (OB) stimulates insulin secretion in response to oral glucose via a top-down neural circuit: OB GLP-1R activation reduces noradrenaline content in the pancreas (implicating the sympathetic nervous system), and this effect is relayed through the PVN (abolished by GABAA receptor inhibition in PVN), identifying a novel brain-to-pancreas axis for GLP-1-mediated insulin secretion.","method":"Stereotaxic OB GLP-1R activation in lean and diet-induced obese male mice; measurement of pancreatic noradrenaline; α2-adrenergic receptor agonist blockade; GABAA receptor inhibition in PVN; glucose-stimulated insulin secretion assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — pharmacological circuit dissection with multiple anatomical interventions and mechanistic readouts in vivo","pmids":["39138162"],"is_preprint":false},{"year":2024,"finding":"Orforglipron, a nonpeptide oral GLP-1R agonist, binds human GLP-1R with high affinity (Ki = 1 nM), has low intrinsic efficacy for effector activation and negligible β-arrestin recruitment; low GLP-1R occupancy by orforglipron is sufficient to yield a full biological response in vivo; target engagement in pancreas and brain is confirmed using CRISPR-Cas9-sensitized rat Glp1r, demonstrating the pharmacological basis for nonpeptide GLP-1R agonism.","method":"Radioligand competition binding ([125I]GLP-1 and [3H]orforglipron); signal transduction assays; receptor occupancy modeling; glucose tolerance tests in humanized GLP-1R mice; CRISPR-Cas9 Glp1r-sensitized rats; DIO weight loss studies","journal":"Science translational medicine","confidence":"High","confidence_rationale":"Tier 1 — binding assays, signaling assays, CRISPR genetic validation, and in vivo pharmacology","pmids":["39693407"],"is_preprint":false}],"current_model":"The GCG gene encodes preproglucagon, which is processed tissue-specifically into glucagon (in pancreatic α-cells, acting through glucagon receptors to promote hepatic glycogenolysis, gluconeogenesis, and counterregulation), GLP-1 (in intestinal L-cells and brain NTS neurons, acting through a single class B GPCR—GLP-1R—to stimulate insulin secretion via cAMP/PKA and β-arrestin-1, inhibit glucagon secretion, slow gastric emptying, and suppress feeding through hypothalamic and brainstem circuits including DMHGLP-1R and PPGNTS neurons), GLP-2 (promoting intestinal growth via a distinct GLP-2R), and oxyntomodulin (suppressing appetite and ghrelin); GLP-1 secretion from L-cells is triggered by luminal glucose through α-gustducin/sweet taste receptor machinery and modulated by leptin (via STAT3/Ob-Rb) and IL-6 (upregulating proglucagon and PC1/3 in α-cells), while GLP-1R signaling efficacy is determined by β-arrestin recruitment (promoting receptor internalization and desensitization), GLP-1R trafficking, and intracellular cAMP generation, with structural mechanisms revealed by crystal and cryo-EM structures of GLP-1R in inactive, peptide-bound, non-peptide-agonist-bound, and positive allosteric modulator-bound states."},"narrative":{"teleology":[{"year":1983,"claim":"Cloning the preproglucagon cDNA established that glucagon, GLP-1, and GLP-2 arise from a single gene through exon duplication, resolving how structurally related gut-pancreas peptides share a common genetic origin.","evidence":"cDNA cloning and sequence analysis of the human preproglucagon gene","pmids":["6877358"],"confidence":"High","gaps":["Tissue-specific post-translational processing mechanisms were not defined","Functions of GLP-1 and GLP-2 were unknown at this point"]},{"year":1993,"claim":"Demonstration that GLP-1(7-36 amide) and GLP-1(7-37) are equipotent insulinotropic agents with identical clearance kinetics established the active forms of the peptide and its extremely short plasma half-life (~5–6 min), framing a key pharmacological challenge.","evidence":"Intravenous infusion of both GLP-1 forms in healthy humans with insulin, glucagon, glucose, and clearance measurements","pmids":["8482423"],"confidence":"High","gaps":["The proteolytic enzyme responsible for rapid degradation was not identified in this study","Receptor binding kinetics of each form were not compared"]},{"year":1996,"claim":"Two foundational mechanisms of GLP-1 action were defined: central GLP-1 was shown to suppress feeding via hypothalamic circuits (blocked by exendin(9-39)), and peripheral GLP-1 was shown to stimulate insulin secretion through both KATP channel closure and nonselective cation channel activation in beta cells.","evidence":"ICV injection with c-fos mapping and receptor antagonist epistasis in rats; patch-clamp electrophysiology and Ca²⁺ imaging in beta cells","pmids":["8538742","8993395"],"confidence":"High","gaps":["Identity of the specific neuronal populations mediating central anorexia was unresolved","Relative contribution of KATP vs. NSCC pathways to insulin secretion in vivo was unclear"]},{"year":1999,"claim":"Cloning of the GLP-2 receptor demonstrated that each major proglucagon-derived peptide acts through a distinct GPCR, with GLP-2R showing exquisite ligand specificity (nanomolar affinity for GLP-2 but not GLP-1), explaining how a single precursor gene controls diverse tissue-specific functions.","evidence":"cDNA cloning of rat and human GLP-2R; cAMP and radioligand binding assays; in vivo intestinotrophic activity","pmids":["9990065"],"confidence":"High","gaps":["Downstream signaling cascades of GLP-2R were not characterized","Intestinotrophic mechanism beyond receptor activation was unknown"]},{"year":2003,"claim":"A cluster of discoveries established GLP-1 as a trophic and antiapoptotic factor for beta cells (via caspase-3/bcl-2), identified leptin-STAT3 signaling as a regulator of L-cell GLP-1 secretion, confirmed DPP-IV as the primary GLP-1-degrading enzyme in vivo, and showed oxyntomodulin suppresses appetite and ghrelin in humans, broadening the functional repertoire of proglucagon-derived peptides.","evidence":"Human islet culture with apoptosis markers; L-cell secretion assays with leptin/STAT3; IV pharmacokinetics in renal insufficiency patients; double-blind crossover OXM infusion in humans","pmids":["12960095","12540594","14988249","14557443"],"confidence":"High","gaps":["Whether GLP-1 antiapoptotic effects are sustained in vivo long-term was untested","Molecular mechanism of oxyntomodulin anorexia (receptor identity) was unclear"]},{"year":2005,"claim":"GLP-1R signaling was linked to cardioprotection via cAMP/PI3K/MAPK prosurvival pathways phosphorylating BAD, and to beta-cell mass expansion via IRS-2/PI3K/Akt-mediated Foxo1 nuclear exclusion upregulating PDX-1, revealing GLP-1 actions beyond acute insulin secretion.","evidence":"Isolated perfused heart ischemia/reperfusion with kinase inhibitors; cell-based studies of GLP-1R → IRS-2 → PI3K → Akt → Foxo1 → PDX-1","pmids":["15616022","15671479"],"confidence":"High","gaps":["In vivo relevance of GLP-1 cardioprotection in humans was not established","Whether Foxo1 exclusion is necessary or sufficient for beta-cell proliferation was not genetically tested"]},{"year":2007,"claim":"The luminal glucose-sensing mechanism for GLP-1 secretion was identified: intestinal L cells employ sweet taste receptors and α-gustducin, the same molecular machinery as lingual taste cells, to detect sugars and trigger GLP-1 release.","evidence":"α-Gustducin knockout mice; NCI-H716 cells; siRNA knockdown; lactisole pharmacological antagonism; glucose tolerance tests","pmids":["17724330"],"confidence":"High","gaps":["Whether this mechanism accounts for the majority of nutrient-stimulated GLP-1 secretion in humans was untested","Lipid and amino acid sensing pathways in L cells were not addressed"]},{"year":2007,"claim":"Structural understanding of incretin-receptor engagement advanced through binding studies showing GLP-1R uses a two-domain mechanism (ECD captures peptide C-terminus; TMD binds N-terminus for activation), and crystallography of the related GIP receptor ECD–GIP complex confirmed a conserved binding mode across class B GPCRs.","evidence":"Radioligand competition with GLP-1R truncation constructs; crystal structure of GIP receptor ECD bound to GIP(1-42)","pmids":["17635131","17715056"],"confidence":"High","gaps":["Full-length GLP-1R structure was not yet available","Activation-associated conformational transitions were not visualized"]},{"year":2008,"claim":"β-Arrestin-1 was identified as a direct GLP-1R interactor that positively mediates GLP-1-stimulated cAMP, ERK/CREB, and insulin secretion, adding an arrestin-dependent signaling arm beyond classical G-protein coupling.","evidence":"Co-immunoprecipitation in INS-1 cells; siRNA knockdown of β-arrestin-1 with cAMP, ERK/CREB, IRS-2, and insulin secretion readouts","pmids":["18445652"],"confidence":"High","gaps":["Whether β-arrestin-1 signals from endosomes vs. plasma membrane was not resolved","Contribution to in vivo insulin secretion was not tested"]},{"year":2011,"claim":"IL-6 was shown to stimulate GLP-1 production from pancreatic alpha cells by upregulating both proglucagon and prohormone convertase 1/3 expression, providing a mechanism linking exercise-induced IL-6 to enhanced incretin availability.","evidence":"IL-6 administration in mice; alpha-cell and L-cell line culture; IL-6 neutralization; proglucagon and PC1/3 mRNA quantification","pmids":["22037645"],"confidence":"High","gaps":["Quantitative contribution of alpha-cell-derived GLP-1 vs. L-cell-derived GLP-1 to systemic GLP-1 pools was unresolved","Signaling pathway downstream of IL-6 receptor in alpha cells was not fully mapped"]},{"year":2018,"claim":"Biased agonism at GLP-1R was functionally validated: agonists that reduce β-arrestin recruitment and receptor internalization while retaining cAMP signaling produce superior long-term insulin secretion compared to FDA-approved GLP-1 mimetics, establishing receptor trafficking as a determinant of therapeutic efficacy.","evidence":"Series of biased agonists tested for GLP-1R internalization, recycling, β-arrestin recruitment, and insulin secretion in primary islets; glycemic studies in mice","pmids":["29686402"],"confidence":"High","gaps":["Long-term in vivo consequences of biased agonism on beta-cell mass were not assessed","Whether biased agonism also affects central anorectic actions was unknown"]},{"year":2020,"claim":"Structural resolution of GLP-1R in multiple states—inactive (crystal structure showing closed ECD), non-peptide agonist-bound (cryo-EM revealing an extracellular binding pocket for TT-OAD2), and positive allosteric modulator-bound (cryo-EM showing molecular-glue mechanism at TM1/TM2)—provided a comprehensive conformational framework for receptor activation and drug design.","evidence":"X-ray crystallography at 3.2 Å; cryo-EM structures; disulfide cross-linking; MD simulations; signaling and insulin secretion assays","pmids":["32152292","31915381","32690941"],"confidence":"High","gaps":["Dynamics of GLP-1R activation at the single-molecule level in membranes remain unresolved","How allosteric modulator engagement affects receptor trafficking and desensitization in vivo is unknown"]},{"year":2020,"claim":"Tirzepatide was shown to exhibit biased agonism at GLP-1R favoring cAMP over β-arrestin, and β-arrestin-1 knockout islets confirmed that β-arrestin-1 limits insulin responses to GLP-1 but not GIP, explaining how dual-agonist design achieves enhanced insulinotropic efficacy.","evidence":"cAMP and β-arrestin signaling assays; GLP-1R internalization; insulin secretion in β-arrestin-1 KO vs. WT mouse islets; receptor occupancy modeling","pmids":["32730231"],"confidence":"High","gaps":["In vivo receptor-level occupancy at therapeutic doses in humans was modeled but not directly measured","Contribution of GIP vs. GLP-1 receptor pathways to weight loss was not dissected"]},{"year":2021,"claim":"Circuit dissection revealed that preproglucagon NTS neurons (PPGNTS) encode satiation from vagal gastrointestinal distension input and suppress eating independently of peripheral GLP-1R agonist action, demonstrating that central and peripheral GLP-1 systems constitute parallel anorectic pathways.","evidence":"Optogenetic manipulation of PPGNTS neurons; calcium imaging; vagal circuit mapping; genetic ablation; comparison with semaglutide in mice","pmids":["33589843"],"confidence":"High","gaps":["Whether combined PPGNTS activation and GLP-1RA treatment produces synergistic weight loss long-term was untested","Molecular identity of the PPGNTS-released mediator(s) (GLP-1 vs. other peptides) was not resolved"]},{"year":2024,"claim":"Multiple new GLP-1R-expressing neural circuits were functionally mapped: DMH GLP-1R neurons encode preingestive satiation and interact with ARC NPY/AgRP neurons, while olfactory bulb GLP-1R activation stimulates insulin secretion via a sympathetic brain-to-pancreas axis relayed through the PVN, expanding the known central targets of GLP-1 signaling.","evidence":"Optogenetics and calcium imaging of DMHGLP-1R neurons; stereotaxic OB GLP-1R activation; PVN GABAA inhibition; pancreatic noradrenaline measurement in lean and DIO mice","pmids":["38935778","39138162"],"confidence":"High","gaps":["Whether DMH or OB circuits are engaged by therapeutic GLP-1RA doses in humans is unknown","Molecular mediators downstream of PVN relay to sympathetic outflow are not identified"]},{"year":2024,"claim":"Orforglipron, a nonpeptide oral GLP-1R agonist, was shown to bind GLP-1R with 1 nM affinity yet with low intrinsic efficacy and negligible β-arrestin recruitment; CRISPR-Cas9-sensitized Glp1r rats confirmed target engagement in pancreas and brain, establishing the pharmacological basis for oral nonpeptide GLP-1R agonism.","evidence":"Radioligand binding; signal transduction assays; receptor occupancy modeling; humanized GLP-1R mice; CRISPR-sensitized rats; DIO weight loss","pmids":["39693407"],"confidence":"High","gaps":["Long-term metabolic and safety profiles of low-efficacy nonpeptide agonism are not established","Whether orforglipron engages the same allosteric site as TT-OAD2 is not resolved"]},{"year":null,"claim":"Key unresolved questions include the quantitative contribution of alpha-cell-derived vs. L-cell-derived GLP-1 to systemic incretin pools, the molecular mediators through which PPGNTS neurons suppress eating, the in vivo relevance of GLP-1R allosteric modulator and biased agonist mechanisms to long-term beta-cell mass and weight regulation, and how Gpr17 and other inhibitory GPCRs coordinate with nutrient-sensing pathways to fine-tune GLP-1 secretion.","evidence":"","pmids":[],"confidence":"Low","gaps":["Alpha-cell vs. L-cell GLP-1 contributions not quantified in humans","PPGNTS neuron-released mediator identity unresolved","Long-term in vivo consequences of biased agonism on beta-cell mass unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[1,2,3,6,10,11]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,13]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[1,9,10,13]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,11,18,22,24,25,26,31]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[5,13,14]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[2,21,27,29,30]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,14,19]}],"complexes":[],"partners":["GLP1R","GCGR","GLP2R","ARRB1","IDE","DPP4","GNAT3","IL6"],"other_free_text":[]},"mechanistic_narrative":"GCG encodes preproglucagon, a precursor that is tissue-specifically processed into glucagon, GLP-1, GLP-2, and oxyntomodulin, each acting through distinct receptors to coordinate glucose homeostasis, appetite, and intestinal growth [PMID:6877358, PMID:12554744]. Glucagon promotes hepatic glucose output via glycogenolysis and gluconeogenesis [PMID:12626323]; GLP-1 stimulates glucose-dependent insulin secretion through cAMP/PKA-mediated closure of KATP channels and activation of nonselective cation channels in beta cells [PMID:8993395], protects beta cells from apoptosis via caspase-3 downregulation and bcl-2 upregulation [PMID:12960095], and suppresses feeding through hypothalamic (DMH) and brainstem (NTS) GLP-1R-expressing circuits that encode satiation independently of peripheral preproglucagon neurons [PMID:8538742, PMID:38935778, PMID:33589843]. GLP-1 secretion from intestinal L cells is triggered by luminal glucose via α-gustducin/sweet taste receptor machinery [PMID:17724330] and modulated by leptin (STAT3/Ob-Rb) and IL-6-driven upregulation of proglucagon and PC1/3 in alpha cells [PMID:12540594, PMID:22037645], while GLP-1R signaling efficacy is determined by β-arrestin-1 recruitment and receptor trafficking, with biased agonists that reduce β-arrestin engagement and GLP-1R internalization producing greater insulinotropic responses [PMID:18445652, PMID:29686402, PMID:32730231]."},"prefetch_data":{"uniprot":{"accession":"P01275","full_name":"Pro-glucagon","aliases":[],"length_aa":180,"mass_kda":20.9,"function":"Plays a key role in glucose metabolism and homeostasis. Regulates blood glucose by increasing gluconeogenesis and decreasing glycolysis. A counterregulatory hormone of insulin, raises plasma glucose levels in response to insulin-induced hypoglycemia. Plays an important role in initiating and maintaining hyperglycemic conditions in diabetes. Binds to and activates the glucagon receptor GCGR, which couples to the G(s) G protein and elevates intracellular cAMP, triggering downstream metabolic responses (PubMed:32193322) Potent stimulator of glucose-dependent insulin release (PubMed:22037645, PubMed:40446798). Also stimulates insulin release in response to IL6 (PubMed:22037645). Plays important roles on gastric motility and the suppression of plasma glucagon levels (PubMed:10605628, PubMed:14719035, PubMed:12554744). May be involved in the suppression of satiety and stimulation of glucose disposal in peripheral tissues, independent of the actions of insulin (PubMed:10605628, PubMed:14719035, PubMed:12554744). Has growth-promoting activities on intestinal epithelium (PubMed:10605628, PubMed:14719035, PubMed:12554744). May also regulate the hypothalamic pituitary axis (HPA) via effects on LH, TSH, CRH, oxytocin, and vasopressin secretion (PubMed:10605628, PubMed:14719035, PubMed:12554744). Increases islet mass through stimulation of islet neogenesis and pancreatic beta cell proliferation (PubMed:10605628, PubMed:14719035, PubMed:12554744). Inhibits beta cell apoptosis (PubMed:10605628, PubMed:14719035, PubMed:12554744) Stimulates intestinal growth and up-regulates villus height in the small intestine, concomitant with increased crypt cell proliferation and decreased enterocyte apoptosis. The gastrointestinal tract, from the stomach to the colon is the principal target for GLP-2 action. Plays a key role in nutrient homeostasis, enhancing nutrient assimilation through enhanced gastrointestinal function, as well as increasing nutrient disposal. Stimulates intestinal glucose transport and decreases mucosal permeability Significantly reduces food intake. Inhibits gastric emptying in humans. Suppression of gastric emptying may lead to increased gastric distension, which may contribute to satiety by causing a sensation of fullness May modulate gastric acid secretion and the gastro-pyloro-duodenal activity. May play an important role in intestinal mucosal growth in the early period of life","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P01275/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GCG","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/GCG","total_profiled":1310},"omim":[{"mim_id":"620346","title":"MAJOR FACILITATOR SUPERFAMILY DOMAIN-CONTAINING PROTEIN 11; MFSD11","url":"https://www.omim.org/entry/620346"},{"mim_id":"620340","title":"NIPA-LIKE DOMAIN-CONTAINING PROTEIN 1; NIPAL1","url":"https://www.omim.org/entry/620340"},{"mim_id":"612880","title":"SYNAPTOTAGMIN-LIKE 2; SYTL2","url":"https://www.omim.org/entry/612880"},{"mim_id":"608981","title":"ACTIVIN A RECEPTOR, TYPE IC; ACVR1C","url":"https://www.omim.org/entry/608981"},{"mim_id":"608145","title":"NIPA MAGNESIUM TRANSPORTER 1; NIPA1","url":"https://www.omim.org/entry/608145"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Endoplasmic reticulum","reliability":"Approved"},{"location":"Vesicles","reliability":"Additional"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"pancreas","ntpm":1834.3}],"url":"https://www.proteinatlas.org/search/GCG"},"hgnc":{"alias_symbol":["GLP1","GLP2","GRPP","GLP-1"],"prev_symbol":[]},"alphafold":{"accession":"P01275","domains":[{"cath_id":"-","chopping":"107-144","consensus_level":"medium","plddt":71.8647,"start":107,"end":144},{"cath_id":"1.20.5","chopping":"151-180","consensus_level":"medium","plddt":81.338,"start":151,"end":180}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P01275","model_url":"https://alphafold.ebi.ac.uk/files/AF-P01275-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P01275-F1-predicted_aligned_error_v6.png","plddt_mean":68.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GCG","jax_strain_url":"https://www.jax.org/strain/search?query=GCG"},"sequence":{"accession":"P01275","fasta_url":"https://rest.uniprot.org/uniprotkb/P01275.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P01275/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P01275"}},"corpus_meta":[{"pmid":"17498508","id":"PMC_17498508","title":"Biology of incretins: GLP-1 and GIP.","date":"2007","source":"Gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/17498508","citation_count":2797,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"31767182","id":"PMC_31767182","title":"Glucagon-like peptide 1 (GLP-1).","date":"2019","source":"Molecular metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/31767182","citation_count":1327,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24843404","id":"PMC_24843404","title":"GIP and GLP-1, the two incretin hormones: Similarities and differences.","date":"2010","source":"Journal of diabetes investigation","url":"https://pubmed.ncbi.nlm.nih.gov/24843404","citation_count":551,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"34216571","id":"PMC_34216571","title":"SGLT2 inhibitors and GLP-1 receptor agonists: established and emerging indications.","date":"2021","source":"Lancet (London, England)","url":"https://pubmed.ncbi.nlm.nih.gov/34216571","citation_count":418,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"26177483","id":"PMC_26177483","title":"Adverse Effects of GLP-1 Receptor Agonists.","date":"2015","source":"The review of diabetic studies : RDS","url":"https://pubmed.ncbi.nlm.nih.gov/26177483","citation_count":397,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"25834231","id":"PMC_25834231","title":"Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease.","date":"2015","source":"Physiological reviews","url":"https://pubmed.ncbi.nlm.nih.gov/25834231","citation_count":392,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"32730231","id":"PMC_32730231","title":"Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist.","date":"2020","source":"JCI insight","url":"https://pubmed.ncbi.nlm.nih.gov/32730231","citation_count":362,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"32396843","id":"PMC_32396843","title":"How May GIP Enhance the Therapeutic Efficacy of GLP-1?","date":"2020","source":"Trends in endocrinology and metabolism: TEM","url":"https://pubmed.ncbi.nlm.nih.gov/32396843","citation_count":324,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"34626851","id":"PMC_34626851","title":"GLP-1 physiology informs the pharmacotherapy of obesity.","date":"2021","source":"Molecular metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/34626851","citation_count":318,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"28869249","id":"PMC_28869249","title":"GLP-1 and the kidney: from physiology to pharmacology and outcomes in diabetes.","date":"2017","source":"Nature reviews. 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humans.","date":"2006","source":"Gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/16401467","citation_count":206,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"23377698","id":"PMC_23377698","title":"Secretion of glucagon-like peptide-1 in patients with type 2 diabetes mellitus: systematic review and meta-analyses of clinical studies.","date":"2013","source":"Diabetologia","url":"https://pubmed.ncbi.nlm.nih.gov/23377698","citation_count":201,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"20682687","id":"PMC_20682687","title":"Common variants in 40 genes assessed for diabetes incidence and response to metformin and lifestyle intervention in the diabetes prevention program.","date":"2010","source":"Diabetes","url":"https://pubmed.ncbi.nlm.nih.gov/20682687","citation_count":196,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"17715056","id":"PMC_17715056","title":"Crystal structure of the incretin-bound extracellular domain of a G protein-coupled receptor.","date":"2007","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/17715056","citation_count":194,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"18445652","id":"PMC_18445652","title":"Beta-Arrestin-1 mediates glucagon-like peptide-1 signaling to insulin secretion in cultured pancreatic beta cells.","date":"2008","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/18445652","citation_count":188,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":45652,"output_tokens":4874,"usd":0.105033},"stage2":{"model":"claude-opus-4-6","input_tokens":8424,"output_tokens":3280,"usd":0.18618},"total_usd":0.887237,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":60130,"output_tokens":8815,"usd":0.156307},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":12664,"output_tokens":9193,"usd":0.439717}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2007,\n      \"finding\": \"GLP-1 (encoded by GCG) acts through a specific G protein-coupled receptor (GLP-1R) to produce glucose-dependent insulin secretion, induction of beta-cell proliferation, and enhanced resistance to apoptosis; it also inhibits glucagon secretion and slows gastric emptying through receptor-mediated mechanisms.\",\n      \"method\": \"Review of multiple experimental studies including receptor binding, in vivo pharmacology, and genetic models\",\n      \"journal\": \"Gastroenterology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 + Strong — foundational findings replicated across many independent labs, multiple orthogonal methods\",\n      \"pmids\": [\"17498508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"GLP-1 is rapidly degraded by the enzyme dipeptidyl peptidase-4 (DPP-4), which cleaves the peptide and terminates its insulinotropic activity; the major circulating forms are the DPP-4-generated metabolites accounting for 60–80% of total immunoreactive GLP-1 in peripheral plasma.\",\n      \"method\": \"Immunoassay characterization, biochemical degradation studies\",\n      \"journal\": \"Gastroenterology / Best practice & research. Clinical endocrinology & metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 + Strong — replicated across multiple labs with biochemical assays\",\n      \"pmids\": [\"17498508\", \"19748060\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"GLP-1 receptor activation in pancreatic beta cells leads to closure of ATP-sensitive K+ channels and activation of nonselective cation channels (NSCCs), producing membrane depolarization, rise of intracellular [Ca2+], and exocytosis of insulin-containing secretory granules; NSCCs are dually regulated by intracellular cAMP and [Ca2+]i.\",\n      \"method\": \"Electrophysiology, patch-clamp recordings, intracellular calcium imaging in pancreatic beta cells\",\n      \"journal\": \"Annals of the New York Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct electrophysiological and calcium imaging measurements in beta cells, replicated\",\n      \"pmids\": [\"8993395\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"GLP-1 receptor (a GPCR) activation leads to increased IRS-2, PI3K, and PKB signaling in pancreatic beta cells, promoting nuclear exclusion of Foxo1 in a PKB-mediated manner, which up-regulates PDX-1 expression to increase beta-cell mass and insulin secretory capacity.\",\n      \"method\": \"Cellular signaling assays, nuclear localization studies, gene expression analysis in beta cells\",\n      \"journal\": \"Science's STKE\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — multiple signaling pathway measurements in beta cells, single-lab mechanistic study\",\n      \"pmids\": [\"15671479\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GLP-1 receptor agonists activate and internalize within NTS astrocytes (not only neurons); GLP-1R agonist exendin-4 increases cAMP levels in astrocytes, and pharmacological inhibition of NTS astrocytes attenuates the anorectic and body weight-suppressive effects of intra-NTS GLP-1R activation, demonstrating a role for astrocytic GLP-1R signaling in energy balance.\",\n      \"method\": \"Fluorophore-labeled agonist localization, live cell calcium signaling (ex vivo), cAMP assay (in vitro), GLP-1R antagonist blocking, pharmacological inhibition with behavioral readout\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — multiple orthogonal methods (imaging, calcium signaling, cAMP, pharmacological inhibition with behavioral readout) in single study\",\n      \"pmids\": [\"27013681\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The crystal structure of the full-length human GLP-1 receptor in a peptide-free inactive state reveals a unique closed conformation of the extracellular domain (ECD); disulfide cross-linking validates the physiological relevance of this closed conformation, and MD simulations indicate large conformational dynamics of ECD necessary for GLP-1 binding.\",\n      \"method\": \"X-ray crystallography (3.2 Å resolution), disulfide cross-linking, electron microscopy, molecular dynamics simulation\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 + Moderate — crystal structure with orthogonal validation by disulfide cross-linking and EM\",\n      \"pmids\": [\"32152292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The non-peptide agonist TT-OAD2 binds to an unpredicted pocket in the GLP-1 receptor distinct from the peptide agonist site; structural analysis (cryo-EM) shows it drives receptor activation through reorganization of extracellular loop 3 and transmembrane helices 6 and 7 independently of direct interaction within the deep transmembrane domain pocket, and exhibits biased agonism with distinct G-protein activation kinetics compared to peptide agonists.\",\n      \"method\": \"Cryo-EM structure determination, biased agonism signaling assays, kinetics of G-protein activation\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 + Moderate — cryo-EM structure combined with pharmacological signaling characterization\",\n      \"pmids\": [\"31915381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The positive allosteric modulator LSN3160440 of GLP-1R acts as a 'molecular glue' that cooperatively binds both the receptor (at TM1–TM2 interface high in the helical bundle) and the orthosteric ligand GLP-1(9-36), stabilizing an active state conformation; it enhances insulin secretion in a glucose-, ligand-, and GLP-1R-dependent manner, with pharmacological probe dependence driven by a single residue.\",\n      \"method\": \"Cryo-EM structure of GLP-1R/GLP-1/Gs/LSN3160440 complex, pharmacological characterization, insulin secretion assay\",\n      \"journal\": \"Nature chemical biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 + Moderate — cryo-EM structure combined with functional assays and mutagenesis-informed pharmacology\",\n      \"pmids\": [\"32690941\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"GLP-1R undergoes agonist-mediated endocytosis; biased GLP-1R agonists that reduce β-arrestin recruitment and retain GLP-1R at the plasma membrane produce greater long-term insulin release and glycemic benefits in mice without increased nausea signs compared to FDA-approved mimetics.\",\n      \"method\": \"GLP-1R trafficking assays, β-arrestin recruitment assays, insulin secretion measurement, in vivo glucose tolerance tests, nausea behavioral assays in mice\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — multiple orthogonal methods (trafficking, signaling, in vivo) in single study\",\n      \"pmids\": [\"29686402\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Tirzepatide shows biased agonism at the GLP-1 receptor, favoring cAMP generation over β-arrestin recruitment, coincident with weaker GLP-1R internalization compared to native GLP-1; experiments in primary islets reveal that β-arrestin1 limits the insulin response to GLP-1 but not GIP or tirzepatide, suggesting that biased agonism enhances insulin secretion.\",\n      \"method\": \"Receptor occupancy calculations, cAMP and β-arrestin signaling assays, GLP-1R internalization assay, primary islet insulin secretion experiments with β-arrestin1 manipulation\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — multiple signaling assays plus primary islet functional experiments\",\n      \"pmids\": [\"32730231\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The GLP-1 receptor uses a two-domain peptide binding mechanism: the extracellular N-terminal domain provides most binding energy while the core transmembrane domain binds the N-terminal region of agonist peptides and transmits signal to the G-protein; GLP-1 requires its full length for high affinity whereas exendin-4 can bind the isolated N-domain with affinity matching the full-length receptor.\",\n      \"method\": \"Binding assays with truncated ligands and truncated receptors (full-length vs. isolated N-domain constructs)\",\n      \"journal\": \"Biochemical Society transactions\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — systematic binding studies with truncated receptor and ligand variants\",\n      \"pmids\": [\"17635131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"GLP-1 activates sensory afferents in the gastrointestinal mucosa (with cell bodies in the nodose ganglion) before being degraded, signaling to the brain stem and hypothalamus; approximately 90% of GLP-1 is degraded before reaching systemic circulation, supporting a neural/paracrine rather than purely endocrine mechanism for many of its physiological actions.\",\n      \"method\": \"In vivo pharmacokinetic measurements, neural recording studies, GLP-1 degradation assays\",\n      \"journal\": \"Hormone and metabolic research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — replicated pharmacokinetic and neural data supporting paracrine/neural model\",\n      \"pmids\": [\"15655703\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Central (brain-derived, PPG neurons in NTS) and peripheral (gut-derived) GLP-1 systems suppress eating via independent gut-brain circuits; PPGNTS neurons predominantly receive vagal input from oxytocin-receptor-expressing vagal neurons rather than GLP-1R-expressing neurons, and PPGNTS neurons are not necessary for eating suppression by GLP-1R agonists.\",\n      \"method\": \"Optogenetics, calcium imaging, circuit tracing, chemogenetic manipulation, behavioral feeding assays in mice\",\n      \"journal\": \"Nature metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 + Strong — multiple independent circuit-dissection methods with clear functional readouts\",\n      \"pmids\": [\"33589843\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1 receptor (GLP-1R) neurons in the dorsomedial hypothalamus (DMH) encode preingestive satiation; optogenetic activation of DMH GLP-1R neurons causes satiation, and GLP-1RA administration selectively increases activity of DMH GLP-1R neurons during eating; these neurons interact with NPY/AgRP neurons of the arcuate nucleus to regulate food intake.\",\n      \"method\": \"Optogenetics, calcium imaging, chemogenetics, GLP-1RA administration in mice, human brain sample analysis\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — multiple orthogonal neural circuit methods with behavioral and physiological readouts\",\n      \"pmids\": [\"38935778\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1 receptor activation in the olfactory bulb (OB) stimulates insulin secretion in response to oral glucose via a top-down neural mechanism: OB GLP-1R activation reduces noradrenaline content in the pancreas, implicating the sympathetic nervous system (SNS); this effect is relayed through GABAA receptor signaling in the paraventricular nucleus of the hypothalamus (PVN).\",\n      \"method\": \"Stereotaxic GLP-1R agonist injection in OB, oral glucose tolerance test, noradrenaline measurement in pancreas, pharmacological blockade (α2-adrenergic agonist, GABAA antagonist in PVN) in mice\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — pharmacological circuit dissection with multiple intervention points and functional readouts\",\n      \"pmids\": [\"39138162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Intestinal Gpr17 (an orphan GPCR co-expressed in GLP-1-expressing enteroendocrine cells) acts as an inhibitory pathway for GLP-1 secretion: Gpr17 ablation increases GLP-1 (but not GIP) secretion in response to glucose or lipid; in vitro, Gpr17 overexpression or agonism reduces voltage-gated calcium currents and cAMP production in EEC models, two critical regulators of GLP-1 secretion.\",\n      \"method\": \"Conditional knockout mice, intestinal organoids, in vitro EEC model with overexpression/agonism, calcium current measurements, cAMP assays, glucose tolerance tests\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — genetic KO plus in vitro mechanistic studies with multiple readouts in single study\",\n      \"pmids\": [\"34986353\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GPR40 AgoPAMs stimulate GLP-1 secretion from enteroendocrine L-cells; the food intake-reducing and weight-lowering effects of GPR40 AgoPAMs require both GPR40 and GLP-1 receptor signaling, as demonstrated in GPR40 and GLP-1 receptor-null mice, placing GLP-1 secretion downstream of GPR40 activation in the regulation of food intake.\",\n      \"method\": \"GPR40 and GLP-1R knockout mouse studies, active GLP-1 level measurement, food intake and body weight measurement, gastric motility assay\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — genetic epistasis with receptor-null mice plus pharmacological intervention\",\n      \"pmids\": [\"28292762\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"GLP-1 receptor activation leads to adenylyl cyclase-mediated cAMP elevation and subsequent activation of cAMP-dependent protein kinases, which stimulate insulin release in conjunction with enhanced Ca2+ and ATP levels in beta cells; ERK1/2 activation is also a downstream pathway of GLP-1R signaling.\",\n      \"method\": \"Signal transduction assays, cAMP measurement, kinase activity assays in beta cells and other cell types\",\n      \"journal\": \"Peptides\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 + Strong — replicated signaling pathway measurements across multiple labs\",\n      \"pmids\": [\"29412835\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"GLP-1R activation (by exendin-4) in prostate cancer cells decreases ERK-MAPK phosphorylation and attenuates cancer cell proliferation in a GLP-1R expression-dependent manner; GLP-1R knockdown with siRNA abolishes the antiproliferative effect, and the receptor-mediated mechanism is confirmed in vivo in athymic mouse xenograft models.\",\n      \"method\": \"siRNA knockdown, GLP-1R antagonist, ERK-MAPK phosphorylation assay, cell proliferation assay, in vivo xenograft tumor model\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 + Moderate — siRNA knockdown + antagonist + in vivo confirmation with mechanistic readout\",\n      \"pmids\": [\"24879833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The non-peptide GLP-1R agonist orforglipron binds the human GLP-1R with high affinity (Ki = 1 nM), shows low intrinsic efficacy for effector activation, and negligible β-arrestin recruitment; in CRISPR-Cas9 humanized rat GLP-1R models, orforglipron engages the receptor in pancreas and brain consistent with peptide agonists, and low GLP-1R occupancy is sufficient for a full biological glucose-lowering response.\",\n      \"method\": \"Competition binding with [125I]GLP-1(7-36)NH2 and [3H]orforglipron, signal transduction assays, β-arrestin recruitment assay, CRISPR-Cas9 humanized rat model, glucose tolerance test, receptor occupancy calculation\",\n      \"journal\": \"Science translational medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 + Moderate — radioligand binding, signaling assays, and in vivo CRISPR model in single comprehensive study\",\n      \"pmids\": [\"39693407\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GCG-encoded GLP-1 is secreted from intestinal L-cells, pancreatic α-cells, and brain NTS neurons upon nutrient ingestion; it binds its cognate class B GPCR (GLP-1R) via a two-domain mechanism, activating adenylyl cyclase/cAMP/PKA signaling to close K-ATP channels, activate nonselective cation channels, raise intracellular Ca2+, and stimulate glucose-dependent insulin secretion, while also engaging IRS-2/PI3K/PKB/Foxo1 pathways to promote beta-cell survival; receptor internalization is regulated by β-arrestin recruitment, and biased agonism favoring cAMP over β-arrestin enhances insulin secretion; peripheral GLP-1 is rapidly degraded by DPP-4 and acts partly through vagal sensory afferents, while central PPG neurons in the NTS and DMH GLP-1R neurons form independent circuits controlling satiety and food intake.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper and extract mechanistic discoveries only from KEEP papers about the canonical GCG gene product (preproglucagon and its peptide products: glucagon, GLP-1, GLP-2, oxyntomodulin, etc.).\n\n**Classification notes:**\n- Papers about GLP-1/glucagon physiology, signaling, receptor biology → KEEP\n- Papers primarily about GLP-1 receptor agonist drugs (clinical pharmacology, outcomes trials, safety reviews) with no mechanistic findings about GCG products → EXCLUDE (clinical/pharmacological reviews)\n- Papers about GIP without mechanistic GCG content → EXCLUDE\n- MGC cDNA collection papers (PMID 12477932, 15489334) → EXCLUDE (not mechanistic)\n- BioPlex interactome (PMID 33961781) → EXCLUDE (no specific GCG mechanism)\n- Gene Ontology paper (PMID 21873635) → EXCLUDE\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1983,\n      \"finding\": \"The human preproglucagon gene (GCG) encodes multiple peptides through exon duplication and divergence, establishing that glucagon, GLP-1, and GLP-2 are all products of a single preproglucagon precursor gene.\",\n      \"method\": \"cDNA cloning and sequence analysis of the human preproglucagon gene\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — foundational molecular cloning, foundational discovery replicated across species\",\n      \"pmids\": [\"6877358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"The two bioactive forms of GLP-1 (7-36 amide and 7-37) are equipotent in stimulating insulin secretion, suppressing glucagon, and lowering plasma glucose in healthy humans, and share similar metabolic clearance rates (~12–15 pmol/kg·min) and plasma half-lives (~5–6 min), indicating C-terminal amidation does not significantly alter GLP-1 biological activity or metabolism.\",\n      \"method\": \"Intravenous infusion of GLP-1 7-36 amide vs. GLP-1 7-37 in healthy human subjects with measurement of insulin, C-peptide, glucagon, glucose, and metabolic clearance rates\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vivo pharmacokinetic and pharmacodynamic comparison with multiple orthogonal endpoints\",\n      \"pmids\": [\"8482423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Central GLP-1 (ICV injection) potently inhibits feeding in fasted rats and activates c-fos expression in the paraventricular nucleus (PVN) of the hypothalamus and central nucleus of the amygdala; the specific GLP-1 receptor antagonist exendin(9-39) blocks these effects and augments feeding in satiated animals, establishing central GLP-1 as a physiological mediator of satiety acting through hypothalamic circuits.\",\n      \"method\": \"Intracerebroventricular (ICV) injection of GLP-1 and exendin(9-39) in rats; food intake measurement; c-fos immunohistochemistry\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — receptor antagonist epistasis plus functional readout (food intake) plus neuronal activation marker; highly cited foundational study\",\n      \"pmids\": [\"8538742\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"GLP-1 and PACAP depolarize pancreatic beta cells and stimulate insulin secretion through at least two mechanisms: closure of ATP-sensitive K+ channels and activation of nonselective cation channels (NSCCs) carrying predominantly Na+-dependent current; NSCCs are dually regulated by intracellular cAMP and Ca2+, and activation of GLP-1 signaling raises [Ca2+]i and promotes exocytosis of insulin-containing granules.\",\n      \"method\": \"Electrophysiology (patch-clamp), intracellular Ca2+ imaging, and pharmacological manipulation in pancreatic beta cells\",\n      \"journal\": \"Annals of the New York Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct electrophysiological recordings with pharmacological dissection of ion channel mechanisms\",\n      \"pmids\": [\"8993395\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"GLP-2 (a proglucagon-derived peptide) exerts its intestinotrophic actions through a specific G protein-coupled receptor (GLP-2R) expressed in the gut; GLP-2R activation by GLP-2 but not GLP-1 increases cAMP (EC50 = 0.58 nM) and shows saturable high-affinity radioligand binding (Kd = 0.57 nM), demonstrating ligand specificity among proglucagon-derived peptides.\",\n      \"method\": \"cDNA cloning of rat and human GLP-2R; cAMP assays; radioligand binding; in vivo intestinotrophic activity of GLP-2 analogs\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — receptor cloning with functional reconstitution, binding assays, and in vivo validation\",\n      \"pmids\": [\"9990065\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Glucagon is the principal counterregulatory hormone that increases hepatic glucose output by simultaneously promoting glycogenolysis and gluconeogenesis while inhibiting glycogenesis and glycolysis, acting through the glucagon receptor; diabetic hyperglucagonemia contributes to pathological hyperglycemia.\",\n      \"method\": \"Review integrating in vivo and in vitro mechanistic studies of glucagon receptor signaling and hepatic glucose metabolism\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — synthesis of multiple mechanistic studies with well-established in vivo and in vitro experimental support\",\n      \"pmids\": [\"12626323\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"GLP-1 inhibits apoptosis in freshly isolated human islets by downregulating active caspase-3 and upregulating bcl-2 at both mRNA and protein levels, while enhancing intracellular insulin content and glucose-dependent insulin secretion; this establishes a direct antiapoptotic mechanism of GLP-1 in human beta cells.\",\n      \"method\": \"Culture of human islets with GLP-1 (10 nM); measurement of apoptotic nuclei, caspase-3 and bcl-2 mRNA and protein; insulin content and secretion assays\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (histology, RT-PCR, Western blot, secretion assay) in human tissue\",\n      \"pmids\": [\"12960095\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"GLP-1 stimulates pancreatic beta-cell proliferation, induces islet neogenesis, promotes differentiation toward a beta-cell phenotype from exocrine or progenitor cells, and exerts antiapoptotic actions preserving beta-cell mass in vivo; GLP-2 analogously stimulates gastrointestinal mucosal cell proliferation and exerts antiapoptotic actions on gut epithelium.\",\n      \"method\": \"Review integrating in vivo genetic models, cell culture experiments, and proglucagon-derived peptide biology\",\n      \"journal\": \"Molecular endocrinology (Baltimore, Md.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple experimental systems cited; well-established mechanistic framework\",\n      \"pmids\": [\"12554744\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Leptin directly stimulates GLP-1 secretion from intestinal L cells via the long form of the leptin receptor (Ob-Rb), inducing STAT3 phosphorylation; leptin resistance in diet-induced obese mice is associated with reduced basal GLP-1 and diminished GLP-1 response to oral glucose, linking leptin signaling to GLP-1 secretory regulation.\",\n      \"method\": \"GLP-1 secretion assays in fetal rat intestinal cells, GLUTag and NCI-H716 L-cell lines, and in vivo in rats and ob/ob mice; Western blot for STAT3 phosphorylation; high-fat diet mouse model\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple cell models plus in vivo validation with receptor expression and downstream signaling confirmed\",\n      \"pmids\": [\"12540594\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"GLP-1 rapidly degrades in plasma with a half-life of ~1–2 min primarily through DPP-IV cleavage; kidneys are important for final elimination of the metabolite GLP-1(9-36 amide), as patients with chronic renal insufficiency show significantly longer half-lives for the metabolite (5.3 vs. 3.3 min) but not for intact GLP-1.\",\n      \"method\": \"Intravenous infusion of GLP-1 in patients with chronic renal insufficiency vs. healthy controls; specific immunoassays for intact and total GLP-1; pharmacokinetic modeling\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — controlled intravenous pharmacokinetic study with specific immunoassays distinguishing intact vs. metabolite forms\",\n      \"pmids\": [\"14988249\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Oxyntomodulin (a proglucagon-derived peptide) suppresses appetite and reduces food intake in humans when infused intravenously, and significantly suppresses circulating ghrelin levels (by ~44% of postprandial decrease), identifying oxyntomodulin as a physiological appetite-regulating signal from the GCG locus.\",\n      \"method\": \"Randomized, double-blind, placebo-controlled crossover study with IV OXM infusion in healthy subjects; ad libitum food intake measurement; plasma ghrelin, appetite VAS scores\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — double-blind crossover RCT with multiple physiological endpoints\",\n      \"pmids\": [\"14557443\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"GLP-1 directly protects the heart against ischemia/reperfusion injury via GLP-1 receptor-dependent activation of cAMP, PI3-kinase, and p42/44 MAPK prosurvival pathways, leading to phosphorylation of the proapoptotic peptide BAD; protection is abolished by the GLP-1 receptor antagonist exendin(9-39), Rp-cAMP, LY294002 (PI3K inhibitor), or UO126 (MEK inhibitor).\",\n      \"method\": \"Isolated perfused rat heart and whole animal ischemia/reperfusion models; infarct size measurement; pharmacological inhibition with receptor antagonist and kinase inhibitors; Western blot for phospho-BAD\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted organ model with multiple orthogonal pharmacological antagonists and downstream signaling readout\",\n      \"pmids\": [\"15616022\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"GLP-1 receptor activation in pancreatic beta cells promotes nuclear exclusion of Foxo1 through PKB (Akt)-mediated phosphorylation, downstream of IRS-2 and PI3K signaling; this upregulates PDX-1, a master regulator of beta-cell growth and differentiation, providing a mechanism for GLP-1-induced expansion of beta-cell mass.\",\n      \"method\": \"Mechanistic review integrating cell-based studies of GLP-1R → IRS-2 → PI3K → PKB → Foxo1 phosphorylation → PDX-1 upregulation in pancreatic beta cells\",\n      \"journal\": \"Science's STKE : signal transduction knowledge environment\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — synthesis of multiple cell-based mechanistic experiments; single review perspective\",\n      \"pmids\": [\"15671479\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"GLP-2 (a proglucagon-derived peptide) stimulates glucagon secretion in humans both in the fasting state and postprandially, enhances intestinal lipid absorption (elevated postprandial triglycerides and free fatty acids), and inhibits pentagastrin-stimulated gastric acid secretion by ~15%, without affecting gastric emptying.\",\n      \"method\": \"Randomized crossover studies in 15 healthy male volunteers each; IV infusion of GLP-2 vs. placebo; plasma glucagon, lipid, and gastric acid measurements; 13C-octanoate breath test for gastric emptying\",\n      \"journal\": \"Gastroenterology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — controlled crossover human studies with multiple physiological endpoints\",\n      \"pmids\": [\"16401467\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Insulin-degrading enzyme (IDE) degrades glucagon (as well as insulin, amyloid-β, and amylin) by encapsulating it within an enclosed catalytic chamber formed by IDE-N and IDE-C domains; substrate access requires repositioning of these domains, and the substrate undergoes conformational changes to form beta-sheets with IDE for degradation.\",\n      \"method\": \"Crystal structures of human IDE in complex with substrates including glucagon; in vitro activity assays; mutagenesis disrupting IDE-N/IDE-C contacts increases catalytic activity 40-fold\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus mutagenesis plus in vitro reconstitution\",\n      \"pmids\": [\"17051221\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Intestinal L cells express sweet taste receptors and the taste G protein α-gustducin; glucose-stimulated GLP-1 secretion is defective in α-gustducin null mice and in isolated intestinal tissue from these mice; GLP-1 release from NCI-H716 cells is promoted by sugars and the noncaloric sweetener sucralose and blocked by the sweet receptor antagonist lactisole or siRNA for α-gustducin, establishing that L cells 'taste' luminal glucose through the same machinery as lingual taste cells to trigger GLP-1 secretion.\",\n      \"method\": \"α-Gustducin knockout mice; glucose tolerance tests; isolated small bowel GLP-1 secretion assays; NCI-H716 cell line; siRNA knockdown of α-gustducin; pharmacological antagonism with lactisole\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout plus in vitro siRNA knockdown plus pharmacological antagonism across multiple systems\",\n      \"pmids\": [\"17724330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"GLP-1 binds to its class B GPCR through a two-domain mechanism: the N-terminal extracellular domain (ECD) provides most binding energy for the C-terminal region of GLP-1, while the core transmembrane domain binds the N-terminal region of GLP-1 to transmit the activation signal; in contrast, exendin-4 can bind the isolated ECD with full-receptor affinity, indicating mechanistic differences between GLP-1 and exendin-4 receptor engagement.\",\n      \"method\": \"Peptide binding studies using full-length and truncated GLP-1 receptor variants and truncated ligands; radioligand competition assays\",\n      \"journal\": \"Biochemical Society transactions\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic binding studies with receptor and ligand truncations; mechanistic model supported by multiple constructs\",\n      \"pmids\": [\"17635131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Crystal structure of the GIP receptor extracellular domain (ECD) bound to GIP(1-42) reveals that the incretin hormone binds in an alpha-helical conformation within a surface groove of the ECD through predominantly hydrophobic interactions; the N-terminal residues of GIP remain free, consistent with a two-domain binding model conserved across class B GPCRs including GLP-1R.\",\n      \"method\": \"X-ray crystallography of human GIP receptor ECD in complex with GIP(1-42); thermodynamic binding analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with thermodynamic validation; provides structural framework for incretin-receptor binding\",\n      \"pmids\": [\"17715056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"β-Arrestin-1 physically associates with the GLP-1 receptor (demonstrated by co-immunoprecipitation in INS-1 beta cells) and mediates GLP-1 signaling to insulin secretion; β-arrestin-1 knockdown reduces cAMP production, ERK and CREB activation, IRS-2 expression, and insulin secretion in response to GLP-1, without affecting GLP-1R surface expression or ligand-induced internalization.\",\n      \"method\": \"Co-immunoprecipitation in INS-1 cells; siRNA knockdown of β-arrestin-1; measurement of cAMP, ERK/CREB phosphorylation, IRS-2 expression, and insulin secretion\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus siRNA knockdown with multiple downstream signaling readouts\",\n      \"pmids\": [\"18445652\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"IL-6 stimulates GLP-1 secretion from both intestinal L cells and pancreatic alpha cells; in alpha cells, IL-6 increases GLP-1 production by upregulating proglucagon (GCG) gene expression and prohormone convertase 1/3 expression, establishing a mechanism by which exercise-elevated IL-6 enhances GLP-1 availability to improve insulin secretion.\",\n      \"method\": \"IL-6 administration in mice and in vitro in L-cell and alpha-cell lines; measurement of GLP-1 secretion, proglucagon mRNA, and prohormone convertase 1/3 expression; IL-6 neutralization experiments; exercise models\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple cell types, in vivo and in vitro, with gene expression and secretion endpoints, and loss-of-function (IL-6 neutralization)\",\n      \"pmids\": [\"22037645\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A major lineage of enteroendocrine cells co-expresses proglucagon (GLP-1), CCK, GIP, PYY, neurotensin, and secretin but not somatostatin throughout the intestine; cell ablation driven by the proglucagon promoter (diphtheria toxin receptor model) markedly reduces not only GLP-1 cells but also PYY, neurotensin, GIP, CCK, and secretin cells, demonstrating that GCG-expressing L cells represent a broad shared lineage of enteroendocrine cells.\",\n      \"method\": \"Transgenic mice with CCK-eGFP reporter; FACS purification and quantitative PCR; LC-MS proteomics; immunohistochemistry; proglucagon-promoter-driven diphtheria toxin receptor cell ablation; single-cell qPCR\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including genetic cell ablation, proteomics, and single-cell analysis\",\n      \"pmids\": [\"23064014\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"NTS astrocytes express functional GLP-1 receptors; systemic or central administration of fluorophore-labeled exendin-4 localizes within NTS astrocytes and neurons in a GLP-1R-dependent manner (blocked by exendin(9-39)); GLP-1R agonists activate NTS astrocytes (live-cell calcium signaling) and increase cAMP in astrocytes in vitro; pharmacological inhibition of NTS astrocytes attenuates the anorectic and body-weight-suppressive effects of intra-NTS GLP-1R activation.\",\n      \"method\": \"Fluorophore-labeled GLP-1R agonist localization in vivo; ex vivo and in vitro calcium signaling; cAMP assays in astrocytes; immunohistochemistry; pharmacological inhibition of NTS astrocytes with behavioral readout\",\n      \"journal\": \"The Journal of neuroscience : the official journal of the Society for Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods establishing astrocytic GLP-1R localization and functional consequence on food intake\",\n      \"pmids\": [\"27013681\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"GLP-1 receptor agonists that retain GLP-1R at the plasma membrane (reduced internalization and slower agonist dissociation rates, with reduced β-arrestin recruitment) produce greater long-term insulin release compared to FDA-approved GLP-1 mimetics; this enhanced insulin secretion is dependent on reduced β-arrestin recruitment, establishing a mechanism by which GLP-1R trafficking determines insulinotropic efficacy.\",\n      \"method\": \"Series of biased GLP-1R agonists tested for GLP-1R internalization, recycling, β-arrestin recruitment, and insulin secretion in primary islets; glycemic studies in mice; comparison to panel of FDA-approved GLP-1 mimetics\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — structure-activity series with multiple signaling and functional endpoints including in vivo glycemic testing\",\n      \"pmids\": [\"29686402\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Crystal structure of the full-length human GLP-1R in peptide-free inactive state at 3.2 Å reveals a unique closed conformation of the extracellular domain (ECD); disulfide cross-linking validates the physiological relevance of this closed conformation; EM and MD simulations show large ECD conformational dynamics necessary for GLP-1 binding, providing a mechanistic framework for receptor activation.\",\n      \"method\": \"X-ray crystallography (3.2 Å); disulfide cross-linking; electron microscopy; molecular dynamics simulation\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with multiple orthogonal validation methods\",\n      \"pmids\": [\"32152292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"A non-peptide agonist (TT-OAD2) binds the GLP-1 receptor at an unpredicted extracellular pocket distinct from the peptide-binding site, causing reorganization of ECL3 and TM helices 6 and 7 independently of direct interaction with the deep transmembrane pocket; TT-OAD2 shows biased agonism with distinct G-protein activation kinetics compared to peptide agonists, and protrudes beyond the receptor core to interact with lipid/detergent, explaining its distinct activation kinetics.\",\n      \"method\": \"Cryo-EM structure of GLP-1R bound to TT-OAD2; signaling assays (cAMP, β-arrestin); binding competition studies\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure plus pharmacological characterization; reveals novel binding site\",\n      \"pmids\": [\"31915381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"A positive allosteric modulator (LSN3160440) of GLP-1R acts as a 'molecular glue' that cooperatively binds both the receptor and orthosteric ligand GLP-1(9-36), stabilizing the active-state conformation; cryo-EM structure reveals the modulator binds at an interface between TM1 and TM2 high in the helical bundle, allowing access to the peptide ligand; LSN3160440 enhances insulin secretion in a glucose-, ligand-, and GLP-1R-dependent manner.\",\n      \"method\": \"Cryo-EM structure of GLP-1R bound to LSN3160440 + GLP-1 + Gs; insulin secretion assays; pharmacological characterization with multiple ligands; probe-dependence analysis\",\n      \"journal\": \"Nature chemical biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure plus functional insulin secretion assay and pharmacological characterization\",\n      \"pmids\": [\"32690941\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Tirzepatide shows biased agonism at the GLP-1 receptor, favoring cAMP generation over β-arrestin recruitment, and drives less GLP-1R internalization compared to native GLP-1; β-arrestin1 limits the insulin response to GLP-1 but not GIP or tirzepatide, and tirzepatide engages the GIP receptor more than the GLP-1R at clinically efficacious doses; these signaling properties together enhance insulin secretion.\",\n      \"method\": \"Receptor occupancy calculations; cAMP and β-arrestin recruitment signaling assays; GLP-1R internalization assays; insulin secretion in primary islets from β-arrestin1 knockout and wild-type mice\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple signaling assays with genetic validation (β-arrestin1 KO islets)\",\n      \"pmids\": [\"32730231\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PPGNTS neurons (preproglucagon-expressing neurons in the nucleus tractus solitarii) encode satiation through vagal signaling of gastrointestinal distension and receive vagal input predominantly from oxytocin-receptor-expressing vagal neurons rather than GLP-1 receptor-expressing vagal neurons; PPGNTS neurons are not necessary for eating suppression by GLP-1 receptor agonists, and concurrent PPGNTS neuron activation suppresses eating more potently than semaglutide alone, demonstrating that central and peripheral GLP-1 systems suppress eating via independent gut-brain circuits.\",\n      \"method\": \"Optogenetic manipulation of PPGNTS neurons; calcium imaging; vagal circuit mapping; genetic deletion of PPGNTS neurons; comparison with semaglutide treatment in mice\",\n      \"journal\": \"Nature metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — optogenetics plus calcium imaging plus genetic circuit dissection with behavioral readout\",\n      \"pmids\": [\"33589843\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Intestinal Gpr17 (an orphan GPCR) is co-expressed in GLP-1-expressing enteroendocrine cells and functions as an inhibitory regulator of GLP-1 secretion; Gpr17 knockout or acute intestinal epithelial ablation improves glucose tolerance and GSIS; Gpr17 agonism in vitro reduces voltage-gated calcium currents and decreases cAMP production, the two critical factors regulating GLP-1 secretion; Gpr17 null intestinal organoids show increased GLP-1 (but not GIP) secretion in response to glucose or lipid.\",\n      \"method\": \"Inducible intestinal-specific Gpr17 knockout mice; intestinal organoids; electrophysiology (voltage-gated Ca2+ currents); cAMP assays; GSIS and glucose tolerance tests\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO plus organoid model plus electrophysiology and cAMP mechanistic studies\",\n      \"pmids\": [\"34986353\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1 receptor neurons in the dorsomedial hypothalamus (DMH) encode preingestive satiation; optogenetic activation of DMHGLP-1R neurons causes satiation; calcium imaging shows these neurons are activated specifically during eating behavior; GLP-1RA administration selectively increases DMHGLP-1R neuron activity, and these neurons interact with NPY/AgRP neurons of the arcuate nucleus to regulate food intake.\",\n      \"method\": \"Optogenetic manipulation and calcium imaging of DMHGLP-1R neurons in mice; GLP-1RA administration; human and mouse brain sample analysis; food intake measurements\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — optogenetics plus calcium imaging plus pharmacological GLP-1RA treatment with defined circuit mapping\",\n      \"pmids\": [\"38935778\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GLP-1 receptor activation in the olfactory bulb (OB) stimulates insulin secretion in response to oral glucose via a top-down neural circuit: OB GLP-1R activation reduces noradrenaline content in the pancreas (implicating the sympathetic nervous system), and this effect is relayed through the PVN (abolished by GABAA receptor inhibition in PVN), identifying a novel brain-to-pancreas axis for GLP-1-mediated insulin secretion.\",\n      \"method\": \"Stereotaxic OB GLP-1R activation in lean and diet-induced obese male mice; measurement of pancreatic noradrenaline; α2-adrenergic receptor agonist blockade; GABAA receptor inhibition in PVN; glucose-stimulated insulin secretion assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological circuit dissection with multiple anatomical interventions and mechanistic readouts in vivo\",\n      \"pmids\": [\"39138162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Orforglipron, a nonpeptide oral GLP-1R agonist, binds human GLP-1R with high affinity (Ki = 1 nM), has low intrinsic efficacy for effector activation and negligible β-arrestin recruitment; low GLP-1R occupancy by orforglipron is sufficient to yield a full biological response in vivo; target engagement in pancreas and brain is confirmed using CRISPR-Cas9-sensitized rat Glp1r, demonstrating the pharmacological basis for nonpeptide GLP-1R agonism.\",\n      \"method\": \"Radioligand competition binding ([125I]GLP-1 and [3H]orforglipron); signal transduction assays; receptor occupancy modeling; glucose tolerance tests in humanized GLP-1R mice; CRISPR-Cas9 Glp1r-sensitized rats; DIO weight loss studies\",\n      \"journal\": \"Science translational medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — binding assays, signaling assays, CRISPR genetic validation, and in vivo pharmacology\",\n      \"pmids\": [\"39693407\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"The GCG gene encodes preproglucagon, which is processed tissue-specifically into glucagon (in pancreatic α-cells, acting through glucagon receptors to promote hepatic glycogenolysis, gluconeogenesis, and counterregulation), GLP-1 (in intestinal L-cells and brain NTS neurons, acting through a single class B GPCR—GLP-1R—to stimulate insulin secretion via cAMP/PKA and β-arrestin-1, inhibit glucagon secretion, slow gastric emptying, and suppress feeding through hypothalamic and brainstem circuits including DMHGLP-1R and PPGNTS neurons), GLP-2 (promoting intestinal growth via a distinct GLP-2R), and oxyntomodulin (suppressing appetite and ghrelin); GLP-1 secretion from L-cells is triggered by luminal glucose through α-gustducin/sweet taste receptor machinery and modulated by leptin (via STAT3/Ob-Rb) and IL-6 (upregulating proglucagon and PC1/3 in α-cells), while GLP-1R signaling efficacy is determined by β-arrestin recruitment (promoting receptor internalization and desensitization), GLP-1R trafficking, and intracellular cAMP generation, with structural mechanisms revealed by crystal and cryo-EM structures of GLP-1R in inactive, peptide-bound, non-peptide-agonist-bound, and positive allosteric modulator-bound states.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GCG encodes proglucagon, the precursor of glucagon-like peptide-1 (GLP-1), a peptide hormone secreted by intestinal L-cells, pancreatic α-cells, and brainstem preproglucagon (PPG) neurons that serves as a master integrator of nutrient-stimulated insulin secretion, β-cell survival, gastric motility, and satiety. GLP-1 binds the class B GPCR GLP-1R via a two-domain mechanism, activating adenylyl cyclase/cAMP/PKA signaling that closes K-ATP channels, opens nonselective cation channels, raises intracellular Ca²⁺, and triggers glucose-dependent insulin exocytosis, while parallel IRS-2/PI3K/PKB/Foxo1 signaling promotes β-cell proliferation and resistance to apoptosis [PMID:17498508, PMID:8993395, PMID:15671479]. Peripheral GLP-1 is rapidly degraded by DPP-4 (60–80% of circulating GLP-1 is inactive metabolite) and acts largely through vagal sensory afferents rather than as a classical endocrine hormone, whereas central PPG neurons in the NTS and GLP-1R-expressing neurons in the DMH form independent circuits that encode preingestive satiation and suppress food intake [PMID:15655703, PMID:33589843, PMID:38935778]. Biased agonism at GLP-1R—favoring cAMP over β-arrestin recruitment—reduces receptor internalization and enhances sustained insulin secretion, a principle exploited by next-generation agonists including tirzepatide and the non-peptide orforglipron [PMID:29686402, PMID:32730231, PMID:39693407].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Establishing the ionic mechanism by which GLP-1 triggers insulin exocytosis resolved how a gut peptide achieves glucose-dependent insulinotropism: GLP-1R activation closes K-ATP channels and opens cAMP/Ca²⁺-regulated nonselective cation channels, depolarizing β-cells and raising [Ca²⁺]i.\",\n      \"evidence\": \"Patch-clamp electrophysiology and intracellular calcium imaging in pancreatic β-cells\",\n      \"pmids\": [\"8993395\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the specific NSCC channel subunits was not determined\", \"Relative contribution of K-ATP closure vs. NSCC opening to total depolarization was not quantified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Demonstrating that GLP-1 acts partly through vagal sensory afferents rather than systemic circulation, given ~90% DPP-4-mediated degradation before reaching the systemic bloodstream, shifted the conceptual model from purely endocrine to paracrine/neural signaling.\",\n      \"evidence\": \"In vivo pharmacokinetic measurements of GLP-1 degradation and neural recording from nodose ganglion afferents\",\n      \"pmids\": [\"15655703\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relative quantitative contribution of neural vs. endocrine GLP-1 to insulin secretion in humans remains imprecise\", \"Molecular identity of mucosal GLP-1 sensors on vagal afferents not identified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identifying the IRS-2/PI3K/PKB/Foxo1/PDX-1 pathway downstream of GLP-1R explained how GLP-1 promotes β-cell proliferation and survival beyond its acute insulinotropic effects.\",\n      \"evidence\": \"Signaling assays, nuclear localization of Foxo1, and gene expression analysis in pancreatic β-cells\",\n      \"pmids\": [\"15671479\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab mechanistic study; independent replication in primary human islets was limited\", \"Relative importance of this pathway vs. cAMP/CREB for β-cell mass expansion was not resolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Defining the two-domain peptide binding mechanism of GLP-1R (ECD captures the ligand C-terminus; transmembrane core contacts the ligand N-terminus to activate G-protein coupling) provided the structural framework for all subsequent drug design.\",\n      \"evidence\": \"Binding assays using truncated ligands and truncated receptor constructs (isolated N-domain vs. full-length)\",\n      \"pmids\": [\"17635131\", \"17498508\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No atomic-resolution structure of the full peptide–receptor complex was available at that time\", \"Thermodynamic contributions of each domain interaction were approximated, not precisely measured\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Discovering that NTS astrocytes express functional GLP-1R and are required for the anorectic effects of intra-NTS GLP-1R activation expanded the cellular targets of GLP-1 signaling in the brain beyond neurons.\",\n      \"evidence\": \"Fluorophore-labeled agonist localization, ex vivo calcium imaging, cAMP assay, and pharmacological astrocyte inhibition with behavioral readout\",\n      \"pmids\": [\"27013681\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether astrocytic GLP-1R signaling is relevant to peripherally administered GLP-1R agonists was untested\", \"Downstream effector pathways in astrocytes beyond cAMP were not characterized\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrating that biased GLP-1R agonists with reduced β-arrestin recruitment retain receptor at the plasma membrane and produce greater insulin release with fewer nausea-like side effects established biased agonism as a viable therapeutic strategy.\",\n      \"evidence\": \"GLP-1R trafficking and β-arrestin recruitment assays, insulin secretion measurements, in vivo glucose tolerance tests and nausea behavioral assays in mice\",\n      \"pmids\": [\"29686402\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Translation of reduced nausea from mouse conditioned taste aversion to human clinical nausea was unconfirmed\", \"Structural basis for β-arrestin bias was not resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Structural resolution of GLP-1R in both inactive (crystal structure) and agonist-bound (cryo-EM) states—including a non-peptide agonist binding at an unpredicted pocket—revealed the conformational dynamics underlying receptor activation and enabled rational design of allosteric modulators.\",\n      \"evidence\": \"X-ray crystallography (3.2 Å), cryo-EM structures of GLP-1R with non-peptide agonist TT-OAD2 and with PAM LSN3160440, disulfide cross-linking validation, MD simulations\",\n      \"pmids\": [\"32152292\", \"31915381\", \"32690941\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full activation trajectory from closed ECD to open signaling complex not captured dynamically\", \"PAM mechanism validated only for GLP-1(9-36); generalization to other truncated or intact GLP-1 peptides unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Tirzepatide was shown to exhibit biased agonism at GLP-1R (favoring cAMP over β-arrestin), and β-arrestin1 was identified as a specific negative regulator of GLP-1-stimulated (but not GIP-stimulated) insulin secretion in primary islets, providing a mechanistic rationale for dual-agonist efficacy.\",\n      \"evidence\": \"cAMP and β-arrestin signaling assays, GLP-1R internalization assay, β-arrestin1 manipulation in primary islets\",\n      \"pmids\": [\"32730231\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"β-arrestin1 manipulation was pharmacological/genetic in rodent islets; human islet data limited\", \"How dual GIP/GLP-1 agonism circumvents β-arrestin-mediated desensitization at the structural level is unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Dissecting central vs. peripheral GLP-1 circuits revealed that brainstem PPG neurons and gut-derived GLP-1 suppress eating independently; PPG_NTS neurons receive vagal oxytocin-receptor neuron input and are dispensable for the anorectic effect of exogenous GLP-1R agonists.\",\n      \"evidence\": \"Optogenetics, calcium imaging, viral circuit tracing, chemogenetic manipulation, and behavioral feeding assays in mice\",\n      \"pmids\": [\"33589843\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether peripheral GLP-1R agonists act exclusively on area postrema/hindbrain vs. penetrating the blood-brain barrier was not resolved\", \"Functional redundancy between central PPG circuits and peripherally derived GLP-1 in chronic treatment contexts is unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identification of DMH GLP-1R neurons as preingestive satiation encoders and demonstration of an olfactory bulb→PVN→SNS→pancreas circuit for GLP-1R-mediated insulin secretion expanded the known central targets of GLP-1 signaling to higher-order brain regions.\",\n      \"evidence\": \"Optogenetic and chemogenetic activation of DMH GLP-1R neurons with feeding assays; stereotaxic OB GLP-1R agonist injection with oral glucose tolerance and pharmacological blockade in mice\",\n      \"pmids\": [\"38935778\", \"39138162\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"DMH GLP-1R neuron molecular identity beyond GLP-1R expression is incomplete\", \"OB→PVN circuit relies on pharmacological dissection without genetic specificity for the relay neurons\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Characterization of orforglipron showed that a non-peptide oral GLP-1R agonist with negligible β-arrestin recruitment and low intrinsic efficacy achieves full glucose-lowering at low receptor occupancy, validating biased agonism principles for oral drug design.\",\n      \"evidence\": \"Radioligand binding, signal transduction and β-arrestin recruitment assays, CRISPR-Cas9 humanized rat GLP-1R model with glucose tolerance tests\",\n      \"pmids\": [\"39693407\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term metabolic and CNS effects of chronic low-occupancy non-peptide agonism are not characterized\", \"Whether orforglipron accesses brainstem and hypothalamic GLP-1R populations at therapeutic doses is unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How GLP-1R signaling kinetics differ across tissue-specific contexts (β-cells, NTS neurons, astrocytes, DMH neurons, olfactory bulb) and how biased agonism is shaped by local receptor microenvironments remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No tissue-resolved single-cell signaling kinetics across all GLP-1R-expressing populations\", \"Structural basis for how partial vs. full agonists produce distinct β-arrestin/G-protein bias ratios is incomplete\", \"Whether central and peripheral GLP-1 systems have synergistic or redundant therapeutic effects during chronic agonist treatment is untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [0, 2, 8, 9]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 2, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 1, 11]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 2, 3, 5, 6, 7, 8, 9, 17]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [3, 17]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [4, 12, 13, 14]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 1, 15, 16]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"GLP1R\",\n      \"DPP4\",\n      \"ARRB1\",\n      \"GPR17\",\n      \"FFAR1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"GCG encodes preproglucagon, a precursor that is tissue-specifically processed into glucagon, GLP-1, GLP-2, and oxyntomodulin, each acting through distinct receptors to coordinate glucose homeostasis, appetite, and intestinal growth [PMID:6877358, PMID:12554744]. Glucagon promotes hepatic glucose output via glycogenolysis and gluconeogenesis [PMID:12626323]; GLP-1 stimulates glucose-dependent insulin secretion through cAMP/PKA-mediated closure of KATP channels and activation of nonselective cation channels in beta cells [PMID:8993395], protects beta cells from apoptosis via caspase-3 downregulation and bcl-2 upregulation [PMID:12960095], and suppresses feeding through hypothalamic (DMH) and brainstem (NTS) GLP-1R-expressing circuits that encode satiation independently of peripheral preproglucagon neurons [PMID:8538742, PMID:38935778, PMID:33589843]. GLP-1 secretion from intestinal L cells is triggered by luminal glucose via α-gustducin/sweet taste receptor machinery [PMID:17724330] and modulated by leptin (STAT3/Ob-Rb) and IL-6-driven upregulation of proglucagon and PC1/3 in alpha cells [PMID:12540594, PMID:22037645], while GLP-1R signaling efficacy is determined by β-arrestin-1 recruitment and receptor trafficking, with biased agonists that reduce β-arrestin engagement and GLP-1R internalization producing greater insulinotropic responses [PMID:18445652, PMID:29686402, PMID:32730231].\",\n  \"teleology\": [\n    {\n      \"year\": 1983,\n      \"claim\": \"Cloning the preproglucagon cDNA established that glucagon, GLP-1, and GLP-2 arise from a single gene through exon duplication, resolving how structurally related gut-pancreas peptides share a common genetic origin.\",\n      \"evidence\": \"cDNA cloning and sequence analysis of the human preproglucagon gene\",\n      \"pmids\": [\"6877358\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific post-translational processing mechanisms were not defined\", \"Functions of GLP-1 and GLP-2 were unknown at this point\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Demonstration that GLP-1(7-36 amide) and GLP-1(7-37) are equipotent insulinotropic agents with identical clearance kinetics established the active forms of the peptide and its extremely short plasma half-life (~5–6 min), framing a key pharmacological challenge.\",\n      \"evidence\": \"Intravenous infusion of both GLP-1 forms in healthy humans with insulin, glucagon, glucose, and clearance measurements\",\n      \"pmids\": [\"8482423\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The proteolytic enzyme responsible for rapid degradation was not identified in this study\", \"Receptor binding kinetics of each form were not compared\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Two foundational mechanisms of GLP-1 action were defined: central GLP-1 was shown to suppress feeding via hypothalamic circuits (blocked by exendin(9-39)), and peripheral GLP-1 was shown to stimulate insulin secretion through both KATP channel closure and nonselective cation channel activation in beta cells.\",\n      \"evidence\": \"ICV injection with c-fos mapping and receptor antagonist epistasis in rats; patch-clamp electrophysiology and Ca²⁺ imaging in beta cells\",\n      \"pmids\": [\"8538742\", \"8993395\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the specific neuronal populations mediating central anorexia was unresolved\", \"Relative contribution of KATP vs. NSCC pathways to insulin secretion in vivo was unclear\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Cloning of the GLP-2 receptor demonstrated that each major proglucagon-derived peptide acts through a distinct GPCR, with GLP-2R showing exquisite ligand specificity (nanomolar affinity for GLP-2 but not GLP-1), explaining how a single precursor gene controls diverse tissue-specific functions.\",\n      \"evidence\": \"cDNA cloning of rat and human GLP-2R; cAMP and radioligand binding assays; in vivo intestinotrophic activity\",\n      \"pmids\": [\"9990065\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling cascades of GLP-2R were not characterized\", \"Intestinotrophic mechanism beyond receptor activation was unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"A cluster of discoveries established GLP-1 as a trophic and antiapoptotic factor for beta cells (via caspase-3/bcl-2), identified leptin-STAT3 signaling as a regulator of L-cell GLP-1 secretion, confirmed DPP-IV as the primary GLP-1-degrading enzyme in vivo, and showed oxyntomodulin suppresses appetite and ghrelin in humans, broadening the functional repertoire of proglucagon-derived peptides.\",\n      \"evidence\": \"Human islet culture with apoptosis markers; L-cell secretion assays with leptin/STAT3; IV pharmacokinetics in renal insufficiency patients; double-blind crossover OXM infusion in humans\",\n      \"pmids\": [\"12960095\", \"12540594\", \"14988249\", \"14557443\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether GLP-1 antiapoptotic effects are sustained in vivo long-term was untested\", \"Molecular mechanism of oxyntomodulin anorexia (receptor identity) was unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"GLP-1R signaling was linked to cardioprotection via cAMP/PI3K/MAPK prosurvival pathways phosphorylating BAD, and to beta-cell mass expansion via IRS-2/PI3K/Akt-mediated Foxo1 nuclear exclusion upregulating PDX-1, revealing GLP-1 actions beyond acute insulin secretion.\",\n      \"evidence\": \"Isolated perfused heart ischemia/reperfusion with kinase inhibitors; cell-based studies of GLP-1R → IRS-2 → PI3K → Akt → Foxo1 → PDX-1\",\n      \"pmids\": [\"15616022\", \"15671479\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of GLP-1 cardioprotection in humans was not established\", \"Whether Foxo1 exclusion is necessary or sufficient for beta-cell proliferation was not genetically tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The luminal glucose-sensing mechanism for GLP-1 secretion was identified: intestinal L cells employ sweet taste receptors and α-gustducin, the same molecular machinery as lingual taste cells, to detect sugars and trigger GLP-1 release.\",\n      \"evidence\": \"α-Gustducin knockout mice; NCI-H716 cells; siRNA knockdown; lactisole pharmacological antagonism; glucose tolerance tests\",\n      \"pmids\": [\"17724330\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this mechanism accounts for the majority of nutrient-stimulated GLP-1 secretion in humans was untested\", \"Lipid and amino acid sensing pathways in L cells were not addressed\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Structural understanding of incretin-receptor engagement advanced through binding studies showing GLP-1R uses a two-domain mechanism (ECD captures peptide C-terminus; TMD binds N-terminus for activation), and crystallography of the related GIP receptor ECD–GIP complex confirmed a conserved binding mode across class B GPCRs.\",\n      \"evidence\": \"Radioligand competition with GLP-1R truncation constructs; crystal structure of GIP receptor ECD bound to GIP(1-42)\",\n      \"pmids\": [\"17635131\", \"17715056\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length GLP-1R structure was not yet available\", \"Activation-associated conformational transitions were not visualized\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"β-Arrestin-1 was identified as a direct GLP-1R interactor that positively mediates GLP-1-stimulated cAMP, ERK/CREB, and insulin secretion, adding an arrestin-dependent signaling arm beyond classical G-protein coupling.\",\n      \"evidence\": \"Co-immunoprecipitation in INS-1 cells; siRNA knockdown of β-arrestin-1 with cAMP, ERK/CREB, IRS-2, and insulin secretion readouts\",\n      \"pmids\": [\"18445652\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether β-arrestin-1 signals from endosomes vs. plasma membrane was not resolved\", \"Contribution to in vivo insulin secretion was not tested\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"IL-6 was shown to stimulate GLP-1 production from pancreatic alpha cells by upregulating both proglucagon and prohormone convertase 1/3 expression, providing a mechanism linking exercise-induced IL-6 to enhanced incretin availability.\",\n      \"evidence\": \"IL-6 administration in mice; alpha-cell and L-cell line culture; IL-6 neutralization; proglucagon and PC1/3 mRNA quantification\",\n      \"pmids\": [\"22037645\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative contribution of alpha-cell-derived GLP-1 vs. L-cell-derived GLP-1 to systemic GLP-1 pools was unresolved\", \"Signaling pathway downstream of IL-6 receptor in alpha cells was not fully mapped\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Biased agonism at GLP-1R was functionally validated: agonists that reduce β-arrestin recruitment and receptor internalization while retaining cAMP signaling produce superior long-term insulin secretion compared to FDA-approved GLP-1 mimetics, establishing receptor trafficking as a determinant of therapeutic efficacy.\",\n      \"evidence\": \"Series of biased agonists tested for GLP-1R internalization, recycling, β-arrestin recruitment, and insulin secretion in primary islets; glycemic studies in mice\",\n      \"pmids\": [\"29686402\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term in vivo consequences of biased agonism on beta-cell mass were not assessed\", \"Whether biased agonism also affects central anorectic actions was unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Structural resolution of GLP-1R in multiple states—inactive (crystal structure showing closed ECD), non-peptide agonist-bound (cryo-EM revealing an extracellular binding pocket for TT-OAD2), and positive allosteric modulator-bound (cryo-EM showing molecular-glue mechanism at TM1/TM2)—provided a comprehensive conformational framework for receptor activation and drug design.\",\n      \"evidence\": \"X-ray crystallography at 3.2 Å; cryo-EM structures; disulfide cross-linking; MD simulations; signaling and insulin secretion assays\",\n      \"pmids\": [\"32152292\", \"31915381\", \"32690941\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of GLP-1R activation at the single-molecule level in membranes remain unresolved\", \"How allosteric modulator engagement affects receptor trafficking and desensitization in vivo is unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Tirzepatide was shown to exhibit biased agonism at GLP-1R favoring cAMP over β-arrestin, and β-arrestin-1 knockout islets confirmed that β-arrestin-1 limits insulin responses to GLP-1 but not GIP, explaining how dual-agonist design achieves enhanced insulinotropic efficacy.\",\n      \"evidence\": \"cAMP and β-arrestin signaling assays; GLP-1R internalization; insulin secretion in β-arrestin-1 KO vs. WT mouse islets; receptor occupancy modeling\",\n      \"pmids\": [\"32730231\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo receptor-level occupancy at therapeutic doses in humans was modeled but not directly measured\", \"Contribution of GIP vs. GLP-1 receptor pathways to weight loss was not dissected\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Circuit dissection revealed that preproglucagon NTS neurons (PPGNTS) encode satiation from vagal gastrointestinal distension input and suppress eating independently of peripheral GLP-1R agonist action, demonstrating that central and peripheral GLP-1 systems constitute parallel anorectic pathways.\",\n      \"evidence\": \"Optogenetic manipulation of PPGNTS neurons; calcium imaging; vagal circuit mapping; genetic ablation; comparison with semaglutide in mice\",\n      \"pmids\": [\"33589843\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether combined PPGNTS activation and GLP-1RA treatment produces synergistic weight loss long-term was untested\", \"Molecular identity of the PPGNTS-released mediator(s) (GLP-1 vs. other peptides) was not resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Multiple new GLP-1R-expressing neural circuits were functionally mapped: DMH GLP-1R neurons encode preingestive satiation and interact with ARC NPY/AgRP neurons, while olfactory bulb GLP-1R activation stimulates insulin secretion via a sympathetic brain-to-pancreas axis relayed through the PVN, expanding the known central targets of GLP-1 signaling.\",\n      \"evidence\": \"Optogenetics and calcium imaging of DMHGLP-1R neurons; stereotaxic OB GLP-1R activation; PVN GABAA inhibition; pancreatic noradrenaline measurement in lean and DIO mice\",\n      \"pmids\": [\"38935778\", \"39138162\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether DMH or OB circuits are engaged by therapeutic GLP-1RA doses in humans is unknown\", \"Molecular mediators downstream of PVN relay to sympathetic outflow are not identified\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Orforglipron, a nonpeptide oral GLP-1R agonist, was shown to bind GLP-1R with 1 nM affinity yet with low intrinsic efficacy and negligible β-arrestin recruitment; CRISPR-Cas9-sensitized Glp1r rats confirmed target engagement in pancreas and brain, establishing the pharmacological basis for oral nonpeptide GLP-1R agonism.\",\n      \"evidence\": \"Radioligand binding; signal transduction assays; receptor occupancy modeling; humanized GLP-1R mice; CRISPR-sensitized rats; DIO weight loss\",\n      \"pmids\": [\"39693407\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term metabolic and safety profiles of low-efficacy nonpeptide agonism are not established\", \"Whether orforglipron engages the same allosteric site as TT-OAD2 is not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the quantitative contribution of alpha-cell-derived vs. L-cell-derived GLP-1 to systemic incretin pools, the molecular mediators through which PPGNTS neurons suppress eating, the in vivo relevance of GLP-1R allosteric modulator and biased agonist mechanisms to long-term beta-cell mass and weight regulation, and how Gpr17 and other inhibitory GPCRs coordinate with nutrient-sensing pathways to fine-tune GLP-1 secretion.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Alpha-cell vs. L-cell GLP-1 contributions not quantified in humans\", \"PPGNTS neuron-released mediator identity unresolved\", \"Long-term in vivo consequences of biased agonism on beta-cell mass unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [1, 2, 3, 6, 10, 11]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [1, 9, 10, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0162582\", \"supporting_discovery_ids\": [3, 11, 18, 22, 24, 25, 26, 31]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 11, 18, 22, 24, 25, 26, 31]},\n      {\"term_id\": \"R-HSA-422551\", \"supporting_discovery_ids\": [1, 5, 9]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [5, 13, 14]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [2, 21, 27, 29, 30]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 14, 19]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"GLP1R\",\n      \"GCGR\",\n      \"GLP2R\",\n      \"ARRB1\",\n      \"IDE\",\n      \"DPP4\",\n      \"GNAT3\",\n      \"IL6\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\nWait, I need to fix the pathway term_ids. Let me correct those to valid Reactome IDs:\n\n```json\n{\n  \"mechanistic_narrative\": \"GCG encodes preproglucagon, a precursor that is tissue-specifically processed into glucagon, GLP-1, GLP-2, and oxyntomodulin, each acting through distinct receptors to coordinate glucose homeostasis, appetite, and intestinal growth [PMID:6877358, PMID:12554744]. Glucagon promotes hepatic glucose output via glycogenolysis and gluconeogenesis [PMID:12626323]; GLP-1 stimulates glucose-dependent insulin secretion through cAMP/PKA-mediated closure of KATP channels and activation of nonselective cation channels in beta cells [PMID:8993395], protects beta cells from apoptosis via caspase-3 downregulation and bcl-2 upregulation [PMID:12960095], and suppresses feeding through hypothalamic (DMH) and brainstem (NTS) GLP-1R-expressing circuits that encode satiation independently of peripheral preproglucagon neurons [PMID:8538742, PMID:38935778, PMID:33589843]. GLP-1 secretion from intestinal L cells is triggered by luminal glucose via α-gustducin/sweet taste receptor machinery [PMID:17724330] and modulated by leptin (STAT3/Ob-Rb) and IL-6-driven upregulation of proglucagon and PC1/3 in alpha cells [PMID:12540594, PMID:22037645], while GLP-1R signaling efficacy is determined by β-arrestin-1 recruitment and receptor trafficking, with biased agonists that reduce β-arrestin engagement and GLP-1R internalization producing greater insulinotropic responses [PMID:18445652, PMID:29686402, PMID:32730231].\",\n  \"teleology\": [\n    {\n      \"year\": 1983,\n      \"claim\": \"Cloning the preproglucagon cDNA established that glucagon, GLP-1, and GLP-2 arise from a single gene through exon duplication, resolving how structurally related gut-pancreas peptides share a common genetic origin.\",\n      \"evidence\": \"cDNA cloning and sequence analysis of the human preproglucagon gene\",\n      \"pmids\": [\"6877358\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific post-translational processing mechanisms were not defined\", \"Functions of GLP-1 and GLP-2 were unknown at this point\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Demonstration that GLP-1(7-36 amide) and GLP-1(7-37) are equipotent insulinotropic agents with identical clearance kinetics established the active forms of the peptide and its extremely short plasma half-life (~5–6 min), framing a key pharmacological challenge.\",\n      \"evidence\": \"Intravenous infusion of both GLP-1 forms in healthy humans with insulin, glucagon, glucose, and clearance measurements\",\n      \"pmids\": [\"8482423\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The proteolytic enzyme responsible for rapid degradation was not identified in this study\", \"Receptor binding kinetics of each form were not compared\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Two foundational mechanisms of GLP-1 action were defined: central GLP-1 was shown to suppress feeding via hypothalamic circuits (blocked by exendin(9-39)), and peripheral GLP-1 was shown to stimulate insulin secretion through both KATP channel closure and nonselective cation channel activation in beta cells.\",\n      \"evidence\": \"ICV injection with c-fos mapping and receptor antagonist epistasis in rats; patch-clamp electrophysiology and Ca²⁺ imaging in beta cells\",\n      \"pmids\": [\"8538742\", \"8993395\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the specific neuronal populations mediating central anorexia was unresolved\", \"Relative contribution of KATP vs. NSCC pathways to insulin secretion in vivo was unclear\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Cloning of the GLP-2 receptor demonstrated that each major proglucagon-derived peptide acts through a distinct GPCR, with GLP-2R showing exquisite ligand specificity (nanomolar affinity for GLP-2 but not GLP-1), explaining how a single precursor gene controls diverse tissue-specific functions.\",\n      \"evidence\": \"cDNA cloning of rat and human GLP-2R; cAMP and radioligand binding assays; in vivo intestinotrophic activity\",\n      \"pmids\": [\"9990065\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling cascades of GLP-2R were not characterized\", \"Intestinotrophic mechanism beyond receptor activation was unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"A cluster of discoveries established GLP-1 as a trophic and antiapoptotic factor for beta cells (via caspase-3/bcl-2), identified leptin-STAT3 signaling as a regulator of L-cell GLP-1 secretion, confirmed DPP-IV as the primary GLP-1-degrading enzyme in vivo, and showed oxyntomodulin suppresses appetite and ghrelin in humans, broadening the functional repertoire of proglucagon-derived peptides.\",\n      \"evidence\": \"Human islet culture with apoptosis markers; L-cell secretion assays with leptin/STAT3; IV pharmacokinetics in renal insufficiency patients; double-blind crossover OXM infusion in humans\",\n      \"pmids\": [\"12960095\", \"12540594\", \"14988249\", \"14557443\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether GLP-1 antiapoptotic effects are sustained in vivo long-term was untested\", \"Molecular mechanism of oxyntomodulin anorexia (receptor identity) was unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"GLP-1R signaling was linked to cardioprotection via cAMP/PI3K/MAPK prosurvival pathways phosphorylating BAD, and to beta-cell mass expansion via IRS-2/PI3K/Akt-mediated Foxo1 nuclear exclusion upregulating PDX-1, revealing GLP-1 actions beyond acute insulin secretion.\",\n      \"evidence\": \"Isolated perfused heart ischemia/reperfusion with kinase inhibitors; cell-based studies of GLP-1R → IRS-2 → PI3K → Akt → Foxo1 → PDX-1\",\n      \"pmids\": [\"15616022\", \"15671479\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of GLP-1 cardioprotection in humans was not established\", \"Whether Foxo1 exclusion is necessary or sufficient for beta-cell proliferation was not genetically tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The luminal glucose-sensing mechanism for GLP-1 secretion was identified: intestinal L cells employ sweet taste receptors and α-gustducin, the same molecular machinery as lingual taste cells, to detect sugars and trigger GLP-1 release.\",\n      \"evidence\": \"α-Gustducin knockout mice; NCI-H716 cells; siRNA knockdown; lactisole pharmacological antagonism; glucose tolerance tests\",\n      \"pmids\": [\"17724330\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this mechanism accounts for the majority of nutrient-stimulated GLP-1 secretion in humans was untested\", \"Lipid and amino acid sensing pathways in L cells were not addressed\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Structural understanding of incretin-receptor engagement advanced through binding studies showing GLP-1R uses a two-domain mechanism (ECD captures peptide C-terminus; TMD binds N-terminus for activation), and crystallography of the related GIP receptor ECD–GIP complex confirmed a conserved binding mode across class B GPCRs.\",\n      \"evidence\": \"Radioligand competition with GLP-1R truncation constructs; crystal structure of GIP receptor ECD bound to GIP(1-42)\",\n      \"pmids\": [\"17635131\", \"17715056\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length GLP-1R structure was not yet available\", \"Activation-associated conformational transitions were not visualized\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"β-Arrestin-1 was identified as a direct GLP-1R interactor that positively mediates GLP-1-stimulated cAMP, ERK/CREB, and insulin secretion, adding an arrestin-dependent signaling arm beyond classical G-protein coupling.\",\n      \"evidence\": \"Co-immunoprecipitation in INS-1 cells; siRNA knockdown of β-arrestin-1 with cAMP, ERK/CREB, IRS-2, and insulin secretion readouts\",\n      \"pmids\": [\"18445652\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether β-arrestin-1 signals from endosomes vs. plasma membrane was not resolved\", \"Contribution to in vivo insulin secretion was not tested\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"IL-6 was shown to stimulate GLP-1 production from pancreatic alpha cells by upregulating both proglucagon and prohormone convertase 1/3 expression, providing a mechanism linking exercise-induced IL-6 to enhanced incretin availability.\",\n      \"evidence\": \"IL-6 administration in mice; alpha-cell and L-cell line culture; IL-6 neutralization; proglucagon and PC1/3 mRNA quantification\",\n      \"pmids\": [\"22037645\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative contribution of alpha-cell-derived GLP-1 vs. L-cell-derived GLP-1 to systemic GLP-1 pools was unresolved\", \"Signaling pathway downstream of IL-6 receptor in alpha cells was not fully mapped\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Biased agonism at GLP-1R was functionally validated: agonists that reduce β-arrestin recruitment and receptor internalization while retaining cAMP signaling produce superior long-term insulin secretion compared to FDA-approved GLP-1 mimetics, establishing receptor trafficking as a determinant of therapeutic efficacy.\",\n      \"evidence\": \"Series of biased agonists tested for GLP-1R internalization, recycling, β-arrestin recruitment, and insulin secretion in primary islets; glycemic studies in mice\",\n      \"pmids\": [\"29686402\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term in vivo consequences of biased agonism on beta-cell mass were not assessed\", \"Whether biased agonism also affects central anorectic actions was unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Structural resolution of GLP-1R in multiple states—inactive (crystal structure showing closed ECD), non-peptide agonist-bound (cryo-EM revealing an extracellular pocket for TT-OAD2), and positive allosteric modulator-bound (cryo-EM showing molecular-glue mechanism at TM1/TM2)—provided a comprehensive conformational framework for receptor activation and drug design.\",\n      \"evidence\": \"X-ray crystallography at 3.2 Å; cryo-EM structures; disulfide cross-linking; MD simulations; signaling and insulin secretion assays\",\n      \"pmids\": [\"32152292\", \"31915381\", \"32690941\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of GLP-1R activation at the single-molecule level in membranes remain unresolved\", \"How allosteric modulator engagement affects receptor trafficking and desensitization in vivo is unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Tirzepatide was shown to exhibit biased agonism at GLP-1R favoring cAMP over β-arrestin, and β-arrestin-1 knockout islets confirmed that β-arrestin-1 limits insulin responses to GLP-1 but not GIP, explaining how dual-agonist design achieves enhanced insulinotropic efficacy.\",\n      \"evidence\": \"cAMP and β-arrestin signaling assays; GLP-1R internalization; insulin secretion in β-arrestin-1 KO vs. WT mouse islets; receptor occupancy modeling\",\n      \"pmids\": [\"32730231\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo receptor-level occupancy at therapeutic doses in humans was modeled but not directly measured\", \"Contribution of GIP vs. GLP-1 receptor pathways to weight loss was not dissected\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Circuit dissection revealed that preproglucagon NTS neurons (PPGNTS) encode satiation from vagal gastrointestinal distension input and suppress eating independently of peripheral GLP-1R agonist action, demonstrating that central and peripheral GLP-1 systems constitute parallel anorectic pathways.\",\n      \"evidence\": \"Optogenetic manipulation of PPGNTS neurons; calcium imaging; vagal circuit mapping; genetic ablation; comparison with semaglutide in mice\",\n      \"pmids\": [\"33589843\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether combined PPGNTS activation and GLP-1RA treatment produces synergistic weight loss long-term was untested\", \"Molecular identity of the PPGNTS-released mediator(s) (GLP-1 vs. other peptides) was not resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Multiple new GLP-1R-expressing neural circuits were functionally mapped: DMH GLP-1R neurons encode preingestive satiation and interact with ARC NPY/AgRP neurons, while olfactory bulb GLP-1R activation stimulates insulin secretion via a sympathetic brain-to-pancreas axis relayed through the PVN, expanding the known central targets of GLP-1 signaling.\",\n      \"evidence\": \"Optogenetics and calcium imaging of DMHGLP-1R neurons; stereotaxic OB GLP-1R activation; PVN GABAA inhibition; pancreatic noradrenaline measurement in lean and DIO mice\",\n      \"pmids\": [\"38935778\", \"39138162\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether DMH or OB circuits are engaged by therapeutic GLP-1RA doses in humans is unknown\", \"Molecular mediators downstream of PVN relay to sympathetic outflow are not identified\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Orforglipron, a nonpeptide oral GLP-1R agonist, was shown to bind GLP-1R with 1 nM affinity yet with low intrinsic efficacy and negligible β-arrestin recruitment; CRISPR-Cas9-sensitized Glp1r rats confirmed target engagement in pancreas and brain, establishing the pharmacological basis for oral nonpeptide GLP-1R agonism.\",\n      \"evidence\": \"Radioligand binding; signal transduction assays; receptor occupancy modeling; humanized GLP-1R mice; CRISPR-sensitized rats; DIO weight loss\",\n      \"pmids\": [\"39693407\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term metabolic and safety profiles of low-efficacy nonpeptide agonism are not established\", \"Whether orforglipron engages the same allosteric site as TT-OAD2 is not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the quantitative contribution of alpha-cell-derived vs. L-cell-derived GLP-1 to systemic incretin pools, the molecular mediators through which PPGNTS neurons suppress eating, the in vivo relevance of GLP-1R allosteric modulator and biased agonist mechanisms to long-term beta-cell mass and weight regulation, and how Gpr17 and other inhibitory GPCRs coordinate with nutrient-sensing pathways to fine-tune GLP-1 secretion.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Alpha-cell vs. L-cell GLP-1 contributions not quantified in humans\", \"PPGNTS neuron-released mediator identity unresolved\", \"Long-term in vivo consequences of biased agonism on beta-cell mass unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [1, 2, 3, 6, 10, 11]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [1, 9, 10, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 11, 18, 22, 24, 25, 26, 31]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [5, 13, 14]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [2, 21, 27, 29, 30]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 14, 19]},\n      {\"term_id\": \"R-HSA-8963743\", \"supporting_discovery_ids\": [4, 13, 15]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"GLP1R\",\n      \"GCGR\",\n      \"GLP2R\",\n      \"ARRB1\",\n      \"IDE\",\n      \"DPP4\",\n      \"GNAT3\",\n      \"IL6\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}