{"gene":"GPR37","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":1997,"finding":"GPR37 (hETBR-LP) was cloned as a novel 614-amino acid, seven-transmembrane domain GPCR enriched in the brain, particularly in Purkinje cells and hippocampal neurons; recombinant expression confirmed it did not bind ET-1, ET-3, bombesin, or neuropeptide Y, nor did it mobilize Ca2+ or alter cAMP, establishing it as an orphan receptor.","method":"cDNA cloning, Northern blot, in situ hybridization, radioligand binding assay, Ca2+/cAMP functional assays in COS7 and HEK293 cells","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 — original cloning with multiple orthogonal functional assays in heterologous cells","pmids":["9144577"],"is_preprint":false},{"year":2002,"finding":"GPR37/Pael-R was identified as a substrate of the E3 ubiquitin ligase Parkin via yeast two-hybrid; when overexpressed in cells, Pael-R becomes unfolded, insoluble, and ubiquitinated, accumulates in the ER, and induces ER stress-induced cell death that is suppressed by Parkin-mediated ubiquitination and degradation.","method":"Yeast two-hybrid, co-immunoprecipitation, ubiquitination assay, cell death assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (Y2H, Co-IP, in vitro ubiquitination, cell death rescue), replicated across multiple studies","pmids":["12150907","14579121"],"is_preprint":false},{"year":2002,"finding":"CHIP forms a complex with Hsp70, Parkin, and Pael-R in vitro and in vivo; CHIP promotes dissociation of Hsp70 from Parkin and Pael-R, thereby facilitating Parkin-mediated ubiquitination of Pael-R and enhancing Parkin's ability to suppress Pael-R-induced cell death.","method":"Co-immunoprecipitation (in vitro and in vivo), in vitro ubiquitination assay, cell death assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted complex, in vitro ubiquitination, and functional cell death assay in a single study","pmids":["12150907"],"is_preprint":false},{"year":2003,"finding":"Panneuronal expression of Pael-R in Drosophila caused age-dependent selective degeneration of dopaminergic neurons; co-expression of Parkin degraded Pael-R and suppressed its toxicity, while interfering with endogenous Drosophila Parkin promoted Pael-R accumulation and augmented toxicity, placing Parkin downstream of Pael-R accumulation in dopaminergic neurodegeneration.","method":"Drosophila transgenic expression, genetic epistasis, dopaminergic neuron counting","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis in an in vivo organismal model with quantified neuronal phenotype","pmids":["12670421"],"is_preprint":false},{"year":2003,"finding":"Pael-R, Parkin, alpha-synuclein, and ubiquitin were found to accumulate in Lewy bodies of Parkinson's disease patients, with Pael-R localized to the core of Lewy bodies.","method":"Immunohistochemistry on post-mortem human brain tissue (Parkinson's disease and MSA cases)","journal":"Annals of neurology","confidence":"Medium","confidence_rationale":"Tier 3 — direct localization in human disease tissue but no functional manipulation","pmids":["14991825"],"is_preprint":false},{"year":2003,"finding":"Glup/PACRG, encoded by a gene adjacent to Parkin, forms a large molecular chaperone complex (containing Hsp70, Hsp90, and chaperonin components) that associates with Pael-R, suppresses Pael-R-induced cell death, and facilitates inclusion body formation when the proteasome is inhibited.","method":"Co-immunoprecipitation, protein complex isolation, cell death assay, siRNA knockdown, immunofluorescence","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP showing complex, functional cell death and knockdown assays in a single study","pmids":["14532270"],"is_preprint":false},{"year":2006,"finding":"HRD1, an ER-resident ubiquitin ligase, interacts with Pael-R through its proline-rich domain, promotes Pael-R ubiquitination and degradation, and siRNA-mediated knockdown of HRD1 causes Pael-R accumulation and caspase-3 activation; ATF6 overexpression induces HRD1 and accelerates Pael-R degradation.","method":"Co-immunoprecipitation, ubiquitination assay, siRNA knockdown, caspase-3 activation measurement","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (Co-IP, ubiquitination assay, siRNA KD with defined phenotype) in a single study","pmids":["17059562"],"is_preprint":false},{"year":2006,"finding":"Adenoviral vector-mediated overexpression of Pael-R in the substantia nigra of mice induced ER stress and selective death of dopaminergic neurons; this neurodegeneration was enhanced in mice deficient in Parkin or the ER chaperone ORP150, and was partially blocked by a dopamine synthesis inhibitor.","method":"In vivo adenoviral vector injection, Parkin/ORP150 knockout mice, dopaminergic neuron counting, pharmacological inhibition","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic epistasis with multiple knockout lines and pharmacological intervention","pmids":["17116640"],"is_preprint":false},{"year":2007,"finding":"Pael-R knockout mice showed ~40% reduction in striatal dopamine, while Pael-R transgenic mice showed increased striatal DOPAC and vesicular dopamine content; Pael-R transgenic nigrostriatal neurons were more vulnerable and knockout neurons less vulnerable to PD-related neurotoxins, establishing GPR37 as a regulator of dopamine metabolism in the nigrostriatal system.","method":"Pael-R knockout and transgenic mouse models, HPLC dopamine measurement, neurotoxin challenge","journal":"Neuroscience research","confidence":"High","confidence_rationale":"Tier 2 — bidirectional genetic manipulation (KO and overexpression) with defined biochemical and pharmacological phenotypes","pmids":["17889953"],"is_preprint":false},{"year":2007,"finding":"Drosophila thioredoxin (TRX), acting primarily as a molecular chaperone, suppressed Pael-R-induced dopaminergic neurotoxicity; redox-defective TRX mutants retained chaperone activity and still partially suppressed Pael-R toxicity, indicating chaperone (not antioxidant) activity is the primary mechanism of suppression.","method":"Drosophila co-expression, site-directed mutagenesis of TRX, dopaminergic neuron counting, locomotor assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — mutagenesis dissecting mechanism in an in vivo Drosophila model","pmids":["17301052"],"is_preprint":false},{"year":2008,"finding":"HRD1's proline-rich domain is necessary to promote Pael-R degradation, while its transmembrane domain is required for transferring Pael-R from the ER to the cytosol for proteasomal degradation; a transmembrane-domain mutant of HRD1 was markedly unstable.","method":"Domain deletion/mutagenesis of HRD1, co-immunoprecipitation, degradation assay","journal":"Journal of pharmacological sciences","confidence":"Medium","confidence_rationale":"Tier 2 — domain mutagenesis with functional degradation assay","pmids":["18344614"],"is_preprint":false},{"year":2008,"finding":"Parkin knockout/Pael-R transgenic double mice exhibited progressive and selective loss of dopaminergic and noradrenergic neurons without inclusion body formation, with persistent unfolded protein response activation, elevated dopamine/metabolites, and later reduction in mitochondrial complex I activity.","method":"Double transgenic/knockout mouse model, catecholaminergic neuron counting, UPR markers, mitochondrial complex I activity assay","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis in vivo with multiple mechanistic readouts replicated across lifetime","pmids":["18691389"],"is_preprint":false},{"year":2009,"finding":"GPR37 surface expression is dramatically enhanced by removal of its N-terminal domain (first ~210 amino acids); coexpression with adenosine A2A receptor or dopamine D2 receptor increases GPR37 surface expression, with co-immunoprecipitation confirming GPR37-D2R physical association that modestly alters D2R agonist/antagonist affinity; interaction with the PDZ scaffold syntenin-1 also dramatically increases GPR37 surface expression.","method":"N-terminal truncation, co-immunoprecipitation, FACS surface expression, radioligand binding","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches (truncation, Co-IP, PDZ interaction) with quantified surface expression","pmids":["19799451"],"is_preprint":false},{"year":2009,"finding":"rAAV2/6-mediated overexpression of Pael-R in the rat nigrostriatal system caused rapid insoluble accumulation of Pael-R, severe and selective loss of nigral dopaminergic neurons and nigrostriatal fibers, striatal dopamine depletion, and spontaneous motor impairments; GABAergic neurons of the globus pallidus were unaffected, demonstrating selective vulnerability of nigral dopaminergic neurons to Pael-R accumulation.","method":"In vivo AAV vector injection in rats, immunohistochemistry, HPLC dopamine measurement, behavioral testing","journal":"Neurobiology of disease","confidence":"High","confidence_rationale":"Tier 2 — in vivo targeted overexpression with selective cellular phenotype and multiple mechanistic readouts","pmids":["19348945"],"is_preprint":false},{"year":2013,"finding":"Prosaptide (the active fragment of prosaposin) and full-length prosaposin activate GPR37 signaling: prosaptide promoted GPR37 endocytosis, bound to GPR37, stimulated ERK phosphorylation in a pertussis toxin-sensitive manner, stimulated 35S-GTPγS binding, and inhibited forskolin-stimulated cAMP production; prosaposin/prosaptide protected primary astrocytes against oxidative stress in a GPR37-dependent manner (attenuated by GPR37 siRNA knockdown), identifying prosaposin/prosaptide as GPR37 ligands.","method":"GPCR endocytosis assay, ligand binding, ERK phosphorylation assay, 35S-GTPγS binding, cAMP assay, pertussis toxin treatment, siRNA knockdown, oxidative stress protection assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal signaling assays, pertussis toxin sensitivity, siRNA validation, and functional protection assay in a single study","pmids":["23690594"],"is_preprint":false},{"year":2013,"finding":"The C-terminal cysteine-rich domain of GPR37 controls receptor plasma membrane expression and function: deletion of six cysteines in this region promoted plasma membrane expression and signaling, while removal of the C-terminal cysteine-rich domain protected against GPR37-mediated apoptosis and cell death.","method":"Site-directed mutagenesis, flow cytometry surface expression, signaling assay, apoptosis/cell death assay","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — mutagenesis with multiple functional readouts in a single study","pmids":["23398388"],"is_preprint":false},{"year":2014,"finding":"PICK1 interacts with GPR37/PAELR via its PDZ domain binding the C-terminal PDZ motif of PAELR; pulldown with GST-ct-PAELR retained recombinant and native PICK1 from rat brain; PICK1 overexpression reduced PAELR expression and attenuated PAELR-induced cell death during rotenone treatment in a proteasome-dependent manner.","method":"GST pulldown, co-immunoprecipitation, cell death assay, proteasome inhibitor treatment","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2–3 — GST pulldown confirmed with native brain tissue and functional cell death assay","pmids":["24749734"],"is_preprint":false},{"year":2015,"finding":"Deletion of GPR37 attenuated pilocarpine-induced tremulous jaw movements (a model of parkinsonian tremor), and the ability of adenosine A2A receptor activation to control tremulous jaw movements was lost in GPR37-null mice, indicating that A2A receptor-mediated modulation of parkinsonian tremor requires GPR37.","method":"GPR37 knockout mice, behavioral tremulous jaw movement assay, pharmacological A2A receptor manipulation","journal":"Behavioural brain research","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis in vivo with defined behavioral phenotype","pmids":["25862943"],"is_preprint":false},{"year":2018,"finding":"GABARAPL2 interacts with the cysteine-rich region (-CCCCCC-EEC) of GPR37's C-terminal tail; affinity chromatography showed Myc-tagged GABARAPL2 was retained by GST-ct-PAELR; transient transfection of GABARAPL2 in HEK293 cells reduced PAELR expression, suggesting autophagy-mediated regulation of GPR37 levels.","method":"Affinity chromatography (GST pulldown), in-silico modelling, transient transfection, Western blot","journal":"Neuroscience letters","confidence":"Low","confidence_rationale":"Tier 3 — single pulldown method with computational modelling, functional data limited to expression changes","pmids":["29496607"],"is_preprint":false},{"year":2019,"finding":"GPR37 is expressed in migrating GnRH neurons and olfactory ensheathing cells (OECs); pharmacological inhibition of GPR37 signaling in nasal explants attenuated GnRH neuronal migration and OEC movement; GPR37-deficient mice showed decreased olfactory bulb nerve layer and attenuated/delayed GnRH neuron maturation and migration into the brain.","method":"PCR, immunocytochemistry, nasal explant inhibition assay, GPR37 knockout mouse analysis","journal":"Frontiers in cellular neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological inhibition and KO mouse with defined developmental/cellular phenotypes","pmids":["31143101"],"is_preprint":false},{"year":2024,"finding":"Osteocalcin (OCN), a bone-derived protein, activates GPR37 in a subpopulation of VTA GABAergic neurons to decrease potassium currents via cAMP reduction and subsequent THIK-1 (K2P13.1) channel suppression, increasing neuronal excitability and priming rapid visual escape; GPR37 KO and conditional GPR37 KO in VTA GABAergic neurons delayed escape, and reconstituting OCN-GPR37 signaling in VTA restored normal response.","method":"Knockout mice, conditional knockout, single-cell transcriptomics, electrophysiology, chemogenetics, ex vivo calcium imaging","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — multiple genetic manipulations with electrophysiological and behavioral readouts, but preprint only","pmids":["bio_10.1101_2024.08.12.607510"],"is_preprint":true},{"year":2024,"finding":"GPR37 mediates the analgesic and pro-resolving effects of protectin DX (PDX) in macrophages: PDX binds GPR37 and induces calcium responses in peritoneal macrophages, promotes GPR37-dependent macrophage phagocytosis and efferocytosis, and GPR37 KO mice showed deficits in fracture-induced postoperative pain resolution and abolished PDX analgesia.","method":"Gpr37 knockout mice, ligand binding assay, calcium imaging in macrophages, efferocytosis assay, behavioral pain assay, lipidomics","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — ligand binding confirmed, GPR37 KO abolishes effect, cellular mechanism defined; preprint only","pmids":["bio_10.1101_2025.11.10.687700"],"is_preprint":true},{"year":2024,"finding":"GPR37 contains endogenous ligands present in brain extracts, as detected using the GzESTY cell-based Gi/o/z signaling assay, supporting ongoing deorphanization of GPR37.","method":"Cell-based GPCR signaling assay (GzESTY) with brain extract stimulation","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 — single assay with crude brain extract, ligand identity not determined; preprint only","pmids":["bio_10.1101_2024.07.26.605282"],"is_preprint":true},{"year":2025,"finding":"Intrathecal activation of spinal GPR37 by agonists TX14A and protectin D1 dose-dependently inhibited capsaicin-induced long-term increase in nociception and produced an 'unpriming' effect in the hyperalgesic priming model; global GPR37 knockout abolished these long-term effects; ex vivo Ca2+ imaging showed i.th. TX14A rescued dorsal horn interneurons from long-term potentiation/depression, suggesting GPR37 agonism erases spinal pain memory.","method":"Intrathecal drug administration, GPR37 knockout mice, behavioral pain assays, ex vivo Ca2+ imaging of spinal dorsal horn","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — GPR37 KO abolishes effect, ex vivo mechanistic imaging, multiple agonists; preprint only","pmids":["bio_10.1101_2025.11.25.690482"],"is_preprint":true},{"year":2025,"finding":"Protectin D1 (PD1) attenuates abdominal aortic aneurysm progression in a GPR37-dependent manner in macrophages: PD1 enhanced macrophage efferocytosis of apoptotic vascular smooth muscle cells via GPR37-dependent calcium signaling and reduced macrophage-derived TNF-α and IL-1β; GPR37-dependent effects were confirmed in isolated macrophages and murine AAA models.","method":"Murine AAA models (elastase+BAPN), GPR37 knockout mice, macrophage efferocytosis assay, calcium signaling, scRNA-seq, cytokine measurement","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — GPR37 KO confirmation, mechanistic cellular assays; preprint only","pmids":["bio_10.1101_2025.10.20.683538"],"is_preprint":true}],"current_model":"GPR37 (Pael-R) is an orphan GPCR expressed predominantly in neurons and glia that acts as a substrate of the E3 ubiquitin ligase Parkin; when misfolded, it accumulates in the ER and induces ER stress-mediated death of dopaminergic neurons, while properly folded GPR37 functions as a receptor for prosaposin/prosaptide and osteocalcin, coupling via Gi/o to inhibit cAMP, activate ERK, and regulate diverse processes including dopamine metabolism, cell migration, spinal pain sensitization, and macrophage efferocytosis; its surface trafficking is controlled by N-terminal sequences, C-terminal cysteine-rich domain, interaction partners (D2R, A2AR, syntenin-1, PICK1, GABARAPL2), and ubiquitin ligases Parkin and HRD1."},"narrative":{"teleology":[{"year":1997,"claim":"Cloning of GPR37 established it as a novel brain-enriched seven-transmembrane orphan GPCR without identifiable endogenous ligands or classical second-messenger coupling, setting the stage for functional deorphanization.","evidence":"cDNA cloning with Northern blot, in situ hybridization, radioligand binding, and Ca²⁺/cAMP assays in COS7/HEK293 cells","pmids":["9144577"],"confidence":"High","gaps":["No ligand identified","No signaling pathway activated in initial assays","Physiological role unknown"]},{"year":2002,"claim":"Identification of GPR37 as a Parkin substrate linked receptor misfolding and ER stress to dopaminergic cell death, providing a direct molecular connection between GPR37 accumulation and Parkinson's disease-related neurodegeneration.","evidence":"Yeast two-hybrid, co-immunoprecipitation, ubiquitination assays, cell death rescue by Parkin; CHIP–Hsp70 complex facilitation of Parkin-mediated Pael-R ubiquitination","pmids":["12150907","14579121"],"confidence":"High","gaps":["Whether endogenous levels of GPR37 (not overexpressed) cause toxicity in neurons","Mechanism by which GPR37 misfolding is sensed"]},{"year":2003,"claim":"In vivo Drosophila and mouse models demonstrated that GPR37 accumulation selectively kills dopaminergic neurons in a Parkin-dependent manner, validating the Parkin–Pael-R axis in an organismal context and establishing cell-type selectivity of toxicity.","evidence":"Drosophila transgenic/genetic epistasis with dopaminergic neuron counting; Pael-R localization to Lewy body cores in human PD brain","pmids":["12670421","14991825"],"confidence":"High","gaps":["Molecular basis for selective vulnerability of dopaminergic neurons","Whether Lewy body accumulation is cause or consequence"]},{"year":2006,"claim":"Discovery that the ER-resident E3 ligase HRD1 independently ubiquitinates and degrades GPR37 revealed a second quality-control pathway (ERAD) beyond Parkin, and mouse models confirmed that ER stress and dopamine synthesis both contribute to GPR37-induced neurodegeneration.","evidence":"Co-IP, ubiquitination assay, siRNA knockdown of HRD1 with caspase-3 activation; in vivo adenoviral overexpression in Parkin-KO and ORP150-KO mice with dopamine synthesis inhibitor rescue","pmids":["17059562","17116640"],"confidence":"High","gaps":["Relative contribution of HRD1 vs. Parkin to endogenous GPR37 turnover","Whether dopamine metabolites directly modify GPR37"]},{"year":2007,"claim":"Bidirectional genetic manipulation in mice established GPR37 as a physiological regulator of striatal dopamine metabolism, moving the gene's role beyond a pure toxicity substrate to a functional modulator of the nigrostriatal system.","evidence":"GPR37 knockout and transgenic mice with HPLC dopamine/DOPAC measurement and neurotoxin challenge","pmids":["17889953"],"confidence":"High","gaps":["Signaling pathway linking GPR37 to dopamine synthesis or release","Receptor activation state of endogenous GPR37 in these models"]},{"year":2009,"claim":"Dissection of GPR37 trafficking determinants revealed that the N-terminal ectodomain retains the receptor intracellularly, while co-expression with D2R or A2AR or the PDZ scaffold syntenin-1 rescues surface delivery, establishing GPR37 as a trafficking-regulated receptor whose functional availability depends on interaction partners.","evidence":"N-terminal truncation, FACS surface expression, co-immunoprecipitation with D2R/A2AR, syntenin-1 interaction","pmids":["19799451"],"confidence":"High","gaps":["Whether N-terminal cleavage occurs endogenously","Stoichiometry and in vivo relevance of GPR37–D2R heteromers"]},{"year":2013,"claim":"Deorphanization of GPR37 by prosaposin/prosaptide established it as a Gi/o-coupled receptor that inhibits cAMP, activates ERK, and protects astrocytes from oxidative stress, transforming understanding from a passive toxicity substrate to an active signaling receptor.","evidence":"Endocytosis assay, ligand binding, ERK phosphorylation, GTPγS binding, cAMP assay, pertussis toxin sensitivity, siRNA knockdown, oxidative stress protection in primary astrocytes","pmids":["23690594"],"confidence":"High","gaps":["Whether prosaposin is the primary endogenous ligand in vivo","Structural basis of ligand–receptor interaction"]},{"year":2013,"claim":"The C-terminal cysteine-rich domain was identified as a negative regulator of GPR37 surface expression and a determinant of receptor-mediated apoptosis, and additional C-terminal interactors PICK1 and GABARAPL2 were shown to modulate GPR37 levels.","evidence":"Cysteine mutagenesis with flow cytometry and apoptosis assay; GST pulldown with PICK1 and GABARAPL2; cell death rescue by PICK1 overexpression","pmids":["23398388","24749734","29496607"],"confidence":"Medium","gaps":["Whether GABARAPL2 mediates autophagic degradation of GPR37 in neurons","Functional interplay between C-terminal partners","GABARAPL2 interaction confirmed by only a single pulldown method"]},{"year":2015,"claim":"GPR37 was shown to be required for A2A receptor-mediated modulation of parkinsonian tremor, establishing a functional epistatic relationship between GPR37 and adenosine signaling in motor circuits.","evidence":"GPR37 KO mice with pilocarpine-induced tremulous jaw movements and pharmacological A2A receptor manipulation","pmids":["25862943"],"confidence":"Medium","gaps":["Whether GPR37–A2AR heteromerization is necessary for the behavioral phenotype","Downstream signaling mediating the interaction"]},{"year":2019,"claim":"GPR37 was found to regulate GnRH neuron and olfactory ensheathing cell migration during development, extending its physiological roles beyond dopaminergic circuits to developmental neurobiology.","evidence":"Nasal explant pharmacological inhibition and GPR37 KO mouse analysis of GnRH neuron migration and olfactory bulb innervation","pmids":["31143101"],"confidence":"Medium","gaps":["Ligand activating GPR37 during GnRH neuron migration","Downstream effectors of GPR37 in migrating cells"]},{"year":null,"claim":"Key unresolved questions include the identity of the primary endogenous ligand(s) in vivo, the structural basis of GPR37 activation, whether GPR37 functions primarily as a monomer or in heteromeric complexes in native tissue, and the mechanism by which GPR37 signaling modulates dopamine metabolism.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No solved receptor structure","No consensus on primary endogenous ligand in brain","Mechanism linking GPR37 signaling to dopamine synthesis/release undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,14]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[8,17]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[12,14,15]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[1,6]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[14,17]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[1,6]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[1,3,7]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[8,17]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[3,7,11]}],"complexes":[],"partners":["PRKN","HRD1","STUB1","DRD2","ADORA2A","SDCBP","PICK1","GABARAPL2"],"other_free_text":[]},"mechanistic_narrative":"GPR37 (Pael-R) is a brain-enriched orphan GPCR that functions at the intersection of protein quality control and neuroprotective signaling, with roles spanning dopaminergic neuron survival, pain modulation, cell migration, and macrophage efferocytosis. Misfolded GPR37 accumulates in the ER and triggers ER stress-mediated death selectively in dopaminergic neurons; this toxicity is counteracted by Parkin-mediated ubiquitination and proteasomal degradation, and by the ER-resident E3 ligase HRD1 [PMID:12150907, PMID:12670421, PMID:17059562]. Properly folded, surface-delivered GPR37 is activated by prosaposin/prosaptide, coupling through Gi/o to inhibit cAMP, activate ERK, and protect astrocytes from oxidative stress [PMID:23690594]. GPR37 knockout mice display reduced striatal dopamine, altered vulnerability to dopaminergic neurotoxins, impaired GnRH neuron migration, and attenuated adenosine A2A receptor-mediated modulation of parkinsonian tremor [PMID:17889953, PMID:31143101, PMID:25862943]."},"prefetch_data":{"uniprot":{"accession":"O15354","full_name":"Prosaposin receptor GPR37","aliases":["Endothelin B receptor-like protein 1","ETBR-LP-1","G-protein coupled receptor 37","Parkin-associated endothelin receptor-like receptor","PAELR"],"length_aa":613,"mass_kda":67.1,"function":"G-protein-coupled receptor that plays a role in several physiological pathways such as resolution of inflammatory pain and oligodendrocyte differentiation (By similarity). Acts as a receptor for several ligands including prosaposin, osteocalcin or neuroprotectin D1. Ligand binding induces endocytosis, followed by an ERK phosphorylation cascade (PubMed:11439185, PubMed:23690594). Acts as a receptor for osteocalcin (OCN) to regulate oligodendrocyte differentiation and central nervous system myelination. Mechanistically, plays a negative role in oligodendrocyte differentiation and myelination during development via activation of the ERK1/2 signaling pathway. Therefore, regulates the stability of myelin or resistance of myelin itself to demyelination. Upon activation by neuroprotectin D1 (NPD1), promotes the activation of phagocytosis in macrophages as well as the shift in cytokine release toward an anti-inflammatory profile, and thus helps to reverse inflammatory pain. In addition, the increased macrophage phagocytosis mediates protection against sepsis upon pathogen infection. Additionally, extracellular vesicles derived from efferocyte express prosaposin, which binds to macrophage GPR37 to increase expression of the efferocytosis receptor TIM4 via an ERK-AP1-dependent signaling axis, leading to increased macrophage efferocytosis efficiency and accelerated resolution of inflammation (By similarity). May also act as a maturation factor of LRP6, protecting LRP6 from the endoplasmic reticulum (ER)-associated protein degradation (ERAD) and thereby promoting the Wnt/beta-catenin signaling pathway (PubMed:28341812)","subcellular_location":"Cell projection, dendrite; Synapse; Cell membrane; Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/O15354/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GPR37","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/GPR37","total_profiled":1310},"omim":[{"mim_id":"621533","title":"NEURODEVELOPMENTAL DISORDER WITH SEIZURES AND IMPAIRED INTELLECTUAL AND LANGUAGE DEVELOPMENT; NEDSIL","url":"https://www.omim.org/entry/621533"},{"mim_id":"618487","title":"G PROTEIN-COUPLED RECEPTOR 151; GPR151","url":"https://www.omim.org/entry/618487"},{"mim_id":"617630","title":"G PROTEIN-COUPLED RECEPTOR 37-LIKE 1; GPR37L1","url":"https://www.omim.org/entry/617630"},{"mim_id":"611176","title":"JNK/MAPK8-ASSOCIATED MEMBRANE PROTEIN; JKAMP","url":"https://www.omim.org/entry/611176"},{"mim_id":"602583","title":"G PROTEIN-COUPLED RECEPTOR 37; GPR37","url":"https://www.omim.org/entry/602583"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Nuclear membrane","reliability":"Uncertain"},{"location":"Nucleoli","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"brain","ntpm":67.2},{"tissue":"retina","ntpm":36.2}],"url":"https://www.proteinatlas.org/search/GPR37"},"hgnc":{"alias_symbol":["EDNRBL","hET(B)R-LP","PAELR"],"prev_symbol":[]},"alphafold":{"accession":"O15354","domains":[{"cath_id":"1.20.1070.10","chopping":"256-400_419-573","consensus_level":"high","plddt":82.7383,"start":256,"end":573}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O15354","model_url":"https://alphafold.ebi.ac.uk/files/AF-O15354-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O15354-F1-predicted_aligned_error_v6.png","plddt_mean":61.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPR37","jax_strain_url":"https://www.jax.org/strain/search?query=GPR37"},"sequence":{"accession":"O15354","fasta_url":"https://rest.uniprot.org/uniprotkb/O15354.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O15354/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O15354"}},"corpus_meta":[{"pmid":"12150907","id":"PMC_12150907","title":"CHIP is associated with Parkin, a gene responsible for familial Parkinson's disease, and enhances its ubiquitin ligase activity.","date":"2002","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/12150907","citation_count":404,"is_preprint":false},{"pmid":"12670421","id":"PMC_12670421","title":"Parkin suppresses dopaminergic neuron-selective neurotoxicity induced by Pael-R in Drosophila.","date":"2003","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/12670421","citation_count":301,"is_preprint":false},{"pmid":"12676955","id":"PMC_12676955","title":"Parkin facilitates the elimination of expanded polyglutamine proteins and leads to preservation of proteasome function.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12676955","citation_count":231,"is_preprint":false},{"pmid":"23690594","id":"PMC_23690594","title":"GPR37 and GPR37L1 are receptors for the neuroprotective and glioprotective factors prosaptide and 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recombinant expression confirmed it did not bind ET-1, ET-3, bombesin, or neuropeptide Y, nor did it mobilize Ca2+ or alter cAMP, establishing it as an orphan receptor.\",\n      \"method\": \"cDNA cloning, Northern blot, in situ hybridization, radioligand binding assay, Ca2+/cAMP functional assays in COS7 and HEK293 cells\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — original cloning with multiple orthogonal functional assays in heterologous cells\",\n      \"pmids\": [\"9144577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"GPR37/Pael-R was identified as a substrate of the E3 ubiquitin ligase Parkin via yeast two-hybrid; when overexpressed in cells, Pael-R becomes unfolded, insoluble, and ubiquitinated, accumulates in the ER, and induces ER stress-induced cell death that is suppressed by Parkin-mediated ubiquitination and degradation.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, ubiquitination assay, cell death assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (Y2H, Co-IP, in vitro ubiquitination, cell death rescue), replicated across multiple studies\",\n      \"pmids\": [\"12150907\", \"14579121\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CHIP forms a complex with Hsp70, Parkin, and Pael-R in vitro and in vivo; CHIP promotes dissociation of Hsp70 from Parkin and Pael-R, thereby facilitating Parkin-mediated ubiquitination of Pael-R and enhancing Parkin's ability to suppress Pael-R-induced cell death.\",\n      \"method\": \"Co-immunoprecipitation (in vitro and in vivo), in vitro ubiquitination assay, cell death assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted complex, in vitro ubiquitination, and functional cell death assay in a single study\",\n      \"pmids\": [\"12150907\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Panneuronal expression of Pael-R in Drosophila caused age-dependent selective degeneration of dopaminergic neurons; co-expression of Parkin degraded Pael-R and suppressed its toxicity, while interfering with endogenous Drosophila Parkin promoted Pael-R accumulation and augmented toxicity, placing Parkin downstream of Pael-R accumulation in dopaminergic neurodegeneration.\",\n      \"method\": \"Drosophila transgenic expression, genetic epistasis, dopaminergic neuron counting\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in an in vivo organismal model with quantified neuronal phenotype\",\n      \"pmids\": [\"12670421\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Pael-R, Parkin, alpha-synuclein, and ubiquitin were found to accumulate in Lewy bodies of Parkinson's disease patients, with Pael-R localized to the core of Lewy bodies.\",\n      \"method\": \"Immunohistochemistry on post-mortem human brain tissue (Parkinson's disease and MSA cases)\",\n      \"journal\": \"Annals of neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — direct localization in human disease tissue but no functional manipulation\",\n      \"pmids\": [\"14991825\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Glup/PACRG, encoded by a gene adjacent to Parkin, forms a large molecular chaperone complex (containing Hsp70, Hsp90, and chaperonin components) that associates with Pael-R, suppresses Pael-R-induced cell death, and facilitates inclusion body formation when the proteasome is inhibited.\",\n      \"method\": \"Co-immunoprecipitation, protein complex isolation, cell death assay, siRNA knockdown, immunofluorescence\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP showing complex, functional cell death and knockdown assays in a single study\",\n      \"pmids\": [\"14532270\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"HRD1, an ER-resident ubiquitin ligase, interacts with Pael-R through its proline-rich domain, promotes Pael-R ubiquitination and degradation, and siRNA-mediated knockdown of HRD1 causes Pael-R accumulation and caspase-3 activation; ATF6 overexpression induces HRD1 and accelerates Pael-R degradation.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, siRNA knockdown, caspase-3 activation measurement\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (Co-IP, ubiquitination assay, siRNA KD with defined phenotype) in a single study\",\n      \"pmids\": [\"17059562\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Adenoviral vector-mediated overexpression of Pael-R in the substantia nigra of mice induced ER stress and selective death of dopaminergic neurons; this neurodegeneration was enhanced in mice deficient in Parkin or the ER chaperone ORP150, and was partially blocked by a dopamine synthesis inhibitor.\",\n      \"method\": \"In vivo adenoviral vector injection, Parkin/ORP150 knockout mice, dopaminergic neuron counting, pharmacological inhibition\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic epistasis with multiple knockout lines and pharmacological intervention\",\n      \"pmids\": [\"17116640\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Pael-R knockout mice showed ~40% reduction in striatal dopamine, while Pael-R transgenic mice showed increased striatal DOPAC and vesicular dopamine content; Pael-R transgenic nigrostriatal neurons were more vulnerable and knockout neurons less vulnerable to PD-related neurotoxins, establishing GPR37 as a regulator of dopamine metabolism in the nigrostriatal system.\",\n      \"method\": \"Pael-R knockout and transgenic mouse models, HPLC dopamine measurement, neurotoxin challenge\",\n      \"journal\": \"Neuroscience research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — bidirectional genetic manipulation (KO and overexpression) with defined biochemical and pharmacological phenotypes\",\n      \"pmids\": [\"17889953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Drosophila thioredoxin (TRX), acting primarily as a molecular chaperone, suppressed Pael-R-induced dopaminergic neurotoxicity; redox-defective TRX mutants retained chaperone activity and still partially suppressed Pael-R toxicity, indicating chaperone (not antioxidant) activity is the primary mechanism of suppression.\",\n      \"method\": \"Drosophila co-expression, site-directed mutagenesis of TRX, dopaminergic neuron counting, locomotor assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis dissecting mechanism in an in vivo Drosophila model\",\n      \"pmids\": [\"17301052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"HRD1's proline-rich domain is necessary to promote Pael-R degradation, while its transmembrane domain is required for transferring Pael-R from the ER to the cytosol for proteasomal degradation; a transmembrane-domain mutant of HRD1 was markedly unstable.\",\n      \"method\": \"Domain deletion/mutagenesis of HRD1, co-immunoprecipitation, degradation assay\",\n      \"journal\": \"Journal of pharmacological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain mutagenesis with functional degradation assay\",\n      \"pmids\": [\"18344614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Parkin knockout/Pael-R transgenic double mice exhibited progressive and selective loss of dopaminergic and noradrenergic neurons without inclusion body formation, with persistent unfolded protein response activation, elevated dopamine/metabolites, and later reduction in mitochondrial complex I activity.\",\n      \"method\": \"Double transgenic/knockout mouse model, catecholaminergic neuron counting, UPR markers, mitochondrial complex I activity assay\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in vivo with multiple mechanistic readouts replicated across lifetime\",\n      \"pmids\": [\"18691389\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"GPR37 surface expression is dramatically enhanced by removal of its N-terminal domain (first ~210 amino acids); coexpression with adenosine A2A receptor or dopamine D2 receptor increases GPR37 surface expression, with co-immunoprecipitation confirming GPR37-D2R physical association that modestly alters D2R agonist/antagonist affinity; interaction with the PDZ scaffold syntenin-1 also dramatically increases GPR37 surface expression.\",\n      \"method\": \"N-terminal truncation, co-immunoprecipitation, FACS surface expression, radioligand binding\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches (truncation, Co-IP, PDZ interaction) with quantified surface expression\",\n      \"pmids\": [\"19799451\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"rAAV2/6-mediated overexpression of Pael-R in the rat nigrostriatal system caused rapid insoluble accumulation of Pael-R, severe and selective loss of nigral dopaminergic neurons and nigrostriatal fibers, striatal dopamine depletion, and spontaneous motor impairments; GABAergic neurons of the globus pallidus were unaffected, demonstrating selective vulnerability of nigral dopaminergic neurons to Pael-R accumulation.\",\n      \"method\": \"In vivo AAV vector injection in rats, immunohistochemistry, HPLC dopamine measurement, behavioral testing\",\n      \"journal\": \"Neurobiology of disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo targeted overexpression with selective cellular phenotype and multiple mechanistic readouts\",\n      \"pmids\": [\"19348945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Prosaptide (the active fragment of prosaposin) and full-length prosaposin activate GPR37 signaling: prosaptide promoted GPR37 endocytosis, bound to GPR37, stimulated ERK phosphorylation in a pertussis toxin-sensitive manner, stimulated 35S-GTPγS binding, and inhibited forskolin-stimulated cAMP production; prosaposin/prosaptide protected primary astrocytes against oxidative stress in a GPR37-dependent manner (attenuated by GPR37 siRNA knockdown), identifying prosaposin/prosaptide as GPR37 ligands.\",\n      \"method\": \"GPCR endocytosis assay, ligand binding, ERK phosphorylation assay, 35S-GTPγS binding, cAMP assay, pertussis toxin treatment, siRNA knockdown, oxidative stress protection assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal signaling assays, pertussis toxin sensitivity, siRNA validation, and functional protection assay in a single study\",\n      \"pmids\": [\"23690594\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The C-terminal cysteine-rich domain of GPR37 controls receptor plasma membrane expression and function: deletion of six cysteines in this region promoted plasma membrane expression and signaling, while removal of the C-terminal cysteine-rich domain protected against GPR37-mediated apoptosis and cell death.\",\n      \"method\": \"Site-directed mutagenesis, flow cytometry surface expression, signaling assay, apoptosis/cell death assay\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis with multiple functional readouts in a single study\",\n      \"pmids\": [\"23398388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"PICK1 interacts with GPR37/PAELR via its PDZ domain binding the C-terminal PDZ motif of PAELR; pulldown with GST-ct-PAELR retained recombinant and native PICK1 from rat brain; PICK1 overexpression reduced PAELR expression and attenuated PAELR-induced cell death during rotenone treatment in a proteasome-dependent manner.\",\n      \"method\": \"GST pulldown, co-immunoprecipitation, cell death assay, proteasome inhibitor treatment\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — GST pulldown confirmed with native brain tissue and functional cell death assay\",\n      \"pmids\": [\"24749734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Deletion of GPR37 attenuated pilocarpine-induced tremulous jaw movements (a model of parkinsonian tremor), and the ability of adenosine A2A receptor activation to control tremulous jaw movements was lost in GPR37-null mice, indicating that A2A receptor-mediated modulation of parkinsonian tremor requires GPR37.\",\n      \"method\": \"GPR37 knockout mice, behavioral tremulous jaw movement assay, pharmacological A2A receptor manipulation\",\n      \"journal\": \"Behavioural brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in vivo with defined behavioral phenotype\",\n      \"pmids\": [\"25862943\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"GABARAPL2 interacts with the cysteine-rich region (-CCCCCC-EEC) of GPR37's C-terminal tail; affinity chromatography showed Myc-tagged GABARAPL2 was retained by GST-ct-PAELR; transient transfection of GABARAPL2 in HEK293 cells reduced PAELR expression, suggesting autophagy-mediated regulation of GPR37 levels.\",\n      \"method\": \"Affinity chromatography (GST pulldown), in-silico modelling, transient transfection, Western blot\",\n      \"journal\": \"Neuroscience letters\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single pulldown method with computational modelling, functional data limited to expression changes\",\n      \"pmids\": [\"29496607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"GPR37 is expressed in migrating GnRH neurons and olfactory ensheathing cells (OECs); pharmacological inhibition of GPR37 signaling in nasal explants attenuated GnRH neuronal migration and OEC movement; GPR37-deficient mice showed decreased olfactory bulb nerve layer and attenuated/delayed GnRH neuron maturation and migration into the brain.\",\n      \"method\": \"PCR, immunocytochemistry, nasal explant inhibition assay, GPR37 knockout mouse analysis\",\n      \"journal\": \"Frontiers in cellular neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological inhibition and KO mouse with defined developmental/cellular phenotypes\",\n      \"pmids\": [\"31143101\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Osteocalcin (OCN), a bone-derived protein, activates GPR37 in a subpopulation of VTA GABAergic neurons to decrease potassium currents via cAMP reduction and subsequent THIK-1 (K2P13.1) channel suppression, increasing neuronal excitability and priming rapid visual escape; GPR37 KO and conditional GPR37 KO in VTA GABAergic neurons delayed escape, and reconstituting OCN-GPR37 signaling in VTA restored normal response.\",\n      \"method\": \"Knockout mice, conditional knockout, single-cell transcriptomics, electrophysiology, chemogenetics, ex vivo calcium imaging\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic manipulations with electrophysiological and behavioral readouts, but preprint only\",\n      \"pmids\": [\"bio_10.1101_2024.08.12.607510\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GPR37 mediates the analgesic and pro-resolving effects of protectin DX (PDX) in macrophages: PDX binds GPR37 and induces calcium responses in peritoneal macrophages, promotes GPR37-dependent macrophage phagocytosis and efferocytosis, and GPR37 KO mice showed deficits in fracture-induced postoperative pain resolution and abolished PDX analgesia.\",\n      \"method\": \"Gpr37 knockout mice, ligand binding assay, calcium imaging in macrophages, efferocytosis assay, behavioral pain assay, lipidomics\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ligand binding confirmed, GPR37 KO abolishes effect, cellular mechanism defined; preprint only\",\n      \"pmids\": [\"bio_10.1101_2025.11.10.687700\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GPR37 contains endogenous ligands present in brain extracts, as detected using the GzESTY cell-based Gi/o/z signaling assay, supporting ongoing deorphanization of GPR37.\",\n      \"method\": \"Cell-based GPCR signaling assay (GzESTY) with brain extract stimulation\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single assay with crude brain extract, ligand identity not determined; preprint only\",\n      \"pmids\": [\"bio_10.1101_2024.07.26.605282\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Intrathecal activation of spinal GPR37 by agonists TX14A and protectin D1 dose-dependently inhibited capsaicin-induced long-term increase in nociception and produced an 'unpriming' effect in the hyperalgesic priming model; global GPR37 knockout abolished these long-term effects; ex vivo Ca2+ imaging showed i.th. TX14A rescued dorsal horn interneurons from long-term potentiation/depression, suggesting GPR37 agonism erases spinal pain memory.\",\n      \"method\": \"Intrathecal drug administration, GPR37 knockout mice, behavioral pain assays, ex vivo Ca2+ imaging of spinal dorsal horn\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — GPR37 KO abolishes effect, ex vivo mechanistic imaging, multiple agonists; preprint only\",\n      \"pmids\": [\"bio_10.1101_2025.11.25.690482\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Protectin D1 (PD1) attenuates abdominal aortic aneurysm progression in a GPR37-dependent manner in macrophages: PD1 enhanced macrophage efferocytosis of apoptotic vascular smooth muscle cells via GPR37-dependent calcium signaling and reduced macrophage-derived TNF-α and IL-1β; GPR37-dependent effects were confirmed in isolated macrophages and murine AAA models.\",\n      \"method\": \"Murine AAA models (elastase+BAPN), GPR37 knockout mice, macrophage efferocytosis assay, calcium signaling, scRNA-seq, cytokine measurement\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — GPR37 KO confirmation, mechanistic cellular assays; preprint only\",\n      \"pmids\": [\"bio_10.1101_2025.10.20.683538\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"GPR37 (Pael-R) is an orphan GPCR expressed predominantly in neurons and glia that acts as a substrate of the E3 ubiquitin ligase Parkin; when misfolded, it accumulates in the ER and induces ER stress-mediated death of dopaminergic neurons, while properly folded GPR37 functions as a receptor for prosaposin/prosaptide and osteocalcin, coupling via Gi/o to inhibit cAMP, activate ERK, and regulate diverse processes including dopamine metabolism, cell migration, spinal pain sensitization, and macrophage efferocytosis; its surface trafficking is controlled by N-terminal sequences, C-terminal cysteine-rich domain, interaction partners (D2R, A2AR, syntenin-1, PICK1, GABARAPL2), and ubiquitin ligases Parkin and HRD1.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GPR37 (Pael-R) is a brain-enriched orphan GPCR that functions at the intersection of protein quality control and neuroprotective signaling, with roles spanning dopaminergic neuron survival, pain modulation, cell migration, and macrophage efferocytosis. Misfolded GPR37 accumulates in the ER and triggers ER stress-mediated death selectively in dopaminergic neurons; this toxicity is counteracted by Parkin-mediated ubiquitination and proteasomal degradation, and by the ER-resident E3 ligase HRD1 [PMID:12150907, PMID:12670421, PMID:17059562]. Properly folded, surface-delivered GPR37 is activated by prosaposin/prosaptide, coupling through Gi/o to inhibit cAMP, activate ERK, and protect astrocytes from oxidative stress [PMID:23690594]. GPR37 knockout mice display reduced striatal dopamine, altered vulnerability to dopaminergic neurotoxins, impaired GnRH neuron migration, and attenuated adenosine A2A receptor-mediated modulation of parkinsonian tremor [PMID:17889953, PMID:31143101, PMID:25862943].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Cloning of GPR37 established it as a novel brain-enriched seven-transmembrane orphan GPCR without identifiable endogenous ligands or classical second-messenger coupling, setting the stage for functional deorphanization.\",\n      \"evidence\": \"cDNA cloning with Northern blot, in situ hybridization, radioligand binding, and Ca²⁺/cAMP assays in COS7/HEK293 cells\",\n      \"pmids\": [\"9144577\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No ligand identified\", \"No signaling pathway activated in initial assays\", \"Physiological role unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identification of GPR37 as a Parkin substrate linked receptor misfolding and ER stress to dopaminergic cell death, providing a direct molecular connection between GPR37 accumulation and Parkinson's disease-related neurodegeneration.\",\n      \"evidence\": \"Yeast two-hybrid, co-immunoprecipitation, ubiquitination assays, cell death rescue by Parkin; CHIP–Hsp70 complex facilitation of Parkin-mediated Pael-R ubiquitination\",\n      \"pmids\": [\"12150907\", \"14579121\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether endogenous levels of GPR37 (not overexpressed) cause toxicity in neurons\", \"Mechanism by which GPR37 misfolding is sensed\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"In vivo Drosophila and mouse models demonstrated that GPR37 accumulation selectively kills dopaminergic neurons in a Parkin-dependent manner, validating the Parkin–Pael-R axis in an organismal context and establishing cell-type selectivity of toxicity.\",\n      \"evidence\": \"Drosophila transgenic/genetic epistasis with dopaminergic neuron counting; Pael-R localization to Lewy body cores in human PD brain\",\n      \"pmids\": [\"12670421\", \"14991825\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis for selective vulnerability of dopaminergic neurons\", \"Whether Lewy body accumulation is cause or consequence\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Discovery that the ER-resident E3 ligase HRD1 independently ubiquitinates and degrades GPR37 revealed a second quality-control pathway (ERAD) beyond Parkin, and mouse models confirmed that ER stress and dopamine synthesis both contribute to GPR37-induced neurodegeneration.\",\n      \"evidence\": \"Co-IP, ubiquitination assay, siRNA knockdown of HRD1 with caspase-3 activation; in vivo adenoviral overexpression in Parkin-KO and ORP150-KO mice with dopamine synthesis inhibitor rescue\",\n      \"pmids\": [\"17059562\", \"17116640\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contribution of HRD1 vs. Parkin to endogenous GPR37 turnover\", \"Whether dopamine metabolites directly modify GPR37\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Bidirectional genetic manipulation in mice established GPR37 as a physiological regulator of striatal dopamine metabolism, moving the gene's role beyond a pure toxicity substrate to a functional modulator of the nigrostriatal system.\",\n      \"evidence\": \"GPR37 knockout and transgenic mice with HPLC dopamine/DOPAC measurement and neurotoxin challenge\",\n      \"pmids\": [\"17889953\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling pathway linking GPR37 to dopamine synthesis or release\", \"Receptor activation state of endogenous GPR37 in these models\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Dissection of GPR37 trafficking determinants revealed that the N-terminal ectodomain retains the receptor intracellularly, while co-expression with D2R or A2AR or the PDZ scaffold syntenin-1 rescues surface delivery, establishing GPR37 as a trafficking-regulated receptor whose functional availability depends on interaction partners.\",\n      \"evidence\": \"N-terminal truncation, FACS surface expression, co-immunoprecipitation with D2R/A2AR, syntenin-1 interaction\",\n      \"pmids\": [\"19799451\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether N-terminal cleavage occurs endogenously\", \"Stoichiometry and in vivo relevance of GPR37–D2R heteromers\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Deorphanization of GPR37 by prosaposin/prosaptide established it as a Gi/o-coupled receptor that inhibits cAMP, activates ERK, and protects astrocytes from oxidative stress, transforming understanding from a passive toxicity substrate to an active signaling receptor.\",\n      \"evidence\": \"Endocytosis assay, ligand binding, ERK phosphorylation, GTPγS binding, cAMP assay, pertussis toxin sensitivity, siRNA knockdown, oxidative stress protection in primary astrocytes\",\n      \"pmids\": [\"23690594\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether prosaposin is the primary endogenous ligand in vivo\", \"Structural basis of ligand–receptor interaction\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The C-terminal cysteine-rich domain was identified as a negative regulator of GPR37 surface expression and a determinant of receptor-mediated apoptosis, and additional C-terminal interactors PICK1 and GABARAPL2 were shown to modulate GPR37 levels.\",\n      \"evidence\": \"Cysteine mutagenesis with flow cytometry and apoptosis assay; GST pulldown with PICK1 and GABARAPL2; cell death rescue by PICK1 overexpression\",\n      \"pmids\": [\"23398388\", \"24749734\", \"29496607\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether GABARAPL2 mediates autophagic degradation of GPR37 in neurons\", \"Functional interplay between C-terminal partners\", \"GABARAPL2 interaction confirmed by only a single pulldown method\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"GPR37 was shown to be required for A2A receptor-mediated modulation of parkinsonian tremor, establishing a functional epistatic relationship between GPR37 and adenosine signaling in motor circuits.\",\n      \"evidence\": \"GPR37 KO mice with pilocarpine-induced tremulous jaw movements and pharmacological A2A receptor manipulation\",\n      \"pmids\": [\"25862943\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether GPR37–A2AR heteromerization is necessary for the behavioral phenotype\", \"Downstream signaling mediating the interaction\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"GPR37 was found to regulate GnRH neuron and olfactory ensheathing cell migration during development, extending its physiological roles beyond dopaminergic circuits to developmental neurobiology.\",\n      \"evidence\": \"Nasal explant pharmacological inhibition and GPR37 KO mouse analysis of GnRH neuron migration and olfactory bulb innervation\",\n      \"pmids\": [\"31143101\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ligand activating GPR37 during GnRH neuron migration\", \"Downstream effectors of GPR37 in migrating cells\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the identity of the primary endogenous ligand(s) in vivo, the structural basis of GPR37 activation, whether GPR37 functions primarily as a monomer or in heteromeric complexes in native tissue, and the mechanism by which GPR37 signaling modulates dopamine metabolism.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No solved receptor structure\", \"No consensus on primary endogenous ligand in brain\", \"Mechanism linking GPR37 signaling to dopamine synthesis/release undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 14]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [8, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [12, 14, 15]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [14, 17]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [1, 3, 7]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [8, 17]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [3, 7, 11]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"PRKN\",\n      \"HRD1\",\n      \"STUB1\",\n      \"DRD2\",\n      \"ADORA2A\",\n      \"SDCBP\",\n      \"PICK1\",\n      \"GABARAPL2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}