{"gene":"GPR151","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2021,"finding":"GPR151 physically couples with P2X3 ion channels in nociceptive DRG neurons, promoting P2X3 functional activity; knockout of Gpr151 suppressed P2X3-mediated calcium elevation, while overexpression enhanced it, establishing a direct functional interaction.","method":"Co-immunoprecipitation, calcium imaging, conditional knockout and overexpression in mouse DRG neurons, chronic constriction injury model","journal":"Brain : a journal of neurology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal functional interaction confirmed by loss-of-function, gain-of-function, and calcium assays with clear phenotypic readout","pmids":["34244727"],"is_preprint":false},{"year":2021,"finding":"GPR151 in DRG neurons is required for nerve injury-induced upregulation of CSF1, which is necessary for downstream microglial activation in the spinal cord; P2X3 knockdown reversed CSF1 upregulation and microglial activation, placing GPR151 upstream of CSF1-mediated neuroinflammation.","method":"Conditional knockout, siRNA knockdown of P2X3, immunohistochemistry for microglial markers, chronic constriction injury mouse model","journal":"Brain : a journal of neurology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis (GPR151 → P2X3 → CSF1 → microglia) established by multiple loss-of-function approaches","pmids":["34244727"],"is_preprint":false},{"year":2021,"finding":"GPR151 couples to Gαi protein (but not Gαq, Gα12, or Gα13) and activates ERK through the Gβγ subunit in trigeminal ganglion neurons after nerve injury; ERK activation and downstream neuroinflammatory chemokines (CCL5, CCL7, CXCL9, CXCL10) were abolished in Gpr151-/- mice.","method":"Co-immunoprecipitation with Gα subunits, phospho-ERK immunostaining, gene microarray, MEK inhibitor (PD98059), global Gpr151 knockout mice, partial infraorbital nerve transection model","journal":"Pain","confidence":"High","confidence_rationale":"Tier 2 — G protein coupling determined by Co-IP with multiple Gα subunits; downstream pathway confirmed by KO and pharmacological inhibition with gene expression readout","pmids":["33239523"],"is_preprint":false},{"year":2019,"finding":"GPR151 is activated under acidic conditions (pH ~5.8–6.0); GPR151-Gαi fusion proteins showed increased [35S]GTPγS binding at low pH, and reporter gene assays in CHO cells expressing GPR151 confirmed activation is maximal around pH 5.8, identifying GPR151 as a proton-sensing GPCR coupled to Gi.","method":"[35S]GTPγS binding assay with GPR151-Gαi fusion proteins, reporter gene assay in CHO cells at varying pH","journal":"Journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 1–2 — in vitro GTPγS assay and cell-based reporter assay, single lab, two orthogonal methods","pmids":["31119277"],"is_preprint":false},{"year":2016,"finding":"Galanin (100 nM–10 µM) did not induce calcium signalling in ND7/23 cells transfected with GPR151, indicating galanin is not an endogenous ligand for GPR151 despite the receptor's sequence homology with galanin receptors.","method":"Calcium signalling assay in GPR151-transfected ND7/23 cells with galanin application","journal":"Molecular and cellular neurosciences","confidence":"Medium","confidence_rationale":"Tier 2 — direct functional assay ruling out a candidate ligand, single lab","pmids":["27913310"],"is_preprint":false},{"year":2014,"finding":"GPR151 protein is highly and specifically expressed in medial and lateral habenula neurons and their efferent axonal projections to the interpeduncular nucleus, rostromedial tegmental area, raphe nuclei, and dorsal tegmental nucleus; GPR151-expressing axons colocalize with cholinergic, substance P-ergic, and glutamatergic markers; this pattern is conserved across rat, mouse, and zebrafish.","method":"Immunohistochemistry, confocal microscopy, quantitative colocalization analysis across species","journal":"The Journal of comparative neurology","confidence":"Medium","confidence_rationale":"Tier 2–3 — direct protein localization in multiple species with colocalization analysis, but no functional manipulation","pmids":["25116430"],"is_preprint":false},{"year":2022,"finding":"Gpr151-/- mice show reduced social preference in the social preference test compared to wild-type controls, indicating that GPR151 in the habenula normally promotes social reward behavior; this parallels the role of habenular mu opioid receptors in social reward.","method":"Social preference test in Gpr151-/- knockout mice versus wild-type controls","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — clean KO with defined behavioral phenotype, single lab, single method","pmids":["36424418"],"is_preprint":false},{"year":2022,"finding":"Three human loss-of-function variants of GPR151 (Arg95Ter, Tyr99Ter, Phe175LeufsTer7) were confirmed as loss-of-function in vitro, and Gpr151-/- mice showed no difference in body weight on normal chow but higher body weight on a high-fat diet compared to wild-type, indicating GPR151 deficiency does not reduce BMI under normal conditions.","method":"In vitro functional confirmation of human LOF variants, mouse Gpr151-/- body weight measurement on normal and high-fat diet","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro LOF confirmation plus mouse KO metabolic phenotyping, single study","pmids":["35381001"],"is_preprint":false}],"current_model":"GPR151 is an orphan Gαi-coupled GPCR that senses extracellular acidification, is highly expressed in habenular neurons and nociceptive DRG neurons, physically associates with P2X3 ion channels to potentiate their activity, signals through Gβγ to activate ERK and downstream neuroinflammatory chemokines, drives CSF1-mediated spinal microglial activation after nerve injury, and promotes social reward behavior via habenular circuitry."},"narrative":{"teleology":[{"year":2014,"claim":"Establishing where GPR151 protein is expressed resolved that this orphan receptor is a habenula-specific marker whose axonal projections reach defined midbrain nuclei, framing it as a potential regulator of habenular circuit function.","evidence":"Immunohistochemistry and confocal colocalization in rat, mouse, and zebrafish brain sections","pmids":["25116430"],"confidence":"Medium","gaps":["No functional consequence of habenular GPR151 expression was demonstrated","Expression in peripheral sensory neurons was not examined in this study"]},{"year":2016,"claim":"Despite sequence homology with galanin receptors, galanin was excluded as a GPR151 ligand, clarifying that the receptor remains an orphan GPCR with an unknown endogenous agonist.","evidence":"Calcium signaling assay in GPR151-transfected ND7/23 cells with galanin","pmids":["27913310"],"confidence":"Medium","gaps":["Only one candidate ligand was tested; no systematic ligand screen was performed","Assay relied on calcium readout, which may miss Gαi-coupled responses"]},{"year":2019,"claim":"Identification of extracellular protons as GPR151 activators solved the receptor's activation stimulus, establishing it as a pH-sensing GPCR coupled to Gi signaling with maximal activity around pH 5.8.","evidence":"[35S]GTPγS binding assay with GPR151-Gαi fusion proteins and CHO cell reporter gene assay at varying pH","pmids":["31119277"],"confidence":"Medium","gaps":["Proton activation demonstrated only in heterologous systems; not confirmed in native neurons","Whether protons are the sole or primary endogenous stimulus is unresolved"]},{"year":2021,"claim":"Defining the GPR151 signaling cascade in sensory neurons — Gαi coupling, Gβγ-ERK activation, and downstream neuroinflammatory chemokine induction — established the intracellular pathway by which this receptor drives neuroinflammation after nerve injury.","evidence":"Co-IP with Gα subunits, phospho-ERK staining, MEK inhibitor blockade, gene microarray in Gpr151-/- mice after partial infraorbital nerve transection","pmids":["33239523"],"confidence":"High","gaps":["Whether the Gβγ-ERK axis operates identically in DRG versus trigeminal neurons is untested","Direct identification of the kinase linking Gβγ to ERK was not performed"]},{"year":2021,"claim":"Demonstrating that GPR151 physically couples to P2X3 channels and is required for CSF1-dependent microglial activation placed GPR151 at the apex of a nociceptive signaling cascade (GPR151→P2X3→CSF1→microglia), explaining its role in neuropathic pain.","evidence":"Co-IP, calcium imaging, conditional KO and overexpression in DRG neurons, P2X3 siRNA, microglial marker immunohistochemistry in chronic constriction injury model","pmids":["34244727"],"confidence":"High","gaps":["Structural basis of the GPR151–P2X3 physical interaction is unknown","Whether GPR151 modulates other ion channels beyond P2X3 has not been tested"]},{"year":2022,"claim":"GPR151 knockout reduced social preference behavior, extending the receptor's functional roles beyond pain into habenula-dependent social reward processing.","evidence":"Social preference test in Gpr151-/- versus wild-type mice","pmids":["36424418"],"confidence":"Medium","gaps":["Circuit-level mechanism (which habenular projection mediates the effect) is unresolved","Rescue experiments restoring GPR151 selectively in habenula were not performed"]},{"year":2022,"claim":"Functional validation of human GPR151 loss-of-function variants and metabolic phenotyping of Gpr151-/- mice showed that GPR151 deficiency increases body weight on high-fat diet, linking the receptor to metabolic regulation under dietary stress.","evidence":"In vitro functional assays for human LOF variants; body weight measurements on normal and high-fat diet in Gpr151-/- mice","pmids":["35381001"],"confidence":"Medium","gaps":["Mechanism connecting GPR151 loss to high-fat diet weight gain is unknown","Whether the metabolic phenotype is habenula-dependent or peripheral is untested"]},{"year":null,"claim":"The endogenous lipid or peptide ligand (if any, beyond protons) of GPR151, the structural basis of its P2X3 interaction, and the neural circuit mechanisms underlying its roles in social behavior and metabolism remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No endogenous non-proton ligand has been identified","No structural or cryo-EM model of GPR151 or the GPR151–P2X3 complex exists","Cell-type-specific rescue or chemogenetic studies in habenula are lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[3]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,3,5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,3]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[1,2]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,5,6]}],"complexes":[],"partners":["P2RX3","GNAI1","CSF1"],"other_free_text":[]},"mechanistic_narrative":"GPR151 is an orphan Gαi-coupled GPCR enriched in habenular neurons and nociceptive dorsal root ganglion (DRG) neurons that functions as a proton sensor and modulator of pain-related neuroinflammatory signaling. It is activated by extracellular acidification (optimal ~pH 5.8), couples exclusively to Gαi, and signals through Gβγ to activate ERK and downstream chemokines (CCL5, CCL7, CXCL9, CXCL10), while physically associating with P2X3 ion channels to potentiate their calcium-conductance activity and drive CSF1-dependent spinal microglial activation after nerve injury [PMID:31119277, PMID:33239523, PMID:34244727]. In the habenula, GPR151 is expressed in cholinergic and glutamatergic projection neurons innervating midbrain targets, and its deletion impairs social reward behavior [PMID:25116430, PMID:36424418]."},"prefetch_data":{"uniprot":{"accession":"Q8TDV0","full_name":"G-protein coupled receptor 151","aliases":["G-protein coupled receptor PGR7","GPCR-2037","Galanin receptor 4","Galanin-receptor-like protein","GalRL"],"length_aa":419,"mass_kda":46.6,"function":"Proton-sensing G-protein coupled receptor","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q8TDV0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GPR151","classification":"Not Classified","n_dependent_lines":12,"n_total_lines":1208,"dependency_fraction":0.009933774834437087},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GPR151","total_profiled":1310},"omim":[{"mim_id":"618487","title":"G PROTEIN-COUPLED RECEPTOR 151; GPR151","url":"https://www.omim.org/entry/618487"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Not detected","tissue_distribution":"Not detected","driving_tissues":[],"url":"https://www.proteinatlas.org/search/GPR151"},"hgnc":{"alias_symbol":["PGR7","GALR4"],"prev_symbol":[]},"alphafold":{"accession":"Q8TDV0","domains":[{"cath_id":"1.20.1070.10","chopping":"17-320","consensus_level":"high","plddt":90.1876,"start":17,"end":320}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8TDV0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8TDV0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8TDV0-F1-predicted_aligned_error_v6.png","plddt_mean":78.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPR151","jax_strain_url":"https://www.jax.org/strain/search?query=GPR151"},"sequence":{"accession":"Q8TDV0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8TDV0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8TDV0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8TDV0"}},"corpus_meta":[{"pmid":"34244727","id":"PMC_34244727","title":"GPR151 in nociceptors modulates neuropathic pain via regulating P2X3 function and microglial activation.","date":"2021","source":"Brain : a journal of neurology","url":"https://pubmed.ncbi.nlm.nih.gov/34244727","citation_count":71,"is_preprint":false},{"pmid":"25116430","id":"PMC_25116430","title":"Conserved expression of the GPR151 receptor in habenular axonal projections of vertebrates.","date":"2014","source":"The Journal of comparative neurology","url":"https://pubmed.ncbi.nlm.nih.gov/25116430","citation_count":42,"is_preprint":false},{"pmid":"33239523","id":"PMC_33239523","title":"G protein-coupled receptor GPR151 is involved in trigeminal neuropathic pain through the induction of Gβγ/extracellular signal-regulated kinase-mediated neuroinflammation in the trigeminal ganglion.","date":"2021","source":"Pain","url":"https://pubmed.ncbi.nlm.nih.gov/33239523","citation_count":30,"is_preprint":false},{"pmid":"31119277","id":"PMC_31119277","title":"GPR31 and GPR151 are activated under acidic conditions.","date":"2019","source":"Journal of biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/31119277","citation_count":25,"is_preprint":false},{"pmid":"28657115","id":"PMC_28657115","title":"Monosynaptic retrograde tracing of neurons expressing the G-protein coupled receptor Gpr151 in the mouse brain.","date":"2017","source":"The Journal of comparative neurology","url":"https://pubmed.ncbi.nlm.nih.gov/28657115","citation_count":22,"is_preprint":false},{"pmid":"20657737","id":"PMC_20657737","title":"Arabidopsis thaliana PGR7 encodes a conserved chloroplast protein that is necessary for efficient photosynthetic electron transport.","date":"2010","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/20657737","citation_count":21,"is_preprint":false},{"pmid":"27913310","id":"PMC_27913310","title":"Targeted disruption of the orphan receptor Gpr151 does not alter pain-related behaviour despite a strong induction in dorsal root ganglion expression in a model of neuropathic pain.","date":"2016","source":"Molecular and cellular neurosciences","url":"https://pubmed.ncbi.nlm.nih.gov/27913310","citation_count":20,"is_preprint":false},{"pmid":"36424418","id":"PMC_36424418","title":"The mu opioid receptor and the orphan receptor GPR151 contribute to social reward in the habenula.","date":"2022","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/36424418","citation_count":13,"is_preprint":false},{"pmid":"31673852","id":"PMC_31673852","title":"Production of 5-aminolevulinic acid from glutamate by overexpressing HemA1 and pgr7 from Arabidopsis thaliana in Escherichia coli.","date":"2019","source":"World journal of microbiology & biotechnology","url":"https://pubmed.ncbi.nlm.nih.gov/31673852","citation_count":9,"is_preprint":false},{"pmid":"35011589","id":"PMC_35011589","title":"Expression of Proton-Sensitive GPR31, GPR151, TASK1 and TASK3 in Common Skin Tumors.","date":"2021","source":"Cells","url":"https://pubmed.ncbi.nlm.nih.gov/35011589","citation_count":7,"is_preprint":false},{"pmid":"37296398","id":"PMC_37296398","title":"Deep learning-based classification model for GPR151 activator activity prediction.","date":"2023","source":"BMC bioinformatics","url":"https://pubmed.ncbi.nlm.nih.gov/37296398","citation_count":4,"is_preprint":false},{"pmid":"22842078","id":"PMC_22842078","title":"Characterization of the theta replication plasmid pGR7 from Acetobacter aceti CCM 3610.","date":"2012","source":"Research in microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/22842078","citation_count":3,"is_preprint":false},{"pmid":"35381001","id":"PMC_35381001","title":"Analyzing human knockouts to validate GPR151 as a therapeutic target for reduction of body mass index.","date":"2022","source":"PLoS genetics","url":"https://pubmed.ncbi.nlm.nih.gov/35381001","citation_count":3,"is_preprint":false},{"pmid":"40295925","id":"PMC_40295925","title":"The Orphan Receptor GPR151: Discovery, Expression, and Emerging Biological Significance.","date":"2025","source":"ACS chemical neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/40295925","citation_count":1,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8021,"output_tokens":1914,"usd":0.026387},"stage2":{"model":"claude-opus-4-6","input_tokens":5261,"output_tokens":2008,"usd":0.114757},"total_usd":0.141144,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2021,\n      \"finding\": \"GPR151 physically couples with P2X3 ion channels in nociceptive DRG neurons, promoting P2X3 functional activity; knockout of Gpr151 suppressed P2X3-mediated calcium elevation, while overexpression enhanced it, establishing a direct functional interaction.\",\n      \"method\": \"Co-immunoprecipitation, calcium imaging, conditional knockout and overexpression in mouse DRG neurons, chronic constriction injury model\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal functional interaction confirmed by loss-of-function, gain-of-function, and calcium assays with clear phenotypic readout\",\n      \"pmids\": [\"34244727\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GPR151 in DRG neurons is required for nerve injury-induced upregulation of CSF1, which is necessary for downstream microglial activation in the spinal cord; P2X3 knockdown reversed CSF1 upregulation and microglial activation, placing GPR151 upstream of CSF1-mediated neuroinflammation.\",\n      \"method\": \"Conditional knockout, siRNA knockdown of P2X3, immunohistochemistry for microglial markers, chronic constriction injury mouse model\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis (GPR151 → P2X3 → CSF1 → microglia) established by multiple loss-of-function approaches\",\n      \"pmids\": [\"34244727\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GPR151 couples to Gαi protein (but not Gαq, Gα12, or Gα13) and activates ERK through the Gβγ subunit in trigeminal ganglion neurons after nerve injury; ERK activation and downstream neuroinflammatory chemokines (CCL5, CCL7, CXCL9, CXCL10) were abolished in Gpr151-/- mice.\",\n      \"method\": \"Co-immunoprecipitation with Gα subunits, phospho-ERK immunostaining, gene microarray, MEK inhibitor (PD98059), global Gpr151 knockout mice, partial infraorbital nerve transection model\",\n      \"journal\": \"Pain\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — G protein coupling determined by Co-IP with multiple Gα subunits; downstream pathway confirmed by KO and pharmacological inhibition with gene expression readout\",\n      \"pmids\": [\"33239523\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"GPR151 is activated under acidic conditions (pH ~5.8–6.0); GPR151-Gαi fusion proteins showed increased [35S]GTPγS binding at low pH, and reporter gene assays in CHO cells expressing GPR151 confirmed activation is maximal around pH 5.8, identifying GPR151 as a proton-sensing GPCR coupled to Gi.\",\n      \"method\": \"[35S]GTPγS binding assay with GPR151-Gαi fusion proteins, reporter gene assay in CHO cells at varying pH\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro GTPγS assay and cell-based reporter assay, single lab, two orthogonal methods\",\n      \"pmids\": [\"31119277\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Galanin (100 nM–10 µM) did not induce calcium signalling in ND7/23 cells transfected with GPR151, indicating galanin is not an endogenous ligand for GPR151 despite the receptor's sequence homology with galanin receptors.\",\n      \"method\": \"Calcium signalling assay in GPR151-transfected ND7/23 cells with galanin application\",\n      \"journal\": \"Molecular and cellular neurosciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional assay ruling out a candidate ligand, single lab\",\n      \"pmids\": [\"27913310\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"GPR151 protein is highly and specifically expressed in medial and lateral habenula neurons and their efferent axonal projections to the interpeduncular nucleus, rostromedial tegmental area, raphe nuclei, and dorsal tegmental nucleus; GPR151-expressing axons colocalize with cholinergic, substance P-ergic, and glutamatergic markers; this pattern is conserved across rat, mouse, and zebrafish.\",\n      \"method\": \"Immunohistochemistry, confocal microscopy, quantitative colocalization analysis across species\",\n      \"journal\": \"The Journal of comparative neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — direct protein localization in multiple species with colocalization analysis, but no functional manipulation\",\n      \"pmids\": [\"25116430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Gpr151-/- mice show reduced social preference in the social preference test compared to wild-type controls, indicating that GPR151 in the habenula normally promotes social reward behavior; this parallels the role of habenular mu opioid receptors in social reward.\",\n      \"method\": \"Social preference test in Gpr151-/- knockout mice versus wild-type controls\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined behavioral phenotype, single lab, single method\",\n      \"pmids\": [\"36424418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Three human loss-of-function variants of GPR151 (Arg95Ter, Tyr99Ter, Phe175LeufsTer7) were confirmed as loss-of-function in vitro, and Gpr151-/- mice showed no difference in body weight on normal chow but higher body weight on a high-fat diet compared to wild-type, indicating GPR151 deficiency does not reduce BMI under normal conditions.\",\n      \"method\": \"In vitro functional confirmation of human LOF variants, mouse Gpr151-/- body weight measurement on normal and high-fat diet\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro LOF confirmation plus mouse KO metabolic phenotyping, single study\",\n      \"pmids\": [\"35381001\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GPR151 is an orphan Gαi-coupled GPCR that senses extracellular acidification, is highly expressed in habenular neurons and nociceptive DRG neurons, physically associates with P2X3 ion channels to potentiate their activity, signals through Gβγ to activate ERK and downstream neuroinflammatory chemokines, drives CSF1-mediated spinal microglial activation after nerve injury, and promotes social reward behavior via habenular circuitry.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GPR151 is an orphan Gαi-coupled GPCR enriched in habenular neurons and nociceptive dorsal root ganglion (DRG) neurons that functions as a proton sensor and modulator of pain-related neuroinflammatory signaling. It is activated by extracellular acidification (optimal ~pH 5.8), couples exclusively to Gαi, and signals through Gβγ to activate ERK and downstream chemokines (CCL5, CCL7, CXCL9, CXCL10), while physically associating with P2X3 ion channels to potentiate their calcium-conductance activity and drive CSF1-dependent spinal microglial activation after nerve injury [PMID:31119277, PMID:33239523, PMID:34244727]. In the habenula, GPR151 is expressed in cholinergic and glutamatergic projection neurons innervating midbrain targets, and its deletion impairs social reward behavior [PMID:25116430, PMID:36424418].\",\n  \"teleology\": [\n    {\n      \"year\": 2014,\n      \"claim\": \"Establishing where GPR151 protein is expressed resolved that this orphan receptor is a habenula-specific marker whose axonal projections reach defined midbrain nuclei, framing it as a potential regulator of habenular circuit function.\",\n      \"evidence\": \"Immunohistochemistry and confocal colocalization in rat, mouse, and zebrafish brain sections\",\n      \"pmids\": [\"25116430\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No functional consequence of habenular GPR151 expression was demonstrated\",\n        \"Expression in peripheral sensory neurons was not examined in this study\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Despite sequence homology with galanin receptors, galanin was excluded as a GPR151 ligand, clarifying that the receptor remains an orphan GPCR with an unknown endogenous agonist.\",\n      \"evidence\": \"Calcium signaling assay in GPR151-transfected ND7/23 cells with galanin\",\n      \"pmids\": [\"27913310\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Only one candidate ligand was tested; no systematic ligand screen was performed\",\n        \"Assay relied on calcium readout, which may miss Gαi-coupled responses\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identification of extracellular protons as GPR151 activators solved the receptor's activation stimulus, establishing it as a pH-sensing GPCR coupled to Gi signaling with maximal activity around pH 5.8.\",\n      \"evidence\": \"[35S]GTPγS binding assay with GPR151-Gαi fusion proteins and CHO cell reporter gene assay at varying pH\",\n      \"pmids\": [\"31119277\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Proton activation demonstrated only in heterologous systems; not confirmed in native neurons\",\n        \"Whether protons are the sole or primary endogenous stimulus is unresolved\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defining the GPR151 signaling cascade in sensory neurons — Gαi coupling, Gβγ-ERK activation, and downstream neuroinflammatory chemokine induction — established the intracellular pathway by which this receptor drives neuroinflammation after nerve injury.\",\n      \"evidence\": \"Co-IP with Gα subunits, phospho-ERK staining, MEK inhibitor blockade, gene microarray in Gpr151-/- mice after partial infraorbital nerve transection\",\n      \"pmids\": [\"33239523\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether the Gβγ-ERK axis operates identically in DRG versus trigeminal neurons is untested\",\n        \"Direct identification of the kinase linking Gβγ to ERK was not performed\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrating that GPR151 physically couples to P2X3 channels and is required for CSF1-dependent microglial activation placed GPR151 at the apex of a nociceptive signaling cascade (GPR151→P2X3→CSF1→microglia), explaining its role in neuropathic pain.\",\n      \"evidence\": \"Co-IP, calcium imaging, conditional KO and overexpression in DRG neurons, P2X3 siRNA, microglial marker immunohistochemistry in chronic constriction injury model\",\n      \"pmids\": [\"34244727\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of the GPR151–P2X3 physical interaction is unknown\",\n        \"Whether GPR151 modulates other ion channels beyond P2X3 has not been tested\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"GPR151 knockout reduced social preference behavior, extending the receptor's functional roles beyond pain into habenula-dependent social reward processing.\",\n      \"evidence\": \"Social preference test in Gpr151-/- versus wild-type mice\",\n      \"pmids\": [\"36424418\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Circuit-level mechanism (which habenular projection mediates the effect) is unresolved\",\n        \"Rescue experiments restoring GPR151 selectively in habenula were not performed\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Functional validation of human GPR151 loss-of-function variants and metabolic phenotyping of Gpr151-/- mice showed that GPR151 deficiency increases body weight on high-fat diet, linking the receptor to metabolic regulation under dietary stress.\",\n      \"evidence\": \"In vitro functional assays for human LOF variants; body weight measurements on normal and high-fat diet in Gpr151-/- mice\",\n      \"pmids\": [\"35381001\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism connecting GPR151 loss to high-fat diet weight gain is unknown\",\n        \"Whether the metabolic phenotype is habenula-dependent or peripheral is untested\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The endogenous lipid or peptide ligand (if any, beyond protons) of GPR151, the structural basis of its P2X3 interaction, and the neural circuit mechanisms underlying its roles in social behavior and metabolism remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No endogenous non-proton ligand has been identified\",\n        \"No structural or cryo-EM model of GPR151 or the GPR151–P2X3 complex exists\",\n        \"Cell-type-specific rescue or chemogenetic studies in habenula are lacking\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 3, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 5, 6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"P2RX3\",\n      \"GNAI1\",\n      \"CSF1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}