{"gene":"GPR25","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":1997,"finding":"GPR25 encodes a 360-amino acid intronless GPCR localized to chromosome 1q32.1, sharing highest sequence identity with GPR15, angiotensin II type 1A receptor, and somatostatin receptor 5; no transcripts were detected in liver or 12 brain regions by Northern analysis.","method":"PCR cloning, genomic library screening, Northern analysis, fluorescence in situ hybridization (FISH)","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct experimental cloning and chromosomal mapping with multiple orthogonal methods (PCR, library screen, FISH, Northern), single lab","pmids":["9020062"],"is_preprint":false},{"year":2015,"finding":"GPR25 constitutively inhibits cAMP-mediated gene expression (both baseline CRE-reporter expression and forskolin-stimulated expression) in transfected CHO cells, indicating constitutive Gi-coupled signaling.","method":"Transient transfection of CHO cells with CRE-luciferase reporter and orphan receptor plasmids; luciferase reporter assay","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based functional assay with reporter gene readout, single lab, tested across many receptors providing internal reference","pmids":["26384023"],"is_preprint":false},{"year":2018,"finding":"In non-mammalian vertebrates (zebrafish, spotted gar, pigeon), GPR25 is activated by Apelin and Apela peptides, leading to inhibition of forskolin-stimulated cAMP production and receptor internalization; human GPR25 was NOT activated by Apelin or Apela under the same conditions.","method":"pGL3-CRE-luciferase reporter assay, cAMP accumulation assay, confocal microscopy of receptor internalization in transfected HEK293 cells","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal functional assays (cAMP, reporter, internalization imaging) in single lab; negative result for human GPR25 explicitly established","pmids":["29727602"],"is_preprint":false},{"year":2024,"finding":"GPR25 is identified as a lymphocyte receptor for the chemoattractant cytokine CXCL17; GPR25 is induced on innate lymphocytes before peripheral emigration and imprinted on activated B and T cells in secondary lymphoid tissues; GPR25 mediates lymphocyte homing to barrier epithelia of airways, oral cavity, stomach, and biliary and genitourinary tracts in mouse models.","method":"Single-cell transcriptomics, omics dataset integration, adoptive transfer/homing assays in mouse models, flow cytometry of human tissues","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple independent omics and functional in vivo homing assays, published in high-impact peer-reviewed journal, independently corroborated by separate labs","pmids":["39293486"],"is_preprint":false},{"year":2025,"finding":"Human CXCL17 activates human GPR25 with EC50 ~100 nM via β-arrestin recruitment; activation is specific (17 other GPCRs not activated); deletion of three conserved C-terminal residues of CXCL17 abolishes activation; alanine substitution of GPR25 residues W95 or R178 (in the predicted orthosteric binding pocket) abolishes response to CXCL17; CXCL17-GPR25 interaction induces TGF-α shedding and chemotactic migration of transfected HEK293T cells.","method":"NanoBiT-based β-arrestin recruitment assay, site-directed mutagenesis of GPR25 (W95A, R178A), C-terminal deletion of CXCL17, TGF-α shedding assay, chemotaxis assay in transfected HEK293T cells, AlphaFold 3 structural modeling","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — mutagenesis of both ligand and receptor, multiple orthogonal functional assays (β-arrestin, TGF-α shedding, chemotaxis), independently replicated by another lab (PMID:39293486)","pmids":["40279398"],"is_preprint":false},{"year":2025,"finding":"GPR25 expression is induced by TGF-β signaling on CD8 T cells; Gpr25-deficient T cells infiltrate tissues normally after viral infection but fail to efficiently develop into tissue-resident memory (TRM) cells in lung and liver; GPR25 deficiency impairs secondary TRM cell differentiation, TCF1 expression, and TGF-β signaling responses; GPR25 promotes TRM cell expansion and tumor control.","method":"Adoptive transfer of Gpr25-deficient vs wild-type T cells, viral infection and tumor challenge mouse models, single-cell transcriptomics, TGF-β signaling gene signature analysis","journal":"Science immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic loss-of-function (Gpr25 KO) with specific cellular phenotype across multiple models (viral infection, tumor), single-cell transcriptomics for pathway placement, peer-reviewed high-impact journal","pmids":["41270189"],"is_preprint":false},{"year":2025,"finding":"Human CXCL17 binds and activates zebrafish and coelacanth GPR25 orthologs; β-arrestin recruitment and chemotaxis are dependent on the three C-terminal residues of CXCL17; a NanoBiT binding assay confirmed direct binding of CXCL17 C-terminal fragment to fish GPR25 orthologs.","method":"NanoBiT-based β-arrestin recruitment assay, NanoBiT-based direct binding assay, chemotaxis assay in transfected HEK293T cells, C-terminal deletion mutagenesis of CXCL17","journal":"Biochimie","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal assays (binding, β-arrestin, chemotaxis) with mutagenesis, single lab, fish orthologs only","pmids":["40972788"],"is_preprint":false},{"year":2025,"finding":"A two-step model of GPR25 activation by CXCL17 is proposed and validated: the GPR25 N-terminus orients CXCL17 for receptor activation via its C-terminus; the chimeric FPR1:GPR25 receptor (replacing GPR25 N-terminus with FPR1 N-terminus) responds to CXCL17 with significantly reduced potency; mutagenesis of GPR25 residues W95, R178, and R264 causes complete loss of chemotactic responsiveness to CXCL17; N-terminally truncated CXCL17 (64–119) retains chemotactic activity at GPR25 but with severely reduced potency.","method":"Site-directed mutagenesis of GPR25 (W95A, R178A, R264A), chimeric FPR1:GPR25 receptor construction, chemotaxis assay in transfected murine pre-B L1.2 cells, N-terminal truncation of CXCL17","journal":"Basic & clinical pharmacology & toxicology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — rigorous mutagenesis of multiple residues plus chimeric receptor approach with functional chemotaxis readout, mechanistic model validated by multiple orthogonal perturbations in single study","pmids":["42207165"],"is_preprint":false},{"year":2026,"finding":"Zebrafish CXCL17 paralogs (Dr-CXCL17 and Dr-CXCL17-like) directly bind and activate zebrafish GPR25 and induce chemotactic migration; deletion of three C-terminal residues of either paralog abolishes binding, activation, and chemotactic effects, confirming conserved C-terminal-dependent activation mechanism.","method":"Bacterial recombinant protein expression and in vitro refolding, NanoBiT-based β-arrestin recruitment assay, NanoBiT-based ligand-receptor binding assay, chemotaxis assay in transfected HEK293T cells, C-terminal deletion mutagenesis","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal assays (binding, β-arrestin, chemotaxis) with deletion mutagenesis, single lab, fish orthologs","pmids":["41521655"],"is_preprint":false}],"current_model":"GPR25 is an orphan class-A GPCR that is deorphanized as a receptor for the chemoattractant cytokine CXCL17: CXCL17 binds the orthosteric pocket of GPR25 (requiring conserved residues W95, R178, and R264 on the receptor and the conserved C-terminal fragment of CXCL17) via a two-step mechanism in which the GPR25 N-terminus first orients CXCL17, which then activates the receptor through its C-terminus, leading to Gi-mediated inhibition of cAMP, β-arrestin recruitment, and directional chemotaxis; in vivo, GPR25 mediates lymphocyte homing to non-intestinal mucosal tissues and lungs, and promotes TRM CD8 T cell development by enhancing TGF-β signaling."},"narrative":{"mechanistic_narrative":"GPR25 is a class-A G protein-coupled receptor that functions as a chemoattractant receptor governing lymphocyte trafficking to mucosal barrier tissues [PMID:39293486, PMID:40279398]. Long an orphan receptor exhibiting constitutive Gi-coupled inhibition of cAMP-driven gene expression [PMID:26384023], GPR25 was deorphanized as the receptor for the cytokine CXCL17, which it binds and is activated by with EC50 ~100 nM, triggering β-arrestin recruitment, TGF-α shedding, and directional chemotaxis [PMID:39293486, PMID:40279398]. Ligand engagement follows a two-step mechanism in which the GPR25 N-terminus first orients CXCL17 and the conserved C-terminus of CXCL17 then activates the receptor through an orthosteric pocket built from residues W95, R178, and R264, all of which are required for the chemotactic response [PMID:40279398, PMID:42207165]. In vivo, GPR25 is induced on innate lymphocytes and imprinted on activated B and T cells to direct homing to airway, oral, gastric, biliary, and genitourinary epithelia [PMID:39293486], and its expression is induced by TGF-β on CD8 T cells where it promotes tissue-resident memory cell development, TCF1 expression, and tumor control [PMID:41270189]. The receptor's ligand recognition is evolutionarily conserved, with CXCL17 orthologs activating fish and coelacanth GPR25 through the same C-terminal-dependent mechanism, though human GPR25 is not activated by the Apelin/Apela peptides that activate non-mammalian orthologs [PMID:29727602, PMID:40972788, PMID:41521655].","teleology":[{"year":1997,"claim":"Establishing that GPR25 is a distinct intronless GPCR gene with a defined chromosomal location and a restricted expression pattern provided the molecular entry point for a then-orphan receptor.","evidence":"PCR cloning, genomic library screening, FISH mapping, and Northern analysis","pmids":["9020062"],"confidence":"Medium","gaps":["No ligand, signaling pathway, or function identified","Tissue expression beyond liver/brain negative result undefined"]},{"year":2015,"claim":"Demonstrating constitutive inhibition of cAMP-driven gene expression placed GPR25 within Gi-coupled signaling even before its ligand was known.","evidence":"CRE-luciferase reporter assay in transfected CHO cells","pmids":["26384023"],"confidence":"Medium","gaps":["Activity was constitutive/ligand-independent","No endogenous agonist identified","G protein coupling inferred from reporter, not direct measurement"]},{"year":2018,"claim":"Cross-species ligand testing showed Apelin/Apela activate non-mammalian GPR25 orthologs but not human GPR25, indicating divergent ligand recognition and ruling these peptides out as the human agonist.","evidence":"cAMP accumulation, CRE-luciferase reporter, and internalization imaging in transfected HEK293 cells across zebrafish, gar, pigeon, and human orthologs","pmids":["29727602"],"confidence":"Medium","gaps":["Human GPR25 agonist still unidentified","Mechanistic basis for the species difference unresolved"]},{"year":2024,"claim":"Identifying GPR25 as a CXCL17 receptor that directs lymphocyte homing deorphanized the receptor in vivo and assigned it a physiological role in mucosal immune surveillance.","evidence":"Single-cell transcriptomics, omics integration, and adoptive transfer/homing assays in mouse models with human tissue flow cytometry","pmids":["39293486"],"confidence":"High","gaps":["Molecular details of CXCL17 binding not resolved here","Downstream signaling consequences in homing lymphocytes not dissected"]},{"year":2025,"claim":"Defining the CXCL17-GPR25 pharmacology with mutagenesis of both ligand and receptor pinpointed the orthosteric determinants of activation and confirmed signaling outputs.","evidence":"NanoBiT β-arrestin recruitment, W95A/R178A receptor mutagenesis, CXCL17 C-terminal deletion, TGF-α shedding and chemotaxis assays, AlphaFold 3 modeling","pmids":["40279398"],"confidence":"High","gaps":["No experimental structure of the complex","Quantitative G protein coupling not directly measured"]},{"year":2025,"claim":"Genetic loss-of-function established that GPR25 is dispensable for tissue infiltration but required for tissue-resident memory CD8 T cell differentiation, linking the receptor to TGF-β signaling and antitumor immunity.","evidence":"Adoptive transfer of Gpr25-deficient T cells in viral infection and tumor models with single-cell transcriptomics and TGF-β signature analysis","pmids":["41270189"],"confidence":"High","gaps":["Mechanistic link between GPR25 signaling and TGF-β pathway enhancement undefined","Whether CXCL17 is the relevant ligand in this context untested"]},{"year":2025,"claim":"A validated two-step activation model resolved how CXCL17 engages GPR25, separating the N-terminal orientation step from C-terminal-driven receptor activation through specific pocket residues.","evidence":"W95A/R178A/R264A mutagenesis, FPR1:GPR25 N-terminal chimera, CXCL17 N-terminal truncation, and chemotaxis in L1.2 and HEK293T cells; conserved binding confirmed in fish and coelacanth orthologs","pmids":["42207165","40972788"],"confidence":"High","gaps":["Atomic-resolution structure still absent","Affinity contributions of N-terminus vs orthosteric pocket not quantified"]},{"year":2026,"claim":"Confirming that endogenous zebrafish CXCL17 paralogs activate GPR25 via the same C-terminal-dependent mechanism established deep evolutionary conservation of the ligand-receptor pair.","evidence":"Recombinant CXCL17 paralog refolding, NanoBiT binding and β-arrestin assays, chemotaxis, and C-terminal deletion mutagenesis","pmids":["41521655"],"confidence":"Medium","gaps":["Restricted to fish orthologs","Physiological role of zebrafish CXCL17-GPR25 axis in vivo untested"]},{"year":null,"claim":"How GPR25 signaling mechanistically enhances TGF-β responses to drive tissue-resident memory differentiation, and the atomic structure of the CXCL17-GPR25 complex, remain unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No experimental structure of the ligand-bound receptor","Signaling crosstalk between GPR25 and TGF-β pathway unmapped","G protein subtype selectivity in immune cells undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[3,4,7]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[4,6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,4]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,4]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[3,5]}],"complexes":[],"partners":["CXCL17"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O00155","full_name":"C-X-C chemokine receptor GPR25","aliases":["G-protein coupled receptor 25"],"length_aa":361,"mass_kda":38.8,"function":"Receptor for the C-X-C chemokine CXCL17, which plays a key role in lymphocyte homing (PubMed:39293486, PubMed:40279398). Ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of downstream effectors, such as RhoA pathway (PubMed:39293486). Activation by CXCL17 on lymphocytes activates the RhoA pathway, which regulates cytoskeletal dynamics and integrins, mediating lymphocyte recruitment into the respiratory, upper gastrointestinal, biliary and genito-urinary tracts (PubMed:39293486)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/O00155/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GPR25","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GPR25","total_profiled":1310},"omim":[{"mim_id":"602174","title":"G PROTEIN-COUPLED RECEPTOR 25; GPR25","url":"https://www.omim.org/entry/602174"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in single","driving_tissues":[{"tissue":"stomach 1","ntpm":1.8}],"url":"https://www.proteinatlas.org/search/GPR25"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"O00155","domains":[{"cath_id":"1.20.1070.10","chopping":"39-182_190-332","consensus_level":"high","plddt":91.0962,"start":39,"end":332}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O00155","model_url":"https://alphafold.ebi.ac.uk/files/AF-O00155-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O00155-F1-predicted_aligned_error_v6.png","plddt_mean":82.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPR25","jax_strain_url":"https://www.jax.org/strain/search?query=GPR25"},"sequence":{"accession":"O00155","fasta_url":"https://rest.uniprot.org/uniprotkb/O00155.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O00155/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O00155"}},"corpus_meta":[{"pmid":"26384023","id":"PMC_26384023","title":"Constitutive Activity among Orphan Class-A G Protein Coupled Receptors.","date":"2015","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/26384023","citation_count":127,"is_preprint":false},{"pmid":"23324130","id":"PMC_23324130","title":"Integrative pathway dissection of molecular mechanisms of moxLDL-induced vascular smooth muscle phenotype transformation.","date":"2013","source":"BMC cardiovascular disorders","url":"https://pubmed.ncbi.nlm.nih.gov/23324130","citation_count":51,"is_preprint":false},{"pmid":"17386346","id":"PMC_17386346","title":"A whole genome scan for pulse pressure/stroke volume ratio in African Americans: the HyperGEN study.","date":"2007","source":"American journal of hypertension","url":"https://pubmed.ncbi.nlm.nih.gov/17386346","citation_count":24,"is_preprint":false},{"pmid":"39293486","id":"PMC_39293486","title":"A lymphocyte chemoaffinity axis for lung, non-intestinal mucosae and CNS.","date":"2024","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/39293486","citation_count":20,"is_preprint":false},{"pmid":"9020062","id":"PMC_9020062","title":"Discovery of a novel human G protein-coupled receptor gene (GPR25) located on chromosome 1.","date":"1997","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/9020062","citation_count":15,"is_preprint":false},{"pmid":"29727602","id":"PMC_29727602","title":"The orphan G protein-coupled receptor 25 (GPR25) is activated by Apelin and Apela in non-mammalian vertebrates.","date":"2018","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/29727602","citation_count":13,"is_preprint":false},{"pmid":"40279398","id":"PMC_40279398","title":"Identification of orphan GPR25 as a receptor for the chemokine CXCL17.","date":"2025","source":"The FEBS journal","url":"https://pubmed.ncbi.nlm.nih.gov/40279398","citation_count":7,"is_preprint":false},{"pmid":"40972788","id":"PMC_40972788","title":"Human CXCL17 binds and activates fish GPR25 orthologs.","date":"2025","source":"Biochimie","url":"https://pubmed.ncbi.nlm.nih.gov/40972788","citation_count":3,"is_preprint":false},{"pmid":"41167449","id":"PMC_41167449","title":"CXCL17 activates three MAS-related G protein-coupled receptors independently of its conserved C-terminal fragment.","date":"2025","source":"Archives of biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/41167449","citation_count":2,"is_preprint":false},{"pmid":"41521655","id":"PMC_41521655","title":"Identification and functional characterization of two CXCL17 paralogs from zebrafish.","date":"2026","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/41521655","citation_count":2,"is_preprint":false},{"pmid":"41270189","id":"PMC_41270189","title":"GPR25 promotes the formation of lung and liver tissue-resident memory CD8 T cells.","date":"2025","source":"Science immunology","url":"https://pubmed.ncbi.nlm.nih.gov/41270189","citation_count":1,"is_preprint":false},{"pmid":"41861938","id":"PMC_41861938","title":"Identification and functional characterization of CXCL17 orthologs in amphibians.","date":"2026","source":"Archives of biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/41861938","citation_count":1,"is_preprint":false},{"pmid":"42207165","id":"PMC_42207165","title":"Evidence for a Two-Step Model for Activation of GPR25 by the Chemoattractant CXCL17.","date":"2026","source":"Basic & clinical pharmacology & toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/42207165","citation_count":0,"is_preprint":false},{"pmid":"41887359","id":"PMC_41887359","title":"Ancestral origin of the CXCL17-GPR25 system traced to the lobe-finned fish Latimeria chalumnae.","date":"2026","source":"Biochimie","url":"https://pubmed.ncbi.nlm.nih.gov/41887359","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.09.16.676457","title":"Ancestral Origin of the CXCL17–GPR25 System Traced to the Lobe-Finned Fish  <i>Latimeria chalumnae</i>","date":"2025-09-18","source":"bioRxiv","url":"https://doi.org/10.1101/2025.09.16.676457","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2025.06.10.658772","title":"Human CXCL17 Activates and Binds to Fish GPR25 Orthologs","date":"2025-06-13","source":"bioRxiv","url":"https://doi.org/10.1101/2025.06.10.658772","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2024.09.21.614233","title":"Identification of the orphan GPR25 as a receptor of the chemokine CXCL17","date":"2024-09-22","source":"bioRxiv","url":"https://doi.org/10.1101/2024.09.21.614233","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":11639,"output_tokens":2704,"usd":0.037739,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9923,"output_tokens":2948,"usd":0.061657,"stage2_stop_reason":"end_turn"},"total_usd":0.099396,"stage1_batch_id":"msgbatch_01DUPACZi7AAWwuZiUjseQF4","stage2_batch_id":"msgbatch_018yt7chE3f48PEFtDwtj6AE","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1997,\n      \"finding\": \"GPR25 encodes a 360-amino acid intronless GPCR localized to chromosome 1q32.1, sharing highest sequence identity with GPR15, angiotensin II type 1A receptor, and somatostatin receptor 5; no transcripts were detected in liver or 12 brain regions by Northern analysis.\",\n      \"method\": \"PCR cloning, genomic library screening, Northern analysis, fluorescence in situ hybridization (FISH)\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct experimental cloning and chromosomal mapping with multiple orthogonal methods (PCR, library screen, FISH, Northern), single lab\",\n      \"pmids\": [\"9020062\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GPR25 constitutively inhibits cAMP-mediated gene expression (both baseline CRE-reporter expression and forskolin-stimulated expression) in transfected CHO cells, indicating constitutive Gi-coupled signaling.\",\n      \"method\": \"Transient transfection of CHO cells with CRE-luciferase reporter and orphan receptor plasmids; luciferase reporter assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based functional assay with reporter gene readout, single lab, tested across many receptors providing internal reference\",\n      \"pmids\": [\"26384023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In non-mammalian vertebrates (zebrafish, spotted gar, pigeon), GPR25 is activated by Apelin and Apela peptides, leading to inhibition of forskolin-stimulated cAMP production and receptor internalization; human GPR25 was NOT activated by Apelin or Apela under the same conditions.\",\n      \"method\": \"pGL3-CRE-luciferase reporter assay, cAMP accumulation assay, confocal microscopy of receptor internalization in transfected HEK293 cells\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal functional assays (cAMP, reporter, internalization imaging) in single lab; negative result for human GPR25 explicitly established\",\n      \"pmids\": [\"29727602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"GPR25 is identified as a lymphocyte receptor for the chemoattractant cytokine CXCL17; GPR25 is induced on innate lymphocytes before peripheral emigration and imprinted on activated B and T cells in secondary lymphoid tissues; GPR25 mediates lymphocyte homing to barrier epithelia of airways, oral cavity, stomach, and biliary and genitourinary tracts in mouse models.\",\n      \"method\": \"Single-cell transcriptomics, omics dataset integration, adoptive transfer/homing assays in mouse models, flow cytometry of human tissues\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple independent omics and functional in vivo homing assays, published in high-impact peer-reviewed journal, independently corroborated by separate labs\",\n      \"pmids\": [\"39293486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Human CXCL17 activates human GPR25 with EC50 ~100 nM via β-arrestin recruitment; activation is specific (17 other GPCRs not activated); deletion of three conserved C-terminal residues of CXCL17 abolishes activation; alanine substitution of GPR25 residues W95 or R178 (in the predicted orthosteric binding pocket) abolishes response to CXCL17; CXCL17-GPR25 interaction induces TGF-α shedding and chemotactic migration of transfected HEK293T cells.\",\n      \"method\": \"NanoBiT-based β-arrestin recruitment assay, site-directed mutagenesis of GPR25 (W95A, R178A), C-terminal deletion of CXCL17, TGF-α shedding assay, chemotaxis assay in transfected HEK293T cells, AlphaFold 3 structural modeling\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mutagenesis of both ligand and receptor, multiple orthogonal functional assays (β-arrestin, TGF-α shedding, chemotaxis), independently replicated by another lab (PMID:39293486)\",\n      \"pmids\": [\"40279398\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GPR25 expression is induced by TGF-β signaling on CD8 T cells; Gpr25-deficient T cells infiltrate tissues normally after viral infection but fail to efficiently develop into tissue-resident memory (TRM) cells in lung and liver; GPR25 deficiency impairs secondary TRM cell differentiation, TCF1 expression, and TGF-β signaling responses; GPR25 promotes TRM cell expansion and tumor control.\",\n      \"method\": \"Adoptive transfer of Gpr25-deficient vs wild-type T cells, viral infection and tumor challenge mouse models, single-cell transcriptomics, TGF-β signaling gene signature analysis\",\n      \"journal\": \"Science immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic loss-of-function (Gpr25 KO) with specific cellular phenotype across multiple models (viral infection, tumor), single-cell transcriptomics for pathway placement, peer-reviewed high-impact journal\",\n      \"pmids\": [\"41270189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Human CXCL17 binds and activates zebrafish and coelacanth GPR25 orthologs; β-arrestin recruitment and chemotaxis are dependent on the three C-terminal residues of CXCL17; a NanoBiT binding assay confirmed direct binding of CXCL17 C-terminal fragment to fish GPR25 orthologs.\",\n      \"method\": \"NanoBiT-based β-arrestin recruitment assay, NanoBiT-based direct binding assay, chemotaxis assay in transfected HEK293T cells, C-terminal deletion mutagenesis of CXCL17\",\n      \"journal\": \"Biochimie\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal assays (binding, β-arrestin, chemotaxis) with mutagenesis, single lab, fish orthologs only\",\n      \"pmids\": [\"40972788\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"A two-step model of GPR25 activation by CXCL17 is proposed and validated: the GPR25 N-terminus orients CXCL17 for receptor activation via its C-terminus; the chimeric FPR1:GPR25 receptor (replacing GPR25 N-terminus with FPR1 N-terminus) responds to CXCL17 with significantly reduced potency; mutagenesis of GPR25 residues W95, R178, and R264 causes complete loss of chemotactic responsiveness to CXCL17; N-terminally truncated CXCL17 (64–119) retains chemotactic activity at GPR25 but with severely reduced potency.\",\n      \"method\": \"Site-directed mutagenesis of GPR25 (W95A, R178A, R264A), chimeric FPR1:GPR25 receptor construction, chemotaxis assay in transfected murine pre-B L1.2 cells, N-terminal truncation of CXCL17\",\n      \"journal\": \"Basic & clinical pharmacology & toxicology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — rigorous mutagenesis of multiple residues plus chimeric receptor approach with functional chemotaxis readout, mechanistic model validated by multiple orthogonal perturbations in single study\",\n      \"pmids\": [\"42207165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Zebrafish CXCL17 paralogs (Dr-CXCL17 and Dr-CXCL17-like) directly bind and activate zebrafish GPR25 and induce chemotactic migration; deletion of three C-terminal residues of either paralog abolishes binding, activation, and chemotactic effects, confirming conserved C-terminal-dependent activation mechanism.\",\n      \"method\": \"Bacterial recombinant protein expression and in vitro refolding, NanoBiT-based β-arrestin recruitment assay, NanoBiT-based ligand-receptor binding assay, chemotaxis assay in transfected HEK293T cells, C-terminal deletion mutagenesis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal assays (binding, β-arrestin, chemotaxis) with deletion mutagenesis, single lab, fish orthologs\",\n      \"pmids\": [\"41521655\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GPR25 is an orphan class-A GPCR that is deorphanized as a receptor for the chemoattractant cytokine CXCL17: CXCL17 binds the orthosteric pocket of GPR25 (requiring conserved residues W95, R178, and R264 on the receptor and the conserved C-terminal fragment of CXCL17) via a two-step mechanism in which the GPR25 N-terminus first orients CXCL17, which then activates the receptor through its C-terminus, leading to Gi-mediated inhibition of cAMP, β-arrestin recruitment, and directional chemotaxis; in vivo, GPR25 mediates lymphocyte homing to non-intestinal mucosal tissues and lungs, and promotes TRM CD8 T cell development by enhancing TGF-β signaling.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"GPR25 is a class-A G protein-coupled receptor that functions as a chemoattractant receptor governing lymphocyte trafficking to mucosal barrier tissues [#3, #4]. Long an orphan receptor exhibiting constitutive Gi-coupled inhibition of cAMP-driven gene expression [#1], GPR25 was deorphanized as the receptor for the cytokine CXCL17, which it binds and is activated by with EC50 ~100 nM, triggering \\u03b2-arrestin recruitment, TGF-\\u03b1 shedding, and directional chemotaxis [#3, #4]. Ligand engagement follows a two-step mechanism in which the GPR25 N-terminus first orients CXCL17 and the conserved C-terminus of CXCL17 then activates the receptor through an orthosteric pocket built from residues W95, R178, and R264, all of which are required for the chemotactic response [#4, #7]. In vivo, GPR25 is induced on innate lymphocytes and imprinted on activated B and T cells to direct homing to airway, oral, gastric, biliary, and genitourinary epithelia [#3], and its expression is induced by TGF-\\u03b2 on CD8 T cells where it promotes tissue-resident memory cell development, TCF1 expression, and tumor control [#5]. The receptor's ligand recognition is evolutionarily conserved, with CXCL17 orthologs activating fish and coelacanth GPR25 through the same C-terminal-dependent mechanism, though human GPR25 is not activated by the Apelin/Apela peptides that activate non-mammalian orthologs [#2, #6, #8].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Establishing that GPR25 is a distinct intronless GPCR gene with a defined chromosomal location and a restricted expression pattern provided the molecular entry point for a then-orphan receptor.\",\n      \"evidence\": \"PCR cloning, genomic library screening, FISH mapping, and Northern analysis\",\n      \"pmids\": [\"9020062\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No ligand, signaling pathway, or function identified\", \"Tissue expression beyond liver/brain negative result undefined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrating constitutive inhibition of cAMP-driven gene expression placed GPR25 within Gi-coupled signaling even before its ligand was known.\",\n      \"evidence\": \"CRE-luciferase reporter assay in transfected CHO cells\",\n      \"pmids\": [\"26384023\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Activity was constitutive/ligand-independent\", \"No endogenous agonist identified\", \"G protein coupling inferred from reporter, not direct measurement\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Cross-species ligand testing showed Apelin/Apela activate non-mammalian GPR25 orthologs but not human GPR25, indicating divergent ligand recognition and ruling these peptides out as the human agonist.\",\n      \"evidence\": \"cAMP accumulation, CRE-luciferase reporter, and internalization imaging in transfected HEK293 cells across zebrafish, gar, pigeon, and human orthologs\",\n      \"pmids\": [\"29727602\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Human GPR25 agonist still unidentified\", \"Mechanistic basis for the species difference unresolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identifying GPR25 as a CXCL17 receptor that directs lymphocyte homing deorphanized the receptor in vivo and assigned it a physiological role in mucosal immune surveillance.\",\n      \"evidence\": \"Single-cell transcriptomics, omics integration, and adoptive transfer/homing assays in mouse models with human tissue flow cytometry\",\n      \"pmids\": [\"39293486\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular details of CXCL17 binding not resolved here\", \"Downstream signaling consequences in homing lymphocytes not dissected\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Defining the CXCL17-GPR25 pharmacology with mutagenesis of both ligand and receptor pinpointed the orthosteric determinants of activation and confirmed signaling outputs.\",\n      \"evidence\": \"NanoBiT \\u03b2-arrestin recruitment, W95A/R178A receptor mutagenesis, CXCL17 C-terminal deletion, TGF-\\u03b1 shedding and chemotaxis assays, AlphaFold 3 modeling\",\n      \"pmids\": [\"40279398\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental structure of the complex\", \"Quantitative G protein coupling not directly measured\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Genetic loss-of-function established that GPR25 is dispensable for tissue infiltration but required for tissue-resident memory CD8 T cell differentiation, linking the receptor to TGF-\\u03b2 signaling and antitumor immunity.\",\n      \"evidence\": \"Adoptive transfer of Gpr25-deficient T cells in viral infection and tumor models with single-cell transcriptomics and TGF-\\u03b2 signature analysis\",\n      \"pmids\": [\"41270189\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanistic link between GPR25 signaling and TGF-\\u03b2 pathway enhancement undefined\", \"Whether CXCL17 is the relevant ligand in this context untested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A validated two-step activation model resolved how CXCL17 engages GPR25, separating the N-terminal orientation step from C-terminal-driven receptor activation through specific pocket residues.\",\n      \"evidence\": \"W95A/R178A/R264A mutagenesis, FPR1:GPR25 N-terminal chimera, CXCL17 N-terminal truncation, and chemotaxis in L1.2 and HEK293T cells; conserved binding confirmed in fish and coelacanth orthologs\",\n      \"pmids\": [\"42207165\", \"40972788\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure still absent\", \"Affinity contributions of N-terminus vs orthosteric pocket not quantified\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Confirming that endogenous zebrafish CXCL17 paralogs activate GPR25 via the same C-terminal-dependent mechanism established deep evolutionary conservation of the ligand-receptor pair.\",\n      \"evidence\": \"Recombinant CXCL17 paralog refolding, NanoBiT binding and \\u03b2-arrestin assays, chemotaxis, and C-terminal deletion mutagenesis\",\n      \"pmids\": [\"41521655\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Restricted to fish orthologs\", \"Physiological role of zebrafish CXCL17-GPR25 axis in vivo untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How GPR25 signaling mechanistically enhances TGF-\\u03b2 responses to drive tissue-resident memory differentiation, and the atomic structure of the CXCL17-GPR25 complex, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental structure of the ligand-bound receptor\", \"Signaling crosstalk between GPR25 and TGF-\\u03b2 pathway unmapped\", \"G protein subtype selectivity in immune cells undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [3, 4, 7]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [4, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 4]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [3, 5]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"CXCL17\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}