{"gene":"GPR174","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2012,"finding":"GPR174 is a constitutively active Gαs-coupled receptor; its expression in CHO cells elevates intracellular cAMP and induces morphological changes and proliferation delay. Lysophosphatidylserine (LysoPS) was identified as a ligand that further stimulates cAMP increase and Erk phosphorylation, both inhibited by the Gαs inhibitor NF449.","method":"Stable expression in CHO cells, intracellular cAMP measurement, Erk phosphorylation assay, pharmacological inhibition with NF449","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal functional assays (cAMP, pErk, pharmacological blockade) in a defined cellular system; foundational deorphanization paper","pmids":["23178570"],"is_preprint":false},{"year":2015,"finding":"GPR174 is abundantly expressed in regulatory T (Treg) cells and constrains Treg generation in the thymus and peripheral CD103+ Treg accumulation. LysoPS acting via GPR174 suppresses T cell proliferation and Treg generation in vitro, and GPR174 deficiency in Treg cells reduces EAE susceptibility in vivo.","method":"Gpr174 knockout mice, flow cytometry, in vitro proliferation assays, EAE model, adoptive transfer","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined cellular phenotypes and multiple in vivo/in vitro readouts, replicated in multiple models","pmids":["26077720"],"is_preprint":false},{"year":2015,"finding":"Structure-activity relationships of synthetic LysoPS analogues established that specific structural modules (fatty acid, glycerol, L-serine head group) are required for potency and selectivity at GPR174 (LPS3) versus GPR34 and P2Y10, identifying GPR174 as a LysoPS-specific GPCR within a receptor family.","method":"Synthetic LysoPS analogue screening, functional GPCR assays for receptor subtype selectivity","journal":"Journal of medicinal chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic SAR with pharmacological assays across receptor subtypes, multiple analogues tested","pmids":["25970039"],"is_preprint":false},{"year":2017,"finding":"GPR174 (LPS3) mediates LysoPS-induced suppression of IL-2 production at the mRNA and protein levels in activated CD4+ T cells; this suppressive effect is absent in Gpr174-deficient splenocytes and CD4+ T cells, and LysoPS species are upregulated upon T cell activation.","method":"Gpr174 knockout mice, qPCR, ELISA, in vitro T cell activation assays","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with receptor-specific mechanistic readout (IL-2 mRNA and protein), replicated by Barnes & Cyster 2018","pmids":["29017923"],"is_preprint":false},{"year":2018,"finding":"LysoPS-mediated suppression of T cell activation via GPR174 requires Gαs proteins; mechanistically, GPR174/Gαs signaling suppresses IL-2 production and limits upregulation of CD25 and CD69 on activated T cells. In vivo, GPR174 constrains T cell proliferation induced by sublethal irradiation or Treg depletion.","method":"Gpr174 KO mice, in vitro Gαs inhibition, in vivo T cell proliferation models (irradiation, Treg depletion), flow cytometry, ELISA","journal":"Immunology and cell biology","confidence":"High","confidence_rationale":"Tier 2 — mechanistic G-protein identification with in vivo and in vitro orthogonal evidence; extends and confirms prior work","pmids":["29457279"],"is_preprint":false},{"year":2019,"finding":"CCL21 is a GPR174 ligand identified by biochemical fractionation of conditioned media. GPR174 responds to CCL21 by triggering calcium flux and inducing B cell migration. GPR174 associates preferentially with Gαi in male B cells in a testosterone-dependent manner, and GPR174 suppresses germinal center formation by positioning B cells toward the T-B border rather than the follicle center.","method":"Biochemical fractionation of conditioned media, calcium flux assay, B cell migration assay, Co-immunoprecipitation (GPR174–Gαi), orchidectomy/testosterone treatment, Gpr174 conditional KO mice, intravital imaging/histology","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1–2 — ligand identified by biochemical fractionation, validated by functional assays; receptor-G protein association by Co-IP with hormonal manipulation; in vivo B cell positioning by genetic KO; multiple orthogonal methods","pmids":["31875850"],"is_preprint":false},{"year":2019,"finding":"GPR174 deficiency in Treg cells promotes CTLA-4 and IL-10 expression, and Gpr174-deficient Tregs promote M2 macrophage polarization while dampening pro-inflammatory cytokines (IL-6, TNF-α), thereby protecting mice from LPS- and CLP-induced septic shock.","method":"Gpr174 KO mice, LPS and CLP sepsis models, flow cytometry, ELISA, in vitro macrophage polarization co-culture assays","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 — KO with defined cellular mechanism (Treg→M2 macrophage axis), single lab","pmids":["30850582"],"is_preprint":false},{"year":2022,"finding":"GPR174 deficiency in Tregs upregulates amphiregulin (AREG) expression by inhibiting nuclear accumulation of EGR1 via the Gαs/cAMP/PKA signaling pathway, enhancing endothelial cell function and reducing pro-inflammatory macrophage polarization to promote blood flow recovery after hindlimb ischemia.","method":"Gpr174 conditional KO in Tregs, hindlimb ischemia model, nuclear-cytoplasmic fractionation, cAMP/PKA pathway assays, AREG ELISA, EGR1 ChIP/reporter assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with defined signaling pathway (Gαs/cAMP/PKA→EGR1→AREG) and multiple mechanistic readouts","pmids":["36473866"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structure of LysoPS-bound human GPR174 in complex with Gs protein reveals: (1) the negatively charged LysoPS head group makes extensive polar interactions with key pocket residues; (2) the L-serine moiety inserts into a positively charged sub-cavity; (3) ligand enters via a partially open lateral portal between TM4 and TM5; (4) Gs is engaged via deep insertion of the αH5 helix with extensive polar interactions.","method":"Cryo-EM structure determination, mutagenesis-based functional validation","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with functional mutagenesis validation; definitive structural mechanism","pmids":["36823105"],"is_preprint":false},{"year":2023,"finding":"GPR174 exhibits high constitutive Gαs/cAMP activity due to copurification of endogenous LysoPS. Cryo-EM structures of ligand-free GPR174-Gs complex were determined. GPR174 mutants with reduced LysoPS affinity respond dose-dependently to exogenous LysoPS but not other lipids, confirming LysoPS selectivity. GPR174 adopts a non-canonical Gs coupling mode.","method":"Cryo-EM without exogenous ligand, cAMP activity assays, affinity-reducing mutant rescue assays, lipid specificity assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — structural and biochemical evidence for constitutive LysoPS binding and non-canonical Gs coupling; orthogonal to Liang et al. 2023","pmids":["37737235"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structures of human GPR34 and GPR174 in complex with LysoPS and G protein elucidated distinct lipid-binding modes for these two receptors and structural features of their active states, providing a comparative framework for LysoPS receptor signaling and drug design.","method":"Cryo-EM structure determination, functional validation by mutagenesis and signaling assays","journal":"PLoS biology","confidence":"High","confidence_rationale":"Tier 1 — independent cryo-EM structure with functional studies; corroborates Liang et al. and Nie et al. 2023","pmids":["38048360"],"is_preprint":false},{"year":2025,"finding":"LysoPS/GPR174 signaling activates the cAMP-PKA-CREB pathway in esophageal squamous cell carcinoma cells: LysoPS stimulates GPR174 expression, GPR174 increases cAMP, active PKA translocates to the nucleus and phosphorylates CREB, promoting invasion and metastasis in vitro and in vivo.","method":"LC-MS for LysoPS quantification, Western blotting, nuclear-cytoplasmic fractionation, wound healing/Transwell assays, GPR174 overexpression, nude mouse metastasis model","journal":"Journal of translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 — defined signaling axis (GPR174→cAMP→PKA→pCREB) with multiple assays; single lab, tumor context","pmids":["40229851"],"is_preprint":false},{"year":2025,"finding":"Molecular dynamics simulations of GPR174 bound to an antagonist (mPS) versus LysoPS reveal that LysoPS engages conserved activation motifs (PIF, DRY, N/DPxxY) to couple the ligand-binding site to the G-protein interface via lateral membrane entry, whereas mPS disrupts these pathways and reduces conformational dynamics. Membrane lipids including PIP2 modulate ligand dynamics and receptor conformational states.","method":"Molecular dynamics simulations, network analysis, protein-lipid interaction analysis","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 4 — computational only, no experimental validation in this study","pmids":["41000684"],"is_preprint":true},{"year":2025,"finding":"High-resolution cryo-EM structures of LysoPS-activated GPR174 bound to Gs (2.0 Å) and Gi (3.4 Å) reveal a continuous hydration-mediated signaling transduction network bridging the sodium-binding pocket, NPxxY and DRY motifs, and the G protein-binding interface. This hydration network stabilizes the active state and enables differential Gs and Gi engagement. Molecular dynamics and functional assays confirmed the hydration network is essential for activation and G protein selectivity.","method":"Cryo-EM structure determination (2.0 Å and 3.4 Å), molecular dynamics simulations, functional signaling assays","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 — high-resolution structures plus functional validation; preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.10.06.680656"],"is_preprint":true}],"current_model":"GPR174 is an X-chromosome-encoded, class A GPCR that is constitutively activated by its endogenous ligand lysophosphatidylserine (LysoPS), which binds via a lateral portal between TM4–TM5 with its serine head group inserting into a positively charged sub-cavity; it couples primarily to Gαs (elevating cAMP via PKA) and also to Gαi in a sex hormone (testosterone)-dependent manner, thereby suppressing IL-2 production and T cell activation, constraining Treg generation and function (via the Gαs/cAMP/PKA→EGR1→AREG axis), controlling germinal center formation by directing B cell positioning in response to CCL21 (a second ligand), and modulating macrophage polarization and dendritic cell maturation—collectively imparting sexual dimorphism to humoral immunity and maintaining immune homeostasis."},"narrative":{"teleology":[{"year":2012,"claim":"Establishing that GPR174 is not an orphan receptor but a constitutively active Gαs-coupled GPCR activated by LysoPS resolved its basic signaling identity and linked it to lipid-mediated signaling.","evidence":"Stable expression in CHO cells with cAMP measurement, Erk phosphorylation assay, and pharmacological Gαs blockade with NF449","pmids":["23178570"],"confidence":"High","gaps":["Physiological context of LysoPS–GPR174 signaling unknown","No in vivo functional data","Structural basis of LysoPS recognition unresolved"]},{"year":2015,"claim":"Demonstrating that GPR174 constrains regulatory T cell generation in thymus and periphery established its first immunological function and linked LysoPS sensing to immune homeostasis.","evidence":"Gpr174 knockout mice with flow cytometry, in vitro Treg differentiation assays, and EAE disease model","pmids":["26077720"],"confidence":"High","gaps":["Downstream signaling pathway in Tregs not delineated","Mechanism by which LysoPS suppresses Treg generation not defined","Ligand SAR for GPR174-specific tools still developing"]},{"year":2015,"claim":"Systematic SAR of synthetic LysoPS analogues defined the pharmacophore requirements for GPR174 selectivity versus related receptors GPR34 and P2Y10, enabling future tool compound development.","evidence":"Synthetic analogue screening with functional GPCR subtype-selective assays","pmids":["25970039"],"confidence":"High","gaps":["No high-affinity selective antagonist developed","In vivo utility of analogues not tested"]},{"year":2017,"claim":"Showing that LysoPS suppresses IL-2 production specifically through GPR174 in activated CD4+ T cells identified the key cytokine output controlled by this receptor during T cell activation.","evidence":"Gpr174 KO splenocytes and CD4+ T cells, qPCR and ELISA for IL-2","pmids":["29017923"],"confidence":"High","gaps":["G protein coupling requirement not yet confirmed for IL-2 suppression","Transcription factor targets downstream of GPR174 in T cells unknown"]},{"year":2018,"claim":"Confirming that Gαs is the required coupling partner for GPR174-mediated suppression of IL-2, CD25, and CD69 on T cells established the proximal signaling mechanism and its in vivo relevance for controlling T cell proliferation.","evidence":"Gpr174 KO mice with in vitro Gαs inhibition, in vivo irradiation and Treg depletion models","pmids":["29457279"],"confidence":"High","gaps":["Downstream effectors of Gαs/cAMP in T cells not fully mapped","Whether Gαi coupling occurs in T cells untested"]},{"year":2019,"claim":"Identification of CCL21 as a second GPR174 ligand, and the discovery that GPR174 couples to Gαi in a testosterone-dependent manner in B cells to suppress germinal center formation, revealed a mechanism for sexual dimorphism in humoral immunity.","evidence":"Biochemical fractionation of conditioned media, calcium flux, B cell migration, Co-IP of GPR174–Gαi, orchidectomy/testosterone treatment, conditional KO mice with intravital imaging","pmids":["31875850"],"confidence":"High","gaps":["Structural basis for CCL21 recognition by GPR174 unknown","Mechanism of testosterone-dependent Gαi coupling preference unresolved","Whether CCL21 and LysoPS compete or cooperate at the receptor unclear"]},{"year":2019,"claim":"Demonstrating that GPR174-deficient Tregs upregulate CTLA-4 and IL-10 and promote M2 macrophage polarization extended the receptor's function to the Treg–macrophage axis and sepsis protection.","evidence":"Gpr174 KO mice in LPS and CLP sepsis models, flow cytometry, ELISA, co-culture assays","pmids":["30850582"],"confidence":"Medium","gaps":["Signaling pathway linking GPR174 to CTLA-4/IL-10 upregulation not delineated","Single-lab finding in sepsis models"]},{"year":2022,"claim":"Mapping the Gαs/cAMP/PKA→EGR1→AREG signaling axis in Tregs resolved how GPR174 constrains tissue-repair functions and connected GPR174 to endothelial recovery and macrophage reprogramming after ischemia.","evidence":"Conditional Treg KO, hindlimb ischemia model, nuclear-cytoplasmic fractionation, cAMP/PKA assays, EGR1 ChIP/reporter assays, AREG ELISA","pmids":["36473866"],"confidence":"High","gaps":["Whether EGR1–AREG axis operates in other GPR174-expressing immune cells unknown","Direct binding of EGR1 to AREG promoter in primary Tregs not shown by ChIP-seq"]},{"year":2023,"claim":"Cryo-EM structures of LysoPS-bound GPR174–Gs complex revealed the atomic basis of LysoPS recognition (lateral TM4–TM5 entry, positively charged sub-cavity for L-serine head group) and non-canonical Gs coupling, establishing the structural framework for drug design.","evidence":"Cryo-EM structure determination by three independent groups with mutagenesis validation and signaling assays","pmids":["36823105","37737235","38048360"],"confidence":"High","gaps":["No GPR174–Gi structure from peer-reviewed source","Structural basis of CCL21 binding unresolved","No co-crystal with antagonist available"]},{"year":2025,"claim":"Extension of GPR174 signaling to the cAMP–PKA–CREB axis in esophageal squamous cell carcinoma showed the receptor promotes invasion and metastasis, indicating a pathological role beyond immune regulation.","evidence":"LC-MS, Western blotting, nuclear fractionation, Transwell/wound healing assays, GPR174 overexpression, nude mouse metastasis model","pmids":["40229851"],"confidence":"Medium","gaps":["Single cancer type examined","No genetic loss-of-function in the tumor model","Clinical relevance of GPR174 expression in cancer not established"]},{"year":null,"claim":"Key unresolved questions include the structural basis of CCL21 recognition by GPR174, the molecular mechanism of testosterone-dependent Gαi versus Gαs coupling bias, whether selective GPR174 antagonists can be developed for therapeutic use, and the in vivo relevance of GPR174 in human autoimmunity and cancer.","evidence":"","pmids":[],"confidence":"High","gaps":["No selective GPR174 antagonist with in vivo efficacy reported","No human genetic studies linking GPR174 variants to disease phenotypes in the timeline","Mechanism of biased G protein coupling by sex hormones unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,5]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,2,8]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,8,9]}],"pathway":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,4,5,7,8]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[1,3,4,5,6]}],"complexes":[],"partners":["GNAS","GNAI1","CCL21","EGR1","AREG"],"other_free_text":[]},"mechanistic_narrative":"GPR174 is an X-linked class A GPCR that functions as a lysophosphatidylserine (LysoPS) receptor to regulate immune cell activation, differentiation, and positioning. LysoPS binds GPR174 via a lateral portal between TM4 and TM5, with its L-serine head group inserting into a positively charged sub-cavity, and the receptor couples primarily to Gαs to elevate cAMP, activating PKA-dependent signaling that suppresses IL-2 production and T cell activation, constrains regulatory T cell generation and function, and controls amphiregulin expression through the cAMP/PKA→EGR1→AREG axis [PMID:23178570, PMID:26077720, PMID:29457279, PMID:36473866, PMID:36823105]. GPR174 also responds to CCL21 to direct B cell positioning toward the T–B border, associates with Gαi in a testosterone-dependent manner in male B cells, and thereby suppresses germinal center formation, imparting sexual dimorphism to humoral immunity [PMID:31875850]. Cryo-EM structures reveal that GPR174 exhibits high constitutive Gαs activity due to co-purified endogenous LysoPS and employs a non-canonical Gs coupling mode involving deep insertion of the αH5 helix and a hydration-mediated signaling network linking the sodium-binding pocket to the G protein interface [PMID:36823105, PMID:37737235, PMID:38048360]."},"prefetch_data":{"uniprot":{"accession":"Q9BXC1","full_name":"Probable G-protein coupled receptor 174","aliases":[],"length_aa":333,"mass_kda":38.5,"function":"G-protein-coupled receptor of lysophosphatidylserine (LysoPS) that plays different roles in immune response (PubMed:36823105). Plays a negative role in regulatory T-cell accumulation and homeostasis. Under inflammatory conditions where LysoPS production increases, contributes to the down-regulation of regulatory T-cell activity to favor effector response. Mediates the suppression of IL-2 production in activated T-lymphocytes leading to inhibition of growth, proliferation and differentiation of T-cells. Mechanistically, acts via G(s)-containing heterotrimeric G proteins to trigger elevated cyclic AMP levels and protein kinase A/PKA activity, which may in turn act to antagonize proximal TCR signaling. Plays an important role in the initial period of sepsis through the regulation of macrophage polarization and pro- and anti-inflammatory cytokine secretions. Upon testosterone treatment, acts as a receptor for CCL21 and subsequently triggers through G(q)-alpha and G(12)/G(13) proteins a calcium flux leading to chemotactic effects on activated B-cells. Signals via GNA13 and PKA to promote CD86 up-regulation by follicular B-cells","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9BXC1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GPR174","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/GPR174","total_profiled":1310},"omim":[{"mim_id":"602737","title":"CHEMOKINE, CC MOTIF, LIGAND 21; CCL21","url":"https://www.omim.org/entry/602737"},{"mim_id":"300903","title":"G PROTEIN-COUPLED RECEPTOR 174; GPR174","url":"https://www.omim.org/entry/300903"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"},{"location":"Vesicles","reliability":"Additional"},{"location":"Centriolar satellite","reliability":"Additional"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":15.7}],"url":"https://www.proteinatlas.org/search/GPR174"},"hgnc":{"alias_symbol":["FKSG79","LPS3"],"prev_symbol":[]},"alphafold":{"accession":"Q9BXC1","domains":[{"cath_id":"1.20.1070.10","chopping":"12-237_283-302","consensus_level":"high","plddt":87.3057,"start":12,"end":302}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BXC1","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BXC1-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BXC1-F1-predicted_aligned_error_v6.png","plddt_mean":84.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPR174","jax_strain_url":"https://www.jax.org/strain/search?query=GPR174"},"sequence":{"accession":"Q9BXC1","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BXC1.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BXC1/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BXC1"}},"corpus_meta":[{"pmid":"31875850","id":"PMC_31875850","title":"A GPR174-CCL21 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standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2012,\n      \"finding\": \"GPR174 is a constitutively active Gαs-coupled receptor; its expression in CHO cells elevates intracellular cAMP and induces morphological changes and proliferation delay. Lysophosphatidylserine (LysoPS) was identified as a ligand that further stimulates cAMP increase and Erk phosphorylation, both inhibited by the Gαs inhibitor NF449.\",\n      \"method\": \"Stable expression in CHO cells, intracellular cAMP measurement, Erk phosphorylation assay, pharmacological inhibition with NF449\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal functional assays (cAMP, pErk, pharmacological blockade) in a defined cellular system; foundational deorphanization paper\",\n      \"pmids\": [\"23178570\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GPR174 is abundantly expressed in regulatory T (Treg) cells and constrains Treg generation in the thymus and peripheral CD103+ Treg accumulation. LysoPS acting via GPR174 suppresses T cell proliferation and Treg generation in vitro, and GPR174 deficiency in Treg cells reduces EAE susceptibility in vivo.\",\n      \"method\": \"Gpr174 knockout mice, flow cytometry, in vitro proliferation assays, EAE model, adoptive transfer\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined cellular phenotypes and multiple in vivo/in vitro readouts, replicated in multiple models\",\n      \"pmids\": [\"26077720\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Structure-activity relationships of synthetic LysoPS analogues established that specific structural modules (fatty acid, glycerol, L-serine head group) are required for potency and selectivity at GPR174 (LPS3) versus GPR34 and P2Y10, identifying GPR174 as a LysoPS-specific GPCR within a receptor family.\",\n      \"method\": \"Synthetic LysoPS analogue screening, functional GPCR assays for receptor subtype selectivity\",\n      \"journal\": \"Journal of medicinal chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic SAR with pharmacological assays across receptor subtypes, multiple analogues tested\",\n      \"pmids\": [\"25970039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GPR174 (LPS3) mediates LysoPS-induced suppression of IL-2 production at the mRNA and protein levels in activated CD4+ T cells; this suppressive effect is absent in Gpr174-deficient splenocytes and CD4+ T cells, and LysoPS species are upregulated upon T cell activation.\",\n      \"method\": \"Gpr174 knockout mice, qPCR, ELISA, in vitro T cell activation assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with receptor-specific mechanistic readout (IL-2 mRNA and protein), replicated by Barnes & Cyster 2018\",\n      \"pmids\": [\"29017923\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"LysoPS-mediated suppression of T cell activation via GPR174 requires Gαs proteins; mechanistically, GPR174/Gαs signaling suppresses IL-2 production and limits upregulation of CD25 and CD69 on activated T cells. In vivo, GPR174 constrains T cell proliferation induced by sublethal irradiation or Treg depletion.\",\n      \"method\": \"Gpr174 KO mice, in vitro Gαs inhibition, in vivo T cell proliferation models (irradiation, Treg depletion), flow cytometry, ELISA\",\n      \"journal\": \"Immunology and cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic G-protein identification with in vivo and in vitro orthogonal evidence; extends and confirms prior work\",\n      \"pmids\": [\"29457279\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CCL21 is a GPR174 ligand identified by biochemical fractionation of conditioned media. GPR174 responds to CCL21 by triggering calcium flux and inducing B cell migration. GPR174 associates preferentially with Gαi in male B cells in a testosterone-dependent manner, and GPR174 suppresses germinal center formation by positioning B cells toward the T-B border rather than the follicle center.\",\n      \"method\": \"Biochemical fractionation of conditioned media, calcium flux assay, B cell migration assay, Co-immunoprecipitation (GPR174–Gαi), orchidectomy/testosterone treatment, Gpr174 conditional KO mice, intravital imaging/histology\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — ligand identified by biochemical fractionation, validated by functional assays; receptor-G protein association by Co-IP with hormonal manipulation; in vivo B cell positioning by genetic KO; multiple orthogonal methods\",\n      \"pmids\": [\"31875850\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"GPR174 deficiency in Treg cells promotes CTLA-4 and IL-10 expression, and Gpr174-deficient Tregs promote M2 macrophage polarization while dampening pro-inflammatory cytokines (IL-6, TNF-α), thereby protecting mice from LPS- and CLP-induced septic shock.\",\n      \"method\": \"Gpr174 KO mice, LPS and CLP sepsis models, flow cytometry, ELISA, in vitro macrophage polarization co-culture assays\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined cellular mechanism (Treg→M2 macrophage axis), single lab\",\n      \"pmids\": [\"30850582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"GPR174 deficiency in Tregs upregulates amphiregulin (AREG) expression by inhibiting nuclear accumulation of EGR1 via the Gαs/cAMP/PKA signaling pathway, enhancing endothelial cell function and reducing pro-inflammatory macrophage polarization to promote blood flow recovery after hindlimb ischemia.\",\n      \"method\": \"Gpr174 conditional KO in Tregs, hindlimb ischemia model, nuclear-cytoplasmic fractionation, cAMP/PKA pathway assays, AREG ELISA, EGR1 ChIP/reporter assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with defined signaling pathway (Gαs/cAMP/PKA→EGR1→AREG) and multiple mechanistic readouts\",\n      \"pmids\": [\"36473866\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structure of LysoPS-bound human GPR174 in complex with Gs protein reveals: (1) the negatively charged LysoPS head group makes extensive polar interactions with key pocket residues; (2) the L-serine moiety inserts into a positively charged sub-cavity; (3) ligand enters via a partially open lateral portal between TM4 and TM5; (4) Gs is engaged via deep insertion of the αH5 helix with extensive polar interactions.\",\n      \"method\": \"Cryo-EM structure determination, mutagenesis-based functional validation\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with functional mutagenesis validation; definitive structural mechanism\",\n      \"pmids\": [\"36823105\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GPR174 exhibits high constitutive Gαs/cAMP activity due to copurification of endogenous LysoPS. Cryo-EM structures of ligand-free GPR174-Gs complex were determined. GPR174 mutants with reduced LysoPS affinity respond dose-dependently to exogenous LysoPS but not other lipids, confirming LysoPS selectivity. GPR174 adopts a non-canonical Gs coupling mode.\",\n      \"method\": \"Cryo-EM without exogenous ligand, cAMP activity assays, affinity-reducing mutant rescue assays, lipid specificity assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural and biochemical evidence for constitutive LysoPS binding and non-canonical Gs coupling; orthogonal to Liang et al. 2023\",\n      \"pmids\": [\"37737235\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of human GPR34 and GPR174 in complex with LysoPS and G protein elucidated distinct lipid-binding modes for these two receptors and structural features of their active states, providing a comparative framework for LysoPS receptor signaling and drug design.\",\n      \"method\": \"Cryo-EM structure determination, functional validation by mutagenesis and signaling assays\",\n      \"journal\": \"PLoS biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — independent cryo-EM structure with functional studies; corroborates Liang et al. and Nie et al. 2023\",\n      \"pmids\": [\"38048360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LysoPS/GPR174 signaling activates the cAMP-PKA-CREB pathway in esophageal squamous cell carcinoma cells: LysoPS stimulates GPR174 expression, GPR174 increases cAMP, active PKA translocates to the nucleus and phosphorylates CREB, promoting invasion and metastasis in vitro and in vivo.\",\n      \"method\": \"LC-MS for LysoPS quantification, Western blotting, nuclear-cytoplasmic fractionation, wound healing/Transwell assays, GPR174 overexpression, nude mouse metastasis model\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined signaling axis (GPR174→cAMP→PKA→pCREB) with multiple assays; single lab, tumor context\",\n      \"pmids\": [\"40229851\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Molecular dynamics simulations of GPR174 bound to an antagonist (mPS) versus LysoPS reveal that LysoPS engages conserved activation motifs (PIF, DRY, N/DPxxY) to couple the ligand-binding site to the G-protein interface via lateral membrane entry, whereas mPS disrupts these pathways and reduces conformational dynamics. Membrane lipids including PIP2 modulate ligand dynamics and receptor conformational states.\",\n      \"method\": \"Molecular dynamics simulations, network analysis, protein-lipid interaction analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational only, no experimental validation in this study\",\n      \"pmids\": [\"41000684\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"High-resolution cryo-EM structures of LysoPS-activated GPR174 bound to Gs (2.0 Å) and Gi (3.4 Å) reveal a continuous hydration-mediated signaling transduction network bridging the sodium-binding pocket, NPxxY and DRY motifs, and the G protein-binding interface. This hydration network stabilizes the active state and enables differential Gs and Gi engagement. Molecular dynamics and functional assays confirmed the hydration network is essential for activation and G protein selectivity.\",\n      \"method\": \"Cryo-EM structure determination (2.0 Å and 3.4 Å), molecular dynamics simulations, functional signaling assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution structures plus functional validation; preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.10.06.680656\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"GPR174 is an X-chromosome-encoded, class A GPCR that is constitutively activated by its endogenous ligand lysophosphatidylserine (LysoPS), which binds via a lateral portal between TM4–TM5 with its serine head group inserting into a positively charged sub-cavity; it couples primarily to Gαs (elevating cAMP via PKA) and also to Gαi in a sex hormone (testosterone)-dependent manner, thereby suppressing IL-2 production and T cell activation, constraining Treg generation and function (via the Gαs/cAMP/PKA→EGR1→AREG axis), controlling germinal center formation by directing B cell positioning in response to CCL21 (a second ligand), and modulating macrophage polarization and dendritic cell maturation—collectively imparting sexual dimorphism to humoral immunity and maintaining immune homeostasis.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GPR174 is an X-linked class A GPCR that functions as a lysophosphatidylserine (LysoPS) receptor to regulate immune cell activation, differentiation, and positioning. LysoPS binds GPR174 via a lateral portal between TM4 and TM5, with its L-serine head group inserting into a positively charged sub-cavity, and the receptor couples primarily to Gαs to elevate cAMP, activating PKA-dependent signaling that suppresses IL-2 production and T cell activation, constrains regulatory T cell generation and function, and controls amphiregulin expression through the cAMP/PKA→EGR1→AREG axis [PMID:23178570, PMID:26077720, PMID:29457279, PMID:36473866, PMID:36823105]. GPR174 also responds to CCL21 to direct B cell positioning toward the T–B border, associates with Gαi in a testosterone-dependent manner in male B cells, and thereby suppresses germinal center formation, imparting sexual dimorphism to humoral immunity [PMID:31875850]. Cryo-EM structures reveal that GPR174 exhibits high constitutive Gαs activity due to co-purified endogenous LysoPS and employs a non-canonical Gs coupling mode involving deep insertion of the αH5 helix and a hydration-mediated signaling network linking the sodium-binding pocket to the G protein interface [PMID:36823105, PMID:37737235, PMID:38048360].\",\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"Establishing that GPR174 is not an orphan receptor but a constitutively active Gαs-coupled GPCR activated by LysoPS resolved its basic signaling identity and linked it to lipid-mediated signaling.\",\n      \"evidence\": \"Stable expression in CHO cells with cAMP measurement, Erk phosphorylation assay, and pharmacological Gαs blockade with NF449\",\n      \"pmids\": [\"23178570\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological context of LysoPS–GPR174 signaling unknown\", \"No in vivo functional data\", \"Structural basis of LysoPS recognition unresolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrating that GPR174 constrains regulatory T cell generation in thymus and periphery established its first immunological function and linked LysoPS sensing to immune homeostasis.\",\n      \"evidence\": \"Gpr174 knockout mice with flow cytometry, in vitro Treg differentiation assays, and EAE disease model\",\n      \"pmids\": [\"26077720\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling pathway in Tregs not delineated\", \"Mechanism by which LysoPS suppresses Treg generation not defined\", \"Ligand SAR for GPR174-specific tools still developing\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Systematic SAR of synthetic LysoPS analogues defined the pharmacophore requirements for GPR174 selectivity versus related receptors GPR34 and P2Y10, enabling future tool compound development.\",\n      \"evidence\": \"Synthetic analogue screening with functional GPCR subtype-selective assays\",\n      \"pmids\": [\"25970039\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-affinity selective antagonist developed\", \"In vivo utility of analogues not tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showing that LysoPS suppresses IL-2 production specifically through GPR174 in activated CD4+ T cells identified the key cytokine output controlled by this receptor during T cell activation.\",\n      \"evidence\": \"Gpr174 KO splenocytes and CD4+ T cells, qPCR and ELISA for IL-2\",\n      \"pmids\": [\"29017923\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"G protein coupling requirement not yet confirmed for IL-2 suppression\", \"Transcription factor targets downstream of GPR174 in T cells unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Confirming that Gαs is the required coupling partner for GPR174-mediated suppression of IL-2, CD25, and CD69 on T cells established the proximal signaling mechanism and its in vivo relevance for controlling T cell proliferation.\",\n      \"evidence\": \"Gpr174 KO mice with in vitro Gαs inhibition, in vivo irradiation and Treg depletion models\",\n      \"pmids\": [\"29457279\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effectors of Gαs/cAMP in T cells not fully mapped\", \"Whether Gαi coupling occurs in T cells untested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identification of CCL21 as a second GPR174 ligand, and the discovery that GPR174 couples to Gαi in a testosterone-dependent manner in B cells to suppress germinal center formation, revealed a mechanism for sexual dimorphism in humoral immunity.\",\n      \"evidence\": \"Biochemical fractionation of conditioned media, calcium flux, B cell migration, Co-IP of GPR174–Gαi, orchidectomy/testosterone treatment, conditional KO mice with intravital imaging\",\n      \"pmids\": [\"31875850\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for CCL21 recognition by GPR174 unknown\", \"Mechanism of testosterone-dependent Gαi coupling preference unresolved\", \"Whether CCL21 and LysoPS compete or cooperate at the receptor unclear\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrating that GPR174-deficient Tregs upregulate CTLA-4 and IL-10 and promote M2 macrophage polarization extended the receptor's function to the Treg–macrophage axis and sepsis protection.\",\n      \"evidence\": \"Gpr174 KO mice in LPS and CLP sepsis models, flow cytometry, ELISA, co-culture assays\",\n      \"pmids\": [\"30850582\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Signaling pathway linking GPR174 to CTLA-4/IL-10 upregulation not delineated\", \"Single-lab finding in sepsis models\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Mapping the Gαs/cAMP/PKA→EGR1→AREG signaling axis in Tregs resolved how GPR174 constrains tissue-repair functions and connected GPR174 to endothelial recovery and macrophage reprogramming after ischemia.\",\n      \"evidence\": \"Conditional Treg KO, hindlimb ischemia model, nuclear-cytoplasmic fractionation, cAMP/PKA assays, EGR1 ChIP/reporter assays, AREG ELISA\",\n      \"pmids\": [\"36473866\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether EGR1–AREG axis operates in other GPR174-expressing immune cells unknown\", \"Direct binding of EGR1 to AREG promoter in primary Tregs not shown by ChIP-seq\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Cryo-EM structures of LysoPS-bound GPR174–Gs complex revealed the atomic basis of LysoPS recognition (lateral TM4–TM5 entry, positively charged sub-cavity for L-serine head group) and non-canonical Gs coupling, establishing the structural framework for drug design.\",\n      \"evidence\": \"Cryo-EM structure determination by three independent groups with mutagenesis validation and signaling assays\",\n      \"pmids\": [\"36823105\", \"37737235\", \"38048360\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No GPR174–Gi structure from peer-reviewed source\", \"Structural basis of CCL21 binding unresolved\", \"No co-crystal with antagonist available\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Extension of GPR174 signaling to the cAMP–PKA–CREB axis in esophageal squamous cell carcinoma showed the receptor promotes invasion and metastasis, indicating a pathological role beyond immune regulation.\",\n      \"evidence\": \"LC-MS, Western blotting, nuclear fractionation, Transwell/wound healing assays, GPR174 overexpression, nude mouse metastasis model\",\n      \"pmids\": [\"40229851\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single cancer type examined\", \"No genetic loss-of-function in the tumor model\", \"Clinical relevance of GPR174 expression in cancer not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of CCL21 recognition by GPR174, the molecular mechanism of testosterone-dependent Gαi versus Gαs coupling bias, whether selective GPR174 antagonists can be developed for therapeutic use, and the in vivo relevance of GPR174 in human autoimmunity and cancer.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No selective GPR174 antagonist with in vivo efficacy reported\", \"No human genetic studies linking GPR174 variants to disease phenotypes in the timeline\", \"Mechanism of biased G protein coupling by sex hormones unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 5]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 2, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 8, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 4, 5, 7, 8]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 3, 4, 5, 6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"GNAS\",\n      \"GNAI1\",\n      \"CCL21\",\n      \"EGR1\",\n      \"AREG\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}