GPR25

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).

1. 1997 Medium

   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

   PMID:9020062

   Open questions at the time

   • No ligand, signaling pathway, or function identified
   • Tissue expression beyond liver/brain negative result undefined
2. 2015 Medium

   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

   PMID:26384023

   Open questions at the time

   • Activity was constitutive/ligand-independent
   • No endogenous agonist identified
   • G protein coupling inferred from reporter, not direct measurement
3. 2018 Medium

   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

   PMID:29727602

   Open questions at the time

   • Human GPR25 agonist still unidentified
   • Mechanistic basis for the species difference unresolved
4. 2024 High

   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

   PMID:39293486

   Open questions at the time

   • Molecular details of CXCL17 binding not resolved here
   • Downstream signaling consequences in homing lymphocytes not dissected
5. 2025 High

   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

   PMID:40279398

   Open questions at the time

   • No experimental structure of the complex
   • Quantitative G protein coupling not directly measured
6. 2025 High

   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

   PMID:41270189

   Open questions at the time

   • Mechanistic link between GPR25 signaling and TGF-β pathway enhancement undefined
   • Whether CXCL17 is the relevant ligand in this context untested
7. 2025 High

   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

   PMID:40972788 PMID:42207165

   Open questions at the time

   • Atomic-resolution structure still absent
   • Affinity contributions of N-terminus vs orthosteric pocket not quantified
8. 2026 Medium

   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

   PMID:41521655

   Open questions at the time

   • Restricted to fish orthologs
   • Physiological role of zebrafish CXCL17-GPR25 axis in vivo untested

• 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.
• 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