ADGRF4

ADGRF4 (GPR115) is an adhesion G protein-coupled receptor that functions as a pH-responsive regulator of epithelial mineralization and differentiation (PMID:32868297, PMID:36929047). In mature ameloblasts it is required for enamel mineralization: its loss produces hypomineralized enamel with an enlarged acidic area reflecting dysregulated ion composition and pH homeostasis, and it is indispensable for acid-inducible expression of carbonic anhydrase 6 (Car6) in dental epithelial cells (PMID:32868297). Together with the co-expressed receptor Gpr111, ADGRF4 acts as a pH-responsive molecule that suppresses calcification in enamel pH cycling (PMID:36929047). In stratified epidermis, ADGRF4 is expressed in suprabasal keratinocytes and promotes keratinocyte stratification and KRT1 expression; endogenous protein localizes intracellularly along KRT1/10 keratin filaments and is neither glycosylated nor evidently proteolytically processed, consistent with a non-canonical intracellular role in epidermal differentiation (PMID:36231117). In cancer, ADGRF4 promotes invasiveness in non-small cell lung cancer cells through downstream regulation of PPP2C, the rescue and silencing of which place PPP2C downstream of the receptor (PMID:33288575). The receptor signaling mechanism, ligand, and structural basis of pH sensing have not been characterized in the available corpus.

1. 2012 Medium

   Establishing the expression pattern and an initial loss-of-function test asked whether Gpr115 has a non-redundant developmental role, and revealed co-expression with Gpr111 in developing skin with no overt phenotype, raising the hypothesis of functional redundancy.

   Evidence LacZ reporter knockin and knockout mice with transcriptional profiling

   PMID:22837050

   Open questions at the time

   • No tissue-specific phenotype identified despite expression mapping
   • Redundancy with Gpr111 inferred, not directly tested by double knockout
2. 2020 High

   A clean knockout defined the first concrete physiological role, showing Gpr115 is required for enamel mineralization and controls Car6 expression and pH homeostasis, establishing it as a pH-linked regulator in ameloblasts.

   Evidence Gpr115-KO mice, transcriptomics, and CLDE dental epithelial cell assays with extracellular pH manipulation

   PMID:32868297

   Open questions at the time

   • The signaling pathway linking the receptor to Car6 transcription is undefined
   • No ligand or direct pH-sensing mechanism identified
3. 2020 Medium

   Knockdown-and-rescue experiments placed ADGRF4 in a cancer invasiveness pathway, identifying PPP2C as a downstream effector in non-small cell lung cancer.

   Evidence siRNA knockdown, RNA-seq, invasion assays, and PPP2C overexpression rescue plus PPP2C knockdown

   PMID:33288575

   Open questions at the time

   • Mechanism connecting ADGRF4 to PPP2C regulation unknown
   • Single cancer cell context, no in vivo validation
4. 2022 Medium

   Knockout in organotypic keratinocyte culture extended ADGRF4 function to epidermal differentiation and uncovered a non-canonical intracellular keratin-associated localization.

   Evidence ADGRF4 knockout in HaCaT cells, immunofluorescence, western blot for glycosylation, transcriptomic profiling

   PMID:36231117

   Open questions at the time

   • How an intracellular receptor lacking glycosylation drives KRT1 expression is unresolved
   • No reconstitution or structural validation of keratin association
5. 2022 Low

   A silencing study broadened the cancer role to pancreatic ductal adenocarcinoma, showing effects on proliferation and migration.

   Evidence siRNA silencing with proliferation and migration assays in PDAC cells

   PMID:36396823

   Open questions at the time

   • Single method without pathway placement
   • No downstream effector identified in this context
6. 2023 Medium

   Dual knockdown experiments refined the pH-responsive model, showing Gpr115 and Gpr111 additively suppress calcification and both respond to extracellular pH in enamel formation.

   Evidence In vitro pH manipulation and dual Gpr111/Gpr115 knockdown with calcification assays in dental epithelial cells

   PMID:36929047

   Open questions at the time

   • Molecular basis of pH sensing not established
   • Functional interaction between the two receptors not biochemically defined

• The receptor's ligand, signal transduction mechanism, and the structural basis for its pH responsiveness remain unknown.
• No ligand identified
• No G protein coupling or downstream signaling cascade defined
• No structural model of pH sensing