ADGRF4

ADGRF4 (GPR115) is an adhesion GPCR with a GPS domain and mucin-like Ser/Thr-rich stalk that functions as a pH-responsive receptor in epithelial tissues, governing differentiation and mineralization programs. In mature ameloblasts, ADGRF4 is required for enamel mineralization by controlling expression of carbonic anhydrase 6 (Car6) and thereby regulating ion composition and pH homeostasis; Gpr115-knockout mice develop hypomineralized enamel with expanded acidic zones, and acidic extracellular pH potentiates Car6 induction through ADGRF4-dependent signaling (PMID:32868297, PMID:36929047). In epidermal keratinocytes, ADGRF4 localizes intracellularly along KRT1/10-positive keratin filaments, is not glycosylated or GPS-cleaved, and is indispensable for KRT1 expression and stratification during terminal differentiation (PMID:36231117). In non-small cell lung cancer cells, ADGRF4 promotes invasiveness via downstream regulation of PPP2C, as demonstrated by knockdown-rescue experiments (PMID:33288575).

1. 2002 Low

   Identification of GPR115 as a novel adhesion GPCR with a GPS domain and mucin-like stalks established its structural classification but left its ligand, signaling, and biological function unknown.

   Evidence Bioinformatic mining of human genome databases with phylogenetic analysis

   PMID:12435584

   Open questions at the time

   • Computational prediction only; no experimental validation of protein expression or function
   • No ligand or signaling pathway identified
   • GPS cleavage status not tested
2. 2012 Medium

   Knockout of Gpr115 in mice revealed co-expression with Gpr111 in developing skin but no overt developmental phenotype, raising the possibility of functional redundancy among cluster members.

   Evidence Gpr115 knockout/LacZ knockin mice with transcriptional profiling

   PMID:22837050

   Open questions at the time

   • No examination of enamel or specialized epithelial phenotypes in KO mice
   • Redundancy with Gpr111/Gpr116 not formally tested by double KO
   • No signaling pathway analysis performed
3. 2013 Low

   Expression profiling placed GPR115 specifically in epidermal keratinocytes and showed regulation by cytokines and differentiation, providing the first tissue-specific functional context.

   Evidence qPCR, western blot, and immunohistochemistry on human skin and skin-derived cell lines

   PMID:23840300

   Open questions at the time

   • Expression data only; no loss-of-function or gain-of-function experiments
   • Cytokine-responsive regulation mechanism not defined
   • Downstream signaling not investigated
4. 2020 High

   Two studies established ADGRF4 as a functional regulator of distinct epithelial programs: in ameloblasts it controls enamel mineralization through Car6 and pH homeostasis, while in lung cancer cells it drives invasiveness via PPP2C, defining the first mechanistic downstream targets.

   Evidence Gpr115-KO mice with enamel phenotype plus in vitro CLDE knockdown/rescue and pH assays (ameloblast study); siRNA knockdown with RNA-seq and PPP2C overexpression rescue in lung cancer cells (cancer study)

   PMID:32868297 PMID:33288575

   Open questions at the time

   • Direct signaling cascade linking ADGRF4 to Car6 transcription not delineated
   • Whether PPP2C regulation is direct or through intermediate signaling unknown
   • No ligand identified for ADGRF4 in either context
5. 2022 Medium

   Characterization in keratinocytes revealed that ADGRF4 localizes intracellularly along KRT1/10 filaments, is not glycosylated or GPS-cleaved, and is required for KRT1 expression and epidermal stratification — establishing an unconventional non-surface-resident mode of action for this adhesion GPCR.

   Evidence ADGRF4 knockout in HaCaT keratinocytes, organotypic culture, immunofluorescence, and western blot for glycosylation

   PMID:36231117

   Open questions at the time

   • Mechanism by which intracellular ADGRF4 regulates KRT1 transcription is unknown
   • Whether the intracellular localization reflects a trafficking defect or true functional site is unresolved
   • Results from a single lab using one keratinocyte cell line
6. 2023 Medium

   Demonstration that ADGRF4 responds to extracellular acidification and cooperates additively with GPR111 in regulating calcification consolidated the model of ADGRF4 as a pH-sensing receptor in enamel formation.

   Evidence Dual knockdown of Gpr111 and Gpr115 in dental epithelial cells with pH manipulation and calcification assays

   PMID:36929047

   Open questions at the time

   • pH-sensing domain or residues on ADGRF4 not mapped
   • Whether pH acts directly on ADGRF4 or through an intermediate sensor is unresolved
   • In vivo confirmation of additive Gpr111/Gpr115 function not performed

• No endogenous ligand, G-protein coupling specificity, or structural basis for pH responsiveness has been defined for ADGRF4, and its unconventional intracellular localization in keratinocytes versus ameloblast function remains mechanistically unexplained.
• No ligand identification study reported
• G-protein coupling preference unknown
• Structural basis for lack of GPS cleavage and intracellular retention in keratinocytes undefined