FOXI1 Promotes Expression of V‐ATPase and GPR116 in M‐1 Cells

Abstract

G protein‐coupled receptors (GPCRs) are a diverse family of integral membrane proteins that have significant roles in numerous physiological systems. We previously reported that GPR116 (ADGRF5), an adhesion‐class GPCR, is a critical regulator of vacuolar‐type H+‐ATPase (V‐ATPase) surface expression in A‐type intercalated cells (AICs) in mouse kidney cortical collecting ducts. The V‐ATPase is a multi‐subunit proton pump that localizes to the plasma membrane in specialized acid‐secreting epithelial cells. FOXI1 is a transcription factor that regulates V‐ATPase expression in ICs and other mitochondria‐rich cells. Recently, FOXI1 was identified as a key regulator of CFTR‐rich pulmonary ionocytes which express both V‐ATPase and GPR116. Therefore, we hypothesized that FOXI1 is a transcription factor in AICs that upregulates V‐ATPase as well as GPR116. To test this hypothesis, we cloned FOXI1 from whole mouse kidney into a pME18s expression vector (pFOXI1). Transfection with pFOXI1 significantly increased GPR116 expression (qPCR) in M‐1 mouse cortical collecting duct cells (n=3, ΔCt ±SEM: untransfected = 13.8±0.6, pFoxi1 = 10.7±0.2, p<0.05), but not in HEK293T cells (n=3, not significant). RNA‐scope hybridization on murine kidney sections and pFOXI1‐transfected M‐1 cells revealed colocalization of FOXI1 and GPR116 mRNA in the same cell. Treating pFOXI1‐transfected M‐1 cells with a synthetic agonist peptide for GPR116 results in an increase of [Ca2+]iin a subset of cells (n=3, ΔF340/380 ±SEM: pFoxi1 = 0.296±0.017). Internal calcium mobilization is never seen with the agonist peptide in untransfected control cells, indicating that pFOXI1 is sufficient for the production of physiologically responsive GPR116 on the cell surface (n=3, ΔF340/380±SEM: untransfected = 0 ±0.0009). Moreover, only M‐1 cells transfected with pFOXI1 express GPR110 (ADGRF1), an adhesion‐GPCR with unknown function in the kidney, as well as three V‐ATPase subunits, ATP6V1B1, ATP6V1G3, ATP6V0D2, indicating a larger transcriptional program initiated by pFOXI1 (n=3, untransfected: Ct ~ 40 for all genes, pFoxi1 ΔCt±SEM: ADGRF1 = 13.2±0.21, ATP6V1B1 = 11.4±0.3, ATP6V1G3 = 10±0.1, ATP6V0D2 = 12.1±0.03). Additionally, these pFOXI1 cells also upregulate the expression of SLC26A4 (pendrin), a known marker of B‐type intercalated cells, suggesting that FOXI1‐driven regulation is upstream of AIC fate determination (n=3, ΔCt ± SEM: untransfected = 19.3±0.7, pFoxi1 = 16.1±0.2, p<0.01). These data reveal that FOXI1 upregulates both GPR116 and V‐ATPase in M‐1 cells, generating IC‐like cells. Finally, our study suggests that GPR116 may be a universal and genetically‐coded regulator of V‐ATPase surface expression in FOXI1‐positive cells.