Na⁺/H⁺ exchange regulatory factor 1 is required for ROMK1 K⁺ channel expression in the surface membrane of cultured M-1 cortical collecting duct cells

The ROMK1 K⁺ channel, a member of the ROMK channel family, is the major candidate for the K⁺ secretion pathway in the renal cortical collecting duct (CCD). ROMK1 possesses a PDZ domain-binding motif at its C-terminus that is considered a modulator of ROMK1 expression via interaction with Na⁺/H⁺ exchange regulatory factor (NHERF) 1 and NHERF2 scaffold protein. Although NHERF1 is a potential binding partner of the ROMK1 K⁺ channel, the interaction between NHERF1 and K⁺ channel activity remains unclear. Therefore, in this study, we knocked down NHERF1 in cultured M-1 cells derived from mouse CCD and investigated the surface expression and K⁺ channel current in these cells after exogenous transfection with EGFP-ROMK1. NHERF1 knockdown resulted in reduced surface expression of ROMK1 as indicated by a cell biotinylation assay. Using the patch-clamp technique, we further found that the number of active channels per patched membrane and the Ba²⁺-sensitive whole-cell K⁺ current were decreased in the knockdown cells, suggesting that reduced K⁺ current was accompanied by decreased surface expression of ROMK1 in the NHERF1 knockdown cells. Our results provide evidence that NHERF1 mediates K⁺ current activity through acceleration of the surface expression of ROMK1 K⁺ channels in M-1 cells.