External Ca regulates polycystin-2 (TRPP2) cation currents in LLC-PK1 renal epithelial cells.

Polycystin-2 (PC2, TRPP2) is a nonselective cation channel whose dysfunction is associated with the onset of autosomal dominant polycystic kidney disease (ADPKD). PC2 contributes to Ca transport and cell signaling in renal epithelia and other tissues. Little is known however, as to the external Ca regulation of PC2 channel function. In this study, we explored the effect of external Ca on endogenous PC2 in wild type LLC-PK1 renal epithelial cells. We obtained whole cell currents at different external Ca concentrations, and observed that the basal whole cell conductance in normal Ca(1.2mM), decreased by 30.2% in zero (nominal) Ca and conversely, increased by 38% in high external Ca(6.2mM). The high Ca-increased whole cell currents were completely inhibited by either PC2 gene silencing, or intracellular dialysis with active, but not denatured by boiling, PC2 antibody. Exposure of cells to high Ca was also associated with relocation of PC2 to the plasma membrane. To explore whether a Ca sensing receptor (CaSR) was implicated in the external Ca modulation of PC2 currents, we tested the effect of the CaSR agonists, spermine and the calcimimetic R-568, which largely mimicked the effect of high Ca under Ca-free conditions. The CaSR agonist gentamicin also increased the PC2 currents in the presence of normal Ca. The presence of CaSR was confirmed by immunocytochemistry, which partially colocalized with the intracellular PC2 protein, in an external Ca-dependent manner. The data support a novel Ca sensing mechanism for PC2 expression and functional regulation in renal epithelial cells.