External Ca2+ regulates polycystin-2 (TRPP2) cation currents in LLC-PK1 renal epithelial cells

- The renal epithelial cell line LLC-PK1 expresses endogenous plasma membrane PC2.
- The endogenous PC2 currents are increased by increasing external calcium.
- Calcium-sensing receptor (CaSR) agonists mimic the effect of high calcium.
- PC2 siRNA silencing abolishes the external calcium response.
- A regulatory mechanism links external calcium and PC2 function via activation of CaSR.

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 Ca2+ transport and cell signaling in renal epithelia and other tissues. Little is known however, as to the external Ca2+ regulation of PC2 channel function. In this study, we explored the effect of external Ca2+ on endogenous PC2 in wild type LLC-PK1 renal epithelial cells. We obtained whole cell currents at different external Ca2+ concentrations, and observed that the basal whole cell conductance in normal Ca2+(1.2 mM), decreased by 30.2% in zero (nominal) Ca2+ and conversely, increased by 38% in high external Ca2+(6.2 mM). The high Ca2+-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 Ca2+ was also associated with relocation of PC2 to the plasma membrane. To explore whether a Ca2+ sensing receptor (CaSR) was implicated in the external Ca2+ 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 Ca2+ under Ca2+-free conditions. The CaSR agonist gentamicin also increased the PC2 currents in the presence of normal Ca2+. The presence of CaSR was confirmed by immunocytochemistry, which partially colocalized with the intracellular PC2 protein, in an external Ca2+-dependent manner. The data support a novel Ca2+ sensing mechanism for PC2 expression and functional regulation in renal epithelial cells.