Regulation of the Epithelial Ca2+ Channel TRPV5 by the NHE Regulating Factor NHERF2 and the Serum and Glucocorticoid Inducible Kinase Isoforms SGK1 and SGK3 Expressed in Xenopus oocytes
The epithelial Ca<sup>2+</sup> channel TRPV5 (ECaC1) plays a key role in renal and intestinal Ca<sup>2+</sup> (re)absorption and is thus regulated by 1,25(OH) <sub>2</sub>D<sub>3</sub>. The present study aims to explore whether TRPV5 is regulated by the serum and glucocorticoid inducible kinase SGK1, a kinase transcriptionally upregulated by 1,25(OH) <sub>2</sub>D<sub>3</sub>. To this end cRNA encoding TRPV5 has been injected into Xenopus oocytes with or without additional injection of SGK1, its isoforms SGK2 and SGK3, constitutively active <sup>S422D</sup>SGK1, inactive <sup>K127N</sup>SGK1, constitutively active <sup>T308D,S473D</sup>PKB and/or the Na<sup>+</sup>/H<sup>+</sup> exchanger regulating factor NHERF2. In Xenopus laevisoocytes expression of TRPV5 increases uptake of tracer Ca<sup>S422D;</sup> and induces a Ca<sup>2+</sup> current (I<sub>Ca</sub>). In the presence of Cl<sup>-</sup>, TRPV5 mediated Ca<sup>2+</sup> entry leads to secondary activation of Ca<sup>2+</sup>-sensitive Cl<sup>-</sup> channels (I<sub>Cl(Ca)</sub>). Coexpression of TRPV5 with both <sup>S422D</sup>SGK1 and NHERF2 stimulates tracer Ca<sup>2+</sup> entry, I<sub>Ca</sub> and I<sub>Cl(Ca)</sub>. The effect of <sup>S422D</sup>SGK1 on TRPV5 and NHERF2 expressing oocytes is mimicked by SGK1 and SGK3, but not by SGK2, constitutively active <sup>T308D,S473D</sup>PKB or inactive <sup>K127N</sup>SGK1. The observations suggest that SGK1, SGK3 and NHERF2 regulate TRPV5 and are thus likely to participate in the regulation of calcium homeostasis.