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

Abstract

The epithelial Ca²⁺ channel TRPV5 (ECaC1) plays a key role in renal and intestinal Ca²⁺ (re)absorption and is thus regulated by 1,25(OH) ₂D₃. 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) ₂D₃. 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 ^S422DSGK1, inactive ^K127NSGK1, constitutively active ^T308D,S473DPKB and/or the Na⁺/H⁺ exchanger regulating factor NHERF2. In Xenopus laevisoocytes expression of TRPV5 increases uptake of tracer Ca^S422D; and induces a Ca²⁺ current (I_Ca). In the presence of Cl^-, TRPV5 mediated Ca²⁺ entry leads to secondary activation of Ca²⁺-sensitive Cl^- channels (I_Cl(Ca)). Coexpression of TRPV5 with both ^S422DSGK1 and NHERF2 stimulates tracer Ca²⁺ entry, I_Ca and I_Cl(Ca). The effect of ^S422DSGK1 on TRPV5 and NHERF2 expressing oocytes is mimicked by SGK1 and SGK3, but not by SGK2, constitutively active ^T308D,S473DPKB or inactive ^K127NSGK1. The observations suggest that SGK1, SGK3 and NHERF2 regulate TRPV5 and are thus likely to participate in the regulation of calcium homeostasis.