Up-Regulation of Kir2.1 (KCNJ2) by the Serum & Glucocorticoid Inducible SGK3
Background/Aims: The serum & glucocorticoid inducible kinase SGK3, an ubiquitously expressed serine/threonine kinase, regulates a variety of ion channels. It has previously been shown that SGK3 upregulates the outwardly rectifying K<sup>+</sup> channel K<sub>V</sub>11.1, which is expressed in cardiomyocytes. Cardiomyocytes further express the inward rectifier K<sup>+</sup> channel K<sub>ir</sub>2.1, which contributes to maintenance of resting cell membrane potential. Loss-of-function mutations of KCNJ2 encoding K<sub>ir</sub>2.1 result in Andersen-Tawil syndrome with periodic paralysis, cardiac arrhythmia and dysmorphic features. The present study explored whether SGK3 participates in the regulation of K<sub>ir</sub>2.1. Methods: cRNA encoding K<sub>ir</sub>2.1 was injected into Xenopus oocytes with and without additional injection of cRNA encoding wild type SGK3, constitutively active <sup>S419D</sup>SGK3 or inactive <sup>K191N</sup>SGK3. K<sub>ir</sub>2.1 activity was determined by two-electrode voltage-clamp and K<sub>ir</sub>2.1 protein abundance in the cell membrane by immunostaining and subsequent confocal imaging or by chemiluminescence. Results: Injection of 10 ng cRNA encoding wild type SGK3 and <sup>S419D</sup>SGK3, but not <sup>K191N</sup>SGK3 significantly enhanced K<sub>ir</sub>2.1-mediated currents. SGK inhibitor EMD638683 (50 µM) abrogated <sup>S419D</sup>SGK3-induced up-regulation of K<sub>ir</sub>2.1. Moreover, wild type SGK3 enhanced the channel protein abundance in the cell membrane. The decay of K<sub>ir</sub>2.1-mediated currents following inhibition of channel insertion into the cell membrane by brefeldin A (5 µM) was similar in oocytes coexpressing K<sub>ir</sub>2.1 and SGK3 as in oocytes expressing K<sub>ir</sub>2.1 alone, suggesting that SGK3 influences channel insertion into rather than channel retrieval from the cell membrane. Conclusions: SGK3 is a novel regulator of K<sub>ir</sub>2.1.