Nuclear Na+/K+-atpase plays an active role in nucleoplasmic Ca2+ homeostasis

Abstract

Na+/K+-ATPase, an integral membrane protein, has been studied for over a half century with respect to its transporter function in the plasma membrane, where it expels three Na+ ions from the cell in exchange for two K+ ions. In this study, we demonstrate a functioning Na+/K+-ATPase within HEK293 cell nuclei. This subcellular localization was confirmed by western blotting, ouabain-sensitive ATPase activity of the nuclear membrane fraction, immunocytochemistry and delivery of fluorescently tagged Na+/K+-ATPase a-and bsubunits. In addition, we observed an overlap between nuclear Na+/K+-ATPase and Na/Ca-exchanger (NCX) when nuclei were immunostained with commercially available Na+/K+-ATPase and NCX antibodies, suggesting a concerted physiological coupling between these transporters. In keeping with this, we observed an ATP-dependent, strophanthidin-sensitive Na+ flux into the nuclear envelope (NE) lumen loaded with the Na-sensitive dye, CoroNa-Green. Analogous experiments using Fluo-5N, a low affinity Ca2+ indicator, demonstrated a similar ATP-dependent and strophanthidin-sensitive Ca2+ flux into the NE lumen. Our results reveal an intracellular physiological role for the coordinated efforts of the Na+/K+-ATPase and NCX to actively remove Ca2+ from the nucleoplasm into the NE lumen (i.e. the nucleoplasmic reticulum). © 2012. Published by The Company of Biologists Ltd.