ERM proteins mediate the effects of Na+/H+ exchanger (NHE1) activation in cardiac myocytes

Ezrin, radixin, and moesin (ERM) proteins have been implicated in regulating signalling molecules. The aim of the present study was to investigate the activity and subcellular distribution of ERM proteins in cardiac myocytes from both Wistar and diabetic Goto-Kakizaki (GK) rats, and the role of these proteins in mediating the downstream effects of the cardiac sarcolemmal Na+/H+ exchanger (NHE1) activation in response to cell acidification. Immunofluorescence microscopy revealed that activated ERM proteins were localized predominantly at the intercalated disc regions in left ventricular (LV) myocytes of both Wistar and GK rats under basal conditions. After acid loading, profound changes in activated ERM distribution were observed in both groups of myocytes, with immunolabelling detected in regions corresponding to the transverse tubules. This correlated with a marked increase in phospho-ERM levels in both groups, which was higher in GK myocytes and blocked by NHE1 inhibitor treatment. Levels of phospho-Akt paralleled those of phospho-ERM under the various experimental conditions used; in particular, the marked acid-induced increase in both phospho-ERM and phospho-Akt in GK myocytes was abolished by an NHE1 inhibitor treatment. Moreover, the pattern of glycogen synthase kinase-3 beta (GSK-3 beta) phosphorylation in these myocytes was strikingly similar to that observed for Akt activity under the conditions used. Activated ERM proteins mediate the effects of acid-induced NHE1 activation in LV myocytes. Akt is a downstream effector in the cascade activated by NHE1-ERM interaction. In addition, GSK-3 beta phosphorylation is required for downstream effects of NHE1/ERM-Akt signalling.