Regulation of Cardiac ATP-sensitive Potassium Channel Surface Expression by Calcium/Calmodulin-dependent Protein Kinase II

Cardiac ATP-sensitive potassium (K-ATP) channels are key sensors and effectors of the metabolic status of cardiomyocytes. Alteration in their expression impacts their effectiveness in maintaining cellular energy homeostasis and resistance to injury. We sought to determine how activation of calcium/calmodulin-dependent protein kinase II (CaMKII), a central regulator of calcium signaling, translates into reduced membrane expression and current capacity of cardiac K-ATP channels. We used real-time monitoring of K-ATP channel current density, immunohistochemistry, and biotinylation studies in isolated hearts and cardiomyocytes from wild-type and transgenic mice as well as HEK cells expressing wild-type and mutant K-ATP channel subunits to track the dynamics of K-ATP channel surface expression. Results showed that activation of CaMKII triggered dynamin-dependent internalization of K-ATP channels. This process required phosphorylation of threonine at 180 and 224 and an intact (YSKF333)-Y-330 endocytosis motif of the K-ATP channel Kir6.2 pore-forming subunit. A molecular model of the mu 2 subunit of the endocytosis adaptor protein, AP2, complexed with Kir6.2 predicted that mu 2 docks by interaction with (YSKF333)-Y-330 and Thr-180 on one and Thr-224 on the adjacent Kir6.2 subunit. Phosphorylation of Thr-180 and Thr-224 would favor interactions with the corresponding arginine-and lysine-rich loops on mu 2. We concluded that calcium-dependent activation of CaMKII results in phosphorylation of Kir6.2, which promotes endocytosis of cardiac K-ATP channel subunits. This mechanism couples the surface expression of cardiac K-ATP channels with calcium signaling and reveals new targets to improve cardiac energy efficiency and stress resistance.