CaMKII-Dependent Regulation of Cardiac Sodium Channel

Voltage-gated Na+ channels are key determinants of conduction, action potential profile and refractoriness in mammalian myocardium. Na+ channels are regulated by a number of protein kinases and alterations in phosphorylation are associated with the phenotypic expression of inherited and acquired heart diseases. Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylates NaV1.5 at multiple sites in the domain I-II linker, with effects on channel gating. The CaMKII phosphorylated sites we identified by mass spectrometry (MS) include serines 459, 460, 484, 539, 571, 664, 667 and threonine 486 in the I-II linker and serines 1925, 1937 and 1969 in the carboxyl-Terminus (CT). In addition we evaluated the effect of CAMKII phosphorylation on NaV1.5-S528A and NaV1.5-R526H channels in which direct phosphorylation by PKA is abolished (Aiba, 2014). In Wild-type NaV1.5 channels, acute exposure to CaMKII increased the peak current, produced a-10 mV shift in the availability curve, slowed recovery from inactivation and increased the late Na current (INa-L). These changes were prevented by inclusion of the CaMKII inhibitor peptide, AIP in the pipette or partially prevented by exposure to the PKA inhibitor PKI. NaV1.5-S571A, NaV1.5-S526H and NaV1.5-S528A channels eliminated the CaMKII induced shift in inactivation, slowing of recovery and increase in INa-L S571D or S528D mimicked the effects of phosphorylation. In NaV1.5-S1969A channels exposure to CAMKII increases INa-L compared to NaV1.5-WT with a further increase in INa-L in the presence of CaMKII; however, the steady state inactivation curve is not shifted. These data suggest the presence of functional phosphorylation sites in the CT and the interaction of CaMKII and PKA in the I-II linker. The cross talk between CaMKII and PKA modulation of the channel may have important implications for electrophysiological properties of the heart.