Calmodulin Binds to the C Terminus of Sodium Channels Na _v1.4 and Na _v1.6 and Differentially Modulates Their Functional Properties

Abstract

Modulation of voltage-gated sodium channels (VGSC) can have a major impact on cell excitability. Analysis of calmodulin (CaM) binding to GST-fusion proteins containing the C-terminal domains of Na _v1.1-Na _v1.9 indicates that some of the tetrodotoxin-sensitive VGSC isoforms, including Na _V1.4 and Na _V1.6, are able to bind CaM in a calcium-independent manner. Here we demonstrate that association with CaM is important for functional expression of Na _V1.4 and Na _V1.6 VGSCs. Disrupting the interaction between CaM and the C terminus of Na _V1.4 and Na _V1.6 channels reduced current amplitude by 99 and 62%, respectively. Overexpression of CaM increased the current generated by Na _v1.4 and Na _v1.6 C-terminal mutant constructs that exhibited intermediate current densities and intermediate binding affinities for CaM, demonstrating that this effect on current density was directly dependent on the ability of the C terminus to bind CaM. In addition to the effects on current density, calmodulin also was able to modulate the inactivation kinetics of Na _v1.6, but not Na _v1.4, currents in a calcium-dependent manner. Our data demonstrate that CaM can regulate the properties of VGSCs via calcium-dependent and calcium-independent mechanisms and suggest that modulation of neuronal sodium channels may play a role in calcium-dependent neuronal plasticity.