Sodium Channel Nav1.4 and Nav1.5 Regulation by Calcium and Calmodulin

Voltage-gated sodium channels (NaV) account for the upstroke of the action potential in all excitable tissues. The skeletal muscle voltage-gated Na+ channel (NaV1.4) and the cardiac muscle Na+ channel (NaV1.5) both contain a calmodulin (CaM) binding motif (IQ-motif) in their cytosolic C-terminal domain (CTerm). Both are subject to activation by apo CaM but only NaV1.4 exhibits CaM mediated Ca2+-dependent inactivation (CDI). Here are presented two crystal structures of the NaV1.4 CTerm bound to CaM, with and without Ca2+ present. Thermodynamic data of CaM binding to NaV1.4 or NaV1.5 CTerm are also presented along with a thermodynamic model that identifies a Ca2+-dependent difference in CaM binding to the two isoforms of NaV CTerm. Together these findings provide a mechanistic rationale for the isoform difference in their physiological Ca2+ response. The C-lobe of CaM remains anchored at low and high Ca2+ in both NaV isoforms. At low Ca2+ the N-lobe of CaM makes contacts with the EF-like hand of the CTerm in both NaV isoforms. At high Ca2+ levels the N-lobe of NaV1.5 binds to a post-IQ motif and this binding causes no observable change in channel activity. In NaV1.4 the N-lobe of CaM has low affinity for the post-IQ motif and binds an as yet unidentified partner at high Ca2+. The N-lobe binding at high Ca2+ is the switch that controls NaV CDI. Consistent with this mechanism, removing this binding site in NaV1.5 unveils robust CDI. These findings suggest that Ca2+-dependent inactivation is effected by CaM’s N-lobe binding to a site outside the NaV C-terminal domain. Also, as the N-lobe binding motif of NaV1.5 is a mutational “hotspot” for inherited arrhythmias, the contributions of mutation- induced changes in CDI to arrhythmia generation is an intriguing possibility. The same effects may be operational in neuronal isoforms that mimic the behavior of NaV1.5 leading to neurological disorders such as seizures and neuropathic pain