Ca2+-dependent modulation of voltage-gated myocyte sodium channels

Samantha C Salvage,Zaki F Habib,Hugh R Matthews,Antony P Jackson,Christopher L.-H Huang

doi:10.1042/bst20200604

Abstract

Voltage-dependent Na+ channel activation underlies action potential generation fundamental to cellular excitability. In skeletal and cardiac muscle this triggers contraction via ryanodine-receptor (RyR)-mediated sarcoplasmic reticular (SR) Ca2+ release. We here review potential feedback actions of intracellular [Ca2+] ([Ca2+]i) on Na+ channel activity, surveying their structural, genetic and cellular and functional implications, translating these to their possible clinical importance. In addition to phosphorylation sites, both Nav1.4 and Nav1.5 possess potentially regulatory binding sites for Ca2+ and/or the Ca2+-sensor calmodulin in their inactivating III–IV linker and C-terminal domains (CTD), where mutations are associated with a range of skeletal and cardiac muscle diseases. We summarize in vitro cell-attached patch clamp studies reporting correspondingly diverse, direct and indirect, Ca2+ effects upon maximal Nav1.4 and Nav1.5 currents (Imax) and their half-maximal voltages (V1/2) characterizing channel gating, in cellular expression systems and isolated myocytes. Interventions increasing cytoplasmic [Ca2+]i down-regulated Imax leaving V1/2 constant in native loose patch clamped, wild-type murine skeletal and cardiac myocytes. They correspondingly reduced action potential upstroke rates and conduction velocities, causing pro-arrhythmic effects in intact perfused hearts. Genetically modified murine RyR2-P2328S hearts modelling catecholaminergic polymorphic ventricular tachycardia (CPVT), recapitulated clinical ventricular and atrial pro-arrhythmic phenotypes following catecholaminergic challenge. These accompanied reductions in action potential conduction velocities. The latter were reversed by flecainide at RyR-blocking concentrations specifically in RyR2-P2328S as opposed to wild-type hearts, suggesting a basis for its recent therapeutic application in CPVT. We finally explore the relevance of these mechanisms in further genetic paradigms for commoner metabolic and structural cardiac disease.

Highlights

Transmembrane action potential initiation and propagation, mediated by surface membrane Na+channel (Nav) proteins, is strategic to activation in excitable cells, of which skeletal and cardiac myocytes constitute important examples
We relate the voltage sensing, and channel opening and inactivation processes in skeletal, Nav1.4 and cardiac Nav1.5 to their potential regulation at direct and indirect Ca2+ binding and phosphorylation sites. This includes its III–IV linker region and its interactions with its C-terminal domain, whose different regions are associated with widespread mutations related to skeletal and cardiac muscle disease
We examine in vitro studies in expression systems exploring for direct and indirect effects of Ca2+ on channel properties, extend these to physiological studies in both skeletal and cardiac myocytes in situ, from experimental platforms using normal hearts, and those modelling genetic Ca2+ homeostatic disease, broadening these to genetic exemplars for more common human disease types

Summary

Introduction

Transmembrane action potential initiation and propagation, mediated by surface membrane Na+channel (Nav) proteins, is strategic to activation in excitable cells, of which skeletal and cardiac myocytes constitute important examples. The IQ motif within helix 6 of the Nav1.5 CTD [22] (Figure 3A) can bind the apo-calmodulin C-lobe [14].

Results

Conclusion