Enhanced Tetrodotoxin-Sensitive Neuronal Na+ Channel Activity Associated with Arrhythmogenic Calmodulin Mutation N98S

Abstract

Cardiac arrhythmias, often stemming from abnormal impulse generation, are one of the leading causes of mortality in the United States. Recent evidence suggests that tetrodotoxin-sensitive neuronal Na+ channels (nNav) are an integral part of catecholaminergic polymorphic ventricular tachycardia (CPVT) as well as long QT (LQT)-associated arrhythmias. With the advent of calmodulin (CaM)-dependent disorders manifesting overlap in CPVT and long QT syndrome (LQT), there is surprisingly little evidence linking arrhythmogenic CaM mutations with abnormalities in Na+ handling. Here we investigated recombinant CaM mutant associated with LQTS/CPVT (N98S). We performed simultaneous recordings of membrane potentials and intracellular Ca2+ handling as well as assessed Na+ currents (INa) during various pharmacological interventions. Electrophysiological analysis revealed that dialysis of CaM-N98S (6.5 µM) induced a gain-of-function in INa by altering steady-state inactivation which manifested in non-inactivating persistent INa in intact wild type cardiomycyotes. Surprisingly, CaM mutant-mediated alterations in INa function were abolished by nNav inhibition with 4,9-anhydro-tetrodotoxin (4,9-ah-TTX; 300nM). In line with these findings, myocytes dialyzed with N98S displayed prolonged action potential duration, aberrant Ca release events and arrhythmogenic after depolarizations that were abolished with 4,9-ah-TTX treatment. Taken together, these findings identify a previously unexplored mechanism for arrhythmogenic CaM mutations based on altered nNav function. Furthermore, this study indicates that nNav inhibition may be a viable therapeutic modality for management of CaM-related arrhythmias.