Proton modulation of residue E1784 and its regulation of fast inactivation

The cardiac voltage-gated sodium channel, NaV1.5, is responsible for the phase 0 depolarization of the ventricular cardiomyocyte action potential. NaV1.5 activates in response to depolarization, passes a transient inward sodium current, and then inactivates within milliseconds. Mutants in NaV1.5 that decrease the peak sodium transient cause Brugada syndrome and those that increase the fraction of channels that fail to inactivate cause long QT syndrome type 3 (LQT3). Some mutants both decrease the peak current and increase the non-inactivating current, leading to an overlapping phenotype of Brugada syndrome and LQT3. Of these mutants, E1784K in the proximal C-terminus is the most prevalent. The E1784K mutant alters channel opening, fast inactivation, and slow inactivation, but the exact mechanism by which it does so is unknown. Nor is it known why patients may experience normal heart function for many years before appearance of an arrhythmia. In these studies, the cut-open voltage-clamp technique is used to record NaV1.5 currents and voltage-sensor fluorescence from residue 1784 mutants expressed in Xenopus laevis oocytes. Experiments are conducted with extracellular pH between 7.4 and 4.0. Based on these data, a novel model of the voltage-gated sodium channel is constructed. The following data show that: (1) the E1784K mutant-dependent loss-of-function and gain-of-function effects are preferentially exacerbated by decreases in extracellular pH; (2) the E1784K mutant disrupts channel fast inactivation; (3) the mutant-dependent effects on channel conductance and the preferential effects of decreasing extracellular pH are due to altered channel fast inactivation; (4) non-inactivating sodium current is conferred by a positive charge at residue 1784. These data provide mechanistic insight into how a single mutant may cause multiple disease phenotypes, paving the way for future therapeutic research.