Gating Pore Currents in DIIS4 Mutations of NaV1.4 Associated with Periodic Paralysis: Saturation of Ion Flux and Implications for Disease Pathogenesis

Abstract

S4 voltage–sensor mutations in CaV1.1 and NaV1.4 channels cause the human muscle disorder hypokalemic periodic paralysis (HypoPP). The mechanism whereby these mutations predispose affected sarcolemma to attacks of sustained depolarization and loss of excitability is poorly understood. Recently, three HypoPP mutations in the domain II S4 segment of NaV1.4 were shown to create accessory ionic permeation pathways, presumably extending through the aqueous gating pore in which the S4 segment resides. However, there are several disparities between reported gating pore currents from different investigators, including differences in ionic selectivity and estimates of current amplitude, which in turn have important implications for the pathological relevance of these aberrant currents. To clarify the features of gating pore currents arising from different DIIS4 mutants, we recorded gating pore currents created by HypoPP missense mutations at position R666 in the rat isoform of Nav1.4 (the second arginine from the outside, at R672 in human NaV1.4). Extensive measurements were made for the index mutation, R666G, which created a gating pore that was permeable to K+ and Na+. This current had a markedly shallow slope conductance at hyperpolarized voltages and robust inward rectification, even when the ionic gradient strongly favored outward ionic flow. These characteristics were accounted for by a barrier model incorporating a voltage-gated permeation pathway with a single cation binding site oriented near the external surface of the electrical field. The amplitude of the R666G gating pore current was similar to the amplitude of a previously described proton-selective current flowing through the gating pore in rNaV1.4-R663H mutant channels. Currents with similar amplitude and cation selectivity were also observed in R666S and R666C mutant channels, while a proton-selective current was observed in R666H mutant channels. These results add support to the notion that HypoPP mutations share a common biophysical profile comprised of a low-amplitude inward current at the resting potential that may contribute to the pathological depolarization during attacks of weakness.

Functionality of the voltage-gated proton channel truncated in S4

“…Such ion conduction is called an "omegacurrent" (9) or "gating pore current" (10). Similar conductances through the VSD are elicited in a mutated form of human voltagegated sodium channels (SCN4a) found in patients with periodic paralysis (11,12) and in flatworm potassium channels (13).…”

A calcium channel mutant mouse model of hypokalemic periodic paralysis

“…Missense mutations of the S4 segment may produce a leak or gating pore current that allows ions to pass through this aberrant conduction pathway (18,19). In frog oocytes, where high levels of membrane expression for Na V 1.4 can be achieved, small gating pore currents have been detected for all 6 Na V 1.4 HypoPP mutations studied to date (16,17,20,21). We also detected a gating pore current in voltage-clamped muscle fibers from a mouse model of HypoPP with a knock-in Na V 1.4 R669H mutation (22).…”

Domain III S4 in closed-state fast inactivation: Insights from a periodic paralysis mutation

“…1,2 In sodium channels, charge-reducing mutations of the outer S4 arginines R1 or R2 in domains I to III produce an inwardly rectifying proton or cation current observed with hyperpolarization of membrane potential as experienced with a drop in serum potassium. [3][4][5][6] Present models of hypokalemic periodic paralysis incorporate this leak or "omega" current as a contributing factor in the pathogenesis of the disorder (for a review see refs). 6,7 Voltage sensor mutations at R3 that produce a depolarization-activated omega current were first identified for normokalemic periodic paralysis mutations in domain II of the sodium channel Na V 1.4 8 and domain IV of the calcium channel Ca V 1.1.…”