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Gating pore currents and the resting state of Nav1.4 voltage sensor domains.

ABSTRACT: Mammalian voltage-gated sodium channels are composed of four homologous voltage sensor domains (VSDs; DI, DII, DIII, and DIV) in which their S4 segments contain a variable number of positively charged residues. We used single histidine (H) substitutions of these charged residues in the Na(v)1.4 channel to probe the positions of the S4 segments at hyperpolarized potentials. The substitutions led to the formation of gating pores that were detected as proton leak currents through the VSDs. The leak currents indicated that the mutated residues are accessible from both sides of the membrane. Leak currents of different magnitudes appeared in the DI/R1H, DII/R1H, and DIII/R2H mutants, suggesting that the resting state position of S4 varies depending on the domain. Here, DI/R1H indicates the first arginine R1, in domain DI, has been mutated to histidine. The single R1H, R2H, and R3H mutations in DIV did not produce appreciable proton currents, indicating that the VSDs had different topologies. A structural model of the resting states of the four VSDs of Na(v)1.4 relaxed in their membrane/solution environment using molecular dynamics simulations is proposed based on the recent Na(v)Ab sodium channel X-ray structure. The model shows that the hydrophobic septa that isolate the intracellular and the extracellular media within the DI, DII, and DIII VSDs are ?2 Å long, similar to those of K(v) channels. However, the septum of DIV is longer, which prevents water molecules from hydrating the center of the VSD, thus breaking the proton conduction pathway. This structural model rationalizes the activation sequence of the different VSDs of the Na(v)1.4 channel.

SUBMITTER: Gosselin-Badaroudine P

PROVIDER: S-EPMC3511134 | biostudies-literature | 2012 Nov

REPOSITORIES: biostudies-literature

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Gating pore currents and the resting state of Nav1.4 voltage sensor domains.

Gosselin-Badaroudine Pascal P Delemotte Lucie L Moreau Adrien A Klein Michael L ML Chahine Mohamed M

Proceedings of the National Academy of Sciences of the United States of America 20121107 47

Mammalian voltage-gated sodium channels are composed of four homologous voltage sensor domains (VSDs; DI, DII, DIII, and DIV) in which their S4 segments contain a variable number of positively charged residues. We used single histidine (H) substitutions of these charged residues in the Na(v)1.4 channel to probe the positions of the S4 segments at hyperpolarized potentials. The substitutions led to the formation of gating pores that were detected as proton leak currents through the VSDs. The leak ...[more]

PMID: 23134726

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Project description: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.

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Gating pore currents and the resting state of Nav1.4 voltage sensor domains.

Publications

Gating pore currents and the resting state of Nav1.4 voltage sensor domains.

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