A Na+ Channel Mutation Linked to Hypokalemic Periodic Paralysis Exposes a Proton-selective Gating Pore

Abstract

The heritable muscle disorder hypokalemic periodic paralysis (HypoPP) is characterized by attacks of flaccid weakness, brought on by sustained sarcolemmal depolarization. HypoPP is genetically linked to missense mutations at charged residues in the S4 voltage-sensing segments of either CaV1.1 (the skeletal muscle L-type Ca2+ channel) or NaV1.4 (the skeletal muscle voltage-gated Na+ channel). Although these mutations alter the gating of both channels, these functional defects have proven insufficient to explain the sarcolemmal depolarization in affected muscle. Recent insight into the topology of the S4 voltage-sensing domain has aroused interest in an alternative pathomechanism, wherein HypoPP mutations might generate an aberrant ionic leak conductance by unblocking the putative aqueous crevice (“gating-pore”) in which the S4 segment resides. We tested the rat isoform of NaV1.4 harboring the HypoPP mutation R663H (human R669H ortholog) at the outermost arginine of S4 in domain II for a gating-pore conductance. We found that the mutation R663H permits transmembrane permeation of protons, but not larger cations, similar to the conductance displayed by histidine substitution at Shaker K+ channel S4 sites. These results are consistent with the notion that the outermost charged residue in the DIIS4 segment is simultaneously accessible to the cytoplasmic and extracellular spaces when the voltage sensor is positioned inwardly. The predicted magnitude of this proton leak in mature skeletal muscle is small relative to the resting K+ and Cl− conductances, and is thus not likely to fully account for the aberrant sarcolemmal depolarization underlying the paralytic attacks. Rather, it is possible that a sustained proton leak may contribute to instability of VREST indirectly, for instance, by interfering with intracellular pH homeostasis.

Voltage sensor charge loss accounts for most cases of hypokalemic periodic paralysis

“…Two recent studies have suggested that a gating pore current may be important in the pathogenesis of HypoPP, 13,14 and our data are fully consistent with this hypothesis. In response to depolarizing voltages, S4 segments undergo a conformational change that moves these segments outwards through a gating pore (omega pore) which leads to the opening of the central pore of the channel (alpha pore).…”

“…However, evidence also exists that at least some of these mutations allow an abnormal cation flux to pass through the aqueous omega pore that lines the voltage sensor. 13,14 The full spectrum of HypoPP mutations has not been defined. In particular, it remains to be determined whether the gating pore cation leak hypothesis accounts for all cases.…”

Gating transitions in the selectivity filter region of a sodium channel are coupled to the domain IV voltage sensor

“…1A). Design of these charge neutralized (CN) Nav1.4 mutants, e.g., domain I (DI-CN) was motivated in part by recent findings that mutation of specific charged residues in the S4 segments allows state-dependent ion flux through the voltage-sensing domains (39)(40)(41)(42). These currents directly report the movement of voltage sensors and are referred to as gating pore currents (also omega currents) to distinguish them from the canonical ionic currents through the central pore.…”

Identification and Functional Characterization of Kir2.6 Mutations Associated with Non-familial Hypokalemic Periodic Paralysis

“…How mutations of cationic amino acids in the voltage sensor cause the paradoxical depolarization of sarcolemma during hypokalemia, however, had been an enigma. Recent studies showing that mutations in the voltage sensor create an aberrant conducting pore ("gating pore") that allows passage of small cations (Na ϩ and H ϩ ) at hyperpolarized resting membrane potentials (15,16) provide a mechanistic explanation for paradoxical depolarization of sarcolemma during hypokalemic attacks.…”