Limits...
Gating pore currents in DIIS4 mutations of NaV1.4 associated with periodic paralysis: saturation of ion flux and implications for disease pathogenesis.

Struyk AF, Markin VS, Francis D, Cannon SC - J. Gen. Physiol. (2008)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, University of Texas-Southwestern Medical Center, Dallas, TX 75390, USA. arie.struyk@utsouthwestern.edu

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.

Show MeSH

Related in: MedlinePlus

Gating pore currents from other HypoPP mutations at site R666. Steady-state gating pore currents were recorded from R666S, -C, and -H mutant channels and compared with currents recorded in WT channels (denoted at top). Representative current traces, after leak correction and normalization to the corresponding maximal gating charge displacement, are shown for recordings made in bath and internal solutions approximating the normal mammalian physiological cation gradient (A) or in bath and internal solutions containing NMDG (B). Scale bars for all traces are shown in the inset to A. The mean I-V relationships of gating pore currents seen in each mutant under these conditions is shown in C. For each panel, the filled circles represent currents recorded in the physiological cation gradient, whereas open circles represent currents recorded when NMDG was present in both internal and external compartments. Number of samples recorded for each condition is denoted in parenthesis in the inset legend for each figure. Gating pore currents presumably carried by monovalent cations and abolished by NMDG substitution are seen in R666S and R666C mutant channels. In contrast, inward gating pore currents of similar magnitude are seen in R666H channels in both ionic conditions, consistent with the notion that the charge carriers of this gating pore current are protons rather than larger monovalent cations.
© Copyright Policy
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC2553391&req=5

fig8: Gating pore currents from other HypoPP mutations at site R666. Steady-state gating pore currents were recorded from R666S, -C, and -H mutant channels and compared with currents recorded in WT channels (denoted at top). Representative current traces, after leak correction and normalization to the corresponding maximal gating charge displacement, are shown for recordings made in bath and internal solutions approximating the normal mammalian physiological cation gradient (A) or in bath and internal solutions containing NMDG (B). Scale bars for all traces are shown in the inset to A. The mean I-V relationships of gating pore currents seen in each mutant under these conditions is shown in C. For each panel, the filled circles represent currents recorded in the physiological cation gradient, whereas open circles represent currents recorded when NMDG was present in both internal and external compartments. Number of samples recorded for each condition is denoted in parenthesis in the inset legend for each figure. Gating pore currents presumably carried by monovalent cations and abolished by NMDG substitution are seen in R666S and R666C mutant channels. In contrast, inward gating pore currents of similar magnitude are seen in R666H channels in both ionic conditions, consistent with the notion that the charge carriers of this gating pore current are protons rather than larger monovalent cations.

Mentions: One notable deviation is the amplitude inflection (hump) between −90 and −60 mV where the inward current amplitude exceeds the model curve. This hump was prominent in recordings with asymmetrical ionic solutions wherein Na+ was the major extracellular cation, and the predominant internal cation was either K+ or an impermeant species (see also Fig. 3 C and Fig. 8). Similarly, the amplitude of the anomalous tail current transients was larger in Na+-containing bath solutions (for instance, Fig. 8). Both of these observations can be reconciled with a modification to our model wherein a second open state can be elicited in the R666G gating pore at intermediate depolarizations (to account for the hump in steady-state behavior) or during recovery after prolonged depolarizations (to account for the anomalous tails). In this scenario, both the permeability and ionic selectivity may differ from the primary open state (e.g., higher Na+ permeability). This possibility is discussed below.


Gating pore currents in DIIS4 mutations of NaV1.4 associated with periodic paralysis: saturation of ion flux and implications for disease pathogenesis.

Struyk AF, Markin VS, Francis D, Cannon SC - J. Gen. Physiol. (2008)

Gating pore currents from other HypoPP mutations at site R666. Steady-state gating pore currents were recorded from R666S, -C, and -H mutant channels and compared with currents recorded in WT channels (denoted at top). Representative current traces, after leak correction and normalization to the corresponding maximal gating charge displacement, are shown for recordings made in bath and internal solutions approximating the normal mammalian physiological cation gradient (A) or in bath and internal solutions containing NMDG (B). Scale bars for all traces are shown in the inset to A. The mean I-V relationships of gating pore currents seen in each mutant under these conditions is shown in C. For each panel, the filled circles represent currents recorded in the physiological cation gradient, whereas open circles represent currents recorded when NMDG was present in both internal and external compartments. Number of samples recorded for each condition is denoted in parenthesis in the inset legend for each figure. Gating pore currents presumably carried by monovalent cations and abolished by NMDG substitution are seen in R666S and R666C mutant channels. In contrast, inward gating pore currents of similar magnitude are seen in R666H channels in both ionic conditions, consistent with the notion that the charge carriers of this gating pore current are protons rather than larger monovalent cations.
© Copyright Policy
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2553391&req=5

fig8: Gating pore currents from other HypoPP mutations at site R666. Steady-state gating pore currents were recorded from R666S, -C, and -H mutant channels and compared with currents recorded in WT channels (denoted at top). Representative current traces, after leak correction and normalization to the corresponding maximal gating charge displacement, are shown for recordings made in bath and internal solutions approximating the normal mammalian physiological cation gradient (A) or in bath and internal solutions containing NMDG (B). Scale bars for all traces are shown in the inset to A. The mean I-V relationships of gating pore currents seen in each mutant under these conditions is shown in C. For each panel, the filled circles represent currents recorded in the physiological cation gradient, whereas open circles represent currents recorded when NMDG was present in both internal and external compartments. Number of samples recorded for each condition is denoted in parenthesis in the inset legend for each figure. Gating pore currents presumably carried by monovalent cations and abolished by NMDG substitution are seen in R666S and R666C mutant channels. In contrast, inward gating pore currents of similar magnitude are seen in R666H channels in both ionic conditions, consistent with the notion that the charge carriers of this gating pore current are protons rather than larger monovalent cations.
Mentions: One notable deviation is the amplitude inflection (hump) between −90 and −60 mV where the inward current amplitude exceeds the model curve. This hump was prominent in recordings with asymmetrical ionic solutions wherein Na+ was the major extracellular cation, and the predominant internal cation was either K+ or an impermeant species (see also Fig. 3 C and Fig. 8). Similarly, the amplitude of the anomalous tail current transients was larger in Na+-containing bath solutions (for instance, Fig. 8). Both of these observations can be reconciled with a modification to our model wherein a second open state can be elicited in the R666G gating pore at intermediate depolarizations (to account for the hump in steady-state behavior) or during recovery after prolonged depolarizations (to account for the anomalous tails). In this scenario, both the permeability and ionic selectivity may differ from the primary open state (e.g., higher Na+ permeability). This possibility is discussed below.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, University of Texas-Southwestern Medical Center, Dallas, TX 75390, USA. arie.struyk@utsouthwestern.edu

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.

Show MeSH
Related in: MedlinePlus