Elevated resting H⁺ current in the R1239H type 1 hypokalaemic periodic paralysis mutated Ca²⁺ channel

Abstract

Missense mutations in the gene encoding the α1 subunit of the skeletal muscle voltage-gated Ca channel induce type 1 hypokalaemic periodic paralysis, a poorly understood neuromuscular disease characterized by episodic attacks of paralysis associated with low serum K . The present study aimed at identifying the changes in muscle fibre electrical properties induced by acute expression of the R1239H hypokalaemic periodic paralysis human mutant α1 subunit of Ca channels in a mature muscle environment to better understand the pathophysiological mechanisms involved in this disorder. We transferred genes encoding wild-type and R1239H mutant human Ca channels into hindlimb mouse muscle by electroporation and combined voltage-clamp and intracellular pH measurements on enzymatically dissociated single muscle fibres. As compared to fibres expressing wild-type α1 subunits, R1239H mutant-expressing fibres displayed Ca currents of reduced amplitude and a higher resting leak inward current that was increased by external acidification. External acidification also produced intracellular acidification at a higher rate in R1239H fibres and inhibited inward rectifier K currents. These data indicate that the R1239H mutation induces an elevated leak H current at rest flowing through a gating pore created by the mutation and that external acidification favours onset of muscle paralysis by potentiating H depolarizing currents and inhibiting resting inward rectifier K currents. Our results could thus explain why paralytic attacks preferentially occur during the recovery period following intense muscle exercise.

When muscle Ca2+ channels carry monovalent cations through gating pores: insights into the pathophysiology of type 1 hypokalaemic periodic paralysis

“…Proton permeation through the extra leak conductance in R1239H fibres was confirmed using intracellular pH measurements, with the pH indicator BCECF showing that the influx of protons induced by external acidification, reflected by the rate of intracellular acidification, was larger in R1239H fibres (Fuster et al . ; Fig. ).…”

“…As described in order model systems for the R1239H or for the R528H mutation, voltage clamp experiments revealed a mild reduction of L‐type voltage‐gated Ca 2+ conductance and a negative shift of the voltage dependence in R1239H compared to WT fibres (Fuster et al . ). The properties of the voltage‐gated Ca 2+ channel were found not to be changed in fibres transfected with the HypoPP1 V876E mutant‐GFP‐tagged Cav1.1 α1 subunits, demonstrating and confirming that the HypoPP1 pathogenesis is not related to any change in the Ca 2+ channel function of Cav1.1 (Fuster et al .…”

“…In transfected mouse muscle fibres, the leak conductance was found to be larger in R1239H fibres for voltages more negative than −40 mV, suggesting that the extra leak conductance in these fibres displayed an inward rectification and abrogated voltages less negative than −40 mV (Fuster et al . ). Interestingly, inward rectification is an intrinsic property of gating pores induced by the neutralization of outer arginines in S4 segments of voltage‐gated K + and Na + channels.…”

“…The gating pore current was found to be carried by H + and Na + for the R1239H and the V876E HypoPP1 mutations, respectively (Fuster et al . , b).…”

“…In order to complement these data and circumvent possible adaptive compensatory mechanisms and/or ultrastructural abnormalities that may develop in these model systems, our group acutely expressed in adult mouse hindlimb muscles wild‐type (WT), HypoPP1 R1239H and V876E mutant‐GFP‐tagged Cav1.1 α1 subunits (Fuster et al . , b). Striated patterns of expression of WT, R1239H and V876E subunits gave evidence for their proper localization in the t‐tubule membrane.…”