Skeletal Muscle Calcium Channel Mutation R528G: Enhanced Channel Inactivation And Omega-current At Hyperpolarization Contribute To Hypokalemic Periodic Paralysis.

Abstract

Autosomal dominant inherited hypokalemic periodic paralysis (HypoPP) is caused by S4 voltage sensor mutations in skeletal muscle Ca V 1.1 calcium or Na V 1.4 sodium channels. In the present study, a small German family with the known Ca V 1.1-R528G is described. The phenotype consists of short and infrequent episodes of limb weakness with ictal respiratory and cardiac involvement. There is incomplete penetrance in women, and acetazolamide is beneficial in two patients also taking daily potassium. Expression of the mutation in the GLT mouse muscle cell line revealed accelerated kinetics of inactivation by twofold, a left-shift of the steady-state inactivation curve by 13mV and a reduced recovery from fast inactivation by up to 39%. These changes suggest a stabilization of the inactivated state. Additional significant slowing of activation may support a second open state with differing ion selectivity or decreased activation of calcium-activated potassium channels and thereby contribute to weakness similar to other Ca V 1.1 mutations. Also, as documented for other HypoPP mutants, we found a hyperpolarization-induced inward guanidinium current of 22nS/nF which can be interpreted as an omega current along the voltage sensor gating pore that leads to a gain-of-function at potentials near the resting membrane potential. This finding can explain the long-lasting depolarizations that are known to lead to paralysis. The omega current is large enough so that a relatively mild hypokalemic trigger of 2.4mM already produces episodes of weakness in vivo.

Stac3 enhances expression of human CaV1.1 in Xenopus oocytes and reveals gating pore currents in HypoPP mutant channels

“…A hyperpolarized shift in the voltage dependence of activation, as detected in our oocyte system ( Fig. 2 D ), has also been reported in oocytes without Stac3 ( Morrill and Cannon, 1999 ), R528G in GLT cells ( Bednarz et al, 2016 ), and human R528H/WT myotubes ( Jurkat-Rott et al, 1998 ). A hyperpolarized shift in the voltage dependence of charge displacement was also detected ( Fig.…”

“…In general, the reduced G max is paralleled by a reduction in Q max , which implies the defect is with the expression at the membrane rather than with the impaired coupling of charge movement to the channel opening. Fewer data are available for R528G, which is a much rarer HypoPP mutation, but a reduced G max was observed in our study and for rabbit Ca V 1.1-R528G expressed in GLT cells ( Bednarz et al, 2016 ). A hyperpolarized shift in the voltage dependence of activation, as detected in our oocyte system ( Fig.…”

“…For example, R1C in Shaker and R2H/G in domain II of Na V 1.4 readily conduct guanidinium. An inwardly rectifying gating pore current, while in a solution of 80 mM TEA plus 38 mM guanidinium, was also reported for the HypoPP mutation Ca V 1.1-R528G expressed in GLT cells ( Bednarz et al, 2016 ). In our oocyte expression system, an inwardly rectifying guanidinium current was observed for hCa V 1.1-R528G, but not for hCa V 1.1-R528H or WT hCa V 1.1 ( Fig.…”

Evaluation of mutant muscle Ca2+ channel properties using two different expression systems

“…As mentioned above, in contrast to voltage-gated K + and Na + channels, the search for a gating pore current in HypoPP1 Cav1.1 has been considerably restrained by the difficulty in expressing skeletal muscle Ca 2+ channels in nonmuscle cells. An elevated leak current at hyperpolarized potentials has nevertheless been found in muscle fibers from HypoPP1 patients, in muscle fibers from a transgenic mouse expressing a HypoPP1 mutant Cav1.1, and in a dysgenic muscle cell line transfected with a HypoPP1 mutant Cav1.1 (Jurkat-Rott et al, 2009; Wu et al, 2012; Bednarz et al, 2016). Recently, three studies have implemented two different strategies to explore a gating pore current in HypoPP1-mutated Cav1.1.…”