Electrophysiological properties of the hypokalaemic periodic paralysis mutation (R528H) of the skeletal muscle α_1S subunit as expressed in mouse L cells

Abstract

Hypokalaemic periodic paralysis (HypoPP) is an autosomal dominant muscle disease which has been linked to point mutations in the skeletal muscle L-type calcium channel cq subunit (~ls). Here, we have introduced one of the point mutations causing HypoPP (R528H) into cDNA of the rabbit Cqs. Expression of either the wild-type ~tsor the mutant R528H Cqs (~ls-Rszsn) subunits was obtained in mouse Ltk cells using a selectable expression vector. The Cqs-Rs28n subunit led to the expression of functional L-type Ca z+ channels. Corresponding whole-cell Ba 2+ currents exhibit very slow activation and inactivation kinetics, typical for recombinant skeletal Ca 2+ channel currents. Voltage-dependent activation and inactivation properties were similar for ~ts-and ~IS-RS2Sn, as well as their sensitivity to the dihydropyridine agonist Bay K 8644. Differences in Cqs-and ~ls-Rs28n-directed channels reside in the Ba 2+ current density, which was significantly reduced 3.2 fold in cells expressing ~lS-RS2Sn It was concluded that the R528H mutation of Cqs results in minor differences in the electrophysiological properties but significantly reduces the whole-cell Ca z÷ channel current in its amplitude.

A calcium channel mutant mouse model of hypokalemic periodic paralysis

“…The relative decrease in ionic current density exceeded the reduction in expression level, as assessed by gating charge displacement, which suggests a partial uncoupling of voltage sensor motion to channel opening. Similar changes have previously been reported from studies using human R528H myotubes (14) or heterologous expression in oocytes (35) or L cells (15). Since the R528H mutation lies in an S4 voltage sensor, the functional consequence was initially predicted to be a shift in the voltage dependence of gating.…”

“…Curiously, all 8 mutations in Na V 1.4 and 6 of 7 mutations in Ca V 1.1 ( Figure 1A) occur at arginine residues in the fourth transmembrane segment (S4) voltage-sensor domains (10,11). Functional expression studies in heterologous systems initially focused on voltage-dependent gating of HypoPP mutant channels, and while modest disruption of Na V 1.4 inactivation (12,13) and slowed Ca V 1.1 activation with reduced current density were observed (14,15), none of these changes readily explained the susceptibility to paradoxical depolarization of V rest in low K + that is the pathological hallmark of HypoPP fibers.…”

Functional Characterization and Neuronal Modeling of the Effects of Childhood Absence Epilepsy Variants ofCACNA1H, a T-Type Calcium Channel

“…The finding that some SNPs (E282K and V831M) decreased simulated firing seemingly contradicts this hypothesis. A similar discrepancy between channel phenotype and genotype was found in Ca v 1.1 SNPs associated with periodic paralysis (Lapie et al, 1996), Ca v 1.4 SNPs associated with night blindness , and Na v 1 channel SNPs associated with GEFSϩ (Lossin et al, 2003). One possibility is that the T-channel SNPs alter activity in a manner that cannot be detected when Ca v 3.2 is expressed alone in heterologous expression systems.…”

Voltage-sensor sodium channel mutations cause hypokalemic periodic paralysis type 2 by enhanced inactivation and reduced current

“…Three mutations localized in D2͞S4 and D4͞S4 have been described, but their effects on channel function have not been clarified. Changes in activation, inactivation, and secondary effects all have been suggested to relate to disease pathogenesis (9)(10)(11)(12)(13).…”