Mechanisms of a Human Skeletal Myotonia Produced by Mutation in the C-Terminus of Na _V1.4: Is Ca ²⁺ Regulation Defective?

Abstract

Mutations in the cytoplasmic tail (CT) of voltage gated sodium channels cause a spectrum of inherited diseases of cellular excitability, yet to date only one mutation in the CT of the human skeletal muscle voltage gated sodium channel (hNa _V1.4 _F1705I) has been linked to cold aggravated myotonia. The functional effects of altered regulation of hNa _V1.4 _F1705I are incompletely understood. The location of the hNa _V1.4 _F1705I in the CT prompted us to examine the role of Ca ²⁺ and calmodulin (CaM) regulation in the manifestations of myotonia. To study Na channel related mechanisms of myotonia we exploited the differences in rat and human Na _V1.4 channel regulation by Ca ²⁺ and CaM. hNa _V1.4 _F1705I inactivation gating is Ca ²⁺-sensitive compared to wild type hNa _V1.4 which is Ca ²⁺ insensitive and the mutant channel exhibits a depolarizing shift of the V _1/2 of inactivation with CaM over expression. In contrast the same mutation in the rNa _V1.4 channel background (rNa _V1.4 _F1698I) eliminates Ca ²⁺ sensitivity of gating without affecting the CaM over expression induced hyperpolarizing shift in steady-state inactivation. The differences in the Ca ²⁺ sensitivity of gating between wild type and mutant human and rat Na _V1.4 channels are in part mediated by a divergence in the amino acid sequence in the EF hand like (EFL) region of the CT. Thus the composition of the EFL region contributes to the species differences in Ca ²⁺/CaM regulation of the mutant channels that produce myotonia. The myotonia mutation F1705I slows I _Na decay in a Ca ²⁺-sensitive fashion. The combination of the altered voltage dependence and kinetics of I _Na decay contribute to the myotonic phenotype and may involve the Ca ²⁺-sensing apparatus in the CT of Na _V1.4.