A C-terminal skeletal muscle sodium channel mutation associated with myotonia disrupts fast inactivation

Gene panel analysis of 119 index patients with suspected periodic paralysis in Japan

Introduction

Genetic factors are recognized as the major reason for patients with periodic paralysis. The goal of this study was to determine the genetic causes of periodic paralysis in Japan.

Methods

We obtained a Japanese nationwide case series of 119 index patients (108 men and 11 women) clinically suspected of periodic paralysis, and a gene panel analysis, targeting CACNA1S, SCN4A, and KCNJ2 genes, was conducted.

Results

From 34 cases, 25 pathogenic/likely pathogenic/unknown significance variants were detected in CACNA1S (nine cases), SCN4A (19 cases), or KCNJ2 (six cases), generating a molecular diagnostic rate of 28.6%. In total, seven variants have yet been found linked to periodic paralysis previously. The diagnostic yield of patients with hypokalemic and hyperkalemic periodic paralyzes was 26.2 (17/65) and 32.7% (17/52), respectively. A considerably higher yield was procured from patients with than without positive family history (18/25 vs. 16/94), onset age ≤20 years (24/57 vs. 9/59), or recurrent paralytic attacks (31/94 vs. 3/25).

Discussion

The low molecular diagnostic rate and specific genetic proportion of the present study highlight the etiological complexity of patients with periodic paralysis in Japan.

Periodic paralysis due to cumulative effects of rare variants in SCN4A with small functional alterations

INTRODUCTION/AIMS

Mutations in the SCN4A gene encoding a voltage-gated sodium channel (Nav1.4) cause hyperkalemic periodic paralysis (HyperPP) and hypokalemic periodic paralysis (HypoPP). Typically, both HyperPP and HypoPP are considered as monogenic disorders caused by a missense mutation with a large functional effect. However, a few cases with atypical periodic paralysis phenotype have been caused by multiple mutations in ion-channel genes expressed in skeletal muscles. In this study we investigated the underlying pathogenic mechanisms in such cases.

METHODS

We clinically assessed two families: proband 1 with HyperPP and proband 2 with atypical periodic paralysis with hypokalemia. Genetic analyses were performed by next-generation sequencing and conventional Sanger sequencing, followed by electrophysiological analyses of the mutant Nav1.4 channels expressed in human embryonic kidney 293T (HEK293T) cells using the whole-cell patch-clamp technique.

RESULTS

In proband 1, K880del was identified in the SCN4A gene. In proband 2, K880del and a novel mutation, R1639H, were identified in the same allele of the SCN4A gene. Functional analyses revealed that the K880del in SCN4A has a weak functional effect on hNav1.4, increasing the excitability of the sarcolemma, which could represent a potential pathogenic factor. Although R1639H alone did not reveal functional changes strong enough to be pathogenic, Nav1.4 with both K880del and R1639H showed enhanced activation compared with K880del alone, indicating that R1639H may modify the hNav1.4 channel function.

DISCUSSION

A cumulative effect of variants with small functional alterations may be considered as the underpinning oligogenic pathogenic mechanisms for the unusual phenotype of periodic paralysis.

Ca2+-dependent modulation of voltage-gated myocyte sodium channels

Voltage-dependent Na⁺ channel activation underlies action potential generation fundamental to cellular excitability. In skeletal and cardiac muscle this triggers contraction via ryanodine-receptor (RyR)-mediated sarcoplasmic reticular (SR) Ca²⁺ release. We here review potential feedback actions of intracellular [Ca²⁺] ([Ca²⁺]_i) on Na⁺ channel activity, surveying their structural, genetic and cellular and functional implications, translating these to their possible clinical importance. In addition to phosphorylation sites, both Nav1.4 and Nav1.5 possess potentially regulatory binding sites for Ca²⁺ and/or the Ca^2+-sensor calmodulin in their inactivating III–IV linker and C-terminal domains (CTD), where mutations are associated with a range of skeletal and cardiac muscle diseases. We summarize in vitro cell-attached patch clamp studies reporting correspondingly diverse, direct and indirect, Ca²⁺ effects upon maximal Nav1.4 and Nav1.5 currents (I_max) and their half-maximal voltages (V_1/2) characterizing channel gating, in cellular expression systems and isolated myocytes. Interventions increasing cytoplasmic [Ca²⁺]_i down-regulated I_max leaving V_1/2 constant in native loose patch clamped, wild-type murine skeletal and cardiac myocytes. They correspondingly reduced action potential upstroke rates and conduction velocities, causing pro-arrhythmic effects in intact perfused hearts. Genetically modified murine RyR2-P2328S hearts modelling catecholaminergic polymorphic ventricular tachycardia (CPVT), recapitulated clinical ventricular and atrial pro-arrhythmic phenotypes following catecholaminergic challenge. These accompanied reductions in action potential conduction velocities. The latter were reversed by flecainide at RyR-blocking concentrations specifically in RyR2-P2328S as opposed to wild-type hearts, suggesting a basis for its recent therapeutic application in CPVT. We finally explore the relevance of these mechanisms in further genetic paradigms for commoner metabolic and structural cardiac disease.

Targeted Therapies for Skeletal Muscle Ion Channelopathies: Systematic Review and Steps Towards Precision Medicine

BACKGROUND

Skeletal muscle ion channelopathies include non-dystrophic myotonias (NDM), periodic paralyses (PP), congenital myasthenic syndrome, and recently identified congenital myopathies. The treatment of these diseases is mainly symptomatic, aimed at reducing muscle excitability in NDM or modifying triggers of attacks in PP.

OBJECTIVE

This systematic review collected the evidences regarding effects of pharmacological treatment on muscle ion channelopathies, focusing on the possible link between treatments and genetic background.

METHODS

We searched databases for randomized clinical trials (RCT) and other human studies reporting pharmacological treatments. Preclinical studies were considered to gain further information regarding mutation-dependent drug effects. All steps were performed by two independent investigators, while two others critically reviewed the entire process.

RESULTS

For NMD, RCT showed therapeutic benefits of mexiletine and lamotrigine, while other human studies suggest some efficacy of various sodium channel blockers and of the carbonic anhydrase inhibitor (CAI) acetazolamide. Preclinical studies suggest that mutations may alter sensitivity of the channel to sodium channel blockers in vitro, which has been translated to humans in some cases. For hyperkalemic and hypokalemic PP, RCT showed efficacy of the CAI dichlorphenamide in preventing paralysis. However, hypokalemic PP patients carrying sodium channel mutations may have fewer benefits from CAI compared to those carrying calcium channel mutations. Few data are available for treatment of congenital myopathies.

CONCLUSIONS

These studies provided limited information about the response to treatments of individual mutations or groups of mutations. A major effort is needed to perform human studies for designing a mutation-driven precision medicine in muscle ion channelopathies.

Structural basis of cytoplasmic NaV1.5 and NaV1.4 regulation

Voltage-gated sodium channels (NaVs) are membrane proteins responsible for the rapid upstroke of the action potential in excitable cells. There are nine human voltage-sensitive NaV1 isoforms that, in addition to their sequence differences, differ in tissue distribution and specific function. This review focuses on isoforms NaV1.4 and NaV1.5, which are primarily expressed in skeletal and cardiac muscle cells, respectively. The determination of the structures of several eukaryotic NaVs by single-particle cryo-electron microscopy (cryo-EM) has brought new perspective to the study of the channels. Alignment of the cryo-EM structure of the transmembrane channel pore with x-ray crystallographic structures of the cytoplasmic domains illustrates the complementary nature of the techniques and highlights the intricate cellular mechanisms that modulate these channels. Here, we review structural insights into the cytoplasmic C-terminal regulation of NaV1.4 and NaV1.5 with special attention to Ca2+ sensing by calmodulin, implications for disease, and putative channel dimerization.

EF hand-like motif mutations of Nav1.4 C-terminus cause myotonic syndrome by impairing fast inactivation

INTRODUCTION

Mutations of the voltage-gated sodium channel gene (SCN4A), which encodes Nav1.4, cause nondystrophic myotonia that occasionally is associated with severe apnea and laryngospasm. There are case reports of nondystrophic myotonia due to mutations in the C-terminal tail (CTerm) of Nav1.4, but the functional analysis is scarce.

METHODS

We present two families with nondystrophic myotonia harboring a novel heterozygous mutation (E1702del) and a known heterozygous mutation (E1702K).

RESULTS

The proband with E1702K exhibited repeated rhabdomyolysis, and the daughter showed laryngospasm and cyanosis. Functional analysis of the two mutations as well as another known heterozygous mutation (T1700_E1703del), all located on EF hand-like motif in CTerm, was conducted with whole-cell recording of heterologously expressed channel. All mutations displayed impaired fast inactivation.

DISCUSSION

The CTerm of Nav1.4 is vital for regulating fast inactivation. The study highlights the importance of accumulating pathological mutations of Nav1.4 and their functional analysis data.

Dysfunction of NaV1.4, a skeletal muscle voltage-gated sodium channel, in sudden infant death syndrome: a case-control study

BACKGROUND

Sudden infant death syndrome (SIDS) is the leading cause of post-neonatal infant death in high-income countries. Central respiratory system dysfunction seems to contribute to these deaths. Excitation that drives contraction of skeletal respiratory muscles is controlled by the sodium channel NaV1.4, which is encoded by the gene SCN4A. Variants in NaV1.4 that directly alter skeletal muscle excitability can cause myotonia, periodic paralysis, congenital myopathy, and myasthenic syndrome. SCN4A variants have also been found in infants with life-threatening apnoea and laryngospasm. We therefore hypothesised that rare, functionally disruptive SCN4A variants might be over-represented in infants who died from SIDS.

METHODS

We did a case-control study, including two consecutive cohorts that included 278 SIDS cases of European ancestry and 729 ethnically matched controls without a history of cardiovascular, respiratory, or neurological disease. We compared the frequency of rare variants in SCN4A between groups (minor allele frequency <0·00005 in the Exome Aggregation Consortium). We assessed biophysical characterisation of the variant channels using a heterologous expression system.

FINDINGS

Four (1·4%) of the 278 infants in the SIDS cohort had a rare functionally disruptive SCN4A variant compared with none (0%) of 729 ethnically matched controls (p=0·0057).

INTERPRETATION

Rare SCN4A variants that directly alter NaV1.4 function occur in infants who had died from SIDS. These variants are predicted to significantly alter muscle membrane excitability and compromise respiratory and laryngeal function. These findings indicate that dysfunction of muscle sodium channels is a potentially modifiable risk factor in a subset of infant sudden deaths.

FUNDING

UK Medical Research Council, the Wellcome Trust, National Institute for Health Research, the British Heart Foundation, Biotronik, Cardiac Risk in the Young, Higher Education Funding Council for England, Dravet Syndrome UK, the Epilepsy Society, the Eunice Kennedy Shriver National Institute of Child Health & Human Development of the National Institutes of Health, and the Mayo Clinic Windland Smith Rice Comprehensive Sudden Cardiac Death Program.

Sodium Channelopathies of Skeletal Muscle

The Na_V1.4 sodium channel is highly expressed in skeletal muscle, where it carries almost all of the inward Na⁺ current that generates the action potential, but is not present at significant levels in other tissues. Consequently, mutations of SCN4A encoding Na_V1.4 produce pure skeletal muscle phenotypes that now include six allelic disorders: sodium channel myotonia, paramyotonia congenita, hyperkalemic periodic paralysis, hypokalemic periodic paralysis, congenital myasthenia, and congenital myopathy with hypotonia. Mutation-specific alternations of Na_V1.4 function explain the mechanistic basis for the diverse phenotypes and identify opportunities for strategic intervention to modify the burden of disease.

Towards a Unified Theory of Calmodulin Regulation (Calmodulation) of Voltage-Gated Calcium and Sodium Channels

Voltage-gated Na and Ca(2+) channels represent two major ion channel families that enable myriad biological functions including the generation of action potentials and the coupling of electrical and chemical signaling in cells. Calmodulin regulation (calmodulation) of these ion channels comprises a vital feedback mechanism with distinct physiological implications. Though long-sought, a shared understanding of the channel families remained elusive for two decades as the functional manifestations and the structural underpinnings of this modulation often appeared to diverge. Here, we review recent advancements in the understanding of calmodulation of Ca(2+) and Na channels that suggest a remarkable similarity in their regulatory scheme. This interrelation between the two channel families now paves the way towards a unified mechanistic framework to understand vital calmodulin-dependent feedback and offers shared principles to approach related channelopathic diseases. An exciting era of synergistic study now looms.

Channelopathies of skeletal muscle excitability

Familial disorders of skeletal muscle excitability were initially described early in the last century and are now known to be caused by mutations of voltage-gated ion channels. The clinical manifestations are often striking, with an inability to relax after voluntary contraction (myotonia) or transient attacks of severe weakness (periodic paralysis). An essential feature of these disorders is fluctuation of symptoms that are strongly impacted by environmental triggers such as exercise, temperature, or serum K(+) levels. These phenomena have intrigued physiologists for decades, and in the past 25 years the molecular lesions underlying these disorders have been identified and mechanistic studies are providing insights for therapeutic strategies of disease modification. These familial disorders of muscle fiber excitability are "channelopathies" caused by mutations of a chloride channel (ClC-1), sodium channel (NaV1.4), calcium channel (CaV1.1), and several potassium channels (Kir2.1, Kir2.6, and Kir3.4). This review provides a synthesis of the mechanistic connections between functional defects of mutant ion channels, their impact on muscle excitability, how these changes cause clinical phenotypes, and approaches toward therapeutics.

Calmodulin and Ca(2+) control of voltage gated Na(+) channels

The structures of the cytosolic portion of voltage activated sodium channels (CTNav) in complexes with calmodulin and other effectors in the presence and the absence of calcium provide information about the mechanisms by which these effectors regulate channel activity. The most studied of these complexes, those of Nav1.2 and Nav1.5, show details of the conformations and the specific contacts that are involved in channel regulation. Another voltage activated sodium channel, Nav1.4, shows significant calcium dependent inactivation, while its homolog Nav1.5 does not. The available structures shed light on the possible localization of the elements responsible for this effect. Mutations in the genes of these 3 Nav channels are associated with several disease conditions: Nav1.2, neurological conditions; Nav1.4, syndromes involving skeletal muscle; and Nav1.5, cardiac arrhythmias. Many of these disease-specific mutations are located at the interfaces involving CTNav and its effectors.

A rendezvous with the queen of ion channels: Three decades of ion channel research by David T Yue and his Calcium Signals Laboratory

David T. Yue was a renowned biophysicist who dedicated his life to the study of Ca(2+) signaling in cells. In the wake of his passing, we are left not only with a feeling of great loss, but with a tremendous and impactful body of work contributed by a remarkable man. David's research spanned the spectrum from atomic structure to organ systems, with a quantitative rigor aimed at understanding the fundamental mechanisms underlying biological function. Along the way he developed new tools and approaches, enabling not only his own research but that of his contemporaries and those who will come after him. While we cannot hope to replicate the eloquence and style we are accustomed to in David's writing, we nonetheless undertake a review of David's chosen field of study with a focus on many of his contributions to the calcium channel field.

Defective fast inactivation recovery of Nav 1.4 in congenital myasthenic syndrome

OBJECTIVE

To describe the unique phenotype and genetic findings in a 57-year-old female with a rare form of congenital myasthenic syndrome (CMS) associated with longstanding muscle fatigability, and to investigate the underlying pathophysiology.

METHODS

We used whole-cell voltage clamping to compare the biophysical parameters of wild-type and Arg1457His-mutant Nav 1.4.

RESULTS

Clinical and neurophysiological evaluation revealed features consistent with CMS. Sequencing of candidate genes indicated no abnormalities. However, analysis of SCN4A, the gene encoding the skeletal muscle sodium channel Nav 1.4, revealed a homozygous mutation predicting an arginine-to-histidine substitution at position 1457 (Arg1457His), which maps to the channel's voltage sensor, specifically D4/S4. Whole-cell patch clamp studies revealed that the mutant required longer hyperpolarization to recover from fast inactivation, which produced a profound use-dependent current attenuation not seen in the wild type. The mutant channel also had a marked hyperpolarizing shift in its voltage dependence of inactivation as well as slowed inactivation kinetics.

INTERPRETATION

We conclude that Arg1457His compromises muscle fiber excitability. The mutant fast-inactivates with significantly less depolarization, and it recovers only after extended hyperpolarization. The resulting enhancement in its use dependence reduces channel availability, which explains the patient's muscle fatigability. Arg1457His offers molecular insight into a rare form of CMS precipitated by sodium channel inactivation defects. Given this channel's involvement in other muscle disorders such as paramyotonia congenita and hyperkalemic periodic paralysis, our study exemplifies how variations within the same gene can give rise to multiple distinct dysfunctions and phenotypes, revealing residues important in basic channel function.

Conservation of Ca2+/calmodulin regulation across Na and Ca2+ channels

Voltage-gated Na and Ca2+ channels comprise distinct ion channel superfamilies, yet the carboxy tails of these channels exhibit high homology, hinting at a long-shared and purposeful module. For different Ca2+ channels, carboxyl-tail interactions with calmodulin do elaborate robust and similar forms of Ca2+ regulation. However, Na channels have only shown subtler Ca2+ modulation that differs among reports, challenging attempts at unified understanding. Here, by rapid Ca2+ photorelease onto Na channels, we reset this view of Na channel regulation. For cardiac-muscle channels (NaV1.5), reported effects from which most mechanistic proposals derive, we observe no Ca2+ modulation. Conversely, for skeletal-muscle channels (NaV1.4), we uncover fast Ca2+ regulation eerily similar to that of Ca2+ channels. Channelopathic myotonia mutations halve NaV1.4 Ca2+ regulation, and transplanting the NaV1.4 carboxy tail onto Ca2+ channels recapitulates Ca2+ regulation. Thus, we argue for the persistence and physiological relevance of an ancient Ca2+ regulatory module across Na and Ca2+ channels.

Mechanisms of a human skeletal myotonia produced by mutation in the C-terminus of NaV1.4: is Ca2+ regulation defective?

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 Ca²⁺ 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 Ca²⁺ and CaM. hNa_V1.4_F1705I inactivation gating is Ca²⁺-sensitive compared to wild type hNa_V1.4 which is Ca²⁺ 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 Ca²⁺ sensitivity of gating without affecting the CaM over expression induced hyperpolarizing shift in steady-state inactivation. The differences in the Ca²⁺ 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 Ca²⁺/CaM regulation of the mutant channels that produce myotonia. The myotonia mutation F1705I slows I_Na decay in a Ca²⁺-sensitive fashion. The combination of the altered voltage dependence and kinetics of I_Na decay contribute to the myotonic phenotype and may involve the Ca²⁺-sensing apparatus in the CT of Na_V1.4.

An algorithm for candidate sequencing in non-dystrophic skeletal muscle channelopathies

Human skeletal muscle channelopathies (HSMCs) are a group of heritable conditions with ion channel-related etiology and similar presentation. To create a comprehensive picture of the phenotypic spectrum for each condition and to devise a strategy that facilitates the differential diagnosis, we collected the genotype and phenotype information from more than 500 previously published HSMC studies. Using these records, we were able to identify clear correlations between particular clinical features and the underlying alteration(s) in the genes SCN4A, CACNA1S, KCNJ2, and CLCN1. This allowed us to develop a clinical, symptom-based, binary decision flow algorithm that predicts the proper genetic origin with high accuracy (0.88-0.93). The algorithm was implemented in a stand-alone online tool ("CGPS"- http://cgps.ddd.co.kr ) to assist with HSCM diagnosis in the clinical practice. The CGPS provides simple, symptom-oriented navigation that guides the user to the most likely molecular basis of the presentation, which permits highly targeted genetic screens and, upon confirmation, tailored pharmacotherapy based on the molecular origin.

Voltage-gated sodium channel-associated proteins and alternative mechanisms of inactivation and block

Voltage-gated sodium channels mediate inward current of action potentials upon membrane depolarization of excitable cells. The initial transient sodium current is restricted to milliseconds through three distinct channel-inactivating and blocking mechanisms. All pore-forming alpha subunits of sodium channels possess structural elements mediating fast inactivation upon depolarization and recovery within milliseconds upon membrane repolarization. Accessory subunits modulate fast inactivation dynamics, but these proteins can also limit current by contributing distinct inactivation and blocking particles. A-type isoforms of fibroblast growth factor homologous factors (FHFs) bear a particle that induces long-term channel inactivation, while sodium channel subunit Navβ4 employs a blocking particle that rapidly dissociates upon membrane repolarization to generate resurgent current. Despite their different physiological functions, the FHF and Navβ4 particles have similarity in amino acid composition and mechanisms for docking within sodium channels. The three competing channel-inactivating and blocking processes functionally interact to regulate a neuron's intrinsic excitability.

The Na conductance in the sarcolemma and the transverse tubular system membranes of mammalian skeletal muscle fibers

Na (and Li) currents and fluorescence transients were recorded simultaneously under voltage-clamp conditions from mouse flexor digitorum brevis fibers stained with the potentiometric dye di-8-ANEPPS to investigate the distribution of Na channels between the surface and transverse tubular system (TTS) membranes. In fibers rendered electrically passive, voltage pulses resulted in step-like fluorescence changes that were used to calibrate the dye response. The effects of Na channel activation on the TTS voltage were investigated using Li, instead of Na, because di-8-ANEPPS transients show anomalies in the presence of the latter. Na and Li inward currents (I(Na), I(Li); using half of the physiological ion concentration) showed very steep voltage dependences, with no reversal for depolarizations beyond the calculated equilibrium potential, suggesting that most of the current originates from a noncontrolled membrane compartment. Maximum peak I(Li) was ∼ 30% smaller than for I(Na), suggesting a Li-blocking effect. I(Li) activation resulted in the appearance of overshoots in otherwise step-like di-8-ANEPPS transients. Overshoots had comparable durations and voltage dependence as those of I(Li). Simultaneously measured maximal overshoot and peak I(Li) were 54 ± 5% and 773 ± 53 µA/cm(2), respectively. Radial cable model simulations predicted the properties of I(Li) and di-8-ANEPPS transients when TTS access resistances of 10-20 Ω cm(2), and TTS-to-surface Na permeability density ratios in the range of 40:60 to 70:30, were used. Formamide-based osmotic shock resulted in incomplete detubulation. However, results from a subpopulation of treated fibers (low capacitance) provide confirmatory evidence that a significant proportion of I(Li), and the overshoot in the optical signals, arises from the TTS in normal fibers. The quantitative evaluation of the distribution of Na channels between the sarcolemma and the TTS membranes, as provided here, is crucial for the understanding of the radial and longitudinal propagation of the action potential, which ultimately govern the mechanical activation of muscle in normal and diseased conditions.

Skeletal muscle na channel disorders

Five inherited human disorders affecting skeletal muscle contraction have been traced to mutations in the gene encoding the voltage-gated sodium channel Na_v1.4. The main symptoms of these disorders are myotonia or periodic paralysis caused by changes in skeletal muscle fiber excitability. Symptoms of these disorders vary from mild or latent disease to incapacitating or even death in severe cases. As new human sodium channel mutations corresponding to disease states become discovered, the importance of understanding the role of the sodium channel in skeletal muscle function and disease state grows.

Mechanisms underlying a life-threatening skeletal muscle Na+ channel disorder

Myotonia is an intrinsic muscular disorder caused by muscle fibre hyperexcitability, which produces a prolonged time for relaxation after voluntary muscle contraction or internal mechanical stimulation. Missense mutations in skeletal muscle genes encoding Cl− or Na+ channels cause non-dystrophic myotonias.Mutations of the SCN4A gene that encodes the skeletal voltage-gated Na+ channel Nav1.4 can produce opposing phenotypes leading to hyperexcitable or inexcitable muscle fibres. Nav1.4 mutations result in different forms of myotonias that can be found in adults. However, the recently reported myotonic manifestations in infants have been shown to be lethal. This was typically the case for children suffering from severe neonatal episodic laryngospasm (SNEL). A novel Nav1.4 channel missense mutation was found in these children that has not yet been analysed. In this study, we characterize the functional consequences of the new A799S Na+ channel mutation that is associated with sodium channel myotonia in newborn babies. We have used mammalian cell expression and patch-clamp techniques to monitor the channel properties.We found that the A799S substitution changes several biophysical properties of the channel by causing a hyperpolarizing shift of the steady-state activation, and slowing the kinetics of fast inactivation and deactivation. In addition, the single channel open probability was dramatically increased, contributing hence to a severe phenotype. We showed that substitutions at position 799 of the Nav1.4 channel favoured the channel open state with sustained activity leading to hyperexcitability of laryngeal muscles that could be lethal during infancy.

Mechanisms underlying insect chill-coma

At their critical thermal minimum (CT(min)) insects enter chill-coma, a reversible state where neuromuscular transmission and movement cease. The physiological mechanisms responsible for the insect CT(min) remain poorly understood despite the regular use of chill-coma onset and recovery as a means to assess evolved or acquired variation in low temperature tolerance. In this review, we summarize the use of chill-coma as a metric of thermal tolerance to date, and synthesise current knowledge on the nature and plasticity of lower thermal limits to present probable physiological mechanisms of cold-induced failure. Chill-coma is likely to be driven by an inability to maintain ionic homeostasis through the effects of temperature on ion-motive ATPases, ion channel gating mechanisms, and/or the lipid membrane, leading to a loss of nerve and muscle excitability.

Open- and closed-state fast inactivation in sodium channels: differential effects of a site-3 anemone toxin

The role of sodium channel closed-state fast inactivation in membrane excitability is not well understood. We compared open- and closed-state fast inactivation, and the gating charge immobilized during these transitions, in skeletal muscle channel hNa(V)1.4. A significant fraction of total charge movement and its immobilization occurred in the absence of channel opening. Simulated action potentials in skeletal muscle fibers were attenuated when pre-conditioned by sub-threshold depolarization. Anthopleurin A, a site-3 toxin that inhibits gating charge associated with the movement of DIVS4, was used to assess the role of this voltage sensor in closed-state fast inactivation. Anthopleurin elicited opposing effects on the gating mode, kinetics and charge immobilized during open- versus closed-state fast inactivation. This same toxin produced identical effects on recovery of channel availability and remobilization of gating charge, irrespective of route of entry into fast inactivation. Our findings suggest that depolarization promoting entry into fast inactivation from open versus closed states provides access to the IFMT receptor via different rate-limiting conformational translocations of DIVS4.

Gating behaviour of sodium currents in adult mouse muscle recorded with an improved two-electrode voltage clamp

Muscle contraction is triggered by the spread of an action potential along the fibre. The ionic current to generate the action potential is conducted through voltage-activated sodium channels, and mutations of these channels are known to cause several human muscle disorders. Mouse models have been created by introducing point mutations into the sodium channel gene. This achievement has created the need for a high-fidelity technique to record sodium currents from intact mouse muscle fibres. We have optimized a two-electrode voltage clamp, using sharp microelectrodes to preserve the myoplasmic contents. The voltage-dependent behaviour of sodium channel activation, inactivation and slow-inactivation were characterized. The voltage range for these gating behaviours was remarkably hyperpolarized, in comparison to studies in artificial expression systems. These results provide normative data for sodium channels natively expressed in mouse muscle and illustrate the need to modify model simulations of muscle excitability to account for the hyperpolarized shift.

Sodium channelopathies of skeletal muscle result from gain or loss of function

Five hereditary sodium channelopathies of skeletal muscle have been identified. Prominent symptoms are either myotonia or weakness caused by an increase or decrease of muscle fiber excitability. The voltage-gated sodium channel NaV1.4, initiator of the muscle action potential, is mutated in all five disorders. Pathogenetically, both loss and gain of function mutations have been described, the latter being the more frequent mechanism and involving not just the ion-conducting pore, but aberrant pores as well. The type of channel malfunction is decisive for therapy which consists either of exerting a direct effect on the sodium channel, i.e., by blocking the pore, or of restoring skeletal muscle membrane potential to reduce the fraction of inactivated channels.

Hereditary channelopathies in neurology

Ion channelopathies are caused by malfunction or altered regulation of ion channel proteins due to hereditary or acquired protein changes. In neurology, main phenotypes include certain forms of epilepsy, ataxia, migraine, neuropathic pain, myotonia, and muscle weakness including myasthenia and periodic paralyses. The total prevalence of monogenic channelopathies in neurology is about 35:100,000. Susceptibility-related mutations further increase the relevance of channel genes in medicine considerably. As many disease mechanisms have been elucidated by functional characterization on the molecular level, the channelopathies are regarded as model disorders for pathogenesis and treatment of non-monogenic forms of epilepsy and migraine. As more than 35% of marketed drugs target ion channels, there is a high chance to identify compounds that counteract the effects of the mutations.

New mutation of the Na channel in the severe form of potassium-aggravated myotonia

Myotonia manifests in several hereditary diseases, including hyperkalemic periodic paralysis (HyperPP), paramyotonia congenita (PMC), and potassium-aggravated myotonia (PAM). These are allelic disorders originating from missense mutations in the gene that codes the skeletal muscle sodium channel, Nav1.4. Moreover, a severe form of PAM has been designated as myotonia permanens. A new mutation of Nav1.4, Q1633E, was identified in a Japanese family presenting with the PAM phenotype. The proband suffered from cyanotic attacks during infancy. The mutated amino acid residue is located on the EF-hand calcium-binding motif in the intracellular C-terminus. A functional analysis of the mutant channel using the voltage-clamp method revealed disruption of fast inactivation, a slower rate of current decay, and a depolarized shift in the voltage dependence of availability. This study has identified a new mutation of PAM with a severe phenotype and emphasizes the importance of the C-terminus for fast inactivation of the sodium channel. Muscle Nerve 39: 666-673, 2009.

Regulation of persistent Na current by interactions between beta subunits of voltage-gated Na channels

The beta subunits of voltage-gated Na channels (Scnxb) regulate the gating of pore-forming alpha subunits, as well as their trafficking and localization. In heterologous expression systems, beta1, beta2, and beta3 subunits influence inactivation and persistent current in different ways. To test how the beta4 protein regulates Na channel gating, we transfected beta4 into HEK (human embryonic kidney) cells stably expressing Na(V)1.1. Unlike a free peptide with a sequence from the beta4 cytoplasmic domain, the full-length beta4 protein did not block open channels. Instead, beta4 expression favored open states by shifting activation curves negative, decreasing the slope of the inactivation curve, and increasing the percentage of noninactivating current. Consequently, persistent current tripled in amplitude. Expression of beta1 or chimeric subunits including the beta1 extracellular domain, however, favored inactivation. Coexpressing Na(V)1.1 and beta4 with beta1 produced tiny persistent currents, indicating that beta1 overcomes the effects of beta4 in heterotrimeric channels. In contrast, beta1(C121W), which contains an extracellular epilepsy-associated mutation, did not counteract the destabilization of inactivation by beta4 and also required unusually large depolarizations for channel opening. In cultured hippocampal neurons transfected with beta4, persistent current was slightly but significantly increased. Moreover, in beta4-expressing neurons from Scn1b and Scn1b/Scn2b null mice, entry into inactivated states was slowed. These data suggest that beta1 and beta4 have antagonistic roles, the former favoring inactivation, and the latter favoring activation. Because increased Na channel availability may facilitate action potential firing, these results suggest a mechanism for seizure susceptibility of both mice and humans with disrupted beta1 subunits.

The dominant cold-sensitive Out-cold mutants of Drosophila melanogaster have novel missense mutations in the voltage-gated sodium channel gene paralytic

Here we report the molecular characterization of Out-cold (Ocd) mutants of Drosophila melanogaster, which produce a dominant, X-linked, cold-sensitive paralytic phenotype. From its initial 1.5-Mb cytological location within 13F1-16A2, P-element and SNP mapping reduced the Ocd critical region to <100 kb and to six candidate genes: hangover, CG9947, CG4420, eIF2a, Rbp2, and paralytic (para). Complementation testing with para null mutations strongly suggests Ocd and para are allelic, as does gene rescue of Ocd semilethality with a wild-type para transgene. Pesticide resistance and electrophysiological phenotypes of Ocd mutants support this conclusion. The para gene encodes a voltage-gated sodium channel. Sequencing the Ocd lines revealed mutations within highly conserved regions of the para coding sequence, in the transmembrane segment S6 of domain III (I1545M and T1551I), and in the linker between domains III and IV (G1571R), the location of the channel inactivation gate. The G1571R mutation is of particular interest as mutations of the orthologous residue (G1306) in the human skeletal muscle sodium channel gene SCN4A are associated with cases of periodic paralysis and myotonia, including the human cold-sensitive disorder paramyotonia congenita. The mechanisms by which sodium channel mutations cause cold sensitivity are not well understood. Therefore, in the absence of suitable vertebrate models, Ocd provides a system in which genetic, molecular, physiological, and behavioral tools can be exploited to determine mechanisms underlying sodium channel periodic paralyses.

Calmodulin regulation of Nav1.4 current: role of binding to the carboxyl terminus

Calmodulin (CaM) regulates steady-state inactivation of sodium currents (Na_V1.4) in skeletal muscle. Defects in Na current inactivation are associated with pathological muscle conditions such as myotonia and paralysis. The mechanisms of CaM modulation of expression and function of the Na channel are incompletely understood. A physical association between CaM and the intact C terminus of Na_V1.4 has not previously been demonstrated. FRET reveals channel conformation-independent association of CaM with the C terminus of Na_V1.4 (CT-Na_V1.4) in mammalian cells. Mutation of the Na_V1.4 CaM-binding IQ motif (Na_V1.4_IQ/AA) reduces cell surface expression of Na_V1.4 channels and eliminates CaM modulation of gating. Truncations of the CT that include the IQ region abolish Na current. Na_V1.4 channels with one CaM fused to the CT by variable length glycine linkers exhibit CaM modulation of gating only with linker lengths that allowed CaM to reach IQ region. Thus one CaM is sufficient to modulate Na current, and CaM acts as an ancillary subunit of Na_V1.4 channels that binds to the CT in a conformation-independent fashion, modulating the voltage dependence of inactivation and facilitating trafficking to the surface membrane.

SCN9A Mutations in Paroxysmal Extreme Pain Disorder: Allelic Variants Underlie Distinct Channel Defects and Phenotypes

Paroxysmal extreme pain disorder (PEPD), previously known as familial rectal pain (FRP, or OMIM 167400), is an inherited condition characterized by paroxysms of rectal, ocular, or submandibular pain with flushing. A genome-wide linkage search followed by mutational analysis of the candidate gene SCN9A, which encodes hNa(v)1.7, identified eight missense mutations in 11 families and 2 sporadic cases. Functional analysis in vitro of three of these mutant Na(v)1.7 channels revealed a reduction in fast inactivation, leading to persistent sodium current. Other mutations in SCN9A associated with more negative activation thresholds are known to cause primary erythermalgia (PE). Carbamazepine, a drug that is effective in PEPD, but not PE, showed selective block of persistent current associated with PEPD mutants, but did not affect the negative activation threshold of a PE mutant. PEPD and PE are allelic variants with distinct underlying biophysical mechanisms and represent a separate class of peripheral neuronal sodium channelopathy.