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

Towards Zebrafish Models of CNS Channelopathies

Channelopathies are a large group of systemic disorders whose pathogenesis is associated with dysfunctional ion channels. Aberrant transmembrane transport of K⁺, Na⁺, Ca²⁺ and Cl^- by these channels in the brain induces central nervous system (CNS) channelopathies, most commonly including epilepsy, but also migraine, as well as various movement and psychiatric disorders. Animal models are a useful tool for studying pathogenesis of a wide range of brain disorders, including channelopathies. Complementing multiple well-established rodent models, the zebrafish (Danio rerio) has become a popular translational model organism for neurobiology, psychopharmacology and toxicology research, and for probing mechanisms underlying CNS pathogenesis. Here, we discuss current prospects and challenges of developing genetic, pharmacological and other experimental models of major CNS channelopathies based on zebrafish.

Kinetic Alterations in Resurgent Sodium Currents of Mutant Nav1.4 Channel in Two Patients Affected by Paramyotonia Congenita

Simple Summary

Paramyotonia congenita (PMC) is a rare muscle disorder that causes myotonia and weakness of facial and limb muscles. The electromyography in PMC shows continual spontaneous, high-frequency spike potentials in skeletal muscles. Genetic mutations in the Na_v1.4 channel that cause hyperexcitability of muscle fibers are responsible for PMC. However, the genotype–phenotype relationship is highly diversified, and the molecular pathology remains unclear. Here, we investigated the electrophysiology in the Na_v1.4 channel with mutations, p.V781I and p.A1737T, which were found in two Taiwanese patients. We identified the distinct changes in gating mechanisms altered by mutations which may underlie the clinical phenotype.

Abstract

Paramyotonia congenita (PMC) is a rare skeletal muscle disorder characterized by muscle stiffness upon repetitive exercise and cold exposure. PMC was reported to be caused by dominant mutations in the SCN4A gene encoding the α subunit of the Na_v1.4 channel. Recently, we identified two missense mutations of the SCN4A gene, p.V781I and p.A1737T, in two PMC families. To evaluate the changes in electrophysiological properties caused by the mutations, both mutant and wild-type (WT) SCN4A genes were expressed in CHO-K1 and HEK-293T cells. Then, whole-cell patch-clamp recording was employed to study the altered gating of mutant channels. The activation curve of transient current showed a hyperpolarizing shift in both mutant Na_v1.4 channels as compared to the WT channel, whereas there was a depolarizing shift in the fast inactivation curve. These changes confer to an increase in window current in the mutant channels. Further investigations demonstrated that the mutated channel proteins generate significantly larger resurgent currents as compared to the WT channel and take longer to attain the peak of resurgent current than the WT channel. In conclusion, the current study demonstrates that p.V781I and p.A1737T mutations in the Na_v1.4 channel increase both the sustained and the resurgent Na⁺ current, leading to membrane hyperexcitability with a lower firing threshold, which may influence the clinical phenotype.

The Clinical, Myopathological, and Genetic Analysis of 20 Patients With Non-dystrophic Myotonia

Introduction

Non-dystrophic myotonias (NDMs) are skeletal muscle ion channelopathies caused by CLCN1 or SCN4A mutations. This study aimed to describe the clinical, myopathological, and genetic analysis of NDM in a large Chinese cohort.

Methods

We reviewed the clinical manifestations, laboratory results, electrocardiogram, electromyography, muscle biopsy, genetic analysis, treatment, and follow-up of 20 patients (from 18 families) with NDM.

Results

Cases included myotonia congenita (MC, 17/20) and paramyotonia congenita (PMC, 3/20). Muscle stiffness and hypertrophy, grip and percussion myotonia, and the warm-up phenomenon were frequently observed in MC and PMC patients. Facial stiffness, eye closure myotonia, and cold sensitivity were more common in PMC patients and could be accompanied by permanent weakness. Nine MC patients and two PMC patients had cardiac abnormalities, mainly manifested as cardiac arrhythmia, and the father of one patient died of sudden cardiac arrest. Myotonic runs in electromyography were found in all patients, and seven MC patients had mild myopathic changes. There was no difference in muscle pathology between MC and PMC patients, most of whom had abnormal muscle fiber type distribution or selective muscle fiber atrophy. Nineteen CLCN1 variants were found in 17 MC patients, among which c.795T>G (p.D265E) was a new variant, and two SCN4A variants were found in three PMC patients. The patients were treated with mexiletine and/or carbamazepine, and the symptoms of myotonia were partially improved.

Conclusions

MC and PMC have considerable phenotypic overlap. Genetic investigation contributes to identifying the subtype of NDM. The muscle pathology of NDM lacks specific changes.

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.

Changes in Resurgent Sodium Current Contribute to the Hyperexcitability of Muscles in Patients with Paramyotonia Congenita

Paramyotonia congenita (PMC) is a rare hereditary skeletal muscle disorder. The major symptom, muscle stiffness, is frequently induced by cold exposure and repetitive exercise. Mutations in human SCN4A gene, which encodes the α-subunit of Na_v1.4 channel, are responsible for PMC. Mutation screening of SCN4A gene from two PMC families identified two missense mutations, p.T1313M and p.R1448H. To elucidate the electrophysiological abnormalities caused by the mutations, the p.T1313M, p.R1448H, and wild-type (WT) SCN4A genes were transient expressed on Chinese hamster ovary (CHO-K1) cells. The detailed study on the gating defects of the mutant channels using the whole-cell patch clamping technique was performed. The mutant Na_v1.4 channels impaired the basic gating properties with increasing sustained and window currents during membrane depolarization and facilitated the genesis of resurgent currents during repolarization. The mutations caused a hyperpolarization shift in the fast inactivation and slightly enhanced the slow inactivation with an increase in half-maximal inactivation voltage. No differences were found in the decay kinetics of the tail current between mutant and WT channels. In addition to generating the larger resurgent sodium current, the time to peak in the mutant channels was longer than that in the WT channels. In conclusion, our results demonstrated that the mutations p.T1313M and p.R1448H in Na_v1.4 channels can enhance fast inactivation, slow inactivation, and resurgent current, revealing that subtle changes in gating processes can influence the clinical phenotype.

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.

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.

Ryanodine receptor modulation by caffeine challenge modifies Na+ current properties in intact murine skeletal muscle fibres

We investigated effects of the ryanodine receptor (RyR) modulator caffeine on Na⁺ current (I_Na) activation and inactivation in intact loose-patch clamped murine skeletal muscle fibres subject to a double pulse procedure. I_Na activation was examined using 10-ms depolarising, V₁, steps to varying voltages 0–80 mV positive to resting membrane potential. The dependence of the subsequent, I_Na inactivation on V₁ was examined by superimposed, V₂, steps to a fixed depolarising voltage. Current-voltage activation and inactivation curves indicated that adding 0.5 and 2 mM caffeine prior to establishing the patch seal respectively produced decreased (within 1 min) and increased (after ~2 min) peak I_Na followed by its recovery to pretreatment levels (after ~40 and ~30 min respectively). These changes accompanied negative shifts in the voltage dependence of I_Na inactivation (within 10 min) and subsequent superimposed positive activation and inactivation shifts, following 0.5 mM caffeine challenge. In contrast, 2 mM caffeine elicited delayed negative shifts in both activation and inactivation. These effects were abrogated if caffeine was added after establishing the patch seal or with RyR block by 10 μM dantrolene. These effects precisely paralleled previous reports of persistently (~10 min) increased cytosolic [Ca²⁺] with 0.5 mM, and an early peak rapidly succeeded by persistently reduced [Ca²⁺] likely reflecting gradual RyR inactivation with ≥1.0 mM caffeine. The latter findings suggested inhibitory effects of even resting cytosolic [Ca²⁺] on I_Na. They suggest potentially physiologically significant negative feedback regulation of RyR activity on Na_v1.4 properties through increased or decreased local cytosolic [Ca²⁺], Ca²⁺-calmodulin and FKBP12.

A zebrafish model of nondystrophic myotonia with sodium channelopathy

Nondystrophic myotonias are disorders of Na⁺ (Na_v1.4 or SCN4A) and Cl^- (CLCN1) channels in skeletal muscles, and frequently show phenotype heterogeneity. The molecular mechanism underlying their pathophysiology and phenotype heterogeneity remains unclear. As zebrafish models have been recently exploited for studies of the pathophysiology and phenotype heterogeneity of various human genetic diseases, a zebrafish model may be useful for delineating nondystrophic myotonias. Here, we generated transgenic zebrafish expressing a human mutant allele of SCN4A, referred to as Tg(mylpfa:N440K), and needle electromyography revealed increased number of myotonic discharges and positive sharp waves in the muscles of Tg(mylpfa:N440K) than in controls. In addition, forced exercise test at a water temperature of 24 °C showed a decrease in the distance moved, time spent in and number of visits to the zone with stronger swimming resistance. Finally, a forced exercise test at a water temperature of 18 °C exhibited a higher number of dive-bombing periods and drifting-down behavior than in controls. These findings indicate that Tg(mylpfa:N440K) is a good vertebrate model of exercise- and cold-induced human nondystrophic myotonias. This zebrafish model may contribute to provide insight into the pathophysiology of myotonia in sodium channelopathy and could be used to explore a new therapeutic avenue.

Pathological findings in a patient with non-dystrophic myotonia with a mutation of the SCN4A gene; a case report

Background

Non-dystrophic myotonias (NDMs) are skeletal muscle disorders involving myotonia distinct from myotonic dystrophy. It has been reported that the muscle pathology is usually normal or comprises mild myopathic changes in NDMs. We describe various pathological findings mimicking those of myotonic dystrophy (DM) in biopsied muscle specimens from a patient with NDMs with a long disease duration.

Case presentation

A 66-year-old Japanease man presented eye closure myotonia, percussion myotonia and grip myotonia together with the warm-up phenomenon and cold aggravation from early childhood. On genetic analysis, a heterozygous mutation of the SCN4A gene (c.2065 C > T, p.L689F), with no mutation of the CLCN1, DMPK, or ZNF9/CNBP gene, was detected. He was diagnosed as having NDMs. A biopsy of the biceps brachii muscle showed increasing fiber size variation, internal nuclei, chained nuclei, necrotic fibers, fiber splitting, endomysial fibrosis, pyknotic nuclear clumps and disorganized intermyofibrillar networks. Sarcoplasmic masses, tubular aggregates and ragged-red fibers were absent.

Conclusion

It is noteworthy that the present study revealed various pathological findings resembling those seen in DM, although the pathology is usually normal or mild in NDMs. The pathological similarities may be due to muscular modification with long-standing myotonia or excessive muscle contraction based on abnormal channel activity.

Electronic supplementary material

The online version of this article (10.1186/s12883-019-1360-0) contains supplementary material, which is available to authorized users.

Sodium current inhibition following stimulation of exchange protein directly activated by cyclic-3',5'-adenosine monophosphate (Epac) in murine skeletal muscle

We investigated effects of pharmacological triggering of exchange protein directly activated by cyclic-3',5'-adenosine monophosphate (Epac) on Nav1.4 currents from intact murine (C67BL6) skeletal muscle fibres for the first time. This employed a loose patch clamp technique which examined ionic currents in response to superimposed 10-ms V1 steps to varying degrees of depolarisation, followed by V2 steps to a fixed, +100 mV depolarisation relative to resting membrane potential following 40 mV hyperpolarising prepulses of 50 ms duration. The activation and inactivation properties of the resulting Na+ membrane current densities revealed reduced maximum currents and steepnesses in their voltage dependences after addition of the Epac activator 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate (1 µM) to the bathing Krebs-Henseleit solutions. Contrastingly, voltages at half-maximal current and timecourses of currents obtained in response to the V1 depolarising steps were unchanged. These effects were abolished by further addition of the RyR-inhibitor dantrolene (10 µM). In contrast, challenge by dantrolene alone left both currents and their parameters intact. These effects of Epac activation in inhibiting skeletal muscle, Nav1.4, currents, complement similar effects previously reported in the homologous Nav1.5 in murine cardiomyocytes. They are discussed in terms of a hypothesis implicating Epac actions in increasing RyR-mediated SR Ca2+ release resulting in a Ca2+-mediated inhibition of Nav1.4. The latter effect may form the basis for Ca2+-dependent Na+ channel dysregulation in SCN4A channelopathies associated with cold- and K+-aggravated myotonias.

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.

Physiology and Pathophysiology of Sodium Channel Inactivation

Voltage-gated sodium channels are present in different tissues within the human body, predominantly nerve, muscle, and heart. The sodium channel is composed of four similar domains, each containing six transmembrane segments. Each domain can be functionally organized into a voltage-sensing region and a pore region. The sodium channel may exist in resting, activated, fast inactivated, or slow inactivated states. Upon depolarization, when the channel opens, the fast inactivation gate is in its open state. Within the time frame of milliseconds, this gate closes and blocks the channel pore from conducting any more sodium ions. Repetitive or continuous stimulations of sodium channels result in a rate-dependent decrease of sodium current. This process may continue until the channel fully shuts down. This collapse is known as slow inactivation. This chapter reviews what is known to date regarding, sodium channel inactivation with a focus on various mutations within each NaV subtype and with clinical implications.

Translational approach to address therapy in myotonia permanens due to a new SCN4A mutation

Objective:

We performed a clinical, functional, and pharmacologic characterization of the novel p.P1158L Nav1.4 mutation identified in a young girl presenting a severe myotonic phenotype.

Methods:

Wild-type hNav1.4 channel and P1158L mutant were expressed in tsA201 cells for functional and pharmacologic studies using patch-clamp.

Results:

The patient shows pronounced myotonia, slowness of movements, and generalized muscle hypertrophy. Because of general discomfort with mexiletine, she was given flecainide with satisfactory response. In vitro, mutant channels show a slower current decay and a rightward shift of the voltage dependence of fast inactivation. The voltage dependence of activation and slow inactivation were not altered. Mutant channels were less sensitive to mexiletine, whereas sensitivity to flecainide was not altered. The reduced inhibition of mutant channels by mexiletine was also observed using clinically relevant drug concentrations in a myotonic-like condition.

Conclusions:

Clinical phenotype and functional alterations of P1158L support the diagnosis of myotonia permanens. Impairment of fast inactivation is consistent with the possible role of the channel domain III S4-S5 loop in the inactivation gate docking site. The reduced sensitivity of P1158L to mexiletine may have contributed to the unsatisfactory response of the patient. The success of flecainide therapy underscores the usefulness of in vitro functional studies to help in the choice of the best drug for each individual.

Flecainide-Responsive Myotonia Permanens With SNEL Onset: A New Case and Literature Review

Sodium channel myotonias are inherited muscle diseases linked to mutations in the voltage-gated sodium channel. These diseases may also affect newborns with variable symptoms. More recently, severe neonatal episodic laryngospasm (SNEL) has been described in a small number of patients. A timely diagnosis of SNEL is crucial because a specific treatment is now available that will likely reduced laryngospasm and improve vital and cerebral outcomes. We report here on an 8-year-old girl who had presented, at birth, with SNEL who subsequently developed myotonia permanens starting at age 3 years. Results of molecular analysis revealed a de novo SCN4A G1306E mutation. The girl was treated with carbamazepine, acetazolamide, and mexiletine, with little improvement; after switching her treatment to flecainide, she experienced a dramatic reduction in muscle stiffness and myotonic symptoms as well as an improvement in behavior.

Novel SCN9A mutations underlying extreme pain phenotypes: unexpected electrophysiological and clinical phenotype correlations

The importance of NaV1.7 (encoded by SCN9A) in the regulation of pain sensing is exemplified by the heterogeneity of clinical phenotypes associated with its mutation. Gain-of-function mutations are typically pain-causing and have been associated with inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). IEM is usually caused by enhanced NaV1.7 channel activation, whereas mutations that alter steady-state fast inactivation often lead to PEPD. In contrast, nonfunctional mutations in SCN9A are known to underlie congenital insensitivity to pain (CIP). Although well documented, the correlation between SCN9A genotypes and clinical phenotypes is still unclear. Here we report three families with novel SCN9A mutations. In a multiaffected dominant family with IEM, we found the heterozygous change L245 V. Electrophysiological characterization showed that this mutation did not affect channel activation but instead resulted in incomplete fast inactivation and a small hyperpolarizing shift in steady-state slow inactivation, characteristics more commonly associated with PEPD. In two compound heterozygous CIP patients, we found mutations that still retained functionality of the channels, with two C-terminal mutations (W1775R and L1831X) exhibiting a depolarizing shift in channel activation. Two mutations (A1236E and L1831X) resulted in a hyperpolarizing shift in steady-state fast inactivation. To our knowledge, these are the first descriptions of mutations with some retained channel function causing CIP. This study emphasizes the complex genotype-phenotype correlations that exist for SCN9A and highlights the C-terminal cytoplasmic region of NaV1.7 as a critical region for channel function, potentially facilitating analgesic drug development studies.

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.

Painful cramps and giant myotonic discharges in a family with the Nav1.4-G1306A mutation

INTRODUCTION

Two previously reported Norwegian patients with painful muscle cramps and giant myotonic discharges were genotyped and compared with those of members of 21 families harboring the same mutation.

METHODS

Using primers specific for SCN4A and CLCN1, the DNA of the Norwegian family members was amplified and bidirectionally sequenced. Clinical and neurophysiological features of other families harboring the same mutation were studied.

RESULTS

A G1306A mutation in the Nav1.4 voltage-gated sodium channel of skeletal muscle was identified. This mutation is known to cause myotonia fluctuans. No giant myotonic discharges or painful muscle cramps were found in the other G1306A families.

CONCLUSIONS

Ephaptic transmission between neighboring muscle fibers may not only cause the unusual size of the myotonic discharges in this family, but also a more severe type of potassium-aggravated myotonia than myotonia fluctuans.

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.

Diagnosis of skeletal muscle channelopathies

INTRODUCTION

Skeletal muscle channelopathies are rare disorders of muscle membrane excitability. Their episodic nature may result in diagnostic difficulty and delays in diagnosis. Advances in diagnostic clinical electrophysiology combined with DNA-based diagnosis have improved diagnostic accuracy and efficiency. Ascribing pathogenic status to identified genetic variants in muscle channel genes may be complex and functional analysis, including molecular expression, may help with this. Accurate clinical and genetic diagnosis enables genetic counselling, advice regarding prognosis and aids treatment selection.

AREAS COVERED

An approach to accurate and efficient diagnosis is outlined. The importance of detailed clinical evaluation including careful history, examination and family history is emphasised. The role of specialised electrodiagnostics combined with DNA testing and molecular expression is considered. New potential biomarkers including muscle MRI using MRC Centre protocols are discussed.

EXPERT OPINION

A combined diagnostic approach using careful clinical assessment, specialised neurophysiology and DNA testing will now achieve a clear diagnosis in most patients with muscle channelopathies. An accurate diagnosis enables genetic counselling and provides information regarding prognosis and treatment selection. Genetic analysis often identifies new variants of uncertain significance. In this situation, functional expression studies as part of a diagnostic service will enable determination of pathogenic status of novel genetic variants.

Biophysical characterization of M1476I, a sodium channel founder mutation associated with cold-induced myotonia in French Canadians

M1476I, a French Canadian founder mutation of Na⁺ channel Nav1.4, causes potassium-aggravated myotonia, with cold-induced myotonia as the most distinctive clinical feature. Mexiletine, a class 1B local anaesthetic, relieves the myotonic symptoms of patients carrying the M1476I mutation. We used the patch-clamp method to investigate the functional characteristics of this mutation by heterologous expression in tsA201 cells. The M1476I mutation caused an increased persistent Na⁺ current, a 2- to 3-fold slower fast inactivation, a 6.4 mV depolarizing shift in the midpoint of steady-state inactivation, and an accelerated recovery from fast inactivation compared to the wild-type (WT) channel. Cooling slowed the kinetics of both channel types and increased the amplitude of the persistent current in M1476I channels.Mexiletine suppressed the persistent Na⁺ current generated by the M1476I mutation and blocked both WT and M1476I channels in a use- dependent manner. The inactivation-deficient M1476I channels were less susceptible to mexiletine during repetitive pulses. The decreased use-dependent block of M1476I channels might have resulted from the slower onset of mexiletine block, and/or the faster recovery from mexiletine block, given that the affinity of mexiletine for the inactivated state of the WT and mutant channels was similar. Increased extracellular concentrations of potassium had no effect on either M1476I or WT currents. These results indicated that cooling can augment the disruption of the voltage dependence of fast inactivation by M1476I channels.

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.

A mutation in a rare type of intron in a sodium-channel gene results in aberrant splicing and causes myotonia

Many mutations in the skeletal-muscle sodium-channel gene SCN4A have been associated with myotonia and/or periodic paralysis, but so far all of these mutations are located in exons. We found a patient with myotonia caused by a deletion/insertion located in intron 21 of SCN4A, which is an AT-AC type II intron. This is a rare class of introns that, despite having AT-AC boundaries, are spliced by the major or U2-type spliceosome. The patient's skeletal muscle expressed aberrantly spliced SCN4A mRNA isoforms generated by activation of cryptic splice sites. In addition, genetic suppression experiments using an SCN4A minigene showed that the mutant 5' splice site has impaired binding to the U1 and U6 snRNPs, which are the cognate factors for recognition of U2-type 5' splice sites. One of the aberrantly spliced isoforms encodes a channel with a 35-amino acid insertion in the cytoplasmic loop between domains III and IV of Nav1.4. The mutant channel exhibited a marked disruption of fast inactivation, and a simulation in silico showed that the channel defect is consistent with the patient's myotonic symptoms. This is the first report of a disease-associated mutation in an AT-AC type II intron, and also the first intronic mutation in a voltage-gated ion channel gene showing a gain-of-function defect.

Structure of the EF-hand domain of polycystin-2 suggests a mechanism for Ca2+-dependent regulation of polycystin-2 channel activity

The C-terminal cytoplasmic tail of polycystin-2 (PC2/TRPP2), a Ca(2+)-permeable channel, is frequently mutated or truncated in autosomal dominant polycystic kidney disease. We have previously shown that this tail consists of three functional regions: an EF-hand domain (PC2-EF, 720-797), a flexible linker (798-827), and an oligomeric coiled coil domain (828-895). We found that PC2-EF binds Ca(2+) at a single site and undergoes Ca(2+)-dependent conformational changes, suggesting it is an essential element of Ca(2+)-sensitive regulation of PC2 activity. Here we describe the NMR structure and dynamics of Ca(2+)-bound PC2-EF. Human PC2-EF contains a divergent non-Ca(2+)-binding helix-loop-helix (HLH) motif packed against a canonical Ca(2+)-binding EF-hand motif. This HLH motif may have evolved from a canonical EF-hand found in invertebrate PC2 homologs. Temperature-dependent steady-state NOE experiments and NMR R(1) and R(2) relaxation rates correlate with increased molecular motion in the EF-hand, possibly due to exchange between apo and Ca(2+)-bound states, consistent with a role for PC2-EF as a Ca(2+)-sensitive regulator. Structure-based sequence conservation analysis reveals a conserved hydrophobic surface in the same region, which may mediate Ca(2+)-dependent protein interactions. We propose that Ca(2+)-sensing by PC2-EF is responsible for the cooperative nature of PC2 channel activation and inhibition. Based on our results, we present a mechanism of regulation of the Ca(2+) dependence of PC2 channel activity by PC2-EF.

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.

Skeletal muscle channelopathies: new insights into the periodic paralyses and nondystrophic myotonias

PURPOSE OF REVIEW

To summarize advances in our understanding of the clinical phenotypes, genetics, and molecular pathophysiology of the periodic paralyses, the nondystrophic myotonias, and other muscle channelopathies.

RECENT FINDINGS

The number of pathogenic mutations causing periodic paralysis, nondystrophic myotonias, and ryanodinopathies continues to grow with the advent of exon hierarchy analysis strategies for genetic screening and better understanding and recognition of disease phenotypes. Recent studies have expanded and clarified the role of gating pore current in channelopathy pathogenesis. It has been shown that the gating pore current can account for the molecular and phenotypic diseases observed in the muscle sodium channelopathies, and, given that homologous residues are affected in mutations of calcium channels, it is possible that pore leak represents a pathomechanism applicable to many channel diseases. Improvements in treatment of the muscle channelopathies are on the horizon. A randomized controlled trial has been initiated for the study of mexiletine in nondystrophic myotonias. The class IC antiarrhythmia drug flecainide has been shown to depress ventricular ectopy and improve exercise capacity in patients with Andersen-Tawil syndrome.

SUMMARY

Recent studies have expanded our understanding of gating pore current as a disease-causing mechanism in the muscle channelopathies and have allowed new correlations to be drawn between disease genotype and phenotype.