Sodium channel mutations in paramyotonia congenita exhibit similar biophysical phenotypes in vitro

Muscle excitability testing

Conventional electrophysiological methods, i.e. nerve conduction studies and electromyography are suitable methods for the diagnosis of neuromuscular disorders, however, they provide limited information about muscle fibre membrane properties and underlying disease mechanisms. Muscle excitability testing is a technique that provides in vivo information about muscle fibre membrane properties such as membrane potential and ion channel function. Since the 1960s, various methodologies have been suggested to examine muscle membrane properties but technical difficulties have limited its use. In 2009, an automated, fast and simple application, the so-called multi-fibre muscle velocity recovery cycles (MVRC) has accelerated the use of muscle excitability testing. Later, frequency ramp and repetitive stimulation protocols have been developed. Though this method has been used mainly in research for revealing disease mechanisms across a broad range of neuromuscular disorders, it may have additional diagnostic uses; value has been shown particularly in muscle channelopathies. This review will provide a description of the state-of-the art of methodological and clinical studies for muscle excitability testing.

Periodic paralysis

Periodic paralysis is a rare, dominantly inherited disorder of skeletal muscle in which episodic attacks of weakness are caused by a transient impairment of fiber excitability. Attacks of weakness are often elicited by characteristic environmental triggers, which were the basis for clinically delineating subtypes of periodic paralysis and are an important distinction for optimal disease management. All forms of familial periodic paralysis are caused by mutations of ion channels, often selectively expressed in skeletal muscle, that destabilize the resting potential. The missense mutations usually alter channel function through gain-of-function changes rather than producing a complete loss-of-function null. The knowledge of which channel gene harbors a variant, whether that variant is expected to (or known to) alter function, and how altered function impairs fiber excitability aides in the interpretation of patient signs and symptoms, the interpretation of gene test results, and how to optimize therapeutic intervention for symptom management and improve quality of life.

Hyperkalemic periodic paralysis associated with a novel missense variant located in the inner pore of Nav1.4

BACKGROUND

Hyperkalemic periodic paralysis (HyperPP) is an autosomal dominantly inherited disease characterized by episodic paralytic attacks with hyperkalemia, and is caused by mutations of the SCN4A gene encoding the skeletal muscle type voltage-gated sodium channel Nav1.4. The pathological mechanism of HyperPP was suggested to be associated with gain-of-function changes for Nav1.4 gating, some of which are defects of slow inactivation.

CASE PRESENTATION & METHODS

We identified a HyperPP family consisting of the proband and his mother, who showed a novel heterozygous SCN4A variant, p.V792G, in an inner pore lesion of segment 6 in Domain II of Nav1.4. Clinical and neurophysiological evaluations were conducted for the proband and his mother. We explored the pathogenesis of the variant by whole-cell patch clamp technique using HEK293T cells expressing the mutant Nav1.4 channel.

RESULTS

Functional analysis of Nav1.4 with the V792G mutation revealed a hyperpolarized shift of voltage-dependent activation and fast inactivation. Moreover, steady-state slow inactivation in V792G was impaired with larger residual currents in comparison with wild-type Nav1.4.

CONCLUSION

V792G in SCN4A is a pathogenic variant associated with the HyperPP phenotype and the inner pore lesion of Nav1.4 plays a crucial role in slow inactivation.

Sodium channelopathies of skeletal muscle and brain

Voltage-gated sodium channels initiate action potentials in nerve, skeletal muscle, and other electrically excitable cells. Mutations in them cause a wide range of diseases. These channelopathy mutations affect every aspect of sodium channel function, including voltage sensing, voltage-dependent activation, ion conductance, fast and slow inactivation, and both biosynthesis and assembly. Mutations that cause different forms of periodic paralysis in skeletal muscle were discovered first and have provided a template for understanding structure, function, and pathophysiology at the molecular level. More recent work has revealed multiple sodium channelopathies in the brain. Here we review the well-characterized genetics and pathophysiology of the periodic paralyses of skeletal muscle and then use this information as a foundation for advancing our understanding of mutations in the structurally homologous α-subunits of brain sodium channels that cause epilepsy, migraine, autism, and related comorbidities. We include studies based on molecular and structural biology, cell biology and physiology, pharmacology, and mouse genetics. Our review reveals unexpected connections among these different types of sodium channelopathies.

Functional and Structural Characterization of ClC-1 and Nav1.4 Channels Resulting from CLCN1 and SCN4A Mutations Identified Alone and Coexisting in Myotonic Patients

Non-dystrophic myotonias have been linked to loss-of-function mutations in the ClC-1 chloride channel or gain-of-function mutations in the Na_v1.4 sodium channel. Here, we describe a family with members diagnosed with Thomsen’s disease. One novel mutation (p.W322*) in CLCN1 and one undescribed mutation (p.R1463H) in SCN4A are segregating in this family. The CLCN1-p.W322* was also found in an unrelated family, in compound heterozygosity with the known CLCN1-p.G355R mutation. One reported mutation, SCN4A-p.T1313M, was found in a third family. Both CLCN1 mutations exhibited loss-of-function: CLCN1-p.W322* probably leads to a non-viable truncated protein; for CLCN1-p.G355R, we predict structural damage, triggering important steric clashes. The SCN4A-p.R1463H produced a positive shift in the steady-state inactivation increasing window currents and a faster recovery from inactivation. These gain-of-function effects are probably due to a disruption of interaction R1463-D1356, which destabilizes the voltage sensor domain (VSD) IV and increases the flexibility of the S4-S5 linker. Finally, modelling suggested that the p.T1313M induces a strong decrease in protein flexibility on the III-IV linker. This study demonstrates that CLCN1-p.W322* and SCN4A-p.R1463H mutations can act alone or in combination as inducers of myotonia. Their co-segregation highlights the necessity for carrying out deep genetic analysis to provide accurate genetic counseling and management of patients.

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.

Heterologous Expression of Ion Channels in Mammalian Cell Lines

Heterologous expression of recombinant ion channel subunits in mammalian cell lines allows for characterization of their functional properties and pharmacological regulation. In this chapter, we describe methods for thawing, refreezing, passaging, cell culture, and transfection of tsA201 cells suitable for electrophysiology and imaging experiments. Furthermore, we discuss the strengths and limitations of using these methods.

Non-dystrophic myotonias: clinical and mutation spectrum of 70 German patients

Introduction

Non-dystrophic myotonias (NDM) are heterogeneous diseases caused by mutations in CLCN1 and SCN4A. The study aimed to describe the clinical and genetic spectrum of NDM in a large German cohort.

Methods

We retrospectively identified all patients with genetically confirmed NDM diagnosed in our center. The following data were analyzed: demographics, family history, muscular features, cardiac involvement, CK, EMG, genotype, other tested genes, treatment perceived efficacy.

Results

70 patients (age 40.2 years ± 14.9; 52.8% males) were included in our study (48 NDM-CLCN1, 22 NDM-SCN4A). The most frequent presenting symptoms were myotonia (NDM-CLCN1 83.3%, NDM-SCN4A 72.2%) and myalgia (NDM-CLCN1 57.4%, NDM-SCN4A 52.6%). Besides a more prominent facial involvement in NDM-SCN4A and cold-sensitivity in NDM-CLCN1, no other significant differences were observed between groups. Cardiac arrhythmia or conduction defects were documented in sixNDM-CLCN1 patients (three of them requiring a pacemaker) and one patient with NDM-SCN4A. CK was normal in 40% of patients. Myotonic runs in EMG were detected in 89.1% of CLCN1 and 78.9% of SCN4A. 50% of NDM-CLCN1 patients had the classic c.2680C>T (p.Arg894*) mutation. 12 new genetic variants are reported. About 50% of patients were not taking any anti-myotonic drug at the last follow-up. The anti-myotonic drugs with the best patient’s perceived efficacy were mexiletine and lamotrigine.

Conclusion

This study highlights the relevant clinical overlap between NDM-CLCN1 and NDM-SCN4A patients and warrants the use of early and broad genetic investigation for the precise identification of the NDM subtype. Besides the clinical and genetic heterogeneity, the limited response to current anti-myotonic drugs constitutes a continuing challenge.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00415-020-10328-1.

Guidelines on clinical presentation and management of nondystrophic myotonias

The nondystrophic myotonias are rare muscle hyperexcitability disorders caused by gain-of-function mutations in the SCN4A gene or loss-of-function mutations in the CLCN1 gene. Clinically, they are characterized by myotonia, defined as delayed muscle relaxation after voluntary contraction, which leads to symptoms of muscle stiffness, pain, fatigue, and weakness. Diagnosis is based on history and examination findings, the presence of electrical myotonia on electromyography, and genetic confirmation. In the absence of genetic confirmation, the diagnosis is supported by detailed electrophysiological testing, exclusion of other related disorders, and analysis of a variant of uncertain significance if present. Symptomatic treatment with a sodium channel blocker, such as mexiletine, is usually the first step in management, as well as educating patients about potential anesthetic complications.

Identification of a SCN4A mutation in a large Chinese family with atypical normokalemic periodic paralysis using whole-exome sequencing

OBJECTIVES

Normokalemic periodic paralysis (NormoKPP) of skeletal muscle is an autosomal dominant disorder caused by mutations in the gene encoding voltage-gated sodium channel protein type 4 subunit alpha (SCN4A), which leads to ion channel dysfunction. Little is known about the relationship between genotype and the clinical symptoms of NormoKPP. The present study aimed to evaluate the genetic variation in a large Chinese family with NormoKPP. The patients in this pedigree did not respond to saline treatment, but calcium gluconate treatment was effective.

METHODS

We performed a series of clinical examinations and genetic analyses, using whole-exome and Sanger sequencing, to examine the mutation status of SCN4A in a Chinese family segregating for NormoKPP.

RESULTS

Whole-exome sequencing revealed a c.2111C>T substitution in SCN4A in most of the affected family members. This mutation results in the amino acid substitution p.T704M.

CONCLUSIONS

These results support a causative role of this mutation in SCN4A in NormoKPP, and provide information about the relationship between genotype and atypical clinical symptoms.

Role of the voltage sensor module in Nav domain IV on fast inactivation in sodium channelopathies: The implication of closed-state inactivation

The segment 4 (S4) voltage sensor in voltage-gated sodium channels (Na_vs) have domain-specific functions, and the S4 segment in domain DIV (DIVS4) plays a key role in the activation and fast inactivation processes through the coupling of arginine residues in DIVS4 with residues of putative gating charge transfer center (pGCTC) in DIVS1-3. In addition, the first four arginine residues (R1-R4) in Na_v DIVS4 have position-specific functions in the fast inactivation process, and mutations in these residues are associated with diverse phenotypes of Na_v-related diseases (sodium channelopathies). R1 and R2 mutations commonly display a delayed fast inactivation, causing a gain-of-function, whereas R3 and R4 mutations commonly display a delayed recovery from inactivation and profound use-dependent current attenuation, causing a severe loss-of-function. In contrast, mutations of residues of pGCTC in Na_v DIVS1-3 can also alter fast inactivation. Such alterations in fast inactivation may be caused by disrupted interactions of DIVS4 with DIVS1-3. Despite fast inactivation of Na_vs occurs from both the open-state (open-state inactivation; OSI) and closed state (closed-state inactivation; CSI), changes in CSI have received considerably less attention than those in OSI in the pathophysiology of sodium channelopathies. CSI can be altered by mutations of arginine residues in DIVS4 and residues of pGCTC in Na_vs, and altered CSI can be an underlying primary biophysical defect of sodium channelopathies. Therefore, CSI should receive focus in order to clarify the pathophysiology of sodium channelopathies.

Sodium Channel Myotonia Due to Novel Mutations in Domain I of Nav1.4

Sodium channel myotonia is a form of muscle channelopathy due to mutations that affect the Nav1.4 channel. We describe seven families with a series of symptoms ranging from asymptomatic to clearly myotonic signs that have in common two novel mutations, p.Ile215Thr and p.Gly241Val, in the first domain of the Nav1.4 channel. The families described have been clinically and genetically evaluated. p.Ile215Thr and p.Gly241Val lie, respectively, on extracellular and intracellular loops of the first domain of the Nav1.4 channel. We assessed that the p.Ile215Thr mutation can be related to a founder effect in people from Southern Italy. Electrophysiological evaluation of the channel function showed that the voltage dependence of the activation for both the mutant channels was significantly shifted toward hyperpolarized potentials (Ile215Thr: -28.6 ± 1.5 mV and Gly241Val: -30.2 ± 1.3 mV vs. WT: -18.5 ± 1.3 mV). The slow inactivation was also significantly affected, whereas fast inactivation showed a different behavior in the two mutants. We characterized two novel mutations of the SCN4A gene expanding the knowledge about genetics of mild forms of myotonia, and we present, to our knowledge, the first homozygous patient with sodium channel myotonia.

Changes of Resurgent Na+ Currents in the Nav1.4 Channel Resulting from an SCN4A Mutation Contributing to Sodium Channel Myotonia

Myotonia congenita (MC) is a rare disorder characterized by stiffness and weakness of the limb and trunk muscles. Mutations in the SCN4A gene encoding the alpha-subunit of the voltage-gated sodium channel Na_v1.4 have been reported to be responsible for sodium channel myotonia (SCM). The Na_v1.4 channel is expressed in skeletal muscles, and its related channelopathies affect skeletal muscle excitability, which can manifest as SCM, paramyotonia and periodic paralysis. In this study, the missense mutation p.V445M was identified in two individual families with MC. To determine the functional consequences of having a mutated Na_v1.4 channel, whole-cell patch-clamp recording of transfected Chinese hamster ovary cells was performed. Evaluation of the transient Na⁺ current found that a hyperpolarizing shift occurs at both the activation and inactivation curves with an increase of the window currents in the mutant channels. The Na_v1.4 channel's co-expression with the Na_vβ4 peptide can generate resurgent Na⁺ currents at repolarization following a depolarization. The magnitude of the resurgent currents is higher in the mutant than in the wild-type (WT) channel. Although the decay kinetics are comparable between the mutant and WT channels, the time to the peak of resurgent Na⁺ currents in the mutant channel is significantly protracted compared with that in the WT channel. These findings suggest that the p.V445M mutation in the Na_v1.4 channel results in an increase of both sustained and resurgent Na⁺ currents, which may contribute to hyperexcitability with repetitive firing and is likely to facilitate recurrent myotonia in SCM patients.

Myotonic Myopathy With Secondary Joint and Skeletal Anomalies From the c.2386C>G, p.L769V Mutation in SCN4A

The phenotypic spectrum associated with the skeletal muscle voltage-gated sodium channel gene (SCN4A) has expanded with advancements in genetic testing. Autosomal dominant SCN4A mutations were first linked to hyperkalemic periodic paralysis, then subsequently included paramyotonia congenita, several variants of myotonia, and finally hypokalemic periodic paralysis. Biallelic recessive mutations were later identified in myasthenic myopathy and in infants showing a severe congenital myopathy with hypotonia. We report a patient with a pathogenic de novo SCN4A variant, c.2386C>G p.L769V at a highly conserved leucine. The phenotype was manifest at birth with arthrogryposis multiplex congenita, severe episodes of bronchospasm that responded immediately to carbamazepine therapy, and electromyographic evidence of widespread myotonia. Another de novo case of p.L769V has been reported with hip dysplasia, scoliosis, myopathy, and later paramyotonia. Expression studies of L796V mutant channels showed predominantly gain-of-function changes, that included defects of slow inactivation. Computer simulations of muscle excitability reveal a strong predisposition to myotonia with exceptionally prolonged bursts of discharges, when the L796V defects are included. We propose L769V is a pathogenic variant, that along with other cases in the literature, defines a new dominant SCN4A disorder of myotonic myopathy with secondary congenital joint and skeletal involvement.

In vivo assessment of interictal sarcolemmal membrane properties in hypokalaemic and hyperkalaemic periodic paralysis

OBJECTIVE

Hypokalaemic periodic paralysis (HypoPP) is caused by mutations of Ca_v1.1, and Na_v1.4 which result in an aberrant gating pore current. Hyperkalaemic periodic paralysis (HyperPP) is due to a gain-of-function mutation of the main alpha pore of Na_v1.4. This study used muscle velocity recovery cycles (MVRCs) to investigate changes in interictal muscle membrane properties in vivo.

METHODS

MVRCs and responses to trains of stimuli were recorded in tibialis anterior and compared in patients with HyperPP(n = 7), HypoPP (n = 10), and normal controls (n = 26).

RESULTS

Muscle relative refractory period was increased, and early supernormality reduced in HypoPP, consistent with depolarisation of the interictal resting membrane potential. In HyperPP the mean supernormality and residual supernormality to multiple conditioning stimuli were increased, consistent with increased inward sodium current and delayed repolarisation, predisposing to spontaneous myotonic discharges.

CONCLUSIONS

The in vivo findings suggest the interictal resting membrane potential is depolarized in HypoPP, and mostly normal in HyperPP. The MVRC findings in HyperPP are consistent with presence of a window current, previously proposed on the basis of in vitro expression studies. Although clinically similar, HyperPP was electrophysiologically distinct from paramyotonia congenita.

SIGNIFICANCE

MVRCs provide important in vivo data that complements expression studies of ion channel mutations.

Possible role of SCN4A skeletal muscle mutation in apnea during seizure

SCN4A gene mutations cause a number of neuromuscular phenotypes including myotonia. A subset of infants with myotonia-causing mutations experience severe life-threatening episodic laryngospasm with apnea. We have recently identified similar SCN4A mutations in association with sudden infant death syndrome. Laryngospasm has also been proposed as a contributory mechanism to some cases of sudden unexpected death in epilepsy (SUDEP). We report an infant with EEG-confirmed seizures and recurrent apneas. Whole-exome sequencing identified a known pathogenic mutation in the SCN4A gene that has been reported in several unrelated families with myotonic disorder. We propose that the SCN4A mutation contributed to the apneas in our case, irrespective of the underlying cause of the epilepsy. We suggest this supports the notion that laryngospasm may contribute to some cases of SUDEP, and implicates a possible shared mechanism between a proportion of sudden infant deaths and sudden unexpected deaths in epilepsy.

Clinical electrophysiology of muscle diseases and episodic muscle disorders

The electrodiagnostic tests performed in a patient with suspected muscle disease should provide reliable answers to the addressed questions: (1) differentiate a myopathic disorder from a neuropathic one and (2) precise the nature and cause of the myopathy. Answer to the first question mainly requires needle electromyography (EMG) of 4-6 muscles. Recordings may include extraction and measurements of motor unit potentials (MUPs). Reduced MUP spike duration indicates a lack of active muscle fibers within the motor units, and is the most reliable sign of myopathy. Needle EMG will also guide toward the etiology of the myopathy through the topographical distribution (proximal, distal, etc.) of abnormal EMG tracings and the identification of electrical activity at rest, especially fibrillation and myotonic discharges which guide toward evolutive myopathies and myotonic syndromes, respectively. The study of sensory nerve conduction should involve two to three nerves in order to disclose the coexistence of a sensory neuropathy (particularly in mitochondrial myopathies). If the diagnosis remains uncertain, functional provocative tests should be performed: 3Hz repetitive nerve stimulation to search for a myasthenic syndrome, repeated short exercise (combined with cooling if necessary) in the case of myotonic syndrome; long exercise test if periodic paralysis is suspected.

Hypokalaemic periodic paralysis and myotonia in a patient with homozygous mutation p.R1451L in NaV1.4

Dominantly inherited channelopathies of the skeletal muscle voltage-gated sodium channel Na_V1.4 include hypokalaemic and hyperkalaemic periodic paralysis (hypoPP and hyperPP) and myotonia. HyperPP and myotonia are caused by Na_V1.4 channel overactivity and overlap clinically. Instead, hypoPP is caused by gating pore currents through the voltage sensing domains (VSDs) of Na_V1.4 and seldom co-exists clinically with myotonia. Recessive loss-of-function Na_V1.4 mutations have been described in congenital myopathy and myasthenic syndromes. We report two families with the Na_V1.4 mutation p.R1451L, located in VSD-IV. Heterozygous carriers in both families manifest with myotonia and/or hyperPP. In contrast, a homozygous case presents with both hypoPP and myotonia, but unlike carriers of recessive Na_V1.4 mutations does not manifest symptoms of myopathy or myasthenia. Functional analysis revealed reduced current density and enhanced closed state inactivation of the mutant channel, but no evidence for gating pore currents. The rate of recovery from inactivation was hastened, explaining the myotonia in p.R1451L carriers and the absence of myasthenic presentations in the homozygous proband. Our data suggest that recessive loss-of-function Na_V1.4 variants can present with hypoPP without congenital myopathy or myasthenia and that myotonia can present even in carriers of homozygous Na_V1.4 loss-of-function mutations.

Substitutions of the S4DIV R2 residue (R1451) in NaV1.4 lead to complex forms of paramyotonia congenita and periodic paralyses

Mutations in Na_V1.4, the skeletal muscle voltage-gated Na⁺ channel, underlie several skeletal muscle channelopathies. We report here the functional characterization of two substitutions targeting the R1451 residue and resulting in 3 distinct clinical phenotypes. The R1451L is a novel pathogenic substitution found in two unrelated individuals. The first individual was diagnosed with non-dystrophic myotonia, whereas the second suffered from an unusual phenotype combining hyperkalemic and hypokalemic episodes of periodic paralysis (PP). The R1451C substitution was found in one individual with a single attack of hypoPP induced by glucocorticoids. To elucidate the biophysical mechanism underlying the phenotypes, we used the patch-clamp technique to study tsA201 cells expressing WT or R1451C/L channels. Our results showed that both substitutions shifted the inactivation to hyperpolarized potentials, slowed the kinetics of inactivation, slowed the recovery from slow inactivation and reduced the current density. Cooling further enhanced these abnormalities. Homology modeling revealed a disruption of hydrogen bonds in the voltage sensor domain caused by R1451C/L. We concluded that the altered biophysical properties of R1451C/L well account for the PMC-hyperPP cluster and that additional factors likely play a critical role in the inter-individual differences of clinical expression resulting from R1451C/L.

In vivo assessment of muscle membrane properties in the sodium channel myotonias

INTRODUCTION

The gain-of-function mutations that underlie sodium channel myotonia (SCM) and paramyotonia congenital (PMC) produce differing clinical phenotypes. We used muscle velocity recovery cycles (MVRCs) to investigate membrane properties.

METHODS

MVRCs and responses to trains of stimuli were compared in patients with SCM (n = 9), PMC (n = 8), and normal controls (n = 26).

RESULTS

The muscle relative refractory period was reduced in SCM, consistent with faster recovery of the mutant sodium channels from inactivation. Both SCM and PMC showed an increased early supernormality and increased mean supernormality following multiple conditioning stimuli, consistent with slowed sodium channel inactivation. Trains of fast impulses caused a loss of amplitude in PMC, after which only half of the muscle fibers recovered, suggesting that the remainder stayed depolarized by persistent sodium currents.

DISCUSSION

The differing effects of mutations on sodium channel function can be demonstrated in human subjects in vivo using this technique. Muscle Nerve 57: 586-594, 2018.

Sodium channel NaV1.9 mutations associated with insensitivity to pain dampen neuronal excitability

Voltage-gated sodium channel (NaV) mutations cause genetic pain disorders that range from severe paroxysmal pain to a congenital inability to sense pain. Previous studies on NaV1.7 and NaV1.8 established clear relationships between perturbations in channel function and divergent clinical phenotypes. By contrast, studies of NaV1.9 mutations have not revealed a clear relationship of channel dysfunction with the associated and contrasting clinical phenotypes. Here, we have elucidated the functional consequences of a NaV1.9 mutation (L1302F) that is associated with insensitivity to pain. We investigated the effects of L1302F and a previously reported mutation (L811P) on neuronal excitability. In transfected heterologous cells, the L1302F mutation caused a large hyperpolarizing shift in the voltage-dependence of activation, leading to substantially enhanced overlap between activation and steady-state inactivation relationships. In transfected small rat dorsal root ganglion neurons, expression of L1302F and L811P evoked large depolarizations of the resting membrane potential and impaired action potential generation. Therefore, our findings implicate a cellular loss of function as the basis for impaired pain sensation. We further demonstrated that a U-shaped relationship between the resting potential and the neuronal action potential threshold explains why NaV1.9 mutations that evoke small degrees of membrane depolarization cause hyperexcitability and familial episodic pain disorder or painful neuropathy, while mutations evoking larger membrane depolarizations cause hypoexcitability and insensitivity to pain.

Predominantly myalgic phenotype caused by the c.3466G>A p.A1156T mutation in SCN4A gene

Objective:

To characterize the clinical phenotype in patients with p.A1156T sodium channel mutation.

Methods:

Twenty-nine Finnish patients identified with the c.3466G>A p.A1156T mutation in the SCN4A gene were extensively examined. In a subsequent study, 63 patients with similar myalgic phenotype and with negative results in myotonic dystrophy type 2 genetic screening (DM2-neg group) and 93 patients diagnosed with fibromyalgia were screened for the mutation. Functional consequences of the p.A1156T mutation were studied in HEK293 cells with whole-cell patch clamp.

Results:

The main clinical manifestation in p.A1156T patients was not myotonia or periodic paralysis but exercise- and cold-induced muscle cramps, muscle stiffness, and myalgia. EMG myotonic discharges were detected in most but not all. Electrophysiologic compound muscle action potentials exercise test showed variable results. The p.A1156T mutation was identified in one patient in the DM2-neg group but not in the fibromyalgia group, making a total of 30 patients so far identified. Functional studies of the p.A1156T mutation showed mild attenuation of channel fast inactivation.

Conclusions:

The unspecific symptoms of myalgia stiffness and exercise intolerance without clinical myotonia or periodic paralysis in p.A1156T patients make the diagnosis challenging. The symptoms of milder SCN4A mutations may be confused with other similar myalgic syndromes, including fibromyalgia and myotonic dystrophy type 2.

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.

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.

Mice with an NaV1.4 sodium channel null allele have latent myasthenia, without susceptibility to periodic paralysis

Over 60 mutations of SCN4A encoding the NaV1.4 sodium channel of skeletal muscle have been identified in patients with myotonia, periodic paralysis, myasthenia, or congenital myopathy. Most mutations are missense with gain-of-function defects that cause susceptibility to myotonia or periodic paralysis. Loss-of-function from enhanced inactivation or null alleles is rare and has been associated with myasthenia and congenital myopathy, while a mix of loss and gain of function changes has an uncertain relation to hypokalaemic periodic paralysis. To better define the functional consequences for a loss-of-function, we generated NaV1.4 null mice by deletion of exon 12. Heterozygous null mice have latent myasthenia and a right shift of the force-stimulus relation, without evidence of periodic paralysis. Sodium current density was half that of wild-type muscle and no compensation by retained expression of the foetal NaV1.5 isoform was detected. Mice null for NaV1.4 did not survive beyond the second postnatal day. This mouse model shows remarkable preservation of muscle function and viability for haploinsufficiency of NaV1.4, as has been reported in humans, with a propensity for pseudo-myasthenia caused by a marginal Na(+) current density to support sustained high-frequency action potentials in muscle.

Physiological and Pathophysiological Insights of Nav1.4 and Nav1.5 Comparison

Mutations in Nav1.4 and Nav1.5 α-subunits have been associated with muscular and cardiac channelopathies, respectively. Despite intense research on the structure and function of these channels, a lot of information is still missing to delineate the various physiological and pathophysiological processes underlying their activity at the molecular level. Nav1.4 and Nav1.5 sequences are similar, suggesting structural and functional homologies between the two orthologous channels. This also suggests that any characteristics described for one channel subunit may shed light on the properties of the counterpart channel subunit. In this review article, after a brief clinical description of the muscular and cardiac channelopathies related to Nav1.4 and Nav1.5 mutations, respectively, we compare the knowledge accumulated in different aspects of the expression and function of Nav1.4 and Nav1.5 α-subunits: the regulation of the two encoding genes (SCN4A and SCN5A), the associated/regulatory proteins and at last, the functional effect of the same missense mutations detected in Nav1.4 and Nav1.5. First, it appears that more is known on Nav1.5 expression and accessory proteins. Because of the high homologies of Nav1.5 binding sites and equivalent Nav1.4 sites, Nav1.5-related results may guide future investigations on Nav1.4. Second, the analysis of the same missense mutations in Nav1.4 and Nav1.5 revealed intriguing similarities regarding their effects on membrane excitability and alteration in channel biophysics. We believe that such comparison may bring new cues to the physiopathology of cardiac and muscular diseases.

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.

Ion channel gene expression predicts survival in glioma patients

Ion channels are important regulators in cell proliferation, migration, and apoptosis. The malfunction and/or aberrant expression of ion channels may disrupt these important biological processes and influence cancer progression. In this study, we investigate the expression pattern of ion channel genes in glioma. We designate 18 ion channel genes that are differentially expressed in high-grade glioma as a prognostic molecular signature. This ion channel gene expression based signature predicts glioma outcome in three independent validation cohorts. Interestingly, 16 of these 18 genes were down-regulated in high-grade glioma. This signature is independent of traditional clinical, molecular, and histological factors. Resampling tests indicate that the prognostic power of the signature outperforms random gene sets selected from human genome in all the validation cohorts. More importantly, this signature performs better than the random gene signatures selected from glioma-associated genes in two out of three validation datasets. This study implicates ion channels in brain cancer, thus expanding on knowledge of their roles in other cancers. Individualized profiling of ion channel gene expression serves as a superior and independent prognostic tool for glioma patients.

Phenotypic variation of Val1589Met mutation in a four-generation Chinese pedigree with mild paramyotonia congenitia: case report

Four generations of a Chinese family with a mild form of paramyotonia congenital was characterized in phenotype and genotype. For each member, clinical history, physical examination, laboratory tests, electrophysiological and gene analyses were recorded and carried out. A potassium loading, exercise and cold provocation were further tested to diagnose the clinical differentiation. All members shared the characteristics of mild muscle cramp and stiffness induced by exercise or exposed to cold. The symptoms were relieved after rest and warming. A Val1589Met mutation at exon 24 of the SCN4A gene appears in affected subjects, while healthy members had a point mutation at position 1513 at exon 24 of the SCN4A gene. The mild phenotype of the paramyotonia congenital in the family had a Val1589Met mutation in the SCN4A gene. Various phenotypes can exist among different families, indicating that family, individual, genetic or environmental factors influence symptoms.

A gating model for wildtype and R1448H Nav1.4 channels in paramyotonia

We studied the consequences of the Nav1.4 mutation R1448H that is situated in the fourth voltage sensor of the channel and causes paramyotonia, a cold-induced myotonia followed by weakness. Previous work showed that the mutation uncouples inactivation from activation. We measured whole-cell Na(+) currents at 10, 15, 20, and 25°C using HEK293 cells stably transfected with wildtype (WT) and R1448H Na(+) channels. A Markov model was developed the parameters of which reproduced the data measured on WT and R1448H channels in the whole voltage and temperature range. It required an additional transient inactivated state and an additional closed-state inactivation transition not previously described. The model was used to predict single-channel properties, free energy barriers and temperature dependence of rates. It allowed us to draw the following conclusions: i) open-state inactivation results from a two-step process; ii) the channel re-openings that cause paramyotonia originate from enhanced deactivation/reactivation and not from destabilized inactivation; iii) the closed-state inactivation of R1448H is strikingly enhanced. We assume that latter explains the episodic weakness following cold-induced myotonia.

The voltage sensor module in sodium channels

The mechanism by which voltage-gated ion channels respond to changes in membrane polarization during action potential signaling in excitable cells has been the subject of research attention since the original description of voltage-dependent sodium and potassium flux in the squid giant axon. The cloning of ion channel genes and the identification of point mutations associated with channelopathy diseases in muscle and brain has facilitated an electrophysiological approach to the study of ion channels. Experimental approaches to the study of voltage gating have incorporated the use of thiosulfonate reagents to test accessibility, fluorescent probes, and toxins to define domain-specific roles of voltage-sensing S4 segments. Crystallography, structural and homology modeling, and molecular dynamics simulations have added computational approaches to study the relationship of channel structure to function. These approaches have tested models of voltage sensor translocation in response to membrane depolarization and incorporate the role of negative countercharges in the S1 to S3 segments to define our present understanding of the mechanism by which the voltage sensor module dictates gating particle permissiveness in excitable cells.

Seeing the forest through the trees: towards a unified view on physiological calcium regulation of voltage-gated sodium channels

Voltage-gated sodium channels (Na(V)s) underlie the upstroke of the action potential in the excitable tissues of nerve and muscle. After opening, Na(V)s rapidly undergo inactivation, a crucial process through which sodium conductance is negatively regulated. Disruption of inactivation by inherited mutations is an established cause of lethal cardiac arrhythmia, epilepsy, or painful syndromes. Intracellular calcium ions (Ca(2+)) modulate sodium channel inactivation, and multiple players have been suggested in this process, including the cytoplasmic Na(V) C-terminal region including two EF-hands and an IQ motif, the Na(V) domain III-IV linker, and calmodulin. Calmodulin can bind to the IQ domain in both Ca(2+)-bound and Ca(2+)-free conditions, but only to the DIII-IV linker in a Ca(2+)-loaded state. The mechanism of Ca(2+) regulation, and its composite effect(s) on channel gating, has been shrouded in much controversy owing to numerous apparent experimental inconsistencies. Herein, we attempt to summarize these disparate data and propose a novel, to our knowledge, physiological mechanism whereby calcium ions promote sodium current facilitation due to Ca(2+) memory at high-action-potential frequencies where Ca(2+) levels may accumulate. The available data suggest that this phenomenon may be disrupted in diseases where cytoplasmic calcium ion levels are chronically high and where targeted phosphorylation may decouple the Ca(2+) regulatory machinery. Many Na(V) disease mutations associated with electrical dysfunction are located in the Ca(2+)-sensing machinery and misregulation of Ca(2+)-dependent channel modulation is likely to contribute to disease phenotypes.

The diagnosis and treatment of myotonic disorders

Myotonia is a defining clinical symptom and sign common to a relatively small group of muscle diseases, including the myotonic dystrophies and the nondystrophic myotonic disorders. Myotonia can be observed on clinical examination, as can its electrical correlate, myotonic discharges, on electrodiagnostic testing. Research interest in the myotonic disorders continues to expand rapidly, which justifies a review of the scientific bases, clinical manifestations, and numerous therapeutic approaches associated with these disorders. We review the pathomechanisms of myotonia, the clinical features of the dystrophic and nondystrophic myotonic disorders, and the diagnostic approach and treatment options for patients with symptomatic myotonia.

Nav 1.4 slow-inactivation: is it a player in the warm-up phenomenon of myotonic disorders?

Myotonia is a heritable disorder in which patients are unable to willfully relax their muscles. The physiological basis for myotonia lies in well-established deficiencies of skeletal muscle chloride and sodium conductances. What is unclear is how normal muscle function can temporarily return with repeated movement, the so-called "warm-up" phenomenon. Electrophysiological analyses of the skeletal muscle voltage-gated sodium channel Nav 1.4 (gene name SCN4A), a key player in myotonia, have revealed several parallels between the Nav 1.4 biophysical signature, specifically slow-inactivation, and myotonic warm-up, which suggest that Nav 1.4 is critical not only in producing the myotonic reaction, but also in mediating the warm-up.

Paramyotonia congenita: from clinical diagnosis to in silico protein modeling analysis

BACKGROUND

Paramyotonia congenita (PMC) is an autosomal dominant disorder characterized by cold- or exercise-induced myotonia. PMC is caused by a mutation in SCN4A which encodes the α-subunit of the skeletal muscle sodium channel.

METHODS

The patient was an 11-year-old Japanese girl who was diagnosed as having PMC. To confirm the diagnosis, an orbital ice-pack test and blinking tests were performed. Next, to identify the mutation, genetic analysis of SCN4A was performed. Finally, to evaluate the mutation effect on the protein structure, in silico protein modeling analysis was performed.

RESULTS

Cold- and exercise-induced myotonia was reproduced in the patient with non-invasive bedside tests: ice-pack and blinking tests. In the genetic analysis, a missense mutation, c.4343G>A in SCN4A, was identified, which may result in an arginine to histidine substitution at 1448 in the protein sequence (p.Arg1448His). According to the protein modeling analysis, the mutation neutralized the positive electrostatic charge at 1448 in the DIV/S4 segment and disrupted the beginning of the helical structure in the DIV/S3-S4 linker of the SCN4A protein.

CONCLUSIONS

Diagnostic physical interventions in the patient confirmed the phenotype presentation consistent with PMC, and the in silico protein modeling analysis of p.Arg1448His predicted structural changes which can affect function of the protein. All the data confirmed the diagnosis of PMC in the patient and added to existing literature emphasizing the important role of arginine residue at 1448.

Pathophysiological role of omega pore current in channelopathies

In voltage-gated cation channels, a recurrent pattern for mutations is the neutralization of positively charged residues in the voltage-sensing S4 transmembrane segments. These mutations cause dominant ion channelopathies affecting many tissues such as brain, heart, and skeletal muscle. Recent studies suggest that the pathogenesis of associated phenotypes is not limited to alterations in the gating of the ion-conducting alpha pore. Instead, aberrant so-called omega currents, facilitated by the movement of mutated S4 segments, also appear to contribute to symptoms. Surprisingly, these omega currents conduct cations with varying ion selectivity and are activated in either a hyperpolarized or depolarized voltage range. This review gives an overview of voltage sensor channelopathies in general and focuses on pathogenesis of skeletal muscle S4 disorders for which current knowledge is most advanced.

Voltage sensor of ion channels and enzymes

Placed in the cell membrane (a two-dimensional environment), ion channels and enzymes are able to sense voltage. How these proteins are able to detect the difference in the voltage across membranes has attracted much attention, and at times, heated debate during the last few years. Sodium, Ca2+ and K+ voltage-dependent channels have a conserved positively charged transmembrane (S4) segment that moves in response to changes in membrane voltage. In voltage-dependent channels, S4 forms part of a domain that crystallizes as a well-defined structure consisting of the first four transmembrane (S1-S4) segments of the channel-forming protein, which is defined as the voltage sensor domain (VSD). The VSD is tied to a pore domain and VSD movements are allosterically coupled to the pore opening to various degrees, depending on the type of channel. How many charges are moved during channel activation, how much they move, and which are the molecular determinants that mediate the electromechanical coupling between the VSD and the pore domains are some of the questions that we discuss here. The VSD can function, however, as a bona fide proton channel itself, and, furthermore, the VSD can also be a functional part of a voltage-dependent phosphatase.

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.

Leaky sodium channels from voltage sensor mutations in periodic paralysis, but not paramyotonia

BACKGROUND

Hypokalemic periodic paralysis (HypoPP) is associated with mutations in either the Ca(V)1.1 calcium channel or the Na(V)1.4 sodium channel. Some Na(V)1.4 HypoPP mutations have been shown to cause an anomalous inward current that may contribute to the attacks of paralysis. Herein, we test whether disease-associated Na(V)1.4 mutations in previously untested homologous regions of the channel also give rise to the anomalous current.

METHODS

The functional properties of mutant Na(V)1.4 channels were studied with voltage-clamp techniques in an oocyte expression system.

RESULTS

The HypoPP mutation Na(V)1.4-R1132Q conducts an anomalous gating pore current, but the homologous R1448C mutation in paramyotonia congenita does not.

CONCLUSIONS

Gating pore currents arising from missense mutations at arginine residues in the voltage sensor domains of Na(V)1.4 are a common feature of HypoPP mutant channels and contribute to the attacks of paralysis.

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.

Molecular determinants of state-dependent block of voltage-gated sodium channels by pilsicainide

BACKGROUND AND PURPOSE

Pilsicainide, an anti-arrhythmic drug used in Japan, is described as a pure sodium channel blocker. We examined the mechanisms by which it is able to block open channels, because these properties may be especially useful to reduce hyperexcitability in pathologies characterized by abnormal sodium channel opening.

EXPERIMENTAL APPROACH

The effects of pilsicainide on various heterologously expressed human sodium channel subtypes and mutants were investigated using the patch clamp technique.

KEY RESULTS

Pilsicainide exhibited tonic and use-dependent effects comparable to those of mexiletine and flecainide on hNav1.4 channels. These use-dependent effects were abolished in the mutations F1586C and Y1593C within segment 6 of domain IV, suggesting that the interaction of pilsicainide with these residues is critical for its local anaesthetic action. Its affinity constants for closed channels (K(R)) and channels inactivated from the closed state (K(I)) were high, suggesting that its use-dependent block (UDB) requires the channel to be open for it to reach a high-affinity blocking site. Accordingly, basic pH, which slightly increased the proportion of neutral drug, dramatically decreased K(R) and K(I) values. Effects of pilsicainide were similar on skeletal muscle hNav1.4, brain hNav1.1 and heart hNav1.5 channels. The myotonic R1448C and G1306E hNav1.4 mutants were more and less sensitive to pilsicainide, respectively, due to mutation-induced gating modifications.

CONCLUSIONS AND IMPLICATIONS

Although therapeutic concentrations of pilsicainide may have little effect on resting and closed-state inactivated channels, it induces a strong UDB due to channel opening, rendering the drug ideally suited for inhibition of high-frequency action potential firing.

The tarantula toxins ProTx-II and huwentoxin-IV differentially interact with human Nav1.7 voltage sensors to inhibit channel activation and inactivation

The voltage-gated sodium channel Na(v)1.7 plays a crucial role in pain, and drugs that inhibit hNa(v)1.7 may have tremendous therapeutic potential. ProTx-II and huwentoxin-IV (HWTX-IV), cystine knot peptides from tarantula venoms, preferentially block hNa(v)1.7. Understanding the interactions of these toxins with sodium channels could aid the development of novel pain therapeutics. Whereas both ProTx-II and HWTX-IV have been proposed to preferentially block hNa(v)1.7 activation by trapping the domain II voltage-sensor in the resting configuration, we show that specific residues in the voltage-sensor paddle of domain II play substantially different roles in determining the affinities of these toxins to hNa(v)1.7. The mutation E818C increases ProTx-II's and HWTX-IV's IC(50) for block of hNa(v)1.7 currents by 4- and 400-fold, respectively. In contrast, the mutation F813G decreases ProTx-II affinity by 9-fold but has no effect on HWTX-IV affinity. It is noteworthy that we also show that ProTx-II, but not HWTX-IV, preferentially interacts with hNa(v)1.7 to impede fast inactivation by trapping the domain IV voltage-sensor in the resting configuration. Mutations E1589Q and T1590K in domain IV each decreased ProTx-II's IC(50) for impairment of fast inactivation by ~6-fold. In contrast mutations D1586A and F1592A in domain-IV increased ProTx-II's IC(50) for impairment of fast inactivation by ~4-fold. Our results show that whereas ProTx-II and HWTX-IV binding determinants on domain-II may overlap, domain II plays a much more crucial role for HWTX-IV, and contrary to what has been proposed to be a guiding principle of sodium channel pharmacology, molecules do not have to exclusively target the domain IV voltage-sensor to influence sodium channel inactivation.

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.

Voltage-sensor mutations in channelopathies of skeletal muscle

Mutations of voltage-gated ion channels cause several channelopathies of skeletal muscle, which present clinically with myotonia, periodic paralysis, or a combination of both. Expression studies have revealed both loss-of-function and gain-of-function defects for the currents passed by mutant channels. In many cases, these functional changes could be mechanistically linked to the defects of fibre excitability underlying myotonia or periodic paralysis. One remaining enigma was the basis for depolarization-induced weakness in hypokalaemic periodic paralysis (HypoPP) arising from mutations in either sodium or calcium channels. Curiously, 14 of 15 HypoPP mutations are at arginines in S4 voltage sensors, and recent observations show that these substitutions support an alternative pathway for ion conduction, the gating pore, that may be the source of the aberrant depolarization during an attack of paralysis.