Slow Na+ channel inactivation must be disrupted to evoke prolonged depolarization-induced paralysis

Novel compound heterozygous mutations in SCN4A as a potential genetic cause contributing to myopathic manifestations: A case report and literature review

Background

SCN4A mutations account for a diverse array of clinical manifestations, encompassing periodic paralysis, myotonia, and newly recognized symptoms like classical congenital myopathy or congenital myasthenic syndromes. We describe the initial occurrence of myopathic features mimic with recessive classical CM in a Korean infant presenting with novel compound heterozygous SCN4A mutations. The infant exhibited profound hypotonia after birth, thereby expanding the spectrum of SCN4A-related channelopathy.

Methods

The genetic analyses comprised targeted exome sequencing, employing a Celemics G-Mendeliome DES Panel, along with Sanger sequencing.

Results

Considering the clinical manifestations observed in the proband, SCN4A variants emerged as the primary contenders for autosomal recessive (AR) congenital myopathy 22a, classic (#620351). Sanger sequencing validated the association of SCN4A variants with the phenotype, affirming the AR nature of the compound heterozygous variants in both the carrier mother (c.3533G > T/p.Gly1178Val) and the father (c.4216G > A/p.Ala1406Thr).

Conclusion

Our report emphasizes the association of novel compound heterozygous mutations in SCN4A with myopathic features resembling CM, as supporting by muscle biopsy. It is essential to note that pathogenic SCN4A LoF mutations are exceedingly rare. This study contributes to our understanding of SCN4A mutations and their role in myopathic features mimic with classical CM.

A Novel Therapeutic Formulation for the Improved Treatment of Indian Red Scorpion (Mesobuthus tamulus) Venom-Induced Toxicity-Tested in Caenorhabditis elegans and Rodent Models

Indian Red Scorpion (Mesobuthus tamulus) stings are a neglected public health problem in tropical and sub-tropical countries, including India. The drawbacks of conventional therapies using commercial anti-scorpion antivenom (ASA) and α1-adrenoreceptor antagonists (AAA) have prompted us to search for an adequate formulation to improve treatment against M. tamulus stings. Novel therapeutic drug formulations (TDF) of low doses of commercial ASA, AAA, and ascorbic acid have remarkably improved in neutralising the in vivo toxic effects of M. tamulus venom (MTV) tested in Caenorhabditis elegans and Wistar strain albino rats in vivo models. The neutralisation of MTV-induced production of free radicals, alteration of the mitochondrial transmembrane potential, and upregulated expression of genes involved in apoptosis, detoxification, and stress response in C. elegans by TDF surpassed the same effect shown by individual components of the TDF. Further, TDF efficiently neutralized the MTV-induced increase in blood glucose level within 30 to 60 min post-treatment, organ tissue damage, necrosis, and pulmonary oedema in Wistar rats, indicating its clinical application for effecting treating M. tamulus envenomation. This study demonstrates for the first time that C. elegans can be a model organism for screening the neutralization potency of the drug molecules against a neurotoxic scorpion venom.

Correlation of Venom Toxinome Composition of Indian Red Scorpion (Mesobuthus tamulus) with Clinical Manifestations of Scorpion Stings: Failure of Commercial Antivenom to Immune-Recognize the Abundance of Low Molecular Mass Toxins of This Venom

The Indian red scorpion (Mesobuthus tamulus), with its life-threatening sting, is the world's most dangerous species of scorpion. The toxinome composition of M. tamulus venom was determined by tandem mass spectrometry (MS) analysis of venom protein bands separated by SDS-PAGE. A total of 110 venom toxins were identified from searching the MS data against the Buthidae family (taxid: 6855) of toxin entries in nonredundant protein databases. The Na⁺ and K⁺ ion channel toxins taken together are the most abundant toxins (76.7%) giving rise to the neurotoxic nature of this venom. The other minor toxin classes in the M. tamulus venom proteome are serine protease-like protein (2.9%), serine protease inhibitor (2.2%), antimicrobial peptide (2.3%), hyaluronidase (2.2%), makatoxin (2.1%), lipolysis potentiating peptides (1.2%), neurotoxin affecting Cl^- channel (1%), parabutoporin (0.6%), Ca²⁺ channel toxins (0.8%), bradykinin potentiating peptides (0.2%), HMG CoA reductase inhibitor (0.1%), and other toxins with unknown pharmacological activity (7.7%). Several of these toxins have been shown to be promising drug candidates. M. tamulus venom does not show enzymatic activity (phospholipase A₂, l-amino acid oxidase, adenosine tri-, di-, and monophosphatase, hyaluronidase, metalloproteinase, and fibrinogenolytic), in vitro hemolytic activity, interference with blood coagulation, or platelet modulation properties. The clinical manifestations post M. tamulus sting have been described in the literature and are well correlated with its venom proteome composition. An abundance of low molecular mass toxins (3-15 kDa) are responsible for exerting the major pharmacological effects of M. tamulus venom, though they are poorly immune-recognized by commercial scorpion antivenom. This is a major concern for the development of effective antivenom therapy against scorpion stings.

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.

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.

Effects of S906T polymorphism on the severity of a novel borderline mutation I692M in Nav 1.4 cause periodic paralysis

Hyperkalemic periodic paralysis (HyperPP) is a dominantly inherited muscle disease caused by mutations in SCN4A gene encoding skeletal muscle voltage gated Na_v 1.4 channels. We identified a novel Na_v 1.4 mutation I692M in 14 families out of the 104 genetically identified HyperPP families in the Neuromuscular Centre Ulm and is therefore as frequent as I693T (13 families out of 14 HyperPP families) in Germany. Surprisingly, in 13 families, a known polymorphism S906T was also present. It was on the affected allele in at least 10 families compatible with a possible founder effect in central Europe. All affected members suffered from episodic weakness; myotonia was also common. Compared with I692M patients, I692M-S906T patients had longer weakness episodes, more affected muscles, CK elevation and presence of permanent weakness. Electrophysiological investigation showed that both mutants had incomplete slow inactivation and a hyperpolarizing shift of activation which contribute to membrane depolarization and weakness. Additionally, I692M-S906T significantly enhanced close-state fast inactivation compared with I692M alone, suggesting a higher proportion of inactivated I692M-S906T channels upon membrane depolarization which may facilitate the initiation of weakness episodes and therefore clinical manifestation. Our results suggest that polymorphism S906T has effects on the clinical phenotypic and electrophysiological severity of a novel borderline Na_v 1.4 mutation I692M, making the borderline mutation fully penetrant.

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.

A recessive Nav1.4 mutation underlies congenital myasthenic syndrome with periodic paralysis

OBJECTIVE

To determine the molecular basis of a complex phenotype of congenital muscle weakness observed in an isolated but consanguineous patient.

METHODS

The proband was evaluated clinically and neurophysiologically over a period of 15 years. Genetic testing of candidate genes was performed. Functional characterization of the candidate mutation was done in mammalian cell background using whole cell patch clamp technique.

RESULTS

The proband had fatigable muscle weakness characteristic of congenital myasthenic syndrome with acute and reversible attacks of most severe muscle weakness as observed in periodic paralysis. We identified a novel homozygous SCN4A mutation (p.R1454W) linked to this recessively inherited phenotype. The p.R1454W substitution induced an important enhancement of fast and slow inactivation, a slower recovery for these inactivated states, and a frequency-dependent regulation of Nav1.4 channels in the heterologous expression system.

CONCLUSION

We identified a novel loss-of-function mutation of Nav1.4 that leads to a recessive phenotype combining clinical symptoms and signs of congenital myasthenic syndrome and periodic paralysis, probably by decreasing channel availability for muscle action potential genesis at the neuromuscular junction and propagation along the sarcolemma.

Enhanced slow inactivation of the human skeletal muscle sodium channel causing normokalemic periodic paralysis

Normokalemic periodic paralysis (normoPP) is a type of skeletal muscle function disorder which is characterized by paralysis attack with concomitant normal serum potassium level. We previously reported that R675Q mutation of human skeletal muscle voltage-gated sodium channel α subunit (SCN4A) may be the novel mutation which caused normoPP in Chinese families. However, it is still not clear how this mutation affects the SCN4A channel function. In this study, we used patch-clamp recording to study the function of wild type (WT) and R675Q mutant of SCN4A channels expressed in human embryonic kidney (HEK293) cells. We found that R675Q mutation did not affect the voltage dependence of sodium channel activation. The fast inactivation was also not significantly affected by R675Q mutation. However, R675Q mutation of SCN4A channels exhibited an 11.1 mV hyperpolarized shift in the voltage dependence of slow inactivation and significantly prolonged the recovery from prolonged inactivation state. Our results thus indicate that SCN4A was functionally affected by R675Q mutation, suggesting a possible reason for causing normoPP in Chinese patients.

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.

Cold-induced disruption of Na+ channel slow inactivation underlies paralysis in highly thermosensitive paramyotonia

The Q270K mutation of the skeletal muscle Na(+) channel alpha subunit (Nav1.4) causes atypical paramyotonia with a striking sensitivity to cold. Attacks of paralysis and a drop in the compound muscle action potential (CMAP) are exclusively observed at cold. To understand the pathogenic process, we studied the consequences of this mutation on channel gating at different temperatures. WT or Q270K recombinant Nav1.4 channels fused at their C-terminal end to the enhanced green fluorescent protein (EGFP) were expressed in HEK-293 cells. Whole-cell Na(+) currents were recorded using the patch clamp technique to examine channel gating at 30 degrees C and after cooling the bathing solution to 20 degrees C. Mutant channel fast inactivation was impaired at both temperatures. Cooling slowed the kinetics and enhanced steady-state fast inactivation of both mutant and WT channels. Mutant channel slow inactivation was fairly comparable to that of the WT at 30 degrees C, but became clearly abnormal at 20 degrees C. Cooling enhanced slow inactivation in the WT by shifting the voltage dependence toward hyperpolarization, but induced the opposite effect in the mutant. Destabilization of mutant channel slow inactivation in combination with defective fast inactivation is expected to increase the susceptibility to prolonged membrane depolarization, and can ultimately lead to membrane inexcitability and paralysis at cold. Thus, abnormal temperature sensitivity of slow inactivation can be a determinant pathogenic factor, and should therefore be more widely considered in thermosensitive Na(+) channelopathies.

Genotype-phenotype correlation and therapeutic rationale in hyperkalemic periodic paralysis

Familial hyperkalemic periodic paralysis (PP) is a dominantly inherited muscle disease characterized by attacks of flaccid weakness and intermittent myotonia. Some patients experience muscle stiffness that is aggravated by cold and exercise, bordering on the diagnosis of paramyotonia congenita. Hyperkalemic PP and paramyotonia congenita are allelic diseases caused by gain-of-function mutations of the skeletal muscle sodium channel, Nav1.4, which is essential for the generation of skeletal muscle action potentials. In this review, the functional and clinical consequences of the mutations and therapeutic strategies are reported and the differential diagnoses discussed. Also, the question is addressed of whether hyperkalemic PP is truly a different entity than normokalemic PP. Additionally, the differential diagnosis of Andersen-Tawil syndrome in which hyperkalemic PP attacks may occur will be briefly introduced. Last, because hyperkalemic PP has been described to be associated with an R83H mutation of a MiRP2 potassium channel subunit, evidence refuting disease-causality in this case will be discussed.

Inherited disorders of voltage-gated sodium channels

A variety of inherited human disorders affecting skeletal muscle contraction, heart rhythm, and nervous system function have been traced to mutations in genes encoding voltage-gated sodium channels. Clinical severity among these conditions ranges from mild or even latent disease to life-threatening or incapacitating conditions. The sodium channelopathies were among the first recognized ion channel diseases and continue to attract widespread clinical and scientific interest. An expanding knowledge base has substantially advanced our understanding of structure-function and genotype-phenotype relationships for voltage-gated sodium channels and provided new insights into the pathophysiological basis for common diseases such as cardiac arrhythmias and epilepsy.

Electrophysiological properties of mutant Nav1.7 sodium channels in a painful inherited neuropathy

Although the physiological basis of erythermalgia, an autosomal dominant painful neuropathy characterized by redness of the skin and intermittent burning sensation of extremities, is not known, two mutations of Na(v)1.7, a sodium channel that produces a tetrodotoxin-sensitive, fast-inactivating current that is preferentially expressed in dorsal root ganglia (DRG) and sympathetic ganglia neurons, have recently been identified in patients with primary erythermalgia. Na(v)1.7 is preferentially expressed in small-diameter DRG neurons, most of which are nociceptors, and is characterized by slow recovery from inactivation and by slow closed-state inactivation that results in relatively large responses to small, subthreshold depolarizations. Here we show that these mutations in Na(v)1.7 produce a hyperpolarizing shift in activation and slow deactivation. We also show that these mutations cause an increase in amplitude of the current produced by Na(v)1.7 in response to slow, small depolarizations. These observations provide the first demonstration of altered sodium channel function associated with an inherited painful neuropathy and suggest that these physiological changes, which confer hyperexcitability on peripheral sensory and sympathetic neurons, contribute to symptom production in hereditary erythermalgia.

An expanding view for the molecular basis of familial periodic paralysis

The periodic paralyses are rare disorders of skeletal muscle characterized by episodic attacks of weakness due to intermittent failure of electrical excitability. Familial forms of periodic paralysis are all caused by mutations in genes coding for voltage-gated ion channels. New discoveries in the past 2 years have broadened our views on the diversity of phenotypes produced by mutations of a single channel gene and have led to the identification of potassium channel mutations, in addition to those previously found in sodium and calcium channels. This review focuses on the clinical features, molecular genetic defects, and pathophysiologic mechanisms that underlie familial periodic paralysis.

Impairment of slow inactivation as a common mechanism for periodic paralysis in DIIS4-S5

BACKGROUND

Mutations in the human skeletal muscle sodium channels are associated with hyperKPP, hypoKPP, paramyotonia congenita, and potassium-aggravated myotonia. This article describes the clinical manifestations of a patient with hyperKPP carrying a mutation (L689I) occurring in the linker DIIS4-S5 and its functional expression in a mammalian system.

OBJECTIVE

To correlate the clinical manifestations of hyperkalemic periodic paralysis (hyperKPP) with the functional expression of a sodium channel mutation.

METHODS

The mutation was introduced into a mammalian expression vector and expressed in the human embryonic kidney 293 cells. The functional expression of the L689I and that of the wild-type channels was monitored using the whole cell voltage-clamp technique.

RESULTS

There was no change in the kinetics of fast inactivation, and inactivation curves were indistinguishable from that of wild-type channels. However, the L689I mutation caused a hyperpolarizing shift in the voltage dependence of activation and the mutant channels showed an impaired slow inactivation process. In addition, the mutant channels have a larger persistent current at -40 mV where window current may occur.

CONCLUSIONS

The L689I mutation has similar effects to the T704M mutation and causes hyperKPP in this family. Because both of these hyperKPP mutations cause episodic muscle weakness, and because patients harboring another mutation (I693T) also can have episodic weakness, it is hypothesized that mutations occurring in this region of the sodium channel may cause episodic weakness through an impaired slow inactivation process coupled with enhanced activation.

The human skeletal muscle Na channel mutation R669H associated with hypokalemic periodic paralysis enhances slow inactivation

Missense mutations of the human skeletal muscle voltage-gated Na channel (hSkM1) underlie a variety of diseases, including hyperkalemic periodic paralysis (HyperPP), paramyotonia congenita, and potassium-aggravated myotonia. Another disorder of sarcolemmal excitability, hypokalemic periodic paralysis (HypoPP), which is usually caused by missense mutations of the S4 voltage sensors of the L-type Ca channel, was associated recently in one family with a mutation in the outermost arginine of the IIS4 voltage sensor (R669H) of hSkM1 (Bulman et al., 1999). Intriguingly, an arginine-to-histidine mutation at the homologous position in the L-type Ca(2+) channel (R528H) is a common cause of HypoPP. We have studied the gating properties of the hSkM1-R669H mutant Na channel experimentally in human embryonic kidney cells and found that it has no significant effects on activation or fast inactivation but does cause an enhancement of slow inactivation. R669H channels exhibit an approximately 10 mV hyperpolarized shift in the voltage dependence of slow inactivation and a twofold to fivefold prolongation of recovery after prolonged depolarization. In contrast, slow inactivation is often disrupted in HyperPP-associated Na channel mutants. These results demonstrate that, in R669H-associated HypoPP, enhanced slow inactivation does not preclude, and may contribute to, prolonged attacks of weakness and add support to previous evidence implicating the IIS4 voltage sensor in slow-inactivation gating.

The human skeletal muscle Na channel mutation R669H associated with hypokalemic periodic paralysis enhances slow inactivation

Missense mutations of the human skeletal muscle voltage-gated Na channel (hSkM1) underlie a variety of diseases, including hyperkalemic periodic paralysis (HyperPP), paramyotonia congenita, and potassium-aggravated myotonia. Another disorder of sarcolemmal excitability, hypokalemic periodic paralysis (HypoPP), which is usually caused by missense mutations of the S4 voltage sensors of the L-type Ca channel, was associated recently in one family with a mutation in the outermost arginine of the IIS4 voltage sensor (R669H) of hSkM1 (Bulman et al., 1999). Intriguingly, an arginine-to-histidine mutation at the homologous position in the L-type Ca(2+) channel (R528H) is a common cause of HypoPP. We have studied the gating properties of the hSkM1-R669H mutant Na channel experimentally in human embryonic kidney cells and found that it has no significant effects on activation or fast inactivation but does cause an enhancement of slow inactivation. R669H channels exhibit an approximately 10 mV hyperpolarized shift in the voltage dependence of slow inactivation and a twofold to fivefold prolongation of recovery after prolonged depolarization. In contrast, slow inactivation is often disrupted in HyperPP-associated Na channel mutants. These results demonstrate that, in R669H-associated HypoPP, enhanced slow inactivation does not preclude, and may contribute to, prolonged attacks of weakness and add support to previous evidence implicating the IIS4 voltage sensor in slow-inactivation gating.

A double mutation in families with periodic paralysis defines new aspects of sodium channel slow inactivation

Hyperkalemic periodic paralysis (HyperKPP) is an autosomal dominant skeletal muscle disorder caused by single mutations in the SCN4A gene, encoding the human skeletal muscle voltage-gated Na(+) channel. We have now identified one allele with two novel mutations occurring simultaneously in the SCN4A gene. These mutations are found in two distinct families that had symptoms of periodic paralysis and malignant hyperthermia susceptibility. The two nucleotide transitions predict phenylalanine 1490-->leucine and methionine 1493-->isoleucine changes located in the transmembrane segment S5 in the fourth repeat of the alpha-subunit Na(+) channel. Surprisingly, this mutation did not affect fast inactivation parameters. The only defect produced by the double mutant (F1490L-M1493I, expressed in human embryonic kidney 293 cells) is an enhancement of slow inactivation, a unique behavior not seen in the 24 other disease-causing mutations. The behavior observed in these mutant channels demonstrates that manifestation of HyperKPP does not necessarily require disruption of slow inactivation. Our findings may also shed light on the molecular determinants and mechanism of Na(+) channel slow inactivation and help clarify the relationship between Na(+) channel defects and the long-term paralytic attacks experienced by patients with HyperKPP.

Spectrum of sodium channel disturbances in the nondystrophic myotonias and periodic paralyses

Several heritable forms of myotonia and periodic paralysis are caused by missense mutations in the voltage-gated sodium channel of skeletal muscle. Mutations produce gain-of-function defects, either disrupted inactivation or enhanced activation. Both defects result in too much inward Na current which may either initiate pathologic bursts of action potentials (myotonia) or cause flaccid paralysis by depolarizing fibers to a refractory inexcitable state. Myotonic stiffness and periodic paralysis occur as paroxysmal attacks often triggered by environmental factors such as serum K+, cold, or exercise. Many gaps remain in our understanding of the interactions between genetic predisposition and these environmental influences. Targeted gene manipulation in animals may provide the tools to fill in these gaps.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Activation and inactivation of the voltage-gated sodium channel: role of segment S5 revealed by a novel hyperkalaemic periodic paralysis mutation

Hyperkalaemic periodic paralysis, paramyotonia congenita, and potassium-aggravated myotonia are three autosomal dominant skeletal muscle disorders linked to the SCN4A gene encoding the alpha-subunit of the human voltage-sensitive sodium channel. To date, approximately 20 point mutations causing these disorders have been described. We have identified a new point mutation, in the SCN4A gene, in a family with a hyperkalaemic periodic paralysis phenotype. This mutation predicts an isoleucine-to-phenylalanine substitution at position 1495 located in the transmembrane segment S5 in the fourth homologous domain of the human alpha-subunit sodium channel. Introduction of the I1495F mutation into the wild-type channels disrupted the macroscopic current inactivation decay and shifted both steady-state activation and inactivation to the hyperpolarizing direction. The recovery from fast inactivation was slowed, and there was no effect on channel deactivation. Additionally, a significant enhancement of slow inactivation was observed in the I1495F mutation. In contrast, the T704M mutation, a hyperkalaemic periodic paralysis mutation located in the cytoplasmic interface of the S5 segment of the second domain, also shifted activation in the hyperpolarizing direction but had little effect on fast inactivation and dramatically impaired slow inactivation. These results, showing that the I1495F and T704M hyperkalaemic periodic paralysis mutations both have profound effects on channel activation and fast-slow inactivation, suggest that the S5 segment maybe in a location where fast and slow inactivation converge.

Activation and inactivation of the voltage-gated sodium channel: role of segment S5 revealed by a novel hyperkalaemic periodic paralysis mutation

Hyperkalaemic periodic paralysis, paramyotonia congenita, and potassium-aggravated myotonia are three autosomal dominant skeletal muscle disorders linked to the SCN4A gene encoding the alpha-subunit of the human voltage-sensitive sodium channel. To date, approximately 20 point mutations causing these disorders have been described. We have identified a new point mutation, in the SCN4A gene, in a family with a hyperkalaemic periodic paralysis phenotype. This mutation predicts an isoleucine-to-phenylalanine substitution at position 1495 located in the transmembrane segment S5 in the fourth homologous domain of the human alpha-subunit sodium channel. Introduction of the I1495F mutation into the wild-type channels disrupted the macroscopic current inactivation decay and shifted both steady-state activation and inactivation to the hyperpolarizing direction. The recovery from fast inactivation was slowed, and there was no effect on channel deactivation. Additionally, a significant enhancement of slow inactivation was observed in the I1495F mutation. In contrast, the T704M mutation, a hyperkalaemic periodic paralysis mutation located in the cytoplasmic interface of the S5 segment of the second domain, also shifted activation in the hyperpolarizing direction but had little effect on fast inactivation and dramatically impaired slow inactivation. These results, showing that the I1495F and T704M hyperkalaemic periodic paralysis mutations both have profound effects on channel activation and fast-slow inactivation, suggest that the S5 segment maybe in a location where fast and slow inactivation converge.

Defective slow inactivation of sodium channels contributes to familial periodic paralysis

OBJECTIVE

To evaluate the effects of missense mutations within the skeletal muscle sodium (Na) channel on slow inactivation (SI) in periodic paralysis and related myotonic disorders.

BACKGROUND

Na channel mutations in hyperkalemic periodic paralysis and the nondystrophic myotonias interfere with the normally rapid inactivation of muscle Na currents following an action potential. This defect causes persistent inward Na currents that produce muscle depolarization, myotonia, or onset of weakness. Distinct from fast inactivation is the process called SI, which limits availability of Na channels on a time scale of seconds to minutes, thereby influencing muscle excitability.

METHODS

Human Na channel cDNAs containing mutations associated with paralytic and nonparalytic phenotypes were transiently expressed in human embryonic kidney cells for whole-cell Na current recording. Extent of SI over a range of conditioning voltages (-120 to +20 mV) was defined as the fraction of Na current that failed to recover within 20 ms at - 100 mV. The time course of entry to SI at -30 mV was measured using a conditioning pulse duration of 20 ms to 60 seconds. Recovery from SI at -100 mV was assessed over 20 ms to 10 seconds.

RESULTS

The two most common hyperkalemic periodic paralysis (HyperPP) mutations responsible for episodic attacks of weakness or paralysis, T704M and M1592V, showed clearly impaired SI, as we and others have observed previously for the rat homologs of these mutations. In addition, a new paralysis-associated mutant, I693T, with cold-induced weakness, exhibited a comparable defect in SI. However, SI remained intact for both the HyperPP/paramyotonia congenita (PMC) mutant, A1156T, and the nonparalytic potassium-aggravated myotonia (PAM) mutant, V1589M.

CONCLUSIONS

SI is defective in a subset of mutant Na channels associated with episodic weakness (HyperPP or PMC) but remains intact for mutants studied so far that cause myotonia without weakness (PAM).

Enhanced slow inactivation by V445M: a sodium channel mutation associated with myotonia

Over 20 different missense mutations in the alpha subunit of the adult skeletal muscle Na channel have been identified in families with either myotonia (muscle stiffness) or periodic paralysis, or both. The V445M mutation was recently found in a family with myotonia but no weakness. This mutation in transmembrane segment IS6 is novel because no other disease-associated mutations are in domain I. Na currents were recorded from V445M and wild-type channels transiently expressed in human embryonic kidney cells. In common with other myotonic mutants studied to date, fast gating behavior was altered by V445M in a manner predicted to increase excitability: an impairment of fast inactivation increased the persistent Na current at 10 ms and activation had a hyperpolarized shift (4 mV). In contrast, slow inactivation was enhanced by V445M due to both a slower recovery (10 mV left shift in beta(V)) and an accelerated entry rate (1.6-fold). Our results provide additional evidence that IS6 is crucial for slow inactivation and show that enhanced slow inactivation cannot prevent myotonia, whereas previous studies have shown that disrupted slow inactivation predisposes to episodic paralysis.

Effects of local anesthetics on Na+ channels containing the equine hyperkalemic periodic paralysis mutation

We examined the ability of local anesthetics to correct altered inactivation properties of rat skeletal muscle Na+ channels containing the equine hyperkalemic periodic paralysis (eqHPP) mutation when expressed in Xenopus oocytes. Increased time constants of current decay in eqHPP channels compared with wild-type channels were restored by 1 mM benzocaine but were not altered by lidocaine or mexiletine. Inactivation curves, which were determined by measuring the dependence of the relative peak current amplitude after depolarization to -10 mV on conditioning prepulse voltages, could be shifted in eqHPP channels back toward that observed for wild-type (WT) channels using selected concentrations of benzocaine, lidocaine, and mexiletine. Recovery from inactivation at -80 mV (50-ms conditioning pulse) in eqHPP channels followed a monoexponential time course and was markedly accelerated compared with wild-type channels (tauWT = 10.8 +/- 0.9 ms; taueqHPP = 2.9 +/- 0.4 ms). Benzocaine slowed the time course of recovery (taueqHPP,ben = 9.6 +/- 0.4 ms at 1 mM) in a concentration-dependent manner. In contrast, the recovery from inactivation with lidocaine and mexiletine had a fast component (taufast,lid = 3.2 +/- 0.2 ms; taufast,mex = 3.1 +/- 0.2 ms), which was identical to the recovery in eqHPP channels without drug, and a slow component (tauslow,lid = 1,688 +/- 180 ms; tauslow,mex = 2,323 +/- 328 ms). The time constant of the slow component of the recovery from inactivation was independent of the drug concentration, whereas the fraction of current recovering slowly depended on drug concentrations and conditioning pulse durations. Our results show that local anesthetics are generally incapable of fully restoring normal WT behavior in inactivation-deficient eqHPP channels.

Functional expression of the Ile693Thr Na+ channel mutation associated with paramyotonia congenita in a human cell line

1. The Ile693Thr mutation of the skeletal muscle Na+ channel alpha-subunit is associated with an unusual phenotype of paramyotonia congenita characterized by cold-induced muscle weakness but no stiffness. This mutation occurs in the S4-S5 linker of domain II, a region that has not been previously implicated in paramyotonia congenita. 2. The Ile693Thr mutation was introduced into the human skeletal muscle Na+ gene for functional expression in human embryonic kidney (HEK) cells. The currents expressed were recorded with the whole-cell voltage-clamp technique. 3. In comparison with wild-type currents, Ile693Thr mutant currents showed a clear shift of about -9 mV in the voltage dependence of activation. 4. In contrast to other mutations of the Na+ channel known to cause paramyotonia congenita, the Ile693Thr mutation did not induce any significant change in the kinetics, nor in the voltage dependence, of fast inactivation. 5. In conclusion, this study provides further evidence of the involvement of the S4-S5 linker in the voltage dependence of Na+ channel activation. The negative shift in the voltage dependence found in this mutation must be associated to other defects, plausibly an impairment of the slow inactivation, to account for the long periods of muscle weakness experienced by the patients.

Ion channel mutations and diseases of skeletal muscle

Voltage-gated ion channels play a critical role in coupling excitation at the neuromuscular junction to activation of contractile elements within a muscle fiber. Abnormal channel function can lead to either muscle paralysis or delayed relaxation. Recent advances in the molecular characterization of these ion channels have provided the tools needed to investigate the relationship between channel mutations and disorders of muscle excitability. This article reviews our current understanding of muscle sodium, calcium, and chloride channels and their role in the pathogenesis of myotonia and periodic paralysis.

Effect of alkali metal cations on slow inactivation of cardiac Na+ channels

Human heart Na+ channels were expressed transiently in both mammalian cells and Xenopus oocytes, and Na+ currents measured using 150 mM intracellular Na+. The kinetics of decaying outward Na+ current in response to 1-s depolarizations in the F1485Q mutant depends on the predominant cation in the extracellular solution, suggesting an effect on slow inactivation. The decay rate is lower for the alkali metal cations Li+, Na+, K+, Rb+, and Cs+ than for the organic cations Tris, tetramethylammonium, N-methylglucamine, and choline. In whole cell recordings, raising [Na+]zero from 10 to 150 mM increases the rate of recovery from slow inactivation at -140 mV, decreases the rate of slow inactivation at relatively depolarized voltages, and shifts steady-state slow inactivation in a depolarized direction. Single channel recordings of F1485Q show a decrease in the number of blank (i.e., null) records when [Na+]0 is increased. Significant clustering of blank records when depolarizing at a frequency of 0.5 Hz suggests that periods of inactivity represent the sojourn of a channel in a slow-inactivated state. Examination of the single channel kinetics at +60 mV during 90-ms depolarizations shows that neither open time, closed time, nor first latency is significantly affected by [Na+]0. However raising [Na+]0 decreases the duration of the last closed interval terminated by the end of the depolarization, leading to an increased number of openings at the depolarized voltage. Analysis of single channel data indicates that at a depolarized voltage a single rate constant for entry into a slow-inactivated state is reduced in high [Na+]0, suggesting that the binding of an alkali metal cation, perhaps in the ion-conducting pore, inhibits the closing of the slow inactivation gate.

From mutation to myotonia in sodium channel disorders

Hyperkalemic periodic paralysis, paramyotonia congenita, and the potassium-aggravated myotonias are all caused by point mutations in the alpha-subunit of a sodium channel expressed selectively in skeletal muscle. This review updates the growing list of genotype-phenotype correlations for these mutations and summarizes the alterations in channel function they produce. A toxin-based in vitro model demonstrates that subtle defects in sodium channel inactivation are sufficient to cause myotonia and computer modeling suggests that specific types of inactivation defect may predispose to paralysis or myotonia.

Human Na+ channel fast and slow inactivation in paramyotonia congenita mutants expressed in Xenopus laevis oocytes

1. Paramyotonia congenita (PC) is a human hereditary disease caused by one or more amino acid substitutions in skeletal muscle sodium channels. Using macropatches, the effect of PC mutations R1448C and T1313M were compared with wild-type (WT) in Xenopus oocytes coinjected with both alpha- and beta-subunits of human skeletal muscle (SkM1) sodium channels. 2. Slow inactivation in either T1313M or R1448C was not different from WT. Fast inactivation in both PC mutants, however, was significantly altered. 3. Commonly used biophysical protocols (such as I-V curves, steady-state inactivation curves, and measurements of inactivation rates) did not uniformly indicate that hyperexcitability should result from T1313M or R1448C. In fact, the only alteration of fast inactivation common to T1313M and R1448C that predicted cellular hyperexcitability was slowed open-state inactivation, compared with WT. 4. To test whether this alteration was sufficient to cause the phenotypic hyperexcitability, we used a novel voltage command that simulated muscle membrane activity. With this protocol, we found that R1448C and T1313M were similar in that they maintained a significantly higher channel availability during high frequency activity, compared with WT.

Slow inactivation differs among mutant Na channels associated with myotonia and periodic paralysis

Several heritable forms of myotonia and hyperkalemic periodic paralysis (HyperPP) are caused by missense mutations in the alpha subunit of the skeletal muscle Na channel (SkM1). These mutations impair fast inactivation or shift activation toward hyperpolarized potentials, inducing persistent Na currents that may cause muscle depolarization, myotonia, and onset of weakness. It has been proposed that the aberrant Na current and resulting weakness will be sustained only if Na channel slow inactivation is also impaired. We therefore measured slow inactivation for wild-type and five mutant Na channels constructed in the rat skeletal muscle isoform (rSkM1) and expressed in HEK cells. Two common HyperPP mutations (T698M in domain II-S5 and M1585V in IV-S6) had defective slow inactivation. This defect reduced use-dependent inhibition of Na currents elicited during 50-Hz stimulation. A rare HyperPP mutation (M1353V in IV-S1) and mutations within the domain III-IV linker that cause myotonia (G1299E) or myotonia plus weakness (T1306M) did not impair slow inactivation. We also observed that slow inactivation of wild-type rSkM1 was incomplete; therefore it is possible that stable membrane depolarization and subsequent muscle weakness may be caused solely by defects in fast inactivation or activation. Model simulations showed that abnormal slow inactivation, although not required for expression of a paralytic phenotype, may accentuate muscle membrane depolarization, paralysis, and sensitivity to hyperkalemia.

Impaired slow inactivation in mutant sodium channels

Hyperkalemic periodic paralysis (HyperPP) is a disorder in which current through Na+ channels causes a prolonged depolarization of skeletal muscle fibers, resulting in membrane inexcitability and muscle paralysis. Although HyperPP mutations can enhance persistent sodium currents, unaltered slow inactivation would effectively eliminate any sustained currents through the mutant channels. We now report that rat skeletal muscle channels containing the mutation T698M, which corresponds to the human T704M HyperPP mutation, recover very quickly from prolonged depolarizations. Even after holding at -20 mV for 20 min, approximately 25% of the maximal sodium current is available subsequent to a 10-ms hyperpolarization (-100 mV). Under the same conditions, recovery is less than 3% in wild-type channels and in the F1304Q mutant, which has impaired fast inactivation. This effect of the T698M mutation on slow inactivation, in combination with its effects on activation, is expected to result in persistent currents such as that seen in HyperPP muscle.

The kinetics of voltage-gated ion channels

When Hodgkin & Huxley (1952) first embarked on the analysis of their voltageclamp data on the ionic currents in the squid giant axon, they hoped to be able to deduce a mechanism from it, but it soon became clear that the electrical data would by themselves yield only very general information about the class of system likely to be involved.