From mutation to myotonia in sodium channel disorders

Clinico-Genotypic Correlation: Recurrent Attacks of Paralysis and Skeletal Muscle SCN4A Mutation (p.Ile693Thr)

Skeletal sodium channel mutations have been known to demonstrate a multitude of clinical manifestations of which one such commonly known entity is paramyotonia congenita. We describe the clinical features of proband in our case report and the various phenotypic manifestations described with the mentioned mutation from different centres. Our case serves to highlight the heterogeneity that exists in SCN4A mutations and the possible effect of other genetic/environmental factors in determining the final phenotype.

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

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

Selective Voltage-Gated Sodium Channel Peptide Toxins from Animal Venom: Pharmacological Probes and Analgesic Drug Development

Voltage-gated sodium channels (Navs) play critical roles in action potential generation and propagation. Nav channelopathy as well as pathological sensitization contribute to allodynia and hyperalgesia. Recent evidence has demonstrated the significant roles of Nav subtypes (Nav1.3, 1.7, 1.8, and 1.9) in nociceptive transduction, and therefore these Navs may represent attractive targets for analgesic drug discovery. Animal toxins are structurally diverse peptides that are highly potent yet selective on ion channel subtypes and therefore represent valuable probes to elucidate the structures, gating properties, and cellular functions of ion channels. In this review, we summarize recent advances on peptide toxins from animal venom that selectively target Nav1.3, 1.7, 1.8, and 1.9, along with their potential in analgesic drug discovery.

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

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

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.

Normokalemic periodic paralysis is not a distinct disease

INTRODUCTION

Recent molecular studies of the original cases of normokalemic periodic paralysis (normoKPP) have raised suspicions that these families actually had hyperkalemic periodic paralysis (hyperKPP) due to mutations in the skeletal muscle sodium channel gene SCN4A. However, there is still a debate about the existence of normoKPP.

METHODS

We screened 230 individuals with primary periodic paralysis for mutations in the SCN4A, CACNA1S, and KCNJ2 genes. All patients had either a hyperKPP or a hypoKPP phenotype, and none had a normoKPP phenotype.

RESULTS

In 4 hyperKPP patients from 2 families, molecular analyses revealed Arg675Gly and Arg675Gln mutations of SCN4A, which were previously reported to cause normoKPP. Each patient exhibited the characteristic clinical and laboratory features (including hyperkalemia during spontaneous attacks) of hyperKPP.

CONCLUSION

Our findings support the notion that normoKPP is not a distinct disease.

Voltage-gated sodium channels: biophysics, pharmacology, and related channelopathies

Voltage-gated sodium channels (VGSC) are multi-molecular protein complexes expressed in both excitable and non-excitable cells. They are primarily formed by a pore-forming multi-spanning integral membrane glycoprotein (α-subunit) that can be associated with one or more regulatory β-subunits. The latter are single-span integral membrane proteins that modulate the sodium current (I_Na) and can also function as cell adhesion molecules. In vitro some of the cell-adhesive functions of the β-subunits may play important physiological roles independently of the α-subunits. Other endogenous regulatory proteins named “channel partners” or “channel interacting proteins” (ChiPs) like caveolin-3 and calmodulin/calmodulin kinase II (CaMKII) can also interact and modulate the expression and/or function of VGSC. In addition to their physiological roles in cell excitability and cell adhesion, VGSC are the site of action of toxins (like tetrodotoxin and saxitoxin), and pharmacologic agents (like antiarrhythmic drugs, local anesthetics, antiepileptic drugs, and newly developed analgesics). Mutations in genes that encode α- and/or β-subunits as well as the ChiPs can affect the structure and biophysical properties of VGSC, leading to the development of diseases termed sodium “channelopathies”.  This review will outline the structure, function, and biophysical properties of VGSC as well as their pharmacology and associated channelopathies and highlight some of the recent advances in this field.

A hot topic: temperature sensitive sodium channelopathies

Perturbations to body temperature affect almost all cellular processes and, within certain limits, results in minimal effects on overall physiology. Genetic mutations to ion channels, or channelopathies, can shift the fine homeostatic balance resulting in a decreased threshold to temperature induced disturbances. This review summarizes the functional consequences of currently identified voltage-gated sodium (NaV) channelopathies that lead to disorders with a temperature sensitive phenotype. A comprehensive knowledge of the relationships between genotype and environment is not only important for understanding the etiology of disease, but also for developing safe and effective treatment paradigms.

Novel molecular determinants in the pore region of sodium channels regulate local anesthetic binding

The pore of the Na+ channel is lined by asymmetric loops formed by the linkers between the fifth and sixth transmembrane segments (S5-S6). We investigated the role of the N-terminal portion (SS1) of the S5-S6 linkers in channel gating and local anesthetic (LA) block using site-directed cysteine mutagenesis of the rat skeletal muscle (Na(V)1.4) channel. The mutants examined have variable effects on voltage dependence and kinetics of fast inactivation. Of the cysteine mutants immediately N-terminal to the putative DEKA selectivity filter in four domains, only Q399C in domain I and F1236C in domain III exhibit reduced use-dependent block. These two mutations also markedly accelerated the recovery from use-dependent block. Moreover, F1236C and Q399C significantly decreased the affinity of QX-314 for binding to its channel receptor by 8.5- and 3.3-fold, respectively. Oddly enough, F1236C enhanced stabilization of slow inactivation by both hastening entry into and delaying recovery from slow inactivation states. It is noteworthy that symmetric applications of QX-314 on both external and internal sides of F1236C mutant channels reduced recovery from use-dependent block, indicating an allosteric effect of external QX-314 binding on the recovery of availability of F1236C. These observations suggest that cysteine mutation in the SS1 region, particularly immediate adjacent to the DEKA ring, may lead to a structural rearrangement that alters binding of permanently charged QX-314 to its receptor. The results lend further support for a role for the selectivity filter region as a structural determinant for local anesthetic block.

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

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

Constrained analogues of tocainide as potent skeletal muscle sodium channel blockers towards the development of antimyotonic agents

1-Benzyl-N-(2,6-dimethylphenyl)piperidine-3-carboxamide and 4-benzyl-N-(2,6-dimethylphenyl)piperazine-2-carboxamide, two conformationally restricted analogues of tocainide, were designed and synthesized as voltage-gated skeletal muscle sodium channel blockers. They showed, with respect to tocainide, a marked increase in both potency and use-dependent block.

Reduced muscle-fiber conduction but normal slowing after cold exposure in paramyotonia congenita

In this study we investigated a family with paramyotonia (PC) congenita caused by a Gly1306Val mutation in the voltage-gated sodium-channel gene SCN4A. A previous study showed that exposure to cold aggravates the muscle stiffness in patients with this mutation. However, the mechanism behind cold sensitivity and the sodium-channel defect remained unclear. In order to gain a better understanding of sarcolemmal propagation in these patients, we measured muscle-fiber conduction velocity (MFCV) invasively. We studied four PC patients and four healthy subjects at room temperature. After the muscle was cooled, MFCV was measured again in the two PC patients and four control subjects. MFCV was significantly lower in the PC patients at room temperature, compatible with dysfunctional sodium channels. After cooling, MFCV was significantly lower in both groups as compared with room temperature. The relative slowing was 1.4% per degrees C for PC patients and 1.5% per degrees C for healthy subjects. These results indicate that, in these PC patients, mutant and wild-type sodium channels respond equally to cold exposure. Thus, MFCV is abnormal in these patients, but the aggravation of muscle stiffness cannot be explained by an abnormal sarcolemmal response to cold.

SCN1A mutations and epilepsy

SCN1A is part of the SCN1A-SCN2A-SCN3A gene cluster on chromosome 2q24 that encodes for alpha pore forming subunits of sodium channels. The 26 exons of SCN1A are spread over 100 kb of genomic DNA. Genetic defects in the coding sequence lead to generalized epilepsy with febrile seizures plus (GEFS+) and a range of childhood epileptic encephalopathies of varied severity (e.g., SMEI). All published mutations are collated. More than 100 novel mutations are spread throughout the gene with the more debilitating usually de novo. Some clustering of mutations is observed in the C-terminus and the loops between segments 5 and 6 of the first three domains of the protein. Functional studies so far show no consistent relationship between changes to channel properties and clinical phenotype. Of all the known epilepsy genes SCN1A is currently the most clinically relevant, with the largest number of epilepsy related mutations so far characterized.

A new case of autosomal dominant myotonia associated with the V1589M missense mutation in the muscle sodium channel gene and its phenotypic classification

We report a phenotype associated with the Val1589Met substitution in SCN4A gene in a French family which would be better classified as paramyotonia congenita. The proband was a 48-year-old woman, who described muscle stiffness and occasional flaccid weakness, both symptoms being induced by exercise, cold and heat. Severe muscle stiffness affected facial, oropharyngeal and limb muscles leading to transient paralysis of these muscles. One sister, two nephews and the son of the proband had similar symptoms. Molecular analysis of the muscle sodium channel gene (SCN4A) by nucleotide sequencing revealed a G-to-A transition of cDNA nucleotide at position 4765 predicting a substitution of methionine for valine at position 1589. This shows that the Val1589Met mutation in the SCN4 gene may cause different phenotypes, either potassium-aggravated myotonia or paramyotonia congenita. Familial or individual factors other than the missense mutation per se influence the expression of the disease in sodium channel disorders.

Oxaliplatin induces hyperexcitability at motor and autonomic neuromuscular junctions through effects on voltage-gated sodium channels

Oxaliplatin, an effective cytotoxic treatment in combination with 5-fluorouracil for colorectal cancer, is associated with sensory, motor and autonomic neurotoxicity. Motor symptoms include hyperexcitability while autonomic effects include urinary retention, but the cause of these side-effects is unknown. We examined the effects on motor nerve function in the mouse hemidiaphragm and on the autonomic system in the vas deferens. In the mouse diaphragm, oxaliplatin (0.5 mM) induced multiple endplate potentials (EPPs) following a single stimulus, and was associated with an increase in spontaneous miniature EPP frequency. In the vas deferens, spontaneous excitatory junction potential frequency was increased after 30 min exposure to oxaliplatin; no changes in resting Ca(2+) concentration in nerve terminal varicosities were observed, and recovery after stimuli trains was unaffected. In both tissues, an oxaliplatin-induced increase in spontaneous activity was prevented by the voltage-gated Na(+) channel blocker tetrodotoxin (TTX). Carbamazepine (0.3 mM) also prevented multiple EPPs and the increase in spontaneous activity in both tissues. In diaphragm, beta-pompilidotoxin (100 microM), which slows Na(+) channel inactivation, induced multiple EPPs similar to oxaliplatin's effect. By contrast, blockers of K(+) channels (4-aminopyridine and apamin) did not replicate oxaliplatin-induced hyperexcitability in the diaphragm. The prevention of hyperexcitability by TTX blockade implies that oxaliplatin acts on nerve conduction rather than by effecting repolarisation. The similarity between beta-pompilidotoxin and oxaliplatin suggests that alteration of voltage-gated Na(+) channel kinetics is likely to underlie the acute neurotoxic actions of oxaliplatin.

K-aggravated myotonia mutations at residue G1306 differentially alter deactivation gating of human skeletal muscle sodium channels

Fast inactivation and deactivation gating were compared between wild-type human voltage-gated skeletal muscle sodium channel (hNaV1.4) and potassium-aggravated myotonia (PAM) mutations G1306A, G1306E, and G1306V. Cell-attached macropatches were used to compare wild-type and PAM-gating properties in normal extracellular K+ (4 mM), decreased K+ (1 mM), and increased K+ (10 mM). G1306E/A increased the apparent valence of the conductance (g(V)) curve. Compared to hNaV1.4, the steady-state inactivation (h infinity) curve was depolarized for G1306E/A but hyperpolarized by G1306V, and this mutation increased apparent valence. G1306A/E slowed the rate of current rise towards peak activation. G1306V slowed open-state deactivation, inactivated-state deactivation, and recovery from fast inactivation. G1306A/E abbreviated open-state deactivation at negative commands. These mutants slowed open-state deactivation at more positive commands, at voltages for which fast inactivation might influence tail current decay. G1306E abbreviated recovery delay without affecting recovery rate. Low K+ increased peak current in hNaV1.4 and in G1306V. For G1306E, low K+ increased the rate of entry into fast inactivation, hyperpolarized the g(V) and h(infinity) curves, and increased recovery delay. Biophysical underpinnings of PAM caused by mutations of G1306 thus vary with the specific mutation, and hyperkalemic exacerbation of effects of mutations at this residue are not direct.

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.

The neurobiology of antiepileptic drugs for the treatment of nonepileptic conditions

Antiepileptic drugs (AEDs) are commonly prescribed for nonepileptic conditions, including migraine headache, chronic neuropathic pain, mood disorders, schizophrenia and various neuromuscular syndromes. In many of these conditions, as in epilepsy, the drugs act by modifying the excitability of nerve (or muscle) through effects on voltage-gated sodium and calcium channels or by promoting inhibition mediated by gamma-aminobutyric acid (GABA) A receptors. In neuropathic pain, chronic nerve injury is associated with the redistribution and altered subunit compositions of sodium and calcium channels that predispose neurons in sensory pathways to fire spontaneously or at inappropriately high frequencies, often from ectopic sites. AEDs may counteract this abnormal activity by selectively affecting pain-specific firing; for example, many AEDs suppress high-frequency action potentials by blocking voltage-activated sodium channels in a use-dependent fashion. Alternatively, AEDs may specifically target pathological channels; for example, gabapentin is a ligand of alpha2delta voltage-activated calcium channel subunits that are overexpressed in sensory neurons after nerve injury. Emerging evidence suggests that effects on signaling pathways that regulate neuronal plasticity and survival may be a factor in the delayed clinical efficacy of AEDs in some neuropsychiatric conditions, including bipolar affective disorder.

Age-related changes in the distribution of Na(v)1.1 and Na(v)1.2 in rat cerebellum

Modification of sodium channel availability and behavior is obviously a good candidate for alteration of action potential observed during aging. In the present study, we revealed age-related alterations in the expression of voltage-gated Na+ (Na(v)) channel in rat cerebellum by immunohistochemistry. In the cerebellar cortex of aged rats, Na(v)1.1 immunoreactivity in Purkinje cell bodies was highly increased, whereas granule cells showed lower staining intensity. In the cerebellar nuclei of aged rats, Na(v)1.1 and Na(v)1.2 expression was specifically increased in the cerebellar output neurons, which was confirmed by image analysis. The first demonstration of age-related changes in Na(v) channel expression contributes to our understanding of the mechanisms responsible for alteration in synaptic transmission during aging.

Normokalemic periodic paralysis revisited: does it exist?

Normokalemic periodic paralysis (normoKPP) is well established in the literature, but there are doubts as to whether it exists as a discrete entity. Retrospective clinical and molecular analysis has confirmed suspicions that most normoKPP families actually have a variant of hyperkalemic periodic paralysis (hyperKPP) due to a mutation of the muscle-specific sodium channel gene (SCN4A). However, the original normoKPP family described by Poskanzer and Kerr (Poskanzer DC, Kerr DNS. A third type of periodic paralysis, with normokalemia and favourable response to sodium chloride. Am J Med 1961;31:328-342) has remained unchallenged. We identified the Met1592Val mutation of SCN4A in an affected descendent of this original normoKPP family. This is the final piece in the puzzle: normoKPP is actually a variant of hyperKPP and is not a distinct disorder.

Force assessment in periodic paralysis after electrical muscle stimulation

OBJECTIVE

To obtain an objective measure of muscle force in periodic paralysis, we studied ankle dorsiflexion torque during induced paralytic attacks in hyperkalemic and hypokalemic patients. SUBJECTS, PATIENTS, AND METHODS: Dorsiflexor torque after peroneal nerve stimulation was recorded during provocative tests on 5 patients with hypokalemic or hyperkalemic disorders and on 2 control subjects (1995-2001). Manual strength assessment was simultaneously performed in a blinded fashion. Standardized provocation procedures were used.

RESULTS

The loss of torque in hyperkalemic patients roughly paralleled the loss of clinically detectable strength, whereas in the hypokalemic patients, pronounced torque loss occurred well before observed clinical effects. No dramatic changes occurred in the control subjects. Torque amplitude decreased more than 70% in all patients during the provocation tests; such decreases were associated with alterations induced in serum potassium concentrations.

CONCLUSIONS

Stimulated torque measurement offers several advantages in characterizing muscle dysfunction in periodic paralysis: (1) it is independent of patient effort; (2) it can show a definitely abnormal response early during provocative maneuvers; and (3) characteristics of muscle contraction can be measured that are unobservable during voluntary contraction. Stimulated torque measurements can characterize phenotypic muscle function in neuromuscular diseases.

Mexiletine block of disease-associated mutations in S6 segments of the human skeletal muscle Na(+) channel

1. Over twenty different missense mutations in the alpha-subunit of the adult skeletal muscle Na(+) channel (hSkM1) have been identified as a cause of myotonia or periodic paralysis. We examined state-dependent mexiletine block for mutations involving the putative binding site in S6 segments (V445M, S804F, V1293I, V1589M and M1592V). Whole-cell Na(+) currents were measured from wild-type (WT) and mutant channels transiently expressed in HEK cells. 2. Use-dependent block (10 ms pulses to -10 mV, at 20 Hz) in 100 microM mexiletine was reduced modestly by mutations in IVS6 (V1589M, M1592V) and enhanced by the mutation in IS6 (V445M). For mutations in IIS6 (S804F) and IIIS6 (V1293I) use-dependent block was not statistically different from that of wild-type channels. 3. Resting-state block (10 ms pulses to -10 mV from -150 mV, at 0.1 Hz) of S6 mutants was comparable to that of WT (dissociation constant for resting channels, K(R) = 650 +/- 40 microM, n = 9). The S6 mutant with the greatest change in K(R) was V445M (K(R) = 794 +/- 45 microM, n = 5), but this difference was only marginally significant (P = 0.047). 4. A modified technique for estimating local anaesthetic affinity of inactivated channels was developed to reduce errors due to slow inactivation and to failure of drug binding to reach equilibrium. Mexiletine affinity for inactivated channels was reduced by mutations in IVS6 (V1589M: dissociation constant for the inactivated state (K(I)) = 44.7 microM; M1592V: K(I) = 40.0 microM) and increased by the mutation in IS6 (V445M: K(I) = 15.0 microM), compared to wild-type channels (K(I) = 28.3 microM). 5. We conclude that the disease-associated S6 mutations in domains I-IV cause at most a 2-fold change in inactivated state affinity and have even less of an effect on resting block. Model simulations show that the reduced use-dependent block of IVS6 mutants derives primarily from an increased off-rate at hyperpolarized potentials, whereas the enhanced use-dependent block of the IS6 mutant was due to a higher affinity for inactivated V445M channels.

Increased rigidity of the chiral centre of tocainide favours stereoselectivity and use-dependent block of skeletal muscle Na(+) channels enhancing the antimyotonic activity in vivo

1. Searching for the structural requirements improving the potency and the stereoselectivity of Na(+) channel blockers as antimyotonic agents, new derivatives of tocainide, in which the chiral carbon atom is constrained in a rigid alpha-proline or pyrrolo-imidazolic cycle, were synthesized as pure enantiomers. 2. Their ability to block Na(+) currents, elicited from -100 to -20 mV at 0.3 Hz (tonic block) and 2-10 Hz (use-dependent block) frequencies, was investigated in vitro on single fibres of frog semitendinosus muscle using the vaseline-gap voltage-clamp method. 3. The alpha-proline derivative, To5, was 5 and 21 fold more potent than tocainide in producing tonic and 10 Hz-use-dependent block, respectively. Compared to To5, the presence of one methyl group on the aminic (To6) or amidic (To7) nitrogen atom decreased use-dependence by 2- and 6-times, respectively. When methylene moieties were present on both nitrogen atoms (To8), both tonic and use-dependent block were reduced. 4. Contrarily to tocainide, all proline derivatives were stereoselective in relation to an increased rigidity. A further increase in the molecular rigidity as in pyrrolo-imidazolic derivatives markedly decreased the drug potency with respect to tocainide. 5. Antimyotonic activity, evaluated as the shortening of the time of righting reflexes of myotonic adr/adr mice upon acute drug in vivo administration was 3 fold more effective for R-To5 than for R-Tocainide. 6. Thus, constraining the chiral centre of tocainide in alpha-proline cycle leads to more potent and stereoselective use-dependent Na(+) channel blockers with improved therapeutic potential.

Resurgence of sodium channel research

A variety of isoforms of mammalian voltage-gated sodium channels have been described. Ten genes encoding sodium channel alpha subunits have been identified, and nine of those isoforms have been functionally expressed in exogenous systems. The alpha subunit is associated with accessory beta subunits in some tissues, and three genes encoding different beta subunits have been identified. The alpha subunit isoforms have distinct patterns of development and localization in the nervous system, skeletal and cardiac muscle. In addition, many of the isoforms demonstrate subtle differences in their functional properties. However, there are no clear subfamilies of the channels, unlike the situation with potassium and calcium channels. The subtle differences in the functional properties of the sodium channel isoforms result in unique conductances in specific cell types, which have important physiological effects for the organism. Small alterations in the electrophysiological properties of the channel resulting from mutations in specific isoforms cause human diseases such as periodic paralysis, long QT syndrome, and epilepsy.

Enhanced inactivation and acceleration of activation of the sodium channel associated with epilepsy in man

Generalized epilepsy with febrile seizures-plus (GEFS+) is a benign Mendelian syndrome characterized by childhood-onset febrile and afebrile seizures. Three point mutations within two voltage-gated sodium channel genes have been identified so far: in GEFS+ type 1 a mutation in the beta1-subunit gene SCN1B, and in GEFS+ type 2 two mutations within the neuronal alpha-subunit gene SCN1A. Functional expression of the SCN1B and one of the SCN1A mutations revealed defects in fast channel inactivation which are in line with previous findings on myotonia causing mutations in SCN4A, the skeletal muscle sodium channel alpha-subunit gene, all showing an impaired fast inactivation. We now studied the second GEFS+ mutation (T875M in SCN1A), using the highly homologous SCN4A gene (mutation T685M). Unexpectedly, the experiments revealed a pronounced enhancement of both fast and slow inactivation and a defect of channel activation for T685M compared to wild-type channels. Steady-state fast and slow inactivation curves were shifted in the hyperpolarizing direction, entry into slow inactivation was threefold accelerated, recovery from slow inactivation was slowed by threefold and the time course of activation was slightly but significantly accelerated. In contrast to other disease-causing mutations in SCN1A, SCN1B and SCN4A, the only mechanism that could explain hyperexcitability of the cell membrane would be the acceleration of activation. Because the enhancement of slow inactivation was the most obvious alteration in gating found for T685M, this might be the disease-causing mechanism for that mutation. In this case, the occurrence of epileptic seizures could be explained by a decrease of excitability of inhibitory neurons.

Fibroblast Growth Factor Homologous Factor 1B Binds to the C Terminus of the Tetrodotoxin-resistant Sodium Channel rNav1.9a (NaN)

In this study we demonstrate a direct interaction between a cytosolic fibroblast growth factor family member and a sodium channel. A yeast two-hybrid screen for proteins that associate with the cytoplasmic domains of the tetrodotoxin-resistant sodium channel rNa(v)1.9a (NaN) led to the identification of fibroblast growth factor homologous factor 1B (FHF1B), a member of the fibroblast growth factor family, as an interacting partner of rNa(v)1.9a. FHF1B selectively interacts with the C-terminal region but not the other four intracellular segments of rNa(v)1.9a. FHF1B binds directly to the C-terminal polypeptide of rNa(v)1.9a both in vitro and in mammalian cell lines. The N-terminal 5-77 amino acid residues of FHF1B are essential for binding to rNa(v)1.9a. FHF1B did not interact with C termini of two other sodium channels hNa(v)1.7a (hNaNE) and rNa(v)1.8a (SNS), which share 50% similarity to the C-terminal polypeptide of rNa(v)1.9a. FHF1B is the first growth factor found to bind specifically to a sodium channel. Although the functional significance of this interaction is not clear, FHF1B may affect the rNa(v)1.9a channel directly or by recruiting other proteins to the channel complex. Alternatively, it is possible that rNa(v)1.9a may help deliver this factor to the cell membrane, where it exerts its function.

A sodium channel mutation causing epilepsy in man exhibits subtle defects in fast inactivation and activation in vitro

Generalized epilepsy with febrile seizures plus (GEFS+) is a benign epileptic syndrome of humans. It is characterized by febrile and afebrile generalized seizures that occur predominantly in childhood and respond well to standard antiepileptic therapy. A mutation in the b1-subunit of the voltage-gated sodium channel, linked to chromosome 19q13 (GEFS+ type 1) has been found in one family. For four other families, linkage was found to chromosome 2q21-33 (GEFS+ type 2) where three genes encoding neuronal sodium channel a-subunits are located (SCN1-3A). Recently, the first two mutations were identified in SCN1A. We introduced one of these mutations, which is highly conserved to SCN1A, into the cDNA of the gene SCN4A encoding the a-subunit of the human skeletal muscle sodium channel (hSkm1). The mutation is located in the S4 voltage sensor of domain IV, predicting substitution of histidine for the fifth of eight arginines (R1460H in hSkm1). Functional studies were performed by expressing the a-subunit alone in the mammalian tsA201 cell line using the whole-cell patch clamp technique. Compared to wild-type (WT), mutant R1460H channels showed small defects in fast inactivation. The time course of inactivation was slightly (1.5-fold) slowed and its voltage dependence reduced, and recovery from inactivation was accelerated 3-fold. However, there was no increase in persistent sodium current as observed for SCN4A mutations causing myotonia or periodic paralysis. The activation time course of R1460H channels was slightly accelerated. Slow inactivation was slightly but significantly stabilized, confirming the importance of this region for slow inactivation. The combination of activation and fast inactivation defects can explain the occurrence of epileptic seizures, but the effects were much more subtle than the inactivation defects described previously for mutations in SCN4A causing disease in skeletal muscle. Hence, with regard to pathological excitability, our results suggest a greater vulnerability of the central nervous system compared to muscle tissue.

Clinical and genetic heterogeneity in myotonic dystrophies

This review of myotonic dystrophies primarily concentrates on the clinical and genetic findings that can distinguish a novel form of myotonic dystrophy, myotonic dystrophy type 2 (DM2); proximal myotonic myopathy (PROMM); and proximal myotonic dystrophy (PDM) from myotonic dystrophy type 1 (DM1). The multisystemic nature of these disorders leads to a spectrum of symptoms and signs. Careful clinical evaluation of patients with DM2/PROMM shows that the similarities among the multisystemic myotonic disorders outweigh the differences. An important point in the comparison of the phenotypes of DM1 and DM2/PROMM is that no severe congenital type of DM2/PROMM has yet been described. Genetic linkage analyses show that myotonic dystrophies can be divided into three types: the conventional Steinert type linked to chromosome 19q13.3 (DM1); DM2/PROMM and PDM linked to chromosome 3q21.3; and families not linked to either chromosomal site. Although the diagnosis may be clinically suspected, it depends on DNA analysis.

Physiologic basis of potentials recorded in electromyography

The extracellularly recorded configuration of a single muscle fiber discharge is generally appreciated to be triphasic with an initially positive deflection. However, careful attention to waveform appearance during the electrodiagnostic medicine examination reveals that both innervated and denervated muscle waveforms may display a pantheon of configurations. Further, despite the fact that innervated and denervated single muscle fiber discharges arise from distinctly different intracellular action potential (IAP) configurations, their extracellularly recorded waveforms can appear quite similar, leading to potential misidentification and, hence, the possibility of an erroneous diagnostic conclusion. The least appreciated, but nevertheless critical, aspect of explanations for muscle waveform configurations is the relationship between the muscle fiber and recording electrode. Additionally, it is important to appreciate both the near-field and far-field aspects of single fiber and compound muscle action potentials. In this review, the leading/trailing dipole model is used to explain muscle waveform configurations in both innervated and denervated tissues.

Sodium channel Na(v)1.6 is localized at nodes of ranvier, dendrites, and synapses

Voltage-gated sodium channels perform critical roles for electrical signaling in the nervous system by generating action potentials in axons and in dendrites. At least 10 genes encode sodium channels in mammals, but specific physiological roles that distinguish each of these isoforms are not known. One possibility is that each isoform is expressed in a restricted set of cell types or is targeted to a specific domain of a neuron or muscle cell. Using affinity-purified isoform-specific antibodies, we find that Na(v)1.6 is highly concentrated at nodes of Ranvier of both sensory and motor axons in the peripheral nervous system and at nodes in the central nervous system. The specificity of this antibody was also demonstrated with the Na(v)1.6-deficient mouse mutant strain med, whose nodes were negative for Na(v)1.6 immunostaining. Both the intensity of labeling and the failure of other isoform-specific antibodies to label nodes suggest that Na(v)1.6 is the predominant channel type in this structure. In the central nervous system, Na(v)1.6 is localized in unmyelinated axons in the retina and cerebellum and is strongly expressed in dendrites of cortical pyramidal cells and cerebellar Purkinje cells. Ultrastructural studies indicate that labeling in dendrites is both intracellular and on dendritic shaft membranes. Remarkably, Na(v)1.6 labeling was observed at both presynaptic and postsynaptic membranes in the cortex and cerebellum. Thus, a single sodium channel isoform is targeted to different neuronal domains and can influence both axonal conduction and synaptic responses.

Hypothyroidism unmasking proximal myotonic myopathy

No specific diagnostic test is available to identify patients with proximal myotonic myopathy and to distinguish them from common disorders causing similar complaints. We describe three patients from three separate families who were initially diagnosed as having hypothyroid myopathy. Proximal weakness, stiffness and myotonia have persisted in each patient (2-10 years) despite the restoration of the euthyroid state. A familial pattern of autosomal dominant inheritance for proximal weakness, myotonia, and cataracts was clearly identified in one family and was likely in the other two families. DNA testing showed normal size of CTG repeat in the gene for myotonic dystrophy. The clinical presentation of these three patients strongly suggests that hypothyroidism can unmask PROMM in asymptomatic individuals who carry the genetic abnormality. Other cases of 'hypothyroid myopathy' may represent examples of unmasked PROMM.

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.

Channelopathies

In patients with mutations in the genes that encode the chloride, sodium, and calcium channels in skeletal muscle, there is abnormal function of the muscle membrane, which can cause myotonia or attacks of weakness. Mutations in the chloride and sodium channels can lead to myotonia, which typically begins in early childhood. Mexiletine is usually effective in controlling myotonia in these patients. Mexiletine is also effective in preventing attacks of cold-provoked muscle paralysis in patients with paramyotonia congenita, a sodium channel disorder. Certain mutations in the sodium channel cause attacks of hyperkalemic periodic paralysis; these attacks are often controlled with thiazide diuretics. Mutations in the skeletal muscle calcium channel cause periodic attacks of weakness, but hypokalemia (not hyperkalemia) occurs during these episodes. The carbonic anhydrase inhibitors acetazolamide and dichlorphenamide prevent attacks of hypokalemic periodic paralysis, although the mechanism by which they produce this protective effect remains a mystery. Interestingly, the hypokalemic attacks with periodic weakness that occur in some thyrotoxic patients are made worse by acetazolamide. This undesirable response to treatment emphasizes that not all disorders associated with hypokalemic periodic paralysis will benefit from carbonic anhydrase inhibitor therapy. DNA analysis to search for a mutation in the genes that encode for chloride, sodium, or calcium channels in skeletal muscle is helpful to establish the diagnosis. Some patients may eventually require provocative testing, however, to evaluate the attack of weakness and to reach a final diagnosis. Fortunately, there are effective treatments for the channelopathies that affect the skeletal muscle membrane.

Effects of temperature on slow and fast inactivation of rat skeletal muscle Na(+) channels

Patch-clamp studies of mammalian skeletal muscle Na(+) channels are commonly done at subphysiological temperatures, usually room temperature. However, at subphysiological temperatures, most Na(+) channels are inactivated at the cell resting potential. This study examined the effects of temperature on fast and slow inactivation of Na(+) channels to determine if temperature changed the fraction of Na(+) channels that were excitable at resting potential. The loose patch voltage clamp recorded Na(+) currents (I(Na)) in vitro at 19, 25, 31, and 37 degrees C from the sarcolemma of rat type IIb fast-twitch omohyoid skeletal muscle fibers. Temperature affected the fraction of Na(+) channels that were excitable at the resting potential. At 19 degrees C, only 30% of channels were excitable at the resting potential. In contrast, at 37 degrees C, 93% of Na(+) channels were excitable at the resting potential. Temperature did not alter the resting potential or the voltage dependencies of activation or fast inactivation. I(Na) available at the resting potential increased with temperature because the steady-state voltage dependence of slow inactivation shifted in a depolarizing direction with increasing temperature. The membrane potential at which half of the Na(+) channels were in the slow inactivated state was shifted by +16 mV at 37 degrees C compared with 19 degrees C. Consequently, the low availability of excitable Na(+) channels at subphysiological temperatures resulted from channels being in the slow, inactivated state at the resting potential.

Increased hindrance on the chiral carbon atom of mexiletine enhances the block of rat skeletal muscle Na+ channels in a model of myotonia induced by ATX

1 The antiarrhythmic drug mexiletine (Mex) is also used against myotonia. Searching for a more efficient drug, a new compound (Me5) was synthesized substituting the methyl group on the chiral carbon atom of Mex by an isopropyl group. Effects of Me5 on Na+ channels were compared to those of Mex in rat skeletal muscle fibres using the cell-attached patch clamp method. 2 Me5 (10 microM) reduced the maximal sodium current (INa) by 29.7+/-4.4 % (n=6) at a frequency of stimulation of 0.3 Hz and 65.7+/-4.4 % (n=6) at 1 Hz. At same concentration (10 microM), Mex was incapable of producing any effect (n=3). Me5 also shifted the steady-state inactivation curves by -7. 9+/-0.9 mV (n=6) at 0.3 Hz and -12.2+/-1.0 mV (n=6) at 1 Hz. 3 In the presence of sea anemone toxin II (ATX; 5 microM), INa decayed more slowly and no longer to zero, providing a model of sodium channel myotonia. The effects of Me5 on peak INa were similar whatever ATX was present or not. Interestingly, Me5 did not modify the INa decay time constant nor the steady-state INa to peak INa ratio. 4 Analysis of ATX-induced late Na+ channel activity shows that Me5 did not affect mean open times and single-channel conductance, thus excluding open channel block property. 5 These results indicate that increasing hindrance on the chiral atom of Mex increases drug potency on wild-type and ATX-induced noninactivating INa and that Me5 might improve the prophylaxis of myotonia.

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.

Coding sequence, genomic organization, and conserved chromosomal localization of the mouse gene Scn11a encoding the sodium channel NaN

Previous studies have shown that sodium channel alpha-subunit NaN is preferentially expressed in small-diameter sensory neurons of dorsal root ganglia and trigeminal ganglia. These neurons include high-threshold nociceptors that are involved in transduction of pain associated with tissue and nerve injury. In this study, we show that mouse NaN is a 1765-amino-acid peptide that is predicted to produce a current that is resistant to tetrodotoxin (TTX-R). Mouse and rat NaN are 80 and 89% identical at the nucleotide and amino acid levels, respectively. The Scn11a gene encoding this cDNA is organized into 24 exons. Unlike some alpha-subunits, Scn11a does not have an alternative exon 5 in domain I. Introns of the U2 and U12 spliceosome types are present at conserved positions relative to other members of this family. Scn11a is located on mouse chromosome 9, close to the two other TTX-R sodium channel genes, Scn5a and Scn10a. The human gene, SCN11A, was mapped to the conserved linkage group on chromosome 3p21-p24, close to human SCN5A and SCN10A. The colocalization of the three sodium channel genes supports a common lineage of the TTX-R sodium channels.

Effect of mexiletine on sea anemone toxin-induced non-inactivating sodium channels of rat skeletal muscle: a model of sodium channel myotonia

The sea anemone toxin ATX II impairs skeletal muscle sodium channel inactivation, mimicking the persistent inward current observed in patients suffering from sodium channel myotonia. Mexiletine has beneficial effects on myotonia. To verify the efficiency of the drug on persistent inward current, we investigated the effect of 50 microM R(-)-mexiletine on sodium channels in cell-attached patches of rat skeletal muscle fibres, in the absence or presence of 2 microM ATX II. With the toxin, a proportion of channels displayed remarkable abnormal activity lasting the entire depolarisation, which resulted in a persistent inward current that represented up to 2.0% of the peak current. Mexiletine reduced by 75% the peak current elicited by depolarisation from -100 to -20 mV. This was due to the reduction by 60% of the maximal available peak current Imax and to the negative shift by -7 mV of steady-state inactivation. Mexiletine also greatly decreased the late current, but the effect was limited to 60% of reduction, comparable to that on Imax. Therefore mexiletine was able to block the ATX II-modified sodium channels, inhibiting the myotonia-producing persistent inward current.

Diversity of mammalian voltage-gated sodium channels

A variety of different isoforms of mammalian voltage-gated sodium channels have been identified. These channels can be classified into three different types. Eight type 1 isoforms have been identified in the CNS, PNS, skeletal muscle, and heart. All of these channels have been expressed in exogenous systems, and all of the genes have been mapped. Three type 2 isoforms have been identified in heart, uterus, and muscle. These channels diverge from the type 1 channels in critical regions, and have not been functionally expressed, so their significance is unknown. A single isoform identified in the PNS may represent a third class of channels, in that it diverges from both type 1 and 2 channels. The type 3 channel has not been functionally expressed.

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).

Functional consequences of a domain 1/S6 segment sodium channel mutation associated with painful congenital myotonia

An unusual form of painful congenital myotonia is associated with a novel SCN4A mutation causing a valine to methionine substitution in the domain 1/S6 segment of the skeletal muscle sodium channel. We studied the functional characteristics of this mutant allele using a recombinant channel to gain understanding about the nature of the biophysical defect responsible for this unique phenotype. When expressed heterologously in a cultured mammalian cell line (tsA201), the mutant channel exhibits subtle defects in its gating properties similar, but not identical, to other myotonia-producing sodium channel mutations. The main abnormalities are the presence of a small non-inactivating current that occurs during short test depolarizations, a shift in the voltage-dependence of channel activation to more negative potentials, and a slowing of the time course of recovery from inactivation. Flecainide, a potent sodium channel blocker previously reported to benefit patients affected by this form of myotonia, effectively inhibits the abnormal sodium current associated with expression of the mutant channel. Our findings demonstrate the unique pattern of sodium channel dysfunction associated with a D1/S6 myotonia-producing sodium channel mutation, and provide a mechanism for the beneficial effects of flecainide in this setting.

Comparison of slow inactivation in human heart and rat skeletal muscle Na+ channel chimaeras

1. Voltage-gated Na+ channels undergo two types of inactivation in response to depolarization. One type, fast inactivation, occurs with a time scale of milliseconds. The other, slow inactivation, occurs over seconds to minutes. In addition, these two processes appear to be distinct at the molecular level. However, the molecular mechanism of Na+ channel slow inactivation is unknown. 2. We used patch clamp techniques to study slow inactivation, activation and fast inactivation in alpha-subunit cDNA clones for wild-type human heart Na+ channels (hH1) and rat skeletal muscle Na+ channels (mu1) transiently expressed in human embryonic kidney (HEK) cells. Our experiments showed that the Na+ channel slow inactivation phenotype (development, steady state and recovery) differed dramatically between hH1 and mu1. Slow inactivation in mu1 had a faster onset, a steeper voltage dependence, and was more complete compared with hH1. In addition, recovery from slow inactivation was much slower for mu1 than for hH1. Activation and fast inactivation kinetics were also different in hH1 and mu1. In hH1, fast inactivation was slower and V values of activation and steady-state fast inactivation (hthorn ) were more negative than in mu1. 3. To better understand the molecular basis of Na+ channel slow inactivation, Na+ channel chimaeras were constructed with domains from hH1 and mu1. The slow inactivation phenotype in the chimaeras (domains denoted by subscripts) mu1(1)hH1(2,3,4), mu1(1,2)hH1(3,4) and mu1(1,2,3)hH1(4) was intermediate compared with that of wild-type. However, the chimaera mu1(1)hH1(2,3,4) was more like wild-type hH1, while the chimaeras mu1(1,2)hH1(3,4) and mu1(1,2,3)hH1(4) were more similar to wild-type mu1. In the chimaeras, activation resembled that of mu1, fast inactivation resembled that of hH1, and steady-state fast inactivation fell between that of hH1 and mu1. 4. The data demonstrate that all four domains can modulate the Na+ channel slow inactivation phenotype. However, domains D1 and D2 may play a more prominent role in determining Na+ channel slow inactivation phenotype than D3 and D4. The results also support previous conclusions that D3 and D4 (and the D3-D4 linker) play an important role in Na+ channel fast inactivation, and that activation may require non-equivalent contributions from all four domains.

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.

Blockade by cAMP of native sodium channels of adult rat skeletal muscle fibers

Although the skeletal muscle sodium channel is a good substrate for cAMP-dependent protein kinase (PKA), no functional consequence was observed for this channel expressed in heterologous systems. Therefore, we investigated the effect of 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP), a membrane-permeable cAMP analog, on the native sodium channels of freshly dissociated rat skeletal muscle fibers by means of the cell-attached patch-clamp technique. Externally applied CPT-cAMP (0.5 mM) reduced peak ensemble average currents by approximately 75% with no change in kinetics. Single-channel conductance and normalized activation curves were unchanged by CPT-cAMP. In contrast, steady-state inactivation curves showed a reduction of the maximal available current and a negative shift of the half-inactivation potential. Similar effects were observed with dibutyryl adenosine 3',5'-cyclic monophosphate but not with cAMP, which does not easily permeate the cell membrane. Incubation of fibers for 1 h with 10 microM H-89, a PKA inhibitor, did not prevent the effect of CPT-cAMP. Finally, the beta-adrenoreceptor agonist isoproterenol mimicked CPT-cAMP when applied at 0.5 mM but had no effect at 0.1 mM. These results indicate that cAMP inhibits native skeletal muscle sodium channels by acting within the fiber, independently of PKA activation.