K(+)-aggravated myotonia: destabilization of the inactivated state of the human muscle Na+ channel by the V1589M mutation

Muscle excitability testing

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

Endogenous ion channels expressed in human embryonic kidney (HEK-293) cells

Mammalian expression systems, particularly the human embryonic kidney (HEK-293) cells, combined with electrophysiological studies, have greatly benefited our understanding of the function, characteristic, and regulation of various ion channels. It was previously assumed that the existence of endogenous ion channels in native HEK-293 cells could be negligible. Still, more and more ion channels are gradually reported in native HEK-293 cells, which should draw our attention. In this regard, we summarize the different ion channels that are endogenously expressed in HEK-293 cells, including voltage-gated Na⁺ channels, Ca²⁺ channels, K⁺ channels, Cl^- channels, nonselective cation channels, TRP channels, acid-sensitive ion channels, and Piezo channels, which may complicate the recording of the heterogeneously expressed ion channels to a certain degree. We noted that the expression patterns and channel profiles varied with different studies, which may be due to the distinct originality of the cells, cell culture conditions, passage numbers, and different recording protocols. Therefore, a better knowledge of endogenous ion channels may help minimize potential problems in characterizing heterologously expressed ion channels. Based on this, it is recommended that HEK-293 cells from unknown sources should be examined before transfection for the characterization of their functional profile, especially when the expression level of exogenous ion channels does not overwhelm the endogenous ion channels largely, or the current amplitude is not significantly higher than the native currents.

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

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

Predicting functional effects of missense variants in voltage-gated sodium and calcium channels

Malfunctions of voltage-gated sodium and calcium channels (encoded by SCNxA and CACNA1x family genes, respectively) have been associated with severe neurologic, psychiatric, cardiac, and other diseases. Altered channel activity is frequently grouped into gain or loss of ion channel function (GOF or LOF, respectively) that often corresponds not only to clinical disease manifestations but also to differences in drug response. Experimental studies of channel function are therefore important, but laborious and usually focus only on a few variants at a time. On the basis of known gene-disease mechanisms of 19 different diseases, we inferred LOF (n = 518) and GOF (n = 309) likely pathogenic variants from the disease phenotypes of variant carriers. By training a machine learning model on sequence- and structure-based features, we predicted LOF or GOF effects [area under the receiver operating characteristics curve (ROC) = 0.85] of likely pathogenic missense variants. Our LOF versus GOF prediction corresponded to molecular LOF versus GOF effects for 87 functionally tested variants in SCN1/2/8A and CACNA1I (ROC = 0.73) and was validated in exome-wide data from 21,703 cases and 128,957 controls. We showed respective regional clustering of inferred LOF and GOF nucleotide variants across the alignment of the entire gene family, suggesting shared pathomechanisms in the SCNxA/CACNA1x family genes.

Mechanisms of Drug Binding to Voltage-Gated Sodium Channels

Voltage-gated sodium (Na⁺) channels are expressed in virtually all electrically excitable tissues and are essential for muscle contraction and the conduction of impulses within the peripheral and central nervous systems. Genetic disorders that disrupt the function of these channels produce an array of Na⁺ channelopathies resulting in neuronal impairment, chronic pain, neuromuscular pathologies, and cardiac arrhythmias. Because of their importance to the conduction of electrical signals, Na⁺ channels are the target of a wide variety of local anesthetic, antiarrhythmic, anticonvulsant, and antidepressant drugs. The voltage-gated family of Na⁺ channels is composed of α-subunits that encode for the voltage sensor domains and the Na⁺-selective permeation pore. In vivo, Na⁺ channel α-subunits are associated with one or more accessory β-subunits (β₁-β₄) that regulate gating properties, trafficking, and cell-surface expression of the channels. The permeation pore of Na⁺ channels is divided in two parts: the outer mouth of the pore is the site of the ion selectivity filter, while the inner cytoplasmic pore serves as the channel activation gate. The cytoplasmic lining of the permeation pore is formed by the S6 segments that include highly conserved aromatic amino acids important for drug binding. These residues are believed to undergo voltage-dependent conformational changes that alter drug binding as the channels cycle through the closed, open, and inactivated states. The purpose of this chapter is to broadly review the mechanisms of Na⁺ channel gating and the models used to describe drug binding and Na⁺ channel inhibition.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

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

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

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

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

A combination of topical antiseptics for the treatment of sore throat blocks voltage-gated neuronal sodium channels

Amylmetacresol and dichloro-benzylalcohol are ingredients of lozenges used for the treatment of sore throat. In a former in vitro study, a local anaesthetic-like effect of these substances has been described. Since amylmetacresol and dichloro-benzylalcohol are co-administered in over-the-counter lozenges, the intention of this study is to evaluate the in vitro effects of the combination of these compounds on the voltage-gated sodium channel. We analysed the block of inward sodium currents induced by the combination of amylmetacresol, dichloro-benzylalcohol and the local anaesthetic lidocaine. Tonic and use-dependent block and effects on the inactivated channel state of the neuronal sodium channel were examined. Therefore, the α-subunit of the voltage-gated NaV1.2 sodium channel was heterologously expressed in HEK 293 cells in vitro. Inward sodium currents were investigated in the whole-cell configuration of the patch-clamp technique. The combination of amylmetacresol and dichloro-benzylalcohol and the combination of amylmetacresol and lidocaine induced a block of resting and inactivated sodium channels both displaying a pronounced block at the inactivated channel state. In addition, the combination of all three compounds also resulted in a voltage-dependent block of inward sodium currents. While use-dependent block by co-application of amylmetacresol and dichloro-benzylalcohol was moderate (<20 %), lidocaine and amylmetacresol induced a robust use-dependent block (up to 50 %). This study demonstrates local anaesthetic-like effects of a combination of amylmetacresol and dichloro-benzylalcohol as established ingredients of lozenges. In the presence of amylmetacresol, dichloro-benzylalcohol and lidocaine, a prominent block of inward sodium currents is apparent.

Interaction of alfaxalone with the neuronal and the skeletal muscle sodium channel

The neurosteroid alfaxalone exerts potent anesthetic activity in humans and animals. In former studies on myelinated axons, alfaxalone was assumed to produce a local anesthetic-like effect on the peripheral nervous system. Therefore,the present in vitro study aimed to characterize possible modulatory actions of alfaxalone on voltage-gated sodium channels. -Subunits of voltage-gated neuronal (Nav1.2)and skeletal muscle (Nav1.4) sodium channels were stably expressed in human embryonic kidney cells, and in vitro effects of alfaxalone were compared with lidocaine by means of the patch clamp technique. Alfaxalone preferentially blocked slow inactivated channels and therefore could provide membrane-stabilizing effects in ischemic/hypoxic tissues where slow inactivation is regarded to play a crucial role.

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

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

Skeletal muscle na channel disorders

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

A novel N440K sodium channel mutation causes myotonia with exercise-induced weakness--exclusion of CLCN1 exon deletion/duplication by MLPA

We report a 4-generation Turkish family with 10 affected members presenting with myotonia and potassium- and exercise-induced paralytic attacks. The clinical presentation was neither typical for the chloride channel myotonias Thomsen and Becker nor for the separate sodium channel myotonia entities potassium-aggravated myotonia, paramyotonia congenita, and hyperkalemic periodic paralysis. It is best described by a combination of potassium-aggravated myotonia and hyperkalemic periodic paralysis. We excluded exonic chloride channel mutations including CLCN1 exon deletion/duplication by MLPA. Instead we identified a novel p.N440K sodium channel mutation that is located at the inner end of segment S6 of repeat I. We discuss the genotype phenotype relation.

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

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

Hereditary channelopathies in neurology

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

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

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

Topical antiseptics for the treatment of sore throat block voltage-gated neuronal sodium channels in a local anaesthetic-like manner

Lozenges for the treatment of sore throat provide relief of discomfort in cases of oral inflammation. This effect has not been fully explained so far. Here, we have examined the proposition that key components of pharmaceutical preparations for the treatment of sore throat which are routinely regarded antiseptics might have sodium channel-blocking, i.e. local anaesthetic-like effects. We investigated the effects of hexylresorcinol, amylmetacresol and dichloro-benzylalcohol on voltage-operated neuronal (Na(V)1.2) sodium channels heterologously expressed in HEK 293 cells in vitro. Hexylresorcinol, amylmetacresol and dichloro-benzylalcohol reversibly blocked depolarisation-induced whole-cell sodium inward currents. The half-maximum blocking concentrations (EC(50)) at -150 mV were 23.1, 53.6 and 661.6 microM, respectively. Block induced by hexylresorcinol and amylmetacresol was increased at depolarised potentials and use-dependent during trains of depolarisations applied at high frequency (100 Hz) indicating that both drugs bind more tightly to inactivated conformations of the channel. Estimates for the inactivated state affinity were 1.88 and 35 microM for hexylresorcinol and amylmetacresol, respectively. Hexylresorcinol and amylmetacresol are 10-20 fold more potent than the local anaesthetic lidocaine in blocking sodium inward current. Both drugs show an increased effect at depolarised membrane potentials or in conditions of high-frequency discharges.

High-affinity blockade of voltage-operated skeletal muscle and neuronal sodium channels by halogenated propofol analogues

BACKGROUND AND PURPOSE

Voltage-operated sodium channels constitute major target sites for local anaesthetic-like action. The clinical use of local anaesthetics is still limited by severe side effects, in particular, arrhythmias and convulsions. These side effects render the search for new local anaesthetics a matter of high interest.

EXPERIMENTAL APPROACH

We have investigated the effects of three halogenated structural analogues of propofol on voltage-operated human skeletal muscle sodium channels (Na(V)1.4) and the effect of one compound (4-chloropropofol) on neuronal sodium channels (Na(V)1.2) heterologously expressed in human embryonic kidney cell line 293.

KEY RESULTS

4-Iodo-, 4-bromo- and 4-chloropropofol reversibly suppressed depolarization-induced whole-cell sodium inward currents with high potency. The IC(50) for block of resting channels at -150 mV was 2.3, 3.9 and 11.3 microM in Na(V)1.4, respectively, and 29.2 microM for 4-chloropropofol in Na(V)1.2. Membrane depolarization inducing inactivation strongly increased the blocking potency of all compounds. Estimated affinities for the fast-inactivated channel state were 81 nM, 312 nM and 227 nM for 4-iodopropofol, 4-bromopropofol and 4-chloropropofol in Na(V)1.4, and 450 nM for 4-chloropropofol in Na(V)1.2. Recovery from fast inactivation was prolonged in the presence of drug leading to an accumulation of block during repetitive stimulation at high frequencies (100 Hz).

CONCLUSIONS AND IMPLICATIONS

Halogenated propofol analogues constitute a novel class of sodium channel-blocking drugs possessing almost 100-fold higher potency compared with the local anaesthetic and anti-arrhythmic drug lidocaine. Preferential drug binding to inactivated channel states suggests that halogenated propofol analogues might be especially effective in suppressing ectopic discharges in a variety of pathological conditions.

Endotoxin reduces availability of voltage-gated human skeletal muscle sodium channels at depolarized membrane potentials

OBJECTIVE

Critical illness myopathy is a common cause for difficulties in weaning from the respirator and prolonged rehabilitation of patients recovering from sepsis. Several studies have shown that the primary cause of acute generalized muscle weakness is loss of muscle membrane excitability. This study was designed to investigate a potential direct interaction of lipopolysaccharides from Escherichia coli with voltage-gated human skeletal muscle sodium channels (NaV1.4) in vitro.

DESIGN

In vitro laboratory investigation.

SETTING

University laboratory.

SUBJECTS

NaV1.4 sodium channel alpha-subunits stably expressed in human embryonic kidney (HEK293) cells.

INTERVENTIONS

We investigated the effect of lipopolysaccharide on voltage-dependent sodium channel gating by using two distinct modes of application: 1) acute perfusion (pharmacologic lipopolysaccharide concentrations between 5 ng/mL and 50 microg/mL) in order to establish a concentration-effect relationship; and 2) incubation with a clinically relevant concentration of lipopolysaccharide (300 pg/mL).

MEASUREMENTS AND MAIN RESULTS

Lipopolysaccharide did not alter the kinetics of sodium current activation or inactivation when depolarizations were started from hyperpolarized holding potentials. However, when either fast or slow inactivation was induced by membrane depolarization before the test pulse, lipopolysaccharide reversibly reduced channel availability during the test pulse at concentrations of > or = 50 ng/mL revealed by a maximum hyperpolarizing shift of -25 mV in the voltage dependence of fast and slow inactivation, respectively. Incubation with a lipopolysaccharide concentration of 300 pg/mL for 1 hr reproduced the effects on slow but not on fast inactivation. After 20 hrs of low-dose lipopolysaccharide, the peak sodium current was significantly reduced.

CONCLUSIONS

Our results show that lipopolysaccharide interacts with voltage-gated sodium channels, reducing channel availability at depolarized membrane potentials during acute application, independent of the membrane potential after chronic exposure. These effects may contribute to reduced muscle membrane excitability in sepsis.

High-affinity blockade of voltage-operated skeletal muscle sodium channels by 2,6-dimethyl-4-chlorophenol

BACKGROUND AND OBJECTIVE

The aromatic alcohol most closely resembling the aromatic tail of lidocaine is 2,6-dimethylphenol. This agent is as potent as lidocaine in blocking voltage-operated sodium channels. The aim of this study was to show the effect of halogenation in the para-position on the potency of this compound to block voltage-operated sodium channels.

METHODS

Insertion of the halogen chloride into the para-position of the molecule 2,6-dimethylphenol yielded 2,6-dimethyl-4-chlorophenol. Block of sodium currents by this compound was studied using heterologously expressed voltage-operated rat neuronal (rat IIa) sodium channels.

RESULTS

2,6-dimethyl-4-chlorophenol reversibly suppressed depolarization-induced whole-cell sodium inward currents. The ECR50 for block of resting channels at a hyperpolarized holding potential (-150 mV) was 127 micromol, the Hill coefficient nH 1.7. Membrane depolarization inducing either fast or slow-inactivation strongly increased the blocking potency. This is an important feature of a local-anaesthetic-like action. The estimated half-maximum effect concentration for the fast-inactivated channel state ECI50 was 28 micromol, the Hill coefficient nH 3.8. When 20-30% of channels were slow-inactivated using long (2.5 s) prepulses, followed by a 10 ms repolarization period to allow recovery from fast inactivation, the IC50 at -100 mV holding potential was reduced to 53 micromol.

CONCLUSION

These results, which show that 2,6-dimethyl-4-chlorophenol blocks voltage-operated sodium channels in a lidocaine-like manner while having a several fold higher potency than the non-halogenated parent compound, highlight a potentially meaningful principle of increasing the sodium channel blocking potency of phenol derivatives.

Tramadol, fentanyl and sufentanil but not morphine block voltage-operated sodium channels

Lidocaine-like sodium channel blocking drugs provide pain relief either by interrupting impulse conduction in neurons when applied locally in high concentrations or, when given systemically, by suppressing high-frequency ectopic discharges due to preferential drug binding to inactivated channel states. Lidocaine-like actions of opioids have frequently been demonstrated clinically. However, drug binding to resting and inactivated channel conformations has been studied systematically only in the case of meperidine. The aim of this in vitro study was to investigate the effects of four currently used opioids on heterologously expressed neuronal (NaV(1.2)) voltage-gated sodium channels. Block of sodium currents was studied at hyperpolarized holding potentials and at depolarized potentials inducing either fast- or slow-inactivation. Sufentanil, fentanyl and tramadol but not morphine reversibly suppressed sodium inward currents at high concentrations (half-maximum blocking concentrations (IC50) 49+/-4, 141+/-6 and 103+/-8 microM) when depolarizations were started from hyperpolarized holding potentials. Short depolarizations inducing fast-inactivation and long prepulses inducing slow-inactivation significantly (*p < or = 0.001) increased the blocking potency for these opioids. 15% slow inactivated channels reduced the respective IC50 values to 5+/-3, 12+/-2 and 21+/-2 microM. These results show that: (1) Sufentanil, fentanyl and tramadol block voltage-gated sodium channels with half-maximum inhibitory concentrations similar to the IC50 reported for meperidine. (2) Slow inactivation--a physiological mechanism to suppress ectopic activity in response to slow shifts in membrane potential--increases binding affinity for sufentanil, fentanyl and tramadol. (3) Morphine has no such effects.

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.

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.

Preferred mexiletine block of human sodium channels with IVS4 mutations and its pH-dependence

The effects of extracellular pH (6.2, 7.4 and 8.2) and 0.1 mM mexiletine, a channel blocker of the lidocaine type, are studied on two mutations of the fourth voltage sensor of the Nav1.4 sodium channel, R1448H/C. The fast inactivated channel state to which mexiletine preferentially binds is destabilized by the mutations. By contrast to the expected low response of R1448H/C carriers, mexiletine is particularly effective in preventing exercise-induced stiffness and paralysis from which these patients suffer. Our measurements performed in the whole-cell mode on stably transfected HEK cells show for the first time that the mutations strikingly accelerate closed-state inactivation and, as steady-state fast inactivation is shifted to more negative potentials, stabilize the fast inactivated channel state in the potential range around the resting potential. At pH 7.4 and 8.2, the phasic mexiletine block is larger for R1448C (55%) and R1448H (47%) than for wild-type channels (31%) due to slowed recovery from block (tau is approximately 520 ms for R1448C versus 270 ms for wild-type at pH 7.4) although the recovery from inactivation is slightly faster for the mutants (tau is approximately 1.9 ms for R1448C versus 3.8 ms for wild-type at pH 7.4). At pH 6.2, recovery from block is relatively fast (tau is approximately 35 ms for R1448H/C and 14 ms for wild-type) and thus shows no use-dependence. We conclude that enhanced closed-state inactivation expands the concept of a mutation-induced uncoupling of channel inactivation from activation to a new potential range and that the higher mexiletine efficacy in R1448H/C carriers compared to other myotonic patients offers a pharmacogenetic strategy for mutation-specific treatment.

An improved model for the binding of lidocaine and structurally related local anaesthetics to fast-inactivated voltage-operated sodium channels, showing evidence of cooperativity

1. The interaction of lidocaine-like local anaesthetics with voltage-operated sodium channels is traditionally assumed to be characterized by tighter binding of the drugs to depolarized channels. As inactivated and drug-bound channels are both unavailable on depolarization, an indirect approach is required to yield estimates for the dissociation constants from channels in inactivated states. The established model, originally described by Bean et al., describes the difference in affinity between resting and inactivated states in terms of the concentration dependence of the voltage shift in the availability curve. We have tested the hypothesis that this model, which assumes a simple Langmuir relationship, could be improved by introducing a Hill-type exponent, which would take into account potential sources of cooperativity. 2. Steady-state block by lidocaine was studied in heterologously (HEK 293) expressed human skeletal muscle sodium channels and compared with experimental data previously obtained for 2,6-dimethylphenol, 3,5-dimethyl-4-chlorophenol, and 4-chlorophenol. Cells were clamped to membrane potentials from -150 to -5 mV, and a subsequent test pulse was used to assess the number of channels available to open. 3. All compounds shifted the voltage dependence of channel availability in the direction of negative prepulse potentials. Prediction of the concentration dependence of the voltage shift in the availability curve was improved by the modified model, as shown by a marked reduction in the residual sum of squares. 4. For all compounds, the Hill-type exponent was significantly greater than one. These results could be interpreted in the light of the contemporary hypothesis that lidocaine functions as an allosteric gating effector to enhance sodium channel inactivation by strengthening the latch mechanism of inactivation, which is considered to be a particle-binding process allosterically coupled to activation. Alternatively, they could be interpreted by postulating additional binding sites for lidocaine on fast-inactivated sodium channels.

Nanotechnology for neuronal ion channels

Ion channels provide the basis for the regulation of electrical excitability in the central and peripheral nervous systems. This review deals with the techniques that make the study of structure and function of single channel molecules in living cells possible. These are the patch clamp technique, which was derived from the conventional voltage clamp method and is currently being developed for automated and high throughput measurements; and fluorescence and nano-techniques, which were originally applied to non-biological surfaces and are only recently being used to study cell membranes and their proteins, especially in combination with the patch clamp technique. The characterisation of the membrane channels by techniques that resolve their morphological and physical properties and dynamics in space and time in the nano range is termed nanoscopy.

Blockade of voltage-operated neuronal and skeletal muscle sodium channels by S(+)- and R(-)-ketamine

Besides its general anesthetic effect, ketamine has local anesthetic-like actions. We studied the voltage- and use-dependent interaction of S(+)- and R(-)-ketamine with two different isoforms of voltage-operated sodium channels, with a special emphasis on the difference in affinity between resting and inactivated channel states. Rat brain IIa and human skeletal muscle sodium channels were heterologously expressed in human embryonic kidney 293 cells. S(+)- and R(-)-ketamine reversibly suppressed whole-cell sodium inward currents; the 50% inhibitory concentration values at -70 mV holding potential were 240 +/- 60 microM and 333 +/- 93 microM for the neuronal isoform and 59 +/- 10 microM and 181 +/- 49 microM for the skeletal muscle isoform. S(+)-ketamine was significantly more potent than R(-)-ketamine in the skeletal muscle isoform only. Ketamine had a higher affinity to inactivated than to resting channels. However, the estimated difference in affinity between inactivated and resting channels was only 8- to 10-fold, and the time course of drug equilibration between inactivated and resting channels was too fast to cause use-dependent block at 10 Hz up to a concentration of 300 microM. These results suggest that ketamine is less effective than lidocaine-like local anesthetics in stabilizing the inactivated channel state.

IMPLICATIONS

Blockade of sodium channels by ketamine shows voltage dependency, an important feature of local anesthetic action. However, ketamine is less effective than lidocaine-like local anesthetics in stabilizing the inactivated state. Because it does not elicit phasic blockade at small concentrations, its ability to reduce the firing frequency of action potentials may be small.

Genetic disorders of neuromuscular ion channels

Ion channels are complex proteins that span the lipid bilayer of the cell membrane, where they orchestrate the electrical signals necessary for normal function of the central nervous system, peripheral nerve, and both skeletal and cardiac muscle. The role of ion channel defects in the pathogenesis of numerous disorders, many of them neuromuscular, has become increasingly apparent over the last decade. Progress in molecular biology has allowed cloning and expression of genes that encode channel proteins, while comparable advances in biophysics, including patch-clamp electrophysiology and related techniques, have made the study of expressed proteins at the level of single channel molecules possible. Understanding the molecular basis of ion channel function and dysfunction will facilitate both the accurate classification of these disorders and the rational development of specific therapeutic interventions. This review encompasses clinical, genetic, and pathophysiological aspects of ion channels disorders, focusing mainly on those with neuromuscular manifestations.

Block of voltage-operated sodium channels by 2,6-dimethylphenol, a structural analogue of lidocaine's aromatic tail

1. The structural features that determine the state-dependent interaction of local anaesthetics with voltage-operated sodium channels are still a matter of debate. We have studied the blockade of sodium channels by 2,6-dimethylphenol, a phenol derivative which resembles the aromatic tail of lidocaine, etidocaine, and bupivacaine. 2. The effects of 2,6-dimethylphenol were studied on heterologously (HEK 293) expressed rat neuronal (rat brain IIA) and human skeletal muscle (hSkM1) sodium channels using whole-cell voltage-clamp experiments. 3. 2,6-Dimethylphenol was effective in blocking whole-cell sodium inward currents. Its potency was comparable to the potency of lidocaine previously obtained with similar protocols by others. The IC(50) at -70 mV holding potential was 150 and 187 microM for the skeletal muscle and the neuronal isoform, respectively. In both isoforms, the blocking potency increased with the fraction of inactivated channels at depolarized holding potentials. However, the block achieved at -70 mV with respect to -150 mV holding potential was significantly higher only in the skeletal muscle isoform. The estimated dissociation constant K(d) from the inactivated state was 25 microM and 28 microM in the skeletal muscle and the neuronal isoform, respectively. The kinetics of drug equilibration between resting and inactivated channel states were about 10 fold faster compared with lidocaine. 4. Our results show that the blockade induced by 2,6-dimethylphenol retains voltage-dependency, a typical feature of lidocaine-like local anaesthetics. This is consistent with the hypothesis that the 'aromatic tail' determines the state-dependent interaction of local anaesthetics with the sodium channel.

Enhanced inactivation and pH sensitivity of Na(+) channel mutations causing hypokalaemic periodic paralysis type II

Hypokalaemic periodic paralysis (hypoPP) is a dominantly inherited muscle disorder characterized by episodes of flaccid weakness. Previous genetic studies revealed mutations in the voltage-gated calcium channel alpha1-subunit (CACNA1S gene) in families with hypoPP (type I). Electrophysiological studies on these mutants in different expression systems could not explain the pathophysiology of the disease. In addition, several mutations (Arg669His, Arg672His, Arg672Gly and Arg672Ser) in the voltage sensor of the skeletal muscle sodium channel alpha-subunit (SCN4A gene) have been found in families with hypoPP (type II). For Arg672Gly/His a fast inactivation defect was described, and for Arg669His an impairment of slow inactivation was reported. Except for the substitution for serine, we have now expressed all mutants in a human cell-line and studied them electrophysiologically. Patch-clamp recordings show an enhanced fast inactivation for all three mutations, whereas two of them reveal enhanced slow inactivation. This may reduce the number of functional sodium channels at resting membrane potential and contribute to the long-lasting periods of paralysis experienced by hypoPP patients. The gating of both histidine mutants (Arg669His, Arg672His) can be modulated by changes of extra- or intracellular pH. The inactivation defects of Arg669His and Arg672His can be alleviated by low pH to a significant degree, suggesting that the decrease of pH in muscle cells (e.g. during muscle work) might lead to an auto-compensation of functional defects. This may explain a delay or prevention of paralytic attacks in patients by slight physical activity. Moreover, the histidine residues may be the target for a potential therapeutic action by acetazolamide.

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.

The anesthetic propofol modulates gating in paramyotonia congenita mutant muscle sodium channels

We examined the effects of propofol on a paramyotonia congenita mutant skeletal muscle sodium channel in vitro, because life-threatening complications resulting from severe muscle rigidity during induction of anesthesia have been observed using other anesthetics in patients with hereditary sodium channel myopathies. Our hypothesis was that propofol might interact directly with mutant channels, causing enhanced muscle excitability in affected patients. Whole-cell voltage-clamp experiments were performed on HEK 293 cells expressing R1448H mutant sodium channels. Propofol blocked sodium inward current at clinical concentrations (5 micromol/L) when depolarizing pulses were started from holding potentials close to the physiological resting potential (-70 mV). Higher propofol concentrations (>/=25 micromol/L) accelerated pathologically delayed inactivation kinetics and delayed pathologically enhanced recovery from inactivation. Our in vitro results show that inactivation-deficient sodium channels are specifically targeted and blocked by propofol. This might reduce enhanced muscle excitability experienced by affected patients in vivo.

Propofol blocks human skeletal muscle sodium channels in a voltage-dependent manner

Propofol decreases muscle tone in the absence of neuromuscular blocking drugs. This effect probably cannot be attributed solely to central nervous depression. We studied the effects of propofol on heterologously expressed skeletal muscle sodium channels. Our hypothesis was that the decrease in muscle tone may partly be attributed to an interaction of propofol with sarcolemmal sodium channels. Cells were voltage clamped and whole-cell sodium inward currents were recorded in the absence and presence of propofol. When depolarizing pulses to 0 mV were started from a holding potential close to the normal resting potential of muscle (-70 mV), or when a 2.5-s prepulse inducing slow inactivation was applied before the test pulse at -100 mV, a significant reduction in the peak current amplitude was achieved by 10 and 5 microM propofol, respectively (P < 0.001). Half-maximum blocking concentrations with these protocols were 23 and 22 microM. Blocking potency increased at depolarized membrane potentials with the fraction of inactivated channels; the estimated dissociation constant K(d) from the inactivated state was 4.6 microM. These results suggest that propofol significantly blocks sarcolemmal sodium channels at clinically relevant concentrations while maintaining potentials close to the physiological resting potential.

IMPLICATIONS

Voltage-gated sodium channels mediate the initiation and propagation of action potentials along the sarcolemma. Results from our study show that those channels are targeted and blocked in a concentration- and voltage-dependent manner by propofol. This mechanism may contribute to the reduction in muscle excitability.

A new mutation in a family with cold-aggravated myotonia disrupts Na(+) channel inactivation

OBJECTIVE

To identify the molecular and physiologic abnormality in familial myotonia with cold sensitivity, hypertrophy, and no weakness.

BACKGROUND

Sodium channel mutations were previously identified as the cause of several allelic disorders with varying combinations of myotonia and periodic paralysis. A three-generation family with dominant myotonia aggravated by cooling, but no weakness, was screened for mutations in the skeletal muscle sodium channel alpha-subunit gene (SCN4A).

METHODS

Single-strand conformation polymorphism was used to screen all 24 exons of SCN4A and abnormal conformers were sequenced to confirm the presence of mutations. The functional consequence of a SCN4A mutation was explored by recording sodium currents from human embryonic kidney cells transiently transfected with an expression construct that was mutated to reproduce the genetic defect.

RESULTS

A three-generation Italian family with myotonia is presented, in which a novel SCN4A mutation (leucine 266 substituted by valine, L266V) is identified. This change removes only a single methylene group from the 1,836-amino-acid protein, and is present in a region of the protein previously not known to be critical for channel function (domain I transmembrane segment 5). Electrophysiologic studies of the L266V mutation showed defects in fast inactivation, consistent with other disease-causing SCN4A mutations studied to date. Slow inactivation was not impaired.

CONCLUSIONS

This novel mutation of the sodium channel indicates that a single carbon change in a transmembrane alpha-helix of domain I can alter channel inactivation and cause cold-sensitive myotonia.

Structural requirements for voltage-dependent block of muscle sodium channels by phenol derivatives

We have studied the effects of four different phenol derivatives, with methyl and halogen substituents, on heterologously expressed human skeletal muscle sodium channels, in order to find structural determinants of blocking potency. All compounds blocked skeletal muscle sodium channels in a concentration-dependent manner. The methylated phenol 3-methylphenol and the halogenated phenol 4-chlorophenol blocked sodium currents on depolarization from -100 mV to 0 mV with IC(50) values of 2161 and 666 microM respectively. Methylation of the halogenated compound further increased potency, reducing the IC(50) to 268 microM in 2-methyl-4-chlorophenol and to 150 microM in 3,5-dimethyl-4-chlorophenol. Membrane depolarization before the test depolarization increased sodium channel blockade. When depolarizations were started from -70 mV or when a 2.5 s prepulse was introduced before the test pulse inducing slow inactivation, the IC(50) was reduced more than 3 fold in all compounds. The values of K(D) for the fast-inactivated state derived from drug-induced shifts in steady-state availability curves were 14 microM for 3,5-dimethyl-4-chlorophenol, 19 microM for 2-methyl-4-chlorophenol, 26 microM for 4-chlorophenol and 115 microM for 3-methylphenol. All compounds accelerated the current decay during depolarization and slowed recovery from fast inactivation. No relevant frequency-dependent block after depolarizing pulses applied at 10, 50 and 100 Hz was detected for any of the compounds. All the phenol derivatives that we examined are effective blockers of skeletal muscle sodium channels, especially in conditions that are associated with membrane depolarization. Blocking potency is increased by halogenation and by methylation with increasing numbers of methyl groups.

Phenol derivatives accelerate inactivation kinetics in one inactivation-deficient mutant human skeletal muscle Na(+) channel

Altered inactivation kinetics in skeletal muscle Na(+) channels due to mutations in the encoding gene are causal for the alterations in muscle excitability in nondystrophic myotonia. Na(+) channel blockers like lidocaine and mexiletine, suggested for therapy of myotonia, do not reconstitute inactivation in channels with defective inactivation in vitro. We examined the effects of four methylated and/or halogenated phenol derivatives on one heterologously expressed inactivation-deficient Paramyotonia congenita-mutant (R1448H) muscle Na(+) channel in vitro. All these compounds accelerated delayed inactivation of R1448H-whole-cell currents during a depolarization and delayed accelerated recovery from inactivation. The potency of these effects paralleled the potency of the drugs to block the peak current amplitude. We conclude that the investigated phenol derivatives affect inactivation-deficient Na(+) channels more specifically than lidocaine and mexiletine. However, for all compounds, the effect on inactivation was accompanied by a substantial block of the peak current amplitude.

Patch clamp studies of the thr1313met mutant sodium channel causing paramyotonia congenita

Paramyotonia congenita (PC) is an autosomal-dominant disorder due to a point mutation in the adult skeletal muscle Na channel gene. Muscle fibers from PC patients have normal membrane properties at 32 degrees C. At 27 degrees C, they are inexcitable, have increased Na conductance, and have a reduced resting membrane potential of -40 mV. To define the biophysical basis for the muscle membrane abnormalities, we performed patch clamp whole-cell and outside-out single Na channel studies at 22 degrees C on cultured human muscle cells from 4 control patients and 2 sisters with PC and the thr1313met mutant Na channel. The whole-cell studies showed no difference in window currents. Unlike cells transfected with the thr1313met mutant Na channel, the inactivation time constant, tau(h), for PC cells was similar to control cells. For PC recordings containing long-duration single Na channel openings, mean open time was prolonged at -60, -40, and -20 mV. The long-duration Na channel openings occurred randomly with no evidence of modal gating. The number of channel openings, occurrence of late openings, and the prolonged mean open time resulted in a sustained inward Na current at -40 mV. We suggest that the biophysical marker of the thr1313met mutant Na channel is a voltage- and temperature-dependent abnormality in mutant single Na channel behavior.

Voltage-dependent blockade of normal and mutant muscle sodium channels by benzylalcohol

1. We studied the effects of benzylalcohol on heterologously expressed wild type (WT), paramyotonia congenita (R1448H) and hyperkalaemic periodic paralysis (M1360V) mutant alpha-subunits of human skeletal muscle sodium channels. 2. Benzylalcohol blocked rested channels at -150 mV membrane potential, with an ECR(50) of 5.3 mM in wild type, 5.1 mM in R1448H, and 6.2 mM in M1360V. When blockade was assessed at -100 mV, the ECR(50) was reduced in R1448H (2 mM) compared with both wild type (4.3 mM; P<0.01) and M1360V (4.3 mM). 3. Membrane depolarization before the test depolarization significantly promoted benzylalcohol-induced sodium channel blockade. The values of K(D) for the fast-inactivated state derived from benzylalcohol-induced shifts in steady-state availability curves were 0.66 mM in wild type and 0.58 mM in R1448H. In the presence of slow inactivation induced by 2.5 s depolarizing prepulses, the ECI(50) for benzylalcohol-induced current inhibition was 0.59 mM in wild type and 0.53 mM in R1448H. 4. Recovery from fast inactivation was prolonged in the presence of drug in all clones. 5. Benzylalcohol induced significant frequency-dependent block at stimulating frequencies of 10, 50, and 100 Hz in all clones. 6. Our results clearly show that benzylalcohol is an effective blocker of muscle sodium channels in conditions that are associated with membrane depolarization. Mutants that enter voltage-dependent inactivation at more hyperpolarized membrane potentials compared with wild type are more sensitive to inhibitory effects at the normal resting potential.

Endogenous expression of the beta1A sodium channel subunit in HEK-293 cells

The expression of the sole alpha-subunit of muscle or brain sodium channels in frog oocytes mediates currents with a bimodal inactivation with an abnormal slow mode that is strongly depressed only by co-expression of the beta1-subunit. In contrast, in the expression of the alpha-subunit in the human embryonic kidney cell line, HEK-293, the slow mode is almost absent, suggesting an endogenous expression of the beta1-subunit. We have tested this hypothesis by reverse transcriptase-polymerase chain reaction. We found an abundant expression of mRNA encoding the beta1A splicing of the putative regulatory sodium channel subunit but no mRNA encoding the beta1-subunit in HEK cells. This finding is consistent with the idea that the endogenous beta1A-subunit is sufficient for suppressing the slowly inactivating mode of sodium currents by co-assembly with alpha-subunits, and calls attention to the reliability of effects attributed in HEK cells to alpha-beta1 co-expression.

Different effects of mexiletine on two mutant sodium channels causing paramyotonia congenita and hyperkalemic periodic paralysis

Effects of the antiarrhythmic and antimyotonic drug mexiletine were studied on two sodium channel mutants causing paramyotonia congenita (R1448H) and an overlap paramyotonic and hyperkalemic paralytic syndrome (M1360V). Channels were expressed in human embryonic kidney cells and studied electrophysiologically, using the whole-cell patch-clamp technique. Compared to the wild-type, channel, both mutants showed alterations of inactivation, i.e. slower inactivation, left shift of steady-state inactivation and faster recovery from inactivation. Mexiletine caused a significantly larger use-dependent block of the R1448H mutant when compared to M1360V and wild-type channels. This can be explained by a prolonged recovery from mexiletine block as observed for R1448H channels, since the affinity of mexiletine for the inactivated state was similar for all three clones. The use-dependent block of sodium channels by mexiletine reduces repetitive series of action potentials and therefore improves muscle stiffness in myotonic patients. The enhanced use-dependent block as seen with R1448H may explain the extraordinary therapeutic efficacy of mexiletine in most patients with paramyotonia congenita.

Myopathic mutations affect differently the inactivation of the two gating modes of sodium channels

Three groups of mutations of the alpha subunit of the rat skeletal muscle sodium channel (rSkM1), homologous to mutations linked to human muscle hereditary diseases, have been studied by heterologous expression in frog oocytes: S798F, G1299E, G1299V, and G1299A, linked with potassium-aggravated myotonia (PAM); T1306M, R1441C and R1441P, linked with paramyotonia congenita (PC); T698M and M1353V, linked with the hyperkalemic periodic paralysis (HyPP). Wild-type rSkM1 channels (WT) show two gating modes, M1 and M2, which differ mainly in the process of inactivation. The naturally most representative mode M1 is tenfold faster and develops at approximately 30 mV less depolarized potentials. A common feature of myopathy-linked mutants is an increase in the mode M2 probability, P(M2), but phenotype-specific alterations of voltage-dependence and kinetics of inactivation of both modes are also observed. The coexpression of the sodium channel beta1 subunit, which has been studied for WT and for the five best expressing mutants, generally caused a threefold reduction of P(M2) without changing the properties of the individual modes. This indicates that the mutations do not affect the alpha - beta1 interaction and that the phenotypic changes in P(M2) observed for the enhanced mode M2 behavior of the sole alpha subunits, although largely depressed in the native tissue, are likely to be the most important functional modification that causes the muscle hyperexcitability observed in all patients carrying the myotonic mutations. The interpretation of the more phenotype-specific changes revealed by our study is not obvious, but it may offer clues for understanding the different clinical manifestations of the diseases associated with the various mutations.

Voltage-dependent block of normal and mutant muscle sodium channels by 4-Chloro-m-Cresol

1 The effects of 4-Chloro-m-Cresol (4-CmC) were examined on heterologously expressed wild type (WT), Paramyotonia Congenita (R1448H) and Hyperkalemic Periodic Paralysis (M1360V) mutant alpha-subunits of human muscle sodium channels. 2 Block of rested sodium channels caused by 4-CmC was concentration-dependent with an ECR50 of 0.40 mM in WT, 0.45 mM in R1448H and 0.49 mM in M1360V. 3 Inactivation significantly promoted 4-CmC-induced sodium channel block in all clones indicated by 4-CmC-induced shifts of steady-state availability curves, reflecting a higher proportion of channel block at depolarized membrane potentials. Channel block was almost complete (>90%) at concentrations close to the ECR50 (0.5 mM) on application of an inactivating prepulse before the test pulse. 4 4-CmC accelerated the current decay following depolarization and prolonged recovery from inactivation in all clones. Of these, R1448H, the mutant which displayed severely impaired inactivation in the controls, responded to 4-CmC with the most pronounced acceleration of inactivation. Control experiments revealed enhanced recovery from inactivation in the mutants, which was restored to normal in 0.1 mM 4-CmC. 5 4-CmC induced no additional frequency-dependent block. 6 Our results clearly demonstrate that 4-CmC is as effective as lidocaine (Fan et al., 1996) in blocking muscle sodium channels. Low concentrations of the compound (</=ECR50) were able to restore pathologically accelerated recovery from inactivation and impaired inactivation in the mutants to the WT value.

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.

A human muscle Na+ channel mutation in the voltage sensor IV/S4 affects channel block by the pentapeptide KIFMK

1. Whole cell patch clamping of transfected HEK293 cells was used to examine the effects of a pentapeptide (KIFMK) containing the proposed inactivation particle of the Na+ channel on two mutations causing myotonia. One mutation (R1448P) is located in the voltage sensor IV/S4, and the other one (G1306E) near the postulated inactivation gate within the III-IV linker. 2. In the absence of peptide, currents of wild-type (WT) and mutant human muscle Na+ channels decayed monoexponentially with inactivation time constants that were 5-fold (R1448P) and 3-fold (G1306E) larger for the mutants. Upon intracellular application of KIFMK (0.3-1 mM) the current decay became biexponential with an additional fast decaying component that increased in amplitude with depolarization. 3. Furthermore, the peptide induced large tail currents upon repolarization, indicating that KIFMK prevents inactivation by blocking open Na+ channels. The peak of this tail current decreased only slowly with depolarizations of increasing duration. The voltage dependence of this decline indicated that the dissociation rate of the charged peptide decreased with depolarization. Increased external [Na+] ([Na+]e) antagonized block by KIFMK, consistent with a pore-blocking mechanism. 4. The results are discussed with regard to a three-state model for one open, an absorbing inactivated and one blocked state with voltage-dependent on- and off-rates for peptide binding. The peptide had qualitatively similar effects on WT and both mutants, indicating that the freely diffusible peptide accelerates the current decay in all three clones. However, for the R1448P mutation the affinity for KFIMK was decreased and the voltage dependence of peptide block was changed in a similar way to the voltage dependence of inactivation. These data suggest that the mutation R1448P affects the voltage-dependent formation of a receptor site for both the inactivation particle and KIFMK.

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.

Hyperkalemic periodic paralysis M1592V mutation modifies activation in human skeletal muscle Na+ channel

Mutations in the human skeletal muscle Na+ channel underlie the autosomal dominant disease hyperkalemic periodic paralysis (HPP). Muscle fibers from affected individuals exhibit sustained Na+ currents thought to depolarize the sarcolemma and thus inactivate normal Na+ channels. We expressed human wild-type or M1592V mutant alpha-subunits with the beta1-subunit in Xenopus laevis oocytes and recorded Na+ currents using two-electrode and cut-open oocyte voltage-clamp techniques. The most prominent functional difference between M1592V mutant and wild-type channels is a 5- to 10-mV shift in the hyperpolarized direction of the steady-state activation curve. The shift in the activation curve for the mutant results in a larger overlap with the inactivation curve than that observed for wild-type channels. Accordingly, the current through M1592V channels displays a larger noninactivating component than does that through wild-type channels at membrane potentials near -40 mV. The functional properties of the M1592V mutant resemble those of the previously characterized HPP T704M mutant. Both clinically similar phenotypes arise from mutations located at a distance from the putative voltage sensor of the channel.