Search

Mutations that disrupt the TBC1D24 presynaptic protein have been implicated in various neurological disorders including epilepsy, chronic encephalopathy, DOORS (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures) syndrome, nonsyndromic hearing loss, and myoclonus. We present the case of a 22-month-old male with infantile-onset paroxysmal episodes of facial and limb myoclonus. The episodes were linked to biallelic variants in exon 2 of the TBC1D24 gene that lead to amino acid changes (c.304C >T/p.Pro102Ser and c.410T > C/p.Val137Ala), each variant being inherited from a parent. Follow-up imaging in adolescence revealed widened right cerebellar sulci. We discuss the evolving landscape of TBC1D24 associated phenotypes; this case adds to a growing body of evidence linking this gene to movement disorders in children.

KEY POINTS: During myotonia congenita, reduced chloride (Cl(−)) conductance results in impaired muscle relaxation and increased muscle stiffness after forceful voluntary contraction. Repetitive contraction of myotonic muscle decreases or even abolishes myotonic muscle stiffness, a phenomenon called ‘warm up’. Pharmacological inhibition of low Cl(−) channels by anthracene‐9‐carboxylic acid in muscle from mice and ADR (‘arrested development of righting response’) muscle from mice showed a relaxation deficit under physiological conditions compared to wild‐type muscle. At increased osmolarity up to 400 mosmol L(–1), the relaxation deficit of myotonic muscle almost reached that of control muscle. These effects were mediated by the cation and anion cotransporter, NKCC1, and anti‐myotonic effects of hypertonicity were at least partly antagonized by the application of bumetanide. ABSTRACT: Low chloride‐conductance myotonia is caused by mutations in the skeletal muscle chloride (Cl(−)) channel gene type 1 (CLCN1). Reduced Cl(−) conductance of the mutated channels results in impaired muscle relaxation and increased muscle stiffness after forceful voluntary contraction. Exercise decreases muscle stiffness, a phenomena called ‘warm up’. To gain further insight into the patho‐mechanism of impaired muscle stiffness and the warm‐up phenomenon, we characterized the effects of increased osmolarity on myotonic function. Functional force and membrane potential measurements were performed on muscle specimens of ADR (‘arrested development of righting response’) mice (an animal model for low gCl(–) conductance myotonia) and pharmacologically‐induced myotonia. Specimens were exposed to solutions of increasing osmolarity at the same time as force and membrane potentials were monitored. In the second set of experiments, ADR muscle and pharmacologically‐induced myotonic muscle were exposed to an antagonist of NKCC1. Upon osmotic stress, ADR muscle was depolarized to a lesser extent than control wild‐type muscle. High osmolarity diminished myotonia and facilitated the warm‐up phenomenon as depicted by a faster muscle relaxation time (T (90/10)). Osmotic stress primarily resulted in the activation of the NKCC1. The inhibition of NKCC1 with bumetanide prevented the depolarization and reversed the anti‐myotonic effect of high osmolarity. Increased osmolarity decreased signs of myotonia and facilitated the warm‐up phenomenon in different in vitro models of myotonia. Activation of NKCC1 activity promotes warm‐up and reduces the number of contractions required to achieve normal relaxation kinetics.

In myotonia, reduced Cl- conductance of the mutated ClC-1 channels causes hindered muscle relaxation after forceful voluntary contraction due to muscle membrane hyperexcitability. Repetitive contraction temporarily decreases myotonia, a phenomena called "warm up." The underlying mechanism for the reduction of hyperexcitability in warm-up is currently unknown. Since potassium displacement is known to reduce excitability in, for example, muscle fatigue, we characterized the role of potassium in native myotonia congenita (MC) muscle. Muscle specimens of ADR mice (an animal model for low gCl- conductance myotonia) were exposed to increasing K+ concentrations. To characterize functional effects of potassium ion current, the muscle of ADR mice was exposed to agonists and antagonists of the big conductance Ca2+-activated K+ channel (BK) and the voltage-gated Kv7 channel. Effects were monitored by functional force and membrane potential measurements. By increasing [K+]0 to 5 mM, the warm-up phenomena started earlier and at [K+]0 7 mM only weak myotonia was detected. The increase of [K+]0 caused a sustained membrane depolarization accompanied with a reduction of myotonic bursts in ADR mice. Retigabine, a Kv7.2-Kv7.5 activator, dose-dependently reduced relaxation deficit of ADR myotonic muscle contraction and promoted the warm-up phenomena. In vitro results of this study suggest that increasing potassium conductivity via activation of voltage-gated potassium channels enhanced the warm-up phenomena, thereby offering a potential therapeutic treatment option for myotonia congenita.

Hypokalemic periodic paralysis is a skeletal muscle disease characterized by episodic weakness associated with low serum potassium. We compared clinical and biophysical effects of R222W, the first hNaV1.4 domain I mutation linked to this disease. R222W patients exhibited a higher density of fibers with depolarized resting membrane potentials and produced action potentials that were attenuated compared to controls. Functional characterization of the R222W mutation in heterologous expression included the inactivation deficient IFM/QQQ background to isolate activation. R222W decreased sodium current and slowed activation without affecting probability. Consistent with the phenotype of muscle weakness, R222W shifted fast inactivation to hyperpolarized potentials, promoted more rapid entry, and slowed recovery. R222W increased the extent of slow inactivation and slowed its recovery. A two-compartment skeletal muscle fiber model revealed that defects in fast inactivation sufficiently explain action potential attenuation in patients. Molecular dynamics simulations showed that R222W disrupted electrostatic interactions within the gating pore, supporting the observation that R222W promotes omega current at hyperpolarized potentials. Sodium channel inactivation defects produced by R222W are the primary driver of skeletal muscle fiber action potential attenuation, while hyperpolarization-induced omega current produced by that mutation promotes muscle fiber depolarization.

ZusammenfassungIn der Behandlung der myotonen Dystrophien, nicht dystrophen Myotonien und periodischen Paralysen haben sich in den letzten Jahren einige neue Aspekte ergeben, die in der aktualisierten Leitlinie zu myotonen Dystrophien, nicht dystrophen Myotonien und periodischen Paralysen zusammenfassend dargestellt sind.Nach wie vor besteht eine europaweit nur sehr eingeschränkte Verfügbarkeit von Mexiletin, welches in Deutschland nur noch über die Auslandsapotheke aus z. B. Japan, den USA oder Kanada in einer Dosierung von 100 mg oder 200 mg bezogen werden kann, wenngleich die Wirksamkeit von Mexiletin bei myotoner Dystrophie Typ 1 in einer amerikanischen Studie erneut bestätigt wurde (Logigian et al., 2010).In einer rezenten Studie konnte gezeigt werden, dass der Carboanhydrasehemmer Dichlorphenamid sowohl bei hypokaliämischer als auch bei hyperkaliämischer periodischer Paralyse die Attackenfrequenz senkt, allerdings war der Unterschied bei der hyperkaliämischen Lähmung nicht signifikant (Sansone et al., 2016). Bei dieser Studie handelt es sich um die Kombination zweier randomisierter Untersuchungen über 9 Wochen, kombiniert mit einer einjährigen Extensionsphase, bei der alle Teilnehmer Dichlorphenamid erhielten. Die Studie wurde gegen Placebo durchgeführt; der ursprünglich geplante Vergleich mit Acetazolamid wurde abgebrochen, weil die an der Studie teilnehmenden Patienten Dichlorphenamid aufgrund subjektiv besserer Wirksamkeit vorzogen und deshalb keine Gruppe für einen Vergleich Dichlorphenamid/Acetazolamid gebildet werden konnte. Die Hauptnebenwirkungen waren Parästhesien, Nierensteinbildung und eine Verlangsamung des Denkens. Die Studie erlaubte keine Rückschlüsse auf die Beziehung zwischen Wirksamkeit und Genotyp; die häufigste Mutation war T704 M (Nav1.4) bei der hyperkaliämischen Lähmung und R528H (Cav1.1) und R1239H (Cav1.1) bei der hypokaliämischen Lähmung. Ein Patient in der hypokaliämischen Gruppe mit der Mutation pR222 W (Nav1.4) verschlechterte sich. Dichlorphenamid ist inzwischen als Keveyis in den USA im Handel. In der Europäischen Union ist Dichlorphenamid (noch) nicht zugelassen; es hat aber den Status einer „orphan drug“ und ist somit verordnungsfähig.In einer doppelblinden randomisierten placebokontrollierten Studie mit 22 Patienten mit nicht dystrophen Myotonien konnte gezeigt werden, dass Lamotrigin in einer Dosierung von 300 mg/d die myotone Symptomatik signifikant gegenüber dem Ausgangsbefund verbesserte (Anderson G et al., 2017).Eine offene Behandlungsstudie mit Ranolazin, einem Piperazinderivat, in einer Dosierung von 2 × 500 mg bei 13 Patienten mit Chloridkanalmyotonie ergab eine signifikant reduzierte EMG-Myotonie, eine nach Patientenangaben signifikant reduzierte Muskelsteifigkeit und geringer auch reduzierte Muskelschwäche und eine reduzierte Myotonie in den klinischen Tests (Arnold WD et al., 2017).

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

The original article contains an error during online publication. Table 2 was included during production round and now deleted. The Original article has been corrected.

Hypokalemic periodic paralysis is a skeletal muscle disease characterized by episodic weakness associated with low serum potassium. We compared clinical and biophysical effects of R222W, the first hNaV1.4 domain I mutation linked to this disease. R222W patients exhibited a higher density of fibers with depolarized resting membrane potentials and produced action potentials that were attenuated compared to controls. Functional characterization of the R222W mutation in heterologous expression included the inactivation deficient IFM/QQQ background to isolate activation. R222W decreased sodium current and slowed activation without affecting probability. Consistent with the phenotype of muscle weakness, R222W shifted fast inactivation to hyperpolarized potentials, promoted more rapid entry, and slowed recovery. R222W increased the extent of slow inactivation and slowed its recovery. A two-compartment skeletal muscle fiber model revealed that defects in fast inactivation sufficiently explain action potential attenuation in patients. Molecular dynamics simulations showed that R222W disrupted electrostatic interactions within the gating pore, supporting the observation that R222W promotes omega current at hyperpolarized potentials. Sodium channel inactivation defects produced by R222W are the primary driver of skeletal muscle fiber action potential attenuation, while hyperpolarization-induced omega current produced by that mutation promotes muscle fiber depolarization.

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

Eplerenone, an aldosterone antagonist, repolarizes muscle membrane in-vitro and increases strength in-vivo in channelopathies. In Duchenne dystrophy, it is administered for cardiomyopathy. We studied its mechanism of action on skeletal muscle to test its suitability for increasing strength in Duchenne dystrophy. Using membrane potential measurements, quantitative PCR, ELISA, and Western blots, we examined the effects of eplerenone on skeletal muscle Na,K-ATPase. The repolarizing effect of eplerenone in muscle fibres was counteracted by oubain, an ATPase blocker. In our experiment, ATPA1A mRNA and total ATPase protein were not elevated. Instead, Tyr10 of the α1 subunit was dephosphorylated which would agree with ATPase activation. Dephosporylation of the coupled Akt kinase corroborated our findings. We conclude that eplerenone repolarizes muscle membrane by Na,K-ATPase activation by dephosphorylation at Tyr10. Since ATPase protein is known to be compensatorily increased in Duchenne patients without activity change, eplerenone treatment may be beneficial.

Primary periodic paralyses (PPs) are autosomal dominant ion channel disorders characterized by episodic flaccid weakness associated with variations in serum potassium level. The main prophylactic therapy of choice for PPsis carbonic anhydrase inhibitors that are not always effective. In this report, we described two PP patients who were successfully treated with coenzyme Q10. They remained asymptomatic since initiation of treatment, which may be associated with promotion of energy synthesis, anti-oxidant activity, influence of the fiber type composition and regulation of the expression of gene. To our knowledge, this is the first report of primary periodic paralyses which have been successfully treated with CoQ10. More observations need to substantiate this clinical finding in PPs.

Heterologous expression of sodium channel mutations in hypokalemic periodic paralysis reveals 2 variants on channel dysfunction. Charge-reducing mutations of voltage sensing S4 arginine residues alter channel gating as typically studied with expression in mammalian cells. These mutations also produce leak currents through the voltage sensor module, as typically studied with expression in Xenopus oocytes. DIIIS4 mutations at R3 in the skeletal muscle sodium channel produce gating defects and omega current consistent with the phenotype of reduced excitability. Here, we confirm DIIIS4 R3C gating defects in the oocyte expression system for fast inactivation and its recovery. We provide novel data for the effects of the cysteine mutation on voltage sensor movement, to further our understanding of sodium channel defects in hypokalemic periodic paralysis. Gating charge movement and its remobilization are selectively altered by the mutation at hyperpolarized membrane potential, as expected with reduced serum potassium.

Malignant hyperthermia is a dreaded complication of general anaesthesia. Predisposed individuals can be identified using the standardised caffeine/halothane in-vitro contracture test on a surgically dissected skeletal muscle specimen. Skeletal muscle is composed of muscle fibres and interwoven fascial components. Several malignant hyperthermia-associated neuromuscular diseases are associated with an altered connective tissue composition. We analysed adjacent fascial components of skeletal muscle histologically and physiologically. We investigated whether the fascial tissue is sensitive to electrical or pharmacological stimulation in a way similar to the in-vitro contracture test for diagnosing malignant hyperthermia. Using immunohistochemical staining, α-smooth muscle actin-positive cells (myofibroblasts) were detected in the epi-, endo- and perimysium of human fascial tissue. Force measurements on isolated fascial strips after pharmacological challenge with mepyramin revealed that myofascial tissue is actively regulated by myofibroblasts, thereby influencing the biomechanical properties of skeletal muscle. Absence of electrical reactivity and insensitivity to caffeine and halothane suggests that, reassuringly, the malignant hyperthermia diagnostic in-vitro contracture test is not influenced by the muscular fascial tissue.

To implement chlorine 35 ((35)Cl) magnetic resonance (MR) at a 7-T whole-body MR system and evaluate its feasibility for imaging humans.All examinations were performed with ethical review board approval; written informed consent was obtained from all volunteers. Seven examinations each of brain and muscle in healthy volunteers and four examinations of patients were performed. Two patients with histologically confirmed glioblastoma multiforme underwent brain imaging. (35)Cl MR and (35)Cl inversion-recovery (IR) MR were performed. Two patients with genetically confirmed hypokalemic periodic paralysis underwent calf muscle imaging. Seven multiecho sequences (acquisition time, 5 minutes; voxel dimension, 11 mm(3)) were applied to determine transverse relaxation time as affected by magnetic field heterogeneity (T2*) and chlorine concentration. (35)Cl and sodium 23 ((23)Na) MR were conducted with a 7-T whole-body MR system. (35)Cl longitudinal relaxation time (T1) and T2* of healthy human brain and muscle were determined with a three-dimensional density-adapted-projection reconstruction technique to achieve short echo times and high signal-to-noise ratio (SNR) efficiency. A nonlinear least squares routine and mono- (T1) and biexponential (T2*) models were used for curve fitting.Phantom imaging revealed 15-fold lower SNR and much shorter relaxation times for (35)Cl than (23)Na. In vivo T2* was biexponential and extremely short. Monoexponential fits of T1 revealed 9.2 and 4.0 milliseconds ± 0.7 (standard deviation) for brain and muscle, respectively. In glioblastoma tissue, increased Cl(-) concentrations and increased Cl(-) IR signal intensities were detected. Voxel dimension and acquisition time, respectively, were 6 mm(3) and 9 minutes 45 seconds ((35)Cl MR) and 10 mm(3) and 10 minutes ((35)Cl IR MR). In patients with hypokalemic periodic paralysis versus healthy volunteers, Cl(-) and Na(+) concentrations were increased. Cl(-) concentration of muscle could be determined (voxel size, 11 mm(3); total acquisition time, 35 minutes).MR at 7 T enables in vivo imaging of (35)Cl in human brain and muscle in clinically feasible acquisition times (10-35 minutes) and voxel volumes (0.2-1.3 cm(3)). Pathophysiological changes of Cl(-) homeostasis due to cancer or muscular ion channel disease can be visualized.

Cations leaking through the voltage sensor of mutant sodium or calcium channels underlie hypokalaemic periodic paralysis. Groome et al. use muscle fibre recordings, voltage clamp, and molecular dynamics, to investigate recently discovered Nav1.4 channel mutations. They identify a novel voltage sensor movement that may explain the muscle pathology., Hypokalaemic periodic paralysis is typically associated with mutations of voltage sensor residues in calcium or sodium channels of skeletal muscle. To date, causative sodium channel mutations have been studied only for the two outermost arginine residues in S4 voltage sensor segments of domains I to III. These mutations produce depolarization of skeletal muscle fibres in response to reduced extracellular potassium, owing to an inward cation-selective gating pore current activated by hyperpolarization. Here, we describe mutations of the third arginine, R3, in the domain III voltage sensor i.e. an R1135H mutation which was found in two patients in separate families and a novel R1135C mutation identified in a third patient in another family. Muscle fibres from a patient harbouring the R1135H mutation showed increased depolarization tendency at normal and reduced extracellular potassium compatible with the diagnosis. Additionally, amplitude and rise time of action potentials were reduced compared with controls, even for holding potentials at which all NaV1.4 are fully recovered from inactivation. These findings may be because of an outward omega current activated at positive potentials. Expression of R1135H/C in mammalian cells indicates further gating defects that include significantly enhanced entry into inactivation and prolonged recovery that may additionally contribute to action potential inhibition at the physiological resting potential. After S4 immobilization in the outward position, mutant channels produce an inward omega current that most likely depolarizes the resting potential and produces the hypokalaemia-induced weakness. Gating current recordings reveal that mutations at R3 inhibit S4 deactivation before recovery, and molecular dynamics simulations suggest that this defect is caused by disrupted interactions of domain III S2 countercharges with S4 arginines R2 to R4 during repolarization of the membrane. This work reveals a novel mechanism of disrupted S4 translocation for hypokalaemic periodic paralysis mutations at arginine residues located below the gating pore constriction of the voltage sensor module.

Zusammenfassung Muskuläre Kanalopathien wie Myotonien, dyskaliämische periodische Paralysen (PP), maligne Hyperthermie (MH) und Core-Myopathien sind durch Mutationen in Na+-, K+-, Ca2+- und Cl−-Kanälen bedingt. Eine leichte Membrandepolarisation verursacht myotone Aktivität. Eine vermehrte Depolarisation kann die Über- in Untererregbarkeit umkehren und eine transiente Schwäche hervorrufen. Eine länger anhaltende Depolarisation der Plasma- und T-tubulären Membran ist für die dominanten dyskaliämischen PP die gemeinsame Basis der Schwäche. Der Kaliumspiegel im Serum beeinflusst das Ruhemembranpotenzial, weshalb Abweichungen vom physiologischen Bereich (z. B. durch Schilddrüsendysfunktion) Membrandepolarisation und Muskelschwäche begünstigen. Bei der Anlage zu MH, einer potenziell lebensbedrohlichen hypermetabolen Krise, erhöhen dominante Mutationen im zytoplasmatischen Teil des Ca2+-Freisetzungskanals des sarkoplasmatischen Retikulums die Empfindlichkeit für volatile Anästhetika. Dagegen führen dominante oder rezessive Mutationen im sarkoplasmatischen Teil zur Entleerung der Ca2+-Speicher und so zu Schwäche und Core-Myopathie.

We studied a two-generation family presenting with conditions that included progressive permanent weakness, myopathic myopathy, exercise-induced contracture before normokalaemic periodic paralysis or, if localized to the tibial anterior muscle group, transient compartment-like syndrome (painful acute oedema with neuronal compression and drop foot). 23Na and 1H magnetic resonance imaging displayed myoplasmic sodium overload, and oedema. We identified a novel familial Ca(v)1.1 calcium channel mutation, R1242G, localized to the third positive charge of the domain IV voltage sensor. Functional expression of R1242G in the muscular dysgenesis mouse cell line GLT revealed a 28% reduced central pore inward current and a -20 mV shift of the steady-state inactivation curve. Both changes may be at least partially explained by an outward omega (gating pore) current at positive potentials. Moreover, this outward omega current of 27.5 nS/nF may cause the reduction of the overshoot by 13 mV and slowing of the upstroke of action potentials by 36% that are associated with muscle hypoexcitability (permanent weakness and myopathic myopathy). In addition to the outward omega current, we identified an inward omega pore current of 95 nS/nF at negative membrane potentials after long depolarizing pulses that shifts the R1242G residue above the omega pore constriction. A simulation reveals that the inward current might depolarize the fibre sufficiently to trigger calcium release in the absence of an action potential and therefore cause an electrically silent depolarization-induced muscle contracture. Additionally, evidence of the inward current can be found in 23Na magnetic resonance imaging-detected sodium accumulation and 1H magnetic resonance imaging-detected oedema. We hypothesize that the episodes are normokalaemic because of depolarization-induced compensatory outward potassium flux through both delayed rectifiers and omega pore. We conclude that the position of the R1242G residue before elicitation of the omega current is decisive for its conductance: if the residue is located below the gating pore as in the resting state then outward currents are observed; if the residue is above the gating pore because of depolarization, as in the inactivated state, then inward currents are observed. This study shows for the first time that functional characterization of omega pore currents is possible using a cultured cell line expressing mutant Ca(v)1.1 channels. Likewise, it is the first calcium channel mutation for complicated normokalaemic periodic paralysis.

This exploratory study aims to create an evidence-based comprehensive characterization of hyperkalemic periodic paralysis (hyperPP). HyperPP is a rare genetic disorder that causes episodes of flaccid paralysis. Disease descriptions in the literature are based upon isolated clinical encounters and case reports. We describe the experience of a large cohort of genetically diagnosed individuals with hyperPP. We surveyed genetically characterized individuals age 18 and over to assess disease comorbidities, diagnostic testing, management, and quality of life issues relevant to hyperPP. Myotonia was reported by 55.8 % of subjects and paramyotonia by 45.3 %. There is a relative risk of 3.6 (p

conductance of the mutated channels, the patients may develop generalized muscle rigidity and hypermetabolism during general anaesthesia. The clinical symptoms resemble malignant hyperthermia (MH), which may lead to mistreatment of the patient. Methods: Muscle specimens of ADR mice (an animal model of MC) as well as of human individuals were used and exposed to potent ryanodine receptor type 1 (RyR1) activators and increasing K + concentration. Muscle force was monitored by a standardized diagnostic method for MH, the so-called in vitro contracture test. Results: Neither muscle of ADR mice nor MC muscle (murine and human myotonic muscle) showed pathological contractures after exposure to the potent RyR1 agonists caffeine and halothane. Increasing concentrations of K + had a dose-dependent preventive effect on myotonic stiffness. Conclusion: We conclude that the adverse anaesthetic MH-like episodes observed in MC patients do not primarily originate from an altered Ca 2+ release in skeletal muscle. In MC muscle, this hypermetabolism is facilitated by a (pharmacologically induced) sustained depolarization due to an instable membrane potential. The in vitro results suggest that these patients benefit from tight K + monitoring because of the membrane potential stabilizing effect of K

High-frequency tetanic stimulation is associated with an increase in extracellular and T-tubular K(+) and changes of Na(+) and Cl(-) concentrations, membrane depolarization as well as inactivation of voltage-gated Na(+) channels. These alterations are expected to lead to fiber inexcitability, which is largely prevented by mechanisms intrinsic or extrinsic to muscle fibers. They act by adapting electrical membrane properties or by accelerating the reconstitution of ionic homeostasis. The high Cl(-) conductance of muscle fibers supports the K(+) conductance in fast and complete repolarization and creates a mechanism for the fast reuptake of K(+), thereby reducing the T-tubular K(+) accumulation. Excitability is increased by a Ca(2+) and proteinkinase C dependent inhibition of the Cl(-) conductance which is efficient especially in the T-tubular system. Several mediators activate the Na(+)/K(+)-ATPase and thus enhance the restoration of ionic homeostasis. Examples are purines (ATP, ADP), calcitonin-gene related peptide and adrenaline. It is also necessary to adapt the strength of the sarcoplasmic Ca(2+) concentration to the requirements of tetanic contractions. An overwhelming Ca(2+) signal leads to enzymatically driven excitation-contraction uncoupling. This process is most likely driven by the Ca(2+) dependent protease μ-calpain and might lead to the long-lasting fatigue observed after excessive physical activity.

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle metabolism which is characterized by generalized muscle rigidity, increased body temperature, rhabdomyolysis, and severe metabolic acidosis. The underlying mechanism of MH involves excessive Ca2+ release in myotubes via the ryanodine receptor type 1 (RyR1). As RyR1 is also expressed in B–lymphocytes, this study investigated whether cellular metabolism of native B–lymphocytes was also altered in MH susceptible (MHS) individuals. A potent activator of RyR1, 4–chloro–m–cresol (4-CmC) was used to challenge native B-lymphocytes in a real–time, metabolic assay based on a pH–sensitive silicon biosensor chip. At the cellular level, a dose–dependent, phasic acidification occurred with 4–CmC. The acidification rate, an indicator of metabolic activation, was significantly higher in B–lymphocytes from MHS patients and required 3 to 5 fold lower concentrations of 4–CmC to evoke similar acidification rates to MHN. Native B–lymphocytes from MHS individuals are more sensitive to 4–CmC than those from MHN, reflecting a greater Ca2+ turnover. The acidification response, however, was less pronounced than in muscle cells, presumably reflecting the lower expression of RyR1 in B–lymphocytes.

Introduction Familial hemiplegic migraine (FHM) is a rare autosomal dominant subtype of migraine with aura. The FHM3 subtype is caused by mutations in SCN1A, which is also the most frequent epilepsy gene encoding the voltage-gated Na+ channel NaV1.1. The aim of this study was to explore the clinical, genetic and pathogenetic features of a pure FHM3 family. Methods A three-generation family was enrolled in this study for genetic testing and assessment of clinical features. Whole cell patch-clamp was performed to determine the functions of identified mutant NaV1.1 channels, which were transiently expressed in human tsA201 cells together with β1 and β2 subunits. Results and conclusions We identified a novel SCN1A (p.Leu1624Pro) mutation in a pure FHM family with notably early-onset attacks at mean age of 7. L1624P locates in S3 of domain IV, the same domain as two of four known pure FHM3 mutations. Compared to WT channels, L1624P displayed an increased threshold-near persistent current in addition to other gain-of-function features such as: a slowing of fast inactivation, a positive shift in steady-state inactivation, a faster recovery and higher channel availability during repetitive stimulation. Similar to the known FHM3 mutations, this novel mutation predicts hyperexcitability of GABAergic inhibitory neurons.

Myotonia congenita is a genetic condition that is caused by mutations in the muscle chloride channel gene CLCN1 and characterized by delayed muscle relaxation and muscle stiffness. We here investigate the functional consequences of two novel disease-causing missense mutations, C277R and C277Y, using heterologous expression in HEK293T cells and patch clamp recording.Bothmutationsreducemacroscopicanioncurrentsintransfectedcells.SincehClC-1is a double-barrelled anion channel, this reduction in current amplitude might be caused by altered gating of individual protopores or of joint openings and closing of both protopores. We used non-stationary noise analysis and single channel recordings to separate the mutants' effects on individual and common gating processes. We found that C277Y inverts the voltage dependence and reduces the open probabilities of protopore and common gates resulting in decreases of absolute open probabilities of homodimeric channels to values below 3%. In heterodimeric channels, C277R and C277Y also reduce open probabilities and shift the common gate activation curvetowardspositivepotentials.Moreover,C277YmodifiesporepropertiesofhClC-1.Itreduces singleprotoporecurrentamplitudestoabouttwo-thirdsofwild-typevalues,andinvertstheanion permeability sequence to I − =NO3 − >B r − >Cl − . Our findings predict a dramatic reduction of the muscle fibre resting chloride conductance and thus fully explain the disease-causing effects

To assess the presence and persistence of muscular edema and increased myoplasmic sodium (Na(+)) concentration in Duchenne muscular dystrophy (DMD). We examined eight DMD patients (mean age 9.5 ± 5.4 years) and eight volunteers (mean age 9.5 ± 3.2 years) with 3-tesla proton ((1)H) and (23)Na density-adapted 3D-radial MR sequences. Seven DMD patients were re-examined about 7 months later without change of therapy. The eighth DMD patient was re-examined after 5 and 11 months under medication with eplerenone. We quantified muscle edema on STIR images with background noise as reference and fatty degeneration on T1-weighted images using subcutaneous fat as reference. Na(+) was quantified by a muscular tissue Na(+) concentration (TSC) sequence employing a reference containing 51.3 mM Na(+) with 5 % agarose. With an inversion-recovery (IR) sequence, we determined mainly the myoplasmic Na(+). The normalized muscular (23)Na IR signal intensity was higher in DMD than in volunteers (n = 8, 0.75 ± 0.07 vs. 0.50 ± 0.05, p < 0.001) and persisted at second measurement (n = 7, 1st 0.75 ± 0.07, 2nd 0.73 ± 0.06, p = 0.50). When compared to volunteers (25.6 ± 2.0 mmol/l), TSC was markedly increased in DMD (38.0 ± 5.9 mmol/l, p < 0.001) and remained constant (n = 7, 1st 37.9 ± 6.4 mmol/l, 2nd 37.0 ± 4.0 mmol/l, p = 0.49). Muscular edema (15.6 ± 3.5 vs. 6.9 ± 0.7, p < 0.001) and fat content (0.48 ± 0.08 vs. 0.38 ± 0.01, p = 0.003) were elevated in DMD when compared to volunteers. This could also be confirmed during follow-up (n = 7, p = 0.91, p = 0.12). Eplerenone slightly improved muscle strength and reduced muscular sodium and edema. The permanent muscular Na(+) overload in all DMD patients is likely osmotically relevant and responsible for the persisting, mainly intracellular muscle edema that may contribute to the progressive muscle degeneration.

Becker myotonia is a recessive muscle disease with prevalence of > 1:50,000. It is caused by markedly reduced function of the chloride channel encoded by CLCN1. We describe a Polish patient with severe myotonia, transient weakness, and muscle cramps who only responds to lidocaine. In addition, the patient has Prinzmetal angina pectoris and multiple lipomatosis. He is compound heterozygeous for a novel p.W303X and a frequent p.R894X CLCN1 mutation. CLCN1 exon number variation was excluded by MLPA. His son with latent myotonia was heterozygeous for p.R894X. We discuss the potential relations of the three rare diseases and the inheritance of p.R894X.

Objective: The therapeutic effects of 4-aminopyridine (4AP) were investigated in a randomized, double-blind, crossover trial in 10 subjects with familial episodic ataxia with nystagmus. Methods: After randomization, placebo or 4AP (5 mg 3 times daily) was administered for 2 3-month-long treatment periods separated by a 1-month-long washout period. The primary outcome measure was the number of ataxia attacks per month; the secondary outcome measures were the attack duration and patient-reported quality of life (Vestibular Disorders Activities of Daily Living Scale [VDADL]). Nonparametric tests and a random-effects model were used for statistical analysis. Results: The diagnosis of episodic ataxia type 2 (EA2) was genetically confirmed in 7 subjects. Patients receiving placebo had a median monthly attack frequency of 6.50, whereas patients taking 4AP had a frequency of 1.65 ( p = 0.03). Median monthly attack duration decreased from 13.65 hours with placebo to 4.45 hours with 4AP ( p = 0.08). The VDADL score decreased from 6.00 to 1.50 ( p = 0.02). 4AP was well-tolerated. Conclusions: This controlled trial on EA2 and familial episodic ataxia with nystagmus demonstrated that 4AP decreases attack frequency and improves quality of life. Level of evidence: This crossover study provides Class II evidence that 4AP decreases attack frequency and improves the patient-reported quality of life in patients with episodic ataxia and related familial ataxias.

The myotonias and familial periodic paralyses are muscle channelopathies. They have in common an impaired muscle excitation that is caused by mutations in voltage-gated Na + , K + , Ca 2+ , and Cl − channels. Membrane hyperexcitability usually results in myotonic stiffness; with increasing membrane depolarization hyperexcitability can be transiently turned into hypoexcitability causing transient weakness as in severe myotonia. Hypoexcitability due to long-lasting depolarization that inhibits action potential generation is the common mechanism for the periodic paralyses. Interictally, the ion channel malfunction may be compensated, so that specific exogenous or endogenous provocative factors are required to produce symptoms in the patients. An especially obvious triggering agent is the level of serum potassium, the ion decisive for resting membrane potential and degree of excitability. Periodic paralysis mutations for which the ion channel malfunction is not fully compensated interictally cause progressive myopathy.

Mechano-electrical transduction (MET) in the stereocilia of outer hair cells (OHCs) was studied in newborn Wistar rats using scanning electron microscopy to investigate the stereociliar cross-links, Nomarski laser differential interferometry to investigate stereociliar stiffness and by testing the functionality of the MET channels by recording the entry of fluorescent dye, FM1-43, into stereocilia. Preparations were taken from rats on their day of birth (P0) or 1–4 days later (P1–P4). Hair bundles developed from the base to the apex and from the inner to outer OHC rows. MET channel responses were detected in apical coil OHCs on P1. To study the possible recovery of MET after disrupting the cross-links, the same investigations were performed after the application of Ca 2+ chelator 1,2-bis(o-aminophenoxy)ethane- N , N , N ′, N ′-tetraacetic acid (BAPTA) and allowing the treated samples to recover in culture medium for 0–20 h. We found that the structure and function were abolished by BAPTA. In P0–P1 samples, structural recovery was complete and the open probability of MET channels reached control values. In P3–P4 samples, complete recovery only occurred in OHCs of the outermost row. Although our results demonstrate an enormous recovery potential of OHCs in the postnatal period, the structural component restricts the potential for therapy in patients.

In this article, we analyze myopathies with cores, for which an association to malignant hyperthermia (MH) has been suggested. We discuss the clinical features, the underlying genetic defects, subsequent effects on cellular calcium metabolism, and in vitro muscle responses to MH triggers. We describe in detail central core disease, multiminicore disease, and nemaline rod myopathy. We categorize the diseases according to the affected proteins and discuss the risk for MH, which is high or theoretically possible when the calcium-conducting proteins are affected.

Normal resting potential (P1) of myofibers follows the Nernst equation, exhibiting about −85 mV at a normal extracellular K + concentration ([K + ] o ) of 4 mM. Hyperpolarization occurs with decreased [K + ] o , although at [K + ] o < 1.0 mM, myofibers paradoxically depolarize to a second stable potential of −60 mV (P2). In rat myofiber bundles, P2 also was found at more physiological [K + ] o and was associated with inexcitability. To increase the relative frequency of P2 to 50%, [K + ] o needed to be lowered to 1.5 mM. In the presence of the ionophore gramicidin, [K + ] o reduction to only 2.5 mM yielded the same effect. Acetazolamide normalized this increased frequency of P2 fibers. The findings mimic hypokalemic periodic paralysis (HypoPP), a channelopathy characterized by hypokalemia-induced weakness. Of myofibers from 7 HypoPP patients, up to 25% were in P2 at a [K + ] o of 4 mM, in accordance with their permanent weakness, and up to 99% were in P2 at a [K + ] o of 1.5 mM, in accordance with their paralytic attacks. Of 36 HypoPP patients, 25 had permanent weakness and myoplasmic intracellular Na + ([Na + ] i ) overload (up to 24 mM) as shown by in vivo 23 Na-MRI. Acetazolamide normalized [Na + ] i and increased muscle strength. HypoPP myofibers showed a nonselective cation leak of 12–19.5 μS/cm 2 , which may explain the Na + overload. The leak sensitizes myofibers to reduced serum K + , and the resulting membrane depolarization causes the weakness. We postulate that the principle of paradoxical depolarization and loss of function upon [K + ] o reduction may apply to other tissues, such as heart or brain, when they become leaky (e.g., because of ischemia).

Paroxysmal dyskinesias are episodic movement disorders that can be inherited or are sporadic in nature. The pathophysiology underlying these disorders remains largely unknown but may involve disrupted ion homeostasis due to defects in cell-surface channels or nutrient transporters. In this study, we describe a family with paroxysmal exertion-induced dyskinesia (PED) over 3 generations. Their PED was accompanied by epilepsy, mild developmental delay, reduced CSF glucose levels, hemolytic anemia with echinocytosis, and altered erythrocyte ion concentrations. Using a candidate gene approach, we identified a causative deletion of 4 highly conserved amino acids (Q282_S285del) in the pore region of the glucose transporter 1 (GLUT1). Functional studies in Xenopus oocytes and human erythrocytes revealed that this mutation decreased glucose transport and caused a cation leak that alters intracellular concentrations of sodium, potassium, and calcium. We screened 4 additional families, in which PED is combined with epilepsy, developmental delay, or migraine, but not with hemolysis or echinocytosis, and identified 2 additional GLUT1 mutations (A275T, G314S) that decreased glucose transport but did not affect cation permeability. Combining these data with brain imaging studies, we propose that the dyskinesias result from an exertion-induced energy deficit that may cause episodic dysfunction of the basal ganglia, and that the hemolysis with echinocytosis may result from alterations in intracellular electrolytes caused by a cation leak through mutant GLUT1.

The voltage-gated potassium channels KV7.2 and KV7.3 (genes KCNQ2 and KCNQ3) constitute a major component of the M-current controlling the firing rate in many neurons. Mutations within these two channel subunits cause benign familial neonatal convulsions (BFNC). Here we identified a novel BFNC-causing mutation (E119G) in the S1–S2 region of KV7.2. Electrophysiological investigations in Xenopus oocytes using two-microelectrode voltage clamping revealed that the steady-state activation curves for E119G alone and its coexpressions with KV7.2 and/or KV7.3 wild-type (WT) channels were significantly shifted in the depolarizing direction compared to KV7.2 or KV7.2/KV7.3. These shifts reduced the relative current amplitudes for mutant channels particularly in the subthreshold range of an action potential (about 45% reduction at −50 mV for E119G compared to KV7.2, and 33% for E119G/KV7.3 compared to KV7.2/KV7.3 channels). Activation kinetics were significantly slowed for mutant channels. Our results indicate that small changes in channel gating at subthreshold voltages are sufficient to cause neonatal seizures and demonstrate the importance of the M-current for this voltage range. This was confirmed by a computer model predicting an increased burst duration for the mutation. On a molecular level, these results reveal a critical role in voltage sensing of the negatively charged E119 in S1–S2 of KV7.2, a region that – according to molecular modelling – might interact with a positive charge in the S4 segment.

Background: Peripheral nerve hyperexcitability (PNH) is characterized by muscle overactivity due to spontaneous discharges of lower motor neurons usually associated with antibodies against voltage-gated potassium channels. PNH may also occur in combination with episodic ataxia or epilepsy caused by mutations in K V 1.1 or K V 7.2 channels. Only one PNH-associated mutation has been described so far in K V 7.2 (R207W), in a family with both PNH and neonatal seizures. Methods: PNH was characterized by video and electromyography. The KCNQ2 gene was sequenced and K V 7.2 channels were functionally characterized using two-microelectrode voltage-clamping in Xenopus oocytes. Results: In a patient with PNH without other neurologic symptoms, we identified a novel KCNQ2 mutation predicting loss of a charged residue within the voltage sensor of K V 7.2 (R207Q). Functional analysis of both PNH-associated mutants revealed large depolarizing shifts of the conductance-voltage relationships and marked slowing of the activation time course compared to wild type (WT) channels, less pronounced for R207Q than R207W. Co-expression of both mutant with WT channels revealed a dominant negative effect reducing the relative current amplitudes after short depolarizations by >70%. The anticonvulsant retigabine, an activator of neuronal K V 7 channels, reversed the depolarizing shift. Conclusions: Mutations in KCNQ2 can cause idiopathic PNH alone and should be considered in sporadic cases. Both K V 7.2 mutants produce PNH by changing voltage-dependent activation with a dominant negative effect on the WT channel. This distinguishes them from all hitherto examined Kv7.2 or K V 7.3 mutations which cause neonatal seizures by haploinsufficiency. Retigabine may be beneficial in treating PNH.

An increasing number of human diseases have been found to result from mutations in ion channels, including voltage-gated cation channels. Though the mutations are known, the pathophysiological mechanisms underlying many of these channelopathies remain unclear. In this issue of the Journal, Struyk

Two previously reported Norwegian patients with painful muscle cramps and giant myotonic discharges were genotyped and compared with those of members of 21 families harboring the same mutation.Using primers specific for SCN4A and CLCN1, the DNA of the Norwegian family members was amplified and bidirectionally sequenced. Clinical and neurophysiological features of other families harboring the same mutation were studied.A G1306A mutation in the Nav1.4 voltage-gated sodium channel of skeletal muscle was identified. This mutation is known to cause myotonia fluctuans. No giant myotonic discharges or painful muscle cramps were found in the other G1306A families.Ephaptic transmission between neighboring muscle fibers may not only cause the unusual size of the myotonic discharges in this family, but also a more severe type of potassium-aggravated myotonia than myotonia fluctuans.

The myotonias and familial periodic paralyses are hereditary muscle channelopathies with onset at birth or second decade, at the latest. Both have impaired muscle excitation caused by mutations in voltage-gated Na+, K+, Ca2+, and Cl− channels. Membrane hyperexcitability usually results in myotonic stiffness. Severe myotonia may cause laryngospasm resulting in cyanosis and loss of consciousness, misinterpreted as epileptic seizure. With increasing membrane depolarization, hyperexcitability can be transiently turned into hypoexcitability causing flaccid weakness. Hypoexcitability due to long-lasting depolarization that inhibits action potential generation is the common mechanism for the periodic paralyses. Episodes of weakness can be confused with episodic ataxia. Interictally, the ion channel malfunction may be compensated so that specific exogenous or endogenous provocative factors are required to produce symptoms in the patients. An especially obvious and therefore name-giving triggering agent is the level of serum potassium, the ion decisive for resting membrane potential and degree of excitability.

Malignant hyperthermia (MH) is a rare pharmacogenetic disorder of skeletal muscle metabolism, inherited in an autosomal dominant manner. In the majority of cases, mutations are found in the gene encoding ryanodine receptor type 1 (RyR1), the calcium release channel in the sarcoplasmic reticulum. MH is typically triggered by volatile anesthetics; crises outside general anesthesia are extremely rare. More than half of reported adverse anesthetic MH events are allotted to children. Clinical crises are characterized by tachycardia, acidosis, generalized muscle stiffness, and high body temperature originating from unleashed calcium release from the sarcoplasmic reticulum. Life-threatening complications include cardiac arrhythmia, renal failure, disseminated intravascular coagulation, and rhabdomyolysis. Clinical treatment is based on rapid infusion of dantrolene, which inhibits myoplasmic calcium increase and might also be beneficial in other hypermetabolic MH-like events, especially when calcium turnover is augmented. Increasing knowledge on genetic causes and pathophysiologic pathways allows specific handling of these patients.

The familial periodic paralyses (PP) were commonly considered to be benign diseases since frequency and severity of the paralytic attacks decrease in adulthood. However, with increasing age, a third of the patients develop permanent weakness and muscle degeneration with fatty replacement. Another complication, cardiac arrhythmia, can result from the dyskalemia during paralytic attacks. The familial PP are typical dominant ion channelopathies: the function of the mutant muscular channel is compensated in the interictal state but defective under certain conditions which then cause flaccid weakness. A triggering factor is the level of serum potassium, the extracellular ion decisive for membrane excitability. In hyper- and hypokalemic periodic paralysis, the mutations are specifically located in the voltage-gated sodium and calcium channels which are essential for action potential generation or excitation-contraction coupling. The common mechanism for the membrane inexcitability during paralytic attacks is a transient membrane depolarization that inactivates the sodium channels which are then no longer available for action potential generation. For the third PP type, the Andersen syndrome, the responsible gene is also expressed in cardiac muscle, and, independently of paralytic attacks, the hazard of ventricular arrhythmias is inherent. The gene product, an inwardly rectifying potassium channel, is responsible for maintaining the resting membrane potential, and all known mutations cause dominant-negative effects on the tetrameric channel complexes. In this article the clinical consequences of the mutations and the therapeutic strategies for all three types of PP are reported.

This review focuses on the electrical properties of the transverse (T) tubular membrane of skeletal muscle, with reference to the contribution of the T-tubular system (TTS) to the surface action potential, the radial spread of excitation and its role in excitation-contraction coupling. Particularly, the most important ion channels and ion transporters that enable proper depolarization and repolarization of the T-tubular membrane are described. Since propagation of excitation along the TTS into the depth of the fibers is a delicate balance between excitatory and inhibitory currents, the composition of channels and transporters is specific to the TTS and different from the surface membrane. The TTS normally enables the radial spread of excitation and the signal transfer to the sarcoplasmic reticulum to release calcium that activates the contractile apparatus. However, due to its structure, even slight shifts of ions may alter its volume, Nernstian potentials, ion permeabilities, and consequently T-tubular membrane potential and excitability.

Background: Muscle channelopathies such as paramyotonia, hyperkalemic periodic paralysis, and potassium-aggravated myotonia are caused by gain-of-function Na + channel mutations. Methods: Implementation of a three-dimensional radial 23 Na magnetic resonance (MR) sequence with ultra-short echo times allowed the authors to quantify changes in the total muscular 23 Na signal intensity. By this technique and T2-weighted 1 H MRI, the authors studied whether the affected muscles take up Na + and water during episodes of myotonic stiffness or of cold- or exercise-induced weakness. Results: A 22% increase in the 23 Na signal intensity and edema-like changes on T2-weighted 1 H MR images were associated with cold-induced weakness in all 10 paramyotonia patients; signal increase and weakness disappeared within 1 day. A 10% increase in 23 Na, but no increase in the T2-weighted 1 H signal, occurred during cold- or exercise-induced weakness in seven hyperkalemic periodic paralysis patients, and no MR changes were observed in controls or exercise-induced stiffness in six potassium-aggravated myotonia patients. Measurements on native muscle fibers revealed provocation-induced, intracellular Na + accumulation and membrane depolarization by −41 mV for paramyotonia, by −30 mV for hyperkalemic periodic paralysis, and by −20 mV for potassium-aggravated myotonia. The combined in vivo and in vitro approach showed a close correlation between the increase in 23 Na MR signal intensity and the membrane depolarization (r = 0.92). Conclusions: The increase in the total 23 Na signal intensity reflects intracellular changes, the cold-induced Na + shifts are greatest and osmotically relevant in paramyotonia patients, and even osmotically irrelevant Na + shifts can be detected by the implemented 23 Na MR technique.

The modulation of cell proliferation in neural progenitor cells (NPCs) is believed to play a role in neuronal regeneration. Recent studies showed that K(+) channel activity influenced cell proliferation. Therefore, we examined NPCs for K(+) channels and tested whether NPC self renewing can be modulated by synthetic K(+) channel modulators. The whole-cell K(+) current was partly K(+) dependent and showed a cumulative inactivating component. Two tetra-ethyl-ammonium ion (TEA)-sensitive K(+) currents with different voltage dependencies ( = 65 microm, E(50) = -0.3 +/- 1.3 mV and = 8 mm, E(50) = -15.2 +/- 2.8 mV) and an almost TEA-insensitive current were identified. Kaliotoxin blocked approximately 50% of the entire K(+) currents (IC(50) = 0.25 nm). These properties resembled functional characteristics of K(v)1.4, K(v)1.3 and K(v)3.1 channels. Transcripts for these channels, as well as proteins for K(v)1.3 and K(v)3.1, were identified. Immunocytochemical staining revealed K(v)1.3 and K(v)3.1 K(+) channel expression in almost all NPCs. The blockage of K(v)3.1 by low concentrations of TEA, as well as the blockage of K(v)1.3 by Psora-4, increased NPC proliferation. These findings underline the regulatory role of K(+) channels on the cell cycle and imply that K(+) channel modulators might be used therapeutically to activate endogenous NPCs.