ClC-1 chloride channels: state-of-the-art research and future challenges

Case report: Multiple approach analysis in a case of clinically assessed myotonia congenita

Myotonia congenita, both in a dominant (Thomsen disease) and recessive form (Becker disease), is caused by molecular defects in CLCN1 that encodes the major skeletal muscle chloride channel, ClC-1. This channel is important for the normal repolarization of muscle action potentials and consequent relaxation of the muscle, and its dysfunction leads to impaired muscle relaxation after voluntary or evoked contraction and muscle stiffness. More than 300 CLCN1 pathogenic variants have been found in association with congenital myotonia, inherited as recessive or dominant traits (with complete or incomplete penetrance). In this study, we describe the case of a 44-year-old woman complaining of "leg stiffness" since the age of 20 years and presenting with transient muscle weakness, especially after sitting for several minutes, with grip myotonia and feet myotonia, cold-sensitive and warm-up. The strength was normal, but muscle hypertrophy in the lower limbs was evident. EMG myotonia was detected in all explored muscles. The patient's father had precocious cataract correction but did not show myotonic discharges at EMG. Examination of the patient's sons (aged 18 years and 12 years) was unremarkable. The patient started treatment with mexiletine, with improvement in grip myotonia and limb stiffness, but it was soon interrupted due to gastrointestinal disturbances. Direct sequencing of CLCN1 identified the previously described heterozygous intronic variant c.1471 + 1G > A, which resulted in the skipping of exon 13 in the CLCN1 muscle transcript. In addition, the rare heterozygous synonymous nucleotide change c.762C > T p.Cys254Cys was identified and predicted to alter physiological splicing. The detection of multiple splicing abnormalities leading to premature termination codons supported the in silico prediction. We developed a Western blot assay to assess the ClC-1 protein in muscle biopsy, and we observed that ClC-1 levels were consistently reduced in the patient's muscle, supporting the pathogenic behavior of the variants disclosed. Overall, we report a novel case of Becker myotonia and highlight the importance of multiple levels of analysis to achieve a firm molecular diagnosis.

The Role of Ion-Transporting Proteins on Crosstalk Between the Skeletal Muscle and Central Nervous Systems Elicited by Physical Exercise

A paradigm shift in the understanding of bidirectional interactions between peripheral and central nervous systems is essential for development of rehabilitation and preventive interventions based on physical exercise. Although a causal relationship has not been completely established, modulation of voltage-dependent ion channels (Ca2+, Cl-, K+, Na+, lactate-, H+) in skeletal and neuronal cells provides opportunities to maintain force production during exercise and reduce the risk of disease. However, there are caveats to consider when interpreting the effects of physical exercise on this bidirectional axis, since exercise protocol details (e.g., duration and intensity) have variable effects on this crosstalk. Therefore, an integrative perspective of the skeletal muscle and brain's communication pathway is discussed, and the role of physical exercise on such communication highway is explained in this review.

Myotonia congenita in a Greek cohort: Genotype spectrum and impact of the CLCN1:c.501C > G variant as a genetic modifier

INTRODUCTION/AIMS

Myotonia congenita (MC) is the most common hereditary channelopathy in humans. Characterized by muscle stiffness, MC may be transmitted as either an autosomal dominant (Thomsen) or a recessive (Becker) disorder. MC is caused by variants in the voltage-gated chloride channel 1 (CLCN1) gene, important for the normal repolarization of the muscle action potential. More than 250 disease-causing variants in the CLCN1 gene have been reported. This study provides an MC genotype-phenotype spectrum in a large cohort of Greek patients and focuses on novel variants and disease epidemiology, including additional insights for the variant CLCN1:c.501C > G.

METHODS

Sanger sequencing for the entire coding region of the CLCN1 gene was performed. Targeted segregation analysis of likely candidate variants in additional family members was performed. Variant classification was based on American College of Medical Genetics (ACMG) guidelines.

RESULTS

Sixty-one patients from 47 unrelated families were identified, consisting of 51 probands with Becker MC (84%) and 10 with Thomsen MC (16%). Among the different variants detected, 11 were novel and 16 were previously reported. The three most prevalent variants were c.501C > G, c.2680C > T, and c.1649C > G. Additionally, c.501C > G was detected in seven Becker cases in-cis with the c.1649C > G.

DISCUSSION

The large number of patients in whom a diagnosis was established allowed the characterization of genotype-phenotype correlations with respect to both previously reported and novel findings. For the c.501C > G (p.Phe167Leu) variant a likely nonpathogenic property is suggested, as it only seems to act as an aggravating modifying factor in cases in which a pathogenic variant triggers phenotypic expression.

Clinical and molecular characterization of myotonia congenita using whole-exome sequencing in Egyptian patients

BACKGROUND

Myotonia Congenita (MC) is a rare disease classified into two major forms; Thomsen and Becker disease caused by mutations in the CLCN1 gene, which affects muscle excitability and encodes voltage-gated chloride channels (CLC-1). While, there are no data regarding the clinical and molecular characterization of myotonia in Egyptian patients.

METHODS

Herein, we report seven Egyptian MC patients from six unrelated families. Following the clinical diagnosis, whole-exome sequencing (WES) was performed for genetic diagnosis. Various in silico prediction tools were utilized to interpret variant pathogenicity. The candidate variants were then validated using Sanger sequencing technique.

RESULTS

In total, seven cases were recruited. The ages at the examination were ranged from eight months to nineteen years. Clinical manifestations included warm-up phenomenon, hand grip, and percussion myotonia. Electromyography was performed in all patients and revealed myotonic discharges. Molecular genetic analysis revealed five different variants. Of them, we identified two novel variants in the CLCN1 gene ( c.1583G > C; p.Gly528Ala and c.2203_2216del;p.Thr735ValfsTer57) and three known variants in the CLCN1 and SCN4A gene. According to in silico tools, the identified novel variants were predicted to have deleterious effects.

CONCLUSIONS

As the first study to apply WES among Egyptian MC patients, our findings reported two novel heterozygous variants that expand the CLCN1 mutational spectrum for MC diagnosis. These results further confirm that genetic testing is essential for early diagnosis of MC, which affects follow-up treatment and prognostic assessment in clinical practice.

Targeting neuromuscular junction to treat neuromuscular disorders

The integrity and preservation of the neuromuscular junction (NMJ), the interface between the motor neuron and skeletal muscle, is critical for maintaining a healthy skeletal muscle. The structural and/or functional defects in the three cellular components of NMJ, namely the pre-synaptic terminal, synaptic cleft, and post-synaptic region, negatively affect skeletal muscle mass and/or strength. Therefore, NMJ repair appears to be an appropriate therapy for muscle disorders. Mouse models provide a detailed molecular characterization of various cellular components of NMJ with relevance to human diseases. This review discusses different molecular targets on the three cellular components of NMJ for treating muscle diseases. The potential effects of these therapies on NMJ morphology and motor performance, their therapeutic efficacy, and clinical relevance are discussed. Collectively, the available data supports targeting NMJ alone or as an adjunct therapy in treating muscle disorders. However, the potential impact of such interventions on human patients with muscle disorders requires further investigation.

Transcriptome analysis reveals genes associated with stem cell activation by physical exercise in the dentate gyrus of aged p16Ink4a knockout mice

Throughout adulthood neural stem cells divide in neurogenic niches-the dentate gyrus of the hippocampus and the subventricular zone-producing progenitor cells and new neurons. Stem cells self-renew, thus preserving their pool. Furthermore, the number of stem/progenitor cells in the neurogenic niches decreases with age. We have previously demonstrated that the cyclin-dependent kinase inhibitor p16Ink4a maintains, in aged mice, the pool of dentate gyrus stem cells by preventing their activation after a neurogenic stimulus such as exercise (running). We showed that, although p16Ink4a ablation by itself does not activate stem/progenitor cells, exercise strongly induced stem cell proliferation in p16Ink4a knockout dentate gyrus, but not in wild-type. As p16Ink4a regulates stem cell self-renewal during aging, we sought to profile the dentate gyrus transcriptome from p16Ink4a wild-type and knockout aged mice, either sedentary or running for 12 days. By pairwise comparisons of differentially expressed genes and by correlative analyses through the DESeq2 software, we identified genes regulated by p16Ink4a deletion, either without stimulus (running) added, or following running. The p16Ink4a knockout basic gene signature, i.e., in sedentary mice, involves upregulation of apoptotic, neuroinflammation- and synaptic activity-associated genes, suggesting a reactive cellular state. Conversely, another set of 106 genes we identified, whose differential expression specifically reflects the pattern of proliferative response of p16 knockout stem cells to running, are involved in processes that regulate stem cell activation, such as synaptic function, neurotransmitter metabolism, stem cell proliferation control, and reactive oxygen species level regulation. Moreover, we analyzed the regulation of these stem cell-specific genes after a second running stimulus. Surprisingly, the second running neither activated stem cell proliferation in the p16Ink4a knockout dentate gyrus nor changed the expression of these genes, confirming that they are correlated to the stem cell reactivity to stimulus, a process where they may play a role regulating stem cell activation.

ClC-1 Chloride Channel: Inputs on the Structure-Function Relationship of Myotonia Congenita-Causing Mutations

Myotonia congenita is a hereditary muscle disease mainly characterized by muscle hyperexcitability, which leads to a sustained burst of discharges that correlates with the magnitude and duration of involuntary aftercontractions, muscle stiffness, and hypertrophy. Mutations in the chloride voltage-gated channel 1 (CLCN1) gene that encodes the skeletal muscle chloride channel (ClC-1) are responsible for this disease, which is commonly known as myotonic chloride channelopathy. The biophysical properties of the mutated channel have been explored and analyzed through in vitro approaches, providing important clues to the general function/dysfunction of the wild-type and mutated channels. After an exhaustive search for CLCN1 mutations, we report in this review more than 350 different mutations identified in the literature. We start discussing the physiological role of the ClC-1 channel in skeletal muscle functioning. Then, using the reported functional effects of the naturally occurring mutations, we describe the biophysical and structural characteristics of the ClC-1 channel to update the knowledge of the function of each of the ClC-1 helices, and finally, we attempt to point out some patterns regarding the effects of mutations in the different helices and loops of the protein.

Diagnostics in skeletal muscle channelopathies

INTRODUCTION

Skeletal muscle channelopathies (SMCs) are a heterogenous group of disorders, caused by mutations in skeletal ion channels leading to abnormal muscle excitability, resulting in either delayed muscle relaxation (myotonia) which characterizes non-dystrophic myotonias (NDMs), or membrane transient inactivation, causing episodic weakness, typical of periodic paralyses (PPs).

AREAS COVERED

SMCs include myotonia congenita, paramyotonia congenita, and sodium-channel myotonia among NDMs, and hyper-normokalemic, hypokalemic, or late-onset periodic paralyses among PPs. When suspecting an SMC, a structured diagnostic approach is required. Detailed personal and family history and clinical examination are essential, while neurophysiological tests should confirm myotonia and rule out alternative diagnosis. Moreover, specific electrodiagnostic studies are important to further define the phenotype of de novo cases and drive molecular analyses together with clinical data. Definite diagnosis is achieved through genetic testing, either with Sanger sequencing or multigene next-generation sequencing panel. In still unsolved patients, more advanced techniques, as exome-variant sequencing or whole-genome sequencing, may be considered in expert centers.

EXPERT OPINION

The diagnostic approach to SMC is still mainly based on clinical data; moreover, definite diagnosis is sometimes complicated by the difficulty to establish a proper genotype-phenotype correlation. Lastly, further studies are needed to allow the genetic characterization of unsolved patients.

Cryo-EM structures of ClC-2 chloride channel reveal the blocking mechanism of its specific inhibitor AK-42

ClC-2 transports chloride ions across plasma membranes and plays critical roles in cellular homeostasis. Its dysfunction is involved in diseases including leukodystrophy and primary aldosteronism. AK-42 was recently reported as a specific inhibitor of ClC-2. However, experimental structures are still missing to decipher its inhibition mechanism. Here, we present cryo-EM structures of apo ClC-2 and its complex with AK-42, both at 3.5 Å resolution. Residues S162, E205 and Y553 are involved in chloride binding and contribute to the ion selectivity. The side-chain of the gating glutamate E205 occupies the putative central chloride-binding site, indicating that our structure represents a closed state. Structural analysis, molecular dynamics and electrophysiological recordings identify key residues to interact with AK-42. Several AK-42 interacting residues are present in ClC-2 but not in other ClCs, providing a possible explanation for AK-42 specificity. Taken together, our results experimentally reveal the potential inhibition mechanism of ClC-2 inhibitor AK-42.

Natural Compound Boldine Lessens Myotonic Dystrophy Type 1 Phenotypes in DM1 Drosophila Models, Patient-Derived Cell Lines, and HSALR Mice

Myotonic dystrophy type 1 (DM1) is a complex rare disorder characterized by progressive muscle dysfunction, involving weakness, myotonia, and wasting, but also exhibiting additional clinical signs in multiple organs and systems. Central dysregulation, caused by an expansion of a CTG trinucleotide repeat in the DMPK gene's 3' UTR, has led to exploring various therapeutic approaches in recent years, a few of which are currently under clinical trial. However, no effective disease-modifying treatments are available yet. In this study, we demonstrate that treatments with boldine, a natural alkaloid identified in a large-scale Drosophila-based pharmacological screening, was able to modify disease phenotypes in several DM1 models. The most significant effects include consistent reduction in nuclear RNA foci, a dynamic molecular hallmark of the disease, and noteworthy anti-myotonic activity. These results position boldine as an attractive new candidate for therapy development in DM1.

Progressive development of melanoma-induced cachexia differentially impacts organ systems in mice

Cachexia is a systemic wasting syndrome that increases cancer-associated mortality. How cachexia progressively and differentially impacts distinct tissues is largely unknown. Here, we find that the heart and skeletal muscle undergo wasting at early stages and are the tissues transcriptionally most impacted by cachexia. We also identify general and organ-specific transcriptional changes that indicate functional derangement by cachexia even in tissues that do not undergo wasting, such as the brain. Secreted factors constitute a top category of cancer-regulated genes in host tissues, and these changes include upregulation of the angiotensin-converting enzyme (ACE). ACE inhibition with the drug lisinopril improves muscle force and partially impedes cachexia-induced transcriptional changes, although wasting is not prevented, suggesting that cancer-induced host-secreted factors can regulate tissue function during cachexia. Altogether, by defining prevalent and temporal and tissue-specific responses to cachexia, this resource highlights biomarkers and possible targets for general and tissue-tailored anti-cachexia therapies.

Case report: Incomplete penetrance of autosomal dominant myotonia congenita caused by a rare CLCN1 variant c.1667T>A (p.I556N) in a Malaysian family

Myotonia congenita (MC) is a rare neuromuscular disease caused by mutations within the CLCN1 gene encoding skeletal muscle chloride channels. MC is characterized by delayed muscle relaxation during contraction, resulting in muscle stiffness. There is a lack of MC case reports and data on the prevalence among Malaysians. We report a clinical case of a 50-year-old woman presents with muscle stiffness and cramp episodes that started in early childhood. She had difficulty initiating muscle movement and presented with transient muscle weakness after rest, which usually improved after repeated contraction (warm-up phenomenon). She was diagnosed with MC after myotonic discharge on electromyography (EMG). Her brother had similar symptoms; however, no additional family members showed MC symptoms. Serum creatine kinase levels were elevated in both the proband and her brother with 447 U/L and 228 U/L recorded, respectively. Genetic analysis by whole-exome sequencing (WES) revealed a previously reported pathogenic CLCN1 gene variant c.1667T>A (p.I556N). Genetic screening of all family members revealed that the same variant was observed in the children of both the proband and her brother; however, the children did not present with either clinical or electrophysiological MC symptoms. The multiplex ligation-dependent probe amplification (MLPA) analysis conducted identified neither exon deletion nor duplication in CLCN1. In conclusion, this report describes the first case of MC in Malaysia in which incomplete penetrance observed in this family is caused by a known pathogenic CLCN1 variant.

Statins Induce Locomotion and Muscular Phenotypes in Drosophila melanogaster That Are Reminiscent of Human Myopathy: Evidence for the Role of the Chloride Channel Inhibition in the Muscular Phenotypes

The underlying mechanisms for statin-induced myopathy (SIM) are still equivocal. In this study, we employ Drosophila melanogaster to dissect possible underlying mechanisms for SIM. We observe that chronic fluvastatin treatment causes reduced general locomotion activity and climbing ability. In addition, transmission microscopy of dissected skeletal muscles of fluvastatin-treated flies reveals strong myofibrillar damage, including increased sarcomere lengths and Z-line streaming, which are reminiscent of myopathy, along with fragmented mitochondria of larger sizes, most of which are round-like shapes. Furthermore, chronic fluvastatin treatment is associated with impaired lipid metabolism and insulin signalling. Mechanistically, knockdown of the statin-target Hmgcr in the skeletal muscles recapitulates fluvastatin-induced mitochondrial phenotypes and lowered general locomotion activity; however, it was not sufficient to alter sarcomere length or elicit myofibrillar damage compared to controls or fluvastatin treatment. Moreover, we found that fluvastatin treatment was associated with reduced expression of the skeletal muscle chloride channel, ClC-a (Drosophila homolog of CLCN1), while selective knockdown of skeletal muscle ClC-a also recapitulated fluvastatin-induced myofibril damage and increased sarcomere lengths. Surprisingly, exercising fluvastatin-treated flies restored ClC-a expression and normalized sarcomere lengths, suggesting that fluvastatin-induced myofibrillar phenotypes could be linked to lowered ClC-a expression. Taken together, these results may indicate the potential role of ClC-a inhibition in statin-associated muscular phenotypes. This study underlines the importance of Drosophila melanogaster as a powerful model system for elucidating the locomotion and muscular phenotypes, promoting a better understanding of the molecular mechanisms underlying SIM.

Case report: Coexistence of myotonia congenita and Brugada syndrome in one family

Myotonia congenita is a rare neuromuscular disorder caused by CLCN1 mutations resulting in delayed muscle relaxation. Extramuscular manifestations are not considered to be present in chloride skeletal channelopathies, although recently some cardiac manifestations have been described. We report a family with autosomal dominant myotonia congenita and Brugada syndrome. Bearing in mind the previously reported cases of cardiac arrhythmias in myotonia congenita patients, we discuss the possible involvement of the CLCN1-gene mutations in primary cardiac arrhythmia.

The Clinical, Myopathological, and Genetic Analysis of 20 Patients With Non-dystrophic Myotonia

Introduction

Non-dystrophic myotonias (NDMs) are skeletal muscle ion channelopathies caused by CLCN1 or SCN4A mutations. This study aimed to describe the clinical, myopathological, and genetic analysis of NDM in a large Chinese cohort.

Methods

We reviewed the clinical manifestations, laboratory results, electrocardiogram, electromyography, muscle biopsy, genetic analysis, treatment, and follow-up of 20 patients (from 18 families) with NDM.

Results

Cases included myotonia congenita (MC, 17/20) and paramyotonia congenita (PMC, 3/20). Muscle stiffness and hypertrophy, grip and percussion myotonia, and the warm-up phenomenon were frequently observed in MC and PMC patients. Facial stiffness, eye closure myotonia, and cold sensitivity were more common in PMC patients and could be accompanied by permanent weakness. Nine MC patients and two PMC patients had cardiac abnormalities, mainly manifested as cardiac arrhythmia, and the father of one patient died of sudden cardiac arrest. Myotonic runs in electromyography were found in all patients, and seven MC patients had mild myopathic changes. There was no difference in muscle pathology between MC and PMC patients, most of whom had abnormal muscle fiber type distribution or selective muscle fiber atrophy. Nineteen CLCN1 variants were found in 17 MC patients, among which c.795T>G (p.D265E) was a new variant, and two SCN4A variants were found in three PMC patients. The patients were treated with mexiletine and/or carbamazepine, and the symptoms of myotonia were partially improved.

Conclusions

MC and PMC have considerable phenotypic overlap. Genetic investigation contributes to identifying the subtype of NDM. The muscle pathology of NDM lacks specific changes.

Chloride channels with ClC-1-like properties differentially regulate the excitability of dopamine receptor D1- and D2-expressing striatal medium spiny neurons

Dynamic chloride (Cl^-) regulation is critical for synaptic inhibition. In mature neurons, Cl^- influx and extrusion are primarily controlled by ligand-gated anion channels (GABA_A and glycine receptors) and the potassium chloride cotransporter K⁺-Cl^- cotransporter 2 (KCC2), respectively. Here, we report for the first time, to our knowledge, a presence of a new source of Cl^- influx in striatal neurons with properties similar to chloride voltage-gated channel 1 (ClC-1). Using whole cell patch-clamp recordings, we detected an outwardly rectifying voltage-dependent current that was impermeable to the large anion methanesulfonate (MsO^-). The anionic current was sensitive to the ClC-1 inhibitor 9-anthracenecarboxylic acid (9-AC) and the nonspecific blocker phloretin. The mean fractions of anionic current inhibition by MsO^-, 9-AC, and phloretin were not significantly different, indicating that anionic current was caused by active ClC-1-like channels. In addition, we found that Cl^- current was not sensitive to the transmembrane protein 16A (TMEM16A; Ano1) inhibitor Ani9 and that the outward Cl^- rectification was preserved even at a very high intracellular Ca²⁺ concentration (2 mM), indicating that TMEM16B (Ano2) did not contribute to the total current. Western blotting and immunohistochemical analyses confirmed the presence of ClC-1 channels in the striatum mainly localized to the somata of striatal neurons. Finally, we found that 9-AC decreased action potential firing frequencies and increased excitability in medium spiny neurons (MSNs) expressing dopamine type 1 (D1) and type 2 (D2) receptors in the brain slices, respectively. We conclude that ClC-1-like channels are preferentially located at the somata of MSNs, are functional, and can modulate neuronal excitability.

The Donnan-dominated resting state of skeletal muscle fibers contributes to resilience and longevity in dystrophic fibers

Duchenne muscular dystrophy (DMD) is an X-linked dystrophin-minus muscle-wasting disease. Ion homeostasis in skeletal muscle fibers underperforms as DMD progresses. But though DMD renders these excitable cells intolerant of exertion, sodium overloaded, depolarized, and spontaneously contractile, they can survive for several decades. We show computationally that underpinning this longevity is a strikingly frugal, robust Pump-Leak/Donnan (P-L/D) ion homeostatic process. Unlike neurons, which operate with a costly “Pump-Leak–dominated” ion homeostatic steady state, skeletal muscle fibers operate with a low-cost “Donnan-dominated” ion homeostatic steady state that combines a large chloride permeability with an exceptionally small sodium permeability. Simultaneously, this combination keeps fiber excitability low and minimizes pump expenditures. As mechanically active, long-lived multinucleate cells, skeletal muscle fibers have evolved to handle overexertion, sarcolemmal tears, ischemic bouts, etc.; the frugality of their Donnan dominated steady state lets them maintain the outsized pump reserves that make them resilient during these inevitable transient emergencies. Here, P-L/D model variants challenged with DMD-type insult/injury (low pump-strength, overstimulation, leaky Nav and cation channels) show how chronic “nonosmotic” sodium overload (observed in DMD patients) develops. Profoundly severe DMD ion homeostatic insult/injury causes spontaneous firing (and, consequently, unwanted excitation–contraction coupling) that elicits cytotoxic swelling. Therefore, boosting operational pump-strength and/or diminishing sodium and cation channel leaks should help extend DMD fiber longevity.

Muscle Glycogen Metabolism and High-Intensity Exercise Performance: A Narrative Review

Muscle glycogen is the main substrate during high-intensity exercise and large reductions can occur after relatively short durations. Moreover, muscle glycogen is stored heterogeneously and similarly displays a heterogeneous and fiber-type specific depletion pattern with utilization in both fast- and slow-twitch fibers during high-intensity exercise, with a higher degradation rate in the former. Thus, depletion of individual fast- and slow-twitch fibers has been demonstrated despite muscle glycogen at the whole-muscle level only being moderately lowered. In addition, muscle glycogen is stored in specific subcellular compartments, which have been demonstrated to be important for muscle function and should be considered as well as global muscle glycogen availability. In the present review, we discuss the importance of glycogen metabolism for single and intermittent bouts of high-intensity exercise and outline possible underlying mechanisms for a relationship between muscle glycogen and fatigue during these types of exercise. Traditionally this relationship has been attributed to a decreased ATP resynthesis rate due to inadequate substrate availability at the whole-muscle level, but emerging evidence points to a direct coupling between muscle glycogen and steps in the excitation-contraction coupling including altered muscle excitability and calcium kinetics.

Functional analysis of the F337C mutation in the CLCN1 gene associated with dominant myotonia congenita reveals an alteration of the macroscopic conductance and voltage dependence

Background

Myotonia congenita (MC) is a common channelopathy affecting skeletal muscle and which is due to pathogenic variants within the CLCN1 gene. Various alterations in the function of the channel have been reported and we here illustrate a novel one.

Methods

A patient presenting the symptoms of myotonia congenita was shown to bear a new heterozygous missense variant in exon 9 of the CLCN1 gene (c.1010 T > G, p.(Phe337Cys)). Confocal imaging and patch clamp recordings of transiently transfected HEK293 cells were used to functionally analyze the effect of this variant on channel properties.

Results

Confocal imaging showed that the F337C mutant incorporated as well as the WT channel into the plasma membrane. However, in patch clamp, we observed a smaller conductance for F337C at −80 mV. We also found a marked reduction of the fast gating component in the mutant channels, as well as an overall reduced voltage dependence.

Conclusion

To our knowledge, this is the first report of a mixed alteration in the biophysical properties of hClC‐1 consisting of a reduced conductance at resting potential and an almost abolished voltage dependence.

Association of Three Different Mutations in the CLCN1 Gene Modulating the Phenotype in a Consanguineous Family with Myotonia Congenita

Myotonia congenita is a genetic disease caused by mutations in the CLCN1 gene, which encodes for the major chloride skeletal channel ClC-1, involved in the normal repolarization of muscle action potentials and consequent relaxation of the muscle after contraction. Two allelic forms are recognized, depending on the phenotype and the inheritance pattern: the autosomal dominant Thomsen disease with milder symptoms and the autosomal recessive Becker disorder with a severe phenotype. Before the recent advances of molecular testing, the diagnosis and genetic counseling of families was a challenge due to the large number of mutations in the CLCN1 gene, found both in homozygous or in heterozygous state. Here, we studied a consanguineous family in which three members presented a variable phenotype of myotonia, associated to a combination of three different mutations in the CLCN1 gene. A pathogenic splicing site mutation which causes the skipping of exon 17 was present in homozygosis in one very severely affected son. This mutation was present in compound heterozygosis in the consanguineous parents, but interestingly it was associated to a different second variant in the other allele: c.1453 A > G in the mother and c.1842 G > C in the father. Both displayed variable, but less severe phenotypes than their homozygous son. These results highlight the importance of analyzing the combination of different variants in the same gene in particular in families with patients displaying different phenotypes. This approach may improve the diagnosis, prognosis, and genetic counseling of the involved families.

Guidelines on clinical presentation and management of nondystrophic myotonias

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

Improving genetic diagnostics of skeletal muscle channelopathies

INTRODUCTION

Skeletal muscle channelopathies are rare inherited conditions that cause significant morbidity and impact on quality of life. Some subsets have a mortality risk. Improved genetic methodology and understanding of phenotypes have improved diagnostic accuracy and yield.

AREAS COVERED

We discuss diagnostic advances since the advent of next-generation sequencing and the role of whole exome and genome sequencing. Advances in genotype-phenotype-functional correlations have improved understanding of inheritance and phenotypes. We outline new phenotypes, particularly in the pediatric setting and consider co-existing mutations that may act as genetic modifiers. We also discuss four newly identified genes associated with skeletal muscle channelopathies.

EXPERT OPINION

Next-generation sequencing using gene panels has improved diagnostic rates, identified new mutations, and discovered patients with co-existing pathogenic mutations ('double trouble'). This field has previously focussed on single genes, but we are now beginning to understand interactions between co-existing mutations, genetic modifiers, and their role in pathomechanisms. New genetic observations in pediatric presentations of channelopathies broadens our understanding of the conditions. Genetic and mechanistic advances have increased the potential to develop treatments.

Changes in Expression and Cellular Localization of Rat Skeletal Muscle ClC-1 Chloride Channel in Relation to Age, Myofiber Phenotype and PKC Modulation

The ClC-1 chloride channel 1 is important for muscle function as it stabilizes resting membrane potential and helps to repolarize the membrane after action potentials. We investigated the contribution of ClC-1 to adaptation of skeletal muscles to needs induced by the different stages of life. We analyzed the ClC-1 gene and protein expression as well as mRNA levels of protein kinase C (PKC) alpha and theta involved in ClC-1 modulation, in soleus (SOL) and extensor digitorum longus (EDL) muscles of rats in all stage of life. The cellular localization of ClC-1 in relation to age was also investigated. Our data show that during muscle development ClC-1 expression differs according to phenotype. In fast-twitch EDL muscles ClC-1 expression increased 10-fold starting at 7 days up to 8 months of life. Conversely, in slow-twitch SOL muscles ClC-1 expression remained constant until 33 days of life and subsequently increased fivefold to reach the adult value. Aging induced a downregulation of gene and protein ClC-1 expression in both muscle types analyzed. The mRNA of PKC-theta revealed the same trend as ClC-1 except in old age, whereas the mRNA of PKC-alpha increased only after 2 months of age. Also, we found that the ClC-1 is localized in both membrane and cytoplasm, in fibers of 12-day-old rats, becoming perfectly localized on the membrane in 2-month-old rats. This study could represent a point of comparison helpful for the identification of accurate pharmacological strategies for all the pathological situations in which ClC-1 protein is altered.

Myotonia congenita and periodic hypokalemia paralysis in a consanguineous marriage pedigree: Coexistence of a novel CLCN1 mutation and an SCN4A mutation

Myotonia congenita and hypokalemic periodic paralysis type 2 are both rare genetic channelopathies caused by mutations in the CLCN1 gene encoding voltage-gated chloride channel CLC-1 and the SCN4A gene encoding voltage-gated sodium channel Na_v1.4. The patients with concomitant mutations in both genes manifested different unique symptoms from mutations in these genes separately. Here, we describe a patient with myotonia and periodic paralysis in a consanguineous marriage pedigree. By using whole-exome sequencing, a novel F306S variant in the CLCN1 gene and a known R222W mutation in the SCN4A gene were identified in the pedigree. Patch clamp analysis revealed that the F306S mutant reduced the opening probability of CLC-1 and chloride conductance. Our study expanded the CLCN1 mutation database. We emphasized the value of whole-exome sequencing for differential diagnosis in atypical myotonic patients.

Skeletal muscle ClC-1 chloride channels in health and diseases

In 1970, the study of the pathomechanisms underlying myotonia in muscle fibers isolated from myotonic goats highlighted the importance of chloride conductance for skeletal muscle function; 20 years later, the human ClC-1 chloride channel has been cloned; last year, the crystal structure of human protein has been solved. Over the years, the efforts of many researchers led to significant advances in acknowledging the role of ClC-1 in skeletal muscle physiology and the mechanisms through which ClC-1 dysfunctions lead to impaired muscle function. The wide spectrum of pathophysiological conditions associated with modification of ClC-1 activity, either as the primary cause, such as in myotonia congenita, or as a secondary adaptive mechanism in other neuromuscular diseases, supports the idea that ClC-1 is relevant to preserve not only for skeletal muscle excitability, but also for skeletal muscle adaptation to physiological or harmful events. Improving this understanding could open promising avenues toward the development of selective and safe drugs targeting ClC-1, with the aim to restore normal muscle function. This review summarizes the most relevant research on ClC-1 channel physiology, associated diseases, and pharmacology.

Census of halide-binding sites in protein structures

Motivation

Halides are negatively charged ions of halogens, forming fluorides (F⁻), chlorides (Cl⁻), bromides (Br⁻) and iodides (I⁻). These anions are quite reactive and interact both specifically and non-specifically with proteins. Despite their ubiquitous presence and important roles in protein function, little is known about the preferences of halides binding to proteins. To address this problem, we performed the analysis of halide–protein interactions, based on the entries in the Protein Data Bank.

Results

We have compiled a pipeline for the quick analysis of halide-binding sites in proteins using the available software. Our analysis revealed that all of halides are strongly attracted by the guanidinium moiety of arginine side chains, however, there are also certain preferences among halides for other partners. Furthermore, there is a certain preference for coordination numbers in the binding sites, with a correlation between coordination numbers and amino acid composition. This pipeline can be used as a tool for the analysis of specific halide–protein interactions and assist phasing experiments relying on halides as anomalous scatters.

Availability and implementation

All data described in this article can be reproduced via complied pipeline published at https://github.com/rostkick/Halide_sites/blob/master/README.md.

Supplementary information

Supplementary data are available at Bioinformatics online.

Safinamide's potential in treating nondystrophic myotonias: Inhibition of skeletal muscle voltage-gated sodium channels and skeletal muscle hyperexcitability in vitro and in vivo

The antiarrhythmic sodium-channel blocker mexiletine is used to treat patients with myotonia. However, around 30% of patients do not benefit from mexiletine due to poor tolerability or suboptimal response. Safinamide is an add-on therapy to levodopa for Parkinson's disease. In addition to MAOB inhibition, safinamide inhibits neuronal sodium channels, conferring anticonvulsant activity in models of epilepsy. Here, we investigated the effects of safinamide on skeletal muscle hNa_v1.4 sodium channels and in models of myotonia, in-vitro and in-vivo. Using patch-clamp, we showed that safinamide reversibly inhibited sodium currents in HEK293T cells transfected with hNav1.4. At the holding potential (hp) of -120 mV, the half-maximum inhibitory concentrations (IC₅₀) were 160 and 33 μM at stimulation frequencies of 0.1 and 10 Hz, respectively. The calculated affinity constants of safinamide were dependent on channel state: 420 μM for closed channels and 9 μM for fast-inactivated channels. The p.F1586C mutation in hNav1.4 greatly impaired safinamide inhibition, suggesting that the drug binds to the local anesthetic receptor site in the channel pore. In a condition mimicking myotonia, i.e. hp. of -90 mV and 50-Hz stimulation, safinamide inhibited I_Na with an IC₅₀ of 6 μM, being two-fold more potent than mexiletine. Using the two-intracellular microelectrodes current-clamp method, action potential firing was recorded in vitro in rat skeletal muscle fibers in presence of the chloride channel blocker, 9-anthracene carboxylic acid (9-AC), to increase excitability. Safinamide counteracted muscle fiber hyperexcitability with an IC₅₀ of 13 μM. In vivo, oral safinamide was tested in the rat model of myotonia. In this model, intraperitoneal injection of 9-AC greatly increased the time of righting reflex (TRR) due to development of muscle stiffness. Safinamide counteracted 9-AC induced TRR increase with an ED₅₀ of 1.2 mg/kg, which is 7 times lower than that previously determined for mexiletine. In conclusion, safinamide is a potent voltage and frequency dependent blocker of skeletal muscle sodium channels. Accordingly, the drug was able to counteract abnormal muscle hyperexcitability induced by 9-AC, both in vitro and in vivo. Thus, this study suggests that safinamide may have potential in treating myotonia and warrants further preclinical and human studies to fully evaluate this possibility.

Defective Gating and Proteostasis of Human ClC-1 Chloride Channel: Molecular Pathophysiology of Myotonia Congenita

The voltage-dependent ClC-1 chloride channel, whose open probability increases with membrane potential depolarization, belongs to the superfamily of CLC channels/transporters. ClC-1 is almost exclusively expressed in skeletal muscles and is essential for stabilizing the excitability of muscle membranes. Elucidation of the molecular structures of human ClC-1 and several CLC homologs provides important insight to the gating and ion permeation mechanisms of this chloride channel. Mutations in the human CLCN1 gene, which encodes the ClC-1 channel, are associated with a hereditary skeletal muscle disease, myotonia congenita. Most disease-causing CLCN1 mutations lead to loss-of-function phenotypes in the ClC-1 channel and thus increase membrane excitability in skeletal muscles, consequently manifesting as delayed relaxations following voluntary muscle contractions in myotonic subjects. The inheritance pattern of myotonia congenita can be autosomal dominant (Thomsen type) or recessive (Becker type). To date over 200 myotonia-associated ClC-1 mutations have been identified, which are scattered throughout the entire protein sequence. The dominant inheritance pattern of some myotonia mutations may be explained by a dominant-negative effect on ClC-1 channel gating. For many other myotonia mutations, however, no clear relationship can be established between the inheritance pattern and the location of the mutation in the ClC-1 protein. Emerging evidence indicates that the effects of some mutations may entail impaired ClC-1 protein homeostasis (proteostasis). Proteostasis of membrane proteins comprises of biogenesis at the endoplasmic reticulum (ER), trafficking to the surface membrane, and protein turn-over at the plasma membrane. Maintenance of proteostasis requires the coordination of a wide variety of different molecular chaperones and protein quality control factors. A number of regulatory molecules have recently been shown to contribute to post-translational modifications of ClC-1 and play critical roles in the ER quality control, membrane trafficking, and peripheral quality control of this chloride channel. Further illumination of the mechanisms of ClC-1 proteostasis network will enhance our understanding of the molecular pathophysiology of myotonia congenita, and may also bring to light novel therapeutic targets for skeletal muscle dysfunction caused by myotonia and other pathological conditions.

CLCN1 Molecular Characterization in 19 South-Italian Patients With Dominant and Recessive Type of Myotonia Congenita

Myotonia congenita is a genetic disease characterized by impaired muscle relaxation after forceful contraction (myotonia). It is caused by mutations in the CLCN1 gene, encoding the voltage-gated chloride channel of skeletal muscle, ClC-1. According to the pattern of inheritance, two distinct clinical forms have been described, Thomsen disease, inherited as an autosomal dominant trait and Becker disease inherited as an autosomal recessive trait. We report genetic and clinical data concerning 19 patients−13 familial and six isolated cases—all but one originating from the Campania Region, in southern Italy. Twelve patients (63.2%) present Becker type myotonia and 7 (36.8%) Thomsen type. Sex ratio M:F in Becker type is 6:6, while in Thomsen myotonia 4:3. The age of onset of the disease ranged from 2 to 15 years in Becker patients, and from 4 to 20 years in Thomsen. Overall 18 mutations were identified, 10 located in the coding part of the gene (exons 1, 3, 4, 5, 7, 8, 13, 15, 21, 22), and four in the intron part (introns 1, 2, 10, 18). All the exon mutations but two were missense mutations. Some of them, such as c.2551 G > A, c.817G > A and c.86A > C recurred more frequently. About 70% of mutations was inherited with an autosomal recessive pattern, two (c.86A and c.817G>A) with both mechanisms. Three novel mutations were identified, never described in the literature: p.Gly276Ser, p.Phe486Ser, and p.Gln812^*, associated with Becker phenotype. Furthermore, we identified three CLCN1 mutations—c.86A>C + c.2551G > A, c.313C > T + c.501C > G and 899G > A + c.2284+5C > T, two of them inherited in cis on the same allele, in three unrelated families. The concomitant occurrence of both clinical pictures—Thomsen and Becker—was observed in one family. Intra-familial phenotypic variability was observed in two families, one with Becker phenotype, and one with Thomsen disease. In the latter an incomplete penetrance was hypothesized.

An Up-to-Date Overview of the Complexity of Genotype-Phenotype Relationships in Myotonic Channelopathies

Myotonic disorders are inherited neuromuscular diseases divided into dystrophic myotonias and non-dystrophic myotonias (NDM). The latter is a group of dominant or recessive diseases caused by mutations in genes encoding ion channels that participate in the generation and control of the skeletal muscle action potential. Their altered function causes hyperexcitability of the muscle membrane, thereby triggering myotonia, the main sign in NDM. Mutations in the genes encoding voltage-gated Cl⁻ and Na⁺ channels (respectively, CLCN1 and SCN4A) produce a wide spectrum of phenotypes, which differ in age of onset, affected muscles, severity of myotonia, degree of hypertrophy, and muscle weakness, disease progression, among others. More than 200 CLCN1 and 65 SCN4A mutations have been identified and described, but just about half of them have been functionally characterized, an approach that is likely extremely helpful to contribute to improving the so-far rather poor clinical correlations present in NDM. The observed poor correlations may be due to: (1) the wide spectrum of symptoms and overlapping phenotypes present in both groups (Cl⁻ and Na⁺ myotonic channelopathies) and (2) both genes present high genotypic variability. On the one hand, several mutations cause a unique and reproducible phenotype in most patients. On the other hand, some mutations can have different inheritance pattern and clinical phenotypes in different families. Conversely, different mutations can be translated into very similar phenotypes. For these reasons, the genotype-phenotype relationships in myotonic channelopathies are considered complex. Although the molecular bases for the clinical variability present in myotonic channelopathies remain obscure, several hypotheses have been put forward to explain the variability, which include: (a) differential allelic expression; (b) trans-acting genetic modifiers; (c) epigenetic, hormonal, or environmental factors; and (d) dominance with low penetrance. Improvements in clinical tests, the recognition of the different phenotypes that result from particular mutations and the understanding of how a mutation affects the structure and function of the ion channel, together with genetic screening, is expected to improve clinical correlation in NDMs.

Identification of two novel compound heterozygous CLCN1 mutations associated with autosomal recessive myotonia congenita

Objectives: Myotonia congenita (MC) is a rare genetic muscular disorder caused by CLCN1 mutations, which codes for skeletal muscle chloride channel CLC1. MC is characterized by impaired muscle relaxation after contraction resulting in muscle stiffness. This study aimed to identify the genetic etiology of a Chinese family affected with recessive MC. Methods: Whole exome sequencing was performed to identify the disease-associated variants. The candidate causal genes discovered by WES were then confirmed by Sanger sequencing and co-segregation analyses were also conducted. Results: Two novel compound heterozygous mutations in CLCN1 gene, p.D94Y (paternal allele) and p.Y206* (maternal allele), were successfully identified as the pathogenic mutations by whole-exome sequencing (WES). The mutations were confirmed with Sanger sequencing in the family members and cosegregated with the MC phenotype. The two mutations have not been reported in the HGMD, dbSNP, 1000 Genomes project, ClinVar database, ExAC, and gnomAD previously. Mutation p.D94Y is predicted to be deleterious by using in silico tools and p.Y206* is a nonsense mutation, causing protein synthesis termination. Conclusions: Molecular genetics analysis offers an accurate method for diagnosing MC. Our results expand the mutational spectrum of recessive MC.

Dietary reference values for chloride

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) has derived dietary reference values (DRVs) for chloride. There are no appropriate biomarkers of chloride status, no balance studies and no adequate evidence on the relationship between chloride intake and health outcomes that can be used to set DRVs for chloride. There is a close relationship between sodium and chloride balances in the body. Sodium chloride is the main source of both electrolytes in European diets and similar urinary excretion levels of sodium and chloride (on a molar basis) are typically observed in Western populations. Hence, the Panel considered that reference values for chloride can be set at values equimolar to the reference values for sodium for all population groups, and are as follows: 1.7 g/day for children aged 1-3 years, 2.0 g/day for children aged 4-6 years, 2.6 g/day for children aged 7-10 years, 3.1 g/day for children aged 11-17 years and 3.1 g/day for adults including pregnant and lactating women. Consistent with the reference values for sodium, these levels of chloride intake are considered to be safe and adequate for the general EU population, under the consideration that the main dietary source of chloride intake is sodium chloride. For infants aged 7-11 months, an adequate intake of 0.3 g/day is set.

Dietary reference values for sodium

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) derived dietary reference values (DRVs) for sodium. Evidence from balance studies on sodium and on the relationship between sodium intake and health outcomes, in particular cardiovascular disease (CVD)-related endpoints and bone health, was reviewed. The data were not sufficient to enable an average requirement (AR) or population reference intake (PRI) to be derived. However, by integrating the available evidence and associated uncertainties, the Panel considers that a sodium intake of 2.0 g/day represents a level of sodium for which there is sufficient confidence in a reduced risk of CVD in the general adult population. In addition, a sodium intake of 2.0 g/day is likely to allow most of the general adult population to maintain sodium balance. Therefore, the Panel considers that 2.0 g sodium/day is a safe and adequate intake for the general EU population of adults. The same value applies to pregnant and lactating women. Sodium intakes that are considered safe and adequate for children are extrapolated from the value for adults, adjusting for their respective energy requirement and including a growth factor, and are as follows: 1.1 g/day for children aged 1-3 years, 1.3 g/day for children aged 4-6 years, 1.7 g/day for children aged 7-10 years and 2.0 g/day for children aged 11-17 years, respectively. For infants aged 7-11 months, an Adequate Intake (AI) of 0.2 g/day is proposed based on upwards extrapolation of the estimated sodium intake in exclusively breast-fed infants aged 0-6 months.

Becker's myotonia: novel mutations and clinical variability in patients born to consanguineous parents

Myotonia congenita is an inherited muscle disease present from childhood that is characterized by impaired muscle relaxation after contraction resulting in muscle stiffness; moreover, skeletal striated muscle groups may be involved. Myotonia congenita occurs due to chloride (Cl) channel mutations that reduce the stabilizing Cl conductance, and it is caused by mutations in the CLCN1 gene. This paper describes four patients from two different healthy consanguineous Turkish families with muscle stiffness and easy fatigability. A genetic investigation was performed. Mutation analyses showed a homozygous p.Tyr150* (c.450C > A) mutation in patients 1, 2 and 3 and a homozygous p.Leu159Cysfs*11 (c.475delC) mutation in patient 4 in the CLCN1 gene. These mutations have never been reported before and in silico analyses showed that the mutations were disease causing. They may be predicted to cause nonsense-mediated mRNA decay. Our data expand the spectrum of CLCN1 mutations and provide insights for genotype-phenotype correlations of myotonia congenita.

Kir2.2 p.Thr140Met: a genetic susceptibility to sporadic periodic paralysis

INTRODUCTION

Periodic paralyses (PP) are recurrent episodes of flaccid limb muscle weakness. Next to autosomal dominant forms, sporadic PP (SPP) cases are known but their genetics are unclear.

METHODS

In a patient with hypokalemic SPP, we performed exome sequencing to identify a candidate gene. We sequenced this gene in 263 unrelated PP patients without any known causative mutations. Then we performed functional analysis of all variants found and molecular modelling for interpretation.

RESULTS

Exome sequencing in the proband yielded three heterozygous variants predicted to be linked to disease. These encoded p.Thr140Met in the Kir2.2 potassium channel, p.Asp229Asn in protein kinase C theta, and p.Thr15943Ile in titin. Since all hitherto known causative PP genes code for ion channels, we studied the Kir2.2-encoding gene, KCNJ12, for involvement in PP pathogenesis. KCNJ12 screening in 263 PP patients revealed three further variants, each in a single individual and coding for p.Gly419Ser, p.Cys75Tyr, and p.Ile283Val. All four Kir2.2 variants were functionally expressed. Only p.Thr140Met displayed relevant functional alterations, i.e. homo-tetrameric channels produced almost no current, and hetero-tetrameric channels suppressed co-expressed wildtype Kir2.1 in a dominant-negative manner. Molecular modelling showed Kir2.2 p.Thr140Met to reduce movement of potassium ions towards binding sites in the hetero-tetramer pore compatible with a reduced maximal current. MD simulations revealed loss of hydrogen bonding with the p.Thr140Met substitution.

DISCUSSION

The electrophysiological findings of p.Thr140Met are similar to those found in thyrotoxic PP caused by Kir2.6 mutations. Also, the homologous Thr140 residue is mutated in Kir2.6. This supports the idea that Kir2.2 p.Thr140Met conveys susceptibility to SPP and should be included in genetic screening.

The analysis of myotonia congenita mutations discloses functional clusters of amino acids within the CBS2 domain and the C-terminal peptide of the ClC-1 channel

Myotonia congenita (MC) is a skeletal-muscle hyperexcitability disorder caused by loss-of-function mutations in the ClC-1 chloride channel. Mutations are scattered over the entire sequence of the channel protein, with more than 30 mutations located in the poorly characterized cytosolic C-terminal domain. In this study, we characterized, through patch clamp, seven ClC-1 mutations identified in patients affected by MC of various severities and located in the C-terminal region. The p.Val829Met, p.Thr832Ile, p.Val851Met, p.Gly859Val, and p.Leu861Pro mutations reside in the CBS2 domain, while p.Pro883Thr and p.Val947Glu are in the C-terminal peptide. We showed that the functional properties of mutant channels correlated with the clinical phenotypes of affected individuals. In addition, we defined clusters of ClC-1 mutations within CBS2 and C-terminal peptide subdomains that share the same functional defect: mutations between 829 and 835 residues and in residue 883 induced an alteration of voltage dependence, mutations between 851 and 859 residues, and in residue 947 induced a reduction of chloride currents, whereas mutations on 861 residue showed no obvious change in ClC-1 function. This study improves our understanding of the mechanisms underlying MC, sheds light on the role of the C-terminal region in ClC-1 function, and provides information to develop new antimyotonic drugs.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Chloride Homeostasis in Neurons With Special Emphasis on the Olivocerebellar System: Differential Roles for Transporters and Channels

The intraneuronal ionic composition is an important determinant of brain functioning. There is growing evidence that aberrant homeostasis of the intracellular concentration of Cl- ([Cl-]i) evokes, in addition to that of Na+ and Ca2+, robust impairments of neuronal excitability and neurotransmission and thereby neurological conditions. More specifically, understanding the mechanisms underlying regulation of [Cl-]i is crucial for deciphering the variability in GABAergic and glycinergic signaling of neurons, in both health and disease. The homeostatic level of [Cl-]i is determined by various regulatory mechanisms, including those mediated by plasma membrane Cl- channels and transporters. This review focuses on the latest advances in identification, regulation and characterization of Cl- channels and transporters that modulate neuronal excitability and cell volume. By putting special emphasis on neurons of the olivocerebellar system, we establish that Cl- channels and transporters play an indispensable role in determining their [Cl-]i and thereby their function in sensorimotor coordination.

A selective class of inhibitors for the CLC-Ka chloride ion channel

CLC proteins are a ubiquitously expressed family of chloride-selective ion channels and transporters. A dearth of pharmacological tools for modulating CLC gating and ion conduction limits investigations aimed at understanding CLC structure/function and physiology. Herein, we describe the design, synthesis, and evaluation of a collection of N-arylated benzimidazole derivatives (BIMs), one of which (BIM1) shows unparalleled (>20-fold) selectivity for CLC-Ka over CLC-Kb, the two most closely related human CLC homologs. Computational docking to a CLC-Ka homology model has identified a BIM1 binding site on the extracellular face of the protein near the chloride permeation pathway in a region previously identified as a binding site for other less selective inhibitors. Results from site-directed mutagenesis experiments are consistent with predictions of this docking model. The residue at position 68 is 1 of only ∼20 extracellular residues that differ between CLC-Ka and CLC-Kb. Mutation of this residue in CLC-Ka and CLC-Kb (N68D and D68N, respectively) reverses the preference of BIM1 for CLC-Ka over CLC-Kb, thus showing the critical role of residue 68 in establishing BIM1 selectivity. Molecular docking studies together with results from structure-activity relationship studies with 19 BIM derivatives give insight into the increased selectivity of BIM1 compared with other inhibitors and identify strategies for further developing this class of compounds.

Mapping ligand binding pockets in chloride ClC-1 channels through an integrated in silico and experimental approach using anthracene-9-carboxylic acid and niflumic acid

BACKGROUND AND PURPOSE

Although chloride channels are involved in several physiological processes and acquired diseases, the availability of compounds selectively targeting CLC proteins is limited. ClC-1 channels are responsible for sarcolemma repolarization after an action potential in skeletal muscle and have been associated with myotonia congenita and myotonic dystrophy as well as with other muscular physiopathological conditions. To date only a few ClC-1 blockers have been discovered, such as anthracene-9-carboxylic acid (9-AC) and niflumic acid (NFA), whereas no activator exists. The absence of a ClC-1 structure and the limited information regarding the binding pockets in CLC channels hamper the identification of improved modulators.

EXPERIMENTAL APPROACH

Here we provide an in-depth characterization of drug binding pockets in ClC-1 through an integrated in silico and experimental approach. We first searched putative cavities in a homology model of ClC-1 built upon an eukaryotic CLC crystal structure, and then validated in silico data by measuring the blocking ability of 9-AC and NFA on mutant ClC-1 channels expressed in HEK 293 cells.

KEY RESULTS

We identified four putative binding cavities in ClC-1. 9-AC appears to interact with residues K231, R421 and F484 within the channel pore. We also identified one preferential binding cavity for NFA and propose R421 and F484 as critical residues.

CONCLUSIONS AND IMPLICATIONS

This study represents the first effort to delineate the binding sites of ClC-1. This information is fundamental to discover compounds useful in the treatment of ClC-1-associated dysfunctions and might represent a starting point for specifically targeting other CLC proteins.

Loop diuretics affect skeletal myoblast differentiation and exercise-induced muscle hypertrophy

Muscle wasting or sarcopenia contributes to morbidity and mortality in patients with cancer, renal failure, or heart failure, and in elderly individuals. Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1) is highly expressed in mammalian skeletal muscle, where it contributes to the generation of membrane ion currents and potential. However, the physiologic function of NKCC1 in myogenesis is unclear. We investigated this issue using the NKCC1 inhibitors bumetanide and furosemide, which are commonly used loop diuretics. NKCC1 protein levels increased during C2C12 murine skeletal myoblast differentiation, similarly to those of the myogenic markers myogenin and myosin heavy chain (MHC). NKCC1 inhibitors markedly suppressed myoblast fusion into myotubes and the expression of myogenin and MHC. Furthermore, phosphorylated and total NKCC1 levels were elevated in mouse skeletal muscles after 6 weeks’ voluntary wheel running. Immunofluorescence analyses of myofiber cross-sections revealed more large myofibers after exercise, but this was impaired by daily intraperitoneal bumetanide injections (0.2 or 10 mg/kg/day). NKCC1 plays an essential role in myogenesis and exercise-induced skeletal muscle hypertrophy, and sarcopenia in patients with renal or heart failure may be attributable to treatment with loop diuretics.

Targeted Next Generation Sequencing in patients with Myotonia Congenita

INTRODUCTION

Myotonia Congenita (MC) is a nondystrophic skeletal muscle disease characterized by muscle stiffness, weakness, delayed skeletal relaxation and hypertrophic muscle. The disease can be inherited as dominant or recessive. More than 130 mutations in CLCN1 gene have been identified.

MATERIALS AND METHODS

We analyzed the entire coding region and exon-intron boundaries of the CLCN1 gene in 40 MC patients. Samples already Sanger-sequenced were successively evaluated by Next Generation Sequencing (NGS), on Ion Torrent PGM. Moreover, additional 15 patients were sequenced directly by NGS.

RESULTS

NGS allowed us to identify all CLCN1 mutations except those located within exon 3, demonstrating a 96% of sensitivity. Due to primer design, one SNP (exactly rs7794560) also failed to be detected. Our results enlarge the spectrum of CLCN1 mutations and showed a novel approach for molecular analysis of MC.

ClC Channels and Transporters: Structure, Physiological Functions, and Implications in Human Chloride Channelopathies

The discovery of ClC proteins at the beginning of the 1990s was important for the development of the Cl^- transport research field. ClCs form a large family of proteins that mediate voltage-dependent transport of Cl^- ions across cell membranes. They are expressed in both plasma and intracellular membranes of cells from almost all living organisms. ClC proteins form transmembrane dimers, in which each monomer displays independent ion conductance. Eukaryotic members also possess a large cytoplasmic domain containing two CBS domains, which are involved in transport modulation. ClC proteins function as either Cl^- channels or Cl^-/H⁺ exchangers, although all ClC proteins share the same basic architecture. ClC channels have two gating mechanisms: a relatively well-studied fast gating mechanism, and a slow gating mechanism, which is poorly defined. ClCs are involved in a wide range of physiological processes, including regulation of resting membrane potential in skeletal muscle, facilitation of transepithelial Cl^- reabsorption in kidneys, and control of pH and Cl^- concentration in intracellular compartments through coupled Cl^-/H⁺ exchange mechanisms. Several inherited diseases result from C1C gene mutations, including myotonia congenita, Bartter's syndrome (types 3 and 4), Dent's disease, osteopetrosis, retinal degeneration, and lysosomal storage diseases. This review summarizes general features, known or suspected, of ClC structure, gating and physiological functions. We also discuss biophysical properties of mammalian ClCs that are directly involved in the pathophysiology of several human inherited disorders, or that induce interesting phenotypes in animal models.

The Caenorhabditis elegans Excretory System: A Model for Tubulogenesis, Cell Fate Specification, and Plasticity

The excretory system of the nematode Caenorhabditis elegans is a superb model of tubular organogenesis involving a minimum of cells. The system consists of just three unicellular tubes (canal, duct, and pore), a secretory gland, and two associated neurons. Just as in more complex organs, cells of the excretory system must first adopt specific identities and then coordinate diverse processes to form tubes of appropriate topology, shape, connectivity, and physiological function. The unicellular topology of excretory tubes, their varied and sometimes complex shapes, and the dynamic reprogramming of cell identity and remodeling of tube connectivity that occur during larval development are particularly fascinating features of this organ. The physiological roles of the excretory system in osmoregulation and other aspects of the animal's life cycle are only beginning to be explored. The cellular mechanisms and molecular pathways used to build and shape excretory tubes appear similar to those used in both unicellular and multicellular tubes in more complex organs, such as the vertebrate vascular system and kidney, making this simple organ system a useful model for understanding disease processes.

The choroid plexus as a sex hormone target: Functional implications

The choroid plexuses (CPs) are highly vascularized branched structures that protrude into the ventricles of the brain, and form a unique interface between the blood and the cerebrospinal fluid (CSF). In recent years, novel functions have been attributed to this tissue such as in immune and chemical surveillance of the central nervous system, brain development, adult neurogenesis and circadian rhythm regulation. Sex hormones (SH) are widely recognized as modulators in several neurodegenerative diseases, and there is evidence that estrogens and androgens regulate several fundamental biological functions in the CPs. Therefore, SH are likely to affect the composition of the CSF impacting on brain homeostasis. This review will look at implications of the CPs' sex-related specificities.

Helix O modulates voltage dependency of CLC-1

The chloride channel (CLC) family of proteins consists of channels and transporters that share similarities in architecture and play essential roles in physiological functions. Among the CLC family, CLC-1 channels have the representative homodimeric double-barreled structure carrying two gating processes. One is protopore gating that acts on each pore independently by glutamate residue (E_ext). The other is common gating that closes both pores simultaneously in association with large conformational changes across each subunit. In skeletal muscle, CLC-1 is associated with maintaining normal sarcolemmal excitability, and a number of myotonic mutants were reported to modify the channel gating of CLC-1. In this study, we characterized highly conserved helix O as a key determinant of structural stability in CLC-1. Supporting this hypothesis, myotonic mutant (G523D) at N-terminal of helix O showed the activation at hyperpolarizing membrane potentials with a reversed voltage dependency. However, introducing glutamate at serine residue (S537) at the C-terminal of the helix O on G523D restored WT-like voltage dependency of the common gate and showed proton insensitive voltage dependency. To further validate this significant site, site-specific mutagenesis experiments was performed on V292 that is highly conserved as glutamate in antiporter and closely located to S537 and showed that this area is essential for channel function. Taken together, the results of our study suggest the importance of helix O as the main contributor for stable structure of evolutionary conserved CLC proteins and its key role in voltage dependency of the CLC-1. Furthermore, the C-terminal of the helix O can offer a clue for possible proton involvement in CLC-1 channel.

Thomsen disease with ptosis and abnormal MR findings

Myotonia congenita is a non-dystrophic skeletal muscle disorder characterized by muscle stiffness and an inability of the muscle to relax after voluntary contraction caused by a mutation in the gene encoding skeletal muscle chloride channel-1 (CLCN1). We encountered a case of Thomsen disease with ptosis. A short tau inversion recovery MR imaging demonstrated high-intensity lesions in the levator palpebrae superioris muscles. Molecular genetic testing revealed a heterozygosity for the c.1439C>A (p.P480H) mutation in the CLCN1 gene. The expression level of ClC-1 was significantly reduced on the sarcolemma of the biceps brachii muscle from the patient, compared with that from healthy volunteer. Functional analysis of the p.P480H mutation is required for further elucidating the pathogenesis of Thomsen disease.