Myotonia congenita mutation enhances the degradation of human CLC-1 chloride channels

Molecular mechanism of hyperactivation conferred by a truncation of TRPA1

A drastic TRPA1 mutant (R919*) identified in CRAMPT syndrome patients has not been mechanistically characterized. Here, we show that the R919* mutant confers hyperactivity when co-expressed with wild type (WT) TRPA1. Using functional and biochemical assays, we reveal that the R919* mutant co-assembles with WT TRPA1 subunits into heteromeric channels in heterologous cells that are functional at the plasma membrane. The R919* mutant hyperactivates channels by enhancing agonist sensitivity and calcium permeability, which could account for the observed neuronal hypersensitivity-hyperexcitability symptoms. We postulate that R919* TRPA1 subunits contribute to heteromeric channel sensitization by altering pore architecture and lowering energetic barriers to channel activation contributed by the missing regions. Our results expand the physiological impact of nonsense mutations, reveal a genetically tractable mechanism for selective channel sensitization, uncover insights into the process of TRPA1 gating, and provide an impetus for genetic analysis of patients with CRAMPT or other stochastic pain syndromes.

Chaperone activity of niflumic acid on ClC-1 chloride channel mutants causing myotonia congenita

Myotonia congenita (MC) is an inherited rare disease characterized by impaired muscle relaxation after contraction, resulting in muscle stiffness. It is caused by loss-of-function mutations in the skeletal muscle chloride channel ClC-1, important for the stabilization of resting membrane potential and for the repolarization phase of action potentials. Thanks to in vitro functional studies, the molecular mechanisms by which ClC-1 mutations alter chloride ion influx into the cell have been in part clarified, classifying them in “gating-defective” or “expression-defective” mutations. To date, the treatment of MC is only palliative because no direct ClC-1 activator is available. An ideal drug should be one which is able to correct biophysical defects of ClC-1 in the case of gating-defective mutations or a drug capable to recover ClC-1 protein expression on the plasma membrane for trafficking-defective ones. In this study, we tested the ability of niflumic acid (NFA), a commercial nonsteroidal anti-inflammatory drug, to act as a pharmacological chaperone on trafficking-defective MC mutants (A531V, V947E). Wild-type (WT) or MC mutant ClC-1 channels were expressed in HEK293 cells and whole-cell chloride currents were recorded with the patch-clamp technique before and after NFA incubation. Membrane biotinylation assays and western blot were performed to support electrophysiological results. A531V and V947E mutations caused a decrease in chloride current density due to a reduction of ClC-1 total protein level and channel expression on the plasma membrane. The treatment of A531V and V947E-transfected cells with 50 µM NFA restored chloride currents, reaching levels similar to those of WT. Furthermore, no significant difference was observed in voltage dependence, suggesting that NFA increased protein membrane expression without altering the function of ClC-1. Indeed, biochemical experiments confirmed that V947E total protein expression and its plasma membrane distribution were recovered after NFA incubation, reaching protein levels similar to WT. Thus, the use of NFA as a pharmacological chaperone in trafficking defective ClC-1 channel mutations could represent a good strategy in the treatment of MC. Because of the favorable safety profile of this drug, our study may easily open the way for confirmatory human pilot studies aimed at verifying the antimyotonic activity of NFA in selected patients carrying specific ClC-1 channel mutations.

First Two Case Reports of Becker's Type Myotonia Congenita in Colombia: Clinical and Genetic Features

Background

Becker's type myotonia congenita is an autosomal recessive nondystrophic skeletal muscle disorder characterized by muscle stiffness and the inability of muscle relaxation after voluntary contraction. It is caused by mutations in the CLCN1 gene, which encodes for a chloride channel mainly expressed in the striated muscle. Most cases have been reported in the European population, and only mexiletine has demonstrated a randomized placebo-controlled, double-blinded effectiveness.

Case Presentation

We present two male siblings from Colombia with Latino ancestry, without parental consanguinity, with myotonia during voluntary movements, muscle hypertrophy of lower extremities, transient weakness, and severe muscle fatigue after exercise from three years of age. A genetic panel for dystrophic muscle disorders and a muscle biopsy were both negative. Genetic testing was performed in their second decade of life. Both patients' exomic sequencing test reported the mutation c.1129C >T (p.Arg377*) affecting exon 10 of the CLCN1, generating a premature stop codon. This mutation was described as pathogenic and observed in only one other patient in the United Kingdom.

Conclusion

To our knowledge, these are the first cases of Becker's type myotonia congenita reported in Colombia. Increasing awareness of healthcare providers for this type of disease in the region could lead to the identification of undiagnosed patients. Limited availability of medical geneticists as well as genetic testing may be the cause of the lack of previous description of cases, in addition to the delay in the diagnosis of the patients. Further epidemiological studies can reveal underdiagnosed myotonias in the country and in the Latin-American region.

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.

Functional Study of Novel Bartter's Syndrome Mutations in ClC-Kb and Rescue by the Accessory Subunit Barttin Toward Personalized Medicine

Type III and IV Bartter syndromes (BS) are rare kidney tubulopathies caused by loss-of-function mutations in the CLCNKB and BSND genes coding respectively for the ClC-Kb chloride channels and accessory subunit barttin. ClC-K channels are expressed in the Henle's loop, distal convoluted tubule, and cortical collecting ducts of the kidney and contribute to chloride absorption and urine concentration. In our Italian cohort, we identified two new mutations in CLCNKB, G167V and G289R, in children affected by BS and previously reported genetic variants, A242E, a chimeric gene and the deletion of the whole CLCNKB. All the patients had hypokalemia and metabolic alkalosis, increased serum renin and aldosterone levels and were treated with a symptomatic therapy. In order to define the molecular mechanisms responsible for BS, we co-expressed ClC-Kb wild type and channels with point mutations with barttin in HEK 293 cells and characterized chloride currents through the patch-clamp technique. In addition, we attempted to revert the functional defect caused by BS mutations through barttin overexpression. G167V and A242E channels showed a drastic current reduction compared to wild type, likely suggesting compromised expression of mutant channels at the plasma membrane. Conversely, G289R channel was similar to wild type raising the doubt that an additional mutation in another gene or other mechanisms could account for the clinical phenotype. Interestingly, increasing ClC-K/barttin ratio augmented G167V and A242E mutants' chloride current amplitudes towards wild type levels. These results confirm a genotype-phenotype correlation in BS and represent a preliminary proof of concept that molecules functioning as molecular chaperones can restore channel function in expression-defective ClC-Kb mutants.

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.

A large intragenic deletion in the CLCN1 gene causes Hereditary Myotonia in pigs

Mutations in the CLCN1 gene are the primary cause of non-dystrophic Hereditary Myotonia in several animal species. However, there are no reports of Hereditary Myotonia in pigs to date. Therefore, the objective of the present study was to characterize the clinical and molecular findings of Hereditary Myotonia in an inbred pedigree. The clinical, electromyographic, histopathological, and molecular findings were evaluated. Clinically affected pigs presented non-dystrophic recessive Hereditary Myotonia. Nucleotide sequence analysis of the entire coding region of the CLCN1 gene revealed the absence of the exons 15 and 16 in myotonic animals. Analysis of the genomic region flanking the deletion unveiled a large intragenic deletion of 4,165 nucleotides. Interestingly, non-related, non-myotonic pigs expressed transcriptional levels of an alternate transcript (i.e., X2) that was identical to the deleted X1 transcript of myotonic pigs. All myotonic pigs and their progenitors were homozygous recessive and heterozygous, respectively, for the 4,165-nucleotide deletion. This is the first study reporting Hereditary Myotonia in pigs and characterizing its clinical and molecular findings. Moreover, to the best of our knowledge, Hereditary Myotonia has never been associated with a genomic deletion in the CLCN1 gene in any other species.

FKBP8 Enhances Protein Stability of the CLC-1 Chloride Channel at the Plasma Membrane

Mutations in the skeletal muscle-specific CLC-1 chloride channel are associated with the human hereditary disease myotonia congenita. The molecular pathophysiology underlying some of the disease-causing mutations can be ascribed to defective human CLC-1 protein biosynthesis. CLC-1 protein folding is assisted by several molecular chaperones and co-chaperones, including FK506-binding protein 8 (FKBP8). FKBP8 is generally considered an endoplasmic reticulum- and mitochondrion-resident membrane protein, but is not thought to contribute to protein quality control at the cell surface. Herein, we aim to test the hypothesis that FKBP8 may regulate CLC-1 protein at the plasma membrane. Surface biotinylation and subcellular fractionation analyses reveal that a portion of FKBP8 is present at the plasma membrane, and that co-expression with CLC-1 enhances surface localization of FKBP8. Immunoblotting analyses of plasma membrane proteins purified from skeletal muscle further confirm surface localization of FKBP8. Importantly, FKBP8 promotes CLC-1 protein stability at the plasma membrane. Together, our data underscore the importance of FKBP8 in the peripheral quality control of CLC-1 channel.

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.

Electrophysiological characteristics of R47W and A298T mutations in CLC-1 of myotonia congenita patients and evaluation of clinical features

Myotonia congenita (MC) is a genetic disease that displays impaired relaxation of skeletal muscle and muscle hypertrophy. This disease is mainly caused by mutations of CLCN1 that encodes human skeletal muscle chloride channel (CLC-1). CLC-1 is a voltage gated chloride channel that activates upon depolarizing potentials and play a major role in stabilization of resting membrane potentials in skeletal muscle. In this study, we report 4 unrelated Korean patients diagnosed with myotonia congenita and their clinical features. Sequence analysis of all coding regions of the patients was performed and mutation, R47W and A298T, was commonly identified. The patients commonly displayed transient muscle weakness and only one patient was diagnosed with autosomal dominant type of myotonia congenita. To investigate the pathological role of the mutation, electrophysiological analysis was also performed in HEK 293 cells transiently expressing homo- or heterodimeric mutant channels. The mutant channels displayed reduced chloride current density and altered channel gating. However, the effect of A298T on channel gating was reduced with the presence of R47W in the same allele. This analysis suggests that impaired CLC-1 channel function can cause myotonia congenita and that R47W has a protective effect on A298T in relation to channel gating. Our results provide clinical features of Korean myotonia congenita patients who have the heterozygous mutation and reveal underlying pathophyological consequences of the mutants by taking electrophysiological approach.

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.

Regulation of CLC-1 chloride channel biosynthesis by FKBP8 and Hsp90β

Mutations in human CLC-1 chloride channel are associated with the skeletal muscle disorder myotonia congenita. The disease-causing mutant A531V manifests enhanced proteasomal degradation of CLC-1. We recently found that CLC-1 degradation is mediated by cullin 4 ubiquitin ligase complex. It is currently unclear how quality control and protein degradation systems coordinate with each other to process the biosynthesis of CLC-1. Herein we aim to ascertain the molecular nature of the protein quality control system for CLC-1. We identified three CLC-1-interacting proteins that are well-known heat shock protein 90 (Hsp90)-associated co-chaperones: FK506-binding protein 8 (FKBP8), activator of Hsp90 ATPase homolog 1 (Aha1), and Hsp70/Hsp90 organizing protein (HOP). These co-chaperones promote both the protein level and the functional expression of CLC-1 wild-type and A531V mutant. CLC-1 biosynthesis is also facilitated by the molecular chaperones Hsc70 and Hsp90β. The protein stability of CLC-1 is notably increased by FKBP8 and the Hsp90β inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) that substantially suppresses cullin 4 expression. We further confirmed that cullin 4 may interact with Hsp90β and FKBP8. Our data are consistent with the idea that FKBP8 and Hsp90β play an essential role in the late phase of CLC-1 quality control by dynamically coordinating protein folding and degradation.

Impaired surface membrane insertion of homo- and heterodimeric human muscle chloride channels carrying amino-terminal myotonia-causing mutations

Mutations in the muscle chloride channel gene (CLCN1) cause myotonia congenita, an inherited condition characterized by muscle stiffness upon sudden forceful movement. We here studied the functional consequences of four disease-causing mutations that predict amino acid substitutions Q43R, S70L, Y137D and Q160H. Wild-type (WT) and mutant hClC-1 channels were heterologously expressed as YFP or CFP fusion protein in HEK293T cells and analyzed by whole-cell patch clamp and fluorescence recordings on individual cells. Q43R, Y137D and Q160H, but not S70L reduced macroscopic current amplitudes, but left channel gating and unitary current amplitudes unaffected. We developed a novel assay combining electrophysiological and fluorescence measurements at the single-cell level in order to measure the probability of ion channel surface membrane insertion. With the exception of S70L, all tested mutations significantly reduced the relative number of homodimeric hClC-1 channels in the surface membrane. The strongest effect was seen for Q43R that reduced the surface insertion probability by more than 99% in Q43R homodimeric channels and by 92 ± 3% in heterodimeric WT/Q43R channels compared to homodimeric WT channels. The new method offers a sensitive approach to investigate mutations that were reported to cause channelopathies, but display only minor changes in ion channel function.

The Cullin 4A/B-DDB1-Cereblon E3 Ubiquitin Ligase Complex Mediates the Degradation of CLC-1 Chloride Channels

Voltage-gated CLC-1 chloride channels play a critical role in controlling the membrane excitability of skeletal muscles. Mutations in human CLC-1 channels have been linked to the hereditary muscle disorder myotonia congenita. We have previously demonstrated that disease-associated CLC-1 A531V mutant protein may fail to pass the endoplasmic reticulum quality control system and display enhanced protein degradation as well as defective membrane trafficking. Currently the molecular basis of protein degradation for CLC-1 channels is virtually unknown. Here we aim to identify the E3 ubiquitin ligase of CLC-1 channels. The protein abundance of CLC-1 was notably enhanced in the presence of MLN4924, a specific inhibitor of cullin-RING E3 ligases. Subsequent investigation with dominant-negative constructs against specific subtypes of cullin-RING E3 ligases suggested that CLC-1 seemed to serve as the substrate for cullin 4A (CUL4A) and 4B (CUL4B). Biochemical examinations further indicated that CUL4A/B, damage-specific DNA binding protein 1 (DDB1), and cereblon (CRBN) appeared to co-exist in the same protein complex with CLC-1. Moreover, suppression of CUL4A/B E3 ligase activity significantly enhanced the functional expression of the A531V mutant. Our data are consistent with the idea that the CUL4A/B-DDB1-CRBN complex catalyses the polyubiquitination and thus controls the degradation of CLC-1 channels.

CLC channel function and dysfunction in health and disease

CLC channels and transporters are expressed in most tissues and fulfill diverse functions. There are four human CLC channels, ClC-1, ClC-2, ClC-Ka, and ClC-Kb, and five CLC transporters, ClC-3 through −7. Some of the CLC channels additionally associate with accessory subunits. Whereas barttin is mandatory for the functional expression of ClC-K, GlialCam is a facultative subunit of ClC-2 which modifies gating and thus increases the functional variability within the CLC family. Isoform-specific ion conduction and gating properties optimize distinct CLC channels for their cellular tasks. ClC-1 preferentially conducts at negative voltages, and the resulting inward rectification provides a large resting chloride conductance without interference with the muscle action potential. Exclusive opening at voltages negative to the chloride reversal potential allows for ClC-2 to regulate intracellular chloride concentrations. ClC-Ka and ClC-Kb are equally suited for inward and outward currents to support transcellular chloride fluxes. Every human CLC channel gene has been linked to a genetic disease, and studying these mutations has provided much information about the physiological roles and the molecular basis of CLC channel function. Mutations in the gene encoding ClC-1 cause myotonia congenita, a disease characterized by sarcolemmal hyperexcitability and muscle stiffness. Loss-of-function of ClC-Kb/barttin channels impairs NaCl resorption in the limb of Henle and causes hyponatriaemia, hypovolemia and hypotension in patients suffering from Bartter syndrome. Mutations in CLCN2 were found in patients with CNS disorders but the functional role of this isoform is still not understood. Recent links between ClC-1 and epilepsy and ClC-Ka and heart failure suggested novel cellular functions of these proteins. This review aims to survey the knowledge about physiological and pathophysiological functions of human CLC channels in the light of recent discoveries from biophysical, physiological, and genetic studies.

Activation and inhibition of TMEM16A calcium-activated chloride channels

Calcium-activated chloride channels (CaCC) encoded by family members of transmembrane proteins of unknown function 16 (TMEM16) have recently been intensely studied for functional properties as well as their physiological roles as chloride channels in various tissues. One technical hurdle in studying these channels is the well-known channel rundown that frequently impairs the precision of electrophysiological measurements for the channels. Using experimental protocols that employ fast-solution exchange, we circumvented the problem of channel rundown by normalizing the Ca²⁺-induced current to the maximally-activated current obtained within a time period in which the channel rundown was negligible. We characterized the activation of the TMEM16A-encoded CaCC (also called ANO1) by Ca²⁺, Sr²⁺, and Ba²⁺, and discovered that Mg²⁺ competes with Ca²⁺ in binding to the divalent-cation binding site without activating the channel. We also studied the permeability of the ANO1 pore for various anions and found that the anion occupancy in the pore–as revealed by the permeability ratios of these anions–appeared to be inversely correlated with the apparent affinity of the ANO1 inhibition by niflumic acid (NFA). On the other hand, the NFA inhibition was neither affected by the degree of the channel activation nor influenced by the types of divalent cations used for the channel activation. These results suggest that the NFA inhibition of ANO1 is likely mediated by altering the pore function but not through changing the channel gating. Our study provides a precise characterization of ANO1 and documents factors that can affect divalent cation activation and NFA inhibition of ANO1.

Functional characterization of ClC-1 mutations from patients affected by recessive myotonia congenita presenting with different clinical phenotypes

Myotonia congenita (MC) is caused by loss-of-function mutations of the muscle ClC-1 chloride channel. Clinical manifestations include the variable association of myotonia and transitory weakness. We recently described a cohort of recessive MC patients showing, at a low rate repetitive nerves stimulation protocol, different values of compound muscle action potential (CMAP) transitory depression, which is considered the neurophysiologic counterpart of transitory weakness. From among this cohort, we studied the chloride currents generated by G190S (associated with pronounced transitory depression), F167L (little or no transitory depression), and A531V (variable transitory depression) hClC-1 mutants in transfected HEK293 cells using patch-clamp. While F167L had no effect on chloride currents, G190S dramatically shifts the voltage dependence of channel activation and A531V reduces channel expression. Such variability in molecular mechanisms observed in the hClC-1 mutants may help to explain the different clinical and neurophysiologic manifestations of each ClCN1 mutation. In addition we examined five different mutations found in compound heterozygosis with F167L, including the novel P558S, and we identified additional molecular defects. Finally, the G190S mutation appeared to impair acetazolamide effects on chloride currents in vitro.

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Myotonia congenita is a muscle disorder due to mutations in ClC-1 chloride channel.

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Eight ClC-1 channel mutants were studied using patch-clamp technique.

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Mutations induce a variety of molecular defects in ClC-1 channel function.

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We discuss the relationship between genotype and clinical phenotype.