Absence of the skeletal muscle sarcolemma chloride channel ClC-1 in myotonic mice

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.

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.

Abundance of ClC-1 chloride channel in human skeletal muscle: fiber type specific differences and effect of training

Cl− channel protein 1 (ClC-1) may be important for excitability and contractility in skeletal muscle, but ClC-1 abundance has not been examined in human muscle. The aim of the present study was to examine ClC-1 abundance in human skeletal muscle, including fiber type specific differences and the effect of exercise training. A commercially available antibody was tested with positive and negative control tissue, and it recognized specifically ClC-1 in the range from 100 to 150 kDa. Abundance of ClC-1 was 38% higher ( P < 0.01) in fast twitch Type IIa muscle fibers than in slow twitch Type I. Muscle ClC-1 abundance did not change with 4 wk of training consisting of 30 min cycling at 85% of maximal heart rate (HRmax) and 3 × 30-s all out sprints or during a 7-wk training period with 10–12 × 30 s uphill cycling and 4–5 × ~4 min cycling at 90%–95% of HRmax. ClC-1 abundance correlated negatively ( P < 0.01) with maximal oxygen consumption ( r = –0.552) and incremental exercise performance ( r = –0.546). In addition, trained cyclists had lower ( P < 0.01) ClC-1 abundance than lesser trained individuals. The present observations indicate that a low abundance of muscle ClC-1 may be beneficial for exercise performance, but the role of abundance and regulation of ClC-1 in skeletal muscle of humans with respect to exercise performance and trainability need to be elucidated.

NEW & NOTEWORTHY Abundance of the Cl− channel protein 1 (ClC-1) chloride channel may be important for excitability and contractility in human skeletal muscle and may therefore have implications for fatigue development. In this study, we confirmed ClC-1 specificity for a commercially available antibody, and this study is first to our knowledge to determine ClC-1 protein abundance in human muscle by Western blotting. We observed that abundance of ClC-1 was higher in fast compared with slow twitch fibers and lower in trained individuals than in recreationally active.

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.

Role of physiological ClC-1 Cl- ion channel regulation for the excitability and function of working skeletal muscle

Electrical membrane properties of skeletal muscle fibers have been thoroughly studied over the last five to six decades. This has shown that muscle fibers from a wide range of species, including fish, amphibians, reptiles, birds, and mammals, are all characterized by high resting membrane permeability for Cl⁻ ions. Thus, in resting human muscle, ClC-1 Cl⁻ ion channels account for ∼80% of the membrane conductance, and because active Cl⁻ transport is limited in muscle fibers, the equilibrium potential for Cl⁻ lies close to the resting membrane potential. These conditions—high membrane conductance and passive distribution—enable ClC-1 to conduct membrane current that inhibits muscle excitability. This depressing effect of ClC-1 current on muscle excitability has mostly been associated with skeletal muscle hyperexcitability in myotonia congenita, which arises from loss-of-function mutations in the CLCN1 gene. However, given that ClC-1 must be drastically inhibited (∼80%) before myotonia develops, more recent studies have explored whether acute and more subtle ClC-1 regulation contributes to controlling the excitability of working muscle. Methods were developed to measure ClC-1 function with subsecond temporal resolution in action potential firing muscle fibers. These and other techniques have revealed that ClC-1 function is controlled by multiple cellular signals during muscle activity. Thus, onset of muscle activity triggers ClC-1 inhibition via protein kinase C, intracellular acidosis, and lactate ions. This inhibition is important for preserving excitability of working muscle in the face of activity-induced elevation of extracellular K⁺ and accumulating inactivation of voltage-gated sodium channels. Furthermore, during prolonged activity, a marked ClC-1 activation can develop that compromises muscle excitability. Data from ClC-1 expression systems suggest that this ClC-1 activation may arise from loss of regulation by adenosine nucleotides and/or oxidation. The present review summarizes the current knowledge of the physiological factors that control ClC-1 function in active muscle.

REVIEW ■ : Chloride Channel Myotonias

Myotonia (muscle stiffness) is a symptom of several inherited diseases in humans and also in animals. It is due to muscle membrane hyperexcitability, which, in turn, can be caused by mutations in plasma membrane ion channels. The skeletal muscle chloride channel CLC-1 provides the major part of muscle membrane conductance and is important for keeping this membrane close to its resting voltage. Mutations in CLC-1 can cause both recessive (Becker) and dominant (Thomsen) forms of myotonia. Some of these mutations have been introduced into the functional cDNA and analyzed in the Xenopus oocyte expression system. From these studies, it was concluded that CLC-1 functions as a homooligomer with probably four subunits. Dominant mutant subunits are assumed to associate with wild-type ones, leading to their inactivation. The principle disease-causing mechanism of dominant mutations is a drastic alteration in the voltage dependence of CLC-1 gating. Some mutations in CLC-1 can be inherited either recessively or dominantly, probably depending on the genetic background. These studies point to the important role of CLC-1 in muscle physiology and provide interesting insights into the structure and function of this gene family of voltage-gated chloride channels. NEUROSCIENTIST 2:225-232, 1996

Channelopathies of skeletal muscle excitability

Familial disorders of skeletal muscle excitability were initially described early in the last century and are now known to be caused by mutations of voltage-gated ion channels. The clinical manifestations are often striking, with an inability to relax after voluntary contraction (myotonia) or transient attacks of severe weakness (periodic paralysis). An essential feature of these disorders is fluctuation of symptoms that are strongly impacted by environmental triggers such as exercise, temperature, or serum K(+) levels. These phenomena have intrigued physiologists for decades, and in the past 25 years the molecular lesions underlying these disorders have been identified and mechanistic studies are providing insights for therapeutic strategies of disease modification. These familial disorders of muscle fiber excitability are "channelopathies" caused by mutations of a chloride channel (ClC-1), sodium channel (NaV1.4), calcium channel (CaV1.1), and several potassium channels (Kir2.1, Kir2.6, and Kir3.4). This review provides a synthesis of the mechanistic connections between functional defects of mutant ion channels, their impact on muscle excitability, how these changes cause clinical phenotypes, and approaches toward therapeutics.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

LARGE glycans on dystroglycan function as a tunable matrix scaffold to prevent dystrophy

The dense glycan coat that surrounds every cell is essential for cellular development and physiological function, and it is becoming appreciated that its composition is highly dynamic. Post-translational addition of the polysaccharide repeating unit [-3-xylose-α1,3-glucuronic acid-β1-]n by like-acetylglucosaminyltransferase (LARGE) is required for the glycoprotein dystroglycan to function as a receptor for proteins in the extracellular matrix. Reductions in the amount of [-3-xylose-α1,3-glucuronic acid-β1-]n (hereafter referred to as LARGE-glycan) on dystroglycan result in heterogeneous forms of muscular dystrophy. However, neither patient nor mouse studies has revealed a clear correlation between glycosylation status and phenotype. This disparity can be attributed to our lack of knowledge of the cellular function of the LARGE-glycan repeat. Here we show that coordinated upregulation of Large and dystroglycan in differentiating mouse muscle facilitates rapid extension of LARGE-glycan repeat chains. Using synthesized LARGE-glycan repeats we show a direct correlation between LARGE-glycan extension and its binding capacity for extracellular matrix ligands. Blocking Large upregulation during muscle regeneration results in the synthesis of dystroglycan with minimal LARGE-glycan repeats in association with a less compact basement membrane, immature neuromuscular junctions and dysfunctional muscle predisposed to dystrophy. This was consistent with the finding that patients with increased clinical severity of disease have fewer LARGE-glycan repeats. Our results reveal that the LARGE-glycan of dystroglycan serves as a tunable extracellular matrix protein scaffold, the extension of which is required for normal skeletal muscle function.

Excitation-induced exchange of Na+, K+, and Cl- in rat EDL muscle in vitro and in vivo: physiology and pathophysiology

In skeletal muscle, excitation leads to increased [Na(+)](i), loss of K(+), increased [K(+)](o), depolarization, and Cl(-) influx. This study quantifies these changes in rat extensor digitorum longus (EDL) muscles in vitro and in vivo using flame photometric determination of Na(+) and K(+) and (36)Cl as a tracer for Cl(-). In vitro, 5-Hz stimulation for 300 s increased intracellular Na(+) content by 4.6 ± 1.2 µmol/g wet wt (P < 0.002) and decreased intracellular K(+) content by 5.5 ± 2.3 µmol/g wet wt (P < 0.03). This would increase [K(+)](o) by 28 ± 12 mM, sufficient to cause severe loss of excitability as the result of inactivation of Na(+) channels. In rat EDL, in vivo stimulation at 5 Hz for 300 s or 60 Hz for 60 s induced significant loss of K(+) (P < 0.01), sufficient to increase [K(+)](o) by 71 ± 22 mM and 73 ± 15 mM, respectively. In spite of this, excitability may be maintained by the rapid and marked stimulation of the electrogenic Na(+),K(+) pumps already documented. This may require full utilization of the transport capacity of Na(+),K(+) pumps, which then becomes a limiting factor for physical performance. In buffer containing (36)Cl, depolarization induced by increasing [K(+)](o) to 40-80 mM augmented intracellular (36)Cl by 120-399% (P < 0.001). Stimulation for 120-300 s at 5-20 Hz increased intracellular (36)Cl by 100-188% (P < 0.001). In rats, Cl(-) transport in vivo was examined by injecting (36)Cl, where electrical stimulation at 5 Hz for 300 s or 60 Hz for 60 s increased (36)Cl uptake by 81% (P < 0.001) and 84% (P < 0.001), respectively, indicating excitation-induced depolarization. Cl(-) influx favors repolarization, improving K(+) clearance and maintenance of excitability. In conclusion, excitation-induced fluxes of Na(+), K(+), and Cl(-) can be quantified in vivo, providing new evidence that in working muscles, extracellular accumulation of K(+) is considerably higher than previously observed and the resulting depression of membrane excitability may be a major cause of muscle fatigue.

Physiology and pathophysiology of CLC-1: mechanisms of a chloride channel disease, myotonia

The CLC-1 chloride channel, a member of the CLC-channel/transporter family, plays important roles for the physiological functions of skeletal muscles. The opening of this chloride channel is voltage dependent and is also regulated by protons and chloride ions. Mutations of the gene encoding CLC-1 result in a genetic disease, myotonia congenita, which can be inherited as an autosmal dominant (Thomsen type) or an autosomal recessive (Becker type) pattern. These mutations are scattered throughout the entire protein sequence, and no clear relationship exists between the inheritance pattern of the mutation and the location of the mutation in the channel protein. The inheritance pattern of some but not all myotonia mutants can be explained by a working hypothesis that these mutations may exert a “dominant negative” effect on the gating function of the channel. However, other mutations may be due to different pathophysiological mechanisms, such as the defect of protein trafficking to membranes. Thus, the underlying mechanisms of myotonia are likely to be quite diverse, and elucidating the pathophysiology of myotonia mutations will require the understanding of multiple molecular/cellular mechanisms of CLC-1 channels in skeletal muscles, including molecular operation, protein synthesis, and membrane trafficking mechanisms.

Dystrophin deficiency exacerbates skeletal muscle pathology in dysferlin-null mice

Background

Mutations in the genes coding for either dystrophin or dysferlin cause distinct forms of muscular dystrophy. Dystrophin links the cytoskeleton to the sarcolemma through direct interaction with β-dystroglycan. This link extends to the extracellular matrix by β-dystroglycan's interaction with α-dystroglycan, which binds extracellular matrix proteins, including laminin α2, agrin and perlecan, that possess laminin globular domains. The absence of dystrophin disrupts this link, leading to compromised muscle sarcolemmal integrity. Dysferlin, on the other hand, plays an important role in the Ca²⁺-dependent membrane repair of damaged sarcolemma in skeletal muscle. Because dysferlin and dystrophin play different roles in maintaining muscle cell integrity, we hypothesized that disrupting sarcolemmal integrity with dystrophin deficiency would exacerbate the pathology in dysferlin-null mice and allow further characterization of the role of dysferlin in skeletal muscle.

Methods

To test our hypothesis, we generated dystrophin/dysferlin double-knockout (DKO) mice by breeding mdx mice with dysferlin-null mice and analyzed the effects of a combined deficiency of dysferlin and dystrophin on muscle pathology and sarcolemmal integrity.

Results

The DKO mice exhibited more severe muscle pathology than either mdx mice or dysferlin-null mice, and, importantly, the onset of the muscle pathology occurred much earlier than it did in dysferlin-deficient mice. The DKO mice showed muscle pathology of various skeletal muscles, including the mandible muscles, as well as a greater number of regenerating muscle fibers, higher serum creatine kinase levels and elevated Evans blue dye uptake into skeletal muscles. Lengthening contractions caused similar force deficits, regardless of dysferlin expression. However, the rate of force recovery within 45 minutes following lengthening contractions was hampered in DKO muscles compared to mdx muscles or dysferlin-null muscles, suggesting that dysferlin is required for the initial recovery from lengthening contraction-induced muscle injury of the dystrophin-glycoprotein complex-compromised muscles.

Conclusions

The results of our study suggest that dysferlin-mediated membrane repair helps to limit the dystrophic changes in dystrophin-deficient skeletal muscle. Dystrophin deficiency unmasks the function of dysferlin in membrane repair during lengthening contractions. Dystrophin/dysferlin-deficient mice provide a very useful model with which to evaluate the effectiveness of therapies designed to treat dysferlin deficiency.

Impaired Wheel Running Exercise in CLC-1 Chloride Channel-Deficient Myotonic Mice

Background: Genetic deficiency of the muscle CLC-1 chloride channel leads to myotonia, which is manifested most prominently by slowing of muscle relaxation. Humans experience this as muscle stiffness upon initiation of contraction, although this can be overcome with repeated efforts (the “warm-up” phenomenon). The extent to which CLC-1 deficiency impairs exercise activity is controversial. We hypothesized that skeletal muscle CLC-1 chloride channel deficiency leads to severe reductions in spontaneous exercise. Methodology/Principal Findings: To examine this quantitatively, myotonic CLC-1 deficient mice were provided access to running wheels, and their spontaneous running activity was quantified subsequently. Differences between myotonic and normal mice in running were not present soon after introduction to the running wheels, but were fully established during week 2. During the eighth week, myotonic mice were running significantly less than normal mice (322 ± 177 vs 5058 ± 1253 m/day, P = 0.025). Furthermore, there were considerable reductions in consecutive running times (18.8 ± 1.5 vs 59.0 ± 3.7 min, P < 0.001) and in the distance per consecutive running period (58 ± 38 vs 601 ± 174 m, P = 0.048) in myotonic compared with normal animals. Conclusion/Significance: These findings indicate that CLC-1 chloride deficient myotonia in mice markedly impairs spontaneous exercise activity, with reductions in both total distance and consecutive running times.

Fatigue-inducing stimulation resolves myotonia in a drug-induced model

Background

Slowed muscle relaxation is the contractile hallmark of myotonia congenita, a disease caused by genetic CLC-1 chloride channel deficiency, which improves with antecedent brief contractions ("warm-up phenomenon"). It is unclear to what extent the myotonia continues to dissipate during continued repetitive contractions and how this relates temporally to muscle fatigue. Diaphragm, EDL, and soleus muscles were examined in vitro during repetitive 20 Hz and 50 Hz train stimulation in a drug-induced (9-AC) rat myotonia model.

Results

At the onset of stimulation, 9-AC treated diaphragm and EDL muscle had markedly prolonged half relaxation and late relaxation times (range 147 to 884 ms, 894 to 1324 ms). Half relaxation and late relaxation times reached near-normal values over the 5-10 and 10-40 subsequent contractions, respectively. In both muscles myotonia declined faster during repetitive 50 Hz than 20 Hz stimulation, and much faster than the rate of force loss during fatigue at both frequencies. Soleus muscle was resistant to the myotonic effects of 9-AC.

Conclusions

In a drug-induced model of mechanical myotonia, fatigue-inducing stimulation resolves the myotonia, which furthermore appears to be independent from the development of muscle fatigue.

Chloride currents from the transverse tubular system in adult mammalian skeletal muscle fibers

Chloride fluxes are the main contributors to the resting conductance of mammalian skeletal muscle fibers. ClC-1, the most abundant chloride channel isoform in this preparation, is believed to be responsible for this conductance. However, the actual distribution of ClC-1 channels between the surface and transverse tubular system (TTS) membranes has not been assessed in intact muscle fibers. To investigate this issue, we voltageclamped enzymatically dissociated short fibers using a two-microelectrode configuration and simultaneously recorded chloride currents (I_Cl) and di-8-ANEPPS fluorescence signals to assess membrane potential changes in the TTS. Experiments were conducted in conditions that blocked all but the chloride conductance. Fibers were equilibrated with 40 or 70 mM intracellular chloride to enhance the magnitude of inward I_Cl, and the specific ClC-1 blocker 9-ACA was used to eliminate these currents whenever necessary. Voltage-dependent di-8-ANEPPS signals and I_Cl acquired before (control) and after the addition of 9-ACA were comparatively assessed. Early after the onset of stimulus pulses, di-8-ANEPPS signals under control conditions were smaller than those recorded in the presence of 9-ACA. We defined as attenuation the normalized time-dependent difference between these signals. Attenuation was discovered to be I_Cl dependent since its magnitude varied in close correlation with the amplitude and time course of I_Cl. While the properties of I_Cl, and those of the attenuation seen in optical records, could be simultaneously predicted by model simulations when the chloride permeability (P_Cl) at the surface and TTS membranes were approximately equal, the model failed to explain the optical data if P_Cl was precluded from the TTS membranes. Since the ratio between the areas of TTS membranes and the sarcolemma is large in mammalian muscle fibers, our results demonstrate that a significant fraction of the experimentally recorded I_Cl arises from TTS contributions.

Sarcolemmal-restricted localization of functional ClC-1 channels in mouse skeletal muscle

Skeletal muscle fibers exhibit a high resting chloride conductance primarily determined by ClC-1 chloride channels that stabilize the resting membrane potential during repetitive stimulation. Although the importance of ClC-1 channel activity in maintaining normal muscle excitability is well appreciated, the subcellular location of this conductance remains highly controversial. Using a three-pronged multidisciplinary approach, we determined the location of functional ClC-1 channels in adult mouse skeletal muscle. First, formamide-induced detubulation of single flexor digitorum brevis (FDB) muscle fibers from 15-16-day-old mice did not significantly alter macroscopic ClC-1 current magnitude (at -140 mV; -39.0 +/- 4.5 and -42.3 +/- 5.0 nA, respectively), deactivation kinetics, or voltage dependence of channel activation (V(1/2) was -61.0 +/- 1.7 and -64.5 +/- 2.8 mV; k was 20.5 ± 0.8 and 22.8 +/- 1.2 mV, respectively), despite a 33% reduction in cell capacitance (from 465 +/- 36 to 312 +/- 23 pF). In paired whole cell voltage clamp experiments, where ClC-1 activity was measured before and after detubulation in the same fiber, no reduction in ClC-1 activity was observed, despite an approximately 40 and 60% reduction in membrane capacitance in FDB fibers from 15-16-day-old and adult mice, respectively. Second, using immunofluorescence and confocal microscopy, native ClC-1 channels in adult mouse FDB fibers were localized within the sarcolemma, 90 degrees out of phase with double rows of dihydropyridine receptor immunostaining of the T-tubule system. Third, adenoviral-mediated expression of green fluorescent protein-tagged ClC-1 channels in adult skeletal muscle of a mouse model of myotonic dystrophy type 1 resulted in a significant reduction in myotonia and localization of channels to the sarcolemma. Collectively, these results demonstrate that the majority of functional ClC-1 channels localize to the sarcolemma and provide essential insight into the basis of myofiber excitability in normal and diseased skeletal muscle.

CLC chloride channels and transporters: from genes to protein structure, pathology and physiology

CLC genes are expressed in species from bacteria to human and encode Cl(-)-channels or Cl(-)/H(+)-exchangers. CLC proteins assemble to dimers, with each monomer containing an ion translocation pathway. Some mammalian isoforms need essential beta -subunits (barttin and Ostm1). Crystal structures of bacterial CLC Cl(-)/H(+)-exchangers, combined with transport analysis of mammalian and bacterial CLCs, yielded surprising insights into their structure and function. The large cytosolic carboxy-termini of eukaryotic CLCs contain CBS domains, which may modulate transport activity. Some of these have been crystallized. Mammals express nine CLC isoforms that differ in tissue distribution and subcellular localization. Some of these are plasma membrane Cl(-) channels, which play important roles in transepithelial transport and in dampening muscle excitability. Other CLC proteins localize mainly to the endosomal-lysosomal system where they may facilitate luminal acidification or regulate luminal chloride concentration. All vesicular CLCs may be Cl(-)/H(+)-exchangers, as shown for the endosomal ClC-4 and -5 proteins. Human diseases and knockout mouse models have yielded important insights into their physiology and pathology. Phenotypes and diseases include myotonia, renal salt wasting, kidney stones, deafness, blindness, male infertility, leukodystrophy, osteopetrosis, lysosomal storage disease and defective endocytosis, demonstrating the broad physiological role of CLC-mediated anion transport.

Chloride conductance in the transverse tubular system of rat skeletal muscle fibres: importance in excitation-contraction coupling and fatigue

Contraction in skeletal muscle fibres is governed by excitation of the transverse-tubular (t-) system, but the properties of the t-system and their importance in normal excitability are not well defined. Here we investigate the properties of the t-system chloride conductance using rat skinned muscle fibres in which the sarcolemma has been mechanically removed but the normal excitation-contraction coupling mechanism kept functional. When the t-system chloride conductance was eliminated, either by removal of all Cl(-) or by block of the chloride channels with 9-anthracene carboxylic acid (9-AC) or by treating muscles with phorbol 12,13-dibutyrate, there was a marked reduction in the threshold electric field intensity required to elicit a t-system action potential (AP) and twitch response. Calculations of the t-system chloride conductance indicated that it constitutes a large proportion of the total chloride conductance observed in intact fibres. Blocking the chloride conductance increased the size of the twitch response and was indicative that Cl(-) normally carries part of the repolarizing current across the t-system membrane on each AP. Block of the t-system chloride conductance also reduced tetanic force responses at higher frequency stimulation (100 Hz) and greatly reduced twitch responses in the period shortly after a brief tetanus, owing to rapid loss of t-system excitability during the AP train. Blocking activity of the Na(+)-K(+) pump in the t-system membrane caused loss of excitability owing to K(+) build-up in the sealed t-system, and this occurred approximately 3-4 times faster when the chloride conductance was blocked. These findings show that the t-system chloride conductance plays a vital role during normal activity by countering the effects of K(+) accumulation in the t-system and maintaining muscle excitability.

Isotonic contractile impairment due to genetic CLC-1 chloride channel deficiency in myotonic mouse diaphragm muscle

The hallmark of genetic CLC-1 chloride channel deficiency in myotonic humans, goats and mice is delayed muscle relaxation resulting from persistent electrical discharges. In addition to the ion channel defect, muscles from myotonic humans and mice also have major changes in fibre type and myosin isoform composition, but the extent to which this affects isometric contractions remains controversial. Many muscles, including the diaphragm, shorten considerably during normal activities, but shortening contractions have never been assessed in myotonic muscle. The present study tested the hypothesis that CLC-1 deficiency leads to an impairment of muscle isotonic contractile performance. This was tested in vitro on diaphragm muscle from SWR/J-Clcn1(adr-mto)/J myotonic mice. The CLC-1-deficient muscle demonstrated delayed relaxation, as expected. During the contractile phase, there were significant reductions in power and work across a number of stimulation frequencies and loads in CLC-1-deficient compared with normal muscle, the magnitude of which in many instances exceeded 50%. Reductions in shortening and velocity of shortening occurred, and were more pronounced when calculated as a function of absolute than relative load. However, the maximal unloaded shortening velocity calculated from Hill's equation was not altered significantly. The impaired isotonic contractile performance of CLC-1-deficient muscle persisted during fatigue-inducing stimulation. These data indicate that genetic CLC-1 chloride channel deficiency in mice not only produces myotonia but also substantially worsens the isotonic contractile performance of diaphragm muscle.

CLC chloride channels and transporters: a biophysical and physiological perspective

Chloride-transporting proteins play fundamental roles in many tissues in the plasma membrane as well as in intracellular membranes. They have received increasing attention in the last years because crucial, and often unexpected and novel, physiological functions have been disclosed with gene-targeting approaches, X-ray crystallography, and biophysical analysis. CLC proteins form a gene family that comprises nine members in mammals, at least four of which are involved in human genetic diseases. The X-ray structure of the bacterial CLC homolog, ClC-ec1, revealed a complex fold and confirmed the anticipated homodimeric double-barreled architecture of CLC-proteins with two separate Cl-ion transport pathways, one in each subunit. Four of the mammalian CLC proteins, ClC-1, ClC-2, ClC-Ka, and ClC-Kb, are chloride ion channels that fulfill their functional roles-stabilization of the membrane potential, transepithelial salt transport, and ion homeostasisin the plasma membrane. The other five CLC proteins are predominantly expressed in intracellular organelles like endosomes and lysosomes, where they are probably important for a proper luminal acidification, in concert with the V-type H+-ATPase. Surprisingly, ClC-4, ClC-5, and probably also ClC-3, are not Cl- ion channels but exhibit significant Cl-/H+ antiporter activity, as does the bacterial homolog ClC-ec1 and the plant homolog AtCLCa. The physiological significance of the Cl-/H+ antiport activity remains to be established.

ClC-1 CHLORIDE CHANNEL: MATCHING ITS PROPERTIES TO A ROLE IN SKELETAL MUSCLE

1. ClC-1 is a Cl- channel in mammalian skeletal muscle that plays an important role in membrane repolarization following muscular contraction. Reduction of ClC-1 conductance results in myotonia, a state characterized by muscle hyperexcitability. 2. As is the case for other members of the ClC family, ClC-1 exists as a dimer that forms a double-barrelled channel. Each barrel, or pore, of ClC-1 is gated by its own gate ('fast' or 'single pore' gate), whereas both pores are gated simultaneously by another mechanism ('slow' or 'common' gate). 3. Comparison of the biophysical and pharmacological properties of heterologously expressed ClC-1 with the properties of the Cl- conductance measured in skeletal muscle strongly suggests that ClC-1 is the major Cl- channel responsible for muscle repolarization. However, not all results obtained in experiments on whole muscle or muscle fibres support this notion. 4. In the present review we attempt to bring together the current knowledge of ClC-1 with the physiology of skeletal muscle.

Genetic CLC-1 chloride channel deficiency modifies diaphragm muscle isometric contractile properties

Genetic deficiency of the muscle chloride channel CLC-1 leads to myotonia congenita in humans as well as myotonia in mice and goats. The hallmark of myotonia is delayed muscle relaxation due to persistent electrical discharges in the muscle. The present study tested the hypothesis that performance of CLC-1 deficient diaphragm muscle is also altered during the contractile phase of the contraction-relaxation cycle. Diaphragm of CLC-1 deficient and wild type mice underwent in vitro isometric contractility testing. Myotonia was easily demonstrable during contractions elicited by train stimulation, but was not seen during twitch stimulation or during train stimulation preceded by a series of twitch stimulations. Twitch force was reduced from 16.7+/-2.5 N/cm(2) in normal muscle to 7.2+/-1.9 N/cm(2) in CLC-1 deficient muscle (P<0.002). Isometric twitch contraction time was shortened from 19.6+/-0.9 to 15.7+/-1.0 ms (P<0.002). During repetitive 25 Hz stimulation, force/area was lower for diseased than normal muscle, whereas force as a percent of initial values declined at a faster rate for normal than diseased muscle. The latter could be accounted for by a rightward shift in the force-frequency relationship of CLC-1 deficient relative to normal muscle, as use of stimulation frequencies which elicited comparable force levels as a percentage of maximum 100 Hz tetanic force led to similar rates of fatigue. These findings indicate that genetic CLC-1 deficiency not only affects muscle relaxation (myotonia) but also modulates diaphragm performance during the contractile phase of the contraction-relaxation cycle.

Ion channels and ion transporters of the transverse tubular system of skeletal muscle

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

The effect of dehydroepiandrosterone sulfate (DHEAS) on myotonia: intracellular studies

OBJECTIVE

In order to find some appropriate medicine to suppress myotonia without decreasing muscle strength experiments were performed on myotonic (mto) mice whose Cl channel does not develop due to stop codon and serves as an animal model of myotonia. In myotonic dystrophy dehydroepiandrosterone is low in the serum and it has been reported that intravenous injections of DHEAS to human cases improves myotonia and activities of daily living.

MATERIALS AND METHODS

Three pairs of heterozygote mto mice, SWR/J-Clcn1(adr-mto/+) and ten Wistar rats were used. We performed intracellular recordings of myotonia from mto mice and the drug effects on insertion myotonia were recorded from the hemidiaphragm preparations of mto mice with different concentrations of DHEAS. Isometric twitch tension was recorded from rat hemidiaphragm preparations in Tyrode's solution and the effect of DHEAS on the muscle twitch tension was measured at different concentrations of DHEAS from 100 mg/l to 300 mg/l. The effect of mexiletine on ITT was also measured.

RESULTS

In mto mice insertion myotonia was recorded as soon as the microelectrode was inserted in the muscle cells. When DHEAS was added to Tyrode's solution, insertion myotonia was suppressed. DHEAS decreased ITT up to 70% of the original value, though mexiletine decreased ITT to 30% of the original value. Therefore, the decrement of the muscle strength in DHEAS solution is much smaller than that of mexiletine.

CONCLUSION

Since myotonic dystrophy shows progressive muscle weakness in addition to myotonia, medications like DHEAS are more favorable than the typical Na channel blocker.

Cytoplasmic ATP-sensing domains regulate gating of skeletal muscle ClC-1 chloride channels

ClC proteins are a family of chloride channels and transporters that are found in a wide variety of prokaryotic and eukaryotic cell types. The mammalian voltage-gated chloride channel ClC-1 is important for controlling the electrical excitability of skeletal muscle. Reduced excitability of muscle cells during metabolic stress can protect cells from metabolic exhaustion and is thought to be a major factor in fatigue. Here we identify a novel mechanism linking excitability to metabolic state by showing that ClC-1 channels are modulated by ATP. The high concentration of ATP in resting muscle effectively inhibits ClC-1 activity by shifting the voltage gating to more positive potentials. ADP and AMP had similar effects to ATP, but IMP had no effect, indicating that the inhibition of ClC-1 would only be relieved under anaerobic conditions such as intense muscle activity or ischemia, when depleted ATP accumulates as IMP. The resulting increase in ClC-1 activity under these conditions would reduce muscle excitability, thus contributing to fatigue. We show further that the modulation by ATP is mediated by cystathionine beta-synthase-related domains in the cytoplasmic C terminus of ClC-1. This defines a function for these domains as gating-modulatory domains sensitive to intracellular ligands, such as nucleotides, a function that is likely to be conserved in other ClC proteins.

Physiological functions of CLC Cl- channels gleaned from human genetic disease and mouse models

The CLC gene family encodes nine different Cl() channels in mammals. These channels perform their functions in the plasma membrane or in intracellular organelles such as vesicles of the endosomal/lysosomal pathway or in synaptic vesicles. The elucidation of their cellular roles and their importance for the organism were greatly facilitated by mouse models and by human diseases caused by mutations in their respective genes. Human mutations in CLC channels are known to cause diseases as diverse as myotonia (muscle stiffness), Bartter syndrome (renal salt loss) with or without deafness, Dent's disease (proteinuria and kidney stones), osteopetrosis and neurodegeneration, and possibly epilepsy. Mouse models revealed blindness and infertility as further consequences of CLC gene disruptions. These phenotypes firmly established the roles CLC channels play in stabilizing the plasma membrane voltage in muscle and possibly in neurons, in the transport of salt and fluid across epithelia, in the acidification of endosomes and synaptic vesicles, and in the degradation of bone by osteoclasts.

Regulated sarcolemmal localization of the muscle-specific ClC-1 chloride channel

The skeletal muscle-specific ClC-1 is a voltage-gated chloride channel protein. Specific antibodies against ClC-1 revealed in muscle sections a sarcolemmal staining that was absent in the myotonic arrested development of righting response (ADR) mouse muscle. The intensity of the sarcolemmal staining varied from one type of muscle to another and in lateral sections showed a typical mosaic pattern that colocalized with beta-dystroglycan and left the transverse tubule openings clear. Surprisingly, in isolated myofibers, the ClC-1 protein was absent from the sarcolemma. Instead, it localized to intracellular I band areas as soon as the myofibers were isolated. When the isolated myofibers were incubated with the kinase inhibitor staurosporine, the ClC-1 protein shifted back to the sarcolemma. Electric stimulation of the cultivated fibers had a similar effect. Also, myofibers infected with a recombinant Semliki Forest virus (SFV) expressing myc-tagged ClC-1 showed intracellular localization of the protein. The virally expressed mycClC-1 reached the Golgi apparatus but sarcolemmal staining remained nondetectable, and addition of staurosporine into the growth medium recruited the mycClC-1 to the sarcolemma. These data indicate that sarcolemmal targeting of the ClC-1 requires specific signals that are provided by the physiological environment.

Reducing chloride conductance prevents hyperkalaemia-induced loss of twitch force in rat slow-twitch muscle

Exercise-induced loss of skeletal muscle K(+) can seriously impede muscle performance through membrane depolarization. Thus far, it has been assumed that the negative equilibrium potential and large membrane conductance of Cl(-) attenuate the loss of force during hyperkalaemia. We questioned this idea because there is some evidence that Cl(-) itself can exert a depolarizing influence on membrane potential (V(m)). With this study we tried to identify the possible roles played by Cl(-) during hyperkalaemia. Isolated rat soleus muscles were kept at 25 degrees C and twitch contractions were evoked by current pulses. Reducing [Cl(-)](o) to 5 mM, prior to introducing 12.5 mM K(o), prevented the otherwise occurring loss of force. Reversing the order of introducing these two solutions revealed an additional effect, i.e. the ongoing hyperkalaemia-related loss of force was sped up tenfold after reducing [Cl(-)](o). However, hereafter twitch force recovered completely. The recovery of force was absent at [K(+)](o) exceeding 14 mM. In addition, reducing [Cl(-)](o) increased membrane excitability by 24%, as shown by a shift in the relationship between force and current level. Measurements of V(m) indicated that the antagonistic effect of reducing [Cl(-)](o) on hyperkalaemia-induced loss of force was due to low-Cl(-)-induced membrane hyperpolarization. The involvement of specific Cl(-) conductance was established with 9-anthracene carboxylic acid (9-AC). At 100 microm, 9-AC reduced the loss of force due to hyperkalaemia, while at 200 microm, 9-AC completely prevented loss of force. To study the role of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) in this matter, we added 400 microm of the NKCC inhibitor bumetanide to the incubation medium. This did not affect the hyperkalaemia-induced loss of force. We conclude that Cl(-) exerts a permanent depolarizing influence on V(m). This influence of Cl(-) on V(m), in combination with a large membrane conductance, can apparently have two different effects on hyperkalaemia-induced loss of force. It might exert a stabilizing influence on force production during short periods of hyperkalaemia, but it can add to the loss of force during prolonged periods of hyperkalaemia.

Transgenic overexpression of human DMPK accumulates into hypertrophic cardiomyopathy, myotonic myopathy and hypotension traits of myotonic dystrophy

Abnormal expression of human myotonic dystrophy protein kinase (hDMPK) gene products has been implicated in myotonic dystrophy type 1 (DM1), yet the impact of distress accumulation produced by persistent overexpression of this poorly understood member of the Rho kinase-related protein kinase gene-family remains unknown. Here, in the aged transgenic murine line carrying approximately 25 extra copies of a complete hDMPK gene with all exons and an intact promoter region (Tg26-hDMPK), overexpression of mRNA and protein transgene products in cardiac, skeletal and smooth muscles resulted in deficient exercise endurance, an integrative index of muscle systems underperformance. In contrast to age-matched (11-15 months) wild-type controls, hearts from Tg26-hDMPK developed cardiomyopathic remodeling with myocardial hypertrophy, myocyte disarray and interstitial fibrosis. Hypertrophic cardiomyopathy was associated with a propensity for dysrhythmia and characterized by overt intracellular calcium overload promoting nuclear translocation of transcription factors responsible for maladaptive gene reprogramming. Skeletal muscles in distal limbs of Tg26-hDMPK showed myopathy with myotonic discharges coupled with deficit in sarcolemmal chloride channels, required regulators of hyperexcitability. Fiber degeneration in Tg26-hDMPK resulted in sarcomeric disorganization, centralization of nuclei and tubular aggregation. Moreover, the reduced blood pressure in Tg26-hDMPK indicated deficient arterial smooth muscle tone. Thus, the cumulative stress induced by permanent overexpression of hDMPK gene products translates into an increased risk for workload intolerance, hypertrophic cardiomyopathy with dysrhythmia, myotonic myopathy and hypotension, all distinctive muscle traits of DM1. Proper expression of hDMPK is, therefore, mandatory in supporting the integral balance among cytoarchitectural infrastructure, ion-homeostasis and viability control in various muscle cell types.

Exon 17 skipping in CLCN1 leads to recessive myotonia congenita

Mutations in CLCN1, the gene encoding the ClC-1 chloride channel in skeletal muscle, lead to myotonia congenita. The effects on the intramembranous channel forming domains have been investigated more than that at the intracellular C-terminus. We have performed a mutation screen involving the whole CLCN1 gene of patients with myotonia congenita by polymerase chain reaction (PCR), single-strand conformation polymorphism studies, and sequencing. Two unrelated patients harbored the same homozygous G-to-T mutation on the donor splice site of intron 17. This led to the skipping of exon 17, as evidenced by the reverse transcriptase PCR. When the exon 17-deleted CLCN1 was expressed in Xenopus oocytes, no chloride current was measurable. This function could be restored by coexpression with the wild-type channel. Our data suggest an important role of this C-terminal region and that exon 17 skipping resulting from a homozygous point mutation in CLCN1 can lead to recessive myotonia congenita.

Protective role of extracellular chloride in fatigue of isolated mammalian skeletal muscle

A possible role of extracellular Cl(-) concentration ([Cl(-)](o)) in fatigue was investigated in isolated skeletal muscles of the mouse. When [Cl(-)](o) was lowered from 128 to 10 mM, peak tetanic force was unchanged, fade was exacerbated (wire stimulation electrodes), and a hump appeared during tetanic relaxation in both nonfatigued slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles. Low [Cl(-)](o) increased the rate of fatigue 1) with prolonged, continuous tetanic stimulation in soleus, 2) with repeated intermittent tetanic stimulation in soleus or EDL, and 3) to a greater extent with repeated tetanic stimulation when wire stimulation electrodes were used rather than plate stimulation electrodes in soleus. In nonfatigued soleus muscles, application of 9 mM K(+) with low [Cl(-)](o) caused more rapid and greater tetanic force depression, along with greater depolarization, than was evident at normal [Cl(-)](o). These effects of raised [K(+)](o) and low [Cl(-)](o) were synergistic. From these data, we suggest that normal [Cl(-)](o) provides protection against fatigue involving high-intensity contractions in both fast- and slow-twitch mammalian muscle. This phenomenon possibly involves attenuation of the depolarization caused by stimulation- or exercise-induced run-down of the transsarcolemmal K(+) gradient.

Loss of the muscle-specific chloride channel in type 1 myotonic dystrophy due to misregulated alternative splicing

Myotonic dystrophy type 1 (DM1) is a dominant multisystemic disorder caused by a CTG expansion in the 3' untranslated region of the DMPK gene. A predominant characteristic of DM1 is myotonia resulting from skeletal muscle membrane hyperexcitability. Here we demonstrate loss of the muscle-specific chloride channel (ClC-1) mRNA and protein in DM1 skeletal muscle tissue due to aberrant splicing of the ClC-1 pre-mRNA. The splicing regulator, CUG binding protein (CUG-BP), which is elevated in DM1 striated muscle, binds to the ClC-1 pre-mRNA, and overexpression of CUG-BP in normal cells reproduces the aberrant pattern of ClC-1 splicing observed in DM1 skeletal muscle. We propose that disruption of alternative splicing regulation causes a predominant pathological feature of DM1.

Expanded CUG repeats trigger aberrant splicing of ClC-1 chloride channel pre-mRNA and hyperexcitability of skeletal muscle in myotonic dystrophy

In myotonic dystrophy (dystrophia myotonica, DM), expression of RNAs that contain expanded CUG or CCUG repeats is associated with degeneration and repetitive action potentials (myotonia) in skeletal muscle. Using skeletal muscle from a transgenic mouse model of DM, we show that expression of expanded CUG repeats reduces the transmembrane chloride conductance to levels well below those expected to cause myotonia. The expanded CUG repeats trigger aberrant splicing of pre-mRNA for ClC-1, the main chloride channel in muscle, resulting in loss of ClC-1 protein from the surface membrane. We also have identified a similar defect in ClC-1 splicing and expression in two types of human DM. We propose that a transdominant effect of mutant RNA on RNA processing leads to chloride channelopathy and membrane hyperexcitability in DM.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Aging-associated down-regulation of ClC-1 expression in skeletal muscle: phenotypic-independent relation to the decrease of chloride conductance

In order to clarify the mechanism underlying the reduction of resting membrane chloride conductance (gCl) during aging, the levels of mRNA encoding the principal skeletal muscle chloride channel, ClC-1, were measured. Total RNA samples isolated from tibialis anterior muscles of aged (24-29 months old) and adult (3-4 months old) rats were examined for ClC-1 expression using Northern blot analysis, and macroscopic gCl was recorded from extensor digitorum longus muscle fibers from each adult and aged rat in vitro using a two intracellular microelectrode technique. Although interindividual variability was observed, aged rats exhibited a parallel reduction of both gCl and ClC-1 mRNA expression as compared to adult rats. A linear correlation exists between individual values of ClC-1 mRNA and gCl. These results provide evidence that ClC-1 is the main determinant of sarcolemmal gCl and demonstrate that the decrease of gCl observed during aging is associated with a down-regulation of ClC-1 expression in muscle.

Myotonic ADR-MDX mutant mice show less severe muscular dystrophy than MDX mice

In Duchenne muscular dystrophy (DMD) and its murine model, the dystrophic mouse (MDX), the skeletal musculature lacks dystrophin. The presumed function of this cytoskeletal protein is to protect the sarcolemma against mechanical stress during muscle activity. To test this hypothesis in vivo, we bred a double mutant mouse that combines two genetic defects: the dystrophin-deficiency of the MDX mouse and the Cl- channel myotonia of the arrested development of righting response (ADR) mouse. We hypothesized that high mechanical muscle activity would aggravate muscular dystrophy in double mutant ADR-MDX mice. On the contrary, ADR-MDX mice showed fewer signs of muscle fiber necrosis and fibrosis than MDX mice at all ages. Plasma creatine kinase levels were slightly increased in ADR-MDX, but significantly lower when compared to MDX mice. Sections of ADR-MDX muscle showed a uniform pattern of oxidative muscle fibers. Similar findings have been obtained in dystrophin-positive ADR mice, they result from a complete fiber-type IIB to IIA transformation in myotonic muscle. Our results suggest that small, oxidative fibers of myotonic mice are less sensitive to dystrophin deficiency. Therefore, ADR-MDX mice develop less severe muscular dystrophy than MDX mice do, although their muscles are continually stressed. The new ADR-MDX double mutant mouse is the first animal model combining both a dystrophinopathy and a channelopathy. The results presented here give new insights into the pathomechanism of muscular dystrophy and may be helpful for the therapeutic management of DMD.

Effect of transverse-tubular chloride conductance on excitability in skinned skeletal muscle fibres of rat and toad

1. The influence of the transverse-tubular (T-) system Cl- conductance on membrane excitability in skeletal muscle fibres of toad and rat was examined because of conflicting conclusions of previous studies on Cl- conductance. A mechanically skinned fibre preparation was used that permitted investigation of Ca2+ release via the normal T-system voltage-sensor mechanism after complete removal of the surface membrane, which thereby allowed estimation of the T-system potential from force measurements. 2. When a skinned fibre was bathed in a high-[K+] solution, the sealed T-system became polarized and could be rapidly depolarized by replacing the K+ with Na+, thereby eliciting Ca2+ release from the sarcoplasmic reticulum. In rat skinned fibres, addition of 20 mM Cl- to the 'myoplasm' (i.e. bathing solution) partially depolarized the T-system, inducing Ca2+ release and subsequent voltage-sensor inactivation. These effects were completely abolished with 100 microM of the Cl- channel blocker 9-anthracene carboxylic acid (9-AC). Voltage-sensor inactivation increased in a graded manner over the range 3-20 mM myoplasmic Cl-. 3. In toad fibres, voltage-sensor inactivation was only detectable at > 10 mM myoplasmic Cl-, and 20 mM Cl- was only able to depolarize the T-system sufficiently to trigger Ca2+ release if the myoplasmic [K+] was reduced by 50 %. In toad fibres, 100 microM 9-AC caused little if any block of the T-system Cl- conductance. 4. It was also found that when skinned fibres were obtained from muscles that had been bathed in a zero Cl- extracellular solution, the initial Na+ substitutions were more effective at depolarizing the T-system. This is consistent with Cl- trapped in the sealed T-system exerting a polarizing effect on T-system potential. 5. These results unequivocally demonstrate that there is a large 9-AC-sensitive Cl- conductance in the T-system of rat fibres, and a smaller, though still appreciable, Cl- conductance in the T-system of toad fibres, which is relatively insensitive to 9-AC. The results are important for understanding the basis of the Cl- channel aberration in myotonia.

Chloride conductance in mouse muscle is subject to post-transcriptional compensation of the functional Cl- channel 1 gene dosage

1. In mature mammalian muscle, the muscular chloride channel ClC-1 contributes about 75% of the sarcolemmal resting conductance (Gm). In mice carrying two defective alleles of the corresponding Clc1 gene, chloride conductance (GCl) is reduced to less than 10% of that of wild-type, and this causes hyperexcitability, the salient feature of the disease myotonia. Potassium conductance (GK) values in myotonic mouse muscle fibres are lowered by about 60% compared with wild-type. 2. The defective Clcadr allele causes loss of the 4.5 kb ClC-1 mRNA. Mice heterozygous for the defective Clc1adr allele contain about 50% functional mRNA in their muscles compared with homozygous wild-type mice. 3. Despite a halved functional gene dosage, heterozygous muscles display an average GCl which is not significantly different from that of homozygous wild-type animals. The GK values in heterozygotes are also indistinguishable from homozygous wild-type animals. 4. These results indicate that a regulatory mechanism acting at the post-transcriptional level limits the density of ClC-1 channels. GK is probably indirectly regulated by muscle activity.

Extracellular interaction of the voltage-dependent Ca2+ channel alpha2delta and alpha1 subunits

The role of the extracellular domain of the voltage-dependent Ca2+ channel alpha2delta subunit in assembly with the alpha1C subunit was investigated. Transiently transfected tsA201 cells processed the alpha2delta subunit properly as disulfide linkages and cleavage sites between the alpha2 and delta subunits were shown to be similar to native channel protein. Coimmunoprecipitation experiments demonstrated that in the absence of delta subunits, alpha2 subunits do not assemble with alpha1 subunits. Furthermore, the transmembrane and cytoplasmic sequences in delta can be exchanged with those of an unrelated protein without any effect on the association between the alpha2delta and alpha1 proteins. Extracellular domains of the alpha2delta subunit are also shown to be responsible for increasing the binding affinity of [3H]PN200-110 (isopropyl-4-(2,1, 3-benzoxadiazol-4-yl)-1,4-dihydro-2, 6-dimethyl-5-([3H]methoxycarbonyl)-pyridine-3-carboxylate) for the alpha1C subunit. Investigation of the corresponding interaction site on the alpha1 subunit revealed that although tryptic peptides containing repeat III of native alpha1S subunit remain in association with the alpha2delta subunit during wheat germ agglutinin chromatography, repeat III by itself is not sufficient for assembly with the alpha2delta subunit. Our results suggest that the alpha2delta subunit likely interacts with more than one extracellular loop of the alpha1 subunit.

Subunit stoichiometry of human muscle chloride channels

Voltage-gated Cl- channels belonging to the ClC family appear to function as homomultimers, but the number of subunits needed to form a functional channel is controversial. To determine subunit stoichiometry, we constructed dimeric human skeletal muscle Cl- channels in which one subunit was tagged by a mutation (D136G) that causes profound changes in voltage-dependent gating. Sucrose-density gradient centrifugation experiments indicate that both monomeric and dimeric hClC-1 channels in their native configurations exhibit similar sedimentation properties consistent with a multimeric complex having a molecular mass of a dimer. Expression of the heterodimeric channel in a mammalian cell line results in a homogenous population of Cl- channels exhibiting novel gating properties that are best explained by the formation of heteromultimeric channels with an even number of subunits. Heteromultimeric channels were not evident in cells cotransfected with homodimeric WT-WT and D136G-D136G constructs excluding the possibility that functional hClC-1 channels are assembled from more than two subunits. These results demonstrate that the functional hClC-1 unit consists of two subunits.

Purification of the CIC-0 chloride channel from Torpedo california electroplax identification of a phosphorylation site for cAMP-dependent protein kinase

The voltage-gated chloride channel (CIC-0) from the electric organ of Torpedo californica was purified by immunoaffinity chromatography. A polyclonal antibody was shown to specifically recognize the CIC-0 channel (M(r) 85,000-90,000) in a Western blot of total membrane proteins. As monitored by immunoprecipitation, the formation of antibody-antigen complexes in solution strongly depends on the detergent composition. The highest yield of precipitated CIC-0 was obtained from an incubation mixture containing both an anionic detergent, cholate or lauryl sulfate, and the zwitterionic detergent CHAPS. In contrast, immuno-precipitation of CIC-0 was largely reduced when cholate was exchanged for the nonionic detergent Triton x-100, suggesting that the efficient formation of immuno-complexes is favored by negatively charged detergent. In initial immunopurification experiments, in addition to CIC-0 a major contaminating polypeptide of M(r) approximately 115,000 was copurified, which represents the SITS-binding protein [Jentsch et al. (1989) Biochem. J. 261, 155]. Purification of CIC-0 could be increased from about 35% up to 60% homogeneity when immunoaffinity chromatography was performed in the presence of N-acetylglucosamine. Therefore the highly glycosylated SITS-binding protein most likely interacts with the CIC-0 protein via its carbohydrate parts. The purified CIC-0 channel was found to be phosphorylated by PKA in vitro to a level of 0.35-0.4 mol of phosphate incorporated per mol of CIC-0. Proteolytic digestion with endoproteinase GluC and HPLC-separation revealed two major phosphopeptides, which could be identified by amino acid sequence analysis as different size fragments of the same consensus phosphorylation site. Comparison of the peptide sequences with the deduced protein sequence of CIC-0 [Jentsch et al. (1990) Nature 348, 510; O'Neill et al. (1991) Biochem. Biophys. Acta 1129, 131] indicates serine 600 as the phosphorylated residue. Therefore, our results provide strong evidence that CIC-0 is phosphorylated at this single site by PKA in vitro.

Characteristics of skeletal muscle chloride channel C1C-1 and point mutant R304E expressed in Sf-9 insect cells

Using the baculovirus system, the skeletal muscle chloride channel, CIC-1 (rat), and a point mutant replacing arginine 304 with glutamic acid were expressed at high levels in cultured Sf-9 insect cells. Whole-cell patch-clamping revealed large inwardly rectifying currents with maxima up to 15 nA inward and 2.5 nA outward. Saturation was evident at voltage steps positive to +40 mV whilst steps negative to -60 mV produced inactivating currents made up of a steady state component and two exponentially decaying components with tau 1 = 6.14+/- 0.92 ms, tau 2 = 36.5+/- 3.29 ms (S.D) n = 7 for steps to -120 mV. Currents recorded in the outside-out patch configuration were often unexpectedly large and up to 5% of whole-cell currents obtained in the same cell, suggesting an uneven channel distribution in the plasmalemma of Sf-9 cells. The pharmacology of a number of chloride channel blockers, including anthracene-9-carboxylate (A9C), niflumate, and perrhenate, was investigated and showed for the first time that perrhenate is an effective blocker of C1C-1 and that it has a complex mechanism of action. Further, the potency of A9C was found to be dependent on external chloride concentration. As in studies on muscle cells themselves, blockade was rapidly effective and easily reversible, except when applying the indanyloxyacetate derivative, IAA94/95, which took up to 10 min to act, and, consistent with an intracellular site of action, was difficult to reverse by washing. Mutation of the highly conserved arginine at position 304 to a glutamic acid did not significantly alter the behaviour of the channel.

Dual function of the voltage-dependent Ca2+ channel alpha 2 delta subunit in current stimulation and subunit interaction

Voltage-dependent Ca2+ channels are modulated by complex interactions with the alpha 2 delta subunit. In vitro translation was used to demonstrate a single transmembrane topology of the alpha 2 delta subunit in which all but the transmembrane sequence and 5 carboxy-terminal amino acids are extracellular. The glycosylated extra-cellular domain is required for current stimulation, as shown by coexpression of truncated alpha 2 delta subunits with alpha 1A and beta 4 subunits in Xenopus oocytes and deglycosylation with peptide-N-glycosidase F. However, coexpression of the transmembrane domain-containing delta subunit reduced the stimulatory effects of full-length alpha 2 delta subunits and substitution of a different transmembrane domain resulted in a loss of current stimulation. These results support a model whereby the alpha 2 delta transmembrane domain mediates subunit interactions and the glycosylated extracellular domain enhances current amplitude.