Role of innervation, excitability, and myogenic factors in the expression of the muscular chloride channel ClC-1. A study on normal and myotonic muscle

CaMKIIβ deregulation contributes to neuromuscular junction destabilization in Myotonic Dystrophy type I

BACKGROUND

Myotonic Dystrophy type I (DM1) is the most common muscular dystrophy in adults. Previous reports have highlighted that neuromuscular junctions (NMJs) deteriorate in skeletal muscle from DM1 patients and mouse models thereof. However, the underlying pathomechanisms and their contribution to muscle dysfunction remain unknown.

METHODS

We compared changes in NMJs and activity-dependent signalling pathways in HSALR and Mbnl1ΔE3/ΔE3 mice, two established mouse models of DM1.

RESULTS

Muscle from DM1 mouse models showed major deregulation of calcium/calmodulin-dependent protein kinases II (CaMKIIs), which are key activity sensors regulating synaptic gene expression and acetylcholine receptor (AChR) recycling at the NMJ. Both mouse models exhibited increased fragmentation of the endplate, which preceded muscle degeneration. Endplate fragmentation was not accompanied by changes in AChR turnover at the NMJ. However, the expression of synaptic genes was up-regulated in mutant innervated muscle, together with an abnormal accumulation of histone deacetylase 4 (HDAC4), a known target of CaMKII. Interestingly, denervation-induced increase in synaptic gene expression and AChR turnover was hampered in DM1 muscle. Importantly, CaMKIIβ/βM overexpression normalized endplate fragmentation and synaptic gene expression in innervated Mbnl1ΔE3/ΔE3 muscle, but it did not restore denervation-induced synaptic gene up-regulation.

CONCLUSIONS

Our results indicate that CaMKIIβ-dependent and -independent mechanisms perturb synaptic gene regulation and muscle response to denervation in DM1 mouse models. Changes in these signalling pathways may contribute to NMJ destabilization and muscle dysfunction in DM1 patients.

Chloride channel inhibition improves neuromuscular function under conditions mimicking neuromuscular disorders

AIM

The skeletal muscle Cl^- channels, the ClC-1 channels, stabilize the resting membrane potential and dampen muscle fibre excitability. This study explored whether ClC-1 inhibition can recover nerve-stimulated force in isolated muscle under conditions of compromised neuromuscular transmission akin to disorders of myasthenia gravis and Lambert-Eaton syndrome.

METHODS

Nerve-muscle preparations were isolated from rats. Preparations were exposed to pre-or post-synaptic inhibitors (ω-agatoxin, elevated extracellular Mg²⁺ , α-bungarotoxin or tubocurarine). The potential of ClC-1 inhibition (9-AC or reduced extracellular Cl^- ) to recover nerve-stimulated force under these conditions was assessed.

RESULTS

ClC-1 inhibition recovered force in both slow-twitch soleus and fast-twitch EDL muscles exposed to 0.2 µmol/L tubocurarine or 3.5 mmol/L Mg²⁺ . Similarly, ClC-1 inhibition recovered force in soleus muscles exposed to α-bungarotoxin or ω-agatoxin. Moreover, the concentrations of tubocurarine and Mg²⁺ required for reducing force to 50% rose from 0.14 ± 0.02 µmol/L and 4.2 ± 0.2 mmol/L in control muscles to 0.45 ± 0.03 µmol/L and 4.7 ± 0.3 mmol/L in muscles with 9-AC respectively (P < .05, paired T test). Inhibition of acetylcholinesterase (neostigmine) and inhibition of voltage-gated K⁺ channels (4-AP) relieve symptoms in myasthenia gravis and Lambert-Eaton syndrome, respectively. Neostigmine and 9-AC additively increased the tubocurarine concentration required to reduce nerve-stimulated force to 50% (0.56 ± 0.05 µmol/L with 9-AC and neostigmine) and, similarly, 4-AP and 9-AC additively increased the Mg²⁺ concentration required to reduce nerve-stimulated force to 50% (6.5 ± 0.2 mmol/L with 9-AC and 4-AP).

CONCLUSION

This study shows that ClC-1 inhibition can improve neuromuscular function in pharmacological models of compromised neuromuscular transmission.

Self-Assembly of Size-Controlled m-Pyridine-Urea Oligomers and Their Biomimetic Chloride Ion Channels

The m-pyridine urea (mPU) oligomer was constructed by using the intramolecular hydrogen bond formed by the pyridine nitrogen atom and the NH of urea and the intermolecular hydrogen bond of the terminal carbonyl group and the NH of urea. Due to the synergistic effect of hydrogen bonds, mPU oligomer folds and exhibits strong self-assembly behaviour. Affected by folding, mPU oligomer generates a twisted plane, and one of its important features is that the carbonyl group of the urea group orientates outwards from the twisted plane, while the NHs tend to direct inward. This feature is beneficial to NH attraction for electron-rich species. Among them, the trimer self-assembles into helical nanotubes, and can efficiently transport chloride ions. This study provides a novel and efficient strategy for constructing self-assembled biomimetic materials for electron-rich species transmission.

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.

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.

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

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

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

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

Chloride Channels are Involved in the Development of Atrial Fibrillation - A Transcriptomic and proteomic Study

Electrical and structural remodeling processes are contributors to the self-perpetuating nature of atrial fibrillation (AF). However, their correlation has not been clarified. In this study, human atrial tissues from the patients with rheumatic mitral valve disease in either sinus rhythm or persistent AF were analyzed using a combined transcriptomic and proteomic approach. An up-regulation in chloride intracellular channel (CLIC) 1, 4, 5 and a rise in type IV collagen were revealed. Combined with the results from immunohistochemistry and electron microscope analysis, the distribution of type IV collagen and effects of fibrosis on myocyte membrane indicated the possible interaction between CLIC and type IV collagen, confirmed by protein structure prediction and co-immunoprecipitation. These results indicate that CLICs play an important role in the development of atrial fibrillation and that CLICs and structural type IV collagen may interact on each other to promote the development of AF in rheumatic mitral valve disease.

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.

Huntington disease skeletal muscle is hyperexcitable owing to chloride and potassium channel dysfunction

Huntington disease is a progressive and fatal genetic disorder with debilitating motor and cognitive defects. Chorea, rigidity, dystonia, and muscle weakness are characteristic motor defects of the disease that are commonly attributed to central neurodegeneration. However, no previous study has examined the membrane properties that control contraction in Huntington disease muscle. We show primary defects in ex vivo adult skeletal muscle from the R6/2 transgenic mouse model of Huntington disease. Action potentials in diseased fibers are more easily triggered and prolonged than in fibers from WT littermates. Furthermore, some action potentials in the diseased fibers self-trigger. These defects occur because of decreases in the resting chloride and potassium conductances. Consistent with this, the expression of the muscle chloride channel, ClC-1, in Huntington disease muscle was compromised by improper splicing and a corresponding reduction in total Clcn1 (gene for ClC-1) mRNA. Additionally, the total Kcnj2 (gene for the Kir2.1 potassium channel) mRNA was reduced in disease muscle. The resulting muscle hyperexcitability causes involuntary and prolonged contractions that may contribute to the chorea, rigidity, and dystonia that characterize Huntington disease.

Novel insights into the pathomechanisms of skeletal muscle channelopathies

The nondystrophic myotonias and primary periodic paralyses are an important group of genetic muscle diseases characterized by dysfunction of ion channels that regulate membrane excitability. Clinical manifestations vary and include myotonia, hyperkalemic and hypokalemic periodic paralysis, progressive myopathy, and cardiac arrhythmias. The severity of myotonia ranges from severe neonatal presentation causing respiratory compromise through to mild later-onset disease. It remains unclear why the frequency of attacks of paralysis varies greatly or why many patients develop a severe permanent fixed myopathy. Recent detailed characterizations of human genetic mutations in voltage-gated muscle sodium (gene: SCN4A), chloride (gene: CLCN1), calcium (gene: CACNA1S), and inward rectifier potassium (genes: KCNJ2, KCNJ18) channels have resulted in new insights into disease mechanisms, clinical phenotypic variation, and therapeutic options.

Fibrillation potentials of denervated rat skeletal muscle are associated with expression of cardiac-type voltage-gated sodium channel isoform Nav1.5

OBJECTIVE

The molecular mechanisms underlying fibrillation potentials are still unclear. We hypothesised that expression of the cardiac-type voltage-gated sodium channel isoform Nav1.5 in denervated rat skeletal muscle is associated with the generation of such potentials.

METHODS

Muscle samples were extracted and analysed biologically from surgically denervated rat extensor digitorum longus muscle after concentric needle electromyographic recording at various time points after denervation (4h to 6days).

RESULTS

Both nav1.5 messenger RNA (mRNA) signal on northern blotting and Nav1.5 protein expression on immunohistochemistry appeared on the second day after denervation, exactly when fibrillation potentials appeared. Administration of lidocaine, which has much stronger affinity for sodium channels in cardiac muscle than for those in skeletal muscle, dramatically decreased fibrillation potentials, but had no effect on contralateral compound muscle action potentials.

CONCLUSIONS

Expression of Nav1.5 participates in the generation of fibrillation potentials in denervated rat skeletal muscle.

SIGNIFICANCE

We proposed an altered expression of voltage-gated sodium channel isoforms as a novel mechanism to explain the occurrence of fibrillation potentials following skeletal muscle denervation.

Identification of secondary effects of hyperexcitability by proteomic profiling of myotonic mouse muscle

Myotonia is a symptom of various genetic and acquired skeletal muscular disorders and is characterized by hyperexcitability of the sarcolemma. Here, we have performed a comparative proteomic study of the genetic mouse models ADR, MTO and MTO*5J of human congenital myotonia in order to determine myotonia-specific changes in the global protein complement of gastrocnemius muscle. Proteomic analyses of myotonia in the mouse, which is caused by mutations in the gene encoding the muscular chloride channel Clc1, revealed a generally perturbed protein expression pattern in severely affected ADR and MTO muscle, but less pronounced alterations in mildly diseased MTO*5J mice. Alterations were found in major metabolic pathways, the contractile machinery, ion handling elements, the cellular stress response and cell signaling mechanisms, clearly confirming a glycolytic-to-oxidative transformation process in myotonic fast muscle. In the long-term, a detailed biomarker signature of myotonia will improve our understanding of the pathobiochemical processes underlying this disorder and be helpful in determining how a single mutation in a tissue-specific gene can trigger severe downstream effects on the expression levels of a very large number of genes in contractile tissues.

Antioxidant treatment of hindlimb-unloaded mouse counteracts fiber type transition but not atrophy of disused muscles

Oxidative stress was proposed as a trigger of muscle impairment in various muscle diseases. The hindlimb-unloaded (HU) rodent is a model of disuse inducing atrophy and slow-to-fast transition of postural muscles. Here, mice unloaded for 14 days were chronically treated with the selective antioxidant trolox. After HU, atrophy was more pronounced in the slow-twitch soleus muscle (Sol) than in the fast-twitch gastrocnemius and tibialis anterior muscles, and was absent in extensor digitorum longus muscle. In accord with the phenotype transition, HU Sol showed a reduced expression of myosin heavy chain type 2A (MHC-2A) and increase in MHC-2X and MHC-2B isoforms. In parallel, HU Sol displayed an increased sarcolemma chloride conductance related to an increased expression of ClC-1 channels, changes in excitability parameters, a positive shift of the mechanical threshold, and a decrease of the resting cytosolic calcium concentration. Moreover, the level of lipoperoxidation increased proportionally to the degree of atrophy of each muscle type. As expected, trolox treatment fully prevented oxidative stress in HU mice. Atrophy was not prevented but the drug significantly attenuated Sol phenotypic transition and excitability changes. Trolox treatment had no effect on control mice. These results suggest possible benefits of antioxidants in protecting muscle against disuse.

Altered RNA splicing contributes to skeletal muscle pathology in Kennedy disease knock-in mice

Here, we used a mouse model of Kennedy disease, a degenerative disorder caused by an expanded CAG repeat in the androgen receptor (AR) gene, to explore pathways leading to cellular dysfunction. We demonstrate that male mice containing a targeted Ar allele with 113 CAG repeats (AR113Q mice) exhibit hormone- and glutamine length-dependent missplicing of Clcn1 RNA in skeletal muscle. Changes in RNA splicing are associated with increased expression of the RNA-binding protein CUGBP1. Furthermore, we show that skeletal muscle denervation in the absence of a repeat expansion leads to increased CUGBP1 expression. However, this induction of CUGBP1 is not sufficient to alter Clcn1 RNA splicing, indicating that changes mediated by both denervation and AR113Q toxicity contribute to altered RNA processing. To test this notion directly, we exogenously expressed the AR in vitro and observed hormone-dependent changes in the splicing of pre-mRNAs from a human cardiac troponin T minigene. These effects were notably similar to changes mediated by RNA with expanded CUG tracts, but not CAG tracts, highlighting unanticipated similarities between CAG and CUG repeat diseases. The expanded glutamine AR also altered hormone-dependent splicing of a calcitonin/calcitonin gene-related peptide minigene, suggesting that toxicity of the mutant protein additionally affects RNA processing pathways that are distinct from those regulated by CUGBP1. Our studies demonstrate the occurrence of hormone-dependent alterations in RNA splicing in Kennedy disease models, and they indicate that these changes are mediated by both the cell-autonomous effects of the expanded glutamine AR protein and by alterations in skeletal muscle that are secondary to denervation.

Recovery of function in a myogenic mouse model of spinal bulbar muscular atrophy

With this paper, we deliberately challenge the prevailing neurocentric theory of the etiology of spinal bulbar muscular atrophy (SBMA). We offer data supporting an alternative view that androgen receptor (AR) acts in skeletal muscles to cause the symptoms of SBMA. While SBMA has been linked to a CAG repeat expansion in the AR gene and mutant AR is presumed to act in motoneurons to cause SBMA, we find that over-expression of wild type AR solely in skeletal muscle fibers results in the same androgen-dependent disease phenotype as when mutant AR is broadly expressed. Like other recent SBMA mouse models, transgenic (tg) females in our model exhibit a motor phenotype only when exposed to androgens, and this motor dysfunction is independent of motoneuronal or muscle fiber cell death. Muscles from symptomatic females also show denervation-like changes in gene expression comparable to a knock-in model of SBMA. Furthermore, once androgen treatment ends, tg females rapidly recover motor function and muscle gene expression, demonstrating the strict androgen-dependence of the disease phenotype in our model. Our results argue that SBMA may be caused by AR acting in 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 channels as drug targets

Chloride channels represent a relatively under-explored target class for drug discovery as elucidation of their identity and physiological roles has lagged behind that of many other drug targets. Chloride channels are involved in a wide range of biological functions, including epithelial fluid secretion, cell-volume regulation, neuroexcitation, smooth-muscle contraction and acidification of intracellular organelles. Mutations in several chloride channels cause human diseases, including cystic fibrosis, macular degeneration, myotonia, kidney stones, renal salt wasting and hyperekplexia. Chloride-channel modulators have potential applications in the treatment of some of these disorders, as well as in secretory diarrhoeas, polycystic kidney disease, osteoporosis and hypertension. Modulators of GABA(A) (gamma-aminobutyric acid A) receptor chloride channels are in clinical use and several small-molecule chloride-channel modulators are in preclinical development and clinical trials. Here, we discuss the broad opportunities that remain in chloride-channel-based drug discovery.

Ribonuclear foci at the neuromuscular junction in myotonic dystrophy type 1

In myotonic dystrophy type 1 (DM1) the muscle fibers express RNA containing an expanded CUG repeat (CUG(exp)). The CUG(exp) RNA is retained in the nucleus, forming ribonuclear foci. Splicing factors in the muscleblind (MBNL) family are sequestered in ribonuclear foci, resulting in abnormal regulation of alternative splicing. In extrajunctional nuclei, these effects on splicing regulation lead to reduced chloride conductance and altered insulin receptor signaling. Here we show that CUG(exp) RNA is also expressed in subsynaptic nuclei of muscle fibers and in motor neurons in DM1, causing sequestration of MBNL1 protein in both locations. In a transgenic mouse model, expression of CUG(exp) RNA at high levels in extrajunctional nuclei replicates many features of DM1, but the toxic RNA is poorly expressed in subsynaptic nuclei and the mice fail to develop denervation-like features of DM1 myopathology. Our findings indicate that subsynaptic nuclei and motor neurons are at risk for DM1-induced spliceopathy, which may affect function or stability of the neuromuscular junction.

Muscle chloride channel dysfunction in two mouse models of myotonic dystrophy

Muscle degeneration and myotonia are clinical hallmarks of myotonic dystrophy type 1 (DM1), a multisystemic disorder caused by a CTG repeat expansion in the 3' untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Transgenic mice engineered to express mRNA with expanded (CUG)(250) repeats (HSA(LR) mice) exhibit prominent myotonia and altered splicing of muscle chloride channel gene (Clcn1) transcripts. We used whole-cell patch clamp recordings and nonstationary noise analysis to compare and biophysically characterize the magnitude, kinetics, voltage dependence, and single channel properties of the skeletal muscle chloride channel (ClC-1) in individual flexor digitorum brevis (FDB) muscle fibers isolated from 1-3-wk-old wild-type and HSA(LR) mice. The results indicate that peak ClC-1 current density at -140 mV is reduced >70% (-48.5 +/- 3.6 and -14.0 +/- 1.6 pA/pF, respectively) and the kinetics of channel deactivation increased in FDB fibers obtained from 18-20- d-old HSA(LR) mice. Nonstationary noise analysis revealed that the reduction in ClC-1 current density in HSA(LR) FDB fibers results from a large reduction in ClC-1 channel density (170 +/- 21 and 58 +/- 11 channels/pF in control and HSA(LR) fibers, respectively) and a modest decrease in maximal channel open probability(0.91 +/- 0.01 and 0.75 +/- 0.03, respectively). Qualitatively similar results were observed for ClC-1 channel activity in knockout mice for muscleblind-like 1 (Mbnl1(DeltaE3/DeltaE3)), a second murine model of DM1 that exhibits prominent myotonia and altered Clcn1 splicing (Kanadia et al., 2003). These results support a molecular mechanism for myotonia in DM1 in which a reduction in both the number of functional sarcolemmal ClC-1 and maximal channel open probability, as well as an acceleration in the kinetics of channel deactivation, results from CUG repeat-containing mRNA molecules sequestering Mbnl1 proteins required for proper CLCN1 pre-mRNA splicing and chloride channel function.

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.

Transforming growth factor beta (TGF-beta) signaling is regulated by electrical activity in skeletal muscle cells. TGF-beta type I receptor is transcriptionally regulated by myotube excitability

Transforming growth factor (TGF-beta) is involved in several cellular processes such as cell proliferation, differentiation, and apoptosis. At the cell surface, TGF-beta binds to serine-threonine kinase transmembrane receptors (type II and type I) to initiate Smad-dependent intracellular signaling cascades. During the early stages of skeletal muscle differentiation, myotubes start to evoke spontaneous electrical activity in association with contractions that arise following the maturation of the excitation-contraction apparatus. In this work, we report that TGF-beta-dependent signaling is regulated by electrical activity in developing rat primary myotubes, as determined by Smad2 phosphorylation, Smad4 nuclear translocation, and p3TPLux reporter activity. This electrical activity-dependent regulation is associated with changes in TGF-beta type I receptor (TbetaRI) levels, correlated with changes in transducing receptors at the cell membrane (measured through radiolabeling binding assays). The inhibition of electrical activity with tetrodotoxin, a voltage-dependent sodium channel blocker, increases TbetaRI levels via a transcription-dependent mechanism. In contrast, the promotion of electrical activity in myotube cultures, induced by the up-regulation of voltage-dependent sodium channels or by direct stimulation with extracellular electrodes, causes TbetaRI levels to decrease. Similar results were obtained in denervated adult muscles, suggesting that electrical activity-dependent regulation of TbetaRI also occurs in vivo. Additional results suggest that this activity-dependent regulation is mediated by myogenin. Altogether, these findings support the possibility for a novel regulatory mechanism acting on TGF-beta signaling cascade in skeletal muscle cells.

Identification and functional characterization of EhClC-A, an Entamoeba histolytica ClC chloride channel located at plasma membrane

ClC chloride channels perform a wide variety of physiological functions and they had been characterized in animals, yeast, plants and bacteria but not in protozoa. By blast search we found in Entamoeba histolytica, the protozoan responsible for human amoebiasis, two genes (Ehclc-A and Ehclc-B) encoding for putative polypeptides with 25-30% identity to ClC chloride channels of several organisms. Reverse transcription polymerase chain reaction (RT-PCR) experiments showed that both genes are transcribed in trophozoites. Phylogenetic analysis revealed that EhClC-A and EhClC-B polypeptides belong to the eukaryotic branch of plasma membrane ClCs. Specific antibodies against EhClC-A confirmed that it is located at the trophozoite plasma membrane. Xenopus laevis oocytes microinjected with Ehclc-A cRNA elicited anion currents not detected in oocytes microinjected with water. Induced currents were inwardly rectifying and had a permeability sequence of Cl->Br->I->F->>NO3-. The chloride channel blocker 4-acetamido-4'isothiocyanostilbene-2, 2'-disulphonic acid (SITS) strongly inhibited the oocytes anion currents and trophozoites growth. Experiments at diverse pHs suggested that EhClC-A is not a Cl-/H+ exchanger, but it is an ion channel that could be involved in pH regulation. EhClC-A may also participate in cell volume regulation. As far as we know, EhClC-A is the first chloride channel characterized in protozoa.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

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.

Alteration of cyclopiazonic acid-mediated contracture of mouse diaphragm after denervation

As a major Ca(2+) source for muscle contraction, the sarcoplasmic reticulum (SR) of skeletal muscle maintains its Ca(2+) content by uptake of myoplasmic Ca(2+) and by replenishment with extracellular Ca(2+). Since transection of motor nerve alters the functions of SR Ca(2+) pump and sarcolemma ion channels, this study explored the effect of denervation on the contracture evoked by cyclopiazonic acid (CPA), an inhibitor of SR Ca(2+) pump. In innervated hemidiaphragm, CPA elicited a bimodal elevation of muscle tone, which was dependent on extracellular Ca(2+) and differentially inhibited by pretreatment with 2-aminoethoxydiphenylborane (APB) and U73122. Activation of muscle Na(+) channels to simulate denervation-induced membrane depolarization did not change the contracture profile. After denervation for 5-14 days when the contracture induced by caffeine was not yet depressed, CPA elicited only APB-sensitive monophasic contracture. Stimulation of ATP-regulated K(+) channels with lemakalim hyperpolarized muscle membrane and attenuated CPA contracture in denervated, but not innervated, hemidiaphragm. The effects of lemakalim were antagonized by glybenclamide. It is inferred that the bimodal CPA contracture is resulted from distinct recruitments of Ca(2+) entry and that denervation alters the voltage dependence and down-regulates CPA-mediated Ca(2+) influx.

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.

Reduced expression of aquaporin 4 in human muscles with amyotrophic lateral sclerosis and other neurogenic atrophies

Aquaporin 4 (AQP4) is a water channel protein that is widely distributed in human tissues. However, the precise functional role of AQP4 in skeletal muscle tissue has not yet been determined. Expression of AQP4 was reported to be reduced in muscle tissue from Duchenne muscular dystrophy patients. In the regenerating phase of skeletal muscle, AQP4 expression was reduced when nerve supply was not present. However, in diseased human muscles with neurogenic atrophy including amyotrophic lateral sclerosis, there has been no data on the changes in AQP4 expression. In the present study, we investigated the expression of AQP4 at mRNA and protein levels in human muscles with neurogenic atrophy. The mean level of AQP4 mRNA was significantly lower in muscles with neurogenic atrophy than that in muscles from normal controls. The myofiber surface immunostaining with anti-AQP4 antibody in muscles with neurogenic atrophy was reduced on the surface of scattered myofibers, small angulated myofibers, and myofibers in small- and large-group atrophy despite the presence of dystrophin. Based on the present findings, we conclude that the expression of AQP4 is affected by nerve supply and is down-regulated in human muscles with neurogenic atrophy.

Enhanced dystrophic progression in mdx mice by exercise and beneficial effects of taurine and insulin-like growth factor-1

A preclinical screening for prompt-to-use drugs that are safer than steroids and beneficial in Duchenne muscular dystrophy was performed. Compounds able to reduce calcium-induced degeneration (taurine or creatine 10% in chow) or to stimulate regeneration [insulin-like growth factor-1 (IGF-1); 50 or 500 microg/kg s.c.] were administered for 4 to 8 weeks to mdx mice undergoing chronic exercise on a treadmill, a protocol to worsen dystrophy progression. alpha-Methyl-prednisolone (PDN; 1 mg/kg) was used as positive control. The effects were evaluated in vivo on forelimb strength and in vitro electrophysiologically on the macroscopic chloride conductance (gCl), an index of degeneration-regeneration events in mdx muscles, and on the mechanical threshold, a calcium-sensitive index of excitation-contraction coupling. The exercise produced a significant weakness and an impairment of gCl, by further decreasing the already low value of degenerating diaphragm (DIA) and fully hampering the increase of gCl typical of regenerating extensor digitorum longus (EDL) mdx muscle. The already negative voltage threshold for contraction of mdx EDL was also slightly worsened. Taurine > creatine > IGF-1 counteracted the exercise-induced weakness. The amelioration of gCl was drug- and muscle-specific: taurine was effective in EDL, but not in DIA muscle; IGF-1 and PDN were fully restorative in both muscles, whereas creatine was ineffective. An acute effect of IGF-1 on gCl was observed in vitro in untreated, but not in IGF-1-treated exercised mdx muscles. Taurine > PDN > IGF-1, but not creatine, significantly ameliorated the negative threshold voltage values of the EDL fibers. The results predict a potential benefit of taurine and IGF-1 for treating human dystrophy.

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.

Establishment and characterization of immortal hepatocytes derived from various transgenic mouse lines

The potential of three genetic changes introduced into mice by the transgenic or knockout technology aimed at immortalizing hepatocytes in vitro and concomitantly preserving their differentiated hepatic functions was analyzed. Six hepatocyte lines were isolated from neonatal and adult transgenic mice expressing either IgEGF (a secretable variant of hEGF) or SV40 T antigen in the liver and from neonatal and adult p53 knockout (KO) mice and have been subcultured >150 times in serum-free, arginine-deficient medium. Only in SV40 T antigen transgenic lines profiles of mRNAs encoding serum proteins, transcription factors, and liver-specific enzymes were similar to those found in livers and primary hepatocytes. Accordingly, these cells displayed basal and inducible expression of CYP proteins as well as testosterone metabolizing activities. Thus, either knockout of the p53 gene or expression of SV40 T antigen or of IgEGF imparts immortality to hepatocytes in vitro, but only SV40 T antigen expression is compatible with the concomitant long-term preservation of differentiated liver functions.

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.

Altered gene expression in steroid-treated denervated muscle

In rats treated with high-dose corticosteroids, skeletal muscle that is denervated in vivo (steroid-denervated) develops electrical inexcitability similar to that seen in patients with acute quadriplegic myopathy. To determine whether changes in muscle gene transcription might underlie inexcitability of steroid-denervated muscle we performed RNase protection assays to quantitate adult (SkM1) and embryonic (SkM2) sodium channel isoforms and chloride channel (CLC-1) mRNA levels in control, denervated, steroid-innervated, and steroid-denervated skeletal muscle. While SkM1 mRNA levels were relatively unaffected by denervation or steroid treatment, SkM2 mRNA levels were increased by both. These effects were synergistic and high levels of SkM2 mRNA were expressed in denervated muscle exposed to corticosteroids. Skeletal muscle CLC-1 mRNA levels were decreased by denervation. To better understand the marked upregulation of SkM2 in steroid-denervated muscle we examined changes in myogenin and glucocorticoid receptor mRNA levels. However, changes in these mRNA levels cannot account for the upregulation of SkM2 in steroid-denervated muscle.

Role of phosphorylation and physiological state in the regulation of the muscular chloride channel ClC-1: a voltage-clamp study on isolated M. interosseus fibers

Chloride currents (I(Cl)) were investigated with the two-electrode voltage-clamp technique in enzymatically isolated fibers from interosseus muscles of wild-type (WT), denervated WT, and myotonic (ADR, ClC-1-deficient) mice. Characteristics of I(Cl) were consistent with previous observations on rat muscle fibers and cultured nonmuscle cells transfected with hClC-1 cDNA. In the presence of 0.1 mM anthracene-9-carboxylic acid and in ADR fibers, I(Cl) was reduced by >90%. WT interosseus fibers denervated 6-7 days prior to isolation showed approximately 50% I(Cl) compared to control fibers. Addition of 3.3 microM staurosporine, a nonspecific inhibitor of protein kinases, increased I(Cl) in WT interosseus fibers by a factor of approximately two and altered its kinetic characteristics. We conclude that in dissociated fibers cultured for 1-2 days, in contrast to freshly isolated muscles, chloride conductance is downregulated by a mechanism involving protein phosphorylation. In situ, this short-term regulation may complement transcriptional long-term regulation of ClC-1.

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.

Higher content of insulin-like growth factor-I in dystrophic mdx mouse: potential role in the spontaneous regeneration through an electrophysiological investigation of muscle function

Insulin-like growth factor-I (IGF-I) is known to promote proliferation and differentiation of muscle cells during growth and regeneration. Both these conditions are characterized by acquisition of specialized muscle functions, such as a large macroscopic chloride conductance (GCl), a parameter that is a target of growth hormone (GH)/IGF-I axis action on skeletal muscle. The present study has been aimed at evaluating the role of IGF-I in the spontaneous regeneration occurring in hind limb muscle of dystrophic mdx mouse. IGF-I levels have been measured in hind limb muscles, plasma and liver of mdx and control mice of 8-10 weeks and 5 months of age by radioimmunoassay. In parallel the biophysical and pharmacological properties of muscle chloride channels of extensor digitorum longus (EDL) muscle fibers of mice belonging to the same age-group have been measured electrophysiologically in vitro. At 8-10 weeks of age, significantly greater amounts of IGF-I were found in plasma and hind limb muscles of mdx mice with respect to controls. Such a difference was only just detectable and no longer statistically significant at 5 months of age. No differences were found in hepatic IGF-I levels at either age. The EDL muscle fibers of mdx mice at 8-10 weeks of age were characterized by higher GCl values and by a different pharmacological sensitivity to the enantiomers of 2-(p-chlorophenoxy)-propionic acid (CPP), specific chloride channel ligands, with respect to age-matched controls. However, these differences were no longer detected at 5 months of age. Our results suggest a role of IGF-I in the high regenerative potential of muscles from mdx mice and support the hypothesis that the biophysical and pharmacological properties of chloride channels of EDL muscle fibers are sensitive indices of the action of regeneration-promoting factors on muscle function.

Expression of the potassium channel KV3.4 in mouse skeletal muscle parallels fiber type maturation and depends on excitation pattern

We report the detailed expression pattern of the voltage-dependent potassium channel KV3.4 (rat homologue, Raw3) in mouse skeletal muscle. Using semi-quantitative RT-PCR, we show that its expression is detectable at embryonic day 17 and rises to adult levels within 2 weeks after birth. Expression is fiber type-dependent, with mRNA levels being 5-6-fold lower in the mixed slow/fast soleus muscle than in the fast tibialis anterior and extensor digitorum longus muscles. Fast muscles from myotonic mice exhibit low KV3.4 mRNA levels similar to those of wild-type soleus. In denervated extensor digitorum longus, KV3.4 expression declines to perinatal levels. We conclude that KV3.4 expression in mouse skeletal muscle is regulated by the pattern of excitation.

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.

Opposite effect of intracellular Ca2+ and protein kinase C on the expression of inwardly rectifying K+ channel 1 in mouse skeletal muscle

The level of inwardly rectifying K+ channel 1 (IRK1) mRNA decreased upon denervation and increased during muscle differentiation in mouse skeletal muscle. To identify the mechanism(s) underlying the regulation of IRK1 mRNA expression, we examined its expression using the well differentiated C2C12 mouse skeletal muscle cell line as a model system. Since nerve-induced muscle activity results in contraction, it was questioned whether the changes in IRK1 expression might be relevant to the increased intracellular calcium that functions as a cytoplasmic messenger in excitation-contraction coupling. Indeed, activation of either L-type calcium channels or ryanodine receptors increased the level of IRK1 mRNA. More directly, ionomycin activated the IRK1 expression in time- and dose-dependent manners, which was abolished by treatment with EGTA. Genistein, a tyrosine kinase inhibitor, also abolished the stimulating effect of ionomycin. Meanwhile, activation of protein kinase C by 12-O-tetradecanoylphorbol acetate (TPA) markedly decreased the level of IRK1 mRNA, which required ongoing protein synthesis. Actinomycin D experiments revealed that ionomycin increased the half-life of IRK1 mRNA from 0.86 to 1.97 h, but TPA decreased it to 0.38 h. However, neither ionomycin nor TPA appreciably altered the rate of IRK1 gene transcription. Based on these observations, we conclude that intracellular calcium and protein kinase C are oppositely involved in the muscle activity-dependent regulation of IRK1 gene expression and that both act at the level of mRNA stability.

Effects of chronic growth hormone treatment in aged rats on the biophysical and pharmacological properties of skeletal muscle chloride channels

1. The effects of a 4-month daily treatment with recombinant human growth hormone (GH) (150 micrograms kg-1) to aged rats were evaluated on the passive and active membrane electrical properties of extensor digitorum longus (EDL) muscle fibres in vitro by means of a two intracellular microelectrode technique. 2. Chronic GH treatment completely restored the diameter and the membrane capacitance of aged EDL muscle fibres and significantly lowered the membrane resistance towards the adult value. There was also an increase of the threshold current, a shortening of the latency and an increase of the amplitude of the action potential and a significant amelioration of the membrane firing capability. 3. The effects were almost fully attributable to a significant 50% increase of resting conductance to chloride ions (GCl), although an observed restoration of potassium conductance and a possible effect on voltage-activated sodium channels could contribute to the effects. 4. EDL muscle fibres of untreated aged rats showed a different pharmacological response to 2-(p-chlorophenoxy) propionic acid (CPP) enantiomers from that seen in adult rats; the S-(-) isomer was less potent in blocking GCl and the R-(+) isomer always increased GCl instead of producing the typical biphasic effect observed in adult fibres (an increase of GCl at 1-10 microM and a decrease at higher concentrations). The 4-month-GH-treated aged rats showed a pharmacological sensitivity to CPP enantiomers similar to that of adults. 5. The in vitro application of insulin-like growth factor I (IGF-I), the peripheral mediator of GH, produced a significant and irreversible increase of GCl of EDL muscle of EDL muscle of untreated aged rats, an effect not observed in adults. This effect was completely inhibited by preincubation with 0.5 microM okadaic acid, suggesting that the IGF-I receptor transduction pathway can act on the phosphorylation state of the chloride channel through a serine-threonine protein phosphatase. 6. The results show that the skeletal muscle chloride channel is a target of the impairment of GH/IGF-I axis occurring in aged subjects. The acute and chronic effects observed on GCl of aged muscle fibres suggest that the GH/IGF-I stimuli act through a modulation of channel phosphorylation state and through the synthesis of 'adult'-like type chloride channels.

Expression of nerve-regulated genes in muscles of mouse mutants affected by spinal muscular atrophies and muscular dystrophies

The expression of the genes for the alpha-subunit of AChR (AChR alpha), for the myogenic factors myogenin and MyoD, for the calcium-binding protein parvalbumin (PV), and for the muscular chloride channel CIC-1 was studied in the three mouse spinal muscular atrophies (SMAs). These were the mutants "wobbler" (WR), "muscle deficient" (MDF) and "progressive motor neuronopathy" (PMN). Murine myopathies "muscular dystrophy with myositis" (MDM) and "X-linked muscular dystrophy" (MDX) were used as controls. AChR alpha and myogenin mRNA levels were strongly elevated in muscles affected by SMAs (reflecting denervation), whereas only myogenin mRNA was moderately elevated in MDX and MDM muscles, probably due to fiber regeneration. As in denervated muscle, CIC-1 and PV mRNA levels were lowered in SMAs. No changes were seen in muscles of up to 222-day-old symptomless ciliary neurotrophic factor (CNTF) knockout mice. The patterns of gene expression were characteristic for the type of muscle disease, indicating their possible usefulness for clinical diagnosis.

Electrical properties of diaphragm and EDL muscles during the life of dystrophic mice

The membrane electrical properties of diaphragm and extensor digitorum longus (EDL) muscle fibers of dystrophic mdx and control mice from 4 wk to 14-19 mo of age were recorded with the intracellular microelectrode technique. Up to 8 wk of age, the diaphragm and EDL muscles did not differ between the two strains. From 8 up to 20 wk, the mdx diaphragm fibers showed a higher membrane resistance (Rm), which was due to significantly lower values of resting chloride conductance (GCl) and an overexcitability with respect to age-matched controls. Oppositely, the mdx EDL muscle fibers had significantly lower Rm and higher GCl values than age-related controls at 8, 10, and 13 wk, along with a decreased membrane excitability. These differences were no longer detectable at 20 wk. The diaphragm and EDL muscles from 14- to 19-mo-old controls showed a decrease of GCl and an increase of potassium conductance with respect to adult animals. In aged mdx animals, these changes were very dramatic in diaphragm fibers, whereas no differences, with respect to adults, were found in the EDL muscle. Thus GCl is an index of the dystrophic condition of mdx muscles. In the degenerating diaphragm, the impairment of GCl can account for some of the pathological features of the muscle. In the EDL muscle, the changes of GCl can follow the high regenerative potential of the hindlimb muscles of the mdx phenotype.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.