Muscle chloride channel dysfunction in two mouse models of myotonic dystrophy

Characterization of ClC-1 chloride channels in zebrafish: a new model to study myotonia

The function of the chloride channel ClC-1 is crucial for the control of muscle excitability. Thus, reduction of ClC-1 functions by CLCN1 mutations leads to myotonia congenita. Many different animal models have contributed to understanding the myotonia pathophysiology. However, these models do not allow in vivo screening of potentially therapeutic drugs, as the zebrafish model does. In this work, we identified and characterized the two zebrafish orthologues (clc-1a and clc-1b) of the ClC-1 channel. Both channels are mostly expressed in the skeletal muscle as revealed by RT-PCR, western blot, and electrophysiological recordings of myotubes, and clc-1a is predominantly expressed in adult stages. Characterization in Xenopus oocytes shows that the zebrafish channels display similar anion selectivity and voltage dependence to their human counterparts. However, they show reduced sensitivity to the inhibitor 9-anthracenecarboxylic acid (9-AC), and acidic pH inverts the voltage dependence of activation. Reduction of clc-1a/b expression hampers spontaneous and mechanically stimulated movement, which could be reverted by expression of human ClC-1 but not by some ClC-1 containing myotonia mutations. Treatment of clc-1-depleted zebrafish with mexiletine, a typical drug used in human myotonia, improves the motor behaviour. Our work extends the repertoire of ClC channels to evolutionary structure-function studies and proposes the zebrafish clcn1 crispant model as a simple tool to find novel therapies for myotonia. KEY POINTS: We have identified two orthologues of ClC-1 in zebrafish (clc-1a and clc-1b) which are mostly expressed in skeletal muscle at different developmental stages. Functional characterization of the activity of these channels reveals many similitudes with their mammalian counterparts, although they are less sensitive to 9-AC and acidic pH inverts their voltage dependence of gating. Reduction of clc-1a/b expression hampers spontaneous and mechanically stimulated movement which could be reverted by expression of human ClC-1. Myotonia-like symptoms caused by clc-1a/b depletion can be reverted by mexiletine, suggesting that this model could be used to find novel therapies for myotonia.

Verapamil mitigates chloride and calcium bi-channelopathy in a myotonic dystrophy mouse model

Myotonic dystrophy type 1 (DM1) involves misregulated alternative splicing for specific genes. We used exon or nucleotide deletion to mimic altered splicing of genes central to muscle excitation-contraction coupling in mice. Mice with forced skipping of exon 29 in the CaV1.1 calcium channel combined with loss of ClC-1 chloride channel function displayed markedly reduced lifespan, whereas other combinations of splicing mimics did not affect survival. The Ca2+/Cl- bi-channelopathy mice exhibited myotonia, weakness, and impairment of mobility and respiration. Chronic administration of the calcium channel blocker verapamil rescued survival and improved force generation, myotonia, and respiratory function. These results suggest that Ca2+/Cl- bi-channelopathy contributes to muscle impairment in DM1 and is potentially mitigated by common clinically available calcium channel blockers.

Choroid plexus mis-splicing and altered cerebrospinal fluid composition in myotonic dystrophy type 1

Myotonic dystrophy type 1 is a dominantly inherited multisystemic disease caused by CTG tandem repeat expansions in the DMPK 3' untranslated region. These expanded repeats are transcribed and produce toxic CUG RNAs that sequester and inhibit activities of the MBNL family of developmental RNA processing factors. Although myotonic dystrophy is classified as a muscular dystrophy, the brain is also severely affected by an unusual cohort of symptoms, including hypersomnia, executive dysfunction, as well as early onsets of tau/MAPT pathology and cerebral atrophy. To address the molecular and cellular events that lead to these pathological outcomes, we recently generated a mouse Dmpk CTG expansion knock-in model and identified choroid plexus epithelial cells as particularly affected by the expression of toxic CUG expansion RNAs. To determine if toxic CUG RNAs perturb choroid plexus functions, alternative splicing analysis was performed on lateral and hindbrain choroid plexi from Dmpk CTG knock-in mice. Choroid plexus transcriptome-wide changes were evaluated in Mbnl2 knockout mice, a developmental-onset model of myotonic dystrophy brain dysfunction. To determine if transcriptome changes also occurred in the human disease, we obtained post-mortem choroid plexus for RNA-seq from neurologically unaffected (two females, three males; ages 50-70 years) and myotonic dystrophy type 1 (one female, three males; ages 50-70 years) donors. To test that choroid plexus transcriptome alterations resulted in altered CSF composition, we obtained CSF via lumbar puncture from patients with myotonic dystrophy type 1 (five females, five males; ages 35-55 years) and non-myotonic dystrophy patients (three females, four males; ages 26-51 years), and western blot and osmolarity analyses were used to test CSF alterations predicted by choroid plexus transcriptome analysis. We determined that CUG RNA induced toxicity was more robust in the lateral choroid plexus of Dmpk CTG knock-in mice due to comparatively higher Dmpk and lower Mbnl RNA levels. Impaired transitions to adult splicing patterns during choroid plexus development were identified in Mbnl2 knockout mice, including mis-splicing previously found in Dmpk CTG knock-in mice. Whole transcriptome analysis of myotonic dystrophy type 1 choroid plexus revealed disease-associated RNA expression and mis-splicing events. Based on these RNA changes, predicted alterations in ion homeostasis, secretory output and CSF composition were confirmed by analysis of myotonic dystrophy type 1 CSF. Our results implicate choroid plexus spliceopathy and concomitant alterations in CSF homeostasis as an unappreciated contributor to myotonic dystrophy type 1 CNS pathogenesis.

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

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

Combinatorial chloride and calcium channelopathy in myotonic dystrophy

Myotonic dystrophy type 1 (DM1) involves misregulated alternative splicing for specific genes. We used exon or nucleotide deletion to mimic altered splicing of genes central to muscle excitation-contraction coupling processes in mice. Mice with forced-skipping of exon 29 in CaV1.1 calcium channel combined with loss of ClC-1 chloride channel function showed a markedly reduced lifespan, whereas other combinations of splicing mimics did not affect survival. The Ca2+/Cl- bi-channelopathy mice exhibited myotonia, weakness, and impairment of mobility and respiration. Chronic administration of the calcium channel blocker verapamil rescued survival and improved force generation, myotonia, and respiratory function. These results suggest that Ca2+/Cl- bi-channelopathy contributes to muscle impairment in DM1 and is potentially mitigated by common clinically available calcium channel blockers.

Correction of Clcn1 alternative splicing reverses muscle fiber type transition in mice with myotonic dystrophy

In myotonic dystrophy type 1 (DM1), deregulated alternative splicing of the muscle chloride channel Clcn1 causes myotonia, a delayed relaxation of muscles due to repetitive action potentials. The degree of weakness in adult DM1 is associated with increased frequency of oxidative muscle fibers. However, the mechanism for glycolytic-to-oxidative fiber type transition in DM1 and its relationship to myotonia are uncertain. Here we cross two mouse models of DM1 to create a double homozygous model that features progressive functional impairment, severe myotonia, and near absence of type 2B glycolytic fibers. Intramuscular injection of an antisense oligonucleotide for targeted skipping of Clcn1 exon 7a corrects Clcn1 alternative splicing, increases glycolytic 2B levels to ≥ 40% frequency, reduces muscle injury, and improves fiber hypertrophy relative to treatment with a control oligo. Our results demonstrate that fiber type transitions in DM1 result from myotonia and are reversible, and support the development of Clcn1-targeting therapies for DM1.

Application of Antisense Conjugates for the Treatment of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is one of the most common muscular dystrophies and can be potentially treated with antisense therapy decreasing mutant DMPK, targeting miRNAs or their binding sites or via a blocking mechanism for MBNL1 displacement from the repeats. Unconjugated antisense molecules are able to correct the disease phenotype in mouse models, but they show poor muscle penetration upon systemic delivery in DM1 patients. In order to overcome this challenge, research has focused on the improvement of the therapeutic window and biodistribution of antisense therapy using bioconjugation to lipids, cell penetrating peptides or antibodies. Antisense conjugates are able to induce the long-lasting correction of DM1 pathology at both molecular and functional levels and also efficiently penetrate hard-to-reach tissues such as cardiac muscle. Delivery to the CNS at clinically relevant levels remains challenging and the use of alternative administration routes may be necessary to ameliorate some of the symptoms experienced by DM1 patients. With several antisense therapies currently in clinical trials, the outlook for achieving a clinically approved treatment for patients has never looked more promising.

CaV1.1 Calcium Channel Signaling Complexes in Excitation-Contraction Coupling: Insights from Channelopathies

In skeletal muscle, excitation-contraction (EC) coupling relies on the mechanical coupling between two ion channels: the L-type voltage-gated calcium channel (CaV1.1), located in the sarcolemma and functioning as the voltage sensor of EC coupling, and the ryanodine receptor 1 (RyR1), located on the sarcoplasmic reticulum serving as the calcium release channel. To this day, the molecular mechanism by which these two ion channels are linked remains elusive. However, recently, skeletal muscle EC coupling could be reconstituted in heterologous cells, revealing that only four proteins are essential for this process: CaV1.1, RyR1, and the cytosolic proteins CaVβ1a and STAC3. Due to the crucial role of these proteins in skeletal muscle EC coupling, any mutation that affects any one of these proteins can have devastating consequences, resulting in congenital myopathies and other pathologies.Here, we summarize the current knowledge concerning these four essential proteins and discuss the pathophysiology of the CaV1.1, RyR1, and STAC3-related skeletal muscle diseases with an emphasis on the molecular mechanisms. Being part of the same signalosome, mutations in different proteins often result in congenital myopathies with similar symptoms or even in the same disease.

Acute exposure to extracellular BTP2 does not inhibit Ca2+ release during EC coupling in intact skeletal muscle fibers

The inhibitor of store-operated Ca2+ entry (SOCE) BTP2 was reported to inhibit ryanodine receptor Ca2+ leak and electrically evoked Ca2+ release from the sarcoplasmic reticulum when introduced into mechanically skinned muscle fibers. However, it is unclear how effects of intracellular application of a highly lipophilic drug like BTP2 on Ca2+ release during excitation-contraction (EC) coupling compare with extracellular exposure in intact muscle fibers. Here, we address this question by quantifying the effect of short- and long-term exposure to 10 and 20 µM BTP2 on the magnitude and kinetics of electrically evoked Ca2+ release in intact mouse flexor digitorum brevis muscle fibers. Our results demonstrate that neither the magnitude nor the kinetics of electrically evoked Ca2+ release evoked during repetitive electrical stimulation were altered by brief exposure (2 min) to either BTP2 concentration. However, BTP2 did reduce the magnitude of electrically evoked Ca2+ release in intact fibers when applied extracellularly for a prolonged period of time (30 min at 10 µM or 10 min at 20 µM), consistent with slow diffusion of the lipophilic drug across the plasma membrane. Together, these results indicate that the time course and impact of BTP2 on Ca2+ release during EC coupling in skeletal muscle depends strongly on whether the drug is applied intracellularly or extracellularly. Further, these results demonstrate that electrically evoked Ca2+ release in intact muscle fibers is unaltered by extracellular application of 10 µM BTP2 for <25 min, validating this use to assess the role of SOCE in the absence of an effect on EC coupling.

Advanced Gene-Targeting Therapies for Motor Neuron Diseases and Muscular Dystrophies

Gene therapy is a revolutionary, cutting-edge approach to permanently ameliorate or amend many neuromuscular diseases by targeting their genetic origins. Motor neuron diseases and muscular dystrophies, whose genetic causes are well known, are the frontiers of this research revolution. Several genetic treatments, with diverse mechanisms of action and delivery methods, have been approved during the past decade and have demonstrated remarkable results. However, despite the high number of genetic treatments studied preclinically, those that have been advanced to clinical trials are significantly fewer. The most clinically advanced treatments include adeno-associated virus gene replacement therapy, antisense oligonucleotides, and RNA interference. This review provides a comprehensive overview of the advanced gene therapies for motor neuron diseases (i.e., amyotrophic lateral sclerosis and spinal muscular atrophy) and muscular dystrophies (i.e., Duchenne muscular dystrophy, limb-girdle muscular dystrophy, and myotonic dystrophy) tested in clinical trials. Emphasis has been placed on those methods that are a few steps away from their authoritative approval.

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

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

TNF Signaling Acts Downstream of MiR-322/-503 in Regulating DM1 Myogenesis

Myotonic dystrophy type 1 (DM1) is caused by the expanded CUG repeats and usually displays defective myogenesis. Although we previously reported that ectopic miR-322/-503 expression improved myogenesis in DM1 by targeting the toxic RNA, the underlying pathways regulating myogenesis that were aberrantly altered in DM1 and rescued by miR-322/-503 were still unknown. Here, we constructed DM1 and miR-322/-503 overexpressing DM1 myoblast models, which were subjected to in vitro myoblast differentiation along with their corresponding controls. Agreeing with previous findings, DM1 myoblast showed remarkable myogenesis defects, while miR-322/-503 overexpression successfully rescued the defects. By RNA sequencing, we noticed that Tumor necrosis factor (TNF) signaling was the only pathway that was significantly and oppositely altered in these two experimental sets, with it upregulated in DM1 and inhibited by miR-322/-503 overexpression. Consistently, hyperactivity of TNF signaling was detected in two DM1 mouse models. Blocking TNF signaling significantly rescued the myogenesis defects in DM1. On the contrary, TNF-α treatment abolished the rescue effect of miR-322/-503 on DM1 myogenesis. Taking together, these results implied that TNF signaling mediated the myogenesis defects in DM1 and might act downstream of miR-322/-503 in regulating the myogenesis in DM1. Moreover, the inhibition of TNF signaling benefiting myogenesis in DM1 provided us with a novel therapeutic strategy for DM1.

Biophysical mechanisms for QRS- and QTc-interval prolongation in mice with cardiac expression of expanded CUG-repeat RNA

Myotonic dystrophy type 1 (DM1), the most common form of muscular dystrophy in adults, results from the expression of toxic gain-of-function transcripts containing expanded CUG-repeats. DM1 patients experience cardiac electrophysiological defects, including prolonged PR-, QRS-, and QT-intervals, that increase susceptibility to sudden cardiac death (SCD). However, the specific biophysical and molecular mechanisms that underlie the electrocardiograph (ECG) abnormalities and SCD in DM1 are unclear. Here, we addressed this issue using a novel transgenic mouse model that exhibits robust cardiac expression of expanded CUG-repeat RNA (LC15 mice). ECG measurements in conscious LC15 mice revealed significantly prolonged QRS- and corrected QT-intervals, but a normal PR-interval. Although spontaneous arrhythmias were not observed in conscious LC15 mice under nonchallenged conditions, acute administration of the sodium channel blocker flecainide prolonged the QRS-interval and unveiled an increased susceptibility to lethal ventricular arrhythmias. Current clamp measurements in ventricular myocytes from LC15 mice revealed significantly reduced action potential upstroke velocity at physiological pacing (9 Hz) and prolonged action potential duration at all stimulation rates (1–9 Hz). Voltage clamp experiments revealed significant rightward shifts in the voltage dependence of sodium channel activation and steady-state inactivation, as well as a marked reduction in outward potassium current density. Together, these findings indicate that expression of expanded CUG-repeat RNA in the murine heart results in reduced sodium and potassium channel activity that results in QRS- and QT-interval prolongation, respectively.

Zebrafish mbnl mutants model physical and molecular phenotypes of myotonic dystrophy

The muscleblind RNA-binding proteins (MBNL1, MBNL2 and MBNL3) are highly conserved across vertebrates and are important regulators of RNA alternative splicing. Loss of MBNL protein function through sequestration by CUG or CCUG RNA repeats is largely responsible for the phenotypes of the human genetic disorder myotonic dystrophy (DM). We generated the first stable zebrafish (Danio rerio) models of DM-associated MBNL loss of function through mutation of the three zebrafish mbnl genes. In contrast to mouse models, zebrafish double and triple homozygous mbnl mutants were viable to adulthood. Zebrafish mbnl mutants displayed disease-relevant physical phenotypes including decreased body size and impaired movement. They also exhibited widespread alternative splicing changes, including the misregulation of many DM-relevant exons. Physical and molecular phenotypes were more severe in compound mbnl mutants than in single mbnl mutants, suggesting partially redundant functions of Mbnl proteins. The high fecundity and larval optical transparency of this complete series of zebrafish mbnl mutants will make them useful for studying DM-related phenotypes and how individual Mbnl proteins contribute to them, and for testing potential therapeutics. This article has an associated First Person interview with the first author of the paper.

Degradation of Toxic RNA in Myotonic Dystrophy Using Gapmer Antisense Oligonucleotides

Myotonic dystrophy (DM) types 1 (DM1) and 2 (DM2) are caused by autosomal dominant gain-of-function RNA which are, in turn, created by the expansion of repeat sequences in the DMPK and ZNF9 genes, respectively. The expansions are highly unstable and biased for further expansion in somatic cells and across generations. Despite the different genes involved, DM1 and DM2 share several clinical features due to having the similar underlying mechanism of repetitive RNA-mediated toxicity. Both disorders manifest as multisystemic conditions with features including myotonia, cataract development, and abnormalities in cardiac conduction. At present, there is no cure for DM and treatments mostly aim at symptom management. Among the therapeutics being developed, antisense therapy using gapmers is one of the most promising. Compared to other antisense oligonucleotides, gapmers maintain the ability to induce RNase H cleavage while having enhanced target binding affinity and nuclease resistance. This chapter will consolidate the different strategies studied thus far to develop a treatment for DM1 through the targeting of toxic repetitive RNA using gapmers.

Targeted splice sequencing reveals RNA toxicity and therapeutic response in myotonic dystrophy

Biomarker-driven trials hold promise for therapeutic development in chronic diseases, such as muscular dystrophy. Myotonic dystrophy type 1 (DM1) involves RNA toxicity, where transcripts containing expanded CUG-repeats (CUGexp) accumulate in nuclear foci and sequester splicing factors in the Muscleblind-like (Mbnl) family. Oligonucleotide therapies to mitigate RNA toxicity have emerged but reliable measures of target engagement are needed. Here we examined muscle transcriptomes in mouse models of DM1 and found that CUGexp expression or Mbnl gene deletion cause similar dysregulation of alternative splicing. We selected 35 dysregulated exons for further study by targeted RNA sequencing. Across a spectrum of mouse models, the individual splice events and a composite index derived from all events showed a graded response to decrements of Mbnl or increments of CUGexp. Antisense oligonucleotides caused prompt reduction of CUGexp RNA and parallel correction of the splicing index, followed by subsequent elimination of myotonia. These results suggest that targeted splice sequencing may provide a sensitive and reliable way to assess therapeutic impact in DM1.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

An Overview of Alternative Splicing Defects Implicated in Myotonic Dystrophy Type I

Myotonic dystrophy type I (DM1) is the most common form of adult muscular dystrophy, caused by expansion of a CTG triplet repeat in the 3′ untranslated region (3′UTR) of the myotonic dystrophy protein kinase (DMPK) gene. The pathological CTG repeats result in protein trapping by expanded transcripts, a decreased DMPK translation and the disruption of the chromatin structure, affecting neighboring genes expression. The muscleblind-like (MBNL) and CUG-BP and ETR-3-like factors (CELF) are two families of tissue-specific regulators of developmentally programmed alternative splicing that act as antagonist regulators of several pre-mRNA targets, including troponin 2 (TNNT2), insulin receptor (INSR), chloride channel 1 (CLCN1) and MBNL2. Sequestration of MBNL proteins and up-regulation of CELF1 are key to DM1 pathology, inducing a spliceopathy that leads to a developmental remodelling of the transcriptome due to an adult-to-foetal splicing switch, which results in the loss of cell function and viability. Moreover, recent studies indicate that additional pathogenic mechanisms may also contribute to disease pathology, including a misregulation of cellular mRNA translation, localization and stability. This review focuses on the cause and effects of MBNL and CELF1 deregulation in DM1, describing the molecular mechanisms underlying alternative splicing misregulation for a deeper understanding of DM1 complexity. To contribute to this analysis, we have prepared a comprehensive list of transcript alterations involved in DM1 pathogenesis, as well as other deregulated mRNA processing pathways implications.

Depletion of ATP Limits Membrane Excitability of Skeletal Muscle by Increasing Both ClC1-Open Probability and Membrane Conductance

Activation of skeletal muscle contractions require that action potentials can be excited and propagated along the muscle fibers. Recent studies have revealed that muscle fiber excitability is regulated during repeated firing of action potentials by cellular signaling systems that control the function of ion channel that determine the resting membrane conductance (G_m). In fast-twitch muscle, prolonged firing of action potentials triggers a marked increase in G_m, reducing muscle fiber excitability and causing action potential failure. Both ClC-1 and K_ATP ion channels contribute to this G_m rise, but the exact molecular regulation underlying their activation remains unclear. Studies in expression systems have revealed that ClC-1 is able to bind adenosine nucleotides, and that low adenosine nucleotide levels result in ClC-1 activation. In three series of experiments, this study aimed to explore whether ClC-1 is also regulated by adenosine nucleotides in native skeletal muscle fibers, and whether the adenosine nucleotide sensitivity of ClC-1 could explain the rise in G_m muscle fibers during prolonged action potential firing. First, whole cell patch clamping of mouse muscle fibers demonstrated that ClC-1 activation shifted in the hyperpolarized direction when clamping pipette solution contained 0 mM ATP compared with 5 mM ATP. Second, three-electrode G_m measurement during muscle fiber stimulation showed that glycolysis inhibition, with 2-deoxy-glucose or iodoacetate, resulted in an accelerated and rapid >400% G_m rise during short periods of repeated action potential firing in both fast-twitch and slow-twitch rat, and in human muscle fibers. Moreover, ClC-1 inhibition with 9-anthracenecarboxylic acid resulted in either an absence or blunted G_m rise during action potential firing in human muscle fibers. Third, G_m measurement during repeated action potential firing in muscle fibers from a murine McArdle disease model suggest that the rise in G_m was accelerated in a subset of fibers. Together, these results are compatible with ClC-1 function being regulated by the level of adenosine nucleotides in native tissue, and that the channel operates as a sensor of skeletal muscle metabolic state, limiting muscle excitability when energy status is low.

Skeletal muscle CaV1.1 channelopathies

Ca_V1.1 is specifically expressed in skeletal muscle where it functions as voltage sensor of skeletal muscle excitation-contraction (EC) coupling independently of its functions as L-type calcium channel. Consequently, all known Ca_V1.1-related diseases are muscle diseases and the molecular and cellular disease mechanisms relate to the dual functions of Ca_V1.1 in this tissue. To date, four types of muscle diseases are known that can be linked to mutations in the CACNA1S gene or to splicing defects. These are hypo- and normokalemic periodic paralysis, malignant hyperthermia susceptibility, Ca_V1.1-related myopathies, and myotonic dystrophy type 1. In addition, the Ca_V1.1 function in EC coupling is perturbed in Native American myopathy, arising from mutations in the Ca_V1.1-associated protein STAC3. Here, we first address general considerations concerning the possible roles of Ca_V1.1 in disease and then discuss the state of the art regarding the pathophysiology of the Ca_V1.1-related skeletal muscle diseases with an emphasis on molecular disease mechanisms.

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.

Central Role of Subthreshold Currents in Myotonia

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than the action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction. ANN NEUROL 2020;87:175-183.

Myoediting: Toward Prevention of Muscular Dystrophy by Therapeutic Genome Editing

Muscular dystrophies represent a large group of genetic disorders that significantly impair quality of life and often progress to premature death. There is no effective treatment for these debilitating diseases. Most therapies, developed to date, focus on alleviating the symptoms or targeting the secondary effects, while the underlying gene mutation is still present in the human genome. The discovery and application of programmable nucleases for site-specific DNA double-stranded breaks provides a powerful tool for precise genome engineering. In particular, the CRISPR/Cas system has revolutionized the genome editing field and is providing a new path for disease treatment by targeting the disease-causing genetic mutations. In this review, we provide a historical overview of genome-editing technologies, summarize the most recent advances, and discuss potential strategies and challenges for permanently correcting genetic mutations that cause muscular dystrophies.

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 Take Center Stage in Acute Regulation of Excitability in Skeletal Muscle: Implications for Fatigue

Initiation and propagation of action potentials in muscle fibers is a key element in the transmission of activating motor input from the central nervous system to their contractile apparatus, and maintenance of excitability is therefore paramount for their endurance during work. Here, we review current knowledge about the acute regulation of ClC-1 channels in active muscles and its importance for muscle excitability, function, and fatigue.

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.

A Potent Inhibitor of Protein Sequestration by Expanded Triplet (CUG) Repeats that Shows Phenotypic Improvements in a Drosophila Model of Myotonic Dystrophy

Myotonic dystrophy is the most common form of adult-onset muscular dystrophy, originating in a CTG repeat expansion in the DMPK gene. The expanded CUG transcript sequesters MBNL1, a key regulator of alternative splicing, leading to the misregulation of numerous pre-mRNAs. We report an RNA-targeted agent as a possible lead compound for the treatment of myotonic dystrophy type 1 (DM1) that reveals both the promise and challenges for this type of small-molecule approach. The agent is a potent inhibitor of the MBNL1-rCUG complex with an inhibition constant (Ki ) of 25±8 nm, and is also relatively nontoxic to HeLa cells, able to dissolve nuclear foci, and correct the insulin receptor splicing defect in DM1 model cells. Moreover, treatment with this compound improves two separate disease phenotypes in a Drosophila model of DM1: adult external eye degeneration and larval crawling defect. However, the compound has a relatively low maximum tolerated dose in mice, and its cell uptake may be limited, providing insight into directions for future development.

Restricting calcium currents is required for correct fiber type specification in skeletal muscle

Skeletal muscle excitation-contraction (EC) coupling is independent of calcium influx. In fact, alternative splicing of the voltage-gated calcium channel CaV1.1 actively suppresses calcium currents in mature muscle. Whether this is necessary for normal development and function of muscle is not known. However, splicing defects that cause aberrant expression of the calcium-conducting developmental CaV1.1e splice variant correlate with muscle weakness in myotonic dystrophy. Here, we deleted CaV1.1 (Cacna1s) exon 29 in mice. These mice displayed normal overall motor performance, although grip force and voluntary running were reduced. Continued expression of the developmental CaV1.1e splice variant in adult mice caused increased calcium influx during EC coupling, altered calcium homeostasis, and spontaneous calcium sparklets in isolated muscle fibers. Contractile force was reduced and endurance enhanced. Key regulators of fiber type specification were dysregulated and the fiber type composition was shifted toward slower fibers. However, oxidative enzyme activity and mitochondrial content declined. These findings indicate that limiting calcium influx during skeletal muscle EC coupling is important for the secondary function of the calcium signal in the activity-dependent regulation of fiber type composition and to prevent muscle disease.

Computational investigation of RNA CUG repeats responsible for myotonic dystrophy 1

Despite the importance of the knowledge of molecular hydration entropy (ΔShyd) in chemical and biological processes, the exact calculation of ΔShyd is very difficult, because of the complexity in solute–water interactions. Although free-energy perturbation (FEP) methods have been employed quite widely in the literature, the poor convergent behavior of the van der Waals interaction term in the potential function limited the accuracy and robustness. In this study, we propose a new method for estimating ΔShyd by means of combining the FEP approach and the scaled particle theory (or information theory) to separately calculate the electrostatic solute–water interaction term (ΔSelec) and the hydrophobic contribution approximated by the cavity formation entropy (ΔScav), respectively. Decomposition of ΔShyd into ΔScav and ΔSelec terms is found to be very effective with a substantial accuracy enhancement in ΔShyd estimation, when compared to the conventional full FEP calculations. ΔScav appears to dominate over ΔSelec in magnitude, even in the case of polar solutes, implying that the major contribution to the entropic cost for hydration comes from the formation of a solvent-excluded volume. Our hybrid scaled particle theory and FEP method is thus found to enhance the accuracy of ΔShyd prediction by effectively complementing the conventional full FEP method.

Muscleblind-Like 1 and Muscleblind-Like 3 Depletion Synergistically Enhances Myotonia by Altering Clc-1 RNA Translation

Loss of Muscleblind-like 1 (Mbnl1) is known to alter Clc-1 splicing to result in myotonia. Mbnl1^ΔE3/ΔE3/Mbnl3^ΔE2 mice, depleted of Mbnl1 and Mbnl3, demonstrate a profound enhancement of myotonia and an increase in the number of muscle fibers with very low Clc-1 currents, where gCl_max values approach ~ 1 mS/cm², with the absence of a further enhancement in Clc-1 splice errors, alterations in polyA site selection or Clc-1 localization. Significantly, Mbnl1^ΔE3/ΔE3/Mbnl3^ΔE2 muscles demonstrate an aberrant accumulation of Clc-1 RNA on monosomes and on the first polysomes. Mbnl1 and Mbnl3 bind Clc-1 RNA and both proteins bind Hsp70 and eEF1A, with these associations being reduced in the presence of RNA. Thus binding of Mbnl1 and Mbnl3 to Clc-1 mRNA engaged with ribosomes can facilitate an increase in the local concentration of Hsp70 and eEF1A to assist Clc-1 translation. Dual depletion of Mbnl1 and Mbnl3 therefore initiates both Clc-1 splice errors and translation defects to synergistically enhance myotonia. As the HSA^LR model for myotonic dystrophy (DM1) shows similar Clc-1 defects, this study demonstrates that both splice errors and translation defects are required for DM1 pathology to manifest.

Research in context

Research in context: Myotonic Dystrophy type 1 (DM1) is a dominant disorder resulting from the expression of expanded CUG repeat RNA, which aberrantly sequesters and inactivates the muscleblind-like (MBNL) family of proteins. In mice, inactivation of Mbnl1 is known to alter Clc-1 splicing to result in myotonia. We demonstrate that concurrent depletion of Mbnl1 and Mbnl3 results in a synergistic enhancement of myotonia, with an increase in muscle fibers showing low chloride currents. The observed synergism results from the aberrant accumulation of Clc-1 mRNA on monosomes and the first polysomes. This translation error reflects the ability of Mbnl1 and Mbnl3 to act as adaptors that recruit Hsp70 and eEF1A to the Clc-1 mRNA engaged with ribosomes, to facilitate translation. Thus our study demonstrates that Clc-1 RNA translation defects work coordinately with Clc-1 splice errors to synergistically enhance myotonia in mice lacking Mbnl1 and Mbnl3.

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Mbnl 1 & 3 loss enhances myotonia and increases fibers with low chloride currents.

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Clc-1 RNA increase in lighter polysome fractions results in low chloride currents.

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Mbnl 1 & 3 interact with Hsp70 and eEF1A in an RNA moderated manner.

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Mbnl 1 & 3 recruitment of Hsp70 and eEF1A to Clc-1 RNA facilitates translation.

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The HSA^LR DM1 mouse model shows similar Clc-1 RNA translation defects.

Targeting PKCθ in skeletal muscle and muscle diseases: good or bad?

Protein kinase Cθ (PKCθ) is a member of the novel calcium-independent PKC family, with a relatively selective tissue distribution. Most studies have focused on its unique role in T-lymphocyte activation and suggest that inhibition of PKCθ could represent a novel therapeutic approach in the treatment of chronic inflammation, autoimmunity and allograft rejection. However, considering that PKCθ is also expressed in other cell types, including skeletal muscle cells, it is important to understand its function in different tissues before proposing it as a molecular target for the treatment of immune-mediated diseases. A number of studies have highlighted the role of PKCθ in mediating several intracellular pathways, regulating muscle cell development, homoeostasis and remodelling, although a comprehensive picture is still lacking. Moreover, we recently showed that lack of PKCθ in a mouse model of Duchenne muscular dystrophy (DMD) ameliorates the progression of the disease. In the present article, we review new developments in our understanding of the involvement of PKCθ in intracellular mechanisms regulating skeletal muscle development, growth and maintenance under physiological conditions and recent advances showing a hitherto unrecognized role of PKCθ in promoting muscular dystrophy.

Developmental insights into the pathology of and therapeutic strategies for DM1: Back to the basics

Myotonic Dystrophy type 1 (DM1), the most prevalent adult onset muscular dystrophy, is a trinucleotide repeat expansion disease caused by CTG expansion in the 3'-UTR of DMPK gene. This expansion results in the expression of toxic gain-of-function RNA that forms ribonuclear foci and disrupts normal activities of RNA-binding proteins belonging to the MBNL and CELF families. Changes in alternative splicing, translation, localization, and mRNA stability due to sequestration of MBNL proteins and up-regulation of CELF1 are key to DM1 pathology. However, recent discoveries indicate that pathogenic mechanisms of DM1 involves many other factors as well, including repeat associated translation, activation of PKC-dependent signaling pathway, aberrant polyadenylation, and microRNA deregulation. Expression of the toxic repeat RNA culminates in the developmental remodeling of the transcriptome, which produces fetal isoforms of proteins that are unable to fulfill the physiological requirements of adult tissues. This review will describe advances in the understanding of DM1 pathogenesis as well as current therapeutic developments for DM1.

Role of Mitofusin-2 in mitochondrial localization and calcium uptake in skeletal muscle

As muscle contraction requires ATP and Ca(2+), skeletal muscle function is highly dependent on communication between two major intracellular organelles: mitochondria and sarcoplasmic reticulum (SR). In adult skeletal muscle, mitochondria located within the I-band of the sarcomere are connected to the SR by small ∼10 nm electron dense tethers that bridge the outer mitochondrial membrane to the region of SR that is ∼130 nm from the site of Ca(2+) release. However, the molecular composition of tethers and their precise impact on mitochondrial Ca(2+) uptake in skeletal muscle is unclear. Mitofusin-2 (Mfn2) is a transmembrane GTPase present in both mitochondria and ER/SR membranes that forms trans-dimers and participates in mitochondrial fusion. Here we evaluated the role Mfn2 plays in mitochondrial morphology, localization, and functional SR-mitochondrial Ca(2+) crosstalk in adult skeletal muscle. Compared to a non-targeting (CTRL) siRNA, in vivo electroporation of 400 nM Mfn2 siRNA (Mfn2 KD) into mouse footpads resulted in a marked acute reduction (67±3%) in Mfn2 protein levels in flexor digitorum brevis (FDB) muscles that occurred without a change in other key Ca(2+) regulatory proteins. Electron microscopy analyses revealed that Mfn2 knockdown resulted in a change in mitochondria morphology and mis-localization of some mitochondria from the I-band to the A-band region of the sarcomere. To assess the role of Mfn2 in SR-mitochondrial crosstalk, we measured mitochondrial Ca(2+) uptake and myoplasmic Ca(2+) transients with rhod-2 and mag-fluo-4, respectively, during repetitive high frequency tetanic stimulation (5×100 Hz tetani, 500 ms/tetani, 0.2 duty cycle) in CTRL and Mfn2 KD fibers. Mitochondrial Ca(2+) uptake during repetitive tetanic stimulation was significantly reduced (40%) in Mfn2 KD FDB fibers, which was accompanied by a parallel elevation in the global electrically evoked myoplasmic Ca(2+) transient. Mfn2 KD also resulted in a reduction of the mitochondrial membrane potential, which contributed to the observed decrease in activity-dependent mitochondrial Ca(2+) uptake. Consistent with this idea, a similar decrease in mitochondrial Ca(2+) uptake was also observed in wild type fibers following a comparable reduction in mitochondrial membrane potential induced by acute exposure to a low concentration (50 nM) of carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP). In addition, both global and mitochondrial Ca(2+) transients during repetitive tetanic stimulation were similarly reduced by both slow (EGTA) and fast (BAPTA) Ca(2+) chelating agents. Together, these results indicate that Mfn2 promotes proper mitochondrial morphology, localization, and membrane potential required for optimal activity-dependent mitochondrial Ca(2+) uptake and buffering of the global myoplasmic Ca(2+) transient in adult skeletal muscle.

Myotonic dystrophy

Myotonic dystrophy (dystrophia myotonica, DM) is one of the most common lethal monogenic disorders in populations of European descent. DM type 1 was first described over a century ago. More recently, a second form of the disease, DM type 2 was recognized, which results from repeat expansion in a different gene. Both disorders have autosomal dominant inheritance and multisystem features, including myotonic myopathy, cataract, and cardiac conduction disease. This article reviews the clinical presentation and pathophysiology of DM and discusses current management and future potential for developing targeted therapies.

Myotonic dystrophies: An update on clinical aspects, genetic, pathology, and molecular pathomechanisms

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. However, it is now clear that additional pathogenic mechanism like changes in gene expression, protein translation and micro-RNA metabolism may also contribute to disease pathology. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. This review is an update on the recent advances in the understanding of the molecular mechanisms behind myotonic dystrophies. This article is part of a Special Issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

Protein kinase C theta (PKCθ) modulates the ClC-1 chloride channel activity and skeletal muscle phenotype: a biophysical and gene expression study in mouse models lacking the PKCθ

In skeletal muscle, the resting chloride conductance (gCl), due to the ClC-1 chloride channel, controls the sarcolemma electrical stability. Indeed, loss-of-function mutations in ClC-1 gene are responsible of myotonia congenita. The ClC-1 channel can be phosphorylated and inactivated by protein kinases C (PKC), but the relative contribution of each PKC isoforms is unknown. Here, we investigated on the role of PKCθ in the regulation of ClC-1 channel expression and activity in fast- and slow-twitch muscles of mouse models lacking PKCθ. Electrophysiological studies showed an increase of gCl in the PKCθ-null mice with respect to wild type. Muscle excitability was reduced accordingly. However, the expression of the ClC-1 channel, evaluated by qRT-PCR, was not modified in PKCθ-null muscles suggesting that PKCθ affects the ClC-1 activity. Pharmacological studies demonstrated that although PKCθ appreciably modulates gCl, other isoforms are still active and concur to this role. The modification of gCl in PKCθ-null muscles has caused adaptation of the expression of phenotype-specific genes, such as calcineurin and myocyte enhancer factor-2, supporting the role of PKCθ also in the settings of muscle phenotype. Importantly, the lack of PKCθ has prevented the aging-related reduction of gCl, suggesting that its modulation may represent a new strategy to contrast the aging process.

Most expression and splicing changes in myotonic dystrophy type 1 and type 2 skeletal muscle are shared with other muscular dystrophies

The prevailing pathomechanistic paradigm for myotonic dystrophy (DM) is that aberrant expression of embryonic/fetal mRNA/protein isoforms accounts for most aspects of the pleiotropic phenotype. To identify aberrant isoforms in skeletal muscle of DM1 and DM2 patients, we performed exon-array profiling and RT-PCR validation on the largest DM sample set to date, including Duchenne, Becker and tibial muscular dystrophy (NMD) patients as disease controls, and non-disease controls. Strikingly, most expression and splicing changes in DM patients were shared with NMD controls. Comparison between DM and NMD identified almost no significant differences. We conclude that DM1 and DM2 are essentially identical for dysregulation of gene expression, and DM expression changes represent a subset of broader spectrum dystrophic changes. We found no evidence for qualitative splicing differences between DM1 and DM2. While some DM-specific splicing differences exist, most of the DM splicing differences were also seen in NMD controls. SSBP3 exon 6 missplicing was observed in all diseased muscle and led to reduced protein. We conclude there is no widespread DM-specific spliceopathy in skeletal muscle and suggest that missplicing in DM (and NMD) may not be the driving mechanism for the muscle pathology, since the same pathways show expression changes unrelated to splicing.

Smaug/SAMD4A restores translational activity of CUGBP1 and suppresses CUG-induced myopathy

We report the identification and characterization of a previously unknown suppressor of myopathy caused by expansion of CUG repeats, the mutation that triggers Myotonic Dystrophy Type 1 (DM1). We screened a collection of genes encoding RNA-binding proteins as candidates to modify DM1 pathogenesis using a well established Drosophila model of the disease. The screen revealed smaug as a powerful modulator of CUG-induced toxicity. Increasing smaug levels prevents muscle wasting and restores muscle function, while reducing its function exacerbates CUG-induced phenotypes. Using human myoblasts, we show physical interactions between human Smaug (SMAUG1/SMAD4A) and CUGBP1. Increased levels of SMAUG1 correct the abnormally high nuclear accumulation of CUGBP1 in myoblasts from DM1 patients. In addition, augmenting SMAUG1 levels leads to a reduction of inactive CUGBP1-eIF2α translational complexes and to a correction of translation of MRG15, a downstream target of CUGBP1. Therefore, Smaug suppresses CUG-mediated muscle wasting at least in part via restoration of translational activity of CUGBP1.

RNA toxicity in human disease and animal models: from the uncovering of a new mechanism to the development of promising therapies

Mutant ribonucleic acid (RNA) molecules can be toxic to the cell, causing human disease through trans-acting dominant mechanisms. RNA toxicity was first described in myotonic dystrophy type 1, a multisystemic disorder caused by the abnormal expansion of a non-coding trinucleotide repeat sequence. The development of multiple and complementary animal models of disease has greatly contributed to clarifying the complex disease pathways mediated by toxic RNA molecules. RNA toxicity is not limited to myotonic dystrophy and spreads to an increasing number of human conditions, which share some unifying pathogenic events mediated by toxic RNA accumulation and disruption of RNA-binding proteins. The remarkable progress in the dissection of disease pathobiology resulted in the rational design of molecular therapies, which have been successfully tested in animal models. Toxic RNA diseases, and in particular myotonic dystrophy, clearly illustrate the critical contribution of animal models of disease in translational research: from gene mutation to disease mechanisms, and ultimately to therapy development. This article is part of a Special Issue entitled: Animal Models of Disease.

Age-dependent chloride channel expression in skeletal muscle fibres of normal and HSA(LR) myotonic mice

Abstract  We combine electrophysiological and optical techniques to investigate the role that the expression of chloride channels (ClC-1) plays on the age-dependent electrical properties of mammalian muscle fibres. To this end, we comparatively evaluate the magnitude and voltage dependence of chloride currents (ICl), as well as the resting resistance, in fibres isolated from control and human skeletal actin (HSA)(LR) mice (a model of myotonic dystrophy) of various ages. In control mice, the maximal peak chloride current ([peak-ICl]max) increases from -583 ± 126 to -956 ± 260 μA cm(-2) (mean ± SD) between 3 and 6 weeks old. Instead, in 3-week-old HSA(LR) mice, ICl are significantly smaller (-153 ± 33 μA cm(-2)) than in control mice, but after a long period of ∼14 weeks they reach statistically comparable values. Thus, the severe ClC-1 channelopathy in young HSA(LR) animals is slowly reversed with aging. Frequency histograms of the maximal chloride conductance (gCl,max) in fibres of young HSA(LR) animals are narrow and centred in low values; alternatively, those from older animals show broad distributions, centred at larger gCl,max values, compatible with mosaic expressions of ClC-1 channels. In fibres of both animal strains, optical data confirm the age-dependent increase in gCl, and additionally suggest that ClC-1 channels are evenly distributed between the sarcolemma and transverse tubular system membranes. Although gCl is significantly depressed in fibres of young HSA(LR) mice, the resting membrane resistance (Rm) at -90 mV is only slightly larger than in control mice due to upregulation of a Rb-sensitive resting conductance (gK,IR). In adult animals, differences in Rm are negligible between fibres of both strains, and the contributions of gCl and gK,IR are less altered in HSA(LR) animals. We surmise that while hyperexcitability in young HSA(LR) mice can be readily explained on the basis of reduced gCl, myotonia in adult HSA(LR) animals may be explained on the basis of a mosaic expression of ClC-1 channels in different fibres and/or on alterations of other conductances.

RNA interference targeting CUG repeats in a mouse model of myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is an RNA dominant disease caused by expression of DM protein kinase (DMPK) transcripts that contain an expanded CUG repeat (CUG(exp)). The toxic mRNA localizes to nuclear foci and sequesters proteins involved in the regulation of alternative splicing, such as, muscleblind-like 1 (MBNL1). Here, we used synthetic short interfering RNAs (siRNAs) to target CUG repeats and test the concept that inhibiting the expression of CUG(exp) RNA can mitigate features of DM1 in transgenic mice. Intramuscular injection and electroporation of siRNA resulted in ~70-80% downregulation of CUG(exp) transcripts. A limited survey of endogenous mouse transcripts that contain nonexpanded CUG or CAG repeats showed that most were not affected, though Txlnb containing (CUG)(9) was significantly reduced. By this strategy, the number and intensity of CUG(exp) nuclear foci were reduced and splicing of MBNL1-dependent exons was improved. These data suggest that the expanded CUG repeats are a potential target for allele-selective RNA interference.

Relationship between membrane Cl- conductance and contractile endurance in isolated rat muscles

Resting skeletal muscle fibres have a large membrane Cl(-) conductance (G(Cl)) that dampens their excitability. Recently, however, muscle activity was shown to induce PKC-mediated reduction in G(Cl) in rat muscles of 40-90%. To examine the physiological significance of this PKC-mediated G(Cl) reduction for the function of muscles, this study explored effects of G(Cl) reductions on contractile endurance in isolated rat muscles. Contractile endurance was assessed from the ability of muscle to maintain force during prolonged stimulation under conditions when G(Cl) was manipulated by: (i) inhibition of PKC, (ii) reduction of solution Cl(-) or (iii) inhibition of ClC-1 Cl(-) channels using 9-anthracene-carboxylic acid (9-AC). Experiments showed that contractile endurance was optimally preserved by reductions in G(Cl) similar to what occurs in active muscle. Contrastingly, further G(Cl) reductions compromised the endurance. The experiments thus show a biphasic relationship between G(Cl) and contractile endurance in which partial G(Cl) reduction improves endurance while further G(Cl) reduction compromises endurance. Intracellular recordings of trains of action potentials suggest that this biphasic dependency of contractile endurance on G(Cl) reflects that lowering G(Cl) enhances muscle excitability but low G(Cl) also increases the depolarisation of muscle fibres during excitation and reduces their ability to re-accumulate K(+) lost during excitation. If G(Cl) becomes very low, the latter actions dominate causing reduced endurance. It is concluded that the PKC-mediated ClC-1 channel inhibition in active muscle reduces G(Cl) to a level that optimises contractile endurance during intense exercise.

ClC1 chloride channel in myotonic dystrophy type 2 and ClC1 splicing in vitro

Myotonic dystrophy type 2 (DM2) is caused by CCTG-repeat expansions. Occurrence of splicing and mutations in the muscle chloride channel gene CLCN1 have been reported to contribute to the phenotype. To examine the effect of CLCN1 in DM2 in Germany, we determined the frequency of a representative ClC1 mutation, R894X, and its effect on DM2 clinical features. Then, we examined CLCN1 mRNA splice variants in patient muscle functionally expressed the most abundant variant, and determined its subcellular localization. Finally, we established a cellular system for studying mouse clcn1 pre-mRNA splicing and tested effects of expression of (CCUG)₁₈, (CUG)₂₄ and (AAG)₂₄ RNAs. The R894X mutation was present in 7.7% of DM2 families. DM2 R894X-carriers had more myotonia and myalgia than non-carriers. The most abundant CLCN1 splice variant in DM2 (80% of all transcripts) excluded exons 6-7 and lead to a truncated ClC1(236X) protein. Heterologous ClC1(236X) expression did not yield functional channels. Co-expression with ClC1 did not show a dominant negative effect, but a slightly suppressive effect. In C₂C₁₂ cells, the clc1 splice variants generated by (CCUG)₁₈-RNA resembled those in DM2 muscle and differed from those generated by (CUG)₂₄ and (AAG)₂₄. We conclude that ClC1 mutations exert gene dose effects and enhance myotonia and pain in DM2 in Germany. Additionally, the ClC1(236X) splice variant may contribute to myotonia in DM2. Since splice variants depend on the types of repeats expressed in the cellular C₂C₁₂ model, similar cell models of other tissues may be useful for studying repeatdependent pathogenetic mechanisms more easily than in transgenic animals.

Repression of nuclear CELF activity can rescue CELF-regulated alternative splicing defects in skeletal muscle models of myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is caused by the expansion of CUG repeats in the 3’ UTR of DMPK transcripts. DM1 pathogenesis has been attributed in part to alternative splicing dysregulation via elevation of CUG-BP, Elav-like family member 1 (CELF1). Several therapeutic approaches have been tested in cells and mice, but no previous studies had specifically targeted CELF1. Here, we show that repressing CELF activity rescues CELF-dependent alternative splicing in cell culture and transgenic mouse models of DM1. CELF-independent splicing, however, remained dysregulated. These data highlight both the potential and limitations of targeting CELF1 for the treatment of DM1.

Recent advances in developing small molecules targeting RNA

RNAs are underexploited targets for small molecule drugs or chemical probes of function. This may be due, in part, to a fundamental lack of understanding of the types of small molecules that bind RNA specifically and the types of RNA motifs that specifically bind small molecules. In this review, we describe recent advances in the development and design of small molecules that bind to RNA and modulate function that aim to fill this void.

Muscle weakness in myotonic dystrophy associated with misregulated splicing and altered gating of Ca(V)1.1 calcium channel

Myotonic dystrophy type 1 and type 2 (DM1 and DM2) are genetic diseases in which mutant transcripts containing expanded CUG or CCUG repeats cause cellular dysfunction by altering the processing or metabolism of specific mRNAs and miRNAs. The toxic effects of mutant RNA are mediated partly through effects on proteins that regulate alternative splicing. Here we show that alternative splicing of exon 29 (E29) of Ca(V)1.1, a calcium channel that controls skeletal muscle excitation-contraction coupling, is markedly repressed in DM1 and DM2. The extent of E29 skipping correlated with severity of weakness in tibialis anterior muscle of DM1 patients. Two splicing factors previously implicated in DM1, MBNL1 and CUGBP1, participated in the regulation of E29 splicing. In muscle fibers of wild-type mice, the Ca(V)1.1 channel conductance and voltage sensitivity were increased by splice-shifting oligonucleotides that induce E29 skipping. In contrast to human DM1, expression of CUG-expanded RNA caused only a modest increase in E29 skipping in mice. However, forced skipping of E29 in these mice, to levels approaching those observed in human DM1, aggravated the muscle pathology as evidenced by increased central nucleation. Together, these results indicate that DM-associated splicing defects alter Ca(V)1.1 function, with potential for exacerbation of myopathy.

Triplet repeat RNA structure and its role as pathogenic agent and therapeutic target

This review presents detailed information about the structure of triplet repeat RNA and addresses the simple sequence repeats of normal and expanded lengths in the context of the physiological and pathogenic roles played in human cells. First, we discuss the occurrence and frequency of various trinucleotide repeats in transcripts and classify them according to the propensity to form RNA structures of different architectures and stabilities. We show that repeats capable of forming hairpin structures are overrepresented in exons, which implies that they may have important functions. We further describe long triplet repeat RNA as a pathogenic agent by presenting human neurological diseases caused by triplet repeat expansions in which mutant RNA gains a toxic function. Prominent examples of these diseases include myotonic dystrophy type 1 and fragile X-associated tremor ataxia syndrome, which are triggered by mutant CUG and CGG repeats, respectively. In addition, we discuss RNA-mediated pathogenesis in polyglutamine disorders such as Huntington's disease and spinocerebellar ataxia type 3, in which expanded CAG repeats may act as an auxiliary toxic agent. Finally, triplet repeat RNA is presented as a therapeutic target. We describe various concepts and approaches aimed at the selective inhibition of mutant transcript activity in experimental therapies developed for repeat-associated diseases.

Molecular Effects of the CTG Repeats in Mutant Dystrophia Myotonica Protein Kinase Gene

Myotonic Dystrophy type 1 (DM1) is a multi-system disorder characterized by muscle wasting, myotonia, cardiac conduction defects, cataracts, and neuropsychological dysfunction. DM1 is caused by expansion of a CTG repeat in the 3´untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene. A body of work demonstrates that DMPK mRNAs containing abnormally expanded CUG repeats are toxic to several cell types. A core mechanism underlying symptoms of DM1 is that mutant DMPK RNA interferes with the developmentally regulated alternative splicing of defined pre-mRNAs. Expanded CUG repeats fold into ds(CUG) hairpins that sequester nuclear proteins including human Muscleblind-like (MBNL) and hnRNP H alternative splicing factors. DM1 cells activate CELF family member CUG-BP1 protein through hyperphosphorylation and stabilization in the cell nucleus. CUG-BP1 and MBNL1 proteins act antagonistically in exon selection in several pre-mRNA transcripts, thus MBNL1 sequestration and increase in nuclear activity of CUG-BP1 both act synergistically to missplice defined transcripts. Mutant DMPK-mediated effect on subcellular localization, and defective phosphorylation of cytoplasmic CUG-BP1, have additionally been linked to defective translation of p21 and MEF2A in DM1, possibly explaining delayed differentiation of DM1 muscle cells. Mutant DMPK transcripts bind and sequester transcription factors such as Specificity protein 1 leading to reduced transcription of selected genes. Recently, transcripts containing long hairpin structures of CUG repeats have been shown to be a Dicer ribonuclease target and Dicer-induced downregulation of the mutant DMPK transcripts triggers silencing effects on RNAs containing long complementary repeats. In summary, mutant DMPK transcripts alter gene transcription, alternative splicing, and translation of specific gene transcripts, and have the ability to trigger gene-specific silencing effects in DM1 cells. Therapies aimed at reversing these gene expression alterations should prove effective ways to treat DM1.