[Show abstract][Hide abstract] ABSTRACT: Disease processes and trauma affecting nerve-evoked muscle activity, motorneurons, synapses and myofibers cause different levels of muscle weakness, i.e. reduced maximal force production in response to voluntary activation or nerve stimulation. However, the mechanisms of muscle weakness are not well known. Using murine models of amyotrophic lateral sclerosis (SOD1G93A transgenic mice), congenital myasthenic syndrome (AChE knockout mice and MuskV789M/- mutant mice), Schwartz-Jampel syndrome (Hspg2C1532YNEO/C1532YNEO mutant mice) and traumatic nerve injury (Neurotomized wild-type mice), we show that the reduced maximal activation capacity (the ability of the nerve to maximally activate the muscle) explains 52%, 58% and 100% of severe weakness in respectively SOD1G93A, Neurotomized and Musk mice, whereas muscle atrophy only explains 37%, 27% and 0%. We also demonstrate that the impaired maximal activation capacity observed in SOD1, Neurotomized, and Musk mice is not highly related to Hdac4 gene upregulation. Moreover, in SOD1 and Neurotomized mice our results suggest LC3, Fn14, Bcl3 and Gadd45a as candidate genes involved in the maintenance of the severe atrophic state. In conclusion, our study indicates that muscle weakness can result from the triggering of different signaling pathways. This knowledge may be helpful in designing therapeutic strategies and finding new drug targets for amyotrophic lateral sclerosis, congenital myasthenic syndrome, Schwartz-Jampel syndrome and nerve injury.
[Show abstract][Hide abstract] ABSTRACT: Myostatin (Mstn) participates in the regulation of skeletal muscle size and emerges as a reg-ulator of muscle metabolism. We here hypothesized that lack of myostatin profoundly de-presses oxidative phosphorylation dependent muscle function. For this extent, we explored Mstn-/- mice as a model for the constitutive absence of myostatin and AAV-mediated over-expression of myostatin propeptide as a model of myostatin blockade in adult wildtype mice. We show that muscles from Mstn-/- mice, although larger and stronger, fatigue ex-tremely rapidly. Myostatin deficiency shifts muscle from aerobic towards anaerobic energy metabolism as evidenced by decreased mitochondrial respiration, reduced expression of PPAR transcriptional regulators, increased enolase activity, and exercise induced lactic acido-sis. In consequence, constitutively reduced myostatin signaling diminishes exercise capacity, while the hypermuscular state of Mstn-/- mice increases oxygen consumption and the energy cost of running. We wondered whether these results are the mere consequence of the con-genital fiber-type switch towards a glycolytic phenotype of constitutive Mstn-/- mice. Hence we over-expressed myostatin propeptide in adult mice, which did not affect fiber-type dis-tribution, while nonetheless causing increased muscle fatigability, diminished exercise ca-pacity and decreased Pparb/d and Pgc1a expression. In conclusion, our results suggest that myostatin endows skeletal muscle with high oxidative capacity and low fatigability, thus reg-ulating the delicate balance between muscle mass, muscle force, energy metabolism and endurance capacity.
[Show abstract][Hide abstract] ABSTRACT: Myostatin regulates skeletal muscle size via the activin receptor IIB (ActRIIB). However, its effect on muscle energy metabolism and energy dependent muscle function remains largely unexplored. This question needs to be solved urgently since various therapies for neuromuscular diseases based on blockade of ActRIIB signaling are being developed. Here we show in mice that four months of pharmacological abrogation of ActRIIB signaling by treatment with soluble ActRIIB-Fc triggers extreme muscle fatigability. This is associated with elevated serum lactate levels and a severe metabolic myopathy in the mdx mouse, an animal model of Duchenne muscular dystrophy. Blockade of ActRIIB signaling down-regulates Porin, a crucial ADP/ATP shuttle between cytosol and mitochondrial matrix leading to a consecutive deficiency of oxidative phosphorylation as measured by in vivo Phophorus Magnetic Resonance Spectroscopy ((31)P-MRS). Further, ActRIIB blockade reduces muscle capillarization, which further compounds the metabolic stress. We show that ActRIIB regulates key determinants of muscle metabolism, such as Pparß, Pgc1a, and Pdk4 thereby optimizing different components of muscle energy metabolism. In conclusion, ActRIIB signaling endows skeletal muscle with high oxidative capacity and low fatigability. The severe metabolic side effects following ActRIIB blockade caution against deploying this strategy, at least in isolation, for treatment of neuromuscular disorders.Molecular Therapy (2014); doi:10.1038/mt.2014.90.
[Show abstract][Hide abstract] ABSTRACT: As androgens might have rapid androgen-receptor (AR) independent action on muscle cells, we analyzed the in vivo acute effect of androgens on maximal force generation capacity and electrically evoked calcium transient responsible for the excitation-contraction coupling in skeletal muscle from wild-type male mice and muscle fibre androgen receptor (AR) deficient (AR(skm-/y)) male mice. We tested the hypothesis that acute in vivo androgen treatment improves contractility and modifies calcium transient in mouse hindlimb muscles. In addition, we determined whether the reduced maximal force generation capacity of AR(skm-/y) mice is caused by an alteration in calcium transient. We found that acute dehydrotestosterone (DHT) and testosterone treatment of mice does not change in situ maximal force, power or fatigue resistance of tibialis anterior muscles. In agreement with this observation, maximal force and twitch kinetics also remained unchanged when both whole extensor digitorum longus (EDL) muscle or fibre bundles were incubated in vitro with DHT. Electrically evoked calcium transient, i.e. calcium amplitude, time to peak and decay, was also not modified by DHT treatment of EDL muscle fibre bundles. Finally, we found no difference in calcium transient between AR(skm-/y) and wild-type mice despite the reduced maximal force in EDL fibre bundles of AR(skm-/y) mice. In conclusion, acute androgen treatment has no ergogenic effect on muscle contractility and does not affect calcium transient in response to stimulation. In addition, the reduced maximal force of AR(skm-/y) mice is not related to calcium transient dysfunction.
[Show abstract][Hide abstract] ABSTRACT: Loss-of-function mutations in the myotubularin gene (MTM1) cause X-linked myotubular myopathy (XLMTM), a fatal, congenital pediatric disease that affects the entire skeletal musculature. Systemic administration of a single dose of a recombinant serotype 8 adeno-associated virus (AAV8) vector expressing murine myotubularin to Mtm1-deficient knockout mice at the onset or at late stages of the disease resulted in robust improvement in motor activity and contractile force, corrected muscle pathology, and prolonged survival throughout a 6-month study. Similarly, single-dose intravascular delivery of a canine AAV8-MTM1 vector in XLMTM dogs markedly improved severe muscle weakness and respiratory impairment, and prolonged life span to more than 1 year in the absence of toxicity or a humoral or cell-mediated immune response. These results demonstrate the therapeutic efficacy of AAV-mediated gene therapy for myotubular myopathy in small- and large-animal models, and provide proof of concept for future clinical trials in XLMTM patients.
Science translational medicine 01/2014; 6(220):220ra10. · 14.41 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Myotonic dystrophy is the most common adult muscle dystrophy. In view of emerging therapies, which use animal models as a proof of principle, the development of reliable outcome measures for in vivo longitudinal study of mouse skeletal muscle function is becoming crucial. To satisfy this need, we have developed a device to measure ankle dorsi- and plantarflexion torque in rodents. We present an in vivo 8-month longitudinal study of the contractile properties of the skeletal muscles of the DMSXL mouse model of myotonic dystrophy type 1. Between 4 and 12months of age, we observed a reduction in muscle strength in the ankle dorsi- and plantarflexors of DMSXL compared to control mice although the strength per muscle cross-section was normal. Mild steady myotonia but no abnormal muscle fatigue was also observed in the DMSXL mice. Magnetic resonance imaging and histological analysis performed at the end of the study showed respectively reduced muscle cross-section area and smaller muscle fibre diameter in DMSXL mice. In conclusion, our study demonstrates the feasibility of carrying out longitudinal in vivo studies of muscle function over several months in a mouse model of myotonic dystrophy confirming the feasibility of this method to test preclinical therapeutics.
[Show abstract][Hide abstract] ABSTRACT: Manipulation of the mouse genome by site-specific mutagenesis has been extensively used to study gene function and model human disorders. Mouse models of myotubular myopathy (XLMTM), a severe congenital muscular disorder due to loss-of-function mutations in the MTM1 gene, have been generated by homologous recombination and shown that myotubularin is essential for skeletal muscle. However, since the Mtm1 deletion occurred constitutively or shortly after birth in these mice, it is not known whether myotubularin is required during adulthood, an important issue not only in the context of muscle biology but also therapies. To delete the Mtm1 gene in adult muscle fibers, we constructed a recombinant adeno-associated vector (AAV) that expresses the Cre recombinase under the muscle-specific desmin promoter. We report that a single injection of this vector into muscles of 3 month-old Mtm1 conditional mice leads to a myotubular myopathy phenotype with myofiber atrophy, disorganization of organelle positioning, such as mitochondria and nuclei, T-tubule defects and severe muscle weakness. In addition, our results show that MTM1-related atrophy and dysfunction correlates with abnormalities in satellite cell number and markers of autophagy, protein synthesis and neuromuscular junction transmission. The expression level of atrogenes was also analyzed. Therefore, we provide a valuable tissue model that recapitulates the main features of the disease, and it is useful to study pathogenesis and evaluate therapeutic strategies. We establish the proof-of-concept that myotubularin is required for the proper function of skeletal muscle during adulthood, suggesting that therapies will be required for the entire life of XLMTM patients.
Human Molecular Genetics 02/2013; · 6.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Myotonic dystrophy type 1 (DM1) is caused by an unstable CTG repeat expansion in the 3'UTR of the DM protein kinase (DMPK) gene. DMPK transcripts carrying CUG expansions form nuclear foci and affect splicing regulation of various RNA transcripts. Furthermore, bidirectional transcription over the DMPK gene and non-conventional RNA translation of repeated transcripts have been described in DM1. It is clear now that this disease may involve multiple pathogenic pathways including changes in gene expression, RNA stability and splicing regulation, protein translation, and micro-RNA metabolism. We previously generated transgenic mice with 45-kb of the DM1 locus and >300 CTG repeats (DM300 mice). After successive breeding and a high level of CTG repeat instability, we obtained transgenic mice carrying >1,000 CTG (DMSXL mice). Here we described for the first time the expression pattern of the DMPK sense transcripts in DMSXL and human tissues. Interestingly, we also demonstrate that DMPK antisense transcripts are expressed in various DMSXL and human tissues, and that both sense and antisense transcripts accumulate in independent nuclear foci that do not co-localize together. Molecular features of DM1-associated RNA toxicity in DMSXL mice (such as foci accumulation and mild missplicing), were associated with high mortality, growth retardation, and muscle defects (abnormal histopathology, reduced muscle strength, and lower motor performances). We have found that lower levels of IGFBP-3 may contribute to DMSXL growth retardation, while increased proteasome activity may affect muscle function. These data demonstrate that the human DM1 locus carrying very large expansions induced a variety of molecular and physiological defects in transgenic mice, reflecting DM1 to a certain extent. As a result, DMSXL mice provide an animal tool to decipher various aspects of the disease mechanisms. In addition, these mice can be used to test the preclinical impact of systemic therapeutic strategies on molecular and physiological phenotypes.
[Show abstract][Hide abstract] ABSTRACT: MTM1 gene mutations cause X-linked myotubular myopathy (XLMTM), a fatal congenital myopathy characterized by small myofibers with frequent central nuclei. XLMTM patients typically present with severe hypotonia, muscle weakness and respiratory failure. Previous intramuscular (IM) injection studies in Mtm1 KO mice demonstrated potential efficacy of gene therapy to treat the disease. We extended these results to a large animal model by testing IM delivery of an AAV8-canine MTM1 vector into the cranial tibialis muscle of dogs with XLMTM. We observed dramatic improvement in strength of treated limb muscles to levels approaching that of age-matched normal littermates. This response was detectable at 1 week post-injection and improvement continued until a terminal measurement at 6 weeks post-injection. Concomitantly, the AAV-injected XLMTM muscles showed substantial increase in mass and myofiber size, and decreases in pathological features. We also utilized the Mtm1 KO mouse to determine the response to systemic intravenous delivery of an AAV serotype 9 vector expressing myotubularin under the muscle-specific desmin promoter. Myotubularin was rapidly and persistently expressed in muscles scattered throughout the body and this translated into robust improvement of skeletal muscle pathology and contractile force, and normalized motor activity of treated mice. Importantly, the lifespans of treated mice, which normally survive ∼7–8 weeks, were prolonged to >6 months. We then evaluated the effects of a targeted protein-replacement agent (3E10Fv-MTM1) injected into the tibialis anterior muscle of Mtm1 KO mice. Injection of 3E10Fv-MTM1 over 2 weeks increased contractile function (p < 0.05) and improved ultrastructural organization. These promising findings suggest that gene and protein-replacement strategies that produce even low doses of myotubularin can significantly attenuate the pathophysiology and muscle weakness that characterize XLMTM.
[Show abstract][Hide abstract] ABSTRACT: Blockade of myostatin/ActRIIB signaling has been proposed as a therapeutic strategy for Duchenne muscular dystrophy (DMD) because of its supposed stimulating effect on skeletal muscle size and function. Here we show that constitutive lack of myostatin in Mstn−/− mice resulted in increased muscle fatigability, decreased exercise performance, exercise-induced abnormal lactic acidemia and decreased mitochondrial respiration. However, Mstn−/− mice develop a fiber type disproportion towards a glycolytic muscle phenotype with less mitochondria and decreased capillary density, and these developmental abnormalities may account for the observed changes in muscle function and energy metabolism. To determine the role of myostatin/ActRIIB signaling on adult muscle physiology and metabolism, we treated adult wild type mice and mdx mice for 4 months with soluble Activin IIB receptor (sActRIIB-Fc) in order to block signaling by myostatin and homologous proteins. In wild type mice, sAcRIIB-Fc strongly increased muscle size and tetanic force, whereas fatigue resistance decreased. Mice exhausted precociously during treadmill exercise and this was associated with grossly elevated serum lactate levels. Treatment of mdx mice with sActRIIB-Fc, however, resulted in largely decreased specific force. Mdx mice developed extreme exercise intolerance and exercise induced lactate acidosis. Mitochondrial enzyme activities and fiber type distribution remained unchanged, thus a primary mitochondrial deficit was unlikely the cause of the exercise failure. Importantly, capillary density of muscle strongly decreased following treatment with sActRIIB-Fc and this was associated with marked dystrophic changes. Our results suggest that myostatin optimizes muscle metabolism by adapting muscle vascularization to oxygen requirements. Blockade of myostatin/ActRIIB signaling causes a capillary-fiber mismatch especially in de- and regenerating dystrophic muscle, provoking a secondary metabolic myopathy.
[Show abstract][Hide abstract] ABSTRACT: Duchenne Muscular Dystrophy (DMD) is characterized by the lack of dystrophin that leads to severe myofiber degeneration. We have shown that endomysial fibrosis is correlated with age at ambulation loss in DMD patients. However, the dystrophin-deficient mdx mouse does not have fibrotic lesions in adult limb muscles. Here, we describe a model of chronic mechanical muscle injury that triggers chronic lesions in mdx hindlimb muscle.
Micromechanical injuries were performed daily in tibialis anterior muscles for 2 weeks.
Endomysial fibrosis appeared beginning 1 week post-injury, remained stable for 3 months and was associated with loss of specific maximal force. Fibrosis was associated with an increased expression of factors involved in fibrogenesis including α-smooth muscle actin, connective tissue growth factor, and lysyl oxidase, which colocalized with collagen deposits.
This induced fibrotic dystrophic model may be useful to study mechanisms of fibrosis in dystrophinopathies and to evaluate antifibrotic treatments.
[Show abstract][Hide abstract] ABSTRACT: Dystrophin contributes to force transmission and has a protein-scaffolding role for a variety of signaling complexes in skeletal muscle. In the present study, we tested the hypothesis that the muscle adaptive response following mechanical overloading (ML) would be decreased in MDX dystrophic muscle lacking dystrophin. We found that the gains in muscle maximal force production and fatigue resistance in response to ML were both reduced in MDX mice as compared to healthy mice. MDX muscle also exhibited decreased cellular and molecular muscle remodeling (hypertrophy and promotion of slower/oxidative fiber type) in response to ML, and altered intracellular signalings involved in muscle growth and maintenance (mTOR, myostatin, follistatin, AMPKα1, REDD1, atrogin-1, Bnip3). Moreover, dystrophin rescue via exon skipping restored the adaptive response to ML. Therefore our results demonstrate that the adaptive response in response to ML is impaired in dystrophic MDX muscle, most likely because of the dystrophin crucial role.
PLoS ONE 04/2012; 7(4):e35346. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effects of locomotor training (LT) on skeletal muscle after peripheral nerve injury and acetylcholinesterase deficiency are not well documented.
We determined the effects of LT on mouse soleus muscle performance after sciatic nerve transection with excision (full and permanent denervation), nerve transection (partial functional reinnervation), nerve crush (full denervation with full functional reinnervation), and acetylcholinesterase deficiency (alteration in neuromuscular junction functioning).
We found no significant effect of LT on the recovery of soleus muscle weight, maximal force in response to muscle stimulation, and fatigue resistance after nerve transection with or without excision. However, LT significantly increased soleus muscle fatigue resistance after nerve crush and acetylcholinesterase deficiency. Moreover, hindlimb immobilization significantly aggravated the deficit in soleus muscle maximal force production and atrophy after nerve crush.
LT is beneficial, and reduced muscle use is detrimental for intrinsic muscle performance in the context of disturbed nerve-muscle communication.
[Show abstract][Hide abstract] ABSTRACT: Selenium is an essential trace element and selenoprotein N (SelN) was the first selenium-containing protein shown to be directly involved in human inherited diseases. Mutations in the SEPN1 gene, encoding SelN, cause a group of muscular disorders characterized by predominant affection of axial muscles. SelN has been shown to participate in calcium and redox homeostasis, but its pathophysiological role in skeletal muscle remains largely unknown. To address SelN function in vivo, we generated a Sepn1-null mouse model by gene targeting. The Sepn1(-/-) mice had normal growth and lifespan, and were macroscopically indistinguishable from wild-type littermates. Only minor defects were observed in muscle morphology and contractile properties in SelN-deficient mice in basal conditions. However, when subjected to challenging physical exercise and stress conditions (forced swimming test), Sepn1(-/-) mice developed an obvious phenotype, characterized by limited motility and body rigidity during the swimming session, as well as a progressive curvature of the spine and predominant alteration of paravertebral muscles. This induced phenotype recapitulates the distribution of muscle involvement in patients with SEPN1-Related Myopathy, hence positioning this new animal model as a valuable tool to dissect the role of SelN in muscle function and to characterize the pathophysiological process.
PLoS ONE 08/2011; 6(8):e23094. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Myotonic dystrophy is the most common muscular dystrophy in adults and the first recognized example of an RNA-mediated disease. Congenital myotonic dystrophy (CDM1) and myotonic dystrophy of type 1 (DM1) or of type 2 (DM2) are caused by the expression of mutant RNAs containing expanded CUG or CCUG repeats, respectively. These mutant RNAs sequester the splicing regulator Muscleblind-like-1 (MBNL1), resulting in specific misregulation of the alternative splicing of other pre-mRNAs. We found that alternative splicing of the bridging integrator-1 (BIN1) pre-mRNA is altered in skeletal muscle samples of people with CDM1, DM1 and DM2. BIN1 is involved in tubular invaginations of membranes and is required for the biogenesis of muscle T tubules, which are specialized skeletal muscle membrane structures essential for excitation-contraction coupling. Mutations in the BIN1 gene cause centronuclear myopathy, which shares some histopathological features with myotonic dystrophy. We found that MBNL1 binds the BIN1 pre-mRNA and regulates its alternative splicing. BIN1 missplicing results in expression of an inactive form of BIN1 lacking phosphatidylinositol 5-phosphate-binding and membrane-tubulating activities. Consistent with a defect of BIN1, muscle T tubules are altered in people with myotonic dystrophy, and membrane structures are restored upon expression of the normal splicing form of BIN1 in muscle cells of such individuals. Finally, reproducing BIN1 splicing alteration in mice is sufficient to promote T tubule alterations and muscle weakness, a predominant feature of myotonic dystrophy.
Nature medicine 06/2011; 17(6):720-5. · 28.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Autosomal dominant centronuclear myopathy (AD-CNM) is due to mutations in the gene encoding dynamin 2 (DNM2) involved in endocytosis and intracellular membrane trafficking. To understand the pathomechanisms resulting from a DNM2 mutation, we generated a knock-in mouse model expressing the most frequent AD-CNM mutation (KI-Dnm2(R465W)). Heterozygous (HTZ) mice developed a myopathy showing a specific spatial and temporal muscle involvement. In the primarily and prominently affected tibialis anterior muscle, impairment of the contractile properties was evidenced at weaning and was progressively associated with atrophy and histopathological abnormalities mainly affecting mitochondria and reticular network. Expression of genes involved in ubiquitin-proteosome and autophagy pathways was up-regulated during DNM2-induced atrophy. In isolated muscle fibers from wild-type and HTZ mice, Dnm2 localized in regions of intense membrane trafficking (I-band and perinuclear region), emphasizing the pathophysiological hypothesis in which DNM2-dependent trafficking would be altered. In addition, HTZ fibers showed an increased calcium concentration as well as an intracellular Dnm2 and dysferlin accumulation. A similar dysferlin retention, never reported so far in congenital myopathies, was also demonstrated in biopsies from DNM2-CNM patients and can be considered as a new marker to orientate direct genetic testing. Homozygous (HMZ) mice died during the first hours of life. Impairment of clathrin-mediated endocytosis, demonstrated in HMZ embryonic fibroblasts, could be the cause of lethality. Overall, this first mouse model of DNM2-related myopathy shows the crucial role of DNM2 in muscle homeostasis and will be a precious tool to study DNM2 functions in muscle, pathomechanisms of DNM2-CNM and developing therapeutic strategies.
Human Molecular Genetics 12/2010; 19(24):4820-36. · 6.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Excessive muscle growth of myostatin deficient mice is accompa-nied by a fibre type conversion towards fast glycolytic fibres. In the present study, we determined the energetic and functional conse-quences of such altered muscle phenotype. Mitochondrial respira-tion rates, measured on skinned fibres from mstnÀ/À mice, showed a 40% decrease in complexes II and IV activities compared to controls, whereas the activity of complex I remained unchanged. Non-invasive muscle energetic measurements (31P MR spectros-copy), using a rest-electrical stimulation-recovery protocol, revealed a larger energy consumption per specific workload in mstnÀ/À ani-mals, whereas the initial rates of [PCr] consumption and [PCr] resyn-thesis were normal. In addition, the end-of-stimulation intracellular pH was more acidotic in mstnÀ/À animals. Contractility measure-ments revealed higher maximal tetanic forces of mstnÀ/À muscle compared to wildtype (WT) muscle. However, mstnÀ/À muscle fati-gued much faster than WT muscle and force subsided to WT levels following 3 min of repetitive tetanic stimulations. This increased muscle fatigability was further confirmed by a 25% shorter run-to-exhaustion time following treadmill exercise, and this earlier exhaustion was associated with a pronounced lactic acidosis. Inter-estingly, the maximal rate of oxygen consumption (VO 2max) was increased by 15% in mstnÀ/À mice but this increase was dispropor-tionately low considering the more than twofold greater muscle mass. Moreover, both groups reached VO 2max at similar running velocities indicating that the corresponding energetic cost of running was larger in mstnÀ/À animals. MstnÀ/À mice displayed also a strongly reduced spontaneous motor activity in addition to the exer-cise induced fatigability. In conclusion, lack of myostatin compromises mitochondrial activity on one side and increases total energy cost on the other side which renders hypertrophied mstnÀ/À muscle more fatigable as well as energetically less efficient.