X-linked myotubular myopathy (XLMTM) due to MTM1 mutations is a rare and often lethal congenital myopathy. Its downstream molecular and cellular mechanisms are currently incompletely understood. The most abundant protein in muscle, myosin, has been implicated in the pathophysiology of other congenital myopathies. Hence, in the present study, we aimed to define whether myosin is also dysfunctional in XLMTM and whether it, thus, may constitute a potential drug target. To this end, we used skeletal muscle tissue from patients and canine/mouse models; we performed Mant-ATP chase experiments coupled with x-ray diffraction analyses and LC/MS-based proteomics studies. In patients with XLMTM, we found that myosin molecules are structurally disordered and preferably adopt their ATP-consuming biochemical state. This phosphorylation-related (mal)adaptation was mirrored by a striking remodeling of the myofiber energetic proteome in XLMTM dogs. In line with these, we confirmed an accrued myosin ATP consumption in mice lacking MTM1. Hence, we treated these with a myosin ATPase inhibitor, mavacamten. After a 4-week treatment period, we observed a partial restoration of the myofiber proteome, especially proteins involved in cytoskeletal, sarcomeric, and energetic pathways. Altogether, our study highlights myosin inhibition as a potentially new drug mechanism for the complex XLMTM muscle phenotype.

Centronuclear myopathies (CNM) are rare inherited muscle disorders characterized by muscle atrophy, weakness, and altered muscle fiber structure, primarily due to mutations in genes like MTM1 , DNM2 , and BIN1 . The pathomechanisms implicated in CNM are only partially understood, and no curative therapies are available for patients. This study exploits a unique multi-omics dataset and network-based analyses to elucidate the molecular pathways involved in CNM. First, we performed Weighted Gene Co-expression Network Analysis (WGCNA) to identify gene modules correlated with CNM phenotypes. We find that modules correlated to a positive muscular phenotype were enriched for genes involved in muscle contraction, RNA processes and oxidative phosphorylation, while modules associated with impaired muscle structure and function were enriched in immune response, innervation, vascularization processes, and fatty acid oxidation. Next, we integrated transcriptomic, proteomic, and metabolomic data from the Mtm1 -/y mouse model with public knowledge bases using a multilayer network approach and explored the network using a random walk with restart approach. This study revealed novel metabolites that could be targeted through dietary supplementation as potential therapeutic strategies. Our findings demonstrate how multi-omics network analyses can reveal new aspects of CNM pathomechanisms and identify avenues for intervention. ### Competing Interest Statement The authors have declared no competing interest.

Centronuclear myopathies (CNMs) are rare congenital muscle disorders with no effective treatment. Previous studies showed that tamoxifen improved muscle function in mice modeling CNMs caused by variants in MTM1, BIN1 and DNM2. Here, we investigated whether tamoxifen administration improves muscle function and pathology in the severe recessive Ryr1TM/indel mouse model of RYR1-related CNM. Contractile performance, histological analyses and protein levels were assessed in Ryr1TM/indel mice and control littermates (wild type) treated with either a tamoxifen-enriched diet (65 mg/kg of food) or a control diet for 5 weeks, beginning at 3 weeks of age. Ryr1TM/indel mice displayed muscle weakness, reduced myofiber size and a high number of fibers with nuclei in abnormal position, regardless of the treatment. Force production during repeated contractions was reduced in tamoxifen-treated Ryr1TM/indel mice compared to that in untreated Ryr1TM/indel mice. The levels of CNM proteins (DNM2 and BIN1) were unchanged following the treatment. Tamoxifen did not improve muscle dysfunction, atrophy or histological hallmarks in Ryr1TM/indel mice. Our data indicate that tamoxifen supplementation is not beneficial and may negatively impact muscle function in this model of CNM, suggesting limited therapeutic value for patients with RYR1 mutations.

Centronuclear and myotubular myopathies (CNMs) are rare, inherited muscle disorders characterized by muscle atrophy, weakness, and altered muscle fiber structure, primarily caused by mutations in MTM1, DNM2, or BIN1. The molecular mechanisms driving CNM are only partially understood, and no curative therapies are available. To elucidate molecular pathways involved in CNMs, we present an integrative multi-omics analysis across several CNM mouse models untreated or treated with pre-clinical strategies, combining transcriptomic, proteomic, and metabolomic datasets with curated interaction, metabolic, tissue, and phenotype knowledge using network-based approaches. Weighted Gene Co-expression Network Analysis (WGCNA) identified gene modules commonly altered in three CNM genetic forms. Modules correlated with improved muscle function were enriched for processes such as muscle contraction, RNA metabolism, and oxidative phosphorylation, whereas modules linked to disease severity were enriched for immune response, innervation, vascularization, and fatty acid oxidation. We further integrated transcriptomic, proteomic, and metabolomic data from the Mtm1-/y mouse model with public knowledge bases into a multilayer network, and explored it using a random walk with restart approach. These analyses highlighted metabolites closely connected to CNM phenotypes, some of which may represent candidates for nutritional or pharmacological modulation. Our findings illustrate how integrative multi-omics and network analyses reveal both pathogenic and protective pathways in CNM and provide a foundation for identifying novel therapeutic opportunities.

Les myopathies centronucléaires sont des maladies congénitales rares, caractérisées par une faiblesse musculaire et un positionnement anormal des organites. Les principaux gènes associés à ces pathologies sont MTM1 (codant la myotubularine 1), DNM2 (codant la dynamine 2), BIN1 (codant le bridging integrator 1) et RYR1 (codant le récepteur à la ryanodine 1). À ce jour, aucun traitement n’est approuvé. Cependant, le tamoxifène, une molécule déjà utilisée en clinique dans le cancer du sein, a montré des effets positifs sur le phénotype musculaire de modèles murins qui reproduisent trois des formes principales de myopathies centronucléaires. Dans cet article, les effets du tamoxifène sont comparés pour les différentes formes de ces pathologies.

X-linked myotubular myopathy (XL-MTM) is a rare and often lethal congenital myopathy, clinically characterized by muscle weakness. Its underlying molecular mechanisms remain incompletely understood. As myosin has been implicated in the pathophysiology of other forms of congenital myopathies, in the present study, we aimed to define whether muscle myosin is dysfunctional in the context of XL-MTM. For that, we used muscle tissue from human patients and a canine model. We isolated individual muscle fibres from these two species and performed loaded Mant-ATP chase experiments. Our preliminary results indicate a shift of myosin metabolic/biochemical states towards highly energy-consuming conformations in human and canine XL-MTM. Subsequently, to reverse this alteration potentially contributing to muscle fatigue, we investigated the effects of an ATP-conserving drug targeting myosin, mavacamten, using a well-defined mouse model of XL-MTM lacking myotubularin. The results are currently being analysed and include histology as well as untargeted global proteomics. Potentially, taken together, these data will lead to a better understanding of XL-MTM and the potency of myosin as a drug target.

Men taking antioxidant vitamin E supplements have increased prostate cancer (PC) risk. However, whether pro-oxidants protect from PC remained unclear. In this work, we show that a pro-oxidant vitamin K precursor [menadione sodium bisulfite (MSB)] suppresses PC progression in mice, killing cells through an oxidative cell death: MSB antagonizes the essential class III phosphatidylinositol (PI) 3-kinase VPS34—the regulator of endosome identity and sorting—through oxidation of key cysteines, pointing to a redox checkpoint in sorting. Testing MSB in a myotubular myopathy model that is driven by loss of MTM1 —the phosphatase antagonist of VPS34—we show that dietary MSB improved muscle histology and function and extended life span. These findings enhance our understanding of pro-oxidant selectivity and show how definition of the pathways they impinge on can give rise to unexpected therapeutic opportunities.

Les myopathies centronucleaires sont des maladies congenitales rares, caracterisees par une faiblesse musculaire et un positionnement anormal des organites. Les principaux genes associes a ces pathologies sont MTM1 (codant la myotubularine 1), DNM2 (codant la dynamine 2), BIN1 (codant le bridging integrator 1) et RYR1 (codant le recepteur a la ryanodine 1). ce jour, aucun traitement n'est approuve. Cependant, le tamoxifene, une molecule deja utilisee en clinique dans le cancer du sein, a montre des effets positifs sur le phenotype musculaire de modeles murins qui reproduisent trois des formes principales de myopathies centronucleaires. Dans cet article, les effets du tamoxifene sont compares pour les differentes formes de ces pathologies. Centronuclear myopathies are rare congenital disorders characterized by muscle weakness and mislocalization of organelles. The main genes associated to these muscle diseases are MTM1, DNM2, BIN1 and RYR1. To date, no therapy is available. Nevertheless, tamoxifen, a pharmacological compound already used in clinics for breast cancer, showed beneficial effects on the muscle phenotypes in mouse models for centronuclear myopathies. Here, the effects of tamoxifen on muscle phenotypes will be compared in the various forms of this muscle disease.

INTRODUCTION AND AIMS:Mitochondrial myopathies are rare genetic disorders for which no effective treatment exists. We previously showed that the pharmacological cyclophilin inhibitor cyclosporine A (CsA) extends the lifespan of fast-twitch skeletal muscle-specific mitochondrial transcription factor A knockout (Tfam KO) mice, lacking the ability to transcribe mitochondrial DNA and displaying lethal mitochondrial myopathy. Our present aim was to assess whether the positive effect of CsA was associated with improved in vivo mitochondrial energy production. METHODS:Mice were treated with CsA for 4 weeks, beginning at 12 weeks (i.e., before the terminal disease phase). Hindlimb plantar flexor muscles were fatigued by 80 contractions (40 Hz, 1.5 s on, 6 s off) while measuring force and energy metabolism using phosphorus-31 magnetic resonance spectroscopy. RESULTS:Force decreased at similar rates in Tfam KO mice with and without the CsA treatment, reaching 50% of the baseline value after ~14 ± 1 contractions, which was faster than in control mice (25 ± 1 contractions). Phosphocreatine (PCr) decreased to ~10% of the control concentration in Tfam KO mice, independent of the treatment, which was larger than the ~20% observed in control mice. The time constant of PCr recovery was higher in untreated Tfam KO than that in control muscle (+100%) and similar in untreated and CsA-treated Tfam KO mice. DISCUSSION:The results do not support improved mitochondrial energy production as a mechanism underlying the prolonged lifespan of Tfam KO mitochondrial myopathy mice treated with CsA. Thus, other mechanisms must be involved, such as the previously observed CsA-mediated protection against excessive mitochondrial Ca2+ accumulation.

Nemaline myopathies are the most common form of congenital myopathies. Variants in ACTA1 (NEM3) comprise 15-25% of all nemaline myopathy cases. Patients harboring variants in ACTA1 present with a heterogeneous disease course characterized by stable or progressive muscle weakness and, in severe cases, respiratory failure and death. To date, no specific treatments are available. Since NEM3 is an actin-based thin filament disease, we tested the ability of tirasemtiv, a fast skeletal muscle troponin activator, to improve skeletal muscle function in a mouse model of NEM3, harboring the patient-based p.Asp286Gly variant in Acta1. Acute and long-term tirasemtiv treatment significantly increased muscle contractile capacity at submaximal stimulation frequencies in both fast-twitch extensor digitorum longus and gastrocnemius muscle, and intermediate-twitch diaphragm muscle in vitro and in vivo. Additionally, long-term tirasemtiv treatment in NEM3 mice resulted in a decreased respiratory rate with preserved minute volume, suggesting more efficient respiration. Altogether, our data support the therapeutic potential of fast skeletal muscle troponin activators in alleviating skeletal muscle weakness in a mouse model of NEM3 caused by the Acta1:p.Asp286Gly variant.

Congenital myopathies are rare and severe genetic diseases affecting the skeletal muscle function in children and adults. They present a variable spectrum of phenotypes and a genetic heterogeneity. Subgroups are defined according to the clinical and histopathological features and encompass core myopathy, centronuclear myopathy, nemaline myopathy and other rare congenital myopathies. No approved treatment exists to date for any congenital myopathies. To tackle this important unmet need, an increased number of proof-of-concept studies recently assessed the therapeutic potential of various strategies, either pharmacological or genetic-based, aiming at counteracting muscle weakness or/and cure the pathology. Here, we list the implicated genes and cellular pathways, and review the therapeutic approaches preclinically tested and the ongoing/completed clinical trials for the different types of congenital myopathies.

Congenital myopathies define a genetically heterogeneous group of disorders associated with severe muscle weakness, for which no therapies are currently available. Here we investigated the repurposing of tamoxifen in mouse models of mild or severe forms of centronuclear myopathies due to mutations in BIN1 (encoding amphiphysin 2) or DNM2 (encoding dynamin 2), respectively. Exposure to a tamoxifen-enriched diet from 3 weeks of age resulted in significant improvement in muscle contractility without increase in fibre size in both models, underlying an increase in the capacity of the muscle fibres to produce more force. In addition, the histological alterations were fully rescued in the BIN1-centronuclear myopathies mouse model. To assess the mechanism of the rescue, transcriptome analyses and targeted protein studies were performed. Although tamoxifen is known to modulate the transcriptional activity of the oestrogen receptors, correction of the disease transcriptomic signature was marginal on tamoxifen treatment. Conversely, tamoxifen lowered the abnormal increase in dynamin 2 protein level in both centronuclear myopathies models. Of note, it was previously reported that dynamin 2 increase is a main pathological cause of centronuclear myopathies. The Akt/mTOR muscle hypertrophic pathway and protein markers of the ubiquitin-proteasome system (the E3 ubiquitin ligase cullin 3) and autophagy (p62) were increased in both models of centronuclear myopathies. Normalization of dynamin 2 level mainly correlated with the normalization of cullin 3 protein level on tamoxifen treatment, supporting the idea that the ubiquitin-proteasome system is a main target for the tamoxifen effect in the amelioration of these diseases. Overall, our data suggest that tamoxifen antagonizes disease development probably through dynamin 2 level regulation. In conclusion, the beneficial effect of tamoxifen on muscle function supports the suggestion that tamoxifen may serve as a common therapy for several autosomal forms of centronuclear myopathies. Gineste et al. show that tamoxifen improves skeletal muscle function and structure in mouse models of BIN1- and DNM2-related centronuclear myopathies, likely through a reduction in dynamin 2 levels. The findings suggest potential for tamoxifen to be repurposed for the treatment of autosomal forms of centronuclear myopathies.

Cells rapidly lose their physiological phenotype upon disruption of their extracellular matrix (ECM)-intracellular cytoskeleton interactions. By comparing adult mouse skeletal muscle fibers, isolated either by mechanical dissection or by collagenase-induced ECM digestion, we investigated acute effects of ECM disruption on cellular and mitochondrial morphology, transcriptomic signatures, and Ca2+ handling. RNA-sequencing showed striking differences in gene expression patterns between the two isolation methods with enzymatically dissociated fibers resembling myopathic phenotypes. Mitochondrial appearance was grossly similar in the two groups, but 3D electron microscopy revealed shorter and less branched mitochondria following enzymatic dissociation. Repeated contractions resulted in a prolonged mitochondrial Ca2+ accumulation in enzymatically dissociated fibers, which was partially prevented by cyclophilin inhibitors. Of importance, muscle fibers of mice with severe mitochondrial myopathy show pathognomonic mitochondrial Ca2+ accumulation during repeated contractions and this accumulation was concealed with enzymatic dissociation, making this an ambiguous method in studies of native intracellular Ca2+ fluxes.

AbstractMitochondrial myopathies are rare genetic disorders characterized by muscle weakness and exercise intolerance. Currently, no effective treatment exists for these myopathies. Interestingly, the pharmacological cyclophilin inhibitor cyclosporine A (CsA) extended lifespan and prevented loss of force and mitochondrial Ca2+overload in muscle fibers in the skeletal muscle-specificTfamknockout mouse model of lethal mitochondrial myopathy (TfamKO). The unaffected expression of proteins involved in mitochondrial energy metabolism suggests that these improvements occurred without improvement in metabolism. In this study, we aimed at investigating the effects of four weeks of CsA administration onin vivocontractile function and mitochondrial energy production inTfamKO mice. The treatment started before the terminal phase with severe muscle weakness and weight loss. Our results show that CsA treatment delayed progression into the terminal disease phase. This occurred without any obvious positive effects on mitochondrial energy production at rest or during fatigue induced by repeated contractions. In conclusion, cyclophilin inhibitors may have the potential of counteracting devastating muscle weakness in patients with mitochondrial myopathies most probably by preventing deleterious effects triggered by excessive mitochondrial Ca2+uptake rather than by improving mitochondrial energy production.

Nemaline myopathy, a disease of the actin-based thin filament, is one of the most frequent congenital myopathies. To date, no specific therapy is available to treat muscle weakness in nemaline myopathy. We tested the ability of tirasemtiv, a fast skeletal troponin activator that targets the thin filament, to augment muscle force-both in vivo and in vitro-in a nemaline myopathy mouse model with a mutation (H40Y) in Acta1. In Acta1(H40Y) mice, treatment with tirasemtiv increased the force response of muscles to submaximal stimulation frequencies. This resulted in a reduced energetic cost of force generation, which increases the force production during a fatigue protocol. The inotropic effects of tirasemtiv were present in locomotor muscles and, albeit to a lesser extent, in respiratory muscles, and they persisted during chronic treatment, an important finding as respiratory failure is the main cause of death in patients with congenital myopathy. Finally, translational studies on permeabilized muscle fibers isolated from a biopsy of a patient with the ACTA1(H40Y) mutation revealed that at physiological Ca2+ concentrations, tirasemtiv increased force generation to values that were close to those generated in muscle fibers of healthy subjects. These findings indicate the therapeutic potential of fast skeletal muscle troponin activators to improve muscle function in nemaline myopathy due to the ACTA1(H40Y) mutation, and future studies should assess their merit for other forms of nemaline myopathy and for other congenital myopathies.

Cells rapidly lose their physiological phenotype upon isolation from their native microenvironment. Here, we investigated the role of the extracellular matrix (ECM) for mitochondrial morphology and Ca2+ handling in adult mouse skeletal muscle fibres. Adult skeletal muscle fibres were isolated from mouse toe muscle either by collagenase-induced dissociation of the ECM or by mechanical dissection that leaves the proximate ECM intact. Experiments were generally performed four hours after cell isolation. At this time, the expression of genes encoding for structural proteins was lower in enzymatically dissociated than in mechanically dissected fibres. Mitochondrial appearance was grossly similar in the two groups, but 3D electron microscopy revealed shorter and less branched mitochondria in enzymatically dissociated than in mechanically dissected fibres. The increase in free cytosolic [Ca2+] during repeated tetanic stimulation was similar in the two groups of fibres, but this was accompanied by an excessive mitochondrial Ca2+ uptake only in enzymatically dissociated muscle fibres. The aberrant mitochondrial Ca2+ uptake was partially prevented by the mitochondrial Ca2+ uniporter inhibitor Ru360 and by cyclosporine A and NV556, which inhibit the mitochondrial matrix protein PPIF (also called cyclophilin D). Importantly, inhibition of PPIF with NV556 significantly improved survival of mice with mitochondrial myopathy in which muscle mitochondria take up excessive amounts of Ca2+ also with intact ECM. In conclusion, skeletal muscle fibres isolated by collagenase-induced dissociation of the ECM display aberrant mitochondrial Ca2+ uptake, which involves a PPIF-dependent mitochondrial Ca2+ influx resembling that observed in mitochondrial myopathies.

Mitochondrial diseases are genetic disorders that lead to impaired mitochondrial function, resulting in exercise intolerance and muscle weakness. In patients, muscle fatigue due to defects in mitochondrial oxidative capacities commonly precedes muscle weakness. In mice, deletion of the fast-twitch skeletal muscle-specific Tfam gene (Tfam KO) leads to a deficit in respiratory chain activity, severe muscle weakness and early death. Here, we performed a time-course study of mitochondrial and muscular dysfunctions in 11-and 14-week-old Tfam KO mice, i.e. before and when mice are about to enter the terminal stage, respectively. Although force in the unfatigued state was reduced in Tfam KO mice compared to control littermates (wild type) only at 14 weeks, during repeated submaximal contractions fatigue was faster at both ages. During fatiguing stimulation, total phosphocreatine breakdown was larger in Tfam KO muscle than in wild-type muscle at both ages, whereas phosphocreatine consumption was faster only at 14 weeks. In conclusion, the Tfam KO mouse model represents a reliable model of lethal mitochondrial myopathy in which impaired mitochondrial energy production and premature fatigue occur before muscle weakness and early death.

Summary Cells rapidly lose their physiological phenotype upon disruption of their extracellular matrix (ECM)-intracellular cytoskeleton interactions. Here, we investigated acute effects of ECM disruption on cellular and mitochondrial morphology, transcriptomic signatures, and Ca 2+ handling in adult mouse skeletal muscle fibers. Adult skeletal muscle fibers were isolated from mouse toe muscle either by collagenase-induced dissociation of the ECM or by mechanical dissection that leaves the contiguous ECM intact. Experiments were generally performed four hours after cell isolation. At this time, there were striking differences in the gene expression patterns between fibers isolated with the two methods; 24h after cell isolation, enzymatically dissociated fibers had transcriptomic signatures resembling dystrophic phenotypes. Mitochondrial appearance was grossly similar in the two groups, but 3D electron microscopy revealed shorter and less branched mitochondria in enzymatically dissociated than in mechanically dissected fibers. Similar increases in free cytosolic [Ca 2+ ] during repeated tetanic stimulation were accompanied by marked mitochondrial Ca 2+ uptake only in enzymatically dissociated muscle fibers. The aberrant mitochondrial Ca 2+ uptake was partially prevented by the mitochondrial Ca 2+ uniporter inhibitor Ru360 and by cyclosporine A and NV556, which inhibit the mitochondrial protein Ppif (also called cyclophilin D). Importantly, inhibition of Ppif with NV556 significantly improved survival of mice with mitochondrial myopathy in which muscle mitochondria take up excessive amounts of Ca 2+ even with an intact ECM. In conclusion, skeletal muscle fibers isolated by collagenase-induced dissociation of the ECM display aberrant mitochondrial Ca 2+ uptake, which involves a Ppif-dependent mitochondrial Ca 2+ influx resembling that observed in mitochondrial myopathies.

INTRODUCTION:The conditional nebulin knockout mouse is a new model mimicking nemaline myopathy, a rare disease characterized by muscle weakness and rods within muscle fibers. We investigated the impact of nebulin (NEB) deficiency on muscle function in vivo. METHODS:Conditional nebulin knockout mice and control littermates were studied at 10 to 12 months. Muscle function (force and fatigue) and anatomy (muscles volume and fat content) were measured in vivo. Myosin heavy chain (MHC) composition and nebulin (NEB) protein expression were assessed by protein electrophoresis. RESULTS:Conditional nebulin knockout mice displayed a lower NEB level (-90%) leading to a 40% and 45% reduction in specific maximal force production and muscles volume, respectively. Nebulin deficiency was also associated with higher resistance to fatigue and increased MHC I content. DISCUSSION:Adult nebulin-deficient mice displayed severe muscle atrophy and weakness in vivo related to a low NEB content but an improved fatigue resistance due to a slower contractile phenotype.

L-tyrosine supplementation may provide benefit to nemaline myopathy (NM) patients, however previous studies are inconclusive, with no elevation of L-tyrosine levels in blood or tissue reported. We evaluated the ability of L-tyrosine treatments to improve skeletal muscle function in all three published animal models of NM caused by dominant skeletal muscle α-actin ( ACTA1 ) mutations. Highest safe L-tyrosine concentrations were determined for dosing water and feed of wildtype zebrafish and mice respectively. NM Tg ACTA1 D286G - eGFP zebrafish treated with 10 μM L-tyrosine from 24 hours to 6 days post fertilization displayed no improvement in swimming distance. NM Tg ACTA1 D286G mice consuming 2% L-tyrosine supplemented feed from preconception had significant elevations in free L-tyrosine levels in sera (57%) and quadriceps muscle (45%) when examined at 6–7 weeks old. However indicators of skeletal muscle integrity (voluntary exercise, bodyweight, rotarod performance) were not improved. Additionally no benefit on the mechanical properties, energy metabolism, or atrophy of skeletal muscles of 6–7 month old Tg ACTA1 D286G and KI Acta1 H40Y mice eventuated from consuming a 2% L-tyrosine supplemented diet for 4 weeks. Therefore this study yields important information on aspects of the clinical utility of L-tyrosine for ACTA1 NM.