Defects in mitochondrial ATP synthesis in dystrophin-deficient mdx skeletal muscles may be caused by complex I insufficiency

Improved mitochondrial function in the hearts of sarcolipin-deficient dystrophin and utrophin double-knockout mice

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease associated with cardiomyopathy. DMD cardiomyopathy is characterized by abnormal intracellular Ca2+ homeostasis and mitochondrial dysfunction. We used dystrophin and utrophin double-knockout (mdx:utrn-/-) mice in a sarcolipin (SLN) heterozygous-knockout (sln+/-) background to examine the effect of SLN reduction on mitochondrial function in the dystrophic myocardium. Germline reduction of SLN expression in mdx:utrn-/- mice improved cardiac sarco/endoplasmic reticulum (SR) Ca2+ cycling, reduced cardiac fibrosis, and improved cardiac function. At the cellular level, reducing SLN expression prevented mitochondrial Ca2+ overload, reduced mitochondrial membrane potential loss, and improved mitochondrial function. Transmission electron microscopy of myocardial tissues and proteomic analysis of mitochondria-associated membranes showed that reducing SLN expression improved mitochondrial structure and SR-mitochondria interactions in dystrophic cardiomyocytes. These findings indicate that SLN upregulation plays a substantial role in the pathogenesis of cardiomyopathy and that reducing SLN expression has clinical implications in the treatment of DMD cardiomyopathy.

The slow-release adiponectin analog ALY688-SR modifies early-stage disease development in the D2.mdx mouse model of Duchenne muscular dystrophy

Fibrosis is associated with respiratory and limb muscle atrophy in Duchenne muscular dystrophy (DMD). Current standard of care partially delays the progression of this myopathy but there remains an unmet need to develop additional therapies. Adiponectin receptor agonism has emerged as a possible therapeutic target to lower inflammation and improve metabolism in mdx mouse models of DMD but the degree to which fibrosis and atrophy are prevented remain unknown. Here, we demonstrate that the recently developed slow-release peptidomimetic adiponectin analog, ALY688-SR, remodels the diaphragm of murine model of DMD on DBA background (D2.mdx) mice treated from days 7-28 of age during early stages of disease. ALY688-SR also lowered interleukin-6 (IL-6) mRNA but increased IL-6 and transforming growth factor-β1 (TGF-β1) protein contents in diaphragm, suggesting dynamic inflammatory remodeling. ALY688-SR alleviated mitochondrial redox stress by decreasing complex I-stimulated H2O2 emission. Treatment also attenuated fibrosis, fiber type-specific atrophy, and in vitro diaphragm force production in diaphragm suggesting a complex relationship between adiponectin receptor activity, muscle remodeling, and force-generating properties during the very early stages of disease progression in murine model of DMD on DBA background (D2.mdx) mice. In tibialis anterior, the modest fibrosis at this young age was not altered by treatment, and atrophy was not apparent at this young age. These results demonstrate that short-term treatment of ALY688-SR in young D2.mdx mice partially prevents fibrosis and fiber type-specific atrophy and lowers force production in the more disease-apparent diaphragm in relation to lower mitochondrial redox stress and heterogeneous responses in certain inflammatory markers. These diverse muscle responses to adiponectin receptor agonism in early stages of DMD serve as a foundation for further mechanistic investigations.NEW & NOTEWORTHY There are limited therapies for the treatment of Duchenne muscular dystrophy. As fibrosis involves an accumulation of collagen that replaces muscle fibers, antifibrotics may help preserve muscle function. We report that the novel adiponectin receptor agonist ALY688-SR prevents fibrosis in the diaphragm of D2.mdx mice with short-term treatment early in disease progression. These responses were related to altered inflammation and mitochondrial functions and serve as a foundation for the development of this class of therapy.

Mitochondria and Reactive Oxygen Species: The Therapeutic Balance of Powers for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a genetic progressive muscle-wasting disorder that leads to rapid loss of mobility and premature death. The absence of functional dystrophin in DMD patients reduces sarcolemma stiffness and increases contraction damage, triggering a cascade of events leading to muscle cell degeneration, chronic inflammation, and deposition of fibrotic and adipose tissue. Efforts in the last decade have led to the clinical approval of novel drugs for DMD that aim to restore dystrophin function. However, combination therapies able to restore dystrophin expression and target the myriad of cellular events found impaired in dystrophic muscle are desirable. Muscles are higher energy consumers susceptible to mitochondrial defects. Mitochondria generate a significant source of reactive oxygen species (ROS), and they are, in turn, sensitive to proper redox balance. In both DMD patients and animal models there is compelling evidence that mitochondrial impairments have a key role in the failure of energy homeostasis. Here, we highlighted the main aspects of mitochondrial dysfunction and oxidative stress in DMD and discussed the recent findings linked to mitochondria/ROS-targeted molecules as a therapeutic approach. In this respect, dual targeting of both mitochondria and redox homeostasis emerges as a potential clinical option in DMD.

MicroRNAs as a Tool for Differential Diagnosis of Neuromuscular Disorders

Neuromuscular disorders (NMD) are a class of progressive disorders that are characterized by wasting of the muscles. Some of the disorders like Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), congenital muscular dystrophies (CMDs), limb-girdle muscular dystrophies (LGMD), and mild spinal muscular atrophy (SMA) type III share several presenting clinical features, and hence, diagnosis is usually a challenging task. In this study, the diagnostic potential of some species of microRNAs (miRNAs) that are known to play roles in normal and pathological contexts of myocytes (myomiRs) were evaluated to assess their potential in differential diagnosis of NMDs. In this study, seventy-four patients with different neuromuscular disorders along with thirty age-matched healthy control subjects were enrolled. Peripheral blood samples were collected from enrolled subjects followed by miRNA extraction and reverse transcription followed by quantification of the circulating levels of the studied miRNAs (miR-499, miR-206, miR-208a, miR-223, miR-191, miR-103a-3p, miR-103a-5p), by real-time PCR and statistical analysis. The data indicated that miR-499 level showed high circulating levels in DMD patients as well as in patients with other related disorders such as BMD. However, the levels of miR-499 were much higher in DMD patients and it can be used to diagnose DMD. In addition, miR-206 can selectively differentiate between DMD and all other disorders. The results also revealed that miR-208a and miR-223 were significantly dysregulated in SMA patients, and miR-103a-3p could distinguish DMD from BMD. The expression levels of some miRNA species can be utilized in the process of differential diagnosis of NMDs and can serve as a diagnostic biomarker, and such findings will pave the way towards generating targeted therapies.

Dimethyl fumarate modulates the dystrophic disease program following short-term treatment

New medicines are urgently required to treat the fatal neuromuscular disease Duchenne muscular dystrophy (DMD). Dimethyl fumarate (DMF) is a potent immunomodulatory small molecule nuclear erythroid 2-related factor 2 activator with current clinical utility in the treatment of multiple sclerosis and psoriasis that could be effective for DMD and rapidly translatable. Here, we tested 2 weeks of daily 100 mg/kg DMF versus 5 mg/kg standard-care prednisone (PRED) treatment in juvenile mdx mice with early symptomatic DMD. Both drugs modulated seed genes driving the DMD disease program and improved force production in fast-twitch muscle. However, only DMF showed pro-mitochondrial effects, protected contracting muscles from fatigue, improved histopathology, and augmented clinically compatible muscle function tests. DMF may be a more selective modulator of the DMD disease program than PRED, warranting follow-up longitudinal studies to evaluate disease-modifying impact.

Adenylosuccinic Acid: An Orphan Drug with Untapped Potential

Adenylosuccinic acid (ASA) is an orphan drug that was once investigated for clinical application in Duchenne muscular dystrophy (DMD). Endogenous ASA participates in purine recycling and energy homeostasis but might also be crucial for averting inflammation and other forms of cellular stress during intense energy demand and maintaining tissue biomass and glucose disposal. This article documents the known biological functions of ASA and explores its potential application for the treatment of neuromuscular and other chronic diseases.

Modeling Duchenne Muscular Dystrophy Cardiomyopathy with Patients' Induced Pluripotent Stem-Cell-Derived Cardiomyocytes

Duchenne muscular dystrophy (DMD) is an X-linked progressive muscle degenerative disease caused by mutations in the dystrophin gene, resulting in death by the end of the third decade of life at the latest. A key aspect of the DMD clinical phenotype is dilated cardiomyopathy, affecting virtually all patients by the end of the second decade of life. Furthermore, despite respiratory complications still being the leading cause of death, with advancements in medical care in recent years, cardiac involvement has become an increasing cause of mortality. Over the years, extensive research has been conducted using different DMD animal models, including the mdx mouse. While these models present certain important similarities to human DMD patients, they also have some differences which pose a challenge to researchers. The development of somatic cell reprograming technology has enabled generation of human induced pluripotent stem cells (hiPSCs) which can be differentiated into different cell types. This technology provides a potentially endless pool of human cells for research. Furthermore, hiPSCs can be generated from patients, thus providing patient-specific cells and enabling research tailored to different mutations. DMD cardiac involvement has been shown in animal models to include changes in gene expression of different proteins, abnormal cellular Ca²⁺ handling, and other aberrations. To gain a better understanding of the disease mechanisms, it is imperative to validate these findings in human cells. Furthermore, with the recent advancements in gene-editing technology, hiPSCs provide a valuable platform for research and development of new therapies including the possibility of regenerative medicine. In this article, we review the DMD cardiac-related research performed so far using human hiPSCs-derived cardiomyocytes (hiPSC-CMs) carrying DMD mutations.

Nuclear Factor Erythroid 2-Related Factor 2 and Its Targets in Skeletal Muscle Repair and Regeneration

Significance: Skeletal muscles have a robust regenerative capacity in response to acute and chronic injuries. Muscle repair and redox homeostasis are intimately linked; increased generation of reactive oxygen species leads to cellular dysfunction and contributes to muscle wasting and progression of muscle diseases. In exemplary muscle disease, Duchenne muscular dystrophy (DMD), caused by mutations in the DMD gene that encodes the muscle structural protein dystrophin, the regeneration machinery is severely compromised, while oxidative stress contributes to the progression of the disease. The nuclear factor erythroid 2-related factor 2 (NRF2) and its target genes, including heme oxygenase-1 (HO-1), provide protective mechanisms against oxidative insults. Recent Advances: Relevant advances have been evolving in recent years in understanding the mechanisms by which NRF2 regulates processes that contribute to effective muscle regeneration. To this end, pathways related to muscle satellite cell differentiation, oxidative stress, mitochondrial metabolism, inflammation, fibrosis, and angiogenesis have been studied. The regulatory role of NRF2 in skeletal muscle ferroptosis has been also suggested. Animal studies have shown that NRF2 pathway activation can stop or reverse skeletal muscle pathology, especially when endogenous stress defence mechanisms are imbalanced. Critical Issues: Despite the growing recognition of NRF2 as a factor that regulates various aspects of muscle regeneration, the mechanistic impact on muscle pathology in various models of muscle injury remains imprecise. Future Directions: Further studies are necessary to fully uncover the role of NRF2 in muscle regeneration, both in physiological and pathological conditions, and to investigate the possibilities for development of new therapeutic modalities. Antioxid. Redox Signal. 38, 619-642.

Effect of Large-Conductance Calcium-Dependent K+ Channel Activator NS1619 on Function of Mitochondria in the Heart of Dystrophin-Deficient Mice

Dystrophin-deficient muscular dystrophy (Duchenne dystrophy) is characterized by impaired ion homeostasis, in which mitochondria play an important role. In the present work, using a model of dystrophin-deficient mdx mice, we revealed decrease in the efficiency of potassium ion transport and total content of this ion in the heart mitochondria. We evaluated the effect of chronic administration of the benzimidazole derivative NS1619, which is an activator of the large-conductance Ca2+-dependent K+ channel (mitoBK_Ca), on the structure and function of organelles and the state of the heart muscle. It was shown that NS1619 improves K+ transport and increases content of the ion in the heart mitochondria of mdx mice, but this is not associated with the changes in the level of mitoBK_Ca protein and expression of the gene encoding this protein. The effect of NS1619 was accompanied by the decrease in the intensity of oxidative stress, assessed by the level of lipid peroxidation products (MDA products), and normalization of the mitochondrial ultrastructure in the heart of mdx mice. In addition, we found positive changes in the tissue manifested by the decrease in the level of fibrosis in the heart of dystrophin-deficient animals treated with NS1619. It was noted that NS1619 had no significant effect on the structure and function of heart mitochondria in the wild-type animals. The paper discusses mechanisms of influence of NS1619 on the function of mouse heart mitochondria in Duchenne muscular dystrophy and prospects for applying this approach to correct pathology.

Ion Channels of the Sarcolemma and Intracellular Organelles in Duchenne Muscular Dystrophy: A Role in the Dysregulation of Ion Homeostasis and a Possible Target for Therapy

Duchenne muscular dystrophy (DMD) is caused by the absence of the dystrophin protein and a properly functioning dystrophin-associated protein complex (DAPC) in muscle cells. DAPC components act as molecular scaffolds coordinating the assembly of various signaling molecules including ion channels. DMD shows a significant change in the functioning of the ion channels of the sarcolemma and intracellular organelles and, above all, the sarcoplasmic reticulum and mitochondria regulating ion homeostasis, which is necessary for the correct excitation and relaxation of muscles. This review is devoted to the analysis of current data on changes in the structure, functioning, and regulation of the activity of ion channels in striated muscles in DMD and their contribution to the disruption of muscle function and the development of pathology. We note the prospects of therapy based on targeting the channels of the sarcolemma and organelles for the correction and alleviation of pathology, and the problems that arise in the interpretation of data obtained on model dystrophin-deficient objects.

A deep redox proteome profiling workflow and its application to skeletal muscle of a Duchenne Muscular Dystrophy model

Perturbation to the redox state accompanies many diseases and its effects are viewed through oxidation of biomolecules, including proteins, lipids, and nucleic acids. The thiol groups of protein cysteine residues undergo an array of redox post-translational modifications (PTMs) that are important for regulation of protein and pathway function. To better understand what proteins are redox regulated following a perturbation, it is important to be able to comprehensively profile protein thiol oxidation at the proteome level. Herein, we report a deep redox proteome profiling workflow and demonstrate its application in measuring the changes in thiol oxidation along with global protein expression in skeletal muscle from mdx mice, a model of Duchenne Muscular Dystrophy (DMD). In-depth coverage of the thiol proteome was achieved with >18,000 Cys sites from 5,608 proteins in muscle being quantified. Compared to the control group, mdx mice exhibit markedly increased thiol oxidation, where a ∼2% shift in the median oxidation occupancy was observed. Pathway analysis for the redox data revealed that coagulation system and immune-related pathways were among the most susceptible to increased thiol oxidation in mdx mice, whereas protein abundance changes were more enriched in pathways associated with bioenergetics. This study illustrates the importance of deep redox profiling in gaining greater insight into oxidative stress regulation and pathways/processes that are perturbed in an oxidizing environment.

BKCa Activator NS1619 Improves the Structure and Function of Skeletal Muscle Mitochondria in Duchenne Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive hereditary disease caused by the absence of the dystrophin protein. This is secondarily accompanied by a dysregulation of ion homeostasis, in which mitochondria play an important role. In the present work, we show that mitochondrial dysfunction in the skeletal muscles of dystrophin-deficient mdx mice is accompanied by a reduction in K+ transport and a decrease in its content in the matrix. This is associated with a decrease in the expression of the mitochondrial large-conductance calcium-activated potassium channel (mitoBKCa) in the muscles of mdx mice, which play an important role in cytoprotection. We observed that the BKCa activator NS1619 caused a normalization of mitoBKCa expression and potassium homeostasis in the muscle mitochondria of these animals, which was accompanied by an increase in the calcium retention capacity, mitigation of oxidative stress, and improvement in mitochondrial ultrastructure. This effect of NS1619 contributed to the reduction of degeneration/regeneration cycles and fibrosis in the skeletal muscles of mdx mice as well as a normalization of sarcomere size, but had no effect on the leakage of muscle enzymes and muscle strength loss. In the case of wild-type mice, we noted the negative effect of NS1619 manifested in the inhibition of the functional activity of mitochondria and disruption of their structure, which, however, did not significantly affect the state of the skeletal muscles of the animals. This article discusses the role of mitoBKCa in the development of DMD and the prospects of the approach associated with the correction of its function in treatments of this secondary channelopathy.

Mitochondrial Dysfunction as an Underlying Cause of Skeletal Muscle Disorders

Mitochondria are an important energy source in skeletal muscle. A main function of mitochondria is the generation of ATP for energy through oxidative phosphorylation (OXPHOS). Mitochondrial defects or abnormalities can lead to muscle disease or multisystem disease. Mitochondrial dysfunction can be caused by defective mitochondrial OXPHOS, mtDNA mutations, Ca2+ imbalances, mitochondrial-related proteins, mitochondrial chaperone proteins, and ultrastructural defects. In addition, an imbalance between mitochondrial fusion and fission, lysosomal dysfunction due to insufficient biosynthesis, and/or defects in mitophagy can result in mitochondrial damage. In this review, we explore the association between impaired mitochondrial function and skeletal muscle disorders. Furthermore, we emphasize the need for more research to determine the specific clinical benefits of mitochondrial therapy in the treatment of skeletal muscle disorders.

The Effect of Uridine on the State of Skeletal Muscles and the Functioning of Mitochondria in Duchenne Dystrophy

Duchenne muscular dystrophy is caused by the loss of functional dystrophin that secondarily causes systemic metabolic impairment in skeletal muscles and cardiomyocytes. The nutraceutical approach is considered as a possible complementary therapy for this pathology. In this work, we have studied the effect of pyrimidine nucleoside uridine (30 mg/kg/day for 28 days, i.p.), which plays an important role in cellular metabolism, on the development of DMD in the skeletal muscles of dystrophin deficient mdx mice, as well as its effect on the mitochondrial dysfunction that accompanies this pathology. We found that chronic uridine administration reduced fibrosis in the skeletal muscles of mdx mice, but it had no effect on the intensity of degeneration/regeneration cycles and inflammation, pseudohypetrophy, and muscle strength of the animals. Analysis of TEM micrographs showed that uridine also had no effect on the impaired mitochondrial ultrastructure of mdx mouse skeletal muscle. The administration of uridine was found to lead to an increase in the expression of the Drp1 and Parkin genes, which may indicate an increase in the intensity of organelle fission and the normalization of mitophagy. Uridine had little effect on OXPHOS dysfunction in mdx mouse mitochondria, and moreover, it was suppressed in the mitochondria of wild type animals. At the same time, uridine restored the transport of potassium ions and reduced the production of reactive oxygen species; however, this had no effect on the impaired calcium retention capacity of mdx mouse mitochondria. The obtained results demonstrate that the used dose of uridine only partially prevents mitochondrial dysfunction in skeletal muscles during Duchenne dystrophy, though it mitigates the development of destructive processes in skeletal muscles.

Bioenergetic and Metabolic Impairments in Induced Pluripotent Stem Cell-Derived Cardiomyocytes Generated from Duchenne Muscular Dystrophy Patients

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene and dilated cardiomyopathy (DCM) is a major cause of morbidity and mortality in DMD patients. We tested the hypothesis that DCM is caused by metabolic impairments by employing induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) generated from four DMD patients; an adult male, an adult female, a 7-year-old (7y) male and a 13-year-old (13y) male, all compared to two healthy volunteers. To test the hypothesis, we measured the bioenergetics, metabolomics, electrophysiology, mitochondrial morphology and mitochondrial activity of CMs, using respirometry, LC–MS, patch clamp, electron microscopy (EM) and confocal microscopy methods. We found that: (1) adult DMD CMs exhibited impaired energy metabolism and abnormal mitochondrial structure and function. (2) The 7y CMs demonstrated arrhythmia-free spontaneous firing along with “healthy-like” metabolic status, normal mitochondrial morphology and activity. In contrast, the 13y CMs were mildly arrhythmogenic and showed adult DMD-like bioenergetics deficiencies. (3) In DMD adult CMs, mitochondrial activities were attenuated by 45–48%, whereas the 7y CM activity was similar to that of healthy CMs. (4) In DMD CMs, but not in 7y CMs, there was a 75% decrease in the mitochondrial ATP production rate compared to healthy iPSC-CMs. In summary, DMD iPSC-CMs exhibit bioenergetic and metabolic impairments that are associated with rhythm disturbances corresponding to the patient’s phenotype, thereby constituting novel targets for alleviating cardiomyopathy in DMD patients.

Mitochondrial Stress Responses in Duchenne muscular dystrophy: Metabolic Dysfunction or Adaptive Reprogramming?

Mitochondrial stress may be a secondary contributor to muscle weakness in inherited muscular dystrophies. Duchenne muscular dystrophy has received the majority of attention whereby most discoveries suggest mitochondrial ATP synthesis may be reduced. However, not all studies support this finding. Furthermore, some studies have reported increased mitochondrial reactive oxygen species and propensity for permeability transition pore formation as an inducer of apoptosis, although divergent findings have also been described. A closer examination of the literature suggests the degree and direction of mitochondrial stress responses may depend on the progression of the disease, the muscle type examined, the mouse model employed with regards to pre-clinical research, the precise metabolic pathways in consideration, and in some cases the in vitro technique used to assess a given mitochondrial bioenergetic function. One intent of this review is to provide careful considerations for future experimental designs to resolve the heterogeneous nature of mitochondrial stress during the progression of Duchenne muscular dystrophy. Such considerations have implications for other muscular dystrophies as well which are addressed briefly herein. A renewed perspective of the term 'mitochondrial dysfunction' is presented whereby stress responses might be re-explored in future investigations as direct contributors to myopathy vs an adaptive 'reprogramming' intended to maintain homeostasis in the face of disease stressors themselves. In so doing, the prospective development of mitochondrial enhancement therapies can be driven by advances in perspectives as much as experimental approaches when resolving the precise relationships between mitochondrial remodelling and muscle weakness in Duchenne and, indeed, other muscular dystrophies.

Bidirectional roles of skeletal muscle fibro-adipogenic progenitors in homeostasis and disease

Sarcopenia and myopathies cause progressive muscle weakness and degeneration, which are closely associated with fat infiltration and fibrosis in muscle. Recently, experimental research has shed light on fibro-adipogenic progenitors (FAPs), also known as muscle-resident mesenchymal progenitors with multiple differentiation potential for adipogenesis, fibrosis, osteogenesis and chondrogenesis. They are considered key regulators of muscle homeostasis and integrity. They play supportive roles in muscle development and repair by orchestrating the regulatory interplay between muscle stem cells (MuSCs) and immune cells. Interestingly, FAPs also contribute to intramuscular fat infiltration, fibrosis and other pathologies when the functional integrity of the network is compromised. In this review, we summarize recent insights into the roles of FAPs in maintenance of skeletal muscle homeostasis, and discuss the underlying mechanisms regulating FAPs behavior and fate, highlighting their roles in participating in efficient muscle repair and fat infiltrated muscle degeneration as well as during muscle atrophy. We suggest that controlling and predicting FAPs differentiation may become a promising strategy to improve muscle function and prevent irreparable muscle damage.

Preserved Left Ventricular Function despite Myocardial Fibrosis and Myopathy in the Dystrophin-Deficient D2.B10-Dmdmdx/J Mouse

Duchenne muscular dystrophy involves an absence of dystrophin, a cytoskeletal protein which supports cell structural integrity and scaffolding for signalling molecules in myocytes. Affected individuals experience progressive muscle degeneration that leads to irreversible loss of ambulation and respiratory diaphragm function. Although clinical management has greatly advanced, heart failure due to myocardial cell loss and fibrosis remains the major cause of death. We examined cardiac morphology and function in D2.B10-Dmd^mdx/J (D2-mdx) mice, a relatively new mouse model of muscular dystrophy, which we compared to their wild-type background DBA/2J mice (DBA/2). We also tested whether drug treatment with a specific blocker of mitochondrial permeability transition pore opening (Debio-025), or ACE inhibition (Perindopril), had any effect on dystrophy-related cardiomyopathy. D2-mdx mice were treated for six weeks with Vehicle control, Debio-025 (20 mg/kg/day), Perindopril (2 mg/kg/day), or a combination (n = 8/group). At 18 weeks, compared to DBA/2, D2-mdx hearts displayed greater ventricular collagen, lower cell density, greater cell diameter, and greater protein expression levels of IL-6, TLR4, BAX/Bcl2, caspase-3, PGC-1α, and notably monoamine oxidases A and B. Remarkably, these adaptations in D2-mdx mice were associated with preserved resting left ventricular function similar to DBA/2 mice. Compared to vehicle, although Perindopril partly attenuated the increase in heart weight and collagen at 18 weeks, the drug treatments had no marked impact on dystrophic cardiomyopathy.

Effects of muscle damage on 31 phosphorus magnetic resonance spectroscopy indices of energetic status and sarcolemma integrity in young mdx mice

31 Phosphorus magnetic resonance spectroscopy (31 P-MRS) has been shown to detect altered energetic status (e.g. the ratio of inorganic phosphate to phosphocreatine: Pi/PCr), intracellular acid-base status, and free intracellular magnesium ([Mg2+ ]) in dystrophic muscle compared with unaffected muscle; however, the causes of these differences are not well understood. The purposes of this study were to examine 31 P-MRS indices of energetic status and sarcolemma integrity in young mdx mice compared with wild-type and to evaluate the effects of downhill running to induce muscle damage on 31 P-MRS indices in dystrophic muscle. In vivo 31 P-MRS spectra were acquired from the posterior hindlimb muscles in young (4-10 weeks of age) mdx (C57BL/10ScSn-DMDmdx) and wild-type (C57BL/10ScSnJ) mice using an 11.1-T MR system. The flux of phosphate from PCr to ATP was estimated by 31 P-MRS saturation transfer experiments. Relative concentrations of high-energy phosphates were measured, and intracellular pH and [Mg2+ ] were calculated. 1 H2 O-T2 was measured using single-voxel 1 H-MRS from the gastrocnemius and soleus using a 4.7-T MR system. Downhill treadmill running was performed in a subset of mice. Young mdx mice were characterized by elevated 1 H2 O-T2 (p < 0.01), Pi/PCr (p = 0.02), PCr to ATP flux (p = 0.04) and histological inflammatory markers (p < 0.05) and reduced (p < 0.01) [Mg2+ ] compared with wild-type. Furthermore, 24 h after downhill running, an increase (p = 0.02) in Pi/PCr was observed in mdx and wild-type mice compared with baseline, and a decrease (p < 0.001) in [Mg2+ ] and a lower (p = 0.048) intracellular [H+ ] in damaged muscle regions of mdx mice were observed, consistent with impaired sarcolemma integrity. Overall, our findings demonstrate that 31 P-MRS markers of energetic status and sarcolemma integrity are altered in young mdx compared with wild-type mice, and these indices are exacerbated following downhill running.

Reducing sarcolipin expression improves muscle metabolism in mdx mice

Duchenne muscular dystrophy (DMD) is an inherited muscle wasting disease. Metabolic impairments and oxidative stress are major secondary mechanisms that severely worsen muscle function in DMD. Here, we sought to determine whether germline reduction or ablation of sarcolipin (SLN), an inhibitor of sarco/endoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA), improves muscle metabolism and ameliorates muscle pathology in the mdx mouse model of DMD. Glucose and insulin tolerance tests show that glucose clearance rate and insulin sensitivity were improved in the SLN haploinsufficient mdx (mdx:sln^+/-) and SLN-deficient mdx (mdx:sln^-/-) mice. The histopathological analysis shows that fibrosis and necrosis were significantly reduced in muscles of mdx:sln^+/- and mdx:sln^-/- mice. SR Ca²⁺ uptake, mitochondrial complex protein levels, complex activities, mitochondrial Ca²⁺ uptake and release, and mitochondrial metabolism were significantly improved, and lipid peroxidation and protein carbonylation were reduced in the muscles of mdx:sln^+/- and mdx:sln^-/- mice. These data demonstrate that reduction or ablation of SLN expression can improve muscle metabolism, reduce oxidative stress, decrease muscle pathology, and protects the mdx mice from glucose intolerance.

Sensitivity to behavioral stress impacts disease pathogenesis in dystrophin-deficient mice

Mutation to the gene encoding dystrophin can cause Duchenne muscular dystrophy (DMD) and increase the sensitivity to stress in vertebrate species, including the mdx mouse model of DMD. Behavioral stressors can exacerbate some dystrophinopathy phenotypes of mdx skeletal muscle and cause hypotension-induced death. However, we have discovered that a subpopulation of mdx mice present with a wildtype-like response to mild (forced downhill treadmill exercise) and moderate (scruff restraint) behavioral stressors. These "stress-resistant" mdx mice are more physically active, capable of super-activating the hypothalamic-pituitary-adrenal and renin-angiotensin-aldosterone pathways following behavioral stress and they express greater levels of mineralocorticoid and glucocorticoid receptors in striated muscle relative to "stress-sensitive" mdx mice. Stress-resistant mdx mice also presented with a less severe striated muscle histopathology and greater exercise and skeletal muscle oxidative capacity at rest. Most interestingly, female mdx mice were more physically active following behavioral stressors compared to male mdx mice; a response abolished after ovariectomy and rescued with estradiol. We demonstrate that the response to behavioral stress greatly impacts disease severity in mdx mice suggesting the management of stress in patients with DMD be considered as a therapeutic approach to ameliorate disease progression.

Alteration of skeletal and cardiac muscles function in DBA/2J mdx mice background: a focus on high intensity interval training

Duchenne muscular dystrophy (DMD) is a recessive hereditary myopathy due to deficiency of functional dystrophin. Current therapeutic interventions need more investigation to slow down the progression of skeletal and cardiac muscle weakness. In humans, there is a lack of an adapted training program. In animals, the murine Mdx model with a DBA/2J background (D2-mdx) was recently suggested to present pathological features closer to that of humans. In this study, we characterized skeletal and cardiac muscle functions in males and females D2-mdx mice compared to control groups. We also evaluated the impact of high intensity interval training (HIIT) in these muscles in females and males. HIIT was performed 5 times per week during a month on a motorized treadmill. Specific maximal isometric force production and weakness were measured in the tibialis anterior muscle (TA). Sedentary male and female D2-mdx mice produced lower absolute and specific maximal force compared to control mice. Dystrophic mice showed a decline of force generation during repetitive stimulation compared to controls. This reduction was greater for male D2-mdx mice than females. Furthermore, trained D2-mdx males showed an improvement in force generation after the fifth lengthening contraction compared to sedentary D2-mdx males. Moreover, echocardiography measures revealed a decrease in left ventricular end-diastolic volume, left ventricular ejection volume and left ventricular end-diastolic diameter in sedentary male and female D2-mdx mice. Overall, our results showed a serious muscle function alteration in female and male D2-mdx mice compared to controls. HIIT may delay force loss especially in male D2-mdx mice.

Dystrophin Dp71 Subisoforms Localize to the Mitochondria of Human Cells

Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disease caused by deficiency in dystrophin, a protein product encoded by the DMD gene. Mitochondrial dysfunction is now attracting much attention as a central player in DMD pathology. However, dystrophin has never been explored in human mitochondria. Here, we analyzed dystrophin in cDNAs and mitochondrial fractions of human cells. Mitochondrial fraction was obtained using a magnetic-associated cell sorting (MACS) technology. Dystrophin was analyzed by reverse transcription (RT)-PCR and western blotting using an antibody against the dystrophin C-terminal. In isolated mitochondrial fraction from HEK293 cells, dystrophin was revealed as a band corresponding to Dp71b and Dp71ab subisoforms. Additionally, in mitochondria from HeLa, SH-SY5Y, CCL-136 and HepG2 cells, signals for Dp71b and Dp71ab were revealed as well. Concomitantly, dystrophin mRNAs encoding Dp71b and Dp71ab were disclosed by RT-PCR in these cells. Primary cultured myocytes from three dystrophinopathy patients showed various levels of mitochondrial Dp71 expression. Coherently, levels of mRNA were different in all cells reflecting the protein content, which indicated predominant accumulation of Dp71. Dystrophin was demonstrated to be localized to human mitochondrial fraction, specifically as Dp71 subisoforms. Myocytes derived from dystrophinopathy patients manifested different levels of mitochondrial Dp71, with higher expression revealed in myocytes from Becker muscular dystrophy (BMD) patient-derived myocytes.

Effect of the Non-Immunosuppressive MPT Pore Inhibitor Alisporivir on the Functioning of Heart Mitochondria in Dystrophin-Deficient mdx Mice

Supporting mitochondrial function is one of the therapeutic strategies that improve the functioning of skeletal muscle in Duchenne muscular dystrophy (DMD). In this work, we studied the effect of a non-immunosuppressive inhibitor of mitochondrial permeability transition pore (MPTP) alisporivir (5 mg/kg/day), reducing the intensity of the necrotic process and inflammation in skeletal muscles on the cardiac phenotype of dystrophin-deficient mdx mice. We found that the heart mitochondria of mdx mice show an increase in the intensity of oxidative phosphorylation and an increase in the resistance of organelles to the MPT pore opening. Alisporivir had no significant effect on the hyperfunctionalization of the heart mitochondria of mdx mice, and the state of the heart mitochondria of wild-type animals did not affect the dynamics of organelles but significantly suppressed mitochondrial biogenesis and reduced the amount of mtDNA in the heart muscle. Moreover, alisporivir suppressed mitochondrial biogenesis in the heart of wild-type mice. Alisporivir treatment resulted in a decrease in heart weight in mdx mice, which was associated with a significant modification of the transmission of excitation in the heart. The latter was also noted in the case of WT mice treated with alisporivir. The paper discusses the prospects for using alisporivir to correct the function of heart mitochondria in DMD.

Reduced mitochondrial fission and impaired energy metabolism in human primary skeletal muscle cells of Megaconial Congenital Muscular Dystrophy

Megaconial Congenital Muscular Dystrophy (CMD) is a rare autosomal recessive disorder characterized by enlarged mitochondria located mainly at the periphery of muscle fibers and caused by mutations in the Choline Kinase Beta (CHKB) gene. Although the pathogenesis of this disease is not well understood, there is accumulating evidence for the presence of mitochondrial dysfunction. In this study, we aimed to investigate whether imbalanced mitochondrial dynamics affects mitochondrial function and bioenergetic efficiency in skeletal muscle cells of Megaconial CMD. Immunofluorescence, confocal and transmission electron microscopy studies revealed impaired mitochondrial network, morphology, and localization in primary skeletal muscle cells of Megaconial CMD. The organelle disruption was specific only to skeletal muscle cells grown in culture. The expression levels of mitochondrial fission proteins (DRP1, MFF, FIS1) were found to be decreased significantly in both primary skeletal muscle cells and tissue sections of Megaconial CMD by Western blotting and/or immunofluorescence analysis. The metabolomic and fluxomic analysis, which were performed in Megaconial CMD for the first time, revealed decreased levels of phosphonucleotides, Krebs cycle intermediates, ATP, and altered energy metabolism pathways. Our results indicate that reduced mitochondrial fission and altered mitochondrial energy metabolism contribute to mitochondrial dysmorphology and dysfunction in the pathogenesis of Megaconial CMD.

Alisporivir Improves Mitochondrial Function in Skeletal Muscle of mdx Mice but Suppresses Mitochondrial Dynamics and Biogenesis

Mitigation of calcium-dependent destruction of skeletal muscle mitochondria is considered as a promising adjunctive therapy in Duchenne muscular dystrophy (DMD). In this work, we study the effect of intraperitoneal administration of a non-immunosuppressive inhibitor of calcium-dependent mitochondrial permeability transition (MPT) pore alisporivir on the state of skeletal muscles and the functioning of mitochondria in dystrophin-deficient mdx mice. We show that treatment with alisporivir reduces inflammation and improves muscle function in mdx mice. These effects of alisporivir were associated with an improvement in the ultrastructure of mitochondria, normalization of respiration and oxidative phosphorylation, and a decrease in lipid peroxidation, due to suppression of MPT pore opening and an improvement in calcium homeostasis. The action of alisporivir was associated with suppression of the activity of cyclophilin D and a decrease in its expression in skeletal muscles. This was observed in both mdx mice and wild-type animals. At the same time, alisporivir suppressed mitochondrial biogenesis, assessed by the expression of Ppargc1a, and altered the dynamics of organelles, inhibiting both DRP1-mediated fission and MFN2-associated fusion of mitochondria. The article discusses the effects of alisporivir administration and cyclophilin D inhibition on mitochondrial reprogramming and networking in DMD and the consequences of this therapy on skeletal muscle health.

Transcriptome changes during the initiation and progression of Duchenne muscular dystrophy in Caenorhabditis elegans

Duchenne muscular dystrophy (DMD) is a lethal, X-linked disease characterized by progressive muscle degeneration. The condition is driven by nonsense and missense mutations in the dystrophin gene, leading to instability of the sarcolemma and skeletal muscle necrosis and atrophy. Resulting changes in muscle-specific gene expression that take place in dystrophin's absence remain largely uncharacterized, as they are potentially obscured by the chronic inflammation elicited by muscle damage in humans. Caenorhabditis elegans possess a mild inflammatory response that is not active in the muscle, and lack a satellite cell equivalent. This allows for the characterization of the transcriptome rearrangements affecting disease progression independently of inflammation and regeneration. In effort to better understand these dynamics, we have isolated and sequenced body muscle-specific transcriptomes from C. elegans lacking functional dystrophin at distinct stages of disease progression. We have identified an upregulation of genes involved in mitochondrial function early in disease progression, and an upregulation of genes related to muscle repair in later stages. Our results suggest that in C. elegans, dystrophin may have a signaling role early in development, and its absence may activate compensatory mechanisms that counteract muscle degradation caused by loss of dystrophin. We have also developed a temperature-based screening method for synthetic paralysis that can be used to rapidly identify genetic partners of dystrophin. Our results allow for the comprehensive identification of transcriptome changes that potentially serve as independent drivers of disease progression and may in turn allow for the identification of new therapeutic targets for the treatment of DMD.

Givinostat as metabolic enhancer reverting mitochondrial biogenesis deficit in Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a rare disorder characterized by progressive muscle wasting, weakness, and premature death. Remarkable progress has been made in genetic approaches, restoring dystrophin, or its function. However, the targeting of secondary pathological mechanisms, such as increasing muscle blood flow or stopping fibrosis, remains important to improve the therapeutic benefits, that depend on tackling both the genetic disease and the downstream consequences. Mitochondrial dysfunctions are one of the earliest deficits in DMD, arise from multiple cellular stressors and result in less than 50% of ATP content in dystrophic muscles. Here we establish that there are two temporally distinct phases of mitochondrial damage with depletion of mitochondrial mass at early stages and an accumulation of dysfunctional mitochondria at later stages, leading to a different oxidative fibers pattern, in young and adult mdx mice. We also observe a progressive mitochondrial biogenesis impairment associated with increased deacetylation of peroxisome proliferator-activated receptor-gamma coactivator 1 α (PGC-1α) promoter. Such histone deacetylation is inhibited by givinostat that positively modifies the epigenetic profile of PGC-1α promoter, sustaining mitochondrial biogenesis and oxidative fiber type switch. We, therefore, demonstrate that givinostat exerts relevant effects at mitochondrial level, acting as a metabolic remodeling agent capable of efficiently promoting mitochondrial biogenesis in dystrophic muscle.

Urolithin A improves muscle function by inducing mitophagy in muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy, and despite advances in genetic and pharmacological disease-modifying treatments, its management remains a major challenge. Mitochondrial dysfunction contributes to DMD, yet the mechanisms by which this occurs remain elusive. Our data in experimental models and patients with DMD show that reduced expression of genes involved in mitochondrial autophagy, or mitophagy, contributes to mitochondrial dysfunction. Mitophagy markers were reduced in skeletal muscle and in muscle stem cells (MuSCs) of a mouse model of DMD. Administration of the mitophagy activator urolithin A (UA) rescued mitophagy in DMD worms and mice and in primary myoblasts from patients with DMD, increased skeletal muscle respiratory capacity, and improved MuSCs' regenerative ability, resulting in the recovery of muscle function and increased survival in DMD mouse models. These data indicate that restoration of mitophagy alleviates symptoms of DMD and suggest that UA may have potential therapeutic applications for muscular dystrophies.

Skeletal Muscle Mitochondria Dysfunction in Genetic Neuromuscular Disorders with Cardiac Phenotype

Mitochondrial dysfunction is considered the major contributor to skeletal muscle wasting in different conditions. Genetically determined neuromuscular disorders occur as a result of mutations in the structural proteins of striated muscle cells and therefore are often combined with cardiac phenotype, which most often manifests as a cardiomyopathy. The specific roles played by mitochondria and mitochondrial energetic metabolism in skeletal muscle under muscle-wasting conditions in cardiomyopathies have not yet been investigated in detail, and this aspect of genetic muscle diseases remains poorly characterized. This review will highlight dysregulation of mitochondrial representation and bioenergetics in specific skeletal muscle disorders caused by mutations that disrupt the structural and functional integrity of muscle cells.

The Interplay of Mitophagy and Inflammation in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease caused by a pathogenic disruption of the DYSTROPHIN gene that results in non-functional dystrophin protein. DMD patients experience loss of ambulation, cardiac arrhythmia, metabolic syndrome, and respiratory failure. At the molecular level, the lack of dystrophin in the muscle results in myofiber death, fibrotic infiltration, and mitochondrial dysfunction. There is no cure for DMD, although dystrophin-replacement gene therapies and exon-skipping approaches are being pursued in clinical trials. Mitochondrial dysfunction is one of the first cellular changes seen in DMD myofibers, occurring prior to muscle disease onset and progresses with disease severity. This is seen by reduced mitochondrial function, abnormal mitochondrial morphology and impaired mitophagy (degradation of damaged mitochondria). Dysfunctional mitochondria release high levels of reactive oxygen species (ROS), which can activate pro-inflammatory pathways such as IL-1β and IL-6. Impaired mitophagy in DMD results in increased inflammation and further aggravates disease pathology, evidenced by increased muscle damage and increased fibrosis. This review will focus on the critical interplay between mitophagy and inflammation in Duchenne muscular dystrophy as a pathological mechanism, as well as describe both candidate and established therapeutic targets that regulate these pathways.

Differential Effects of Halofuginone Enantiomers on Muscle Fibrosis and Histopathology in Duchenne Muscular Dystrophy

Progressive loss of muscle and muscle function is associated with significant fibrosis in Duchenne muscular dystrophy (DMD) patients. Halofuginone, an analog of febrifugine, prevents fibrosis in various animal models, including those of muscular dystrophies. Effects of (+)/(−)-halofuginone enantiomers on motor coordination and diaphragm histopathology in mdx mice, the mouse model for DMD, were examined. Four-week-old male mice were treated with racemic halofuginone, or its separate enantiomers, for 10 weeks. Controls were treated with saline. Racemic halofuginone-treated mice demonstrated better motor coordination and balance than controls. However, (+)-halofuginone surpassed the racemic form’s effect. No effect was observed for (−)-halofuginone, which behaved like the control. A significant reduction in collagen content and degenerative areas, and an increase in utrophin levels were observed in diaphragms of mice treated with racemic halofuginone. Again, (+)-halofuginone was more effective than the racemic form, whereas (−)-halofuginone had no effect. Both racemic and (+)-halofuginone increased diaphragm myofiber diameters, with no effect for (−)-halofuginone. No effects were observed for any of the compounds tested in an in-vitro cell viability assay. These results, demonstrating a differential effect of the halofuginone enantiomers and superiority of (+)-halofuginone, are of great importance for future use of (+)-halofuginone as a DMD antifibrotic therapy.

Mutational analysis of mitochondrial tRNA genes in 138 patients with Leber's hereditary optic neuropathy

INTRODUCTION

Mutations in mitochondrial DNA (mtDNA) are the most important causes for Leber's hereditary optic neuropathy (LHON). Of these, three primary mtDNA mutations account for more than 90% cases of this disease. However, to date, little is known regarding the relationship between mitochondrial tRNA (mt-tRNA) variants and LHON.

AIM

In this study, we aimed to investigate the association between mt-tRNA variants and LHON.

METHODOLOGY

One hundred thirty-eight LHON patients lacking three primary mutations (ND1 3460G > A, ND4 11778Gxs > A, and ND6 14484 T > C), as well as 266 controls were enrolled in this study. PCR-Sanger sequencing was performed to screen the mt-tRNA variants. Moreover, the phylogenetic analysis, pathogenicity scoring system, as well as mitochondrial functions were performed.

RESULTS

We identified 8 possible pathogenic variants: tRNA^Phe 593 T > C, tRNA^Leu(UUR) 3275C > T, tRNA^Gln 4363 T > C, tRNA^Met 4435A > G, tRNA^Ala 5587 T > C, tRNA^Glu 14693A > G, tRNA^Thr 15927G > A, and 15951A > G, which may change the structural and functional impact on the corresponding tRNAs, and subsequently lead to a failure in tRNA metabolism. Furthermore, significant reductions in mitochondrial ATP and MMP levels and an overproduction of ROS were observed in cybrid cells containing these mt-tRNA variants, suggesting that these variants may lead to mitochondrial dysfunction which was responsible for LHON.

CONCLUSION

Our study indicated that mt-tRNA variants were associated with LHON, and screening for mt-tRNA variants were recommended for early detection, diagnosis, and prevention of maternally inherited LHON.

Striated muscle activator of Rho signalling (STARS) overexpression in the mdx mouse enhances muscle functional capacity and regulates the actin cytoskeleton and oxidative phosphorylation pathways

NEW FINDINGS

What is the central question of this study? Striated muscle activator of rho signalling (STARS) is an actin-binding protein that regulates transcriptional pathways controlling muscle function, growth and myogenesis, processes that are impaired in dystrophic muscle: what is the regulation of the STARS pathway in Duchenne muscular dystrophy (DMD)? What is the main finding and its importance? Members of the STARS signalling pathway are reduced in the quadriceps of patients with DMD and in mouse models of muscular dystrophy. Overexpression of STARS in the dystrophic deficient mdx mouse model increased maximal isometric specific force and upregulated members of the actin cytoskeleton and oxidative phosphorylation pathways. Regulating STARS may be a therapeutic approach to enhance muscle health.

ABSTRACT

Duchenne muscular dystrophy (DMD) is characterised by impaired cytoskeleton organisation, cytosolic calcium handling, oxidative stress and mitochondrial dysfunction. This results in progressive muscle damage, wasting and weakness and premature death. The striated muscle activator of rho signalling (STARS) is an actin-binding protein that activates the myocardin-related transcription factor-A (MRTFA)/serum response factor (SRF) transcriptional pathway, a pathway regulating cytoskeletal structure and muscle function, growth and repair. We investigated the regulation of the STARS pathway in the quadriceps muscle from patients with DMD and in the tibialis anterior (TA) muscle from the dystrophin-deficient mdx and dko (utrophin and dystrophin null) mice. Protein levels of STARS, SRF and RHOA were reduced in patients with DMD. STARS, SRF and MRTFA mRNA levels were also decreased in DMD muscle, while Stars mRNA levels were decreased in the mdx mice and Srf and Mrtfa mRNAs decreased in the dko mice. Overexpressing human STARS (hSTARS) in the TA muscles of mdx mice increased maximal isometric specific force by 13% (P < 0.05). This was not associated with changes in muscle mass, fibre cross-sectional area, fibre type, centralised nuclei or collagen deposition. Proteomics screening followed by pathway enrichment analysis identified that hSTARS overexpression resulted in 31 upregulated and 22 downregulated proteins belonging to the actin cytoskeleton and oxidative phosphorylation pathways. These pathways are impaired in dystrophic muscle and regulate processes that are vital for muscle function. Increasing the STARS protein in dystrophic muscle improves muscle force production, potentially via synergistic regulation of cytoskeletal structure and energy production.

β-Glucans as Dietary Supplement to Improve Locomotion and Mitochondrial Respiration in a Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular childhood disorder that causes progressive muscle weakness and degeneration. A lack of dystrophin in DMD leads to inflammatory response, autophagic dysregulation, and oxidative stress in skeletal muscle fibers that play a key role in the progression of the pathology. β-glucans can modulate immune function by modifying the phagocytic activity of immunocompetent cells, notably macrophages. Mitochondrial function is also involved in an important mechanism of the innate and adaptive immune responses, owing to high need for energy of immune cells. In the present study, the effects of 1,3-1,6 β-glucans on five-day-old non-dystrophic and dystrophic (sapje) zebrafish larvae were investigated. The effects of the sonication of β-glucans and the dechorionation of embryos were also evaluated. The results showed that the incidence of dystrophic phenotypes was reduced when dystrophic embryos were exposed to 2 and 4 mg L^-1 of 1,3-1,6 β-glucans. Moreover, when the dystrophic larvae underwent 8 mg L^-1 treatment, an improvement of the locomotor performances and mitochondrial respiration were observed. In conclusion, the observed results demonstrated that 1,3-1,6 β-glucans improve locomotor performances and mitochondrial function in dystrophic zebrafish. Therefore, for ameliorating their life quality, 1,3-1,6 β-glucans look like a promising diet supplement for DMD patients, even though further investigations are required.

Discovery and mechanism of action studies of 4,6-diphenylpyrimidine-2-carbohydrazides as utrophin modulators for the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy is a fatal disease with no cure, caused by lack of the cytoskeletal protein dystrophin. Upregulation of utrophin, a dystrophin paralogue, offers a potential therapy independent of mutation type. The failure of first-in-class utrophin modulator ezutromid/SMT C1100 in Phase II clinical trials necessitates development of compounds with better efficacy, physicochemical and ADME properties and/or complementary mechanisms. We have discovered and performed a preliminary optimisation of a novel class of utrophin modulators using an improved phenotypic screen, where reporter expression is derived from the full genomic context of the utrophin promoter. We further demonstrate through target deconvolution studies, including expression analysis and chemical proteomics, that this compound series operates via a novel mechanism of action, distinct from that of ezutromid.

Targeting intramuscular adipose tissue expansion to preserve contractile function in volumetric muscle loss: A potentially novel therapy?

In volumetric muscle loss (VML), the severity of trauma exceeds a muscle's regenerative capacity. VML causes permanent functional impairments for which there are no rehabilitative, pharmacological, or regenerative medicine interventions. Driving failed regeneration in VML is a hostile microenvironment characterized by heightened inflammation, fibrosis, and denervation, which may reduce the remaining muscle tissue's quality, and stimulate intramuscular adipose tissue (IMAT) expansion. IMAT is increased in various muscle disease states, and has known lipotoxic effects on regeneration and contractile function. The contribution of ectopic fat deposition to the hostile VML microenvironment at the injury site and in the remaining tissue warrants further investigation. Targeting IMAT may lead to novel therapeutic strategies for improving functional outcomes in VML.

Abnormal Calcium Handling in Duchenne Muscular Dystrophy: Mechanisms and Potential Therapies

Duchenne muscular dystrophy (DMD) is an X-linked muscle-wasting disease caused by the loss of dystrophin. DMD is associated with muscle degeneration, necrosis, inflammation, fatty replacement, and fibrosis, resulting in muscle weakness, respiratory and cardiac failure, and premature death. There is no curative treatment. Investigations on disease-causing mechanisms offer an opportunity to identify new therapeutic targets to treat DMD. An abnormal elevation of the intracellular calcium (Cai2+) concentration in the dystrophin-deficient muscle is a major secondary event, which contributes to disease progression in DMD. Emerging studies have suggested that targeting Ca²⁺-handling proteins and/or mechanisms could be a promising therapeutic strategy for DMD. Here, we provide an updated overview of the mechanistic roles the sarcolemma, sarcoplasmic/endoplasmic reticulum, and mitochondria play in the abnormal and sustained elevation of Cai2+ levels and their involvement in DMD pathogenesis. We also discuss current approaches aimed at restoring Ca²⁺ homeostasis as potential therapies for DMD.

Beneficial Role of Exercise in the Modulation of mdx Muscle Plastic Remodeling and Oxidative Stress

Duchenne muscular dystrophy (DMD) is an X-linked recessive progressive lethal disorder caused by the lack of dystrophin, which determines myofibers mechanical instability, oxidative stress, inflammation, and susceptibility to contraction-induced injuries. Unfortunately, at present, there is no efficient therapy for DMD. Beyond several promising gene- and stem cells-based strategies under investigation, physical activity may represent a valid noninvasive therapeutic approach to slow down the progression of the pathology. However, ethical issues, the limited number of studies in humans and the lack of consistency of the investigated training interventions generate loss of consensus regarding their efficacy, leaving exercise prescription still questionable. By an accurate analysis of data about the effects of different protocol of exercise on muscles of mdx mice, the most widely-used pre-clinical model for DMD research, we found that low intensity exercise, especially in the form of low speed treadmill running, likely represents the most suitable exercise modality associated to beneficial effects on mdx muscle. This protocol of training reduces muscle oxidative stress, inflammation, and fibrosis process, and enhances muscle functionality, muscle regeneration, and hypertrophy. These conclusions can guide the design of appropriate studies on human, thereby providing new insights to translational therapeutic application of exercise to DMD patients.

Skeletal muscle mitochondria in health and disease

Mitochondrial activity warrants energy supply to oxidative myofibres to sustain endurance workload. The maintenance of mitochondrial homeostasis is ensured by the control of fission and fusion processes and by the mitophagic removal of aberrant organelles. Many diseases are due to or characterized by dysfunctional mitochondria, and altered mitochondrial dynamics or turnover trigger myopathy per se. In this review, we will tackle the role of mitochondrial dynamics, turnover and metabolism in skeletal muscle, both in health and disease.

Metformin Reverses the Enhanced Myocardial SR/ER-Mitochondria Interaction and Impaired Complex I-Driven Respiration in Dystrophin-Deficient Mice

Besides skeletal muscle dysfunction, Duchenne muscular dystrophy (DMD) exhibits a progressive cardiomyopathy characterized by an impaired calcium (Ca²⁺) homeostasis and a mitochondrial dysfunction. Here we aimed to determine whether sarco-endoplasmic reticulum (SR/ER)–mitochondria interactions and mitochondrial function were impaired in dystrophic heart at the early stage of the pathology. For this purpose, ventricular cardiomyocytes and mitochondria were isolated from 3-month-old dystrophin-deficient mice (mdx mice). The number of contacts points between the SR/ER Ca²⁺ release channels (IP3R1) and the porine of the outer membrane of the mitochondria, VDAC1, measured using in situ proximity ligation assay, was greater in mdx cardiomyocytes. Expression levels of IP3R1 as well as the mitochondrial Ca²⁺ uniporter (MCU) and its regulated subunit, MICU1, were also increased in mdx heart. MICU2 expression was however unchanged. Furthermore, the mitochondrial Ca²⁺ uptake kinetics and the mitochondrial Ca²⁺ content were significantly increased. Meanwhile, the Ca²⁺-dependent pyruvate dehydrogenase phosphorylation was reduced, and its activity significantly increased. In Ca²⁺-free conditions, pyruvate-driven complex I respiration was decreased whereas in the presence of Ca²⁺, complex I-mediated respiration was boosted. Further, impaired complex I-mediated respiration was independent of its intrinsic activity or expression, which remains unchanged but is accompanied by an increase in mitochondrial reactive oxygen species production. Finally, mdx mice were treated with the complex I modulator metformin for 1 month. Metformin normalized the SR/ER-mitochondria interaction, decreased MICU1 expression and mitochondrial Ca²⁺ content, and enhanced complex I-driven respiration. In summary, before any sign of dilated cardiomyopathy, the DMD heart displays an aberrant SR/ER-mitochondria coupling with an increase mitochondrial Ca²⁺ homeostasis and a complex I dysfunction. Such remodeling could be reversed by metformin providing a novel therapeutic perspective in DMD.

Treatment with a triazole inhibitor of the mitochondrial permeability transition pore fully corrects the pathology of sapje zebrafish lacking dystrophin

High-throughput screening identified isoxazoles as potent but metabolically unstable inhibitors of the mitochondrial permeability transition pore (PTP). Here we have studied the effects of a metabolically stable triazole analog, TR001, which maintains the PTP inhibitory properties with an in vitro potency in the nanomolar range. We show that TR001 leads to recovery of muscle structure and function of sapje zebrafish, a severe model of Duchenne muscular dystrophy (DMD). PTP inhibition fully restores the otherwise defective respiration in vivo, allowing normal development of sapje individuals in spite of lack of dystrophin. About 80 % sapje zebrafish treated with TR001 are alive and normal at 18 days post fertilization (dpf), a point in time when not a single untreated sapje individual survives. Time to 50 % death of treated zebrafish increases from 5 to 28 dpf, a sizeable number of individuals becoming young adults in spite of the persistent lack of dystrophin expression. TR001 improves respiration of myoblasts and myotubes from DMD patients, suggesting that PTP-dependent dysfunction also occurs in the human disease and that mitochondrial therapy of DMD with PTP-inhibiting triazoles is a viable treatment option.

Targeting Nrf2 for the treatment of Duchenne Muscular Dystrophy

Imbalances in redox homeostasis can result in oxidative stress, which is implicated in various pathological conditions including the fatal neuromuscular disease Duchenne Muscular Dystrophy (DMD). DMD is a complicated disease, with many druggable targets at the cellular and molecular level including calcium-mediated muscle degeneration; mitochondrial dysfunction; oxidative stress; inflammation; insufficient muscle regeneration and dysregulated protein and organelle maintenance. Previous investigative therapeutics tended to isolate and focus on just one of these targets and, consequently, therapeutic activity has been limited. Nuclear erythroid 2-related factor 2 (Nrf2) is a transcription factor that upregulates many cytoprotective gene products in response to oxidants and other toxic stressors. Unlike other strategies, targeted Nrf2 activation has the potential to simultaneously modulate separate pathological features of DMD to amplify therapeutic benefits. Here, we review the literature providing theoretical context for targeting Nrf2 as a disease modifying treatment against DMD.

Nitric Oxide (NO) and Duchenne Muscular Dystrophy: NO Way to Go?

The discordance between pre-clinical success and clinical failure of treatment options for Duchenne Muscular Dystrophy (DMD) is significant. The termination of clinical trials investigating the phosphodiesterase inhibitors, sildenafil and tadalafil (which prolong the second messenger molecule of nitric oxide (NO) signaling), are prime examples of this. Both attenuated key dystrophic features in the mdx mouse model of DMD yet failed to modulate primary outcomes in clinical settings. We have previously attempted to modulate NO signaling via chronic nitrate supplementation of the mdx mouse but failed to demonstrate beneficial modulation of key dystrophic features (i.e., metabolism). Instead, we observed increased muscle damage and nitrosative stress which exacerbated MD. Here, we highlight that acute nitrite treatment of human DMD myoblasts is also detrimental and suggest strategies for moving forward with NO replacement therapy in DMD.

The Failed Clinical Story of Myostatin Inhibitors against Duchenne Muscular Dystrophy: Exploring the Biology behind the Battle

Myostatin inhibition therapy has held much promise for the treatment of muscle wasting disorders. This is particularly true for the fatal myopathy, Duchenne Muscular Dystrophy (DMD). Following on from promising pre-clinical data in dystrophin-deficient mice and dogs, several clinical trials were initiated in DMD patients using different modality myostatin inhibition therapies. All failed to show modification of disease course as dictated by the primary and secondary outcome measures selected: the myostatin inhibition story, thus far, is a failed clinical story. These trials have recently been extensively reviewed and reasons why pre-clinical data collected in animal models have failed to translate into clinical benefit to patients have been purported. However, the biological mechanisms underlying translational failure need to be examined to ensure future myostatin inhibitor development endeavors do not meet with the same fate. Here, we explore the biology which could explain the failed translation of myostatin inhibitors in the treatment of DMD.

The Effect of Deflazacort Treatment on the Functioning of Skeletal Muscle Mitochondria in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe hereditary disease caused by a lack of dystrophin, a protein essential for myocyte integrity. Mitochondrial dysfunction is reportedly responsible for DMD. This study examines the effect of glucocorticoid deflazacort on the functioning of the skeletal-muscle mitochondria of dystrophin-deficient mdx mice and WT animals. Deflazacort administration was found to improve mitochondrial respiration of mdx mice due to an increase in the level of ETC complexes (complexes III and IV and ATP synthase), which may contribute to the normalization of ATP levels in the skeletal muscle of mdx animals. Deflazacort treatment improved the rate of Ca2+ uniport in the skeletal muscle mitochondria of mdx mice, presumably by affecting the subunit composition of the calcium uniporter of organelles. At the same time, deflazacort was found to reduce the resistance of skeletal mitochondria to MPT pore opening, which may be associated with a change in the level of ANT2 and CypD. In this case, deflazacort also affected the mitochondria of WT mice. The paper discusses the mechanisms underlying the effect of deflazacort on the functioning of mitochondria and contributing to the improvement of the muscular function of mdx mice.

Altered dynamics of lipid metabolism in muscle cells from patients with idiopathic inflammatory myopathy is ameliorated by 6 months of training

KEY POINTS

Regular exercise improves muscle functional capacity and clinical state of patients with idiopathic inflammatory myopathy (IIM). In our study, we used an in vitro model of human primary muscle cell cultures, derived from IIM patients before and after a 6-month intensive supervised training intervention to assess the impact of disease and exercise on lipid metabolism dynamics. We provide evidence that muscle cells from IIM patients display altered dynamics of lipid metabolism and impaired adaptive response to saturated fatty acid load compared to healthy controls. A 6-month intensive supervised exercise training intervention in patients with IIM mitigated disease effects in their cultured muscle cells, improving or normalizing their capacity to handle lipids. These findings highlight the putative role of intrinsic metabolic defects of skeletal muscle in the pathogenesis of IIM and the positive impact of exercise, maintained in vitro by yet unknown epigenetic mechanisms.

ABSTRACT

Exercise improves skeletal muscle function, clinical state and quality of life in patients with idiopathic inflammatory myopathy (IIM). Our aim was to identify disease-related metabolic perturbations and the impact of exercise in skeletal muscle cells of IIM patients. Patients underwent a 6-month intensive supervised training intervention. Muscle function, anthropometric and metabolic parameters were examined and muscle cell cultures were established (m. vastus lateralis; Bergström needle biopsy) before and after training from patients and sedentary age/sex/body mass index-matched controls. [¹⁴ C]Palmitate was used to determine fat oxidation and lipid synthesis (thin layer chromatography). Cells were exposed to a chronic (3 days) and acute (3 h) metabolic challenge (the saturated fatty acid palmitate, 100 μm). Reduced oxidative (intermediate metabolites, -49%, P = 0.034) and non-oxidative (diglycerides, -38%, P = 0.013) lipid metabolism was identified in palmitate-treated muscle cells from IIM patients compared to controls. Three days of palmitate exposure elicited distinct regulation of oxidative phosphorylation (OxPHOS) complex IV and complex V/ATP synthase (P = 0.012/0.005) and adipose triglyceride lipase in patients compared to controls (P = 0.045) (immunoblotting). Importantly, 6 months of training in IIM patients improved lipid metabolism (CO₂ , P = 0.010; intermediate metabolites, P = 0.041) and activation of AMP kinase (P = 0.007), and nearly normalized palmitate-induced changes in OxPHOS proteins in myotubes from IIM patients, in parallel with improvements of patients' clinical state. Myotubes from IIM patients displayed altered dynamics of lipid metabolism and impaired response to metabolic challenge with saturated fatty acid. Our observations suggest that metabolic defects intrinsic to skeletal muscle could represent non-immune pathomechanisms, which can contribute to muscle weakness in IIM. A 6-month training intervention mitigated disease effects in muscle cells in vitro, indicating the existence of epigenetic regulatory mechanisms.

The Impact of Mitochondrial Deficiencies in Neuromuscular Diseases

Neuromuscular diseases (NMDs) are a heterogeneous group of acquired or inherited rare disorders caused by injury or dysfunction of the anterior horn cells of the spinal cord (lower motor neurons), peripheral nerves, neuromuscular junctions, or skeletal muscles leading to muscle weakness and waste. Unfortunately, most of them entail serious or even fatal consequences. The prevalence rates among NMDs range between 1 and 10 per 100,000 population, but their rarity and diversity pose difficulties for healthcare and research. Some molecular hallmarks are being explored to elucidate the mechanisms triggering disease, to set the path for further advances. In fact, in the present review we outline the metabolic alterations of NMDs, mainly focusing on the role of mitochondria. The aim of the review is to discuss the mechanisms underlying energy production, oxidative stress generation, cell signaling, autophagy, and inflammation triggered or conditioned by the mitochondria. Briefly, increased levels of inflammation have been linked to reactive oxygen species (ROS) accumulation, which is key in mitochondrial genomic instability and mitochondrial respiratory chain (MRC) dysfunction. ROS burst, impaired autophagy, and increased inflammation are observed in many NMDs. Increasing knowledge of the etiology of NMDs will help to develop better diagnosis and treatments, eventually reducing the health and economic burden of NMDs for patients and healthcare systems.

Safety issues and harmful pharmacological interactions of nutritional supplements in Duchenne muscular dystrophy: considerations for Standard of Care and emerging virus outbreaks

At the moment, little treatment options are available for Duchenne muscular dystrophy (DMD). The absence of the dystrophin protein leads to a complex cascade of pathogenic events in myofibres, including chronic inflammation and oxidative stress as well as altered metabolism. The attention towards dietary supplements in DMD is rapidly increasing, with the aim to counteract pathology-related alteration in nutrient intake, the consequences of catabolic distress or to enhance the immunological response of patients as nowadays for the COVID-19 pandemic emergency. By definition, supplements do not exert therapeutic actions, although a great confusion may arise in daily life by the improper distinction between supplements and therapeutic compounds. For most supplements, little research has been done and little evidence is available concerning their effects in DMD as well as their preventing actions against infections. Often these are not prescribed by clinicians and patients/caregivers do not discuss the use with their clinical team. Then, little is known about the real extent of supplement use in DMD patients. It is mistakenly assumed that, since compounds are of natural origin, if a supplement is not effective, it will also do no harm. However, supplements can have serious side effects and also have harmful interactions, in terms of reducing efficacy or leading to toxicity, with other therapies. It is therefore pivotal to shed light on this unclear scenario for the sake of patients. This review discusses the supplements mostly used by DMD patients, focusing on their potential toxicity, due to a variety of mechanisms including pharmacodynamic or pharmacokinetic interactions and contaminations, as well as on reports of adverse events. This overview underlines the need for caution in uncontrolled use of dietary supplements in fragile populations such as DMD patients. A culture of appropriate use has to be implemented between clinicians and patients' groups.