Peroxisome proliferator-activated receptor γ coactivator1- gene α transfer restores mitochondrial biomass and improves mitochondrial calcium handling in post-necrotic mdx mouse skeletal muscle

Inhibition of Mitochondrial Fission Protein Drp1 Ameliorates Myopathy in the D2-mdx Model of Duchenne Muscular Dystrophy

Although current treatments for Duchenne Muscular Dystrophy (DMD) have proven to be effective in delaying myopathy, there remains a strong need to identify novel targets to develop additional therapies. Mitochondrial dysfunction is an early pathological feature of DMD. A fine balance of mitochondrial dynamics (fission and fusion) is crucial to maintain mitochondrial function and skeletal muscle health. Excessive activation of Dynamin-Related Protein 1 (Drp1)-mediated mitochondrial fission was reported in animal models of DMD. However, whether Drp1-mediated mitochondrial fission is a viable target for treating myopathy in DMD remains unknown. Here, we treated a D2-mdx model of DMD (9-10 weeks old) with Mdivi-1, a selective Drp1 inhibitor, every other day (i.p. injection) for 5 weeks. We demonstrated that Mdivi-1 effectively improved skeletal muscle strength and reduced serum creatine kinase concentration. Mdivi-1 treatment also effectively inhibited mitochondrial fission regulatory protein markers, Drp1(Ser616) phosphorylation and Fis1 in skeletal muscles from D2-mdx mice, which resulted in reduced content of damaged and fragmented mitochondria. Furthermore, Mdivi-1 treatment attenuated lipid peroxidation product, 4-HNE, in skeletal muscle from D2-mdx mice, which was inversely correlated with muscle grip strength. Finally, we revealed that Mdivi-1 treatment downregulated Alpha 1 Type I Collagen (Col1a1) protein expression, a marker of fibrosis, and Interleukin-6 (IL-6) mRNA expression, a marker of inflammation. In summary, these results demonstrate that inhibition of Drp1-mediated mitochondrial fission by Mdivi-1 is effective in improving muscle strength and alleviating muscle damage in D2-mdx mice. These improvements are associated with improved skeletal muscle mitochondrial integrity, leading to attenuated lipid peroxidation.

Molecular pathways involved in the control of contractile and metabolic properties of skeletal muscle fibers as potential therapeutic targets for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is caused by mutations in the gene encoding dystrophin, a subsarcolemmal protein whose absence results in increased susceptibility of the muscle fiber membrane to contraction-induced injury. This results in increased calcium influx, oxidative stress, and mitochondrial dysfunction, leading to chronic inflammation, myofiber degeneration, and reduced muscle regenerative capacity. Fast glycolytic muscle fibers have been shown to be more vulnerable to mechanical stress than slow oxidative fibers in both DMD patients and DMD mouse models. Therefore, remodeling skeletal muscle toward a slower, more oxidative phenotype may represent a relevant therapeutic approach to protect dystrophic muscles from deterioration and improve the effectiveness of gene and cell-based therapies. The resistance of slow, oxidative myofibers to DMD pathology is attributed, in part, to their higher expression of Utrophin; there are, however, other characteristics of slow, oxidative fibers that might contribute to their enhanced resistance to injury, including reduced contractile speed, resistance to fatigue, increased capillary density, higher mitochondrial activity, decreased cellular energy requirements. This review focuses on signaling pathways and regulatory factors whose genetic or pharmacologic modulation has been shown to ameliorate the dystrophic pathology in preclinical models of DMD while promoting skeletal muscle fiber transition towards a slower more oxidative phenotype.

Cardiomyopathy in Duchenne Muscular Dystrophy and the Potential for Mitochondrial Therapeutics to Improve Treatment Response

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease caused by mutations to the dystrophin gene, resulting in deficiency of dystrophin protein, loss of myofiber integrity in skeletal and cardiac muscle, and eventual cell death and replacement with fibrotic tissue. Pathologic cardiac manifestations occur in nearly every DMD patient, with the development of cardiomyopathy-the leading cause of death-inevitable by adulthood. As early cardiac abnormalities are difficult to detect, timely diagnosis and appropriate treatment modalities remain a challenge. There is no cure for DMD; treatment is aimed at delaying disease progression and alleviating symptoms. A comprehensive understanding of the pathophysiological mechanisms is crucial to the development of targeted treatments. While established hypotheses of underlying mechanisms include sarcolemmal weakening, upregulation of pro-inflammatory cytokines, and perturbed ion homeostasis, mitochondrial dysfunction is thought to be a potential key contributor. Several experimental compounds targeting the skeletal muscle pathology of DMD are in development, but the effects of such agents on cardiac function remain unclear. The synergistic integration of small molecule- and gene-target-based drugs with metabolic-, immune-, or ion balance-enhancing compounds into a combinatorial therapy offers potential for treating dystrophin deficiency-induced cardiomyopathy, making it crucial to understand the underlying mechanisms driving the disorder.

PINK1 deficiency alters muscle stem cell fate decision and muscle regenerative capacity

Maintenance of mitochondrial function plays a crucial role in the regulation of muscle stem cell (MuSC), but the underlying mechanisms remain ill defined. In this study, we monitored mitophagy in MuSCS under various myogenic states and examined the role of PINK1 in maintaining regenerative capacity. Results indicate that quiescent MuSCs actively express mitophagy genes and exhibit a measurable mitophagy flux and prominent mitochondrial localization to autophagolysosomes, which become rapidly decreased during activation. Genetic disruption of Pink1 in mice reduces PARKIN recruitment to mitochondria and mitophagy in quiescent MuSCs, which is accompanied by premature activation/commitment at the expense of self-renewal and progressive loss of muscle regeneration, but unhindered proliferation and differentiation capacity. Results also show that impaired fate decisions in PINK1-deficient MuSCs can be restored by scavenging excess mitochondrial ROS. These data shed light on the regulation of mitophagy in MuSCs and position PINK1 as an important regulator of their mitochondrial properties and fate decisions.

Myonectin protects against skeletal muscle dysfunction in male mice through activation of AMPK/PGC1α pathway

To maintain and restore skeletal muscle mass and function is essential for healthy aging. We have found that myonectin acts as a cardioprotective myokine. Here, we investigate the effect of myonectin on skeletal muscle atrophy in various male mouse models of muscle dysfunction. Disruption of myonectin exacerbates skeletal muscle atrophy in age-associated, sciatic denervation-induced or dexamethasone (DEX)-induced muscle atrophy models. Myonectin deficiency also contributes to exacerbated mitochondrial dysfunction and reduces expression of mitochondrial biogenesis-associated genes including PGC1α in denervated muscle. Myonectin supplementation attenuates denervation-induced muscle atrophy via activation of AMPK. Myonectin also reverses DEX-induced atrophy of cultured myotubes through the AMPK/PGC1α signaling. Furthermore, myonectin treatment suppresses muscle atrophy in senescence-accelerated mouse prone (SAMP) 8 mouse model of accelerated aging or mdx mouse model of Duchenne muscular dystrophy. These data indicate that myonectin can ameliorate skeletal muscle dysfunction through AMPK/PGC1α-dependent mechanisms, suggesting that myonectin could represent a therapeutic target of muscle atrophy.

Low-Level Photobiomodulation Therapy Modulates H2O2 Production, TRPC-6, and PGC-1α Levels in the Dystrophic Muscle

Objective: This study evaluated photobiomodulation therapy (PBMT) effects on the factors involved in mitochondrial biogenesis, on the mitochondrial respiratory complexes, and on the transient receptor potential canonical channels (such as TRPC-1 and TRPC-6) in in vitro (mdx muscle cells) and in vivo studies (gastrocnemius muscle) from mdx mice, the dystrophin-deficient model of Duchenne muscular dystrophy (DMD). Background: There is no successful treatment for DMD, therefore demanding search for new therapies that can improve the muscle role, the quality of life, and the survival of dystrophic patients. Methods: The dystrophic primary muscle cells received PBMT at 0.6 J and 5 J, and the dystrophic gastrocnemius muscle received PBMT at 0.6 J. Results: The dystrophic muscle cells treated with PBMT (0.6 J and 5 J) showed no cytotoxicity and significantly lower levels in hydrogen peroxide (H2O2) production. We also demonstrated, for the first time, the capacity of PBMT, at a low dose (0.6 J), in reducing the TRPC-6 content and in raising the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) content in the dystrophic gastrocnemius muscle. Conclusions: PBMT modulates H2O2 production, TRPC-6, and PGC-1α content in the dystrophic muscle. These results suggest that laser therapy could act as an auxiliary therapy in the treatment of dystrophic patients.

High-intensity interval training in the form of isometric contraction improves fatigue resistance in dystrophin-deficient muscle

Duchenne muscular dystrophy is a genetic muscle-wasting disorder characterized by progressive muscle weakness and easy fatigability. Here we examined whether high-intensity interval training (HIIT) in the form of isometric contraction improves fatigue resistance in skeletal muscle from dystrophin-deficient mdx52 mice. Isometric HIIT was performed on plantar flexor muscles in vivo with supramaximal electrical stimulation every other day for 4 weeks (a total of 15 sessions). In the non-trained contralateral gastrocnemius muscle from mdx52 mice, the decreased fatigue resistance was associated with a reduction in the amount of peroxisome proliferator-activated receptor γ coactivator 1-α, citrate synthase activity, mitochondrial respiratory complex II, LC3B-II/I ratio, and mitophagy-related gene expression (i.e. Pink1, parkin, Bnip3 and Bcl2l13) as well as an increase in the phosphorylation levels of Src Tyr416 and Akt Ser473, the amount of p62, and the percentage of Evans Blue dye-positive area. Isometric HIIT restored all these alterations and markedly improved fatigue resistance in mdx52 muscles. Moreover, an acute bout of HIIT increased the phosphorylation levels of AMP-activated protein kinase (AMPK) Thr172, acetyl CoA carboxylase Ser79, unc-51-like autophagy activating kinase 1 (Ulk1) Ser555, and dynamin-related protein 1 (Drp1) Ser616 in mdx52 muscles. Thus, our data show that HIIT with isometric contractions significantly mitigates histological signs of pathology and improves fatigue resistance in dystrophin-deficient muscles. These beneficial effects can be explained by the restoration of mitochondrial function via AMPK-dependent induction of the mitophagy programme and de novo mitochondrial biogenesis. KEY POINTS: Skeletal muscle fatigue is often associated with Duchenne muscular dystrophy (DMD) and leads to an inability to perform daily tasks, profoundly decreasing quality of life. We examined the effect of high-intensity interval training (HIIT) in the form of isometric contraction on fatigue resistance in skeletal muscle from the mdx52 mouse model of DMD. Isometric HIIT counteracted the reduced fatigue resistance as well as dystrophic changes in skeletal muscle of mdx52 mice. This beneficial effect could be explained by the restoration of mitochondrial function via AMP-activated protein kinase-dependent mitochondrial biogenesis and the induction of the mitophagy programme in the dystrophic muscles.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

AMPK is mitochondrial medicine for neuromuscular disorders

Duchenne muscular dystrophy (DMD), myotonic dystrophy type 1 (DM1), and spinal muscular atrophy (SMA) are the most prevalent neuromuscular disorders (NMDs) in children and adults. Central to a healthy neuromuscular system are the processes that govern mitochondrial turnover and dynamics, which are regulated by AMP-activated protein kinase (AMPK). Here, we survey mitochondrial stresses that are common between, as well as unique to, DMD, DM1, and SMA, and which may serve as potential therapeutic targets to mitigate neuromuscular disease. We also highlight recent advances that leverage a mutation-agnostic strategy featuring physiological or pharmacological AMPK activation to enhance mitochondrial health in these conditions, as well as identify outstanding questions and opportunities for future pursuit.

Dysfunctional mitochondria accumulate in a skeletal muscle knockout model of Smn1, the causal gene of spinal muscular atrophy

The approved gene therapies for spinal muscular atrophy (SMA), caused by loss of survival motor neuron 1 (SMN1), greatly ameliorate SMA natural history but are not curative. These therapies primarily target motor neurons, but SMN1 loss has detrimental effects beyond motor neurons and especially in muscle. Here we show that SMN loss in mouse skeletal muscle leads to accumulation of dysfunctional mitochondria. Expression profiling of single myofibers from a muscle specific Smn1 knockout mouse model revealed down-regulation of mitochondrial and lysosomal genes. Albeit levels of proteins that mark mitochondria for mitophagy were increased, morphologically deranged mitochondria with impaired complex I and IV activity and respiration and that produced excess reactive oxygen species accumulated in Smn1 knockout muscles, because of the lysosomal dysfunction highlighted by the transcriptional profiling. Amniotic fluid stem cells transplantation that corrects the SMN knockout mouse myopathic phenotype restored mitochondrial morphology and expression of mitochondrial genes. Thus, targeting muscle mitochondrial dysfunction in SMA may complement the current gene therapy.

Glucose supplementation improves intestinal amino acid transport and muscle amino acid pool in pigs during chronic cold exposure

Mammals in northern regions chronically suffer from low temperatures during autumn-winter seasons. The aim of this study was to investigate the response of intestinal amino acid transport and the amino acid pool in muscle to chronic cold exposure via Min pig models (cold adaptation) and Yorkshire pig models (non-cold adaptation). Furthermore, this study explored the beneficial effects of glucose supplementation on small intestinal amino acid transport and amino acid pool in muscle of cold-exposed Yorkshire pigs. Min pigs (Exp. 1) and Yorkshire pigs (Exp. 2) were divided into a control group (17 °C, n = 6) and chronic cold exposure group (7 °C, n = 6), respectively. Twelve Yorkshire pigs (Exp. 3) were divided into a cold control group and cold glucose supplementation group (8 °C). The results showed that chronic cold exposure inhibited peptide transporter protein 1 (PepT1) and excitatory amino acid transporter 3 (EAAT3) expression in ileal mucosa and cationic amino acid transporter-1 (CAT-1) in the jejunal mucosa of Yorkshire pigs (P < 0.05). In contrast, CAT-1, PepT1 and EAAT3 expression was enhanced in the duodenal mucosa of Min pigs (P < 0.05). Branched amino acids (BCAA) in the muscle of Yorkshire pigs were consumed by chronic cold exposure, accompanied by increased muscle RING-finger protein-1 (MuRF1) and muscle atrophy F-box (atrogin-1) expression (P < 0.05). More importantly, reduced concentrations of dystrophin were detected in the muscle of Yorkshire pigs (P < 0.05). However, glycine concentration in the muscle of Min pigs was raised (P < 0.05). In the absence of interaction between chronic cold exposure and glucose supplementation, glucose supplementation improved CAT-1 expression in the jejunal mucosa and PepT1 expression in the ileal mucosa of cold-exposed Yorkshire pigs (P < 0.05). It also improved BCAA and inhibited MuRF1 and atrogin-1 expression in muscle (P < 0.05). Moreover, dystrophin concentration was improved by glucose supplementation (P < 0.05). In summary, chronic cold exposure inhibits amino acid absorption in the small intestine, depletes BCAA and promotes protein degradation in muscle. Glucose supplementation ameliorates the negative effects of chronic cold exposure on amino acid transport and the amino acid pool in muscle.

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.

Cardiovascular Disease in Duchenne Muscular Dystrophy

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Cardiomyopathy is the leading cause of death in patients with DMD.

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DMD has no cure, and there is no current consensus for treatment of DMD cardiomyopathy.

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This review discusses therapeutic strategies to potentially reduce or prevent cardiac dysfunction in DMD patients.

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Additional studies are needed to firmly establish optimal treatment modalities for DMD cardiomyopathy.

Duchenne muscular dystrophy (DMD) is a devastating disease affecting approximately 1 in every 3,500 male births worldwide. Multiple mutations in the dystrophin gene have been implicated as underlying causes of DMD. However, there remains no cure for patients with DMD, and cardiomyopathy has become the most common cause of death in the affected population. Extensive research is under way investigating molecular mechanisms that highlight potential therapeutic targets for the development of pharmacotherapy for DMD cardiomyopathy. In this paper, the authors perform a literature review reporting on recent ongoing efforts to identify novel therapeutic strategies to reduce, prevent, or reverse progression of cardiac dysfunction in DMD.

Cross-talk between TRPC-1, mTOR, PGC-1α and PPARδ in the dystrophic muscle cells treated with tempol

Background Ca2+ dysregulation and oxidative damage appear to have a central role in Duchenne muscular dystrophy (DMD) progression. The current study provides muscle cell-specific insights into the effect of Tempol on the TRPC 1 channel; on the positive and negative regulators of muscle cell differentiation; on the antioxidant enzymatic system; on the activators of mitochondrial biogenesis; and on the inflammatory process in the dystrophic primary muscle cells in culture.

METHODS

Mdx myotubes were treated with Tempol (5 mM) for 24 h. Untreated mdx myotubes and C57BL/10 myotubes were used as controls.

RESULTS

The Trypan Blue, MTT and Live/Dead Cell assays showed that Tempol (5 mM) presented no cytotoxic effect on the dystrophic muscle cells. The Tempol treated-mdx muscle cells showed significantly lower levels in the fluorescence intensity of intracellular calcium; TRPC-1 channel; MyoD; H₂O₂ and O₂^•- production; 4-HNE levels; SOD2, CAT and GPx levels; and TNF levels. On the other hand, SOD, CAT and GR mRNA relative expression were significantly higher in Tempol treated-mdx muscle cells. In addition, higher levels of Myogenin, MHC-Slow, mTOR, PGC-1α and PPARδ were also observed in Tempol treated-mdx muscle cells.

CONCLUSION

Our findings demonstrated that Tempol decreased intracellular calcium and oxidative stress in primary dystrophic muscle cells, promoting a cross-talk between TRPC-1, mTOR, PGC-1α and PPARδ.

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.

Theaflavin Promotes Mitochondrial Abundance and Glucose Absorption in Myotubes by Activating the CaMKK2-AMPK Signal Axis via Calcium-Ion Influx

Drinking tea has been proven to have a positive biological effect in regulating human glucose and lipid metabolism and preventing type 2 diabetes (T2D). Skeletal muscle (SkM) is responsible for 70% of the sugar metabolism in the human body, and its dysfunction is an important factor leading to the development of obesity, T2D, and muscle diseases. As one of the four known theaflavins (TFs) in black tea, the biological role of theaflavin (TF1) in regulating SkM metabolism has not been reported. In this study, mature myotubes induced by C2C12 cells in vitro were used as models. The results showed that TF1 (20 μM) promoted mitochondrial abundance and glucose absorption in myotubes by activating the CaMKK2-AMPK signaling axis via Ca²⁺ influx. Moreover, it promoted the expression of slow muscle fiber marker genes (Myh7, Myl2, Tnnt1, and Tnnc1) and PGC-1α/SIRT1, as well as enhanced the oxidative phosphorylation capacity of myotubes. In conclusion, this study preliminarily clarified the potential role of TF1 in regulating SkM glucose absorption as well as promoting SkM mitochondrial biosynthesis and slow muscle fiber formation. It has potential research and application values for the prevention/alleviation of SkM-related T2D and Ca²⁺-related skeletal muscle diseases through diet.

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.

Mss51 deletion increases endurance and ameliorates histopathology in the mdx mouse model of Duchenne muscular dystrophy

Mitochondrial derangement is an important contributor to the pathophysiology of muscular dystrophies and may be among the earliest cellular deficits. We have previously shown that disruption of Mss51, a mammalian skeletal muscle protein that localizes to the mitochondria, results in enhanced muscle oxygen consumption rate, increased endurance capacity, and improved limb muscle strength in mice with wildtype background. Here, we investigate whether Mss51 deletion in the mdx murine model of Duchenne muscular dystrophy (mdx-Mss51 KO) counteracts the muscle pathology and mitochondrial irregularities observed in mdx mice. We found that mdx-Mss51 KO mice had increased myofiber oxygen consumption rates and an amelioration of muscle histopathology compared to mdx counterparts. This corresponded with greater treadmill endurance and less percent fatigue in muscle physiology, but no improvement in forelimb grip strength or limb muscle force production. These findings suggest that although Mss51 deletion ameliorates the skeletal muscle mitochondrial respiration defects in mdx and improves fatigue resistance in vivo, the lack of improvement in force production suggests that this target alone may be insufficient for a therapeutic effect.

Oxidative Stress, Inflammation, and Activators of Mitochondrial Biogenesis: Tempol Targets in the Diaphragm Muscle of Exercise Trained-mdx Mice

The mdx mouse phenotype aggravated by chronic exercise on a treadmill makes this murine model more reliable for the study of muscular dystrophy. Thus, to better assess the Tempol effect on dystrophic pathways, the analyses in this study were performed in the blood samples and diaphragm muscle from treadmill trained adult (7–11-weeks old) mdx animals. The mdx mice were divided into three groups: mdxSed, sedentary controls (n = 28); mdxEx, exercise-trained animals (n = 28); and mdxEx+T, exercise-trained animals with the Tempol treatment (n = 28). The results demonstrated that the Tempol treatment promoted muscle strength gain, prevented muscle damage, reduced the inflammatory process, oxidative stress, and angiogenesis regulator, and up regulated the activators of mitochondrial biogenesis. The main new findings of this study are that Tempol reduced the NF-κB and increased the PGC1-α and PPARδ levels in the exercise-trained-mdx mice, which are probably related to the ability of this antioxidant to scavenge excessive ROS. These results reinforce the use of Tempol as a potential therapeutic strategy in DMD.

Twenty-one days of low-intensity eccentric training improve morphological characteristics and function of soleus muscles of mdx mice

Duchene muscular dystrophy (DMD) is caused by the absence of the protein dystrophin, which leads to muscle weakness, progressive degeneration, and eventually death due to respiratory failure. Low-intensity eccentric training (LIET) has been used as a rehabilitation method in skeletal muscles after disuse. Recently, LIET has also been used for rehabilitating dystrophic muscles, but its effects are still unclear. The purpose of this study was to investigate the effects of 21 days of LIET in dystrophic soleus muscle. Thirty-six male mdx mice were randomized into six groups (n = 6/each): mdx sedentary group; mdx training group-3 days; mdx training group-21 days; wild-type sedentary group; wild-type training group-3 days and wild-type training group-21 days. After the training sessions, animals were euthanized, and fragments of soleus muscles were removed for immunofluorescence and histological analyses, and measurements of active force and Ca²⁺ sensitivity of the contractile apparatus. Muscles of the mdx training group-21 days showed an improvement in morphological characteristics and an increase of active force when compared to the sedentary mdx group. The results show that LIET can improve the functionality of dystrophic soleus muscle in mice.

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.

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.

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.

Mitochondrial bioenergetic dysfunction in the D2.mdx model of Duchenne muscular dystrophy is associated with microtubule disorganization in skeletal muscle

In Duchenne muscular dystrophy, a lack of dystrophin leads to extensive muscle weakness and atrophy that is linked to cellular metabolic dysfunction and oxidative stress. This dystrophinopathy results in a loss of tethering between microtubules and the sarcolemma. Microtubules are also believed to regulate mitochondrial bioenergetics potentially by binding the outer mitochondrial membrane voltage dependent anion channel (VDAC) and influencing permeability to ADP/ATP cycling. The objective of this investigation was to determine if a lack of dystrophin causes microtubule disorganization concurrent with mitochondrial dysfunction in skeletal muscle, and whether this relationship is linked to altered binding of tubulin to VDAC. In extensor digitorum longus (EDL) muscle from 4-week old D2.mdx mice, microtubule disorganization was observed when probing for α-tubulin. This cytoskeletal disorder was associated with a reduced ability of ADP to stimulate respiration and attenuate H₂O₂ emission relative to wildtype controls. However, this was not associated with altered α-tubulin-VDAC2 interactions. These findings reveal that microtubule disorganization in dystrophin-deficient EDL is associated with impaired ADP control of mitochondrial bioenergetics, and suggests that mechanisms alternative to α-tubulin’s regulation of VDAC2 should be examined to understand how cytoskeletal disruption in the absence of dystrophin may cause metabolic dysfunctions in skeletal muscle.

Anti-Inflammatory and General Glucocorticoid Physiology in Skeletal Muscles Affected by Duchenne Muscular Dystrophy: Exploration of Steroid-Sparing Agents

In Duchenne muscular dystrophy (DMD), the activation of proinflammatory and metabolic cellular pathways in skeletal muscle cells is an inherent characteristic. Synthetic glucocorticoid intake counteracts the majority of these mechanisms. However, glucocorticoids induce burdensome secondary effects, including hypertension, arrhythmias, hyperglycemia, osteoporosis, weight gain, growth delay, skin thinning, cushingoid appearance, and tissue-specific glucocorticoid resistance. Hence, lowering the glucocorticoid dosage could be beneficial for DMD patients. A more profound insight into the major cellular pathways that are stabilized after synthetic glucocorticoid administration in DMD is needed when searching for the molecules able to achieve similar pathway stabilization. This review provides a concise overview of the major anti-inflammatory pathways, as well as the metabolic effects of glucocorticoids in the skeletal muscle affected in DMD. The known drugs able to stabilize these pathways, and which could potentially be combined with glucocorticoid therapy as steroid-sparing agents, are described. This could create new opportunities for testing in DMD animal models and/or clinical trials, possibly leading to smaller glucocorticoids dosage regimens for DMD patients.

Quantitative fluorescence imaging of mitochondria in body wall muscles of Caenorhabditis elegans under hyperglycemic conditions using a microfluidic chip

Type 2 diabetes is the most common metabolic disease, and insulin resistance plays a role in the pathogenesis of the disease. Because completely functional mitochondria are necessary to obtain glucose-stimulated insulin from pancreatic beta cells, dysfunction of mitochondrial oxidative pathway could be involved in the development of diabetes. As a simple animal model, Caenorhabditis elegans renders itself to investigate such metabolic mechanisms because it possesses insulin/insulin-like growth factor-1 signaling pathway similar to that in humans. Currently, the widely spread agarose pad-based immobilization technique for fluorescence imaging of the mitochondria in C. elegans is laborious, batchwise, and does not allow for facile handling of the worm. To overcome these technical challenges, we have developed a single-channel microfluidic device that can trap a C. elegans and allow to image the mitochondria in body wall muscles accurately and in higher throughput than the traditional approach. In specific, our microfluidic device took advantage of the proprioception of the worm to rotate its body in a microfluidic channel with an aspect ratio above one to gain more space for its undulation motion that was favorable for quantitative fluorescence imaging of mitochondria in the body wall muscles. Exploiting this unique feature of the microfluidic chip-based immobilization and fluorescence imaging, we observed a significant decrease in the mitochondrial fluorescence intensity under hyperglycemic conditions, whereas the agarose pad-based approach did not show any significant change under the same conditions. A machine learning model trained with these fluorescence images from the microfluidic device could classify healthy and hyperglycemic worms at high accuracy. Given this significant technological advantage, its easiness of use and low cost, our microfluidic imaging chip could become a useful immobilization tool for quantitative fluorescence imaging of the body wall muscles in C. elegans.

PGC-1α overexpression increases transcription factor EB nuclear localization and lysosome abundance in dystrophin-deficient skeletal muscle

Duchenne muscular dystrophy (DMD) is caused by the absence of functional dystrophin protein and results in progressive muscle wasting. Dystrophin deficiency leads to a host of dysfunctional cellular processes including impaired autophagy. Autophagic dysfunction appears to be due, at least in part, to decreased lysosomal abundance mediated by decreased nuclear localization of transcription factor EB (TFEB), a transcription factor responsible for lysosomal biogenesis. PGC-1α overexpression decreased disease severity in dystrophin-deficient skeletal muscle and increased PGC-1α has been linked to TFEB activation in healthy muscle. The purpose of this study was to determine the extent to which PGC-1α overexpression increased nuclear TFEB localization, increased lysosome abundance, and increased autophagosome degradation. We hypothesized that overexpression of PGC-1α would drive TFEB nuclear translocation, increase lysosome biogenesis, and improve autophagosome degradation. To address this hypothesis, we delivered PGC-1α via adeno-associated virus (AAV) vector injected into the right limb of 3-week-old mdx mice and the contralateral limbs received a sham injection. At 6 weeks of age, this approach increased PGC-1α transcript by 60-fold and increased TFEB nuclear localization in gastrocnemii from PGC-1α treated limbs by twofold compared to contralateral controls. Furthermore, lamp2, a marker of lysosome abundance, was significantly elevated in muscles from limbs overexpressing PGC-1α. Lastly, increased LC3II and similar p62 in PGC-1α overexpressing-limbs compared to contralateral limbs are supportive of increased degradation of autophagosomes. These data provide mechanistic insight into PGC-1α-mediated benefits to dystrophin-deficient muscle, such that increased TFEB nuclear localization in dystrophin-deficient muscle leads to increased lysosome biogenesis and autophagy.

Duchenne muscular dystrophy is associated with the inhibition of calcium uniport in mitochondria and an increased sensitivity of the organelles to the calcium-induced permeability transition

Duchenne muscular dystrophy (DMD) is characterized by a pronounced and progressive degradation of the structure of skeletal muscles, which decreases their strength and lowers endurance of the organism. At muscular dystrophy, mitochondria are known to undergo significant functional changes, which is manifested in a decreased efficiency of oxidative phosphorylation and impaired energy metabolism of the cell. It is believed that the DMD-induced functional changes of mitochondria are mainly associated with the dysregulation of Ca²⁺ homeostasis. This work examines the kinetic parameters of Ca²⁺ transport and the opening of the Ca²⁺-dependent MPT pore in the skeletal-muscle mitochondria of the dystrophin-deficient C57BL/10ScSn-mdx mice. As compared to the organelles of wild-type animals, skeletal-muscle mitochondria of mdx mice have been found to be much less efficient in respect to Ca²⁺ uniport, with the kinetics of Na⁺-dependent Ca²⁺ efflux not changing. The data obtained indicate that the decreased rate of Ca²⁺ uniport in the mitochondria of mdx mice may be associated with the increased level of the dominant negative subunit of Ca²⁺ uniporter (MCUb). The experiments have also shown that in mdx mice, skeletal-muscle mitochondria have low resistance to the induction of MPT, which may be related to a significantly increased expression of adenylate translocator (ANT2), a possible structural element of the MPT pore. The paper discusses how changes in the expression of calcium uniporter and putative components of the MPT pore caused by the development of DMD can affect Ca²⁺ homeostasis of skeletal-muscle mitochondria.

Autophagy in the heart is enhanced and independent of disease progression in mus musculus dystrophinopathy models

Background

Duchenne muscular dystrophy is a muscle wasting disease caused by dystrophin gene mutations resulting in dysfunctional dystrophin protein. Autophagy, a proteolytic process, is impaired in dystrophic skeletal muscle though little is known about the effect of dystrophin deficiency on autophagy in cardiac muscle. We hypothesized that with disease progression autophagy would become increasingly dysfunctional based upon indirect autophagic markers.

Methods

Markers of autophagy were measured by western blot in 7-week-old and 17-month-old control (C57) and dystrophic (mdx) hearts.

Results

Counter to our hypothesis, markers of autophagy were similar between groups. Given these surprising results, two independent experiments were conducted using 14-month-old mdx mice or 10-month-old mdx/Utrn^± mice, a more severe model of Duchenne muscular dystrophy. Data from these animals suggest increased autophagosome degradation.

Conclusion

Together these data suggest that autophagy is not impaired in the dystrophic myocardium as it is in dystrophic skeletal muscle and that disease progression and related injury is independent of autophagic dysfunction.

Early myopathy in Duchenne muscular dystrophy is associated with elevated mitochondrial H2 O2 emission during impaired oxidative phosphorylation

BACKGROUND

Muscle wasting and weakness in Duchenne muscular dystrophy (DMD) causes severe locomotor limitations and early death due in part to respiratory muscle failure. Given that current clinical practice focuses on treating secondary complications in this genetic disease, there is a clear need to identify additional contributions in the aetiology of this myopathy for knowledge-guided therapy development. Here, we address the unresolved question of whether the complex impairments observed in DMD are linked to elevated mitochondrial H2 O2 emission in conjunction with impaired oxidative phosphorylation. This study performed a systematic evaluation of the nature and degree of mitochondrial-derived H2 O2 emission and mitochondrial oxidative dysfunction in a mouse model of DMD by designing in vitro bioenergetic assessments that attempt to mimic in vivo conditions known to be critical for the regulation of mitochondrial bioenergetics.

METHODS

Mitochondrial bioenergetics were compared with functional and histopathological indices of myopathy early in DMD (4 weeks) in D2.B10-DMDmdx /2J mice (D2.mdx)-a model that demonstrates severe muscle weakness. Adenosine diphosphate's (ADP's) central effect of attenuating H2 O2 emission while stimulating respiration was compared under two models of mitochondrial-cytoplasmic phosphate exchange (creatine independent and dependent) in muscles that stained positive for membrane damage (diaphragm, quadriceps, and white gastrocnemius).

RESULTS

Pathway-specific analyses revealed that Complex I-supported maximal H2 O2 emission was elevated concurrent with a reduced ability of ADP to attenuate emission during respiration in all three muscles (mH2 O2 : +17 to +197% in D2.mdx vs. wild type). This was associated with an impaired ability of ADP to stimulate respiration at sub-maximal and maximal kinetics (-17 to -72% in D2.mdx vs. wild type), as well as a loss of creatine-dependent mitochondrial phosphate shuttling in diaphragm and quadriceps. These changes largely occurred independent of mitochondrial density or abundance of respiratory chain complexes, except for quadriceps. This muscle was also the only one exhibiting decreased calcium retention capacity, which indicates increased sensitivity to calcium-induced permeability transition pore opening. Increased H2 O2 emission was accompanied by a compensatory increase in total glutathione, while oxidative stress markers were unchanged. Mitochondrial bioenergetic dysfunctions were associated with induction of mitochondrial-linked caspase 9, necrosis, and markers of atrophy in some muscles as well as reduced hindlimb torque and reduced respiratory muscle function.

CONCLUSIONS

These results provide evidence that Complex I dysfunction and loss of central respiratory control by ADP and creatine cause elevated oxidant generation during impaired oxidative phosphorylation. These dysfunctions may contribute to early stage disease pathophysiology and support the growing notion that mitochondria are a potential therapeutic target in this disease.

Impairments in left ventricular mitochondrial bioenergetics precede overt cardiac dysfunction and remodelling in Duchenne muscular dystrophy

KEY POINTS

Ninety-eight per cent of patients with Duchenne muscular dystrophy (DMD) develop cardiomyopathy, with 40% developing heart failure. While increased propensity for mitochondrial induction of cell death has been observed in left ventricle, it remains unknown whether this is linked to impaired mitochondrial respiratory control and elevated H₂ O₂ emission prior to the onset of cardiomyopathy. Classic mouse models of DMD demonstrate hyper-regeneration in skeletal muscle which may mask mitochondrial abnormalities. Using a model with less regenerative capacity that is more akin to DMD patients, we observed elevated left ventricular mitochondrial H₂ O₂ and impaired oxidative phosphorylation in the absence of cardiac remodelling or overt cardiac dysfunction at 4 weeks. These impairments were associated with dysfunctions at complex I, governance by ADP and creatine-dependent phosphate shuttling, which results in a less efficient response to energy demands. Mitochondria may be a therapeutic target for the treatment of cardiomyopathy in DMD.

ABSTRACT

In Duchenne muscular dystrophy (DMD), mitochondrial dysfunction is predicted as a response to numerous cellular stressors, yet the contribution of mitochondria to the onset of cardiomyopathy remains unknown. To resolve this uncertainty, we designed in vitro assessments of mitochondrial bioenergetics to model mitochondrial control parameters that influence cardiac function. Both left ventricular mitochondrial responsiveness to the central bioenergetic controller ADP and the ability of creatine to facilitate mitochondrial-cytoplasmic phosphate shuttling were assessed. These measurements were performed in D2.B10-DMD^mdx /2J mice - a model that demonstrates skeletal muscle atrophy and weakness due to limited regenerative capacities and cardiomyopathy more akin to people with DMD than classic models. At 4 weeks of age, there was no evidence of cardiac remodelling or cardiac dysfunction despite impairments in ADP-stimulated respiration and ADP attenuation of H₂ O₂ emission. These impairments were seen at both submaximal and maximal ADP concentrations despite no reductions in mitochondrial content markers. The ability of creatine to enhance ADP's control of mitochondrial bioenergetics was also impaired, suggesting an impairment in mitochondrial creatine kinase-dependent phosphate shuttling. Susceptibly to permeability transition pore opening and the subsequent activation of cell death pathways remained unchanged. Mitochondrial H₂ O₂ emission was elevated despite no change in markers of irreversible oxidative damage, suggesting alternative redox signalling mechanisms should be explored. These findings demonstrate that selective mitochondrial dysfunction precedes the onset of overt cardiomyopathy in D2.mdx mice, suggesting that improving mitochondrial bioenergetics by restoring ADP, creatine-dependent phosphate shuttling and complex I should be considered for treating DMD patients.

Long-Term Morpholino Oligomers in Hexose Elicits Long-Lasting Therapeutic Improvements in mdx Mice

Approval of antisense oligonucleotide eteplirsen highlights the promise of exon-skipping therapeutics for Duchenne muscular dystrophy patients. However, the limited efficacy of eteplirsen underscores the importance to improve systemic delivery and efficacy. Recently, we demonstrated that a glucose and fructose (GF) delivery formulation effectively potentiates phosphorodiamidate morpholino oligomer (PMO). Considering the clinical potential of GF, it is important to determine the long-term compatibility and efficacy with PMO in mdx mice prior to clinical translation. Here, we report that yearlong administration of a clinically applicable PMO dose (50 mg/kg/week for 3 weeks followed by 50 mg/kg/month for 11 months) with GF elicited sustainably high levels of dystrophin expression in mdx mice, with up to 45% of the normal level of dystrophin restored in most peripheral muscles without any detectable toxicity. Importantly, PMO-GF resulted in phenotypical rescue and mitochondrial biogenesis with functional improvement. Carbohydrate metabolites measurements revealed improved metabolic and energetic conditions after PMO-GF treatment in mdx mice without metabolic anomaly. Collectively, our study shows PMO-GF’s ability to elicit long-lasting therapeutic effects with tolerable toxicity and represents a new treatment modality for Duchenne muscular dystrophy, and provides guidelines for antisense oligonucleotides with GF in clinical use.

Skeletal Muscle Metabolism in Duchenne and Becker Muscular Dystrophy-Implications for Therapies

The interactions between nutrition and metabolism and skeletal muscle have long been known. Muscle is the major metabolic organ&mdash;it consumes more calories than other organs&mdash;and therefore, there is a clear need to discuss these interactions and provide some direction for future research areas regarding muscle pathologies. In addition, new experiments and manuscripts continually reveal additional highly intricate, reciprocal interactions between metabolism and muscle. These reciprocal interactions include exercise, age, sex, diet, and pathologies including atrophy, hypoxia, obesity, diabetes, and muscle myopathies. Central to this review are the metabolic changes that occur in the skeletal muscle cells of muscular dystrophy patients and mouse models. Many of these metabolic changes are pathogenic (inappropriate body mass changes, mitochondrial dysfunction, reduced adenosine triphosphate (ATP) levels, and increased Ca²⁺) and others are compensatory (increased phosphorylated AMP activated protein kinase (pAMPK), increased slow fiber numbers, and increased utrophin). Therefore, reversing or enhancing these changes with therapies will aid the patients. The multiple therapeutic targets to reverse or enhance the metabolic pathways will be discussed. Among the therapeutic targets are increasing pAMPK, utrophin, mitochondrial number and slow fiber characteristics, and inhibiting reactive oxygen species. Because new data reveals many additional intricate levels of interactions, new questions are rapidly arising. How does muscular dystrophy alter metabolism, and are the changes compensatory or pathogenic? How does metabolism affect muscular dystrophy? Of course, the most profound question is whether clinicians can therapeutically target nutrition and metabolism for muscular dystrophy patient benefit? Obtaining the answers to these questions will greatly aid patients with muscular dystrophy.

Protective role of Parkin in skeletal muscle contractile and mitochondrial function

KEY POINTS

Parkin, an E3 ubiquitin ligase encoded by the Park2 gene, has been implicated in the regulation of mitophagy, a quality control process in which defective mitochondria are degraded. The exact physiological significance of Parkin in regulating mitochondrial function and contractility in skeletal muscle remains largely unexplored. Using Park2^-/- mice, we show that Parkin ablation causes a decrease in muscle specific force, a severe decrease in mitochondrial respiration, mitochondrial uncoupling and an increased susceptibility to opening of the permeability transition pore. These results demonstrate that Parkin plays a protective role in the maintenance of normal mitochondrial and contractile functions in skeletal muscles.

ABSTRACT

Parkin is an E3 ubiquitin ligase encoded by the Park2 gene. Parkin has been implicated in the regulation of mitophagy, a quality control process in which defective mitochondria are sequestered in autophagosomes and delivered to lysosomes for degradation. Although Parkin has been mainly studied for its implication in neuronal degeneration in Parkinson disease, its role in other tissues remains largely unknown. In the present study, we investigated the skeletal muscles of Park2 knockout (Park2^-/- ) mice to test the hypothesis that Parkin plays a physiological role in mitochondrial quality control in normal skeletal muscle, a tissue highly reliant on mitochondrial content and function. We first show that the tibialis anterior (TA) of Park2^-/- mice display a slight but significant decrease in its specific force. Park2^-/- muscles also show a trend for type IIB fibre hypertrophy without alteration in muscle fibre type proportion. Compared to Park2^+/+ muscles, the mitochondrial function of Park2^-/- skeletal muscles was significantly impaired, as indicated by the significant decrease in ADP-stimulated mitochondrial respiratory rates, uncoupling, reduced activities of respiratory chain complexes containing mitochondrial DNA (mtDNA)-encoded subunits and increased susceptibility to opening of the permeability transition pore. Muscles of Park2^-/- mice also displayed a decrease in the content of the mitochondrial pro-fusion protein Mfn2 and an increase in the pro-fission protein Drp1 suggesting an increase in mitochondrial fragmentation. Finally, Park2 ablation resulted in an increase in basal autophagic flux in skeletal muscles. Overall, the results of the present study demonstrate that Parkin plays a protective role in the maintenance of normal mitochondrial and contractile functions in normal skeletal muscles.

Autophagic dysfunction and autophagosome escape in the mdx mus musculus model of Duchenne muscular dystrophy

AIM

Duchenne muscular dystrophy is caused by the absence of functional dystrophin protein and results in a host of secondary effects. Emerging evidence suggests that dystrophic pathology includes decreased pro-autophagic signalling and suppressed autophagic flux in skeletal muscle, but the relationship between autophagy and disease progression is unknown. The purpose of this investigation was to determine the extent to which basal autophagy changes with disease progression. We hypothesized that autophagy impairment would increase with advanced disease.

METHODS

To test this hypothesis, 7-week-old and 17-month-old dystrophic diaphragms were compared to each other and age-matched controls.

RESULTS

Changes in protein markers of autophagy indicate impaired autophagic stimulation through AMPK, however, robust pathway activation in dystrophic muscle, independent of disease severity. Relative protein abundance of p62, an inverse correlate of autophagic degradation, was dramatically elevated with disease regardless of age. Likewise, relative protein abundance of Lamp2, a lysosome marker, was decreased twofold at 17 months of age in dystrophic muscle and was confirmed, along with mislocalization, in histological samples, implicating lysosomal dysregulation in this process. In dystrophic muscle, autophagosome-sized p62-positive foci were observed in the extracellular space. Moreover, we found that autophagosomes were released from both healthy and dystrophic diaphragms into the extracellular environment, and the occurrence of autophagosome escape was more frequent in dystrophic muscle.

CONCLUSION

These findings suggest autophagic dysfunction proceeds independent of disease progression and blunted degradation of autophagosomes is due in part to decreased lysosome abundance, and contributes to autophagosomal escape to the extracellular space.

Loss of hepatic LRPPRC alters mitochondrial bioenergetics, regulation of permeability transition and trans-membrane ROS diffusion

The French-Canadian variant of Leigh Syndrome (LSFC) is an autosomal recessive oxidative phosphorylation (OXPHOS) disorder caused by a mutation in LRPPRC, coding for a protein involved in the stability of mitochondrially-encoded mRNAs. Low levels of LRPPRC are present in all patient tissues, but result in a disproportionately severe OXPHOS defect in the brain and liver, leading to unpredictable subacute metabolic crises. To investigate the impact of the OXPHOS defect in the liver, we analyzed the mitochondrial phenotype in mice harboring an hepatocyte-specific inactivation of Lrpprc. Loss of LRPPRC in the liver caused a generalized growth delay, and typical histological features of mitochondrial hepatopathy. At the molecular level, LRPPRC deficiency caused destabilization of polyadenylated mitochondrial mRNAs, altered mitochondrial ultrastructure, and a severe complex IV (CIV) and ATP synthase (CV) assembly defect. The impact of LRPPRC deficiency was not limited to OXPHOS, but also included impairment of long-chain fatty acid oxidation, a striking dysregulation of the mitochondrial permeability transition pore, and an unsuspected alteration of trans-membrane H2O2 diffusion, which was traced to the ATP synthase assembly defect, and to changes in the lipid composition of mitochondrial membranes. This study underscores the value of mitochondria phenotyping to uncover complex and unexpected mechanisms contributing to the pathophysiology of mitochondrial disorders.

Reversal of Defective Mitochondrial Biogenesis in Limb-Girdle Muscular Dystrophy 2D by Independent Modulation of Histone and PGC-1α Acetylation

Mitochondrial dysfunction occurs in many muscle degenerative disorders. Here, we demonstrate that mitochondrial biogenesis was impaired in limb-girdle muscular dystrophy (LGMD) 2D patients and mice and was associated with impaired OxPhos capacity. Two distinct approaches that modulated histones or peroxisome proliferator-activated receptor-gamma coactivator 1 α (PGC-1α) acetylation exerted equivalent functional effects by targeting different mitochondrial pathways (mitochondrial biogenesis or fatty acid oxidation[FAO]). The histone deacetylase inhibitor Trichostatin A (TSA) changed chromatin assembly at the PGC-1α promoter, restored mitochondrial biogenesis, and enhanced muscle oxidative capacity. Conversely, nitric oxide (NO) triggered post translation modifications of PGC-1α and induced FAO, recovering the bioenergetics impairment of muscles but shunting the defective mitochondrial biogenesis. In conclusion, a transcriptional blockade of mitochondrial biogenesis occurred in LGMD-2D and could be recovered by TSA changing chromatin conformation, or it could be overcome by NO activating a mitochondrial salvage pathway.

Supplementation action with ascorbic acid in the morphology of the muscular layer and reactive acetylcholinesterase neurons of ileum of mdx mice

The Duchenne Muscular Dystrophy (DMD) is a genetic disorder characterized by the absence of dystrophin protein, causing severe myopathy from increases of oxidative stress. Injuries of intestinal muscle can compromise the myenteric plexus. This study aimed to evaluate the disorders occurred in the muscular layer and in the acetylcholinesterase myenteric neurons (ACHE-r) of ileum of mdx mice, and the effects of supplementation with ascorbic acid (AA) in both components. 30 male mice C57BL/10, and 30 male mice C57BL/10Mdx were separated according to the age and treatment (n=10/group): 30-days-old control group (C30); 30-days-old dystrophic group (D30); 60-days-old control group (C60); 60-days-old dystrophic group (D60); 60-days-old control group supplemented with AA (CS60); and 60-days-old dystrophic group supplemented with AA (DS60). The animals were euthanized and the ileum was collected and processed. Semi-serial sections were stained by Masson's trichrome, and acetylcholinesterase histochemical technique in whole-mounts preparations to identify the myenteric neurons. The muscular layer thickness and the area of smooth muscle of ileum were lower in dystrophic groups, especially in D30 group. The DS60 group showed the muscular layer thickness similar to C60. The density of ACHE-r neurons of myenteric plexus of ileum was lower in D30 animals; however, it was similar in animals of 60-days-old without treatment (C60 and D60) and, higher in DS60. The cell body profile area of ACHE-r neurons was similar in C30-D30 and C60-D60; however, it was higher in DS60. DMD caused damage to the ileum's musculature and myenteric plexus, and the AA prevented the ACHE-r neuronal loss.

Lifelong quercetin enrichment and cardioprotection in Mdx/Utrn+/− mice

Duchenne Muscular Dystrophy (DMD) is associated with progressive cardiac pathology; however, the SIRT1/PGC1-α activator quercetin may cardioprotect dystrophic hearts. We tested the extent to which long-term 0.2% dietary quercetin enrichment attenuates dystrophic cardiopathology in Mdx/Utrn+/− mice. At 2 mo, Mdx/Utrn+/− mice were fed quercetin-enriched (Mdx/Utrn+/−-Q) or control diet (Mdx/Utrn+/−) for 8 mo. Control C57BL/10 (C57) animals were fed a control diet for 10 mo. Cardiac function was quantified by MRI at 2 and 10 mo. Spontaneous physical activity was quantified during the last week of treatment. At 10 mo hearts were excised for histological and biochemical analysis. Quercetin feeding improved various physiological indexes of cardiac function in diseased animals. Mdx/Utrn+/−-Q also engaged in more high-intensity physical activity than controls. Histological analyses of heart tissues revealed higher expression and colocalization of utrophin and α-sarcoglycan. Lower abundance of fibronectin, cardiac damage (Hematoxylin Eosin-Y), and MMP9 were observed in quercetin-fed vs. control Mdx/Utrn+/− mice. Quercetin evoked higher protein abundance of PGC-1α, cytochrome c, ETC complexes I–V, citrate synthase, SOD2, and GPX compared with control-fed Mdx/Utrn+/−. Quercetin decreased abundance of inflammatory markers including NFκB, TGF-β1, and F4/80 compared with Mdx/Utrn+/−; however, P-NFκB, P-IKBα, IKBα, CD64, and COX2 were similar between groups. Dietary quercetin enrichment improves cardiac function in aged Mdx/Utrn+/− mice and increases mitochondrial protein content and dystrophin glycoprotein complex formation. Histological analyses indicate a marked attenuation in pathological cardiac remodeling and indicate that long-term quercetin consumption benefits the dystrophic heart.

NEW & NOTEWORTHY The current investigation provides first-time evidence that quercetin provides physiological cardioprotection against dystrophic pathology and is associated with improved spontaneous physical activity. Secondary findings suggest that quercetin-dependent outcomes are in part due to PGC-1α pathway activation.

Long-Term Quercetin Dietary Enrichment Partially Protects Dystrophic Skeletal Muscle

Duchenne muscular dystrophy (DMD) results from a genetic lesion in the dystrophin gene and leads to progressive muscle damage. PGC-1α pathway activation improves muscle function and decreases histopathological injury. We hypothesized that mild disease found in the limb muscles of mdx mice may be responsive to quercetin-mediated protection of dystrophic muscle via PGC-1α pathway activation. To test this hypothesis muscle function was measured in the soleus and EDL from 14 month old C57, mdx, and mdx mice treated with quercetin (mdxQ; 0.2% dietary enrichment) for 12 months. Quercetin reversed 50% of disease-related losses in specific tension and partially preserved fatigue resistance in the soleus. Specific tension and resistance to contraction-induced injury in the EDL were not protected by quercetin. Given some functional gain in the soleus it was probed with histological and biochemical approaches, however, in dystrophic muscle histopathological outcomes were not improved by quercetin and suppressed PGC-1α pathway activation was not increased. Similar to results in the diaphragm from these mice, these data suggest that the benefits conferred to dystrophic muscle following 12 months of quercetin enrichment were underwhelming. Spontaneous activity at the end of the treatment period was greater in mdxQ compared to mdx indicating that quercetin fed mice were more active in addition to engaging in more vigorous activity. Hence, modest preservation of muscle function (specific tension) and elevated spontaneous physical activity largely in the absence of tissue damage in mdxQ suggests dietary quercetin may mediate protection.

Mitochondria mediate cell membrane repair and contribute to Duchenne muscular dystrophy

Dystrophin deficiency is the genetic basis for Duchenne muscular dystrophy (DMD), but the cellular basis of progressive myofiber death in DMD is not fully understood. Using two dystrophin-deficient mdx mouse models, we find that the mitochondrial dysfunction is among the earliest cellular deficits of mdx muscles. Mitochondria in dystrophic myofibers also respond poorly to sarcolemmal injury. These mitochondrial deficits reduce the ability of dystrophic muscle cell membranes to repair and are associated with a compensatory increase in dysferlin-mediated membrane repair proteins. Dysferlin deficit in mdx mice further compromises myofiber cell membrane repair and enhances the muscle pathology at an asymptomatic age for dysferlin-deficient mice. Restoring partial dystrophin expression by exon skipping improves mitochondrial function and offers potential to improve myofiber repair. These findings identify that mitochondrial deficit in muscular dystrophy compromises the repair of injured myofibers and show that this repair mechanism is distinct from and complimentary to the dysferlin-mediated repair of injured myofibers.

Forty years later: Mitochondria as therapeutic targets in muscle diseases

The hypothesis that mitochondrial dysfunction can be a general mechanism for cell death in muscle diseases is 40 years old. The key elements of the proposed pathogenetic sequence (cytosolic Ca2+ overload followed by excess mitochondrial Ca2+ uptake, functional and then structural damage of mitochondria, energy shortage, worsened elevation of cytosolic Ca2+ levels, hypercontracture of muscle fibers, cell necrosis) have been confirmed in amazing detail by subsequent work in a variety of models. The explicit implication of the hypothesis was that it "may provide the basis for a more rational treatment for some conditions even before their primary causes are known" (Wrogemann and Pena, 1976, Lancet, 1, 672-674). This prediction is being fulfilled, and the potential of mitochondria as pharmacological targets in muscle diseases may soon become a reality, particularly through inhibition of the mitochondrial permeability transition pore and its regulator cyclophilin D.

ActRIIB blockade increases force-generating capacity and preserves energy supply in exercising mdx mouse muscle in vivo

Postnatal blockade of the activin type IIB receptor (ActRIIB) represents a promising therapeutic strategy for counteracting dystrophic muscle wasting. However, its impact on muscle function and bioenergetics remains poorly documented in physiologic conditions. We have investigated totally noninvasively the effect of 8-wk administration of either soluble ActRIIB signaling inhibitor (sActRIIB-Fc) or vehicle PBS (control) on gastrocnemius muscle force-generating capacity, energy metabolism, and anatomy in dystrophic mdx mice using magnetic resonance (MR) imaging and dynamic [³¹P]-MR spectroscopy ([³¹P]-MRS) in vivo ActRIIB inhibition increased muscle volume (+33%) without changing fiber-type distribution, and increased basal animal oxygen consumption (+22%) and energy expenditure (+23%). During an in vivo standardized fatiguing exercise, maximum and total absolute contractile forces were larger (+40 and 24%, respectively) in sActRIIB-Fc treated animals, whereas specific force-generating capacity and fatigue resistance remained unaffected. Furthermore, sActRIIB-Fc administration did not alter metabolic fluxes, ATP homeostasis, or contractile efficiency during the fatiguing bout of exercise, although it dramatically reduced the intrinsic mitochondrial capacity for producing ATP. Overall, sActRIIB-Fc treatment increased muscle mass and strength without altering the fundamental weakness characteristic of dystrophic mdx muscle. These data support the clinical interest of ActRIIB blockade for reversing dystrophic muscle wasting.-Béchir, N., Pecchi, E., Vilmen, C., Le Fur, Y., Amthor, H., Bernard, M., Bendahan, D., Giannesini, B. ActRIIB blockade increases force-generating capacity and preserves energy supply in exercising mdx mouse muscle in vivo.

Oral quercetin administration transiently protects respiratory function in dystrophin-deficient mice

KEY POINT

PGC-1α pathway activation has been shown to decrease disease severity and can be driven by quercetin. Oral quercetin supplementation protected respiratory function for 4-6 months during a 12 month dosing regimen. This transient protection was probably due to a failure to sustain elevated SIRT1 activity and downstream PGC-1α signalling. Quercetin supplementation may be a beneficial treatment as part of a cocktail provided continued SIRT1 activity elevation is achieved.

ABSTRACT

Duchenne muscular dystrophy (DMD) impacts 1 : 3500 boys and leads to muscle dysfunction culminating in death due to respiratory or cardiac failure. There is an urgent need for effective therapies with the potential for immediate application for this patient population. Quercetin, a flavonoid with an outstanding safety profile, may provide therapeutic relief to DMD patients as the wait for additional therapies continues. This study evaluated the capacity of orally administered quercetin (0.2%) in 2 month old mdx mice to improve respiratory function and end-point functional and histological outcomes in the diaphragm following 12 months of treatment. Respiratory function was protected for the first 4-6 months of treatment but appeared to become insensitive to quercetin thereafter. Consistent with this, end-point functional measures were decreased and histopathological measures were more severe in dystrophic muscle compared to C57 and similar between control-fed and quercetin-fed mdx mice. To better understand the transient nature of improved respiratory function, we measured PGC-1α pathway activity, which is suggested to be up-regulated by quercetin supplementation. This pathway was largely suppressed in dystrophic muscle compared to healthy muscle, and at the 14 month time point dietary quercetin enrichment did not increase expression of downstream effectors. These data support the efficacy of quercetin as an intervention for DMD in skeletal muscle, and also indicate the development of age-dependent quercetin insensitivity when continued supplementation fails to drive the PGC-1α pathway. Continued study is needed to determine if this is related to disease severity, age or other factors.

Hypoxia-induced cardiac injury in dystrophic mice

Duchenne muscular dystrophy (DMD) is a disease of progressive destruction of striated muscle, resulting in muscle weakness with progressive respiratory and cardiac failure. Respiratory and cardiac disease are the leading causes of death in DMD patients. Previous studies have suggested an important link between cardiac dysfunction and hypoxia in the dystrophic heart; these studies aim to understand the mechanism underlying this connection. Here we demonstrate that anesthetized dystrophic mice display significant mortality following acute exposure to hypoxia. This increased mortality is associated with a significant metabolic acidosis, despite having significantly higher levels of arterial Po2 Chronic hypoxia does not result in mortality, but rather is characterized by marked cardiac fibrosis. Studies in isolated hearts reveal that the contractile function of dystrophic hearts is highly susceptible to short bouts of ischemia, but these hearts tolerate prolonged acidosis better than wild-type hearts, indicating an increased sensitivity of the dystrophic heart to hypoxia. Dystrophic hearts display decreased cardiac efficiency and oxygen extraction. Isolated dystrophic cardiomyocytes and hearts have normal levels of FCCP-induced oxygen consumption, and mitochondrial morphology and content are normal in the dystrophic heart. These studies demonstrate reductions in cardiac efficiency and oxygen extraction of the dystrophic heart. The underlying cause of this reduced oxygen extraction is not clear; however, the current studies suggest that large disruptions of mitochondrial respiratory function or coronary flow regulation are not responsible. This finding is significant, as hypoxia is a common and largely preventable component of DMD that may contribute to the progression of the cardiac disease in DMD patients.

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.