Nitrosative stress elicited by nNOSµ delocalization inhibits muscle force in dystrophin-null mice

Uncovering the Embryonic Origins of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe degenerative muscle disease caused by mutations in the DMD gene, which encodes dystrophin. Despite its initial description in the late 19th century by French neurologist Guillaume Duchenne de Boulogne, and identification of causal DMD genetic mutations in the 1980s, therapeutics remain challenging. The current standard of care is corticosteroid treatment, which delays the progression of muscle dysfunction but is associated with significant adverse effects. Emerging therapeutic approaches, including AAV-mediated gene transfer, CRISPR gene editing, and small molecule interventions, are under development but face considerable obstacles. Although DMD is viewed as a progressive muscle disease, muscle damage and abnormal molecular signatures are already evident during fetal myogenesis. This early onset of pathology suggests that the limited success of current therapies may partly be due to their administration after aberrant embryonic myogenesis has occurred in the absence of dystrophin. Consequently, identifying optimal therapeutic strategies and intervention windows for DMD may depend on a better understanding of the earliest DMD disease mechanisms. As newer techniques are applied, the field is gaining increasingly detailed insights into the early muscle developmental abnormalities in DMD. A comprehensive understanding of the initial events in DMD pathogenesis and progression will facilitate the generation and testing of effective therapeutic interventions.

The Gut Microbiota Involvement in the Panorama of Muscular Dystrophy Pathogenesis

Muscular dystrophies (MDs) are genetically heterogeneous diseases characterized by primary skeletal muscle atrophy. The collapse of muscle structure and irreversible degeneration of tissues promote the occurrence of comorbidities, including cardiomyopathy and respiratory failure. Mitochondrial dysfunction leads to inflammation, fibrosis, and adipogenic cellular infiltrates that exacerbate the symptomatology of MD patients. Gastrointestinal disorders and metabolic anomalies are common in MD patients and may be determined by the interaction between the intestine and its microbiota. Therefore, the gut-muscle axis is one of the actors involved in the spread of inflammatory signals to all muscles. In this review, we aim to examine in depth how intestinal dysbiosis can modulate the metabolic state, the immune response, and mitochondrial biogenesis in the course and progression of the most investigated MDs such as Duchenne Muscular Dystrophy (DMD) and Myotonic Dystrophy (MD1), to better identify gut microbiota metabolites working as therapeutic adjuvants to improve symptoms of MD.

Acute microtubule changes linked to DMD pathology are insufficient to impair contractile function or enhance contraction-induced injury in healthy muscle

Duchenne muscular dystrophy (DMD) is marked by the genetic deficiency of the dystrophin protein in striated muscle whose consequence is a cascade of cellular changes that predispose the susceptibility to contraction injury central to DMD pathology. Recent evidence identified the proliferation of microtubules enriched in post-translationally modified tubulin as a consequence of dystrophins absence that increases the passive mechanics of the muscle fiber and the excess mechanotransduction elicited reactive oxygen species and calcium signals that promote contraction injury. Motivated by evidence that acutely normalizing the disease microtubule alterations reduced contraction injury in murine DMD muscle (mdx), here we sought the direct impact of these microtubule alterations independent of dystrophins absence and the multitude of other changes consequent to dystrophic disease. To this end we used acute pharmacologic (epithiolone-D, EpoD; 4 hours) or genetic (vashohibin-2 and small vasohibin binding protein overexpression via AAV9; 2 weeks) strategies to effectively model the proliferation of detyrosination enriched microtubules in the mdx muscle. Quantifying in vivo nerve evoked plantarflexor function we find no alteration in peak torque nor contraction kinetics in WT mice modeling these DMD relevant MT alterations. Quantifying the susceptibility to eccentric contraction injury we show EpoD treatment proffered a small but significant protection from contraction injury while VASH/SVBP had no discernable impact. We conclude that the disease dependent MT alterations act in concert with additional cellular changes to predispose contraction injury in DMD.

A review on mechanistic insights into structure and function of dystrophin protein in pathophysiology and therapeutic targeting of Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder characterized by progressive and severe muscle weakening and degeneration. Among the various forms of muscular dystrophy, it stands out as one of the most common and impactful, predominantly affecting boys. The condition arises due to mutations in the dystrophin gene, a key player in maintaining the structure and function of muscle fibers. The manuscript explores the structural features of dystrophin protein and their pivotal roles in DMD. We present an in-depth analysis of promising therapeutic approaches targeting dystrophin and their implications for the therapeutic management of DMD. Several therapies aiming to restore dystrophin protein or address secondary pathology have obtained regulatory approval, and many others are ongoing clinical development. Notably, recent advancements in genetic approaches have demonstrated the potential to restore partially functional dystrophin forms. The review also provides a comprehensive overview of the status of clinical trials for major therapeutic genetic approaches for DMD. In addition, we have summarized the ongoing therapeutic approaches and advanced mechanisms of action for dystrophin restoration and the challenges associated with DMD therapeutics.

Down-regulation of neuronal form of Nitric oxide synthase in the Nurse cell of Trichinella spiralis

The free radical nitric oxide (NO) and Ca2+ are critical regulators of skeletal muscle exercise performance and fatigue. The major source of NO in skeletal muscle cells is the neuronal form of the enzyme Nitric oxide synthase (nNOS). One of the most peculiar characteristics of the Nurse cell of Trichinella spiralis (T. spiralis) is the complete loss of the contractile capabilities of its derivative striated muscle fiber. The aim of the present study was to clarify the expression of nNOS protein and mRNA in striated muscles during the muscle phase of T. spiralis infection in mice. Muscle tissue samples were collected from mice at days 0, 14, 24, and 35 post infection (d.p.i.). The expression of nNOS was investigated by immunohistochemistry, and the expression levels of mRNA of mouse Nitric oxide synthase 1 (Nos1) by real-time PCR. The presence of nNOS protein was still well observable in the disintegrated sarcoplasm at the early stage of infection. The cytoplasm of the developing and mature Nurse cell showed the absence of this protein. At least at the beginning of the Nurse cell development, Trichinella uses the same repairing process of skeletal muscle cell, induced after any trauma and this corroborates very well our results concerning the nNOS expression on day 14 p.i. At a later stage, however, we could suggest that the down-regulation of nNOS in the Nurse cell of T. spiralis either serves a protective function or is an outcome of the genetic identity of the Nurse cell.

Microbes, metabolites and muscle: Is the gut-muscle axis a plausible therapeutic target in Duchenne muscular dystrophy?

NEW FINDINGS

What is the topic of this review? The contribution of gut microbial signalling to skeletal muscle maintenance and development and identification of potential therapeutic targets in progressive muscle degenerative diseases such as Duchenne muscular dystrophy. What advances does it highlight? Gut microbe-derived metabolites are multifaceted signalling molecules key to muscle function, modifying pathways contributing to skeletal muscle wasting, making them a plausible target for adjunctive therapy in muscular dystrophy.

ABSTRACT

Skeletal muscle is the largest metabolic organ making up ∼50% of body mass. Because skeletal muscle has both metabolic and endocrine properties, it can manipulate the microbial populations within the gut. In return, microbes exert considerable influence on skeletal muscle via numerous signalling pathways. Gut bacteria produce metabolites (i.e., short chain fatty acids, secondary bile acids and neurotransmitter substrates) that act as fuel sources and modulators of inflammation, influencing host muscle development, growth and maintenance. The reciprocal interactions between microbes, metabolites and muscle establish a bidirectional gut-muscle axis. The muscular dystrophies constitute a broad range of disorders with varying disabilities. In the profoundly debilitating monogenic disorder Duchenne muscular dystrophy (DMD), skeletal muscle undergoes a reduction in muscle regenerative capacity leading to progressive muscle wasting, resulting in fibrotic remodelling and adipose infiltration. The loss of respiratory muscle in DMD culminates in respiratory insufficiency and eventually premature death. The pathways contributing to aberrant muscle remodelling are potentially modulated by gut microbial metabolites, thus making them plausible targets for pre- and probiotic supplementation. Prednisone, the gold standard therapy for DMD, drives gut dysbiosis, inducing a pro-inflammatory phenotype and leaky gut barrier contributing to several of the well-known side effects associated with chronic glucocorticoid treatment. Several studies have observed that gut microbial supplementation or transplantation exerts positive effects on muscle, including mitigating the side effects of prednisone. There is growing evidence in support of the potential for an adjunctive microbiota-directed regimen designed to optimise gut-muscle axis signalling, which could alleviate muscle wasting in DMD.

Curcumin Administration Improves Force of mdx Dystrophic Diaphragm by Acting on Fiber-Type Composition, Myosin Nitrotyrosination and SERCA1 Protein Levels

The vegetal polyphenol curcumin displays beneficial effects against skeletal muscle derangement induced by oxidative stress, disuse or aging. Since oxidative stress and inflammation are involved in the progression of muscle dystrophy, the effects of curcumin administration were investigated in the diaphragm of mdx mice injected intraperitoneally or subcutaneously with curcumin for 4-12-24 weeks. Curcumin treatment independently of the way and duration of administration (i) ameliorated myofiber maturation index without affecting myofiber necrosis, inflammation and degree of fibrosis; (ii) counteracted the decrease in type 2X and 2B fiber percentage; (iii) increased about 30% both twitch and tetanic tensions of diaphragm strips; (iv) reduced myosin nitrotyrosination and tropomyosin oxidation; (v) acted on two opposite nNOS regulators by decreasing active AMP-Kinase and increasing SERCA1 protein levels, the latter effect being detectable also in myotube cultures from mdx satellite cells. Interestingly, increased contractility, decreased myosin nitrotyrosination and SERCA1 upregulation were also detectable in the mdx diaphragm after a 4-week administration of the NOS inhibitor 7-Nitroindazole, and were not improved further by a combined treatment. In conclusion, curcumin has beneficial effects on the dystrophic muscle, mechanistically acting for the containment of a deregulated nNOS activity.

Microdystrophin Gene Addition Significantly Improves Muscle Functionality and Diaphragm Muscle Histopathology in a Fibrotic Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a rare neuromuscular disease affecting 1:5000 newborn males. No cure is currently available, but gene addition therapy, based on the adeno-associated viral (AAV) vector-mediated delivery of microdystrophin transgenes, is currently being tested in clinical trials. The muscles of DMD boys present significant fibrotic and adipogenic tissue deposition at the time the treatment starts. The presence of fibrosis not only worsens the disease pathology, but also diminishes the efficacy of gene therapy treatments. To gain an understanding of the efficacy of AAV-based microdystrophin gene addition in a relevant, fibrotic animal model of DMD, we conducted a systemic study in juvenile D2.mdx mice using the single intravenous administration of an AAV8 system expressing a sequence-optimized murine microdystrophin, named MD1 (AAV8-MD1). We mainly focused our study on the diaphragm, a respiratory muscle that is crucial for DMD pathology and that has never been analyzed after treatment with AAV-microdystrophin in this mouse model. We provide strong evidence here that the delivery of AAV8-MD1 provides significant improvement in body-wide muscle function. This is associated with the protection of the hindlimb muscle from contraction-induced damage and the prevention of fibrosis deposition in the diaphragm muscle. Our work corroborates the observation that the administration of gene therapy in DMD is beneficial in preventing muscle fibrosis.

The Implication of Hinge 1 and Hinge 4 in Micro-Dystrophin Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a fatal muscle disease caused by dystrophin deficiency. Dystrophin consists of the amino terminus, central rod domain with 24 spectrin-like repeats and four hinges (H), cysteine-rich domain, and carboxyl terminus. Several highly abbreviated micro-dystrophins (μDys) are currently in clinical trials. They all carry H1 and H4. In this study, we investigated whether these two hinges are essential for μDy function in murine DMD models. Three otherwise identical μDys were engineered to contain H1 and/or H4 and were named H1/H4 (with both H1 and H4), ΔH1 (without H1), and ΔH4 (without H4). These constructs were packaged in adeno-associated virus serotype-9 and delivered to the tibialis anterior muscle of 3-month-old male mdx4cv mice (1E12 vector genome particles/muscle). Three months later, we detected equivalent μDys expression in total muscle lysate. However, only H1/H4 and ΔH1 showed correct sarcolemmal localization. ΔH4 mainly existed as sarcoplasmic aggregates. H1/H4 and ΔH1, but not ΔH4, fully restored the dystrophin-associated protein complex and significantly improved the specific muscle force. Eccentric contraction-induced force decline was best protected by H1/H4, followed by ΔH1, but not by ΔH4. Next, we compared H1/H4 and ΔH1 in 6-week-old male mdx mice by intravenous injection (1E13 vector genome particles/mouse). Four months postinjection, H1/H4 significantly outperformed ΔH1 in extensor digitorum longus muscle force measurements but two constructs yielded comparable electrocardiography improvements. We conclude that H4 is essential for μDys function and H1 facilitates force production. Our findings will help develop next-generation μDys gene therapy.

Lifelong Outcomes of Systemic Adeno-Associated Virus Micro-Dystrophin Gene Therapy in a Murine Duchenne Muscular Dystrophy Model

Adeno-associated virus (AAV)-mediated systemic micro-dystrophin (μDys) therapy is currently in clinical trials. The hope is to permanently improve the life quality of Duchenne muscular dystrophy (DMD) patients. Numerous preclinical studies have been conducted to support these trials. However, none examined whether a single therapy at a young age can lead to lifelong disease amelioration. To address this critical question, we injected 1 × 1013 vg particles/mouse of an AAV serotype-9 μDys vector to 3-month-old mdx mice through the tail vein. Therapeutic outcomes were evaluated at the age of 11 months (adulthood, 8 months postinjection) and 21 months (terminal age, 18 months postinjection). Immunostaining and Western blot showed saturated supraphysiological levels of μDys expression in skeletal muscle and heart till the end of the study. Treatment significantly improved grip force and treadmill running, and significantly reduced the serum creatine kinase level at both time points. Since cardiac death is a major threat in late-stage patients, we evaluated cardiac electrophysiology and hemodynamics by ECG and the closed-chest cardiac catheter assay, respectively. Significant improvements were observed in these assays. Importantly, many ECG and hemodynamic parameters (heart rate, PR interval, QRS duration, QTc interval, end-diastolic/systolic volume, dP/dt max and min, max pressure, and ejection fraction) were completely normalized at 21 months of age. Our results have provided direct evidence that a single systemic AAV μDys therapy has the potential to provide lifelong benefits in the murine DMD model.

nNOS-derived NO modulates force production and iNO-derived NO the excitability in C2C12-derived 3D tissue engineering skeletal muscle via different NO signaling pathways

Nitric oxide (NO) is a bioactive gas produced by one of the three NO synthases: neuronal NOS (nNOS), inducible (iNOS), and endothelial NOS (eNOS). NO has a relevant modulatory role in muscle contraction; this takes place through two major signaling pathways: (i) activation of soluble guanylate cyclase and, thus, protein kinase G or (ii) nitrosylation of sulfur groups of cysteine. Although it has been suggested that nNOS-derived NO is the responsible isoform in muscle contraction, the roles of eNOS and iNOS and their signaling pathways have not yet been clarified. To elucidate the action of each pathway, we optimized the generation of myooids, an engineered skeletal muscle tissue based on the C2C12 cell line. In comparison with diaphragm strips from wild-type mice, 180 myooids were analyzed, which expressed all relevant excitation–contraction coupling proteins and both nNOS and iNOS isoforms. Along with the biochemical results, myooids treated with NO donor (SNAP) and unspecific NOS blocker (L-NAME) revealed a comparable NO modulatory effect on force production as was observed in the diaphragm strips. Under the effects of pharmacological tools, we analyzed the myooids in response to electrical stimulation of two possible signaling pathways and NO sources. The nNOS-derived NO exerted its negative effect on force production via the sGG-PKG pathway, while iNOS-derived NO increased the excitability in response to sub-threshold electrical stimulation. These results strengthen the hypotheses of previous reports on the mechanism of action of NO during force production, showed a novel function of iNOS-derived NO, and establish the myooid as a novel and robust alternative model for pathophysiological skeletal muscle research.

Multiomic Approaches to Uncover the Complexities of Dystrophin-Associated Cardiomyopathy

Despite major progress in treating skeletal muscle disease associated with dystrophinopathies, cardiomyopathy is emerging as a major cause of death in people carrying dystrophin gene mutations that remain without a targeted cure even with new treatment directions and advances in modelling abilities. The reasons for the stunted progress in ameliorating dystrophin-associated cardiomyopathy (DAC) can be explained by the difficulties in detecting pathophysiological mechanisms which can also be efficiently targeted within the heart in the widest patient population. New perspectives are clearly required to effectively address the unanswered questions concerning the identification of authentic and effectual readouts of DAC occurrence and severity. A potential way forward to achieve further therapy breakthroughs lies in combining multiomic analysis with advanced preclinical precision models. This review presents the fundamental discoveries made using relevant models of DAC and how omics approaches have been incorporated to date.

Dystrophin deficiency impairs vascular structure and function in the canine model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a muscle-wasting disease caused by dystrophin deficiency. Vascular dysfunction has been suggested as an underlying pathogenic mechanism in DMD. However, this has not been thoroughly studied in a large animal model. Here we investigated structural and functional changes in the vascular smooth muscle and endothelium of the canine DMD model. The expression of dystrophin and endothelial nitric oxide synthase (eNOS), neuronal NOS (nNOS), and the structure and function of the femoral artery from 15 normal and 16 affected adult dogs were evaluated. Full-length dystrophin was detected in the endothelium and smooth muscle in normal but not affected dog arteries. Normal arteries lacked nNOS but expressed eNOS in the endothelium. NOS activity and eNOS expression were reduced in the endothelium of dystrophic dogs. Dystrophin deficiency resulted in structural remodeling of the artery. In affected dogs, the maximum tension induced by vasoconstrictor phenylephrine and endothelin-1 was significantly reduced. In addition, acetylcholine-mediated vasorelaxation was significantly impaired, whereas exogenous nitric oxide-induced vasorelaxation was significantly enhanced. Our results suggest that dystrophin plays a crucial role in maintaining the structure and function of vascular endothelium and smooth muscle in large mammals. Vascular defects may contribute to DMD pathogenesis. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Tetrahydrobiopterin synthesis and metabolism is impaired in dystrophin-deficient mdx mice and humans

AIM

Loss of dystrophin causes oxidative stress and affects nitric oxide synthase-mediated vascular function in striated muscle. Because tetrahydrobiopterin is an antioxidant and co-factor for nitric oxide synthase, we tested the hypothesis that tetrahydrobiopterin would be low in mdx mice and humans deficient for dystrophin.

METHODS

Tetrahydrobiopterin and its metabolites were measured at rest and in response to exercise in Duchenne and Becker muscular dystrophy patients, age-matched male controls as well as wild-type, mdx and mdx mice transgenically overexpressing skeletal muscle-specific dystrophins. Mdx mice were also supplemented with tetrahydrobiopterin and pathophysiology was assessed.

RESULTS

Duchenne muscular dystrophy patients had lower urinary dihydrobiopterin + tetrahydrobiopterin/specific gravity1.020 compared to unaffected age-matched males and Becker muscular dystrophy patients. Mdx mice had low urinary and skeletal muscle dihydrobiopterin + tetrahydrobiopterin compared to wild-type mice. Overexpression of dystrophins that localize neuronal nitric oxide synthase restored dihydrobiopterin + tetrahydrobiopterin in mdx mice to wild-type levels while utrophin overexpression did not. Mdx mice and Duchenne muscular dystrophy patients did not increase tetrahydrobiopterin during exercise and in mdx mice tetrahydrobiopterin deficiency was likely because of lower levels of sepiapterin reductase in skeletal muscle. Tetrahydrobiopterin supplementation improved skeletal muscle strength, resistance to fatiguing and injurious contractions in vivo, increased utrophin and capillary density of skeletal muscle and lowered cardiac muscle fibrosis and left ventricular wall thickness in mdx mice.

CONCLUSION

These data demonstrate that impaired tetrahydrobiopterin synthesis is associated with dystrophin loss and treatment with tetrahydrobiopterin improves striated muscle histopathology and skeletal muscle function in mdx mice.

Duchenne muscular dystrophy

Duchenne muscular dystrophy is a severe, progressive, muscle-wasting disease that leads to difficulties with movement and, eventually, to the need for assisted ventilation and premature death. The disease is caused by mutations in DMD (encoding dystrophin) that abolish the production of dystrophin in muscle. Muscles without dystrophin are more sensitive to damage, resulting in progressive loss of muscle tissue and function, in addition to cardiomyopathy. Recent studies have greatly deepened our understanding of the primary and secondary pathogenetic mechanisms. Guidelines for the multidisciplinary care for Duchenne muscular dystrophy that address obtaining a genetic diagnosis and managing the various aspects of the disease have been established. In addition, a number of therapies that aim to restore the missing dystrophin protein or address secondary pathology have received regulatory approval and many others are in clinical development.

Master Regulators of Muscle Atrophy: Role of Costamere Components

The loss of muscle mass and force characterizes muscle atrophy in several different conditions, which share the expression of atrogenes and the activation of their transcriptional regulators. However, attempts to antagonize muscle atrophy development in different experimental contexts by targeting contributors to the atrogene pathway showed partial effects in most cases. Other master regulators might independently contribute to muscle atrophy, as suggested by our recent evidence about the co-requirement of the muscle-specific chaperone protein melusin to inhibit unloading muscle atrophy development. Furthermore, melusin and other muscle mass regulators, such as nNOS, belong to costameres, the macromolecular complexes that connect sarcolemma to myofibrils and to the extracellular matrix, in correspondence with specific sarcomeric sites. Costameres sense a mechanical load and transduce it both as lateral force and biochemical signals. Recent evidence further broadens this classic view, by revealing the crucial participation of costameres in a sarcolemmal “signaling hub” integrating mechanical and humoral stimuli, where mechanical signals are coupled with insulin and/or insulin-like growth factor stimulation to regulate muscle mass. Therefore, this review aims to enucleate available evidence concerning the early involvement of costamere components and additional putative master regulators in the development of major types of muscle atrophy.

Rapid, redox-mediated mechanical susceptibility of the cortical microtubule lattice in skeletal muscle

The highly ordered cortical microtubule lattice of skeletal muscle is disorganized in dystrophin-deficient mdx mice. Implicated mechanisms include loss of dystrophin binding, altered α-tubulin posttranslational modification, expression of a β-tubulin involved in regeneration, and reactive oxygen species (ROS). Here we show that the transverse microtubules in mdx muscle expressing miniaturized dystrophins are rapidly lost after eccentric contraction. Analysis of mdx lines expressing different dystrophin constructs demonstrate that spectrin-like repeats R4-15 and R20-23 were required for mechanically stable microtubules. Microtubule loss was prevented by the non-specific antioxidant N-acetylcysteine while inhibition of NADPH oxidase 2 had only a partial effect, suggesting that ROS from multiple sources mediate the rapid loss of transverse microtubules after eccentric contraction. Finally, ablation of α-dystrobrevin, β- or γ-cytoplasmic actin phenocopied the transverse microtubule instability of miniaturized dystrophins. Our data demonstrate that multiple dystrophin domains, α-dystrobrevin and cytoplasmic actins are necessary for mechanically stable microtubules.

iNOS is not responsible for RyR1 S-nitrosylation in mdx mice with truncated dystrophin

Background

Previous research indicated that nitric oxide synthase (NOS) is the key molecule for S-nitrosylation of ryanodine receptor 1 (RyR1) in DMD model mice (mdx mice) and that both neuronal NOS (nNOS) and inducible NOS (iNOS) might contribute to the reaction because nNOS is mislocalized in the cytoplasm and iNOS expression is higher in mdx mice. We investigated the effect of iNOS on RyR1 S-nitrosylation in mdx mice and whether transgenic expression of truncated dystrophin reduced iNOS expression in mdx mice or not.

Methods

Three- to 4-month-old C57BL/6 J, mdx, and transgenic mdx mice expressing exon 45–55-deleted human dystrophin (Tg/mdx mice) were used. We also generated two double mutant mice, mdx iNOS KO and Tg/mdx iNOS KO to reveal the iNOS contribution to RyR1 S-nitrosylation. nNOS and iNOS expression levels in skeletal muscle of these mice were assessed by immunohistochemistry (IHC), qRT-PCR, and Western blotting. Total NOS activity was measured by a citrulline assay. A biotin-switch method was used for detection of RyR1 S-nitrosylation. Statistical differences were assessed by one-way ANOVA with Tukey-Kramer post-hoc analysis.

Results

mdx and mdx iNOS KO mice showed the same level of RyR1 S-nitrosylation. Total NOS activity was not changed in mdx iNOS KO mice compared with mdx mice. iNOS expression was undetectable in Tg/mdx mice expressing exon 45–55-deleted human dystrophin, but the level of RyR1 S-nitrosylation was the same in mdx and Tg/mdx mice.

Conclusion

Similar levels of RyR1 S-nitrosylation and total NOS activity in mdx and mdx iNOS KO demonstrated that the proportion of iNOS in total NOS activity was low, even in mdx mice. Exon 45–55-deleted dystrophin reduced the expression level of iNOS, but it did not correct the RyR1 S-nitrosylation. These results indicate that iNOS was not involved in RyR1 S-nitrosylation in mdx and Tg/mdx mice muscles.

Single SERCA2a Therapy Ameliorated Dilated Cardiomyopathy for 18 Months in a Mouse Model of Duchenne Muscular Dystrophy

Loss of dystrophin leads to Duchenne muscular dystrophy (DMD). A pathogenic feature of DMD is the significant elevation of cytosolic calcium. Supraphysiological calcium triggers protein degradation, membrane damage, and eventually muscle death and dysfunction. Sarcoplasmic/endoplasmic reticulum (SR) calcium ATPase (SERCA) is a calcium pump that transports cytosolic calcium to the SR during excitation-contraction coupling. We hypothesize that a single systemic delivery of SERCA2a with adeno-associated virus (AAV) may improve calcium recycling and provide long-lasting benefits in DMD. To test this, we injected an AAV9 human SERCA2a vector (6 × 10¹² viral genome particles/mouse) intravenously to 3-month-old mdx mice, the most commonly used DMD model. Immunostaining and western blot showed robust human SERCA2a expression in the heart and skeletal muscle for 18 months. Concomitantly, SR calcium uptake was significantly improved in these tissues. SERCA2a therapy significantly enhanced grip force and treadmill performance, completely prevented myocardial fibrosis, and normalized electrocardiograms (ECGs). Cardiac catheterization showed normalization of multiple systolic and diastolic hemodynamic parameters in treated mice. Importantly, chamber dilation was completely prevented, and ejection fraction was restored to the wild-type level. Our results suggest that a single systemic AAV9 SERCA2a therapy has the potential to provide long-lasting benefits for DMD.

Membrane recruitment of nNOSµ in microdystrophin gene transfer to enhance durability

Several gene transfer clinical trials are currently ongoing with the common aim of delivering a shortened version of dystrophin, termed a microdystrophin, for the treatment of Duchenne muscular dystrophy (DMD). However, one of the main differences between these trials is the microdystrophin protein produced following treatment. Each gene transfer product is based on different selections of dystrophin domain combinations to assemble microdystrophin transgenes that maintain functional dystrophin domains and fit within the packaging limits of an adeno-associated virus (AAV) vector. While domains involved in mechanical function, such as the actin-binding domain and β-dystroglycan binding domain, have been identified for many years and included in microdystrophin constructs, more recently the neuronal nitric oxide synthase (nNOS) domain has also been identified due to its role in enhancing nNOS membrane localization. As nNOS membrane localization has been established as an important requirement for prevention of functional ischemia in skeletal muscle, inclusion of the nNOS domain into a microdystrophin construct represents an important consideration. The aim of this mini review is to highlight what is currently known about the nNOS domain of dystrophin and to describe potential implications of this domain in a microdystrophin gene transfer clinical trial.

Dystrophin R16/17 protein therapy restores sarcolemmal nNOS in trans and improves muscle perfusion and function

Background

Delocalization of neuronal nitric oxide synthase (nNOS) from the sarcolemma leads to functional muscle ischemia. This contributes to the pathogenesis in cachexia, aging and muscular dystrophy. Mutations in the gene encoding dystrophin result in Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD). In many BMD patients and DMD patients that have been converted to BMD by gene therapy, sarcolemmal nNOS is missing due to the lack of dystrophin nNOS-binding domain.

Methods

Dystrophin spectrin-like repeats 16 and 17 (R16/17) is the sarcolemmal nNOS localization domain. Here we explored whether R16/17 protein therapy can restore nNOS to the sarcolemma and prevent functional ischemia in transgenic mice which expressed an R16/17-deleted human micro-dystrophin gene in the dystrophic muscle. The palmitoylated R16/17.GFP fusion protein was conjugated to various cell-penetrating peptides and produced in the baculovirus-insect cell system. The best fusion protein was delivered to the transgenic mice and functional muscle ischemia was quantified.

Results

Among five candidate cell-penetrating peptides, the mutant HIV trans-acting activator of transcription (TAT) protein transduction domain (mTAT) was the best in transferring the R16/17.GFP protein to the muscle. Systemic delivery of the mTAT.R16/17.GFP protein to micro-dystrophin transgenic mice successfully restored sarcolemmal nNOS without inducing T cell infiltration. More importantly, R16/17 protein therapy effectively prevented treadmill challenge-induced force loss and improved muscle perfusion during contraction.

Conclusions

Our results suggest that R16/17 protein delivery is a highly promising therapy for muscle diseases involving sarcolemmal nNOS delocalizaton.

Electronic supplementary material

The online version of this article (10.1186/s10020-019-0101-6) contains supplementary material, which is available to authorized users.

Variable rescue of microtubule and physiological phenotypes in mdx muscle expressing different miniaturized dystrophins

Delivery of miniaturized dystrophin genes via adeno-associated viral vectors is one leading approach in development to treat Duchenne muscular dystrophy. Here we directly compared the functionality of five mini- and micro-dystrophins via skeletal muscle-specific transgenic expression in dystrophin-deficient mdx mice. We evaluated their ability to rescue defects in the microtubule network, passive stiffness and contractility of skeletal muscle. Transgenic mdx mice expressing the short dystrophin isoform Dp116 served as a negative control. All mini- and micro-dystrophins restored elevated detyrosinated α-tubulin and microtubule density of mdx muscle to values not different from C57BL/10, however, only mini-dystrophins restored the transverse component of the microtubule lattice back to C57BL/10. Passive stiffness values in mdx muscles expressing mini- or micro-dystrophins were not different from C57BL/10. While all mini- and micro-dystrophins conferred significant protection from eccentric contraction-induced force loss in vivo and ex vivo compared to mdx, removal of repeats two and three resulted in less protection from force drop caused by eccentric contraction ex vivo. Our data reveal subtle yet significant differences in the relative functionalities for different therapeutic constructs of miniaturized dystrophin in terms of protection from ex vivo eccentric contraction-induced force loss and restoration of an organized microtubule lattice.

Dystrophin Deficiency Leads to Genomic Instability in Human Pluripotent Stem Cells via NO Synthase-Induced Oxidative Stress

Recent data on Duchenne muscular dystrophy (DMD) show myocyte progenitor's involvement in the disease pathology often leading to the DMD patient's death. The molecular mechanism underlying stem cell impairment in DMD has not been described. We created dystrophin-deficient human pluripotent stem cell (hPSC) lines by reprogramming cells from two DMD patients, and also by introducing dystrophin mutation into human embryonic stem cells via CRISPR/Cas9. While dystrophin is expressed in healthy hPSC, its deficiency in DMD hPSC lines induces the release of reactive oxygen species (ROS) through dysregulated activity of all three isoforms of nitric oxide synthase (further abrev. as, NOS). NOS-induced ROS release leads to DNA damage and genomic instability in DMD hPSC. We were able to reduce both the ROS release as well as DNA damage to the level of wild-type hPSC by inhibiting NOS activity.

Isometric resistance training increases strength and alters histopathology of dystrophin-deficient mouse skeletal muscle

Mutation to the dystrophin gene causes skeletal muscle weakness in patients with Duchenne muscular dystrophy (DMD) or Becker muscular dystrophy (BMD). Deliberation continues regarding implications of prescribing exercise for these patients. The purpose of this study was to determine whether isometric resistance exercise (~10 tetanic contractions/session) improves skeletal muscle strength and histopathology in the mdx mouse model of DMD. Three isometric training sessions increased in vivo isometric torque (22%) and contractility rates (54%) of anterior crural muscles of mdx mice. Mice expressing a BMD-causing missense mutated dystrophin on the mdx background showed comparable increases in torque (22%), while wild-type mice showed less change (11%). Increases in muscle function occurred within 1 h and peaked 3 days posttraining; however, the adaptation was lost after 7 days unless retrained. Six isometric training sessions over 4 wk caused increased isometric torque (28%) and contractility rates (22-28%), reduced fibrosis, as well as greater uniformity of fiber cross-sectional areas, fewer embryonic myosin heavy-chain-positive fibers, and more satellite cells in tibialis anterior muscle compared with the contralateral untrained muscle. Ex vivo functional analysis of isolated extensor digitorum longus (EDL) muscle from the trained hindlimb revealed greater absolute isometric force, lower passive stiffness, and a lower susceptibility to eccentric contraction-induced force loss compared with untrained EDL muscle. Overall, these data support the concept that exercise training in the form of isometric tetanic contractions can improve contractile function of dystrophin-deficient muscle, indicating a potential role for enhancing muscle strength in patients with DMD and BMD. NEW & NOTEWORTHY We focused on adaptive responses of dystrophin-deficient mouse skeletal muscle to isometric contraction training and report that in the absence of dystrophin (or in the presence of a mutated dystrophin), strength and muscle histopathology are improved. Results suggest that the strength gains are associated with fiber hypertrophy, reduced fibrosis, increased number of satellite cells, and blunted eccentric contraction-induced force loss in vitro. Importantly, there was no indication that the isometric exercise training was deleterious to dystrophin-deficient muscle.

Xanthine oxidase is hyper-active in Duchenne muscular dystrophy

Generation of superoxide by xanthine oxidase can be stimulated under ischemic and aberrant calcium homeostasis. Because patients and mice with Duchenne muscular dystrophy (DMD) suffer from ischemia and excessive calcium influx, we tested the hypothesis that xanthine oxidase activity is elevated and contributes to disease pathology. Xanthine oxidase activity was measured by urinary isoxanthopterin in DMD patients at rest and in response to exercise. Urinary isoxanthopterin/creatinine was elevated compared to age-matched controls and Becker muscular dystrophy (BMD) patients. Concentrations were also increased after a six minute walk test in ambulatory patients. We also measured urinary isoxanthopterin in wildtype mice and a number of dystrophic mouse models; the DMD mouse model (mdx), mdx mice overexpressing a variety of transgenic miniaturized and chimeric skeletal muscle-specific dystrophins and utrophin and the β-sarcoglycan deficient (Scgb-/-) mouse which represents type 2E human limb-girdle muscular dystrophy. Mdx and Scgb-/-mice had greater urinary isoxanthopterin/creatinine than wildtype mice while mdx mice expressing dystrophin or utrophin linking the extracellular matrix to the actin cytoskeleton were not different than wildtype. We also measured higher levels of urinary ortho-tyrosine in humans and mice deficient for dystrophin to confirm elevated oxidative stress. Surprisingly, mdx had lower xanthine oxidase protein levels and higher mRNA in gastrocnemius muscle compared to wildtype mice, however, the enzymatic activity of skeletal muscle xanthine oxidase was elevated above wildtype and a transgenic rescued mdx mouse (DysΔMTB-mdx). Downhill treadmill running also caused significant increases in mdx urinary isoxanthopterin that was prevented with the xanthine oxidase inhibitor allopurinol. Similarly, in vitro eccentric contraction-induced force drop of mdx muscle was attenuated by the allopurinol metabolite, oxypurinol. Together, our data suggests hyper-activity of xanthine oxidase in DMD, identifies xanthine oxidase activity as a contributing factor in eccentric contraction-induced force drop of dystrophin-deficient skeletal muscle and highlights the potential of isoxanthopterin as a noninvasive biomarker in DMD.

Neuronal nitric oxide synthase (nNOS) splice variant function: Insights into nitric oxide signaling from skeletal muscle

Defects in neuronal nitric oxide synthase (nNOS) splice variant localization and signaling in skeletal muscle are a firmly established pathogenic characteristic of many neuromuscular diseases, including Duchenne and Becker muscular dystrophy (DMD and BMD, respectively). Therefore, substantial efforts have been made to understand and therapeutically target skeletal muscle nNOS isoform signaling. The purpose of this review is to summarize recent salient advances in understanding of the regulation, targeting, and function of nNOSμ and nNOSβ splice variants in normal and dystrophic skeletal muscle, primarily using findings from mouse models. The first focus of this review is how the differential targeting of nNOS splice variants creates spatially and functionally distinct nitric oxide (NO) signaling compartments at the sarcolemma, Golgi complex, and cytoplasm. Particular attention is given to the functions of sarcolemmal nNOSμ and limitations of current nNOS knockout models. The second major focus is to review current understanding of cGMP-mediated nNOS signaling in skeletal muscle and its emergence as a therapeutic target in DMD and BMD. Accordingly, we address the preclinical and clinical successes and setbacks with the testing of phosphodiesterase 5 inhibitors to redress nNOS signaling defects in DMD and BMD. In summary, this review of nNOS function in normal and dystrophic muscle aims to advance understanding how the messenger NO is harnessed for cellular signaling from a skeletal muscle perspective.

Dystrophin R16/17-syntrophin PDZ fusion protein restores sarcolemmal nNOSμ

Background

Loss of sarcolemmal nNOSμ is a common manifestation in a wide variety of muscle diseases and contributes to the dysregulation of multiple muscle activities. Given the critical role sarcolemmal nNOSμ plays in muscle, restoration of sarcolemmal nNOSμ should be considered as an important therapeutic goal.

Methods

nNOSμ is anchored to the sarcolemma by dystrophin spectrin-like repeats 16 and 17 (R16/17) and the syntrophin PDZ domain (Syn PDZ). To develop a strategy that can independently restore sarcolemmal nNOSμ, we engineered an R16/17-Syn PDZ fusion construct and tested whether this construct alone is sufficient to anchor nNOSμ to the sarcolemma in three different mouse models of Duchenne muscular dystrophy (DMD).

Results

Membrane-associated nNOSμ is completely lost in DMD. Adeno-associated virus (AAV)-mediated delivery of the R16/17-Syn PDZ fusion construct successfully restored sarcolemmal nNOSμ in all three models. Further, nNOS restoration was independent of the dystrophin-associated protein complex.

Conclusions

Our results suggest that the R16/17-Syn PDZ fusion construct is sufficient to restore sarcolemmal nNOSμ in the dystrophin-null muscle.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0182-x) contains supplementary material, which is available to authorized users.

Sarcolemmal loss of active nNOS (Nos1) is an oxidative stress-dependent, early event driving disuse atrophy

Skeletal muscle atrophy following unloading or immobilization represents a major invalidating event in bedridden patients. Among mechanisms involved in atrophy development, a controversial role is played by neuronal NOS (nNOS; NOS1), whose dysregulation at the protein level and/or subcellular distribution also characterizes other neuromuscular disorders. This study aimed to investigate unloading-induced changes in nNOS before any evidence of myofiber atrophy, using vastus lateralis biopsies obtained from young healthy subjects after a short bed-rest and rat soleus muscles after exposure to short unloading periods. Our results showed that (1) changes in nNOS subcellular distribution using NADPH-diaphorase histochemistry to detect enzyme activity were observed earlier than using immunofluorescence to visualize the protein; (2) loss of active nNOS from the physiological subsarcolemmal localization occurred before myofiber atrophy, i.e. in 8-day bed-rest biopsies and in 6 h-unloaded rat soleus, and was accompanied by increased nNOS activity in the sarcoplasm; (3) nNOS (Nos1) transcript and protein levels decreased significantly in the rat soleus after 6 h and 1 day unloading, respectively, to return to ambulatory levels after 4 and 7 days of unloading, respectively; (4) unloading-induced nNOS redistribution appeared dependent on mitochondrial-derived oxidant species, indirectly measured by tropomyosin disulfide bonds which had increased significantly in the rat soleus already after a 6 h-unloading bout; (5) activity of displaced nNOS molecules is required for translocation of the FoxO3 transcription factor to myofiber nuclei. FoxO3 nuclear localization in rat soleus increased after 6 h unloading (about four-fold the ambulatory level), whereas it did not when nNOS expression and activity were inhibited in vivo before and during 6 h unloading. In conclusion, this study demonstrates that the redistribution of active nNOS molecules from sarcolemma to sarcoplasm not only is ahead of the atrophy of unloaded myofibers, and is induced by increased production of mitochondrial superoxide anion, but also drives FoxO3 activation to initiate muscle atrophy. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Nitric oxide-dependent attenuation of noradrenaline-induced vasoconstriction is impaired in the canine model of Duchenne muscular dystrophy

KEY POINTS

We developed a novel method to study sympatholysis in dogs. We showed abolishment of sarcolemmal nNOS, and reduction of total nNOS and total eNOS in the canine Duchenne muscular dystrophy (DMD) model. We showed sympatholysis in dogs involving both nNOS-derived NO-dependent and NO-independent mechanisms. We showed that the loss of sarcolemmal nNOS compromised sympatholysis in the canine DMD model. We showed that NO-independent sympatholysis was not affected in the canine DMD model.

ABSTRACT

The absence of dystrophin in Duchenne muscular dystrophy (DMD) leads to the delocalization of neuronal nitric oxide synthase (nNOS) from the sarcolemma. Sarcolemmal nNOS plays an important role in sympatholysis, a process of attenuating reflex sympathetic vasoconstriction during exercise to ensure blood perfusion in working muscle. Delocalization of nNOS compromises sympatholysis resulting in functional ischaemia and muscle damage in DMD patients and mouse models. Little is known about the contribution of membrane-associated nNOS to blood flow regulation in dystrophin-deficient DMD dogs. We tested the hypothesis that the loss of sarcolemmal nNOS abolishes protective sympatholysis in contracting muscle of affected dogs. Haemodynamic responses to noradrenaline in the brachial artery were evaluated at rest and during contraction in the absence and presence of NOS inhibitors. We found sympatholysis was significantly compromised in DMD dogs, as well as in normal dogs treated with a selective nNOS inhibitor, suggesting that the absence of sarcolemmal nNOS underlies defective sympatholysis in the canine DMD model. Surprisingly, inhibition of all NOS isoforms did not completely abolish sympatholysis in normal dogs, suggesting sympatholysis in canine muscle also involves NO-independent mechanism(s). Our study established a foundation for using the dog model to test therapies aimed at restoring nNOS homeostasis in DMD.

Truncated dystrophin ameliorates the dystrophic phenotype of mdx mice by reducing sarcolipin-mediated SERCA inhibition

Duchenne muscular dystrophy (DMD) and the less severe Becker muscular dystrophy (BMD) are due to mutations in the DMD gene. Previous reports show that in-frame deletion of exons 45-55 produces an internally shorted, but functional, dystrophin protein resulting in a very mild BMD phenotype. In order to elucidate the molecular mechanism leading to this phenotype, we generated exon 45-55 deleted dystrophin transgenic/mdx (Tg/mdx) mice. Muscular function of Tg/mdx mice was restored close to that of wild type (WT) mice but the localization of the neuronal type of nitric oxide synthase was changed from the sarcolemma to the cytosol. This led to hyper-nitrosylation of the ryanodine receptor 1 causing increased Ca²⁺ release from the sarcoplasmic reticulum. On the other hand, Ca²⁺ reuptake by the sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA) was restored to the level of WT mice, suggesting that the Ca²⁺ dysregulation had been compensated by SERCA activation. In line with this, expression of sarcolipin (SLN), a SERCA-inhibitory peptide, was upregulated in mdx mice, but strongly reduced in Tg/mdx mice. Furthermore, knockdown of SLN ameliorated the cytosolic Ca²⁺ homeostasis and the dystrophic phenotype in mdx mice. These findings suggest that SLN may be a novel target for DMD therapy.

Role of neuronal nitric oxide synthase (nNOS) in Duchenne and Becker muscular dystrophies - Still a possible treatment modality?

Neuronal nitric oxide synthase (nNOS) is involved in nitric oxide (NO) production and suggested to play a crucial role in blood flow regulation of skeletal muscle. During activation of the muscle, NO helps attenuate the sympathetic vasoconstriction to accommodate increased metabolic demands, a phenomenon known as functional sympatholysis. In inherited myopathies such as the dystrophinopathies Duchenne and Becker muscle dystrophies (DMD and BMD), nNOS is lost from the sarcolemma. The loss of nNOS may cause functional ischemia contributing to skeletal and cardiac muscle cell injury. Effects of NO is augmented by inhibiting degradation of the second messenger cyclic guanosine monophosphate (cGMP) using sildenafil and tadalafil, both of which inhibit the enzyme phosphodiesterase 5 (PDE5). In animal models of DMD, PDE5-inhibitors prevent functional ischemia, reduce post-exercise skeletal muscle pathology and fatigue, show amelioration of cardiac muscle cell damage and increase cardiac performance. However, effect on clinical outcomes in DMD and BMD patients have been disappointing with minor effects on upper limb performance and none on ambulation. This review aims to summarize the current knowledge of nNOS function related to functional sympatholysis in skeletal muscle and studies on PDE5-inhibitor treatment in nNOS-deficient animal models and patients.

Systemic AAV Micro-dystrophin Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by dystrophin gene mutation. Conceptually, replacing the mutated gene with a normal one would cure the disease. However, this task has encountered significant challenges due to the enormous size of the gene and the distribution of muscle throughout the body. The former creates a hurdle for viral vector packaging and the latter begs for whole-body therapy. To address these obstacles, investigators have invented the highly abbreviated micro-dystrophin gene and developed body-wide systemic gene transfer with adeno-associated virus (AAV). Numerous microgene configurations and various AAV serotypes have been explored in animal models in many laboratories. Preclinical data suggests that intravascular AAV micro-dystrophin delivery can significantly ameliorate muscle pathology, enhance muscle force, and attenuate dystrophic cardiomyopathy in animals. Against this backdrop, several clinical trials have been initiated to test the safety and tolerability of this promising therapy in DMD patients. While these trials are not powered to reach a conclusion on clinical efficacy, findings will inform the field on the prospects of body-wide DMD therapy with a synthetic micro-dystrophin AAV vector. This review discusses the history, current status, and future directions of systemic AAV micro-dystrophin therapy.

Micro-Dystrophin Gene Therapy Goes Systemic in Duchenne Muscular Dystrophy Patients

Whole-body systemic gene therapy is likely the most effective way to reduce greatly the disease burden of Duchenne muscular dystrophy (DMD), an X-linked inherited muscle disease that leads to premature death in early adulthood. Genetically, DMD is due to null mutation of the dystrophin gene, one of the largest genes in the genome. Recent studies have shown highly promising improvements in animal models with intravascular delivery of the engineered micro-dystrophin gene by adeno-associated virus (AAV). Several human trials are now started to advance AAV micro-dystrophin therapy to DMD patients. This is a historical moment for the entire field. Results from these trials will shape the future of neuromuscular disease gene therapy.

Cardiac-Specific Expression of ΔH2-R15 Mini-Dystrophin Normalized All Electrocardiogram Abnormalities and the End-Diastolic Volume in a 23-Month-Old Mouse Model of Duchenne Dilated Cardiomyopathy

Heart disease is a major health threat for Duchenne/Becker muscular dystrophy patients and carriers. Expression of a 6-8 kb mini-dystrophin gene in the heart holds promise to change the disease course dramatically. However, the mini-dystrophin gene cannot be easily studied with adeno-associated virus (AAV) gene delivery because the size of the minigene exceeds AAV packaging capacity. Cardiac protection of the ΔH2-R19 minigene was previously studied using the cardiac-specific transgenic approach. Although this minigene fully normalized skeletal muscle force, it only partially corrected electrocardiogram and heart hemodynamics in dystrophin-null mdx mice that had moderate cardiomyopathy. This study evaluated the ΔH2-R15 minigene using the same transgenic approach in mdx mice that had more severe cardiomyopathy. In contrast to the ΔH2-R19 minigene, the ΔH2-R15 minigene carries dystrophin spectrin-like repeats 16 to 19 (R16-19), a region that has been suggested to protect the heart in clinical studies. Cardiac expression of the ΔH2-R15 minigene normalized all aberrant electrocardiogram changes and improved hemodynamics. Importantly, it corrected the end-diastolic volume, an important diastolic parameter not rescued by ΔH2-R19 mini-dystrophin. It is concluded that that ΔH2-R15 mini-dystrophin is a superior candidate gene for heart protection. This finding has important implications in the design of the mini/micro-dystrophin gene for Duchenne cardiomyopathy therapy.

Transient receptor potential channel 6 regulates abnormal cardiac S-nitrosylation in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked disorder with dystrophin loss that results in skeletal and cardiac muscle weakening and early death. Loss of the dystrophin-sarcoglycan complex delocalizes nitric oxide synthase (NOS) to alter its signaling, and augments mechanosensitive intracellular Ca²⁺ influx. The latter has been coupled to hyperactivation of the nonselective cation channel, transient receptor potential canonical channel 6 (Trpc6), in isolated myocytes. As Ca²⁺ also activates NOS, we hypothesized that Trpc6 would help to mediate nitric oxide (NO) dysregulation and that this would be manifest in increased myocardial S-nitrosylation, a posttranslational modification increasingly implicated in neurodegenerative, inflammatory, and muscle disease. Using a recently developed dual-labeling proteomic strategy, we identified 1,276 S-nitrosylated cysteine residues [S-nitrosothiol (SNO)] on 491 proteins in resting hearts from a mouse model of DMD (dmd^mdx:utrn^+/-). These largely consisted of mitochondrial proteins, metabolic regulators, and sarcomeric proteins, with 80% of them also modified in wild type (WT). S-nitrosylation levels, however, were increased in DMD. Genetic deletion of Trpc6 in this model (dmd^mdx:utrn^+/-:trpc6^-/-) reversed ∼70% of these changes. Trpc6 deletion also ameliorated left ventricular dilation, improved cardiac function, and tended to reduce fibrosis. Furthermore, under catecholamine stimulation, which also increases NO synthesis and intracellular Ca²⁺ along with cardiac workload, the hypernitrosylated state remained as it did at baseline. However, the impact of Trpc6 deletion on the SNO proteome became less marked. These findings reveal a role for Trpc6-mediated hypernitrosylation in dmd^mdx:utrn^+/- mice and support accumulating evidence that implicates nitrosative stress in cardiac and muscle disease.

Link between MHC Fiber Type and Restoration of Dystrophin Expression and Key Components of the DAPC by Tricyclo-DNA-Mediated Exon Skipping

Exon skipping mediated by tricyclo-DNA (tc-DNA) antisense oligonucleotides has been shown to induce significant levels of dystrophin restoration in mdx, a mouse model of Duchenne muscular dystrophy. This translates into significant improvement in key disease indicators in muscle, cardio-respiratory function, heart, and the CNS. Here we examine the relationship between muscle fiber type, based on myosin heavy chain (MHC) profile, and the ability of tc-DNA to restore not only dystrophin but also other members of the dystrophin-associated glycoprotein complex (DAPC). We first profiled this relationship in untreated mdx muscle, and we found that all fiber types support reversion events to a dystrophin-positive state, in an unbiased manner. Importantly, we show that only a small fraction of revertant fibers expressed other members of the DAPC. Immunoblot analysis of protein levels, however, revealed robust expression of these components, which failed to correctly localize to the sarcolemma. We then show that tc-DNA treatment leads to nearly all fibers expressing not only dystrophin but also other key components of the DAPC. Of significance, our work shows that MHC fiber type does not bias the expression of any of these important proteins. This work also highlights that the improved muscle physiology following tc-DNA treatment reported previously results from the complete restoration of the dystrophin complex in all MHCII fibers with equal efficiencies.

A Five-Repeat Micro-Dystrophin Gene Ameliorated Dystrophic Phenotype in the Severe DBA/2J-mdx Model of Duchenne Muscular Dystrophy

Micro-dystrophins are highly promising candidates for treating Duchenne muscular dystrophy, a lethal muscle disease caused by dystrophin deficiency. Here, we report robust disease rescue in the severe DBA/2J-mdx model with a neuronal nitric oxide synthase (nNOS)-binding micro-dystrophin vector. 2 × 10¹³ vector genome particles/mouse of the vector were delivered intravenously to 10-week-old mice and were evaluated at 6 months of age. Saturated micro-dystrophin expression was detected in all skeletal muscles and the heart and restored the dystrophin-associated glycoprotein complex and nNOS. In skeletal muscle, therapy substantially reduced fibrosis and calcification and significantly attenuated inflammation. Centronucleation was significantly decreased in the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles but not in the quadriceps. Muscle function was normalized in the TA and significantly improved in the EDL muscle. Heart histology and function were also evaluated. Consistent with the literature, DBA/2J-mdx mice showed myocardial calcification and fibrosis and cardiac hemodynamics was compromised. Surprisingly, similar myocardial pathology and hemodynamic defects were detected in control DBA/2J mice. As a result, interpretation of the cardiac data proved difficult due to the confounding phenotype in control DBA/2J mice. Our results support further development of this microgene vector for clinical translation. Further, DBA/2J-mdx mice are not good models for Duchenne cardiomyopathy.

Nanotherapy for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal X-linked childhood muscle wasting disease caused by mutations in the dystrophin gene. Nanobiotechnology-based therapies (such as synthetic nanoparticles and naturally existing viral and nonviral nanoparticles) hold great promise to replace and repair the mutated dystrophin gene and significantly change the disease course. While a majority of DMD nanotherapies are still in early preclinical development, several [such as adeno-associated virus (AAV)-mediated systemic micro-dystrophin gene therapy] are advancing for phase I clinical trials. Recent regulatory approval of Ataluren (a nonsense mutation read-through chemical) in Europe and Exondys51 (an exon-skipping antisense oligonucleotide drug) in the United States shall offer critical insight in how to move DMD nanotherapy to human patients. Progress in novel, optimized nano-delivery systems may further improve emerging molecular therapeutic modalities for DMD. Despite these progresses, DMD nanotherapy faces a number of unique challenges. Specifically, the dystrophin gene is one of the largest genes in the genome while nanoparticles have an inherent size limitation per definition. Furthermore, muscle is the largest tissue in the body and accounts for 40% of the body mass. How to achieve efficient bodywide muscle targeting in human patients with nanomedication remains a significant translational hurdle. New creative approaches in the design of the miniature micro-dystrophin gene, engineering of muscle-specific synthetic AAV capsids, and novel nanoparticle-mediated exon-skipping are likely to result in major breakthroughs in DMD therapy. WIREs Nanomed Nanobiotechnol 2018, 10:e1472. doi: 10.1002/wnan.1472 This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Attempting to Compensate for Reduced Neuronal Nitric Oxide Synthase Protein with Nitrate Supplementation Cannot Overcome Metabolic Dysfunction but Rather Has Detrimental Effects in Dystrophin-Deficient mdx Muscle

Duchenne muscular dystrophy arises from the loss of dystrophin and is characterized by calcium dysregulation, muscular atrophy, and metabolic dysfunction. The secondary reduction of neuronal nitric oxide synthase (nNOS) from the sarcolemma reduces NO production and bioavailability. As NO modulates glucose uptake, metabolism, and mitochondrial bioenergetics, we investigated whether an 8-week nitrate supplementation regimen could overcome metabolic dysfunction in the mdx mouse. Dystrophin-positive control (C57BL/10) and dystrophin-deficient mdx mice were supplemented with sodium nitrate (85 mg/l) in drinking water. Following the supplementation period, extensor digitorum longus and soleus were excised and radioactive glucose uptake was measured at rest (basal) and during contraction. Gastrocnemius was excised and mitochondrial respiration was measured using the Oroboros Oxygraph. Tibialis anterior was analyzed immunohistochemically for the presence of dystrophin, nNOS, nitrotyrosine, IgG and CD45+ cells, and histologically to assess areas of damage and regeneration. Glucose uptake in the basal and contracting states was normal in unsupplemented mdx muscles but was reduced following nitrate supplementation in mdx muscles only. The mitochondrial utilization of substrates was also impaired in mdx gastrocnemius during phosphorylating and maximal uncoupled respiration, and nitrate could not improve respiration in mdx muscle. Although nitrate supplementation reduced mitochondrial hydrogen peroxide emission, it induced mitochondrial uncoupling in red gastrocnemius, increased muscle fiber peroxynitrite (nitrotyrosine), and promoted skeletal muscle damage. Our novel data suggest that despite lower nNOS protein expression and likely lower NO production in mdx muscle, enhancing NO production with nitrate supplementation in these mice has detrimental effects on skeletal muscle. This may have important relevance for those with DMD.

Local inhibition of nitrergic activity in tenotomized rats accelerates muscle regeneration by increasing fiber area and decreasing central core lesions

Muscular atrophy is a progressive degeneration characterized by muscular proteolysis, loss of mass and decrease in fiber area. Tendon rupture induces muscular atrophy due to an intrinsic functional connection. Local inhibition of nitric oxide synthase (NOS) by Nω-nitro-L-arginine methyl ester (L-NAME) accelerates tendon histological recovery and induces functional improvement. Here we evaluate the effects of such local nitrergic inhibition on the pattern of soleus muscle regeneration after tenotomy. Adult male Wistar rats (240 to 280 g) were divided into four experimental groups: control (n=4), tenotomized (n=6), vehicle (n=6), and L-NAME (n=6). Muscular atrophy was induced by calcaneal tendon rupture in rats. Changes in muscle wet weight and total protein levels were determined by the Bradford method, and muscle fiber area and central core lesion (CCL) occurrence were evaluated by histochemical assays. Compared to tenotomized (69.3±22%) and vehicle groups (68.1%±17%), L-NAME treatment induced an increase in total protein level (108.3±21%) after 21 days post-injury. A reduction in fiber areas was observed in tenotomized (56.3±1.3%) and vehicle groups (53.9±3.9%). However, L-NAME treatment caused an increase in this parameter (69.3±1.6%). Such events were preceded by a remarkable reduction in the number of fibers with CCL in L-NAME-treated animals (12±2%), but not in tenotomized (21±2.5%) and vehicle groups (19.6±2.8%). Altogether, our data reveal that inhibition of tendon NOS contributed to the attenuation of atrophy and acceleration of muscle regeneration.

GSNOR Deficiency Enhances In Situ Skeletal Muscle Strength, Fatigue Resistance, and RyR1 S-Nitrosylation Without Impacting Mitochondrial Content and Activity

AIM

Nitric oxide (NO) plays important, but incompletely defined roles in skeletal muscle. NO exerts its regulatory effects partly though S-nitrosylation, which is balanced by denitrosylation by enzymes such as S-nitrosoglutathione reductase (GSNOR), whose functions in skeletal muscle remain to be fully deciphered.

RESULTS

GSNOR null (GSNOR^-/-) tibialis anterior (TA) muscles showed normal growth and were stronger and more fatigue resistant than controls in situ. However, GSNOR^-/- lumbrical muscles showed normal contractility and Ca²⁺ handling in vitro, suggesting important differences in GSNOR function between muscles or between in vitro and in situ environments. GSNOR^-/- TA muscles exhibited normal mitochondrial content, and capillary densities, but reduced type IIA fiber content. GSNOR inhibition did not impact mitochondrial respiratory complex I, III, or IV activities. These findings argue that enhanced GSNOR^-/- TA contractility is not driven by changes in mitochondrial content or activity, fiber type, or blood vessel density. However, loss of GSNOR led to RyR1 hypernitrosylation, which is believed to increase muscle force output under physiological conditions. cGMP synthesis by soluble guanylate cyclase (sGC) was decreased in resting GSNOR^-/- muscle and was more responsive to agonist (DETANO, BAY 41, and BAY 58) stimulation, suggesting that GSNOR modulates cGMP production in skeletal muscle.

INNOVATION

GSNOR may act as a "brake" on skeletal muscle contractile performance under physiological conditions by modulating nitrosylation/denitrosylation balance.

CONCLUSIONS

GSNOR may play important roles in skeletal muscle contractility, RyR1 S-nitrosylation, fiber type specification, and sGC activity. Antioxid. Redox Signal. 26, 165-181.

Uniform low-level dystrophin expression in the heart partially preserved cardiac function in an aged mouse model of Duchenne cardiomyopathy

Dystrophin deficiency results in Duchenne cardiomyopathy, a primary cause of death in Duchenne muscular dystrophy (DMD). Gene therapy has shown great promise in ameliorating the cardiac phenotype in mouse models of DMD. However, it is not completely clear how much dystrophin is required to treat dystrophic heart disease. We and others have shown that mosaic dystrophin expression at the wild-type level, depending on the percentage of dystrophin positive cardiomyocytes, can either delay the onset of or fully prevent cardiomyopathy in dystrophin-null mdx mice. Many gene therapy strategies will unlikely restore dystrophin to the wild-type level in a cardiomyocyte. To determine whether low-level dystrophin expression can reduce the cardiac manifestations in DMD, we examined heart histology, ECG and hemodynamics in 21-m-old normal BL6 and two strains of BL6-background dystrophin-deficient mice. Mdx3cv mice show uniform low-level expression of a near full-length dystrophin protein in every myofiber while mdx4cv mice have no dystrophin expression. Immunostaining and western blot confirmed marginal level dystrophin expression in the heart of mdx3cv mice. Although low-level expression did not reduce myocardial histopathology, it significantly ameliorated QRS prolongation and normalized diastolic hemodynamic deficiencies. Our study demonstrates for the first time that low-level dystrophin can partially preserve heart function.

Nutraceuticals and Their Potential to Treat Duchenne Muscular Dystrophy: Separating the Credible from the Conjecture

In recent years, complementary and alternative medicine has become increasingly popular. This trend has not escaped the Duchenne Muscular Dystrophy community with one study showing that 80% of caregivers have provided their Duchenne patients with complementary and alternative medicine in conjunction with their traditional treatments. These statistics are concerning given that many supplements are taken based on purely "anecdotal" evidence. Many nutraceuticals are thought to have anti-inflammatory or anti-oxidant effects. Given that dystrophic pathology is exacerbated by inflammation and oxidative stress these nutraceuticals could have some therapeutic benefit for Duchenne Muscular Dystrophy (DMD). This review gathers and evaluates the peer-reviewed scientific studies that have used nutraceuticals in clinical or pre-clinical trials for DMD and thus separates the credible from the conjecture.

Small Fractions of Muscular Dystrophy Embryonic Stem Cells Yield Severe Cardiac and Skeletal Muscle Defects in Adult Mouse Chimeras

Duchenne muscular dystrophy (DMD) is characterized by the loss of the protein dystrophin, leading to muscle fragility, progressive weakening, and susceptibility to mechanical stress. Although dystrophin-negative mdx mouse models have classically been used to study DMD, phenotypes appear mild compared to patients. As a result, characterization of muscle pathology, especially in the heart, has proven difficult. We report that injection of mdx embryonic stem cells (ESCs) into Wild Type blastocysts produces adult mouse chimeras with severe DMD phenotypes in the heart and skeletal muscle. Inflammation, regeneration and fibrosis are observed at the whole organ level, both in dystrophin-negative and dystrophin-positive portions of the chimeric tissues. Skeletal and cardiac muscle function are also decreased to mdx levels. In contrast to mdx heterozygous carriers, which show no significant phenotypes, these effects are even observed in chimeras with low levels of mdx ESC incorporation (10%-30%). Chimeric mice lack typical compensatory utrophin upregulation, and show pathological remodeling of Connexin-43. In addition, dystrophin-negative and dystrophin-positive isolated cardiomyocytes show augmented calcium response to mechanical stress, similar to mdx cells. These global effects highlight a novel role of mdx ESCs in triggering muscular dystrophy even when only low amounts are present. Stem Cells 2017;35:597-610.

Optimization of Internally Deleted Dystrophin Constructs

Duchenne muscular dystrophy (DMD) is a severe, genetic muscle disease caused by the absence of the sarcolemmal protein dystrophin. Gene replacement therapy is considered a potential strategy for the treatment of DMD, aiming to restore the missing protein. Although the elements of the dystrophin molecule have been identified and studies in transgenic mdx mice have explored the importance of a number of these structural domains, the resulting modified dystrophin protein products that have been developed so far are only partially characterized in relation to their structure and function in vivo. To optimize a dystrophin cDNA construct for therapeutic application we designed and produced four human minidystrophins within the packaging capacity of lentiviral vectors. Two novel minidystrophins retained the centrally located neuronal nitric oxide synthase (nNOS)-anchoring domain in order to achieve sarcolemmal nNOS restoration, which is lost in most internally deleted dystrophin constructs. Functionality of the resulting truncated dystrophin proteins was investigated in muscle of adult dystrophin-deficient mdx mice followed by a battery of detailed immunohistochemical and morphometric tests. This initial assessment aimed to determine the overall suitability of various constructs for cloning into lentiviral vectors for ex vivo gene delivery to stem cells for future preclinical studies.

Dystrophin restoration therapy improves both the reduced excitability and the force drop induced by lengthening contractions in dystrophic mdx skeletal muscle

Background

The greater susceptibility to contraction-induced skeletal muscle injury (fragility) is an important dystrophic feature and tool for testing preclinic dystrophin-based therapies for Duchenne muscular dystrophy. However, how these therapies reduce the muscle fragility is not clear.

Methods

To address this question, we first determined the event(s) of the excitation-contraction cycle which is/are altered following lengthening (eccentric) contractions in the mdx muscle.

Results

We found that the immediate force drop following lengthening contractions, a widely used measure of muscle fragility, was associated with reduced muscle excitability. Moreover, the force drop can be mimicked by an experimental reduction in muscle excitation of uninjured muscle. Furthermore, the force drop was not related to major neuromuscular transmission failure, excitation-contraction uncoupling, and myofibrillar impairment. Secondly, and importantly, the re-expression of functional truncated dystrophin in the muscle of mdx mice using an exon skipping strategy partially prevented the reductions in both force drop and muscle excitability following lengthening contractions.

Conclusion

We demonstrated for the first time that (i) the increased susceptibility to contraction-induced muscle injury in mdx mice is mainly attributable to reduced muscle excitability; (ii) dystrophin-based therapy improves fragility of the dystrophic skeletal muscle by preventing reduction in muscle excitability.

Dystrophin Threshold Level Necessary for Normalization of Neuronal Nitric Oxide Synthase, Inducible Nitric Oxide Synthase, and Ryanodine Receptor-Calcium Release Channel Type 1 Nitrosylation in Golden Retriever Muscular Dystrophy Dystrophinopathy

At present, the clinically most advanced strategy to treat Duchenne muscular dystrophy (DMD) is the exon-skipping strategy. Whereas antisense oligonucleotide-based clinical trials are underway for DMD, it is essential to determine the dystrophin restoration threshold needed to ensure improvement of muscle physiology at the molecular level. A preclinical trial has been conducted in golden retriever muscular dystrophy (GRMD) dogs treated in a forelimb by locoregional delivery of rAAV8-U7snRNA to promote exon skipping on the canine dystrophin messenger. Here, we exploited rAAV8-U7snRNA-transduced GRMD muscle samples, well characterized for their percentage of dystrophin-positive fibers, with the aim of defining the threshold of dystrophin rescue necessary for normalization of the status of neuronal nitric oxide synthase mu (nNOSμ), inducible nitric oxide synthase (iNOS), and ryanodine receptor-calcium release channel type 1 (RyR1), crucial actors for efficient contractile function. Results showed that restoration of dystrophin in 40% of muscle fibers is needed to decrease abnormal cytosolic nNOSμ expression and to reduce overexpression of iNOS, these two parameters leading to a reduction in the NO level in the muscle fibers. Furthermore, the same percentage of dystrophin-positive fibers of 40% was associated with the normalization of RyR1 nitrosylation status and with stabilization of the RyR1-calstabin1 complex that is required to facilitate coupled gating. We concluded that a minimal threshold of 40% of dystrophin-positive fibers is necessary for the reinstatement of central proteins needed for proper muscle contractile function, and thus identified a rate of dystrophin expression significantly improving, at the molecular level, the dystrophic muscle physiology.

Absence of Dystrophin Disrupts Skeletal Muscle Signaling: Roles of Ca2+, Reactive Oxygen Species, and Nitric Oxide in the Development of Muscular Dystrophy

Dystrophin is a long rod-shaped protein that connects the subsarcolemmal cytoskeleton to a complex of proteins in the surface membrane (dystrophin protein complex, DPC), with further connections via laminin to other extracellular matrix proteins. Initially considered a structural complex that protected the sarcolemma from mechanical damage, the DPC is now known to serve as a scaffold for numerous signaling proteins. Absence or reduced expression of dystrophin or many of the DPC components cause the muscular dystrophies, a group of inherited diseases in which repeated bouts of muscle damage lead to atrophy and fibrosis, and eventually muscle degeneration. The normal function of dystrophin is poorly defined. In its absence a complex series of changes occur with multiple muscle proteins showing reduced or increased expression or being modified in various ways. In this review, we will consider the various proteins whose expression and function is changed in muscular dystrophies, focusing on Ca(2+)-permeable channels, nitric oxide synthase, NADPH oxidase, and caveolins. Excessive Ca(2+) entry, increased membrane permeability, disordered caveolar function, and increased levels of reactive oxygen species are early changes in the disease, and the hypotheses for these phenomena will be critically considered. The aim of the review is to define the early damage pathways in muscular dystrophy which might be appropriate targets for therapy designed to minimize the muscle degeneration and slow the progression of the disease.

Animal models of Duchenne muscular dystrophy: from basic mechanisms to gene therapy

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disorder. It is caused by loss-of-function mutations in the dystrophin gene. Currently, there is no cure. A highly promising therapeutic strategy is to replace or repair the defective dystrophin gene by gene therapy. Numerous animal models of DMD have been developed over the last 30 years, ranging from invertebrate to large mammalian models. mdx mice are the most commonly employed models in DMD research and have been used to lay the groundwork for DMD gene therapy. After ~30 years of development, the field has reached the stage at which the results in mdx mice can be validated and scaled-up in symptomatic large animals. The canine DMD (cDMD) model will be excellent for these studies. In this article, we review the animal models for DMD, the pros and cons of each model system, and the history and progress of preclinical DMD gene therapy research in the animal models. We also discuss the current and emerging challenges in this field and ways to address these challenges using animal models, in particular cDMD dogs.