The susceptibility of muscle cells to oxidative stress is independent of nitric oxide synthase expression

Redox Control of Skeletal Muscle Regeneration

Skeletal muscle shows high plasticity in response to external demand. Moreover, adult skeletal muscle is capable of complete regeneration after injury, due to the properties of muscle stem cells (MuSCs), the satellite cells, which follow a tightly regulated myogenic program to generate both new myofibers and new MuSCs for further needs. Although reactive oxygen species (ROS) and reactive nitrogen species (RNS) have long been associated with skeletal muscle physiology, their implication in the cell and molecular processes at work during muscle regeneration is more recent. This review focuses on redox regulation during skeletal muscle regeneration. An overview of the basics of ROS/RNS and antioxidant chemistry and biology occurring in skeletal muscle is first provided. Then, the comprehensive knowledge on redox regulation of MuSCs and their surrounding cell partners (macrophages, endothelial cells) during skeletal muscle regeneration is presented in normal muscle and in specific physiological (exercise-induced muscle damage, aging) and pathological (muscular dystrophies) contexts. Recent advances in the comprehension of these processes has led to the development of therapeutic assays using antioxidant supplementation, which result in inconsistent efficiency, underlying the need for new tools that are aimed at precisely deciphering and targeting ROS networks. This review should provide an overall insight of the redox regulation of skeletal muscle regeneration while highlighting the limits of the use of nonspecific antioxidants to improve muscle function. Antioxid. Redox Signal. 27, 276-310.

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.

Bmi1 enhances skeletal muscle regeneration through MT1-mediated oxidative stress protection in a mouse model of dystrophinopathy

Enhanced polycomb complex protein Bmi1 expression in adult stem cells of the skeletal muscle leads to improved muscle function in a model of Duchenne Muscular Dystrophy via metallothionein1-mediated protection from oxidative stress.

The Polycomb group (PcG) protein Bmi1 is an essential epigenetic regulator of stem cell function during normal development and in adult organ systems. We show that mild up-regulation of Bmi1 expression in the adult stem cells of the skeletal muscle leads to a remarkable improvement of muscle function in a mouse model of Duchenne muscular dystrophy. The molecular mechanism underlying enhanced physiological function of Bmi1 depends on the injury context and it is mediated by metallothionein 1 (MT1)–driven modulation of resistance to oxidative stress in the satellite cell population. These results lay the basis for developing Bmi1 pharmacological activators, which either alone or in combination with MT1 agonists could be a powerful novel therapeutic approach to improve regeneration in muscle wasting conditions.

AAV-directed muscular dystrophy gene therapy

IMPORTANCE OF THE FIELD

Muscle-directed gene therapy for genetic muscle diseases can be performed by the recombinant adeno-associated viral (rAAV) vector delivery system to achieve long-term therapeutic gene transfer in all affected muscles.

AREAS COVERED IN THIS REVIEW

Recent progress in rAAV-vector-mediated muscle-directed gene transfer and associated techniques for the treatment of muscular dystrophies (MD). The review covers literature from the past 2 - 3 years.

WHAT THE READER WILL GAIN

rAAV-directed muscular dystrophy gene therapy can be achieved by mini-dystrophin replacement and exon-skipping strategies. The additional strategies of enhancing muscle regeneration and reducing inflammation in the muscle micro-environment should be useful to optimize therapeutic efficacy. This review compares the merits and shortcomings of different administration methods, promoters and experimental animals that will guide the choice of the appropriate strategy for clinical trials.

TAKE HOME MESSAGE

Restoration of muscle histopathology and function has been performed using rAAV systemic gene delivery. In addition, the combination of gene replacement and adjuvant therapies in the future may be beneficial with regard to improving muscle regeneration and decreasing myofiber necrosis. The challenges faced by large animal model studies and in human trials arise from gene transfer efficiency and immune response, which may be overcome by optimizing the rAAV vectors utilized and the administration methods.

Isoprostanes in dystrophinopathy: Evidence of increased oxidative stress

Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are degenerative disorders of muscle. Although the mechanisms underlying muscle degeneration are still uncertain, oxidative-damage has been proposed to play a key role. Isoprostanes are markers of free radical-catalyzed lipid peroxidation; the aim of our study was to evaluate plasma isoprostane levels in group of patients affected by Duchenne and Becker muscular dystrophies. PF(2)-isoprostane levels were measured by colorimetric enzyme immunoassay in the plasma of 17 patients with DMD and 24 with BMD. When compared to a group of healthy controls, affected patients showed significantly higher plasma levels of isoprostanes (p=0.001). When patients were stratified according to the clinical diagnosis, isoprostane levels were not statistically different between DMD and BMD patients. In conclusion whether the condition of oxidative stress found in plasma depends on the degenerative process occurring in muscles or on different mechanisms, such as the release of myoglobin in the blood, should be ascertained. However, our study confirms that oxidative stress findings in DMD/BMD patients are effectively present at the plasma levels. The condition of oxidative stress might act as an adjunctive cause of extra-muscular cell damage to which these patients are exposed for their entire life.

L-arginine decreases inflammation and modulates the nuclear factor-kappaB/matrix metalloproteinase cascade in mdx muscle fibers

Duchenne muscular dystrophy (DMD) is a lethal, X-linked disorder associated with dystrophin deficiency that results in chronic inflammation, sarcolemma damage, and severe skeletal muscle degeneration. Recently, the use of L-arginine, the substrate of nitric oxide synthase (nNOS), has been proposed as a pharmacological treatment to attenuate the dystrophic pattern of DMD. However, little is known about signaling events that occur in dystrophic muscle with l-arginine treatment. Considering the implication of inflammation in dystrophic processes, we asked whether L-arginine inhibits inflammatory signaling cascades. We demonstrate that L-arginine decreases inflammation and enhances muscle regeneration in the mdx mouse model. Classic stimulatory signals, such as proinflammatory cytokines interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha, are significantly decreased in mdx mouse muscle, resulting in lower nuclear factor (NF)-kappaB levels and activity. NF-kappaB serves as a pivotal transcription factor with multiple levels of regulation; previous studies have shown perturbation of NF-kappaB signaling in both mdx and DMD muscle. Moreover, L-arginine decreases the activity of metalloproteinase (MMP)-2 and MMP-9, which are transcriptionally activated by NF-kappaB. We show that the inhibitory effect of L-arginine on the NF-kappaB/MMP cascade reduces beta-dystroglycan cleavage and translocates utrophin and nNOS throughout the sarcolemma. Collectively, our results clarify the molecular events by which L-arginine promotes muscle membrane integrity in dystrophic muscle and suggest that NF-kappaB-related signaling cascades could be potential therapeutic targets for DMD management.

A canine minidystrophin is functional and therapeutic in mdx mice

Duchenne muscular dystrophy (DMD) is the most common and lethal genetic muscle disorder lacking a curative treatment. We wish to use the dystrophin-deficient golden retriever muscular dystrophy (GRMD) dog, a canine model of DMD, to investigate adeno-associated virus (AAV) vector-mediated minidystrophin gene therapy. The dog model is useful in evaluating vector dose requirement and immunological consequences owing to its large size and outbred nature. In this study, we have cloned and constructed a canine minidystrophin gene vector. Owing to limited availability of the GRMD dogs, here we first examined the functions and therapeutic effects of the canine minidystrophin in the mdx mouse model. We observed efficient minigene expression without cellular immune responses in mdx mice after AAV1-cMinidys vector intramuscular injection. We also observed restoration of the missing dystrophin-associated protein complex (DPC) onto the sarcolemma, including sarcoglycans and dystrobrevin, and a partial restoration of alpha-syntrophin and neural nitric oxide synthase (nNOS). In addition, minidystrophin treatment ameliorated dystrophic pathology, such as fibrosis and myofiber central nucleation (CN). CN remained minimal (<2%) after AAV injection in the neonatal mdx mice and was reduced from more than 75% to about 25% after AAV injection in adult mdx mice. Finally, in vivo cell membrane leakage test with Evans blue dye showed that the canine minidystrophin could effectively protect the myofiber plasma membrane integrity. Our results, thus, demonstrated the functionality and therapeutic potential of the canine minidystrophin and paved its way for further testing in the GRMD dog model.

The role of free radicals in the pathophysiology of muscular dystrophy

Null mutation of any one of several members of the dystrophin protein complex can cause progressive, and possibly fatal, muscle wasting. Although these muscular dystrophies arise from mutation of a single gene that is expressed primarily in muscle, the resulting pathology is complex and multisystemic, which shows a broader disruption of homeostasis than would be predicted by deletion of a single-gene product. Before the identification of the deficient proteins that underlie muscular dystrophies, such as Duchenne muscular dystrophy (DMD), oxidative stress was proposed as a major cause of the disease. Now, current knowledge supports the likelihood that interactions between the primary genetic defect and disruptions in the normal production of free radicals contribute to the pathophysiology of muscular dystrophies. In this review, we focus on the pathophysiology that results from dystrophin deficiency in humans with DMD and the mdx mouse model of DMD. Current evidence indicates three general routes through which free radical production can be disrupted in dystrophin deficiency to contribute to the ensuing pathology. First, constitutive differences in free radical production can disrupt signaling processes in muscle and other tissues and thereby exacerbate pathology. Second, tissue responses to the presence of pathology can cause a shift in free radical production that can promote cellular injury and dysfunction. Finally, behavioral differences in the affected individual can cause further changes in the production and stoichiometry of free radicals and thereby contribute to disease. Unfortunately, the complexity of the free radical-mediated processes that are perturbed in complex pathologies such as DMD will make it difficult to develop therapeutic approaches founded on systemic administration of antioxidants. More mechanistic knowledge of the specific disruptions of free radicals that underlie major features of muscular dystrophy is needed to develop more targeted and successful therapeutic approaches.

Early onset of lipofuscin accumulation in dystrophin-deficient skeletal muscles of DMD patients and mdx mice

Lipofuscin, the so-called ageing pigment, is formed by the oxidative degradation of cellular macromolecules by oxygen-derived free radicals and redox-active metal ions. Usually it accumulates in post-mitotic, long-lived cells such as neurons and cardiac muscle cells. In contrast, it is rarely seen in either normal or diseased skeletal muscle fibres. In this paper, we report that lipofuscin accumulates at an early age in both human and murine dystrophic muscles. Autofluorescent lipofuscin granules were localized, using confocal laser scanning microscopy and electron microscopy, in dystrophin-deficient skeletal muscles of X chromosome-linked young Duchenne muscular dystrophy (DMD) patients and of mdx mice at various ages after birth. Age-matched normal controls were studied similarly. Autofluorescent lipofuscin granules were observed in dystrophic biceps brachii muscles of 2-7-year-old DMD patients where degeneration and regeneration of myofibres are active, but they were rarely seen in age-matched normal controls. In normal mice, lipofuscin first appears in diaphragm muscles nearly 20 weeks after birth but in mdx muscles it occurs much earlier, 4 weeks after birth, when the primary degeneration of dystrophin-deficient myofibres is at a peak. Lipofuscin accumulation increases with age in both mdx and normal controls and is always higher in dystrophic muscles than in age-matched normal controls. At the electron microscopical level, it was confirmed that the localisation of autofluorescent granules observed by light microscopy in dystrophin-deficient skeletal muscles coincided with lipofuscin granules in myofibres and myosatellite cells, and in macrophages accumulating around myofibres and in interstitial connective tissue. Our results agree with previous biochemical and histochemical data implying increased oxidative damages in DMD and mdx muscles. They indicate that dystrophin-deficient myofibres are either more susceptible to oxidative stress, or are subjected to higher intra- or extracellular oxidative stress than normal controls, or both.

Nitric Oxide Induces Apoptosis in Mouse C2C12 Myoblast Cells

To investigate whether nitric oxide (NO) induces apoptosis in myoblast cells, the effect of the sodium nitroprusside (SNP), NO donor, on the apoptosis of mouse C2C12 myoblast cells was examined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, flow cytometry, 4,6-diamidino-2-phenylindole (DAPI) staining, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) assay, DNA fragmentation assay, reverse transcription-polymerase chain reaction (RT-PCR), Western blot analysis, and caspase-3 enzyme assay. Mouse C2C12 myoblast cells treated with SNP exhibited several apoptotic features. SNP increased p53 expression and bax expression. SNP also enhanced caspase-3 enzyme activity. The data show that NO may induce apoptotic cell death in myoblast cells through the activation of p53-, bax-, and caspase-dependent intracellular death-related pathways.

Evolving therapeutic strategies for Duchenne muscular dystrophy: targeting downstream events

Duchenne muscular dystrophy (DMD) is a progressive, lethal, muscle wasting disease that affects 1 of 3500 boys born worldwide. The disease results from mutation of the dystrophin gene that encodes a cytoskeletal protein associated with the muscle cell membrane. Although gene therapy will likely provide the cure for DMD, it remains on the distant horizon, emphasizing the need for more rapid development of palliative treatments that build on improved understanding of the complex pathology of dystrophin deficiency. In this review, we have focused on therapeutic strategies that target downstream events in the pathologic progression of DMD. Much of this work has been developed initially using the dystrophin-deficient mdx mouse to explore basic features of the pathophysiology of dystrophin deficiency and to test potential therapeutic interventions to slow, reverse, or compensate for functional losses that occur in muscular dystrophy. In some cases, the initial findings in the mdx model have led to clinical treatments for DMD boys that have produced improvements in muscle function and quality of life. Many of these investigations have concerned interventions that can affect protein balance in muscle, by inhibiting specific proteases implicated in the DMD pathology, or by providing anabolic factors or depleting catabolic factors that can contribute to muscle wasting. Other investigations have exploited the use of anti-inflammatory agents that can reduce the contribution of leukocytes to promoting secondary damage to dystrophic muscle. A third general strategy is designed to increase the regenerative capacity of dystrophic muscle and thereby help retain functional muscle mass. Each of these general approaches to slowing the pathology of dystrophin deficiency has yielded encouragement and suggests that targeting downstream events in dystrophinopathy can yield worthwhile, functional improvements in DMD.

The contribution of reactive oxygen species to sarcopenia and muscle ageing

Ageing is a complex process that in muscle in usually associated with a decrease in mass, strength, and velocity of contraction. One of the most striking effects of ageing on muscle is known as sarcopenia, a process that is the result of many cellular changes, such as a reduction in the number of motor units coupled with an increase in motor unit size, progressive denervation, decreased synthesis of myofibrillar components, atrophy due to disuse, accumulation of connective tissue, etc. It has been suggested that sarcopenia may be triggered by reactive oxygen species (ROS) that have accumulated throughout one's lifetime. ROS, which are generated by the addition of a single electron to the oxygen molecule, are formed in all tissues including muscle fibres and, especially, in the mitochondrial respiratory chain. Such reactive elements are usually quite harmful and result in oxidative stress that can damage other cellular components such as DNA, proteins, lipids, etc. resulting in further damage to the cells and tissues. As a consequence, the intra and intercellular membranes of the muscle fibers, in particular those of the Sarcoplasmic reticulum, may be modified and the Ca(2+) transport mechanism altered. During the ageing process ROS production may drastically increase because of an altered function of the respiratory chain and an insufficient functioning of the antioxidant cellular defences. How such an oxidative insult plays a role in the age-related decrease of muscle performance and mass has yet to be defined. What does have a clear role in the progression of sarcopenia is the significant reduction of the regenerative potential of muscle fibres. This reduction is due to a reduced pool of satellite cells that are usually recruited to replace damaged fibres and promote their regeneration. Exercise as a method to prevent or at least delay sarcopenia has been discussed in many scientific reports. While on the one hand, it seems clear that exercise is effective in reducing the loss of muscle mass, on the other it appears that physical activity increases both the mechanical damage and the accumulation of free radicals as a result of an increase in the aerobic metabolism of the muscles involved.

Correlated NOS-Imu and myf5 expression by satellite cells in mdx mouse muscle regeneration during NOS manipulation and deflazacort treatment

Satellite cells, muscle precursor cells in skeletal muscle, are normally quiescent and become activated by disease or injury. A lack of dystrophin and changes in the expression or activity of neuronal nitric oxide synthase (NOS-I) affect the timing of activation in vivo. Nitric oxide synthase inhibition delays muscle repair in normal mice, and worsens muscular dystrophy in the mdx mouse, a genetic homologue of Duchenne muscular dystrophy. However, the potential role of activation and repair events mediated by nitric oxide in determining the outcome of steroid or other treatments for muscular dystrophy is not clear. We tested the hypothesis that the extent of repair in dystrophic muscles of mdx mice is partly dependent on NOS-Imu expression and activity. Myotube formation in regenerating muscle was promoted by deflazacort treatment of mdx dystrophic mice (P<0.05), and improved by combination with the nitric oxide synthase substrate, L-arginine, especially in the diaphragm. NOS-Imu mRNA expression and activity were present in satellite cells and very new myotubes of regenerating and dystrophic muscle. Deflazacort treatment resulted in increased NOS-Imu expression in regenerating muscles in a strong and specific correlation with myf5 expression (r=0.95, P<0.01), a marker for muscle repair. Nitric oxide synthase inhibition prevented the deflazacort-induced rise in NOS-Imu and myf5 expression in the diaphragm without affecting the diameter of non-regenerating fibres. These in vivo studies suggest that gains in NOS-Imu expression and nitric oxide synthase activity in satellite cells can increase the extent and speed of repair, even in the absence of dystrophin in muscle fibres. NOS-Imu may be a useful therapeutic target to augment the effects of steroidal or other treatments of muscular dystrophy.

Interactions between neutrophils and macrophages promote macrophage killing of rat muscle cells in vitro

Current evidence indicates that the physiological functions of inflammatory cells are highly sensitive to their microenvironment, which is partially determined by the inflammatory cells and their potential targets. In the present investigation, interactions between neutrophils, macrophages and muscle cells that may influence muscle cell death are examined. Findings show that in the absence of macrophages, neutrophils kill muscle cells in vitro by superoxide-dependent mechanisms, and that low concentrations of nitric oxide (NO) protect against neutrophil-mediated killing. In the absence of neutrophils, macrophages kill muscle cells through a NO-dependent mechanism, and the presence of target muscle cells causes a three-fold increase in NO production by macrophages, with no change in the concentration of inducible nitric oxide synthase. Muscle cells that are co-cultured with both neutrophils and macrophages in proportions that are observed in injured muscle show cytotoxicity through a NO-dependent, superoxide-independent mechanism. Furthermore, the concentration of myeloid cells that is necessary for muscle killing is greatly reduced in assays that use mixed myeloid cell populations, rather than uniform populations of neutrophils or macrophages. These findings collectively show that the magnitude and mechanism of muscle cell killing by myeloid cells are modified by interactions between muscle cells and neutrophils, between muscle cells and macrophages and between macrophages and neutrophils.

The dystrophin-glycoprotein complex, cellular signaling, and the regulation of cell survival in the muscular dystrophies

Mutations of different components of the dystrophin-glycoprotein complex (DGC) cause muscular dystrophies that vary in terms of severity, age of onset, and selective involvement of muscle groups. Although the primary pathogenetic processes in the muscular dystrophies have clearly been identified as apoptotic and necrotic muscle cell death, the pathogenetic mechanisms that lead to cell death remain to be determined. Studies of components of the DGC in muscle and in nonmuscle tissues have revealed that the DGC is undoubtedly a multifunctional complex and a highly dynamic structure, in contrast to the unidimensional concept of the DGC as a mechanical component in the cell. Analysis of the DGC reveals compelling analogies to two other membrane-associated protein complexes, namely integrins and caveolins. Each of these complexes mediates signal transduction cascades in the cell, and disruption of each complex causes muscular dystrophies. The signal transduction cascades associated with the DGC, like those associated with integrins and caveolins, play important roles in cell survival signaling, cellular defense mechanisms, and regulation of the balance between cell survival and cell death. This review focuses on the functional components of the DGC, highlighting the evidence of their participation in cellular signaling processes important for cell survival. Elucidating the link between these functional components and the pathogenetic processes leading to cell death is the foremost challenge to understanding the mechanisms of disease expression in the muscular dystrophies due to defects in the DGC.

Role of nitric oxide in the pathogenesis of muscular dystrophies: a "two hit" hypothesis of the cause of muscle necrosis

Although the genetic and biochemical bases of many of the muscular dystrophies have been elucidated, the pathophysiological mechanisms leading to muscle cell death and degeneration remain elusive. Among the most well studied of the dystrophies are those due to defects in proteins that make up the dystrophin-glycoprotein complex (DGC). There has been much interest in the role of nitric oxide (NO(*)) in the pathogenesis of these diseases because the enzyme that synthesizes NO(*), nitric oxide synthase (NOS), is associated with the DGC. Recent studies of dystrophies related to DGC defects suggest that one mechanism of cellular injury is functional ischemia related to alterations in cellular NOS and disruption of a normal protective action of NO(*). This protective action is the prevention of local ischemia during contraction-induced increases in sympathetic vasoconstriction. However, the loss of this protection, alone, does not explain the subsequent muscle cell death and degeneration since mice lacking neuronal NOS (the predominant isoform expressed in muscle) do not develop a muscular dystrophy. Thus, there must be additional biochemical changes conferred upon the cells by these DGC defects, and these changes are discussed in terms of a proposed "two hit" hypothesis of the pathogenetic mechanisms that underlie the muscular dystrophies. According to this hypothesis, pathogenic defects in the DGC have at least two biochemical consequences: a reduction in NO(*)-mediated protection against ischemia, and an increase in cellular susceptibility to metabolic stress. Either one alone may be insufficient to lead to muscle cell death. However, in combination, the biochemical consequences are sufficient to cause muscle degeneration. The role of oxidative stress as a final common pathophysiologic pathway is discussed in terms of data showing that oxidative injury precedes pathologic changes and that muscle cells with defects in the DGC have an increased susceptibility to oxidant challenges. Accordingly, this "two hit" hypothesis may explain many of the complex spatial and temporal variations in disease expression that characterize the muscular dystrophies, such as grouped necrosis, a pre-necrotic phase of the disease, and selective muscle involvement.