How does dystrophin deficiency lead to muscle degeneration?--evidence from the mdx mouse

The role of mechanobiology in progression of rotator cuff muscle atrophy and degeneration

Rotator cuff (RC) muscles undergo several detrimental changes following mechanical unloading resulting from RC tendon tear. In this review, we highlight the pathological causes and consequences of mechanical alterations at the whole muscle, muscle fiber, and muscle resident cell level as they relate to RC disease progression. In brief, the altered mechanical loads associated with RC tear lead to architectural, structural, and compositional changes at the whole-muscle and muscle fiber level. At the cellular level, these changes equate to direct disruption of mechanobiological signaling, which is exacerbated by mechanically regulated biophysical and biochemical changes to the cellular and extra-cellular environment (also known as the stem cell "niche"). Together, these data have important implications for both pre-clinical models and clinical practice. In pre-clinical models, it is important to recapitulate both the atrophic and degenerative muscle loss found in humans using clinically relevant modes of injury. Clinically, understanding the mechanics and underlying biology of the muscle will impact both surgical decision-making and rehabilitation protocols, as interventions that may be good for atrophic muscle will have a detrimental effect on degenerating muscle, and vice versa. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:546-556, 2018.

Critical Role of Intracellular RyR1 Calcium Release Channels in Skeletal Muscle Function and Disease

The skeletal muscle Ca²⁺ release channel, also known as ryanodine receptor type 1 (RyR1), is the largest ion channel protein known and is crucial for effective skeletal muscle contractile activation. RyR1 function is controlled by Ca_v1.1, a voltage gated Ca²⁺ channel that works mainly as a voltage sensor for RyR1 activity during skeletal muscle contraction and is also fine-tuned by Ca²⁺, several intracellular compounds (e.g., ATP), and modulatory proteins (e.g., calmodulin). Dominant and recessive mutations in RyR1, as well as acquired channel alterations, are the underlying cause of various skeletal muscle diseases. The aim of this mini review is to summarize several current aspects of RyR1 function, structure, regulation, and to describe the most common diseases caused by hereditary or acquired RyR1 malfunction.

The role of cell transplantation in modifying the course of limb girdle muscular dystrophy: a longitudinal 5-year study

Limb girdle muscular dystrophy (LGMD), a group of progressive degenerative disorders, causes functional limitation affecting the quality of life. Cell therapy is being widely explored and preliminary studies have shown beneficial effects. Cell therapy induces trophic-factors release, angiogenesis, anti-inflammation, and protein synthesis, which helps in the reparative process at the microcellular level. In this 5-year longitudinal study, the effect of autologous bone marrow mononuclear cells is studied on the natural course of 65 patients with LGMD. Functional Independence Measure and manual muscle testing showed statistically significant improvement, post-cell transplantation. The key finding of this study was demonstration of a plateau phase in the disease progression of the patients. No adverse events were noted. Autologous bone marrow mononuclear cells may be a novel, safe, and effective treatment approach to control the rate of progression of LGMD, thus improving the functional outcomes. Further randomized controlled trials are required.

The first exon duplication mouse model of Duchenne muscular dystrophy: A tool for therapeutic development

Exon duplication mutations account for up to 11% of all cases of Duchenne muscular dystrophy (DMD), and a duplication of exon 2 is the most common duplication in patients. For use as a platform for testing of duplication-specific therapies, we developed a mouse model that carries a Dmd exon 2 duplication. By using homologous recombination we duplicated exon 2 within intron 2 at a location consistent with a human duplication hotspot. mRNA analysis confirms the inclusion of a duplicated exon 2 in mouse muscle. Dystrophin expression is essentially absent by immunofluorescent and immunoblot analysis, although some muscle specimens show very low-level trace dystrophin expression. Phenotypically, the mouse shows similarities to mdx, the standard laboratory model of DMD. In skeletal muscle, areas of necrosis and phagocytosis are seen at 3 weeks, with central nucleation prominent by four weeks, recapitulating the "crisis" period in mdx. Marked diaphragm fibrosis is noted by 6 months, and remains unchanged at 12 months. Our results show that the Dup2 mouse is both pathologically (in degree and distribution) and physiologically similar to mdx. As it recapitulates the most common single exon duplication found in DMD patients, this new model will be a useful tool to assess the potential of duplicated exon skipping.

Spatio-Temporal Differences in Dystrophin Dynamics at mRNA and Protein Levels Revealed by a Novel FlipTrap Line

Dystrophin (Dmd) is a structural protein that links the extracellular matrix to actin filaments in muscle fibers and is required for the maintenance of muscles integrity. Mutations in Dmd lead to muscular dystrophies in humans and other vertebrates. Here, we report the characterization of a zebrafish gene trap line that fluorescently labels the endogenous Dmd protein (Dmd-citrine, Gt(dmd-citrine) ^ct90a). We show that the Dmd-citrine line recapitulates endogenous dmd transcript expression and Dmd protein localization. Using this Dmd-citrine line, we follow Dmd localization to the myosepta in real-time using time-lapse microscopy, and find that the accumulation of Dmd protein at the transverse myosepta coincides with the onset of myotome formation, a critical stage in muscle maturation. We observed that Dmd protein localizes specifically to the myosepta prior to dmd mRNA localization. Additionally, we demonstrate that the Dmd-citrine line can be used to assess muscular dystrophy following both genetic and physical disruptions of the muscle.

Dysfunctional muscle and liver glycogen metabolism in mdx dystrophic mice

Background

Duchenne muscular dystrophy (DMD) is a severe, genetic muscle wasting disorder characterised by progressive muscle weakness. DMD is caused by mutations in the dystrophin (dmd) gene resulting in very low levels or a complete absence of the dystrophin protein, a key structural element of muscle fibres which is responsible for the proper transmission of force. In the absence of dystrophin, muscle fibres become damaged easily during contraction resulting in their degeneration. DMD patients and mdx mice (an animal model of DMD) exhibit altered metabolic disturbances that cannot be attributed to the loss of dystrophin directly. We tested the hypothesis that glycogen metabolism is defective in mdx dystrophic mice.

Results

Dystrophic mdx mice had increased skeletal muscle glycogen (79%, (P<0.01)). Skeletal muscle glycogen synthesis is initiated by glycogenin, the expression of which was increased by 50% in mdx mice (P<0.0001). Glycogen synthase activity was 12% higher (P<0.05) but glycogen branching enzyme activity was 70% lower (P<0.01) in mdx compared with wild-type mice. The rate-limiting enzyme for glycogen breakdown, glycogen phosphorylase, had 62% lower activity (P<0.01) in mdx mice resulting from a 24% reduction in PKA activity (P<0.01). In mdx mice glycogen debranching enzyme expression was 50% higher (P<0.001) together with starch-binding domain protein 1 (219% higher; P<0.01). In addition, mdx mice were glucose intolerant (P<0.01) and had 30% less liver glycogen (P<0.05) compared with control mice. Subsequent analysis of the enzymes dysregulated in skeletal muscle glycogen metabolism in mdx mice identified reduced glycogenin protein expression (46% less; P<0.05) as a possible cause of this phenotype.

Conclusion

We identified that mdx mice were glucose intolerant, and had increased skeletal muscle glycogen but reduced amounts of liver glycogen.

Intestine of dystrophic mice presents enhanced contractile resistance to stretching despite morphological impairment

Protein dystrophin is a component of the dystrophin-associated protein complex, which links the contractile machinery to the plasma membrane and to the extracellular matrix. Its absence leads to a condition known as Duchenne muscular dystrophy (DMD), a disease characterized by progressive skeletal muscle degeneration, motor disability, and early death. In mdx mice, the most common DMD animal model, loss of muscle cells is observed, but the overall disease alterations are less intense than in DMD patients. Alterations in gastrointestinal tissues from DMD patients and mdx mice are not yet completely understood. Thus, we investigated the possible relationships between morphological (light and electron microscopy) and contractile function (by recording the isometric contractile response) with alterations in Ca²⁺ handling in the ileum of mdx mice. We evidenced a 27% reduction in the ileal muscular layer thickness, a partial damage to the mucosal layer, and a partial damage to mitochondria of the intestinal myocytes. Functionally, the ileum from mdx presented an enhanced responsiveness during stretch, a mild impairment in both the electromechanical and pharmacomechanical signaling associated with altered calcium influx-induced contraction, with no alterations in the sarcoplasmic reticulum Ca²⁺ storage (maintenance of the caffeine and thapsigargin-induced contraction) compared with control animals. Thus, it is evidenced that the protein dystrophin plays an important role in the preservation of both the microstructure and ultrastructure of mice intestine, while exerting a minor but important role concerning the intestinal contractile responsiveness and calcium handling.

Generation of skeletal muscle cells from embryonic and induced pluripotent stem cells as an in vitro model and for therapy of muscular dystrophies

Muscular dystrophies (MDs) are a heterogeneous group of inherited disorders characterized by progressive muscle wasting and weakness likely associated with exhaustion of muscle regeneration potential. At present, no cures or efficacious treatments are available for these diseases, but cell transplantation could be a potential therapeutic strategy. Transplantation of myoblasts using satellite cells or other myogenic cell populations has been attempted to promote muscle regeneration, based on the hypothesis that the donor cells repopulate the muscle and contribute to its regeneration. Embryonic stem cells (ESCs) and more recently induced pluripotent stem cells (iPSCs) could generate an unlimited source of differentiated cell types, including myogenic cells. Here we review the literature regarding the generation of myogenic cells considering the main techniques employed to date to elicit efficient differentiation of human and murine ESCs or iPSCs into skeletal muscle. We also critically analyse the possibility of using these cellular populations as an alternative source of myogenic cells for cell therapy of MDs.

Alpha sarcoglycan is required for FGF-dependent myogenic progenitor cell proliferation in vitro and in vivo

Mice deficient in α-sarcoglycan (Sgca-null mice) develop progressive muscular dystrophy and serve as a model for human limb girdle muscular dystrophy type 2D. Sgca-null mice suffer a more severe myopathy than that of mdx mice, the model for Duchenne muscular dystrophy. This is the opposite of what is observed in humans and the reason for this is unknown. In an attempt to understand the cellular basis of this severe muscular dystrophy, we isolated clonal populations of myogenic progenitor cells (MPCs), the resident postnatal muscle progenitors of dystrophic and wild-type mice. MPCs from Sgca-null mice generated much smaller clones than MPCs from wild-type or mdx dystrophic mice. Impaired proliferation of Sgca-null myogenic precursors was confirmed by single fiber analysis and this difference correlated with Sgca expression during MPC proliferation. In the absence of dystrophin and associated proteins, which are only expressed after differentiation, SGCA complexes with and stabilizes FGFR1. Deficiency of Sgca leads to an absence of FGFR1 expression at the membrane and impaired MPC proliferation in response to bFGF. The low proliferation rate of Sgca-null MPCs was rescued by transduction with Sgca-expressing lentiviral vectors. When transplanted into dystrophic muscle, Sgca-null MPCs exhibited reduced engraftment. The reduced proliferative ability of Sgca-null MPCs explains, at least in part, the severity of this muscular dystrophy and also why wild-type donor progenitor cells engraft efficiently and consequently ameliorate disease.

The expression of myogenic regulatory factors and muscle growth factors in the masticatory muscles of dystrophin-deficient (mdx) mice

The activities of myogenic regulatory factors (MRF) and muscle growth factors increase in muscle that is undergoing regeneration, and may correspond to some specific changes. Little is known about the role of MRFs in masticatory muscles in mdx mice (the model of Duchenne muscular dystrophy) and particularly about their mRNA expression during the process of muscle regeneration. Using Taqman RT-PCR, we examined the mRNA expression of the MRFs myogenin and MyoD1 (myogenic differentiation 1), and of the muscle growth factors myostatin, IGF1 (insulin-like growth factor) and MGF (mechano-growth factor) in the masseter, temporal and tongue masticatory muscles of mdx mice (n = 6 to 10 per group). The myogenin mRNA expression in the mdx masseter and temporal muscle was found to have increased (P < 0.05), whereas the myostatin mRNA expressions in the mdx masseter (P < 0.005) and tongue (P < 0.05) were found to have diminished compared to those for the controls. The IGF and MGF mRNA amounts in the mdx mice remained unchanged. Inside the mdx animal group, gender-related differences in the mRNA expressions were also found. A higher mRNA expression of myogenin and MyoD1 in the mdx massterer and temporal muscles was found in females in comparison to males, and the level of myostatin was higher in the masseter and tongue muscle (P < 0.001 for all comparisons). Similar gender-related differences were also found within the control groups. This study reveals the intermuscular differences in the mRNA expression pattern of myogenin and myostatin in mdx mice. The existence of these differences implies that dystrophinopathy affects the skeletal muscles differentially. The finding of gender-related differences in the mRNA expression of the examined factors may indicate the importance of hormonal influences on muscle regeneration.

Spatial and temporal expression of hypoxia-inducible factor-1α during myogenesis in vivo and in vitro

We investigated the spatial and temporal expression patterns of hypoxia-inducible factor-1α (HIF-1α) during muscle regeneration and myogenesis in a C2C12 cell culture system. The expression of HIF-1α synchronized with that of myogenic regulatory genes during muscle regeneration at both the mRNA and protein levels. The HIF-1α protein was localized in the nuclei of newly formed regenerating myofibers in three different muscle injury models, including freezing, bupivacaine injection, and muscular dystrophy. In myogenic cell culture, the HIF-1α protein was localized in the nucleus and cytoplasm of the majority of myoblasts and myotubes. HIF-1α protein expression decreased concomitant with the increased expression of MyoD and myogenin proteins after the induction of myogenic differentiation. We investigated the adaptive response of myoblasts to hypoxia-like conditions induced by treatment of cobalt chloride. This treatment allowed HIF-1α to accumulate and translocate to the nucleus to activate transcription of its target genes, suggesting that myoblasts adapted to acute hypoxia-like conditions through enhancing an HIF-1-dependent pathway. Our results provide insight into the possible involvement of HIF-1α in myogenesis in vivo and in vitro.

Oxidative stress regulation of stem and progenitor cells

In recent years, it has become clear that balanced regulation of reactive oxygen species is of critical significance for cell-fate determination as well as for stem cell development, function, and survival. Although many questions regarding intracellular redox status regulation of stem cell fate remain, we review here what is known regarding the impact of cell-fate signaling as shown with a variety of human cancer cells and more recently on cancer-initiating cells and on the regenerative capacity of skeletal muscle and hematopoietic tissue and their stem cells. We also discuss the role of altered intracellular redox status as a potential primary pathogenic mechanism in muscular dystrophy and hematopoietic pathologies. Studies discussed here illustrate how understanding altered redox regulation of stem cell behavior may contribute to the development of novel stem cell therapies.

Long-term improvement in mdx cardiomyopathy after therapy with peptide-conjugated morpholino oligomers

AIMS

The cardiomyopathy found in Duchenne muscular dystrophy (DMD) is responsible for death due to heart failure in approximately 30% of patients and additionally contributes to many DMD morbidities. Strategies to bypass DMD-causing mutations to allow an increase in body-wide dystrophin have proved promising, but increasing cardiac dystrophin continues to be challenging. The purpose of this study was to determine if therapeutic restoration of cardiac dystrophin improved the significant cardiac hypertrophy and diastolic dysfunction identified in X-linked muscular dystrophy (mdx) dystrophin-null mouse due to a truncation mutation over time after treatment.

METHODS AND RESULTS

Mice lacking dystrophin due to a truncation mutation (mdx) were given an arginine-rich, cell-penetrating, peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO) that delivered a splice-switching oligonucleotide-mediated exon skipping therapy to restore dystrophin in mdx mice before the development of detectable cardiomyopathy. PPMO successfully restored cardiac dystrophin expression, preserved cardiac sarcolemma integrity, and prevented the development of cardiac pathology that develops in mdx-null mice over time. By echocardiography and Doppler analysis of the mitral valve, we identified that PPMO treatment of mdx mice prevented the cardiac hypertrophy and diastolic dysfunction identified in sham-treated, age-matched mdx mice, characteristic of DMD patients early in the disease process, in as little as 5-6 weeks after the initiation of treatment. Surprisingly, despite the short-term replacement of cardiac dystrophin (<1% present after 12 weeks by immunodetection), PPMO therapy also provided a durable cardiac improvement in cardiac hypertrophy and diastolic dysfunction for up to 7 months after the initiation of treatment.

CONCLUSION

These results demonstrate for the first time that PPMO-mediated exon skipping therapy early in the course of DMD may effectively prevent or slow down associated cardiac hypertrophy and diastolic dysfunction with significant long-term impact.

Dysregulated intracellular signaling and inflammatory gene expression during initial disease onset in Duchenne muscular dystrophy

Duchenne muscular dystrophy is a debilitating genetic disorder characterized by severe muscle wasting and early death in affected boys. The primary cause of this disease is mutations in the dystrophin gene that result in the absence of the protein dystrophin and the associated dystrophin-glycoprotein complex in the plasma membrane of muscle fibers. In normal muscle, this complex forms a link between the extracellular matrix and the cytoskeleton that is thought to protect muscle fibers from contraction-induced membrane lesions and to regulate cell signaling cascades. Although the primary defect is known, the mechanisms that initiate disease onset have not been characterized. Data collected during early maturation suggest that inflammatory and immune responses are key contributors to disease pathogenesis and may be initiated by aberrant signaling in dystrophic muscle. However, detailed time course studies of the inflammatory and immune processes are incomplete and need to be characterized further to understand the disease progression. The purposes of this review are to examine the possibility that initial disease onset in dystrophin-deficient muscle results from aberrant inflammatory signaling pathways and to highlight the potential clinical relevance of targeting these pathways to treat Duchenne muscular dystrophy.

Comparison of the myoplasmic calcium transient elicited by an action potential in intact fibres of mdx and normal mice

The myoplasmic free [Ca2+] transient elicited by an action potential (Delta[Ca2+]) was compared in fast-twitch fibres of mdx (dystrophin null) and normal mice. Methods were used that maximized the likelihood that any detected differences apply in vivo. Small bundles of fibres were manually dissected from extensor digitorum longus muscles of 7- to 14-week-old mice. One fibre within a bundle was microinjected with furaptra, a low-affinity rapidly responding fluorescent calcium indicator. A fibre was accepted for study if it gave a stable, all-or-nothing fluorescence response to an external shock. In 18 normal fibres, the peak amplitude and the full-duration at half-maximum (FDHM) of Delta[Ca2+] were 18.4 +/- 0.5 microm and 4.9 +/- 0.2 ms, respectively (mean +/- s.e.m.; 16 degrees C). In 13 mdx fibres, the corresponding values were 14.5 +/- 0.6 microm and 4.7 +/- 0.2 ms. The difference in amplitude is statistically highly significant (P = 0.0001; two-tailed t test), whereas the difference in FDHM is not (P = 0.3). A multi-compartment computer model was used to estimate the amplitude and time course of the sarcoplasmic reticulum (SR) calcium release flux underlying Delta[Ca2+]. Estimates were made based on several differing assumptions: (i) that the resting myoplasmic free Ca2+ concentration ([Ca2+]R) and the total concentration of parvalbumin ([Parv(T)]) are the same in mdx and normal fibres, (ii) that [Ca2+](R) is larger in mdx fibres, (iii) that [Parv(T)] is smaller in mdx fibres, and (iv) that [Ca2+]R is larger and [Parv(T)] is smaller in mdx fibres. According to the simulations, the 21% smaller amplitude of Delta[Ca2+] in mdx fibres in combination with the unchanged FDHM of Delta[Ca2+] is consistent with mdx fibres having a approximately 25% smaller flux amplitude, a 6-23% larger FDHM of the flux, and a 9-20% smaller total amount of released Ca2+ than normal fibres. The changes in flux are probably due to a change in the gating of the SR Ca2+-release channels and/or in their single channel flux. The link between these changes and the absence of dystrophin remains to be elucidated.

Sustained dystrophin expression induced by peptide-conjugated morpholino oligomers in the muscles of mdx mice

Cell-penetrating peptides (CPPs), containing arginine (R), 6-aminohexanoic acid (X), and/or beta-alanine (B) conjugated to phosphorodiamidate morpholino oligomers (PMOs), enhance their delivery in cell culture. In this study, the potency, functional biodistribution, and toxicity of these conjugates were evaluated in vivo, in EGFP-654 transgenic mice that ubiquitously express the aberrantly spliced EGFP-654 pre-mRNA reporter. Correct splicing and enhanced green fluorescence protein (EGFP) upregulation serve as a positive readout for peptide-PMO (PPMO) entry into cells and access to EGFP-654 pre-mRNA in the nucleus. Intraperitoneal injections of a series of PPMOs, A-N (12 mg/kg), administered once a day for four successive days resulted in splicing correction in numerous tissues. PPMO-B was highly potent in the heart, diaphragm, and quadriceps, which are key muscles in the treatment of Duchenne muscular dystrophy. We therefore investigated PPMO M23D-B, designed to force skipping of stop-codon containing dystrophin exon 23, in an mdx mouse model of the disease. Systemic delivery of M23D-B yielded persistent exon 23 skipping, yielding high and sustained dystrophin protein expression in body-wide muscles, including cardiac muscle, without detectable toxicity. The rescued dystrophin reduced serum creatinine kinase to near-wild-type levels, indicating improvement in muscle integrity. This is the first report of oligonucleotide-mediated exon skipping and dystrophin protein induction in the heart of treated animals.

Mdx respiratory impairment following fibrosis of the diaphragm

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease that causes respiratory or cardiac failure and results in death at about 20 years of age. An animal model of DMD, the mdx mouse, is commonly used to estimate dystrophic pathology. The pathological features of limb muscles are relatively mild, however the diaphragm is severely affected and exhibits a degenerative pattern similar to that observed in human DMD. Although, the muscle strength assay of the dystrophic diaphragm has been used to estimate mdx respiratory impairment, systemic functional assessments compared with histopathological analysis have not been demonstrated. Here, we report a sensitive procedure using whole-body plethysmography to monitor respiratory parameters detected during early respiratory insufficiency in the mdx mouse. The dystrophic changes in the diaphragm lead to respiratory dysfunctions. These methods may be useful to assess the therapeutic approaches for the mdx mouse.

VEGF overexpression via adeno-associated virus gene transfer promotes skeletal muscle regeneration and enhances muscle function in mdx mice

Vascular endothelial growth factor (VEGF) is a major regulator of physiological and pathological angiogenesis. Recently it was reported that the delivery of VEGF using recombinant adeno-associated virus (rAAV) vectors reduces muscle damage and promotes muscle regeneration in different experimental models of muscle necrosis. We demonstrate that intramuscular administration of rAAV-VEGF improved pathophysiology of the mdx mouse, a model of Duchenne muscular dystrophy (DMD). One month after injection, rAAV-VEGF-treated muscles showed augmented expression of VEGF and immunolocalization of its receptor, VEGFR-2. VEGF-treated mdx mice showed increased forelimb strength and strength normalized to weight. Treatment reduced necrotic fibers area and increased regenerating fibers area with an augmented number of myogenin-positive satellite cells and myonuclei, and of developmental myosin heavy chain-positive fibers. Only the regenerating area showed increased capillary density. This study provides novel evidence of a VEGF beneficial effect in mdx mice that is exerted mainly by a proregenerative and angiogenic effect. It opens new therapeutic prospectives in DMD and other types of muscular disorders.

Lipid peroxidation inhibition blunts nuclear factor-kappaB activation, reduces skeletal muscle degeneration, and enhances muscle function in mdx mice

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease resulting from lack of the sarcolemmal protein dystrophin. However, the mechanism leading to the final disease status is not fully understood. Several lines of evidence suggest a role for nuclear factor (NF)-kappaB in muscle degeneration as well as regeneration in DMD patients and mdx mice. We investigated the effects of blocking NF-kappaB by inhibition of oxidative stress/lipid peroxidation on the dystrophic process in mdx mice. Five-week-old mdx mice received three times a week for 5 weeks either IRFI-042 (20 mg/kg), a strong antioxidant and lipid peroxidation inhibitor, or its vehicle. IRFI-042 treatment increased forelimb strength (+22%, P < 0.05) and strength normalized to weight (+23%, P < 0.05) and decreased fatigue (-45%, P < 0.05). It also reduced serum creatine kinase levels (P < 0.01) and reduced muscle-conjugated diene content and augmented muscle-reduced glutathione (P < 0.01). IRFI-042 blunted NF-kappaB DNA-binding activity and tumor necrosis factor-alpha expression in the dystrophic muscles (P < 0.01), reducing muscle necrosis (P < 0.01) and enhancing regeneration (P < 0.05). Our data suggest that oxidative stress/lipid peroxidation represents one of the mechanisms activating NF-kappaB and the consequent pathogenetic cascade in mdx muscles. Most importantly, these new findings may have clinical implications for the pharmacological treatment of patients with DMD.

Common pathological mechanisms in mouse models for muscular dystrophies

Duchenne/Becker and limb-girdle muscular dystrophies share clinical symptoms like muscle weakness and wasting but differ in clinical presentation and severity. To get a closer view on the differentiating molecular events responsible for the muscular dystrophies, we have carried out a comparative gene expression profiling of hindlimb muscles of the following mouse models: dystrophin-deficient (mdx, mdx(3cv)), sarcoglycan-deficient (Sgca null, Sgcb null, Sgcg null, Sgcd null), dysferlin-deficient (Dysf null, SJL(Dysf)), sarcospan-deficient (Sspn null), and wild-type (C57Bl/6, C57Bl/10) mice. The expression profiles clearly discriminated between severely affected (dystrophinopathies and sarcoglycanopathies) and mildly or nonaffected models (dysferlinopathies, sarcospan-deficiency, wild-type). Dystrophin-deficient and sarcoglycan-deficient profiles were remarkably similar, sharing inflammatory and structural remodeling processes. These processes were also ongoing in dysferlin-deficient animals, albeit at lower levels, in agreement with the later age of onset of this muscular dystrophy. The inflammatory proteins Spp1 and S100a9 were up-regulated in all models, including sarcospan-deficient mice, which points, for the first time, at a subtle phenotype for Sspn null mice. In conclusion, we identified biomarker genes for which expression correlates with the severity of the disease, which can be used for monitoring disease progression. This comparative study is an integrating step toward the development of an expression profiling-based diagnostic approach for muscular dystrophies in humans.

Propagation in the transverse tubular system and voltage dependence of calcium release in normal and mdx mouse muscle fibres

Using a two-microelectrode voltage clamp technique, we investigated possible mechanisms underlying the impaired excitation-contraction coupling in skeletal muscle fibres of the mdx mouse, a model of the human disease Duchenne muscular dystrophy. We evaluated the role of the transverse tubular system (T-system) by using the potentiometric indicator di-8 ANEPPS, and that of the sarcoplasmic reticulum (SR) Ca2+ release by measuring Ca2+ transients with a low affinity indicator in the presence of high EGTA concentrations under voltage clamp conditions. We observed minimal differences in the T-system structure and the T-system electrical propagation was not different between normal and mdx mice. Whereas the maximum Ca2+ release elicited by voltage pulses was reduced by approximately 67% in mdx fibres, in agreement with previous results obtained using AP stimulation, the voltage dependence of SR Ca2+ release was identical to that seen in normal fibres. Taken together, our data suggest that the intrinsic ability of the sarcoplasmic reticulum to release Ca2+ may be altered in the mdx mouse.

Muscle-fiber apoptosis in neuromuscular diseases

Muscle-fiber loss is a characteristic of many progressive neuromuscular disorders. Over the past decade, identification of a growing number of apoptosis-associated factors and events in pathological skeletal muscle provided increasing evidence that apoptotic cell-death mechanisms account significantly for muscle-fiber atrophy and loss in a wide spectrum of neuromuscular disorders. It became obvious that there is not one specific pathway for muscle fibers to undergo apoptotic degradation. In contrast, certain neuromuscular diseases seem to involve characteristic expression patterns of apoptosis-related factors and pathways. Furthermore, there are some characteristics of muscle-fiber apoptosis that rely on the muscle fiber itself as an extremely specified cell type. Multinucleated muscle fibers with successive muscle-fiber segments controlled by individual nuclei display some specifics different from apoptosis of mononucleated cells. This review focuses on the expression patterns of apoptosis-associated factors in different primary and secondary neuromuscular disorders and gives a synopsis of current knowledge.

Gadolinium reduces short-term stretch-induced muscle damage in isolated mdx mouse muscle fibres

Duchenne muscular dystrophy is a lethal muscle disease caused by absence of the protein dystrophin which is part of a glycoprotein complex located on the intracellular surface of the surface membrane. The precise function of dystrophin and the reason why its absence causes severe muscle damage are unclear. Stretch-induced muscle damage is well recognised in normal muscle and is more severe in muscles from animals lacking dystrophin (mdx mice). It has been proposed that stretch-induced damage underlies the progression of damage in muscular dystrophy. In the present study we confirm that single fibres from mdx muscle are more susceptible to stretch-induced damage and show that there is an associated rise in intracellular sodium concentration ([Na+]i) which is greater than in wild-type mice. We show that this rise in [Na+]i can be prevented by Gd3+, which is an established blocker of stretch-activated channels. mdx fibres have a higher than normal resting [Na+]i and this is also reduced by Gd3+. If Gd3+ is applied over the period in which [Na+]i rises following stretched contraction, it prevents one component of the reduced force. The other component of reduced force is caused by inhomogeneity of sarcomeres and can be minimised by stretching the muscle to its new optimum length. These experiments show that part of the short-term damage caused by stretch in mdx fibres can be prevented by blocking stretch-activated channels.

The action potential-evoked sarcoplasmic reticulum calcium release is impaired in mdx mouse muscle fibres

The mdx mouse, a model of the human disease Duchenne muscular dystrophy, has skeletal muscle fibres which display incompletely understood impaired contractile function. We explored the possibility that action potential-evoked Ca(2+) release is altered in mdx fibres. Action potential-evoked Ca(2+)-dependent fluorescence transients were recorded, using both low and high affinity Ca(2+) indicators, from enzymatically isolated fibres obtained from extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles of normal and mdx mice. Fibres were immobilized using either intracellular EGTA or N-benzyl-p-toluene sulphonamide, an inhibitor of the myosin II ATPase. We found that the amplitude of the action potential-evoked Ca(2+) transients was significantly decreased in mdx mice with no measured difference in that of the surface action potential. In addition, Ca(2+) transients recorded from mdx fibres in the absence of EGTA also displayed a marked prolongation of the slow decay phase. Model simulations of the action potential-evoked transients in the presence of high EGTA concentrations suggest that the reduction in the evoked sarcoplasmic reticulum Ca(2+) release flux is responsible for the decrease in the peak of the Ca(2+) transient in mdx fibres. Since the myoplasmic Ca(2+) concentration is a critical regulator of muscle contraction, these results may help to explain the weakness observed in skeletal muscle fibres from mdx mice and, possibly, Duchenne muscular dystrophy patients.

TNF-alpha is a mitogen in skeletal muscle

Emerging evidence suggests that tumor necrosis factor (TNF)-alpha plays a role in muscle repair. To determine whether TNF-alpha modulates satellite cell proliferation, the current study evaluated TNF-alpha effects on DNA synthesis in primary myoblasts and on satellite cell activation in adult mouse muscle. Exposure to recombinant TNF-alpha increased total DNA content in rat primary myoblasts dose-dependently over a 24-h period and increased the number of primary myoblasts incorporating 5-bromo-2'-deoxyuridine (BrdU) during a 30-min pulse labeling. Systemic injection of TNF-alpha stimulated BrdU incorporation by satellite cells in muscles of adult mice, whereas no BrdU was incorporated by satellite cells in control mice. TNF-alpha stimulated serum response factor (SRF) binding to the serum response element (SRE) present in the c-fos gene promoter and stimulated reporter gene expression controlled by the same element. Our data suggest that TNF-alpha activates satellite cells to enter the cell cycle and accelerates G1-to-S phase transition, and these actions may involve activation of early response genes via SRF.

Oxidative stress and the pathogenesis of muscular dystrophies

The muscular dystrophies represent a diverse group of diseases differing in underlying genetic basis, age of onset, mode of inheritance, and severity of progression, but they share certain common pathologic features. Most prominent among these features is the necrotic degeneration of muscle fibers. Although the genetic basis of many of the dystrophies has been known for over a decade and new disease genes continue to be discovered, the pathogenetic mechanisms leading to muscle cell death in the dystrophies remain a mystery. This review focuses on the oxidative stress theory, which states that the final common pathway of muscle cell death in these diseases involves oxidative damage.

Eccentric contraction injury in dystrophic canine muscle

OBJECTIVE

To test the hypothesis that eccentric contractions induce greater injury in dystrophic compared with normal canine muscle.

DESIGN

Blinded cohort study.

SETTING

Animal laboratory.

ANIMALS

Ten dogs with a homologue to Duchenne muscular dystrophy (Golden retriever muscular dystrophy [GRMD]) and 10 normal littermates.

INTERVENTIONS

Contractions induced in tibiotarsal flexors and extensors by sciatic nerve stimulation. Because more powerful extensors overrode flexors, eccentric contractions occurred in flexors. Concentric contractions were induced in contralateral flexors by peroneal nerve stimulation.

MAIN OUTCOME MEASURE

Tibiotarsal flexion force 3 days after contractions. Muscle was examined for injury (esterase activity, Evans blue dye penetration) and regeneration (embryonic myosin isoform expression).

RESULTS

Mean force deficit after eccentric flexor contractions was 43.3%+/-25.7% in GRMD dogs compared with 25.0%+/-18.4% in controls (P=.04, Wilcoxon rank-sum test). Concentric contractions induced force deficits in GRMD but not normal dogs; however, the difference between the 2 groups was not significant (P=.08, Wilcoxon rank-sum test). After concentric contractions in controls, force decrements correlated with esterase activity measured by area (r=.794, P=.006) and intensity (r=.697, P=.025, Spearman rank correlation). No other significant correlation was detected between force and biopsy data.

CONCLUSIONS

Force data support the hypothesis that eccentric contractions induce greater injury in dystrophic compared with normal canine muscle. Phenotypic features of the dystrophic canine model used here are similar to those of humans with Duchenne's.

Apoptosis and muscle fibre loss in neuromuscular disorders

The past decade has witnessed increasing evidence that besides necrosis, apoptotic cell death mechanisms contribute to muscle fibre loss in various neuromuscular conditions, including the muscular dystrophies, metabolic myopathies, and cases of denervation. The up-regulation of bax and bcl-2, both members of the bcl-2 family, indicate that the predominant effectors involve permeability transition pores in the mitochondrial membrane and subsequent caspase activation which confers the typical morphological and biochemical features of apoptosis such as DNA-fragmentation. It is likely that apoptotic degradation of nuclei and contractile elements is a localized event in muscle fibre segments leading to muscle fibre atrophy and finally loss in these disorders. Essential triggers of apoptosis seem to be homeostatic dysregulation as well as oxidative stress, with increased generation of free oxygen radicals and nitric oxide. In the absence of effective primary treatments, there is hope that interventions in muscle fibre apoptosis will bear promising therapeutic strategies.

Effect of injecting primary myoblasts versus putative muscle-derived stem cells on mass and force generation in mdx mice

It is well established that the injection of normal myoblasts or of muscle-derived stem cells (MDSCs) into the muscle of dystrophin-deficient mdx mice results in the incorporation of a number of donor myoblasts into the host muscle. However, the effect of the injected exogenous cells on mdx muscle mass and functional capacity has not been evaluated. This study evaluates the mass and functional capacity of the extensor digitorum longus (EDL) muscles of adult, male mdx mice that received intramuscular injections of primary myoblasts or of MDSCs (isolated by a preplating technique; Qu, Z., Balkir, L., van Deutekom, J.C., Robbins, P.D., Pruchnic, R., and Huard, J., J. Cell Biol. 1998;142:1257-1267) derived from normal mice. Evaluations were made 9 weeks after cell transplantation. Uninjected mdx EDL muscles have a mass 50% greater than that of age-matched C57BL/10J (normal) EDL muscles. Injections of either primary myoblasts or MDSCs have no effect on the mass of mdx EDL muscles. EDL muscles of mdx mice generate 43% more absolute twitch tension and 43% less specific tetanic tension then do EDL muscles of C57BL/10J mice. However, the absolute tetanic and specific twitch tension of mdx and C57BL/10J EDL muscles are similar. Injection of either primary myoblasts or MDSCs has no effect on the absolute or specific twitch and tetanic tensions of mdx muscle. Approximately 25% of the myofibers in mdx EDL muscles that received primary myoblasts react positively with antibody to dystrophin. There is no significant difference in the number of dystrophin-positive myofibers when MDSCs are injected. Regardless of the source of donor cells, dystrophin is limited to short distances (60-900 microm) along the length of the myofibers. This may, in part, explain the failure of cellular therapy to alter the contractile properties of murine dystrophic muscle.

Changes in mechanosensitive channel gating following mechanical stimulation in skeletal muscle myotubes from the mdx mouse

We studied the effects of membrane stretch and voltage on the gating of single mechanosensitive (MS) channels in myotubes from dystrophin-deficient mdx mice. In earlier studies of MS channels in mdx myotubes, we found a novel class of stretch-inactivated channels. In the present experiments, we used a gentle suction protocol to determine whether seal formation damaged the membrane and altered MS channel gating, since dystrophin-deficiency is known to be associated with an increased susceptibility to mechanically induced damage. In some recordings from mdx myotubes, MS channel open probability gradually increased to levels approaching unity following seal formation. In these recordings, channels remained open for the duration of the recording. In other recordings, MS channel open probability remained low after seal formation and applying weak suction evoked conventional stretch-activated gating. Applying strong suction or very positive voltages, however, caused some channels to enter a high open probability gating mode. The shift to a high open probability gating mode coincided with the appearance of stretch-inactivated gating. These findings suggested that mechanical stimulation altered the mechanical properties of the patch causing some MS channels to enter a novel gating mode. In support of this idea, stretch-activated and stretch-inactivated channels were not detected in the same membrane patch and channel inactivation occurred at lower pressures than activation (P(1/2,) = -13 and -26.5 mmHg, respectively). Other experiments showed that stretch-inactivated gating was not due to a simple loss of MS channel activity from a non-random process such as vesiculation or bleb formation: channel inactivation by suction was readily reversible, stable over tens of minutes, and followed the predictions of the binomial theorem for independent, randomly gating channels. In addition, the voltage-dependent gating of stretch-inactivated channels was similar to that of stretch-activated channels. The results show that MS channels in dystrophin-deficient muscle exist in two distinct gating modes and that mechanical stimuli cause an irreversible conversion between modes. We discuss possible mechanisms for the changes in MS channel gating in relation to the known cytoskeletal abnormalities of mdx muscle and its possible implications for the pathogenesis of Duchenne dystrophy.

Comparative evolution of muscular dystrophy in diaphragm, gastrocnemius and masseter muscles from old male mdx mice

X chromosome-linked muscular dystrophic mdx mouse lacks the sarcolemmal protein dystrophin and represents a genetic homologue of human Duchenne muscular dystrophy (DMD). The present study analysed some aspects of pathological processes such as fibrosis, frequency of centralized nuclei, presence of degenerative or regenerative fibres, expression of utrophin and associated protein complexes, and myosin heavy chain isoforms in three muscles [diaphragm (DIA), gastrocnemius (GTC) and masseter (MAS)] from old male mdx mice. All parameters investigated comparatively in these pathological muscles provided evidence that the MAS mdx muscle presents a slight deterioration pattern in comparison to that of DIA and GTC muscles. Utrophin and associated proteins are present in many cell clusters with continuous membrane labelling in MAS muscle. Respective proportions of myosin heavy chain isoforms, measured by electrophoresis/densitometry, showed only slight change in GTC muscle, significant evolution in DIA muscle but drastic isoform conversions in MAS muscle. These results highlighted the difference in deterioration susceptibility of various muscles to muscular dystrophy. The reason why this occurs in MAS muscles is still obscure and discussed in terms of the comparative developmental origins of these muscles.

Velocity of actomyosin sliding in vitro is reduced in dystrophic mouse diaphragm

It has recently been suggested that dystrophin deficiency in mdx diaphragm muscle is associated with quantitative changes in the myosin molecular motor. In vitro motility assays were used to study the kinetics of actomyosin interactions between purified actin filaments and myosin molecules. Monomeric myosin was obtained from the diaphragm and limb (semitendinosus) muscles of 9-mo-old male mdx (mdx) and age-matched control mice. The sliding velocity (vo, microm/s) of fluorescent-labeled actin filaments moving over a myosin-coated surface (40 microg/ml) was measured. In diaphragm, vo was significantly slower in mdx than in control mice (1.2 +/- 0.1 microm s(-1) versus 1.9 +/- 0.1 microm s(-1), p < 0.001). Conversely, there was no significant difference in vo between control and mdx semitendinous muscles (2.4 +/- 0.1 microm s(-1) versus 2.5 +/- 0.1 micro(-1)). As compared with control mice, mdx diaphragm exhibited a shift from IIX-MHC to IIA-MHC (p < 0.001) and a reduction in IIB-MHC (p < 0.01). Semitendinous muscle from control and mdx mice contained almost exclusively type IIB MHC. Our results are in good agreement with the proposal that myosin is altered in dystrophic mouse diaphragm.

A fluorescence-based method for measuring nitric oxide in extracts of skeletal muscle

We describe here a fluorescence assay for nitric oxide synthase activity in skeletal muscle based on a new indicator, 4,5-diaminofluorescein (DAF-2). The rapid and irreversible binding of DAF-2 to oxidized NO allows real-time measurement of NO production. The method is safer and more convenient than the usual citrulline radioassay and can be used with crude muscle extracts. Rabbit fast tibialis anterior (TA) muscle had a nitric oxide synthase (NOS) activity of 44.3 +/- 3.5 pmol/min/mg muscle. Addition of NOS blocker N(G)-allyl-L-arginine reduced this activity by 43%. Slow soleus muscle displayed NOS activity of 7.3 +/- 2.5 pmol/min/mg muscle, 16% that of the TA muscle. Continuous stimulation of TA muscle at 10 Hz for 3 weeks reduced NOS activity by 47% to an intermediate value consistent with the associated conversion of the muscle phenotype from fast to slow.

Development-dependent disappearance of caspase-3 in skeletal muscle is post-transcriptionally regulated

Caspase-3, a major player in apoptosis, engages apoptosis-activated cells into an irreversible pathway leading to cell death. In this article, we report that caspase-3 protein is absent from rat and mouse adult skeletal muscles, despite the abundant presence of its mRNA. During skeletal muscle development, caspase-3 protein is present in neonatal animals, but its expression gradually decreases, and disappears completely by 1 month of age, when there is still abundant caspase-3 mRNA. This discordance between caspase-3 message and protein expression is unique to skeletal muscle, as in all other analyzed tissues the protein presence correlates with the presence of the mRNA. The only circumstance in which caspase-3 protein appears in adults is in regenerating muscles; once regeneration is complete, however, it again becomes undetectable in repaired muscles. We conclude that caspase-3 protein in skeletal muscle is uniquely regulated at the post-transcriptional level, unseen in other tissues such as brain, heart, lung, kidney, thymus, spleen, liver, or testis. The post-transcriptional regulation of caspase-3 might serve as a fail-safe mechanism to avoid accidental cell death.

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.

Three mouse models of muscular dystrophy: the natural history of strength and fatigue in dystrophin-, dystrophin/utrophin-, and laminin alpha2-deficient mice

To optimize and evaluate treatments for muscular dystrophy, it is important to know the natural history of the disease in the absence of therapeutic intervention. Here we characterized disease progression of three mutant mouse strains of muscular dystrophy: mdx mice, which lack dystrophin; mdx:utrn-/- mice, which also lack utrophin; and dy/dy mice, which are deficient in laminin alpha2. Normal mice show a marked increase in forelimb strength over the first 10 weeks of life and little fatigue (<5%) over five consecutive strength trials. Mdx and mdx:utrn-/- mice demonstrate less strength then normal mice and approximately 40% fatigue at each age. Mdx mice become obese but mdx:utrn-/- mice do not. Dy/dy mice remain small and are much weaker than mdx and mdx:utrn-/- mice at all ages even when normalized to weight; however, they show only minimal fatigue (10%). This work demonstrates a distinct pattern of disease progression in each model and provides a foundation for assessing strategies for improving strength in each model.

Dystrophin in adult zebrafish muscle

Mutations in the human dystrophin gene are implicated in the fatal muscle wasting disease Duchenne Muscular Dystrophy (DMD). This gene expresses a sarcolemmal-associated protein that is evolutionarily conserved, underpinning its important role in the architecture of muscle. In terms of DMD modelling, the mouse has served as a suitable vertebrate species but the pathophysiology of the disease in the mouse does not entirely mimic human DMD. We have examined the zebrafish in order to expand the repertoire of vertebrate species for muscle disease modelling, and to dissect further the functional interactions of dystrophin. We report here the identification of an apparent zebrafish orthologue of the human dystrophin gene that expresses a 400-kDa protein that is localised to the muscle membrane surface. These data suggest that the zebrafish may prove to be a beneficial vertebrate model to examine the role and functional interactions of dystrophin in disease and development.

Transfer of full-length Dmd to the diaphragm muscle of Dmd(mdx/mdx) mice through systemic administration of plasmid DNA

Mutations in the gene encoding dystrophin, a large cytoskeletal protein in muscle, lead to Duchenne muscular dystrophy (DMD). Affected individuals often die of respiratory failure resulting primarily from diaphragm muscle degeneration. Here we report a new procedure to transfer the full-length dystrophin cDNA into the diaphragm muscle of Dmd(mdx/mdx) mice, which carry a mutation in the dystrophin gene (Dmd). Significant gene transfer was found after intravenous injection of naked plasmid DNA followed by a brief (eight second) occlusion of blood flow at the vena cava. This is the first demonstration of gene transfer into the diaphragm muscle through systemic administration of naked plasmid DNA. The approach has potential application for treatment of DMD.

Alteration of excitation-contraction coupling mechanism in extensor digitorum longus muscle fibres of dystrophic mdx mouse and potential efficacy of taurine

No clear data is available about functional alterations in the calcium-dependent excitation-contraction (e-c) coupling mechanism of dystrophin-deficient muscle of mdx mice. By means of the intracellular microelectrode "point" voltage clamp method, we measured the voltage threshold for contraction (mechanical threshold; MT) in intact extensor digitorum longus (EDL) muscle fibres of dystrophic mdx mouse of two different ages: 8 - 12 weeks, during the active regeneration of hind limb muscles, and 6 - 8 months, when regeneration is complete. The EDL muscle fibres of 8 - 12-week-old wildtype animals had a more negative rheobase voltage (potential of equilibrium for contraction- and relaxation-related calcium movements) with respect to control mice of 6 - 8 months. However, at both ages, the EDL muscle fibres of mdx mice contracted at more negative potentials with respect to age-matched controls and had markedly slower time constants to reach the rheobase. The in vitro application of 60 mM taurine, whose normally high intracellular muscle levels play a role in e-c coupling, was without effect on 6 - 8-month-old wildtype EDL muscle, while it significantly ameliorated the MT of mdx mouse. HPLC determination of taurine content at 6 - 8 months showed a significant 140% rise of plasma taurine levels and a clear trend toward a decrease in amino acid levels in hind limb muscles, brain and heart, suggesting a tissue difficulty in retaining appropriate levels of the amino acid. The data is consistent with a permanent alteration of e-c coupling in mdx EDL muscle fibres. The alteration could be related to the proposed increase in intracellular calcium, and can be ameliorated by taurine, suggesting a potential therapeutic role of the amino acid.

Mast cell proliferation and alterations in bFGF amount and localization are involved in the response of muscle to dystrophin deficiency in hypertrophic feline dystrophy

To test the hypothesis that basic fibroblast growth factor and mast cells play a key role in the phenotypic differences between human dystrophinopathies and hypertrophic feline muscular dystrophy, serial sections of dystrophin-deficient, carrier and normal cat muscle biopsy specimens were examined. They were stained immunohistochemically for dystrophin and different markers of differentiation such as desmin, vimentin and utrophin. Basic fibroblast growth factor was increased in the myofibers of dystrophic cats compared to normal controls and carriers. An association of basic fibroblast growth factor with fiber regeneration and necrosis was shown. The amount of mast cells was markedly increased in muscle tissue of dystrophic cats with a clear predominance of tryptase-positive cells present in large amounts in the endomysium. Mast cells, like basic fibroblast growth factor, were concentrated in areas of muscle fiber regeneration and necrosis. Our data concerning basic fibroblast growth factor and mast cells are consistent with a highly abnormal cellular environment in feline dystrophic muscle with very high levels of basic fibroblast growth factor which is likely modulated by mast cells.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Rescue of dystrophin expression in mdx mouse muscle by RNA/DNA oligonucleotides

Chimeric RNA/DNA oligonucleotides ("chimeraplasts") have been shown to induce single base alterations in genomic DNA both in vitro and in vivo. The mdx mouse strain has a point mutation in the dystrophin gene, the consequence of which is a muscular dystrophy resulting from deficiency of the dystrophin protein in skeletal muscle. To test the feasibility of chimeraplast-mediated gene therapy for muscular dystrophies, we used a chimeraplast (designated "MDX1") designed to correct the point mutation in the dystrophin gene in mdx mice. After direct injection of MDX1 into muscles of mdx mice, immunohistochemical analysis revealed dystrophin-positive fibers clustered around the injection site. Two weeks after single injections into tibialis anterior muscles, the maximum number of dystrophin-positive fibers (approximately 30) in any muscle represented 1-2% of the total number of fibers in that muscle. Ten weeks after single injections, the range of the number of dystrophin-positive fibers was similar to that seen after 2 wk, suggesting that the expression was stable, as would be predicted for a gene-conversion event. Staining with exon-specific antibodies showed that none of these were "revertant fibers." Furthermore, dystrophin from MDX1-injected muscles was full length by immunoblot analysis. No dystrophin was detectable by immunohistochemical or immunoblot analysis after control chimeraplast injections. Finally, reverse transcription-PCR analysis demonstrated the presence of transcripts with the wild-type dystrophin sequence only in mdx muscles injected with MDX1 chimeraplasts. These results provide the foundation for further studies of chimeraplast-mediated gene therapy as a therapeutic approach to muscular dystrophies and other genetic disorders of muscle.

Dystrophin mutations predict cellular susceptibility to oxidative stress

Mutations in the dystrophin gene that lead to the expression of truncated forms of the dystrophin protein cause muscular dystrophies of varying severities both in humans and in mice. We have shown previously that dystrophin-deficient muscle is more susceptible to oxidative injury than is normal muscle. In this report, we have used muscle cells derived from mdx mice, which express no dystrophin, and mdx-transgenic strains that express full-length dystrophin or truncated forms of dystrophin to explore further the relationship between dystrophin expression and susceptibility of muscle to oxidative injury. We show that, when differentiated into myotubes, the relative susceptibility of the cell populations to oxidative stress correlates with the severity of the dystrophy in the strain from which the cells were isolated. The most susceptible populations exhibited the greatest oxidative damage as assessed by protein oxidation. Thus, the relative efficacy of truncated dystrophin proteins to protect muscle from necrotic degeneration in vivo is predicted by their ability to protect muscle cells from free radical mediated injury. These findings support the hypothesis that the dystrophin protein complex may have important regulatory or signaling properties in terms of cell survival and antioxidant defense mechanisms.