Poloxamer 188 reduces the contraction-induced force decline in lumbrical muscles from mdx mice

Duchenne Muscular Dystrophy is a genetic disease caused by the lack of the protein dystrophin. Dystrophic muscles are highly susceptible to contraction-induced injury, and following contractile activity, have disrupted plasma membranes that allow leakage of calcium ions into muscle fibers. Because of the direct relationship between increased intracellular calcium concentration and muscle dysfunction, therapeutic outcomes may be achieved through the identification and restriction of calcium influx pathways. Our purpose was to determine the contribution of sarcolemmal lesions to the force deficits caused by contraction-induced injury in dystrophic skeletal muscles. Using isolated lumbrical muscles from dystrophic (mdx) mice, we demonstrate for the first time that poloxamer 188 (P188), a membrane-sealing poloxamer, is effective in reducing the force deficit in a whole mdx skeletal muscle. A reduction in force deficit was also observed in mdx muscles that were exposed to a calcium-free environment. These results, coupled with previous observations of calcium entry into mdx muscle fibers during a similar contraction protocol, support the interpretation that extracellular calcium enters through sarcolemmal lesions and contributes to the force deficit observed in mdx muscles. The results provide a basis for potential therapeutic strategies directed at membrane stabilization of dystrophin-deficient skeletal muscle fibers.     

Localized Treatment with a Novel FDA-Approved Proteasome Inhibitor Blocks the Degradation of Dystrophin and Dystrophin-Associated Proteins in mdx Mice

Duchenne Muscular Dystrophy (DMD) is an incurable inherited disease ofchildhood, characterized by progressive muscle degeneration and weakness. Our previousfindings supported the idea that dystrophin and associated proteins, absent or greatlyreduced in DMD, are degraded in dystrophin-deficient muscle by the proteasomaldependentpathway. Indeed, treatment with the proteasome inhibitor MG-132 of skeletalmuscles from mdx mice --a spontaneous mouse model of DMD-- as well as from DMDpatients, effectively rescued the expression and correct cellular localization of dystrophinand associated proteins. These promising results led us to further explore the use ofproteasome inhibitors as a therapy for DMD. Therefore, we directed our attentiontowards two new dipeptide boronic acid inhibitors blocking the proteasomal-dependentdegradation pathway: Velcade (bortezomib or PS-341) and MLN273 (PS-273). Theexciting aspect of this development is that these drugs have already progressed to preclinicaland clinical trials, in different fields than muscular dystrophy. Indeed, Velcadehas been already FDA-approved for treatment of multiple myeloma and its side effectshad been already explored and managed. Promisingly, MLN273 is currently in thepreclinical trial phase. Here, we test the effectiveness of Velcade and MLN273 by localinjection into the gastrocnemius muscle of mdx mice. We show the rescue of expressionand membrane localization of 􀀁-dystroglycan, 􀀂-dystroglycan, 􀀁-sarcoglycan, anddystrophin after Velcade and MLN273 localized treatment, versus untreated (PBS only)mdx mice. Intriguingly, we also show that localized treatment with Velcade and MLN273reduces the activation of Nuclear Factor-kappaB (NF-kB). Because NF-kB pathway hasbeen shown to be involved in inflammation responses in myopathies and DMD, ourcurrent results may have important clinical implications. Clearly, more investigations areneeded, but our results emphasize the effectiveness of the pharmacological approach as apotential treatment for Duchenne muscular dystrophy.     

A role for nitric oxide in muscle repair: nitric oxide-mediated activation of muscle satellite cells

Muscle satellite cells are quiescent precursors interposed between myofibers and a sheath of external lamina. Although their activation and recruitment to cycle enable muscle repair and adaptation, the activation signal is not known. Evidence is presented that nitric oxide (NO) mediates satellite cell activation, including morphological hypertrophy and decreased adhesion in the fiber-lamina complex. Activation in vivo occurred within 1 min after injury. Cell isolation and histology showed that pharmacological inhibition of nitric oxide synthase (NOS) activity prevented the immediate injury-induced myogenic cell release and delayed the hypertrophy of satellite cells in that muscle. Transient activation of satellite cells in contralateral muscles 10 min later suggested that a circulating factor may interact with NO-mediated signaling. Interestingly, satellite cell activation in muscles of mdx dystrophic mice and NOS-I knockout mice quantitatively resembled NOS-inhibited release of normal cells, in agreement with reports of displaced and reduced NOS expression in dystrophin-deficient mdx muscle and the complete loss of NOS-I expression in knockout mice. Brief NOS inhibition in normal and mdx mice during injury produced subtle alterations in subsequent repair, including apoptosis in myotube nuclei and myotube formation inside laminar sheaths. Longer NOS inhibition delayed and restricted the extent of repair and resulted in fiber branching. A model proposes the hypothesis that NO release mediates satellite cell activation, possibly via shear-induced rapid increases in NOS activity that produce "NO transients."     

IFN-γ promotes muscle damage in the mdx mouse model of Duchenne muscular dystrophy by suppressing M2 macrophage activation and inhibiting muscle cell proliferation

Duchenne muscular dystrophy is a degenerative disorder that leads to death by the third decade of life. Previous investigations have shown that macrophages that invade dystrophic muscle are a heterogeneous population consisting of M1 and M2 macrophages that promote injury and repair, respectively. In the present investigation, we tested whether IFN-γ worsens the severity of mdx dystrophy by activating macrophages to a cytolytic M1 phenotype and by suppressing the activation of proregenerative macrophages to an M2 phenotype. IFN-γ is a strong inducer of the M1 phenotype and is elevated in mdx dystrophy. Contrary to our expectations, null mutation of IFN-γ caused no reduction of cytotoxicity of macrophages isolated from mdx muscle and did not reduce muscle fiber damage in vivo or improve gross motor function of mdx mice at the early, acute peak of pathology. In contrast, ablation of IFN-γ reduced muscle damage in vivo during the regenerative stage of the disease and increased activation of the M2 phenotype and improved motor function of mdx mice at that later stage of the disease. IFN-γ also inhibited muscle cell proliferation and differentiation in vitro, and IFN-γ mutation increased MyoD expression in mdx muscle in vivo, showing that IFN-γ can have direct effects on muscle cells that could impair repair. Taken together, the findings show that suppression of IFN-γ signaling in muscular dystrophy reduces muscle damage and improves motor performance by promoting the M2 macrophage phenotype and by direct actions on muscle cells.     

Induction of galectin-1 by TGF-β1 accelerates fibrosis through enhancing nuclear retention of Smad2

Fibrosis is one of the most serious side effects in cancer patients undergoing radio-/ chemo-therapy, especially of the lung, pancreas or kidney. Based on our previous finding that galectin-1 (Gal-1) was significantly increased during radiation-induced lung fibrosis in areas of pulmonary fibrosis, we herein clarified the roles and action mechanisms of Gal-1 during fibrosis. Our results revealed that treatment with TGF-β1 induced the differentiation of fibroblast cell lines (NIH3T3 and IMR-90) to myofibroblasts, as evidenced by increased expression of the fibrotic markers smooth muscle actin-alpha (α-SMA), fibronectin, and collagen (Col-1). We also observed marked and time-dependent increases in the expression level and nuclear accumulation of Gal-1. The TGF-β1-induced increases in Gal-1, α-SMA and Col-1 were decreased by inhibitors of PI3-kinase and p38 MAPK, but not ERK. Gal-1 knockdown using shRNA decreased the phosphorylation and nuclear retention of Smad2, preventing the differentiation of fibroblasts. Gal-1 interacted with Smad2 and phosphorylated Smad2, which may accelerate fibrotic processes. In addition, up-regulation of Gal-1 expression was demonstrated in a bleomycin (BLM)-induced mouse model of lung fibrosis in vivo. Together, our results indicate that Gal-1 may promote the TGF-β1-induced differentiation of fibroblasts by sustaining nuclear localization of Smad2, and could be a potential target for the treatment of pulmonary fibrotic diseases.     

Oxidative stress and calcium homeostasis in dystrophic skin fibroblasts

Muscular dystrophy is a genetic disease that affects primarily skeletal muscle. The dystrophin absence has been related to the degeneration of muscle fibres. Indirect evidences suggest that oxidative stress may play a role in the pathogenesis of the disease, but the significance and precise extent of this contribution is poorly understood. In this paper we show that Becker Muscular Dystrophy (BMD) and Duchenne Muscular Dystrophy (DMD) skin fibroblasts are more susceptible to H2O2 treatment than are fibroblasts from unaffected persons. In particular, we found that, in growing DMD skin fibroblasts, the oxidative treatment resulted in significantly reduced growing capacity. We also investigated the concentrations of intracellular calcium during H2O2 treatment. The intracellular free calcium concentration increased by 22%, 35%, and 40% in unaffected, BMD, and DMD fibroblasts, respectively. However, the increase of the intracellular free calcium concentration is not related, as previously hypothesized, to a reduction of acylphosphatase concentrations, which seem to be unaffected by the H2O2 treatment, but rather to reduced enzyme activity.     

A nitric oxide synthase transgene ameliorates muscular dystrophy in mdx mice

Dystrophin-deficient muscles experience large reductions in expression of nitric oxide synthase (NOS), which suggests that NO deficiency may influence the dystrophic pathology. Because NO can function as an antiinflammatory and cytoprotective molecule, we propose that the loss of NOS from dystrophic muscle exacerbates muscle inflammation and fiber damage by inflammatory cells. Analysis of transgenic mdx mice that were null mutants for dystrophin, but expressed normal levels of NO in muscle, showed that the normalization of NO production caused large reductions in macrophage concentrations in the mdx muscle. Expression of the NOS transgene in mdx muscle also prevented the majority of muscle membrane injury that is detectable in vivo, and resulted in large decreases in serum creatine kinase concentrations. Furthermore, our data show that mdx muscle macrophages are cytolytic at concentrations that occur in dystrophic, NOS-deficient muscle, but are not cytolytic at concentrations that occur in dystrophic mice that express the NOS transgene in muscle. Finally, our data show that antibody depletions of macrophages from mdx mice cause significant reductions in muscle membrane injury. Together, these findings indicate that macrophages promote injury of dystrophin-deficient muscle, and the loss of normal levels of NO production by dystrophic muscle exacerbates inflammation and membrane injury in muscular dystrophy.     

rAAV6-microdystrophin rescues aberrant Golgi complex organization in mdx skeletal muscles

Muscular dystrophies are a diverse group of severe degenerative muscle diseases. Recent interest in the role of the Golgi complex (GC) in muscle disease has been piqued by findings that several dystrophies result from mutations in putative Golgi-resident glycosyltransferases. Given this new role of the Golgi in sarcolemmal stability, we hypothesized that abnormal Golgi distribution, regulation and/or function may constitute part of the pathology of other dystrophies, where the primary defect is independent of Golgi function. Thus, we investigated GC organization in the dystrophin-deficient muscles of mdx mice, a mouse model for Duchenne muscular dystrophy. We report aberrant organization of the synaptic and extrasynaptic GC in skeletal muscles of mdx mice. The GC is mislocalized and improperly concentrated at the surface and core of mdx myofibers. Golgi complex localization is disrupted after the onset of necrosis and normal redistribution is impaired during regeneration of mdx muscle fibers. Disruption of the microtubule cytoskeleton may account in part for aberrant GC localization in mdx myofibers. Golgi complex distribution is restored to wild type and microtubule cytoskeleton organization is significantly improved by recombinant adeno-associated virus 6-mediated expression of DeltaR4-R23/DeltaCT microdystrophin showing a novel mode of microdystrophin functionality. In summary, GC distribution abnormalities are a novel component of mdx skeletal muscle pathology rescued by microdystrophin expression.     

Programmed cell death in dystrophic (mdx) muscle is inhibited by IGF-II

The pathology of Duchenne Muscular Dystrophy (DMD) is characterised by unstable muscle fibres and by increased cell turnover due to the absence of functional dystrophin protein. We have used skeletal muscle, primary muscle stem cell cultures (Smith and Schofield, 1994; Smith et al., paper submitted) and clonal cell lines of the mouse DMD model (mdx) and its congenic control (C57BI) to demonstrate that programmed cell death (PCD) and apoptotic morphology is increased in dystrophic (mdx) muscle and in cultured muscle cells. We also show that the peptide growth factor (IGF-II), which is thought to play a role in mammalian myogenesis, reduces PCD in mammalian skeletal muscle myoblasts both in vivo and in vitro. This is the first time that apoptosis or PCD have been demonstrated in normal mammalian skeletal muscle. We discuss the potential of this system in determining the role of PCD in mammalian myogenesis and skeletal muscle maturation, its significance in dystrophic muscle, and suggest a novel therapeutic route whereby the pathology of DMD may be alleviated using the survival properties of IGF-II.     

Early metabolic changes measured by 1H MRS in healthy and dystrophic muscle after injury

Skeletal muscle injury is often assessed by clinical findings (history, pain, tenderness, strength loss), by imaging, or by invasive techniques. The purpose of this work was to determine if in vivo proton magnetic resonance spectroscopy ((1)H MRS) could reveal metabolic changes in murine skeletal muscle after contraction-induced injury. We compared findings in the tibialis anterior muscle from both healthy wild-type (WT) muscles (C57BL/10 mice) and dystrophic (mdx mice) muscles (an animal model for human Duchenne muscular dystrophy) before and after contraction-induced injury. A mild in vivo eccentric injury protocol was used due to the high susceptibility of mdx muscles to injury. As expected, mdx mice sustained a greater loss of force (81%) after injury compared with WT (42%). In the uninjured muscles, choline (Cho) levels were 47% lower in the mdx muscles compared with WT muscles. In mdx mice, taurine levels decreased 17%, and Cho levels increased 25% in injured muscles compared with uninjured mdx muscles. Intramyocellular lipids and total muscle lipid levels increased significantly after injury but only in WT. The increase in lipid was confirmed using a permeable lipophilic fluorescence dye. In summary, loss of torque after injury was associated with alterations in muscle metabolite levels that may contribute to the overall injury response in mdx mice. These results show that it is possible to obtain meaningful in vivo (1)H MRS regarding skeletal muscle injury.     

Taurine deficiency,synthesis and transport in the mdx mouse model for Duchenne Muscular Dystrophy

The amino acid taurine is essential for the function of skeletal muscle and administration is proposed as a treatment for Duchenne Muscular Dystrophy (DMD). Taurine homeostasis is dependent on multiple processes including absorption of taurine from food, endogenous synthesis from cysteine and reabsorption in the kidney. This study investigates the cause of reported taurine deficiency in the dystrophic mdx mouse model of DMD. Levels of metabolites (taurine, cysteine, cysteine sulfinate and hypotaurine) and proteins (taurine transporter [TauT], cysteine deoxygenase and cysteine sulfinate dehydrogenase) were quantified in juvenile control C57 and dystrophic mdx mice aged 18 days, 4 and 6 weeks. In C57 mice, taurine content was much higher in both liver and plasma at 18 days, and both cysteine and cysteine deoxygenase were increased. As taurine levels decreased in maturing C57 mice, there was increased transport (reabsorption) of taurine in the kidney and muscle. In mdx mice, taurine and cysteine levels were much lower in liver and plasma at 18 days, and in muscle cysteine was low at 18 days, whereas taurine was lower at 4: these changes were associated with perturbations in taurine transport in liver, kidney and muscle and altered metabolism in liver and kidney. These data suggest that the maintenance of adequate body taurine relies on sufficient dietary intake of taurine and cysteine availability and metabolism, as well as retention of taurine by the kidney. This research indicates dystrophin deficiency not only perturbs taurine metabolism in the muscle but also affects taurine metabolism in the liver and kidney, and supports targeting cysteine and taurine deficiency as a potential therapy for DMD.     

Lack of correlation between outcomes of membrane repair assay and correction of dystrophic changes in experimental therapeutic strategy in dysferlinopathy

Mutations in the dysferlin gene are the cause of Limb-girdle Muscular Dystrophy type 2B and Miyoshi Myopathy. The dysferlin protein has been implicated in sarcolemmal resealing, leading to the idea that the pathophysiology of dysferlin deficiencies is due to a deficit in membrane repair. Here, we show using two different approaches that fulfilling membrane repair as asseyed by laser wounding assay is not sufficient for alleviating the dysferlin deficient pathology. First, we generated a transgenic mouse overexpressing myoferlin to test the hypothesis that myoferlin, which is homologous to dysferlin, can compensate for the absence of dysferlin. The myoferlin overexpressors show no skeletal muscle abnormalities, and crossing them with a dysferlin-deficient model rescues the membrane fusion defect present in dysferlin-deficient mice in vitro. However, myoferlin overexpression does not correct muscle histology in vivo. Second, we report that AAV-mediated transfer of a minidysferlin, previously shown to correct the membrane repair deficit in vitro, also fails to improve muscle histology. Furthermore, neither myoferlin nor the minidysferlin prevented myofiber degeneration following eccentric exercise. Our data suggest that the pathogenicity of dysferlin deficiency is not solely related to impairment in sarcolemmal repair and highlight the care needed in selecting assays to assess potential therapies for dysferlinopathies.     

Myotubes from transgenic mdx mice expressing full-length dystrophin show normal calcium regulation.

A lack of dystrophin results in muscle degeneration in Duchenne muscular dystrophy. Dystrophin-deficient human and mouse muscle cells have higher resting levels of intracellular free calcium ([Ca2+]i) and show a related increase in single-channel open probabilities of calcium leak channels. Elevated [Ca2+]i results in high levels of calcium-dependent proteolysis, which in turn increases calcium leak channel activity. This process could initiate muscle degeneration by further increasing [Ca2+]i and proteolysis in a positive feedback loop. Here, we tested the direct effect of restoration of dystrophin on [Ca2+]i and channel activity in primary myotubes from mdx mice made transgenic for full-length dystrophin. Transgenic mdx mice have been previously shown to have normal dystrophin localization and no muscle degeneration. Fura-2 calcium measurements and single-channel patch recordings showed that resting [Ca2+]i levels and open probabilities of calcium leak channels of transgenic mdx myotubes were similar to normal levels and significantly lower than mdx littermate controls (mdx) that lack dystrophin. Thus, restoration of normal calcium regulation in transgenic mdx mice may underlie the resulting absence of degeneration.     

Improvement of calcium handling and changes in calcium-release properties after mini- or full-length dystrophin forced expression in cultured skeletal myotubes

Dystrophin is a cytoskeletal protein normally expressed underneath the sarcolemma of muscle fibers. The lack of dystrophin in Duchenne muscular Dystrophy (DMD) muscles results in fiber necrosis, which was proposed to be mediated by chronic calcium mishandling. The extensive comparison of dystrophic cells from human or mdx mice with normal muscles have suggested that the lack of dystrophin may alter the resting calcium permeability and steady-state levels of calcium, but this latter observation remains controversial. It is also not clear, whether calcium mishandling is resulting from the dystrophic process or if dystrophin can directly regulate calcium handling in muscle cells. This prompted us to determine if transfection of full-length dystrophin or Becker Muscular Dystrophy (BMD) minidystrophin, a candidate for viral-mediated gene therapy, could change calcium handling properties. We took advantage of specific properties of Sol8 cell line showing the absence of dystrophin expression together with a drastic calcium mishandling. Here, we show that full-length dystrophin allowed the recovery of a low resting intracellular-free calcium concentration together with lower calcium transients. We also show for the first time that stable expression of minidystrophin was able to restore normal calcium handling in Sol8 myotubes through a better control of steady-state levels, calcium transients, and subcellular calcium events. It suggests that dystrophin could play a regulatory role on calcium homeostasis apparatus and that functional links exist between calcium signaling and cytoskeleton.     

Menstrual blood-derived cells confer human dystrophin expression in the murine model of Duchenne muscular dystrophy via cell fusion and myogenic transdifferentiation

Duchenne muscular dystrophy (DMD), the most common lethal genetic disorder in children, is an X-linked recessive muscle disease characterized by the absence of dystrophin at the sarcolemma of muscle fibers. We examined a putative endometrial progenitor obtained from endometrial tissue samples to determine whether these cells repair muscular degeneration in a murine mdx model of DMD. Implanted cells conferred human dystrophin in degenerated muscle of immunodeficient mdx mice. We then examined menstrual blood–derived cells to determine whether primarily cultured nontransformed cells also repair dystrophied muscle. In vivo transfer of menstrual blood–derived cells into dystrophic muscles of immunodeficient mdx mice restored sarcolemmal expression of dystrophin. Labeling of implanted cells with enhanced green fluorescent protein and differential staining of human and murine nuclei suggest that human dystrophin expression is due to cell fusion between host myocytes and implanted cells. In vitro analysis revealed that endometrial progenitor cells and menstrual blood–derived cells can efficiently transdifferentiate into myoblasts/myocytes, fuse to C2C12 murine myoblasts by in vitro coculturing, and start to express dystrophin after fusion. These results demonstrate that the endometrial progenitor cells and menstrual blood–derived cells can transfer dystrophin into dystrophied myocytes through cell fusion and transdifferentiation in vitro and in vivo.     

成肌调节因子MyoD和myogenin在不同月龄DMD模型鼠mdx小鼠的表达

目的探讨成肌调节因子MyoD和myogenin在不同月龄DMD模型鼠mdx鼠的表达情况。方法取不同月龄DMD模型鼠mdx鼠以及相应的同龄正常C57鼠的腓肠肌,冰冻切片后用HE染色显示肌肉病理,SABC-DAB染色检测成肌调节因子MyoD和myogenin的表达。结果不同月龄mdx鼠肌肉坏死和再生程度不同,MyoD和myogenin在1月龄mdx鼠表达最强,在13月龄mdx鼠仍有表达,在正常同龄C57鼠不表达。结论MyoD与Myogenin在肌肉损伤后的再生修复过程中起作用,可作为鉴定肌肉前体细胞和反映肌肉再生的指标。     

Caenorhabditis elegans as a chemical screening tool for the study o f neuromuscular disorders. Manual and semi-automated methods

We previously reported the use of the cheap and fast-growing nematode Caenorhabditis elegans to search for molecules, which reduce muscle degeneration in a model for Duchenne Muscular Dystrophy (DMD). We showed that Prednisone, a steroid that is generally prescribed as a palliative treatment to DMD patients, also reduced muscle degeneration in the C. elegans DMD model. We further showed that this strategy could lead to the discovery of new and unsuspected small molecules, which have been further validated in a mammalian model of DMD, i.e. the mdx mouse model. These proof-of-principles demonstrate that C. elegans can serve as a screening tool to search for drugs against neuromuscular disorders. Here, we report and discuss two methodologies used to screen chemical libraries for drugs against muscle disorders in C. elegans. We first describe a manual method used to find drugs against DMD. We further present a semi-automated method, which is currently in use for the search of drugs against the Schwartz-Jampel Syndrome (SJS). Both assays are simple to implement and can be readily transposed and/or adapted to screens against other muscle/neuromuscular diseases, which can be modeled in the worm. Finally we discuss, with respect to our experience and knowledge, the different parameters that have to be taken into account before choosing one or the other method.     

Genetic disruption of calcineurin improves skeletal muscle pathology and cardiac disease in a mouse model of limb-girdle muscular dystrophy

Calcineurin (Cn) is a Ca(2+)/calmodulin-dependent serine/threonine phosphatase that regulates differentiation-specific gene expression in diverse tissues, including the control of fiber-type switching in skeletal muscle. Recent studies have implicated Cn signaling in diminishing skeletal muscle pathogenesis associated with muscle injury or disease-related muscle degeneration. For example, use of the Cn inhibitor cyclosporine A has been shown to delay muscle regeneration following toxin-induced injury and inhibit regeneration in the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy. In contrast, transgenic expression of an activated mutant of Cn in skeletal muscle was shown to increase utrophin expression and reduce overall disease pathology in mdx mice. Here we examine the effect of altered Cn activation in the context of the delta-sarcoglycan-null (scgd(-/-)) mouse model of limb-girdle muscular dystrophy. In contrast to results discussed in mdx mice, genetic deletion of a loxP-targeted calcineurin B1 (CnB1) gene using a skeletal muscle-specific cre allele in the scgd(-/-) background substantially reduced skeletal muscle degeneration and histopathology compared with the scgd(-/-) genotype alone. A similar regression in scgd-dependent disease manifestation was also observed in calcineurin Abeta (CnAbeta) gene-targeted mice in both skeletal muscle and heart. Conversely, increased Cn expression using a muscle-specific transgene increased cardiac fibrosis, decreased cardiac ventricular shortening, and increased muscle fiber loss in the quadriceps. Our results suggest that inhibition of Cn may benefit select types of muscular dystrophy.     

Functional characteristics of dystrophic skeletal muscle: insights from animal models.

Muscular dystrophies are a clinically and genetically heterogeneous group of disorders that show myofiber degeneration and regeneration. Identification of animal models of muscular dystrophy has been instrumental in research on the pathogenesis, pathophysiology, and treatment of these disorders. We review our understanding of the functional status of dystrophic skeletal muscle from selected animal models with a focus on 1) the mdx mouse model of Duchenne muscular dystrophy, 2) the Bio 14.6 delta-sarcoglycan-deficient hamster model of limb-girdle muscular dystrophy, and 3) transgenic null mutant murine lines of sarcoglycan (alpha, beta, delta, and gamma) deficiencies. Although biochemical data from these models suggest that the dystrophin-sarcoglycan-dystroglycan-laminin network is critical for structural integrity of the myofiber plasma membrane, emerging studies of muscle physiology suggest a more complex picture, with specific functional deficits varying considerably from muscle to muscle and model to model. It is likely that changes in muscle structure and function, downstream of the specific, primary biochemical deficiency, may alter muscle contractile properties.