Dystrophin-deficient mdx muscle fibers are preferentially vulnerable to necrosis induced by experimental lengthening contractions

Absence of aquaporin-4 in skeletal muscle alters proteins involved in bioenergetic pathways and calcium handling

Aquaporin-4 (AQP4) is a water channel expressed at the sarcolemma of fast-twitch skeletal muscle fibers, whose expression is altered in several forms of muscular dystrophies. However, little is known concerning the physiological role of AQP4 in skeletal muscle and its functional and structural interaction with skeletal muscle proteome. Using AQP4-null mice, we analyzed the effect of the absence of AQP4 on the morphology and protein composition of sarcolemma as well as on the whole skeletal muscle proteome. Immunofluorescence analysis showed that the absence of AQP4 did not perturb the expression and cellular localization of the dystrophin-glycoprotein complex proteins, aside from those belonging to the extracellular matrix, and no alteration was found in sarcolemma integrity by dye extravasation assay. With the use of a 2DE-approach (BN/SDS-PAGE), protein maps revealed that in quadriceps, out of 300 Coomassie-blue detected and matched spots, 19 proteins exhibited changed expression in AQP4(-/-) compared to WT mice. In particular, comparison of the protein profiles revealed 12 up- and 7 down-regulated protein spots in AQP4-/- muscle. Protein identification by MS revealed that the perturbed expression pattern belongs to proteins involved in energy metabolism (i.e. GAPDH, creatine kinase), as well as in Ca(2+) handling (i.e. parvalbumin, SERCA1). Western blot analysis, performed on some significantly changed proteins, validated the 2D results. Together these findings suggest AQP4 as a novel determinant in the regulation of skeletal muscle metabolism and better define the role of this water channel in skeletal muscle physiology.

Voluntary exercise induces structural remodeling in the hearts of dystrophin-deficient mice

In this exploratory study, we test the hypothesis that voluntary exercise affects the progression of dystrophic changes in the left ventricle of the heart. Wild-type (C57BL/10ScSn) and dystrophin-deficient (mdx) mice, aged 7 weeks, were divided into sedentary and exercise-treated groups and tested for differences in cardiac histomorphometry. Exercised mdx mice were found to exhibit significantly enlarged ventricles and thinner lateral ventricular walls than sedentary mdx mice (P < 0.05). Trichrome staining indicated the presence of fibrotic lesions in the left ventricular myocardium in 20% of the exercised mdx group. Fibrotic lesions were not found in control or sedentary mdx mice. No histomorphometric differences were found between treatment groups in wild-type mice. Our findings suggest voluntary exercise may accelerate the progression of ventricular dilation and fibrosis in young mdx mice. The effects of exercise on cardiac remodeling should be considered during the treatment of cardiac disease in dystrophin-deficient patients.

The chondrogenic response to exercise in the proximal femur of normal and mdx mice

Background

Submaximal exercise is used in the management of muscular dystrophy. The effects of mechanical stimulation on skeletal development are well understood, although its effects on cartilage growth have yet to be investigated in the dystrophic condition. The objective of this study was to investigate the chondrogenic response to voluntary exercise in dystrophin-deficient mice.

Methods

Control and dystrophin-deficient (mdx) mice were divided into sedentary and exercise-treated groups and tested for chondral histomorphometric differences at the proximal femur.

Results

Control mice ran 7 km/week further than mdx mice on average, but this difference was not statistically significant (P > 0.05). However, exercised control mice exhibited significantly enlarged femur head diameter, articular cartilage thickness, articular cartilage tissue area, and area of calcified cartilage relative to sedentary controls and exercised mdx mice (P < 0.05). No differences were found between other treatment groups.

Conclusions

Mdx mice exhibit a reduced chondrogenic response to increased mechanical stimulation relative to controls. However, no significant reduction in articular dimensions was found, indicating loss of chondral tissue may not be a clinical concern with dystrophinopathy.

Oxidative stress by monoamine oxidases is causally involved in myofiber damage in muscular dystrophy

Several studies documented the key role of oxidative stress and abnormal production of reactive oxygen species (ROS) in the pathophysiology of muscular dystrophies (MDs). The sources of ROS, however, are still controversial as well as their major molecular targets. This study investigated whether ROS produced in mitochondria by monoamine oxidase (MAO) contributes to MD pathogenesis. Pargyline, an MAO inhibitor, reduced ROS accumulation along with a beneficial effect on the dystrophic phenotype of Col6a1(-/-) mice, a model of Bethlem myopathy and Ullrich congenital MD, and mdx mice, a model of Duchenne MD. Based on our previous observations on oxidative damage of myofibrillar proteins in heart failure, we hypothesized that MAO-dependent ROS might impair contractile function in dystrophic muscles. Indeed, oxidation of myofibrillar proteins, as probed by formation of disulphide cross-bridges in tropomyosin, was detected in both Col6a1(-/-) and mdx muscles. Notably, pargyline significantly reduced myofiber apoptosis and ameliorated muscle strength in Col6a1(-/-) mice. This study demonstrates a novel and determinant role of MAO in MDs, adding evidence of the pivotal role of mitochondria and suggesting a therapeutic potential for MAO inhibition.

Disease-causing missense mutations in actin binding domain 1 of dystrophin induce thermodynamic instability and protein aggregation

Mutations in the dystrophin gene cause Duchenne muscular dystrophy (DMD) most commonly through loss of protein expression. In a small subpopulation of patients, missense mutations can cause DMD, Becker muscular dystrophy, or X-linked cardiomyopathy. Nearly one-half of disease-causing missense mutations are located in actin-binding domain 1 (ABD1) of dystrophin. To test the hypothesis that ABD1 missense mutations cause disease by impairing actin-binding activity, we engineered the K18N, L54R, D165V, A168D, L172H, and Y231N mutations into the full-length dystrophin cDNA and characterized the biochemical properties of each mutant protein. The K18N and L54R mutations are associated with the most severe diseases in humans and each caused a small but significant 4-fold decrease in actin-binding affinity, while the affinities of the other four mutant proteins were not significantly different from WT dystrophin. More interestingly, WT dystrophin was observed to unfold in a single-step, highly cooperative manner. In contrast, all six mutant proteins were significantly more prone to thermal denaturation and aggregation. Our results suggest that missense mutations in ABD1 may all cause loss of dystrophin function via protein instability and aggregation rather than through loss of ligand binding function. However, more severe disease progressions may be due to the combinatorial effects of some mutations on both protein aggregation and impaired actin-binding activity.

Use of Evans blue dye to compare limb muscles in exercised young and old mdx mice

Evans blue dye (EBD) is used to mark damaged and permeable muscle fibers in mouse models of muscular dystrophy and as an endpoint in therapeutic trials. We counted EBD-positive muscle fibers and extracted EBD from muscles sampled throughout the hindlimbs in young adult and old mdx mice to determine if the natural variability in morphology would allow measurement of a functional improvement in one limb compared to the contralateral limb. Following one bout of rotarod or treadmill exercise that greatly increased serum creatine kinase levels, the number of EBD(+) muscle fibers in 12-19-month-old mdx mice increased 3-fold, EBD in the muscles increased, and, importantly, contralateral pairs of muscles contained similar amounts of EBD. In contrast, the intra- and interlimb amounts of EBD in 2-7-month-old mdx mice were much too variable. A therapeutic effect can more readily be measured in old mdx mice. These results will be useful in the design of therapy protocols using the mdx mouse.

Talin 1 and 2 are required for myoblast fusion, sarcomere assembly and the maintenance of myotendinous junctions

Talin 1 and 2 connect integrins to the actin cytoskeleton and regulate the affinity of integrins for ligands. In skeletal muscle, talin 1 regulates the stability of myotendinous junctions (MTJs), but the function of talin 2 in skeletal muscle is not known. Here we show that MTJ integrity is affected in talin 2-deficient mice. Concomitant ablation of talin 1 and 2 leads to defects in myoblast fusion and sarcomere assembly, resembling defects in muscle lacking beta1 integrins. Talin 1/2-deficient myoblasts express functionally active beta1 integrins, suggesting that defects in muscle development are not primarily caused by defects in ligand binding, but rather by disruptions of the interaction of integrins with the cytoskeleton. Consistent with this finding, assembly of integrin adhesion complexes is perturbed in the remaining muscle fibers of talin 1/2-deficient mice. We conclude that talin 1 and 2 are crucial for skeletal muscle development, where they regulate myoblast fusion, sarcomere assembly and the maintenance of MTJs.

Effects of prednisolone on skeletal muscle contractility in mdx mice

Current treatment for Duchenne muscular dystrophy (DMD) is chronic administration of the glucocorticoid prednisolone. Prednisolone improves muscle strength in boys with DMD, but the mechanism is unknown. The purpose of this study was to determine how prednisolone improves muscle strength by examining muscle contractility in dystrophic mice over time and in conjunction with eccentric injury. Mdx mice began receiving prednisolone (n = 23) or placebo (n = 16) at 5 weeks of age. Eight weeks of prednisolone increased specific force of the extensor digitorum longus muscle 26%, but other parameters of contractility were not affected. Prednisolone also improved the histological appearance of muscle by decreasing the number of centrally nucleated fibers. Prednisolone treatment did not affect force loss during eccentric contractions or recovery of force following injury. These data are of clinical relevance, because the increase in muscle strength in boys with DMD taking prednisolone does not appear to occur via the same mechanism in dystrophic mice.

Applications of metabolomics and proteomics to the mdx mouse model of Duchenne muscular dystrophy: lessons from downstream of the transcriptome

Functional genomic studies are dominated by transcriptomic approaches, in part reflecting the vast amount of information that can be obtained, the ability to amplify mRNA and the availability of commercially standardized functional genomic DNA microarrays and related techniques. This can be contrasted with proteomics, metabolomics and metabolic flux analysis (fluxomics), which have all been much slower in development, despite these techniques each providing a unique viewpoint of what is happening in the overall biological system. Here, we give an overview of developments in these fields 'downstream' of the transcriptome by considering the characterization of one particular, but widely used, mouse model of human disease. The mdx mouse is a model of Duchenne muscular dystrophy (DMD) and has been widely used to understand the progressive skeletal muscle wasting that accompanies DMD, and more recently the associated cardiomyopathy, as well as to unravel the roles of the other isoforms of dystrophin, such as those found in the brain. Studies using proteomics, metabolomics and fluxomics have characterized perturbations in calcium homeostasis in dystrophic skeletal muscle, provided an understanding of the role of dystrophin in skeletal muscle regeneration, and defined the changes in substrate energy metabolism in the working heart. More importantly, they all point to perturbations in proteins, metabolites and metabolic fluxes reflecting mitochondrial energetic alterations, even in the early stage of the dystrophic pathology. Philosophically, these studies also illustrate an important lesson relevant to both functional genomics and the mouse phenotyping in that the knowledge generated has advanced our understanding of cell biology and physiological organization as much as it has advanced our understanding of the disease.

Sarcospan reduces dystrophic pathology: stabilization of the utrophin-glycoprotein complex

Mutations in the dystrophin gene cause Duchenne muscular dystrophy and result in the loss of dystrophin and the entire dystrophin–glycoprotein complex (DGC) from the sarcolemma. We show that sarcospan (SSPN), a unique tetraspanin-like component of the DGC, ameliorates muscular dystrophy in dystrophin-deficient mdx mice. SSPN stabilizes the sarcolemma by increasing levels of the utrophin–glycoprotein complex (UGC) at the extrasynaptic membrane to compensate for the loss of dystrophin. Utrophin is normally restricted to the neuromuscular junction, where it replaces dystrophin to form a functionally analogous complex. SSPN directly interacts with the UGC and functions to stabilize utrophin protein without increasing utrophin transcription. These findings reveal the importance of protein stability in the prevention of muscular dystrophy and may impact the future design of therapeutics for muscular dystrophies.

Progressive myopathy and defects in the maintenance of myotendinous junctions in mice that lack talin 1 in skeletal muscle

The development and function of skeletal muscle depend on molecules that connect the muscle fiber cytoskeleton to the extracellular matrix (ECM). beta1 integrins are ECM receptors in skeletal muscle, and mutations that affect the alpha7beta1 integrin cause myopathy in humans. In mice, beta1 integrins control myoblast fusion, the assembly of the muscle fiber cytoskeleton, and the maintenance of myotendinous junctions (MTJs). The effector molecules that mediate beta1 integrin functions in muscle are not known. Previous studies have shown that talin 1 controls the force-dependent assembly of integrin adhesion complexes and regulates the affinity of integrins for ligands. Here we show that talin 1 is essential in skeletal muscle for the maintenance of integrin attachment sites at MTJs. Mice with a skeletal muscle-specific ablation of the talin 1 gene suffer from a progressive myopathy. Surprisingly, myoblast fusion and the assembly of integrin-containing adhesion complexes at costameres and MTJs advance normally in the mutants. However, with progressive ageing, the muscle fiber cytoskeleton detaches from MTJs. Mechanical measurements on isolated muscles show defects in the ability of talin 1-deficient muscle to generate force. Collectively, our findings show that talin 1 is essential for providing mechanical stability to integrin-dependent adhesion complexes at MTJs, which is crucial for optimal force generation by skeletal muscle.

A common disease-associated missense mutation in alpha-sarcoglycan fails to cause muscular dystrophy in mice

Limb-girdle muscular dystrophy type 2D (LGMD2D) is caused by autosomal recessive mutations in the alpha-sarcoglycan gene. An R77C substitution is the most prevalent cause of the disease, leading to disruption of the sarcoglycan-sarcospan complex. To model this common mutation, we generated knock-in mice with an H77C substitution in alpha-sarcoglycan. The floxed neomycin (Neo)-cassette retained at the targeted H77C alpha-sarcoglycan locus caused a loss of alpha-sarcoglycan expression, resulting in muscular dystrophy in homozygotes, whereas Cre-mediated deletion of the floxed Neo-cassette led to recovered H77C alpha-sarcoglycan expression. Contrary to expectations, mice homozygous for the H77C-encoding allele expressed both this mutant alpha-sarcoglycan and the other components of the sarcoglycan-sarcospan complex in striated muscle, and did not develop muscular dystrophy. Accordingly, conditional rescued expression of the H77C protein in striated muscle of the alpha-sarcoglycan-deficient mice prevented the disease. Adding to the case that the behavior of mutant alpha-sarcoglycan is different between humans and mice, mutant human R77C alpha-sarcoglycan restored the expression of the sarcoglycan-sarcospan complex when introduced by adenoviral vector into the skeletal muscle of previously created alpha-sarcoglycan null mice. These findings indicate that the alpha-sarcoglycan with the most frequent missense mutation in LGMD2D is correctly processed, is transported to the sarcolemma, and is fully functional in mouse muscle. Our study presents an unexpected difference in the behavior of a missense-mutated protein in mice versus human patients, and emphasizes the need to understand species-specific protein quality control systems.

Diaphragm muscle strip preparation for evaluation of gene therapies in mdx mice

1. Duchenne muscular dystrophy (DMD), a severe muscle wasting disease of young boys with an incidence of one in every 3000, results from a mutation in the gene that encodes dystrophin. The absence of dystrophin expression in skeletal muscles and heart results in the degeneration of muscle fibres and, consequently, severe muscle weakness and wasting. The mdx mouse discovered in 1984, with some adjustments for differences, has proven to be an invaluable model for scientific investigations of dystrophy. 2. The development of the diaphagm strip preparation provided an ideal experimental model for investigations of skeletal muscle impairments in structure and function induced by interactions of disease- and age-related factors. Unlike the limb muscles of the mdx mouse, which show adaptive changes in structure and function, the diaphragm strip preparation reflects accurately the deterioration in muscle structure and function observed in boys with DMD. 3. The advent of sophisticated servo motors and force transducers interfaced with state-of-the-art software packages to drive complex experimental designs during the 1990s greatly enhanced the capability of the mdx mouse and the diaphragm strip preparation to evaluate more accurately the impact of the disease on the structure-function relationships throughout the life span of the mouse. 4. Finally, during the 1990s and through the early years of the 21st century, many promising, sophisticated genetic techniques have been designed to ameliorate the devastating impact of muscular dystrophy on the structure and function of skeletal muscles. During this period of rapid development of promising genetic therapies, the combination of the mdx mouse and the diaphragm strip preparation has provided an ideal model for the evaluation of the success, or failure, of these genetic techniques to improve dystrophic muscle structure, function or both. With the 2 year life span of the mdx mouse, the impact of age-related effects can be studied in this model.

Biology of the striated muscle dystrophin-glycoprotein complex

Since its first description in 1990, the dystrophin-glycoprotein complex has emerged as a critical nexus for human muscular dystrophies arising from defects in a variety of distinct genes. Studies in mammals widely support a primary role for the dystrophin-glycoprotein complex in mechanical stabilization of the plasma membrane in striated muscle and provide hints for secondary functions in organizing molecules involved in cellular signaling. Studies in model organisms confirm the importance of the dystrophin-glycoprotein complex for muscle cell viability and have provided new leads toward a full understanding of its secondary roles in muscle biology.

Primary role of functional ischemia, quantitative evidence for the two-hit mechanism, and phosphodiesterase-5 inhibitor therapy in mouse muscular dystrophy

BACKGROUND

Duchenne Muscular Dystrophy (DMD) is characterized by increased muscle damage and an abnormal blood flow after muscle contraction: the state of functional ischemia. Until now, however, the cause-effect relationship between the pathogenesis of DMD and functional ischemia was unclear. We examined (i) whether functional ischemia is necessary to cause contraction-induced myofiber damage and (ii) whether functional ischemia alone is sufficient to induce the damage.

METHODOLOGY/PRINCIPAL FINDINGS

In vivo microscopy was used to document assays developed to measure intramuscular red blood cell flux, to quantify the amount of vasodilatory molecules produced from myofibers, and to determine the extent of myofiber damage. Reversal of functional ischemia via pharmacological manipulation prevented contraction-induced myofiber damage in mdx mice, the murine equivalent of DMD. This result indicates that functional ischemia is required for, and thus an essential cause of, muscle damage in mdx mice. Next, to determine whether functional ischemia alone is enough to explain the disease, the extent of ischemia and the amount of myofiber damage were compared both in control and mdx mice. In control mice, functional ischemia alone was found insufficient to cause a similar degree of myofiber damage observed in mdx mice. Additional mechanisms are likely contributing to cause more severe myofiber damage in mdx mice, suggestive of the existence of a "two-hit" mechanism in the pathogenesis of this disease.

CONCLUSIONS/SIGNIFICANCE

Evidence was provided supporting the essential role of functional ischemia in contraction-induced myofiber damage in mdx mice. Furthermore, the first quantitative evidence for the "two-hit" mechanism in this disease was documented. Significantly, the vasoactive drug tadalafil, a phosphodiesterase 5 inhibitor, administered to mdx mice ameliorated muscle damage.

Potential of oligonucleotide-mediated exon-skipping therapy for Duchenne muscular dystrophy

Many of the mutations associated with Duchenne muscular dystrophy can potentially be rescued by exon-skipping therapy, targeting selected exons of prespliced mRNA for the dystrophin gene with antisense oligonucleotides, thereby restoring reading frames. The recent development of antisense oligonucleotides with higher stability and lower toxicity, such as morpholinos, has made it possible to restore dystrophin efficiently in dystrophic mice in vivo with no obvious side effects. There seems little doubt that such exon-skipping therapy is destined to proceed to the clinical application stage in patients with Duchenne muscular dystrophy. One of the remaining issues to be addressed is the skipping of multiple exons because such multi-exon skipping therapy could expand the potential patient target population to include 80% of those with duplication mutations and 90% of those with deletion mutations. At present, this multi-exon skipping strategy is being investigated in dystrophic dogs as well as dystrophic mice. There are several challenges that still need to be overcome, including the low uptake of antisense oligonucleotides into the heart and the need to design efficient, nontoxic, cost-effective oligonucleotides. This review summarizes recent progress in exon-skipping therapy and discusses future perspectives with regard to human clinical trials.

Effect of head-down hanging on the course of degenerative process in the hind paw muscles of 12-month-old MDX mice

Effect of functional unloading on the course of degenerative process in muscles was studied in dystrophin-deficient mdx mice. Head down-hanging of animals led to a significant decrease in the cross-section area of muscle fibers, increase in the percentage of fibers with centrally located nuclei and of Evans blue-stained fibers. Gravitation unloading of 12-month-old animals with pronounced manifestations of muscular dystrophy did not inhibit this pathological process.

Disrupted mechanical stability of the dystrophin-glycoprotein complex causes severe muscular dystrophy in sarcospan transgenic mice

The dystrophin-glycoprotein complex spans the muscle plasma membrane and provides a mechanical linkage between laminin in the extracellular matrix and actin in the intracellular cytoskeleton. Within the dystrophin-glycoprotein complex, the sarcoglycans and sarcospan constitute a subcomplex of transmembrane proteins that stabilize α-dystroglycan, a receptor for laminin and other components of the extracellular matrix. In order to elucidate the function of sarcospan, we generated transgenic mice that overexpress sarcospan in skeletal muscle. Sarcospan transgenic mice with moderate (tenfold) levels of sarcospan overexpression exhibit a severe phenotype that is similar to mouse models of laminin-deficient congenital muscular dystrophy (MD). Sarcospan transgenic mice display severe kyphosis and die prematurely between 6 and 10 weeks of age. Histological analysis reveals that sarcospan expression causes muscle pathology marked by increased muscle fiber degeneration and/or regeneration. Sarcospan transgenic muscle does not display sarcolemma damage, which is distinct from dystrophin- and sarcoglycan-deficient muscular dystrophies. We show that sarcospan clusters the sarcoglycans into insoluble protein aggregates and causes destabilization of α-dystroglycan. Evidence is provided to demonstrate abnormal extracellular matrix assembly, which represents a probable pathological mechanism for the severe and lethal dystrophic phenotype. Taken together, these data suggest that sarcospan plays an important mechanical role in stabilizing the dystrophin-glycoprotein complex.

Dystrophin-deficient mdx mice display a reduced life span and are susceptible to spontaneous rhabdomyosarcoma

Duchenne muscular dystrophy (DMD) is the most common, lethal genetic disorder of children. A number of animal models of muscular dystrophy exist, but the most effective model for characterizing the structural and functional properties of dystrophin and therapeutic interventions has been the mdx mouse. Despite the approximately 20 years of investigations of the mdx mouse, the impact of the disease on the life span of mdx mice and the cause of death remain unresolved. Consequently, a life span study of the mdx mouse was designed that included cohorts of male and female mdx and wild-type C57BL/10 mice housed under specific pathogen-free conditions with deaths restricted to natural causes and with examination of the carcasses for pathology. Compared with wild-type mice, both mdx male and female mice had reduced life spans and displayed a progressively dystrophic muscle histopathology. Surprisingly, old mdx mice were prone to develop muscle tumors that resembled the human form of alveolar rhabdomyosarcoma, a cancer associated with poor prognosis. Rhabdomyosarcomas have not been observed previously in nontransgenic mice. The results substantiate the mdx mouse as an important model system for studies of the pathogenesis of and potential remedies for DMD.

Calcium misregulation and the pathogenesis of muscular dystrophy

Although the exact nature of the relationship between calcium and the pathogenesis of Duchenne muscular dystrophy (DMD) is not fully understood, this is an important issue which has been addressed in several recent reviews (Alderton and Steinhardt, 2000a, Gailly, 2002, Allen et al., 2005). A key question when trying to understand the cellular basis of DMD is how the absence or low level of expression of dystrophin, a cytoskeletal protein, results in the slow but progressive necrosis of muscle fibres. Although loss of cytoskeletal and sarcolemmal integrity which results from the absence of dystrophin clearly plays a key role in the pathogenesis associated with DMD, a number of lines of evidence also establish a role for misregulation of calcium ions in the DMD pathology, particularly in the cytoplasmic space just under the sarcolemma. A number of calcium-permeable channels have been identified which can exhibit greater activity in dystrophic muscle cells, and exIsting evidence suggests that these may represent different variants of the same channel type (perhaps the transient receptor potential channel, TRPC). In addition, a prominent role for calcium-activated proteases in the DMD pathology has been established, as well as modulation of other intracellular regulatory proteins and signaling pathways. Whether dystrophin and its associated proteins have a direct role in the regulation of calcium ions, calcium channels or intracellular calcium stores, or indirectly alters calcium regulation through enhancement of membrane tearing, remains unclear. Here we focus on areas of consensus or divergence amongst the existing literature, and propose areas where future research would be especially valuable.

Structural and functional analysis of the sarcoglycan-sarcospan subcomplex

Sarcospan is a component of the dystrophin-glycoprotein complex that forms a tight subcomplex with the sarcoglycans. The sarcoglycan-sarcospan subcomplex functions to stabilize alpha-dystroglycan at the plasma membrane and perturbations of this subcomplex are associated with autosomal recessive limb-girdle muscular dystrophy. In order to characterize protein interactions within this subcomplex, we first demonstrate that sarcospan forms homo-oligomers within the membrane. Experiments with a panel of site-directed mutants reveal that proper structure of the large extracellular loop is an important determinant of oligo formation. Furthermore, the intracellular N- and C-termini contribute to stability of sarcospan-mediated webs. Point mutation of each cysteine residue reveals that Cys 162 and Cys 164 within the large extracellular loop form disulfide bridges, which are critical for proper sarcospan structure. The extracellular domain of sarcospan also forms the main binding site for the sarcoglycans. We propose a model whereby sarcospan forms homo-oligomers that cluster the components of the dystrophin-glycoprotein complex within the membrane.

Poloxamer 188 failed to prevent exercise-induced membrane breakdown in mdx skeletal muscle fibers

We sought to determine the effectiveness of poloxamer 188 (P188) in protecting dystrophin-deficient, mdx skeletal muscle fiber membrane against exercise-induced breaches. mdx mice were treated with either P188 or placebo via intraperitoneal injections and run on a treadmill for 60-90 min. Membrane breakdown was quantified in cross-sections of rectus femoris muscle pretreated with Evans blue dye (in vivo). The mean % dye-penetrated muscle in the P188 and placebo groups was not significantly different in each of three trials. These results contrast with a recent report of P188 being highly effective in protecting the stretch- and dobutamine-stressed mdx heart muscle. The most likely explanations for the disparity are: (1) the exercise stress we used was beyond the protective range of P188, (2) P188 delivery and serum concentration were sub-optimal, or (3) the mdx skeletal myopathy and cardiomyopathy have fundamentally different responses to treatment.

Passive mechanical properties of maturing extensor digitorum longus are not affected by lack of dystrophin

Mechanical weakness of skeletal muscle is thought to contribute to onset and early progression of Duchenne muscular dystrophy, but this has not been systematically assessed. The purpose of this study was to determine in mice: (1) whether the passive mechanical properties of maturing dystrophic (mdx) muscles were different from control; and (2) if different, the time during maturation when these properties change. Prior to and following the overt onset of the dystrophic process (14-35 days), control and dystrophic extensor digitorum longus (EDL) muscles were subjected to two passive stretch protocols in vitro (5% strain at instantaneous and 1.5 L(0)/s strain rates). Force profiles were fit to a viscoelastic muscle model to determine stiffness and damping. The mdx and control EDL muscles exhibited similar passive mechanical properties at each age, suggesting a functional threshold for dystrophic muscle below which damage may be minimized. Determining this threshold may have important clinical implications for treatments of muscular dystrophy involving physical activity.

MECHANOSENSITIVE ION CHANNELS IN SKELETAL MUSCLE: A LINK IN THE MEMBRANE PATHOLOGY OF MUSCULAR DYSTROPHY

1. Mechanosensitive (MS) channels are expressed abundantly in skeletal muscle at all stages of development. In recordings from membrane patches, MS channels are constitutively active at the resting potential. The channels are selective for cations and have a large single-channel conductance (approximately 25 pS in physiological saline) and a high Ca2+ permeability (relative permeability of Ca2+ to K+ (PCa/PK) = 7). 2. Mechanosensitive channel activity recorded from the surface of myotubes from dystrophic mdx mice was substantially greater than the activity recorded from wild-type myotubes. Increased channel activity in the mutant results from the induction in a subpopulation of channels of a novel MS gating mode characterized by markedly prolonged channel openings and inactivation in response to membrane stretch. 3. Membrane stretch or a strong depolarization causes an irreversible switch to the stretch-inactivated gating mode in mdx myotubes. A stretch-induced shift in MS channel gating mode may contribute to stretch-induced elevations in [Ca2+]i during the early stages of disease pathogenesis. 4. Abnormalities of MS channel behaviour are also detected in recordings from patches on flexor digitorum brevis fibres acutely isolated from mdx mice. Mechanosensitive channel opening probability is higher in mdx fibres at all developmental stages. In addition, channel numbers are persistently elevated during postnatal development, failing to undergo a normal process of downregulation during the first 3 postnatal weeks. 5. Two distinct mechanisms may contribute to elevations of [Ca2+]i in dystrophin-deficient skeletal muscle: (i) a membrane stress-dependent switch of MS channels into to a prolonged opening mode; and (ii) a loss of developmental downregulation leading to persistent MS channel expression during postnatal muscle development.

Cromolyn administration (to block mast cell degranulation) reduces necrosis of dystrophic muscle in mdx mice

Duchenne muscular dystrophy is a lethal muscle wasting disorder, resulting from mutations in the gene encoding for the skeletal muscle protein dystrophin. The absence of functional dystrophin leaves the muscle membrane vulnerable to damage during contraction. Damage initially occurs as 'tears' in the membrane, this damage can be exacerbated by the inflammatory response leading to myofibre necrosis rather than repair. Mast cells resident within skeletal muscle represent an immediate source of pro-inflammatory cytokines. We hypothesise that blockade of mast cell degranulation would reduce the extent of myofibre necrosis in the mdx mouse. Daily cromolyn injections were performed on young and exercised adult mdx mice and histological analysis confirmed that mast cell degranulation contributes to myofibre necrosis. This research identified high biological variation between individual mdx mice in the severity of the dystrophic pathology, and supported a relationship between extent of muscle damage in adult mdx mice and their individual enthusiasm for voluntary wheel running.

Dystrophin, its interactions with other proteins, and implications for muscular dystrophy

Duchenne muscular dystrophy is the most prevalent and severe form of human muscular dystrophy. Investigations into the molecular basis for Duchenne muscular dystrophy were greatly facilitated by seminal studies in the 1980s that identified the defective gene and its major protein product, dystrophin. Biochemical studies revealed its tight association with a multi-subunit complex, the so-named dystrophin-glycoprotein complex. Since its description, the dystrophin-glycoprotein complex has emerged as an important structural unit of muscle and also as a critical nexus for understanding a diverse array of muscular dystrophies arising from defects in several distinct genes. The dystrophin homologue utrophin can compensate at the cell/tissue level for dystrophin deficiency, but functions through distinct molecular mechanisms of protein-protein interaction.

Chronic treatment with agents that stabilize cytosolic IkappaB-alpha enhances survival and improves resting membrane potential in MDX muscle fibers subjected to chronic passive stretch

The potential pathogenic role of increased NFkappaB signaling in passively stretched dystrophic skeletal muscle was examined by treating adult mdx mice with an agent that stabilized cytosolic IkappaB-alpha (pyrollidine dithiocarbamate, PDTC)and examining the effects of this treatment on the chronically stretched mdx triangularis sterni (TS) muscle. Daily PDTC treatment significantly increased the number of surviving striated TS fibers regardless of age. TS fibers from untreated mdx mice had significantly lower resting potentials (RPs) than nondystrophic mice. Treatment with GdCl3 to block resting Ca2+ influx had no effect on RP in either nondystrophic or mdx preparations. Daily treatment with PDTC significantly improved the RP regardless of age. These results are consistent with the hypothesis that passive stretch activates an NFkappaB-mediated pathogenic mechanism in dystrophic muscle and suggest that agents which stabilize cytosolic IkappaB-alpha levels may be useful for treating Duchenne and related muscular dystrophies.

Role of contraction-induced injury in the mechanisms of muscle damage in muscular dystrophy

1. Duchenne muscular dystrophy (DMD) is a severe disease of skeletal muscle, characterized by an X-linked recessive inheritance and a lack of dystrophin in muscle fibres. It is associated with progressive and severe wasting and weakness of nearly all muscles and premature death by cardiorespiratory failure. 2. Studies investigating the susceptibility of dystrophic skeletal muscles to contraction-mediated damage, especially after lengthening actions where activated muscles are stretched forcibly, have concluded that dystrophin may confer protection to muscle fibres by providing a mechanical link between the contractile apparatus and the plasma membrane. In the absence of dystrophin, there is disruption to normal force transmission and greater stress placed upon myofibrillar and membrane proteins, leading to muscle damage. 3. Contraction protocols (involving activation and stretch of isolated muscles or muscle fibres) have been developed to assess the relative susceptibility of dystrophic (and otherwise healthy) muscles to contraction-induced injury. These protocols have been used successfully to determine the relative efficacy of different (gene, cell or pharmacological) interventions designed to ameliorate or cure the dystrophic pathology. More research is needed to develop specific 'contraction assays' that will assist in the evaluation of the clinical significance of different therapeutic strategies for muscular dystrophy.

Muscle-bone interactions in dystrophin-deficient and myostatin-deficient mice

We have investigated muscle-bone interactions using two mouse mutants that are known to differ from normal mice in skeletal muscle growth and development: mice lacking myostatin (GDF8) and mice lacking dystrophin (mdx). Myostatin-deficient mice show increased muscle size and strength compared to normal mice, whereas the mdx mouse is a well-established animal model for Duchenne muscular dystrophy. The mdx mice have significantly larger hindlimb muscles than controls, and histological sections of the quadriceps muscles show dystrophic changes with extensive fibrosis. Femoral bone mineral density (BMD) and fracture strength (Fu) are significantly greater in mdx mice than controls, and these variables are more strongly correlated with quadriceps muscle mass than with body mass. In contrast, mdx mice do not shower high bone mineral density in the spine relative to controls, whereas myostatin-deficient mice have significantly increased BMD in the lumbar spine compared to normal mice. Both mdx mice and myostatin-deficient mice have expanded femoral trochanters for attachment of large hindlimb muscles, and both mutant strains show increased cross-sectional area moments of inertia mediolaterally (Iyy) but not anteroposteriorly (Ixx) compared to normal mice. These data suggest that lean (muscle) mass is a significant determinant of bone mineral density and strength in the limb skeleton, even when accompanied by a dystrophic phenotype. Likewise, increased muscle mass produces a marked increase in the external dimensions of muscle attachment sites, even when increased muscle size is accompanied by extensive fibrosis and muscle weakness.

Chronic exercise accelerates the degeneration–regeneration cycle and downregulates insulin-like growth factor-1 in muscle ofmdx mice

The aim of this study was to examine the effects of chronic running exercise on degenerative-regenerative processes in the hindlimb muscles of dystrophin-deficient mdx mice. The number of large-sized degenerative-regenerative groups (DRGs) was markedly decreased, whereas that of small-sized DRGs was unchanged by exercise. Expression of insulin-like growth factor-1 (Igf1), as well as a myogenic factor MyoD (Myod1), was downregulated in mdx muscles by exercise. The downregulation of Igf1 may well correlate with the decrease in the population of early regenerating fibers, which existed predominantly in DRGs, because IGF-1 was mainly localized in these fibers. Our data indicate that chronic exercise may accelerate the active cycle of degeneration-regeneration in mdx skeletal muscles. This means that mdx skeletal muscles can temporarily cope with work-induced injury by enhancing muscle regeneration and repair, but we speculate that an early decline of IGF-1 will accelerate age-dependent muscle wasting and weakness in the later stage of life in mdx mice.

EDL and soleus muscles of the C57BL6J/dy2jlaminin-α2-deficient dystrophic mouse are not vulnerable to eccentric contractions

Many muscular dystrophies arise as a consequence of mutations in a series of interconnected proteins associated with the sarcolemma. This group of proteins is collectively referred to as the 'dystrophin-associated complex'. We used the C57BL6J/dy(2j), dystrophia muscularis, dystrophic mouse, in which the laminin-alpha(2) component of the dystrophin-associated complex is mutated, to test the hypothesis that the disruption of this complex will destabilize the lipid bilayer, rendering it more susceptible to damage during eccentric contractions. We demonstrated that neither slow- nor fast-twitch dystrophic muscles were more susceptible to eccentric contractions when compared with controls. Only fast-twitch extensor digitorum longus (EDL) muscles (from both dystrophic and control mice) showed an irreversible loss of force with our eccentric contraction protocol, suggesting that it is the fast 11b fibres (not present in slow-twitch soleus) which are most susceptible to eccentric damage. We used the general anaesthetic halothane to increase the fluidity of the lipid bilayer to see if this would uncover any greater susceptibility of the dystrophic muscle to eccentric damage. This also did not reveal any greater fragility of fast- and slow-twitch dystrophic muscles. We did, however, demonstrate that halothane made both control and dystrophic fast- and slow-twitch muscles more susceptible to eccentric contraction damage. The C57BL6J/dy(2j) dystrophic laminopathy produced the pathophysiological and pathohistological characteristics associated with muscular dystrophy: the fast- and slow-twitch dystophic muscles produced only 55 and 53%, respectively, of the force of control muscles and 34 and 40%, respectively, of the dystrophic muscle fibres were branched. The presence of the branched fibres in the dystrophic muscles did not make them more susceptible to eccentric damage but may have contributed to the reduction in maximal force in the dystrophic muscles. We conclude that our data do not support the structural hypothesis that the dystrophin-associated complex acts as a scaffolding to support the lipid bilayer, but are consistent with channel-based hypotheses put forward to explain the dystrophic process.

Purinoceptor expression in regenerating skeletal muscle in the mdx mouse model of muscular dystrophy and in satellite cell cultures

ATP is an important extracellular signaling molecule mediating its effects by activation of P2X and P2Y receptors. P2 receptors are expressed during muscle development, and recent findings demonstrate that ATP can regulate myoblast proliferation and differentiation in vitro. However, the role of purinergic signaling during regeneration of injured skeletal muscle has not been investigated. To examine this process in a clinically relevant system, we used the mouse model of muscular dystrophy (mdx), in which muscle degeneration is rapidly followed by regeneration. The latter process, in vivo muscle regeneration, was the focus of this study, and to study the cellular mechanisms involved in it, a parallel study on normal rat skeletal myoblast cultures was conducted. Using immunohistochemistry, RT-PCR, and electrophysiology, we investigated the expression of the P2X1-7 receptor subtypes and the P2Y1,2,4,6 receptors. Experiments in vitro and in vivo demonstrated the sequential expression of the P2X5, P2Y1, and P2X2 receptors during the process of muscle regeneration. The P2X5 and P2Y1 receptors were expressed first on activated satellite cells, and the P2Y1 receptor was also expressed on infiltrating immune cells. Subsequent P2X2 receptor expression on newly formed myotubes showed significant colocalization with AChRs, suggesting a role in regulation of muscle innervation. Thus, this study provides the first evidence for a role for purinergic signaling in muscle regeneration and raises the possibility of new therapeutic strategies in the treatment of muscle disease.

Cellular and molecular adaptations to injurious mechanical stress

Transient disruptions of plasma membrane integrity--'wounds'--are frequently suffered by cells of gut, skin, muscle and the aorta, organs that are normally subjected to mechanical stress in vivo. As a protection against such potentially fatal mechanically induced injuries, cells may employ specialized submembranous proteins that mechanically reinforce the plasma membrane and thus prevent wounding or, should wounding occur, they may assemble a cytoskeletal structure to aid wound healing. Membrane wounds may provide a route out of the cytoplasm for basic fibroblast growth factor, explaining how a growth factor that lacks a conventional signal peptide sequence can act extracellularly.

Sustained Improvement of Muscle Function One Year After Full-Length Dystrophin Gene Transfer intomdxMice by a Gutted Helper-Dependent Adenoviral Vector

Dystrophin gene transfer using helper-dependent adenoviral vectors (HDAd) deleted of all viral genes is a promising option to treat muscles in Duchenne muscular dystrophy (DMD). Previously, we reported high-level dystrophin expression and functional correction of dystrophin-deficient (mdx) mouse muscle 60 days after gene transfer with an HDAd encoding two full-length murine dystrophin cDNAs (referred to as HDCBDysM). In the present study, we tested the long-term efficacy of HDCBDysM by examining muscle contractility parameters and the stability of dystrophin expression 1 year after injection into neonatal mdx muscles. At this point, HDCBDysM-treated muscles averaged 52% dystrophin-positive fibers. Treated muscles also displayed significantly greater isometric force production as well as greater resistance to the force deficits and damage caused by eccentric contractions. The level of protection against eccentric contraction-induced force deficits correlated with the percentage of dystrophin-positive fibers. Furthermore, HDCBDysM treatment restored the dystrophin-glycoprotein complex (DGC) to the sarcolemma and improved other aspects of mdx muscle histopathology examined (central nucleation, muscle hypertrophy, and mononuclear [phagocytic] cell infiltration). These improvements occurred despite the induction of a humoral response against murine dystrophin. Our results indicate that major therapeutic benefits of HDCBDysM are maintained for a long period of the animals' lifespan and suggest that HDCBDys holds promise for treating DMD by gene therapy.

Extrajunctional resting Ca2+ influx is not increased in a severely dystrophic expiratory muscle (triangularis sterni) of the mdx mouse

Freshly isolated adult mdx and nondystrophic (C57B110SnJ) muscle fibers were used to examine the potential role of resting Ca2+ influx in the pathogenesis of Duchenne and related dystrophies. Microfluorimetric determinations of resting divalent cation influx were obtained from undissociated intact muscle fibers in the triangularis sterni (TS), a thin expiratory muscle. Morphological evidence indicated severe dystrophic alterations in the mdx TS at 5 months, and a pronounced loss of fibers with connective tissue infiltration in older animals. To examine resting Ca2+ influx, fibers were loaded with FURA PE3 and the rate of quenching of intracellular signal following the extracellular addition of Mn2+ was determined from extrajunctional regions. There was no significant difference in quench rate between nondystrophic and mdx TS fibers. These results indicate that severe dystrophic pathology in the absence of dystrophin is not due to generalized increases in resting Ca2+ influx.

Reduced mobility of fibroblast growth factor (FGF)-deficient myoblasts might contribute to dystrophic changes in the musculature of FGF2/FGF6/mdx triple-mutant mice

Development and regeneration of muscle tissue is a highly organized, multistep process that requires cell proliferation, migration, differentiation, and maturation. Previous data implicate fibroblast growth factors (FGFs) as critical regulators of these processes, although their precise role in vivo is still not clear. We have explored the consequences of the loss of multiple FGFs (FGF2 and FGF6 in particular) for muscle regeneration in mdx mice, which serve as a model for chronic muscle damage. We show that the combined loss of FGF2 and FGF6 leads to severe dystrophic changes in the musculature. We found that FGF6 mutant myoblasts had decreased migration ability in vivo, whereas wild-type myoblasts migrated normally in a FGF6 mutant environment after transplantation of genetically labeled myoblasts from FGF6 mutants in wild-type mice and vice versa. In addition, retrovirus-mediated expression of dominant-negative versions of Ras and Ral led to a reduced migration of transplanted myoblasts in vivo. We propose that FGFs are critical components of the muscle regeneration machinery that enhance skeletal muscle regeneration, probably by stimulation of muscle stem cell migration.

The cranial sartorius muscle undergoes true hypertrophy in dogs with golden retriever muscular dystrophy

The degree of atrophy or hypertrophy of selected pelvic limb muscles was determined in the canine homologue of Duchenne muscular dystrophy. While most muscles were atrophied, the caudal and cranial sartorius were hypertrophied. Cranial sartorius weights were corrected for body weight and endomysial space to determine true muscle weights (g/kg; mean+/-SD) in three golden retriever muscular dystrophy age groups, 4-10 (Group 1; n=15), 13-26 (Group 2; n=4), and 33-66 (Group 3; n=4) months and grouped normal dogs (6-20 months; n=12). Group 1 golden retriever muscular dystrophy weights (2.2063+/-0.6884) were greater than those of normal dogs (1.2699+/-0.1966), indicating that young golden retriever muscular dystrophy dogs have true cranial sartorius muscle hypertrophy. Values of Group 2 (1.3758+/-0.5078) and Group 3 (0.5720+/-0.2423) golden retriever muscular dystrophy dogs were less than those of Group 1, suggesting that the cranial sartorius muscle atrophies over time. Given that cranial sartorius muscle weight correlated with tarsal joint angle in affected dogs (r=-0.817), the hypertrophied muscle may play a role analogous to iliotibial band tightness in Duchenne muscular dystrophy.

Muscular dystrophies: influence of physical conditioning on the disease evolution

PURPOSE OF REVIEW

To summarize the current knowledge of the effects of physical activity on muscular dystrophies.

RECENT FINDINGS

Although the usefulness of exercise training in muscular dystrophy patients has been debated for many years, only a limited number of articles addressing this issue have been published to date. Existing studies on the effects of strength training in patients with muscular dystrophies have shown promising results, but interpretations are hampered by several methodological shortcomings.

SUMMARY

The scientific basis for solid recommendations of different exercise regimens in muscular dystrophies is poor, but existing data suggest beneficial effects of adopting an active lifestyle. Low- to moderate-intensity resistance and aerobic training may be recommended in slowly progressive myopathic disorders. To date, there is no evidence to support the recommendation of high-resistance exercise regimens over low-moderate intensity exercise. In rapidly progressive myopathies, which are due to aberrant structural proteins such as Duchenne muscular dystrophy, the use of high-resistance and eccentric training should be avoided. There is still, however, no evidence that physical training can influence the evolution of muscular dystrophies in the long term.

Consequence of parvalbumin deficiency in the mdx mouse: histological, biochemical and mechanical phenotype of a new double mutant

We tested the hypothesis whether the mild dystrophy in mdx mice could result from the contribution of the cytosolic calcium buffer parvalbumin in maintaining a normal cytosolic [Ca2+]i, in spite of an increased passive Ca2+ influx. By crossing mdx mice with parvalbumin-deficient mice, double mutant mice, lacking both dystrophin and parvalbumin, were obtained. Though resting cytosolic [Ca2+]i and total calcium content were similar to that of mdx muscles, this new animal model presented a slightly more severe phenotype than the mdx mouse. Muscle pseudo-hypertrophy, the density of myotubes and of centronucleated fibres as well as the loss of IIB fibres were all increased in parvalbumin-deficient mdx mice. Many of these deficits were overcome in late adulthood, albeit fibrosis was clearly more pronounced than in mdx muscles. At 90 days, parvalbumin-deficient mdx mice showed higher levels of creatine phosphokinase and lower muscle strength, in vivo, than mdx mice. Isometric tension of isolated muscle was reduced, but the susceptibility to eccentric contraction was not increased. The slight aggravation of muscle dystrophy observed in mdx mice deprived of parvalbumin cannot explain the severity of the affection observed in xmd dogs and Duchenne dystrophy patients where parvalbumin is constitutively not expressed.

Attenuation of stretch-induced histopathologic changes of skeletal muscles by quinacrine

Quinacrine is an inhibitor of phospholipase A(2), an enzyme thought to be involved in activity-related injury of skeletal muscles. Histopathologic changes after injury by stretches of activated plantar-flexor muscles were measured in untreated and quinacrine-treated rats. On day 4 of treatment (50 mg.kg(-1) intraperitoneally for 5 days), 30 stretches were induced by ankle rotation after muscles reached a maximal isometric force. During the stretch protocol, peak stretch forces and isometric force deficits after each stretch [total deficits 56.7 +/- 2.8% (untreated rats) and 59.6 +/- 1.7% (quinacrine-treated rats)] were similar for both groups (n = 6 each). Two days after the stretch protocol, histopathologic changes were evaluated using antibody staining on cross-sections of gastrocnemius medialis muscles. Swollen myofibers devoid of desmin were identified. Similar cells, but not all swollen myofibers, in adjacent sections stained for albumin. Quinacrine reduced the number of desmin-negative and albumin-positive cells by 88% (P < 0.05) and 84% (P < 0.05), indicating that it attenuated histopathologic changes that follow stretch injury of activated skeletal muscles. Histopathologic changes following muscle injury or myopathic disease may thus be reduced or even prevented by selective drug intervention, thereby reducing the risk of muscle fibrosis. Muscle Nerve 27: 65-71, 2003

Loss of sarcolemma nNOS in sarcoglycan-deficient muscle

nNOS, anchored to the sarcolemma through its interactions with the dystrophin-glycoprotein complex, is dramatically reduced in dystrophin-deficient mdx mice and Duchenne muscular dystrophy patients. Recent evidence suggests that loss of nNOS in dystrophin-deficient muscle may contribute significantly to the progression of muscle pathology through a variety of mechanisms. To investigate whether nNOS plays a role in other forms of muscular dystrophy, we analyzed protein expression of nNOS in several sarcoglycan-deficient animal models of muscular dystrophy as well as patients with primary mutations in the sarcoglycan genes. Primary mutations in alpha-, beta-, delta-, and gamma-sarcoglycan result in autosomal recessive limb girdle muscular dystrophy (AR-LGMD). We report that loss of the sarcoglycan-sarcospan complex in muscle causes a dramatic reduction in the levels of nNOS expression at the membrane, even in the presence of normal dystrophin and syntrophin expression. Furthermore, we show that expression of three out of four sarcoglycans is not sufficient to maintain nNOS at the sarcolemma. Our data suggest that loss of nNOS may contribute to muscle pathology in AR-LGMD with primary mutations in the sarcoglycans.

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.

Adaptations to exercise training and contraction-induced muscle injury in animal models of muscular dystrophy

This article reviews the current status of exercise training and contraction-induced muscle-injury investigations in animal models of muscular dystrophy. Most exercise-training studies have compared the adaptations of normal and dystrophic muscles with exercise. Adaptation of diseased muscle to exercise occurs at many levels, starting with the extracellular matrix, but also involves cytoskeletal architecture, muscle contractility, repair mechanisms, and gene regulation. The majority of exercise-injury investigations have attempted to determine the susceptibility of dystrophin-deficient muscles to contraction-induced injury. There is some evidence in animal models that diseased muscle can adapt and respond to mechanical stress. However, exercise-injury studies show that dystrophic muscles have an increased susceptibility to high mechanical forces. Most of the studies involving exercise training have shown that muscle adaptations in dystrophic animals were qualitatively similar to the adaptations observed in control muscle. Deleterious effects of the dystrophy usually occur only in older animals with advanced muscle fiber degeneration or after high-resistive eccentric training. The main limitations in applying these conclusions to humans are the differences in phenotypic expression between humans and genetically homologous animal models and in the significant biomechanical differences between humans and these animal models.