Small-caliber skeletal muscle fibers do not suffer necrosis in mdx mouse dystrophy

Total ion content of skeletal and cardiac muscle in the mdx mouse dystrophy: Ca2+ is elevated at all ages

The mdx mouse has been shown to have a gene defect at the locus which is homologous to that which is defective in Duchenne muscular dystrophy and they both lack dystrophin, the protein product of this defective gene. The exact cause of myofibre necrosis in DMD is not known but there is evidence to support a causal relationship between elevated calcium and tissue necrosis. Since the mdx mouse exhibits age-dependent changes in the proportion of tissue necrosis, we have measured total ion content (Ca2+, Na+, K+, and Mg2+) in the heart and skeletal muscle of animals at different ages to determine if ionic changes correlate with reported periods of necrosis. Total calcium is elevated throughout the ages studied (10 days, 30 days and 254-347 days) in both tissues and does not correlate with necrosis, although it appears that pre-necrotic tissues do not exhibit such a wide variation in calcium content as is observed in tissues from older animals. These changes are discussed with reference to the other ions measured and to the regulation of intracellular calcium.

Dystrophic changes in mdx muscle regenerating from denervation and devascularization

The regenerative capacity of young mdx muscle after a denervating and devascularizing injury (DD) was examined in extensor digitorum longus (EDL) and compared with that of age-matched control mouse EDL. DD of the right EDL was produced at the age approximating the onset of dystrophy in the mdx model, and mice recovered for 2 weeks. Contralateral unoperated EDLs from mdx and control mice served as internal controls for histopathology, myofiber cross-sectional area (CSA), and ultrastructure of fiber regeneration in DD-EDL. Mdx DD-EDL were composed of small, uniformly mature myofibers with mostly peripheral nuclei. This contrasted with control DD-EDL in which fibers were centrally nucleated. In addition, the unoperated mdx EDL exhibited the central nucleation of spontaneous recovery from dystrophy. The CSA distribution of mdx DD-EDL myofibers was significantly shifted toward smaller CSA compared with unoperated mdx EDL, although mean CSA did not differ between the two mdx muscle groups. The CSA distribution of control DD-EDL was significantly different and shifted toward smaller CSA from both unoperated control EDL and from mdx DD-EDL distributions. Ultrastructural features of dystrophy were present in both mdx DD-EDL and in the unoperated mdx EDL, although they appeared more prevalent in the latter. These results suggest that short-term plasticity of mdx muscle recovery from imposed injury may be greater than that of normal muscle in establishing a regenerating fiber population.

Invited review: myoblast transfer: a possible therapy for inherited myopathies?

A potential therapeutic strategy for genetic diseases is to alter the genetic constitution of the affected tissues by means of grafts of normal precursor or stem cells. Over several years, evidence has accumulated to suggest that primary diseases of skeletal muscle, such as Duchenne muscular dystrophy, may be susceptible to this approach. This review makes a critical examination of such background evidence, and also of more recent data directly addressing the concept of therapy by means of grafts of normal myogenic cells. It is concluded that the data establish the principle that such grafts effect an alteration of the genetic constitution and phenotype of skeletal muscle and, therefore, might be used to alleviate recessively inherited myopathies. Several obstacles to the therapeutic application of this method to human disease are also identified; these seem to be problems of a technical nature rather than of basic principle, and none appears insuperable.

Dystrophin expression in myotubes formed by the fusion of normal and dystrophic myoblasts

Mdx mouse dystrophy is characterized by the absence in the muscle cytoplasmic membrane of a high molecular weight protein called dystrophin. A possible avenue for treatment of muscular dystrophies is to inject normal myoblasts in a dystrophic muscle to form hybrid muscle fibers. Hybrid myotubes were formed in vitro by the fusion of normal rat and dystrophic mouse (mdx) myoblasts. Staining with Hoechst dye 33258 permitted the clear distinction of mouse and rat nuclei. Immunostaining demonstrated that dystrophin was present over the entire membrane of all hybrid myotubes even when nuclei ratio normal/dystrophic was low.

X-irradiation improves mdx mouse muscle as a model of myofiber loss in DMD

The mdx mouse, although a genetic and biochemical homologue of human Duchenne muscular dystrophy (DMD), presents a comparatively mild histopathological and clinical phenotype. These differences are partially attributable to the greater efficacy of regeneration in the mdx mouse than in DMD muscle. To lessen this disparity, we have used a single dose of X-irradiation (16 Gy) to inhibit regeneration in one leg of mdx mice. The result is an almost complete block of muscle fiber regeneration leading to progressive loss of muscle fibers and their replacement by loose connective tissue. Surviving fibers are mainly peripherally nucleated and, surprisingly, of large diameter. Thus, X-irradiation converts mdx muscle to a model system in which the degenerative process can be studied in isolation from the complicating effect of myofiber regeneration. This system should be of use for testing methods of alleviating the myofiber degeneration which is common to mdx and DMD.

Developmental and tissue-specific regulation of mouse dystrophin: the embryonic isoform in muscular dystrophy

Dystrophin, the protein product of the Duchenne muscular dystrophy locus, is encoded by a 14 kb transcript of over 65 exons. A point mutation in the homologous mouse gene causes muscular dystrophy in mdx mice. We have examined the developmental regulation of transcription of this gene in skeletal mouse muscle and also the tissue specificity of the transcript in muscle and brain, by using the polymerase chain reaction to amplify overlapping segments of dystrophin mRNA spanning the entire coding sequence and 5'-untranslated region. We have characterised a specific embryonic transcript that would encode dystrophin with a different C-terminus and have shown that this persists from the earliest stages to the adult in mdx skeletal muscle. The brain transcript shows striking sequence homology to rat and human, being highly conserved at the 5'-untranslated region and is present in both wild-type and mdx mice.

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

Lengthening contractions were induced in the right anterior tibialis muscles (ATM) of anaesthetized normal and mdx (dystrophic) mice by supramaximal, nonfatiguing stimulation of the sciatic nerve for 180 min. In the left ATM of the same animals identical stimulation caused shortening contractions because of a prestimulation Achilles tenotomy. The prevalence of recently necrotic fibers was determined in all stimulated ATM by demonstrating the presence of IgG in the necrotic fibers using immunoperoxidase staining of cryostat sections. The results were compared to unstimulated normal and mdx ATM. A significantly higher rate of necrosis was demonstrated after lengthening contractions in the mdx ATM than normal ATM. Unstimulated normal and mdx ATM have either no or extremely infrequent necrotic fibers. We suggest that the enhanced vulnerability of mdx muscle fibers to lengthening contractions is related to the deficiency of dystrophin, which renders the sarcolemma more susceptible to suffer focal breaks. A similar situation may occur in Duchenne muscular dystrophy.

Randomized, double-blind trial of mazindol in Duchenne dystrophy

There is evidence that growth hormone may be related to the progression of weakness in Duchenne dystrophy. We conducted a 12-month controlled trial of mazindol, a putative growth hormone secretion inhibitor, in 83 boys with Duchenne dystrophy. Muscle strength, contractures, functional ability and pulmonary function were tested at baseline, and 6 and 12 months after treatment with mazindol (3 mg/d) or placebo. The study was designed to have a power of greater than 0.90 to detect a slowing to 25% of the expected rate of progression of weakness at P less than 0.05. Mazindol did not benefit strength at any point in the study. Side effects attributable to mazindol included decreased appetite (36%), dry mouth (10%), behavioral change (22%), and gastrointestinal symptoms (18%); mazindol dosage was reduced in 43% of patients. The effect of mazindol on GH secretion was estimated indirectly by comparing the postabsorptive IGF-I levels obtained following 3, 6, 9, and 12 months in the mazindol treated to those in the placebo groups. Although mazindol-treated patients gained less weight and height than placebo-treated patients, no significant effect on IGF-I levels was observed. Mazindol doses not slow the progression of weakness in Duchenne dystrophy.

Canine X-linked muscular dystrophy: morphologic lesions

Gross pathologic lesions and light microscopic and ultrastructural features of skeletal muscle lesions in canine X-linked muscular dystrophy (CXMD) were studied in dogs from 3 months to 6 years of age. Necrosis and regeneration were present at all ages, but were most prominent in the youngest dogs studied. Increased intracytoplasmic calcium, as evidenced by positive alizarin red S staining, was associated with fiber necrosis, but was also seen in small numbers of otherwise normal fibers. Progressive changes included development of severe fiber size variation, endomysial and perimysial fibrosis, prominent cytoplasmic disorganization, internalization of myonuclei, mitochondrial proliferation, mild fat infiltration, and alterations in the fiber-type pattern. The most consistent early ultrastructural changes were dilatation of the sarcoplasmic reticulum and focal subsarcolemmal areas of degeneration. Convincing sarcolemmal defects were not found. Z-band streaming was present at all ages, and Z-band duplication and nemaline rods were seen in older dogs. Evidence for abnormal regeneration was found in the oldest dog, and was associated with extensive fibrosis. These findings document the progression of lesions in CXMD, and illustrate the profound alterations in fiber organization and fiber type that may occur in late stages of dystrophin-deficient muscular dystrophy.

Dystrophin: a clinical perspective

Dystrophin, the protein product of the gene related to Duchenne and Becker muscular dystrophies, is a large cytoskeletal protein associated with the muscle fiber membrane. Recently identified dystrophin-related myopathies affecting animals can serve as experimental models for human disease. Immunologic detection of dystrophin in clinical muscle biopsies provides a direct biochemical test for both Duchenne and Becker muscular dystrophies. Applications of dystrophin testing include improved diagnostic accuracy, carrier detection, fetal diagnosis, and evaluation of asymptomatic male infants identified as a result of neonatal screening for increased serum creatine kinase levels. Identification of dystrophin has brought us to the point of addressing rational therapies.

Mouse chimeras and genetic rescue of mosaic muscle

The nuclear-cytoplasmic relationships existing within mosaic muscle will likely determine whether myoblast transfer can effectively rescue diseased muscle. The mouse chimera preparation is one source of such mosaic muscle in which that in vivo relationship can be investigated in the complete absence of complicating immunological or surgical trauma. For several metabolic enzymes, the mature muscle fiber appears to contain a homogeneous mix of the proteins encoded by multiple myonuclei. This relationship is clearly not representative of all muscle proteins, as several examples of proteins highly localized to "nuclear territories" have now been described. Nonetheless, the intrafiber distribution of certain enzymes, particularly GP1-1, is appropriate for the basis of a genotype marking system applicable to mosaic fibers. In vitro rescue of mdg myotubes is readily achievable by incorporation of few normal myonuclei and possibly by only one. In vivo requirements are apparently far more stringent and an hypothesis in which the mdg gene product, a Ca+(+) channel subunit, is restricted to nuclear territories would be consistent with the disparate results obtained in vitro and in vivo. Finally, chimeras containing mdx/mdx cells may show a partial amelioration of muscle pathology and may provide a means of determining the minimum genetically normal myonuclear compliment required to prevent degeneration of dystrophin-deficient fibers.

Dystrophin distribution in heterozygote MDX mice

The distribution of dystrophin in myofibers from normal, mdx hemizygous, and mdx heterozygous mice was studied at various times in development. While normal mice exhibit dystrophin immunostaining around the entire fiber periphery regardless of age, mdx hemizygous mice exhibit no staining (0-35 days). In contrast, young (10 day) heterozygous mdx mice showed neighboring dystrophin-negative and dystrophin-positive fibers as well as fibers with a discontinuous or patchy dystrophin labelling. Older heterozygotes displayed very few negative fibers, with most fibers exhibiting apparently complete dystrophin immunostaining. This, coupled with the absence of muscle fiber degeneration at any age point, and the apparently normal levels of dystrophin in older heterozygous mice, indicates that myonuclei containing the dystrophin gene can compensate for myonuclei which do not contain the dystrophin gene within the same myofiber.