The mutant mdx: inherited myopathy in the mouse. Morphological studies of nerves, muscles and end-plates

Structure and function of the neuromuscular junction in young adult mdx mice

Dystrophin, the protein product of the gene responsible for X-linked muscular dystrophies, shares structural features with the cytoskeletal proteins spectrin and alpha-actinin. Like spectrin, it is localized at the cytoplasmic surface of the sarcolemma and is particularly concentrated in the subsynaptic region of the neuromuscular junction. Mdx mice have a profound deficiency of dystrophin and develop a necrotizing myopathy in the first weeks of life. Abnormalities of the neuromuscular junction, including a redistribution of postsynaptic molecules and reduction in synaptic folding, are also observed. We have studied these mice to see whether the lack of dystrophin has a specific effect on the structure and function of their neuromuscular junctions. Using a fore-limb muscle from 8 week old mdx mice we confirm the previously described postsynaptic structural changes and in addition show that many nerve terminals are abnormally complex. We demonstrate that these structural abnormalities are found exclusively at neuromuscular junctions on regenerated muscle fibres. Despite these structural abnormalities, miniature endplate potential frequency, the quantal content of endplate potentials, the amplitude and time course of miniature endplate currents and the number of acetylcholine receptors at the postsynaptic membrane are normal in mdx mice of this age. We conclude that in the mdx mouse the absence of dystrophin from the postsynaptic membrane has little direct effect on the function of the neuromuscular junction but that degeneration and regeneration of muscle fibres leads to remodelling of both its pre- and postsynaptic components.

Dystrophin is a component of the subsynaptic membrane

A subsynaptic protein of Mr approximately 300 kD is a major component of Torpedo electric organ postsynaptic membranes and copurifies with the AChR and the 43-kD subsynaptic protein. mAbs against this protein react with neuromuscular synapses in higher vertebrates, but not at synapses in dystrophic muscle. The Torpedo 300-kD protein comigrates in SDS-PAGE with murine dystrophin and reacts with antibodies against murine dystrophin. The sequence of a partial cDNA isolated by screening an expression library with mAbs against the Torpedo 300-kD protein shows striking homology to mammalian dystrophin, and in particular to the b isoform of dystrophin. These results indicate that dystrophin is a component of the postsynaptic membrane at neuromuscular synapses and raise the possibility that loss of dystrophin from synapses in dystrophic muscle may have consequences that contribute to muscular dystrophy.

Fiber regeneration is not persistent in dystrophic (MDX) mouse skeletal muscle

Fiber replacement has been measured in adult mdx mouse limb skeletal muscles. During the first 10 days after birth all fibers appear normal; between Week 3 and 4 there is massive fiber degeneration followed by regeneration in which close to 100% of the fibers are repaired or replaced. New fibers arising in adult mice are characterized by expression of fetal myosin mRNAs in whole muscle extracts, and by staining of individual fibers with an embryonic myosin heavy chain-specific antibody. By 10 weeks of age new fiber replacement rate, indicated by frequency of fibers reacting with antibody, is reduced to about 10%, and by 1 year of age less than 1% of the fibers are being replaced at rates above control. Total fiber number also remains fairly constant. We conclude that the fibers regenerating up to 10 weeks of age become stabilized and do not undergo further rounds of degeneration and regeneration. This is consistent with the observed benign phenotype of adult mdx animals and with the idea that once-regenerated fibers escape the catastrophic dystrophic phenotype by acquiring a function that compensates for their mdx mutation. The mechanism by which regenerated mdx fibers restore adequate function in the absence of dystrophin may, when understood, provide clues to effective nongenetic interventions for muscular dystrophy in humans where regenerated fibers continue to degenerate and where the disease is often fatal.

The subcellular distribution of dystrophin in mouse skeletal, cardiac, and smooth muscle

We use a highly specific and sensitive antibody to further characterize the distribution of dystrophin in skeletal, cardiac, and smooth muscle. No evidence for localization other than at the cell surface is apparent in skeletal muscle and no 427-kD dystrophin labeling was detected in sciatic nerve. An elevated concentration of dystrophin appears at the myotendinous junction and the neuromuscular junction, labeling in the latter being more intense specifically in the troughs of the synaptic folds. In cardiac muscle the distribution of dystrophin is limited to the surface plasma membrane but is notably absent from the membrane that overlays adherens junctions of the intercalated disks. In smooth muscle, the plasma membrane labeling is considerably less abundant than in cardiac or skeletal muscle and is found in areas of membrane underlain by membranous vesicles. As in cardiac muscle, smooth muscle dystrophin seems to be excluded from membrane above densities that mark adherens junctions. Dystrophin appears as a doublet on Western blots of skeletal and cardiac muscle, and as a single band of lower abundance in smooth muscle that corresponds most closely in molecular weight to the upper band of the striated muscle doublet. The lower band of the doublet in striated muscle appears to lack a portion of the carboxyl terminus and may represent a dystrophin isoform. Isoform differences and the presence of dystrophin on different specialized membrane surfaces imply multiple functional roles for the dystrophin protein.

Animal models of muscular dystrophy--what can they teach us?

The discovery and characterization of the X-linked gene which is defective in Duchenne muscular dystrophy (DMD) and of its protein product, dystrophin, has led to the identification of biochemical homologues of this disease in the mouse, the dog and the cat. All three animal models resemble DMD in that they lack dystrophin and that their skeletal muscle fibres undergo spontaneous necrosis and regeneration. In the dog and man, the degenerative and fibrotic aspects predominate, leading to a progressive loss of muscle structure and function, and to severe clinical disability. By contrast, in the mouse and the cat there is little fibrosis and the regenerative process seems to overcompensate, producing a true muscle hypertrophy and little or no clinical deficit. This interspecies variation in pathological response limits the usefulness of these animals as models for therapeutic testing, calling into question the strength of linkage between a given biochemical lesion and a particular pattern of pathology. However, these differences do give a valuable perspective to the pathology of the dystrophin-deficiency diseases, permitting identification of the immediate and secondary consequences of the lack of dystrophin. Moreover, the dystrophic mouse and dog are readily bred as colonies, thus providing consistent material for investigating the function of dystrophin and for testing methods of replacing its function or compensating for the absence of this function in the muscles of DMD patients. The fact that a lack of dystrophin is compatible, in some species, with only minor muscle dysfunction, raises hopes for an effective therapy in man.

Negative halothane-caffeine contracture test in mdx (dystrophin-deficient) mice

The genetics of malignant hyperthermia (MH) are ill-understood; however, the association of Duchenne muscular dystrophy (DMD) with MH is well known. A deficiency of dystrophin is common to both the DMD and mdx mouse, an animal model for DMD. Using muscle contracture tests for MH, we have shown that in the mdx mouse there is no MH susceptibility, suggesting the lack of a direct role of the dystrophin in the development of MH syndrome.

Human dystrophin expression in mdx mice after intramuscular injection of DNA constructs

Duchenne's muscular dystrophy (DMD), which affects one in 3,500 males, causes progressive myopathy of skeletal and cardiac muscles and premature death. One approach to treatment would be to introduce the normal dystrophin gene into diseased muscle cells. When pure plasmid DNA is injected into rodent skeletal or cardiac muscle, the cells express reporter genes. We now show that a 12-kilobase full-length human dystrophin complementary DNA gene and a 6.3-kilobase Becker-like gene can be expressed in cultured cells and in vivo. When the human dystrophin expression plasmids are injected intramuscularly into dystrophin-deficient mdx mice, the human dystrophin proteins are present in the cytoplasm and sarcolemma of approximately 1% of the myofibres. Myofibres expressing human dystrophin contain an increased proportion of peripheral nuclei. The results indicate that transfer of the dystrophin gene into the myofibres of DMD patients could be beneficial, but a larger number of genetically modified myofibres will be necessary for clinical efficacy.

Changes in muscle plasma membranes in young mice with X chromosome-linked muscular dystrophy: a freeze-fracture study

The structure of the muscle plasma membrane of tibialis anterior muscles of X chromosome-linked muscular dystrophy (mdx) mice was studied by the freeze-fracture technique at 3, 7 and 14 days after birth. The ultrastructural features of the freeze-fracture replicas of the muscle plasma membrane alterations in young mdx mice showed a decrease of orthogonal array, orthogonal array subunit particle and intramembranous particle densities on the protoplasmic face. The results are consistent with the previous studies which have shown that the orthogonal arrays are significantly decreased in number in muscle plasma membranes of adult mdx mice and in those of Duchenne dystrophy. However, the immature mdx mouse membranes at 3 days after birth contained as many orthogonal arrays as controls and did not show a statistically significant decrease (P greater than 0.1 by the Wilcoxon rank-sum test). Moreover, the orthogonal arrays were also numerous in young mdx mouse muscle plasma membranes at 7 and 14 days after birth, although the density was less than that of the control mice (P less than 0.01 by the Wilcoxon rank-sum test). These changes in young mdx mouse plasma membranes may precede the later muscle fibre degeneration in this mouse dystrophy and may provide us with an additional clue to the mechanism why mdx mice scarcely show any disability despite the absence of dystrophyn.

Localization of dystrophin relative to acetylcholine receptor domains in electric tissue and adult and cultured skeletal muscle

Two high-affinity mAbs were prepared against Torpedo dystrophin, an electric organ protein that is closely similar to human dystrophin, the gene product of the Duchenne muscular dystrophy locus. The antibodies were used to localize dystrophin relative to acetylcholine receptors (AChR) in electric organ and in skeletal muscle, and to show identity between Torpedo dystrophin and the previously described 270/300-kD Torpedo postsynaptic protein. Dystrophin was found in both AChR-rich and AChR-poor regions of the innervated face of the electroplaque. Immunogold experiments showed that AChR and dystrophin were closely intermingled in the AChR domains. In contrast, dystrophin appeared to be absent from many or all AChR-rich domains of the rat neuromuscular junction and of AChR clusters in cultured muscle (Xenopus laevis). It was present, however, in the immediately surrounding membrane (deep regions of the junctional folds, membrane domains interdigitating with and surrounding AChR domains within clusters). These results suggest that dystrophin may have a role in organization of AChR in electric tissue. Dystrophin is not, however, an obligatory component of AChR domains in muscle and, at the neuromuscular junction, its roles may be more related to organization of the junctional folds.

Duchenne's cardiomyopathy in a canine model: electrocardiographic and echocardiographic studies

Thirteen dogs affected with X-linked Duchenne's muscular dystrophy and 11 female carrier dogs were studied by electrocardiography (ECG) and echocardiography. Twelve of the affected dogs were studied as immature animals and followed at 1 to 6 month intervals until they were 7 to 46 months of age. Compared with control dogs, affected dogs had significantly increased (p less than 0.02) Q/R ratios in ECG leads II, III, aVF, CV6LL (V2) and CV6LU (V4). Carrier dogs had significantly increased (p less than 0.02) Q/R ratios in leads V2 and V4. The Q/R ratio increased in three of six dogs followed up from age 6 months to greater than 2 years. The PR intervals were significantly shorter (p less than 0.02) in affected dogs. Ventricular arrhythmias were identified in four of six mature affected dogs. Two-dimensional echocardiography revealed distinctive hyperechoic lesions in 12 of the 13 affected dogs and in 6 of the 11 carrier dogs. Hyperechoic lesions corresponded to calcified myocardium and surrounding dense connective tissue. This study establishes the dog affected with Duchenne's muscular dystrophy as an animal model of Duchenne's cardiomyopathy and demonstrates that the heart in carrier dogs is affected by the dystrophic process.

Passive avoidance behaviour deficit in the mdx mouse

Thirty per cent of boys with Duchenne muscular dystrophy (DMD) suffer from various degrees of mental retardation. Since dystrophin, the protein absent in muscles of boys with DMD, is produced also in the brain, it was postulated that the deficiency of brain dystrophin might account for the mental retardation found in DMD boys. The mdx mouse, a mouse model of DMD, fails to produce dystrophin in muscle and brain. This prompted us to study the cognitive function of these animals. Learning and memory processes were studied in 10 mdx females and 9 genetically matched controls using the passive avoidance test. Statistically significant differences in the retention of the passive avoidance response was detected between mdx and control mice, indicating an impairment in passive avoidance learning in mdx mice. Our data reinforce the view that brain dystrophin deficiency is correlated with cognitive dysfunction and indicate that mdx mice might be a model for the mental retardation found in DMD boys.

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.

Immunolocalization and developmental expression of dystrophin related protein in skeletal muscle

Dystrophin Related Protein is the recently identified protein product of a large autosomal transcript, showing significant similarity to dystrophin at the carboxyl terminus. Dystrophin related protein and dystrophin share a similar abundance and molecular weight, however, they differ both in their tissue distribution and expression in Duchenne/Becker muscular dystrophy. Here we define the immunolocalization of dystrophin related protein to neuromuscular and myotendinous junctions, along with peripheral nerves and vasculature of skeletal muscle. Groups of regenerating muscle fibres as well as embryonic and neonatal muscle express far greater amounts of dystrophin related protein compared with adult mdx mice. These findings may explain the paradoxical labelling seen using dystrophin antibodies in Duchenne patients and dystrophin deficient mdx mice. Finally, no abnormalities of dystrophin related protein expression were detected in three patients with Duchenne-like autosomal recessive 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.

Neuromuscular transmission in the mdx mouse

In both disorders, the muscle fiber plasma membrane is rendered selectively vulnerable by dystrophin deficiency. In both disorders there are also ultrastructural abnormalities involving the postsynaptic membrane of the neuromuscular junction. The object of this electrophysiologic study was to determine whether the observed ultrastructural abnormalities at the mdx neuromuscular junction are associated with an abnormality of neuromuscular transmission. In comparison with age-matched control mice, the mdx mice show an abnormal, age-dependent decrease of the amplitude of the miniature end-plate potential and a concomitant increase in the quantal content of the end-plate potential. Consequently, the safety margin of neuromuscular transmission is not impaired.

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.

A noninvasive procedure to detect muscle weakness in the mdx mouse

The forward pulling tension exerted by individual mice was measured nearly isometrically in a simple apparatus designed to determine whole body tension (WBT). WBT determinations on control (C57Bl10/SnJ) and experimental (C57Bl10-mdx) mice indicate a muscle weakness which lasts throughout the lifespan of mdx mice. Direct muscle stimulation experiments in vivo also showed significant decreases in peak twitch and tetanic tensions in adult mdx muscle with no obvious alterations in twitch time course or in twitch: tetanus ratios. We suggest that the noninvasive WBT procedure may be used to partially assess various therapies on this murine model of Duchenne muscular dystrophy.

Ultrastructural localization of dystrophin in human muscle by using gold immunolabelling

Immunolabelling with a 5 nm gold probe was used to localize dystrophin at the ultrastructural level in human muscle. The primary antibody was monoclonal, raised against a segment (amino acids 1181-1388) from the rod domain of dystrophin. The antibody (Dy4/6D3) is specific for dystrophin and shows no immunoreactivity with any protein from mdx mouse muscle or from patients with a gene deletion spanning part of the molecule recognized by the antibody (Nicholson et al. 1989 a; England et al. 1990). Using this antibody, labelling was almost entirely confined to a narrow 75 nm rim at the periphery of the muscle fibres. Histograms of the distance from the gold probe to the cytoplasmic face of the plasma membrane and of the distance between gold probes (nearest neighbour in a plane parallel with the plasma membrane) displayed modes at approximately 15 nm and 120 nm, respectively. The distribution of the probe was the same in longitudinal and transverse sections of the muscle. These observations suggest that the rod portion of the dystrophin molecule is normally arranged close to the cytoplasmic face of the plasma membrane and that the molecules form an interconnecting network. Labelling was not associated with the transverse tubular system.

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.

Analysis of dystrophin in fast- and slow-twitch skeletal muscles from mdx and dy2J mice at different ages

Muscles from mdx, control, and dy2J/dy2J mice at different ages were analyzed for dystrophin in an attempt to relate the chronology of the protein expression with the final phenotypes in regenerated, normal, and dystrophic muscle, respectively. Immunostaining and gold staining of electrophoresis gels were carried out in the investigation. At 5, 25, and 219 days of age, control muscles exhibited dystrophin bands in both the fast-twitch extensor digitorum longus (EDL) and the slow-twitch soleus (SOL) muscles. Muscles from the mdx mice at comparable ages (8, 28, and 217 days) never exhibited bands for dystrophin, although titin, nebulin, myosin, and other protein bands were present at intensities comparable to those in control muscles. The dystrophin band was present in both the EDL and SOL from dy2J/dy2J dystrophic mice. As indicated by the present study, the dystrophin deficiency from mdx tissue is not transient. This suggests that dystrophin is not necessary for the success of mdx muscle regeneration.

Excitation contraction coupling in normal and mdx mice

The protein dystrophin is absent from patients with Duchenne muscular dystrophy and from the muscles of mdx mice. Recent studies have shown that dystrophin is located at the surface membrane and at the triadic junction, where it is associated with the transverse tubular membrane. Since the triadic junction is the site of excitation-contraction (EC) coupling, we have investigated whether intramembrane charge movement, a step in EC coupling, is modified by the absence of dystrophin. Charge movements are thought to arise from the transverse tubular membrane and to underlie the dependence of sarcoplasmic reticulum Ca2+ release on transverse tubular membrane potential. We find no differences between intramembrane charge movements or passive membrane electrical properties measured in muscles from mdx mice compared with normal mice. If dystrophin does play a role in EC coupling, that role is likely to be subsequent to the charge movement step.

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.

Molecular biology of Duchenne and Becker's muscular dystrophy: clinical applications

Recent advances in molecular genetics have led to the isolation of the gene defective in patients with Duchenne and Becker's muscular dystrophy and the characterization of its protein product, dystrophin. In this communication, the developments culminating in the identification of the Duchenne muscular dystrophy locus are reviewed. The practical applications of this research and pitfalls that limit prenatal diagnosis and carrier detection are discussed.

The molecular basis of muscular dystrophy in the mdx mouse: a point mutation

The mdx mouse is an X-linked myopathic mutant, an animal model for human Duchenne muscular dystrophy. In both mouse and man the mutations lie within the dystrophin gene, but the phenotypic differences of the disease in the two species confer much interest on the molecular basis of the mdx mutation. The complementary DNA for mouse dystrophin has been cloned, and the sequence has been used in the polymerase chain reaction to amplify normal and mdx dystrophin transcripts in the area of the mdx mutation. Sequence analysis of the amplification products showed that the mdx mouse has a single base substitution within an exon, which causes premature termination of the polypeptide chain.

Fibroblast growth factor in the extracellular matrix of dystrophic (mdx) mouse muscle

Polyclonal antibody F547 reacts with a bovine basic fibroblast growth factor (bFGF) and a human recombinant bFGF, but not with bovine acidic fibroblast growth factor. This antibody localized bFGF in the extracellular matrix of mouse skeletal muscle, primarily in the fiber endomysium, which includes the heparin-containing basal lamina. In mdx mouse muscle, which displays persistent regeneration, FGF levels in the extracellular matrix are higher than those in controls. Overabundance of matrix FGF in mdx muscles may be related to an increase in both satellite cell and regenerative activity in the dystrophic muscle and may help explain the benign phenotype of mdx animals compared with the genetically identical human Duchenne muscular dystrophy.

Mdx muscle grafts retain the mdx phenotype in normal hosts

Whole muscle grafts were made between mdx and normal mice to investigate whether the mdx myopathic lesion is intrinsic to mdx muscle or is a property of its environment. Grafts were examined between 20 and 101 days. Unequivocal necrotic muscle fibers and/or newly formed basophilic myotubes were noted in 8 of 16 grafts of mdx muscle made in normal hosts but in none of 16 grafts of normal muscle made in mdx hosts. In older grafts, the proportion of centrally nucleated fibers and variability of fiber diameter were both higher in mdx muscle grafted into normal hosts than in normal muscle grafted into either mdx or normal hosts. Analysis of the glucose-6-phosphate isomerase (GPI) isoenzyme content of the grafts indicated that the muscle formed was predominantly of donor origin. These findings provide evidence that the mdx lesion is a primary myopathy rather than secondary to an extramuscular primary lesion.

Recovery of induced mutations for X chromosome-linked muscular dystrophy in mice

We have used elevated levels of plasma creatine phosphokinase activity to identify muscular dystrophy phenotypes in mice and to screen the progeny of chemical mutagen-treated male mice for X chromosome-linked muscular dystrophy mutations. We were not successful in identifying heterozygous carriers of these induced muscular dystrophy mutations in greater than 8000 progeny. However, we were highly successful in identifying three additional alleles of the characterized mdx locus. These alleles of mdx were recovered from various mutagen-treated males and they occur on an X chromosome that carries flanking markers that allow us to follow the mutations in genetic crosses and in the development of corresponding mutant stocks. These alleles have been designated as mdx2Cv, mdx3Cv, and mdx4Cv. Preliminary data show that mice with mdx2Cv and mdx3Cv mutations have muscular dystrophic phenotypes that do not grossly differ from the characterized mdx mutation. These additional mdx mutations expand the value of mouse models of X chromosome-linked muscular dystrophy and potentially define additional sites of mutation that impair dystrophin expression.

Distribution of dystrophin, nebulin and Ricinus communis I (RCA-I)-binding glycoprotein in tissues of normal and mdx mice

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene locus, appeared as an immunoreactive triplet of polypeptides in striated muscle tissues from normal mice on Western blot analysis. In smooth muscle tissues, an immunoreactive doublet of corresponding molecular weight was detected. No dystrophin was found in normal mouse brain, not even after enrichment for the Triton X-100 insoluble fraction. Dystrophin was absent from all corresponding tissues from the mdx mutant mouse strain which is known to lack dystrophin. The possibility that these immunoreactive bands represent isoforms is discussed. We have also investigated two other high molecular weight proteins which show secondary abnormalities in DMD muscle, namely nebulin and the 370 kDa Ricinus communis I lectin (RCA I)-binding glycoprotein. Nebulin levels were reduced in skeletal muscle from 6-week-old mdx mice but not in oesophagus from the same animals. By contrast, the RCA I-binding 370 kDa glycoprotein which is greatly reduced in DMD skeletal muscle was present in normal amounts in mdx skeletal muscle. These findings show, for the first time, that mdx myopathy differs from DMD myopathy not only morphologically, but also in its secondary biochemical abnormalities.

Distinct dystrophin mRNA species are expressed in embryonic and adult mouse skeletal muscle

We have examined dystrophin mRNA in embryonic, newborn and adult mouse skeletal muscle. A discrete nerve-independent increase in mRNA size was observed between embryonic and adult stages, indicating that a developmentally regulated mRNA isoform switch occurs in the expression of the Duchenne muscular dystrophy (DMD) gene in skeletal muscle. These distinct mRNAs are most likely generated via selection of alternative transcriptional start sites or RNA processing pathways. In addition, denervation of adult muscle was without effect on the expression pattern.

Functional regeneration in the hindlimb skeletal muscle of the mdx mouse

The pattern of spontaneous skeletal muscle degeneration and clinical recovery hindlimb muscles of the mdx mutant mouse was examined for functional and metabolic confirmation of apparent structural regeneration. The contractile properties, histochemical staining and myosin light chain and parvalbumin contents of extensor digitorum longus (EDL) and soleus (Sol) muscles of mdx and age-matched control mice were studied at 3-4 and 32 weeks. Histochemical staining (myofibrillar ATPase and NADH-tetrazolium reductase) revealed no significant change in slow-twitch-oxidative (SO) or fast-twitch-oxidative-glycolytic (FOG) fibre type proportions in mdx Sol apart from the normal age-related increase in SO fibres. At 32 weeks mdx EDL, however, showed significantly smaller fast-twitch-glycolytic (FG) and larger FOG proportions than those in control EDL. These fibre type distributions were confirmed by differential staining with antibodies to myosin slow-twitch and fast-twitch heavy chain isozymes. Frequency distribution of cross-sectional area for each fibre type showed a wider than normal range of areas especially in FOG fibres of mdx Sol, and FG fibres of mdx EDL, supporting previous observations using autoradiography of myofibre regeneration. Isometric twitch and tetanic tensions in Sol were significantly less than in controls at 4 weeks, but by 32 weeks, values were not different from age-matched controls. In mdx EDL at 3 weeks, twitch and tetanus tensions were significantly less, and time-to-peak twitch tensions were significantly faster than in control EDL. By 32 weeks, mdx EDL twitch and tetanus tensions expressed relative to muscle weight continued to be significantly lower than in age-matched controls, despite normal absolute tensions. The maximum velocity of shortening in 32-week mdx EDL was significantly lower than in control EDL. Myosin light chain distribution in mdx Sol exhibited significantly less light chain 2-slow (LC2s) and more light chain 1b-slow(LC1bs) at 32 weeks than age-matched control Sol. Gels of EDL from 32-week-old mdx mice showed significantly less light chain 2-fast-phosphorylated (LC2f-P) and light chain 3-fast (LC3f) and significantly more light chain 1-fast (LC1f) and light chain 2-fast (LC2f), but normal parvalbumin content compared to age-matched controls. These observations suggest that mdx hindlimb muscles are differentially affected by the disease process as it occurs in murine models of dystrophy. However, the uniqueness of mdx Sol and to a lesser extent EDL is that they also undergo an important degree of functional regeneration which is able to compensate spontaneously for degenerative influences of genetic origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Canine X-linked muscular dystrophy. An animal model of Duchenne muscular dystrophy: clinical studies

The progression of clinical disease and serum creatine kinase (CK) levels in canine X-linked muscular dystrophy (CXMD) was studied in 7 dogs from birth to 12-14 months and in 18 dogs at varying intervals from birth to 8 weeks. One affected male was studied from age 3.5 to 6 years, and all pups were descendants of this dog. A lethal neonatal form was recognized in some pups. In the more typical form, clinical signs of stunting, weakness and gait abnormalities were evident by 6-9 weeks and were progressive, leading to marked muscle atrophy, fibrosis and contractures by 6 months. Serum CK levels were markedly elevated, such that affected pups could be identified by 1 week. CK values increased until 6-8 weeks, then plateaued at approx. 100 times normal. Affected females and beagle-cross dogs were less severely affected than large breed-cross dogs. In the 2 adult dogs with cardiac insufficiency CK levels had decreased to 5-15 times normal. These studies show that CXMD and Duchenne muscular dystrophy have striking phenotypic as well as genotypic similarities. In addition, these studies of CXMD suggest that in females and in smaller dogs the same genetic defect results in a less severe clinical disease.

Satellite cells from dystrophic (mdx) mouse muscle are stimulated by fibroblast growth factor in vitro

Satellite cells cultured from dystrophic (mdx) and from control mouse hindlimb muscles grow and fuse to form muscle fibers within 4-5 days. Total cell number and muscle-fiber formation are stimulated by bovine fibroblast growth factor (FGF). At low FGF levels (0.02-0.20 ng/ml) control satellite cells as well as fibroblasts are unresponsive, while mdx satellite cells show three- to four-fold increases in growth. Control cells do not begin to respond until FGF levels reach 1-5 ng/ml. Heparin, a major constituent of muscle fiber basal lamina, inhibits myogenesis in these mouse muscle cultures. The heightened sensitivity of mdx satellite cells to FGF may permit high rates of new fiber formation in vivo without a parallel hyperplasia in the muscle fibroblast population. This finding may be important in explaining successful regeneration in mdx muscle in vivo and the fact that mdx animals escape the catastrophic symptoms seen in the related human Duchenne muscular dystrophy.

Inherited neuromuscular diseases in the mouse. A review of the literature

There are several neuromuscular disorders affecting the human being. Most of these are poorly understood and lack and effective treatment. Due to the limitation of experimental manipulation in "anima nobili", inherited neuromuscular diseases in laboratory animals constitute a valuable source of scientific information. Amongst several animal species affected by neuromuscular disorders the house mouse is of particular interest because of its small size, short pregnancy and low costs of maintanence. In the present review 20 murine mutants with diseases affecting peripheral nerves, skeletal muscles and motor end-plates are tabulated. Genetic, clinical and pathological aspects are discussed aiming to provide information about these mutants which might be of great interest as animal models for human neuromuscular diseases.

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

The prevalence of internal nuclei in muscle fibers (centronucleation), which is a reliable cumulative index of all prior muscle fiber necrosis, was measured at different ages in different muscles of mdx mice and was correlated with muscle fiber diameter. The prevalence of centronucleated fibers (as percentage of total number of fibers) rose gradually after age 20 days until it reached a peak level of 80% at age 60 days. No significant centronucleation (or necrosis) was observed in the following circumstances: in 4 different limb muscles before age 15 days, in leg muscles that were denervated by peripheral nerve section or rendered immobile by high thoracic cordotomy at 15 days, or in rotator extraocular muscles throughout the animals' life span. In these situations, muscle fiber diameter remained below approximately 20 micron. The mechanism by which small-diameter fibers are resistant to necrosis in mdx dystrophy is unknown, but a similar situation exists in hamster and Duchenne muscular dystrophy.

The homologue of the Duchenne locus is defective in X-linked muscular dystrophy of dogs

Duchenne muscular dystrophy (DMD) is the most common and the most severe of the muscular dystrophies in man. It is inherited as an X-linked recessive trait and is characterized by ongoing necrosis of skeletal muscle fibres with regeneration and eventually fibrosis and fatty infiltration. Although the gene and gene product which are defective in DMD have recently been identified, the pathogenesis of the disease is still poorly understood. A myopathy has been described in the dog which has been shown to be inherited as an X-linked trait and which is therefore a potential model of the human disease. We have studied the phenotypic expression of the disease, canine X-linked muscular dystrophy (CXMD), and have examined the molecular relationship between it and DMD. We report here that dogs with CXMD faithfully mimic the phenotype of Duchenne muscular dystrophy and that they lack the Duchenne gene transcript and its protein product, dystrophin.

Cell and fiber-type distribution of dystrophin

Duchenne muscular dystrophy is the result of dystrophin deficiency. We have determined the cell types likely to express the pathogenic effects of this neuromuscular disease by determining the pattern of dystrophin expression in normal cells. We find that all physiological types of muscle cells express dystrophin at similar levels, and that the dystrophin content of various tissues correlates with the myogenic cell population of each tissue. The dystrophin content of brain and spinal cord, however, is found not to correlate with any type of muscle cell, and it is suggested that neurons express dystrophin. The potential involvement of striated muscle fibers, the vasculature, and the nervous system in the etiology of Duchenne muscular dystrophy makes it likely that the disease is a complex disorder of combined pathogenesis. We also find that the dystrophic chicken does not represent an animal model for dystrophin deficiency.

Ultrastructure of the skeletal muscle in the X chromosome-linked dystrophic (mdx) mouse. Comparison with Duchenne muscular dystrophy

Ultrastructurally there are some clear differences in the pathology of muscle in X chromosome-linked muscular dystrophy of the mouse (mdx) and Duchenne muscular dystrophy (DMD). In particular the mouse muscle does not become infiltrated by large aggregations of connective tissue. It has been proposed that the differences are due to secondary biochemical changes consequent on the absence of dystrophin in both conditions. If this is the case, attention should be directed to the earliest ultrastructural changes held in common by both disorders. The most conspicuous of these, preceding myofibril breakdown, is dilation of the sarcoplasmic reticulum. Any physiological link between this and the absence of dystrophin remains to be determined. We suggest that in the mdx mouse, the widespread myofibre necrosis occurring at 3-4 weeks is triggered by increased mechanical demands causing the lack of dystrophin to become critical at this time. Subsequent regeneration of the myofibres appears to be almost completely successful. The ultimate failure of regeneration in DMD, in contrast, may be due to an additional factors acting in DMD exacerbating the lack of dystrophin. This additional factor may be associated with the plasma membrane lesions (not seen in mdx). Alternatively there may be factors present in the mouse that compensate for the lack of dystrophin. It is pointed out that to understand better the different processes occurring in mdx and DMD we need to learn more about the factors which control the balance between the growth of muscle and the growth of connective tissue in both normal and pathological human and mouse muscle.

Freeze-fracture studies of myofiber plasma membrane in X chromosome-linked muscular dystrophy (mdx) mice

The structure of the muscle plasma membrane of extensor digitorum longus muscles of X chromosome-linked muscular dystrophy (mdx) mice was studied by freeze-fracture technique at several time points after birth. The common denominator of the abnormalities was the decreased density of orthogonal arrays throughout all the time points examined. The results demonstrated that the ultrastructural features of the muscle plasma membrane alterations in mdx mice were similar to those in Duchenne dystrophy.

Dystrophin: the protein product of the Duchenne muscular dystrophy locus

The protein product of the human Duchenne muscular dystrophy locus (DMD) and its mouse homolog (mDMD) have been identified by using polyclonal antibodies directed against fusion proteins containing two distinct regions of the mDMD cDNA. The DMD protein is shown to be approximately 400 kd and to represent approximately 0.002% of total striated muscle protein. This protein is also detected in smooth muscle (stomach). Muscle tissue isolated from both DMD-affected boys and mdx mice contained no detectable DMD protein, suggesting that these genetic disorders are homologous. Since mdx mice present no obvious clinical abnormalities, the identification of the mdx mouse as an animal model for DMD has important implications with regard to the etiology of the lethal DMD phenotype. We have named the protein dystrophin because of its identification via the isolation of the Duchenne muscular dystrophy locus.