Prevention of skeletal muscle fiber necrosis in hamster dystrophy

TAZ as a novel enhancer of MyoD-mediated myogenic differentiation

Myoblast differentiation is indispensable for skeletal muscle formation and is governed by the precisely coordinated regulation of a series of transcription factors, including MyoD and myogenin, and transcriptional coregulators. TAZ (transcriptional coactivator with PDZ-binding motif) has been characterized as a modulator of mesenchymal stem cell differentiation into osteoblasts and adipocytes through its regulation of lineage-specific master transcription factors. In this study, we investigated whether TAZ affects myoblast differentiation, which is one of the differentiated lineages of mesenchymal stem cells. Ectopic overexpression of TAZ in myoblasts increases myogenic gene expression in a MyoD-dependent manner and hastens myofiber formation, whereas TAZ knockdown delays myogenic differentiation. In addition, enforced coexpression of TAZ and MyoD in fibroblasts accelerates MyoD-induced myogenic differentiation. TAZ physically interacts with MyoD through the WW domain and activates MyoD-dependent gene transcription. TAZ additionally enhances the interaction of MyoD with the myogenin gene promoter. These results strongly suggest that TAZ functions as a novel transcriptional modulator of myogenic differentiation by promoting MyoD-mediated myogenic gene expression.

Responses of diseased muscle to electrical and mechanical intervention

It is well established that the properties of muscle fibres are influenced by their neurons and that this is at least in part mediated by the pattern of activity. Application of this knowledge has led to the experimental trial of electrical stimulation in diseased muscle, both in the dystrophic mouse and in children with Duchenne muscular dystrophy. This has shown a beneficial effect of slow frequency stimulation. Another route through which muscle properties can be influenced is by changing the load by procedures such as tenotomy. This has been studied by complete tenotomy in normal animals and recently by selective partial procedures in human disease. Y. Rideau has shown that release of early shortening (contractures) of several muscles, a consistent feature in Duchenne muscular dystrophy, has a beneficial effect on muscle function. From personal observations on a number of Rideau's patients who have undergone this procedure the improvement in function seems disproportionate to what could be explained on simple biomechanical grounds alone and suggests some more fundamental change in the contractile properties of the muscle.

Beneficial effects of training on developing dystrophic muscle

The objective of this study was to determine whether increased contractile activity is beneficial or detrimental to developing dystrophic muscle. Hamsters (20-days-old) were gradually introduced to running at a speed of 14 m/min at 10% grade for 2 (T2) or 4 (T4) h/d for 4 weeks, 5 d/wk. Histological and fiber type properties were determined in the soleus (SOL), plantaris (PL), and extensor digitorum longus and contractile properties in SOL and PL from 5 animals/group, including 5 controls. Experimental animals had normal body and muscle mass. Training for 2 h/d had little effect on SOL contractile properties, whereas 4 h/d resulted in significant increases in force, percentage of type I fibers, and type I hypertrophy. Force also increased in PL. Muscle necrosis was reduced in SOL (T2 and T4) and unchanged in PL. In conclusion, endurance training generally had a beneficial or, at least, no detrimental effect on developing dystrophic muscles.

Transient neonatal denervation alters the proliferative capacity of myosatellite cells in dystrophic (129ReJdy/dy) muscle

It has been previously shown that transiently denervated, neonatal dystrophic muscle fails to undergo the degeneration-regeneration cycle characteristic of murine dystrophy (Moschella and Ontell, 1987). Thus, the myosatellite cells (myogenic stem cells) in these muscles have been spared the mitotic challenge to which dystrophic myosatellite cells are normally subjected early in the time course of the disease. By in vitro evaluation of the proliferative capacity of myosatellite cells derived from extensor digitorum longus (EDL) muscles of 100-day-old genetically normal (+/+) and genetically dystrophic [dy/dy (129ReJdy/dy)] mice and from muscles of age-matched mice that had been neonatally denervated (by sciaticotomy) and allowed to reinnervate, it has been possible to directly determine whether the cessation of spontaneous regeneration in older dy/dy muscles in vivo, is due to an innate defect in the proliferative capacity of the myosatellite cells or exhaustion of the myosatellite cells' mitotic activity during the regenerative phase of the disease. This study demonstrates that transient neonatal denervation of dystrophic muscle (Den.dy/dy) increases the number of muscle colony-forming cells (MCFs) per milligram of wet weight muscle tissue, increases the plating efficiency, and significantly increases the in vitro mitotic activity of dystrophic myosatellite cells toward normal values. The increased mitotic capability of myosatellite cells derived from Den.dy/dy muscle as compared to unoperated dy/dy muscle suggests that there is no innate defect in the proliferative capacity of the myosatellite cells of dy/dy muscles and that the cessation of spontaneous regeneration in the dy/dy muscles is related to the exhaustion of their myosatellite cells' mitotic capability.

Glucocorticoids and immunosuppressants do not change the prevalence of necrosis and regeneration in mdx skeletal muscles

Therapeutic doses of methylprednisolone, azathioprine, cyclosporin A, and cyclophosphamide administered to mdx mice between 15 and 45 days of age failed to significantly influence the time course and prevalence of necrosis and regeneration or serum creatine kinase activity. This finding contrasts with previously reported findings of beneficial effects of glucocorticoids in Duchenne muscular dystrophy (DMD). This may indicate that, mechanisms upon which beneficial effects of glucocorticoids depend in DMD, do not operate (sufficiently) in mdx mice.

Effects of tenotomy on muscle histology and energy metabolism in normal and dystrophic mice

Creatine, phosphocreatine, ATP and phosphate concentrations were measured in extracts of tibialis anterior and gastrocnemius muscles from 42 normal and 39 dystrophic mice from 1 to 16 weeks of age. No differences were observed at 1 week, prior to the onset of histological abnormalities in dystrophic animals. Creatine and phosphocreatine concentrations were significantly reduced in older dystrophic mice, and phosphate levels were higher, while ATP levels generally did not differ. Tenotomy of gastrocnemius at 1 week prevented the development of dystrophy in this muscle but this was not associated with an increase in phosphogen concentrations. Serum creatine kinase levels were significantly higher in dystrophic mice than normal mice but only during the first two weeks of life; levels in older mice were not significantly different. This study shows that the reported deficits in phosphogen concentrations in dystrophic muscles are likely to reflect the results, rather than the cause, of the dystrophic process.

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.

Early immobilization of hindleg muscles of dystrophic mice: short-term and long-term effects

Hindleg muscles of normal and dystrophic mice were immobilized unilaterally early during the postnatal period. After 1 week the casts were removed. Of one group of mice hindleg muscles were processed for histopathological and morphometrical evaluation at the end of the immobilization period. Hindlegs of other groups of mice were remobilized for various periods of time before the muscles were examined. In normal mice immobilization of calf muscles that were fixed in a shortened position resulted in atrophy of about 35% compared with untreated muscles. This was accompanied by a reduction in fibre number of about 15%. The antagonists that had been fixed extended, did not show those effects. Immobilization of dystrophic muscles minimized pathology in both agonists and antagonists, although atrophy developed. Upon remobilization the normal muscles resumed postnatal development. They did not deviate from normal, untreated muscles at the age of 3.5 months. Upon remobilization of dystrophic muscles pathology developed, but less severely than during the second and third postnatal week in untreated dystrophic muscles. Significant differences in morphometrical parameters compared with untreated dystrophic muscles were observed during the 3 months remobilization period studied.

Regeneration of dystrophic muscle following multiple injections of bupivacaine

Regenerated myofibers formed subsequent to orthotopic transplantation of young, dystrophic mouse muscle fail to display the extensive histopathological changes characteristics of murine dystrophy. In order to determine whether this modification of the phenotypic expression of murine dystrophy is unique to the transplantation system or whether it can be found when other extreme trauma induces dystrophic muscle to regenerate, the extensor digitorum longus muscles of 4-6-week-old normal (129 ReJ +/+) and dystrophic (129 ReJ dy/dy) mice were given two series of injections of the myotoxin bupivacaine, spaced 12 hours apart. These injections resulted in necrosis of approximately 90% of the original myofibers. At 100 days after injection, the regenerated normal muscle appeared "healthy," whereas the regenerated dystrophic muscle displayed histopathological changes. It is suggested that the differences in the time course of innervation of the myotubes in the transplantation system as compared with that in the bupivacaine system may be a factor in determining whether regenerated dystrophic myofibers express a dystrophic morphology.

Passive stretch of adult chicken muscle produces a myopathy remarkably similar to hereditary muscular dystrophy

The wings of 10 chickens between 1 and 5 years of age were passively extended. An increase in plasma creatine phosphokinase activity was observed in 30 min, continued to rise for 24 h, and then declined, suggesting mechanically induced damage to muscle fibers. Wing muscles were removed and examined histologically at various times after stretch. Patagialis muscles, but not biceps brachii, showed the development of muscle fiber pathology. The patagialis muscle is less active than the biceps brachii and is stretched to a greater degree by wing extension. Susceptibility of muscles to development of pathology appeared to be correlated with the age of the chickens. Pathology was remarkably similar to that observed in young chickens with hereditary muscular dystrophy. Necrotic fibers exhibiting segmental necrosis, abnormal shapes, enlargement, splitting, vacuolation, and phagocytosis were evident. Of particular interest was the appearance of abnormal clusters of acetylcholinesterase activity along the sarcolemma. These sites were shown to appear on fibers of 2-week-old dystrophic chicks prior to necrosis and increase in plasma creatine phosphokinase activity. It is suggested that aging of inactive muscles may promote adhesions between muscle fibers rendering them susceptible to damage when stretched and that necrosis of dystrophic fibers may be initiated by a similar mechanism. Such could occur if the genetic defect resulted in interfiber adhesions. Support for this hypothesis by other reports in the literature is discussed.

Comparison of fiber size and phenotypic gene expression in muscles of dystrophic C57BL/6J DY2J/DY2J mice

Considerable evidence has shown a correlation between fiber size and degree of necrosis in dystrophic muscles of hamster, X-linked muscular dystrophy (MDX) mice and humans. It has been proposed that small-caliber fibers have an immunity to the phenotypic expression of the dystrophic gene(s). The results from the present study show a discordance between fiber size and necrosis in dystrophic muscles of C57BL/6J dy2J/dy2J mice. Extensor carpi radialis longus and brevis muscles (ECRL and ECRB respectively) were compared in normal and dystrophic 2-week, 4-week and 12-month animals by measuring the mean cross-sectional area of type II fibers, determination of relative proportions of types IIA and IIB fibers and calculation of percentage of fibers exhibiting centronucleation in an entire cross-section of muscle (stained for haematoxylin and eosin or ATPase). The ECRL and ECRB muscles were found to have identical sizes of fiber at each of the 3 ages studied and similar proportions of fiber types, yet the former muscle developed and retained significantly more necrosis (manifest as centronucleation) than the latter.

Electron microscopic and autoradiographic characterization of hindlimb muscle regeneration in the mdx mouse

The pattern of postnatal growth and development of skeletal muscle in mdx mice was studied by light and transmission electron microscopy and by autoradiography and was compared with that in their normal age-matched controls at 4 and 32 weeks of age. The muscle weights of both the extensor digitorum longus (EDL) and soleus muscles of mdx mice were significantly greater than those in control mice at both ages. Body weights of male and female mdx mice were also increased over controls up to 12 weeks of age. At 4 weeks, both the EDL and soleus muscles exhibited focal areas of degeneration, necrosis, and regeneration of centrally nucleated extrafusal fibers resulting in a wide range of fiber sizes. By 32 weeks, the majority of fibers in both muscles were centrally nucleated, and focal areas of recent regeneration were observed. By electron microscopy, the course of macrophage infiltration into areas of degenerating fibers and the ongoing regeneration of myofibers within redundant cylinders of external lamina were noted. This pattern was frequent in 4-week-old mdx muscles and was present to a lesser degree at 32 weeks. A notable lack of both adipose tissue infiltration and fibrotic change in the endomysium were observed in muscles at both ages. Autoradiograms of muscles from 4-week-old mdx mice injected with tritiated thymidine showed an increased proportion of labeled sublaminal nuclei at 24 and 48 hours after injection compared to controls. At 32 weeks of age, labeling of nuclei in muscles of mdx mice was also greater than in controls, but was reduced compared to muscle labeling in 4-week-old mdx mice. The observed features of mdx muscle tissue suggest that this animal model is more applicable to the study of regeneration dynamics than to Duchenne-type human muscular dystrophy.

Small-caliber skeletal muscle fibers do not suffer deleterious consequences of dystrophic gene expression

In Duchenne dystrophy and in the genetic dystrophies of CHF-147 hamsters and MDX mice, the fundamental deleterious consequence of dystrophic gene expression is segmental necrosis of skeletal muscle fibers. The nature of the gene defects and the pathogenesis of muscle fiber damage are not known. However, clinical and experimental evidence indicates that muscle fibers, whose girth is below a certain level (estimated at approximately 20-25 microns in diameter in dystrophic hamsters and MDX mice) are not susceptible to necrosis. This apparent "immunity" has been observed in muscle fibers that are naturally of small girth (such as those in extraocular muscles), and in fibers that were prevented from growing normally by experimental procedures (hamsters and mice) or by pathological processes (Duchenne patients). The cellular or molecular basis by which small-caliber muscle fibers are resistant to the necrotizing effect of the dystrophic gene expression remains unknown. In small-caliber muscle fibers, the normal contraction-related mechanical strains per unit surface area are relatively less than in larger fibers; this could explain their relative resistance to necrosis in some dystrophies.

Skeletal muscle pathology in X chromosome-linked muscular dystrophy (mdx) mouse

Histological, histochemical, and morphometric analyses were performed chronologically on muscles from mutant mice with X chromosome-linked muscular dystrophy (mdx), and the findings were compared with those in nondystrophic control animals (C57BL/10ScSn). Massive grouped muscle fiber destruction, followed by complete regeneration, occurred abruptly at 20 days of age. There were no preceding changes in body weight, the number and mean diameter of fibers, and fiber type differentiation before the initial episode of muscle fiber necrosis. Muscle fiber necrosis decreased in intensity after 60 days of age. Even after repeated muscle fiber necrosis and regeneration, the most striking finding was that interstitial fibrosis and adipose tissue replacement were minimal, and there was no apparent fiber loss. Since the necrosis was probably well compensated by the active regenerative process, the mdx mice developed no obvious muscle weakness and thus differed from human and other animal muscular dystrophies with the exception of the dystrophic hamster.

Phosphorylation of the sarcolemma of dystrophic and normal hamster muscle following denervation

The rapid phosphorylation at 0 degree C of sarcolemma preparations of hamster leg muscle was compared with (Na+,K+)-ATPase activity in sham-operated and 7-day denervated muscle. The phosphorylation appeared to be under the trophic influence of the sciatic nerve since the denervated preparations exhibited a markedly reduced phosphorylation. In similar studies using dystrophic hamsters the sarcolemma preparations from sham-operated and denervated leg muscle both exhibited the same degree of phosphorylation.

Hypophysectomy mitigates skeletal muscle fiber damage in hamster dystrophy

Ablation of the pituitary gland by a radiofrequency lesion markedly retarded the musculoskeletal growth of young dystrophic hamsters. The prevalence of centronucleated muscle fibers, which is a reliable cumulative index of the microscopic pathological expression of dystrophy, was drastically reduced in quadriceps muscles of 35- and 45-day-old hypophysectomized dystrophic hamsters, compared with sham-operated controls. Mitigation of skeletal muscle fiber damage by musculoskeletal growth retardation may also occur in human dystrophy.

Physiologic and histologic features of muscle development in the hamster

This study determined to what extent the hind limb muscles of hamsters resemble those of other mammals in undergoing changes in physiologic, morphologic, and histochemical properties as a function of age. Maximal isometric twitch and tetanic responses were evoked in soleus and plantaris muscles of hamsters aged 13 days to 6 months; all experiments were conducted in vivo under sodium pentobarbital anesthesia. In keeping with findings in the cat and rat, both hamster muscles had relatively prolonged twitches in the youngest animals; the twitches became briefer during development, that of plantaris having a minimum mean contraction time of 15.4 +/- 2.4 ms at 20 days and that of soleus, 28.3 +/- 3.5 ms at 46 days. In both muscles there was a subsequent slight prolongation of the twitch. The two muscles had similar masses at 13 and 20 days; thereafter the plantaris became considerably larger and stronger than the soleus and developed more tetanic tension per unit cross-sectional area. In keeping with its briefer contraction, plantaris had a more rapid rate of rise of tetanic tension than soleus and was more susceptible to fatigue; whereas the soleus developed depression of the twitch after a tetanus, the plantaris exhibited potentiation. Histological and histochemical studies showed that the plantaris had significantly more muscle fibers than the soleus and a much greater proportion of type II fibers (91 and 39%, respectively, in 120- to 180-day-old animals). Whereas the type II fibers had similar cross-sectional areas in the two muscles, the type I fibers were significantly smaller in plantaris than in soleus.

Stretch-induced growth in chicken wing muscles: nerve-muscle interaction in muscular dystrophy

Skeletal muscle growth following denervation and denervation plus passive stretch was characterized in the patagialis muscle of normal and dystrophic chicks until 8 wk of age. In both genotypes, muscles denervated at 1 wk of age grew at reduced rates compared with contralateral control muscles whether or not they were passively stretched. Histograms of fiber size distributions as well as morphological criteria showed that passive stretch of denervated dystropic muscles substantially delayed the development of pathology. Denervation alone provided less protection. There was no evidence of fiber necrosis in any denervated dystrophic muscle, although many fibers did exhibit extreme hypertrophy and abnormal morphology. When denervated dystrophic muscles were allowed to reinnervate, growth and development of pathology was rapid. Because denervation, denervation with passive stretch, or passive stretch alone retards, but does not prevent, the development of pathology, it is concluded that dystrophy in the chick is a myogenic defect that is exacerbated by neurally mediated contractile activity.

Role of mechanical damage in pathogenesis of proximal myopathy in man

No adequate explanation has as yet been provided for the predominant proximal and symmetrical distribution of skeletal muscle weakness and wasting in human myopathies. One obvious difference between proximal and distal muscles is that in their postural antigravity role the former are involved in eccentric contractions to a much greater extent than are the latter. Recent physiological studies have shown that eccentric contractions produce considerable muscle damage in normal healthy subjects. The damage starts in individual sarcomeres but becomes more extensive over 1-2 days, the progression probably being due to the stronger sarcomeres stretching the weaker, damaged sarcomeres during normal activity and/or to the enzyme degradation of myofibrillar proteins when muscle damage results in Ca++ inflow. As a muscle becomes weaker and unable to meet the functional demands made upon it the likelihood of accidental stretch becomes greater. The vicious circle of weakness, stretch, damage, and further weakness may be the reason why the proximal muscles, which normally function eccentrically to some degree, are the most severely affected in a wide range of myopathic disorders.

Reinnervation is followed by necrosis in previously denervated skeletal muscles of dystrophic hamsters

Hind leg muscles of dystrophic hamsters were continually denervated by multiple crushes of the sciatic nerve to as long as 93 days of age. In these muscles, the prevalence of centronucleated fibers which is a cumulative index of prior necrosis, remained very low. In control dystrophic muscles the prevalence of centronucleated fibers increased steadily to approximately 80% where it leveled off. By omitting further crushes in other groups of animals, previously denervated muscles became adequately reinnervated. In the reinnervated muscles the prevalence of centronucleated fibers steadily increased throughout the necrotic phase of dystrophy at a rate that was comparable to corresponding stages of the natural history of the disease. These experiments indicated that continued denervation was effective in negating skeletal muscle fiber necrosis throughout the necrotic phase and that the electromechanical activity of muscle fibers which allows muscle fiber necrosis was not a time-locked factor.