Early changes in muscle fiber size and gene expression in response to spinal cord transection and exercise

Muscles of spinal cord-transected rats exhibit severe atrophy and a shift toward a faster phenotype. Exercise can partially prevent these changes. The goal of this study was to investigate early events involved in regulating the muscle response to spinal transection and passive hindlimb exercise. Adult female Sprague-Dawley rats were anesthetized, and a complete spinal cord transection lesion (T10) was created in all rats except controls. Rats were killed 5 or 10 days after transection or they were exercised daily on motor-driven bicycles starting at 5 days after transection and were killed 0.5, 1, or 5 days after the first bout of exercise. Structural and biochemical features of soleus and extensor digitorum longus (EDL) muscles were studied. Atrophy was decreased in all fiber types of soleus and in type 2a and type 2x fibers of EDL after 5 days of exercise. However, exercise did not appear to affect fiber type that was altered within 5 days of spinal cord transection: fibers expressing myosin heavy chain 2x increased in soleus and EDL, and extensive coexpression of myosin heavy chain in soleus was apparent. Activation of satellite cells was observed in both muscles of transected rats regardless of exercise status, evidenced by increased accumulation of MyoD and myogenin. Increased expression was transient, except for MyoD, which remained elevated in soleus. MyoD and myogenin were detected both in myofiber and in satellite cell nuclei in both muscles, but in soleus, MyoD was preferentially expressed in satellite cell nuclei, and in EDL, MyoD was more readily detectable in myofiber nuclei, suggesting that MyoD and myogenin have different functions in different muscles. Exercise did not affect the level or localization of MyoD and myogenin expression. Similarly, Id-1 expression was transiently increased in soleus and EDL upon spinal cord transection, and no effect of exercise was observed. These results indicate that passive exercise can ameliorate muscle atrophy after spinal cord transection and that satellite cell activation may play a role in muscle plasticity in response to spinal cord transection and exercise. Finally, the mechanisms underlying maintenance of muscle mass are likely distinct from those controlling myosin heavy chain expression.

A role for the ETS domain transcription factor PEA3 in myogenic differentiation

Activation of adult myoblasts called satellite cells during muscle degeneration is an important aspect of muscle regeneration. Satellite cells are believed to be the only myogenic stem cells in adult skeletal muscle and the source of regenerating muscle fibers. Upon activation, satellite cells proliferate, migrate to the site of degeneration, and become competent to fuse and differentiate. We show here that the transcription factor polyomavirus enhancer activator 3 (PEA3) is expressed in adult myoblasts in vitro when they are proliferative and during the early stages of differentiation. Overexpression of PEA3 accelerates differentiation, whereas blocking of PEA3 function delays myoblast fusion. PEA3 activates gene expression following binding to the ets motif most efficiently in conjunction with the transcription factor myocyte enhancer factor 2 (MEF2). In vivo, PEA3 is expressed in satellite cells only after muscle degeneration. Taken together, these results suggest that PEA3 is an important regulator of activated satellite cell function.

Elevated levels of albumin in soleus and diaphragm muscles of mdx mice

Muscle damage is often associated with an influx of extracellular fluid containing albumin into the muscle. Muscles affected by muscular dystrophy undergo severe muscle damage; therefore, the hypothesis was tested that muscles of dystrophic (mdx) mice contain elevated levels of albumin. Albumin levels in diaphragm (DIA) and soleus (SOL) muscles of control and mdx mice were measured at 3 months and 1 year of age. Albumin in mdx DIA at 1 year of age was twice that of control. In mdx SOL at 1 year of age albumin was increased 25% compared with control. The increase in albumin correlates well with the decline in function in mdx DIA and SOL muscles. Electron microscopy of muscles suggests that albumin is co-localized with transverse tubules of muscle fibers and thus may be mainly located in extracellular fluid. We conclude that albumin is elevated in muscles affected by muscular dystrophy and suggest that this may be of clinical importance in view of substances bound to albumin under physiological conditions.

Exercise and clenbuterol as strategies to decrease the progression of muscular dystrophy in mdx mice

The effects of exercise and the combination of exercise and clenbuterol on progression of muscular dystrophy were studied in mdx mice. At 3 wk of age, mdx mice were randomly assigned to sedentary (MS), exercise (ME), or combined exercise and clenbuterol (MEC) groups. Clenbuterol was given in the drinking water (1.0-1.5 mg . kg body weight-1 . day-1), and exercise consisted of spontaneous running activity on exercise wheels. At 3 mo or 1 yr of age, ventilatory function, contractile properties, and morphological characteristics of the soleus (Sol) and diaphragm (Dia) muscles were measured. The mdx mice receiving clenbuterol ran less than the mice without clenbuterol. The combination of clenbuterol and exercise was associated with an increase in Sol muscle weight and a muscle weight-to-body weight ratio of 30-35% compared with the sedentary group and approximately 20% compared to exercise alone. Myosin and total protein concentrations of the Sol and Dia increased in the MEC group at 1 yr of age only. Normalized active tension was increased in the Dia at 1 yr of age in both the ME and MEC groups by approximately 30%. Absolute tetanic tension of the Sol was increased at both 3 mo and 1 yr of age in the MEC compared with the MS group. At 1 yr of age, there was an additional 23% increase compared with the ME group. Fatigability increased in the MEC group by approximately 25% in the Sol and Dia muscles at both ages compared with the MS and ME groups. Results indicate that exercise and exercise plus clenbuterol decrease the progression of muscular dystrophy. However, different mechanisms may be involved because the combination of clenbuterol and exercise resulted in increased fatigability and the development of deformities, whereas exercise alone did not. Therefore, clenbuterol may not be suitable for use in patients with muscular dystrophy.

Beneficial versus adverse effects of long-term use of clenbuterol in mdx mice

Long-term administration of the beta 2-adrenergic agonist clenbuterol in mdx mice was used to test the hypothesis that increasing contractile protein content in skeletal muscle will decrease the progression of muscular dystrophy. C57BL/10SNJ (control) and dystrophic (mdx) mice were given clenbuterol (1.0-1.5 mg/kg body weight/day) in the drinking water. Ventilatory function and morphological and functional characteristics of soleus (SOL) and diaphragm (DIA) muscles were evaluated. Clenbuterol administration was associated with increased SOL muscle weight, and SOL muscle weight to body weight ratio in control and mdx mice at both ages. There was a 22% increase in myosin concentration of mdx DIA at 1 year of age, correlating well with increased normalized active tension in mdx DIA at this age. Also, absolute tetanic tension increased in control and mdx SOL with clenbuterol at both ages. Ventilatory function was significantly impaired in mdx mice at both ages and clenbuterol administration did not alleviate this. Clenbuterol treatment was associated with a 30-40% increase in fatigability in DIA and SOL muscles of control and mdx mice at both ages. Furthermore, 1-year-old mdx mice receiving clenbuterol exhibited deformities in hindlimbs and spine. These results suggest that long-term clenbuterol treatment has a positive effect on muscle growth and force generation, but has adverse side effects such as increased muscle fatigability and development of deformities.

Voluntary exercise decreases progression of muscular dystrophy in diaphragm of mdx mice

Effects of voluntary wheel running on contractile properties of diaphragm (DIA) and soleus (SOL) of dystrophic (mdx) and control (C57BL/10SNJ) mice were evaluated. In particular, we tested the hypothesis that daily voluntary running is not deleterious to muscle function in mdx mice. Both groups of mice ran extensively (control mice approximately 7 km/day, mdx mice approximately 5 km/day). Exercise increased maximal specific tetanus tension of mdx DIA from 1.02 +/- 0.04 to 1.33 +/- 0.06 kg/cm2 but did not restore it to the control level (2.55 +/- 0.17 kg/cm2). Maximal tetanus tension of sedentary mdx SOL (2.41 +/- 0.17 kg/cm2) was reduced compared with control (3.10 +/- 0.15 kg/cm2) and was not altered by running activity. Optimal length was significantly lower in DIA of mdx mice, and exercise did not change this. Fatigability and contractile properties of muscles measured in vitro were not altered by running activity with the exception of increased contraction time in mdx DIA. In conclusion, extensive wheel running is not deleterious to muscle function in mdx mice contrary to predictions of the "work overload" theory of muscular dystrophy. Rather, this exercise is beneficial for active tension generation of mdx DIA, the muscle most closely resembling muscles of patients with Duchenne muscular dystrophy.

Does muscular dystrophy affect metabolic rate? A study in mdx mice

In this study metabolic consequences of muscular dystrophy were investigated using the mdx mouse model. Measurements were performed on C57BL/10SNJ (control) and dystrophic (mdx) mice of ages 4-6 weeks (young) and 1 year (adult), i.e. at times when muscle degeneration and regeneration are known to be high (young) and low (adult). Whole body metabolic rate (MR) was measured indirectly under usual living conditions by recording O2 consumption and CO2 production over 24 h. Physical activity of mice was measured simultaneously. Oxygen consumption of soleus (SOL) and extensor digitorum longus (EDL) muscles of control and mdx mice was recorded in vitro, using polarographic O2 electrodes. MR in young mdx was significantly decreased compared to young control, but no differences were found in adults. Also, food consumption and physical activity of mdx were decreased significantly compared to control in young but not in adult mice. There was no difference in resting oxygen consumption of muscles from young mdx and control mice, but oxygen consumption of EDL from adult mdx was less than control. Results suggest that muscular dystrophy results in decreased rate of energy metabolism mainly as a consequence of decreased physical activity. The extensive muscular degeneration and regeneration characteristic of muscular dystrophy therefore do not appear to lead to an increase in whole body metabolism.

Differential expression of muscular dystrophy in diaphragm versus hindlimb muscles of mdx mice

Contractile properties of diaphragm (DIA) from mdx and control mice were compared with those of hindlimb muscles [soleus (SOL) and extensor digitorum longus (EDL)] in vitro. Mice ranged in age from 2 weeks to 1.5 years. Muscles were directly stimulated and properties measured were: contraction time, half-relaxation time, active tension per unit area, fatigue index, and maximal velocity of shortening (Vmax). Active tension decreased significantly with age in mdx DIA but not in control DIA. SOL and EDL active tensions were less in mdx than control over the whole age range and did not decrease with age. Vmax was decreased in mdx DIA, but not in mdx SOL or EDL. These results demonstrate that DIA is more affected by muscular dystrophy than hindlimb muscles. Since many Duchenne patients exhibit respiratory distress, this differential expression of dystrophy in diaphragm, as compared to limb muscles, may have important clinical implications.