An electron microscopic study of satellite-cells and regeneration in dystrophic mouse muscle

Laminin and Integrin in LAMA2-Related Congenital Muscular Dystrophy: From Disease to Therapeutics

Laminin-α2-related congenital muscular dystrophy (LAMA2-CMD) is a devastating neuromuscular disease caused by mutations in the LAMA2 gene. These mutations result in the complete absence or truncated expression of the laminin-α2 chain. The α2-chain is a major component of the laminin-211 and laminin-221 isoforms, the predominant laminin isoforms in healthy adult skeletal muscle. Mutations in this chain result in progressive skeletal muscle degeneration as early as neonatally. Laminin-211/221 is a ligand for muscle cell receptors integrin-α7β1 and α-dystroglycan. LAMA2 mutations are correlated with integrin-α7β1 disruption in skeletal muscle. In this review, we will summarize laminin-211/221 interactions with integrin-α7β1 in LAMA2-CMD muscle. Additionally, we will summarize recent developments using upregulation of laminin-111 in the sarcolemma of laminin-α2-deficient muscle. We will discuss potential mechanisms of action by which laminin-111 is able to prevent myopathy. These published studies demonstrate that laminin-111 is a disease modifier of LAMA2-CMD through different methods of delivery. Together, these studies show the potential for laminin-111 therapy as a novel paradigm for the treatment of LAMA2-CMD.

Muscle fiber growth and necrosis in dystrophic muscles: a comparative study between dy and mdx mice

Histopathological and morphometric studies of murine dystrophic muscles in the early postnatal period were performed. In dy mice, obtained by in vitro fertilization, scattered necrotic fibers were noted at 10 days of age followed by poorly compensated regeneration leading to muscle fiber loss, marked variation in fiber size, and fibrous and adipose tissue proliferation. In mdx mice, clusters of necrotic muscle fibers seen at 10-15 days were followed by well compensated regeneration with little fibrosis. There appeared to be no delay in muscle fiber growth and fiber type differentiation in both dy and mdx mice up to 10 days of age since there was no distinct difference in muscle fiber size, fiber type differentiation and the incidence of myosatellite cells between dystrophic muscles and those from age-matched control mice. Muscle fibers appeared to undergo necrosis only when they had reached a certain stage of maturation since at early stages of development or regeneration they rarely became necrotic. The difference in clinical symptoms between dy and mdx mice may result from differences in their regenerative response to necrosis.

Effect of high- and low-dose dexamethasone on regeneration of minced skeletal muscle autografts in rats

Minced skeletal muscle autografts were carried out in 96 male Sprague-Dawley rats weighing 150 to 175 g. Starting the day of surgery, dexamethasone (DEX) was administered parenterally, 3 X/day, at one of the following doses: 0, 0.001, 0.01, 0.1, or 1.0 mg/kg. Separate groups received each dosage for either 7 or 21 days (N = 8 to 16 per group). On day 22, rats were killed and the immature grafts were excised and weighed. The total number of muscle cells was counted on transverse sections. Collagen, dry matter content, and total and noncollagen protein were measured. The wet weights and the total number of muscle cells for 1.0 or 0.1 mg/kg DEX grafts were less than most other groups, after treatment of either 7 or 21 days. In contrast, 0.001 mg/kg DEX for 21 days was associated with increased cell counts. For either treatment duration, collagen content was higher for 0.001 mg/kg DEX. After 21 days DEX, 0.1 and 0.01 mg/kg groups also had more collagen compared with controls. (The 1.0 mg/kg DEX grafts were not assayed for collagen due to insufficient tissue regeneration). In a subsequent trial, rats received 0.001 mg/kg DEX (N = 14) or vehicle control (N = 11) for 21 days. On day 60, the rats were killed and the grafts, now near maturity, were again analyzed for total myofiber number and collagen content, neither of which was significantly different from controls. In summary, DEX in pharmacologic doses was associated with inhibition of muscle graft regeneration if administered for 7 or 21 days. There was evidence that DEX at 100 to 1000 X lower concentrations caused transient increases in the number of proliferating muscle cells and the collagen concentration.

Electron microscopic and histochemical studies on dystrophic masseter muscle

Masseter muscles from normal and dystrophic mice were investigated with the electron microscope and by the use of histochemical techniques for myosin ATPase and succinic dehydrogenase activity. The observations show that the muscle fibers are severely affected in the masseter of the dystrophic mouse.

There is selective accumulation of a growth factor in chicken skeletal muscle. II. Transferrin accumulation in dystrophic fast muscle

Transferrin or a transferrin-like protein, with ability to stimulate myogenesis and terminal differentiation in vitro, is found in fast chicken muscle during embryonic development. After hatching, however, transferrin is no longer accumulated or is only weakly accumulated by fast muscles like the pectoralis major and the posterior latissimus dorsi but continues to be accumulated by slow muscles like the anterior latissimus dorsi. In congenic lines of chickens bearing the gene for muscular dystrophy, however, adult fast muscles do not lose the ability to accumulate transferrin. While transferrin is found selectively in adult normal and dystrophic muscle it does not appear to be synthesized by muscle cells. Immunocytochemical localization shows that transferrin is accumulated not so much by muscle fibers as it is by single cells in the muscle interstitial space. The relationship between transferrin presence and growth patterns in adult skeletal muscle is not currently understood but evidence suggests that transferrin stimulation of myogenesis observed in vitro may be mediated in vivo by non-muscle cells dwelling within the muscle interstitial space. These cells may act as transferrin-uptake sources for subsequent satellite cell stimulation.

Myosatellite cells, growth, and regeneration in murine dystrophic muscle: a quantitative study

Patterns of growth and regeneration in 2-, 4-, 8-, and 17-week-old murine dystrophic (129 ReJ dy/dy) extensor digitorum longus muscles have been determined. Necrosis and myofiber loss, hypertrophy, and regeneration result in a reduced population of myofibers whose diameter distribution is more extensive than that found in the extensor digitorum longus muscles of age-matched normal mice. At the onset of dystrophic symptoms (2 weeks postnatal), the ratio of myosatellite cell nuclei to the total sublaminal nuclear population (myonuclei + myosatellite cells) is similar to that found in 2-week-old control muscles. The frequency of finding myosatellite cells decreases with age in both control and dystrophic muscles. Myosatellite cells account for 11%, 6%, 5%, and 3% of the total sublaminal nuclear population in control muscle and 12%, 8%, 6%, and 5% of the total sublaminal nuclear population in dystrophic muscle at 2, 4, 8, and 17 weeks, respectively. No preferential association of myosatellite cells with myofibers of a particular diameter is found in control muscle or in the two youngest dystrophic groups. At 8 and 17 weeks, myosatellite cells are less frequently encountered on small-diameter, regenerating myofibers of dystrophic muscle, and they are preferentially associated with large diameter, hypertrophied myofibers. The labeling index of myosatellite cells decreases with age in both normal and dystrophic muscle. At all ages the myosatellite cell labeling index is higher in dystrophic muscle (23%, 7%, 5%, and 2% at 2, 4, 8, and 17 weeks, respectively) than in normal muscle (5%, less than 1% at 2 and 4 weeks, respectively), with no labeled myosatellite cells being found in 8- and 17-week-old normal muscles. It is suggested that the magnitude of the regenerative response of dystrophic murine muscle decreases with age and that this factor may be responsible for the inability of the regenerative response of dystrophic muscle to keep pace with the rapid muscle deterioration.

A quantitative study of the muscle satellite cells in various neuromuscular disorders

The regenerative ability of muscles was studied in various neuromuscular disorders by quantitative electron microscopy using two indices of both the satellite cell population and the euchromatin percentage of satellite cell nucleus. Both the number of satellite cells and the euchromatin percentage were increased in polymyositis. Duchenne muscular dystrophy and myotonic dystrophy showed only an increased number of satellite cells without increased euchromatin percentage, while amyotrophic lateral sclerosis had only an increased euchromatin percentage without increased satellite cell number. These results suggest that some defects of satellite cell function probably exist in progressive muscular dystrophy and amyotrophic lateral sclerosis, while in polymyositis the muscle fiber may have the ability to regenerate completely. The euchromatin percentages of myonuclei were increased in polymyositis and Duchenne muscular dystrophy, but not in amyotrophic lateral sclerosis or myotonic dystrophy compared to those of controls. This suggests the activated function of the remaining muscle fibers in polymyositis and Duchenne muscular dystrophy.

Restoration of functional continuity in dystrophic murine muscle after crushing

The recovery of functional continuity after crush injury was measured by a simple electrophsiological technique in semitendinosus muscles of normal and dystrophic mice of the C57BL/6J(dy2Jdy2J) strain. In contrast to virtually complete restitution in normal muscles, only one-third of fibers regained continuity in dystrophic muscles. The study also confirmed the lower resting membrane potentials of dystrophic fibers and the presence of "functional" denervation in some of them.

Regeneration and reinnervation of the dystrophic mouse soleus muscle. A light- and electron-microscopic study

The regeneration and reinnervation of the dystrophic mouse soleus muscle was investigated in response to a double crush-lesion, which causes degeneration of muscle fibres leaving the innervation intact. In normal and dystrophic muscles, injury produced degeneration of muscle fibres, proliferation and fusion of muscle satellite cells, and growth and reinnervation of regenerating fibres. Four, 6 and 21 days after injury, regenerating dystrophic fibres were 50% smaller in cross-sectional area than regenerating normal fibres and showed several pathological changes. Nerve terminal morphology was initially unaffected by the crush, and nerve terminals were associated with degenerating muscle fibres 2 days after injury and with regenerating muscle fibres 6-28 days after crushing. In intact muscles dystrophic endplates were longer and showed increased ultraterminal sprouting compared to normal endplates. At 28 days after crushing normal nerve terminal sprouting was significantly increased compared to the contralateral control. The extent of nerve terminal sprouting and endplate length in dystrophic muscles was not affected by the degeneration and subsequent regeneration of the muscle fibres. We conclude that a proportion of dystrophic mouse soleus muscle fibres can regenerate after a crush when the innervation is left intact.

A quantitative assessment of dystrophic mouse (129 ReJ dy/dy) myogenesis in vitro

A quantitative assessment was made of the myogenic capability in vitro of muscle cells from dystrophic (129 Rej dy/dy) and normal mice from birth to 5 months old. Seeding efficiency was increased in dystrophic cells from neonatal and 1-week-old mice compared to age-matched controls. The extent of myogenesis in cultures from neonatal and 1-week-old dystrophic mice did not differ from controls. Muscle colony formation in cultures established from 5-month-old dystrophic mice was reduced by 80% compared with normal cultures. Normal and dystrophic cultures established from 5-month-old mice contained equal numbers of fibroblast colonies. The results suggest that decreased myogenesis in cultures from 5-month-old dystrophic mice is due to a relative absence of myogenic cells rather than a numerical dilution of the cultures by fibroblasts. This may be due to population of nonviable satellite cells or to necrosis of dystrophic myotubes in vitro.