Laminin alpha2 (
merosin)-deficient congenital muscular dystrophy (CMD) patients show progressive muscle fiber
necrosis and ineffective muscle regeneration. This is probably due to decreased formation of multi nucleated myotubes resulting from a myoblast fusion defect. When receiving a mechanical signal from muscle membranes, a cascade of RhoA,
focal adhesion kinase (FAK), and
serum response factor (SRF) positively regulates myogenesis and muscle
hypertrophy associated with functional overload. In contrast,
myostatin, a potent negative regulator of skeletal muscle
hypertrophy, appears to be up-regulated in the muscles of mdx mice, an animal model for
Duchenne muscular dystrophy. Using Western blot and immunohistochemical analyses, we investigated the levels of RhoA, FAK, SRF, and
myostatin in the skeletal muscles of dy mice. The amount of
RhoA protein was increased in the hindlimb muscles of dy mice aged 12 weeks. At 12 weeks, FAK immunoreactivity was observed in the myonuclei and/or satellite cells of normal mice, but not of dy mice. SRF
protein levels decreased markedly in the gastrocnemius and rectus femoris muscles of dy mice at 2 and 12 weeks. Several muscle fibers in normal mice possessed uniform SRF immunoreactivity in the cytoplasm. An SRF immunostaining pattern in muscle was not detected in dy mice. Western blot and the densitometric analysis showed a decreased amount of
myocyte enhancer factor 2C (MEF2C) in hindlimb muscles of dy mice. Although slight
myostatin immunoreactivity was observed in the nuclei of some normal mice, marked
myostatin immunoreactivity was observed in the cytoplasm of mature dy mice myonuclei and/or satellite cells. A low expression of FAK, SRF and MEF2C in muscles of dy mice may inhibit postnatal muscle
hypertrophy by fusing satellite cells with existing fibers. Enhancing
myostatin protein would result in further
atrophy and degeneration of muscle fiber in dy mice.