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* Station Centrale de Microscopiè \x83 lectronique, Institut Pasteur, 75015 Paris, France; A null mutation was introduced into the
mouse desmin gene by homologous recombination.
The desmin knockout mice (Des
Faculté de médecine, 75006 Paris, France; § Department of Anatomy, Umëa University, Umëa, Sweden; and
CNRS URA 1960, Institut Pasteur, 75015 Paris, France
/
) develop normally
and are fertile. However, defects were observed after
birth in skeletal, smooth, and cardiac muscles (Li, Z., E. Colucci-Guyon, M. Pincon-Raymond, M. Mericskay, S. Pournin, D. Paulin, and C. Babinet. 1996. Dev. Biol.
175:362-366; Milner, D.J., G. Weitzer, D. Tran, A. Bradley, and Y. Capetanaki. 1996. J. Cell Biol. 134:1255-
1270). In the present study we have carried out a
detailed analysis of somitogenesis, muscle formation,
maturation, degeneration, and regeneration in Des
/
mice. Our results demonstrate that all early stages of
muscle differentiation and cell fusion occur normally.
However, after birth, modifications were observed essentially in weight-bearing muscles such as the soleus or
continually used muscles such as the diaphragm and the
heart. In the absence of desmin, mice were weaker and
fatigued more easily. The lack of desmin renders these
fibers more susceptible to damage during contraction.
We observed a process of degeneration of myofibers, accompanied by macrophage infiltration, and followed
by a process of regeneration. These cycles of degeneration and regeneration resulted in a relative increase in
slow myosin heavy chain (MHC) and decrease in fast
MHC. Interestingly, this second wave of myofibrillogenesis during regeneration was often aberrant and
showed signs of disorganization. Subsarcolemmal accumulation of mitochondria were also observed in these
muscles. The lack of desmin was not compensated by
an upregulation of vimentin in these mice either during
development or regeneration. Absence of desmin filaments within the sarcomere does not interfere with primary muscle formation or regeneration. However,
myofibrillogenesis in regenerating fibers is often abortive, indicating that desmin may be implicated in this
repair process. The results presented here show that
desmin is essential to maintain the structural integrity
of highly solicited skeletal muscle.
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