MYOD PROTEIN ACCUMULATES IN SATELLITE CELLS AND IS NEURALLY REGULATEDIN REGENERATING MYOTUBES AND SKELETAL-MUSCLE FIBERS

MyoD belongs to a family of helix-loop-helix proteins that control myo genic differentiation. Transfection of various non-myogenic cell lines with MyoD transforms them into myogenic cells. In normal embryonic de velopment MyoD is upregulated at the time when the hypaxial musculatur e begins to form, but its role in the function of adult muscle remains to be elucidated. In this study we examined the cellular locations of MyoD protein in normal and abnormal muscles to see whether the presen ce of MyoD protein is correlated with a particular cellular behaviour and to assess the usefulness of MyoD as a marker for satellite cells. Adult rats were anaesthetised and their tibialis anterior or soleus mu scles either denervated, tenotomised, freeze lesioned, lesioned and de nervated, or lesioned and tenotomised. At various intervals after the operations the rats were killed and their muscles removed, snap frozen , and sectioned with a cryostat along with muscles from unoperated neo natal and adult rats. The sections were processed for immunohistochemi stry using a rabbit affinity-purified antibody to recombinant MyoD. My oD proved to be an excellent marker for active satellite cells; satell ite cells in neonatal and regenerating muscles contained high levels o f MyoD protein. MyoD positive cells were not observed in the muscles o f old adults, in which the satellite cells are fully quiescent. MyoD i mmunoreactivity was rapidly lost from satellite cell nuclei after they fused into myotubes and was not detected in either sub-synaptic or no n-synaptic nuclei of mature fibers. Denervation, and to a lesser exten t tenotomy, of lesioned muscles induced expression of MyoD in myotubal nuclei. Denervation of normal muscles also upregulated MyoD in muscle fiber nuclei, an effect which was maximal after 3 days. We conclude t hat MyoD protein is neurally regulated in both myotubes and muscle fib ers. (C) 1995 Wiley-Liss, Inc.