MRF4 protein expression in regenerating rat muscle

The myogenic transcription factors of the MyoD family are not expressed in normal adult skeletal muscle. They are upregulated at the transcript and pr otein levels in a precisely coordinated manner during regeneration. While t he cellular distribution of MyoD, myf-5, and myogenin expression in regener ating muscle is well documented, little is known about the exact localizati on of MRF4. It was the aim of this study to monitor the cellular distributi on of MRF4 protein during regeneration. The soleus muscle of 6-week-old mal e Wistar rats was devascularized and allowed to regenerate for 2, 5, 10 or 14 days. Immunostaining revealed the presence of MRF4 throughout the time p eriods studied. Expression was detected in the nuclei of myofibers which ha d survived the devascularization procedure 2 days after necrosis was induce d. In nuclei of newly formed myotubes and young myofibers, MRF4 was co-expr essed with MyoD and myogenin. MRF4 protein was absent from satellite cells (SC), with anti-M-cadherin being used as a SC marker. Taken together, our r esults demonstrate that MRF4 protein expression in regenerated fibers is re stricted to the time around and after fusion. The absence of MRF4 protein i n SC suggests that the role of MRF4 during regeneration is distinct from my f-5, MyoD, and myogenin.