Expression and neural control of myogenic regulatory factor genes during regeneration of mouse soleus

Given the importance of the myogenic regulatory factors (MRFs) for myoblast differentiation during development, the aims of this work were to clarify the spatial and temporal expression pattern of the four MRF mRNAs during so leus regeneration in mouse after cardiotoxin injury, using in situ hybridiz ation, and to investigate the influence of innervation on the expression of each MRF during a complete degeneration/regeneration process. For this, we performed cardiotoxin injury-induced regeneration experiments on denervate d soleus muscle. Myf-5, MyoD, and MRF4 mRNAs were detected in satellite cel l-derived myoblasts in the first stages of muscle regeneration analyzed (2- 3 days P-l). The Myf-5 transcript level dramatically decreased in young mul tinucleated myotubes, whereas MyoD and MRF4 transcripts were expressed pers istently throughout the regeneration process. Myogenin mRNA was transiently expressed in forming myotubes. These results are discussed with regard to the potential relationships between MyoD and MRF4 in the satellite cell dif ferentiation pathway. Muscle denervation precociously (at 8 days P-l) upreg ulated both the Myf-5 and the MRF4 mRNA levels, whereas the increase of bot h MyoD and myogenin mRNA levels was observed later, in the late stages of r egeneration (30 days P-l). This significant accumulation of each differenti ally upregulated MRF during soleus regeneration after denervation suggests that each myogenic factor might have a distinct role in the regulatory cont rol of muscle gene expression. This role is discussed in relation to the ex pression of the nerve-regulated genes, such as the nAChR subunit gene famil y.