Denervation induces a rapid nuclear accumulation of MRF4 in mature myofibers

Muscle regulatory factor 4 (MRF4) is a member of the family of myogenic tra nscription factors, including MyoD, myogenin, and myf-5, that are necessary for the commitment and differentiation of mesoderm to skeletal muscle, Alt hough the function of these transcription factors during embryonic developm ent has been demonstrated, their role in adult muscle has remained elusive. Regulation of the MRF4 gene differs from the genes encoding the other myog enic factors in that its transcripts accumulate in neonatal muscle during m aturation and continue to be expressed at relatively high levels in the adu lt, On the basis of its mRNA expression pattern, MRF4 has been suggested to regulate genes encoding adult contractile proteins and acetylcholine recep tor subunits, To test this hypothesis, a specific antiserum was developed t o study MRF4 protein expression in adult innervated and denervated muscle, because MRF4 mRNA levels increase by approximately threefold 1 day after ne rve resection, By using three different immunohistochemical methods that va ry widely in sensitivity, we were unable to detect MRF4 immunoreactivity in adult innervated muscles. The same results were obtained with another MRF4 antiserum generated independently, In contrast, any of these three immunol ogic techniques readily detected MRF4 immunoreactivity in myofiber and sate llite cell nuclei of muscles denervated for 24 hours. The highest proportio n of immunopositive nuclei (80%) was found 2-3 days after denervation, Immu noreactivity was no longer detectable by 14 days. There was no differential accumulation of MRF4 protein in the nuclei of satellite cells nor in sole plate (synaptic) nuclei at any time after denervation. No differences were found in the temporal accumulation of MRF4 in nuclei of type I and type II: denervated myofibers, consistent with the similar distribution of MRF4 mRN As in slow- and fast-twitch muscles. Our results are consistent with the la ck of phenotype observed in the adult muscles of MRF4-null mutant mice obse rved by others and suggest that MRF4 may have important roles in the gene p rograms activated after denervation and during muscle regeneration, Publish ed 2000 Wiley-Liss, Inc.