MRF4 gene expression in Xenopus embryos and aneural myofibers

Vertebrate embryos express the transcription factor MRF4 during skeletal muscle differentiation. Previous studies of MRF4 expression in embryonic Xenopus laevis and its response to muscle denervation in adults have led to the suggestion that its transcription may be activated in myotomes and in multinucleate myofibers through an interaction with the motor nerves. We tested this hypothesis by assaying for MRF4 gene transcripts in early neurula stage embryos, beginning before the appearance of neurons. MRF4 transcripts were detectable by reverse transcriptase-polymerase chain reaction (RT-PCR) from at least stage 13-14, well before the differentiation of either nerves or myocytes. We also tested the nerve-dependence of MRF4 gene expression in multinucleate myofibers by comparing transcript levels between interhyoideus muscles in normal larvae and muscles whose motor innervation had been prevented through surgical removal of the brain before cranial motor axon outgrowth. RT-PCR demonstrated similar MRF4 transcript levels in the aneural muscles and controls. These results fail to support the hypothesis that MRF4 gene expression is triggered or is significantly up-regulated in myogenic cells by signals from motor axons.