UPSTREAM SEQUENCES OF THE MYOGENIN GENE CONVEY RESPONSIVENESS TO SKELETAL-MUSCLE DENERVATION IN TRANSGENIC MICE

Myogenin, as well as other MyoD-related skeletal muscle-specific trans cription factors, regulate a large number of skeletal muscle genes dur ing myogenic differentiation. During later development, innervation su ppresses myogenin expression in the fetal hind limb musculature. Dener vation of skeletal muscle reverses the effects of the nerve, and resul ts in the reactivation of myogenin expression, as well as of other emb ryonic muscle proteins. Here we report that myogenin upstream sequence s confer tissue- and developmental-specific expression in transgenic m ice harboring a myogenin/chloramphenicol acetyltransferase (CAT) repor ter construct. Using in situ hybridization to analyze serial sections of E12.5 embryos, we found co-localization of CAT and endogenous myoge nin transcripts in the primordial muscle of the head and limbs, in the intercostal muscle masses, and in the most caudal somites. Later in d evelopment, we observed that the expression of the transgene and endog enous myogenin gene continued to be restricted to skeletal muscle but decreased shortly after birth; a period that coincides with the innerv ation of secondary myotubes. Furthermore, denervation of the mouse hin d limbs induced a 10-fold accumulation of CAT and endogenous myogenin transcripts by 1 day after sciatic nerve resection; a 25-fold increase was observed by 4 days after denervation. Interestingly, we observed that the accumulation of CAT enzyme activity lagged considerably with respect to the increase in CAT transcripts. Our results indicate that the cis-acting elements that temporally and spatially confine transcri ption of the gene during embryonic development, and that mediate the r esponses to innervation and denervation of muscle, lie within the upst ream sequences analyzed in these studies.