Expression of aldolase A steady-state mRNA is delayed relative to other muscle-specific genes during differentiation of chicken myoblasts.

Expression of several muscle-specific genes was monitored during chicken muscle development and myoblast differentiation in primary cultures. The individual patterns of expression for many muscle-specific genes are well documented in ovo and in other model systems of muscle development. However, comparison of aldolase A to other muscle-specific genes in one system has not been reported. Both sarcomeric and cytosolic genes important for the adult muscle fiber were examined in order to elucidate their timing of expression and its relationship to cell fusion. Steady-state mRNA expression was measured using RNase protection assays with cRNA probes generated from cDNA clones for muscle creatine kinase, fast skeletal troponin-T, embryonic myosin heavy chain, and aldolase A. Nonmuscle genes expressed largely in the embryo, aldolase C and beta-actin, were used as controls. The expression of all six genes revealed differences in temporal expression patterns between limb and axial muscle. The temporal expression patterns of all six genes were also monitored in primary myoblast cultures relative to myoblast fusion. In both axial and limb myoblast cultures most of the muscle-specific genes were expressed prior to fusion. During the differentiation of myoblasts to myotubes there was a biphasic pattern in the expression of the muscle-specific genes. The appearance of measurable mRNA was detected by 16 hr in culture, prior to appreciable fusion of the cells. During further differentiation the expression increased gradually and then more rapidly at 96 hr, once fusion was complete. Meanwhile, the nonmuscle embryonic gene expression declined only slightly. For one gene, aldolase A, expression was delayed relative to the other muscle-specific genes, both in the appearance of measurable mRNA and in the later rapid increase in mRNA.