Noncoordinate changes in the steady-state mRNA expressed from aldolase A and aldolase C genes during differentiation of chicken myoblasts.

In chickens, as in all vertebrates, tissue-specific expression of aldolase isozymes A, B, and C is developmentally coordinated. These developmental transitions in aldolase expression have been studied most extensively by charting enzyme activity during normal and abnormal development of specific vertebrate tissues. Indeed, aldolase expression has been a key marker for normal differentiation and for retrodifferentiation during carcinogenesis. Aldolase expression during chicken myoblast differentiation offers a model for investigating the regulatory mechanisms of these developmental transitions at the level of gene expression. For these studies, cDNAs encoding the most isozyme-specific regions of both chicken aldolase A and C were cloned. The chicken aldolase A cDNA represents the first report of this sequence. Aldolase steady-state mRNA expression was measured during chicken myoblast differentiation in primary cultures using RNase protection assays with cRNA probes generated from these aldolase cDNA clones. Steady-state mRNA for aldolase C, the predominant embryonic aldolase isozyme in chickens, did not significantly change throughout myoblast differentiation. In contrast, expression of steady-state mRNA for aldolase A, the only aldolase isozyme found in adult-skeletal muscle, was not detected until after myoblast fusion was approximately 50% completed. Aldolase A expression gradually increased throughout myoblast differentiation until approximately 48 h after fusion was completed when there was a dramatic increase. These results are contrasted with those of Turner et al. (1974) [Dev Biol 37:63-89] that showed a coordinated switch in isozyme activities between the embryonic aldolase C and the muscle-specific aldolase A. This discordant expression indicates that the aldolase A and C genes may employ different regulatory mechanisms during myoblast differentiation.