Sequential expression of chicken actin genes during myogenesis

Abstract

Embryonic muscle development permits the study of contractile protein gene regulation during cellular differentiation. To distinguish the appearance of particular actin mRNAs during chicken myogenesis, we have constructed DNA probes from the transcribed 3' noncoding region of the single-copy α-skeletal, α-cardiac, and β-cytoplasmic actin genes. Hybridization experiments showed that at day 10 in ovo (stage 36), embryonic hindlimbs contain low levels of actin mRNA, predominantly consisting of the α-cardiac and β-actin isotypes. However, by day 17 in ovo (stage 43), the amount of α-skeletal actin mRNA/μg total RNA increased more than 30-fold and represented ~90% of the assayed actin mRNA. Concomitantly, α-cardiac and β-actin mRNAs decreased by 30% and 70%, respectively, from the levels observed at day 10. In primary myoblast cultures, β-actin mRNA increased sharply during the proliferative phase before fusion and steadily declined thereafter. α-Cardiac actin mRNA increased to levels 15-fold greater than α-skeletal actin mRNA in prefusion myoblasts (36 h), and remained at elevated levels. In contrast, the α-skeletal actin mRNA remained low until fusion had begun (48 h), increased 25-fold over the prefusion level by the completion of fusion, and then decreased at later times in culture. Thus, the sequential accumulation of sarcomeric α-actin mRNAs in culture mimics some of the events observed in embryonic limb development. However, maintenance of high levels of α-cardiac actin mRNA as well as the transient accumulation of appreciable α-skeletal actin mRNA suggests that myoblast cultures lack one or more essential components for phenotypic maturation.