Sequential activation of alpha-actin genes during avian cardiogenesis: vascular smooth muscle alpha-actin gene transcripts mark the onset of cardiomyocyte differentiation.

Abstract

The expression of cytoplasmic .beta.-actin and cardiac, skeletal, and smooth muscle .alpha.-actins during early avian cardiogenesis was analyzed by in situ hybridization with mRNA-specific single-stranded DNA probes. The cytoplasmic .beta.-actin gene was ubiquitously expressed in the early chicken embryo. In contrast, the .alpha.-actin genes were sequentially activated in avian cardiac tissue during the early stages of heart tube formation. The accumulation of large quantities of smooth muscle .alpha.-actin transcripts in epimyocardial cells preceded the expression of the sarcomeric .alpha.-actin genes. The accumulation of skeletal .alpha.-actin mRNAs in the developing heart lagged behind that of cardiac .alpha.-actin by several embryonic stages. At Hamburger-Hamilton stage 12, the smooth muscle .alpha.-actin gene was selectively down-regulated in the heart such that only the conus, which subsequently participates in the formation of the vascular trunks, continued to express this gene. This modulation in smooth muscle .alpha.-actin gene expression correlated with the beginning of coexpression of sarcomeric .alpha.-actin transcripts in the epimyocardium and the onset of circulation in the embryo. The specific expression of the vascular smooth muscle .alpha.-actin gene marks the onset of differentiation of cardiac cells and represents the first demonstration of coexpression of both smooth muscle and striated .alpha.-actin genes within myogenic cells.