Induction of initial heart alpha-actin, smooth muscle alpha-actin, in chick pregastrula epiblast: the role of hypoblast and fibroblast growth factor-8.

During heart development at the gastrula stage, inhibition of bone morphogenetic protein (BMP) activity affects the heart specification but does not impair the expression of smooth muscle alpha-actin (SMA), which is first expressed in the heart mesoderm and recruited into initial heart myofibrils. Interaction of tissues between posterior epiblast and hypoblast at the early blastula stage is necessary to induce the expression of SMA, in which Nodal and Chordin are thought to be involved. Here we investigated the role of fibroblast growth factor-8 (FGF8) in the expression of SMA. In situ hybridization and reverse transcription-polymerase chain reaction showed that Fgf8b is expressed predominantly in the nascent hypoblast. Anti-FGF8b antibody inhibited the expression of SMA, cTNT, and Tbx5, which are BMP-independent heart mesoderm/early cardiomyocyte genes, but not Brachyury in cultured posterior blastoderm, and combined FGF8b and Nodal, but neither factor alone induced the expression of SMA in association with heart specific markers in cultured epiblast. Although FGF8b did not induce the upregulation of phospho-Smad2, anti-FGF8b properties suppressed phospho-Smad2 in cultured blastoderm. FGF8b was able to reverse the BMP-induced inhibition of cardiomyogenesis. The results suggest that FGF8b acts on the epiblast synergistically with Nodal at the pregastrula stage and may play a role in the expression of SMA during early cardiogenesis.