Induction and differentiation of the zebrafish heart requires fibroblast growth factor 8 (fgf8/acerebellar)

Vertebrate heart development is initiated from bilateral lateral plate meso derm that expresses the Nkx2.5 and GATA4 transcription factors, but the ext racellular signals specifying heart precursor gene expression are not known , We describe here that the secreted signaling factor Fgf8 is expressed in and required for development of the zebrafish heart precursors, particularl y during initiation of cardiac gene expression. fgf8 is mutated in acerebel lar (ace) mutants, and homozygous mutant embryos do not establish normal ci rculation, although vessel formation is only mildly affected. In contrast, heart development, in particular of the ventricle, is severely abnormal in acerebellar mutants. Several findings argue that Fgf8 has a direct function in development of cardiac precursor cells: fgf8 is expressed in cardiac pr ecursors and later in the heart ventricle, Fgf8 is required for the earlies t stages of nkx2.5 and gata4, but not gata6, expression in cardiac precurso rs, Cardiac gene expression is restored in acerebellar mutant embryos by in jecting fgf8 RNA, or by implanting a Fgf8-coated bead into the heart primor dium, Pharmacological inhibition of Fgf signalling during formation of the heart primordium phenocopies the acerebellar heart phenotype, confirming th at Fgf signaling is required independently of earlier functions during gast rulation, These findings show that fgf8/acerebellar is required for inducti on and patterning of myocardial precursors.