Cardiac BAF complex promotes cardiac progenitors in the zebrafish embryo

Little is known about the earliest events that regulate the behaviour of cardiac progenitor cells fated to form the heart. BAF (Brg/Brm associated factor) complexes are large protein assemblies with chromatin-remodeling activities. These complexes can engage in a number of cell-specific events via differential use of variant subunits. Expression of baf60c, a cardiac specific subunit of the BAF complex, with gata4 and tbx5 has been shown to promote ectopic differentiation of cardiomyocytes in murine embryos. We have used the zebrafish embryo to further examine this cardiac BAF (cBAF) complex. Transplanted cells overexpressing these three factors migrated to the heart-forming region and contribute to myocardium and endorcardium of host embryos at a high frequency. Remarkably, this occurred independent of the location that the cells were placed in the host. Further transplantation experiments using hosts with defects in various germ layers indicate that signal(s) emitted from the endoderm is essential for cBAF complex-driven migration and cardiac differentiation. To determine the endogenous function of cBAF, baf60c together with gata5 and tbx5 were knocked down in the zebrafish embryo through morpholino injection. This led to massive downregulation of myocardial gene expression, with the morphants displaying severe heart defects. In summary, we have recapitulated many of the hallmarks of the earliest cardiac progenitor in vivo. Future studies will be directed at characterizing the mechanisms that govern the activity of the cBAF complex and the behaviors of cBAF-induced cardiac progenitors.