HDAC activity is required for initiation of primitive hematopoiesis. (A-D) Effect of VPA treatment on the expression of fli1a, lmo2, gata2, and scl. Wild-type embryos at 80% to 90% epiboly stages were incubated in 0.2mM VPA and fixed at indicated stages for examination of markers expression by whole-mount in situ hybridization. Embryos at segmentation stages are shown in posterodorsal view, whereas those at 24 hpf are shown in lateral view. (E) gata1 expression in wild-type embryos or embryos coinjected with 375 pg of scl and 50 pg of mLmo2 mRNAs at the 10-somite stage (posterodorsal view). Fifty percent (14/14) of VPA-treated embryos lacked gata1 expression and the other half showed very weak expression of gata1. In contrast, even in the presence of VPA, co-overexpression of scl and mLmo2 resulted in 15 of 44 of embryos with enhanced gata1 expression and 14 of 44 of embryos with normal expression of gata1, suggesting that scl and lmo2 act downstream of HDAC activity. (F) A model for Mta3-NuRD functions in primitive hematopoiesis. Hematopoietic precursors and angioblasts may be independently specified from the ventrolateral mesodermal cells or may be derived from the common ancestor cell population, hemangioblasts. Mta3-NuRD complex activates the expression of the master hematopoietic transcription factors Scl and Gata2, allowing the commitment of mesodermal cells to the hematopoietic fate. Mta3-NuRD complex may activate Scl and Gata2 expression indirectly by suppressing the expression of their repressors, or directly by remodeling their promoter chromatin or deacetylating repressor proteins.