Hhex and Scl function in parallel to regulate early endothelial and blood differentiation in zebrafish

During embryogenesis, endothelial and blood precursors are hypothesized to arise from a common progenitor, the hemangioblast. Several genes that affec t the differentiation of, or are expressed early in, both the endothelial a nd blood lineages may in fact function at the level of the hemangioblast. F or example, the zebrafish cloche mutation disrupts the differentiation of b oth endothelial and blood cells. The transcription factor gene scl is expre ssed in both endothelial and blood lineages from an early stage and can reg ulate their differentiation. Here we report that in zebrafish the homeobox gene hhex (previously called hex) is also expressed in endothelial and bloo d lineages from an early stage. We find that hhex expression in these linea ges is significantly reduced in cloche mutant embryos, indicating that hhex functions downstream of cloche to regulate endothelial and blood different iation. Ectopic expression of hhex through injection of a DNA construct lea ds to the premature and ectopic expression of early endothelial and blood d ifferentiation genes such as fli1, flk1 and gata1, indicating that Hhex can positively regulate endothelial and blood differentiation. However, analys is of a hhex deficiency allele shows that hhex is not essential for early e ndothelial and blood differentiation, suggesting that another gene, perhaps scl, compensates for the absence of Hhex function, Furthermore, we find th at hhex and scl can induce each other's expression, suggesting that these t wo genes cross-regulate each other during early endothelial and blood diffe rentiation. Together, these data provide the initial framework of a pathway that can be used to further integrate the molecular events regulating hema ngioblast differentiation.