Zebrafish mll is essential for haematopoiesis

Studies implicate an important role for the mixed lineage leukemia (Mll) gene in haematopoiesis, mainly through maintaining Hox gene expression. However, the mechanisms underlying Mll-mediated haematopoiesis during embryogenesis remain largely unclear. Here, we investigate the role of mll during zebrafish embryogenesis, in particular, haematopoiesis. Mll depletion caused severe defects in haematopoiesis as indicated by a lack of blood flow and mature blood cells as well as a significant reduction in expression of hematopoietic progenitor and mature blood cell markers. Furthermore, mll depletion prevented the differentiation of hematopoietic progenitors. In addition, we identified the N-terminal mini-peptide of Mll acted as a dominant negative form to disrupt normal function of mll during embryogenesis. As expected, mll knockdown altered the expression of a subset of hox genes. However, overexpression of these down-regulated hox genes only partially rescued the blood deficiency, suggesting that mll may target additional genes to regulate haematopoiesis. In the mll morphants, microarray analysis revealed a dramatic up-regulation of gadd45αa. Multiple assays indicate that mll inhibited gadd45αa expression and that overexpression of gadd45αa mRNA led to a phenotype similar to the one seen in the mll morphants. Taken together, these findings demonstrate that zebrafish mll plays essential roles in haematopoiesis and that gadd45αa may serve as a potential down-stream target for mediating mll function in haematopoiesis.