Genetic analysis of hematopoiesis in zebrafish: Hemangioblast, stem cell, and erythroid differentiation in the cloche, bloodless, and riesling mutants

SCL is a basic helix-loop-helix (bHLH) transcription factor identified by its involvement in chromosomal translocations associated with acute lymphoblastic leukemia. Targeted disruption of SCL gene in ES cells has established an essential role in hematopoiesis, acting at the stem cell level to specify all blood lineages. Although SCL is also expressed in endothelial and neural progenitors, SCL function in these cells remains unknown. In the zebrafish mutant cloche (clo), SCL expression is abolished in hematopoietic and vascular tissues. Correspondingly, it was previously shown that clo fails to differentiate blood and angioblasts. Genetic analysis demonstrates that the clo mutation is not linked to the SCL locus. Forced expression of SCL in clo embryos rescues the blood and vascular defects, suggesting that SCL acts downstream of clo to specify hematopoietic and vascular differentiation. These results corroborate recent in vivo and in vitro data in, mice that also demonstrate a role for SCL to specify both hematopoietic and endothelial lineages, putatively acting at the hemangioblast level. Moreover, we find that SCL variants unable to bind DNA rescue hematopoiesis from gene-targeted SCL−/− embryonic stem cells and complement hematopoietic and vascular deficits in the zebrafish mutant clo. These results challenge widely accepted dogma that bHLH transcription factors exert their biological function by direct binding to cognate DNA sequences in critical targets genes.