The Flk1-Cre-mediated deletion of ETV2 defines its narrow temporal requirement during embryonic hematopoietic development.

During embryonic development, the emergence of hematopoiesis and vasculogenesis is tightly associated, with many transcription factors implicated in both developmental processes. Among those factors, ETV2 acts at the top of the hierarchy and controls the formation of both lineages. However, it is not known at which stage of mesoderm development ETV2 is acting and whether ETV2 activity is further required once mesodermal precursors have been specified to the hematopoietic and endothelial fates. In this study, we characterize the developmental window during which ETV2 expression is required for hematopoietic and endothelial development. Using cre-mediated deletion of ETV2, we demonstrate that ETV2 is acting prior to or at the time of FLK1 expression in mesodermal precursors to initiate the hematopoietic and endothelial program. Using the in vitro differentiation of embryonic stem cells as a model system, we further show that ETV2 re-expression in Etv2(-/-) Flk1-negative precursors drives hematopoiesis specification and switches on the expression of most genes known to be implicated in hematopoietic and endothelial development. Among the downstream targets of ETV2, we identify the transcription factors SCL, GATA2, and FLI1 known to operate a recursive loop controlling hematopoietic development. Surprisingly, SCL re-expression in Etv2(-/-) cells fully rescues hematopoiesis, while the re-expression of FLI1 or GATA2 promotes only a very limited rescue. Altogether, our data establish that ETV2 is required very transiently to specify mesodermal precursors to hematopoiesis and vasculogenesis and that SCL is one of the key downstream targets of ETV2 in controlling hematopoietic specification.