Hematopoietic stem cells (HSC) are generated in the major vasculature of the mouse embryo at embryonic day (E) 10.5 through an endothelial to hematopoietic cell transition (EHT). The genetic programs driving the processes involved in EHT and HSC generation are incompletely known and the role(s) for the pivotal Gata2 transcription factor (expressed in these endothelial and hematopoietic cells) has yet to be fully described.

 We took a conditional knockout approach to examine in what cells and in what cellular processes Gata2 is required. Knockout of Gata2 in VE-cadherin expressing endothelial cells and their hematopoietic cell progeny shows a large decrease in hematopoietic progenitor formation and an absence of HSCs at midgestation. Embryonic lethality occurs at E12.5, two days later than in the germline Gata2 knockout. The number and size of vascular hematopoietic clusters in the aorta, vitelline and umbilical arteries of Gata2^+/- and ^-/- embryos is severely decreased. Knockout of Gata2 subsequent to progenitor and HSC generation in Vav expressing hematopoietic cells results in a later lethality, beginning at E14 with no apparent fetal liver anemia. However, HSC activity is absent. These results demonstrate an initial role for Gata2 in the generation of hematopoietic progenitors and HSCs, and a secondary role in their maintenance and/or expansion.

 To further examine the role of Gata2 at these different developmental stages, a reporter mouse model was created in which normal levels of Gata2 are maintained while allowing the simultaneous temporal and spatial expression of the Venus fluorochrome. Functional assays on sorted cells from tissues of Gata2 Venus embryos demonstrate that all HSC are Gata2 expressing. In contrast, not all hematopoietic progenitors express Gata2 or are Gata2 dependent. Expression of Gata2 in lateral mesoderm and aortic endothelial cells suggests a role for Gata2 in hemogenic endothelium. Taken together, our results suggest that Gata2 expression is dynamic and has different stage and cell specific roles.