Abstract
Hematopoietic stem cells (HSCs) are known to emerge in the dorsal aorta at E10.5 of mouse embryo development. They derive from hemogenic endothelial cells via a process known as endothelial-to-hematopoietic transition (EHT), which comprises a number of intermediate steps and the molecular details of which have not been fully characterized. Our lab has previously demonstrated that embryos deficient for the transcription factor Gata3 have a defect in HSC generation in the dorsal aorta. This was linked to a lack of catecholamine synthesis in these embryos, which demonstrated that neurotransmitters released from the co-developing sympathetic nervous system promote embryonic HSC production. We now have evidence that Gata3 plays an additional role in EHT. Gata3 expression was found to enrich for hemogenic endothelial cell activity, and an endothelial-specific knockout of Gata3 severely impairs HSC generation in the dorsal aorta. Furthermore, with the use of co-aggregation assays developed in the Medvinsky lab, we were able to determine that Gata3 is expressed in the pro-HSC and pre-HSC type I stage, but is downregulated thereafter. RNA-Seq analysis of Gata3-positive and negative cell populations highlighted a cell cycle signature, with cell cycle inhibitors, such as p57Kip2, specifically enriched in Gata3-expressing cells. This suggests that a cell cycle arrest may be required for endothelial cells to complete the transition to a hematopoietic fate, which is supported by our preliminary results with embryos deficient for p57Kip2. Our results thus provide further insights into the molecular mechanisms of HSC generation from endothelial cells.