2009 – TRACING BACK THE ANCESTORS OF HEMATOPOIETIC STEM AND PROGENITOR CELLS REVEAL THEIR UNIQUE MOLECULAR CHARACTERISTICS

A majority of studies have focused on understanding how hemogenic endothelial cells (HECs) mature into hematopoietic stem and progenitor cells (HSPCs). HECs and nascent HSPCs can be identified by the co-expression of endothelial and hematopoietic markers. However, most of the inductive signals that initiate the switch from endothelial to hematopoietic fate must occur in a subset of endothelial cells prior hematopoietic genes being expressed. Difficulties labelling the endothelial ancestors of HECs in vivo have resulted in uncharacterized endothelial cell populations that serve has the foundation for hematopoietic development. In addition, the spatial coexistence of these developmental intermediates in the mammalian embryo further challenges their characterization. Here, utilizing the rapid embryonic development of the zebrafish, which results in synchronized developmental transitions, we segregated and characterized these intermediates. Using this system, in conjunction with novel reporter lines, we traced back the endothelial ancestry of HECs. Unexpectedly, we found that these endothelial precursors have distinct molecular characteristics from their surrounding endothelial counterparts, indicating that HECs derive from a specific endothelial pool that differs from the rest of developing endothelial cells (ECs). Quiescence was one of the hallmarks of these HEC precursors, and we showed that p65 activation critically mediated it both in vivo and in vitro. In addition, when quiescence was lost by p65 ablation, HECs failed to specify. On the contrary, enforced quiescence increased the pool of HEC precursors available to transdifferentiate. Our work uncovers in vivo the previously enigmatic biology of the ECs that serve as the foundation for the hematopoietic system. This knowledge could be used to optimize in vitro protocols of HSPC generation and their derivatives.