Pathways regulating the endothelial-to-hematopoietic transition

During ontogeny, definitive hematopoietic stem and progenitor cells (HSPCs) are generated de novo from a subset of endothelium, the hemogenic endothelium, via a process called endothelial-to-hematopoietic transition (EHT). In this transition, endothelial cells that are an integral part of the vascular wall round up, leave the vessel wall, and become free moving blood cells. To facilitate the visualisation and isolation of hemogenic endothelium throughout this process, we have generated a Runx1 enhancer-reporter mouse model (23GFP), and have begun to dissect the cellular and molecular events that underlie EHT. Our previous work has highlighted that hemogenic endothelium undergoes dynamic cell fate changes already early in development, and identified two distinct stages: a competent stage in which cells readily give rise to endothelial tubules in culture but are able to respond to blood-inducing factors, and a specifying stage characterized by the initiation of a Runx1-driven hematopoietic program, in which endothelial tubule-forming potential is lost and cells begin to commit to the hematopoietic lineage. The mechanisms underlying the transition from competent to specified hemogenic endothelium and the subsequent differentiation of specified hemogenic endothelium into fully committed HSPCs remains poorly understood. Expression profiling by RNA-Seq of the distinct cellular intermediates spanning the developmental window of EHT in the mouse dorsal aorta has identified candidate players and pathways involved in the generation of blood stem and progenitor cells. Of particular interest are transcription factors, cell surface molecules, and the actin cytoskeleton, with preliminary data suggesting a role for Runx1 in the regulation of the cytoskeletal changes that accompany EHT.