Runx1 is required for the endothelial to haematopoietic cell transition but not thereafter

Abstract

HSCs are the founder cells of the adult hematopoietic system, and thus knowledge of the molecular program directing their generation during development is important for regenerative hematopoietic strategies. Runx1 is a pivotal transcription factor required for HSC generation in the vascular regions of the mouse conceptus - the aorta, vitelline and umbilical arteries, yolk sac and placenta 1, 2. It is thought that HSCs emerge from vascular endothelial cells through the formation of intra-arterial clusters 3 and that Runx1 functions during the transition from ‘hemogenic endothelium’ to HSCs 4, 5. Here we show by conditional deletion that Runx1 activity in vascular endothelial cadherin (VEC) positive endothelial cells is indeed essential for intra-arterial cluster, hematopoietic progenitor, and HSC formation. In contrast, Runx1 is not required in cells expressing Vav, one of the first pan-hematopoietic genes expressed in HSCs. Collectively these data show that Runx1 function is essential in endothelial cells for hematopoietic progenitor and HSC formation from the vasculature, but its requirement ends once or before Vav is expressed.

A short history of hemogenic endothelium

“…However, hematopoietic cell clusters that protrude from the endothelium have been observed not only here, but also in other hemogenic sites, including the vitelline and umbilical arteries, yolk sac, and placenta [23,26,[61][62][63]. Lineage tracing indicates that EHT also occurs in those tissues [27,28,64]. All this brings to mind the suggestion made 100 years ago, that hemogenic activity is an inherent property of all young, undifferentiated embryonic endothelium [22,23,62].…”

“…The first in vivo experimental support came much later, from lineage tracing of endothelium in the chick embryo [25,26]. More recently, lineage-tracing studies in which Cre recombinase was expressed under the regulatory control of the endothelial marker VE-Cadherin (VECadh) lent further support to this model [27,28]. Arguably, the strongest support for an endothelial origin of hematopoietic cells came from the recent time-lapse imaging studies that visualized the transition of endothelium into blood in real-time, both in vitro [29] and in/ex vivo [30][31][32][33].…”

“…In the absence of Runx1, hemogenic endothelium is still made but no definitive hematopoietic cells are formed [51][52][53][54][55][56]. Deletion of Runx1 in VE-Cadh + cells indicated that Runx1 is critically required in this population [28]. Although Runx1 expression continues in the emerging HSPCs and their offspring, spatiotemporal differences in the requirement for Runx1 and other members of the core-binding factor family were reported for these cells [28,57].…”

“…Deletion of Runx1 in VE-Cadh + cells indicated that Runx1 is critically required in this population [28]. Although Runx1 expression continues in the emerging HSPCs and their offspring, spatiotemporal differences in the requirement for Runx1 and other members of the core-binding factor family were reported for these cells [28,57]. To obtain a transgenic Runx1 reporter that would allow for the positive identification of hemogenic endothelium, we had initiated studies into the transcriptional regulation of Runx1 and recently identified the Runx1 +23 enhancer.…”

“…Importantly, SCL is not required once the hemogenic endothelium has formed, placing it upstream of Runx1 [28,50,90]. Recently, loss of SCL was shown to initiate a cardiac-related transcriptional program in the yolk sac, indicating that SCL expression represses the cardiac program [89].…”