Distinct temporal requirements for Runx1 in hematopoietic progenitors and stem cells

Abstract

SUMMARYThe transcription factor Runx1 is essential for the formation of yolk sac-derived erythroid/myeloid progenitors (EMPs) and hematopoietic stem cells (HSCs) from hemogenic endothelium during embryogenesis. However, long-term repopulating HSCs (LT-HSCs) persist when Runx1 is conditionally deleted in fetal liver cells, demonstrating that the requirement for Runx1 changes over time. To define more precisely when Runx1 transitions from an essential factor to a homeostatic regulator of EMPs and HSCs, and whether that transition requires fetal liver colonization, we performed conditional, timed deletions of Runx1 between E7.5 and E13.5. We determined that Runx1 loss reduces the formation or function of EMPs up through E10.5. The Runx1 requirement in HSCs ends later, as deletion up to E11.5 eliminates HSCs. At E11.5, there is an abrupt transition to Runx1 independence in at least a subset of HSCs that does not require fetal liver colonization. The transition to Runx1 independence in EMPs is not mediated by other core binding factors (Runx2 and/or Runx3); however, deleting the common non-DNA-binding β subunit (CBFβ) severely compromises LT-HSC function. Hence, the requirements for Runx1 in EMP and HSC formation are temporally distinct, and LT-HSC function is highly reliant on continued core binding factor activity.

Extrinsic and intrinsic control by EKLF (KLF1) within a specialized erythroid niche

“…8, top). This EMP population arises in the E8.25 yolk sac then seeds the fetal liver by E11.5, providing the first source of definitive erythroid progenitors prior to or coincident with colonization by distinct hematopoietic stem cell (HSC) sources (Chen et al, 2011;Lux et al, 2008;McGrath et al, 2011;Tober et al, 2013).…”

A short history of hemogenic endothelium

“…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.…”

Single-cell resolution of morphological changes in hemogenic endothelium

“…Intra-aortic hematopoietic clusters appear transiently in the AGM region between embryonic days ∼10 and 12 in mouse (de Bruijn et al, 2000) and ∼4 and 6 weeks in human (Tavian et al, 1996). Runx1, a required transcription factor for the conversion of hemogenic endothelial cells to HSPCs (Chen et al, 2009;North et al, 1999), is first noted within a subset of endothelial cells in hemogenic vascular beds but then localizes to intra-aortic cluster cells (Tober et al, 2013). The transcription factor Sox17 has also been shown to be important in EHT (Clarke et al, 2013) and hematopoietic stem cell (HSC) survival (Kim et al, 2007).…”