Runx1+ vascular smooth muscle cells are essential for hematopoietic stem and progenitor cell development in vivo

Hematopoietic stem cells (HSCs) produce all essential cellular components of the blood. Stromal cell lines supporting HSCs follow a vascular smooth muscle cell (vSMC) differentiation pathway, suggesting that some hematopoiesis-supporting cells originate from vSMC precursors. These pericyte-like precursors were recently identified in the aorta-gonad-mesonephros (AGM) region; however, their role in in vivo hematopoietic development remains unknown. Here, we identify a subpopulation of NG2+Runx1+ perivascular cells that display a sclerotome-derived vSMC transcriptomic profile. We show that deleting Runx1 in NG2+ cells impairs in vivo hematopoietic development and causes transcriptional changes in pericytes/vSMCs, endothelial cells and hematopoietic cells in the murine AGM. Importantly, this deletion leads also to a signification reduction of HSC reconstitution potential in the bone marrow in vivo. This defect is developmental, as NG2+Runx1+ cells were not detected in the adult bone marrow, demonstrating the existence of a specialised pericyte population in the HSC-generating niche, unique to the embryo. Overall design: Single-cell transcriptome profiling of E11 AGM samples from NG2 +/+; Runx1 fl/fl (WT) (n=1) and NG2-Cre; Runx1 fl/fl (cKO) (n=1) mice