PDGFRB SIGNALING IS REQUIRED TO GENERATE AORTIC HAEMATOPOIETIC CELLS IN VIVO

The first hematopoietic stem cells (HSCs) are generated in the aorta-gonads-mesonephros (AGM) region of the mid-gestation mouse embryo. Signals from the microenvironment are required for HSC generation. However, the signals and identity of cells releasing them in vivo remain unknown. The AGM microenvironment includes perivascular cells. Pericytes (PCs) support HSC maintenance in the adult bone marrow, but their involvement in HSC birth has not been studied. PCs are recruited to the developing blood vessel wall via PDGFRβ signaling which was recently shown to mediate HSC specification in zebrafish. We hypothesize that PDGFRβ signaling is required to generate the first HSCs in vivo. To answer this question, we used the PDGFRβ knock-out mice which lack PCs and die perinatally. Results from our hematopoietic progenitor assays (CFU-Cs) and transplantations show that the germline deletion of PDGFRβ affects both hematopoietic progenitor numbers and HSC activity in the E11 AGM. HSPCs in other hematopoietic organs are not affected at this stage. Whether PDGFRβ+ cells are HSPC precursors or the effect seen is only due to a defect in the niche is under investigation. Characterization of E11 wild-type embryos by immunohistochemistry and RNA sequencing show that the dorsal aorta is surrounded by three phenotypically and transcriptionally distinct perivascular cell layers that include PCs (PDGFRβ+NG2+) and PDGFRβ+NG2- adjacent cells. PCs are highly enriched in HSC niche genes described in the adult microenvironment, suggesting a putative role in AGM hematopoiesis. In conclusion, our results define PDGFRβ signalling as key component of the HSC generating niche in the mouse embryo in vivo that could be tested in vitro to derive HSCs from hematopoietic and non-hematopoietic cell sources for therapy.