Role of PDGF-B/PDGFR: signalling in definitive haematopoiesis
Item statusRestricted Access
Embargo end date27/06/2021
Sá Da Bandeira, Diana
The first haematopoietic stem cells (HSCs) are generated in the dorsal aorta (DA) of the midgestation mouse embryo. Signals from the microenvironment are required for HSC generation. However, the signals and identity of cells releasing them in vivo remain unknown. In the present work, we identified at least three populations of perivascular cells surrounding the DA based on the expression of the perivascular cell markers NG2, PDGFRβ and αSMA. NG2+ PDGFRβ+ αSMA+ pericytes/vascular smooth muscle cells (PCs/vSMCs) and NG2- PDGFRβ+ αSMA- sub-pericytes (Sub-PCs) were found to be enriched in HSC-supportive genes described in the adult bone marrow (BM). As both populations express PDGFRβ, which has recently been shown to mediate HSC specification in zebrafish, and PDGF-B/PDGFRβ signalling is also required for the recruitment of pericytes to the developing blood vessel wall, we hypothesised that PDGF-B/PDGFRβ signalling is required to generate the first HSCs in vivo. To answer this question, we used PDGFRβ knock-out (KO) and PDGF-Bret KO mice, both of which have a defective pericyte recruitment to blood vessels and defective or absent PDGF-B/PDGFRβ signalling. Results from our haematopoietic progenitor assays (CFU-Cs) and transplantations show that the germline deletion of PDGFRβ affects both haematopoietic progenitor numbers and HSC activity in the E11 AGM. HSPCs in other haematopoietic organs are not affected at this stage. PDGF-Bret KO mice showed no defects in midgestation HSPCs, but AGM HSCs failed to reconstitute secondary recipients. Together, these data suggest that PDGFB/PDGFRβ signalling is required for AGM haematopoiesis. We found that perivascular stromal cells surrounding the DA are not affected by these mutations, nor the integrity of the blood vessel, suggesting that PDGFB/PDGFRβ signalling is not required for PC/vSMCs recruitment to the DA. We therefore hypothesised that PDGF-B/PDGFRβ signalling is either required in the niche for HSC specification and/or generation, and/or that PDGFRβ+ cells are precursors of HSCs. Tracing experiments using PDGFRβ-Cre;TdTomato mice found that a subset of AGM HSPCs derive from PDGFRβ-Cre precursors and that both Tomato- and Tomato+ E14 foetal liver (FL) and BM cells reconstitute irradiated recipients. Together these data suggest that adult HSCs have distinct developmental origins, one of them deriving from PDGFRβ+ cells. In conclusion, our results define PDGFRβ signalling as a key component of the HSC generating niche in the mouse embryo, and that a subset of HSCs derive from PDGFRβ+ precursors.