Mesoderm-derived PDGFRA+ cells regulate emergence of hematopoietic stem cells in the dorsal aorta

During embryonic development, the first hematopoietic stem cells (HSCs) arise from a transient population of endothelial cells lining the ventral surface of the dorsal aorta, via a process of endothelial to hematopoietic transition (EHT) at embryonic day (E) 10.5. This region contains resident mesenchymal stem cell-like cells (MSC-LCs), but their identity and role in HSC generation in the AGM are not well understood. Using a library of compound transgenic mice, we identified a population of PDGFRA+/Nestin-GFP (N-GFP)-/PDGFRB-/CD31- cells with MSC-LC activity in the E10.5 and E11.5 mouse AGM. Freshly isolated MSC-LCs were adept at forming blood vessels with CD31+ luminal endothelium enveloped by PDGFRB+ pericytes, when transplanted subcutaneously into mice. Conditional ablation of PDGFRA+ or Nestin+ cells led to either complete or partial loss of MSC-LCs respectively, with severe loss of endothelial and pericyte-like cells, and concomitant loss of blood formation in the AGM. Lineage tracing studies using tamoxifen induction in PDGFRACreERT2/R26eYFP embryos showed that stromal, sub-endothelial, endothelial and long-term repopulating hematopoietic stem cells (LT-HSCs) cells in the E11.5 AGM were progeny of PDGFRA cells. Using transgenic reporter mice, we showed that MesP1 (mesoderm) derived PDGFRA+ cells dominated the sub-endothelial and deeper ventral stroma in the AGM at E10.5 and E11.5 but were replaced by Wnt1 (neural crest) derived cells at E13.5. Re-aggregation of E11.5 Mesp1 derived MSC-LC cells with E13.5 aortic or adult cardiac non-hemogenic endothelial cells resulted in the generation of endothelial cell derived LT-HSCs. RNA-sequencing analysis of non-hemogenic E13.5 endothelial cells showed up-regulation of EHT genes, WNT, BMP and Notch cell signalling pathways when re-aggregated with E11.5 Mesp1 derived MSC-LCs. LT-HSC generation from these re-aggregates was suppressed by dose-dependent inhibition of PDGF-AA/PDGFRA signalling. Taken together, we report that MSC-LC populations in the AGM are temporally dynamic, and that MesP1-derived MSC-LCs regulate hemogenic potential of the endothelium and that this cooperativity is dependent on PDGF-AA/PDGFRA signalling.This submission represents the RNAseq component of the study.