Markers and Regulators Defining the Development of Hematopoietic Stem Cells and their Niches

The future of personalized regenerative cellular medicine depends on the ability to faithfully differentiate pluripotent cells to tissue stem cells, or reprogram them from other cellular sources. So far, the efforts to generate transplantable multi-potent hematopoietic stem cells (HSC) in vitro have yielded hematopoietic cells with only limited functional potential. Thus, improved understanding of in vivo programs that direct specification of hematopoietic stem versus progenitor cells during embryonic development holds immense clinical implication. However, characterization of the unique properties of these cell populations is currently hindered by the inability to distinguish them prospectively due to the absence of discriminating cell surface markers or genes. Here, we uncovered lymphatic vessel endothelial receptor-1 (LYVE1) as a unique cell surface marker that identifies definitive hematopoietic stem and progenitor cells and their cellular precursor, the hemogenic endothelium, in the yolk sac. Furthermore, we showed that LYVE1-eGFP-hCre knock-in mouse is a powerful tool to separate the primitive erythropoiesis from definitive, even after the cell admixture upon onset of embryonic circulation. Using this tool, we provide in vivo evidence that the earliest progenitors to seed the fetal liver derive from the yolk sac. The mechanisms that establish multilineage differentiation potential in hemogenic endothelium are poorly understood. The vascular endothelial growth factor A (VEGF-A) is essential for endothelial development; however, its role in hemogenic endothelium has not been elucidated. Incorporating several unique mouse models of VEGF-A gene targeting, we document that proper VEGF-A dosage is critical for vascular remodeling and generating multipotent HS/PCs in embryonic hemogenic tissues. Our data support the fact that VEGF-A haploinsufficiency is able to establish primitive erythropoiesis and generate transient myelo-erythroid progenitors from the yolk sac. Moreover, we discover a new cellular source of VEGF-A in placental trophoblasts, and show that trophoblast VEGF-A is important for angiogenesis and hematopoiesis in distant hemogenic organs as well