Smad1 expands the hemangioblast population within a limited developmental window.

Bone morphogenetic protein (BMP) signaling is an important regulator of hematovascular development. However, the progenitor population that responds to BMP signaling is undefined, and the relative role of downstream mediators including Smad1 is unclear. We find that Smad1 shows a distinctive expression profile as embryonic stem (ES) cells undergo differentiation in the embryoid body (EB) system, with peak levels in cell populations enriched for the hemangioblast. To test the functional relevance of this observation, we generated an ES cell line that allows temporal control of ectopic Smad1 expression. Continuous expression of Smad1 from day 2 of EB culture does not disturb hematopoiesis, according to colony assays. In contrast, a pulse of Smad1 expression exclusively between day 2 and day 2.25 expands the population of progenitors for primitive erythroblasts and other hematopoietic lineages. This effect correlates with increased levels of transcripts encoding markers for the hemangioblast, including Runx1, Scl, and Gata2. Indeed, the pulse of Smad1 induction also expands the blast colony-forming cell (BL-CFC) population at a level that is fully sufficient to explain subsequent increases in hematopoiesis. Our data demonstrate that Smad1 expression is sufficient to expand the number of cells that commit to hemangioblast fate.

Development of the hemangioblast defines the onset of hematopoiesis in human ES cell differentiation cultures.

The onset of hematopoiesis in the mouse embryo and in the embryonic stem (ES) cell differentiation model is defined by the emergence of the hemangioblast, a progenitor with both hematopoietic and vascular potential. While there is evidence for the existence of a hemangioblast in the mouse, it is unclear if this progenitor develops during the establishment of the human hematopoietic system. In this report, we have mapped hematopoietic development in human ES cell (hESC) differentiation cultures and demonstrated that a comparable hemangioblast population exists. The human hemangioblasts were identified by their capacity to generate blast colonies that display both hematopoietic and vascular potential. These colony-forming cells express the receptor tyrosine kinase KDR (VEGF receptor 2) and represent a transient population that develops in BMP-4-stimulated embryoid bodies (EBs) between 72 and 96 hours of differentiation, prior to the onset of the primitive erythroid program. Two distinct types of hemangioblasts were identified, those that give rise to primitive erythroid cells, macrophages, and endothelial cells and those that generate only the primitive erythroid population and endothelial cells. These findings demonstrate for the first time the existence of the human hemangioblast and in doing so identify the earliest stage of hematopoietic commitment.