Molecular characterization of the hemangioblast

Abstract

The hemangioblast is the common progenitor of the hematopoietic and endothelial lineages. An in vitro, ES-derived population, known as the Blast Colony Forming Cell (BL-CFC) is the hemangioblast (Choi et al., 1998). The BL-CFC forms blast colonies in response to vascular endothelial growth factor (VEGF). The data presented, herein, provides incite into the biology of the BL-CFC and its progeny. These studies demonstrate that the hemangioblast expresses Flk-1 but does not require the receptor tyrosine kinase, Flk-1, for lineage commitment. The Flk-1/VEGF interaction may play a role in vivo in the migration of early mesodermal precursors to the extraernbryonic blood islands. Transfer of Flk-1+ cells or immature blast colonies into blastocysts shows successful engraftment of donor cells into hematopoietic and endothelial populations of the early embryo. Basic fibroblast growth factor (bFGF) has previously been shown to be important for the proper development of the ventral mesoderm. Signaling through its receptor, fgfr-1, is required for hemangioblast development. Basic FGF increased the frequency of the BL-CFC and the percentage of cells expressing Flk-1. Previous studies have implicated the transcription factor, SCL/TAL-1 as a master regulator of all hematopoietic lineages. Analysis of scl-/- ES lines showed that SCL was essential for blast colony formation. In the absence of SCL, there was a divergence of cells towards the endothelial cell lineage. Overexpression of SCL in wild type ES cells, by retroviral transduction, does not increase the BL-CFC frequency but does increase the frequency of hematopoietic progenitors. Use of a hCD4/SCL knock-in ES line proved that the BL-CFC expresses Flk-1 and SCL but does not express any markers used to identify mature hematopoietic precursors. An attempt was made to make novel antibodies against the BL-CFC. These reagents, although not specific for the BL-CFC, recognize endothelial cells within the embryo and adult mouse. In summary, this study shows some of the requirements and characteristics of the hemangioblast and that an in vitro generated cell population can contribute to hematopoietic and endothelial populations in vivo.