Hemogenic and nonhemogenic endothelium can be distinguished by the activity of fetal liver kinase (Flk)-1 promoter/enhancer during mouse embryogenesis

Accumulating evidence in various species has suggested that the origin of definitive hematopoiesis is associated with a special subset of endothelial cells (ECs) that maintain the potential to give rise to hematopoietic cells (HPCs). In this study, we demonstrated that a combination of 5'-flanking region and 3' portion of the first intron of the Flk-1 gene (Flk-1 p/e) that has been implicated in endothelium-specific gene expression distinguishes prospectively the EC that has lost hemogenic activity. We assessed the activity of this Flk-1 p/e by embryonic stem (ES) cell differentiation culture and transgenic mice by using the GFP gene conjugated to this unit. The expression of GFP differed from that of the endogenous Flk-1 gene in that it is active in undifferentiated ES cells and inactive in Flk-1(+) lateral mesoderm. Flk-1 p/e becomes active after generation of vascular endothelial (VE)-cadherin(+) ECs. Emergence of GFP(-) ECs preceded that of GFP(+) ECs, and, finally, most ECs expressed GFP both in vitro and in vivo. Cell sorting experiments demonstrated that only GFP(-) ECs could give rise to HPCs and preferentially expressed Runx1 and c-Myb genes that are required for the definitive hematopoiesis. Integration of both GFP(+) and GFP(-) ECs was observed in the dorsal aorta, but cell clusters appeared associated only to GFP- ECs. These results indicate that activation of Flk-1 p/e is associated with a process that excludes HPC potential from the EC differentiation pathway and will be useful for investigating molecular mechanisms underlying the divergence of endothelial and hematopoietic lineages. (C) 2003 by The American Society of Hematology.