Identification of resident and circulating endothelial stem cells

Abstract

Blood vessels and circulating blood contain rare immature endothelial cells that display in vitro clonal proliferative potential and in vivo vessel forming ability. However their precise location, origin, surface marker and molecular determinants have yet to be precisely defined. In this research body of work, we have identified ABCG2, an ATP binding cassette drug transporter that is expressed by stem cells of many lineages, to label vascular endothelial stem cells (VESC) and circulating endothelial stem cells (CESC). During development, ABCG2 expressing VESC are distributed in arterial, venous, and capillary vessels of multiple tissues including heart, lung, bone marrow and retina. They possess clonal colony forming potential in vitro and contribute to the growth of arteries, veins and capillaries in vivo. Steady state adult tissues also contain VESC that retain colony forming potential, though their frequency is decreased. In human umbilical vessels, ABCG2+ VSEC represent about 1% of umbilical cord vessel EC and showed higher colony forming potential than ABCG2- EC. In addition, CESC that could form EC colonies in vitro were identified from neonatal murine peripheral blood. About 30% of CESC were labeled by ABCG2. Lineage tracing experiments using hematopoietic (Flk2Cre) and EC (Tie2ERTCre) specific mice showed CESC were derived from vascular EC, not hematopoietic cells. CESC could participate (at a single cell level) in vessel formation in vivo in gel transplantation model. Furthermore, after transplantation, CESC retained secondary colony forming potential in formed blood vessels. Finally, we show that Abcg2 not only labels, but is also critical for the emergence/maintenance of VESC and the production of CESC. These findings provide a solid foundation to identify the critical roles of endothelial stem cells in vascular development, homeostasis and repair.