Distinct Tie2 tyrosine phosphorylation sites dictate phenotypic switching in endothelial progenitor cells.

Angiogenesis is a regulated process involving the proliferation, migration, and remodeling of different cell types particularly mature endothelial and their progenitor cells, nominated as endothelial progenitor cells (EPCs). Tie2/Tek is a tyrosine kinase receptor expressed by endothelial cells that induces signal transduction pathways involved in endothelial biology. To address the potential importance of the various tyrosine residues of Tie2 in EPC development, we generated a series of Tie2 tyrosine mutated (Y1106F, Y1100F, and Y1111F) EPCs and then assess the biological features of these cells. Clonogenic, tubulogenic, proliferative, migratory, and functional properties of these cells were analyzed. Next, GFP-positive EPCs containing Tie2 tyrosine mutations were systemically transplanted into sublethaly irradiated mice to analyze the potency of these cells for marrow reconstitution. We found that mutation in the Tie2 tyrosine 1106 residue directed EPCs toward a mature endothelial phenotype, which was associated with augmented tubulogenic and migratory properties, and increased phosphorylation of the active site (tyrosine 992) as well as increased vascular perfusion in the in vivo Matrigel plug assay. Moreover, transplantation of 1106 Tie2 mutant EPCs failed to reconstitute the bone marrow after myeloablation, whereas transplantation of EPCs with the 1100 or 1111 Tie2 tyrosine mutation resulted in bone marrow engraftment, leading to improved survival of recipient mice. Our findings demonstrate that the tyrosine 1106 residue in Tie2 plays a key role to maintain the stemness features of EPCs.