Journal of Histochemistry and Cytochemistry, Vol. 47, 1643-1643, December 1999, Copyright © 1999, The Histochemical Society, Inc.


PROCEEDINGS

1 The de novo formation of blood vessels

Christopher J. Drakea and Charles D. Littlea
a Department of Cell Biology and the Cardiovascular Developmental Biology Center, Medical University of South Carolina

Molecules that control neovascular processes have received considerable attention. Here we describe experimental data regarding two ligand receptor families that function during vessel morphogenesis. The first is vascular endothelial growth factor (VEGF) and its cognate receptor tyrosine kinases (RTK). The second family is comprised of the Angiopoietin-1 (Ang1) and Ang2 ligands and their respective RTK, Tie2. The VEGF ligand is a mitogen that binds cell surface Flt1/VEGF-R1 and Flk1/-R2; our recent evidence shows that VEGF also regulates important morphogenetic activities associated with the de novo formation of blood vessels. Studies were conducted in vivo using quail embryos, and experimental reagents in the form of soluble VEGF receptor-1/Flt-1 or recombinant VEGF165. Microinjection of soluble receptor sharply inhibited the protrusive activity required for normal vascular morphogenesis; in contrast, microinjection of recombinant VEGF165 markedly upregulated cell protrusive activity. The failure of vasculogenesis in soluble VEGFR1 injected embryos is manifested by incomplete vascular networks and the deletion of large vessels. Endothelial cells of injected embryos failed to form the elongated processes necessary for vessel morphogenesis. These events occur before the onset of circulation. Together the two data sets strongly suggest that VEGF regulates endothelial cell protrusive activity necessary for the establishment of a primary vascular pattern. Thus, VEGF is a vascular morphogen. Unlike VEGF, the Ang/Tie2 signaling system does not appear to function until circulation begins. Injection of Ang2, a naturally-occurring inhibitor of the Tie2 RTK, results in endocardial and aortic anomalies. Injected embryos display malformed endocardial tubes, which appear perforated and discontinuous. Instead of large vessels with a single lumen, Ang2-treated aortic primordia resemble a small plexus of microvessels. Since Ang2 is an inhibitory ligand, our data suggest that it competes with a separate ligand, perhaps Ang1, for Tie2 binding. If that assumption is correct, then a continuous Ang1 signal may be required to maintain the integrity of early endothelial tubes. These data show that early vascular morphogenesis is tightly orchestrated, and that experimental perturbation of two RTK signaling mechanisms leads to vascular malformations including endocardial anomalies.