VEGF and Angiopoietin-1 requisitely collaborate during blood vessel development. While Angiopoietin-1 obligately activates its Tie2 receptor, Angiopoietin-2 can activate Tie2 on some cells, while it blocks Tie2 activation on others. Our analysis of mice lacking Angiopoietin-2 reveals that Angiopoietin-2 is dispensable for embryonic vascular development but is requisite for subsequent angiogenic remodeling. Unexpectedly, mice lacking Angiopoietin-2 also exhibit major lymphatic vessel defects. Genetic rescue with Angiopoietin-1 corrects the lymphatic, but not the angiogenesis, defects, suggesting that Angiopoietin-2 acts as a Tie2 agonist in the former setting, but as an antagonist in the latter setting. Our studies define a vascular growth factor whose primary role is in postnatal angiogenic remodeling and also demonstrate that members of the VEGF and Angiopoietin families collaborate during development of the lymphatic vasculature.

Genetic deletion studies have shown that haploinsufficiency of Deltalike ligand (Dll) 4, a transmembrane ligand for the Notch family of receptors, results in major vascular defects and embryonic lethality. To better define the role of Dll4 during vascular growth and differentiation, we selected the postnatal retina as a model because its vasculature develops shortly after birth in a highly stereotypic manner, during which time it is accessible to experimental manipulation. We report that Dll4 expression is dynamically regulated by VEGF in the retinal vasculature, where it is most prominently expressed at the leading front of actively growing vessels. Deletion of a single Dll4 allele or pharmacologic inhibition of Dll4/Notch signaling by intraocular administration of either soluble Dll4-Fc or a blocking antibody against Dll4 all produced the same set of characteristic abnormalities in the developing retinal vasculature, most notably enhanced angiogenic sprouting and increased endothelial cell proliferation, resulting in the formation of a denser and more highly interconnected superficial capillary plexus. In a model of ischemic retinopathy, Dll4 blockade also enhanced angiogenic sprouting and regrowth of lost retinal vessels while suppressing ectopic pathological neovascularization. Our data demonstrate that Dll4 is induced by VEGF as a negative feedback regulator and acts to prevent overexuberant angiogenic sprouting, promoting the timely formation of a well differentiated vascular network.angiogenesis ͉ retina ͉ Notch ͉ oxygen-induced retinopathy