Angiogenesis, typically known as the formation of new blood vessels from an existing vascular bed, is tightly regulated by a balance of positive and negative metabolic pathways. Because of its central role in neoplasia, non-neoplastic disorders including preeclampsia, pulmonary hypertension, ocular microvascular proliferative disorders, neointima formation and restenosis, and also in normal adult physiology, angiogenesis has recently attracted a great deal of scientific interest. Growth factors possessing angiogenic activity include vascular permeability factor/vascular endothelial growth factor (VEGF-A), placenta growth factor (PIGF), fibroblast growth factor, and platelet derived growth factor B chain. VEGF-A is arguably the most important angiogenic cytokine expressed by tumors and it is expected that blocking VEGF-A signaling will be effective in controlling tumor angiogenesis and therefore, limiting or even reversing tumor growth. However, there are also diseases where we need to promote balanced angiogenesis and these include cardiovascular diseases. A major factor limiting the development of rational anti- or pro-angiogenesis therapy is our incomplete understanding of the basic steps and molecular mechanisms by which VEGF-A signals endothelial cell proliferation and migration through its two high affinity tyrosine kinase receptors VEGFR-1 and VEGFR-2. Current information indicates that these receptors have different roles in vasculogenesis and angiogenesis and mediate different signaling pathways. Also, emerging evidence suggests that signaling through VEGFR- 1 actually inhibits some but not all of the functions mediated through VEGFR-2. The long-term goal of the current application is to elucidate the distinct signaling pathways mediated by VEGFR-1 and VEGFR-2 in endothelial cells. Aim 1 will define an activation mechanism and role for PLC yl and (33isoforms in VEGFR- 2-mediated signaling pathways whereas Aim 2 will distinguish the role of Rho A, B, and C in VEGFR-2- mediated signaling. Finally, Aim 3 will investigate the regulatory role of VEGFR-1 on vascular endothelium and a novel role for PI-3K and CDC42 in vessel maturation. Our experiments will include utilizing an EC tissue culture system, a zebrafish model to detect vascular development, and tumor-induced angiogenic models for all Aims. These studies will map the critical signaling pathways responsible for angiogenesis and in the process, identify key molecules that mediate these pathways. Hence, the results of these proposed studies will help identify potential angiogenic inhibitors or promoters that can be targeted and used to improve future therapies against different diseases.