Angiogenesis is an essential pathological component of numerous ocular diseases that afflicts a large segment of the world's population. Substantial advances have been made in our understanding of the factors that promote angiogenesis. This has led to effective therapies that prevent the formation of new blood vessels. Since many patients have existing pathological blood vessels, agents that induce regression of vessels would be very useful. The major barrier to developing such therapies is our lack of understanding of how vessel stability and regression is controlled at the molecular level. The long-term goal is to elucidate the molecular mechanisms controlling vessel stability and regression. Our findings will enable the development of next generation therapies that reverse angiogenic-based ocular diseases such as ROP, PDR and the wet form of AMD. By studying what regulates the stability and regression of existing vessels, this grant will make a unique and complementary contribution to the field of angiogenesis, which is largely focused on the formation of vessels. The central hypothesis is that signaling enzymes control the fate of existing vessels. More specifically, the amplitude of phosphoinositide 3 kinase (PI3K) activity governs vessel stability. Once vessels are destabilized, then phospholipase C y (PLCy) triggers regression by inducing the production of a soluble regression factor. We will test this hypothesis in the following set of aims. 1. Investigate the role of PI3K in governing vessel stability. We will test whether the magnitude of the PI3K/Akt output is a key determinant of tube stability. Using both mono- and co-culture models, we will compare PI3K/Akt output in stable and unstable tubes. In addition, we will determine whether forcing a change in the PI3K/Akt output results in a corresponding change in stability. 2. Determine how PLCy triggers vessel regression. We have established assays to detect regression factors and identified an excellent candidate: autotaxin (ATX). We will determine the timing of ATX expression, whether it's necessary and/or sufficient for regression, and its dependence on PLCy. These studies will test our working hypothesis that PLCy triggers regression by promoting the secretion of a soluble regression factor. 3. Determine the importance of PLCy and ATX for stability and regression in vivo (mice). The working hypothesis for this aim is that the regulators of stability and regression in the in vitro setting will also play a role in vivo. This hypothesis will be tested by comparing stability and regression of vessels in several vascular beds within eyes of wild type and mice heterozygous for PLCy or ATX.