Angiogenesis, or the formation of new blood vessels via vascular endothelial cell proliferation and sprouting, is essential for the development of all vertebrate organs. This is particularly relevant in the central nervous system (CNS), comprised of the brain, spinal cord and retina, where angiogenesis is regulated by glial cells that secrete various growth factors and extracellular matrix (ECM) proteins. How endothelial cells properly interpret glial-derived cues, however, remains largely uncharacterized. Using Cre/lox strategies in mice we have discovered the first glial-endothelial cell signaling axis that regulates angiogenesis in the developing CNS. Components of this pathway include alphavbeta8 integrin in glial cells, its ECM-bound latent transforming growth factor beta (TGFbeta) protein ligands in the ECM, and canonical TGFbeta receptors (TGFbetaR2 and Alk5) in endothelial cells. Cell type-specific ablation of any component in this pathway leads to CNS-specific angiogenesis defects and premature death. Interestingly, other groups have reported that genetic deletion of Wnt growth factors in glial cells, or the Wnt signaling effector beta-catenin in endothelial cells, also leads to angiogenesis pathologies that largely phenocopy those in alphavbeta8 integrin and TGFbeta receptor knockout mice. These results, as well as unpublished mechanistic data that we present in this application, strongly support cross talk between integrin-activated TGFbetas and Wnts, and in this project we will analyze these events in the developing brain. In particular, a unique set of experimental tools, including mouse and zebrafish genetic models, will be generated to characterize signaling mechanisms and gene regulatory pathways underlying glial control of angiogenesis. We will complement the in vivo models with signaling experiments in primary brain endothelial cells stimulated with Wnts and/or TGFbetas, and characterize how components of these pathways are functionally interconnected using biochemistry and gene expression profiling. These experiments will not only reveal new and important insights into how Wnts and TGFbetas are involved in glial regulation of angiogenesis, but may identify novel therapeutic mechanisms underlying vascular-related neurological disorders.
In this project we will utilize mice and zebrafish as experimental model systems to study how glial cells in the brain regulate blood vessel development. These results will reveal novel mechanisms that control brain angiogenesis and cerebral blood vessel homeostasis, and may also identify new therapeutic targets to treat vascular-related human neurological diseases such as birth defects and stroke.