| Summary: | dissertation The mature capillary network, comprised of a quiescent endothelial cell monolayer, utilizes tight cell-cell interactions to maintain vascular stability and limit vascular leak. An essential component of these intercellular contacts is the adherens junction protein vascular endothelial cadherin (VE-cadherin). In multiple disease settings such as macular degeneration, sepsis, and pandemic influenza, the endothelium is activated and destabilized by cytokines such as vascular endothelial growth factor (VEGF), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α). While an innate immune response is necessary to combat infection, an exuberant cytokine release can paradoxically injure the host endothelium, resulting in vascular damage, tissue edema, and death. In this dissertation we demonstrate that an endogenous, endothelial-specific Roundabout (Robo), Robo4, maintains vascular stability and quiescence. Administration of Slit, an endogenous activator of Robo receptors, strengthens endothelial barrier function and limits vascular leak in a Robo4-dependent manner. Furthermore, Slit treatment enhanced survival during sepsis and avian flu infection, diseases both characterized by hypercytokinemia. We also discovered that Slit strengthens the vascular barrier by increasing VE-cadherin localization at the cell surface. Slit also increases p120-catenin localization at the cell surface and enhances the interaction of p120-catenin with VE-cadherin, preventing the internalization of VE-cadherin. Furthermore, Robo4 activation leads to the recruitment of a paxillin-GIT1 signaling module that inactivates Arf6. Arf6 plays a known role in regulating endocytic recycling, thus perhaps defining Robo4-dependent Slit signaling as a paxillin-GIT1-Arf6-p120-catenin-VE-cadherin stabilization pathway. Our studies fundamentally demonstrate that by specifically blunting the vascular response to hypercytokinemia, mortality can be reduced during severe experimental infections. Activation of a vascular stabilizing pathway such as Robo4 may therefore provide a platform for treating multiple infectious threats characterized by an exuberant cytokine response.
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