Absence of blood and lymphatic vessels in the normal cornea is essential for optical clarity and optimal vision. Numerous pathological disorders result in corneal neovascularization that is responsible for blindness in hundreds of millions of individuals worldwide. The vascular endothelial growth factor (VEGF) family and their receptors (VEGFR) are critical elements of the tightly regulated balance of endogenous angiogenesis stimulators and inhibitors, which when disrupted results in neovascularization. The principal positive regulators of blood (hemangiogenesis) and lymphatic (lymphangiogenesis) in the cornea are VEGF-A and VEGF-C, respectively. Among the many inhibitors of hemangiogenesis that reside in the cornea, we have shown that soluble VEGFR-1 (sVEGFR-1), by acting as a trap for VEGF-A, is essential for corneal avascularity (Ambati et al. Nature 2006). In contrast, the molecular basis of the alymphatic cornea has not been resolved;indeed, no endogenous specific inhibitor of lymphangiogenesis in any tissue has yet been reported. In new and exciting studies, we identified the existence of a novel, secreted splice variant of VEGFR-2 in mouse and human, (sVEGFR-2) (Albuquerque et al. Nature Medicine 2009). Surprisingly, loss of sVEGFR-2 resulted in spontaneous invasion of lymphatic but not blood vessels into the mouse cornea at birth. sVEGFR-2 also specifically inhibited injury-induced corneal lymphangiogenesis and dramatically reduced corneal allograft rejection. This proposal will test the hypothesis that the lymphatic specificity of sVEGFR-2 is due to its existence as a monomer that interacts with and antagonizes VEGF-C but not VEGF-A.
The Specific Aims of this proposal are to define the expression and function of sVEGFR-2 during mouse and human corneal development, decipher the molecular mechanisms by which mouse and human sVEGFR-2 inhibit lymphangiogenesis but not hemangiogenesis, and resolve the spatial and temporal expression and function of sVEGFR-2 in the mouse cornea following injury and allograft rejection. By providing clarifying insights into a critical endogenous regulator of lymphangiogenesis and identifying mechanisms that might serve as potential targets for advancing novel therapies for corneal neovascularization and transplant rejection, these studies directly address the 5-year goals of the NEI Corneal Diseases program. The impact of these studies also extends to pathological states such as tumor metastasis, lymphedema, and lymphangioma, which can be better studied and possibly treated by modulating the endogenous uncoupler of lymphatic and blood vessels we have discovered.
Corneal neovascularization and transplant rejection, which involve abnormal blood and lymphatic vessels, lead to blindness in millions worldwide. We discovered a naturally occurring molecule (sVEGFR-2) that selectively blocks lymphatic but not blood vessels. By molecularly uncoupling these intertwined processes, we introduce a new research tool that can be used to specifically study lymphatic vascular biology and a new target that can lead to novel therapies for corneal blindness.
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