Antiangiogenic Factor Receptor PlexinD1 is Required for Proper Formation of the Periocular Vasculature and Establishment of Corneal Avascularity

The cornea is an avascular component of the visual system located in the anterior eye. Avascularity of the cornea is critical for proper vision since the cornea functions by transmitting light into the eye. Impaired vision from loss of avascularity may occur as a result of tissue damage which induces corneal neovascularization from the highly vascularized tissues of the anterior eye. Neovascularization in adult corneas is regulated by secreted pro- and anti-angiogenic factors. These factors function by maintaining corneal avascularity under healthy conditions while permitting neovascularization in damaged corneas. Several pro- and anti-angiogenic factors that function to maintain corneal avascularity during adult life have been identified, however little is known about how pro- and anti-angiogenic factors function to establish avascularity during corneal development. The purpose of this work was to study the role of pro- and anti-angiogenic factors during corneal development. First, the spatial and temporal expression patterns of numerous secreted pro- and anti-angiogenic factors were determined in the anterior eye during avian corneal development using semi-quantitative RT-PCR and RNA in situ hybridization. These techniques were also used to show that known receptors for secreted pro- and anti-angiogenic factors were simultaneously expressed in angioblasts and blood vessels located in the developing anterior eye. These experiments suggested that pro- and anti-angiogenic factor signaling mechanisms may contribute to the patterning of periocular vasculature and establishment of corneal avascularity. Next, I exemplified the role of pro- and anti-angiogenic factors during avian corneal development by using shRNA to knock down the expression of PlexinD1, an antiangiogenic factor receptor expressed in periocular angioblasts and blood vessels. Knockdown of PlexinD1 resulted in multiple patterning defects of the developing periocular vasculature including corneal neovascularization. These phenotypes implicated PlexinD1 as a critical component of the genetic mechanisms that establish corneal avascularity and were suggestive of the role that other pro- and anti-angiogenic factors may play during anterior eye development. These results demonstrate how pro- and antiangiogenic factors are used to simultaneously promote vascularization of the anterior eye and corneal avascularity during development. This information may lead to the creation of novel therapeutic treatments for vascular patterning abnormalities in the anterior eye and corneal neovascularization