Notch Signaling in Pigment Cells and Generation of Mouse Models for Ocular Diseases

Cellular signaling mediated by Notch receptors results in coordinated regulation of cell growth, survival and differentiation. In melanocytes, Notch signaling is essential for the maintenance of melanocytes and the melanocyte stem cell population. Whereas, in pigmented eye structures, specifically in ciliary body and iris, the role of Notch signaling in their structure and their development is largely unknown. In the first part, I have tried to identify possible Notch target genes in melanocytes which might indicate why the melanocyte stem cell population disappears in the absence of Notch. For that purpose, I carried out microarray analysis on Notch deficient melanoblasts and identified putative Notch targets. I have furthermore established mouse melanocyte cell lines from mice which could be used as a valuable tool to study melanocyte biology. In the second part, I have elucidated the functional significance of the Notch cascade in pigmented eye structures. The ciliary body and iris are pigmented epithelial structures in the anterior eye segment that function to maintain correct intraocular pressure and regulate exposure of the internal eye structures to light respectively. In this part I have analyzed the role of the Notch signaling by performing both loss- and gain-of-function experiments for Notch in the murine ocular pigmented epithelium using Mart-1::Cre mice. Loss of canonical Notch2 signaling results in normal iris development but ciliary body aplasia. In contrast, Notch gain-of-function induces the reciprocal phenotype and causes aniridia and ciliary body hyperplasia. The loss and gain-of-function models I have generated develop ocular hypotony and ocular hypertension respectively, mimicking the human diseases phthisis bulbi and closed angle glaucoma. Importantly, the ocular hypertension that occurs following Notch overexpression is responsive to hypotensive drugs, and thus can serve as a platform to evaluate novel hypotensive agents that may be used to treat closed angle glaucoma. In summary, the present study has identified novel putative Notch target genes which will provide us information to study the pigmentation and related disorder. The data obtained in the second chapter provide novel insight into the role of Notch signaling during development of the ciliary body and iris and thus represent a conceptual advance in our understanding of how pigmented structures of the eye are formed. In addition, the genetic models presented in this study represent robust tools with which to study the etiology, pathology and treatment of diseases related to aberrant intraocular pressure, such as phthisis bulbi and closed angle glaucoma.