The requirement for multiplepRB family members and PDZ domain containing proteins in cell cycle regulation and differentiation in the lens epithelium

To elucidate the molecular mechanisms that regulate the cell cycle and cellular differentiation phase of lens development I sought to determine the roles of three groups of proteins in the establishment and/or maintenance of the cell cycle regulation and differentiation in the lens epithelium. Two of these, Rb family members and p53, have well known functions as tumor suppressors. The third is a group of proteins that contain protein-protein interaction domains called PDZ domains. Among these PDZ proteins are the vertebrate homologs of Drosophila tumor suppressors Dlg and Scrib. To study the roles of pRb family members, p53, and PDZ proteins, I utilized transgenic mice that expressed the human papillomavirus (HPV) viral oncogenes, E6 or E7, under the K14 promoter that directed expression to the lens epithelium and transition zone. E6 has been shown to disrupt the function of p53 and certain PDZ domain containing proteins while E7 has been shown to disrupt pRb family members. Examination of the lenses from mice transgenic for E7WT and E7 mutants that fail to bind or degrade pRb, showed that cell cycle regulation in the lens epithelium requires multiple Rb family members. Expression of E6 in the lens epithelium resulted in increased proliferation, multilayering, and compromises in cell adhesion. These defects were not dependent on p53, but were similar to defects seen in Drosophila mutants of Dlg and Scrib. To determine of E6's interaction with PDZ proteins was the basis for the lens epithelial phenotype, I first showed that many of E6 PDZ protein partners are expressed in the lens. I then analyzed the lenses from mice transgenic for E6 mutants that either lost or retained the ability to bind PDZ domains and lenses from an animal carrying a Dlg genetrap allele. These experiments demonstrated the requirement of functional PDZ domain proteins, specifically Dlg and Scrib, in maintaining proper cell cycle control and normal initiation of differentiation in the lens epithelium. Thus, in my thesis I have shown that cell proliferation and differentiation in the lens epithelium is controlled by multiple factors that include the pRb family and PDZ proteins.