The Disabled-2 (Dab2) gene has been proposed to function as a tumor suppressor because it is commonly down-regulated in human ovarian and mammary carcinomas and over-expression can inhibit carcinoma growth. However, mutations in the Dab2 gene have not been detected in human cancers and the true roles of Dab2 in normal cells are unknown. To determine the mechanisms of Dab2 functions in normal cells, we have been investigating Dab2 protein localization, phosphorylation and interactions, and determining the functions of Dab2 during mouse development. Dab2 is expressed in 2 protein forms, p96 and p67. Preliminary studies show that the p96 Dab2 protein is phosphorylated by MAP kinase, localized to clathrin-coated pits and binds to a non-muscle myosin implicated in endocytosis. p96 Dab2 also associates with the cytoplasinic tails of lipoprotein receptors. Indeed, our conditional knockout mice that lack Dab2 show kidney defects similar to those of mice lacking the lipoprotein receptor, megalin. Thus we hypothesize that Dab2 normally regulates megalin traffic. The Dab2 gene has another important function during early post-implantation development. Dab2 mutant embryos have a defect in the extraembryonic visceral endoderm. The phenotype suggests reduced signaling by Nodal, a TGF beta family member. We propose that Dab2 has at least two functions: one in signaling and one in receptor traffic. These functions may be linked or separate. We will test whether the embryonic and kidney defects are indeed due to reduced signaling and traffic, respectively, and whether the p96 and p67 forms of Dab2 have separate or redundant roles. We will investigate the molecular mechanisms whereby Dab2 regulates traffic, especially the role of myosin VI. We will investigate how Dab2 modulates Nodal/TGFbeta signaling in some cells but not others. We will also investigate the significance of Dab2 phosphorylation in regulating these responses. These studies will provide insights into the molecular mechanisms of action of Dab2 and into the role of a single gene in receptor trafficking and signal transduction.
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