Glioblastoma (GBM) is a primary malignancy of the central nervous system (CNS) that is nearly universally fatal. Our long-term goal is to understand the genomic changes that lead to GBM and to use this information to develop better options for GBM patients. Recently, our group has shown that the protein tyrosine phosphatase receptor D, PTPRD, is one of the most commonly inactivated tumor suppressors in GBM. The central hypothesis of this application is that inactivation of PTPRD is a critical event underlying the development of GBM. We and others have shown that PTPRD is a multisite tumor suppressor, inactivated in a wide range of human cancers. We've shown that PTPRD potently suppresses tumor cell growth, a property that is abrogated by the cancer-specific mutations. However, the molecular mechanisms underlying PTPRD's tumor suppressive activity are poorly understood. The objective of this proposal is to understand the mechanism of PTPRD tumor suppression in GBM pathogenesis by pursuing the following 3 specific aims. In Aim 1, we will elucidate the cellular mechanisms underlying the tumor suppressive activity of PTPRD. We will characterize the cellular mechanisms underlying PTPRD function and the effects of cancer-specific mutations of PTPRD on this function. We will do this using a systematic in vitro and in vivo approach to characterize the effects of PTPRD on glioma cell proliferation, cell cycle regulation, programmed cell death, and invasion. In vitro studies will utilize inducible PTPRD cell lines to perform structure function analyses and decipher the effects of cancer-specific mutations on these processes. In vivo studies will utilize intracranial xenografts to characterize the effects of PTPRD in tumorigenesis. We will characterize the ligands of PTPRD. In Aim 2, we will determine the biological consequences of the regulation of the signal transducer and activator-3 (STAT3) by PTPRD. We found that PTPRD dephosphorylates STAT3, a key oncoprotein involved in the growth of glioma cells. Mutations in PTPRD abrogate the ability of the protein to de-phosphorylate STAT-3. We will determine the functional significance and mechanistic details of this regulation using both cell line models and primary tumor tissue. Specifically, we will (1) determine how PTPRD-mediated dephosphorylation of STAT3 affects STAT3 activation, (2) determine the requirement of STAT3 down-regulation for the tumor suppressive activity of PTPRD, (3) characterize the association between loss of PTPRD protein in primary tumors and increased phospho-STAT3, and (4) define the effects of PTPRD on STAT3 transcription in glioma. In Aim 3, we will define in knockout mice, the role of PTPRD in oncogenesis. Using mouse models, we will characterize the effects of Ptprd loss on cancer formation. We will (1) characterize the effects of Ptprd knockout on cancer formation using constitutive and conditional knockouts, (2) generate Ptprd -/- Cdkn2a -/- mice to evaluate the effects of concomitant loss of both 9p tumor suppressors (Ptprd and p16/Arf) on cancer formation, and (3) using the RCAS-TVA glioma system, determine how Ptprd depletion specifically modulates glioma pathogenicity.