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Nuclear Signaling by TRADD

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The adapter protein TRADD regulates pro- and anti-apoptotic signaling from the Tumor Necrosis Factor alpha (TNF) receptor TNFR1. TNFR1 regulation of cell survival and apoptosis is implicated in diverse diseases including ischemic injury, neurodegeneration, infections, inflammatory disease and cancer. An important pathway leading to TNFR1-induced apoptosis requires binding of TRADD to another adapter called FADD, which then recruits and activates caspase 8 leading to activation of downstream caspases and cell death. This occurs in the cytoplasm when TRADD binds to the intracellular death domain of TNFR1. However, multiple lines of evidence indicate that this is not the only mechanism by which TNFR1 (and thus components of the TNFR1 complex such as TRADD) induce apoptosis. If we can understand the other ways that these molecules induce apoptosis we may be able to develop rational strategies to manipulate these responses to improve treatments for diseases where TNF signaling is important.

With the support of an NINDS R21 (NS42662), we recently made the surprising discovery that TRADD is not just a cytoplasmic protein but instead can shuttle between the cytoplasm and the nucleus where it is associated with promyelocytic leukemia protein (PML) nuclear bodies. Nuclear TRADD stimulates an apoptosis pathway that is mechanistically distinct from the well-established cytoplasmic pathway. The nuclear pathway requires p53 and PML and is inhibited by Bcl-xL. We hypothesize that this pathway induces apoptosis through PML nuclear body- and p53-dependent signals that cause mitochondria dysfunction. We further hypothesize that this pathway explains some of the TNF-induced cell death that cannot be accounted for by the established cytoplasmic pathway. To test our hypothesis, we have the following specific aims. 1) Identify the regulators that are required for apoptosis by nuclear TRADD. 2) Determine the role of p53 in apoptosis by nuclear TRADD. 3) Determine the role of PML nuclear bodies in apoptosis induced by nuclear TRADD. 4) Determine the role of nuclear TRADD in mediating TNF-dependent signals. These studies should provide a detailed investigation of a novel apoptosis pathway that may have a role in various human diseases.

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