Internal death programs play significant roles in many diseases. Pathogenic effects can result from inefficient cell death or from inappropriate or excessive death such as that caused by the human immunodeficiency virus (HIV) during AIDS or the SAR-CoV virus during SARS. In this project, we are taking a multifaceted approach to studying molecular mechanisms of both apoptotic and nonapoptotic death programs in lymphocytes as well as other cell types. A major focus of our investigations are death-inducing cell surface receptors in the tumor necrosis factor receptor (TNFR) superfamily such as TNFR1 and CD95/Fas/APO-1. Both receptors play an important role in stimulating both apoptotic and nonapoptotic death of cells principally in immune processes. Little is known about how these alternative death pathways are entrained to receptor signaling. Interestingly, both receptors can have effects beside death such as the induction of transcription factors. We are trying to understand how these receptors stimulate the intracellular machinery that causes cell death in preference to other cellular outcomes. We have devoted many of our efforts to understanding the activation of a protease called caspase-8 which regulates the death program. We have characterized two death programs that emanate from TNFR1 and the Fas receptor, one which is caspase-8 dependent and has an apoptotic morphology and the other which is caspase-8 independent and involves necrosis. Interestingly, the latter death program is only observed when caspase-8 is inhibited. The regulation and molecular pathways of these two forms of lymphocyte death are distinct. In addition, we have discovered that inhibition of caspase-8 in non-lymphoid cells can lead to another form of cell death exhibiting particular cytoplasmic double membrane structures called autophagy. Although initially controversial, several labs have now shown that this form of death is particularly important for the demise of tumor cells by chemotherapeutic agents. We have now shown that the mechanism of autophagic death program is selective degradation of catalase which leads to a marked overaccumulation of reactive oxygen species leading to cellular damage and death. Furthermore, we have focused on genes that play key roles in this process of death. We have found that the human homologue of the Drosophila spinster protein, called hSpin, is essential for autophagic cell death. We have studied the biochemical function of this protein and found that it is important for proper lysosome biogenesis and vesicle trafficking. In particular, it plays a vital role in lysosomal reformation at the end of autophagy. Autophagy is an evolutionarily conserved process from humans to yeast by which cytoplasmic proteins and organelles are catabolized but very little was known about results at the end of autophagy when cells were selecting between autophagic cell death and survival. During starvation, the protein TOR (target of rapamycin), a nutrient-responsive kinase that controls cellular metabolism, is shut off, and autophagy is activated. Double-membrane autophagosomes sequester intracellular components and then fuse with lysosomes to form autolysosomes, which to catabolize their contents to regenerate nutrients. Ourpresent understanding of autophagy is that it terminates with cargo degradation within autolysosomes, but how autophagy is controlled by nutrients and the subsequent fate of the autolysosome were unknown. We discovered that mTOR signalling in mammalian cells is inhibited during initiation of autophagy, but reactivated during extended starvation. Reactivation of mTOR depends on the degradation of autolysosomal products and release of nutrients. mTOR activity in turn terminates autophagy and stimulates impressive proto-lysosomal tubules and vesicles that extrude from autolysosomes and ultimately mature into functional lysosomes. This process, that we term autophagic lysosome reforation (ALR), restores the full complement of lysosomes in the cell. This evolutionarily conserved cycle in autophagy governs nutrient sensing and lysosome homeostasis during starvation. In parallel, we are exploring how the regulation of cellular death programs may play a role in cytopathicity associated with virus infections in AIDS and SARS. In particular, a critical effect in the onset of AIDS following infection with HIV is the death of T lymphocytes caused by the virus. We have found that this death process is necrotic rather than apoptotic and have now identified two viral gene products, vif and vpr, that are involved in this process. We have found that vpr alters the cell cycle and promote death by binding to cellular proteins that have a role in cell cycle progression. In order to study this process rigorously we have constructed a mathematical model to analyze cell death in tissue culture during HIV infection. Remarkably, both of these cytotoxic gene products cause says cycle arrest at the boundary of the G2 and M phases. The mathematical model reveals that the principal cause of cell loss is cell death rather than cell cycle arrest. We are using molecular genetic approaches to determine if cell cycle arrest actually causes cell death and how this might come about. The HIV vpr protein is a small protein (100 amino acids) with no obvious structural domains or enzymatic motifs other than three alpha helices. We have determined that vpr promotes the formation of an apparently abortive complex between mitotic regulators such as CyclinB and Cdk1, and the theta isoform of the 14-3-3 protein which inhibits the cell cycle in the G2 phase. The complex appears to be nucleated by a particular hydrophobic patch on the third helix of the vpr protein. We have also studied how vif causes cell cycle arrest and found that it is a distinctive mechanism from that induced by vpr. We find that vif can alter the nucleocytoplasmic localization of cyclins and cyclin-dependent kinases which leads to disruption of normal cell cycle progression. We continue to explore how HIV-1 alters to cellular machinery to cause the demise of CD4 T cells.We investigated the functions of the largest of the accessory proteins, the ORF 3a protein, of SARS-CoV and found that ORF 3a accounted for cell death caused by this virus. In addition, ORF 3a causes SARS-CoV-induced Golgi fragmentation and that the 3a protein accumulates and localizes to vesicles containing markers for late endosomes. These results establish an important role for ORF 3a in SARS-CoV-induced cell death, Golgi fragmentation, and the accumulation of intracellular vesicles. The overarching conclusion that we can draw from these studies is that cell death can be programmed in many different ways in the cell depending on the cell-type and conditions. Various cell biological mechanisms can participate which means that the genetic control varies depending on the specific mechanism of death. By understanding the molecular mechanisms of death, we hope to gain better insight into normal cellular homeostasis as well as degenerative disease processes.
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