Death and Destruction: How the Ubiquitin Proteasome System Executes Linker Cell-type Death

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Project Summary The long-term goal of the proposed research is to understand the cell biological mechanisms that execute non-apoptotic cell death. Programmed cell death is essential for organismal development and homeostasis, and its disruption is associated with many human diseases including cancer and neurodegeneration. Apoptosis is a prominent cell death form, however mutations in key apoptotic regulators only cause minor developmental defects. Non-apoptotic programs, therefore, also exist, but their molecular basis is poorly understood. Linker Cell-type Death (LCD) is a non-apoptotic and caspase-independent cell death process operating in C. elegans development, and its morphological hallmarks have also been observed in vertebrate development and disease. The Ubiquitin Proteasome System (UPS) is a key effector of LCD in C. elegans, but how it executes cell death is unknown. Here I will use powerful genetic and molecular tools in C. elegans to identify and characterize the proteolytic targets of the UPS during LCD and determine how their degradation trigger cell demise. In contrast to apoptosis, which uses caspases that transiently bind their substrates and, remarkably, remain poorly understood, the UPS stably interacts with its substrates. Therefore, the proteins I discover that precipitate cell death may unearth general mechanisms of cellular destruction that also function during apoptosis and disease. Indeed, our preliminary data suggest that one candidate substrate degraded by the UPS is an enzyme required for the maintenance of heterochromatin, which is in line with our previous observations that dying linker cells exhibit an open chromatin state. Therefore, this proposal will investigate the exciting hypothesis that chromatin remodeling, precipitated by the UPS, triggers cellular destruction. I will also discover additional proteolytic targets and mechanisms that execute non-apoptotic cell death with yeast 2-hybrid and RNAi screens. Because dysregulation of the UPS and chromatin state are also linked to tumorigenesis and neurodegenerative diseases, my studies can provide greater understanding of cell death programs disrupted in disease that can point towards new therapeutic targets.