Our long-term goal is to understand a novel nonapoptotic developmental cell death program we discovered, and its relationship to polyglutamine-induced neurodegenerative disease. Cell death is a major cell fate during metazoan development. Apoptosis, an extensively studied cell death process, requires caspase proteases and is accompanied by a stereotypical morphological signature. Surprisingly, mice lacking apoptotic effectors survive to adulthood, raising the possibility that non- apoptotic cell death may play key roles in animal development. Thus, a major unsolved question is whether alternative developmental cell death pathways exist, and if so, what molecular mechanisms govern their execution. We recently discovered that the death of the C. elegans male-specific linker cell (LC) is not apoptotic. LC death has neither apoptotic, nor autophagic or necrotic morphological features. Instead, the dying LC displays pronounced indentation (crenellation) of the nuclear envelope, uncondensed chromatin, and swelling of the endoplasmic reticulum and mitochondria. Importantly, LC death is independent of CED-3 caspase, all other caspases, and all other known C. elegans apoptotic proteins, including CED-4/Apaf-1, CED-9/Bcl-2 family, and EGL-1 and CED-13 BH3-domain-only proteins. These exciting findings demonstrate that LC death must occur through a novel mechanism. From a genome-wide RNAi screen for genes promoting LC death we identified two genes, pqn-41, a gene of previously unknown function, and let-70, encoding an E2 ubiquitin conjugating enzyme. Loss of either gene blocks nuclear crenellation, but not organelle swelling. The pqn-41C transcript as well as let-70 promote LC death, and are expressed in the LC only as cell death is initiated. LC death displays striking ultrastructural similarities to nonapoptotic developmental cell death in the vertebrate nervous system. Several observations also suggest similarities to polyQ-induced neurodegeneration. Like polyQ proteins, PQN-41C is highly glutamine rich, and forms coiled-coil secondary structures. Like polyQ proteins, PQN-41C aggregates in cells. Furthermore, ultrastructural studies of polyQ disease tissue reveal changes similar to those seen during LC death, including nuclear envelope crenellation and organelle swelling. Here we propose to understand the mechanisms of action of pqn-41 and let- 70, and related mammalian proteins, and to pursue a small molecule screen to identify inhibitors of LC death. Together, these studies should not only inform us about the process of LC death, but may yield important clues and reagents as to the etiology of polyQ-dependent neurodegenerative disease.
Our long-term goal is to understand how cell death takes place during normal animal development and disease. We recently uncovered a novel cell death process in the nematode C. elegans with similarities to normal and disease-related cell death in humans. Here we aim to understand the details of this novel killing mechanism, and to study its relatedness to human disease. Our studies may, therefore, provide in-roads towards understanding and, perhaps, treatment of degenerative human diseases.
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