To Die or to Differentiate: Apoptotic and Non-Apoptotic Roles of Death Molecules in Drosophila melanogaster

Citation

Huh, Jun Ryul (2005) To Die or to Differentiate: Apoptotic and Non-Apoptotic Roles of Death Molecules in Drosophila melanogaster. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Q00G-B953. https://resolver.caltech.edu/CaltechETD:etd-06032005-145438

Abstract

Virtually every cell, if not all, are ready to die under stressful conditions or by necessity during animal development. In Drosophila, three pro-apoptotic proteins, Rpr/Hid/Grim, had been found to induce cell death by preventing the function of the cell death inhibitor, DIAP1. However, the mechanistic details of this process were largely unknown. We have found that Rpr/Hid/Grim induce DIAP1 destabilization through ubiquitination and general translational inhibition. Moreover, from the in-vitro and in-vivo studies, we also found that ubiquitination of DIAP1 by Hid is dependant on DIAP1’s own ability to ubiquitinate itself. Once the life-or-death decision is made, cells can efficiently start apoptosis by quickly removing pre-existing death inhibitors using these mechanisms.

In addition to the canonical roles of death machinery, we have also studied their roles in non-apoptotic developmental processes. In the testis, germline stem cells ultimately give rise to 64 individual sperms. Spermatocytes, and later, spermatids, develop within a single membranous structure, or syncytium. Formation of free-swimming sperms requires the encapsulation of each spermatid by an independent plasma membrane and the elimination of most of the sperm cytoplasm. We demonstrated that at least three independent caspase activation pathways are likely to be involved in these processes with different spatial and temporal activation patterns, and that a global inhibition of caspase activity results in male sterility.

External stresses such as radiation and heat shock were known to induce large amounts of cell death (up to 60% of the total cell population) in proliferative tissues like Drosophila larval imaginal discs. Interestingly, larvae exposed to such stress ultimately develop into normal adult flies. This is facilitated by the compensatory proliferation of cells that neighbor the dying cells. In order to study the mechanistic basis for this process, we uncoupled cell death from death activation by expressing Hid in the presence of P35, a viral inhibitor of effecter caspases. Interestingly, neighboring cells of clones expressing Hid underwent compensatory proliferation, which was no longer observed when we blocked the activation of initiator caspase, Dronc. Our observations indicate that non-apoptotic Dronc activity is required for the generation of a non-autonomous proliferation signal.

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