Characterization of nuclear degradation dynamics during cell death and modeling laminopathies using CRISPR/Cas9 in Drosophila melanogaster
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Abstract
Characterization of apoptosis, a caspase-dependent cell death program, has led to diverse insights into development, tissue homeostasis, and numerous diseases. About a dozen other distinct modes of regulated cell death have been identified, but our understanding of them is incomplete. Of particular interest is elucidating the process of nuclear degradation in non-apoptotic cell death, as lingering nuclear debris can be toxic. During apoptosis, caspases cleave nuclear components, but caspase-independent mechanisms of nuclear breakdown are not fully understood. The Drosophila melanogaster ovary is an excellent model to address differences in nuclear breakdown, since both apoptotic and non-apoptotic cell death occur during oogenesis. In mid-stages of oogenesis, germline-derived NCs die by apoptosis, while in late stages, germline-derived NCs degenerate by non-apoptotic cell death.
A comparative analysis was performed, focusing on the caspase substrate nuclear Lamin and distinct differences in nuclear degradation events were found. Our data revealed a series of nuclear architecture changes during non-apoptotic cell death. The engulfment receptor Draper and phagocytic machinery were non-autonomously required for Lamin degradation, and the lysosomal protease CP1 facilitated Lamin degradation in non-apoptotic nurse cell (NC) death. We next examined the effects of expressing caspase-resistant Lamin and found that it impaired nuclear degradation during apoptosis. In contrast, nuclear degradation in non-apoptotic cell death was impaired by the overexpression of wild-type Lamin, supporting that Lamin degradation mechanisms are distinct in these two modes of cell death.
Nuclear lamins, specifically isoforms encoded by the LMNA gene, are associated with the greatest number of disease-causing mutations in humans. Interestingly, one of these mutations occurs in the caspase-cleavage site and is associated with familial partial lipodystrophy, type 2 (FPLD2). To develop a Drosophila model of this disease, CRISPR/Cas9 was used to introduce a point mutation in lamC, the Drosophila homolog of LMNA. The fat body cells of mutant larvae were found to have altered lamin organization.
Altogether, the work in this dissertation identifies novel nuclear degradation dynamics in a non-apoptotic cell death. In addition, this work describes a fly model system for laminopathy mutations in humans.