The Role of βFTZ-F1 and MMP2 in Regulating Hormone-Mediated Autophagy and Insulin Signaling in the Drosophila Fat Body

Abstract

All animals transition through several different stages during their development. Three major processes are involved in regulating the progression from one developmental stage to the next: cell growth, nutrient utilization and cell death. In Drosophila melanogaster, distinct developmental changes occur during feeding and non-feeding periods. The larval stage is characterized by extensive feeding, while metamorphosis involves a long period of starvation (Aguila et al., 2007). Insulin signaling is a highly conserved pathway that has long been known to play a role in regulating growth, nutrient storage and metabolism. Nutrient storage during the larval stage occurs primarily in the larval fat body, which functions as a hybrid of the mammalian adipose tissue and liver, and allows for the rapid growth of the animal (Britton et al., 2002). As the animal enters metamorphosis, insulin signaling has to be shut off to allow for the breakdown of stored nutrients via autophagy- a conserved catabolic pathway that targets cytoplasmic constituents for degradation. Even though it is known that autophagy and insulin signaling are antagonistic processes, the molecular mechanisms that regulate these two mutually exclusive processes have not been fully characterized (Scott et al., 2004). Fly metamorphosis is regulated in large part by the steroid hormone 20- hydroxyecdysone (20E). While most larval organs are destroyed by massive cell death during metamorphosis, the larval fat body escapes this fate and is instead remodeled from a sheet of attached polygonal cells into individual cells that disperse throughout the developing body, providing nutrients. Specifically, a proteinase called matrix metalloproteinase-2 (MMP2) is responsible for the proteolytic degradation of the extracellular matrix and dissociation of the fat body cells. Previous studies have suggested that MMP2 is a potential downstream target of the βFTZ-F1 mediated, 20E signaling cascade. βFTZ-F1 is a nuclear receptor that is required to confer competence upon tissues to respond to 20E (Bond et al., 2011). In this study, I examined the role of MMP2 and βFTZ-F1 in regulating hormone-induced autophagy and insulin signaling. I hypothesize that MMP2, in addition to functioning in fat body remodeling, also downregulates insulin signaling during metamorphosis, thus triggering nutrient release by autophagy. Moreover, I hypothesize that βFTZ-F1 promotes autophagy in the fat body during metamorphosis. To test these hypotheses, I performed mosaic analyses with reduced βftz-f1 and MMP2 expression and looked for the presence of autophagosomal structures. I have demonstrated that βFTZ-F1 is required for autophagy during metamorphosis, while MMP2 does not seem to play a significant role in the regulation of autophagy during this stage.