Fasting triggers concerted changes in behavior, physical activity and metabolism that are remarkably well conserved through evolution. In mammals, such responses are often coordinated by transcriptional coactivators that are targets for regulation by environmental cues. The CREB family of transcription factors participates in a variety of cellular functions including energy homeostasis. Recently, a new family of key CREB coactivators, called TORCs, has been identified. Upon activation, TORC proteins translocate to the nucleus where they potentiate gene activation via a direct interaction with CREB. Drosophila has a single TORC family member, dTORC, which is induced upon fasting, and phosphorylated and degraded upon feeding in an insulin-dependent manner. dTORC mutant flies are sensitive to starvation and have significantly lower glycogen and lipid stores compared to wild type. dTORC functions in the brain, where it is active in a subset of neuroendocrine cells, to regulate the release of an unidentified neuronal signal that instructs the fat body, the energy storage organ, to store glycogen and lipids. This proposal aims to use Drosophila to understand how TORC proteins function to regulate energy balance. Using a rescuing assay of the dTORC mutant phenotype, the specific domains of dTORC required for activation, nuclear translocation and binding to CREB in cell culture assays will be mutated and tested for function in vivo. The hypothesis that dTORC functions in the neuroendocrine cells to produce the signal to the fat body will be tested by supplying dTORC specifically in these cells and assaying for rescue of the mutant phenotype. In vivo nuclear shuttling of dTORC within the neuroendocrine cells in response to feeding and fasting will be examined. From a genetic screen to identify new components of the dTORC pathway, 4 discrete genomic regions have been found to enhance or suppress dTORC function, and thus are excellent candidates for containing genes that encode novel components of the dTORC pathway. The genes within these regions will be identified using single-gene mutations and RNA interference. A screen for genes that affect insulin-dependent dTORC phosphorylation and degradation will be carried out to identify dTORC regulators, leading to an understanding of how insulin signaling regulates dTORC activity. Given the conservation of TORC protein structure and function between Drosophila and mammals, we expect our results to provide key insights into the regulation of energy balance by mammalian TORC proteins.
Obesity represents a major health problem facing the nation. TORC proteins have recently been found to regulate glucose and lipid metabolism in an insulin-dependent manner. These studies will provide key insights into how TORC proteins regulate energy balance.
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