| Description |
Nuclear receptors (NRs) are a large family of transcription factors that play important roles in development, homeostasis, and disease. NRs can be activated or repressed by hydrophobic ligands and protein cofactors. To determine how, where, and when NRs are likely to function, we used a ligand sensor system to visualize their spatial activity patterns in developing animals. Transgenic lines were established that express the ligand binding domain of each nuclear receptor fused to the DNA-binding domain of yeast GAL4-responsive reporter gene, these NR ligand sensor revealed a wide variety of spatially and temporally dynamic patters, The activation patterns accurately reflect known NR functions in flies, including regulation by hormones and protein partners. We exploited the ligand sensor system in larval organ culture to identify novel activators for the NR Ultraspiracle (USP). Furthermore, this work led to attractive hypotheses regarding previously unstudied Drosophila NRs. To better understand how NRs function in development and metabolism, we have undertaken a functional analysis of Drosophila HNF4 (dHNF4). dHNF4 is the single Drosophila ortholog in its NR subfamily and, like two mammalian counterparts, dHNF4 is highly expressed in tissues that function in nutrient absorption and metabolism. We show here that dHNF4 is required to maintain normal fat metabolism during development. dHNF4 null mutants die within the first hour of adult life under normal feeing conditions, with reduced levels of glucose, and increased levels of triacylglycerides (TAGs) and long chain fatty acids. Mutant larvae abnormally retain TAGs in their midgut and fat body and are sensitive to starvation. Suggesting that they are unable to efficiently mobilize stored fat for energy production. Microarray analysis indicates that many starvation-regulated genes that function in beta-oxidation are dependent of dHNF4 for their proper expression. Moreover, studies using the GAL4-dHNF4 ligand sensor system indicate that dHNF4 transcriptional activity is regulated by starvation and responsive to long chain fatty acids in culture. Taken together, our results support a model in which fatty acids released from stored TAGs upon starvation ‘feed forward' through dHNF4 to upregulate fatty acid beta-oxidation genes and ultimately generate the energy required for survival |
