Cellular and molecular studies of larval oenocyte function in Drosophila

Lipid metabolism is regulated according to the nutritional status of the body and its energy demands. In vertebrates, the liver plays a central and multifunctional role in this process. Within hepatocytes, lipid metabolites of cytochrome P450-4 (Cyp4) enzymes serve to activate Peroxisome Proliferator-Activated Receptors (PPARs), leading to the induction of a myriad of genes involved in lipid uptake, processing and degradation. In principle, the insect Drosophila should provide a genetic model for studying these processes but several aspects of lipid metabolism, including the identity of the cell-types that fulfil the functions of the vertebrate liver are not yet clear. The larval oenocytes of insects are large lipid-laden cells whose function has been a matter of considerable debate over the last hundred and fifty years. Three longstanding hypotheses are that oenocytes regulate tracheal respiration, hemolymph composition or aspects of moulting. To begin to test these hypotheses, I have used enhancer traps and a large database of expression patterns to identify 35 genes selectively expressed in oenocytes. 17 of these encode products highly related to vertebrate liver proteins playing roles in the uptake, modification and degradation of lipids. These include two lipophorin receptors, a Cyp4 protein and 6 peroxisomal enzymes catabolising fatty acids by ?-oxidation. By using an optimised GAL4/UAS-reaper approach to ablate Drosophila oenocytes, I find that they are required for the completion of the moult from second- to-third larval instars. Arrested oenocyte-less larvae display duplicated exoskeletal elements, suppressed food intake and a premature wandering behaviour. These phenotypes are strikingly similar to those produced by mutations in the E75A variant of Ecdysone-induced protein 75B (Eip75B), an orthologue of vertebrate PPARs and one of the 35 oenocyte genes identified here. Together, these studies demonstrate that oenocytes possess much of the lipid-processing and regulatory circuitry of vertebrate hepatocytes and may use this machinery to trigger developmental transitions during larval life