Molecular mechanisms of neutral lipid uptake in Drosophila

Abstract

The lipophorin receptors are required in tissues such as the imaginal discs, the oenocytes and the egg chambers, for the cellular uptake of neutral lipids from hemolymph. In lipophorin receptor mutants, these tissues are severely deprived on intracellular lipid droplets. Our previous research suggested a model in which the lipophorin receptors interact with circulating lipophorin particles -insect diacylglycerol-rich lipoproteinsstabilizing them on the cell surface. This interaction is required for the transfer of neutral lipids to the cell by a still not completely understood mechanism that does not require the endocytosis of the lipophorin particle and that is reminiscent of the extracellular lipolysis of mammalian lipoproteins in the microcapillaries of peripheral tissues. To better understand the molecular mechanisms mediating lipid uptake, we searched for molecular partners of the lipophorin receptors by tandem affinity purification. A key cofactor identified was a large protein with homology to human ApoB, suggesting it was a novel circulating lipoprotein. Isopicnic gradient ultracentrifugation showed it floated at a density of 1.23 g/ml. We generated null mutations that are larval lethal. Silencing of this novel lipoprotein in adults resulted in a strong accumulation of neutral lipids in the gut and a reduction in the ovaries, a phenotype very similar to that of lipophorin knockdown. This suggests that both lipoproteins are essential for the transfer of neutral lipids from the gut to peripheral tissues. Studies in other insects had similarly identified two circulating lipoproteins: lipophorin, which is the main lipid carrier, and Lipid Transfer Particle (LTP), a high density lipoprotein that catalyzes the transfer of lipids between lipophorin and tissues by an unknown mechanism. Our data suggests that the novel high density lipoprotein we identified corresponds to Drosophila LTP. We are currently examining the functional relevance of the interaction between Drosophila LTP and the lipophorin receptors.