Spectrin Exerts Multiple Effects on Lipid Translocation Pathways in Drosophila melanogaster

The cellular basis of several metabolic diseases, such as obesity, is associated with excess of triacylglycerol (TAG) storage within lipid droplets (LDs). The changes in LD physiology and metabolism depend on the complex process of lipid translocation, which occurs both intracellularly and between various tissues. Unraveling the mechanisms underlying this complex process might provide useful knowledge for designing drugs that combat the metabolic diseases associated with it. The data I present in this thesis bring novel insights into the mechanisms of lipid translocation in Drosophila melanogaster, specifically into the transport of lipids across the plasma membrane, their incorporation into lipid droplets, their mobilization from lipid droplets and their transport between larval tissues. First, I show that the cytoskeletal protein Spectrin exerts unexpected multiple effects on lipid translocation pathways in the fat body: 1) loss of the α subunit of the Spectrin tetramer alters the plasma membrane architecture and eliminates a specialized population of cortical LDs, 2) over-expression of β spectrin perturbs transport of dietary fat (due to blocking the secretion of the lipid carrier lipophorin from fat cells), 3) the toxic effects of β spectrin over-expression are rescued by co-expression of α spectrin. As a result of these data, I propose a model of lipid uptake at the plasma membrane, in which Spectrin connects the machinery responsible for lipid uptake to that of LD growth in the cytoplasm of fat cells. Second, I show that added dietary free fatty acids (FFA) are efficiently absorbed, incorporated into LDs, and mobilized by midgut enterocytes. I alsoo present evidence that complex TAGs are not efficiently absorbed, unless they are hydrolyzed into FFAs. This suggests that lumenal lipases play important roles in lipid processing in insects. Additionally, over-expression of the lipid droplet associated protein, LSD2, leads to increased amount of LDs in the midgut. Because of these lines of evidence, I propose that LSD2 acts as a gatekeeper in translocating lipids to export machinery by directing their flow into pathways of LD formation. Third, I present the results of a forward genetic screen designed to identify novel genes involved in the biological process of dietary lipid transport between tissues in Drosophila larvae.