In holometabolous insects, lipid reserves are accumulated during the larval stage and stored as triacylglycerols in the fat body. These reserves are utilized in adults during flight fueled by fatty acid oxidation. In addition, lipids are essential metabolic reserves in adult insects, such as mosquitoes, that do not fuel flight by fatty acid oxidation. Despite the critical importance of lipid reserves in adult insects, relatively little is known about the mechanisms whereby they are accumulated during larval life. In many cases, this involves digestion of dietary lipids and absorption of fatty acids in the midgut, and transport of lipids from the midgut to the fat body via the hemolymph lipoprotein, lipophorin. Insects also absorb other dietary lipids in the midgut, e.g., sterols and carotenoids, and transport them in the hemolymph via lipophorin. This proposal is concerned with two aspects of the processes involved in the accumulation of lipid reserves during larval life. (1) Determination of the pathway for lipophorin biosynthesis and assembly in larval fat body. In larval Manduca sexta, lipophorin is made in the fat body as a nascent lipoprotein particle containing apolipoproteins and phospholipid, but no core or transported lipids. Until recently, it has not been possible to study the biosynthesis of lipophorin in detail due to lack of information about the apolipoproteins whose biosynthesis is coupled to production of the nascent lipophorin. However, the applicant's laboratory has recently completed sequencing the cDNA for the 3,305 amino acid precursor apolipoprotein and is now in a position to study in detail the biosynthesis and assembly of lipophorin. (2) Determination of the mechanism(s) whereby lipids are transferred between lipophorin and tissues, specifically the fat body and the midgut. Lipophorin serves as a reusable shuttle which moves lipids from one tissue to another without itself entering the tissue. This means that lipid transfer from the from the midgut to lipophorin or from lipophorin to fat body occurs at the interface between the hemolymph and the plasma membrane of the cell. However, the details of the transfer process are unknown.
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