Uptake and Cellular Localization of Exogenous Lipids byGiardia lamblia,a Primitive Eukaryote

Abstract

Giardia lambliatrophozoites are unable to carry outde novolipid synthesis. It is therefore likely that lipids are acquired from the small intestine of the host, in which the trophozoites are exposed to free and conjugated fatty acids, various sterols, phospholipids, bile acids, and bile–lipid mixed micelles. Here we show thatG. lambliais capable of taking up exogenous phosphatidylcholine (PC), phosphatidylinositol (PI), sphingomyelin (SM), cholesterol, ceramide (Cer), and fatty acids. Results from epifluorescence and high-resolution confocal microscopy suggest that fluorescent analogs of SM and PC were accumulated in the plasma membranes, whereas palmitic acid and Cer were localized intracellularly. Interestingly, many of these analogs were also concentrated in perinuclear regions. Similar labeling patterns were observed when the fluorescent analogs were delivered to the parasite via liposomes. To test whetherG. lambliawas capable of esterifying exogenous fatty acids into membrane or cellular phospholipids, trophozoites were pulse-labeled with3H-labeled palmitic or myristic acids and the phospholipids analyzed by thin-layer chromatography. Results document thatG. lambliawas able to incorporate exogenous fatty acids into various phospholipids, i.e., PI, PC, PE, and PG. Interestingly, a major portion of radiolabeled fatty acids was incorporated into PG, a phospholipid characteristic of prokaryotic membranes.