Giardia lamblia is a protozoan parasite that causes various intestinal syndromes, and it is a common cause of water-borne illness worldwide, both in developed and developing countries. Giardia attaches to the mucosal epithelia of the duodenum below the bile duct, where it is exposed to bile salts and dietary lipids. G. lamblia is unable to synthesize lipids de novo and must therefore scavenge necessary lipids from its extracellular environment and remodel them as needed. However, the current lipidomic analysis (presented in this dissertation) has revealed that while the Giardia lipidome is rich in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG), its growth medium contains only PC, lyso-PC, and some diacylglycerol. Therefore, it is quite likely that Giardia has more lipid synthesis abilities than previously reported. The lipid analysis also includes fatty-acid analysis by GC-MS, which has revealed that Giardia trophozoites, encysting cells, and in vitro-derived cysts contain odd carbon-chain fatty acids (OCFAs), as well as a number of fatty acids not present in the medium, indicating that elongases and desaturases are also active in Giardia.

The Giardia Genome Database (giardiaDB.org) was searched for the presence of genes encoding enzymes for the synthesis of PG and PE—i.e., two newly synthesized lipids in Giardia. Analyses identified phosphatidylglycerol phosphate synthase (PGPS) and phosphatidylserine decarboxylase (PSD) genes, which are expressed throughout the Giardia life cycle. Furthermore, I also searched for the genes linked to the synthesis of phospholipid transporters that could be involved in importing phospholipids from outside sources. Interestingly, the search indicated that Giardia has the genes for flippase enzymes, which indicates that this parasite relies on scavenging lipids from the host. In order to further elucidate the mechanisms of lipid uptake and incorporation, I labeled cells with [¹³C]-glycerol isotope to determine whether generation of PG occurs via the base- or head-group exchange reactions between PC and glycerol. Side by side, I have also used fluorescent lipids as reporter molecules to understand whether Giardia uses flippase enzymes to uptake lipid molecules. These studies indicate that PG is indeed generated via head-group exchange and that uptake of PC and PE is completely dependent upon internalization through a flippase-like transporter. The current investigation suggests that Giardia has some capability to synthesize its own phospholipids de novo but that it mostly depends upon the supplies from outside sources. I speculate that lipid synthesis and transport systems that are operative in Giardia are interesting and may serve as potential targets for developing new therapies against this waterborne pathogen that affects millions of children worldwide, especially in poor countries.