Phosphatidylinositol synthesis is essential in bloodstream form Trypanosoma brucei

GPI-anchored proteins and free GPI glycolipids of procyclic form Trypanosoma brucei are nonessential for growth, are required for colonization of the tsetse fly, and are not the only components of the surface coat

The procyclic form of Trypanosoma brucei exists in the midgut of the tsetse fly. The current model of its surface glycocalyx is an array of rod-like procyclin glycoproteins with glycosylphosphatidylinositol (GPI) anchors carrying sialylated poly-N-acetyllactosamine side chains interspersed with smaller sialylated poly-N-acetyllactosamine-containing free GPI glycolipids. Mutants for TbGPI12, deficient in the second step of GPI biosynthesis, were devoid of cell surface procyclins and poly-N-acetyllactosamine-containing free GPI glycolipids. This major disruption to their surface architecture severely impaired their ability to colonize tsetse fly midguts but, surprisingly, had no effect on their morphology and growth characteristics in vitro. Transmission electron microscopy showed that the mutants retained a cell surface glycocalyx. This structure, and the viability of the mutants in vitro, prompted us to look for non-GPI-anchored parasite molecules and/or the adsorption of serum components. Neither were apparent from cell surface biotinylation experiments but [3H]glucosamine biosynthetic labeling revealed a group of previously unidentified high apparent molecular weight glycoconjugates that might contribute to the surface coat. While characterizing GlcNAc-PI that accumulates in the TbGPI12 mutant, we observed inositolphosphoceramides for the first time in this organism.

Adaptations in the lipid metabolism of the protozoan parasite Trypanosoma brucei

Trypanosomes are unicellular parasites and like all decent parasites, they try to obtain from the host as much material as possible, including lipids. However, the needs of a parasite are not always the same as those of the host, and therefore, mostly, some biosynthetic work still has to be done by the parasite itself. Very often at least modifications of the lipid components that are acquired from the host have to be made. Furthermore, next to the lipids Trypanosoma brucei indeed obtains from the host, some other lipid components have to be synthesized de novo. Especially the processes where the metabolism of T. brucei differs from that of the host, will be discussed, as at least some of them are excellent targets for the development of urgently needed new chemotherapeutics.

The glycosylphosphatidylinositol (GPI) biosynthetic pathway of bloodstream-form Trypanosoma brucei is dependent on the de novo synthesis of inositol

In bloodstream-form Trypanosoma brucei (the causative agent of African sleeping sickness) the glycosylphosphatidylinositol (GPI) anchor biosynthetic pathway has been validated genetically and chemically as a drug target. The conundrum that GPI anchors could not be in vivo labelled with [3H]-inositol led us to hypothesize that de novo synthesis was responsible for supplying myo-inositol for phosphatidylinositol (PI) destined for GPI synthesis. The rate-limiting step of the de novo synthesis is the isomerization of glucose 6-phosphate to 1-D-myo-inositol-3-phosphate, catalysed by a 1-D-myo-inositol-3-phosphate synthase (INO1). When grown under non-permissive conditions, a conditional double knockout demonstrated that INO1 is an essential gene in bloodstream-form T. brucei. It also showed that the de novo synthesized myo-inositol is utilized to form PI, which is preferentially used in GPI biosynthesis. We also show for the first time that extracellular myo-inositol can in fact be used in GPI formation although to a limited extent. Despite this, extracellular inositol cannot compensate for the deletion of INO1. Supporting these results, there was no change in PI levels in the conditional double knockout cells grown under non-permissive conditions, showing that perturbation of growth is due to a specific lack of de novo synthesized myo-inositol and not a general inositol-less death. These results suggest that there is a distinction between de novo synthesized myo-inositol and that from the extracellular environment.