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International audience; Trypanosoma brucei is a parasitic protist that undergoes a complex life cycle during transmission from its mammalian host (bloodstream forms) to the midgut of its insect vector (procyclic form). In both parasitic forms, most glycolytic steps take place within specialized peroxisomes, called glycosomes. Here, we studied metabolic adaptations in procyclic trypanosome mutants affected in their maintenance of the glycosomal redox balance. T. brucei can theoretically use three strategies to maintain the glycosomal NAD(+)/NADH balance as follows: (i) the glycosomal succinic fermentation branch; (ii) the glycerol 3-phosphate (Gly-3-P)/dihydroxyacetone phosphate (DHAP) shuttle that transfers reducing equivalents to the mitochondrion; and (iii) the glycosomal glycerol production pathway. We showed a hierarchy in the use of these glycosomal NADH-consuming pathways by determining metabolic perturbations and adaptations in single and double mutant cell lines using a combination of NMR, ion chromatography-MS/MS, and HPLC approaches. Although functional, the Gly-3-P/DHAP shuttle is primarily used when the preferred succinate fermentation pathway is abolished in the Delta pepck knock-out mutant cell line. In the absence of these two pathways (Delta pepck/(RNAi)FAD-GPDH.i mutant), glycerol production is used but with a 16-fold reduced glycolytic flux. In addition, the Delta pepck mutant cell line shows a 3.3-fold reduced glycolytic flux compensated by an increase of proline metabolism. The inability of the Delta pepck mutant to maintain a high glycolytic flux demonstrates that the Gly-3-P/DHAP shuttle is not adapted to the procyclic trypanosome context. In contrast, this shuttle was shown earlier to be the only way used by the bloodstream forms of T. brucei to sustain their high glycolytic flux.

African trypanosomes possess high levels of alanine aminotransferase (EC 2.6.1.2), although the function of their activity remains enigmatic, especially in slender bloodstream forms where the metabolism of ketoacids does not occur. Therefore, the gene for alanine aminotransferase enzyme in Trypanosoma brucei (TbAAT) was characterized and its function assessed using a combination of RNA interference and gene knockout approaches. Surprisingly, as much as 95% or more of the activity appears to be unnecessary for growth of either bloodstream or procyclic forms respiring on glucose. A combination of RNA interference and NMR spectroscopy revealed an important role for the activity in procyclic forms respiring on proline. Under these conditions, the major end product of proline metabolism is alanine, and a reduction in TbAAT activity led to a proportionate decrease in the amount of alanine excreted along with an increase in the doubling time of the cells. These results provide evidence of a role for alanine aminotransferase in the metabolism of proline in African trypanosomes by linking glutamate produced by the initial oxidative steps of the pathway with pyruvate produced by the final oxidative step of the pathway. This step appears to be essential when proline is the primary carbon source, which is likely to be the physiological situation in the tsetse fly vector.

In his own words, it was the most dramatic moment of his scientific career. At the back of an auditorium in Nairobi, Kenya, Cyrus Bacchi met Simon Van Nieuwenhove. Bacchi, at the time, was essentially a biochemist whose interest was the African trypanosome. Van Nieuwenhove was a clinician who had worked for many years in the field in Africa. The topic of their conversation was eflornithine—a small and simple compound that would eventually make a big impact. Bacchi had shown that the compound had an effect on the parasite that causes Human African Trypanosomiasis. Van Nieuwenhove was dosing patients with it. The story of eflornithine did not start with eflornithine.It started because no one knew how to purify an enzyme. Bacchi was working on his thesis, which centred on the α-glycerophosphate shuttle in hemoflagellates. The shuttle acts as a way to transport reducing equivalents from the cytosol to the mitochondrion. α-glycerophosphate dehydrogenase was the enzyme he spent much of his time trying to purify. The reason he could not purify it was it was hidden within another compartment of the trypanosome. It would not be until several years later, and published in 1977, that Fred Opperdoes discovered the glycosome [1]: the organelle that encapsulates glycolysis in trypanosomes, and the organelle that was hiding the enzyme. At every step of the purification process Bacchi lost activity in his enzyme extracts. Magnesium chloride, surprisingly, managed to boost activity. Bacchi looked into other nonmetallic compounds that would act as cations and eventually chose the polyamines: naturally occurring, nitrogen-containing cations whose concentration is closely controlled by the cell. Spermidine and spermine were found to be the best replacements for magnesium, yielding higher activities. With evidence that the enzyme he was trying to purify was most likely locked within the glycosome, Bacchi moved his attention to how the polyamines were made within the trypanosome. This was an area that had amassed a lot of knowledge everywhere apart from in trypanosomes. It was in 1677 that Anton van Leeuwenhoek first observed spermatozoa in humans, dogs, and a host of other organisms, and he later discovered crystals of spermine phosphate in human semen. Modern research had identified the biologically active polyamines—spermidine and spermine—in plants and many types of mammalian cells. The biochemical pathways that make and degrade the polyamines, along with some of their enzymes, had also been identified. At the time, nothing was known about polyamines in protozoa, and in trypanosomes in particular. Did these parasites contain polyamines? Could polyamine metabolism be a useful chemotherapeutic target?

The Trypanosoma brucei procyclic form resides within the digestive tract of its insect vector, where it exploits amino acids as carbon sources. Threonine is the amino acid most rapidly consumed by this parasite, however its role is poorly understood. Here, we show that the procyclic trypanosomes grown in rich medium only use glucose and threonine for lipid biosynthesis, with threonine's contribution being ∼ 2.5 times higher than that of glucose. A combination of reverse genetics and NMR analysis of excreted end-products from threonine and glucose metabolism, shows that acetate, which feeds lipid biosynthesis, is also produced primarily from threonine. Interestingly, the first enzymatic step of the threonine degradation pathway, threonine dehydrogenase (TDH, EC 1.1.1.103), is under metabolic control and plays a key role in the rate of catabolism. Indeed, a trypanosome mutant deleted for the phosphoenolpyruvate decarboxylase gene (PEPCK, EC 4.1.1.49) shows a 1.7-fold and twofold decrease of TDH protein level and activity, respectively, associated with a 1.8-fold reduction in threonine-derived acetate production. We conclude that TDH expression is under control and can be downregulated in response to metabolic perturbations, such as in the PEPCK mutant in which the glycolytic metabolic flux was redirected towards acetate production.

Trypanosoma brucei is a kinetoplastid parasite of medical and veterinary importance. Its digenetic life cycle alternates between the bloodstream form in the mammalian host and the procyclic form (PCF) in the bloodsucking insect vector, the tsetse fly. PCF trypanosomes rely in the glucose-depleted environment of the insect vector primarily on the mitochondrial oxidative phosphorylation of proline for their cellular ATP provision. We previously identified two T. brucei mitochondrial carrier family proteins, TbMCP5 and TbMCP15, with significant sequence similarity to functionally characterized ADP/ATP carriers from other eukaryotes. Comprehensive sequence analysis confirmed that TbMCP5 contains canonical ADP/ATP carrier sequence features, whereas they are not conserved in TbMCP15. Heterologous expression in the ANC-deficient yeast strain JL1Δ2Δ3u(-) revealed that only TbMCP5 was able to restore its growth on the non-fermentable carbon source lactate. Transport studies in yeast mitochondria showed that TbMCP5 has biochemical properties and ADP/ATP exchange kinetics similar to those of Anc2p, the prototypical ADP/ATP carrier of S. cerevisiae. Immunofluorescence microscopy and Western blot analysis confirmed that TbMCP5 is exclusively mitochondrial and is differentially expressed with 4.5-fold more TbMCP5 in the procyclic form of the parasite. Silencing of TbMCP5 expression in PCF T. brucei revealed that this ADP/ATP carrier is essential for parasite growth, particularly when depending on proline for energy generation. Moreover, ADP/ATP exchange in isolated T. brucei mitochondria was eliminated upon TbMCP5 depletion. These results confirmed that TbMCP5 functions as the main ADP/ATP carrier in the trypanosome mitochondrion. The important role of TbMCP5 in the T. brucei energy metabolism is further discussed.

Insect stage trypanosomes use an "acetate shuttle" to transfer mitochondrial acetyl-CoA to the cytosol for the essential fatty acid biosynthesis. The mitochondrial acetate sources are acetate: succinate CoA-transferase (ASCT) and an unknown enzymatic activity. We have identified a gene encoding acetyl-CoA thioesterase (ACH) activity, which is shown to be the second acetate source. First, RNAi-mediated repression of ASCT in the ACH null background abolishes acetate production from glucose, as opposed to both single ASCT and ACH mutants. Second, incorporation of radiolabeled glucose into fatty acids is also abolished in this ACH/ASCT double mutant. ASCT is involved in ATP production, whereas ACH is not, because the ASCT null mutant is similar to 1000 times more sensitive to oligomycin, a specific inhibitor of the mitochondrial F-0/F-1-ATP synthase, than wild-type cells or the ACH null mutant. This was confirmed by RNAi repression of the F-0/F-1-ATP synthase F-1 beta subunit, which is lethal when performed in the ASCT null background but not in the wild-type cells or the ACH null background. We concluded that acetate is produced from both ASCT and ACH; however, only ASCT is responsible, together with the F-0/F-1-ATP synthase, for ATP production in the mitochondrion.

International audience; A single copy gene, encoding a protein highly similar to transketolase from other systems, was identified in the Trypanosoma brucei genome. The gene was expressed in E. coil and the purified protein demonstrated transketolase activity with Km values of 0.2 mM and 0.8 mM respectively for xylulose 5-phosphate and ribose 5-phosphate. A peroxisomal targeting signal (PTS-1) present at the C-terminus of the protein suggested a glycosomal localisation. However, subcellular localisation experiments revealed that while the protein was present in glycosomes it was found mainly within the cytosol and thus has a dual localisation. Transketolase activity was absent from the long slender bloodstream form of the parasite and the protein was not detectable in this life cycle stage, with the RNA present only at low abundance, indicating a strong differential regulation, being present predominantly in the procyclic form. The gene was knocked out from procyclic T. brucei and transketolase activity was lost but no growth phenotype was evident in the null mutants. Metabolite profiling to compare wild type and TKT null mutants revealed substantial increases in transketolase substrate metabolites coupled to loss of sedoheptulose 7-phosphate, a principal product of the transketolase reaction. (C) 2011 Elsevier B.V. All rights reserved.

Efficient synthetic routes have been developed for the preparation of two new polyazamacrocycles tagged with structural motifs recognised by the Trypanosoma brucei P2 aminopurine transporter. Biological testing of these compounds showed highly selective anti-protozoal activity against trypanosomes.

A focused library of C2-substituted-1,4,7,10-tetraazacyclododecanes was synthesised and the compounds were tested for their ability to kill trypanosome and malaria parasites. Several compounds showed significant in vitro activity and were selectively active against the parasites over human embryonic kidney cells used as a counter screen.