Comparative Metabolism of C14-Glucose in Two Strains ofTrypanosoma cruzi*

Gene expression to mitochondrial metabolism: Variability among cultured Trypanosoma cruzi strains

The insect-transmitted protozoan parasite Trypanosoma cruzi experiences changes in nutrient availability and rate of flux through different metabolic pathways across its life cycle. The species encompasses much genetic diversity of both the nuclear and mitochondrial genomes among isolated strains. The genetic or expression variation of both genomes are likely to impact metabolic responses to environmental stimuli, and even steady state metabolic function, among strains. To begin formal characterization these differences, we compared aspects of metabolism between genetically similar strains CL Brener and Tulahuen with less similar Esmeraldo and Sylvio X10 strains in a culture environment. Epimastigotes of all strains took up glucose at similar rates. However, the degree of medium acidification that could be observed when glucose was absent from the medium varied by strain, indicating potential differences in excreted metabolic byproducts. Our main focus was differences related to electron transport chain function. We observed differences in ATP-coupled respiration and maximal respiratory capacity, mitochondrial membrane potential, and mitochondrial morphology between strains, despite the fact that abundances of two nuclear-encoded proteins of the electron transport chain are similar between strains. RNA sequencing reveals strain-specific differences in abundances of mRNAs encoding proteins of the respiratory chain but also other metabolic processes. From these differences in metabolism and mitochondrial phenotypes we have generated tentative models for the differential metabolic fluxes or differences in gene expression that may underlie these results.

Deletion of transketolase triggers a stringent metabolic response in promastigotes and loss of virulence in amastigotes of Leishmania mexicana

Transketolase (TKT) is part of the non-oxidative branch of the pentose phosphate pathway (PPP). Here we describe the impact of removing this enzyme from the pathogenic protozoan Leishmania mexicana. Whereas the deletion had no obvious effect on cultured promastigote forms of the parasite, the Δtkt cells were not virulent in mice. Δtkt promastigotes were more susceptible to oxidative stress and various leishmanicidal drugs than wild-type, and metabolomics analysis revealed profound changes to metabolism in these cells. In addition to changes consistent with those directly related to the role of TKT in the PPP, central carbon metabolism was substantially decreased, the cells consumed significantly less glucose, flux through glycolysis diminished, and production of the main end products of metabolism was decreased. Only minor changes in RNA abundance from genes encoding enzymes in central carbon metabolism, however, were detected although fructose-1,6-bisphosphate aldolase activity was decreased two-fold in the knock-out cell line. We also showed that the dual localisation of TKT between cytosol and glycosomes is determined by the C-terminus of the enzyme and by engineering different variants of the enzyme we could alter its sub-cellular localisation. However, no effect on the overall flux of glucose was noted irrespective of whether the enzyme was found uniquely in either compartment, or in both.

Leishmania parasites endanger over 1 billion people worldwide, infecting 300,000 people and causing 20,000 deaths annually. In this study, we scrutinized metabolism in Leishmania mexicana after deletion of the gene encoding transketolase (TKT), an enzyme involved in sugar metabolism via the pentose phosphate pathway which plays key roles in creating ribose 5-phosphate for nucleotide synthesis and also defence against oxidative stress. The insect stage of the parasite, grown in culture medium, did not suffer from any obvious growth defect after the gene was deleted. However, its metabolism changed dramatically, with metabolomics indicating profound changes to flux through the pentose phosphate pathway: decreased glucose consumption, and generally enhanced efficiency in using metabolic substrates with reduced secretion of partially oxidised end products of metabolism. This ‘stringent’ metabolism is reminiscent of the mammalian stage parasites. The cells were also more sensitive to oxidative stress inducing agents and leishmanicidal drugs. Crucially, mice inoculated with the TKT knock-out parasites did not develop an infection pointing to the enzyme playing a key role in allowing the parasites to remain viable in the host, indicating that TKT may be considered a useful target for development of new drugs against leishmaniasis.

Expression, crystallization and preliminary X-ray crystallographic analysis of glucose-6-phosphate dehydrogenase from the human pathogen Trypanosoma cruzi in complex with substrate

An N-terminally truncated version of the enzyme glucose-6-phosphate dehydrogenase from Trypanosoma cruzi lacking the first 37 residues was crystallized both in its apo form and in a binary complex with glucose 6-phosphate. The crystals both belonged to space group P2(1) and diffracted to 2.85 and 3.35 Å resolution, respectively. Self-rotation function maps were consistent with point group 222. The structure was solved by molecular replacement, confirming a tetrameric quaternary structure.

Glucose-6-phosphate dehydrogenase of trypanosomatids: characterization, target validation, and drug discovery

In trypanosomatids, glucose-6-phosphate dehydrogenase (G6PDH), the first enzyme of the pentosephosphate pathway, is essential for the defense of the parasite against oxidative stress. Trypanosoma brucei, Trypanosoma cruzi, and Leishmania mexicana G6PDHs have been characterized. The parasites' G6PDHs contain a unique 37 amino acid long N-terminal extension that in T. cruzi seems to regulate the enzyme activity in a redox-state-dependent manner. T. brucei and T. cruzi G6PDHs, but not their Leishmania spp. counterpart, are inhibited, in an uncompetitive way, by steroids such as dehydroepiandrosterone and derivatives. The Trypanosoma enzymes are more susceptible to inhibition by these compounds than the human G6PDH. The steroids also effectively kill cultured trypanosomes but not Leishmania and are presently considered as promising leads for the development of new parasite-selective chemotherapeutic agents.

Enzymes of the antioxidant network as novel determiners of Trypanosoma cruzi virulence

Virulence of Trypanosoma cruzi depends on a variety of genetic and biochemical factors. It has been proposed that components of the parasites' antioxidant system may play a key part in this process by pre-adapting the pathogen to the oxidative environment encountered during host cell invasion. Using several isolates (10 strains) belonging to the two major phylogenetic lineages (T. cruzi-I and T. cruzi-II), we investigated whether there was an association between virulence (ranging from highly aggressive to attenuated isolates at the parasitemia and histopathological level) and the antioxidant enzyme content. Antibodies raised against trypanothione synthetase (TcTS), ascorbate peroxidase (TcAPX), mitochondrial and cytosolic tryparedoxin peroxidases (TcMPX and TcCPX) and trypanothione reductase (TcTR) were used to evaluate the antioxidant enzyme levels in epimastigote and metacyclic trypomastigote forms in the T. cruzi strains. Levels of TcCPX, TcMPX and TcTS were shown to increase during differentiation from the non-infective epimastigote to the infective metacyclic trypomastigote stage in all parasite strains examined. Peroxiredoxins were found to be present at higher levels in the metacyclic infective forms of the virulent isolates compared with the attenuated strains. Additionally, an increased resistance of epimastigotes from virulent T. cruzi populations to hydrogen peroxide and peroxynitrite challenge was observed. In mouse infection models, a direct correlation was found between protein levels of TcCPX, TcMPX and TcTS, and the parasitemia elicited by the different isolates studied (Pearson's coefficient: 0.617, 0.771, 0.499; respectively, P<0.01). No correlation with parasitemia was found for TcAPX and TcTR proteins in any of the strains analyzed. Our data support that enzymes of the parasite antioxidant armamentarium at the onset of infection represent new virulence factors involved in the establishment of disease.

The pentose phosphate pathway in Trypanosoma cruzi: a potential target for the chemotherapy of Chagas disease

Trypanosoma cruzi is highly sensitive to oxidative stress caused by reactive oxygen species. Trypanothione, the parasite's major protection against oxidative stress, is kept reduced by trypanothione reductase, using NADPH; the major source of the reduced coenzyme seems to be the pentose phosphate pathway. Its seven enzymes are present in the four major stages in the parasite's biological cycle; we have cloned and expressed them in Escherichia coli as active proteins. Glucose 6-phosphate dehydrogenase, which controls glucose flux through the pathway by its response to the NADP/NADPH ratio, is encoded by a number of genes per haploid genome, and is induced up to 46-fold by hydrogen peroxide in metacyclic trypomastigotes. The genes encoding 6-phosphogluconolactonase, 6-phosphogluconate dehydrogenase, transaldolase and transketolase are present in the CL Brener clone as a single copy per haploid genome. 6-phosphogluconate dehydrogenase is very unstable, but was stabilized introducing two salt bridges by site-directed mutagenesis. Ribose-5-phosphate isomerase belongs to Type B; genes encoding Type A enzymes, present in mammals, are absent. Ribulose-5-phosphate epimerase is encoded by two genes. The enzymes of the pathway have a major cytosolic component, although several of them have a secondary glycosomal localization, and also minor localizations in other organelles.

Trypanosoma cruzi strains, Tulahuen 2 and Y, besides the difference in resistance to oxidative stress, display differential glucose-6-phosphate and 6-phosphogluconate dehydrogenases activities

The drugs currently available for Chagas'disease treatment are unsatisfactory due to limited efficacy and toxic side effects, making the search for more specific pharmacological agents a priority. The components of the Trypanosoma cruzi trypanothione-dependent antioxidant system have been pointed out as potential chemotherapeutic targets for the development of more specific drugs. To work properly, this system must have a current supply of NADPH, provided by glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD). Here, we compare two T. cruzi strains, Tulahuen 2 and Y, regarding growth rate, cytosolic tryparedoxin peroxidase (TcCPX) concentration and pentose phosphate pathway dehydrogenases activities. Tulahuen 2 cells show higher values as compared to the Y strain when the following parameters are compared: TcCPX concentration, resistance to H2O2, growth index and G6PD activity. Different patterns of G6PD and 6PGD activities were observed among strains along the growth curve and when cells were challenged with H2O2. These data reinforce the heterogeneity within T. cruzi populations and also the importance of G6PD in protecting the parasite against reactive oxygen species.

The 6-phosphogluconate dehydrogenase from Trypanosoma cruzi: the absence of two inter-subunit salt bridges as a reason for enzyme instability

The third enzyme of the pentose phosphate pathway (PPP), 6-phosphogluconate dehydrogenase (6PGDH), is present in the four major stages of Trypanosoma cruzi, CL Brener clone. The enzyme was too unstable to be purified from epimastigote cell-free extracts. Two genes encoding 6PGDH were cloned and sequenced; the predicted amino acid sequences differ only in five non-essential residues. Since Southern blots suggested the presence of a single copy per haploid genome, the two genes found are probably alleles. One of these genes, encoding a protein with 78.6% identity with the Trypanosoma brucei 6PGDH, was expressed in Escherichia coli as an active recombinant enzyme, which was as unstable as the native 6PGDH. Modeling of the T. cruzi enzyme using the three-dimensional structure of the T. brucei 6PGDH as template suggested the lack of two out of five salt bridges proposed to strengthen subunit interactions in the active dimer. Restoring of these bridges by site-directed mutagenesis resulted in a more stable recombinant T. cruzi 6PGDH, which was used to determine the kinetic parameters. The K(m) value for 6-phosphogluconate (22.2+/-0.4 microM) was identical to the values reported for 6PGDHs from mammals, but the K(m) for NADP (5.9+/-0.2 microM) was significantly lower than the value reported for the human enzyme, and closer to that for the T. brucei enzyme. This suggests the possibility that inhibitors of the T. brucei 6PGDH, under development as potential drugs against African Trypanosomiasis, might also be successful for the chemotherapy of Chagas disease.

Differential behaviour of glucose 6-phosphate dehydrogenase in two morphological forms of Trypanosoma cruzi

1. Glucose 6-phosphate dehydrogenase activity (EC 1.1.1.49) of two morphological forms of Trypanosoma cruzi, epimastigotes and metacyclics, are reported. 2. The kinetic behaviour and some of the kinetic parameters of the enzyme in both forms were studied. The enzymes showed a simple Michaelis-Menten kinetic. 3. The activity in epimastigote forms was alway higher than the metacyclic ones. At subsaturating concentrations of substrate was almost 10-fold higher, whereas at saturating concentrations was about 2-fold higher. 4. In epimastigote forms the specific activity and Km values, at pH 7.5 and 37 degrees C, was found to be 142 mUnits x mg-1 of protein and 0.23 mM, respectively. 5. In the same conditions, the specific activity and Km values in metacyclic forms was 75 mUnits x mg-1 of protein and 1.06 mM, respectively. 6. A possible role in the carbohydrate metabolism of glucose 6-phosphate dehydrogenase in both forms of Trypanosoma cruzi is discussed.

The aerobic metabolism of metronidazole by Trypanosoma cruzi epimastigotes

Trypanosoma cruzi epimastigotes actively metabolize metronidazole under aerobic conditions to a polar compound tentatively identified as 2-methyl-5-nitroimidazole-1-yl-acetic acid. The rate of metabolite formation is increased by more than 50% by pretreatment with phenobarbital and inhibited by SKF-525A and metyrapone. The reaction is dramatically stimulated by the addition of flavone which suggests that the metabolite is produced via the cytochrome P-450 system. Apparently the nitro group in the metabolite is maintained intact. Detoxication reactions catalyzed by cytochrome P-450 appear to be more important than previously suspected as a basis to explain at least partially the resistance of these organisms to known antimicrobial agents. However, other factors such as the fate of nitro substituent in metronidazole require further evaluation.