Energy metabolism and its compartmentation in Trypanosoma brucei

In silico prediction of the metabolism of Blastocrithidia nonstop, a trypanosomatid with non-canonical genetic code

BACKGROUND

Almost all extant organisms use the same, so-called canonical, genetic code with departures from it being very rare. Even more exceptional are the instances when a eukaryote with non-canonical code can be easily cultivated and has its whole genome and transcriptome sequenced. This is the case of Blastocrithidia nonstop, a trypanosomatid flagellate that reassigned all three stop codons to encode amino acids.

RESULTS

We in silico predicted the metabolism of B. nonstop and compared it with that of the well-studied human parasites Trypanosoma brucei and Leishmania major. The mapped mitochondrial, glycosomal and cytosolic metabolism contains all typical features of these diverse and important parasites. We also provided experimental validation for some of the predicted observations, concerning, specifically presence of glycosomes, cellular respiration, and assembly of the respiratory complexes.

CONCLUSIONS

In an unusual comparison of metabolism between a parasitic protist with a massively altered genetic code and its close relatives that rely on a canonical code we showed that the dramatic differences on the level of nucleic acids do not seem to be reflected in the metabolisms. Moreover, although the genome of B. nonstop is extremely AT-rich, we could not find any alterations of its pyrimidine synthesis pathway when compared to other trypanosomatids. Hence, we conclude that the dramatic alteration of the genetic code of B. nonstop has no significant repercussions on the metabolism of this flagellate.

Clethra fimbriata hexanic extract triggers alteration in the energy metabolism in epimastigotes of Trypanosoma cruzi

Chagas disease (ChD), caused by Trypanosoma cruzi, is endemic in American countries and an estimated 8 million people worldwide are chronically infected. Currently, only two drugs are available for therapeutic use against T. cruzi and their use is controversial due to several disadvantages associated with side effects and low compliance with treatment. Therefore, there is a need to search for new tripanocidal agents. Natural products have been considered a potential innovative source of effective and selective agents for drug development to treat T. cruzi infection. Recently, our research group showed that hexanic extract from Clethra fimbriata (CFHEX) exhibits anti-parasitic activity against all stages of T. cruzi parasite, being apoptosis the main cell death mechanism in both epimastigotes and trypomastigotes stages. With the aim of deepening the understanding of the mechanisms of death induced by CFHEX, the metabolic alterations elicited after treatment using a multiplatform metabolomics analysis (RP/HILIC-LC-QTOF-MS and GC-QTOF-MS) were performed. A total of 154 altered compounds were found significant in the treated parasites corresponding to amino acids (Arginine, threonine, cysteine, methionine, glycine, valine, proline, isoleucine, alanine, leucine, glutamic acid, and serine), fatty acids (stearic acid), glycerophospholipids (phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine), sulfur compounds (trypanothione) and carboxylic acids (pyruvate and phosphoenolpyruvate). The most affected metabolic pathways were mainly related to energy metabolism, which was found to be decrease during the evaluated treatment time. Further, exogenous compounds of the triterpene type (betulinic, ursolic and pomolic acid) previously described in C. fimbriata were found inside the treated parasites. Our findings suggest that triterpene-type compounds may contribute to the activity of CFHEX by altering essential processes in the parasite.

Targeting the alternative oxidase (AOX) for human health and food security, a pharmaceutical and agrochemical target or a rescue mechanism?

Some of the most threatening human diseases are due to a blockage of the mitochondrial electron transport chain (ETC). In a variety of plants, fungi, and prokaryotes, there is a naturally evolved mechanism for such threats to viability, namely a bypassing of the blocked portion of the ETC by alternative enzymes of the respiratory chain. One such enzyme is the alternative oxidase (AOX). When AOX is expressed, it enables its host to survive life-threatening conditions or, as in parasites, to evade host defenses. In vertebrates, this mechanism has been lost during evolution. However, we and others have shown that transfer of AOX into the genome of the fruit fly and mouse results in a catalytically engaged AOX. This implies that not only is the AOX a promising target for combating human or agricultural pathogens but also a novel approach to elucidate disease mechanisms or, in several cases, potentially a therapeutic cure for human diseases. In this review, we highlight the varying functions of AOX in their natural hosts and upon xenotopic expression, and discuss the resulting need to develop species-specific AOX inhibitors.

Oxidative Phosphorylation Is Required for Powering Motility and Development of the Sleeping Sickness Parasite Trypanosoma brucei in the Tsetse Fly Vector

The single-celled parasite Trypanosoma brucei is transmitted by hematophagous tsetse flies. Life cycle progression from mammalian bloodstream form to tsetse midgut form and, subsequently, infective salivary gland form depends on complex developmental steps and migration within different fly tissues. As the parasite colonizes the glucose-poor insect midgut, ATP production is thought to depend on activation of mitochondrial amino acid catabolism via oxidative phosphorylation (OXPHOS). This process involves respiratory chain complexes and F₁F_o-ATP synthase and requires protein subunits of these complexes that are encoded in the parasite's mitochondrial DNA (kDNA). Here, we show that progressive loss of kDNA-encoded functions correlates with a decreasing ability to initiate and complete development in the tsetse. First, parasites with a mutated F₁F_o-ATP synthase with reduced capacity for OXPHOS can initiate differentiation from bloodstream to insect form, but they are unable to proliferate in vitro. Unexpectedly, these cells can still colonize the tsetse midgut. However, these parasites exhibit a motility defect and are severely impaired in colonizing or migrating to subsequent tsetse tissues. Second, parasites with a fully disrupted F₁F_o-ATP synthase complex that is completely unable to produce ATP by OXPHOS can still differentiate to the first insect stage in vitro but die within a few days and cannot establish a midgut infection in vivo. Third, parasites lacking kDNA entirely can initiate differentiation but die soon after. Together, these scenarios suggest that efficient ATP production via OXPHOS is not essential for initial colonization of the tsetse vector but is required to power trypanosome migration within the fly. IMPORTANCE African trypanosomes cause disease in humans and their livestock and are transmitted by tsetse flies. The insect ingests these parasites with its blood meal, but to be transmitted to another mammal, the trypanosome must undergo complex development within the tsetse fly and migrate from the insect's gut to its salivary glands. Crucially, the parasite must switch from a sugar-based diet while in the mammal to a diet based primarily on amino acids when it develops in the insect. Here, we show that efficient energy production by an organelle called the mitochondrion is critical for the trypanosome's ability to swim and to migrate through the tsetse fly. Surprisingly, trypanosomes with impaired mitochondrial energy production are only mildly compromised in their ability to colonize the tsetse fly midgut. Our study adds a new perspective to the emerging view that infection of tsetse flies by trypanosomes is more complex than previously thought.

Heterotrophic euglenid Rhabdomonas costata resembles its phototrophic relatives in many aspects of molecular and cell biology

Euglenids represent a group of protists with diverse modes of feeding. To date, only a partial genomic sequence of Euglena gracilis and transcriptomes of several phototrophic and secondarily osmotrophic species are available, while primarily heterotrophic euglenids are seriously undersampled. In this work, we begin to fill this gap by presenting genomic and transcriptomic drafts of a primary osmotroph, Rhabdomonas costata. The current genomic assembly length of 100 Mbp is 14× smaller than that of E. gracilis. Despite being too fragmented for comprehensive gene prediction it provided fragments of the mitochondrial genome and comparison of the transcriptomic and genomic data revealed features of its introns, including several candidates for nonconventional types. A set of 39,456 putative R. costata proteins was predicted from the transcriptome. Annotation of the mitochondrial core metabolism provides the first data on the facultatively anaerobic mitochondrion of R. costata, which in most respects resembles the mitochondrion of E. gracilis with a certain level of streamlining. R. costata can synthetise thiamine by enzymes of heterogenous provenances and haem by a mitochondrial-cytoplasmic C4 pathway with enzymes orthologous to those found in E. gracilis. The low percentage of green algae-affiliated genes supports the ancestrally osmotrophic status of this species.

Bioenergetic consequences of FoF1-ATP synthase/ATPase deficiency in two life cycle stages of Trypanosoma brucei

Mitochondrial ATP synthase is a reversible nanomotor synthesizing or hydrolyzing ATP depending on the potential across the membrane in which it is embedded. In the unicellular parasite Trypanosoma brucei, the direction of the complex depends on the life cycle stage of this digenetic parasite: in the midgut of the tsetse fly vector (procyclic form), the F_oF₁–ATP synthase generates ATP by oxidative phosphorylation, whereas in the mammalian bloodstream form, this complex hydrolyzes ATP and maintains mitochondrial membrane potential (ΔΨm). The trypanosome F_oF₁–ATP synthase contains numerous lineage-specific subunits whose roles remain unknown. Here, we seek to elucidate the function of the lineage-specific protein Tb1, the largest membrane-bound subunit. In procyclic form cells, Tb1 silencing resulted in a decrease of F_oF₁–ATP synthase monomers and dimers, rerouting of mitochondrial electron transfer to the alternative oxidase, reduced growth rate and cellular ATP levels, and elevated ΔΨm and total cellular reactive oxygen species levels. In bloodstream form parasites, RNAi silencing of Tb1 by ∼90% resulted in decreased F_oF₁–ATPase monomers and dimers, but it had no apparent effect on growth. The same findings were obtained by silencing of the oligomycin sensitivity-conferring protein, a conserved subunit in T. brucei F_oF₁–ATP synthase. However, as expected, nearly complete Tb1 or oligomycin sensitivity-conferring protein suppression was lethal because of the inability to sustain ΔΨm. The diminishment of F_oF₁–ATPase complexes was further accompanied by a decreased ADP/ATP ratio and reduced oxygen consumption via the alternative oxidase. Our data illuminate the often diametrically opposed bioenergetic consequences of F_oF₁–ATP synthase loss in insect versus mammalian forms of the parasite.

The Remarkable Metabolism of Vickermania ingenoplastis: Genomic Predictions

A recently redescribed two-flagellar trypanosomatid Vickermania ingenoplastis is insensitive to the classical inhibitors of respiration and thrives under anaerobic conditions. Using genomic and transcriptomic data, we analyzed its genes of the core metabolism and documented that subunits of the mitochondrial respiratory complexes III and IV are ablated, while those of complexes I, II, and V are all present, along with an alternative oxidase. This explains the previously reported conversion of glucose to acetate and succinate by aerobic fermentation. Glycolytic pyruvate is metabolized to acetate and ethanol by pyruvate dismutation, whereby a unique type of alcohol dehydrogenase (shared only with Phytomonas spp.) processes an excess of reducing equivalents formed under anaerobic conditions, leading to the formation of ethanol. Succinate (formed to maintain the glycosomal redox balance) is converted to propionate by a cyclic process involving three enzymes of the mitochondrial methyl-malonyl-CoA pathway, via a cyclic process, which results in the formation of additional ATP. The unusual structure of the V. ingenoplastis genome and its similarity with that of Phytomonas spp. imply their relatedness or convergent evolution. Nevertheless, a critical difference between these two trypanosomatids is that the former has significantly increased its genome size by gene duplications, while the latter streamlined its genome.

ATP regulates the activity of an alternative oxidase in Trypanosoma brucei

The reduced mitochondrial respiratory chain from the bloodstream forms of Trypanosoma brucei is composed of only a membrane-bound glycerol-3-phosphate dehydrogenase and an alternative oxidase. Since these enzymes are not proton pumps, their functions are restricted to the maintenance of the redox balance in the glycosome by means of the dihydroxyacetone phosphate/glycerol-3-phosphate shuttle. Additionally, an F1 Fo -ATP synthase functions as an ATP-hydrolysing enzyme to establish the proton motive force necessary to maintain the basic functions of mitochondria. In this report, we studied the interplay between the alternative oxidase and ATP synthase, and we found that, in addition to its role as a proton pump, ATP synthase contributes to maintain safe levels of ATP to prevent the inhibition of the alternative oxidase by ATP.

Cell-line specific RNA editing patterns in Trypanosoma brucei suggest a unique mechanism to generate protein variation in a system intolerant to genetic mutations

Trypanosoma brucei possesses a highly complex RNA editing system that uses guide RNAs to direct the insertion and deletion of uridines in mitochondrial mRNAs. These changes extensively alter the target mRNAs and can more than double them in length. Recently, analyses showed that several of the edited genes possess the capacity to encode two different protein products. The overlapped reading frames can be accessed through alternative RNA editing that shifts the translated reading frame. In this study, we analyzed the editing patterns of three putative dual-coding genes, ribosomal protein S12 (RPS12), the 5′ editing domain of NADH dehydrogenase subunit 7 (ND7 5′), and C-rich region 3 (CR3). We found evidence that alternatively 5′-edited ND7 5′ and CR3 transcripts are present in the transcriptome, providing evidence for the use of dual ORFs in these transcripts. Moreover, we found that CR3 has a complex set of editing pathways that vary substantially between cell lines. These findings suggest that alternative editing can work to introduce genetic variation in a system that selects against nucleotide mutations.

Trypanosomatid parasite dynamically changes the transcriptome during infection and modifies honey bee physiology

It is still not understood how honey bee parasite changes the gene expression to adapt to the host environment and how the host simultaneously responds to the parasite infection by modifying its own gene expression. To address this question, we studied a trypanosomatid, Lotmaria passim, which can be cultured in medium and inhabit the honey bee hindgut. We found that L. passim decreases mRNAs associated with protein translation, glycolysis, detoxification of radical oxygen species, and kinetoplast respiratory chain to adapt to the anaerobic and nutritionally poor honey bee hindgut during the infection. After the long term infection, the host appears to be in poor nutritional status, indicated by the increase and decrease of take-out and vitellogenin mRNAs, respectively. Simultaneous gene expression profiling of L. passim and honey bee during infection by dual RNA-seq provided insight into how both parasite and host modify their gene expressions.

Transcriptional responses of Leishmania (Leishmania) amazonensis in the presence of trivalent sodium stibogluconate

Background

In the last decade, resistance to antimonials has become a serious problem due to the emergence of drug-resistant strains. Therefore, understanding the mechanisms used by Leishmania parasites to survive under drug pressure is essential, particularly for species of medical-veterinary importance such as L. amazonensis.

Methods

Here, we used RNA-seq technology to analyse transcriptome profiles and identify global changes in gene expression between antimony-resistant and -sensitive L. amazonensis promastigotes.

Results

A total of 723 differentially expressed genes were identified between resistant and sensitive lines. Comparative transcriptomic analysis revealed that genes encoding proteins involved in metabolism (fatty acids) and stress response, as well as those associated with antimony resistance in other Leishmania species, were upregulated in the antimony-resistant line. Most importantly, we observed upregulation of genes encoding autophagy proteins, suggesting that in the presence of trivalent stibogluconate (Sb^III) L. amazonensis can activate these genes either as a survival strategy or to induce cell death, as has been observed in other parasites.

Conclusions

This work identified global transcriptomic changes in an in vitro-adapted strain in response to Sb^III. Our results provide relevant information to continue understanding the mechanism used by parasites of the subgenus Leishmania (L. amazonensis) to generate an antimony-resistant phenotype.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3603-8) contains supplementary material, which is available to authorized users.

Discovery of Sustainable Drugs for Neglected Tropical Diseases: Cashew Nut Shell Liquid (CNSL)-Based Hybrids Target Mitochondrial Function and ATP Production in Trypanosoma brucei

In the search for effective and sustainable drugs for human African trypanosomiasis (HAT), we developed hybrid compounds by merging the structural features of quinone 4 (2-phenoxynaphthalene-1,4-dione) with those of phenolic constituents from cashew nut shell liquid (CNSL). CNSL is a waste product from cashew nut processing factories, with great potential as a source of drug precursors. The synthesized compounds were tested against Trypanosoma brucei brucei, including three multidrug-resistant strains, T. congolense, and a human cell line. The most potent activity was found against T. b. brucei, the causative agent of HAT. Shorter-chain derivatives 20 (2-(3-(8-hydroxyoctyl)phenoxy)-5-methoxynaphthalene-1,4-dione) and 22 (5-hydroxy-2-(3-(8-hydroxyoctyl)phenoxy)naphthalene-1,4-dione) were more active than 4, displaying rapid micromolar trypanocidal activity, and no human cytotoxicity. Preliminary studies probing their mode of action on trypanosomes showed ATP depletion, followed by mitochondrial membrane depolarization and mitochondrion ultrastructural damage. This was accompanied by reactive oxygen species production. We envisage that such compounds, obtained from a renewable and inexpensive material, might be promising bio-based sustainable hits for anti-trypanosomatid drug discovery.

Mitochondrial DNA is critical for longevity and metabolism of transmission stage Trypanosoma brucei

The sleeping sickness parasite Trypanosoma brucei has a complex life cycle, alternating between a mammalian host and the tsetse fly vector. A tightly controlled developmental programme ensures parasite transmission between hosts as well as survival within them and involves strict regulation of mitochondrial activities. In the glucose-rich bloodstream, the replicative 'slender' stage is thought to produce ATP exclusively via glycolysis and uses the mitochondrial F1FO-ATP synthase as an ATP hydrolysis-driven proton pump to generate the mitochondrial membrane potential (ΔΨm). The 'procyclic' stage in the glucose-poor tsetse midgut depends on mitochondrial catabolism of amino acids for energy production, which involves oxidative phosphorylation with ATP production via the F1FO-ATP synthase. Both modes of the F1FO enzyme critically depend on FO subunit a, which is encoded in the parasite's mitochondrial DNA (kinetoplast or kDNA). Comparatively little is known about mitochondrial function and the role of kDNA in non-replicative 'stumpy' bloodstream forms, a developmental stage essential for disease transmission. Here we show that the L262P mutation in the nuclear-encoded F1 subunit γ that permits survival of 'slender' bloodstream forms lacking kDNA ('akinetoplastic' forms), via FO-independent generation of ΔΨm, also permits their differentiation into stumpy forms. However, these akinetoplastic stumpy cells lack a ΔΨm and have a reduced lifespan in vitro and in mice, which significantly alters the within-host dynamics of the parasite. We further show that generation of ΔΨm in stumpy parasites and their ability to use α-ketoglutarate to sustain viability depend on F1-ATPase activity. Surprisingly, however, loss of ΔΨm does not reduce stumpy life span. We conclude that the L262P γ subunit mutation does not enable FO-independent generation of ΔΨm in stumpy cells, most likely as a consequence of mitochondrial ATP production in these cells. In addition, kDNA-encoded genes other than FO subunit a are important for stumpy form viability.

A plant-like mitochondrial carrier family protein facilitates mitochondrial transport of di- and tricarboxylates in Trypanosoma brucei

The procyclic form of the human parasite Trypanosoma brucei harbors one single, large mitochondrion containing all tricarboxylic acid (TCA) cycle enzymes and respiratory chain complexes present also in higher eukaryotes. Metabolite exchange among subcellular compartments such as the cytoplasm, the mitochondrion, and the peroxisomes is crucial for redox homeostasis and for metabolic pathways whose enzymes are dispersed among different organelles. In higher eukaryotes, mitochondrial carrier family (MCF) proteins transport TCA-cycle intermediates across the inner mitochondrial membrane. Previously, we identified several MCF members that are essential for T. brucei survival. Among these, only one MCF protein, TbMCP12, potentially could transport dicarboxylates and tricarboxylates. Here, we conducted phylogenetic and sequence analyses and functionally characterised TbMCP12 in vivo. Our results suggested that similarly to its homologues in plants, TbMCP12 transports both dicarboxylates and tricarboxylates across the mitochondrial inner membrane. Deleting this carrier in T. brucei was not lethal, while its overexpression was deleterious. Our results suggest that the intracellular abundance of TbMCP12 is an important regulatory element for the NADPH balance and mitochondrial ATP-production.

Transcriptome Profiling of Trypanosoma brucei Development in the Tsetse Fly Vector Glossina morsitans

African trypanosomes, the causative agents of sleeping sickness in humans and nagana in animals, have a complex digenetic life cycle between a mammalian host and an insect vector, the blood-feeding tsetse fly. Although the importance of the insect vector to transmit the disease was first realized over a century ago, many aspects of trypanosome development in tsetse have not progressed beyond a morphological analysis, mainly due to considerable challenges to obtain sufficient material for molecular studies. Here, we used high-throughput RNA-Sequencing (RNA-Seq) to profile Trypanosoma brucei transcript levels in three distinct tissues of the tsetse fly, namely the midgut, proventriculus and salivary glands. Consistent with current knowledge and providing a proof of principle, transcripts coding for procyclin isoforms and several components of the cytochrome oxidase complex were highly up-regulated in the midgut transcriptome, whereas transcripts encoding metacyclic VSGs (mVSGs) and the surface coat protein brucei alanine rich protein or BARP were extremely up-regulated in the salivary gland transcriptome. Gene ontology analysis also supported the up-regulation of biological processes such as DNA metabolism and DNA replication in the proventriculus transcriptome and major changes in signal transduction and cyclic nucleotide metabolism in the salivary gland transcriptome. Our data highlight a small repertoire of expressed mVSGs and potential signaling pathways involving receptor-type adenylate cyclases and members of a surface carboxylate transporter family, called PADs (Proteins Associated with Differentiation), to cope with the changing environment, as well as RNA-binding proteins as a possible global regulators of gene expression.

Ribose 5-phosphate isomerase B knockdown compromises Trypanosoma brucei bloodstream form infectivity

Ribose 5-phosphate isomerase is an enzyme involved in the non-oxidative branch of the pentose phosphate pathway, and catalyzes the inter-conversion of D-ribose 5-phosphate and D-ribulose 5-phosphate. Trypanosomatids, including the agent of African sleeping sickness namely Trypanosoma brucei, have a type B ribose-5-phosphate isomerase. This enzyme is absent from humans, which have a structurally unrelated ribose 5-phosphate isomerase type A, and therefore has been proposed as an attractive drug target waiting further characterization. In this study, Trypanosoma brucei ribose 5-phosphate isomerase B showed in vitro isomerase activity. RNAi against this enzyme reduced parasites' in vitro growth, and more importantly, bloodstream forms infectivity. Mice infected with induced RNAi clones exhibited lower parasitaemia and a prolonged survival compared to control mice. Phenotypic reversion was achieved by complementing induced RNAi clones with an ectopic copy of Trypanosoma cruzi gene. Our results present the first functional characterization of Trypanosoma brucei ribose 5-phosphate isomerase B, and show the relevance of an enzyme belonging to the non-oxidative branch of the pentose phosphate pathway in the context of Trypanosoma brucei infection.

Within the non-oxidative branch of the pentose phosphate pathway, ribose 5-phosphate isomerase catalyzes the inter-conversion of ribose 5-phosphate and ribulose 5-phosphate. There are two types of ribose 5-phosphate isomerase, namely A and B. The presence of type B in Trypanosoma brucei, and its absence in humans, make this protein a promising drug target. African sleeping sickness is a serious parasitic disease that relies on limited chemotherapeutic options for control. In our study, a functional characterization of Trypanosoma brucei ribose 5-phosphate isomerase B is reported. Biochemical studies confirmed enzyme isomerase activity and its downregulation by RNAi affected mainly parasites infectivity in vivo. Overall this study shows that ribose 5-phosphate isomerase depletion is detrimental for parasites infectivity under host pressure.

Fe/S protein biogenesis in trypanosomes - A review

Trypanosoma brucei, the causative agent of the African sleeping sickness of humans, and other kinetoplastid flagellates belong to the eukarytotic supergroup Excavata. This early-branching model protist is known for a broad range of unique features. As it is amenable to most techniques of forward and reverse genetics, T. brucei was subject to several studies of its iron-sulfur (Fe/S) protein biogenesis and thus represents the best studied excavate eukaryote. Here we review what is known about the Fe/S protein biogenesis of T. brucei, and focus especially on the comparative and evolutionary interesting aspects. We also explore the connections between the well-known and quite conserved ISC and CIA machineries and the tRNA thiolation pathway. Moreover, the Fe/S cluster protein biogenesis is dissected in the procyclic stage of T. brucei which has an active mitochondrion, as well as in its pathogenic bloodstream stage with a metabolically repressed organelle. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Mitochondrial and nucleolar localization of cysteine desulfurase Nfs and the scaffold protein Isu in Trypanosoma brucei

Trypanosoma brucei has a complex life cycle during which its single mitochondrion is subjected to major metabolic and morphological changes. While the procyclic stage (PS) of the insect vector contains a large and reticulated mitochondrion, its counterpart in the bloodstream stage (BS) parasitizing mammals is highly reduced and seems to be devoid of most functions. We show here that key Fe-S cluster assembly proteins are still present and active in this organelle and that produced clusters are incorporated into overexpressed enzymes. Importantly, the cysteine desulfurase Nfs, equipped with the nuclear localization signal, was detected in the nucleolus of both T. brucei life stages. The scaffold protein Isu, an interacting partner of Nfs, was also found to have a dual localization in the mitochondrion and the nucleolus, while frataxin and both ferredoxins are confined to the mitochondrion. Moreover, upon depletion of Isu, cytosolic tRNA thiolation dropped in the PS but not BS parasites.

Iron-sulphur clusters, their biosynthesis, and biological functions in protozoan parasites

Fe-S clusters are ensembles of sulphide-linked di-, tri-, and tetra-iron centres of a variety of metalloproteins that play important roles in reduction and oxidation of mitochondrial electron transport, energy metabolism, regulation of gene expression, cell survival, nitrogen fixation, and numerous other metabolic pathways. The Fe-S clusters are assembled by one of four distinct systems: NIF, SUF, ISC, and CIA machineries. The ISC machinery is a house-keeping system conserved widely from prokaryotes to higher eukaryotes, while the other systems are present in a limited range of organisms and play supplementary roles under certain conditions such as stress. Fe-S cluster-containing proteins and the components required for Fe-S cluster biosynthesis are modulated under stress conditions, drug resistance, and developmental stages. It is also known that a defect in Fe-S proteins and Fe-S cluster biogenesis leads to many genetic disorders in humans, which indicates the importance of the systems. In this review, we describe the biological and physiological significance of Fe-S cluster-containing proteins and their biosynthesis in parasitic protozoa including Plasmodium, Trypanosoma, Leishmania, Giardia, Trichomonas, Entamoeba, Cryptosporidium, Blastocystis, and microsporidia. We also discuss the roles of Fe-S cluster biosynthesis in proliferation, differentiation, and stress response in protozoan parasites. The heterogeneity of the systems and the compartmentalization of Fe-S cluster biogenesis in the protozoan parasites likely reflect divergent evolution under highly diverse environmental niches, and influence their parasitic lifestyle and pathogenesis. Finally, both Fe-S cluster-containing proteins and their biosynthetic machinery in protozoan parasites are remarkably different from those in their mammalian hosts. Thus, they represent a rational target for the development of novel chemotherapeutic and prophylactic agents against protozoan infections.

Divergence of Erv1-associated mitochondrial import and export pathways in trypanosomes and anaerobic protists

In yeast (Saccharomyces cerevisiae) and animals, the sulfhydryl oxidase Erv1 functions with Mia40 in the import and oxidative folding of numerous cysteine-rich proteins in the mitochondrial intermembrane space (IMS). Erv1 is also required for Fe-S cluster assembly in the cytosol, which uses at least one mitochondrially derived precursor. Here, we characterize an essential Erv1 orthologue from the protist Trypanosoma brucei (TbERV1), which naturally lacks a Mia40 homolog. We report kinetic parameters for physiologically relevant oxidants cytochrome c and O(2), unexpectedly find O(2) and cytochrome c are reduced simultaneously, and demonstrate that efficient reduction of O(2) by TbERV1 is not dependent upon a simple O(2) channel defined by conserved histidine and tyrosine residues. Massive mitochondrial swelling following TbERV1 RNA interference (RNAi) provides evidence that trypanosome Erv1 functions in IMS protein import despite the natural absence of the key player in the yeast and animal import pathways, Mia40. This suggests significant evolutionary divergence from a recently established paradigm in mitochondrial cell biology. Phylogenomic profiling of genes also points to a conserved role for TbERV1 in cytosolic Fe-S cluster assembly. Conversely, loss of genes implicated in precursor delivery for cytosolic Fe-S assembly in Entamoeba, Trichomonas, and Giardia suggests fundamental differences in intracellular trafficking pathways for activated iron or sulfur species in anaerobic versus aerobic eukaryotes.

Biochemistry and evolution of anaerobic energy metabolism in eukaryotes

Major insights into the phylogenetic distribution, biochemistry, and evolutionary significance of organelles involved in ATP synthesis (energy metabolism) in eukaryotes that thrive in anaerobic environments for all or part of their life cycles have accrued in recent years. All known eukaryotic groups possess an organelle of mitochondrial origin, mapping the origin of mitochondria to the eukaryotic common ancestor, and genome sequence data are rapidly accumulating for eukaryotes that possess anaerobic mitochondria, hydrogenosomes, or mitosomes. Here we review the available biochemical data on the enzymes and pathways that eukaryotes use in anaerobic energy metabolism and summarize the metabolic end products that they generate in their anaerobic habitats, focusing on the biochemical roles that their mitochondria play in anaerobic ATP synthesis. We present metabolic maps of compartmentalized energy metabolism for 16 well-studied species. There are currently no enzymes of core anaerobic energy metabolism that are specific to any of the six eukaryotic supergroup lineages; genes present in one supergroup are also found in at least one other supergroup. The gene distribution across lineages thus reflects the presence of anaerobic energy metabolism in the eukaryote common ancestor and differential loss during the specialization of some lineages to oxic niches, just as oxphos capabilities have been differentially lost in specialization to anoxic niches and the parasitic life-style. Some facultative anaerobes have retained both aerobic and anaerobic pathways. Diversified eukaryotic lineages have retained the same enzymes of anaerobic ATP synthesis, in line with geochemical data indicating low environmental oxygen levels while eukaryotes arose and diversified.

Proliferating bloodstream-form Trypanosoma brucei use a negligible part of consumed glucose for anabolic processes

Our quantitative knowledge of carbon fluxes in the long slender bloodstream form (BSF) Trypanosoma brucei is mainly based on non-proliferating parasites, isolated from laboratory animals and kept in buffers. In this paper we present a carbon balance for exponentially growing bloodstream form trypanosomes. The cells grew with a doubling time of 5.3h, contained 46 μ mol of carbon (10(8) cells)(-1) and had a glucose consumption flux of 160 nmol min(-1) (10(8) cells)(-1). The molar ratio of pyruvate excreted versus glucose consumed was 2.1. Furthermore, analysis of the (13)C label distribution in pyruvate in (13)C-glucose incubations of exponentially growing trypanosomes showed that glucose was the sole substrate for pyruvate production. We conclude that the glucose metabolised in glycolysis was hardly, if at all, used for biosynthetic processes. Carbon flux through glycolysis in exponentially growing trypanosomes was 10 times higher than the incorporation of carbon into biomass. This biosynthetic carbon is derived from other precursors present in the nutrient rich growth medium. Furthermore, we found that the glycolytic flux was unaltered when the culture went into stationary phase, suggesting that most of the ATP produced in glycolysis is used for processes other than growth.

The characterization of a unique Trypanosoma brucei β-hydroxybutyrate dehydrogenase

A putative β-hydroxybutyrate dehydrogenase (βHBDH) ortholog was identified in Trypanosoma brucei, the unicellular eukaryotic parasite responsible for causing African Sleeping Sickness. The trypanosome enzyme has greater sequence similarity to bacterial sources of soluble βHBDH than to membrane-bound Type I βHBDH found in higher eukaryotes. The βHBDH gene was cloned from T. brucei genomic DNA and active, recombinant His-tagged enzyme (His(10)-TbβHBDH) was purified to approximate homogeneity from E. coli. βHBDH catalyzes the reversible NADH-dependent conversion of acetoacetate to D-3-hydroxybutyrate. In the direction of D-3-hydroxybutyrate formation, His(10)-TbβHBDH has a k(cat) value of 0.19 s(-1) and a K(M) value of 0.69 mM for acetoacetate. In the direction of acetoacetate formation, His(10)-TbβHBDH has a k(cat) value of 11.2 s(-1) and a K(M) value of 0.65 mM for D-3-hydroxybutyrate. Cofactor preference was examined and His(10)-TbβHBDH utilizes both NAD(H) and NADP(H) almost equivalently, distinguishing the parasite enzyme from other characterized βHBDHs. Furthermore, His(10)-TbβHBDH binds NAD(P)(+) in a cooperative fashion, another unique characteristic of trypanosome βHBDH. The apparent native molecular weight of recombinant His(10)-TbβHBDH is 112 kDa, corresponding to tetramer, as determined through size exclusion chromatography. RNA interference studies in procyclic trypanosomes were carried out to evaluate the importance of TbβHBDH in vivo. Upon knockdown of TbβHBDH, a small reduction in parasite growth was observed suggesting βHBDH has an important physiological role in T. brucei.

Identification and characterization of genes involved in leishmania pathogenesis: the potential for drug target selection

Identifying and characterizing Leishmania donovani genes and the proteins they encode for their role in pathogenesis can reveal the value of this approach for finding new drug targets. Effective drug targets are likely to be proteins differentially expressed or required in the amastigote life cycle stage found in the patient. Several examples and their potential for chemotherapeutic disruption are presented. A pathway nearly ubiquitous in living cells targeted by anticancer drugs, the ubiquitin system, is examined. New findings in ubiquitin and ubiquitin-like modifiers in Leishmania show how disruption of those pathways could point to additional drug targets. The programmed cell death pathway, now recognized among protozoan parasites, is reviewed for some of its components and evidence that suggests they could be targeted for antiparasitic drug therapy. Finally, the endoplasmic reticulum quality control system is involved in secretion of many virulence factors. How disruptions in this pathway reduce virulence as evidence for potential drug targets is presented.

Comparative systems biology: from bacteria to man

Comparative analyses, as carried out by comparative genomics and bioinformatics, have proven extremely powerful to obtain insight into the identity of specific genes that underlie differences and similarities across species. The central concept developed in this chapter is that important aspects of the functional differences between organisms derive not only from the differences in genetic components (which underlies comparative genomics) but also from dynamic, molecular (physical) interactions. Approaches that aim at identifying such network-based rather than component-based homologies between species we shall call Comparative Systems Biology. It will be illustrated by a number of examples from metabolic networks from prokaryotes, via yeast, to man. The potential for species comparisons, at the genome-scale using classical approaches and at the more detailed level of dynamic molecular networks will be illustrated. In our opinion, comparative systems biology, as a marriage between bioinformatics and systems biology, will offer new insights into the nature of organisms for the benefit of medicine, biotechnology, and drug design. As dynamic modeling is becoming more mainstream in cell biology, the potential of comparative systems biology will become more evident.

Intermediary metabolism in protists: a sequence-based view of facultative anaerobic metabolism in evolutionarily diverse eukaryotes

Protists account for the bulk of eukaryotic diversity. Through studies of gene and especially genome sequences the molecular basis for this diversity can be determined. Evident from genome sequencing are examples of versatile metabolism that go far beyond the canonical pathways described for eukaryotes in textbooks. In the last 2-3 years, genome sequencing and transcript profiling has unveiled several examples of heterotrophic and phototrophic protists that are unexpectedly well-equipped for ATP production using a facultative anaerobic metabolism, including some protists that can (Chlamydomonas reinhardtii) or are predicted (Naegleria gruberi, Acanthamoeba castellanii, Amoebidium parasiticum) to produce H(2) in their metabolism. It is possible that some enzymes of anaerobic metabolism were acquired and distributed among eukaryotes by lateral transfer, but it is also likely that the common ancestor of eukaryotes already had far more metabolic versatility than was widely thought a few years ago. The discussion of core energy metabolism in unicellular eukaryotes is the subject of this review. Since genomic sequencing has so far only touched the surface of protist diversity, it is anticipated that sequences of additional protists may reveal an even wider range of metabolic capabilities, while simultaneously enriching our understanding of the early evolution of eukaryotes.

Evidence that low endocytic activity is not directly responsible for human serum resistance in the insect form of African trypanosomes

BACKGROUND

In Trypanosoma brucei, the African trypanosome, endocytosis is developmentally regulated and substantially more active in all known mammalian infective stages. In both mammalian and insect stages endocytic activity is likely required for nutrient acquisition, but in bloodstream forms increased endocytosis is involved in recycling the variant surface glycoprotein and removing host immune factors from the surface. However, a rationale for low endocytic activity in insect stages has not been explored. Here we asked if endocytic down-regulation in the procyclic form was associated with resistance to innate trypanolytic immune factors in the blood meal or tsetse fly midgut.

FINDINGS

Using a well-characterized procyclic parasite with augmented endocytic flux mediated via TbRab5A overexpression, we found that insect stage parasites were able to grow both in the presence of trypanosome lytic factor (TLF) provided in human serum, and also in tsetse flies. Additionally, by placing blood stage parasites in restricted glucose medium, we observed that enlargement of the flagellar pocket, a key morphology associated with defective endocytosis, manifests in parallel with loss of cellular ATP levels.

CONCLUSIONS

These observations suggest that a high rate of endocytosis per se is insufficient to render insect form parasites sensitive to TLF or tsetse-derived trypanocidal factors. However, the data do suggest that endocytosis is energetically burdensome, as endocytic activity is rapidly compromised on energy depletion in bloodstream stages. Hence an important aspect of endocytic modulation in the nutrient-poor tsetse midgut is likely energetic conservation.

Central carbon metabolism of Leishmania parasites

Leishmania spp. are sandfly-transmitted protozoa parasites that cause a spectrum of diseases in humans. Many enzymes involved in Leishmania central carbon metabolism differ from their equivalents in the mammalian host and are potential drug targets. In this review we summarize recent advances in our understanding of Leishmania central carbon metabolism, focusing on pathways of carbon utilization that are required for growth and pathogenesis in the mammalian host. While Leishmania central carbon metabolism shares many features in common with other pathogenic trypanosomatids, significant differences are also apparent. Leishmania parasites are also unusual in constitutively expressing most core metabolic pathways throughout their life cycle, a feature that may allow these parasites to exploit a range of different carbon sources (primarily sugars and amino acids) rapidly in both the insect vector and vertebrate host. Indeed, recent gene deletion studies suggest that mammal-infective stages are dependent on multiple carbon sources in vivo. The application of metabolomic approaches, outlined here, are likely to be important in defining aspects of central carbon metabolism that are essential at different stages of mammalian host infection.

Acetate formation in the energy metabolism of parasitic helminths and protists

Formation and excretion of acetate as a metabolic end product of energy metabolism occurs in many protist and helminth parasites, such as the parasitic helminths Fasciola hepatica, Haemonchus contortus and Ascaris suum, and the protist parasites, Giardia lamblia, Entamoeba histolytica, Trichomonas vaginalis as well as Trypanosoma and Leishmania spp. In all of these parasites acetate is a main end product of their energy metabolism, whereas acetate formation does not occur in their mammalian hosts. Acetate production might therefore harbour novel targets for the development of new anti-parasitic drugs. In parasites, acetate is produced from acetyl-CoA by two different reactions, both involving substrate level phosphorylation, that are catalysed by either a cytosolic acetyl-CoA synthetase (ACS) or an organellar acetate:succinate CoA-transferase (ASCT). The ACS reaction is directly coupled to ATP synthesis, whereas the ASCT reaction yields succinyl-CoA for ATP formation via succinyl-CoA synthetase (SCS). Based on recent work on the ASCTs of F. hepatica, T. vaginalis and Trypanosoma brucei we suggest the existence of three subfamilies of enzymes within the CoA-transferase family I. Enzymes of these three subfamilies catalyse the ASCT reaction in eukaryotes via the same mechanism, but the subfamilies share little sequence homology. The CoA-transferases of the three subfamilies are all present inside ATP-producing organelles of parasites, those of subfamily IA in the mitochondria of trypanosomatids, subfamily IB in the mitochondria of parasitic worms and subfamily IC in hydrogenosome-bearing parasites. Together with the recent characterisation among non-parasitic protists of yet a third route of acetate formation involving acetate kinase (ACK) and phosphotransacetylase (PTA) that was previously unknown among eukaryotes, these recent developments provide a good opportunity to have a closer look at eukaryotic acetate formation.

Surprising variety in energy metabolism within Trypanosomatidae

The metabolism of Trypanosomatidae differs significantly between distinct species and can even be completely different between various life-cycle stages of the same species. It has been proposed that differences in energy metabolism are related to differences in nutrient supply in the environments of the various trypanosomatids. However, the literature shows that availability of substrates does not dictate the type of energy metabolism of trypanosomatids, as Trypanosoma theileri, Trypanosoma lewisi and African trypanosomes all live in the bloodstream of their mammalian host, but have surprisingly large differences in metabolism. Furthermore, in trypanosomatids no obvious relationship exists between energy metabolism and phylogeny or mode of transmission. We provide an overview of the metabolic capacities in the energy metabolism of distinct trypanosomatids, and suggest that these can be divided into four different metabolic categories of increasing complexity.

Acetate produced in the mitochondrion is the essential precursor for lipid biosynthesis in procyclic trypanosomes

Acetyl-CoA produced in mitochondria from carbohydrate or amino acid catabolism needs to reach the cytosol to initiate de novo synthesis of fatty acids. All eukaryotes analyzed so far use a citrate/malate shuttle to transfer acetyl group equivalents from the mitochondrial matrix to the cytosol. Here we investigate how this acetyl group transfer occurs in the procyclic life cycle stage of Trypanosoma brucei, a protozoan parasite responsible of human sleeping sickness and economically important livestock diseases. Deletion of the potential citrate lyase gene, a critical cytosolic enzyme of the citrate/malate shuttle, has no effect on de novo biosynthesis of fatty acids from (14)C-labeled glucose, indicating that another route is used for acetyl group transfer. Because acetate is produced from acetyl-CoA in the mitochondrion of this parasite, we considered genes encoding cytosolic enzymes producing acetyl-CoA from acetate. We identified an acetyl-CoA synthetase gene encoding a cytosolic enzyme (AceCS), which is essential for cell viability. Repression of AceCS by inducible RNAi results in a 20-fold reduction of (14)C-incorporation from radiolabeled glucose or acetate into de novo synthesized fatty acids. Thus, we demonstrate that the essential cytosolic enzyme AceCS of T. brucei is responsible for activation of acetate into acetyl-CoA to feed de novo biosynthesis of lipids. To date, Trypanosoma is the only known eukaryotic organism that uses acetate instead of citrate to transfer acetyl groups over the mitochondrial membrane for cytosolic lipid synthesis.

Adaptations in the glucose metabolism of procyclic Trypanosoma brucei isolates from tsetse flies and during differentiation of bloodstream forms

Procyclic forms of Trypanosoma brucei isolated from the midguts of infected tsetse flies, or freshly transformed from a strain that is close to field isolates, do not use a complete Krebs cycle. Furthermore, short stumpy bloodstream forms produce acetate and are apparently metabolically preadapted to adequate functioning in the tsetse fly.

Glycerol: an unexpected major metabolite of energy metabolism by the human malaria parasite

Background

Malaria is a global health emergency, and yet our understanding of the energy metabolism of the principle causative agent of this devastating disease, Plasmodium falciparum, remains rather basic. Glucose was shown to be an essential nutritional requirement nearly 100 years ago and since this original observation, much of the current knowledge of Plasmodium energy metabolism is based on early biochemical work, performed using basic analytical techniques (e.g. paper chromatography), carried out almost exclusively on avian and rodent malaria. Data derived from malaria parasite genome and transcriptome studies suggest that the energy metabolism of the parasite may be more complex than hitherto anticipated. This study was undertaken in order to further characterize the fate of glucose catabolism in the human malaria parasite, P. falciparum.

Methods

Products of glucose catabolism were determined by incubating erythrocyte-freed parasites with D-[1-¹³C] glucose under controlled conditions and metabolites were identified using ¹³C-NMR spectroscopy.

Results

Following a 2 h incubation of freed-P. falciparum parasites with 25 mM D-[1-¹³C] glucose (n = 4), the major metabolites identified included; [3-¹³C] lactate, [1,3-¹³C] glycerol, [3-¹³C] pyruvate, [3-¹³C] alanine and [3-¹³C] glycerol-3-phosphate. Control experiments performed with uninfected erythrocytes incubated under identical conditions did not show any metabolism of D-[1-¹³C] glucose to glycerol or glycerol-3-phosphate.

Discussion

The identification of glycerol as a major glucose metabolite confirms the view that energy metabolism in this parasite is more complex than previously proposed. It is hypothesized here that glycerol production by the malaria parasite is the result of a metabolic adaptation to growth in O₂-limited (and CO₂ elevated) conditions by the operation of a glycerol-3-phosphate shuttle for the re-oxidation of assimilatory NADH. Similar metabolic adaptations have been reported previously for other microaerobic/anaerobic organisms, such as yeast, rumen protozoa and human parasitic protozoa.

Conclusion

These data highlight the need to re-evaluate the carbon and redox balance of this important human pathogen, ultimately leading to a better understanding of how the parasite is able to adapt to the variable environments encountered during parasite development and disease progression.

Glucose-induced remodeling of intermediary and energy metabolism in procyclic Trypanosoma brucei

The procyclic form of Trypanosoma brucei is a parasitic protozoan that normally dwells in the midgut of its insect vector. In vitro, this parasite prefers d-glucose to l -proline as a carbon source, although this amino acid is the main carbon source available in its natural habitat. Here, we investigated how l -proline is metabolized in glucose-rich and glucose-depleted conditions. Analysis of the excreted end products of (13)C-enriched l -proline metabolism showed that the amino acid is converted into succinate or l -alanine depending on the presence or absence of d-glucose, respectively. The fact that the pathway of l -proline metabolism was truncated in glucose-rich conditions was confirmed by the analysis of 13 separate RNA interference-harboring or knock-out cell lines affecting different steps of this pathway. For instance, RNA interference studies revealed the loss of succinate dehydrogenase activity to be conditionally lethal only in the absence of d-glucose, confirming that in glucose-depleted conditions, l -proline needs to be converted beyond succinate. In addition, depletion of the F(0)/F(1)-ATP synthase activity by RNA interference led to cell death in glucose-depleted medium, but not in glucose-rich medium. This implies that, in the presence of d-glucose, the importance of the F(0)/F(1)-ATP synthase is diminished and ATP is produced by substrate level phosphorylation. We conclude that trypanosomes develop an elaborate adaptation of their energy production pathways in response to carbon source availability.

Characterization of the mitochondrial inner membrane protein translocator Tim17 from Trypanosoma brucei

Mitochondrial protein translocation machinery in the kinetoplastid parasites, like Trypanosoma brucei, has been characterized poorly. In T. brucei genome database, one homolog for a protein translocator of mitochondrial inner membrane (Tim) has been found, which is closely related to Tim17 from other species. The T. brucei Tim17 (TbTim17) has a molecular mass 16.2kDa and it possesses four characteristic transmembrane domains. The protein is localized in the mitochondrial inner membrane. The level of TbTim17 protein is 6-7-fold higher in the procyclic form that has a fully active mitochondrion, than in the mammalian bloodstream form of T. brucei, where many of the mitochondrial activities are suppressed. Knockdown of TbTim17 expression by RNAi caused a cessation of cell growth in the procyclic form and reduced growth rate in the bloodstream form. Depletion of TbTim17 decreased mitochondrial membrane potential more in the procyclic than bloodstream form. However, TbTim17 knockdown reduced the expression level of several nuclear encoded mitochondrial proteins in both the forms. Furthermore, import of presequence containing nuclear encoded mitochondrial proteins was significantly reduced in TbTim17 depleted mitochondria of the procyclic as well as the bloodstream form, confirming that TbTim17 is critical for mitochondrial protein import in both developmental forms. Together, these show that TbTim17 is the translocator of nuclear encoded mitochondrial proteins and its expression is regulated according to mitochondrial activities in T. brucei.

Energy metabolism of bloodstream form Trypanosoma theileri

Bloodstream form Trypanosoma theileri degrades glucose to acetate (47%) and succinate (45%) and, therefore, does not solely rely on glycolysis for ATP production. This trypanosomatid does not use amino acids for energy metabolism. These results refute the prevailing hypothesis that substrate availability determines the type of energy metabolism of trypanosomatids.

Miltefosine (hexadecylphosphocholine) inhibits cytochrome c oxidase in Leishmania donovani promastigotes

Miltefosine (hexadecylphosphocholine [HePC]) is currently on trial as a first-choice, orally active drug for the treatment of visceral leishmaniasis when resistance to organic pentavalent antimonials becomes epidemic. However, data on the targets involved in its leishmanicidal mechanism have, until now, been only fragmentary. We have carried out a systematic study of the alterations induced on the bioenergetic metabolism of Leishmania donovani promastigotes by HePC. Overnight incubation with HePC caused a significant decline in the intracellular ATP levels of the parasites, together with a reduction in the oxygen consumption rate and mitochondrial depolarization, while the integrity of the plasma membrane remained undamaged. In a further step, the effects of HePC on the respiratory chain were addressed in digitonized parasites. The inhibition of the oxygen consumption rate caused by HePC was not reverted either with the uncoupling agent carbonyl cyanide p-trifluoromethoxyphenylhydrazone or with tetramethyl-p-phenylenediamine plus ascorbate, which feeds the electron transport chain at the level of cytochrome c. These results suggest that cytochrome c oxidase is a likely target in the complex leishmanicidal mechanism of HePC. This was further confirmed from the finding that this enzyme was specifically inhibited in a dose-dependent manner by HePC, but not the cytochrome c reductase, ruling out an unspecific effect of HePC on the respiratory chain.

Metabolic functions of glycosomes in trypanosomatids

Protozoan Kinetoplastida, including the pathogenic trypanosomatids of the genera Trypanosoma and Leishmania, compartmentalize several important metabolic systems in their peroxisomes which are designated glycosomes. The enzymatic content of these organelles may vary considerably during the life-cycle of most trypanosomatid parasites which often are transmitted between their mammalian hosts by insects. The glycosomes of the Trypanosoma brucei form living in the mammalian bloodstream display the highest level of specialization; 90% of their protein content is made up of glycolytic enzymes. The compartmentation of glycolysis in these organelles appears essential for the regulation of this process and enables the cells to overcome short periods of anaerobiosis. Glycosomes of all other trypanosomatid forms studied contain an extended glycolytic pathway catalyzing the aerobic fermentation of glucose to succinate. In addition, these organelles contain enzymes for several other processes such as the pentose-phosphate pathway, beta-oxidation of fatty acids, purine salvage, and biosynthetic pathways for pyrimidines, ether-lipids and squalenes. The enzymatic content of glycosomes is rapidly changed during differentiation of mammalian bloodstream-form trypanosomes to the forms living in the insect midgut. Autophagy appears to play an important role in trypanosomatid differentiation, and several lines of evidence indicate that it is then also involved in the degradation of old glycosomes, while a population of new organelles containing different enzymes is synthesized. The compartmentation of environment-sensitive parts of the metabolic network within glycosomes would, through this way of organelle renewal, enable the parasites to adapt rapidly and efficiently to the new conditions.

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.

Energy metabolism of trypanosomatids: adaptation to available carbon sources

Some development stages of the trypanosomatid protozoan parasites are well adapted to in vitro culture. They can be maintained in rich medium containing large excess of glucose and amino acids, which they use as carbon sources for ATP production. Under these growth conditions, carbon sources are converted into partially oxidized end products by so-called aerobic fermentation. Surprisingly, some species, such as the Trypanosoma brucei, Trypanosoma cruzi and Crithidia insect stages, prefer consuming glucose to amino acids, although their natural habitat is L-proline-rich. This review focuses on recent progress in understanding glucose and l-proline metabolism of insect stages, how these metabolic processes are regulated, and the rationale of the aerobic fermentation strategies developed by these parasites.