A potential hexose transporter gene expressed predominantly in the bloodstream form of Trypanosoma brucei

Choroidal blood perfusion could predict the sensitivity of myopia formation in Guinea pigs

In this study, we explored the predictive role of choroidal blood perfusion (ChBP) and choroidal thickness (ChT) on the development of myopia in guinea pigs. Optical Coherence Tomography Angiography (OCTA) was used to assess the baseline choroidal blood perfusion (ChBP) and choroidal thickness (ChT) in 4-week-old guinea pigs. Refraction and axial length (AL) were measured at baseline. Myopia was induced for one week using form-deprivation (FD) or negative lenses followed by measurements of refraction, axial length and choroidal parameters (ChT and ChBP). The correlations were evaluated between the baseline choroidal values and the magnitude of myopia induced, along with the magnitude of changes in ChT and ChBP after myopia induction. There was a significant correlation between the baseline choroidal parameters and ocular refraction. Myopia induction led to choroidal thinning and less ChBP as well as longer eyes. On the other hand, following exposure to the same non-obstructed visual induction period, the myopic shift was less, and it was associated with thicker choroids and more ChBP at baseline. One week of myopia induction also resulted in thinner choroids and less ChBP, and these declines also correlated with their baseline values. In conclusion, the present study shows that the changes in the baseline choroidal ChT and ChBP parameters are proportional to the magnitude of myopia development and axial elongation in guinea pigs. These significant correlations between baseline ChBP and ChT and myopia development suggest that they may be a viable predictor of this process in guinea pigs.

Protean permeases: Diverse roles for membrane transport proteins in kinetoplastid protozoa

Kinetoplastid parasites such as Trypanosoma brucei, Trypanosoma cruzi, and Leishmania species rely upon their insect and vertebrate hosts to provide a plethora of nutrients throughout their life cycles. Nutrients and ions critical for parasite survival are taken up across the parasite plasma membrane by transporters and channels, polytopic membrane proteins that provide substrate-specific pores across the hydrophobic barrier. However, transporters and channels serve a wide range of biological functions beyond uptake of nutrients. This article highlights the diversity of activities that these integral membrane proteins serve and underscores the emerging complexity of their functions.

The proteome and transcriptome of the infectious metacyclic form of Trypanosoma brucei define quiescent cells primed for mammalian invasion

The infectious metacyclic forms of Trypanosoma brucei result from a complex development in the tsetse fly vector. When they infect mammals, they cause African sleeping sickness in humans. Due to scarcity of biological material and difficulties of the tsetse fly as an experimental system, very limited information is available concerning the gene expression profile of metacyclic forms. We used an in vitro system based on expressing the RNA binding protein 6 to obtain infectious metacyclics and determined their protein and mRNA repertoires by mass-spectrometry (MS) based proteomics and mRNA sequencing (RNA-Seq) in comparison to non-infectious procyclic trypanosomes. We showed that metacyclics are quiescent cells, and propose this influences the choice of a monocistronic variant surface glycoprotein expression site. Metacyclics have a largely bloodstream-form type transcriptome, and thus are programmed to translate a bloodstream-form type proteome upon entry into the mammalian host and resumption of cell division. Genes encoding cell surface components showed the largest changes between procyclics and metacyclics, observed at both the transcript and protein levels. Genes encoding metabolic enzymes exhibited expression in metacyclics with features of both procyclic and bloodstream forms, suggesting that this intermediate-type metabolism is dictated by the availability of nutrients in the tsetse fly vector.

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.

Specializations in a successful parasite: what makes the bloodstream-form African trypanosome so deadly?

Most trypanosomatid parasites have both arthropod and mammalian or plant hosts, and the ability to survive and complete a developmental program in each of these very different environments is essential for life cycle progression and hence being a successful pathogen. For African trypanosomes, where the mammalian stage is exclusively extracellular, this presents specific challenges and requires evasion of both the acquired and innate immune systems, together with adaptation to a specific nutritional environment and resistance to mechanical and biochemical stresses. Here we consider the basis for these adaptations, the specific features of the mammalian infective trypanosome that are required to meet these challenges, and how these processes both inform on basic parasite biology and present potential therapeutic targets.

Glucose transporters in parasitic protozoa

Glucose and related hexoses play central roles in the biochemistry and metabolism of single-cell parasites such as Leishmania, Trypanosoma, and Plasmodium that are the causative agents of leishmaniasis, African sleeping sickness, and malaria. Glucose transporters and the genes that encode them have been identified in each of these parasites and their functional properties have been scrutinized. These transporters are related in sequence and structure to mammalian facilitative glucose transporters of the SLC2 family, but they are nonetheless quite divergent in sequence. Hexose transporters have been shown to be essential for the viability of the infectious stage of each of these parasites and thus may represent targets for development of novel anti-parasitic drugs. The study of these transporters also illuminates many aspects of the basic biology of Leishmania, trypanosomes, and malaria parasites.

Targeting of toxic compounds to the trypanosome's interior

Drugs can be targeted into African trypanosomes by exploiting carrier proteins at the surface of these parasites. This has been clearly demonstrated in the case of the melamine-based arsenical and the diamidine classes of drug that are already in use in the treatment of human African trypanosomiasis. These drugs can enter via an aminopurine transporter, termed P2, encoded by the TbAT1 gene. Other toxic compounds have also been designed to enter via this transporter. Some of these compounds enter almost exclusively through the P2 transporter, and hence loss of the P2 transporter leads to significant resistance to these particular compounds. It now appears, however, that some diamidines and melaminophenylarsenicals may also be taken up by other routes (of yet unknown function). These too may be exploited to target new drugs into trypanosomes. Additional purine nucleoside and nucleobase transporters have also been subverted to deliver toxic agents to trypanosomes. Glucose and amino acid transporters too have been investigated with a view to manipulating them to carry toxins into Trypanosoma brucei, and recent work has demonstrated that aquaglyceroporins may also have considerable potential for drug-targeting. Transporters, including those that carry lipids and vitamins such as folate and other pterins also deserve more attention in this regard. Some drugs, for example suramin, appear to enter via routes other than plasma-membrane-mediated transport. Receptor-mediated endocytosis has been proposed as a possible way in for suramin. Endocytosis also appears to be crucial in targeting natural trypanocides, such as trypanosome lytic factor (TLF) (apolipoprotein L1), into trypanosomes and this offers an alternative means of selectively targeting toxins to the trypanosome's interior. Other compounds may be induced to enter by increasing their capacity to diffuse over cell membranes; in this case depending exclusively on selective activity within the cell rather than selective uptake to impart selective toxicity. This review outlines studies that have aimed to exploit trypanosome nutrient uptake routes to selectively carry toxins into these parasites.

Expressed sequence tags from the plant trypanosomatid Phytomonas serpens

We have generated 2190 expressed sequence tags (ESTs) from a cDNA library of the plant trypanosomatid Phytomonas serpens. Upon processing and clustering the set of 1893 accepted sequences was reduced to 697 clusters consisting of 452 singletons and 245 contigs. Functional categories were assigned based on BLAST searches against a database of the eukaryotic orthologous groups of proteins (KOG). Thirty six percent of the generated sequences showed no hits against the KOG database and 39.6% presented similarity to the KOG classes corresponding to translation, ribosomal structure and biogenesis. The most populated cluster contained 45 ESTs homologous to members of the glucose transporter family. This fact can be immediately correlated to the reported Phytomonas dependence on anaerobic glycolytic ATP production due to the lack of cytochrome-mediated respiratory chain. In this context, not only a number of enzymes of the glycolytic pathway were identified but also of the Krebs cycle as well as specific components of the respiratory chain. The data here reported, including a few hundred unique sequences and the description of tandemly repeated motifs and putative transcript stability motifs at untranslated mRNA ends, represent an initial approach to overcome the lack of information on the molecular biology of this organism.

Glucose and proline transport in kinetoplastids

The parasitic protozoa belonging to the kinetoplastids can use both sugars and amino acids as carbon and energy sources. In this review, Benno ter Kuile discusses nutrient acquisition and utilization and how the metabolic strategies reflect the environment encountered in host and vector. Recent genetic and physiological evidence suggests that facilitated diffusion may be the primary uptake mechanism for glucose, and possibly for proline as well, even though there is biochemical and genetic evidence suggesting that active transport occurs, if not across the plasma membrane, then across the membranes of organelles. Trypanosoma brucei seems to have a metabolic strategy that strives for maximum energy efficiency, making no storage materials and thereby limiting the control over its internal conditions. On the other hand, Leishmania donovani does create a storage buffer, entrapping glucose in the cell. In this manner, it maintains constant internal conditions at the expense of energy, enabling it to survive more adverse conditions in the macrophage and in its vector.

What can GPI do for you?

Various functions for glycosylphosphatidylinositol (GPI) protein anchors have been described in mammalian and protozoan systems. These data suggest that some functions are common to higher and lower eukaryotes, whereas others may represent adaptations that are specifically advantageous to either unicellular or metazoan organisms. In this article, Mike Ferguson discusses the current theories of GPI function that have relevance to protozoan parasites and their mammalian hosts.

Interaction of substituted hexose analogues with the Trypanosoma brucei hexose transporter

Glucose metabolism is essential for survival of bloodstream form Trypanosoma brucei subspecies which cause human African trypanosomiasis (sleeping sickness). Hexose analogues may represent good compounds to inhibit glucose metabolism in these cells. Delivery of such compounds to the parasite is a major consideration in drug development. A series of D-glucose and D-fructose analogues were developed to explore the limits of the structure-activity relationship of the THT1 hexose transporter of bloodstream form African trypanosomes, a portal that might be exploited for drug uptake. D-glucose analogues with substituents at the C2 and C6 position continued to interact with the exofacial hexose binding site of the transporter. There was a limit to the size at C6 which still permitted recognition, although compounds carrying large groups at position C2 were still recognised. However, radiolabelled N-acetyl-D-[1-14C] glucosamine was not internalised by trypanosomes, in spite of the ability of this compound to inhibit glucose uptake, indicating that there is a limit to the size of C2 substituent that allows translocation. Addition of an alkylating group (bromoacetyl) at position C2 in the D-glucose series and at position 6 in the D-fructose set, created two analogues which interact with the transporter and kill trypanosomes in vitro. This indicates that inhibition of the transporter may be a good means of killing trypanosomes.

Synthesis and biological activity of nitro heterocycles analogous to megazol, a trypanocidal lead

As part of our efforts to develop new compounds aimed at the therapy of parasitic infections, we synthesized and assayed analogues of a lead compound megazol, 5-(1-methyl-5-nitro-1H-2-imidazolyl)-1,3,4-thiadiazol-2-amine, CAS no. 19622-55-0), in vitro. We first developed a new route for the synthesis of megazol. Subsequently several structural changes were introduced, including substitutions on the two rings of the basic nucleus, replacement of the thiadiazole by an oxadiazole, replacement of the nitroimidazole part by a nitrofurane or a nitrothiophene, and substitutions on the exocyclic nitrogen atom for evaluation of an improved import by the glucose or the purine transporters. Assays of the series of compounds on the protozoan parasites Trypanosoma brucei, Trypanosoma cruzi, and Leishmania donovani, as either extracellular cells or infected macrophages, indicated that megazol was more active than the derivatives. Megazol was then evaluated on primates infected with Trypanosoma brucei gambiense, including late-stage central nervous system infections in combination with suramin. Full recovery was observed in five monkeys in the study with no relapse of parasitemia within a 2 year follow-up. Because there is a lack of efficacious treatments for sleeping sickness in Africa and Chagas disease in South America, megazol is proposed as a potential alternative. The mutagenicity of this compound is at present being reevaluated, and metabolism is also under investigation prior to possible further developments.

A new, expressed multigene family containing a hot spot for insertion of retroelements is associated with polymorphic subtelomeric regions of Trypanosoma brucei

We describe a novel gene family that forms clusters in subtelomeric regions of Trypanosoma brucei chromosomes and partially accounts for the observed clustering of retrotransposons. The ingi and ribosomal inserted mobile element (RIME) non-LTR retrotransposons share 250 bp at both extremities and are the most abundant putatively mobile elements, with about 500 copies per haploid genome. From cDNA clones and subsequently in the T. brucei genomic DNA databases, we identified 52 homologous gene and pseudogene sequences, 16 of which contain a RIME and/or ingi retrotransposon inserted at exactly the same relative position. Here these genes are called the RHS family, for retrotransposon hot spot. Comparison of the protein sequences encoded by RHS genes (21 copies) and pseudogenes (24 copies) revealed a conserved central region containing an ATP/GTP-binding motif and the RIME/ingi insertion site. The RHS proteins share between 13 and 96% identity, and six subfamilies, RHS1 to RHS6, can be defined on the basis of their divergent C-terminal domains. Immunofluorescence and Western blot analyses using RHS subfamily-specific immune sera show that RHS proteins are constitutively expressed and occur mainly in the nucleus. Analysis of Genome Survey Sequence databases indicated that the Trypanosoma brucei diploid genome contains about 280 RHS (pseudo)genes. Among the 52 identified RHS (pseudo)genes, 48 copies are in three RHS clusters located in subtelomeric regions of chromosomes Ia and II and adjacent to the active bloodstream form expression site in T. brucei strain TREU927/4 GUTat10.1. RHS genes comprise the remaining sequence of the size-polymorphic "repetitive region" described for T. brucei chromosome I, and a homologous gene family is present in the Trypanosoma cruzi genome.

Enzymes of carbohydrate metabolism as potential drug targets

The potential for chemotherapeutic exploitation of carbohydrate metabolism in the Trypanosomatidae is reviewed. This review is based largely on discussions held at a meeting of the COST B9 Action, entitled 'Bioenergetics of Protozoan Parasites'. The major questions posed were: which enzymes are the best to target; what further information is required to allow their use for rational drug development; what compounds would constitute the best inhibitors and which of the enzymes of the pentose-phosphate pathway are present inside the glycosomes, as well? Only partial answers could be obtained in many cases, but the interactive discussion between the multidisciplinary group of participants, comprising chemists, biochemists and molecular biologists, provided thought-provoking ideas and will help direct future research.

Hexose transport in asexual stages of Plasmodium falciparum and kinetoplastidae

The hexose sugar, glucose, is a vital energy source for most organisms and an essential nutrient for asexual stages of Plasmodium falciparum. Kinetoplastid organisms (e.g. Trypanosoma and Leishmania spp) also require glucose at certain critical stages of their life cycles. Although phylogenetically unrelated, these organisms share many common challenges during the mammalian stages of a parasitic life cycle, and possess hexose uptake mechanisms that are amenable to study using similar methods. Defining hexose permeation pathways into parasites might expose an Achilles' heel at which both antidisease and antiparasite measures can be aimed. Understanding the mode of entry of glucose also presents a good general model for substrate acquisition in multicompartment systems. In this review, Sanjeev Krishna and colleagues summarize current understanding of hexose transport processes in P. falciparum and provide a comparison with data obtained from kinetoplastids.

Complementation of a glucose transporter mutant of Schizosaccharomyces pombe by a novel Trypanosoma brucei gene

The African trypanosome Trypanosoma brucei has a digenetic life cycle that involves the insect vector and the mammalian host. This is underscored by biochemical switches in its nutritional requirements. In the insect vector, the parasite relies on amino acid catabolism, but in the mammalian host, it derives its energy exclusively from blood glucose. Glucose transport is facilitated, and constitutes the rate-limiting step in ATP synthesis. Here, we report the cloning of a novel glucose transporter-related gene by heterologous screening of a lambdaEMBL4 genomic library of T. brucei EATRO 164 using a rat liver glucose transporter cDNA clone. Genomic analysis shows that the gene is present as a single copy within the parasite genome. The gene encodes a protein with an estimated molecular mass of 55.9 kDa, which shares only segmental homology with members of the glucose transporter superfamily. Several potential post-translational modification sites including phosphorylation, N-glycosylation, and cotranslational myristoylation sites also punctuate the sequence. It is distinguished from classical transporter proteins by the absence of putative hydrophobic membrane-spanning domains. However, this protein was capable of complementing Schizosaccharomyces pombe glucose transporter mutants. The rescued phenotype conferred the ability of the cells to grow on a broad range of sugars, both monosaccharides and disaccharides. The kinetics of glucose uptake reflected those in T. brucei. In addition to complementation in yeast, we also showed that the gene enhanced glucose uptake in cultured mammalian cells.

Mycobacterium tuberculosis expresses a novel pH-dependent divalent cation transporter belonging to the Nramp family

Mammalian natural resistance-associated macrophage protein (Nramp) homologues are important determinants of susceptibility to infection by diverse intracellular pathogens including mycobacteria. Eukaryotic Nramp homologues transport divalent cations such as Fe(2+), Mn(2+), Zn(2+), and Cu(2+). Mycobacterium tuberculosis and Mycobacterium bovis (bacillus Calmette-Guérin [BCG]) also encode an Nramp homologue (Mramp). RNA encoding Mramp induces approximately 20-fold increases in (65)Zn(2+) and (55)Fe(2+) uptake when injected into Xenopus laevis oocytes. Transport is dependent on acidic extracellular pH and is maximal between pH 5.5 and 6.5. Mramp-mediated (65)Zn(2+) and (55)Fe(2+) transport is abolished by an excess of Mn(2+) and Cu(2+), confirming that Mramp interacts with a broad range of divalent transition metal cations. Using semiquantitative reverse transcription PCR, we show that Mramp mRNA levels in M. tuberculosis are upregulated in response to increases in ambient Fe(2+) and Cu(2+) between <1 and 5 microM concentrations and that this upregulation occurs in parallel with mRNA for y39, a putative metal-transporting P-type ATPase. Using a quantitative ratiometric PCR technique, we demonstrate a fourfold decrease in Mramp/y39 mRNA ratios from organisms grown in 5-70 microM Cu(2+). M. bovis BCG cultured axenically and within THP-1 cells also expresses mRNA encoding Mramp. Mramp exemplifies a novel prokaryotic class of metal ion transporter. Within phagosomes, Mramp and Nramp1 may compete for the same divalent cations, with implications for intracellular survival of mycobacteria.

An easy stereospecific synthesis of 1-amino-2,5-anhydro-1-deoxy-D-mannitol and arylamino derivatives

1-Amino-2,5-anhydro-1-deoxy-D-mannitol and a series of arylamino derivatives were prepared by nitrous acid deamination of 2-amino-2-deoxy-D-glucose and subsequent reductive amination of the resulting 2,5-anhydro-D-mannose. Some of these compounds showed an enhanced affinity for the hexose transporter of Trypanosoma brucei as compared to D-fructose.

Surface receptors and transporters of Trypanosoma brucei

African trypanosomes combine antigenic variation of their surface coat with the ability to take up nutrients from their mammalian hosts. Uptake of small molecules such as glucose or nucleosides is mediated by translocators hidden from host antibodies by the surface coat. The multiple glucose transporters and transporters for nucleobases and nucleosides have been characterized. Receptors for host macromolecules such as transferrin and lipoproteins are visible to antibodies but hidden from the cellular arm of the host immune system in an invagination of the trypanosome surface, the flagellar pocket. The trypanosomal transferrin receptor is a heterodimer that resembles the major component of the surface coat of Trypanosoma brucei. The ability to make several versions of this receptor allows T. brucei to bind transferrins from a range of mammals with high affinity. The proteins required for uptake of nutrients by trypanosomes provide a target for chemotherapy that remains to be fully exploited.

Conserved organization of genes in trypanosomatids

Trypanosomatids are unicellular protozoan parasites which constitute some of the most primitive eukaryotes. Leishmania spp, Trypanosoma cruzi and members of the Trypanosoma brucei group, which cause human diseases, are the most studied representatives of this large family. Here we report a comparative analysis of a large genomic region containing glucose transporter genes in three Salivarian trypanosomes (T. brucei, T. congolense and T. vivax), T. cruzi and Leishmania donovani. In T. brucei, the 8 kb (upstream) and 14 kb (downstream) regions flanking the glucose transporter genes cluster contain two and six new genes, respectively, six of them encoding proteins homologous to known eukaryotic proteins (phosphatidylinositol 3 kinase, ribosomal protein S12, DNAJ and three small G-proteins--Rab1, YPT6 and ARL3). This gene organization is identical in T. brucei, T. congolense and T. vivax suggesting that Salivarian trypanosomes have a high level of conservation in gene organization. In T. cruzi and Leishmania, the overall organization of this cluster is conserved, with insertion of additional genes when compared with T. brucei. Phylogenetic reconstitution based on glucose transporters is in accord with the monophyly of the genus Trypanosoma and the early separation of T. vivax within Salivarian trypanosomes. On the basis of gene organization, biochemical characteristics of isoforms and phylogeny, we discuss the genesis of the glucose transporter multigene family in Salivarian trypanosomes.

Functional and molecular characterization of a glycosomal PPi-dependent enzyme in trypanosomatids: pyruvate, phosphate dikinase

Trypanosomatids are parasitic protists that have an ATP-dependent glycolysis with no indication of PPi-dependent metabolism. Most of the glycolysis takes place in peroxisome-like organelles, the glycosomes. We characterized in Trypanosoma brucei a single-copy gene encoding a PPi-dependent enzyme, pyruvate, phosphate dikinase (PPDK), which was expressed functionally in Escherichia coli. Specific antibodies detected a 100-kDa protein in procyclic forms but not in mammalian forms of T. brucei, indicating a differential expression. Glycosomal localization of PPDK was determined by immunofluorescence analysis and was confirmed by Western blot analysis on glycosomal fractions by using anti-PPDK antibodies. Expression and localization of recombinant PPDKs in procyclic forms of T. brucei showed that the AKL motif at the C-terminal extremity of PPDK is necessary for glycosomal targeting. PPDK was detected in every trypanosomatid tested-Trypanosoma congolense, Trypanosoma vivax, Trypanosoma cruzi, Phytomonas, Crithidia and Leishmania-with a good correlation between amount of protein and enzymatic activity. The precise role of PPDK in trypanosomatid carbohydrate metabolism remains to be clarified.

Analysis of trypanosomal endocytic organelles using preparative free-flow electrophoresis

In this paper we demonstrate the power of preparative free-flow electrophoresis (FFE) for the study of endocytosis by African trypanosomes. Endocytosis of extracellular macromolecules by these parasites occurs through a specialized region of the parasite called the flagella pocket. The uptake of fluid phase markers such as horseradish peroxidase (HRP) into the various compartments of the endocytic pathway of bloodstream forms of Trypanosoma brucei brucei was manipulated by regulating the external environment (e.g., by altering the temperature of incubation). The various subcellular compartments were then separated by free-flow electrophoresis (FFE) or isopycnic density gradient centrifugation and analyzed for marker uptake. At low temperatures, HRP was found predominantly in the flagellar pocket. Increasing the temperature resulted in a time-dependent uptake of HRP into more positively charged endosomal fractions. However, little HRP activity was detected in lysosomal compartments, suggesting that either HRP had not yet entered the lysosome or was degraded immediately upon entry. Through the use of FFE we were able to identify and analyze compartments of the endosomal pathway that were not possible to identify by density gradient centrifugation alone. Although the differences in FFE separation of the endocytic compartments as seen in HRP uptake were striking, the minor changes seen within the lysosomal system were more subtle, as depicted in the protease profiles. In conclusion, we show that preparative FFE is a powerful technique for the analysis and separation of flagellar pocket-derived membranes from other endosomal and lysosomal compartments of African trypanosomes.

Kinetoplastid glucose transporters

Protozoa of the order kinetoplastida have colonized many habitats, and several species are important parasites of humans. Adaptation to different environments requires an associated adaptation at a cell's interface with its environment, i.e. the plasma membrane. Sugar transport by the kinetoplastida as a phylogenetically related group of organisms offers an exceptional model in which to study the ways by which the carrier proteins involved in this process may evolve to meet differing environmental challenges. Seven genes encoding proteins involved in glucose transport have been cloned from several kinetoplastid species. The transporters all belong to the glucose transporter superfamily exemplified by the mammalian erythrocyte transporter GLUT1. Some species, such as the African trypanosome Trypanosoma brucei, which undergo a life cycle where the parasites are exposed to very different glucose concentrations in the mammalian bloodstream and tsetse-fly midgut, have evolved two different transporters to deal with this fluctuation. Other species, such as the South American trypanosome Trypanosoma cruzi, multiply predominantly in conditions of relative glucose deprivation (intracellularly in the mammalian host, or within the reduviid bug midgut) and have a single, relatively high-affinity type, transporter. All of the kinetoplastid transporters can also transport d-fructose, and are relatively insensitive to the classical inhibitors of GLUT1 transport cytochalasin B and phloretin.

Implications of conserved structural motifs in disparate trypanosome surface proteins

Evasion of the host immune system by Trypanosoma brucei is dependent on the sequential expression of individual genes encoding antigenically distinct variant surface glycoproteins (VSG). VSGs are antigenically distinct due to extensive differences in primary sequence; the only obvious conserved feature in the primary sequence is the location of cysteines that form disulphide bridges. Despite this difference, it is believed that VSGs have a conserved tertiary structure which could explain how a range of VSGs with different primary sequences can perform the same apparent function of producing a monolayer barrier that prevents the host antibodies from recognising other cell surface proteins. The main feature of the VSG tertiary structure is two long alpha-helices per monomer that are perpendicular to the cell surface and define the elongated shape of the VSG. The alpha-helices can be identified in the primary sequence by heptad analysis. Here, we briefly review the current understanding of VSG structure and discuss the fact that the cysteine residues and the heptads are conserved in some non-VSG surface proteins from T. brucei, providing strong evidence that these share a similar tertiary structure. These findings suggest that this master structure has evolved to facilitate a range of functions and has implications for understanding the architecture of the trypanosome cell surface and the origins of antigenic variation.

Hexose uptake in Trypanosoma cruzi: structure-activity relationship between substrate and transporter

The gene encoding a hexose transporter, TcrHt1, from Trypanosoma cruzi has been functionally expressed in mammalian Chinese hamster ovary cells. Kinetic parameters of the heterologously expressed protein are very similar to those of the transporter identified in T. cruzi epimastigotes, confirming that TcrHT1 is the major transporter functioning in these parasites. A detailed analysis of substrate recognition using analogues of D-glucose substituted at each carbon position has been performed. The glucose transporter of T. cruzi does not recognize C-3 or C-6 analogues of D-glucose, whereas these analogues were recognized by the glucose transporter of bloodstream-form T. brucei. As for other kinetoplastid transporters, but in stark contrast to the mammalian GLUT family, TcrHT1 can also transport D-fructose, with relatively high affinity (Km = 0.682 +/- 0.003 mM). Amino acid side-chain-modifying reagents were also used to identify residues of the transporter present at the substrate-binding site. While specific modifiers of cysteine, histidine and arginine all inhibited catalytic activity, protection using substrate was only observed using the arginine-specific reagent, phenylglyoxal. Reagents which modify lysine residues had no effect on transport.

Glucose uptake in Trypanosoma vivax and molecular characterization of its transporter gene

A gene, TvHT1, encoding a glucose transporter protein, has been cloned from the haemoflagellate protozoon, Trypanosoma vivax, which has an active Kreb's cycle in the mammalian stage. The deduced polypeptide is similar in amino acid sequence to other kinetoplastid hexose transporters from Trypanosoma brucei (THT1 and THT2), Trypanosoma cruzi (TcrHT1) and Leishmania (Pro-1). The similarity is higher with THT2 (expressed in T. brucei insect forms) than with the other isoforms. The kinetic properties of glucose uptake in Chinese Hamster Ovary (CHO) cells expressing TvHT1 and in trypanosomes show s a saturable transport mechanism typical of a facilitated carrier system, with a similar affinity for D-glucose as that of the T. brucei bloodstream form carrier, THT1 (Km = 0.548 +/- 0.01 mM, Vmax = 4.26 +/- 0.12 nmol.min-1.mg protein-1 in CHO cells and Km = 0.585 +/- 0.068 mM, Vmax = 88.5 +/- 6.2 nmol.min-1.mg protein-1 in T. vivax). The specificity of the TvHT1 protein for various D-glucose analogues, as judged by inhibition of 2-deoxy-D-arabinose-hexose transport, shows properties that are intermediate between those of THT1 on the one hand and TcrHT1 and THT2 on the other. As with the hexose transporters in the other members of Kinetoplastida, the TvHT1-encoded system differs from erythrocyte-type glucose transport by its moderate sensitivity to cytochalasin B and its capacity to transport fructose.

Relationships between bacterial drug resistance pumps and other transport proteins

We have used three reference sequences representative of bacterial drug resistance pumps and sugar transport proteins to collect the 91 most closely related sequences from a composite, nonredundant protein sequence database. Having eliminated certain very close relatives, the remainder were subjected to analysis and alignment by using two different similarity matrices: one of these was a matrix based on structural conservation of amino acid residues in proteins of known conformation and the other was based on the more familiar mutational matrix. Unrooted similarity trees for these proteins were constructed for each matrix and compared. A systematic analysis of the differences between these trees was undertaken and the sequences were analyzed for the presence or absence of certain sequence motifs. The results show that the clades created by the two methods are broadly comparable but that there are some clusters of sequences that are significantly different. Further analysis confirmed that (1) the sequences collected by this objective method are all known or putative 12-helix (in some cases reported as 14-helix) transmembrane proteins, (2) there is evidence for few cases of an origin based on gene duplication, (3) the bacterial drug resistance pumps are distributed in more than one clade and cannot be regarded as a definitive subset of these proteins, and that (4) the diversity is such that there is no evidence of a single ancestral protein. The possible extension of the methods to other cases of divergent protein sequences is discussed.

Functional expression and characterization of the Trypanosoma brucei procyclic glucose transporter, THT2

The gene encoding THT2, one of two hexose-transporter isoforms present in Trypanosoma brucei, has been expressed in both Xenopus laevis oocytes and a stably transfected line of Chinese hamster ovary (CHO) cells. The heterologously expressed gene encodes a protein with pharmacological and kinetic parameters similar to those of the hexose transporter measured in procyclic-culture-form trypanosomes. The substrate recognition of the THT2 transporter differed from that of the THT1 isoform, which is expressed only in bloodstream forms, in that: (i) it has a relatively high affinity for substrate with a Km of 59 microM for 2-deoxy-D-glucose (2-DOG) and a similar high affinity for D-glucose (compared with Km of 0.5 mM for 2-DOG in bloodstream forms); (ii) the affinity for 6-deoxy-D-glucose (6-DOG) is two orders of magnitude lower than that for D-glucose, whereas the bloodstream-form transporter recognizes D-glucose and its 6-DOG analogue with similar affinity; (iii) the bloodstream-form transporter, but not THT2, recognizes 3-fluoro-3-deoxy-D-glucose. D-Fructose-transport capacity and insensitivity to D-galactose was also found in THT2-expressing CHO cells and procyclic trypanosomes. We conclude from these cumulative results that the THT2 gene encodes the transporter responsible for hexose transport in procyclic trypanosomes. The transport of 2-DOG in procyclic organisms was inhibited by both the protonophore, carbonyl cyanide 4-trifluoromethoxy phenylhydrazone (FCCP), and KCN, suggesting a requirement for a protonmotive force. However, sensitivity to these reagents depended on the external substrate concentration, with uptake being unaffected at substrate concentrations higher than 2 mM. THT2 expressed in CHO cells behaved as a facilitated transporter, and was unaffected by FCCP or KCN over the whole substrate concentration range tested.

Role of 3'-untranslated regions in the regulation of hexose transporter mRNAs in Trypanosoma brucei

Trypanosoma brucei is a unicellular parasite that is transmitted from one mammalian host to the next by tsetse flies. The expression of many trypanosome genes is regulated during the life cycle but there is no evidence for developmental control of transcription by RNA polymerase II. T. brucei expresses at least two hexose transporter mRNAs that are developmentally regulated; we show here that specific portions of the 3'-untranslated regions are responsible for the differential expression. Different trypanosome 3'-untranslated regions, from surface protein, phosphoglycerate kinase and aldolase genes as well as the hexose transporter genes, conferred a spectrum of levels of reporter gene expression, and these activities differed between bloodstream forms and the procyclic forms that replicate in the tsetse vector. Experiments with permanently transformed cell lines showed that regulation occurs at the mRNA level. The results suggest that post-transcriptional control of mRNAs in trypanosomatids operates at several levels, and that it will not always be possible to attribute all the regulation to short RNA motifs.

Characterization of glucose transport and cloning of a hexose transporter gene in Trypanosoma cruzi

A gene from Trypanosoma cruzi, TcrHT1, which encodes a member of the glucose transporter superfamily has been cloned. The gene is similar in sequence to the T. brucei hexose transporter THT1 and the Leishmania transporter Pro-1 and is present in the T. cruzi genome as a cluster of at least eight tandemly reiterated copies. Northern blot analysis revealed two mRNA transcripts which differ in size with respect to their 3' untranslated regions. When injected with in vitro transcribed TcrHT1 mRNA, Xenopus oocytes express a hexose transporter with properties similar to those of T. cruzi. Glucose transport in T. cruzi is mediated via a carrier with unique properties when compared with the other glucose transporters already characterized among the Kinetoplastida. It is a facilitated transporter with a high affinity for D-glucose (Km = 84.1 +/- 7.9 microM and Vmax = 46 +/- 9.4 nmol/min per mg of protein) that shares with other kinetoplastid hexose transporters the ability to recognize D-fructose, which distinguishes these carriers from the human erythrocyte glucose transporter GLUT1.

Membrane-related processes and overall energy metabolism in Trypanosoma brucei and other kinetoplastid species

An electrochemical proton gradient exists across the plasma membrane and the mitochondrial membrane of the bloodstream form of Trypanosoma brucei. The membrane potential across the plasma membrane and the regulation of the internal pH depend on the temperature. Leishmania donovani regulates its internal pH and maintains a constant electrochemical proton gradient across its plasma membrane under all conditions examined. The mitochondrion of the T. brucei bloodstream form is energized, even though the reactions taking place in it do not result in net ATP synthesis and the Kreb's cycle and the respiratory chain are absent. Glucose is transported across the plasma membrane of T. brucei by a facilitated diffusion carrier, that can transport a wider range of substrates than its mammalian counterparts. Pyruvate exits the cell via a facilitated diffusion transporter as well. Conflicting evidence exists for the mechanism of glucose transport in L. donovani; biochemical evidence suggests proton/glucose symport, while facilitated diffusion is indicated by physiological data.

Biochemistry and molecular genetics of Leishmania glucose transporters

Glucose is utilized as a significant source of metabolic energy by Leishmania parasites. This sugar is accumulated by the parasite via a specific carrier-mediated transport system located in the parasite membrane. Parasites may also contain another transporter that shuttles glucose between the cytoplasm and the glycosome, a membrane-bound organelle where the early steps of glycolysis occur. The transport systems of both the insect stage promastigotes and the intracellular amastigotes have been characterized and shown to have kinetic properties that are consistent with the different physiological environments of the insect gut and the macrophage phagolysosome. Several genes have been cloned from Leishmania species which encode proteins with substantial sequence similarity to glucose transporters from mammals and lower eukaryotes. Two of these genes are expressed preferentially in the promastigote stage of the life cycle, where glucose is more readily available and more rapidly transported and metabolized than in the intracellular amastigotes. One of these two developmentally-regulated genes has been functionally expressed in Xenopus oocytes and shown to encode a glucose transporter. A third gene encodes a protein that is also a member of the glucose transporter family on the basis of sequence similarity and proposed secondary structure. However, the significant differences between this protein and the other two suggest that it is likely to transport a different substrate. Functional expression will be required to define the specific biochemical role of each gene within the parasite.

Invariant surface proteins in bloodstream forms of Trypanosoma brucei

Antigenic variation of the glycoprotein forming the coat of African trypanosomes has been a dominant field of investigation for many years. The extravagant potential of these parasites to change their surface coat has destroyed hopes for a vaccine based on the variant surface glycoprotein. Recently, there has been a rising interest in the characterization of surface proteins that are not subject to antigenic variation. In this review, Peter Overath, Maliha Chaudhri, Dietmar Steverding and Karl Ziegelbauer summarize the present evidence for the occurrence, cellular localization and function of invariant surface proteins in Trypanosoma brucei.

Organization of two invariant surface glycoproteins in the surface coat of Trypanosoma brucei

The surface coat of Trypanosoma brucei, formed by about 10(7) molecules of the membrane-form variant surface glycoprotein (mfVSG) per cell, is generally considered to constitute a barrier against the access of antibodies directed to invariant surface proteins. The recent characterization of two invariant surface glycoproteins (ISGs) with apparent molecular masses of 65 and 75 kDa (ISG65 and ISG75; 70,000 and 50,000 molecules per cell, respectively), which are both predicted to be composed of large extracellular domains, single transmembrane alpha-helices, and small intracellular domains, enabled a critical test of this hypothesis. Although ISG65 is distributed over the entire surface of the parasites, it is not accessible to antibodies or to the proteinase trypsin in live cells provided the mfVSG is also proteinase resistant. ISG75 is similarly distributed; its accessibility to antibodies depends on the expressed mfVSG, and it is sensitive to trypsin in a variant clone in which the mfVSG is proteinase resistant. Vaccination experiments using recombinant proteins to a mixture of the native ISGs were unsuccessful. ISG65 but not ISG75 elicited an antibody response in chronically infected mice. The results strengthen the view of the protective properties of the variant surface glycoprotein coat by steric hindrance and suggest that additional factors such as low abundance or low immunogenicity of invariant surface proteins may prevent a control of the disease by the humoral immune response.

The glucose transporter family: structure, function and tissue-specific expression

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Uptake and turnover of glucose in Leishmania donovani

Glucose uptake and metabolism by Leishmania donovani promastigotes was studied using D-[14C]glucose in combination with the silicone oil centrifugation technique on organisms preadapted to different growth rates and glucose availability in the chemostat. The uptake step was differentiated from the subsequent metabolism by separation in time rather than by using non-metabolisable analogues. The uptake of glucose was measured as a function of time and/or the external glucose concentration on cells grown at high or low growth rate with glucose either as growth rate-limiting substrate, or present in excess. Glucose uptake as a function of its external concentration could be described as consisting of two components (1) a rapid equilibration owing to facilitated diffusion, followed by (2) a much slower uptake that involves an enzymatic component. This slower accumulation of label could be explained as the conversion of glucose into metabolites and a storage carbohydrate. Uptake experiments in the presence of inhibitors indicated that the conversion of glucose was an energy dependent process. These experiments indicate that the active uptake of glucose by L. donovani, as reported by others does not occur across the plasma membrane and should be reinterpreted as the intracellular conversion of glucose into metabolites and storage carbohydrate.

Transport of D-fructose and its analogues by Trypanosoma brucei

Kinetic parameters for entry of D-fructose into Trypanosoma brucei brucei have been determined. The net uptake of D-fructose was found to be rapid and occurred at a rate which was comparable with that observed for uptake of D-glucose. The Km and Vmax were 3.91 +/- 1.58 mM and 69.1 +/- 7.2 nmol min-1 (mg protein)-1. D-Fructose was metabolized to pyruvate under aerobic conditions and to pyruvate and glycerol under anaerobic conditions in a manner similar to D-glucose. Comparisons of the kinetic parameters for D-fructose transport and metabolism indicated that uptake was rate limiting. Inhibition constants (Ki) for inhibition of 6-deoxy-D-glucose by D-fructose and D-fructose transport by 6-deoxy-D-glucose were consistent with the Km values for these two substrates. These interactions indicate that D-fructose and 6-deoxy-D-glucose share a single common transporter. 1,5-Anhydro-D-glucitol and 1,5 anhydro-D-mannitol (the fused pyranose ring analogues of D-glucose and D-mannose) have been found to interact well with the transporter, while L-sorbose (a D-fructose analogue with a pyranose ring) had only low affinity. However, 2,5-anhydro-D-mannitol (a fused furanose ring analogue of D-fructose) inhibited both 6-deoxy-D-glucose and D-fructose transport with a Ki of approx. 0.8 mM. The high affinity for 2,5-anhydro-D-mannitol (2-deoxy-D-fructofuranose) indicates that D-fructose is transported in the furanose ring form.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional reconstitution of the Trypanosoma brucei plasma-membrane D-glucose transporter

The D-glucose transporter of Trypanosoma brucei was solubilized from the plasma membrane and reconstituted into proteoliposomes. Using the reconstitution of D-glucose transport as the assay and non-specific L-glucose uptake as control, we have purified a membrane protein fraction from T. brucei bloodstream-form ghosts by EDTA/alkali treatment and solubilization with the detergents octylglucoside or octylthioglucoside. Upon removal of the detergent by dialysis, the solubilized protein fraction was reconstituted in sonicated liposomes by a freeze/thaw-sonication step. The reconstituted transporter catalyzed specific D-glucose uptake and was compared in several characteristics with the native facilitated-diffusion transporter as present in live trypanosomes [Seyfang, A. & Duszenko, M. (1991) Eur. J. Biochem. 202, 191-196]. As in vivo, the uptake is time dependent and Na+ independent. Transporter substrate affinity and inhibitor specificity are completely retained and it is inhibited by mercuric ions, phloretin and cytochalasin B, but only partially inhibited by phlorizin. The reconstituted transporter also demonstrates trans-stimulation properties indicative of the carrier-mediated transport of D-glucose. In contrast to the human erythrocyte-type glucose transporter, in T. brucei D-fructose uptake was also catalyzed by the same reconstituted protein fraction and specific D-glucose or D-fructose transport were mutually competitive. Both the inhibitor studies and the fructose transport capacity in the reconstituted system are in good agreement with the native transport in live trypanosomes. The specific activity of D-glucose transport was 1.9 +/- 0.3 nmol.min-1.mg protein-1 at 0.2 mM D-glucose and the yield was about 0.8% of total ghost protein after removal of the variant-surface-glycoprotein coat. The successful functional reconstitution of a protozoan glucose transporter represents an important step towards its purification and detailed characterization. This is especially interesting since bloodstream-form trypanosomes depend entirely upon glycolysis for their ATP production.

Differential regulation of two distinct families of glucose transporter genes in Trypanosoma brucei

A tandemly arranged multigene family encoding putative hexose transporters in Trypanosoma brucei has been characterized. It is composed of two 80% homologous groups of genes called THT1 (six copies) and THT2 (five copies). When Xenopus oocytes are microinjected with in vitro-transcribed RNA from a THT1 gene, they express a glucose transporter with properties similar to those of the trypanosome bloodstream-form protein(s). This THT1-encoded transport system for glucose differs from the human erythrocyte-type glucose transporter by its moderate sensitivity to cytochalasin B and its capacity to transport D-fructose. These properties suggest that the trypanosomal transporter may be a good target for antitrypanosomal drugs. mRNA analysis revealed that expression of these genes was life cycle stage dependent. Bloodstream forms express 40-fold more THT1 than THT2. In contrast, procyclic trypanosomes express no detectable THT1 but demonstrate glucose-dependent expression of THT2.

Differential regulation of two distinct families of glucose transporter genes in Trypanosoma brucei

A tandemly arranged multigene family encoding putative hexose transporters in Trypanosoma brucei has been characterized. It is composed of two 80% homologous groups of genes called THT1 (six copies) and THT2 (five copies). When Xenopus oocytes are microinjected with in vitro-transcribed RNA from a THT1 gene, they express a glucose transporter with properties similar to those of the trypanosome bloodstream-form protein(s). This THT1-encoded transport system for glucose differs from the human erythrocyte-type glucose transporter by its moderate sensitivity to cytochalasin B and its capacity to transport D-fructose. These properties suggest that the trypanosomal transporter may be a good target for antitrypanosomal drugs. mRNA analysis revealed that expression of these genes was life cycle stage dependent. Bloodstream forms express 40-fold more THT1 than THT2. In contrast, procyclic trypanosomes express no detectable THT1 but demonstrate glucose-dependent expression of THT2.

An integral membrane glycoprotein associated with an endocytic compartment of Trypanosoma vivax: identification and partial characterization

A 65-kD glycoprotein (gp65) of Trypanosoma (Duttonella) vivax was identified using a murine monoclonal antibody (mAb 4E1) that had been raised against formalin-fixed, in vitro-propagated, uncoated forms. Intracellular localization studies utilizing the mAb in immunofluorescence on fixed, permeabilized T. vivax bloodstream forms and immunoelectron microscopy on thin sections of Lowicryl K4M-embedded cells revealed labeling of vesicles and tubules in the posterior portion of the parasite. Some mAb-labeled vesicles contained endocytosed 10 nm BSA-gold after incubation of the parasites with the marker for 5-30 min at 37 degrees C, and the greatest degree of colocalization was observed after 5 min. Double labeling experiments using the mAb and a polyclonal anti-variant surface glycoprotein (VSG) antibody to simultaneously localize both gp65 and VSG demonstrated that there was little overlap in the distribution of these antigens. Thus, gp65 is associated with tubules and vesicles that are involved in endocytosis but which appear to be distinct from VSG processing pathways within the cell. Using the mAb for immunoblot analyses, gp65 was shown to be enriched in a fraction of solubilized membrane proteins eluted from either immobilized Con A or Ricinus communis agglutinin and was found to possess carbohydrate linkages cleaved by both endoglycosidase H and O-glycosidase, suggesting the presence of N- and O-linked glycans. Protease protection and crosslinking experiments suggest that gp65 is a transmembrane protein with trypsin cleavage and NH2-crosslinking sites on the lumenal face of the vesicles.

Molecular characterization of two genes encoding members of the glucose transporter superfamily in the parasitic protozoan Leishmania donovani

The polymerase chain reaction was used to clone two genes from Leishmania donovani, each of which encodes a member of a superfamily of membrane transporters which include the mammalian facilitative glucose transporters. One of these transporters, designated D2, is similar in sequence and overall structural features to a previously cloned Leishmania transporter Pro-1. Both D2 and Pro-1 are developmentally regulated genes which are expressed primarily in the insect stage of the parasite life cycle. In contrast, the second novel transporter, D1, is structurally quite different from either D2 or Pro-1, and its expression is not regulated during the parasite life cycle. All three genes are located on different chromosomes in L. donovani.