Benznidazole-resistance in Trypanosoma cruzi is a readily acquired trait that can arise independently in a single population

Benznidazole is the frontline drug used against Trypanosoma cruzi, the causative agent of Chagas disease. However, treatment failures are often reported. Here, we demonstrate that independently acquired mutations in the gene encoding a mitochondrial nitroreductase (TcNTR) can give rise to distinct drug-resistant clones within a single population. Following selection of benznidazole-resistant parasites, all clones examined had lost one of the chromosomes containing the TcNTR gene. Sequence analysis of the remaining TcNTR allele revealed 3 distinct mutant genes in different resistant clones. Expression studies showed that these mutant proteins were unable to activate benznidazole. This correlated with loss of flavin mononucleotide binding. The drug-resistant phenotype could be reversed by transfection with wild-type TcNTR. These results identify TcNTR as a central player in acquired resistance to benznidazole. They also demonstrate that T. cruzi has a propensity to undergo genetic changes that can lead to drug resistance, a finding that has implications for future therapeutic strategies.

Inhibitors of human histone deacetylase with potent activity against the African trypanosome Trypanosoma brucei

A number of hydroxamic acid derivatives which inhibit human histone deacetylases were investigated for efficacy against cultured bloodstream form Trypanosoma brucei. Three out of the four classes tested displayed significant activity. The majority of compounds blocked parasite growth in the submicromolar range. The most potent was a member of the sulphonepiperazine series with an IC(50) of 34nM. These results identify lead compounds with potential for the development of a novel class of trypanocidal agent.

Centromere-associated repeat arrays on Trypanosoma brucei chromosomes are much more extensive than predicted

BACKGROUND

African trypanosomes belong to a eukaryotic lineage which displays many unusual genetic features. The mechanisms of chromosome segregation in these diploid protozoan parasites are poorly understood. Centromeres in Trypanosoma brucei have been localised to chromosomal regions that contain an array of ~147 bp AT-rich tandem repeats. Initial estimates from the genome sequencing project suggested that these arrays ranged from 2 - 8 kb. In this paper, we show that the centromeric repeat regions are much more extensive.

RESULTS

We used a long-range restriction endonuclease mapping approach to more accurately define the sizes of the centromeric repeat arrays on the 8 T. brucei chromosomes where unambiguous assembly data were available. The results indicate that the sizes of the arrays on different chromosomes vary from 20 to 120 kb. In addition, we found instances of length heterogeneity between chromosome homologues. For example, values of 20 and 65 kb were obtained for the arrays on chromosome 1, and 50 and 75 kb for chromosome 5.

CONCLUSIONS

Our results show that centromeric repeat arrays on T. brucei chromosomes are more similar in size to those of higher eukaryotes than previously suspected. This information provides a firmer framework for investigating aspects of chromosome segregation and will allow epigenetic features associated with the process to be more accurately mapped.

Synthesis of benzopolycyclic cage amines: NMDA receptor antagonist, trypanocidal and antiviral activities

The synthesis of several 6,7,8,9,10,11-hexahydro-9-methyl-5,7:9,11-dimethano-5H-benzocyclononen-7-amines is reported. Several of them display low micromolar NMDA receptor antagonist and/or trypanocidal activities. Two compounds are endowed with micromolar anti vesicular stomatitis virus activity, while only one compound shows micromolar anti-influenza activity. The anti-influenza activity of this compound does not seem to be mediated by blocking of the M2 protein.

Novel lipophilic acetohydroxamic acid derivatives based on conformationally constrained spiro carbocyclic 2,6-diketopiperazine scaffolds with potent trypanocidal activity

We describe novel acetohydroxamic acid derivatives with potent activity against cultured bloodstream-form Trypanosoma brucei and selectivity indices of >1000. These analogues were derived from conformationally constrained, lipophilic, spiro carbocyclic 2,6-diketopiperazine (2,6-DKP) scaffolds by attaching acetohydroxamic acid moieties to the imidic nitrogen. Optimal activity was achieved by placing benzyl groups adjacent to the basic nitrogen of the 2,6-DKP core. S-Enantiomer 7d was the most active derivative against T. brucei (IC(50) = 6.8 nM) and T. cruzi (IC(50) = 0.21 μM).

Huprines as a new family of dual acting trypanocidal-antiplasmodial agents

A series of 19 huprines has been evaluated for their activity against cultured bloodstream forms of Trypanosoma brucei and Plasmodium falciparum. Moreover, cytotoxicity against rat myoblast L6 cells was assessed for selected huprines. All the tested huprines are moderately potent and selective trypanocidal agents, exhibiting IC(50) values against T. brucei in the submicromolar to low micromolar range and selectivity indices for T. brucei over L6 cells of approximately 15, thus constituting interesting trypanocidal lead compounds. Two of these huprines were also found to be active against a chloroquine-resistant strain of P. falciparum, thus emerging as interesting trypanocidal-antiplasmodial dual acting compounds, but they exhibited little selectivity for P. falciparum over L6 cells.

Genetic techniques in Trypanosoma cruzi

It is almost 20 years since genetic manipulation of Trypanosoma cruzi was first reported. In this time, there have been steady improvements in the available vector systems, and the applications of the technology have been extended into new areas. Episomal vectors have been modified to enhance the level of expression of transfected genes and to facilitate the sub-cellular location of their products. Integrative vectors have been adapted to allow the development of inducible expression systems and the construction of vectors which enable genome modification through telomere-associated chromosome fragmentation. The uses of reverse genetic approaches to dissect peroxide metabolism and the mechanisms of drug activity and resistance in T. cruzi are illustrated in this chapter as examples of how the technology has been used to investigate biological function. Although there remains scope to improve the flexibility of these systems, they have made valuable contributions towards exploiting the genome sequence data and providing a greater understanding of parasite biology and the mechanisms of infection.

Centromere-associated topoisomerase activity in bloodstream form Trypanosoma brucei

Topoisomerase-II accumulates at centromeres during prometaphase, where it resolves the DNA catenations that represent the last link between sister chromatids. Previously, using approaches including etoposide-mediated topoisomerase-II cleavage, we mapped centromeric domains in trypanosomes, early branching eukaryotes in which chromosome segregation is poorly understood. Here, we show that in bloodstream form Trypanosoma brucei, RNAi-mediated depletion of topoisomerase-IIα, but not topoisomerase-IIβ, results in the abolition of centromere-localized activity and is lethal. Both phenotypes can be rescued by expression of the corresponding enzyme from T. cruzi. Therefore, processes which govern centromere-specific topoisomerase-II accumulation/activation have been functionally conserved within trypanosomes, despite the long evolutionary separation of these species and differences in centromeric DNA organization. The variable carboxyl terminal region of topoisomerase-II has a major role in regulating biological function. We therefore generated T. brucei lines expressing T. cruzi topoisomerase-II truncated at the carboxyl terminus and examined activity at centromeres after the RNAi-mediated depletion of the endogenous enzyme. A region necessary for nuclear localization was delineated to six residues. In other organisms, sumoylation of topoisomerase-II has been shown to be necessary for regulated chromosome segregation. Evidence that we present here suggests that sumoylation of the T. brucei enzyme is not required for centromere-specific cleavage activity.

Comparative outcomes following endoscopic ureteral detachment and formal bladder cuff excision in open nephroureterectomy for upper urinary tract transitional cell carcinoma

PURPOSE

The introduction of laparoscopic nephroureterectomy highlights the need for the critical appraisal of approaches to the distal ureter at surgery for upper tract transitional cell carcinoma. We compared differences after endoscopic ureteral detachment and open bladder cuff excision in nephroureterectomy.

MATERIALS AND METHODS

A total of 138 patients underwent open nephroureterectomy for upper urinary tract transitional cell carcinoma from 1982 to 2005 with a median followup of 43 months. Of these patients 90 underwent endoscopic ureteral detachment and 48 underwent bladder cuff excision. Demographic, perioperative and oncological outcome data were collected in all cases. Statistical analyses were performed using the Student t test, chi-square and log rank tests, and logistic and Cox regression.

RESULTS

Mean operative duration was significantly lower in the endoscopic detachment group than in the bladder cuff group (p <0.01). There were 49 (54.4%) bladder recurrences in the endoscopic detachment group, of which 8 (16.3%) were muscle invasive and 3 (3.3%) developed at the resection site. There were 23 (47.9%) bladder recurrences in the bladder cuff group, of which 3 (13.0%) were muscle invasive and 2 (4.2%) developed at the resection site. All 5 resection site tumors occurred after excision of muscle invasive distal ureteral tumors and 4 of these had positive margins. There were no differences in recurrence-free survival or disease specific survival between the groups. Operation subtype did not predict oncological outcome on univariate or multivariate analysis.

CONCLUSIONS

Endoscopic ureteral detachment reduces operative duration and is associated with equivalent oncological outcomes compared with open bladder cuff excision in nephroureterectomy. Caution should be exercised in patients with low ureteral tumors.

A mechanism for cross-resistance to nifurtimox and benznidazole in trypanosomes

Nifurtimox and benznidazole are the front-line drugs used to treat Chagas disease, the most important parasitic infection in the Americas. These agents function as prodrugs and must be activated within the parasite to have trypanocidal effects. Despite >40 years of research, the mechanism(s) of action and resistance have remained elusive. Here, we report that in trypanosomes, both drugs are activated by a NADH-dependent, mitochondrially localized, bacterial-like, type I nitroreductase (NTR), and that down-regulation of this explains how resistance may emerge. Loss of a single copy of this gene in Trypanosoma cruzi, either through in vitro drug selection or by targeted gene deletion, is sufficient to cause significant cross-resistance to a wide range of nitroheterocyclic drugs. In Trypanosoma brucei, loss of a single NTR allele confers similar cross-resistance without affecting growth rate or the ability to establish an infection. This potential for drug resistance by a simple mechanism has important implications, because nifurtimox is currently undergoing phase III clinical trials against African trypanosomiasis.

The Trypanosoma cruzi metacyclic-specific protein Met-III associates with the nucleolus and contains independent amino and carboxyl terminal targeting elements

Metacyclogenesis in Trypanosoma cruzi involves the differentiation of replicating non-infective epimastigotes into non-replicating metacyclic trypomastigotes. This pre-adapts parasites for infection of the mammalian host and is characterised by several morphological changes and structural alterations to the nucleus, including nucleolar disaggregation. Experimental investigation of these developmental processes has been hampered by a lack of robust molecular markers. Here, we describe the precise temporal expression of the T. cruzi-specific protein Met-III, in the genome reference strain CL Brener. Expression is restricted to metacyclics in the insect stages of the life-cycle and is rapidly down-regulated following invasion of mammalian cells. Met-III localises to dispersed foci typical of the disassembled nucleolus in metacyclics and to the discrete single nucleolus of cells soon after macrophage invasion. To identify elements that target Met-III, we generated a series of tagged green fluorescent protein fusion proteins and examined their sub-nuclear location in transformed parasites. These experiments demonstrated that amino and carboxyl terminal fragments, characterised by clusters of basic residues, could independently mediate nucleolar sequestration. To investigate the function of Met-III, we used gene deletion. This showed that Met-III is not required for the development of metacyclic trypomastigotes and that null mutants can complete the life-cycle in vitro.

pTcINDEX: a stable tetracycline-regulated expression vector for Trypanosoma cruzi

BACKGROUND

Trypanosoma cruzi is a protozoan pathogen of major medical importance in Latin America. It is also an early diverging eukaryote that displays many unusual biochemical features. The completion of the T. cruzi genome project has highlighted the need to extend the range of techniques available to study gene function. To this end we report the development of a stable tetracycline-dependent expression vector applicable to this parasite and describe in detail the parameters of the system.

RESULTS

We first produced T. cruzi cell lines that constitutively expressed bacteriophage T7 RNA polymerase and the tetracycline repressor protein from a multicopy episome. An integrative vector with an inducible expression site under the control of a tetracycline-regulatable T7 promoter (pTcINDEX) was targeted to the transcriptionally silent ribosomal RNA spacer region of these parasites and transformants selected using a T7 RNA polymerase-dependent hygromycin resistance gene. To test the system we used two marker proteins, luciferase and red fluorescent protein (RFP), and an endogenous parasite protein (a mitochondrial superoxide dismutase). In each case we found that induction was both time and dose-dependent. Luciferase mRNA could be induced by at least 100-fold, and luciferase activity up to 60-fold, within 24 hours of the addition of tetracycline. When we examined RFP induction by confocal microscopy and fluorescence activated cell sorter, we observed very high levels of expression (>1000-fold increase in fluorescence intensity), although this was not synchronous throughout clonal populations. Induction of superoxide dismutase resulted in an 18-fold increase in cellular activity. The observation that a tagged version of the enzyme was correctly targeted to the mitochondrion demonstrates that our expression system may also provide a high-throughput strategy for subcellular localisation.

CONCLUSION

Our results show that pTcINDEX represents a valuable addition to the genetic tools available for T. cruzi. The vector system is sufficiently flexible that it should have widespread uses including inducible expression of tagged proteins, generation of conditional knockout cell lines and the application of dominant-negative approaches.

Functional characterisation of the iron superoxide dismutase gene repertoire in Trypanosoma brucei

Superoxide dismutases (SOD) are a family of antioxidant enzymes that function by removing superoxide anions from the cellular environment. Here, we show that the African trypanosome, Trypanosoma brucei, expresses four SOD isoforms, three of which we have validated biochemically as iron dependent, a feature normally associated with prokaryotic SODs. Localisation studies reveal that two of the enzymes are found predominantly in a parasite-specific organelle, the glycosome (TbSODB1 and TbSODB2), while the other two are targeted to the mitochondrion (TbSODA and TbSODC). Functional analysis of the SOD repertoire in bloodstream form parasites was performed using an inducible RNA interference (RNAi) approach. Down-regulation of the glycosomal SOD transcripts corresponded with a significant reduction in the corresponding proteins and a dramatic level of cell death within the population. The importance of one of the mitochondrial enzymes (TbSODA) only became apparent when parasites were exposed to the superoxide-generating agent paraquat following induction of RNAi. These experiments therefore identify essential components of the superoxide metabolising arm of the T. brucei oxidative defence system and validate these enzymes as parasite-specific targets for drug design.

Vitamin C biosynthesis in trypanosomes: a role for the glycosome

The capacity to synthesize vitamin C (ascorbate) is widespread in eukaryotes but is absent from humans. The last step in the biosynthetic pathway involves the conversion of an aldonolactone substrate to ascorbate, a reaction catalyzed by members of an FAD-dependent family of oxidoreductases. Here we demonstrate that both the African trypanosome, Trypanosoma brucei, and the American trypanosome, Trypanosoma cruzi, have the capacity to synthesize vitamin C and show that this reaction occurs in a unique single-membrane organelle, the glycosome. The corresponding T. brucei flavoprotein (TbALO) obeys Michaelis-Menten kinetics and can utilize both L-galactono-gamma-lactone and D-arabinono-gamma-lactone as substrate, properties characteristic of plant and fungal enzymes. We could detect no activity toward the mammalian enzyme substrate L-gulono-gamma-lactone. TbALO null mutants (bloodstream form) were found to display a transient growth defect, a trait that was enhanced when they were cultured in medium in which the essential serum component had been pretreated with ascorbate oxidase to deplete vitamin C. It is implicit, therefore, that bloodstream-form trypanosomes also possess a capacity for ascorbate transport.

Functional mapping of a trypanosome centromere by chromosome fragmentation identifies a 16-kb GC-rich transcriptional "strand-switch" domain as a major feature

Trypanosomatids are an ancient family that diverged from the main eukaryotic lineage early in evolution, which display several unique features of gene organization and expression. Although genome sequencing is now complete, the nature of centromeres in these and other parasitic protozoa has not been resolved. Here, we report the functional mapping of a centromere in the American trypanosome, Trypanosoma cruzi, a parasite with an unusual mechanism of genetic exchange that involves the generation of aneuploidy by nuclear hybridization. Using a telomere-associated chromosome fragmentation approach, we show that the region required for the mitotic stability of chromosome 3 encompasses a transcriptional "strand-switch" domain constituted by a 16-kb GC-rich island. The domain contains several degenerate retrotransposon-like insertions, but atypically, lacks the arrays of satellite repeats normally associated with centromeric regions. This unusual type of organization may represent a paradigm for centromeres in T. cruzi and other primitive eukaryotes.

Distinct trafficking and localization of STEVOR proteins in three stages of the Plasmodium falciparum life cycle

The genome of Plasmodium falciparum harbors three extensive multigene families, var, rif, and stevor (for subtelomeric variable open reading frame), located mainly in the subtelomeric regions of the parasite's 14 chromosomes. STEVOR variants are known to be expressed in asexual parasites, but no function has as yet been ascribed to this protein family. We have examined the expression of STEVOR proteins in intraerythrocytic sexual stages, gametocytes, and extracellular sporozoites isolated from infected Anopheles mosquitoes. In gametocytes, stevor transcripts appear transiently early in development but STEVOR proteins persist for several days and are transported out of the parasite, travel through the host cell cytoplasm, and localize to the erythrocyte plasma membrane. In contrast to asexual parasites, gametocytes move STEVOR to the periphery via a trafficking pathway independent of Maurer's clefts. In sporozoites, STEVOR appear dispersed throughout the cytoplasm in vesicle-like structures. The pattern of STEVOR localization we have observed in gametocytes and sporozoites differs significantly from that in asexual parasite stages. STEVOR variants are therefore likely to perform different functions in each stage of the parasites life cycle in which they occur.

The amino terminal domain of a novel WD repeat protein from Trypanosoma cruzi contains a non-canonical mitochondrial targeting signal

WD (tryptophan/aspartic acid) repeat proteins perform a wide variety of functions in eukaryotic cells. They are characterised by the presence of a number of conserved repeat motifs that contribute to the beta-propeller structures which are the common feature of this large group of proteins. We report here the properties of the first characterised member of this family in the American trypanosome, Trypanosoma cruzi (TcBPP1). In the CL Brener clone the protein is 482 amino acids long and is predicted to contain four WD repeat motifs, flanked by amino and carboxyl terminal extensions. TcBPP1 is a single copy gene present on a 1.0/1.6 Mb pair of homologous chromosomes in a locus that is syntenic with the corresponding regions of Trypanosoma brucei and Leishmania major chromosomes. Consistent with the proposed hybrid nature of the CL Brener clone, the proteins encoded by the two different alleles share only 97% identity at the amino acid level. To determine subcellular location, we examined transfected parasites for the distribution of green fluorescent protein (GFP) fused with different regions of TcBPP1. These studies demonstrated that a 115 amino acid peptide derived from the amino terminal domain of TcBPP1 is able to target GFP to the mitochondrion. Interestingly this region lacks a typical amino terminal presequence suggesting that mitochondrial import is mediated by an alternative targeting signal.

Mechanism of genetic exchange in American trypanosomes

The kinetoplastid Protozoa are responsible for devastating diseases. In the Americas, Trypanosoma cruzi is the agent of Chagas' disease--a widespread disease transmissible from animals to humans (zoonosis)--which is transmitted by exposure to infected faeces of blood-sucking triatomine bugs. The presence of genetic exchange in T. cruzi and in Leishmania is much debated. Here, by producing hybrid clones, we show that T. cruzi has an extant capacity for genetic exchange. The mechanism is unusual and distinct from that proposed for the African trypanosome, Trypanosoma brucei. Two biological clones of T. cruzi were transfected to carry different drug-resistance markers, and were passaged together through the entire life cycle. Six double-drug-resistant progeny clones, recovered from the mammalian stage of the life cycle, show fusion of parental genotypes, loss of alleles, homologous recombination, and uniparental inheritance of kinetoplast maxicircle DNA. There are strong genetic parallels between these experimental hybrids and the genotypes among natural isolates of T. cruzi. In this instance, aneuploidy through nuclear hybridization results in recombination across far greater genetic distances than mendelian genetic exchange. This mechanism also parallels genome duplication.

TcGPXII, a glutathione-dependent Trypanosoma cruzi peroxidase with substrate specificity restricted to fatty acid and phospholipid hydroperoxides, is localized to the endoplasmic reticulum

Until recently, it had been thought that trypanosomes lack glutathione peroxidase activity. Here we report the subcellular localization and biochemical properties of a second glutathione-dependent peroxidase from Trypanosoma cruzi (TcGPXII). TcGPXII is a single-copy gene which encodes a 16 kDa protein that appears to be specifically dependent on glutathione as the source of reducing equivalents. Recombinant TcGPXII was purified and shown to have peroxidase activity towards a narrow substrate range, restricted to hydroperoxides of fatty acids and phospholipids. Analysis of the pathway revealed that TcGPXII activity could be readily saturated by glutathione and that the peroxidase functioned by a Ping Pong mechanism. Enzyme reduction was shown to be the rate-limiting step in this pathway. Using immunofluorescence, TcGPXII was shown to co-localize with a homologue of immunoglobulin heavy-chain binding protein (BiP), a protein restricted to the endoplasmic reticulum and Golgi. As the smooth endoplasmic reticulum is the site of phospholipid and fatty acid biosynthesis, this suggests that TcGPXII may play a specific role in the T. cruzi oxidative defence system by protecting newly synthesized lipids from peroxidation.

The Trypanosoma cruzi enzyme TcGPXI is a glycosomal peroxidase and can be linked to trypanothione reduction by glutathione or tryparedoxin

Trypanosoma cruzi glutathione-dependent peroxidase I (TcGPXI) can reduce fatty acid, phospholipid, and short chain organic hydroperoxides utilizing a novel redox cycle in which enzyme activity is linked to the reduction of trypanothione, a parasite-specific thiol, by glutathione. Here we show that TcGPXI activity can also be linked to trypanothione reduction by an alternative pathway involving the thioredoxin-like protein tryparedoxin. The presence of this new pathway was first detected using dialyzed soluble fractions of parasite extract. Tryparedoxin was identified as the intermediate molecule following purification, sequence analysis, antibody studies, and reconstitution of the redox cycle in vitro. The system can be readily saturated by trypanothione, the rate-limiting step being the interaction of trypanothione with the tryparedoxin. Both tryparedoxin and TcGPXI operate by a ping-pong mechanism. Overexpression of TcGPXI in transfected parasites confers increased resistance to exogenous hydroperoxides. TcGPXI contains a carboxyl-terminal tripeptide (ARI) that could act as a targeting signal for the glycosome, a kinetoplastid-specific organelle. Using immunofluorescence, tagged fluorescent proteins, and biochemical fractionation, we have demonstrated that TcGPXI is localized to both the glycosome and the cytosol. The ability of TcGPXI to use alternative electron donors may reflect their availability at the corresponding subcellular sites.