Overexpression of superoxide dismutase in Trypanosoma cruzi results in increased sensitivity to the trypanocidal agents gentian violet and benznidazole

Molecular Characterization of Four Mexican Isolates of Trypanosoma cruzi and Their Profile Susceptibility to Nifurtimox

PURPOSE

The objective of this study was to molecularly characterize Mexican isolates of T. cruzi obtained from infected triatomine bugs (the vectors of T. cruzi) and to evaluate their susceptibility to Nifurtimox (NFX).

METHODS

Three isolates obtained from Triatoma dimidiata (collected in the State of Veracruz) and one isolate obtained from Triatoma bassolsae (collected in the State of Puebla) were molecularly characterized and the expression of genes associated with natural resistance to NFX was analyzed by qPCR.

RESULTS

Molecular characterization by PCR showed that isolates Zn3, Zn5, and SRB1 belong to the DTU TcI, while isolate Sum3 belongs to TcIV. The latter was also confirmed by sequencing of mitochondrial genes. Isolate Zn5 was the most sensitive to treatment with NFX (IC50, 6.8 μM), isolates SRB1 and Zn3 were partially resistant (IC50, 12.8 μM and 12.7 μM) and isolate Sum3 showed a high degree of resistance to NFX (IC50, 21.4 µM). We also found an association between decreased NTR1 or OYE gene expression with NFX resistance.

CONCLUSION

Our results also evidenced a high variability in the susceptibility to NFX of these T. cruzi isolates Central and Southeastern Mexico, suggesting the presence of naturally resistant isolates circulating in the country. These results have important implications for defining treatment policies for patients with Chagas disease.

Unique thiol metabolism in trypanosomatids: Redox homeostasis and drug resistance

Trypanosomatids are mainly responsible for heterogeneous parasitic diseases: Leishmaniasis, Sleeping sickness, and Chagas disease and control of these diseases implicates serious challenges due to the emergence of drug resistance. Redox-active biomolecules are the endogenous substances in organisms, which play important role in the regulation of redox homeostasis. The redox-active substances like glutathione, trypanothione, cysteine, cysteine persulfides, etc., and other inorganic intermediates (hydrogen peroxide, nitric oxide) are very useful as defence mechanism. In the present review, the suitability of trypanothione and other essential thiol molecules of trypanosomatids as drug targets are described in Leishmania and Trypanosoma. We have explored the role of tryparedoxin, tryparedoxin peroxidase, ascorbate peroxidase, superoxide dismutase, and glutaredoxins in the anti-oxidant mechanism and drug resistance. Up-regulation of some proteins in trypanothione metabolism helps the parasites in survival against drug pressure (sodium stibogluconate, Amphotericin B, etc.) and oxidative stress. These molecules accept electrons from the reduced trypanothione and donate their electrons to other proteins, and these proteins reduce toxic molecules, neutralize reactive oxygen, or nitrogen species; and help parasites to cope with oxidative stress. Thus, a better understanding of the role of these molecules in drug resistance and redox homeostasis will help to target metabolic pathway proteins to combat Leishmaniasis and trypanosomiases.

Mitochondrial Ca2+ and Reactive Oxygen Species in Trypanosomatids

Significance: Millions of people are infected with trypanosomatids and new therapeutic approaches are needed. Trypanosomatids possess one mitochondrion per cell and its study has led to discoveries of general biological interest. These mitochondria, as in their animal counterparts, generate reactive oxygen species (ROS) and have evolved enzymatic and nonenzymatic defenses against them. Mitochondrial calcium ion (Ca2+) overload leads to generation of ROS and its study could lead to relevant information on the biology of trypanosomatids and to novel drug targets. Recent Advances: Mitochondrial Ca2+ is normally involved in maintaining the bioenergetics of trypanosomes, but when Ca2+ overload occurs, it is associated with cell death. Trypanosomes lack key players in the mechanism of cell death described in mammalian cells, although mitochondrial Ca2+ overload results in collapse of their membrane potential, production of ROS, and cytochrome c release. They are also very resistant to mitochondrial permeability transition, and cell death after mitochondrial Ca2+ overload depends on generation of ROS. Critical Issues: In this review, we consider the mechanisms of mitochondrial oxidant generation and removal and the involvement of Ca2+ in trypanosome cell death. Future Directions: More studies are required to determine the reactions involved in generation of ROS by the mitochondria of trypanosomatids, their enzymatic and nonenzymatic defenses against ROS, and the occurrence and composition of a mitochondrial permeability transition pore. Antioxid. Redox Signal. 36, 969-983.

Unraveling the antitrypanosomal mechanism of benznidazole and related 2-nitroimidazoles: From prodrug activation to DNA damage

Nitroheterocycles represent an important class of compound used to treat trypanosomiasis. They often function as prodrugs and can undergo type I nitroreductase (NTR1)-mediated activation before promoting their antiparasitic activities although the nature of these downstream effects has yet to be determined. Here, we show that in an NTR1-dependent process, benznidazole promotes DNA damage in the nuclear genome of Trypanosoma brucei, providing the first direct link between activation of this prodrug and a downstream trypanocidal mechanism. Phenotypic and protein expression studies revealed that components of the trypanosome's homologous recombination (HR) repair pathway (TbMRE11, γH2A, TbRAD51) cooperate to resolve the benznidazole-induced damage, indicating that the prodrug-induced lesions are most likely double stand DNA breaks, while the sequence/recruitment kinetics of these factors parallels that in other eukaryotes HR systems. When extended to other NTR1-activated 2-nitroimidazoles, some were shown to promote DNA damage. Intriguingly, the lesions induced by these required TbMRE11 and TbCSB activities to fix leading us to postulate that TbCSB may operate in systems other than the transcription-coupled nucleotide excision repair pathway. Understanding how existing trypanosomal drugs work will aid future drug design and help unlock novel reactions/pathways that could be exploited as targets for therapeutic intervention.

An Overview on the Therapeutics of Neglected Infectious Diseases-Leishmaniasis and Chagas Diseases

Neglected tropical diseases (NTDs) as termed by WHO include twenty different infectious diseases that are caused by bacteria, viruses, and parasites. Among these NTDs, Chagas disease and leishmaniasis are reported to cause high mortality in humans and are further associated with the limitations of existing drugs like severe toxicity and drug resistance. The above hitches have rendered researchers to focus on developing alternatives and novel therapeutics for the treatment of these diseases. In the past decade, several target-based drugs have emerged, which focus on specific biochemical pathways of the causative parasites. For leishmaniasis, the targets such as nucleoside analogs, inhibitors targeting nucleoside phosphate kinases of the parasite’s purine salvage pathway, 20S proteasome of Leishmania, mitochondria, and the associated proteins are reviewed along with the chemical structures of potential drug candidates. Similarly, in case of therapeutics for Chagas disease, several target-based drug candidates targeting sterol biosynthetic pathway (C14-ademethylase), L-cysteine protease, heme peroxidation, mitochondria, farnesyl pyrophosphate, etc., which are vital and unique to the causative parasite are discussed. Moreover, the use of nano-based formulations towards the therapeutics of the above diseases is also discussed.

Defeating the trypanosomatid trio: proteomics of the protozoan parasites causing neglected tropical diseases

Mass spectrometry-based proteomics enables accurate measurement of the modulations of proteins on a large scale upon perturbation and facilitates the understanding of the functional roles of proteins in biological systems. It is a particularly relevant methodology for studying Leishmania spp., Trypanosoma cruzi and Trypanosoma brucei, as the gene expression in these parasites is primarily regulated by posttranscriptional mechanisms. Large-scale proteomics studies have revealed a plethora of information regarding modulated proteins and their molecular interactions during various life processes of the protozoans, including stress adaptation, life cycle changes and interactions with the host. Important molecular processes within the parasite that regulate the activity and subcellular localisation of its proteins, including several co- and post-translational modifications, are also accurately captured by modern proteomics mass spectrometry techniques. Finally, in combination with synthetic chemistry, proteomic techniques facilitate unbiased profiling of targets and off-targets of pharmacologically active compounds in the parasites. This provides important data sets for their mechanism of action studies, thereby aiding drug development programmes.

Current and Future Prospects of Nitro-compounds as Drugs for Trypanosomiasis and Leishmaniasis

Interest in nitroheterocyclic drugs for the treatment of infectious diseases has undergone a resurgence in recent years. Here we review the current status of monocyclic and bicyclic nitroheterocyclic compounds as existing or potential new treatments for visceral leishmaniasis, Chagas' disease and human African trypanosomiasis. Both monocyclic (nifurtimox, benznidazole and fexinidazole) and bicyclic (pretomanid (PA-824) and delamanid (OPC-67683)) nitro-compounds are prodrugs, requiring enzymatic activation to exert their parasite toxicity. Current understanding of the nitroreductases involved in activation and possible mechanisms by which parasites develop resistance is discussed along with a description of the pharmacokinetic / pharmacodynamic behaviour and chemical structure-activity relationships of drugs and experimental compounds.

Cytosolic Fe-superoxide dismutase safeguards Trypanosoma cruzi from macrophage-derived superoxide radical

Trypanosoma cruzi, the causative agent of Chagas disease (CD), contains exclusively Fe-dependent superoxide dismutases (Fe-SODs). During T. cruzi invasion to macrophages, superoxide radical (O₂ ^•-) is produced at the phagosomal compartment toward the internalized parasite via NOX-2 (gp91-phox) activation. In this work, T. cruzi cytosolic Fe-SODB overexpressers (pRIBOTEX-Fe-SODB) exhibited higher resistance to macrophage-dependent killing and enhanced intracellular proliferation compared with wild-type (WT) parasites. The higher infectivity of Fe-SODB overexpressers compared with WT parasites was lost in gp91-phox ^-/- macrophages, underscoring the role of O₂ ^•- in parasite killing. Herein, we studied the entrance of O₂ ^•- and its protonated form, perhydroxyl radical [(HO₂ ^•); pK_a = 4.8], to T. cruzi at the phagosome compartment. At the acidic pH values of the phagosome lumen (pH 5.3 ± 0.1), high steady-state concentrations of O₂ ^•- and HO₂ ^• were estimated (∼28 and 8 µM, respectively). Phagosomal acidification was crucial for O₂ ^•- permeation, because inhibition of the macrophage H⁺-ATPase proton pump significantly decreased O₂ ^•- detection in the internalized parasite. Importantly, O₂ ^•- detection, aconitase inactivation, and peroxynitrite generation were lower in Fe-SODB than in WT parasites exposed to external fluxes of O₂ ^•- or during macrophage infections. Other mechanisms of O₂ ^•- entrance participate at neutral pH values, because the anion channel inhibitor 5-nitro-2-(3-phenylpropylamino) benzoic acid decreased O₂ ^•- detection. Finally, parasitemia and tissue parasite burden in mice were higher in Fe-SODB-overexpressing parasites, supporting the role of the cytosolic O₂ ^•--catabolizing enzyme as a virulence factor for CD.

Phenotypic diversity and drug susceptibility of Trypanosoma cruzi TcV clinical isolates

Trypanosoma cruzi is a genetically heterogeneous group of organisms that cause Chagas disease. It has been long suspected that the clinical outcome of the disease and response to therapeutic agents are, at least in part, related to the genetic characteristics of the parasite. Herein, we sought to validate the significance of the genotype of T. cruzi isolates recovered from patients with different clinical forms of Chagas disease living in Argentina on their biological behaviour and susceptibility to drugs. Genotype identification of the newly established isolates confirmed the reported predominance of TcV, with a minor frequency of TcI. Epimastigote sensitivity assays demonstrated marked dissimilar responses to benznidazole, nifurtimox, pentamidine and dihydroartemisinin in vitro. Two TcV isolates exhibiting divergent response to benznidazole in epimastigote assays were further tested for the expression of anti-oxidant proteins. Benznidazole-resistant BOL-FC10A epimastigotes had decreased expression of Old Yellow Enzyme and cytosolic superoxide dismutase, and overexpression of mitochondrial superoxide dismutase and tryparedoxin- 1, compared to benznidazole-susceptible AR-SE23C parasites. Drug sensitivity assays on intracellular amastigotes and trypomastigotes reproduced the higher susceptibility of AR-SE23C over BOL-FC10A parasites to benznidazole observed in epimastigotes assays. However, the susceptibility/resistance profile of amastigotes and trypomastigotes to nifurtimox, pentamidine and dihydroartemisinin varied markedly with respect to that of epimastigotes. C3H/He mice infected with AR-SE23C trypomastigotes had higher levels of parasitemia and mortality rate during the acute phase of infection compared to mice infected with BOL-FC10A trypomastigotes. Treatment of infected mice with benznidazole or nifurtimox was efficient to reduce patent parasitemia induced by either isolate. Nevertheless, qPCR performed at 70 dpi revealed parasite DNA in the blood of mice infected with AR-SE23C but not in BOL-FC10A infected mice. These results demonstrate high level of intra-type diversity which may represent an important obstacle for the testing of chemotherapeutic agents.

Current advances in drug discovery for Chagas disease

Chagas disease, also known as American trypanosomiasis, is one of the 17 neglected tropical diseases (NTDs) according to World Health Organization. It is estimated that 8-10 million people are infected worldwide, mainly in Latin America. Chagas disease is caused by the parasite Trypanosoma cruzi and is characterized by two phases: acute and chronic. The current therapy for Chagas disease is limited to drugs such as nifurtimox and benznidazole, which are effective in treating only the acute phase of the disease. In addition, several side effects ranging from hypersensitivity to bone marrow depression and peripheral polyneuropathy have been associated with these drugs. Therefore, the current challenge is to find new effective and safe drugs against this NTD. The aim of this review is to describe the advances in the medicinal chemistry of new anti-chagasic compounds reported in the literature in the last five years. We report promising prototypes for drug discovery identified through target-based and phenotype-based strategies and present some important targets for the development of new synthetic compounds.

Synthetic, small-molecule photoantimicrobials - a realistic approach

The search for suitable, low-molecular weight photoantimicrobials for use in infection control has strong foundations in conventional antiseptic research from the early-mid 20^th Century. Many examples of dyes exist having conventional antimicrobial activity among the azine, acridine and triphenylmethane families which have since also been found to exhibit photosensitising capabilities. The prior employment of these examples in human antisepsis provides a practical basis in terms of low host toxicity, while extant structure-activity relationships for conventional antimicrobial activity can support the development of similar relationships for photoactivated cell killing. The range of chromophores covered allows progress to be made both in topical and deeper, fluid-involved infections.

Prostaglandin F2α synthase in Trypanosoma cruzi plays critical roles in oxidative stress and susceptibility to benznidazole

Nifurtimox (Nfx) and benznidazole (Bz) are the current drugs used for the treatment of Chagas disease. The mechanisms of action and resistance to these drugs in this parasite are poorly known. Prostaglandin F2α synthase or old yellow enzyme (OYE), an NAD(P)H flavin oxidoreductase, has been involved in the activation pathway of other trypanocidal drugs such as Nfx; however, its role in the mechanism of action of Bz is uncertain. In this paper, we performed some experiments of functional genomics in the parasite Trypanosoma cruzi with the aim to test the role of this gene in the resistance to Bz. For this, we overexpressed this gene in sensitive parasites and evaluated the resistance level to the drug and other chemical compounds such as hydrogen peroxide, methyl methanesulfonate and gamma radiation. Interestingly, parasites overexpressing OYE showed alteration of enzymes associated with oxidative stress protection such as superoxide dismutase A and trypanothione reductase. Furthermore, transfected parasites were more sensitive to drugs, genetic damage and oxidative stress. Additionally, transfected parasites were less infective than wild-type parasites and they showed higher alteration in mitochondrial membrane potential and cell cycle after treatment with Bz. These results supply essential information to help further the understanding of the mechanism of action of Bz in T. cruzi.

The Lipid Raft Proteome of African Trypanosomes Contains Many Flagellar Proteins

Lipid rafts are liquid-ordered membrane microdomains that form by preferential association of 3-β-hydroxysterols, sphingolipids and raft-associated proteins often having acyl modifications. We isolated lipid rafts of the protozoan parasite Trypanosoma brucei and determined the protein composition of lipid rafts in the cell. This analysis revealed a striking enrichment of flagellar proteins and several putative signaling proteins in the lipid raft proteome. Calpains and intraflagellar transport proteins, in particular, were found to be abundant in the lipid raft proteome. These findings provide additional evidence supporting the notion that the eukaryotic cilium/flagellum is a lipid raft-enriched specialized structure with high concentrations of sterols, sphingolipids and palmitoylated proteins involved in environmental sensing and cell signaling.

The Trypanosoma cruzi vitamin C dependent peroxidase confers protection against oxidative stress but is not a determinant of virulence

BACKGROUND

The neglected parasitic infection Chagas disease is rapidly becoming a globalised public health issue due to migration. There are only two anti-parasitic drugs available to treat this disease, benznidazole and nifurtimox. Thus it is important to identify and validate new drug targets in Trypanosoma cruzi, the causative agent. T. cruzi expresses an ER-localised ascorbate-dependent peroxidase (TcAPx). This parasite-specific enzyme has attracted interest from the perspective of targeted chemotherapy.

METHODOLOGY/PRINCIPAL FINDINGS

To assess the importance of TcAPx in protecting T. cruzi from oxidative stress and to determine if it is essential for virulence, we generated null mutants by targeted gene disruption. Loss of activity was associated with increased sensitivity to exogenous hydrogen peroxide, but had no effect on susceptibility to the front-line Chagas disease drug benznidazole. This suggests that increased oxidative stress in the ER does not play a significant role in its mechanism of action. Homozygous knockouts could proceed through the entire life-cycle in vitro, although they exhibited a significant decrease in their ability to infect mammalian cells. To investigate virulence, we exploited a highly sensitive bioluminescence imaging system which allows parasites to be monitored in real-time in the chronic stage of murine infections. This showed that depletion of enzyme activity had no effect on T. cruzi replication, dissemination or tissue tropism in vivo.

CONCLUSIONS/SIGNIFICANCE

TcAPx is not essential for parasite viability within the mammalian host, does not have a significant role in establishment or maintenance of chronic infections, and should therefore not be considered a priority for drug design.

Golgi UDP-GlcNAc:polypeptide O-α-N-Acetyl-d-glucosaminyltransferase 2 (TcOGNT2) regulates trypomastigote production and function in Trypanosoma cruzi

All life cycle stages of the protozoan parasite Trypanosoma cruzi are enveloped by mucin-like glycoproteins which, despite major changes in their polypeptide cores, are extensively and similarly O-glycosylated. O-Glycan biosynthesis is initiated by the addition of αGlcNAc to Thr in a reaction catalyzed by Golgi UDP-GlcNAc:polypeptide O-α-N-acetyl-d-glucosaminyltransferases (ppαGlcNAcTs), which are encoded by TcOGNT1 and TcOGNT2. We now directly show that TcOGNT2 is associated with the Golgi apparatus of the epimastigote stage and is markedly downregulated in both differentiated metacyclic trypomastigotes (MCTs) and cell culture-derived trypomastigotes (TCTs). The significance of downregulation was examined by forced continued expression of TcOGNT2, which resulted in a substantial increase of TcOGNT2 protein levels but only modestly increased ppαGlcNAcT activity in extracts and altered cell surface glycosylation in TCTs. Constitutive TcOGNT2 overexpression had no discernible effect on proliferating epimastigotes but negatively affected production of both types of trypomastigotes. MCTs differentiated from epimastigotes at a low frequency, though they were apparently normal based on morphological and biochemical criteria. However, these MCTs exhibited an impaired ability to produce amastigotes and TCTs in cell culture monolayers, most likely due to a reduced infection frequency. Remarkably, inhibition of MCT production did not depend on TcOGNT2 catalytic activity, whereas TCT production was inhibited only by active TcOGNT2. These findings indicate that TcOGNT2 downregulation is important for proper differentiation of MCTs and functioning of TCTs and that TcOGNT2 regulates these functions by using both catalytic and noncatalytic mechanisms.

Trypanosoma cruzi antioxidant enzymes as virulence factors in Chagas disease

SIGNIFICANCE

Chagas disease (CD) affects several million people in Latin America and is spreading beyond its classical boundaries due to the migration of infected host and insect vectors, HIV co-infection, and blood transfusion. The current therapy is not adequate for treatment of the chronic phase of CD, and new drugs are warranted.

RECENT ADVANCES

Trypanosoma cruzi is equipped with a specialized and complex network of antioxidant enzymes that are located at different subcellular compartments which defend the parasite against host oxidative assaults. Recently, strong evidence has emerged which indicates that enzyme components of the T. cruzi antioxidant network (cytosolic and mitochondrial peroxiredoxins and trypanothione synthetase) in naturally occurring strains act as a virulence factor for CD. This precept is recapitulated with the observed increased resistance of T. cruzi peroxirredoxins overexpressers to in vivo or in vitro nitroxidative stress conditions. In addition, the modulation of mitochondrial superoxide radical levels by iron superoxide dismutase (FeSODA) influences parasite programmed cell death, underscoring the role of this enzyme in parasite survival.

CRITICAL ISSUES

The unraveling of the biological significance of FeSODs in T. cruzi programmed cell death in the context of chronic infection in CD is still under examination.

FUTURE DIRECTIONS

The role of the antioxidant enzymes in the pathogenesis of CD, including parasite virulence and persistence, and their feasibility as pharmacological targets justifies further investigation.

Evaluating 5-nitrofurans as trypanocidal agents

The nitroheterocycle nifurtimox, as part of a nifurtimox-eflornithine combination therapy, represents one of a limited number of treatments targeting Trypanosoma brucei, the causative agent of human African trypanosomiasis. The mode of action of this prodrug involves an initial activation reaction catalyzed by a type I nitroreductase (NTR), an enzyme found predominantly in prokaryotes, leading to the formation of a cytotoxic unsaturated open-chain nitrile metabolite. Here, we evaluate the trypanocidal activities of a library of other 5-nitrofurans against the bloodstream form of T. brucei as a preliminary step in the identification of additional nitroaromatic compounds that can potentially partner with eflornithine. Biochemical screening against the purified enzyme revealed that all 5-nitrofurans were effective substrates for T. brucei NTR (TbNTR), with the preferred compounds having apparent kcat/Km values approximately 50-fold greater than those of nifurtimox. For several compounds, in vitro reduction by this nitroreductase yielded products characterized by mass spectrometry as either unsaturated or saturated open-chain nitriles. When tested against the bloodstream form of T. brucei, many of the derivatives displayed significant growth-inhibitory properties, with the most potent compounds generating 50% inhibitory concentrations (IC50s) around 200 nM. The antiparasitic activities of the most potent agents were demonstrated to be NTR dependent, as parasites having reduced levels of the enzyme displayed resistance to the compounds, while parasites overexpressing TbNTR showed hypersensitivity. We conclude that other members of the 5-nitrofuran class of nitroheterocycles have the potential to treat human African trypanosomiasis, perhaps as an alternative partner prodrug to nifurtimox, in the next generation of eflornithine-based combinational therapies.

Vitamin C effects in mice experimentally infected with Trypanosoma cruzi QM2 strain

INTRODUCTION

To evaluate the efficacy of vitamin C in reducing the consequences generated by the production of free radicals in the acute and chronic phases of Chagas disease, two different doses of ascorbic acid were administered orally to 60 mice infected by Trypanosoma cruzi QM2 strain.

METHODS

The animals were divided into six groups: G1, G2, and G3 for the acute phase study, and G'1, G'2, and G'3 for the chronic stage. The groups G1 and G'1 received 8.6 x 10⁻⁴ mg/g of vitamin C daily, whereas G2 and G'2 received 7.14 x 10⁻³ mg/g daily. The other groups, G3 and G'3, were considered placebos and received 10 µL of mineral water.

RESULTS

The study of the acute phase showed statistically significant differences between G1 and the other groups at various count days of the parasitemia evolution. The multiplying parasite was slower in G1 until the 11th day, but on the 22nd day it had greater parasitemia than in G2 and G3, and from the 36th day on, parasitemia stabilized at higher levels. However, when the histopathology of acute and chronic phases is considered, one does not note significant differences.

CONCLUSIONS

The administration of two different doses of vitamin C was not able to protect mice and to contain the oxidative stress caused by free radicals formed by the metabolism of oxygen (reactive oxygen species) and nitrogen (reactive nitrogen species).

Activation of benznidazole by trypanosomal type I nitroreductases results in glyoxal formation

Benznidazole, a 2-nitroimidazole, is the front-line treatment used against American trypanosomiasis, a parasitic infection caused by Trypanosoma cruzi. Despite nearly 40 years of use, the trypanocidal activity of this prodrug is not fully understood. It has been proposed that benznidazole activation leads to the formation of reductive metabolites that can cause a series of deleterious effects, including DNA damage and thiol depletion. Here, we show that the key step in benznidazole activation involves an NADH-dependent trypanosomal type I nitroreductase. This catalyzes an oxygen-insensitive reaction with the interaction of enzyme, reductant, and prodrug occurring through a ping-pong mechanism. Liquid chromatography/mass spectrometry (LC/MS) analysis of the resultant metabolites identified 4,5-dihydro-4,5-dihydroxyimidazole as the major product of a reductive pathway proceeding through hydroxylamine and hydroxy intermediates. The breakdown of this product released the reactive dialdehyde glyoxal, which, in the presence of guanosine, generated guanosine-glyoxal adducts. These experiments indicate that the reduction of benznidazole by type I nitroreductase activity leads to the formation of highly reactive metabolites and that the expression of this enzyme is key to the trypanocidal properties displayed by the prodrug.

Iron metabolism in trypanosomatids, and its crucial role in infection

Iron is almost ubiquitous in living organisms due to the utility of its redox chemistry. It is also dangerous as it can catalyse the formation of reactive free radicals - a classical double-edged sword. In this review, we examine the uptake and usage of iron by trypanosomatids and discuss how modulation of host iron metabolism plays an important role in the protective response. Trypanosomatids require iron for crucial processes including DNA replication, antioxidant defence, mitochondrial respiration, synthesis of the modified base J and, in African trypanosomes, the alternative oxidase. The source of iron varies between species. Bloodstream-form African trypanosomes acquire iron from their host by uptake of transferrin, and Leishmania amazonensis expresses a ZIP family cation transporter in the plasma membrane. In other trypanosomatids, iron uptake has been poorly characterized. Iron-withholding responses by the host can be a major determinant of disease outcome. Their role in trypanosomatid infections is becoming apparent. For example, the cytosolic sequestration properties of NRAMP1, confer resistance against leishmaniasis. Conversely, cytoplasmic sequestration of iron may be favourable rather than detrimental to Trypanosoma cruzi. The central role of iron in both parasite metabolism and the host response is attracting interest as a possible point of therapeutic intervention.

Antioxidant therapy attenuates oxidative insult caused by benzonidazole in chronic Chagas' heart disease

Chronic chagasic cardiac patients are exposed to oxidative stress that apparently contributes to disease progression. Benznidazole (BZN) is the main drug used for the treatment of chagasic patients and its action involves the generation of reactive species. 41 patients with Chagas' heart disease were selected and biomarkers of oxidative stress were measured before and after 2 months of BZN treatment (5 mg/kg/day) and the subsequent antioxidant supplementation with vitamin E (800 UI/day) and C (500 mg/day) during 6 months. Patients were classified according to the modified Los Andes clinical hemodynamic classification in groups IA, IB, II and III, and the activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST) and glutathione reductase (GR), as well as the contents of reduced glutathione (GSH), thiobarbituric acid reactive species (TBARS), protein carbonyl (PC), vitamin E and C and nitric oxide (NO), myeloperoxidase (MPO) and adenosine deaminase (ADA) activities were measured in their blood. Excepting in group III, after BZN treatment SOD, CAT, GPx and GST activities as well as PC levels were enhanced while vitamin E levels were decreased in these groups. After antioxidant supplementation the activities of SOD, GPx and GR were decreased whereas PC, TBARS, NO, and GSH levels were decreased. In conclusion, BZN treatment promoted an oxidative insult in such patients while the antioxidant supplementation was able to attenuate this effect by increasing vitamin E levels, decreasing PC and TBARS levels, inhibiting SOD, GPx and GR activities as well as inflammatory markers, mainly in stages with less cardiac involvement.

Insights into the redox biology of Trypanosoma cruzi: Trypanothione metabolism and oxidant detoxification

Trypanosoma cruzi is the etiologic agent of Chagas' disease, an infection that affects several million people in Latin America. With no immediate prospect of a vaccine and problems associated with current chemotherapies, the development of new treatments is an urgent priority. Several aspects of the redox metabolism of this parasite differ enough from those in the mammalian host to be considered targets for drug development. Here, we review the information about a trypanosomatid-specific molecule centrally involved in redox metabolism, the dithiol trypanothione, and the main effectors of cellular antioxidant defense. We focus mainly on data from T. cruzi, making comparisons with other trypanosomatids whenever possible. In these parasites trypanothione participates in crucial thiol-disulfide exchange reactions and serves as electron donor in different metabolic pathways, from synthesis of DNA precursors to oxidant detoxification. Interestingly, the levels of several enzymes involved in trypanothione metabolism and oxidant detoxification increase during the transformation of T. cruzi to its mammalian-infective form and the overexpression of some of them has been associated with increased resistance to macrophage-dependent oxidative killing. Together, the evidence suggests a central role of the trypanothione-dependent antioxidant systems in the infection process.

Mode of action of natural and synthetic drugs against Trypanosoma cruzi and their interaction with the mammalian host

Current knowledge of the biochemistry of Trypanosoma cruzi has led to the development of new drugs and the understanding of their mode of action. Some trypanocidal drugs such as nifurtimox and benznidazole act through free radical generation during their metabolism. T. cruzi is very susceptible to the cell damage induced by these metabolites because enzymes scavenging free radicals are absent or have very low activities in the parasite. Another potential target is the biosynthetic pathway of glutathione and trypanothione, the low molecular weight thiol found exclusively in trypanosomatids. These thiols scavenge free radicals and participate in the conjugation and detoxication of numerous drugs. Inhibition of this key pathway could render the parasite much more susceptible to the toxic action of drugs such as nifurtimox and benznidazole without affecting the host significantly. Other drugs such as allopurinol and purine analogs inhibit purine transport in T. cruzi, which cannot synthesize purines de novo. Nitroimidazole derivatives such as itraconazole inhibit sterol metabolism. The parasite's respiratory chain is another potential therapeutic target because of its many differences with the host enzyme complexes. The pharmacological modulation of the host's immune response against T. cruzi infection as a possible chemotherapeutic target is discussed. A large set of chemicals of plant origin and a few animal metabolites active against T. cruzi are enumerated and their likely modes of action are briefly discussed.

Increased expression of iron-containing superoxide dismutase-A (TcFeSOD-A) enzyme in Trypanosoma cruzi population with in vitro-induced resistance to benznidazole

Superoxide dismutase (SOD) removes excess superoxide radicals via dismutation to oxygen and hydrogen peroxide. In this work, we have characterized TcFeSOD-A gene from 25 Trypanosoma cruzi populations and clones susceptible, naturally resistant or with in vitro-induced (17 LER) or in vivo-selected resistance to benznidazole (BZR). In the 17 LER T. cruzi population, the levels of TcFeSOD-A mRNA were at least 3-fold higher than its drug-susceptible counterpart 17 WTS. The levels of TcFeSOD-A mRNA were similar among the other T. cruzi populations and clones regardless of the drug-resistance phenotype. We determined whether the increase in mRNA levels was due to gene amplification using Southern blot analysis of the T. cruzi populations and clones. We found that the number of TcFeSOD-A gene copies was similar for all samples tested, except for 17 LER that presented twice as many copies. The chromosomal location of the TcFeSOD-A gene and polymorphisms detected in nucleotide and amino acid sequences of TcFeSOD-A were associated with the zymodeme of the T. cruzi strain but not with drug-resistance phenotype. We observed a 23 kDa TcFeSOD-A polypeptide in all analysed T. cruzi strains. The level of this polypeptide was increased only in the 17 LER population. Specific enzyme activity analysis of TcFeSOD in the T. cruzi samples revealed a correlation between expression and activity. Our findings show an increased expression of the TcFeSOD-A enzyme in the T. cruzi population with in vitro-induced resistance to benznidazole.

Polyamines as a defense mechanism against lipoperoxidation in Trypanosoma cruzi

The polyamines, spermine and spermidine--organic polycations that are absolutely required for eukaryotic cell growth--are shown here to function in Trypanosoma cruzi epimastigotes, as protectors of membrane lipoperoxidation by reactive oxygen species generated either by H2O2/Fe2+ or nifurtimox. In vitro, spermine and spermidine inhibited lipoperoxidation in a dose dependent manner. Spermine was more efficient than spermidine in its inhibitory effect. Lipid peroxidation induced by H2O2 showed an IC50 of 0.55 mM for spermine and 0.9 mM for spermidine while an IC50 of 0.8 mM for spermine and 1.5 mM for spermidine were observed when lipoperoxidation was elicited by nifurtimox. Likewise in vivo, both exogenously added spermine and spermidine or endogenous increase of spermine levels induced by phorbol ester, protected against lipoperoxidation and decreased citotoxicity provoked by nifurtimox. Putrescine and cadaverine, also present in T. cruzi had no effect at all. None of the polyamines had any effect neither on the scavenging of superoxide anion nor on the regulation of antioxidant enzymes such as superoxide dismutase and peroxidases involved in H2O2 detoxification. Here we point out that spermine, by acting as a protector of membrane lipoperoxidation might contribute to survival of T. cruzi continuously exposed to oxidative stress.

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.

A lipid-modified phosphoinositide-specific phospholipase C (TcPI-PLC) is involved in differentiation of trypomastigotes to amastigotes of Trypanosoma cruzi

The phosphoinositide-specific phospholipase C (PI-PLC) is an important component of the inositol phosphate/diacylglycerol signaling pathway. A newly discovered Trypanosoma cruzi PI-PLC (TcPI-PLC) is lipid modified in its N terminus, targeted to its plasma membrane, and believed to play a role in differentiation of the parasite because its expression increases during the differentiation of trypomastigote to amastigote stages. To determine whether TcPI-PLC is involved in this differentiation step, antisense inhibition using phosphorothioate-modified oligonucleotides, and overexpression of the gene were performed. Antisense oligonucleotide-treated parasites showed a reduced rate of differentiation in comparison to controls, as well as accumulation of intermediate forms. Overexpression of TcPI-PLC led to a faster differentiation rate. In contrast, overexpression of a mutant TcPI-PLC that lacked the lipid modification at its N terminus did not affect the differentiation rate. Therefore, TcPI-PLC is involved, when expressed in the plasma membrane, in the differentiation of trypomastigotes to amastigotes, an essential step for the intracellular replication of these parasites.

Trypanosoma cruzi expresses a plant-like ascorbate-dependent hemoperoxidase localized to the endoplasmic reticulum

In most aerobic organisms hemoperoxidases play a major role in H(2)O(2)-detoxification, but trypanosomatids have been reported to lack this activity. Here we describe the properties of an ascorbate-dependent hemoperoxidase (TcAPX) from the American trypanosome Trypanosoma cruzi. The activity of this plant-like enzyme can be linked to the reduction of the parasite-specific thiol trypanothione by ascorbate in a process that involves nonenzymatic interaction. The role of heme in peroxidase activity was demonstrated by spectral and inhibition studies. Ascorbate could saturate TcAPX activity indicating that the enzyme obeys Michaelis-Menten kinetics. Parasites that overexpressed TcAPX activity were found to have increased resistance to exogenous H(2)O(2). To determine subcellular location an epitope-tagged form of TcAPX was expressed in T. cruzi, which was observed to colocalize with endoplasmic reticulum resident chaperone protein BiP. These findings identify an arm of the oxidative defense system of this medically important parasite. The absence of this redox pathway in the human host may be therapeutically exploitable.

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.

Cytotoxicity of prodigiosin and benznidazole on V79 cells

The cytotoxicity of prodigiosin, an antibiotic and potential trypanocide produced by Serratia marcescens, and Benznidazole, a trypanocidal drug, were assayed on V79 fibroblast cell line. Three independent endpoints for cytotoxicity were evaluated; namely, the nucleic acid content (NAC), MTT reduction and neutral red uptake (NRU). IC(50) values of 1-20 microM were obtained for prodigiosin in the NRU, MTT and NAC tests. Prodigiosin had greater trypanocidal activity (IC(50)=5 microM) than Nifurtimox (IC(50)=150 microM) a known trypanocide drug used in Chagas' disease therapy. Benznidazole was less toxic (IC(50)=2000 microM) than prodigiosin (IC(50)=1-20 microM) in V79 cells based on the MTT and NAC assays. Benznidazole stimulated the NRU until 2 mM. Indeed, the cell viability measured with the NRU was higher at all concentrations of benznidazole tested than that measured by MTT reduction and NAC assays.

Evidence for a trypanothione-dependent peroxidase system in Trypanosoma cruzi

Hydroperoxide metabolism in Crithidia fasciculata has recently been shown to be catalyzed by a cascade of three oxidoreductases comprising trypanothione reductase (TR), tryparedoxin (TXN1), and tryparedoxin peroxidase (TXNPx) (Nogoceke et al., Biol. Chem. 378, 827-836, 1997). The existence of this metabolic system in the human pathogen Trypanosoma cruzi is supported here by immunohistochemistry. Epimastigotes of T. cruzi display strong immunoreactivity with antibodies raised against TXN1 and TXNPx of C. fasciculata. In addition, a full-length open reading frame presumed to encode a peroxiredoxin-type protein in T. cruzi (Acc. Nr. AJ 012101) was heterologously expressed in Escherichia coli and shown to exhibit tryparedoxin peroxidase activity. With TXN, TXNPx, trypanothione and TR, T. cruzi possesses all components constituting the crithidial peroxidase system. It is concluded that the antioxidant defense of T. cruzi also depends on the NADPH-fuelled, trypanothione-mediated enzymatic hydroperoxide metabolism.

Glutathione and trypanothione in parasitic hydroperoxide metabolism

Thiol-dependent hydroperoxide metabolism in parasites is reviewed in respect to potential therapeutic strategies. The hydroperoxide metabolism of Crithidia fasciculata has been characterized to comprise a cascade of three enzymes, trypanothione reductase, tryparedoxin, and tryparedoxin peroxidase, plus two supportive enzymes to synthesize the redox mediator trypanothione from glutathione and spermidine. The essentiality of the system in respect to parasite vitality and virulence has been verified by genetic approaches. The system appears to be common to all genera of the Kinetoplastida. The terminal peroxidase of the system belongs to the protein family of peroxiredoxins which is also represented in Entamoeba and a variety of metazoan parasites. Plasmodial hydroperoxide metabolism displays similarities to the mammalian system in comprising glutathione biosynthesis, glutathione reductase, and at least one glutathione peroxidase homolog having the active site selenocysteine replaced by cysteine. Nothing precise is known about the antioxidant defence systems of Giardia, Toxoplasma, and Trichomonas species. Also, the role of ovothiols and mycothiols reportedly present in several parasites remains to be established. Scrutinizing known enzymes of parasitic antioxidant defence for suitability as drug targets leaves only those of the trypanosomatid system as directly or indirectly validated. By generally accepted criteria of target selection and feasibility considerations tryparedoxin and tryparedoxin peroxidase can at present be rated as the most appealing target structures for the development of antiparasitic drugs.