Entamoeba histolytica thioredoxin reductase: molecular and functional characterization of its atypical properties

In Vitro Evaluation of the Antiamoebic Activity of Kaempferol against Trophozoites of Entamoeba histolytica and in the Interactions of Amoebae with Hamster Neutrophils

Entamoeba histolytica (E. histolytica) is a parasite in humans that provokes amoebiasis. The most employed drug is metronidazole (MTZ); however, some studies have reported that this drug induces genotoxic effects. Therefore, it is necessary to explore new compounds without toxicity that can eliminate E. histolytica. Flavonoids are polyphenolic compounds that have demonstrated inhibition of growth and dysregulation of amoebic proteins. Despite the knowledge acquired to date, action mechanisms are not completely understood. The present work evaluates the effect of kaempferol against E. histolytica trophozoites and in the interactions with neutrophils from hamster, which is a susceptibility model. Our study demonstrated a significant reduction in the amoebic viability of trophozoites incubated with kaempferol at 150 μM for 90 min. The gene expression analysis showed a significant downregulation of Pr (peroxiredoxin), Rr (rubrerythrin), and TrxR (thioredoxin reductase). In interactions with amoebae and neutrophils for short times, we observed a reduction in ROS (reactive oxygen species), NO (nitric oxide), and MPO (myeloperoxidase) neutrophil activities. In conclusion, we confirmed that kaempferol is an effective drug against E. histolytica through the decrease in E. histolytica antioxidant enzyme expression and a regulator of several neutrophil mechanisms, such as MPO activity and the regulation of ROS and NO.

Riluzole, a Derivative of Benzothiazole as a Potential Anti-Amoebic Agent against Entamoeba histolytica

Amoebiasis is produced by the parasite Entamoeba histolytica; this disease affects millions of people throughout the world who may suffer from amoebic colitis or amoebic liver abscess. Metronidazole is used to treat this protozoan, but it causes important adverse effects that limit its use. Studies have shown that riluzole has demonstrated activity against some parasites. Thus, the present study aimed, for the first time, to demonstrate the in vitro and in silico anti-amoebic activity of riluzole. In vitro, the results of Entamoeba histolytica trophozoites treated with IC₅₀ (319.5 μM) of riluzole for 5 h showed (i) a decrease of 48.1% in amoeba viability, (ii) ultrastructural changes such as a loss of plasma membrane continuity and alterations in the nuclei followed by lysis, (iii) apoptosis-like cell death, (iv) the triggering of the production of reactive oxygen species and nitric oxide, and (v) the downregulation of amoebic antioxidant enzyme gene expression. Interestingly, docking studies have indicated that riluzole presented a higher affinity than metronidazole for the antioxidant enzymes thioredoxin, thioredoxin reductase, rubrerythrin, and peroxiredoxin of Entamoeba histolytica, which are considered as possible candidates of molecular targets. Our results suggest that riluzole could be an alternative treatment against Entamoeba histolytica. Future studies should be conducted to analyze the in vivo riluzole anti-amoebic effect on the resolution of amebic liver abscess in a susceptible model, as this will contribute to developing new therapeutic agents with anti-amoebic activity.

A Review: Natural and Synthetic Compounds Targeting Entamoeba histolytica and Its Biological Membrane

Amoebiasis is the third most common parasitic cause of morbidity and mortality, particularly in countries with poor hygienic settings in central and south America, Africa, and India. This disease is caused by a protozoan parasite, namely Entamoeba histolytica, which infects approximately 50 million people worldwide, resulting in 70,000 deaths every year. Since the 1960s, E. histolytica infection has been successfully treated with metronidazole. However, there are drawbacks to metronidazole therapy: the side effects, duration of treatment, and need for additional drugs to prevent transmission. Previous interdisciplinary studies, including biophysics, bioinformatics, chemistry, and, more recently, lipidomics studies, have increased biomembranes’ publicity. The biological membranes are comprised of a mixture of membrane and cytosolic proteins. They work hand in hand mainly at the membrane part. They act as dedicated platforms for a whole range of cellular processes, such as cell proliferation, adhesion, migration, and intracellular trafficking, thus are appealing targets for drug treatment. Therefore, this review aims to observe the updated trend of the research regarding the biological membranes of E. histolytica from 2015 to 2021, which may help further research regarding the drug targeting the biological membrane.

Entamoeba histolytica Adaption to Auranofin: A Phenotypic and Multi-Omics Characterization

Auranofin (AF), an antirheumatic agent, targets mammalian thioredoxin reductase (TrxR), an important enzyme controlling redox homeostasis. AF is also highly effective against a diversity of pathogenic bacteria and protozoan parasites. Here, we report on the resistance of the parasite Entamoeba histolytica to 2 µM of AF that was acquired by gradual exposure of the parasite to an increasing amount of the drug. AF-adapted E. histolytica trophozoites (AFAT) have impaired growth and cytopathic activity, and are more sensitive to oxidative stress (OS), nitrosative stress (NS), and metronidazole (MNZ) than wild type (WT) trophozoites. Integrated transcriptomics and redoxomics analyses showed that many upregulated genes in AFAT, including genes encoding for dehydrogenase and cytoskeletal proteins, have their product oxidized in wild type trophozoites exposed to AF (acute AF trophozoites) but not in AFAT. We also showed that the level of reactive oxygen species (ROS) and oxidized proteins (OXs) in AFAT is lower than that in acute AF trophozoites. Overexpression of E. histolytica TrxR (EhTrxR) did not protect the parasite against AF, which suggests that EhTrxR is not central to the mechanism of adaptation to AF.

Revisiting Drug Development Against the Neglected Tropical Disease, Amebiasis

Amebiasis is a neglected tropical disease which is caused by the protozoan parasite Entamoeba histolytica. This disease is one of the leading causes of diarrhea globally, affecting largely impoverished residents in developing countries. Amebiasis also remains one of the top causes of gastrointestinal diseases in returning international travellers. Despite having many side effects, metronidazole remains the drug of choice as an amebicidal tissue-active agent. However, emergence of metronidazole resistance in pathogens having similar anaerobic metabolism and also in laboratory strains of E. histolytica has necessitated the identification and development of new drug targets and therapeutic strategies against the parasite. Recent research in the field of amebiasis has led to a better understanding of the parasite’s metabolic and cellular pathways and hence has been useful in identifying new drug targets. On the other hand, new molecules effective against amebiasis have been mined by modifying available compounds, thereby increasing their potency and efficacy and also by repurposing existing approved drugs. This review aims at compiling and examining up to date information on promising drug targets and drug molecules for the treatment of amebiasis.

Rabeprazole inhibits several functions of Entamoeba histolytica related with its virulence

Amoebiasis is a human parasitic disease caused by Entamoeba histolytica. The parasite can invade the large intestine and other organs such as liver; resistance to the host tissue oxygen is a condition for parasite invasion and survival. Thioredoxin reductase of E. histolytica (EhTrxR) is a critical enzyme mainly involved in maintaining reduced the redox system and detoxifying the intracellular oxygen; therefore, it is necessary for E. histolytica survival under both aerobic in vitro and in vivo conditions. In the present work, it is reported that rabeprazole (Rb), a drug widely used to treat heartburn, was able to inhibit the EhTrxR recombinant enzyme. Moreover, Rb affected amoebic proliferation and several functions required for parasite virulence such as cytotoxicity, oxygen reduction to hydrogen peroxide, erythrophagocytosis, proteolysis, and oxygen and complement resistances. In addition, amoebic pre-incubation with sublethal Rb concentration (600 μM) promoted amoebic death during early liver infection in hamsters. Despite the high Rb concentration used to inhibit amoebic virulence, the wide E. histolytica pathogenic-related functions affected by Rb strongly suggest that its molecular structure can be used as scaffold to design new antiamoebic compounds with lower IC₅₀ values.

The MPO system participates actively in the formation of an oxidative environment produced by neutrophils and activates the antioxidant mechanism of Naegleria fowleri

Naegleria fowleri produces a fatal disease called primary amebic meningoencephalitis (PAM), which is characterized by an extensive inflammatory reaction in the CNS. It is known that the immune response is orchestrated mainly by neutrophils, which activate several defense mechanisms in the host, including phagocytosis, the release of different enzymes such as myeloperoxidase (MPO), and the production of neutrophil extracellular traps. However, the mechanisms by which amoebas evade the neutrophil response are still unknown. In this study, we analyzed the ability of N. fowleri to respond to the stress exerted by MPO. Interestingly, after the interaction of trophozoites with neutrophils, the amoeba viability was not altered; however, ultrastructural changes were observed. To analyze the influence of MPO against N. fowleri and its participation in free radical production, we evaluated its enzymatic activity, expression, and localization with and without the specific 4-aminobenzoic acid hydrazide inhibitor. The production of oxidizing molecules is the principal mechanism used by neutrophils to eliminate pathogens. In this context, we demonstrated an increase in the production of NO, superoxide anion, and reactive oxygen species; in addition, the overexpression of several antioxidant enzymes present in the trophozoites was quantified. The findings strongly suggest that N. fowleri possesses antioxidant machinery that is activated in response to an oxidative environment, allowing it to evade the neutrophil-mediated immune response, which may contribute to the establishment of PAM.

Biological activity of esters of quinoxaline-7-carboxylate 1,4-di-N-oxide against E. histolytica and their analysis as potential thioredoxin reductase inhibitors

Amoebiasis is caused by the protozoan Entamoeba histolytica that affects millions of people throughout the world. The standard treatment is metronidazole, however, this drug causes several side effects, and is also mutagenic and carcinogenic. Therefore, the search for therapeutic alternatives is necessary. Quinoxaline 1,4-di-N-oxides (QdNOs) derivatives have been shown to exhibit activity against different protozoan. In the present study, the effects of esters of quinoxaline-7-carboxylate 1,4-di-N-oxide (7-carboxylate QdNOs) derivatives on E. histolytica proliferation, morphology, ultrastructure, and oxidative stress were evaluated, also their potential as E. histolytica thioredoxin reductase (EhTrxR) inhibitors was analyzed. In vitro tests showed that 12 compounds from n-propyl and isopropyl series, were more active (IC₅₀ = 0.331 to 3.56 μM) than metronidazole (IC₅₀ = 4.5 μM). The compounds with better biological activity have a bulky, trifluoromethyl and isopropyl group at R1-, R2-, and R3-position, respectively. The main alterations found in trophozoites treated with some of these compounds included changes in chromatin, cell granularity, redistribution of vacuoles with cellular debris, and an increase in reactive oxygen species. Interestingly, docking studies suggested that 7-carboxylate QdNOs derivatives could interact with amino acid residues of the NADPH-binding domain and/or the redox-active site of EhTrxR. Enzymatic assays demonstrated that selected 7-carboxylate QdNOs inhibits EhTrxR disulfide reductase activity, and diaphorase activity shows that these compounds could act as electron acceptor substrates for the enzyme. Taken together, these data indicate that among the mechanisms involved in the antiamoebic effect of the 7-carboxylate QdNOs derivatives studied, is the induction of oxidative stress and the inhibition of EhTrxR activity.

Comparative Pathobiology of the Intestinal Protozoan Parasites Giardia lamblia, Entamoeba histolytica, and Cryptosporidium parvum

Protozoan parasites can infect the human intestinal tract causing serious diseases. In the following article, we focused on the three most prominent intestinal protozoan pathogens, namely, Giardia lamblia, Entamoeba histolytica, and Cryptosporidium parvum. Both C. parvum and G. lamblia colonize the duodenum, jejunum, and ileum and are the most common causative agents of persistent diarrhea (i.e., cryptosporidiosis and giardiasis). Entamoeba histolytica colonizes the colon and, unlike the two former pathogens, may invade the colon wall and disseminate to other organs, mainly the liver, thereby causing life-threatening amebiasis. Here, we present condensed information concerning the pathobiology of these three diseases.

Flavonoids as a Natural Treatment Against Entamoeba histolytica

Over the past 20 years, gastrointestinal infections in developing countries have been a serious health problem and are the second leading cause of morbidity among all age groups. Among pathogenic protozoans that cause diarrheal disease, the parasite Entamoeba histolytica produces amebic colitis as well as the most frequent extra-intestinal lesion, an amebic liver abscess (ALA). Usually, intestinal amebiasis and ALA are treated with synthetic chemical compounds (iodoquinol, paromomycin, diloxanide furoate, and nitroimidazoles). Metronidazole is the most common treatment for amebiasis. Although the efficacy of nitroimidazoles in killing amebas is known, the potential resistance of E. histolytica to this treatment is a concern. In addition, controversial studies have reported that metronidazole could induce mutagenic effects and cerebral toxicity. Therefore, natural and safe alternative drugs against this parasite are needed. Flavonoids are natural polyphenolic compounds. Flavonoids depend on malonyl-CoA and phenylalanine to be synthesized. Several flavonoids have anti-oxidant and anti-microbial properties. Since the 1990s, several works have focused on the identification and purification of different flavonoids with amebicidal effects, such as, -(-)epicatechin, kaempferol, and quercetin. In this review, we investigated the effects of flavonoids that have potential amebicidal activity and that can be used as complementary and/or specific therapeutic strategies against E. histolytica trophozoites. Interestingly, it was found that these natural compounds can induce morphological changes in the amebas, such as chromatin condensation and cytoskeletal protein re-organization, as well as the upregulation and downregulation of fructose-1,6-bisphosphate aldolase, glyceraldehyde-phosphate dehydrogenase, and pyruvate:ferredoxin oxidoreductase (enzymes of the glycolytic pathway). Although the specific molecular targets, bioavailability, route of administration, and doses of some of these natural compounds need to be determined, flavonoids represent a very promising and innocuous strategy that should be considered for use against E. histolytica in the era of microbial drug resistance.

In-silico prediction and modeling of the Entamoeba histolytica proteins: Serine-rich Entamoeba histolytica protein and 29 kDa Cysteine-rich protease

BACKGROUND

Amoebiasis is the third most common parasitic cause of morbidity and mortality, particularly in countries with poor hygienic settings. There exists an ambiguity in the diagnosis of amoebiasis, and hence there arises a necessity for a better diagnostic approach. Serine-rich Entamoeba histolyticaprotein (SREHP), peroxiredoxin and Gal/GalNAc lectin are pivotal in E. histolyticavirulence and are extensively studied as diagnostic and vaccine targets. For elucidating the cellular function of these proteins, details regarding their respective quaternary structures are essential. However, studies in this aspect are scant. Hence, this study was carried out to predict the structure of these target proteins and characterize them structurally as well as functionally using appropriate in-silicomethods.

METHODS

The amino acid sequences of the proteins were retrieved from National Centre for Biotechnology Information database and aligned using ClustalW. Bioinformatic tools were employed in the secondary structure and tertiary structure prediction. The predicted structure was validated, and final refinement was carried out.

RESULTS

The protein structures predicted by i-TASSER were found to be more accurate than Phyre2 based on the validation using SAVES server. The prediction suggests SREHP to be an extracellular protein, peroxiredoxin a peripheral membrane protein while Gal/GalNAc lectin was found to be a cell-wall protein. Signal peptides were found in the amino-acid sequences of SREHP and Gal/GalNAc lectin, whereas they were not present in the peroxiredoxin sequence. Gal/GalNAc lectin showed better antigenicity than the other two proteins studied. All the three proteins exhibited similarity in their structures and were mostly composed of loops.

DISCUSSION

The structures of SREHP and peroxiredoxin were predicted successfully, while the structure of Gal/GalNAc lectin could not be predicted as it was a complex protein composed of sub-units. Also, this protein showed less similarity with the available structural homologs. The quaternary structures of SREHP and peroxiredoxin predicted from this study would provide better structural and functional insights into these proteins and may aid in development of newer diagnostic assays or enhancement of the available treatment modalities.

Structural characterization of Giardia duodenalis thioredoxin reductase (gTrxR) and computational analysis of its interaction with NBDHEX

Giardia duodenalis is a microaerophilic parasite that colonizes the upper portions of the small intestine of humans. Giardia infection is a major contributor to diarrheal disease worldwide. Nitroheterocycles (e.g. metronidazole) or benzimidazoles (e.g. albendazole) are the most commonly used therapeutic agents. Unfortunately, their efficacy is reduced by low compliance or resistance phenomena. We recently discovered that the antitumoral drug 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol (NBDHEX) is active against G. duodenalis trophozoites and its mode of action is linked to inhibition of thioredoxin reductase (gTrxR), a key component of Giardia redox system: gTrxR provides efficient defenses against reactive oxygen species (ROS), it is a target of 5-nitroimidazoles antiparasitic drugs and also contributes to their metabolism. However, the exact mechanism responsible for the gTrxR inhibition mediated by this chemical class of antigiardial compounds is yet to be defined. The definition of the structural determinants of activity against gTrxR could be important for the identification of novel drugs endowed with an innovative mode of action. With this aim, we solved the crystal structure of gTrxR and we analyzed in silico the binding mode of NBDHEX. The data presented herein could guide the development of NBDHEX derivatives tailored for selective inhibition of gTrxR as antigiardial agents.

Proteomic and functional analyses reveal pleiotropic action of the anti-tumoral compound NBDHEX in Giardia duodenalis

Giardiasis, a parasitic diarrheal disease caused by Giardia duodenalis, affects one billion people worldwide. Treatment relies only on a restricted armamentarium of drugs. The disease burden and the increase in treatment failure highlight the need for novel, safe and well characterized drug options. The antitumoral compound NBDHEX is effective in vitro against Giardia trophozoites and inhibits glycerol-3-phosphate dehydrogenase. Aim of this work was to search for additional NBDHEX protein targets. The intrinsic NBDHEX fluorescence was exploited in a proteomic analysis to select and detect modified proteins in drug treated Giardia. In silico structural analysis, intracellular localization and functional assays were further performed to evaluate drug effects on the identified targets. A small subset of Giardia proteins was covalently bound to the drug at specific cysteine residues. These proteins include metabolic enzymes, e.g. thioredoxin reductase (gTrxR), as well as elongation factor 1B-γ (gEF1Bγ), and structural proteins, e.g. α-tubulin. We showed that NBDHEX in vitro binds to recombinant gEF1Bγ and gTrxR, but only the last one could nitroreduce NBDHEX leading to drug modification of gTrxR catalytic cysteines, with concomitant disulphide reductase activity inhibition and NADPH oxidase activity upsurge. Our results indicate that NBDHEX reacts with multiple targets whose roles and/or functions are specifically hampered. In addition, NBDHEX is in turn converted to reactive intermediates extending its toxicity. The described NBDHEX pleiotropic action accounts for its antigiardial activity and encourages the use of this drug as a promising alternative for the future treatment of giardiasis.

The Tudor Staphylococcal Nuclease Protein of Entamoeba histolytica Participates in Transcription Regulation and Stress Response

Entamoeba histolytica is the protozoa parasite responsible of human amoebiasis, disease that causes from 40,000 to 100,000 deaths annually worldwide. However, few are known about the expression regulation of molecules involved in its pathogenicity. Transcription of some virulence-related genes is positively controlled by the cis-regulatory element named URE1. Previously we identified the transcription factor that binds to URE1, which displayed a nuclear and cytoplasmic localization. This protein belongs to the Tudor Staphyococcal nuclease (TSN) family, which in other systems participates in virtually all pathways of gene expression, suggesting that this amoebic transcription factor (EhTSN; former EhURE1BP) could also play multiple functions in E. histolytica. The aim of this study was to identify the possible cellular events where EhTSN is involved. Here, we found that EhTSN in nucleus is located in euchromatin and close to, but not into, heterochromatin. We also showed the association of EhTSN with proteins involved in transcription and that the knockdown of EhTSN provokes a diminishing in the mRNA level of the EhRabB gene, which in its promoter region contains the URE1 motif, confirming that EhTSN participates in transcription regulation. In cytoplasm, this protein was found linked to the membrane of small vesicles and to plasma membrane. Through pull-down assays and mass spectrometry we identity thirty two candidate proteins to interact with EhTSN. These proteins participate in transcription, metabolism, signaling, and stress response, among other cellular processes. Interaction of EhTSN with some candidate proteins involved in metabolism, and signaling was validated by co-immunoprecipitation or co-localization. Finally we showed the co-localization of EhTSN and HSP70 in putative stress granules during heat shock and that the knockdown of EhTSN increases the cell death during heat shock treatment, reinforcing the hypothesis that EhTSN has a role during stress response. All data support the proposal that EhTSN is a multifunctional protein of E. histolytica.

The Architecture of Thiol Antioxidant Systems among Invertebrate Parasites

The use of oxygen as the final electron acceptor in aerobic organisms results in an improvement in the energy metabolism. However, as a byproduct of the aerobic metabolism, reactive oxygen species are produced, leaving to the potential risk of an oxidative stress. To contend with such harmful compounds, living organisms have evolved antioxidant strategies. In this sense, the thiol-dependent antioxidant defense systems play a central role. In all cases, cysteine constitutes the major building block on which such systems are constructed, being present in redox substrates such as glutathione, thioredoxin, and trypanothione, as well as at the catalytic site of a variety of reductases and peroxidases. In some cases, the related selenocysteine was incorporated at selected proteins. In invertebrate parasites, antioxidant systems have evolved in a diversity of both substrates and enzymes, representing a potential area in the design of anti-parasite strategies. The present review focus on the organization of the thiol-based antioxidant systems in invertebrate parasites. Differences between these taxa and its final mammal host is stressed. An understanding of the antioxidant defense mechanisms in this kind of parasites, as well as their interactions with the specific host is crucial in the design of drugs targeting these organisms.

Water-Soluble Ruthenium (II) Chiral Heteroleptic Complexes with Amoebicidal in Vitro and in Vivo Activity

Three water-soluble Ru(II) chiral heteroleptic coordination compounds [Ru(en)(pdto)]Cl₂ (1), [Ru(gly)(pdto)]Cl (2), and [Ru(acac)(pdto)]Cl (3), where pdto = 2,2'-[1,2-ethanediylbis-(sulfanediyl-2,1-ethanediyl)]dipyridine, en = ethylendiamine, gly = glycinate, and acac = acetylacetonate, have been synthezised and fully characterized. The crystal structures of compounds 1-3 are described. The IC₅₀ values for compounds 1-3 are within nanomolar range (14, 12, and 6 nM, respectively). The cytotoxicity for human peripheral blood lymphocytes is extremely low (>100 μM). Selectivity indexes for Ru(II) compounds are in the range 700-1300. Trophozoites exposed to Ru(II) compounds die through an apoptotic pathway triggered by ROS production. The orally administration to infected mice induces a total elimination of the parasite charge in mice faeces 1-2-fold faster than metronidazole. Besides, all compounds inhibit the trophozoite proliferation in amoebic liver abscess induced in hamster. All our results lead us to propose these compounds as promising candidates as antiparasitic agents.

Synthesis, antiamoebic and molecular docking studies of furan-thiazolidinone hybrids

In continuation of our previous work, a series of furan-thiazolidinone hybrids was prepared by Knoevenagel condensation of 3-(furan-2-ylmethyl)-2-(phenylimino)-1, 3-thiazolidin-4-one with different aryl aldehydes in presence of strong base. Some members of the series exhibited remarkable antiamoebic activity and cell viability. Three compounds (3, 6 and 11) showed excellent binding energy for Entamoeba histolytica O-acetyle-l-serine sulfohydrolase and Entamoeba histolytica thioredoxin reductase. These compounds demonstrated significant inhibition of O-acetyle-l-serine sulfohydrolase. The promising antiamoebic activity and enzymatic assay of 3, 6 and 11 make them promising molecules for further lead optimization in the development of novel antiamoebic agents.

Functional thioredoxin reductase from pathogenic and free-living Leptospira spp

Low molecular mass thiols and antioxidant enzymes have essential functions to detoxify reactive oxygen and nitrogen species maintaining cellular redox balance. The metabolic pathways for redox homeostasis in pathogenic (Leptospira interrogans) and free-living (Leptospira biflexa) leptospires species were not functionally characterized. We performed biochemical studies on recombinantly produced proteins to in depth analyze kinetic and structural properties of thioredoxin reductase (LinTrxR) and thioredoxin (LinTrx) from L. interrogans, and two TrxRs (LbiTrxR1 and LbiTrxR2) from L. biflexa. All the TrxRs were characterized as homodimeric flavoproteins, with LinTrxR and LbiTrxR1 catalyzing the NADPH dependent reduction of LinTrx and DTNB. The thioredoxin system from L. interrogans was able to use glutathione disulfide, lipoamide disulfide, cystine and bis-γ-glutamyl cysteine and homologous peroxiredoxin as substrates. Classic TrxR activity of LinTrxR2 had not been evidenced in vitro, but recombinant Escherichia coli cells overexpressing LbiTrxR2 showed high tolerance to oxidative stress. The enzymatic systems herein characterized could play a key role for the maintenance of redox homeostasis and the function of defense mechanisms against reactive oxidant species in Leptospira spp. Our results contribute to the general knowledge about redox biochemistry in these bacteria, positioning TrxR as a critical molecular target for the development of new anti-leptospiral drugs.

X-ray structures of thioredoxin and thioredoxin reductase from Entamoeba histolytica and prevailing hypothesis of the mechanism of Auranofin action

The anti-arthritic gold-containing drug Auranofin is lethal to the protozoan intestinal parasite Entamoeba histolytica, the causative agent of human amebiasis, in both culture and animal models of the disease. A putative mechanism of Auranofin action proposes that monovalent gold, Au(I), released from the drug, can bind to the redox-active dithiol group of thioredoxin reductase (TrxR). Au(I) binding in the active site is expected to prevent electron transfer to the downstream substrate thioredoxin (Trx), thus interfering with redox homeostasis in the parasite. To clarify the molecular mechanism of Auranofin action in more detail, we determined a series of atomic resolution X-ray structures for E. histolytica thioredoxin (EhTrx) and thioredoxin reductase (EhTrxR), the latter with and without Auranofin. Only the disulfide-bonded form of the active site dithiol (Cys(140)-Cys(143)) was invariably observed in crystals of EhTrxR in spite of the addition of reductants in various crystallization trials, and no gold was found associated with these cysteines. Non-catalytic Cys(286) was identified as the only site of modification, but further mutagenesis studies using the C286Q mutant demonstrated that this site was not responsible for inhibition of EhTrxR by Auranofin. Interestingly, we obtained both of the catalytically-relevant conformations of this bacterial-like, low molecular weight TrxR in crystals without requiring an engineered disulfide linkage between Cys mutants of TrxR and Trx (as was originally done with Escherichia coli TrxR and Trx). We note that the -CXXC- catalytic motif, even if reduced, would likely not provide space sufficient to bind Au(I) by both cysteines of the dithiol group.

Entamoeba thiol-based redox metabolism: A potential target for drug development

Amebiasis is an intestinal infection widespread throughout the world caused by the human pathogen Entamoeba histolytica. Metronidazole has been a drug of choice against amebiasis for decades despite its low efficacy against asymptomatic cyst carriers and emergence of resistance in other protozoa with similar anaerobic metabolism. Therefore, identification and characterization of specific targets is urgently needed to design new therapeutics for improved treatment against amebiasis. Toward this goal, thiol-dependent redox metabolism is of particular interest. The thiol-dependent redox metabolism in E. histolytica consists of proteins including peroxiredoxin, rubrerythrin, Fe-superoxide dismutase, flavodiiron proteins, NADPH: flavin oxidoreductase, and amino acids including l-cysteine, S-methyl-l-cysteine, and thioprolines (thiazolidine-4-carboxylic acids). E. histolytica completely lacks glutathione and its metabolism, and l-cysteine is the major intracellular low molecular mass thiol. Moreover, this parasite possesses a functional thioredoxin system consisting of thioredoxin and thioredoxin reductase, which is a ubiquitous oxidoreductase system with antioxidant and redox regulatory roles. In this review, we summarize and highlight the thiol-based redox metabolism and its control mechanisms in E. histolytica, in particular, the features of the system unique to E. histolytica, and its potential use for drug development against amebiasis.

New transition metal complexes containing imidazole rings endowed with potential antiamoebic activity

Imidazole-hydrazones were found to be better antiamoebic lead molecules compared to the standard drug metronidazole (MNZ). Chelation of these bioactive ligands with metals enhanced their activity profiles.

New drug target in protozoan parasites: the role of thioredoxin reductase

Amebiasis causes approximately 70,000 deaths annually and is the third cause of death due to parasites worldwide. It is treated primarily with metronidazole, which has adverse side effects, is mutagenic and carcinogenic, and emergence of resistance is an increasing concern. Unfortunately, better therapeutic alternatives are lacking. Re-purposing of older FDA approved drugs is advantageous to drug discovery since safety and pharmacokinetic effects in humans are already known. In high throughput screening studies, we recently demonstrated that auranofin, a gold containing compound originally approved to treat rheumatoid arthritis, has activity against trophozoites of E. histolytica, the causative agent of amebiasis. Auranofin's anti-parasitic activity is attributed to its monovalent gold molecule that readily inhibits E. histolytica thioredoxin reductase. This anti-oxidant enzyme is the only thiol-dependent flavo-reductase present in E. histolytica. Auranofin has also shown promising activity against other protozoans of significant public health importance. Altogether, this evidence suggests that auranofin has the potential to become a broad spectrum alternative therapeutic agent for diseases with a large global burden.

Molecular profiling and functional insights of rock bream (Oplegnathus fasciatus) thioredoxin reductase 3-like molecule: investigation of its transcriptional modulation in response to live pathogen stress

The thioredoxin (Trx) system plays a significant role in cellular antioxidative defense by dismutating the surpluses of reactive oxygen species. Thus, the role of thioredoxin reductase (TrxR) cannot be ignored, owing to its participation in initiating the Trx enzyme cascade. Here, we report the identification and molecular characterization of a teleostean TrxR (RbTrxR-3) ortholog that showed high similarity with the TrxR-3 isoforms of other vertebrates. The complete RbTrxR-3 coding sequence comprised 1800 nucleotides, encoding a 600-amino acid protein with a predicted molecular mass of ~66 kDa. RbTrxR-3 consisted of 16 exons separated by 15 introns and had a total length of 12,658 bp. In silico analysis of the RbTrxR-3 protein sequence revealed that it possesses typical TrxR domain architecture. Moreover, using multiple sequence alignment and pairwise sequence alignment strategies, we showed that RbTrxR-3 has high overall sequence similarity to other teleostean TrxR-3 proteins, including highly conserved active site residues. Phylogenetic reconstruction of RbTrxR-3 affirmed its close evolutionary relationship with fish TrxR-3 orthologs, as indicated by its clustering pattern. RbTrxR-3 transcriptional analysis, performed using quantitative polymerase chain reaction (qPCR), showed that RbTrxR-3 was ubiquitously distributed, with the highest level of mRNA expression in the blood, followed by the gill, and liver. Live bacterial and viral stimuli triggered the modulation of RbTrxR-3 basal transcription in liver tissues that correlated temporally with that of its putative substrate, rock bream thioredoxin1 under the same conditions of pathogenic stress. Finally, resembling the typical function of TrxR protein, purified recombinant RbTrxR-3 showed detectable dose-dependent thiol reductase activity against 5,5'-dithiobis (2-nitrobenzoic) acid. Taken together, these results suggest that RbTrxR-3 plays a role in the host Trx system under conditions of oxidative and pathogenic stress.

New enzymatic pathways for the reduction of reactive oxygen species in Entamoeba histolytica

BACKGROUND

Entamoeba histolytica, an intestinal parasite that is the causative agent of amoebiasis, is exposed to elevated amounts of highly toxic reactive oxygen and nitrogen species during tissue invasion. A flavodiiron protein and a rubrerythrin have been characterized in this human pathogen, although their physiological reductants have not been identified.

METHODS

The present work deals with biochemical studies performed to reach a better understanding of the kinetic and structural properties of rubredoxin reductase and two ferredoxins from E. histolytica.

RESULTS

We complemented the characterization of two different metabolic pathways for O2 and H2O2 detoxification in E. histolytica. We characterized a novel amoebic protein with rubredoxin reductase activity that is able to catalyze the NAD(P)H-dependent reduction of heterologous rubredoxins, amoebic rubrerythrin and flavodiiron protein but not ferredoxins. In addition, the protein exhibited an NAD(P)H oxidase activity, which generates hydrogen peroxide from molecular oxygen. We describe how different ferredoxins were also efficient reducing substrates for both flavodiiron protein and rubrerythrin.

CONCLUSIONS

The enzymatic systems herein characterized could contribute to the in vivo detoxification of O2 and H2O2, playing a key role for the parasite defense against reactive oxidant species.

GENERAL SIGNIFICANCE

To the best of our knowledge this is the first characterization of a eukaryotic rubredoxin reductase, including a novel kinetic study on ferredoxin-dependent reduction of flavodiiron and rubrerythrin proteins.

Kinetic and structural characterization of a typical two-cysteine peroxiredoxin from Leptospira interrogans exhibiting redox sensitivity

Little is known about the mechanisms by which Leptospira interrogans, the causative agent of leptospirosis, copes with oxidative stress at the time it establishes persistent infection within its human host. We report the molecular cloning of a gene encoding a 2-Cys peroxiredoxin (LinAhpC) from this bacterium. After bioinformatic analysis we found that LinAhpC contains the characteristic GGIG and YF motifs present in peroxiredoxins that are sensitive to overoxidation (mainly eukaryotic proteins). These motifs are absent in insensitive prokaryotic enzymes. Recombinant LinAhpC showed activity as a thioredoxin peroxidase with sensitivity to overoxidation by H2O2 (Chyp 1% ~30 µM at pH 7.0 and 30°C). So far, Anabaena 2-Cys peroxiredoxin, Helicobacter pylori AhpC, and LinAhpC are the only prokaryotic enzymes studied with these characteristics. The properties determined for LinAhpC suggest that the protein could be critical for the antioxidant defense capacity in L. interrogans.

In vitro antiamoebic activity evaluation and docking studies of metronidazole-triazole hybrids

An in-house database of 520 compounds was docked against Entamoeba histolytica thioredoxin reductase (EhTrR), a promising target for the treatment of amoebiasis. Amongst these, some metronidazole (MTZ)-triazole hybrids were ranked high, with docking scores from -10.23 to -7.56. Studies of the binding orientations and conformations show that the head groups of MTZ-triazole hybrids interact with the arginine residues within the binding pocket of EhTrR, making it clear that such is the optimal and most reliable orientation for this class of compounds. The top-ten MTZ-triazole hybrids were then selected for evaluation of their activity against the HM1:IMSS strain of amoeba. The most active compound, 2-pyridyl-(1,2,3-triazolyl)metronidazole 10, with an IC50 value of 8.4 nM, was significantly more active than the standard drug MTZ alone. Docking studies revealed that compound 10 may act as an EhTrR inhibitor with activity in the nanomolar range and satisfactory ADME properties; it is a suitable candidate to be carried forward as a potential lead in the discovery of drugs to combat amoebiasis.

Peroxynitrite and peroxiredoxin in the pathogenesis of experimental amebic liver abscess

The molecular mechanisms by which Entamoeba histolytica causes amebic liver abscess (ALA) are still not fully understood. Amebic mechanisms of adherence and cytotoxic activity are pivotal for amebic survival but apparently do not directly cause liver abscess. Abundant evidence indicates that chronic inflammation (resulting from an inadequate immune response) is probably the main cause of ALA. Reports referring to inflammatory mechanisms of liver damage mention a repertoire of toxic molecules by the immune response (especially nitric oxide and reactive oxygen intermediates) and cytotoxic substances released by neutrophils and macrophages after being lysed by amoebas (e.g., defensins, complement, and proteases). Nevertheless, recent evidence downplays these mechanisms in abscess formation and emphasizes the importance of peroxynitrite (ONOO⁻). It seems that the defense mechanism of amoebas against ONOO⁻, namely, the amebic thioredoxin system (including peroxiredoxin), is superior to that of mammals. The aim of the present text is to define the importance of ONOO⁻ as the main agent of liver abscess formation during amebic invasion, and to explain the superior capacity of amoebas to defend themselves against this toxic agent through the peroxiredoxin and thioredoxin system.