2-Cys peroxiredoxin of Plasmodium falciparum is involved in resistance to heat stress of the parasite

Toxoplasma survives the loss of key enzymes of peroxide and glutathione metabolism

Toxoplasma gondii is an obligate intracellular protozoan parasite that replicates rapidly in a variety of host cells. The parasite encodes diverse enzymes of glutathione and peroxide metabolism, but their physiological roles remain poorly understood. Herein, we shed a new perspective on the functions and relevance of the peroxiredoxin and glutathione metabolism in the zoonotic pathogen T. gondii. We show that two cytosolic peroxidases (TgPRX1, TgPRX2), a mitochondrial peroxiredoxin (TgPRX3), and the cytosolic glutathione reductase (TgGR2), glutamate-cysteine ligase (TgGCL), and glutathione synthetase (TgGS) are not required for the lytic cycle of T. gondii under standard growth conditions. However, mutants lacking the gene for either TgPRX1 or TgGR2 exhibited increased susceptibility to exogenous hydrogen peroxide compared to wild-type parasites. Furthermore, we found that the combined deletion of TgPRX1 and TgPRX2 led to a notable impairment of parasite growth, suggesting a functional redundancy between the two peroxidases. Finally, our results show that the apicoplast glutathione reductase (TgGR1) is required for normal parasite growth in vitro and in vivo but is not essential for parasite survival. Our findings highlight that the redox metabolism of Toxoplasma is surprisingly robust and flexible, allowing the parasite to survive under the loss of several key enzymes of peroxide and glutathione metabolism.

Molecular identification of a thioredoxin peroxidase in Babesia gibsoni with potential against oxidative stress

Babesia gibsoni is the infectious agent of canine babesiosis, a vector-borne infection that poses a global threat to the canine health. As B. gibsoni is an erythrocytic intracellular parasite, the completion of its genome and transcriptome sequencing and analysis facilitates the elucidation of the mechanism of B. gibsoni residue in the erythrocyte. The main function of red blood cells (RBCs) is oxygen delivery; thus, B. gibsoni may be exposed to high levels of oxidative stress. To date, no report is available on the mechanism by which B. gibsoni survives oxidative stress inside the RBCs. In this study, the thioredoxin peroxidase, an important type of peroxidoxin, was identified from B. gibsoni, with 255 amino acids and a molecular weight of 27.7 kDa. There are two conserved "VCP" domains at the N- and C-termini, respectively, indicating that this gene was a 2-Cys peroxiredoxin belonging to the PTZ00137 superfamily. It was named BgTPx-2 and was detected to be located in the B. gibsoni-infected erythrocytes through an indirect immunofluorescence assay using the polyclonal antibody against the recombinant TPx-2. Additionally, its antioxidant activity was analyzed by mixed-function oxidation assay, and BgTPx-2 could protect the pBluescript SK ( +) plasmid from oxidative damage, suggesting an antioxidant function of BgTPx-2. Moreover, the immunogenicity of BgTPx-2 was tested by Western blotting and ELISA using the serum of beagle dogs infected with B. gibsoni, and the positive serum exhibited a detectable and significant antibody response against BgTPx-2 on day 4 and day 9 post-infection, respectively.

The cytosolic hyperoxidation-sensitive and -robust Leishmania peroxiredoxins cPRX1 and cPRX2 are both dispensable for parasite infectivity

Typical two-cysteine peroxiredoxins (2-Cys-PRXs) are H2O2-metabolizing enzymes whose activity relies on two cysteine residues. Protists of the family Trypanosomatidae invariably express one cytosolic 2-Cys-PRX (cPRX1). However, the Leishmaniinae sub-family features an additional isoform (cPRX2), almost identical to cPRX1, except for the lack of an elongated C-terminus with a Tyr-Phe (YF) motif. Previously, cytosolic PRXs were considered vital components of the trypanosomatid antioxidant machinery. Here, we shed new light on the properties, functions and relevance of cPRXs from the human pathogen Leishmania infantum. We show first that LicPRX1 is sensitive to inactivation by hyperoxidation, mirroring other YF-containing PRXs participating in redox signaling. Using genetic fusion constructs with roGFP2, we establish that LicPRX1 and LicPRX2 can act as sensors for H2O2 and oxidize protein thiols with implications for signal transduction. Third, we show that while disrupting the LicPRX-encoding genes increases susceptibility of L. infantum promastigotes to external H2O2in vitro, both enzymes are dispensable for the parasites to endure the macrophage respiratory burst, differentiate into amastigotes and initiate in vivo infections. This study introduces a novel perspective on the functions of trypanosomatid cPRXs, exposing their dual roles as both peroxidases and redox sensors. Furthermore, the discovery that Leishmania can adapt to the absence of both enzymes has significant implications for our understanding of Leishmania infections and their treatment. Importantly, it questions the conventional notion that the oxidative response of macrophages during phagocytosis is a major barrier to infection and the suitability of cPRXs as drug targets for leishmaniasis.

Paracrine Signaling Mediated by the Cytosolic Tryparedoxin Peroxidase of Trypanosoma cruzi

Peroxiredoxins are abundant and ubiquitous proteins that participate in different cellular functions, such as oxidant detoxification, protein folding, and intracellular signaling. Under different cellular conditions, peroxiredoxins can be secreted by different parasites, promoting the induction of immune responses in hosts. In this work, we demonstrated that the cytosolic tryparedoxin peroxidase of Trypanosoma cruzi (cTXNPx) is secreted by epimastigotes and trypomastigotes associated with extracellular vesicles and also as a vesicle-free protein. By confocal microscopy, we show that cTXNPx can enter host cells by an active mechanism both through vesicles and as a recombinant protein. Transcriptomic analysis revealed that cTXNPx induces endoplasmic reticulum stress and interleukin-8 expression in epithelial cells. This analysis also suggested alterations in cholesterol metabolism in cTXNPx-treated cells, which was confirmed by immunofluorescence showing the accumulation of LDL and the induction of LDL receptors in both epithelial cells and macrophages. BrdU incorporation assays and qPCR showed that cTXNPx has a mitogenic, proliferative, and proinflammatory effect on these cells in a dose-dependent manner. Importantly, we also demonstrated that cTXNPx acts as a paracrine virulence factor, increasing the susceptibility to infection in cTXNPx-pretreated epithelial cells by approximately 40%. Although the results presented in this work are from in vitro studies and likely underestimate the complexity of parasite-host interactions, our work suggests a relevant role for this protein in establishing infection.

In silico prediction and in vitro assessment of novel heterocyclics with antimalarial activity

The development of new antimalarials is paramount to keep the goals on reduction of malaria cases in endemic regions. The search for quality hits has been challenging as many inhibitory molecules may not progress to the next development stage. The aim of this work was to screen an in-house library of heterocyclic compounds (HCUV) for antimalarial activity combining computational predictions and phenotypic techniques to find quality hits. The physicochemical determinants, pharmacokinetic properties (ADME), and drug-likeness of HCUV were evaluated in silico, and compounds were selected for structure-based virtual screening and in vitro analysis. Seven Plasmodium target proteins were selected from the DrugBank Database, and ligands and receptors were processed using UCSF Chimera and Open Babel before being subjected to docking using Autodock Vina and Autodock 4. Growth inhibition of P. falciparum (3D7) cultures was tested by SYBR Green assays, and toxicity was assessed using hemolytic activity tests and the Galleria mellonella in vivo model. From a total of 792 compounds, 341 with good ADME properties, drug-likeness, and no interference structures were subjected to in vitro analysis. Eight compounds showed IC50 ranging from 0.175 to 0.990 µM, and active compounds included pyridyl-diaminopyrimido-diazepines, pyridyl-N-acetyl- and pyridyl-N-phenyl-pyrazoline derivatives. The most potent compound (UV802, IC50 0.178 µM) showed no toxicophoric and was predicted to interact with P. falciparum 1-cysperoxidredoxin (PfPrx1). For the remaining 7 hits (IC50 < 1 μM), 3 showed in silico binding to PfPrx1, one was predicted to bind the haloacid dehalogenase-like hydrolase and plasmepsin II, and one interacted with the plasmodial heat shock protein 90.

A nuclear redox sensor modulates gene activation and var switching in Plasmodium falciparum

The virulence of Plasmodium falciparum, which causes the deadliest form of human malaria, is attributed to its ability to evade the human immune response. These parasites "choose" to express a single variant from a repertoire of surface antigens called PfEMP1, which are placed on the surface of the infected red cell. Immune evasion is achieved by switches in expression between var genes, each encoding a different PfEMP1 variant. While the mechanisms that regulate mutually exclusive expression of var genes are still elusive, antisense long-noncoding RNAs (lncRNAs) transcribed from the intron of the active var gene were implicated in the "choice" of the single active var gene. Here, we show that this lncRNA colocalizes with the site of var mRNA transcription and is anchored to the var locus via DNA:RNA interactions. We define the var lncRNA interactome and identify a redox sensor, P. falciparum thioredoxin peroxidase I (PfTPx-1), as one of the proteins associated with the var antisense lncRNA. We show that PfTPx-1 localizes to a nuclear subcompartment associated with active transcription on the nuclear periphery, in ring-stage parasite, when var transcription occurs. In addition, PfTPx-1 colocalizes with S-adenosylmethionine synthetase (PfSAMS) in the nucleus, and its overexpression leads to activation of var2csa, similar to overexpression of PfSAMS. Furthermore, we show that PfTPx-1 knockdown alters the var switch rate as well as activation of additional gene subsets. Taken together, our data indicate that nuclear PfTPx-1 plays a role in gene activation possibly by providing a redox-controlled nuclear microenvironment ideal for active transcription.

The interactome of 2-Cys peroxiredoxins in Plasmodium falciparum

Peroxiredoxins (Prxs) are crucially involved in maintaining intracellular H2O2homeostasis via their peroxidase activity. However, more recently, this class of proteins was found to also transmit oxidizing equivalents to selected downstream proteins, which suggests an important function of Prxs in the regulation of cellular protein redox relays. Using a pull-down assay based on mixed disulfide fishing, we characterized the thiol-dependent interactome of cytosolic Prx1a and mitochondrial Prx1m from the apicomplexan malaria parasitePlasmodium falciparum(Pf). Here, 127 cytosolic and 20 mitochondrial proteins that are components of essential cellular processes were found to interact withPfPrx1a andPfPrx1m, respectively. Notably, our data obtained with active-site mutants suggests that reducing equivalents might also be transferred from Prxs to target proteins. Initial functional analyses indicated that the interaction with Prx can strongly impact the activity of target proteins. The results provide initial insights into the interactome of Prxs at the level of a eukaryotic whole cell proteome. Furthermore, they contribute to our understanding of redox regulatory principles and thiol-dependent redox relays of Prxs in subcellular compartments.

Identification of 2-Cys Peroxiredoxin (BmTPx-2) as Antioxidant Active Molecule from Babesia microti

Peroxiredoxins (Prxs) are a family of antioxidant enzymes that reduce peroxides in the presence of thioredoxin, thioredoxin reductase, and nicotinamide adenine dinucleotide phosphate (NADPH) to resist oxidative stress. In this study, we identified and isolated a 2-Cys Prx designated as ‘BmTPx-2’ from Babesia microti, with a full-length cDNA of 826 bp and an open reading frame of 756 bp, which encodes a 251-amino acid protein. BLAST analysis demonstrated that BmTPx-2 shows the typical features of members of the 2-Cys Prx family, which includes harboring two conserved VCP motifs with Cys¹⁰¹ and Cys²²¹ conserved cysteine residues. Recombinant BmTPx-2 was expressed in Escherichia coli and analyzed by western blot. The antioxidant activity of BmTPx-2 was demonstrated using a mixed-function oxidation system and oxidation of NADPH. Furthermore, BmTPx-2 mRNA expression level in parasites at the erythrocytes and tick stages were analyzed by real-time fluorescence quantitative PCR. Peak BmTPx-2 mRNA transcription was detected 8 days after infection at the erythrocyte stage, but not at the tick stage. Taken together, this study characterized BmTPx-2 from B. microti as an antioxidant molecule that was specifically transcribed at the erythrocyte stage.

Knockout of the peroxiredoxin 5 homologue PFAOP does not affect the artemisinin susceptibility of Plasmodium falciparum

Artemisinins are the current mainstay of malaria chemotherapy. Their exact mode of action is an ongoing matter of debate, and several factors have recently been reported to affect an early stage of artemisinin resistance of the most important human malaria parasite Plasmodium falciparum. Here, we identified a locus on chromosome 7 that affects the artemisinin susceptibility of P. falciparum in a quantitative trait locus analysis of a genetic cross between strains 7G8 and GB4. This locus includes the peroxiredoxin gene PFAOP. However, steady-state kinetic data with recombinant PfAOP do not support a direct interaction between this peroxidase and the endoperoxide artemisinin. Furthermore, neither the overexpression nor the deletion of the encoding gene affected the IC₅₀ values for artemisinin or the oxidants diamide and tert-butyl hydroperoxide. Thus, PfAOP is dispensable for blood stage parasite survival, and the correlation between the artemisinin susceptibility and chromosome 7 is probably based on another gene within the identified locus.

H2O2 dynamics in the malaria parasite Plasmodium falciparum

Hydrogen peroxide is an important antimicrobial agent but is also crucially involved in redox signaling and pathogen-host cell interactions. As a basis for systematically investigating intracellular H₂O₂ dynamics and regulation in living malaria parasites, we established the genetically encoded fluorescent H₂O₂ sensors roGFP2-Orp1 and HyPer-3 in Plasmodium falciparum. Both ratiometric redox probes as well as the pH control SypHer were expressed in the cytosol of blood-stage parasites. Both redox sensors showed reproducible sensitivity towards H₂O₂ in the lower micromolar range in vitro and in the parasites. Due to the pH sensitivity of HyPer-3, we used parasites expressing roGFP2-Orp1 for evaluation of short-, medium-, and long-term effects of antimalarial drugs on H₂O₂ levels and detoxification in Plasmodium. None of the quinolines or artemisinins tested had detectable direct effects on the H₂O₂ homeostasis at pharmacologically relevant concentrations. However, pre-treatment of the cells with antimalarial drugs or heat shock led to a higher tolerance towards exogenous H₂O₂. The systematic evaluation and comparison of the two genetically encoded cytosolic H₂O₂ probes in malaria parasites provides a basis for studying parasite-host cell interactions or drug effects with spatio-temporal resolution while preserving cell integrity.

Identification and functional study of a novel 2-cys peroxiredoxin (BmTPx-1) of Babesia microti

Babesia microti is an emerging human pathogen and the primary causative agent of human babesiosis in many regions of the world. Although the peroxiredoxins (Prxs) or thioredoxin peroxidases (TPx) enzymes of this parasite have been sequenced and annotated, their biological properties remain largely unknown. Prxs are a family of antioxidant enzymes that protect biological molecules against metabolically produced reactive oxygen species (ROS) and reduce hydrogen peroxide (H₂O₂) to water in both eukaryotes and prokaryotes. In this study, TPx-1 cDNA was cloned from B. microti (designated BmTPx-1). Recombinant BmTPx-1 (rBmTPx-1) was expressed in Escherichia coli as a histidine fusion protein and purified using Ni-NTA His bind resin. To test the defense capacity of enzymatic antioxidants against the effect of ROS, a mixed-function oxidation system was utilized with the recombinant BmTPx-1 protein. A decreased ability of rBmTPx-1 to donate electrons to the thioredoxin (Trx)/TrxR reductase system was clarified by reaction with H₂O₂. These results suggest that BmTPx-1 has a great impact on protecting parasites from oxidative stress in the erythrocytic stage.

Typical 2-Cys peroxiredoxins in human parasites: Several physiological roles for a potential chemotherapy target

Peroxiredoxins (Prxs) are ubiquitary proteins able to play multiple physiological roles, that include thiol-dependent peroxidase, chaperone holdase, sensor of H2O2, regulator of H2O2-dependent signal cascades, and modulator of the immune response. Prxs have been found in a great number of human pathogens, both eukaryotes and prokaryotes. Gene knock-out studies demonstrated that Prxs are essential for the survival and virulence of at least some of the pathogens tested, making these proteins potential drug targets. However, the multiplicity of roles played by Prxs constitutes an unexpected obstacle to drug development. Indeed, selective inhibitors of some of the functions of Prxs are known (namely of the peroxidase and holdase functions) and are here reported. However, it is often unclear which function is the most relevant in each pathogen, hence which one is most desirable to inhibit. Indeed there are evidences that the main physiological role of Prxs may not be the same in different parasites. We here review which functions of Prxs have been demonstrated to be relevant in different human parasites, finding that the peroxidase and chaperone activities figure prominently, whereas other known functions of Prxs have rarely, if ever, been observed in parasites, or have largely escaped detection thus far.

Effect of thioredoxin peroxidase-1 gene disruption on the liver stages of the rodent malaria parasite Plasmodium berghei

Phenotypic observation of thioredoxin peroxidase-1 (TPx-1) gene-disrupted Plasmodium berghei (TPx-1 KO) in the liver-stage was performed with an in vitro infection system in order to investigate defective liver-stage development in a mouse infection model. Indirect immunofluorescence microscopy assay with anti-circumsporozoite protein antibody revealed that in the liver schizont stage, TPx-1 KO parasite cells were significantly smaller than cells of the wild-type parent strain (WT). Indirect immunofluorescence microscopy assay with anti-merozoite surface protein-1 antibody, which was used to evaluate late schizont-stage development, indicated that TPx-1 KO schizont development was similar to WT strain development towards the merozoite-forming stage (mature schizont). However, fewer merozoites were produced in the mature TPx-1 KO schizont than in the mature WT schizont. Taken together, the results suggest that TPx-1 may be involved in merozoite formation during liver schizont development.

Functional characterization of peroxiredoxins from the human protozoan parasite Giardia intestinalis

The microaerophilic protozoan parasite Giardia intestinalis, causative of one of the most common human intestinal diseases worldwide, infects the mucosa of the proximal small intestine, where it has to cope with O₂ and nitric oxide (NO). Elucidating the antioxidant defense system of this pathogen lacking catalase and other conventional antioxidant enzymes is thus important to unveil novel potential drug targets. Enzymes metabolizing O₂, NO and superoxide anion (O₂^−•) have been recently reported for Giardia, but it is yet unknown how the parasite copes with H₂O₂ and peroxynitrite (ONOO⁻). Giardia encodes two yet uncharacterized 2-cys peroxiredoxins (Prxs), GiPrx1a and GiPrx1b. Peroxiredoxins are peroxidases implicated in virulence and drug resistance in several parasitic protozoa, able to protect from nitroxidative stress and repair oxidatively damaged molecules. GiPrx1a and a truncated form of GiPrx1b (deltaGiPrx1b) were expressed in Escherichia coli, purified and functionally characterized. Both Prxs effectively metabolize H₂O₂ and alkyl-hydroperoxides (cumyl- and tert-butyl-hydroperoxide) in the presence of NADPH and E. coli thioredoxin reductase/thioredoxin as the reducing system. Stopped-flow experiments show that both proteins in the reduced state react with ONOO⁻ rapidly (k = 4×10⁵ M⁻¹ s⁻¹ and 2×10⁵ M⁻¹ s⁻¹ at 4°C, for GiPrx1a and deltaGiPrx1b, respectively). Consistent with a protective role against oxidative stress, expression of GiPrx1a (but not deltaGiPrx1b) is induced in parasitic cells exposed to air O₂ for 24 h. Based on these results, GiPrx1a and deltaGiPrx1b are suggested to play an important role in the antioxidant defense of Giardia, possibly contributing to pathogenesis.

Giardia intestinalis causes one of the most common human intestinal diseases worldwide, called giardiasis. This microorganism infects the small intestine where it has to cope with O₂, nitric oxide (NO) and related reactive species that are toxic for Giardia as it lacks most of the conventional antioxidant enzymes. Understanding how this pathogen survives oxidative stress is thus important because it may help to identify novel drug targets to combat giardiasis. Some enzymes playing a role in the antioxidant defense of Giardia have been recently reported, but it is yet unknown how the parasite copes with two well-known oxidants, hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO⁻). In this study, the Authors show that Giardia expresses two enzymes (called peroxiredoxins), yet uncharacterized, that are able not only to degrade both H₂O₂ and ONOO⁻, but also to repair damaged molecules (called hydroperoxides) that accumulate in the cell under oxidative stress conditions. These results are totally unprecedented because no enzymes with these types of functions have been reported for Giardia to date. If these two enzymes will prove to be essential for Giardia virulence in future studies, a new way will be paved towards the discovery of novel drugs to treat giardiasis.

Identification of a novel and unique transcription factor in the intraerythrocytic stage of Plasmodium falciparum

The mechanisms of stage-specific gene regulation in the malaria parasite Plasmodium falciparum are largely unclear, with only a small number of specific regulatory transcription factors (AP2 family) having been identified. In particular, the transcription factors that function in the intraerythrocytic stage remain to be elucidated. Previously, as a model case for stage-specific transcription in the P. falciparum intraerythrocytic stage, we analyzed the transcriptional regulation of pf1-cys-prx, a trophozoite/schizont-specific gene, and suggested that some nuclear factors bind specifically to the cis-element of pf1-cys-prx and enhance transcription. In the present study, we purified nuclear factors from parasite nuclear extract by 5 steps of chromatography, and identified a factor termed PREBP. PREBP is not included in the AP2 family, and is a novel protein with four K-homology (KH) domains. The KH domain is known to be found in RNA-binding or single-stranded DNA-binding proteins. PREBP is well conserved in Plasmodium species and partially conserved in phylum Apicomplexa. To evaluate the effects of PREBP overexpression, we used a transient overexpression and luciferase assay combined approach. Overexpression of PREBP markedly enhanced luciferase expression under the control of the pf1-cys-prx cis-element. These results provide the first evidence of a novel transcription factor that activates the gene expression in the malaria parasite intraerythrocytic stage. These findings enhance our understanding of the evolution of specific transcription machinery in Plasmodium and other eukaryotes.