Thioredoxin and glutathione systems differ in parasitic and free-living platyhelminths

G6PDH as a key immunometabolic and redox trigger in arthropods

The enzyme glucose-6-phosphate dehydrogenase (G6PDH) plays crucial roles in glucose homeostasis and the pentose phosphate pathway (PPP), being also involved in redox metabolism. The PPP is an important metabolic pathway that produces ribose and nicotinamide adenine dinucleotide phosphate (NADPH), which are essential for several physiologic and biochemical processes, such as the synthesis of fatty acids and nucleic acids. As a rate-limiting step in PPP, G6PDH is a highly conserved enzyme and its deficiency can lead to severe consequences for the organism, in particular for cell growth. Insufficient G6PDH activity can lead to cell growth arrest, impaired embryonic development, as well as a reduction in insulin sensitivity, inflammation, diabetes, and hypertension. While research on G6PDH and PPP has historically focused on mammalian models, particularly human disorders, recent studies have shed light on the regulation of this enzyme in arthropods, where new functions were discovered. This review will discuss the role of arthropod G6PDH in regulating redox homeostasis and immunometabolism and explore potential avenues for further research on this enzyme in various metabolic adaptations.

Trans-splicing in the cestode Hymenolepis microstoma is constitutive across the life cycle and depends on gene structure and composition

Spliced leader (SL) trans-splicing is a key process during mRNA maturation of many eukaryotes, in which a short sequence (SL) is transferred from a precursor SL-RNA into the 5' region of an immature mRNA. This mechanism is present in flatworms, in which it is known to participate in the resolution of polycistronic transcripts. However, most trans-spliced transcripts are not part of operons, and it is not clear if this process may participate in additional regulatory mechanisms in this group. In this work, we present a comprehensive analysis of SL trans-splicing in the model cestode Hymenolepis microstoma. We identified four different SL-RNAs which are indiscriminately trans-spliced to 622 gene models. SL trans-splicing is enriched in constitutively expressed genes and does not appear to be regulated throughout the life cycle. Operons represented at least 20% of all detected trans-spliced gene models, showed conservation to those of the cestode Echinococcus multilocularis, and included complex loci such as an alternative operon (processed as either a single gene through cis-splicing or as two genes of a polycistron). Most insertion sites were identified in the 5' untranslated region (UTR) of monocistronic genes. These genes frequently contained introns in the 5' UTR, in which trans-splicing used the same acceptor sites as cis-splicing. These results suggest that, unlike other eukaryotes, trans-splicing is associated with internal intronic promoters in the 5' UTR, resulting in transcripts with strong splicing acceptor sites without competing cis-donor sites, pointing towards a simple mechanism driving the evolution of novel SL insertion sites.

Evolutionary Adaptations of Parasitic Flatworms to Different Oxygen Tensions

During the evolution of the Earth, the increase in the atmospheric concentration of oxygen gave rise to the development of organisms with aerobic metabolism, which utilized this molecule as the ultimate electron acceptor, whereas other organisms maintained an anaerobic metabolism. Platyhelminthes exhibit both aerobic and anaerobic metabolism depending on the availability of oxygen in their environment and/or due to differential oxygen tensions during certain stages of their life cycle. As these organisms do not have a circulatory system, gas exchange occurs by the passive diffusion through their body wall. Consequently, the flatworms developed several adaptations related to the oxygen gradient that is established between the aerobic tegument and the cellular parenchyma that is mostly anaerobic. Because of the aerobic metabolism, hydrogen peroxide (H₂O₂) is produced in abundance. Catalase usually scavenges H₂O₂ in mammals; however, this enzyme is absent in parasitic platyhelminths. Thus, the architecture of the antioxidant systems is different, depending primarily on the superoxide dismutase, glutathione peroxidase, and peroxiredoxin enzymes represented mainly in the tegument. Here, we discuss the adaptations that parasitic flatworms have developed to be able to transit from the different metabolic conditions to those they are exposed to during their life cycle.

Biochemical and structural characterizations of thioredoxin reductase selenoproteins of the parasitic filarial nematodes Brugia malayi and Onchocerca volvulus

Enzymes in the thiol redox systems of microbial pathogens are promising targets for drug development. In this study we characterized the thioredoxin reductase (TrxR) selenoproteins from Brugia malayi and Onchocerca volvulus, filarial nematode parasites and causative agents of lymphatic filariasis and onchocerciasis, respectively. The two filarial enzymes showed similar turnover numbers and affinities for different thioredoxin (Trx) proteins, but with a clear preference for the autologous Trx. Human TrxR1 (hTrxR1) had a high and similar specific activity versus the human and filarial Trxs, suggesting that, in vivo, hTrxR1 could possibly be the reducing agent of parasite Trxs once they are released into the host. Both filarial TrxRs were efficiently inhibited by auranofin and by a recently described inhibitor of human TrxR1 (TRi-1), but not as efficiently by the alternative compound TRi-2. The enzyme from B. malayi was structurally characterized also in complex with NADPH and auranofin, producing the first crystallographic structure of a nematode TrxR. The protein represents an unusual fusion of a mammalian-type TrxR protein architecture with an N-terminal glutaredoxin-like (Grx) domain lacking typical Grx motifs. Unlike thioredoxin glutathione reductases (TGRs) found in platyhelminths and mammals, which are also Grx–TrxR domain fusion proteins, the TrxRs from the filarial nematodes lacked glutathione disulfide reductase and Grx activities. The structural determinations revealed that the Grx domain of TrxR from B. malayi contains a cysteine (C22), conserved in TrxRs from clade IIIc nematodes, that directly interacts with the C-terminal cysteine-selenocysteine motif of the homo-dimeric subunit. Interestingly, despite this finding we found that altering C22 by mutation to serine did not affect enzyme catalysis. Thus, although the function of the Grx domain in these filarial TrxRs remains to be determined, the results obtained provide insights on key properties of this important family of selenoprotein flavoenzymes that are potential drug targets for treatment of filariasis.

Antiparasitic Effects of Selected Isoflavones on Flatworms

Medicinal plants have been successfully used in the ethno medicine for a wide range of diseases since ancient times. The research on natural products has allowed the discovery of biologically relevant compounds inspired by plant secondary metabolites, what contributed to the development of many chemotherapeutic drugs. Flavonoids represent a group of therapeutically very effective plant secondary metabolites and selected molecules were shown to exert also antiparasitic activity. This work summarizes the recent knowledge generated within past three decades about potential parasitocidal activities of several flavonoids with different chemical structures, particularly on medically important flatworms such as Schistosoma spp., Fasciola spp., Echinococcus spp., Raillietina spp., and model cestode Mesocestoides vogae. Here we focus on curcumin, genistein, quercetin and silymarin complex of flavonolignans. All of them possess a whole spectrum of biological activities on eukaryotic cells which have multi-therapeutic effects in various diseases. In vitro they can induce profound alterations in the tegumental architecture and its functions as well as their activity can significantly modulate or damage worm´s metabolism directly by interaction with enzymes or signaling molecules in dose-dependent manner. Moreover, they seem to differentially regulate the RNA activity in numbers of worm´s genes. This review suggests that examined flavonoids and their derivates are promising molecules for antiparasitic drug research. Due to lack of toxicity, isoflavons could be used directly for therapy, or as adjuvant therapy for diseases caused by medically important cestodes and trematodes.

Effects of Mammalian Thioredoxin Reductase Inhibitors

The mammalian thioredoxin system is driven by NADPH through the activities of isoforms of the selenoprotein thioredoxin reductase (TXNRD, TrxR), which in turn help to keep thioredoxins (TXN, Trx) and further downstream targets reduced. Due to a wide range of functions in antioxidant defense, cell proliferation, and redox signaling, strong cellular aberrations are seen upon the targeting of TrxR enzymes by inhibitors. However, such inhibition can nonetheless have rather unexpected consequences. Accumulating data suggest that inhibition of TrxR in normal cells typically yields a paradoxical effect of increased antioxidant defense, with metabolic pathway reprogramming, increased cellular proliferation, and altered cellular differentiation patterns. Conversely, inhibition of TrxR in cancer cells can yield excessive levels of reactive oxygen species (ROS) resulting in cell death and thus anticancer efficacy. The observed increases in antioxidant capacity upon inhibition of TrxR in normal cells are in part dependent upon activation of the Nrf2 transcription factor, while exaggerated ROS levels in cancer cells can be explained by a non-oncogene addiction of cancer cells to TrxR1 due to their increased endogenous production of ROS. These separate consequences of TrxR inhibition can be utilized therapeutically. Importantly, however, a thorough knowledge of the molecular mechanisms underlying effects triggered by TrxR inhibition is crucial for the understanding of therapy outcomes after use of such inhibitors. The mammalian thioredoxin system is driven by thioredoxin reductases (TXNRD, TrxR), which keeps thioredoxins (TXN, Trx) and further downstream targets reduced. In normal cells, inhibition of TrxR yields a paradoxical effect of increased antioxidant defense upon activation of the Nrf2 transcription factor. In cancer cells, however, inhibition of TrxR yields excessive reactive oxygen species (ROS) levels resulting in cell death and thus anticancer efficacy, which can be explained by a non-oncogene addiction of cancer cells to TrxR1 due to their increased endogenous production of ROS. These separate consequences of TrxR inhibition can be utilized therapeutically.

NADPH-dependent and -independent disulfide reductase systems

Over the past seven decades, research on autotrophic and heterotrophic model organisms has defined how the flow of electrons ("reducing power") from high-energy inorganic sources, through biological systems, to low-energy inorganic products like water, powers all of Life's processes. Universally, an initial major biological recipient of these electrons is nicotinamide adenine dinucleotide-phosphate, which thereby transits from an oxidized state (NADP+) to a reduced state (NADPH). A portion of this reducing power is then distributed via the cellular NADPH-dependent disulfide reductase systems as sequential reductions of disulfide bonds. Along the disulfide reduction pathways, some enzymes have active sites that use the selenium-containing amino acid, selenocysteine, in place of the common but less reactive sulfur-containing cysteine. In particular, the mammalian/metazoan thioredoxin systems are usually selenium-dependent as, across metazoan phyla, most thioredoxin reductases are selenoproteins. Among the roles of the NADPH-dependent disulfide reductase systems, the most universal is that they provide the reducing power for the production of DNA precursors by ribonucleotide reductase (RNR). Some studies, however, have uncovered examples of NADPH-independent disulfide reductase systems that can also support RNR. These systems are summarized here and their implications are discussed.

The Enzymatic and Structural Basis for Inhibition of Echinococcus granulosus Thioredoxin Glutathione Reductase by Gold(I)

AIMS

New drugs are needed to treat flatworm infections that cause severe human diseases such as schistosomiasis. The unique flatworm enzyme thioredoxin glutathione reductase (TGR), structurally different from the human enzyme, is a key drug target. Structural studies of the flatworm Echinococcus granulosus TGR, free and complexed with AuI-MPO, a novel gold inhibitor, together with inhibition assays were performed.

RESULTS

AuI-MPO is a potent TGR inhibitor that achieves 75% inhibition at a 1:1 TGR:Au ratio and efficiently kills E. granulosus in vitro. The structures revealed salient insights: (i) unique monomer-monomer interactions, (ii) distinct binding sites for thioredoxin and the glutaredoxin (Grx) domain, (iii) a single glutathione disulfide reduction site in the Grx domain, (iv) rotation of the Grx domain toward the Sec-containing redox active site, and (v) a single gold atom bound to Cys519 and Cys573 in the AuI-TGR complex. Structural modeling suggests that these residues are involved in the stabilization of the Sec-containing C-terminus. Consistently, Cys→Ser mutations in these residues decreased TGR activities. Mass spectroscopy confirmed these cysteines are the primary binding site.

INNOVATION

The identification of a primary site for gold binding and the structural model provide a basis for gold compound optimization through scaffold adjustments.

CONCLUSIONS

The structural study revealed that TGR functions are achieved not only through a mobile Sec-containing redox center but also by rotation of the Grx domain and distinct binding sites for Grx domain and thioredoxin. The conserved Cys519 and Cys573 residues targeted by gold assist catalysis through stabilization of the Sec-containing redox center. Antioxid. Redox Signal. 27, 1491-1504.

Differential expression of disulfide reductase enzymes in a free-living platyhelminth (Dugesia dorotocephala)

A search of the disulfide reductase activities expressed in the adult stage of the free-living platyhelminth Dugesia dorotocephala was carried out. Using GSSG or DTNB as substrates, it was possible to obtain a purified fraction containing both GSSG and DTNB reductase activities. Through the purification procedure, both disulfide reductase activities were obtained in the same chromatographic peak. By mass spectrometry analysis of peptide fragments obtained after tryptic digestion of the purified fraction, the presence of glutathione reductase (GR), thioredoxin-glutathione reductase (TGR), and a putative thioredoxin reductase (TrxR) was detected. Using the gold compound auranofin to selectively inhibit the GSSG reductase activity of TGR, it was found that barely 5% of the total GR activity in the D. dorotocephala extract can be assigned to GR. Such strategy did allow us to determine the kinetic parameters for both GR and TGR. Although It was not possible to discriminate DTNB reductase activity due to TrxR from that of TGR, a chromatofocusing experiment with a D. dorotocephala extract resulted in the obtention of a minor protein fraction enriched in TrxR, strongly suggesting its presence as a functional protein. Thus, unlike its parasitic counterparts, in the free-living platyhelminth lineage the three disulfide reductases are present as functional proteins, albeit TGR is still the major disulfide reductase involved in the reduction of both Trx and GSSG. This fact suggests the development of TGR in parasitic flatworms was not linked to a parasitic mode of life.

Substrate specificity and mapping of residues critical for transport in the high-affinity glutathione transporter Hgt1p

This study compares the substrate specificity of the yeast glutathione transporter Hgt1p towards reduced glutathione and glutathione conjugates. Mutational analysis of all 13 predicted TMDs identified residues critical for substrate transport and was placed in an ab initio model.

Inhibition of Tapeworm Thioredoxin and Glutathione Pathways by an Oxadiazole N-Oxide Leads to Reduced Mesocestoides vogae Infection Burden in Mice

Parasitic flatworms cause serious infectious diseases that affect humans and livestock in vast regions of the world, yet there are few effective drugs to treat them. Thioredoxin glutathione reductase (TGR) is an essential enzyme for redox homeostasis in flatworm parasites and a promising pharmacological target. We purified to homogeneity and characterized the TGR from the tapeworm Mesocestoides vogae (syn. M. corti). This purification revealed absence of conventional TR and GR. The glutathione reductase activity of the purified TGR exhibits a hysteretic behavior typical of flatworm TGRs. Consistently, M. vogae genome analysis revealed the presence of a selenocysteine-containing TGR and absence of conventional TR and GR. M. vogae thioredoxin and glutathione reductase activities were inhibited by 3,4-bis(phenylsulfonyl)-1,2,5-oxadiazole N²-oxide (VL16E), an oxadiazole N-oxide previously identified as an inhibitor of fluke and tapeworm TGRs. Finally, we show that mice experimentally infected with M. vogae tetrathyridia and treated with either praziquantel, the reference drug for flatworm infections, or VL16E exhibited a 28% reduction of intraperitoneal larvae numbers compared to vehicle treated mice. Our results show that oxadiazole N-oxide is a promising chemotype in vivo and highlights the convenience of M. vogae as a model for rapid assessment of tapeworm infections in vivo.

Insights into the fish thioredoxin system: expression profile of thioredoxin and thioredoxin reductase in rainbow trout (Oncorhynchus mykiss) during infection and in vitro stimulation

Production of reactive oxygen species (ROS) is the first biological response during a disease outbreak and after injury. ROS are highly reactive molecules that can either endanger cell homeostasis or mediate cell signaling in several physiological pathways, including the immune response. Thioredoxin (Trx) and thioredoxin reductase (TrxR) are the essential components of the thioredoxin system, one of the main intracellular redox systems and are therefore important regulators of ROS accumulation. Through the regulation of the intracellular redox milieu, the thioredoxin system plays a key role within the immune system, linking immunology and free radical science. In this study we have firstly identified TrxRs in fish and used this new sequence information to reevaluate the evolution of the thioredoxin system within the vertebrate lineage. We next measured the expression of rainbow trout (Oncorhynchus mykiss) Trx and TrxR transcripts during infection in vivo and in vitro after stimulation of a macrophage cell line and primary macrophage cultures with pathogen associated molecular patterns (PAMPs). Our results showed that both Trx and TrxR were induced during infection at the transcriptional level, confirming their likely involvement in the innate immune response of fish. Since TrxRs are selenium-containing proteins (selenoproteins), we also measured the modulation of their expression upon organic and inorganic selenium exposure in vitro. TrxR was found to be responsive to selenium exposure in vitro, suggesting that it may represent a key mediator in the selenium modulation of innate immunity. In conclusion, our study highlights the need to investigate the involvement of the cell antioxidant pathways, especially the thioredoxin system, within the immune system of vertebrate species.

Expression, crystallization and preliminary X-ray diffraction analysis of thioredoxin glutathione reductase from Schistosoma japonicum in complex with FAD

Thioredoxin glutathione reductase from Schistosoma japonicum (SjTGR), a multifunctional enzyme, plays a vital role in antioxidant pathways and is considered to be a potential drug target for the development of antischistosomal chemotherapy. In this study, two constructs of a truncated form of SjTGR without the last two residues (Sec597-Gly598) were cloned, overexpressed and purified using wild-type and codon-optimized genes. Only SjTGR from the wild-type gene was found to form a complex with flavin adenine dinucleotide (FAD), which could be crystallized in the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 84.185, b = 86.47, c = 183.164 Å, at 295 K using the hanging-drop vapour-diffusion method. One dimer was present in the crystallographic asymmetric unit and the calculated Matthews coefficient (VM) and solvent content were 2.6 Å(3) Da(-1) and 52.8%, respectively. Structural determination of SjTGR is in progress using the molecular-replacement method.

Thioredoxin glutathione reductase-dependent redox networks in platyhelminth parasites

SIGNIFICANCE

Platyhelminth parasites cause chronic infections that are a major cause of disability, mortality, and economic losses in developing countries. Maintaining redox homeostasis is a major adaptive problem faced by parasites and its disruption can shift the biochemical balance toward the host. Platyhelminth parasites possess a streamlined thiol-based redox system in which a single enzyme, thioredoxin glutathione reductase (TGR), a fusion of a glutaredoxin (Grx) domain to canonical thioredoxin reductase (TR) domains, supplies electrons to oxidized glutathione (GSSG) and thioredoxin (Trx). TGR has been validated as a drug target for schistosomiasis.

RECENT ADVANCES

In addition to glutathione (GSH) and Trx reduction, TGR supports GSH-independent deglutathionylation conferring an additional advantage to the TGR redox array. Biochemical and structural studies have shown that the TR activity does not require the Grx domain, while the glutathione reductase and deglutathionylase activities depend on the Grx domain, which receives electrons from the TR domains. The search for TGR inhibitors has identified promising drug leads, notably oxadiazole N-oxides.

CRITICAL ISSUES

A conspicuous feature of platyhelminth TGRs is that their Grx-dependent activities are temporarily inhibited at high GSSG concentrations. The mechanism underlying the phenomenon and its biological relevance are not completely understood.

FUTURE DIRECTIONS

The functional diversity of Trxs and Grxs encoded in platyhelminth genomes remains to be further assessed to thoroughly understand the TGR-dependent redox network. Optimization of TGR inhibitors and identification of compounds targeting other parasite redox enzymes are good options to clinically develop relevant drugs for these neglected, but important diseases.

Current drug targets for helminthic diseases

More than 2 billion people are infected with helminth parasites across the globe. The burgeoning drug resistance against current anthelmintics in parasitic worms of humans and livestock requires urgent attention to tackle these recalcitrant worms. This review focuses on the advancements made in the area of helminth drug target discovery especially from the last few couple of decades. It highlights various approaches made in this field and enlists the potential drug targets currently being pursued to target economically important helminth species both from human as well as livestock to combat disease pathology of schistosomiasis, onchocerciasis, lymphatic filariasis, and other important macroparasitic diseases. Research in the helminths study is trending to identify potential and druggable targets through genomic, proteomic, biochemical, biophysical, in vitro experiments, and in vivo experiments in animal models. The availability of major helminths genome sequences and the subsequent availability of genome-scale functional datasets through in silico search and prioritization are expected to guide the experimental work necessary for target-based drug discovery. Organized and documented list of drug targets from various helminths of economic importance have been systematically covered in this review for further exploring their use and applications, which can give physicians and veterinarians effective drugs in hand to enable them control worm infections.

Oxadiazole 2-oxides are toxic to the human hookworm, Ancylostoma ceylanicum, however glutathione reductase is not the primary target

Hookworm disease, characterized by severe anemia and cognitive and growth delays, currently affects an estimated 740 million people worldwide. Despite the prevalence of this parasitic disease, few effective drug therapies are in use today, and the heavy reliance upon benzimidazoles highlights the need for the development of novel chemotherapies. Recent work with the trematode parasite Schistosoma mansoni has identified oxadiazole 2-oxides as effective antischistosomal compounds that function by targeting and inhibiting the antioxidant enzyme, thioredoxin glutathione reductase. In this study, a related enzyme, glutathione reductase, from the human hookworm Ancylostoma ceylanicum was identified and characterized, and its in vitro activity in the presence of the oxadiazole 2-oxides was analyzed. Ex vivo worm killing assays were also conducted to establish the relationship between a given compound's effect upon worm survival and inhibition of recombinant glutathione reductase (rAceGR). Finally, the in vivo anthelminthic efficacy of furoxan (Fx) was assessed in the hamster model of hookworm infection. The predicted amino acid sequence of AceGR contained a prototypical glutathione reductase active site sequence, but no thioredoxin reductase consensus sequences, suggesting that the glutathione and thioredoxin pathways of A. ceylanicum are distinct. Although ten of the forty-two oxadiazole 2-oxides tested inhibited rAceGR activity by at least fifty percent, and fifteen compounds were toxic to parasites ex vivo, little overlap existed between these two results. We therefore suggest that AceGR is not the primary target of the oxadiazole 2-oxides in effecting parasite death. Lastly, oral treatment of A. ceylanicuminfected hamsters with furoxan resulted in significantly improved weight gains and reduced intestinal worm burdens compared to vehicle treated controls, supporting continued development of this molecule as a novel anthelminthic.

New frontiers in schistosoma genomics and transcriptomics

Schistosomes are digenean blood flukes of aves and mammals comprising 23 species. Some species are causative agents of human schistosomiasis, the second major neglected disease affecting over 230 million people worldwide. Modern technologies including the sequencing and characterization of nucleic acids and proteins have allowed large-scale analyses of parasites and hosts, opening new frontiers in biological research with potential biomedical and biotechnological applications. Nuclear genomes of the three most socioeconomically important species (S. haematobium, S. japonicum, and S. mansoni) have been sequenced and are under intense investigation. Mitochondrial genomes of six Schistosoma species have also been completely sequenced and analysed from an evolutionary perspective. Furthermore, DNA barcoding of mitochondrial sequences is used for biodiversity assessment of schistosomes. Despite the efforts in the characterization of Schistosoma genomes and transcriptomes, many questions regarding the biology and evolution of this important taxon remain unanswered. This paper aims to discuss some advances in the schistosome research with emphasis on genomics and transcriptomics. It also aims to discuss the main challenges of the current research and to point out some future directions in schistosome studies.

Low-molecular-mass antioxidants in parasites

SIGNIFICANCE

Parasitic infections continue to be a major problem for global human health. Vaccines are practically not available and chemotherapy is highly unsatisfactory. One approach toward a novel antiparasitic drug development is to unravel pathways that may be suited as future targets. Parasitic organisms show a remarkable diversity with respect to the nature and functions of their main low-molecular-mass antioxidants and many of them developed pathways that do not have a counterpart in their mammalian hosts.

RECENT ADVANCES

Work of the last years disclosed the individual antioxidants employed by parasites and their distinct pathways. Entamoeba, Trichomonas, and Giardia directly use cysteine as main low-molecular-mass thiol but have divergent cysteine metabolisms. Malarial parasites rely exclusively on cysteine uptake and generate glutathione (GSH) as main free thiol as do metazoan parasites. Trypanosomes and Leishmania have a unique trypanothione-based thiol metabolism but employ individual mechanisms for their cysteine supply. In addition, some trypanosomatids synthesize ovothiol A and/or ascorbate. Various essential parasite enzymes such as trypanothione synthetase and trypanothione reductase in Trypanosomatids and the Schistosoma thioredoxin GSH reductase are currently intensively explored as drug target molecules.

CRITICAL ISSUES

Essentiality is a prerequisite but not a sufficient property of an enzyme to become a suited drug target. The availability of an appropriate in vivo screening system and many other factors are equally important.

FUTURE DIRECTIONS

The current organism-wide RNA-interference and proteome analyses are supposed to reveal many more interesting candidates for future drug development approaches directed against the parasite antioxidant defense systems.

Identification of thioredoxin glutathione reductase inhibitors that kill cestode and trematode parasites

Parasitic flatworms are responsible for serious infectious diseases that affect humans as well as livestock animals in vast regions of the world. Yet, the drug armamentarium available for treatment of these infections is limited: praziquantel is the single drug currently available for 200 million people infected with Schistosoma spp. and there is justified concern about emergence of drug resistance. Thioredoxin glutathione reductase (TGR) is an essential core enzyme for redox homeostasis in flatworm parasites. In this work, we searched for flatworm TGR inhibitors testing compounds belonging to various families known to inhibit thioredoxin reductase or TGR and also additional electrophilic compounds. Several furoxans and one thiadiazole potently inhibited TGRs from both classes of parasitic flatworms: cestoda (tapeworms) and trematoda (flukes), while several benzofuroxans and a quinoxaline moderately inhibited TGRs. Remarkably, five active compounds from diverse families possessed a phenylsulfonyl group, strongly suggesting that this moiety is a new pharmacophore. The most active inhibitors were further characterized and displayed slow and nearly irreversible binding to TGR. These compounds efficiently killed Echinococcus granulosus larval worms and Fasciola hepatica newly excysted juveniles in vitro at a 20 µM concentration. Our results support the concept that the redox metabolism of flatworm parasites is precarious and particularly susceptible to destabilization, show that furoxans can be used to target both flukes and tapeworms, and identified phenylsulfonyl as a new drug-hit moiety for both classes of flatworm parasites.

Thioredoxin glutathione reductase as a novel drug target: evidence from Schistosoma japonicum

Background

Schistosomiasis remains a major public health concern affecting billions of people around the world. Currently, praziquantel is the only drug of choice for treatment of human schistosomiasis. The emergence of drug resistance to praziquantel in schistosomes makes the development of novel drugs an urgent task. Thioredoxin glutathione reductase (TGR) enzymes in Schistosoma mansoni and some other platyhelminths have been identified as alternative targets. The present study was designed to confirm the existense and the potential value of TGR as a target for development of novel antischistosomal agents in Schistosoma japonicum, a platyhelminth endemic in Asia.

Methods and Findings

After cloning the S. japonicum TGR (SjTGR) gene, the recombinant SjTGR selenoprotein was purified and characterized in enzymatic assays as a multifunctional enzyme with thioredoxin reductase (TrxR), glutathione reductase (GR) and glutaredoxin (Grx) activities. Immunological and bioinformatic analyses confirmed that instead of having separate TrxR and GR proteins in mammalian, S. japonicum only encodes TGR, which performs the functions of both enzymes and plays a critical role in maintaining the redox balance in this parasite. These results were in good agreement with previous findings in Schistosoma mansoni and some other platyhelminths. Auranofin, a known inhibitor against TGR, caused fatal toxicity in S. japonicum adult worms in vitro and reduced worm and egg burdens in S. japonicum infected mice.

Conclusions

Collectively, our study confirms that a multifunctional enzyme SjTGR selenoprotein, instead of separate TrxR and GR enzymes, exists in S. japonicum. Furthermore, TGR may be a potential target for development of novel agents against schistosomes. This assumption is strengthened by our demonstration that the SjTGR is an essential enzyme for maintaining the thiol-disulfide redox homeostasis of S. japonicum.

Thioredoxin glutathione reductase: its role in redox biology and potential as a target for drugs against neglected diseases

BACKGROUND

There are two, largely autonomous antioxidant pathways in many organisms, one based on thioredoxin and one based on glutathione, with each pathway having a unique flavoprotein oxidoreductase to maintain them in a reduced state. A recently discovered protein, thioredoxin glutathione reductase (TGR) potentially connects these two pathways. In a large group of parasitic worms, responsible for hundreds of millions of infections in humans and animals, untold morbidity and significant mortality, TGR is the sole enzyme present to maintain redox balance.

SCOPE OF REVIEW

In this review, the current understanding of the biochemical properties of TGR enzymes is compared to the related enzymes thioredoxin reductase and glutathione reductase. The role of the rare amino acid selenocysteine is discussed. An overview of the potential to target TGR for drug development against a range of parasitic worms and preliminary results to identify TGR inhibitors for schistosomiasis treatment is presented.

MAJOR CONCLUSIONS

TGR has properties that are both unique and common to other flavoprotein oxidoreductases. TGR plays a fundamentally different and essential role in the redox biology of parasitic flatworms. Therefore, TGR is a promising target for drug development for schistosomiasis and other trematode and cestode infections.

GENERAL SIGNIFICANCE

TGR may have differing functions in host organisms, but through analyses to understand its ability to reduce both glutathione and thioredoxin we can better understand the reaction mechanisms of an important class of enzymes. The unique properties of TGR in parasitic flatworms provide promising routes to develop new treatments for diseases.

Linked thioredoxin-glutathione systems in platyhelminth parasites: alternative pathways for glutathione reduction and deglutathionylation

In most organisms, thioredoxin (Trx) and/or glutathione (GSH) systems are essential for redox homeostasis and deoxyribonucleotide synthesis. Platyhelminth parasites have a unique and simplified thiol-based redox system, in which the selenoprotein thioredoxin-glutathione reductase (TGR), a fusion of a glutaredoxin (Grx) domain to canonical thioredoxin reductase domains, is the sole enzyme supplying electrons to oxidized glutathione (GSSG) and Trx. This enzyme has recently been validated as a key drug target for flatworm infections. In this study, we show that TGR possesses GSH-independent deglutathionylase activity on a glutathionylated peptide. Furthermore, we demonstrate that deglutathionylation and GSSG reduction are mediated by the Grx domain by a monothiolic mechanism and that the glutathionylated TGR intermediate is resolved by selenocysteine. Deglutathionylation and GSSG reduction via Grx domain, but not Trx reduction, are inhibited at high [GSSG]/[GSH] ratios. We found that Trxs (cytosolic and mitochondrial) provide alternative pathways for deglutathionylation and GSSG reduction. These pathways are operative at high [GSSG]/[GSH] and function in a complementary manner to the Grx domain-dependent one. Despite the existence of alternative pathways, the thioredoxin reductase domains of TGR are an obligate electron route for both the Grx domain- and the Trx-dependent pathways. Overall, our results provide an explanation for the unique array of thiol-dependent redox pathways present in parasitic platyhelminths. Finally, we found that TGR is inhibited by 1-hydroxy-2-oxo-3-(N-3-methyl-aminopropyl)-3-methyl-1-triazene (NOC-7), giving further evidence for NO donation as a mechanism of action for oxadiazole N-oxide TGR inhibitors. Thus, NO donors aimed at TGR could disrupt the entire redox homeostasis of parasitic flatworms.

Mapping the catalytic cycle of Schistosoma mansoni thioredoxin glutathione reductase by X-ray crystallography

Schistosomiasis is the second most widespread human parasitic disease. It is principally treated with one drug, praziquantel, that is administered to 100 million people each year; less sensitive strains of schistosomes are emerging. One of the most appealing drug targets against schistosomiasis is thioredoxin glutathione reductase (TGR). This natural chimeric enzyme is a peculiar fusion of a glutaredoxin domain with a thioredoxin selenocysteine (U)-containing reductase domain. Selenocysteine is located on a flexible C-terminal arm that is usually disordered in the available structures of the protein and is essential for the full catalytic activity of TGR. In this study, we dissect the catalytic cycle of Schistosoma mansoni TGR by structural and functional analysis of the U597C mutant. The crystallographic data presented herein include the following: the oxidized form (at 1.9 Å resolution); the NADPH- and GSH-bound forms (2.3 and 1.9 Å, respectively); and a different crystal form of the (partially) reduced enzyme (3.1 Å), showing the physiological dimer and the entire C terminus of one subunit. Whenever possible, we determined the rate constants for the interconversion between the different oxidation states of TGR by kinetic methods. By combining the crystallographic analysis with computer modeling, we were able to throw further light on the mechanism of action of S. mansoni TGR. In particular, we hereby propose the putative functionally relevant conformational change of the C terminus after the transfer of reducing equivalents from NADPH to the redox sites of the enzyme.