Functional characterization of peroxiredoxins from the human protozoan parasite Giardia intestinalis

Studying the Human Microbiota: Advances in Understanding the Fundamentals, Origin, and Evolution of Biological Timekeeping

The recently observed circadian oscillations of the intestinal microbiota underscore the profound nature of the human-microbiome relationship and its importance for health. Together with the discovery of circadian clocks in non-photosynthetic gut bacteria and circadian rhythms in anucleated cells, these findings have indicated the possibility that virtually all microorganisms may possess functional biological clocks. However, they have also raised many essential questions concerning the fundamentals of biological timekeeping, its evolution, and its origin. This narrative review provides a comprehensive overview of the recent literature in molecular chronobiology, aiming to bring together the latest evidence on the structure and mechanisms driving microbial biological clocks while pointing to potential applications of this knowledge in medicine. Moreover, it discusses the latest hypotheses regarding the evolution of timing mechanisms and describes the functions of peroxiredoxins in cells and their contribution to the cellular clockwork. The diversity of biological clocks among various human-associated microorganisms and the role of transcriptional and post-translational timekeeping mechanisms are also addressed. Finally, recent evidence on metabolic oscillators and host-microbiome communication is presented.

Ultrastructural Alterations of the Human Pathogen Giardia intestinalis after Drug Treatment

This review presents the main cell characteristics altered after in vitro incubation of the parasite with commercial drugs used to treat the disease caused by Giardia intestinalis. This important intestinal parasite primarily causes diarrhea in children. Metronidazole and albendazole are the primary compounds used in therapy against Giardia intestinalis. However, they provoke significant side effects, and some strains have developed resistance to metronidazole. Benzimidazole carbamates, such as albendazole and mebendazole, have shown the best activity against Giardia. Despite their in vitro efficacy, clinical treatment with benzimidazoles has yielded conflicting results, demonstrating lower cure rates. Recently, nitazoxanide has been suggested as an alternative to these drugs. Therefore, to enhance the quality of chemotherapy against this parasite, it is important to invest in developing other compounds that can interfere with key steps of metabolic pathways or cell structures and organelles. For example, Giardia exhibits a unique cell structure called the ventral disc, which is crucial for host adhesion and pathogenicity. Thus, drugs that can disrupt the adhesion process hold promise for future therapy against Giardia. Additionally, this review discusses new drugs and strategies that can be employed, as well as suggestions for developing novel drugs to control the infection caused by this parasite.

Characterization of Metronidazole-Resistant Giardia intestinalis Lines by Comparative Transcriptomics and Proteomics

Metronidazole (MTZ) is a clinically important antimicrobial agent that is active against both bacterial and protozoan organisms. MTZ has been used extensively for more than 60 years and until now resistance has been rare. However, a recent and dramatic increase in the number of MTZ resistant bacteria and protozoa is of great concern since there are few alternative drugs with a similarly broad activity spectrum. To identify key factors and mechanisms underlying MTZ resistance, we utilized the protozoan parasite Giardia intestinalis, which is commonly treated with MTZ. We characterized two in vitro selected, metronidazole resistant parasite lines, as well as one revertant, by analyzing fitness aspects associated with increased drug resistance and transcriptomes and proteomes. We also conducted a meta-analysis using already existing data from additional resistant G. intestinalis isolates. The combined data suggest that in vitro generated MTZ resistance has a substantial fitness cost to the parasite, which may partly explain why resistance is not widespread despite decades of heavy use. Mechanistically, MTZ resistance in Giardia is multifactorial and associated with complex changes, yet a core set of pathways involving oxidoreductases, oxidative stress responses and DNA repair proteins, is central to MTZ resistance in both bacteria and protozoa.

Giardia intestinalis and its Endomembrane System

Giardia intestinalis has unique characteristics, even in the absence of certain organelles. For instance, Golgi and mitochondria are not found. On the other hand, there is a network of peripheral vacuoles (PVs) and mitosomes. The endoplasmic reticulum (ER), nuclear membrane, peroxisomes, and lipid bodies are present. The peripheral vacuole system seems to play several simultaneous roles. It is involved in the endocytic activity of the trophozoite but also has characteristics of early and late endosomes and even lysosomes, establishing a connection with the ER. Some of the PVs contain small vesicles, acting as multivesicular bodies, including the release of exosomes. The mitosomes are surrounded by two membranes, divide during mitosis, and are distributed throughout the cell. They do not contain DNA, enzymes involved in the citric acid cycle, respiratory chain, or ATP synthesis. However, they contain the iron-sulfur complex and transporters as TOM and TIM. Some mitosomes are linked to flagellar axonemes through a fibrillar connection. During encystation, two types of larger cytoplasmic vesicles appear. One originating from the ER contains the cyst wall proteins. Another contains carbohydrates. Both migrate to the cell periphery and fuse with plasma membrane secreting their contents to give rise to the cell wall.

Oxygen levels are key to understanding "Anaerobic" protozoan pathogens with micro-aerophilic lifestyles

Publications abound on the physiology, biochemistry and molecular biology of "anaerobic" protozoal parasites as usually grown under "anaerobic" culture conditions. The media routinely used are poised at low redox potentials using techniques that remove O₂ to "undetectable" levels in sealed containers. However there is growing understanding that these culture conditions do not faithfully resemble the O₂ environments these organisms inhabit. Here we review for protists lacking oxidative energy metabolism, the oxygen cascade from atmospheric to intracellular concentrations and relevant methods of measurements of O₂, some well-studied parasitic or symbiotic protozoan lifestyles, their homeodynamic metabolic and redox balances, organism-drug-oxygen interactions, and the present and future prospects for improved drugs and treatment regimes.

Relevance of peroxiredoxins in pathogenic microorganisms

The oxidative and nitrosative responses generated by animals and plants are important defenses against infection and establishment of pathogenic microorganisms such as bacteria, fungi, and protozoa. Among distinct oxidant species, hydroperoxides are a group of chemically diverse compounds that comprise small hydrophilic molecules, such as hydrogen peroxide and peroxynitrite, and bulky hydrophobic species, such as organic hydroperoxides. Peroxiredoxins (Prx) are ubiquitous enzymes that use a highly reactive cysteine residue to decompose hydroperoxides and can also perform other functions, like molecular chaperone and phospholipase activities, contributing to microbial protection against the host defenses. Prx are present in distinct cell compartments and, in some cases, they can be secreted to the extracellular environment. Despite their high abundance, Prx expression can be further increased in response to oxidative stress promoted by host defense systems, by treatment with hydroperoxides or by antibiotics. In consequence, some isoforms have been described as virulence factors, highlighting their importance in pathogenesis. Prx are very diverse and are classified into six different classes (Prx1-AhpC, BCP-PrxQ, Tpx, Prx5, Prx6, and AhpE) based on structural and biochemical features. Some groups are absent in hosts, while others present structural peculiarities that differentiate them from the host's isoforms. In this context, the intrinsic characteristics of these enzymes may aid the development of new drugs to combat pathogenic microorganisms. Additionally, since some isoforms are also found in the extracellular environment, Prx emerge as attractive targets for the production of diagnostic tests and vaccines. KEY POINTS: • Peroxiredoxins are front-line defenses against host oxidative and nitrosative stress. • Functional and structural peculiarities differ pathogen and host enzymes. • Peroxiredoxins are potential targets to microbicidal drugs.

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.

Click chemistry-facilitated comprehensive identification of proteins adducted by antimicrobial 5-nitroimidazoles for discovery of alternative drug targets against giardiasis

Giardiasis and other protozoan infections are major worldwide causes of morbidity and mortality, yet development of new antimicrobial agents with improved efficacy and ability to override increasingly common drug resistance remains a major challenge. Antimicrobial drug development typically proceeds by broad functional screens of large chemical libraries or hypothesis-driven exploration of single microbial targets, but both strategies have challenges that have limited the introduction of new antimicrobials. Here, we describe an alternative drug development strategy that identifies a sufficient but manageable number of promising targets, while reducing the risk of pursuing targets of unproven value. The strategy is based on defining and exploiting the incompletely understood adduction targets of 5-nitroimidazoles, which are proven antimicrobials against a wide range of anaerobic protozoan and bacterial pathogens. Comprehensive adductome analysis by modified click chemistry and multi-dimensional proteomics were applied to the model pathogen Giardia lamblia to identify dozens of adducted protein targets common to both 5'-nitroimidazole-sensitive and -resistant cells. The list was highly enriched for known targets in G. lamblia, including arginine deiminase, α-tubulin, carbamate kinase, and heat shock protein 90, demonstrating the utility of the approach. Importantly, over twenty potential novel drug targets were identified. Inhibitors of two representative new targets, NADP-specific glutamate dehydrogenase and peroxiredoxin, were found to have significant antigiardial activity. Furthermore, all the identified targets remained available in resistant cells, since giardicidal activity of the respective inhibitors was not impacted by resistance to 5'-nitroimidazoles. These results demonstrate that the combined use of click chemistry and proteomics has the potential to reveal alternative drug targets for overcoming antimicrobial drug resistance in protozoan parasites.

Lateral Acquisitions Repeatedly Remodel the Oxygen Detoxification Pathway in Diplomonads and Relatives

Oxygen and reactive oxygen species (ROS) are important stress factors for cells because they can oxidize many large molecules. Fornicata, a group of flagellated protists that includes diplomonads, have anaerobic metabolism but are still able to tolerate fluctuating levels of oxygen. We identified 25 protein families putatively involved in detoxification of oxygen and ROS in this group using a bioinformatics approach and propose how these interact in an oxygen detoxification pathway. These protein families were divided into a central oxygen detoxification pathway and accessory pathways for the synthesis of nonprotein thiols. We then used a phylogenetic approach to investigate the evolutionary origin of the components of this putative pathway in Diplomonadida and other Fornicata species. Our analyses suggested that the diplomonad ancestor was adapted to low-oxygen levels, was able to reduce O₂ to H₂O in a manner similar to extant diplomonads, and was able to synthesize glutathione and l-cysteine. Several genes involved in the pathway have complex evolutionary histories and have apparently been repeatedly acquired through lateral gene transfer and subsequently lost. At least seven genes were acquired independently in different Fornicata lineages, leading to evolutionary convergences. It is likely that acquiring these oxygen detoxification proteins helped anaerobic organisms (like the parasitic Giardia intestinalis) adapt to low-oxygen environments (such as the digestive tract of aerobic hosts).

Genetic variation in metronidazole metabolism and oxidative stress pathways in clinical Giardia lamblia assemblage A and B isolates

Purpose: Treatment-refractory Giardia cases have increased rapidly within the last decade. No markers of resistance nor a standardized susceptibility test have been established yet, but several enzymes and their pathways have been associated with metronidazole (MTZ) resistant Giardia. Very limited data are available regarding genetic variation in these pathways. We aimed to investigate genetic variation in metabolic pathway genes proposed to be involved in MTZ resistance in recently acquired, cultured clinical isolates.

Methods: Whole genome sequencing of 12 assemblage A2 and 8 assemblage B isolates was done, to decipher genomic variation in Giardia. Twenty-nine genes were identified in a literature search and investigated for their single nucleotide variants (SNVs) in the coding/non-coding regions of the genes, either as amino acid changing (non-synonymous SNVs) or non-changing SNVs (synonymous).

Results: In Giardia assemblage B, several genes involved in MTZ activation or oxidative stress management were found to have higher numbers of non-synonymous SNVs (thioredoxin peroxidase, nitroreductase 1, ferredoxin 2, NADH oxidase, nitroreductase 2, alcohol dehydrogenase, ferredoxin 4 and ferredoxin 1) than the average variation. For Giardia assemblage A2, the highest genetic variability was found in the ferredoxin 2, ferredoxin 6 and in nicotinamide adenine dinucleotide phosphate (NADPH) oxidoreductase putative genes. SNVs found in the ferredoxins and nitroreductases were analyzed further by alignment and homology modeling. SNVs close to the iron-sulfur cluster binding sites in nitroreductase-1 and 2 and ferredoxin 2 and 4 could potentially affect protein function. Flavohemoprotein seems to be a variable-copy gene, due to higher, but variable coverage compared to other genes investigated.

Conclusion: In clinical Giardia isolates, genetic variability is common in important genes in the MTZ metabolizing pathway and in the management of oxidative and nitrosative stress and includes high numbers of non-synonymous SNVs. Some of the identified amino acid changes could potentially affect the respective proteins important in the MTZ metabolism.

Oxygen induces the expression of invasion and stress response genes in the anaerobic salmon parasite Spironucleus salmonicida

Background

Spironucleus salmonicida is an anaerobic parasite that can cause systemic infections in Atlantic salmon. Unlike other diplomonad parasites, such as the human pathogen Giardia intestinalis, Spironucleus species can infiltrate the blood stream of their hosts eventually colonizing organs, skin and gills. How this presumed anaerobe can persist and invade oxygenated tissues, despite having a strictly anaerobic metabolism, remains elusive.

Results

To investigate how S. salmonicida response to oxygen stress, we performed RNAseq transcriptomic analyses of cells grown in the presence of oxygen or antioxidant-free medium. We found that over 20% of the transcriptome is differentially regulated in oxygen (1705 genes) and antioxidant-depleted (2280 genes) conditions. These differentially regulated transcripts encode proteins related to anaerobic metabolism, cysteine and Fe-S cluster biosynthesis, as well as a large number of proteins of unknown function. S. salmonicida does not encode genes involved in the classical elements of oxygen metabolism (e.g., catalases, superoxide dismutase, glutathione biosynthesis, oxidative phosphorylation). Instead, we found that genes encoding bacterial-like oxidoreductases were upregulated in response to oxygen stress. Phylogenetic analysis revealed some of these oxygen-responsive genes (e.g., nadh oxidase, rubrerythrin, superoxide reductase) are rare in eukaryotes and likely derived from lateral gene transfer (LGT) events into diplomonads from prokaryotes. Unexpectedly, we observed that many host evasion- and invasion-related genes were also upregulated under oxidative stress suggesting that oxygen might be an important signal for pathogenesis.

Conclusion

While oxygen is toxic for related organisms, such as G. intestinalis, we find that oxygen is likely a gene induction signal for host invasion- and evasion-related pathways in S. salmonicida. These data provide the first molecular evidence for how S. salmonicida could tolerate oxic host environments and demonstrate how LGT can have a profound impact on the biology of anaerobic parasites.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0634-8) contains supplementary material, which is available to authorized users.

Annotation of the Giardia proteome through structure-based homology and machine learning

Background

Large-scale computational prediction of protein structures represents a cost-effective alternative to empirical structure determination with particular promise for non-model organisms and neglected pathogens. Conventional sequence-based tools are insufficient to annotate the genomes of such divergent biological systems. Conversely, protein structure tolerates substantial variation in primary amino acid sequence and is thus a robust indicator of biochemical function. Structural proteomics is poised to become a standard part of pathogen genomics research; however, informatic methods are now required to assign confidence in large volumes of predicted structures.

Aims

Our aim was to predict the proteome of a neglected human pathogen, Giardia duodenalis, and stratify predicted structures into high- and lower-confidence categories using a variety of metrics in isolation and combination.

Methods

We used the I-TASSER suite to predict structural models for ∼5,000 proteins encoded in G. duodenalis and identify their closest empirically-determined structural homologues in the Protein Data Bank. Models were assigned to high- or lower-confidence categories depending on the presence of matching protein family (Pfam) domains in query and reference peptides. Metrics output from the suite and derived metrics were assessed for their ability to predict the high-confidence category individually, and in combination through development of a random forest classifier.

Results

We identified 1,095 high-confidence models including 212 hypothetical proteins. Amino acid identity between query and reference peptides was the greatest individual predictor of high-confidence status; however, the random forest classifier outperformed any metric in isolation (area under the receiver operating characteristic curve = 0.976) and identified a subset of 305 high-confidence-like models, corresponding to false-positive predictions. High-confidence models exhibited greater transcriptional abundance, and the classifier generalized across species, indicating the broad utility of this approach for automatically stratifying predicted structures. Additional structure-based clustering was used to cross-check confidence predictions in an expanded family of Nek kinases. Several high-confidence-like proteins yielded substantial new insight into mechanisms of redox balance in G. duodenalis-a system central to the efficacy of limited anti-giardial drugs.

Conclusion

Structural proteomics combined with machine learning can aid genome annotation for genetically divergent organisms, including human pathogens, and stratify predicted structures to promote efficient allocation of limited resources for experimental investigation.

Redox Pathways as Drug Targets in Microaerophilic Parasites

The microaerophilic parasites Entamoeba histolytica, Trichomonas vaginalis, and Giardia lamblia jointly cause hundreds of millions of infections in humans every year. Other microaerophilic parasites such as Tritrichomonas foetus and Spironucleus spp. pose a relevant health problem in veterinary medicine. Unfortunately, vaccines against these pathogens are unavailable, but their microaerophilic lifestyle opens opportunities for specifically developed chemotherapeutics. In particular, their high sensitivity towards oxygen can be exploited by targeting redox enzymes. This review focusses on the redox pathways of microaerophilic parasites and on drugs, either already in use or currently in the state of development, which target these pathways.

Nitrosative stress defences of the enterohepatic pathogenic bacterium Helicobacter pullorum

Helicobacter pullorum is an avian bacterium that causes gastroenteritis, intestinal bowel and hepatobiliary diseases in humans. Although H. pullorum has been shown to activate the mammalian innate immunity with release of nitric oxide (NO), the proteins that afford protection against NO and reactive nitrogen species (RNS) remain unknown. Here several protein candidates of H. pullorum, namely a truncated (TrHb) and a single domain haemoglobin (SdHb), and three peroxiredoxin-like proteins (Prx1, Prx2 and Prx3) were investigated. We report that the two haemoglobin genes are induced by RNS, and that SdHb confers resistance to nitrosative stress both in vitro and in macrophages. For peroxiredoxins, the prx2 and prx3 expression is enhanced by peroxynitrite and hydrogen peroxide, respectively. Mutation of prx1 does not alter the resistance to these stresses, while the single ∆prx2 and double ∆prx1∆prx2 mutants have decreased viability. To corroborate the physiological data, the biochemical analysis of the five recombinant enzymes was done, namely by stopped-flow spectrophotometry. It is shown that H. pullorum SdHb reacts with NO much more quickly than TrHb, and that the three Prxs react promptly with peroxynitrite, Prx3 displaying the highest reactivity. Altogether, the results unveil SdHb and Prx3 as major protective systems of H. pullorum against nitrosative stress.

Seroprevalence of five parasitic pathogens in pregnant women in ten Caribbean countries

To date, published epidemiological studies of parasitic infections in humans in the Caribbean region are very limited. Here, we report the seroprevalence of five parasitic pathogens, including Ascaris lumbricoides, Entamoeba histolytica, Giardia lamblia, Schistosoma mansoni, and Toxocara canis in 435 serum samples collected between 2008 and 2011 from pregnant women in ten Caribbean islands. We tested the serum samples for IgG antibodies against the five parasites by enzyme-linked immunosorbent assay (ELISA). Among them, 66.2 % were serologically positive for at least one parasite. The most prevalent parasite was G. lamblia (40.5 %), followed by A. lumbricoides (37.9 %), T. canis (14.5 %), E. histolytica (6.7 %), and S. mansoni (3.0 %). Evidence of infections of G. lamblia and A. lumbricoides were detected in all ten Caribbean countries. Seroprevalence estimates significantly differed between countries for A. lumbricoides, E. histolytica, and T. canis (p values <0.001). For S. mansoni, significance was observed by Fisher's exact test (p = 0.013) but not by multiple comparisons. The prevalence of G. lamblia was not significantly different between countries (p = 0.089). A significant negative correlation between the gross domestic product (GDP) per capita and overall seroprevalence by country was also observed (Pearson's r = -0.9202, p = 0.0002). The data strongly indicates that neglected parasitic infections remain a significant health burden on people in these countries. Thus, justification has been provided to regional health planners to enhance existing public health surveillance programs on parasitic diseases and to heighten the public's awareness through education and outreach programs on how they can minimize the occurrence of parasitic infections.

An up-date on Giardia and giardiasis

Giardia intestinalis is a non-invasive protozoan parasite infecting the upper small intestine causing acute, watery diarrhea or giardiasis in 280 million people annually. Asymptomatic infections are equally common and recent data have suggested that infections even can be protective against other diarrheal diseases. Most symptomatic infections resolve spontaneously but infections can lead to chronic disease and treatment failures are becoming more common world-wide. Giardia infections can also result in irritable bowel syndrome (IBS) and food allergies after resolution. Until recently not much was known about the mechanism of giardiasis or the cause of post-giardiasis syndromes and treatment failures, but here we will describe the recent progress in these areas.

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.

The Biological Fight Against Pathogenic Bacteria and Protozoa

The animal gastrointestinal tract is a tube with two open ends; hence, from the microbial point of view it constitutes an open system, as opposed to the circulatory system that must be a tightly closed microbial-free environment. In particular, the human intestine spans ca. 200 m² and represents a massive absorptive surface composed of a layer of epithelial cells as well as a paracellular barrier. The permeability of this paracellular barrier is regulated by transmembrane proteins known as claudins that play a critical role in tight junctions.

Time-Dependent Transcriptional Changes in Axenic Giardia duodenalis Trophozoites

Giardia duodenalis is the most common gastrointestinal protozoan parasite of humans and a significant contributor to the global burden of both diarrheal disease and post-infectious chronic disorders. Although G. duodenalis can be cultured axenically, significant gaps exist in our understanding of the molecular biology and metabolism of this pathogen. The present study employed RNA sequencing to characterize the mRNA transcriptome of G. duodenalis trophozoites in axenic culture, at log (48 h of growth), stationary (60 h), and declining (96 h) growth phases. Using ~400-times coverage of the transcriptome, we identified 754 differentially transcribed genes (DTGs), mainly representing two large DTG groups: 438 that were down-regulated in the declining phase relative to log and stationary phases, and 281 that were up-regulated. Differential transcription of prominent antioxidant and glycolytic enzymes implicated oxygen tension as a key factor influencing the transcriptional program of axenic trophozoites. Systematic bioinformatic characterization of numerous DTGs encoding hypothetical proteins of unknown function was achieved using structural homology searching. This powerful approach greatly informed the differential transcription analysis and revealed putative novel antioxidant-coding genes, and the presence of a near-complete two-component-like signaling system that may link cytosolic redox or metabolite sensing to the observed transcriptional changes. Motif searching applied to promoter regions of the two large DTG groups identified different putative transcription factor-binding motifs that may underpin global transcriptional regulation. This study provides new insights into the drivers and potential mediators of transcriptional variation in axenic G. duodenalis and provides context for static transcriptional studies.

Albendazole induces oxidative stress and DNA damage in the parasitic protozoan Giardia duodenalis

The control of Giardia duodenalis infections is carried out mainly by drugs, among these albendazole (ABZ) is commonly used. Although the cytotoxic effect of ABZ usually involves binding to β-tubulin, it has been suggested that oxidative stress may also play a role in its parasiticidal mechanism. In this work the effect of ABZ in Giardia clones that are susceptible or resistant to different concentrations (1.35, 8, and 250 μM) of this drug was analyzed. Reactive oxygen species (ROS) were induced by ABZ in susceptible clones and this was associated with a decrease in growth that was alleviated by cysteine supplementation. Remarkably, ABZ-resistant clones exhibited partial cross-resistance to H₂O_2, whereas a Giardia H₂O₂-resistant strain can grow in the presence of ABZ. Lipid oxidation and protein carbonylation in ABZ-treated parasites did not show significant differences as compared to untreated parasites; however, ABZ induced the formation of 8OHdG adducts and DNA degradation, indicating nucleic acid oxidative damage. This was supported by observations of histone H2AX phosphorylation in ABZ-susceptible trophozoites treated with 250 μM ABZ. Flow cytometry analysis showed that ABZ partially arrested cell cycle in drug-susceptible clones at G2/M phase at the expense of cells in G1 phase. Also, ABZ treatment resulted in phosphatidylserine exposure on the parasite surface, an event related to apoptosis. All together these data suggest that ROS induced by ABZ affect Giardia genetic material through oxidative stress mechanisms and subsequent induction of apoptotic-like events.

Advances in understanding Giardia: determinants and mechanisms of chronic sequelae

Giardia lamblia is a flagellated protozoan that is the most common cause of intestinal parasitic infection in children living in resource-limited settings. The pathogenicity of Giardia has been debated since the parasite was first identified, and clinical outcomes vary across studies. Among recent perplexing findings are diametrically opposed associations between Giardia and acute versus persistent diarrhea and a poorly understood potential for long-term sequelae, including impaired child growth and cognitive development. The mechanisms driving these protean clinical outcomes remain elusive, but recent advances suggest that variability in Giardia strains, host nutritional status, the composition of microbiota, co-infecting enteropathogens, host genetically determined mucosal immune responses, and immune modulation by Giardia are all relevant factors influencing disease manifestations after Giardia infection.

Drug resistance in Giardia duodenalis

Giardia duodenalis is a microaerophilic parasite of the human gastrointestinal tract and a major contributor to diarrheal and post-infectious chronic gastrointestinal disease world-wide. Treatment of G. duodenalis infection currently relies on a small number of drug classes. Nitroheterocyclics, in particular metronidazole, have represented the front line treatment for the last 40 years. Nitroheterocyclic-resistant G. duodenalis have been isolated from patients and created in vitro, prompting considerable research into the biomolecular mechanisms of resistance. These compounds are redox-active and are believed to damage proteins and DNA after being activated by oxidoreductase enzymes in metabolically active cells. In this review, we explore the molecular phenotypes of nitroheterocyclic-resistant G. duodenalis described to date in the context of the protist's unusual glycolytic and antioxidant systems. We propose that resistance mechanisms are likely to extend well beyond currently described resistance-associated enzymes (i.e., pyruvate ferredoxin oxidoreductases and nitroreductases), to include NAD(P)H- and flavin-generating pathways, and possibly redox-sensitive epigenetic regulation. Mechanisms that allow G. duodenalis to tolerate oxidative stress may lead to resistance against both oxygen and nitroheterocyclics, with implications for clinical control. The present review highlights the potential for systems biology tools and advanced bioinformatics to further investigate the multifaceted mechanisms of nitroheterocyclic resistance in this important pathogen.

An antioxidant response is involved in resistance of Giardia duodenalis to albendazole

Albendazole (ABZ) is a therapeutic benzimidazole used to treat giardiasis that targets β-tubulin. However, the molecular bases of ABZ resistance in Giardia duodenalis are not understood because β-tubulin in ABZ-resistant clones lacks mutations explaining drug resistance. In previous work we compared ABZ-resistant (1.35, 8, and 250 μM) and ABZ-susceptible clones by proteomic analysis and eight proteins involved in energy metabolism, cytoskeleton dynamics, and antioxidant response were found as differentially expressed among the clones. Since ABZ is converted into sulphoxide (ABZ-SO) and sulphone (ABZ-SOO) metabolites we measured the levels of these metabolites, the antioxidant enzymes and free thiols in the susceptible and resistant clones. Production of reactive oxygen species (ROS) and levels of ABZ-SO/ABZ-SOO induced by ABZ were determined by fluorescein diacetate-based fluorescence and liquid chromatography respectively. The mRNA and protein levels of antioxidant enzymes (NADH oxidase, peroxiredoxin 1a, superoxide dismutase and flavodiiron protein) in these clones were determined by RT-PCR and proteomic analysis. The intracellular sulfhydryl (R-SH) pool was quantified using dinitrobenzoic acid. The results showed that ABZ induced ROS accumulation in the ABZ-susceptible Giardia cultures but not in the resistant ones whilst the accumulation of ABZ-SO and ABZ-SOO was lower in all ABZ-resistant cultures. Consistent with these findings, all the antioxidant enzymes detected and analyzed were upregulated in ABZ-resistant clones. Likewise the R-SH pool increased concomitantly to the degree of ABZ-resistance. These results indicate an association between accumulation of ABZ metabolites and a pro-oxidant effect of ABZ in Giardia-susceptible clones. Furthermore the antioxidant response involving ROS-metabolizing enzymes and intracellular free thiols in ABZ-resistant parasites suggest that this response may contribute to overcome the pro-oxidant cytotoxicity of ABZ.

Antigiardial activity of novel triazolyl-quinolone-based chalcone derivatives: when oxygen makes the difference

Giardiasis is a common diarrheal disease worldwide caused by the protozoan parasite Giardia intestinalis. It is urgent to develop novel drugs to treat giardiasis, due to increasing clinical resistance to the gold standard drug metronidazole (MTZ). New potential antiparasitic compounds are usually tested for their killing efficacy against G. intestinalis under anaerobic conditions, in which MTZ is maximally effective. On the other hand, though commonly regarded as an ‘anaerobic pathogen,’ G. intestinalis is exposed to relatively high O₂ levels in vivo, living attached to the mucosa of the proximal small intestine. It is thus important to test the effect of O₂ when searching for novel potential antigiardial agents, as outlined in a previous study [Bahadur et al. (2014) Antimicrob. Agents Chemother. 58, 543]. Here, 45 novel chalcone derivatives with triazolyl-quinolone scaffold were synthesized, purified, and characterized by high resolution mass spectrometry, ¹H and ¹³C nuclear magnetic resonance and infrared spectroscopy. Efficacy of the compounds against G. intestinalis trophozoites was tested under both anaerobic and microaerobic conditions, and selectivity was assessed in a counter-screen on human epithelial colorectal adenocarcinoma cells. MTZ was used as a positive control in the assays. All the tested compounds proved to be more effective against the parasite in the presence of O₂, with the exception of MTZ that was less effective. Under anaerobiosis eighteen compounds were found to be as effective as MTZ or more (up to three to fourfold); the same compounds proved to be up to >100-fold more effective than MTZ under microaerobic conditions. Four of them represent potential candidates for the design of novel antigiardial drugs, being highly selective against Giardia trophozoites. This study further underlines the importance of taking O₂ into account when testing novel potential antigiardial compounds.

Gene expression changes during Giardia-host cell interactions in serum-free medium

Serial Analysis of Gene Expression (SAGE) was used to quantify transcriptional changes in Giardia intestinalis during its interaction with human intestinal epithelial cells (IECs, HT-29) in serum free M199 medium. Transcriptional changes were compared to those in trophozoites alone in M199 and in TYI-S-33 Giardia growth medium. In total, 90 genes were differentially expressed, mainly those involved in cellular redox homeostasis, metabolism and small molecule transport but also cysteine proteases and structural proteins of the giardin family. Only 29 genes changed their expression due to IEC interaction and the rest were due to M199 medium. Although our findings generated a small dataset, it was consistent with our earlier microarray studies performed under different interaction conditions. This study has confined the number of genes in Giardia to a small subset that specifically change their expression due to interaction with IECs.