Cleavage specificity of recombinant Giardia intestinalis cysteine proteases: Degradation of immunoglobulins and defensins

Giardia intestinalis extracellular vesicles induce changes in gene expression in human intestinal epithelial cells in vitro

Giardiasis is a common waterborne zoonotic disease caused by Giardia intestinalis. Upon infection, Giardia releases excretory and secretory products (ESPs) including secreted proteins (SPs) and extracellular vesicles (EVs). Although the interplay between ESPs and intestinal epithelial cells (IECs) has been previously described, the functions of EVs in these interactions and their differences from those of SPs require further exploration. In the present study, EVs and EV-depleted SPs were isolated from Giardia ESPs. Proteomic analyses of isolated SPs and EVs showed 146 and 91 proteins, respectively. Certain unique and enriched proteins have been identified in SPs and EVs. Transcriptome analysis of Caco-2 cells exposed to EVs showed 96 differentially expressed genes (DEGs), with 56 upregulated and 40 downregulated genes. Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Gene Set Enrichment Analysis (GSEA) indicated that Caco-2 genes related to metabolic processes, the HIF-1 signaling pathway, and the cAMP signaling pathway were affected. This study provides new insights into host-parasite interactions, highlighting the potential significance of EVs on IECs during infections.

Extracellular Cysteine Proteases of Key Intestinal Protozoan Pathogens-Factors Linked to Virulence and Pathogenicity

Intestinal diseases caused by protistan parasites of the genera Giardia (giardiasis), Entamoeba (amoebiasis), Cryptosporidium (cryptosporidiosis) and Blastocystis (blastocystosis) represent a major burden in human and animal populations worldwide due to the severity of diarrhea and/or inflammation in susceptible hosts. These pathogens interact with epithelial cells, promoting increased paracellular permeability and enterocyte cell death (mainly apoptosis), which precede physiological and immunological disorders. Some cell-surface-anchored and molecules secreted from these parasites function as virulence markers, of which peptide hydrolases, particularly cysteine proteases (CPs), are abundant and have versatile lytic activities. Upon secretion, CPs can affect host tissues and immune responses beyond the site of parasite colonization, thereby increasing the pathogens' virulence. The four intestinal protists considered here are known to secrete predominantly clan A (C1- and C2-type) CPs, some of which have been characterized. CPs of Giardia duodenalis (e.g., Giardipain-1) and Entamoeba histolytica (EhCPs 1-6 and EhCP112) degrade mucin and villin, cause damage to intercellular junction proteins, induce apoptosis in epithelial cells and degrade immunoglobulins, cytokines and defensins. In Cryptosporidium, five Cryptopains are encoded in its genome, but only Cryptopains 4 and 5 are likely secreted. In Blastocystis sp., a legumain-activated CP, called Blastopain-1, and legumain itself have been detected in the extracellular medium, and the former has similar adverse effects on epithelial integrity and enterocyte survival. Due to their different functions, these enzymes could represent novel drug targets. Indeed, some promising results with CP inhibitors, such as vinyl sulfones (K11777 and WRR605), the garlic derivative, allicin, and purified amoebic CPs have been obtained in experimental models, suggesting that these enzymes might be useful drug targets.

Regulatory Functions of Hypoxia in Host-Parasite Interactions: A Focus on Enteric, Tissue, and Blood Protozoa

Body tissues are subjected to various oxygenic gradients and fluctuations and hence can become transiently hypoxic. Hypoxia-inducible factor (HIF) is the master transcriptional regulator of the cellular hypoxic response and is capable of modulating cellular metabolism, immune responses, epithelial barrier integrity, and local microbiota. Recent reports have characterized the hypoxic response to various infections. However, little is known about the role of HIF activation in the context of protozoan parasitic infections. Growing evidence suggests that tissue and blood protozoa can activate HIF and subsequent HIF target genes in the host, helping or hindering their pathogenicity. In the gut, enteric protozoa are adapted to steep longitudinal and radial oxygen gradients to complete their life cycle, yet the role of HIF during these protozoan infections remains unclear. This review focuses on the hypoxic response to protozoa and its role in the pathophysiology of parasitic infections. We also discuss how hypoxia modulates host immune responses in the context of protozoan infections.

Mucosal Defense Against Giardia at the Intestinal Epithelial Cell Interface

Human giardiasis, caused by the protozoan parasite Giardia duodenalis (syn. Giardia lamblia, Giardia intestinalis, Lamblia intestinalis), is one of the most commonly-identified parasitic diseases worldwide. Chronic G. duodenalis infections cause a malabsorption syndrome that may lead to failure to thrive and/or stunted growth, especially in children in developing countries. Understanding the parasite/epithelial cell crosstalk at the mucosal surfaces of the small intestine during human giardiasis may provide novel insights into the mechanisms underlying the parasite-induced immunopathology and epithelial tissue damage, leading to malnutrition. Efforts to identify new targets for intervening in the development of intestinal immunopathology and the progression to malnutrition are critical. Translating these findings into a clinical setting will require analysis of these pathways in cells and tissues from humans and clinical trials could be devised to determine whether interfering with unwanted mucosal immune responses developed during human giardiasis provide better therapeutic benefits and clinical outcomes for G. duodenalis infections in humans.

Evaluation of Immune Response and Haematological Parameters in Infected Male Albino Rats by Giardiasis

The present work aimed to study the effects of G. lamblia infection on immunological, haematological studies and to evaluate immunoglobulins and some cytokines. Fifty male albino rats were divided into six groups. The control group including 20 rats and the infected group includes 30 rats. All the estimations were checked all over five checkpoints (CP) (7, 14, 21, 28, and 35 days post-infection). Serum levels of IgA, IgG, IgM and IgE. Cytokines INF-γ, TNF-alpha, IL-4, IL-10, and haematological parameters were determined. Cyst and trophozoite were counted. A considerable increase in the level of immunoglobulins and cytokines in all infected groups compared to the control group was documented. Furthermore, a significant decrease in red blood corpuscles, haemoglobin, and mean corpuscular haemoglobin concentration levels, whereas substantial increases in mean corpuscular volume, mean corpuscular haemoglobin and platelets were observed. Moreover, infected rats had a substantial rise in WBCs, lymphocytes, and eosinophil counts compared to the control group, whereas neutrophils and monocytes had a significant decrease. Number of trophozoites and cysts were significantly increased in infected groups before diminishing after day 28. The current results showed that Th1 and Th2 immune responses, which are characterized by the production of TNF-α, IFN-γ, IL-4 and IL-10, are important for protection against Giardia infections and also verified the balance between these cytokines and the timing of their production was crucial in G. lamblia immune response. Giardia lamblia, Immunity, Antibodies, cytokines, eosinophil.

Intestinal polyparasitism and levels of mucosal anthelmintic SIgA in children from endemic areas in Northeastern Brazil

Interactions between parasites during co-infections are often complex and can impact immunization and treatment programmes, as well as disease outcomes and morbidity. However, little is known about these interactions and the mechanisms involved. In this study, a coproparasitological survey was carried out in school-age children living in endemic areas of parasitic infection in the state of Sergipe, Northeastern Brazil. Anti-helminth-specific and total secretory immunoglobulin-A (SIgA) levels were measured in stool and saliva samples and were compared in children presenting monoparasitism, polyparasitism (helminths and/or intestinal protozoa) and no infections. The survey showed that protozoa were more prevalent than helminths, and that there was a high frequency of polyparasitism in the studied population, mainly from combinations of protozoan species. Although less frequent, combinations between species of protozoa and helminths were also observed. The levels of salivary SIgA in these co-infected individuals were lower than the average observed in infections with helminths alone. Although the children participating in this survey were asymptomatic, and it was, therefore, not possible to evaluate the impact of salivary SIgA reduction on the diseases, and the study highlights the need for further investigations of co-infections by intestinal parasites and the effects on immune response induced by the interactions between different parasites.

Giardia duodenalis Virulence - "To Be, or Not To Be"

Purpose of review

Here, we review recent progress made on the genetic characterization of Giardia duodenalis assemblages and their relationship with virulence. We also discuss the implications of virulence factors in the pathogenesis of giardiasis, and advances in the development of vaccines and drugs based on knowledge of virulence markers.

Recent findings

The use of transcriptomic and proteomic technologies as well as whole genome sequencing (WGS) from single cysts has allowed the assembly of the draft genome sequences for assemblages C and D of G. duodenalis. These findings, along with the published genomes for assemblages A, B, and E, have allowed comparative genomic investigations. In addition, the use of these methodologies for the characterization of the secretomes of trophozoite-epithelial cell interactions for assemblages A/B has led to the identification of virulence markers including energy metabolism enzymes, proteinases, high-cysteine membrane proteins (HCMPs), and variant surface proteins (VSPs). Recently, some drugs and vaccines, targeting virulence factors have been developed, offering possible alternatives to current treatment and prevention options against giardiasis.

Summary

Among the nine recognized species of Giardia, G. duodenalis stands out because of its broad spectrum of hosts and its socio-economic importance. This species comprises eight genetic assemblages (A to H), of which A and B are zoonotic, and the other assemblages have narrow host specificities. Assemblages A and B may be considered as the most virulent ones, but the existence of asymptomatic carriers and considerable genetic variability within and among these assemblages hampers the definition of common virulence factors. The attachment of Giardia trophozoites to epithelial cells and structural cytoskeleton components of the adhesive disk, such as giardins or tubulins, is proposed to play key roles, but toxins have not yet been precisely defined. However, recent transcriptomic and proteomic analyses of the secretomes of trophozoites representing assemblages A and B and interacting with particular epithelial cell lines have defined a series of virulence factors, including glycolytic (e.g., enolase) and arginolytic (e.g., arginine deiminase) enzymes, cysteine proteases (e.g., giardipain-1) and VSPs (e.g., VSP9B10A). Other factors, such as HCMPs and tenascins, have been consistently found to be excreted/secreted, but their role(s) in the pathogenesis of giardiasis has not yet been elucidated. Interestingly, recent investigations of single cysts representing assemblages C and D using advanced sequencing and informatic methods have suggested that the transcription/expression profiles of virulence factors vary both within and between assemblages, thus assemblage-specific molecules might allow adaptation to the microenvironment within the host. Importantly, some drugs active against cysteine-rich proteins of Giardia, including giardipain-1, VSPs and arginine deiminase, have been shown to be targeted by cysteine-modifying compounds as disulfiram, L-canavanin and allicin. On the other hand, VSPs are presently considered as key vaccine candidates because they induce protection against Giardia in rodents and dogs. Overall, this review reveals that much more work is needed to identify, characterize, and understand the roles of virulence factors in Giardia and to assess their validity as drug and vaccine targets. Clear, advanced omics and informatic tools should assist in this future endeavor, with a focus on targeting virulence factors that are common and/or unique to distinct assemblages to develop new and effective interventions against Giardia.

Giardia spp. and the Gut Microbiota: Dangerous Liaisons

Alteration of the intestinal microbiome by enteropathogens is commonly associated with gastrointestinal diseases and disorders and has far-reaching consequences for overall health. Significant advances have been made in understanding the role of microbial dysbiosis during intestinal infections, including infection with the protozoan parasite Giardia duodenalis, one of the most prevalent gut protozoa. Altered species composition and diversity, functional changes in the commensal microbiota, and changes to intestinal bacterial biofilm structure have all been demonstrated during the course of Giardia infection and have been implicated in Giardia pathogenesis. Conversely, the gut microbiota has been found to regulate parasite colonization and establishment and plays a critical role in immune modulation during mono and polymicrobial infections. These disruptions to the commensal microbiome may contribute to a number of acute, chronic, and post-infectious clinical manifestations of giardiasis and may account for variations in disease presentation within and between infected populations. This review discusses recent advances in characterizing Giardia-induced bacterial dysbiosis in the gut and the roles of dysbiosis in Giardia pathogenesis.

High Cysteine Membrane Proteins (HCMPs) Are Up-Regulated During Giardia-Host Cell Interactions

Giardia intestinalis colonizes the upper small intestine of humans and animals, causing the diarrheal disease giardiasis. This unicellular eukaryotic parasite is not invasive but it attaches to the surface of small intestinal epithelial cells (IECs), disrupting the epithelial barrier. Here, we used an in vitro model of the parasite’s interaction with host IECs (differentiated Caco-2 cells) and RNA sequencing (RNAseq) to identify differentially expressed genes (DEGs) in Giardia, which might relate to the establishment of infection and disease induction. Giardia trophozoites interacted with differentiated Caco-2 cells for 1.5, 3, and 4.5 h and at each time point, 61, 89, and 148 parasite genes were up-regulated more than twofold, whereas 209, 265, and 313 parasite genes were down-regulated more than twofold. The most abundant DEGs encode hypothetical proteins and members of the High Cysteine Membrane Protein (HCMP) family. Among the up-regulated genes we also observed proteins associated with proteolysis, cellular redox balance, as well as lipid and nucleic acid metabolic pathways. In contrast, genes encoding kinases, regulators of the cell cycle and arginine metabolism and cytoskeletal proteins were down-regulated. Immunofluorescence imaging of selected, up-regulated HCMPs, using C-terminal HA-tagging, showed localization to the plasma membrane and peripheral vesicles (PVs). The expression of the HCMPs was affected by histone acetylation and free iron-levels. In fact, the latter was shown to regulate the expression of many putative giardial virulence factors in subsequent RNAseq experiments. We suggest that the plasma membrane localized and differentially expressed HCMPs play important roles during Giardia-host cell interactions.

The compact genome of Giardia muris reveals important steps in the evolution of intestinal protozoan parasites

Diplomonad parasites of the genus Giardia have adapted to colonizing different hosts, most notably the intestinal tract of mammals. The human-pathogenic Giardia species, Giardia intestinalis, has been extensively studied at the genome and gene expression level, but no such information is available for other Giardia species. Comparative data would be particularly valuable for Giardia muris, which colonizes mice and is commonly used as a prototypic in vivo model for investigating host responses to intestinal parasitic infection. Here we report the draft-genome of G. muris. We discovered a highly streamlined genome, amongst the most densely encoded ever described for a nuclear eukaryotic genome. G. muris and G. intestinalis share many known or predicted virulence factors, including cysteine proteases and a large repertoire of cysteine-rich surface proteins involved in antigenic variation. Different to G. intestinalis, G. muris maintains tandem arrays of pseudogenized surface antigens at the telomeres, whereas intact surface antigens are present centrally in the chromosomes. The two classes of surface antigens engage in genetic exchange. Reconstruction of metabolic pathways from the G. muris genome suggest significant metabolic differences to G. intestinalis. Additionally, G. muris encodes proteins that might be used to modulate the prokaryotic microbiota. The responsible genes have been introduced in the Giardia genus via lateral gene transfer from prokaryotic sources. Our findings point to important evolutionary steps in the Giardia genus as it adapted to different hosts and it provides a powerful foundation for mechanistic exploration of host-pathogen interaction in the G. muris-mouse pathosystem.

Drug resistance in Giardia: Mechanisms and alternative treatments for Giardiasis

The use of chemotherapeutic drugs is the main resource against clinical giardiasis due to the lack of approved vaccines. Resistance of G. duodenalis to the most used drugs to treat giardiasis, metronidazole and albendazole, is a clinical issue of growing concern and yet unknown impact, respectively. In the search of new drugs, the completion of the Giardia genome project and the use of biochemical, molecular and bioinformatics tools allowed the identification of ligands/inhibitors for about one tenth of ≈150 potential drug targets in this parasite. Further, the synthesis of second generation nitroimidazoles and benzimidazoles along with high-throughput technologies have allowed not only to define overall mechanisms of resistance to metronidazole but to screen libraries of repurposed drugs and new pharmacophores, thereby increasing the known arsenal of anti-giardial compounds to some hundreds, with most demonstrating activity against metronidazole or albendazole-resistant Giardia. In particular, cysteine-modifying agents which include omeprazole, disulfiram, allicin and auranofin outstand due to their pleiotropic activity based on the extensive repertoire of thiol-containing proteins and the microaerophilic metabolism of this parasite. Other promising agents derived from higher organisms including phytochemicals, lactoferrin and propolis as well as probiotic bacteria/fungi have also demonstrated significant potential for therapeutic and prophylactic purposes in giardiasis. In this context the present chapter offers a comprehensive review of the current knowledge, including commonly prescribed drugs, causes of therapeutic failures, drug resistance mechanisms, strategies for the discovery of new agents and alternative drug therapies.

Pathogenesis and post-infectious complications in giardiasis

Giardia is an important cause of diarrhoea, and results in post-infectious and extra-intestinal complications. This chapter presents a state-of-the art of our understanding of how this parasite may cause such abnormalities, which appear to develop at least in part in Assemblage-dependent manner. Findings from prospective longitudinal cohort studies indicate that Giardia is one of the four most prevalent enteropathogens in early life, and represents a risk factor for stunting at 2 years of age. This may occur independently of diarrheal disease, in strong support of the pathophysiological significance of the intestinal abnormalities induced by this parasite. These include epithelial malabsorption and maldigestion, increased transit, mucus depletion, and disruptions of the commensal microbiota. Giardia increases epithelial permeability and facilitates the invasion of gut bacteria. Loss of intestinal barrier function is at the core of the acute and post-infectious complications associated with this infection. Recent findings demonstrate that the majority of the pathophysiological responses triggered by this parasite can be recapitulated by the effects of its membrane-bound and secreted cysteine proteases.

Transcriptomic and proteomic analyses of Giardia intestinalis: Intestinal epithelial cell interactions

Giardia intestinalis is a unicellular protozoan parasite that infects the small intestines of humans and animals. Giardiasis, the disease caused by the parasite, occurs globally across socioeconomic boundaries but is mainly endemic in developing countries and particularly within young children, where pronounced effects manifests in a failure to thrive condition. The molecular pathogenesis of Giardia has been studied using in vitro models of human and rat intestinal epithelial cells (IECs) and parasites from the two major human genotypes or assemblages (A and B). High-quality, genome sequencing of representative isolates from assemblages A (WB) and B (GS) has enabled exploration of these host-parasite models using 'omics' technologies, allowing deep and quantitative analyses of global gene expression changes in IECs and parasites during their interactions, cross-talk and competition. These include a major up-regulation of immune-related genes in the IECs early after the start of interactions, as well as competition between host cells and parasites for nutrients like sugars, amino acids and lipids, which is also reflected in their secretome interactions. Unique parasite proteins dominate these interactions, with many major up-regulated genes being either hypothetical proteins or members of Giardia-specific gene families like the high-cysteine-rich membrane proteins (HCMPs), variable surface proteins (VSPs), alpha-giardins and cysteine proteases. Furthermore, these proteins also dominate in the secretomes, suggesting that they are important virulence factors in Giardia and crucial molecular effectors at the host-parasite interface.

Recent insights into innate and adaptive immune responses to Giardia

Infection with Giardia produces a wide range of clinical outcomes. Acutely infected patients may have no overt symptoms or suffer from severe cramps, diarrhea, nausea and even urticaria. Recently, post-infectious irritable bowel syndrome and chronic fatigue syndrome have been identified as long-term sequelae of giardiasis. Frequently, recurrent and chronic Giardia infection is considered a major contributor to stunting in children from low and middle income countries. Perhaps the most unusual outcome of infection with Giardia is the apparent reduced risk of developing moderate-to-severe diarrhea due to other enteric infections which has been noted in several recent studies. The goal of understanding immune responses against Giardia is therefore to identify protective mechanisms which could become targets for vaccine development, but also to identify mechanisms whereby infections lead to these other diverse outcomes. Giardia induces a robust adaptive immune response in both humans and animals. It has been known for many years that there is production of large amounts of parasite-specific IgA following infection and that CD4⁺ T cell responses contribute to this IgA production and control of the infection. In the past decade, there have been advances in our understanding of the non-antibody effector mechanisms used by the host to fight Giardia infections, in particular the importance of the cytokine interleukin (IL)-17 in orchestrating these responses. There have also been major advances in understanding how the innate response to Giardia infection is initiated and how it contributes to the development of adaptive immunity. Finally, there here have been significant increases in our knowledge of how the resident microbial community influences the immune response and how these responses contribute to the development of some of the symptoms of giardiasis. In this article, we will focus on data generated in the last 10 years and how it has advanced our knowledge about this important parasitic disease.

Giardia intestinalis cystatin is a potent inhibitor of papain, parasite cysteine proteases and, to a lesser extent, human cathepsin B

Cystatins are important regulators of papain-like cysteine proteases. In the protozoan parasite Giardia intestinalis, papain-like cysteine proteases play an essential role in the parasite's biology and pathogenicity. Here, we characterized a cysteine protease inhibitor of G. intestinalis that belongs to type-I-cystatins. The parasite cystatin is shown to be a strong inhibitor of papain (K_i  ≈ 0.3 nm) and three parasite cysteine proteases (CP14019, CP16160 and CP16779, K_i  ≈ 0.9-5.8 nm), but a weaker inhibitor of human cathepsin B (K_i  ≈ 79.9 nm). The protein localizes mainly in the cytoplasm. Together, these data suggest that cystatin of G. intestinalis plays a role in the regulation of cysteine protease activities in the parasite and, possibly, in the interaction with the host.

Giardia duodenalis: Role of secreted molecules as virulent factors in the cytotoxic effect on epithelial cells

During the course of giardiasis in humans and experimental models, G. duodenalis trophozoites express and secrete several proteins (ESPs) affecting structural, cellular and soluble components of the host intestinal milieu. These include the toxin-like molecules CRP136 and ESP58 that induce intestinal hyper-peristalsis. After the completion of the Giardia genome database and using up-to date transcriptomic and proteomic approaches, secreted 'virulence factors' have also been identified and experimentally characterized. This repertoire includes arginine deiminase (ADI) that competes for arginine, an important energy source for trophozoites, some high-cysteine membrane proteins (HCMPs) and VSP88, a versatile variant surface protein (VSP) that functions as an extracellular protease. Another giardial protein, enolase, moonlights as a metabolic enzyme that interacts with the fibrinolytic system and damages host epithelial cells. Other putative Giardia virulence factors are cysteine proteases that degrade multiple host components including mucin, villin, tight junction proteins, immunoglobulins, defensins and cytokines. One of these proteases, named giardipain-1, decreases transepithelial electrical resistance and induces apoptosis in epithelial cells. A putative role for tenascins, present in the Giardia's secretome, is interfering with the host epidermal growth factor. Based on the roles that these molecules play, drugs may be designed to interfere with their functions. This review presents a comprehensive description of secreted Giardia virulence factors. It further describes their cytotoxic mechanisms and roles in the pathophysiology of giardiasis, and then assesses their potential as targets for drug development.