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

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.

Proteases as Therapeutic Targets Against the Parasitic Cnidarian Ceratonova shasta: Characterization of Molecules Key to Parasite Virulence In Salmonid Hosts

Proteases and their inhibitors play critical roles in host-parasite interactions and in the outcomes of infections. Ceratonova shasta is a myxozoan pathogen that causes enteronecrosis in economically important salmonids from the Pacific Northwest of North America. This cnidarian parasite has host-specific genotypes with varying virulence, making it a powerful system to decipher virulence mechanisms in myxozoans. Using C. shasta genome and transcriptome, we identified four proteases of different catalytic types: cathepsin D (aspartic), cathepsin L and Z-like (cysteine) and aminopeptidase-N (metallo); and a stefin (cysteine protease inhibitor), which implied involvement in virulence and hence represent target molecules for the development of therapeutic strategies. We characterized, annotated and modelled their 3D protein structure using bioinformatics and computational tools. We quantified their expression in C. shasta genotype 0 (low virulence, no mortality) and IIR (high virulence and mortality) in rainbow trout Oncorhynchus mykiss, to demonstrate that there are major differences between the genotypes during infection and parasite development. High proliferation of genotype IIR was associated with high expression of the cathepsin D and the stefin, likely correlated with high nutrient demands and to regulate cell metabolism, with upregulation preceding massive proliferation and systemic dispersion. In contrast, upregulation of the cathepsin L and Z-like cysteine proteases may have roles in host immune evasion in genotype 0 infections, which are associated with low proliferation, low inflammation and non-destructive development. In contrast to the other proteases, C. shasta aminopeptidase-N appears to have a prominent role in nematocyst formation in both genotypes, but only during sporogenesis. Homology searches of C. shasta proteases against other myxozoan transcriptomes revealed a high abundance of cathepsin L and aminopeptidase homologs suggesting common gene requirements across species. Our study identified molecules of potential therapeutic significance for aquaculture and serves as a baseline for future research aimed at functional characterisation of these targets.

A Novel Cysteine Protease Inhibitor of Naegleria fowleri That Is Specifically Expressed during Encystation and at Mature Cysts

Naegleria fowleri is a free-living amoeba that is ubiquitous in diverse natural environments. It causes a fatal brain infection in humans known as primary amoebic meningoencephalitis. Despite the medical importance of the parasitic disease, there is a great lack of knowledge about the biology and pathogenicity of N. fowleri. In this study, we identified and characterized a novel cysteine protease inhibitor of N. fowleri (NfCPI). NfCPI is a typical cysteine protease inhibitor belonging to the cystatin family with a Gln-Val-Val-Ala-Gly (QVVAG) motif, a characteristic motif conserved in the cystatin family of proteins. Bacterially expressed recombinant NfCPI has a dimeric structure and exhibits inhibitory activity against several cysteine proteases including cathespin Bs of N. fowleri at a broad range of pH values. Expression profiles of nfcpi revealed that the gene was highly expressed during encystation and cyst of the amoeba. Western blot and immunofluorescence assays also support its high level of expression in cysts. These findings collectively suggest that NfCPI may play a critical role in encystation or cyst formation of N. fowleri by regulating cysteine proteases that may mediate encystation or mature cyst formation of the amoeba. More comprehensive studies to investigate the roles of NfCPI in encystation and its target proteases are necessary to elucidate the regulatory mechanism and the biological significance of NfCPI.

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.

Giardia lamblia Decreases NF-κB p65RelA Protein Levels and Modulates LPS-Induced Pro-Inflammatory Response in Macrophages

The protozoan Giardia lamblia is the most common cause of parasitic gastrointestinal infection worldwide. The parasite developed sophisticated, yet not completely disclosed, mechanisms to escape immune system and growth in the intestine. To further understand the interaction of G. lamblia with host immune cells, we investigated the ability of parasites to modulate the canonical activation of mouse macrophages (Raw 264.7 cell line) and human monocyte-derived macrophages triggered by the TLR4 agonist, lipopolysaccharide (LPS). We observed that G. lamblia impairs LPS-evoked pro-inflammatory status in these macrophage-like cells through inhibition of cyclooxygenase-2 and inducible nitric oxide synthase expression and subsequent NO production. This effect was in part due to the activity of three G. lamblia proteases, a 135 kDa metalloprotease and two cysteine proteases with 75 and 63 kDa, that cleave the p65^RelA subunit of the nuclear factor-kappa B (NF-κB). Moreover, Tnf and Ccl4 transcription was increased in the presence of the parasite. Overall, our data indicates that G. lamblia modulates macrophages inflammatory response through impairment of the NF-κB, thus silencing a crucial signaling pathway of the host innate immune response.

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.

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.