Role of the galactose lectin of Entamoeba histolytica in adherence-dependent killing of mammalian cells

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

The cysteine-rich region of the Entamoeba histolytica adherence lectin (170-kilodalton subunit) is sufficient for high-affinity Gal/GalNAc-specific binding in vitro

Adherence of Entamoeba histolytica trophozoites to colonic mucin, epithelium, and other target cells is mediated by the amebic Gal/GalNAc lectin. We constructed in vitro expression vectors containing full-length (residues 1 to 1280), cysteine-poor (1 to 353 and 1 to 480), and cysteine-rich (356 to 1143 and 480 to 900) fragments of the gene encoding the heavy subunit of the adherence lectin, hgl2. In vitro transcription followed by translation using a nuclease-treated rabbit reticulocyte lysate system was carried out. Immunoreactivity of in vitro-translated Hgl2 was confirmed by immunoprecipitation with lectin-specific monoclonal antibodies (MAbs) 1G7 and 8A3, which recognize linear epitopes. Protein disulfide isomerase (PDI) refolding of Hgl2 enhanced immunoreactivity (P < 0.05) with the conformationally dependent MAb 3F4. Binding of PDI-refolded full-length (P < 0.001) and cysteine-rich (P = 0.005) Hgl2 to CHO cells was galactose dependent and competitively inhibited by native hololectin (50% inhibitory concentration of 39.6 ng/ml). The cysteine-poor region (1 to 353) did not bind CHO cells. Both full-length (1 to 1280) and cysteine-rich (356 to 1143) Hgl2 bound the glyconeoconjugate GalNAc(19)BSA in a GalNAc-specific manner. The smaller cysteine-rich fragment (480 to 900) also exhibited GalNAc-specific binding but to a lesser extent (P < 0.05) than residues 1 to 1280 and 356 to 1143. Neither the cysteine-poor fragment (1 to 480), luciferase (protein control), nor control translation reactions (without hgl2 lectin mRNA) bound GalNAc(19)BSA. Binding to GalNAc(19)BSA was shown to be dependent on the concentration of GalNAc(19)BSA coated in each well or (35)S-lectin added (K(D) = 0.85 +/- 0.37 pM). Binding was competitively inhibited by the terminal GalNAc-containing glycoprotein asialofetuin (P < 0.005). Taken together, these data provide direct evidence that the cysteine-rich region of the Gal/GalNAc lectin heavy subunit contains one or more carbohydrate-binding domains.

Host-pathogen interaction in amebiasis and progress in vaccine development

Entamoeba histolytica, the causative organism of invasive intestinal and extraintestinal amebiasis, infects approximately 50 million people each year, causing an estimated 40 to 100 thousand deaths annually. Because amebae only infect humans and some higher non-human primates, an anti-amebic vaccine could theoretically eradicate the organism. Uncontrolled epidemiologic studies indicate that acquired immunity to amebic infection probably occurs and that such a vaccine might be feasible. Application of molecular biologic techniques has led to rapid progress towards understanding how Entamoeba histolytica causes disease, and to the identification of several amebic proteins associated with virulence. These proteins are now being evaluated as potential vaccine components. Parenteral and oral vaccine preparations containing recombinant amebic proteins have been effective in preventing disease in a gerbil model of amebic liver abscess. Although systemic and mucosal cellular and humoral immunity both appear to play a role in protection against Entamoeba histolytica, the relative importance of each in the human immune response remains unknown. No animal model of intestinal amebiasis currently exists, moreover, so it has been impossible to evaluate protection against colonization and colitis. Further investigation of the fundamental mechanisms by which Entamoeba histolytica causes disease and of the human immune response to amebic infection is necessary to assess the true feasibility of an anti-amebic vaccine.

Involvement of the actin cytoskeleton and p21rho-family GTPases in the pathogenesis of the human protozoan parasite Entamoeba histolytica

It has been estimated that infection with the enteric protozoan parasite Entamoeba histolytica kills more than 50,000 people a year. Central to the pathogenesis of this organism is its ability to directly lyse host cells and cause tissue destruction. Amebic lesions show evidence of cell lysis, tissue necrosis, and damage to the extracellular matrix. The specific molecular mechanisms by which these events are initiated, transmitted, and effected are just beginning to be uncovered. In this article we review what is known about host cell adherence and contact-dependent cytolysis. We cover the involvement of the actin cytoskeleton and small GTP-binding proteins of the p21rho-family in the process of cell killing and phagocytosis, and also look at how amebic interactions with molecules of the extracellular matrix contribute to its cytopathic effects.

The small GTP-binding protein RacG regulates uroid formation in the protozoan parasite Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that invades human intestine leading to ulceration and destruction of tissue. Amoebic movement and phagocytosis of human cells is accompanied by characteristic changes in cell morphology. Amoebae become polarized, developing a frontal pseudopod and a well-defined rear zone of membrane accumulation designated the uroid. In motile eukaryotic cells, a phenomenon that contributes to movement is the capping of receptors at the cell surface. During the capping process, E. histolytica concentrates ligand-receptor complexes in the uroid. Interestingly, some of these surface receptors are involved in the survival of the parasite. While looking for regulators of capping and uroid formation, we identified RacG, an E. histolytica protein that is homologous to human Rac1. This protein belongs to the Rac subfamily of small GTPases implicated in interactions between the actin cytoskeleton and the membrane of mammalian cells. Cloning of the EhracG gene and analysis of the protein activity either in murine fibroblasts or in E. histolytica revealed that EhRacG induces a characteristic Rac phenotype. When expressed in amoebae, an EhRacG-V12 mutant protein not only deregulated cell polarity, but also caused a defect in cytokinesis. Analysis of the cytoskeleton in amoebae bearing this mutant revealed that F-actin concentrated at the periphery of the cell. In addition, the number and localization of uroids were modified. These results suggest a role for EhRacG in amoebic morphogenesis and cytokinesis.

Molecular analysis of the Gal/GalNAc adhesin of Entamoeba histolytica

Attachment of Entamoeba histolytica to colonic epithelium and a variety of other target cells is mediated by a galactose/N-acetyl D-galactosamine (Gal/GalNAc) inhibitable adhesin. Seven monoclonal antibodies specific for nonoverlapping epitopes of the 170 kDa subunit have been shown to have distinct effects on adherence. Four of these monoclonal antibodies inhibit or have no effect on amebic adherence while two others enhance amebic adherence. The epitopes recognized by these seven monoclonal antibodies have been mapped to the extracellular cysteine rich region of the 170 kDa subunit. The conformational nature of the epitopes was examined by testing monoclonal antibody reactivity with isolated regions of the 170 kDa subunit expressed as fusion proteins in E. coli and also with denatured native adhesin. These analyses suggested that three of monoclonal antibodies recognized conformational epitopes while the remaining four recognized linear epitopes. The mapping of these monoclonal antibodies have identified functionally important regions of the Gal/GalNAc adhesin and have also shown that recombinant Gal/GalNAc adhesin, when expressed in E.coli, retained at least some of its native conformation.

Transient expression of luciferase in Entamoeba histolytica driven by the ferredoxin gene 5' and 3' regions

We have successfully transfected Entamoeba histolytica trophozoites with constructs containing ferredoxin sequences fused to the reporter gene luciferase. We have determined the conditions and parameters necessary to maximise transient luciferase expression in our system. Our optimal construct gave values of 268 x 10(3) relative light units per second (RLU s-1) when assayed representing a stimulation of 18,000-fold over the control vector. Comparison of differing constructs allowed us to conclude that the 5' and 3' ferredoxin sequences are both necessary for optimal luciferase expression from our vectors. Transcriptional initiation occurs within the consensus sequence ATTCA in both construct and chromosomal ferredoxin promoters.

Adherence and cytotoxicity of Entamoeba histolytica or how lectins let parasites stick around

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Analysis of the gene family encoding the Entamoeba histolytica galactose-specific adhesin 170-kDa subunit

The 170-kDa or heavy subunit of the galactose binding adhesin of Entamoeba histolytica is seminal in target cell binding and lysis. To determine the existence and complexity of the 170-kDa subunit gene family, hgl, an amebic genomic library in lambda phage was hybridized with DNA fragments from the 5' or 3' ends of hgl1. Termini from three distinct heavy subunit genes were identified including hgl1, hgl2, and a third, unreported gene designated hgl3. The open reading frame of hgl3 was sequenced in its entirety. Non-stringent hybridization of a genomic Southern blot with heavy subunit specific DNA labeled only those bands predicted by hgl1-3. The amino acid sequence of hgl3 was 95.2% identical to hgl1 and 89.4% identical to hgl2. All 97 cysteine residues present in the heavy subunit were conserved in hgl1-3. Analysis of amebic RNA showed that all three heavy subunit genes were expressed in the amebae and that hgl message became less abundant as the amebae entered a stationary growth phase.

Characterization of quail intestinal mucin as a ligand for endogenous quail lectin

The S-type lectins have been shown to be components of mucosal scrapings, and in avian systems these lectins have been localized immunohistochemically to the mucosal surface and goblet cells of the intestine. The interaction of lectin specifically with purified mucin has not, however, been established. Quail intestinal mucin was purified by two subsequent isopycnic density-gradient centrifugations in CsCl and chromatography on Sepharose Cl-2B. Purified mucin, obtained from the void volume of the Sepharose column, was characterized by SDS/PAGE, amino acid and carbohydrate analyses, sensitivity to thiol reduction, and cross-reactivity with antibody preparations to rat and human intestinal mucins on Western blots. Antibody raised against purified quail mucin partially cross-reacts with purified rat, rabbit and human intestinal mucins, and specifically labels the mucosal surface and goblet cells of quail intestine by the immunoperoxidase technique. Protein eluted by lactose from an affinity matrix composed of quail intestinal mucin possessed the same molecular mass on SDS/PAGE as intestinal lectin and reacted on Western blots with a lectin-specific antibody. The data clearly demonstrate the co-localization of lectin and mucin in the quail intestine and also the ability of the lectin to specifically interact with the purified mucin, raising the question of the role of endogenous lectins in secretions.

Serum-independent and serum-dependent cytoadherence in the interaction of Entamoeba histolytica with mammalian target cells

Entamoeba histolytica kills target cells only on direct contact, suggesting that trophozoite-mediated cytolysis is initiated by the contact between trophozoites and target cells. We have shown that adherence between E. histolytica and target cells (polymorphonuclear granulocytes, erythrocytes, Chinese hamster ovary cells, human colon carcinoma cells) was inhibited by specific carbohydrates, and adherence between E. histolytica and polymorphonuclear granulocytes (PMN) was enhanced by preincubation of the trophozoites with serum. Inhibition of adherence clearly paralleled inhibition of cytolysis and phagocytosis of target cells. Cytolysis of PMN, however, was not increased by preincubation of the trophozoites with serum. These results suggest that the effector functions of trophozoites are only dependent on carbohydrate-specific adherence mechanisms mediated by the amoebic Gal/GalNAc-binding lectin. E. histolytica trophozoites themselves can be killed by PMN, depending on the virulence of the trophozoites. PMN could not kill E. histolytica trophozoites more effectively when the adherence was enhanced by preincubation of the trophozoites with serum or when adherence was only mediated by serum-dependent mechanisms.

Neutralizing monoclonal antibody epitopes of the Entamoeba histolytica galactose adhesin map to the cysteine-rich extracellular domain of the 170-kilodalton subunit

Entamoeba histolytica adheres to human colonic mucins and colonic epithelial cells via a galactose-binding adhesin. The adhesin is a heterodimeric glycoprotein composed of 170- and 35-kDa subunits. Fragments of the hgl1 gene encoding the 170-kDa subunit were expressed as recombinant fusion proteins in Escherichia coli and reacted with anti-adhesin monoclonal antibodies (MAbs) or pooled human immune sera. The MAbs tested recognize seven distinct epitopes on the 170-kDa subunit and have distinct effects on the adherence and complement-inhibitory activities of the adhesin. All seven MAbs reacted with a fusion protein containing the cysteine-rich domain of the protein. Pooled human immune sera reacted with the same cysteine-rich domain as the MAbs and also with a construct containing the first 596 amino acids. Reactivity of three MAbs with the surface of intact trophozoites confirmed that the cysteine-rich domain was located extracellularly. The location of individual epitopes was fine mapped by constructing carboxy-terminal deletions in the cysteine-rich region of the fusion protein. The locations of adherence-enhancing and -inhibiting epitopes were partially distinguished, and the epitopes where complement-inhibitory MAbs bound were demonstrated to be near the adhesin's area of sequence identity with the human complement inhibitor CD59.

Interaction between trophozoites of Entamoeba histolytica and the human intestinal cell line HT-29 in the presence or absence of leukocytes

Studies on the interaction between trophozoites of Entamoeba histolytica of pathogenic or non-pathogenic origin and epithelial cells of the human intestine can contribute to the understanding of the pathogenesis of invasive amoebiasis. We have examined the interaction of virulent E. histolytica with the human colonic carcinoma cell line HT-29. Differentiated HT-29 cells are comparable to the mucosa cells to which E. histolytica attaches physiologically. Adherence between E. histolytica trophozoites and HT-29 cells was effectively inhibited by glycoconjugates containing galactose, indicating the importance of the 170-kDa lectin of E. histolytica in binding to intestinal cells. Adherence was not significantly inhibited by glycoconjugates containing N-acetyl-glucosamine, indicating that the 220-kDa lectin of E. histolytica is not involved in binding to HT-29 cells. The destruction of HT-29 cells by pathogenic E. histolytica was dependent on adherence. The destruction was enhanced when polymorphonuclear granulocytes were added to the E. histolytica trophozoites.