Entamoeba histolytica: erythrophagocytosis, collagenolysis, and liver abscess production as virulence markers

Ancestral TALE homeobox protein transcription factor regulates actin dynamics and cellular activities of protozoan parasite Entamoeba invadens

Entamoeba histolytica causes invasive amoebiasis, an important neglected tropical disease with a significant global health impact. The pathogenicity and survival of E. histolytica and its reptilian equivalent, Entamoeba invadens, relies on its ability to exhibit efficient motility, evade host immune responses, and exploit host resources, all of which are governed by the actin cytoskeleton remodeling. Our study demonstrates the early origin and the regulatory role of TALE homeobox protein EiHbox1 in actin-related cellular processes. Several genes involved in different biological pathways, including actin dynamics are differentially expressed in EiHbox1 silenced cells. EiHbox1 silenced parasites showed disrupted F-actin organization and loss of cellular polarity. EiHbox1's presence in the anterior region of migrating cells further suggests its involvement in maintaining cellular polarity. Loss of polarized morphology of EiHbox1 silenced parasites leads to altered motility from fast, directionally persistent, and highly chemotactic to slow, random, and less chemotactic, which subsequently leads to defective aggregation during encystation. EiHbox1 knockdown also resulted in a significant reduction in phagocytic capacity and poor capping response. These findings highlight the importance of EiHbox1 of E. invadens in governing cellular processes crucial for their survival, pathogenicity, and evasion of the host immune system.

Hemoglobin uptake and utilization by human protozoan parasites: a review

The protozoan disease is a major global health concern. Amoebiasis, leishmaniasis, Chagas disease, and African sleeping sickness affect several million people worldwide, leading to millions of deaths annually and immense social and economic problems. Iron is an essential nutrient for nearly all microbes, including invading pathogens. The majority of iron in mammalian hosts is stored intracellularly in proteins, such as ferritin and hemoglobin (Hb). Hb, present in blood erythrocytes, is a very important source of iron and amino acids for pathogenic microorganisms ranging from bacteria to eukaryotic pathogens, such as worms, protozoa, yeast, and fungi. These organisms have developed adequate mechanisms to obtain Hb or its byproducts (heme and globin) from the host. One of the major virulence factors identified in parasites is parasite-derived proteases, essential for host tissue degradation, immune evasion, and nutrient acquisition. The production of Hb-degrading proteases is a Hb uptake mechanism that degrades globin in amino acids and facilitates heme release. This review aims to provide an overview of the Hb and heme-uptake mechanisms utilized by human pathogenic protozoa to survive inside the host.

Low molecular weight protein tyrosine phosphatase (LMW-PTP2) protein can potentially modulate virulence of the parasite Entamoeba histolytica

The Entamoeba histolytica parasite is the causative agent of amebiasis, infecting approximately 1% of the world population and causing 100,000 deaths per year. It binds to Fibronectin (FN), activating signaling pathways regulated by kinases and phosphatases. EhLMW-PTPs genes from E. histolytica encode for Low Molecular Weight Tyrosine Phosphatases expressed in trophozoites and amoebic cysts. The role of these phosphatases in the virulence of the parasite has not yet been well characterized. Our results showed a differential expression of the EhLMW-PTPs, at the mRNA and protein levels, in an asynchronous trophozoites culture. Furthermore, we observed that trophozoites transfected that overexpressed EhLMW-PTP2 phagocytized fewer erythrocytes, possibly due to decreased phagocytic cups, and showed deficiencies in adherence to FN and less cytopathic effect. These analyzes suggest that the parasite's EhLMW-PTPs have an essential role in the mechanisms of proliferation, adhesion, and phagocytosis, regulating its pathogenicity.

Lactoferrin and Its Derived Peptides: An Alternative for Combating Virulence Mechanisms Developed by Pathogens

Due to the emergence of multidrug-resistant pathogens, it is necessary to develop options to fight infections caused by these agents. Lactoferrin (Lf) is a cationic nonheme multifunctional glycoprotein of the innate immune system of mammals that provides numerous benefits. Lf is bacteriostatic and/or bactericidal, can stimulate cell proliferation and differentiation, facilitate iron absorption, improve neural development and cognition, promote bone growth, prevent cancer and exert anti-inflammatory and immunoregulatory effects. Lactoferrin is present in colostrum and milk and is also produced by the secondary granules of polymorphonuclear leukocytes, which store this glycoprotein and release it at sites of infection. Lf is also present in many fluids and exocrine secretions, on the surfaces of the digestive, respiratory and reproductive systems that are commonly exposed to pathogens. Apo-Lf (an iron-free molecule) can be microbiostatic due to its ability to capture ferric iron, blocking the availability of host iron to pathogens. However, apo-Lf is mostly microbicidal via its interaction with the microbial surface, causing membrane damage and altering its permeability function. Lf can inhibit viral entry by binding to cell receptors or viral particles. Lf is also able to counter different important mechanisms evolved by microbial pathogens to infect and invade the host, such as adherence, colonization, invasion, production of biofilms and production of virulence factors such as proteases and toxins. Lf can also cause mitochondrial and caspase-dependent regulated cell death and apoptosis-like in pathogenic yeasts. All of these mechanisms are important targets for treatment with Lf. Holo-Lf (the iron-saturated molecule) can contain up to two ferric ions and can also be microbicidal against some pathogens. On the other hand, lactoferricins (Lfcins) are peptides derived from the N-terminus of Lf that are produced by proteolysis with pepsin under acidic conditions, and they cause similar effects on pathogens to those caused by the parental Lf. Synthetic analog peptides comprising the N-terminus Lf region similarly exhibit potent antimicrobial properties. Importantly, there are no reported pathogens that are resistant to Lf and Lfcins; in addition, Lf and Lfcins have shown a synergistic effect with antimicrobial and antiviral drugs. Due to the Lf properties being microbiostatic, microbicidal, anti-inflammatory and an immune modulator, it represents an excellent natural alternative either alone or as adjuvant in the combat to antibiotic multidrug-resistant bacteria and other pathogens. This review aimed to evaluate the data that appeared in the literature about the effects of Lf and its derived peptides on pathogenic bacteria, protozoa, fungi and viruses and how Lf and Lfcins inhibit the mechanisms developed by these pathogens to cause disease.

Uncovering the Cyclic AMP Signaling Pathway of the Protozoan Parasite Entamoeba histolytica and Understanding Its Role in Phagocytosis

Second messenger signaling controls a surprisingly diverse range of processes in several eukaryotic pathogens. Molecular machinery and pathways involving these messengers thus hold tremendous opportunities for therapeutic interventions. Relative to Ca²⁺ signaling, the knowledge of a crucial second messenger cyclic AMP (cAMP) and its signaling pathway is very scant in the intestinal parasite Entamoeba histolytica. In the current study, mining the available genomic resources, we have for the first time identified the cAMP signal transduction pathway of E. histolytica. Three heptahelical proteins with variable G-protein-coupled receptor domains, heterotrimeric G-proteins (Gα, Gβ, and Gγ subunits), soluble adenylyl cyclase, cyclase-associated protein, and enzyme carbonic anhydrase were identified in its genome. We could also identify several putative candidate genes for cAMP downstream effectors such as protein kinase A, A-kinase anchoring proteins, and exchange protein directly activated by the cAMP pathway. Using specific inhibitors against key identified targets, we could observe changes in the intracellular cAMP levels as well as defect in the rate of phagocytosis of red blood cells by the parasite E. histolytica. We thus strongly believe that characterization of some of these unexplored crucial signaling determinants will provide a paradigm shift in understanding the pathogenicity of this organism.

Differential expression of pathogenic genes of Entamoeba histolytica vs E. dispar in a model of infection using human liver tissue explants

We sought to establish an ex vivo model for examining the interaction of E. histolytica with human tissue, using precision-cut liver slices (PCLS) from donated organs. E. histolytica- or E. dispar-infected PCLS were analyzed at different post-infection times (0, 1, 3, 24 and 48 h) to evaluate the relation between tissue damage and the expression of genes associated with three factors: a) parasite survival (peroxiredoxin, superoxide dismutase and 70 kDa heat shock protein), b) parasite virulence (EhGal/GalNAc lectin, amoebapore, cysteine proteases and calreticulin), and c) the host inflammatory response (various cytokines). Unlike E. dispar (non-pathogenic), E. histolytica produced some damage to the structure of hepatic parenchyma. Overall, greater expression of virulence genes existed in E. histolytica-infected versus E. dispar-infected tissue. Accordingly, there was an increased expression of EhGal/GalNAc lectin, Ehap-a and Ehcp-5, Ehcp-2, ehcp-1 genes with E. histolytica, and a decreased or lack of expression of Ehcp-2, and Ehap-a genes with E. dispar. E. histolytica-infected tissue also exhibited an elevated expression of genes linked to survival, principally peroxiredoxin, superoxide dismutase and Ehhsp-70. Moreover, E. histolytica-infected tissue showed an overexpression of some genes encoding for pro-inflammatory interleukins (ILs), such as il-8, ifn-γ and tnf-α. Contrarily, E. dispar-infected tissue displayed higher levels of il-10, the gene for the corresponding anti-inflammatory cytokine. Additionally, other genes were investigated that are important in the host-parasite relationship, including those encoding for the 20 kDa heat shock protein (HSP-20), the AIG-1 protein, and immune dominant variable surface antigen, as well as for proteins apparently involved in mechanisms for the protection of the trophozoites in different environments (e.g., thioredoxin-reductase, oxido-reductase, and 9 hypothetical proteins). Some of the hypothetical proteins evidenced interesting overexpression rates, however we should wait to their characterization. This finding suggest that the present model could be advantageous for exploring the complex interaction between trophozoites and hepatocytes during the development of ALA, particularly in the initial stages of infection.

Overexpression of Differentially Expressed Genes Identified in Non-pathogenic and Pathogenic Entamoeba histolytica Clones Allow Identification of New Pathogenicity Factors Involved in Amoebic Liver Abscess Formation

We here compared pathogenic (p) and non-pathogenic (np) isolates of Entamoeba histolytica to identify molecules involved in the ability of this parasite to induce amoebic liver abscess (ALA)-like lesions in two rodent models for the disease. We performed a comprehensive analysis of 12 clones (A1–A12) derived from a non-pathogenic isolate HM-1:IMSS-A and 12 clones (B1–B12) derived from a pathogenic isolate HM-1:IMSS-B. “Non-pathogenicity” included the induction of small and quickly resolved lesions while “pathogenicity” comprised larger abscess development that overstayed day 7 post infection. All A-clones were designated as non-pathogenic, whereas 4 out of 12 B-clones lost their ability to induce ALAs in gerbils. No correlation between ALA formation and cysteine peptidase (CP) activity, haemolytic activity, erythrophagocytosis, motility or cytopathic activity was found. To identify the molecular framework underlying different pathogenic phenotypes, three clones were selected for in-depth transcriptome analyses. Comparison of a non-pathogenic clone A1^np with pathogenic clone B2^p revealed 76 differentially expressed genes, whereas comparison of a non-pathogenic clone B8^np with B2^p revealed only 19 differentially expressed genes. Only six genes were found to be similarly regulated in the two non-pathogenic clones A1^np and B8^np in comparison with the pathogenic clone B2^p. Based on these analyses, we chose 20 candidate genes and evaluated their roles in ALA formation using the respective gene-overexpressing transfectants. We conclude that different mechanisms lead to loss of pathogenicity. In total, we identified eight proteins, comprising a metallopeptidase, C2 domain proteins, alcohol dehydrogenases and hypothetical proteins, that affect the pathogenicity of E. histolytica.

The pathogen Entamoeba histolytica can live asymptomatically in the human gut, or it can disrupt the intestinal barrier and induce life-threatening abscesses in different organs, most often in the liver. The molecular framework that enables this invasive, highly pathogenic phenotype is still not well understood. In order to identify factors that are positively or negatively correlated for invasion and destruction of the liver, we used a unique tool, E. histolytica clones that differ dramatically in their pathogenicity, while sharing almost identical genetic background. Based on comprehensive transcriptome studies of these clones, we identified a set of candidate genes that are potentially involved in pathogenicity. Using ectopic overexpression of the most promising candidates, either in pathogenic or in non-pathogenic Entamoeba clones, we identified genes where high expression reduced pathogenicity and only one gene that increased pathogenicity to a certain extend.

Taken together, the current study identifies novel pathogenicity factors of E. histolytica and highlights the observation that various different genes contribute to pathogenicity.

Tyrosine phosphorylation/dephosphorylation of myosin II essential light chains of Entamoeba histolytica trophozoites regulates their motility

Entamoeba histolytica trophozoites dwell in the human intestine as comensals although under still unclear circumstances become invasive and destroy the host tissues. For these activities, trophozoites relay on remarkable motility provided by the cytoskeleton organization. Amebic actin and some of its actin-associated proteins are well known, while components of the myosin II molecule, although predicted from the E. histolytica genome, need biochemical and functional characterization. Recently, an amebic essential light myosin II chain, named EhMLCI, was identified and reported to be phosphorylated in tyrosines. The phosphorylated form of the protein was associated with the soluble assembly incompetent conformation of the heavy myosin chains, while the non-phosphorylated protein was identified with filamentous heavy chains, organized in an assembly competent conformation. It was postulated that EhMLCI tyrosine phosphorylation could act as a negative regulator of myosin II activity by its phosphorylation/dephosphorylation cycles. To test this hypothesis, we constructed an expression vector containing an EhMLCI DNA sequence where two tyrosine residues, with strong probability of phosphorylation and fall within the single EF-hand domain that interacts with the N-terminus of myosin II heavy chains, were replaced by phenylalanines. Transfected trophozoites, expressing the mutant MutEhMLCI protein cannot process it, thereby not incorporated into the phosphorylation/dephosphorylation cycles required for myosin II activity, results in motility defective trophozoites.

Acanthamoeba culbertsoni: Electron-Dense Granules in a Highly Virulent Clinical Isolate

The virulence of various amoebic parasites has been correlated with the presence of electron-dense granules (EDGs) in the cytoplasm of trophozoites. Here, we report the finding by transmission electron microscopy of a large number of EDGs in a recent culture of Acanthamoeba culbertsoni, isolated from a severe case of human keratitis. When this isolate was maintained in culture for 6 mo, the granules almost disappeared. However, after induction of mice brain lesions with the long-term cultured isolate, recovered amoebas had abundant EDGs. Trophozoites of the original isolate, or those recovered from experimental lesions, secreted EDGs into the medium when incubated with MDCK cells. To analyze a possible cytotoxic effect the conditioned medium was incubated with MDCK monolayers. After 5 h, the media containing EDGs produced opening of the tight junctions; at 24 h, cell viability was compromised, and at 48 h most of the cells were detached from the monolayer. In contrast, trophozoites in long-term cultures did not release EDGs to the medium during incubation with MDCK cells, and the corresponding conditioned medium did not have any effect on MDCK monolayers. Our observations further support the hypothesis that EDGs play a role in the cytopathogenic mechanisms of A. culbertsoni.

Binding and Endocytosis of Bovine Hololactoferrin by the Parasite Entamoeba histolytica

Entamoeba histolytica is a human parasite that requires iron (Fe) for its metabolic function and virulence. Bovine lactoferrin (B-Lf) and its peptides can be found in the digestive tract after dairy products are ingested. The aim of this study was to compare virulent trophozoites recently isolated from hamster liver abscesses with nonvirulent trophozoites maintained for more than 30 years in cultures in vitro regarding their interaction with iron-charged B-Lf (B-holo-Lf). We performed growth kinetics analyses of trophozoites in B-holo-Lf and throughout several consecutive transfers. The virulent parasites showed higher growth and tolerance to iron than nonvirulent parasites. Both amoeba variants specifically bound B-holo-Lf with a similar K_d. However, averages of 9.45 × 10⁵ and 6.65 × 10⁶ binding sites/cell were found for B-holo-Lf in nonvirulent and virulent amoebae, respectively. Virulent amoebae bound more efficiently to human and bovine holo-Lf, human holo-transferrin, and human and bovine hemoglobin than nonvirulent amoebae. Virulent amoebae showed two types of B-holo-Lf binding proteins. Although both amoebae endocytosed this glycoprotein through clathrin-coated vesicles, the virulent amoebae also endocytosed B-holo-Lf through a cholesterol-dependent mechanism. Both amoeba variants secreted cysteine proteases cleaving B-holo-Lf. These data demonstrate that the B-Lf endocytosis is more efficient in virulent amoebae.

Iron-Binding Protein Degradation by Cysteine Proteases of Naegleria fowleri

Naegleria fowleri causes acute and fulminant primary amoebic meningoencephalitis. This microorganism invades its host by penetrating the olfactory mucosa and then traveling up the mesaxonal spaces and crossing the cribriform plate; finally, the trophozoites invade the olfactory bulbs. During its invasion, the protozoan obtains nutrients such as proteins, lipids, carbohydrates, and cationic ions (e.g., iron, calcium, and sodium) from the host. However, the mechanism by which these ions are obtained, particularly iron, is poorly understood. In the present study, we evaluated the ability of N. fowleri to degrade iron-binding proteins, including hololactoferrin, transferrin, ferritin, and hemoglobin. Zymography assays were performed for each substrate under physiological conditions (pH 7 at 37°C) employing conditioned medium (CM) and total crude extracts (TCEs) of N. fowleri. Different degradation patterns with CM were observed for hololactoferrin, transferrin, and hemoglobin; however, CM did not cause ferritin degradation. In contrast, the TCEs degraded only hololactoferrin and transferrin. Inhibition assays revealed that cysteine proteases were involved in this process. Based on these results, we suggest that CM and TCEs of N. fowleri degrade iron-binding proteins by employing cysteine proteases, which enables the parasite to obtain iron to survive while invading the central nervous system.

Erythrophagocytosis in Entamoeba histolytica and Entamoeba dispar: a comparative study

Entamoeba histolytica is the causative agent of human intestinal and liver amebiasis. The extraordinary phagocytic activity of E. histolytica trophozoites has been accepted as one of the virulence mechanisms responsible for their invasive capacity. The recognition of the noninvasive Entamoeba dispar as a different species has raised the question as to whether the lack of pathogenic potential of this ameba correlates with a limited phagocytic capacity. We have therefore compared the process of erythrophagocytosis in both species by means of light and video microscopy, hemoglobin measurement, and the estimation of reactive oxygen species (ROS). In the present study, we confirmed that E. dispar has lower erythrophagocytic capacity. We also observed by video microscopy a new event of erythrocyte opsonization-like in both species, being more characteristic in E. histolytica. Moreover, E. dispar showed a lower capacity to produce ROS compared with the invasive species and also showed a large population of amoebae that did not engulf any erythrocyte over time. Our results demonstrate that E. histolytica has a higher phagocytic capacity than E. dispar, including a higher rate of production of ROS in the course of ingesting red blood cells.

Peroxynitrite and peroxiredoxin in the pathogenesis of experimental amebic liver abscess

The molecular mechanisms by which Entamoeba histolytica causes amebic liver abscess (ALA) are still not fully understood. Amebic mechanisms of adherence and cytotoxic activity are pivotal for amebic survival but apparently do not directly cause liver abscess. Abundant evidence indicates that chronic inflammation (resulting from an inadequate immune response) is probably the main cause of ALA. Reports referring to inflammatory mechanisms of liver damage mention a repertoire of toxic molecules by the immune response (especially nitric oxide and reactive oxygen intermediates) and cytotoxic substances released by neutrophils and macrophages after being lysed by amoebas (e.g., defensins, complement, and proteases). Nevertheless, recent evidence downplays these mechanisms in abscess formation and emphasizes the importance of peroxynitrite (ONOO⁻). It seems that the defense mechanism of amoebas against ONOO⁻, namely, the amebic thioredoxin system (including peroxiredoxin), is superior to that of mammals. The aim of the present text is to define the importance of ONOO⁻ as the main agent of liver abscess formation during amebic invasion, and to explain the superior capacity of amoebas to defend themselves against this toxic agent through the peroxiredoxin and thioredoxin system.

Entamoeba histolytica: gene expression analysis of cells invading tissues

Entamoeba histolytica is a protozoan parasite that presents a risk to the health of millions of people worldwide. Due to the existence of different clinical forms caused by the parasite and also different virulence levels presented by one strain, one would expect differences in the profile of gene transcripts between virulent and nonvirulent cultures. In this study we used the differential display to select gene segments related to invasiveness of amoeba. One Brazilian strain of E. histolytica in two conditions, able or not to cause lesions in experimental animals, was used. RNA from this strain, was used to study the differential expression of genes. 29 specific gene fragments differentially expressed in the virulent strain were selected. By real-time PCR, six of these genes had confirmed their differential expression in the virulent culture. These genes may have important roles in triggering invasive amoebiasis and may be related to adaptation of trophozoites to difficulties encountered during colonization of the intestinal epithelium and liver tissue. Future studies with these genes may elucidate its actual role in tissue invasion by E. histolytica generating new pathways for diagnosis and treatment of amoebiasis.

Different clinical outcomes of Entamoeba histolytica in Malaysia: does genetic diversity exist?

The present study was conducted to investigate the clinical outcomes of Entamoeba histolytica infection in symptomatic and asymptomatic Orang Asli (aborigine) communities in Malaysia. Examination was performed on 500 stool samples obtained from Orang Asli communities in 3 different states using formalin-ether concentration, trichrome staining, and single-round PCR techniques. Out of 500 stool samples, single infection of E. histolytica, Entamoeba dispar, and Entamoeba moshkovskii was identified in 3.2%, 13.4%, and 1%, respectively. In addition, 10 samples had mixed infections with E. histolytica and E. dispar. Six samples containing E. dispar were also positive for E. moshkovskii, and only 2 samples had E. histolytica in association with E. dispar and E. moshkovskii. Seventeen E. histolytica-positive samples were from symptomatic subjects, whereas the remaining 11 samples came from asymptomatic subjects. These findings suggest a predominant distribution of pathogenic potential of E. histolytica strains in this community. Therefore, further studies on genotyping of E. histolytica is required, to find out association between E. histolytica genotype and the outcome of the infection.

Cysteine protease-binding protein family 6 mediates the trafficking of amylases to phagosomes in the enteric protozoan Entamoeba histolytica

Phagocytosis plays a pivotal role in nutrient acquisition and evasion from the host defense systems in Entamoeba histolytica, the intestinal protozoan parasite that causes amoebiasis. We previously reported that E. histolytica possesses a unique class of a hydrolase receptor family, designated the cysteine protease-binding protein family (CPBF), that is involved in trafficking of hydrolases to lysosomes and phagosomes, and we have also reported that CPBF1 and CPBF8 bind to cysteine proteases or β-hexosaminidase α-subunit and lysozymes, respectively. In this study, we showed by immunoprecipitation that CPBF6, one of the most highly expressed CPBF proteins, specifically binds to α-amylase and γ-amylase. We also found that CPBF6 is localized in lysosomes, based on immunofluorescence imaging. Immunoblot and proteome analyses of the isolated phagosomes showed that CPBF6 mediates transport of amylases to phagosomes. We also demonstrated that the carboxyl-terminal cytosolic region of CPBF6 is engaged in the regulation of the trafficking of CPBF6 to phagosomes. Our proteome analysis of phagosomes also revealed new potential phagosomal proteins.

Proteases from Entamoeba spp. and Pathogenic Free-Living Amoebae as Virulence Factors

The standard reference for pathogenic and nonpathogenic amoebae is the human parasite Entamoeba histolytica; a direct correlation between virulence and protease expression has been demonstrated for this amoeba. Traditionally, proteases are considered virulence factors, including those that produce cytopathic effects in the host or that have been implicated in manipulating the immune response. Here, we expand the scope to other amoebae, including less-pathogenic Entamoeba species and highly pathogenic free-living amoebae. In this paper, proteases that affect mucin, extracellular matrix, immune system components, and diverse tissues and cells are included, based on studies in amoebic cultures and animal models. We also include proteases used by amoebae to degrade iron-containing proteins because iron scavenger capacity is currently considered a virulence factor for pathogens. In addition, proteases that have a role in adhesion and encystation, which are essential for establishing and transmitting infection, are discussed. The study of proteases and their specific inhibitors is relevant to the search for new therapeutic targets and to increase the power of drugs used to treat the diseases caused by these complex microorganisms.

The Calmodulin-like calcium binding protein EhCaBP3 of Entamoeba histolytica regulates phagocytosis and is involved in actin dynamics

Phagocytosis is required for proliferation and pathogenesis of Entamoeba histolytica and erythrophagocytosis is considered to be a marker of invasive amoebiasis. Ca²⁺ has been found to play a central role in the process of phagocytosis. However, the molecular mechanisms and the signalling mediated by Ca²⁺ still remain largely unknown. Here we show that Calmodulin-like calcium binding protein EhCaBP3 of E. histolytica is directly involved in disease pathomechanism by its capacity to participate in cytoskeleton dynamics and scission machinery during erythrophagocytosis. Using imaging techniques EhCaBP3 was found in phagocytic cups and newly formed phagosomes along with actin and myosin IB. In vitro studies confirmed that EhCaBP3 directly binds actin, and affected both its polymerization and bundling activity. Moreover, it also binds myosin 1B in the presence of Ca²⁺. In cells where EhCaBP3 expression was down regulated by antisense RNA, the level of RBC uptake was reduced, myosin IB was found to be absent at the site of pseudopod cup closure and the time taken for phagocytosis increased, suggesting that EhCaBP3 along with myosin 1B mediate the closure of phagocytic cups. Experiments with EhCaBP3 mutant defective in Ca²⁺ -binding showed that Ca²⁺ binding is required for phagosome formation. Liposome binding assay revealed that EhCaBP3 recruitment and enrichment to membrane is independent of any cellular protein as it binds directly to phosphatidylserine. Taken together, our results suggest a novel pathway mediating phagocytosis in E. histolytica, and an unusual mechanism of modulation of cytoskeleton dynamics by two calcium binding proteins, EhCaBP1 and EhCaBP3 with mostly non-overlapping functions.

Entamoeba histolytica is one of the major causes of morbidity and mortality in developing countries. Phagocytosis plays an important role in both survival and virulence and has been used as a virulence marker. Inhibition of phagocytosis leads to a defect in cellular proliferation. Therefore, the molecules that participate in phagocytosis are good targets for developing new drugs. However, the molecular mechanism of the process is still largely unknown. Here, we demonstrate that Calmodulin-like calcium binding protein EhCaBP3 is involved in erythrophagocytosis. We show this by a number of different approaches including immunostaining of actin, myosin1B, EhCaBP1 and EhCaBP3 during uptake of RBC; over expression and down regulation of EhCaBP3, and over expression of calcium defective mutant of EhCaBP3. Our analysis suggests that EhCaBP3 can regulate actin dynamics. Along with actin and myosin 1B it can participate in both initiation and formation of phagosomes. The Ca²⁺-bound form of this protein is required only for progression from cups into early phagosomes but not for initiation. Our results demonstrate the complex role of Ca²⁺ binding proteins, EhCaBP1 and EhCaBP3 in regulation of phagocytosis in the protist parasite E. histolytica and the novel mechanisms of manipulating actin dynamics at multiple levels.

Host-parasite interaction: parasite-derived and -induced proteases that degrade human extracellular matrix

Parasitic protozoa are among the most important pathogens worldwide. Diseases such as malaria, leishmaniasis, amoebiasis, giardiasis, trichomoniasis, and trypanosomiasis affect millions of people. Humans are constantly threatened by infections caused by these pathogens. Parasites engage a plethora of surface and secreted molecules to attach to and enter mammalian cells. The secretion of lytic enzymes by parasites into host organs mediates critical interactions because of the invasion and destruction of interstitial tissues, enabling parasite migration to other sites within the hosts. Extracellular matrix is a complex, cross-linked structure that holds cells together in an organized assembly and that forms the basement membrane lining (basal lamina). The extracellular matrix represents a major barrier to parasites. Therefore, the evolution of mechanisms for connective-tissue degradation may be of great importance for parasite survival. Recent advances have been achieved in our understanding of the biochemistry and molecular biology of proteases from parasitic protozoa. The focus of this paper is to discuss the role of protozoan parasitic proteases in the degradation of host ECM proteins and the participation of these molecules as virulence factors. We divide the paper into two sections, extracellular and intracellular protozoa.

Novel transmembrane receptor involved in phagosome transport of lysozymes and β-hexosaminidase in the enteric protozoan Entamoeba histolytica

Lysozymes and hexosaminidases are ubiquitous hydrolases in bacteria and eukaryotes. In phagocytic lower eukaryotes and professional phagocytes from higher eukaryotes, they are involved in the degradation of ingested bacteria in phagosomes. In Entamoeba histolytica, which is the intestinal protozoan parasite that causes amoebiasis, phagocytosis plays a pivotal role in the nutrient acquisition and the evasion from the host defense systems. While the content of phagosomes and biochemical and physiological roles of the major phagosomal proteins have been established in E. histolytica, the mechanisms of trafficking of these phagosomal proteins, in general, remain largely unknown. In this study, we identified and characterized for the first time the putative receptor/carrier involved in the transport of the above-mentioned hydrolases to phagosomes. We have shown that the receptor, designated as cysteine protease binding protein family 8 (CPBF8), is localized in lysosomes and mediates transport of lysozymes and β-hexosaminidase α-subunit to phagosomes when the amoeba ingests mammalian cells or Gram-positive bacillus Clostridium perfringens. We have also shown that the binding of CPBF8 to the cargos is mediated by the serine-rich domain, more specifically three serine residues of the domain, which likely contains trifluoroacetic acid-sensitive O-phosphodiester-linked glycan modifications, of CPBF8. We further showed that the repression of CPBF8 by gene silencing reduced the lysozyme and β-hexosaminidase activity in phagosomes and delayed the degradation of C. perfringens. Repression of CPBF8 also resulted in decrease in the cytopathy against the mammalian cells, suggesting that CPBF8 may also be involved in, besides the degradation of ingested bacteria, the pathogenesis against the mammalian hosts. This work represents the first case of the identification of a transport receptor of hydrolytic enzymes responsible for the degradation of microorganisms in phagosomes.

Phagocytosis is the cellular process of engulfing solid particles to form an internal phagosome in protozoa, algae, and professional phagocytes of multicellular eukaryotic organisms. In phagocytic protozoa, phagocytosis is involved in the acquisition of nutrients, and the evasion from the host immune system and inflammation. While hydrolytic enzymes that are essential for the efficient and regulated degradation of phagocytosed particles, such as bacteria, fungi, and eukaryotic organisms, have been characterized, the mechanisms of the transport of these proteins are poorly understood. In the present study, we have demonstrated, for the first time, the molecular mechanisms of how the digestive enzymes are transported to phagosomes. Understanding of such mechanisms of the transport of phagosomal proteins at the molecular level may lead to the identification of a novel target for the development of new preventive measures against parasitic infections caused by phagocytic protozoa.

Effect of bovine lactoferrin in a therapeutic hamster model of hepatic amoebiasis

Entamoeba histolytica is the causative agent of amoebiasis, a disease that produces dysentery as a result of the perforation of the large intestine. This parasite often invades other organs, primarily the liver, leading to an amoebic liver abscess (ALA), which can cause death. Metronidazole is the drug of choice for the treatment of ALA; however, it produces toxic side effects in patients. Lactoferrin (Lf) is a glycoprotein of the innate immune response that sequesters iron in the mucosae. Lf possesses immune-regulatory properties, such as antiinflammatory and antioxidant activities. Moreover, the microbicidal activity of apoLf, which lacks bound iron, has been shown. In this study, we evaluated the therapeutic effect of bovine Lf (bLf) against ALA in a model of hepatic amoebiasis in hamsters. Interestingly, hamsters treated intragastrically with Lf (2.5 mg/100 g mass) over a period of 8 days showed no clinical signs of disease and ALA was effectively decreased, with only 0.63% detectable lesion, compared with 63% in untreated animals. Furthermore, liver function and blood cells approached normal levels among those receiving bLf treatment. These results suggest that bLf may aid in the therapy of amoebiasis, likely without producing undesirable effects in patients.

A C2 domain protein kinase initiates phagocytosis in the protozoan parasite Entamoeba histolytica

Phagocytosis is a process whereby particles are taken in by cells through mechanisms superficially similar to those for endocytosis. It serves a wide range of functions, from providing nutrition in unicellular organisms to initiation of both innate and adaptive immunity in vertebrates. In the protozoan parasite Entamoeba histolytica, it has an essential role in survival and pathogenesis. In this study, we show that EhC2PK, a C2-domain-containing protein kinase, and the Ca²(+) and actin-binding protein, EhCaBP1, are involved in the initiation of phagocytosis in E. histolytica. Conditional suppression of EhC2PK expression and overexpression of a mutant form reveals its role in the initiation of phagocytic cups. EhC2PK binds phosphatidylserine in the presence of Ca²(+) and thereby recruits EhCaBP1 and actin to the membrane. Identification of these proteins in phagocytosis is an important step in amoebic biology and these molecules could be the important targets for developing novel therapies against amoebiasis.

Effect of phosphatidylcholine-cholesterol liposomes on Entamoeba histolytica virulence

Trophozoites of Entamoeba histolytica HM-1:IMSS become less virulent after long-term maintenance in axenic cultures. The factors responsible for the loss of virulence during in vitro cultivation remain unclear. However, it is known that in vitro cultivation of amoeba in culture medium supplemented with cholesterol restores their virulence. In this study, we analyzed the effect of adding phosphatidylcholine-cholesterol (PC-Chol) liposomes to the culture medium and evaluated the effect of this lipid on various biochemical and biological functions of E. histolytica HM-1:IMSS in terms of its virulence. The addition of PC-Chol liposomes to the culture medium maintained the virulence of these parasites against hamster liver at the same level as the original virulent E. histolytica strain, even though these amoebae were maintained without passage through hamster liver for 18 months. The trophozoites also showed increased endocytosis, erythrophagocytosis, and carbohydrate residue expression on the amoebic surface. Protease activities were also modified by the presence of cholesterol in the culture medium. These findings indicate the capacity of cholesterol to preserve amoeba virulence and provide an alternative method for the maintenance of virulent E. histolytica trophozoites without the need for in vivo procedures.

Calmodulin-like protein from Entamoeba histolytica: solution structure and calcium-binding properties of a partially folded protein

The mechanism of Ca(2+)-signaling in the protozoan parasite Entamoeba histolytica is yet to be understood as many of the key regulators are still to be identified. E. histolytica encodes a number of multi-EF-hand Ca(2+)-binding proteins (EhCaBPs). Functionally only one of these molecules, EhCaBP1, has been characterized to date. The calmodulin-like protein from E. histolytica (abbreviated as EhCaM or EhCaBP3) is a 17.23 kDa monomeric protein that shows maximum sequence identity with heterologous calmodulins (CaMs). Though CaM activity has been biochemically shown in E. histolytica, there are no reports on the presence of a typical CaM. In an attempt to understand the structural and functional similarity of EhCaM with CaM, we have determined the three-dimensional (3D) solution structure of EhCaM using NMR. The EhCaM has a well-folded N-terminal domain and an unstructured C-terminal counterpart. Further, it sequentially binds only two calcium ions, an unusual mode of Ca(2+)-binding among the known CaBPs, notably both in the N-terminal domain of EhCaM. Further, EhCaM is present in the nucleus in addition to the cytoplasm as detected by immunofluorescence staining, unlike other EhCaBPs that are detected only in the cytoplasm. Therefore, this protein is likely to have a different function. The presence of unusual and a diverse set of CaBPs in E. histolytica suggests a distinct Ca(2+)-signaling process in E. histolytica. The results reported here help in understanding the structure-function relationship of CaBPs including their Ca(2+)-binding properties.

Electron microscopy and cytochemistry analysis of the endocytic pathway of pathogenic protozoa

Endocytosis is essential for eukaryotic cell survival and has been well characterized in mammal and yeast cells. Among protozoa it is also important for evading from host immune defenses and to support intense proliferation characteristic of some life cycle stages. Here we focused on the contribution of morphological and cytochemical studies to the understanding of endocytosis in Trichomonas, Giardia, Entamoeba, Plasmodium, and trypanosomatids, mainly Trypanosoma cruzi, and also Trypanosoma brucei and Leishmania.

Structural characterization of a novel Ca2+-binding protein from Entamoeba histolytica: structural basis for the observed functional differences with its isoform

A novel Ca(2+)-binding protein (EhCaBP2) was identified from the protozoan parasite Entamoeba histolytica. EhCaBP2 has 79% sequence identity with calcium-binding protein EhCaBP1. The 3D structure of EhCaBP2 was determined using multidimensional nuclear magnetic resonance spectroscopic techniques. The study reveals that the protein consists of two globular domains connected by a short flexible linker region of four residues. On comparison of the 3D structure and dynamics of EhCaBP2 with those of EhCaBP1, it is found that they vary significantly in their N-terminal domains and interdomain linker. Immunofluorescence localization experiments revealed that EhCaBP1 and EhCaBP2 may not carry out similar functions, as their cellular distribution patterns are not the same. The functional differences between the two isoforms are explained on the basis of results obtained from the structural studies. The structural variation in the interdomain linker region and the formation of functionally important hydrophobic clefts in different regions of EhCaBP1 and EhCaBP2 provide interesting insights into the differences in the functionality of these two isoforms.

Entamoeba histolytica: effect on virulence, growth and gene expression in response to monoxenic culture with Escherichia coli 055

Monoxenic cultivation of pathogenic Entamoeba histolytica trophozoites with Escherichia coli serotype 055 which binds strongly to the Gal/GalNAc amoebic lectin, markedly improved the growth of E. histolytica and produced a significant decrease in cysteine proteinase activity and a lower cytopathic activity on monolayer cells after 3 months of monoxenic culture. However, after long term monoxenic culture (12 months) the proteolytic and cytopathic activities were recovered and the amoebic growth reached the maximum yield. Employing the GeneFishing(R) technology and DNA macroarrays we detected differentially gene expression related to the amoebic interaction with bacteria. A number of differentially expressed genes encoding metabolic enzymes, ribosomal proteins, virulence factors and proteins related with cytoskeletal and vesicle trafficking were found. These results suggest that E. coli 055 has a nutritional role that strongly supports the amoebic growth, and is also able to modulate some biological activities related with amoebic virulence.

Transcriptional upregulation of genes related to virulence activation in Entamoeba histolytica

BACKGROUND

To understand the molecular basis of virulence variability in Entamoeba histolytica, this study presents results about differential gene expression induced by E. histolytica trophozoites in liver of hamsters in order to produce experimental amebic liver abscess (ALA) and consequently reactivate its virulence.

METHODS

Amebic cultures were studied before (BALA) and after (AALA) inoculation in hamster peritoneal cavity. Markers of pathogenicity such as the rate of erythrophagocytosis, hemolytic activity, and cytotoxic effects on MDCK cell monolayers were evaluated in order to correlate these phenotypic characteristics to differential gene expression between virulent and non-virulent strains. Genotypic variability was determined by genetic polymorphism using the random-amplified polymorphic DNA (RAPD) technique, which defines the parasite genomic plasticity. mRNA differential display was used in order to identify variable transcripts levels.

RESULTS

The rate of erythrophagocytosis and hemolytic activity were notably increased in AALA in comparison with BALA E. histolytica cultures, as well as the cytotoxic effect on MDCK cells. An increment in the transcription level of several mRNA was shown.

CONCLUSIONS

The RAPD technique allowed us to confirm differences in number and size of polymorphic markers bands between virulent and non-virulent stages, suggesting genomic adaptability in E. histolytica. Eight different genes (membrane-bound acid phosphatase, cysteine proteinase, two different ribosomal proteins, heat shock transcription factor, ribosomal RNA, aldehyde dehydrogenase-1 and patatin-like phospholipase) were sequenced and may be associated with a biological function related to the virulence of E. histolytica. Together these findings show genomic variability between virulent and non-virulent cultures of E. histolytica.

New insights into molecular mechanisms of phagocytosis in Entamoeba histolytica by proteomic analysis

The protozoan parasite Entamoeba histolytica ingests microorganisms and mammalian cells. Phagocytosis is essential for cell growth and is implicated in pathogenesis of E. histolytica. Phagocytosis consists of a number of steps including recognition of and binding to ligands on the target cells via a galactose/N-acetylgalactosamine-specific lectin, activation of a signaling pathway leading to cytoskeletal reorganization, and vesicle trafficking, all of which play distinct but coordinated roles in phagocytosis. Recent studies of proteomic analysis of purified phagosomes or affinity-purified Gal/GalNAc-binding proteins using reversed phase capillary liquid chromatography and ion trap tandem mass spectrometry enabled high throughput identification of proteins involved in phagosome biogenesis. These studies provided a list of proteins involved in the pathway and also shed light on the dynamic process of phagosome maturation. These approaches should provide significant insights into molecular mechanisms of phagosome biogenesis and help to elucidate the pathogenesis of this important parasite.

Entamoeba histolytica: differences in phagosome acidification and degradation between attenuated and virulent strains

Phagocytosis is the important virulent determinant of the enteric protozoan parasite Entamoeba histolytica. We compared the kinetics of phagosome maturation of attenuated and highly-virulent strains of E. histolytica using video microscopy. Phagosomes of attenuated strains were acidified rapidly within 2 min after phagosome formation (at the rate of 0.96 pH/min), persisted at pH 4.46+/-0.13, and degraded ingested GFP-Leishmania very efficiently (90-94% GFP fluorescence was lost in 30 min), while phagosomes of highly-virulent strains were acidified slowly (0.69 pH/min), persisted at 5.11+/-0.23, and degraded GFP less efficiently (60-71% decrease). These results suggest that efficiency of phagosome maturation is most probably inversely correlated with apparent virulence.

Kinetics and strain variation of phagosome proteins of Entamoeba histolytica by proteomic analysis

The protozoan parasite Entamoeba histolytica ingests and feeds on microorganisms and mammalian cells. Phagocytosis is essential for cell growth and implicated in pathogenesis of E. histolytica. We report here the dynamic changes of phagosome proteins during phagosome maturation by proteomic analysis using reversed-phase capillary liquid chromatography and ion trap tandem mass spectrometry. Phagosomes were isolated at various intervals after internalization of latex beads. Immunoblot analysis and electron microscopy verified successful isolation of phagosomes. A total of 159 proteins were identified from the reference strain HM1 at different stages of phagosome maturation. Approximately 70% of them were detected in a time-dependent fashion, suggesting dynamism of phagosome biogenesis. The kinetics of representative proteins were verified by immunoblots and also by video microscopy of live transgenic amebae expressing green fluorescent protein-fused EhRab7A. Furthermore, we observed significant differences in phagosome profiles between HM1 and two recent clinical isolates. Approximately 60% of 229 proteins detected in at least one of these three strains were identified only in one strain, while approximately 20% of these proteins were detected in all three strains. These data should provide significant insights into molecular characterization of phagosome biogenesis, and help to elucidate the pathogenesis of this important infection.

Proteomic analysis of phagocytosis in the enteric protozoan parasite Entamoeba histolytica

Proteomic analysis of phagosomes isolated from Entamoeba histolytica by liquid chromatography and mass spectrometry identified 85 proteins involved in surface recognition, actin cytoskeleton rearrangement, vesicular trafficking, and degradation. Phagosome localization of representative proteins was verified by immunofluorescence assay. This study should provide a basis for molecular identification and characterization of phagosome biogenesis.

Calcium binding protein 1 of the protozoan parasite Entamoeba histolytica interacts with actin and is involved in cytoskeleton dynamics

Blocking expression of EhCaBP1, a calmodulin-like, four EF-hand protein from the protozoan parasite Entamoeba histolytica, resulted in inhibition of cellular proliferation. In this paper we report that EhCaBP1 is involved in dynamic changes of the actin cytoskeleton. Both endocytosis and phagocytosis were severely impaired in cells where EhCaBP1 expression was blocked by inducible expression of the antisense RNA. In wild-type cells both actin and EhCaBP1 were found to co-localize in phagocytic cups and in pseudopods. However, in antisense-blocked cells the phagocytic cup formation is affected. Analysis of the staining patterns in the presence and absence of actin dynamics inhibitors, jasplakinolide and cytochalasin D suggested that EhCaBP1 and polymerized F-actin co-localize on membrane protrusions. Direct interaction between soluble EhCaBP1 and F-actin was further demonstrated by a co-sedimentation assay. A variant of EhCaBP1 did not bind F-actin showing the specificity of the interaction between EhCaBP1 and actin. There is no significant change in the kinetics of in vitro polymerization of actin in presence of EhCaBP1, indicating that EhCaBP1 does not affect filament treadmilling. In addition, using atomic force microscopy; it was found that filaments of F-actin, polymerized in presence of EhCaBP1, were thinner. These results indicate that EhCaBP1 may be involved in dynamic membrane restructuring at the time of cell pseudopod formation, phagocytosis and endocytosis in a process mediated by direct binding of EhCaBP1 to actin, affecting the bundling of actin filaments.

Entamoeba histolytica and Entamoeba dispar: prevalence infection in a rural Mexican community

The frequency of Entamoeba histolytica and Entamoeba dispar infection was analyzed in a rural community in the state of Morelos, Mexico, through PCR technique by using specie specific primer. The E. histolytica specie was detected in 33 of 290 analyzed stool samples (11.4%), E. dispar specie was observed in 21 samples (7.2%) and both species of Entamoeba were detected in seven samples (2.4%). So a higher E. histolytica than E. dispar frequency infection was detected (13.8 versus 9.6%). Even though in our design we did not considered the follow-up of included individuals, the absence of invasive amebiasis cases in the studied population during our stay in town was unexpected.

Antiamoebic and toxicity studies of a carbamic acid derivative and its therapeutic effect in a hamster model of hepatic amoebiasis

Amoebiasis is an important public health problem in developing countries. Entamoeba histolytica, the causative agent of amoebiasis, may develop resistance to nitroimidazoles, a group of drugs considered to be the most effective against this parasitic disease. Therefore, research on new drugs for the treatment of this common infection still constitutes an important therapeutic demand. In the present study we determined the effects of a carbamate derivative, ethyl 4-chlorophenylcarbamate (C4), on trophozoites of E. histolytica strain HM-1:IMSS. C4 was subject to various toxicity tests, including the determination of mutagenicity for bacterial DNA and changes in the enzymatic activities of eukaryotic cells. Genotoxicity studies were performed by the mutagenicity Ames test (plate incorporation and preincubation methods) with Salmonella enterica serovar Typhimurium, with or without metabolic activation produced by the S9 fraction of rat liver. C4 toxicity studies were performed by measuring enzymatic activity in eukaryotic cells by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide-formazan test with Fischer 344 rat hepatocytes. C4 did not induce either frame-shift mutations in S. enterica serovar Typhimurium TA97 or TA98 or base pair substitutions in strains TA100 and TA102. The compound was not toxic for cultured rat hepatic cells. Trophozoites treated with 100 microg of C4 per ml were inhibited 97.88% at 48 h of culture; moreover, damage to the amoebae was also confirmed by electron microscopy. The antiamoebic activity of C4 was evaluated by using an in vivo model of amoebic liver abscess in hamsters. Doses of 75 and 100 mg/100 g of body weight reduced the extent of the amoebic liver abscess by 84 and 94%, respectively. These results justify further studies to clearly validate whether C4 is a new suitable antiamoebic drug.

Entamoeba histolytica: cDNAs cloned as 30kDa collagen-binding proteins (CBP) belong to an antioxidant molecule family. Protection of hamsters from amoebic liver abscess by immunization with recombinant CBP

A cDNA expression library of Entamoeba histolytica was screened with antiserum to native amoebic collagen binding proteins (CBPs), and two clones C13 and C7 which partially encode for the 30 kDa CBP were obtained. The sequenced clones were 90% homologous. C7 had a 69 bp deletion at the 5' end that is present in C13 and encodes for a Glu-Cys-Lys rich region and a four amino acids repeat (Glu-Lys-Glu-Cys). Purified fusion proteins from these cDNA clones were able to bind native type I collagen gels in a pH, calcium, ionic strength, and temperature dependent way. The binding of pgtC13 to collagen gel was time and temperature stable, while pgtC7 binding was not, suggesting that the deleted region in C7 is important for the binding. The clones reported here partially encode a 30 kDa CBP that also belong to an antioxidant molecule family. We demonstrated that the fusion protein pgtC13 is immunogenic and partially protective as a subunit vaccine in the hamster model of amoebic liver abscess.

Entamoeba histolytica: a beta 1 integrin-like fibronectin receptor assembles a signaling complex similar to those of mammalian cells

During tissue invasion, Entamoeba histolytica trophozoites interact with endothelial cells and extracellular matrix (ECM) proteins such as fibronectin (FN), collagen, and laminin. It has been demonstrated that trophozoites interact with FN through a beta1 integrin-like FN receptor (beta 1EhFNR), activating tyrosine kinases. In order to characterize the signaling process triggered by the amoebic receptor, activation, and association of tyrosine kinases and structural proteins were determined. As a result of FN binding by the beta 1EhFNR, the receptor itself, FAK, and paxillin were phosphorylated in tyrosine. Co-immunoprecipitation experiments showed that a multimolecular signaling complex was formed by the amoebic FN receptor, FAK, paxillin, and vinculin. These results strongly suggest that a signaling pathway, similar to the one used in mammalian cells, is activated when E. histolytica trophozoites adhere to FN.

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.

New insights into the role of the cytoskeleton in phagocytosis of Entamoeba histolytica

Entamoeba histolytica, a human parasite, crosses the natural barriers of the intestine and, in turn, spreads into the deeper organs, resulting in amoebiasis. The motility of the parasite and its ability to lyse or phagocytose human cells facilitates passage of the amoeba through the intestinal epithelium. Little is known about the uptake of material by this parasite; nevertheless, the cytoskeleton is believed to play a role in phagocytosis. Myosin IB, an actin-binding protein, localizes to the phagocytic cup and, with time, surrounds the internalized phagosome itself. The role of unconventional myosins in phagocytosis has also been demonstrated in other cell types, suggesting that this molecular mechanism is a common denominator in phagocytic events. Here, we summarize the emerging view of the role of unconventional myosins as well as other cytoskeleton-associated proteins in pseudopod formation at early stages of phagocytosis and during the late step of this process in E. histolytica.

Antisense inhibition of expression of cysteine proteinases affects Entamoeba histolytica-induced formation of liver abscess in hamsters

Trophozoites of virulent Entamoeba histolytica transfected with the antisense gene encoding cysteine proteinase 5 (CP5) have only 10% of the CP activity but retain their cytopathic activity on mammalian monolayers. In the present study we found that the transfected trophozoites with low levels of CP activity were incapable of inducing the formation of liver lesions in hamsters.

Entamoeba histolytica contains a beta 1 integrin-like molecule similar to fibronectin receptors from eukaryotic cells

Entamoeba histolytica trophozoites do interact with extracellular matrix components in order to invade and finally destroy tissue. An important step in this interaction involves the binding of a 140-kDa membrane protein that binds to fibronectin. The similarity of this amoebic receptor to fibronectin receptors from higher eukaryotic cells was defined by indirect immunofluorescence, western blot and immunohistochemistry, using polyclonal monospecific antibodies raised against the amoebic protein. These results suggest that lower eukaryotic cells have and use a beta 1 integrin-like molecule as well as mechanisms similar to those present in higher eukaryotic cells during interaction with extracellular matrix components.

Virulence parameters in the characterization of strains of Entamoeba histolytica

Differences in virulence of strains of Entamoeba histolytica have long been detected by various experimental assays, both in vivo and in vitro. Discrepancies in the strains characterization have been arisen when different biological assays are compared. In order to evaluate different parameters of virulence in the strains characterization, five strains of E. histolytica, kept under axenic culture, were characterized in respect to their, capability to induce hamster liver abscess, erythrophagocytosis rate and cytopathic effect upon VERO cells. It was found significant correlation between in vitro biological assays, but not between in vivo and in vitro assays. Good correlation was found between cytopathic effect and the mean number of uptaken erythrocytes, but not with percentage of phagocytic amoebae, showing that great variability can be observed in the same assay, according to the variable chosen. It was not possible to correlate isoenzyme and restriction fragment pattern with virulence indexes since all studied strains presented pathogenic patterns. The discordant results observed in different virulence assays suggests that virulence itself may not the directly assessed. What is in fact assessed are different biological characteristics or functions of the parasite more than virulence itself. These characteristics or functions may be related or not with pathogenic mechanisms occurring in the development of invasive amoebic disease.