Entamoeba histolytica: tyrosine kinase activity induced by fibronectin through the beta1-integrin-like molecule

The Gal/GalNac lectin as a possible acetylcholine receptor in Entamoeba histolytica

Entamoeba histolytica (E. histolytica) is a protozoan responsible for intestinal amebiasis in at least 500 million people per year, although only 10% of those infected show severe symptoms. It is known that E. histolytica captures molecules released during the host immune response through membrane receptors that favor its pathogenetic mechanisms for the establishment of amebic invasion. It has been suggested that E. histolytica interacts with acetylcholine (ACh) through its membrane. This promotes the increase of virulence factors and diverse mechanisms carried out by the amoeba to produce damage. The aim of this study is to identify a membrane receptor in E. histolytica trophozoites for ACh. Methods included identification by colocalization for the ACh and Gal/GalNAc lectin binding site by immunofluorescence, western blot, bioinformatic analysis, and quantification of the relative expression of Ras 5 and Rab 7 GTPases by RT-qPCR. Results show that the Gal/GalNAc lectin acts as a possible binding site for ACh and this binding may occur through the 150 kDa intermediate subunit. At the same time, this interaction activates the GTPases, Ras, and Rab, which are involved in the proliferation, and reorganization of the amoebic cytoskeleton and vesicular trafficking. In conclusion, ACh is captured by the parasite, and the interaction promotes the activation of signaling pathways involved in pathogenicity mechanisms, contributing to disease and the establishment of invasive amebiasis.

Endocytosis, signal transduction and proteolytic cleaving of human holotransferrin in Entamoeba histolytica

Amoebiasis is a parasitic infection of the human large intestine caused by Entamoeba histolytica; this disease mainly affects people from developing countries. To survive, this primitive protozoan has a high demand for iron, and it uses host iron proteins upon invasion. Transferrin (Tf) is a plasma iron-binding protein that transports and delivers iron to all cells. Iron-loaded Tf (holoTf) in humans can support the proliferation of amoebae in vitro by binding to an amoebic TfR (EhTfR), and amoebae endocytose it inside clathrin-coated vesicles. In this study, it was found that EhTfR phosphorylation is required for human holoTf endocytosis by E. histolytica. Once this complex is endocytosed, human holoTf could be degraded with a nutritional purpose by cysteine proteases. HoloTf endocytosis initiates the activation of the mitogen-activated protein kinases (MAPKs) and focal adhesion kinase (FAK) pathways, which induce cell proliferation with phosphoinositide 3-kinase (PI-3 K) and Ca²⁺ involvement. In the first minutes after holoTf is endocytosed, several proteins are phosphorylated including transketolase, enolase, L-myo-inositol-1-phosphate synthase and phosphoglucomutase, which control carbohydrate metabolism, and heat shock protein-70. The study of these proteins and their signal transduction pathways could be useful for developing future therapies.

Entamoeba histolytica antigenic protein detected in pus aspirates from patients with amoebic liver abscess

Entamoeba histolytica is a causative agent of amoebic liver abscess (ALA) and is endemic in many underdeveloped countries. We investigated antigenic E. histolytica proteins in liver abscess aspirates using proteomics approach. Pus samples were first tested by real-time PCR to confirm the presence of E. histolytica DNA and the corresponding serum samples tested for E. histolytica-specific IgG by a commercial ELISA. Proteins were extracted from three and one pool(s) of pus samples from ALA and PLA (pyogenic liver abscess) patients respectively, followed by analysis using isoelectric focussing, SDS-PAGE and Western blot. Unpurified pooled serum samples from infected hamsters and pooled human amoebic-specific IgG were used as primary antibodies. The antigenic protein band was excised from the gel, digested and analysed by MALDI-TOF/TOF and LC-MS/MS. The results using both primary antibodies showed an antigenic protein band of ∼14kDa. Based on the mass spectrum analysis, putative tyrosine kinase is the most probable identification of the antigenic band.

Pathogenic Naegleria fowleri and non-pathogenic Naegleria lovaniensis exhibit differential adhesion to, and invasion of, extracellular matrix proteins

Naegleria fowleri and Naegleria lovaniensis are closely related free-living amoebae found in the environment. N. fowleri causes primary amoebic meningoencephalitis (PAM), a rapidly fatal disease of the central nervous system, while N. lovaniensis is non-pathogenic. N. fowleri infection occurs when the amoebae access the nasal passages, attach to the nasal mucosa and its epithelial lining, and migrate to the brain. This process involves interaction with components of the host extracellular matrix (ECM). Since the ability to invade tissues can be a characteristic that distinguishes pathogenic from non-pathogenic amoebae, the objective of this study was to assess adhesion to, and invasion of, the ECM by these two related but distinct Naegleria species. N. fowleri exhibited a higher level of adhesion to the ECM components laminin-1, fibronectin and collagen I. Scanning electron microscopy revealed that N. fowleri attached on ECM substrata exhibited a spread-out appearance that included the presence of focal adhesion-like structures. Western immunoblotting revealed two integrin-like proteins for both species, but one of these, with a molecular mass of approximately 70 kDa, was detected at a higher level in N. fowleri. Confocal microscopy indicated that the integrin-like proteins co-localized to the focal adhesion-like structures. Furthermore, anti-integrin antibody decreased adhesion of N. fowleri to ECM components. Finally, N. fowleri disrupted 3D ECM scaffolds, while N. lovaniensis had a minimal effect. Collectively, these results indicate a distinction in adhesion to, and invasion of, ECM proteins between N. fowleri and N. lovaniensis.

Clinical case of cerebral amebiasis caused by E. histolytica

Although amebic brain abscess is a rare form of invasive amebiasis, when present, it is frequently lethal. This disorder always begins with the infection of the colon by Entamoeba histolytica trophozoites, which then travel to extra-intestinal tissues through the bloodstream. Amebic brain abscesses are produced when trophozoites invade the central nervous system. Computerized axial tomography scans can be used to diagnose the presence or absence of a brain abscess with a certainty of 100%. However, this diagnostic tool does not reveal the etiological agent of disease. By analyzing the clinical case of a patient that died due to untimely treatment of this malady, the present study aims to identify a diagnostic tool that can give a precise determination of the etiological agent and therefore permit adequate and opportune treatment. Currently, diagnosis of amebic brain abscess is often done by identification of the ameba in a biopsy or autopsy. By immunohistochemistry and immunofluorescence with specific antibodies, we identified the existence of E. histolytica, which presents proteins similar to Naegleria fowleri in its membrane.

Analysis of the protein kinome of Entamoeba histolytica

Protein kinases play important roles in almost all major signaling and regulatory pathways of eukaryotic organisms. Members in the family of protein kinases make up a substantial fraction of eukaryotic proteome. Analysis of the protein kinase repertoire (kinome) would help in the better understanding of the regulatory processes. In this article, we report the identification and analysis of the repertoire of protein kinases in the intracellular parasite Entamoeba histolytica. Using a combination of various sensitive sequence search methods and manual analysis, we have identified a set of 307 protein kinases in E. histolytica genome. We have classified these protein kinases into different subfamilies originally defined by Hanks and Hunter and studied these kinases further in the context of noncatalytic domains that are tethered to catalytic kinase domain. Compared to other eukaryotic organisms, protein kinases from E. histolytica vary in terms of their domain organization and displays features that may have a bearing in the unusual biology of this organism. Some of the parasitic kinases show high sequence similarity in the catalytic domain region with calmodulin/calcium dependent protein kinase subfamily. However, they are unlikely to act like typical calcium/calmodulin dependent kinases as they lack noncatalytic domains characteristic of such kinases in other organisms. Such kinases form the largest subfamily of kinases in E. histolytica. Interestingly, a PKA/PKG-like subfamily member is tethered to pleckstrin homology domain. Although potential cyclins and cyclin-dependent kinases could be identified in the genome the likely absence of other cell cycle proteins suggests unusual nature of cell cycle in E. histolytica. Some of the unusual features recognized in our analysis include the absence of MEK as a part of the Mitogen Activated Kinase signaling pathway and identification of transmembrane region containing Src kinase-like kinases. Sequences which could not be classified into known subfamilies of protein kinases have unusual domain architectures. Many such unclassified protein kinases are tethered to domains which are Cysteine-rich and to domains known to be involved in protein-protein interactions. Our kinome analysis of E. histolytica suggests that the organism possesses a complex protein phosphorylation network that involves many unusual kinases.

Structure and content of the Entamoeba histolytica genome

The intestinal parasite Entamoeba histolytica is one of the first protists for which a draft genome sequence has been published. Although the genome is still incomplete, it is unlikely that many genes are missing from the list of those already identified. In this chapter we summarise the features of the genome as they are currently understood and provide previously unpublished analyses of many of the genes.

Epithelial cells treated with genistein inhibit adhesion and endocytosis of Paracoccidioides brasiliensis

Paracoccidioidomycosis is caused by Paracoccidioides brasiliensis, which although not formally considered an intracellular pathogen, can be internalized by epithelial cells in vitro and in vivo. The mechanisms used by P. brasiliensis to adhere to and invade non-professional phagocytes have not been identified. The signal-transduction networks, involving protein tyrosine kinase (PTK) and protein phosphatase activities, can modulate crucial events during fungal infections. In this study, the involvement of PTK has been investigated in P. brasiliensis adherence and invasion in mammalian epithelial cells. A significant inhibition of the fungal invasion occurred after the pre-treatment of the epithelial cells with genistein, a specific tyrosine kinase inhibitor, indicating that the tyrosine kinase pathway is involved in P. brasiliensis internalization. In contrast, when the fungus was treated, a slight (not significant) inhibition of PTK was observed, suggesting that PTK might not be the fungus' transduction signal pathway during the invasion process of epithelial cells. An intense PTK immunofluorescence labeling was observed in the periphery of the P. brasiliensis infected cells, little PTK labeling was found in both uninfected cells and yeast cells, at later infection times (8 and 24 h). Moreover, when the epithelial cells were treated with genistein and infected with P. brasiliensis, no labeling was observed, suggesting the importance of the PTK in the infectious process. These results suggest that PTK pathway participates in the transduction signal during the initial events of the adhesion and invasion processes of P. brasiliensis to mammalian epithelial cells.

The Cytoskeleton of Entamoeba histolytica: Structure, Function, and Regulation by Signaling Pathways

Pathogenesis in the parasite Entamoeba histolytica has been related to motility of the trophozoites. Motility is an important feature in amebas as they perform multiple motile functions during invasion of host tissues. As motility depends on the organization and regulation of the cytoskeleton elements, in particular of the actin cytoskeleton, the study of the molecular components of the machinery responsible for movement has been a key aspect to study in this parasite. Although many of the components have high homology in amino acid sequence and function to those characterized in higher eukaryotic cells, there are important differences to suggest that parasitic organisms may have developed adaptative differences that could be useful as targets to stop invasion. The purpose of this review is to evaluate current knowledge about the cytoskeleton of E. histolytica and the ways in which the parasite controls motility.

Rho signaling inEntamoeba histolytica modulates actomyosin-dependent activities stimulated during invasive behavior

Interaction of Entamoeba histolytica trophozoites with target cells and substrates activates signaling pathways in the parasite. Phosphorylation cascades triggered by phospho-inositide and adenyl-cyclase-dependent pathways modulate reorganization of the actin cytoskeleton to form structures that facilitate adhesion. In contrast, little is known about participation of Rho proteins and Rho signaling in actin rearrangements. We report here the in vivo expression of at least one Rho protein in trophozoites, whose activation induced actin reorganization and actin-myosin interaction. Antibodies to EhRhoA1 recombinant protein mainly localized Rho in the cytosol of nonactivated amoebae, but it was translocated to vesicular membranes and to some extent to the plasma membrane after treatment with lysophosphatidic acid (LPA), a specific agonist of Rho activation. Activated Rho was identified in LPA-treated trophozoites. LPA induced striking polymerization of actin into distinct dynamic structures. Disorganization of these structures by inhibition of Rho effector, Rho-kinase (ROCK), and by ML-7, an inhibitor of myosin light chain kinase dependent phosphorylation of myosin light chain, suggested that the actin structures also contained myosin. LPA stimulated concanavalin-A-mediated formation of caps, chemotaxis, invasion of extracellular matrix substrates, and erythrophagocytosis, but not binding to fibronectin. ROCK inhibition impaired LPA-stimulated functions and to some extent adhesion to fibronectin. Similar results were obtained with ML-7. These data suggest the presence and operation of Rho-signaling pathways in E. histolytica, that together with other, already described, signaling routes modulate actomyosin-dependent motile processes, particularly stimulated during invasive behavior.

Involvement of protein kinases on the process of erythrophagocytis by Entamoeba histolytica

Erythrophagocytic capacity of trophozoites of Entamoeba histolytica is considered a factor in the virulence of this pathogenic protozoan. We present evidence showing that such activity resembles the ingestion of microorganisms by highly differentiated phagocytic cells, such as macrophages. Previous treatment of the trophozoites with genistein or tyrphostin, inhibitors of tyrosine protein kinases, with staurosporine, a protein kinase C inhibitor, and wortmannin, a fungal metabolite that inhibits phosphoinositide 3-OH kinase, significantly inhibited their erythrophagocytic capacity.

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.

Signal transduction in Entamoeba histolytica induced by interaction with fibronectin: presence and activation of phosphokinase A and its possible relation to invasiveness

Interaction of Entamoeba histolytica trophozoites with extracellular matrix (ECM) proteins activates signaling pathways through G-protein-coupled receptors. Increments of adenylyl cyclase activity and cAMP produce a striking reorganization of actin into structures that apparently facilitate adhesive, locomotive, and secretory activities. The reorganization of actin is induced by phosphorylation of actin-associated proteins by diverse kinases activated during the signaling process. Although cAMP-dependent kinases have not yet been identified in this parasite, the activation of the adenylyl cyclase route and its effects on particular motility-related functions strongly suggest their presence. Phosphokinase A (PKA) was detected by phosphorylation of the specific substrate, kemptide, its further activation by cAMP, and its inhibition by H89. The catalytic subunit of the enzyme was identified by immunofluorescence microscopy and by immunoprecipitation. Adhesion and damage to cultured cells were monitored by FN-binding and cytotoxicity assays. A cAMP-dependent kinase activated by effectors and agonists of adenylyl cyclase and also during interaction of trophozoites with fibronectin (FN) was found. The enzyme is associated with small granules in the cytoplasm and upon activation, a fraction of its catalytic subunit with an Mr of 100 kDa was translocated to the nucleus, while another fraction was aggregated into big clusters. Activity and translocation were blocked by H89, a specific inhibitor of PKA. Trophozoites stimulated by dBcAMP or forskolin-formed lamellae and restructured actin, but no significant increase in their adhesion to FN was observed and only showed 10% stimulus in their capacity to damage target cells. Treatment with H89 decreased adhesion to 40% and caused 80% inhibition in cell damage. These amebas showed altered organization of the actin structures induced by dBcAMP or FN. Our results support previous suggestions concerning the participation of PKA in the response elicited by the interaction of E. histolytica trophozoites with ECM proteins. They also indicate that adhesion and secretion in conjunction with motile activities are related to invasion processes.

Lipid metabolism in mucous-dwelling amitochondriate protozoa

Entamoeba, Giardia, and trichomonads are the prominent members of a group known as 'mucosal parasites'. While Entamoeba and Giardia trophozoites colonise the small intestine, trichomonads inhabit the genitourinary tracts of humans and animals. These protozoa lack mitochondria, well-developed Golgi complexes, and other organelles typical of higher eukaryotes. Nonetheless, they have developed unique metabolic pathways that allow them to survive and multiply in the small intestine and reproductive tracts by scavenging nutrients from the host. Various investigators have shown that these protozoa are unable to synthesise the majority of their own lipids and cholesterol de novo; rather, they depend mostly on supplies from outside sources. Therefore, questions of how they transport and utilise exogenous lipids for metabolic purposes are extremely important. There is evidence suggesting that these parasites can take up the lipids and cholesterol they need from lipoprotein particles present in the host and/or in the growth medium. Studies also support the idea that individual lipid and fatty acid molecules can be transported without the help of lipoproteins. Exogenous phospholipids have been shown to undergo fatty acid remodelling (by deacylation/reacylation reactions), which allows these protozoa to alter lipids, bypassing the synthesis of entirely new phospholipid molecules. In addition, many of these amitochondriates are, however, capable of elongating/desaturating long-chain fatty acids, and assembling novel glycophospholipid molecules. In this review, progress in various aspects of lipid research on these organisms is discussed. Attempts are also made to identify steps of lipid metabolic pathways that can be used to develop chemotherapeutic agents against these and other mucosal parasites.