Involvement of p21racA, phosphoinositide 3-kinase, and vacuolar ATPase in phagocytosis of bacteria and erythrocytes by Entamoeba histolytica: suggestive evidence for coincidental evolution of amebic invasiveness

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.

EhRacM differentially regulates macropinocytosis and motility in the enteric protozoan parasite Entamoeba histolytica

Macropinocytosis is an evolutionarily conserved endocytic process that plays a vital role in internalizing extracellular fluids and particles in cells. This non-selective endocytic pathway is crucial for various physiological functions such as nutrient uptake, sensing, signaling, antigen presentation, and cell migration. While macropinocytosis has been extensively studied in macrophages and cancer cells, the molecular mechanisms of macropinocytosis in pathogens are less understood. It has been known that Entamoeba histolytica, the causative agent of amebiasis, exploits macropinocytosis for survival and pathogenesis. Since macropinocytosis is initiated by actin polymerization, leading to the formation of membrane ruffles and the subsequent trapping of solutes in macropinosomes, actin cytoskeleton regulation is crucial. Thus, this study focuses on unraveling the role of well-conserved actin cytoskeleton regulators, Rho small GTPase family proteins, in macropinocytosis in E. histolytica. Through gene silencing of highly transcribed Ehrho/Ehrac genes and following flow cytometry analysis, we identified that silencing EhracM enhances dextran macropinocytosis and affects cellular migration persistence. Live imaging and interactome analysis unveiled the cytosolic and vesicular localization of EhRacM, along with its interaction with signaling and membrane traffic-related proteins, shedding light on EhRacM's multiple roles. Our findings provide insights into the specific regulatory mechanisms of macropinocytosis among endocytic pathways in E. histolytica, highlighting the significance of EhRacM in both macropinocytosis and cellular migration.

Live Cells Imaging and Comparative Phosphoproteomics Uncover Proteins from the Mechanobiome in Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite and the causative agent of amoebiasis in humans. This amoeba invades human tissues by taking advantage of its actin-rich cytoskeleton to move, enter the tissue matrix, kill and phagocyte the human cells. During tissue invasion, E. histolytica moves from the intestinal lumen across the mucus layer and enters the epithelial parenchyma. Faced with the chemical and physical constraints of these diverse environments, E. histolytica has developed sophisticated systems to integrate internal and external signals and to coordinate cell shape changes and motility. Cell signalling circuits are driven by interactions between the parasite and extracellular matrix, combined with rapid responses from the mechanobiome in which protein phosphorylation plays an important role. To understand the role of phosphorylation events and related signalling mechanisms, we targeted phosphatidylinositol 3-kinases followed by live cell imaging and phosphoproteomics. The results highlight 1150 proteins, out of the 7966 proteins within the amoebic proteome, as members of the phosphoproteome, including signalling and structural molecules involved in cytoskeletal activities. Inhibition of phosphatidylinositol 3-kinases alters phosphorylation in important members of these categories; a finding that correlates with changes in amoeba motility and morphology, as well as a decrease in actin-rich adhesive structures.

PTEN differentially regulates endocytosis, migration, and proliferation in the enteric protozoan parasite Entamoeba histolytica

PTEN is a lipid phosphatase that is highly conserved and involved in a broad range of biological processes including cytoskeletal reorganization, endocytosis, signal transduction, and cell migration in all eukaryotes. Although regulation of phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] signaling via PTEN has been well established in model organisms and mammals, it remains elusive in the parasitic protist E. histolytica, which heavily relies on PtdIns phosphate(s)-dependent membrane traffic, migration, and phago- and trogocytosis for its pathogenesis. In this study, we characterized the major PTEN from E. histolytica, EhPTEN1, which shows the highest expression at the transcript level in the trophozoite stage among 6 possible PTENs, to understand the significance of PtdIns(3,4,5)P3 signaling in this parasite. Live imaging of GFP-EhPTEN1 expressing amebic trophozoites showed localization mainly in the cytosol with a higher concentration at pseudopods and the extending edge of the phago- and trogocytic cups. Furthermore, quantitative analysis of phago- and trogocytosis using a confocal image cytometer showed that overexpression of EhPTEN1 caused reduction in trogo- and phagocytosis while transcriptional gene silencing of EhPTEN1 gene caused opposite phenotypes. These data suggest that EhPTEN1 has an inhibitory role in these biological processes. Conversely, EhPTEN1 acts as a positive regulator for fluid-phase and receptor-mediated endocytosis in E. histolytica trophozoites. Moreover, we showed that EhPTEN1 was required for optimal growth and migration of this parasite. Finally, the phosphatase activity of EhPTEN1 towards PtdIns(3,4,5)P3 was demonstrated, suggesting that the biological roles of EhPTEN1 are likely linked to its catalytic function. Taken together, these results indicate that EhPTEN1 differentially regulates multiple cellular activities essential for proliferation and pathogenesis of the organism, via PtdIns(3,4,5)P3 signaling. Elucidation of biological roles of PTEN and PtdIns(3,4,5)P3 signaling at the molecular levels promotes our understanding of the pathogenesis of this parasite.

PI Kinase-EhGEF2-EhRho5 axis contributes to LPA stimulated macropinocytosis in Entamoeba histolytica

Entamoeba histolytica is a protozoan responsible for several pathologies in humans. Trophozoites breach the intestinal site to enter the bloodstream and thus traverse to a secondary site. Macropinocytosis and phagocytosis, collectively accounting for heterophagy, are the two major processes responsible for sustenance of Entamoeba histolytica within the host. Both of these processes require significant rearrangements in the structure to entrap the target. Rho GTPases play an indispensable role in mustering proteins that regulate cytoskeletal remodelling. Unlike phagocytosis which has been studied in extensive detail, information on machinery of macropinocytosis in E. histolytica is still limited. In the current study, using site directed mutagenesis and RNAi based silencing, coupled with functional studies, we have demonstrated the involvement of EhRho5 in constitutive and LPA stimulated macropinocytosis. We also report that LPA, a bioactive phospholipid present in the bloodstream of the host, activates EhRho5 and translocates it from cytosol to plasma membrane and endomembrane compartments. Using biochemical and FRAP studies, we established that a PI Kinase acts upstream of EhRho5 in LPA mediated signalling. We further identified EhGEF2 as a guanine nucleotide exchange factor of EhRho5. In the amoebic trophozoites, EhGEF2 depletion leads to reduced macropinocytic efficiency of trophozoites, thus phenocopying its substrate. Upon LPA stimulation, EhGEF2 is found to sequester near the plasma membrane in a wortmannin sensitive fashion, explaining a possible mode for activation of EhRho5 in the amoebic trophozoites. Collectively, we propose that LPA stimulated macropinocytosis in E. histolytica is driven by the PI Kinase-EhGEF2-EhRho5 axis.

Cdc42/Rac Interactive Binding Containing Effector Proteins in Unicellular Protozoans With Reference to Human Host: Locks of the Rho Signaling

Small GTPases are the key to actin cytoskeleton signaling, which opens the lock of effector proteins to forward the signal downstream in several cellular pathways. Actin cytoskeleton assembly is associated with cell polarity, adhesion, movement and other functions in eukaryotic cells. Rho proteins, specifically Cdc42 and Rac, are the primary regulators of actin cytoskeleton dynamics in higher and lower eukaryotes. Effector proteins, present in an inactive state gets activated after binding to the GTP bound Cdc42/Rac to relay a signal downstream. Cdc42/Rac interactive binding (CRIB) motif is an essential conserved sequence found in effector proteins to interact with Cdc42 or Rac. A diverse range of Cdc42/Rac and their effector proteins have evolved from lower to higher eukaryotes. The present study has identified and further classified CRIB containing effector proteins in lower eukaryotes, focusing on parasitic protozoans causing neglected tropical diseases and taking human proteins as a reference point to the highest evolved organism in the evolutionary trait. Lower eukaryotes’ CRIB containing proteins fall into conventional effector molecules, PAKs (p21 activated kinase), Wiskoit-Aldrich Syndrome proteins family, and some have unique domain combinations unlike any known proteins. We also highlight the correlation between the effector protein isoforms and their selective specificity for Cdc42 or Rac proteins during evolution. Here, we report CRIB containing effector proteins; ten in Dictyostelium and Entamoeba, fourteen in Acanthamoeba, one in Trypanosoma and Giardia. CRIB containing effector proteins that have been studied so far in humans are potential candidates for drug targets in cancer, neurological disorders, and others. Conventional CRIB containing proteins from protozoan parasites remain largely elusive and our data provides their identification and classification for further in-depth functional validations. The tropical diseases caused by protozoan parasites lack combinatorial drug targets as effective paradigms. Targeting signaling mechanisms operative in these pathogens can provide greater molecules in combatting their infections.

An atypical EhGEF regulates phagocytosis in Entamoeba histolytica through EhRho1

The parasite Entamoeba histolytica is the etiological agent of amoebiasis, a major cause of morbidity and mortality due to parasitic diseases in developing countries. Phagocytosis is an essential mode of obtaining nutrition and has been associated with the virulence behaviour of E. histolytica. Signalling pathways involved in activation of cytoskeletal dynamics required for phagocytosis remains to be elucidated in this parasite. Our group has been studying initiation of phagocytosis and formation of phagosomes in E. histolytica and have described some of the molecules that play key roles in the process. Here we showed the involvement of non-Dbl Rho Guanine Nucleotide Exchange Factor, EhGEF in regulation of amoebic phagocytosis by regulating activation of EhRho1. EhGEF was found in the phagocytic cups during the progression of cups, until closure of phagosomes, but not in the phagosomes themselves. Our observation from imaging, pull down experiments and down regulating expression of different molecules suggest that EhGEF interacts with EhRho1 and it is required during initiation of phagocytosis and phagosome formation. Also, biophysical, and computational analysis reveals that EhGEF mediates GTP exchange on EhRho1 via an unconventional pathway. In conclusion, we describe a non-Dbl EhGEF of EhRho1 which is involved in endocytic processes of E. histolytica.

Trogocytosis in Unicellular Eukaryotes

Trogocytosis is a mode of internalization of a part of a live cell by nibbling and is mechanistically distinct from phagocytosis, which implies internalization of a whole cell or a particle. Trogocytosis has been demonstrated in a broad range of cell types in multicellular organisms and is also known to be involved in a plethora of functions. In immune cells, trogocytosis is involved in the "cross-dressing" between antigen presenting cells and T cells, and is thus considered to mediate intercellular communication. On the other hand, trogocytosis has also been reported in a variety of unicellular organisms including the protistan (protozoan) parasite Entamoeba histolytica. E. histolytica ingests human T cell line by trogocytosis and acquires complement resistance and cross-dresses major histocompatibility complex (MHC) class I on the cell surface. Furthermore, trogocytosis and trogocytosis-like phenomena (nibbling of a live cell, not previously described as trogocytosis) have also been reported in other parasitic protists such as Trichomonas, Plasmodium, Toxoplasma, and free-living amoebae. Thus, trogocytosis is conserved in diverse eukaryotic supergroups as a means of intercellular communication. It is depicting the universality of trogocytosis among eukaryotes. In this review, we summarize our current understanding of trogocytosis in unicellular organisms, including the history of its discovery, taxonomical distribution, roles, and molecular mechanisms.

Diversity of phosphoinositide binding proteins in Entamoeba histolytica

Phosphatidylinositol phosphates (PIPs, phosphoinositides) are localized to the membranes of all cellular compartments, and play pivotal roles in multiple cellular events. To fulfill their functions, PIPs that are located to specific organelles or membrane domains bind to and recruit various proteins in spatiotemporal specific manner via protein domains that selectively bind to either a single or an array of PIPs. In Entamoeba histolytica, the human intestinal protozoan parasite, PIPs and PIP-binding proteins have been shown to be involved in their virulence-associated mechanisms such as cell motility, vesicular traffic, trogo- and phagocytosis. In silico search of the domains and the signatures implicated in PIP binding in the E. histolytica proteome allows identification of dozens of potential PIP-binding proteins. However, such analysis is often misleading unless the protein domain used as query is cautiously selected and the binding specificity of the proteins are experimentally validated. This is because all the domains initially presumed to bind PIPs in other systems are not always capable of PIP binding, but rather involved in other biological roles. In this review, we carried out in silico survey of proteins which have PIP-binding domains in the E. histolytica genome by utilizing only validated PIP-binding domains that had been experimentally proven to be faithful PIP-binding bioprobes. Our survey has identified that FYVE (Fab1, YOTB1, Vac1, EEA1) and PH (pleckstrin homology) domain containing proteins are the most expanded families in E. histolytica. A few FYVE domain-containing proteins (EhFP4 and 10) and phox homology (PX) domain containing proteins (EhSNX1 and 2) were previously studied in depth in E. histolytica. Furthermore, most of the identified PH domain-containing proteins are annotated as protein kinases and possess protein kinase domains. Overall, PIP-binding domain-containing proteins that can be identified by in silico survey of the genome using the domains from well characterized bioprobes are limited in E. histolytica. However, their domain architectures are often unique, suggesting unique evolution of PIP-binding domain-containing proteins in this organism.

A lysosomal hydrolase receptor, CPBF2, is associated with motility and invasion of the enteric protozoan parasite Entamoeba histolytica

Proper targeting and secretion of lysosomal hydrolases are regulated by transporting receptors. Entamoeba histolytica, the enteric protozoan parasite responsible for human amebiasis, has a unique family of lysosomal hydrolase receptors, cysteine protease binding protein family, CPBF. CPBFs, consisting of 11 members with conserved domain organization, bind to a wide range of cargos including cysteine proteases and glycosidases, which are also known to be involved in pathogenesis of this parasite. In this study, we characterized one of CPBFs, CPBF2, which is involved in cell motility and extracellular matrix invasion. Unexpectedly, these roles of CPBF were not related to its cargo, α-amylase. This is the first demonstration that a putative hydrolase receptor is involved in cell motility and invasion in parasitic protozoa.

Who Needs a Contractile Actomyosin Ring? The Plethora of Alternative Ways to Divide a Protozoan Parasite

Cytokinesis, or the division of the cytoplasm, following the end of mitosis or meiosis, is accomplished in animal cells, fungi, and amoebae, by the constriction of an actomyosin contractile ring, comprising filamentous actin, myosin II, and associated proteins. However, despite this being the best-studied mode of cytokinesis, it is restricted to the Opisthokonta and Amoebozoa, since members of other evolutionary supergroups lack myosin II and must, therefore, employ different mechanisms. In particular, parasitic protozoa, many of which cause significant morbidity and mortality in humans and animals as well as considerable economic losses, employ a wide diversity of mechanisms to divide, few, if any, of which involve myosin II. In some cases, cell division is not only myosin II-independent, but actin-independent too. Mechanisms employed range from primitive mechanical cell rupture (cytofission), to motility- and/or microtubule remodeling-dependent mechanisms, to budding involving the constriction of divergent contractile rings, to hijacking host cell division machinery, with some species able to utilize multiple mechanisms. Here, I review current knowledge of cytokinesis mechanisms and their molecular control in mammalian-infective parasitic protozoa from the Excavata, Alveolata, and Amoebozoa supergroups, highlighting their often-underappreciated diversity and complexity. Billions of people and animals across the world are at risk from these pathogens, for which vaccines and/or optimal treatments are often not available. Exploiting the divergent cell division machinery in these parasites may provide new avenues for the treatment of protozoal disease.

Host Invasion by Pathogenic Amoebae: Epithelial Disruption by Parasite Proteins

The epithelium represents the first and most extensive line of defence against pathogens, toxins and pollutant agents in humans. In general, pathogens have developed strategies to overcome this barrier and use it as an entrance to the organism. Entamoeba histolytica, Naegleriafowleri and Acanthamoeba spp. are amoebae mainly responsible for intestinal dysentery, meningoencephalitis and keratitis, respectively. These amoebae cause significant morbidity and mortality rates. Thus, the identification, characterization and validation of molecules participating in host-parasite interactions can provide attractive targets to timely intervene disease progress. In this work, we present a compendium of the parasite adhesins, lectins, proteases, hydrolases, kinases, and others, that participate in key pathogenic events. Special focus is made for the analysis of assorted molecules and mechanisms involved in the interaction of the parasites with epithelial surface receptors, changes in epithelial junctional markers, implications on the barrier function, among others. This review allows the assessment of initial host-pathogen interaction, to correlate it to the potential of parasite invasion.

Inhibition of Amebic Lysosomal Acidification Blocks Amebic Trogocytosis and Cell Killing

Entamoeba histolytica ingests fragments of live host cells in a nibbling-like process termed amebic trogocytosis. Amebic trogocytosis is required for cell killing and contributes to tissue invasion, which is a hallmark of invasive amebic colitis. Work done prior to the discovery of amebic trogocytosis showed that acid vesicles are required for amebic cytotoxicity. In the present study, we show that acidified lysosomes are required for amebic trogocytosis and cell killing. Interference with lysosome acidification using ammonium chloride, a weak base, or concanamycin A, a vacuolar H⁺ ATPase inhibitor, decreased amebic trogocytosis and amebic cytotoxicity. Our data suggest that the inhibitors do not impair the ingestion of an initial fragment but rather block continued trogocytosis and the ingestion of multiple fragments. The acidification inhibitors also decreased phagocytosis, but not fluid-phase endocytosis. These data suggest that amebic lysosomes play a crucial role in amebic trogocytosis, phagocytosis, and cell killing.IMPORTANCEE. histolytica is a protozoan parasite that is prevalent in low-income countries, where it causes potentially fatal diarrhea, dysentery, and liver abscesses. Tissue destruction is a hallmark of invasive E. histolytica infection. The parasite is highly cytotoxic to a wide range of human cells, and parasite cytotoxic activity is likely to drive tissue destruction. E. histolytica is able to kill human cells through amebic trogocytosis. This process also contributes to tissue invasion. Trogocytosis has been observed in other organisms; however, little is known about the mechanism in any system. We show that interference with lysosomal acidification impairs amebic trogocytosis, phagocytosis, and cell killing, indicating that amebic lysosomes are critically important for these processes.

AGC family kinase 1 participates in trogocytosis but not in phagocytosis in Entamoeba histolytica

The protozoan parasite Entamoeba histolytica is the aetiologic agent of amoebiasis, an endemic infection in developing countries with considerable morbidity and mortality. Recently, trogocytosis has been recognized as the key step in amoebic cytolysis and invasion, a paradigm shift in understanding pathogenicity of this organism. Here we report that AGC family kinase 1 is specifically involved in trogocytosis of live human cells and does not participate in phagocytosis of dead cells. Live imaging reveals localization of this kinase in the long and thin tunnels formed during trogocytosis but not in the trogosomes (endosomes formed after trogocytosis). Silencing of the specific gene leads to a defect in CHO cell destruction and trogocytosis while other endocytic processes remain unaffected. The results suggest that the trogocytic pathway is likely to be different from phagocytosis though many of the steps and molecules involved may be common.

Entamoeba histolytica can kill host cells by trogocytosis, while it ingests dead cells by phagocytosis. Here, Somlata et al. show that EhAGCK1, an AGC family kinase, is specifically involved in trogocytosis, shedding light on the molecular differences between trogocytosis and phagocytosis.

Knockdown of Five Genes Encoding Uncharacterized Proteins Inhibits Entamoeba histolytica Phagocytosis of Dead Host Cells

Entamoeba histolytica is the protozoan parasite that causes invasive amebiasis, which is endemic to many developing countries and characterized by dysentery and liver abscesses. The virulence of E. histolytica correlates with the degree of host cell engulfment, or phagocytosis, and E. histolytica phagocytosis alters amebic gene expression in a feed-forward manner that results in an increased phagocytic ability. Here, we used a streamlined RNA interference screen to silence the expression of 15 genes whose expression was upregulated in phagocytic E. histolytica trophozoites to determine whether these genes actually function in the phagocytic process. When five of these genes were silenced, amebic strains with significant decreases in the ability to phagocytose apoptotic host cells were produced. Phagocytosis of live host cells, however, was largely unchanged, and the defects were surprisingly specific for phagocytosis. Two of the five encoded proteins, which we named E. histolytica ILWEQ (EhILWEQ) and E. histolytica BAR (EhBAR), were chosen for localization via SNAP tag labeling and localized to the site of partially formed phagosomes. Therefore, both EhILWEQ and EhBAR appear to contribute to E. histolytica virulence through their function in phagocytosis, and the large proportion (5/15 [33%]) of gene-silenced strains with a reduced ability to phagocytose host cells validates the previously published microarray data set demonstrating feed-forward control of E. histolytica phagocytosis. Finally, although only limited conclusions can be drawn from studies using the virulence-deficient G3 Entamoeba strain, the relative specificity of the defects induced for phagocytosis of apoptotic cells but not healthy cells suggests that cell killing may play a rate-limiting role in the process of Entamoeba histolytica host cell engulfment.

A surface membrane protein of Entamoeba histolytica functions as a receptor for human chemokine IL-8: its role in the attraction of trophozoites to inflammation sites

Entamoeba histolytica trophozoites respond to the presence of IL-8, moving by chemotaxis towards the source of the chemokine. IL-8 binds to the trophozoite membrane and triggers a response that activates signaling pathways that in turn regulate actin/myosin cytoskeleton organisation to initiate migration towards the chemokine, suggesting the presence of a receptor for IL-8 in the parasite. Antibodies directed to the human IL-8 receptor (CXCR1) specifically recognised a 29 kDa protein in trophozoite membrane fractions. The same protein was immunoprecipitated by this antibody from total amebic extracts. Peptide analysis of the immunoprecipitated protein revealed a sequence with high homology to a previously identified amebic outer membrane peroxiredoxin and a motif within the third loop of human CXCR1, which is an important site for IL-8 binding and activation of signaling processes. Immunodetection assays demonstrated that the anti-human CXCR1 antibody binds to the 29 kDa protein in a different but close site to where IL-8 binds to the trophozoite surface membrane, suggesting that human and amebic receptors for this chemokine share common epitopes. In the context of the human intestinal environment, a receptor for IL-8 could be a great advantage for E. histolytica trophozoite survival, as they could reach an inflammatory milieu containing abundant nutrients. In addition, it has been suggested that the high content of accessible thiol groups of the protein and its peroxidase activity could provide protection in the oxygen rich milieu of colonic lesions, allowing trophozoite invasion of other tissues and escape from the host immune response.

Mycobacterium avium MAV_2941 mimics phosphoinositol-3-kinase to interfere with macrophage phagosome maturation

Mycobacterium avium subsp hominissuis (M. avium) is a pathogen that infects and survives in macrophages. Previously, we have identified the M. avium MAV_2941 gene encoding a 73 amino acid protein exported by the oligopeptide transporter OppA to the macrophage cytoplasm. Mutations in MAV_2941 were associated with significant impairment of M. avium growth in THP-1 macrophages. In this study, we investigated the molecular mechanism of MAV_2941 action and demonstrated that MAV_2941 interacts with the vesicle trafficking proteins syntaxin-8 (STX8), adaptor-related protein complex 3 (AP-3) complex subunit beta-1 (AP3B1) and Archain 1 (ARCN1) in mononuclear phagocytic cells. Sequencing analysis revealed that the binding site of MAV_2941 is structurally homologous to the human phosphatidylinositol 3-kinase (PI3K) chiefly in the region recognized by vesicle trafficking proteins. The β3A subunit of AP-3, encoded by AP3B1, is essential for trafficking cargo proteins, including lysosomal-associated membrane protein 1 (LAMP-1), to the phagosome and lysosome-related organelles. Here, we show that while the heat-killed M. avium when ingested by macrophages co-localizes with LAMP-1 protein, transfection of MAV_2941 in macrophages results in significant decrease of LAMP-1 co-localization with the heat-killed M. avium phagosomes. Mutated MAV_2941, where the amino acids homologous to the binding region of PI3K were changed, failed to interact with trafficking proteins. Inactivation of the AP3B1 gene led to alteration in the trafficking of LAMP-1. These results suggest that M. avium MAV_2941 interferes with the protein trafficking within macrophages altering the maturation of phagosome.

Chew on this: amoebic trogocytosis and host cell killing by Entamoeba histolytica

Entamoeba histolytica was named 'histolytica' (from histo-, 'tissue'; lytic-, 'dissolving') for its ability to destroy host tissues. Direct killing of host cells by the amoebae is likely to be the driving factor that underlies tissue destruction, but the mechanism was unclear. We recently showed that, after attaching to host cells, amoebae bite off and ingest distinct host cell fragments, and that this contributes to cell killing. We review this process, termed 'amoebic trogocytosis' (trogo-, 'nibble'), and how this process interplays with phagocytosis, or whole cell ingestion, in this organism. 'Nibbling' processes have been described in other microbes and in multicellular organisms. The discovery of amoebic trogocytosis in E. histolytica may also shed light on an evolutionarily conserved process for intercellular exchange.

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.

Entamoeba histolytica RacC selectively engages p21-activated kinase effectors

Rho family GTPases modulate actin cytoskeleton dynamics by signaling through multiple effectors, including the p21-activated kinases (PAKs). The intestinal parasite Entamoeba histolytica expresses ∼20 Rho family GTPases and seven isoforms of PAK, two of which have been implicated in pathogenesis-related processes such as amoebic motility and invasion and host cell phagocytosis. Here, we describe two previously unstudied PAK isoforms, EhPAK4 and EhPAK5, as highly specific effectors of EhRacC. A structural model based on 2.35 Å X-ray crystallographic data of a complex between EhRacC(Q65L)·GTP and the EhPAK4 p21 binding domain (PBD) reveals a fairly well-conserved Rho/effector interface despite deviation of the PBD α-helix. A structural comparison with EhRho1 in complex with EhFormin1 suggests likely determinants of Rho family GTPase signaling specificity in E. histolytica. These findings suggest a high degree of Rho family GTPase diversity and specificity in the single-cell parasite E. histolytica. Because PAKs regulate pathogenesis-related processes in E. histolytica, they may be valid pharmacologic targets for anti-amoebiasis drugs.

Regulation of serum-responsive transmembrane kinase EhTMKB1-9 by an unsaturated lipid, oleic acid in protistan parasite Entamoeba histolytica

Transmembrane kinases of Entamoeba histolytica are known to play a wide range of roles from virulence, phagocytosis, and proliferation to stress response. Transmembrane kinase EhTMKB1-9 is thought to be involved in early proliferative response and it was originally identified as a serum inducible gene. Ability to stimulate EhTMKB1 expression of serum starved cells resides in unsaturated fatty acids associated with albumin fraction of serum and the mechanism of stimulation follows activation of EhTMKB1-9 promoter. Gel shift assay showed the presence of proteins that bind to the specific site of EhTMKB1-9 upstream region and the concentration of these protein(s) go down on serum starvation, but level of binding protein(s) go up on serum or fatty acid replenishment. This increase in concentration of binding molecule(s) is due to new synthesis rather than activation of existing molecule(s) as a protein synthesis inhibitor blocked enhanced level of gel shifted material on replenishment. The stimulating activity resides in the fatty acyl chain, but not in the head group. Moreover, the fatty acid initiates signaling through class I PI3 kinases that result in activation of EhTMKB1-9 expression. These results suggest a novel mechanism of gene regulation in E. histolytica, and unsaturated fatty acids as potential new signaling molecules.

A novel alpha kinase EhAK1 phosphorylates actin and regulates phagocytosis in Entamoeba histolytica

Phagocytosis plays a key role in nutrient uptake and virulence of the protist parasite Entamoeba histolytica. Phagosomes have been characterized by proteomics, and their maturation in the cells has been studied. However, there is so far not much understanding about initiation of phagocytosis and formation of phagosomes at the molecular level. Our group has been studying initiation of phagocytosis and formation of phagosomes in E. histolytica, and have described some of the molecules that play key roles in the process. Here we show the involvement of EhAK1, an alpha kinase and a SH3 domain containing protein in the pathway that leads to formation of phagosomes using red blood cell as ligand particle. A number of approaches, such as proteomics, biochemical, confocal imaging using specific antibodies or GFP tagged molecules, expression down regulation by antisense RNA, over expression of wild type and mutant proteins, were used to understand the role of EhAK1 in phagocytosis. EhAK1 was found in the phagocytic cups during the progression of cups, until closure of phagosomes, but not in the phagosomes themselves. It is recruited to the phagosomes through interaction with the calcium binding protein EhCaBP1. A reduction in phagocytosis was observed when EhAK1 was down regulated by antisense RNA, or by over expression of the kinase dead mutant. G-actin was identified as one of the major substrates of EhAK1. Phosphorylated actin preferentially accumulated at the phagocytic cups and over expression of a phosphorylation defective actin led to defects in phagocytosis. In conclusion, we describe an important component of the pathway that is initiated on attachment of red blood cells to E. histolytica cells. The main function of EhAK1 is to couple signalling events initiated after accumulation of EhC2PK to actin dynamics.

Identification of Mycobacterium avium genes expressed during in vivo infection and the role of the oligopeptide transporter OppA in virulence

Mycobacterium avium causes disseminated disease in patients with AIDS and other immunosuppressive conditions and pulmonary infections in individuals with chronic lung diseases. Much still need to be learn about the mechanisms of M. avium pathogenesis. Using a mouse model of disseminated M. avium disease, we applied an in vivo expression technology system and identified M. avium genes up-regulated in different organs of mice during early stage of infection. The M. avium oppA gene, involved in an active transport of oligopeptides across the cell membrane, was found highly expressed in lung, liver and spleen of mice. Mutation in the transport domain of the oppA gene resulted in bacterial attenuation in both macrophages and in mice. Using protein-protein interaction assay, it was determined that two hypothetical small proteins, MAV_2941 (73aa) and MAV_4320 (45aa), interact with OppA. MAV_2941 was shown to be secreted by the bacterium into the macrophage cytoplasm. Mutations in MAV_2941 was associated with significant impairment of growth in macrophages. Understanding the mechanisms involved in the functions of MAV_2941 and MAV_4320 is warranted.

A genomewide overexpression screen identifies genes involved in the phosphatidylinositol 3-kinase pathway in the human protozoan parasite Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery and liver abscess. E. histolytica relies on motility, phagocytosis, host cell adhesion, and proteolysis of extracellular matrix for virulence. In eukaryotic cells, these processes are mediated in part by phosphatidylinositol 3-kinase (PI3K) signaling. Thus, PI3K may be critical for virulence. We utilized a functional genomics approach to identify genes whose products may operate in the PI3K pathway in E. histolytica. We treated a population of trophozoites that were overexpressing genes from a cDNA library with a near-lethal dose of the PI3K inhibitor wortmannin. This screen was based on the rationale that survivors would be overexpressing gene products that directly or indirectly function in the PI3K pathway. We sequenced the overexpressed genes in survivors and identified a cDNA encoding a Rap GTPase, a protein previously shown to participate in the PI3K pathway. This supports the validity of our approach. Genes encoding a coactosin-like protein, EhCoactosin, and a serine-rich E. histolytica protein (SREHP) were also identified. Cells overexpressing EhCoactosin or SREHP were also less sensitive to a second PI3K inhibitor, LY294002. This corroborates the link between these proteins and PI3K. Finally, a mutant cell line with an increased level of phosphatidylinositol (3,4,5)-triphosphate, the product of PI3K activity, exhibited increased expression of SREHP and EhCoactosin. This further supports the functional connection between these proteins and PI3K in E. histolytica. To our knowledge, this is the first forward-genetics screen adapted to reveal genes participating in a signal transduction pathway in this pathogen.

G protein signaling in the parasite Entamoeba histolytica

The parasite Entamoeba histolytica causes amebic colitis and systemic amebiasis. Among the known amebic factors contributing to pathogenesis are signaling pathways involving heterotrimeric and Ras superfamily G proteins. Here, we review the current knowledge of the roles of heterotrimeric G protein subunits, Ras, Rho and Rab GTPase families in E. histolytica pathogenesis, as well as of their downstream signaling effectors and nucleotide cycle regulators. Heterotrimeric G protein signaling likely modulates amebic motility and attachment to and killing of host cells, in part through activation of an RGS-RhoGEF (regulator of G protein signaling-Rho guanine nucleotide exchange factor) effector. Rho family GTPases, as well as RhoGEFs and Rho effectors (formins and p21-activated kinases) regulate the dynamic actin cytoskeleton of E. histolytica and associated pathogenesis-related cellular processes, such as migration, invasion, phagocytosis and evasion of the host immune response by surface receptor capping. A remarkably large family of 91 Rab GTPases has multiple roles in a complex amebic vesicular trafficking system required for phagocytosis and pinocytosis and secretion of known virulence factors, such as amebapores and cysteine proteases. Although much remains to be discovered, recent studies of G protein signaling in E. histolytica have enhanced our understanding of parasitic pathogenesis and have also highlighted possible targets for pharmacological manipulation.

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.

Heterotrimeric G-protein signaling is critical to pathogenic processes in Entamoeba histolytica

Heterotrimeric G-protein signaling pathways are vital components of physiology, and many are amenable to pharmacologic manipulation. Here, we identify functional heterotrimeric G-protein subunits in Entamoeba histolytica, the causative agent of amoebic colitis. The E. histolytica Gα subunit EhGα1 exhibits conventional nucleotide cycling properties and is seen to interact with EhGβγ dimers and a candidate effector, EhRGS-RhoGEF, in typical, nucleotide-state-selective fashions. In contrast, a crystal structure of EhGα1 highlights unique features and classification outside of conventional mammalian Gα subfamilies. E. histolytica trophozoites overexpressing wildtype EhGα1 in an inducible manner exhibit an enhanced ability to kill host cells that may be wholly or partially due to enhanced host cell attachment. EhGα1-overexpressing trophozoites also display enhanced transmigration across a Matrigel barrier, an effect that may result from altered baseline migration. Inducible expression of a dominant negative EhGα1 variant engenders the converse phenotypes. Transcriptomic studies reveal that modulation of pathogenesis-related trophozoite behaviors by perturbed heterotrimeric G-protein expression includes transcriptional regulation of virulence factors and altered trafficking of cysteine proteases. Collectively, our studies suggest that E. histolytica possesses a divergent heterotrimeric G-protein signaling axis that modulates key aspects of cellular processes related to the pathogenesis of this infectious organism.

Entamoeba histolytica Rho1 regulates actin polymerization through a divergent, diaphanous-related formin

Entamoeba histolytica requires a dynamic actin cytoskeleton for intestinal and systemic pathogenicity. Diaphanous-related formins represent an important family of actin regulators that are activated by Rho GTPases. The E. histolytica genome encodes a large family of Rho GTPases and three diaphanous-related formins, of which EhFormin1 is known to regulate mitosis and cytokinesis in trophozoites. We demonstrate that EhFormin1 modulates actin polymerization through its formin homology 2 domain. Despite a highly divergent diaphanous autoinhibitory domain, EhFormin1 is autoinhibited by an N- and C-terminal intramolecular interaction but activated upon binding of EhRho1 to the N-terminal domain tandem. A crystal structure of the EhRho1·GTPγS-EhFormin1 complex illustrates an EhFormin1 conformation that diverges from mammalian mDia1 and lacks a secondary interaction with a Rho insert helix. The structural model also highlights residues required for specific recognition of the EhRho1 GTPase and suggests that the molecular mechanisms of EhFormin1 autoinhibition and activation differ from those of mammalian homologues.

Src and PI3 K inhibitors affect the virulence factors of Entamoeba histolytica

Protein kinases (PKs) of parasitic protozoa are being evaluated as drug targets. A large number of protein kinases within the protein kinome of Entamoeba histolytica strongly suggest that protein phosphorylation is a key component of pathogenesis regulation by this parasite. PI3 K and Src are kinases previously described in this parasite, but their role is poorly understood. Here, the effect of Src-1-inhibitor and PI3 K inhibitor (Wortmannin) on the virulence factors of E. histolytica was evaluated. Results show that both inhibitors affect the actin cytoskeleton and the amoebic movement. Also, the proteolytic activity is diminished by Wortmannin, but not by Src-inhibitor-1; however, the phagocytic capacity is diminished by Wortmannin and Src-1-inhibitor. Finally, we found that the virulence in vivo of E. histolytica is affected by Wortmannin but not by Src-1-inhibitor. This study opens the way for the design of anti-amoebic drugs based on kinase inhibition.

Common pathways for receptor-mediated ingestion of Escherichia coli and LDL cholesterol by Entamoeba histolytica regulated in part by transmembrane kinase 39

The single-celled parasite, Entamoeba histolytica, is an enteric pathogen that ingests bacteria and host cells. Inhibition of phagocytosis renders the parasite avirulent. The ligand/receptor interactions that allow E. histolytica to phagocytose are not well understood. We hypothesised that E. histolytica trophozoites might accomplish ingestion through the utilisation of a scavenger receptor for cholesterol. Here we show that acetylated low density lipoprotein cholesterol was phagocytosed by amoebae via receptor mediated mechanisms. Acetylated low density lipoprotein cholesterol competitively inhibited by 31 ± 1.3% (P < 0.005) the ingestion of Escherichia coli, but not erythrocytes and Jurkat T lymphocytes, suggesting a partially redundant phagocytic pathway for E. coli and cholesterol. Inducible expression ofa signalling-dead dominant-negative version of E. histolytica transmembrane kinase 39 inhibited ingestion of E. coli by 55 ± 3% (P < 0.005) but not LDL particles. We concluded that ingestion of E. coli was regulated by TMK39 and partially shared the acetylated low density lipoprotein cholesterol uptake pathway.

A Multilocus Sequence Typing System (MLST) reveals a high level of diversity and a genetic component to Entamoeba histolytica virulence

Background

The outcome of an Entamoeba histolytica infection is variable and can result in either asymptomatic carriage, immediate or latent disease (diarrhea/dysentery/amebic liver abscess). An E. histolytica multilocus genotyping system based on tRNA gene-linked arrays has shown that genetic differences exist among parasites isolated from patients with different symptoms however, the tRNA gene-linked arrays cannot be located in the current assembly of the E. histolytica Reference genome (strain HM-1:IMSS) and are highly variable.

Results

To probe the population structure of E. histolytica and identify genetic markers associated with clinical outcome we identified in E. histolytica positive samples selected single nucleotide polymorphisms (SNPs) by multiplexed massive parallel sequencing. Profile SNPs were selected which, compared to the reference strain HM-1:IMSS sequence, changed an encoded amino acid at the SNP position, and were present in independent E. histolytica isolates from different geographical origins. The samples used in this study contained DNA isolated from either xenic strains of E. histolytica trophozoites established in culture or E. histolytica positive clinical specimens (stool and amebic liver abscess aspirates). A record of the SNPs present at 16 loci out of the original 21 candidate targets was obtained for 63 of the initial 84 samples (63% of asymptomatically colonized stool samples, 80% of diarrheal stool, 73% of xenic cultures and 84% of amebic liver aspirates). The sequences in all the 63 samples both passed sequence quality control metrics and also had the required greater than 8X sequence coverage for all 16 SNPs in order to confidently identify variants.

Conclusions

Our work is in agreement with previous findings of extensive diversity among E. histolytica isolates from the same geographic origin. In phylogenetic trees, only four of the 63 samples were able to group in two sets of two with greater than 50% confidence. Two SNPs in the cylicin-2 gene (EHI_080100/XM_001914351) were associated with disease (asymptomatic/diarrhea p = 0.0162 or dysentery/amebic liver abscess p = 0.0003). This study demonstrated that there are genetic differences between virulent and avirulent E. histolytica strains and that this approach has the potential to define genetic changes that influence infection outcomes.

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.

NKT cell self-reactivity: evolutionary master key of immune homeostasis?

Complex immune responses have evolved to protect multicellular organisms against the invasion of pathogens. This has exerted strong developmental pressure for specialized functions that can also limit damage to self-tissue. Two arms of immunity, the innate and adaptive immune systems, have evolved for quick, non-specific immune responses to pathogens and more efficient, long-lasting ones upon specific recognition of recurrent pathogens. Specialized cells have arisen as the sentinels of these functions, including macrophages, natural killer (NK), and T and B-lymphocytes. Interestingly, a population of immune cells that can exert both of these complex functions, NKT cells, not only share common functions but also exhibit shared cell surface markers of cells of both arms of the immune system. These features, in combination with sophisticated maintenance of immune homeostasis, will be discussed. The recent finding of self-peptide reactivity of NKT cells in the context of CD1d, with capacity to regulate multiple autoimmune and inflammatory conditions, motivates the current proposal that self-reactive NKT cells might be the ancestral link between present NK and T cells. Their parallel selection through evolution by higher vertebrates could be related to their central function as master regulators of immune homeostasis that in part is shared with regulatory T cells. Hypothetical views on how self-reactive NKT cells secure such a central role will also be proposed.

Mechanisms of adherence, cytotoxicity and phagocytosis modulate the pathogenesis of Entamoeba histolytica

The unicellular parasite Entamoeba histolytica, the causative agent of the human disease amebiasis, has traditionally been distinguished from its nonpathogenic cousin Entamoeba dispar by its propensity for the ingestion of erythrocytes. This classic feature, along with the parasite’s ability to cause extensive host cell death, are critical mechanisms of pathogenesis during human infection. Recent advances have led to a greater understanding of the molecular components that allow E. histolytica to kill and phagocytose extracellular targets during human infection and include detailed studies of the role of the parasite’s cysteine proteinases and other effectors of cytotoxicity, as well as the mechanisms of ligand recognition, signaling and intracellular trafficking during phagocytosis.

Unique structural and nucleotide exchange features of the Rho1 GTPase of Entamoeba histolytica

The single-celled human parasite Entamoeba histolytica possesses a dynamic actin cytoskeleton vital for its intestinal and systemic pathogenicity. The E. histolytica genome encodes several Rho family GTPases known to regulate cytoskeletal dynamics. EhRho1, the first family member identified, was reported to be insensitive to the Rho GTPase-specific Clostridium botulinum C3 exoenzyme, raising the possibility that it may be a misclassified Ras family member. Here, we report the crystal structures of EhRho1 in both active and inactive states. EhRho1 is activated by a conserved switch mechanism, but diverges from mammalian Rho GTPases in lacking a signature Rho insert helix. EhRho1 engages a homolog of mDia, EhFormin1, suggesting a role in mediating serum-stimulated actin reorganization and microtubule formation during mitosis. EhRho1, but not a constitutively active mutant, interacts with a newly identified EhRhoGDI in a prenylation-dependent manner. Furthermore, constitutively active EhRho1 induces actin stress fiber formation in mammalian fibroblasts, thereby identifying it as a functional Rho family GTPase. EhRho1 exhibits a fast rate of nucleotide exchange relative to mammalian Rho GTPases due to a distinctive switch one isoleucine residue reminiscent of the constitutively active F28L mutation in human Cdc42, which for the latter protein, is sufficient for cellular transformation. Nonconserved, nucleotide-interacting residues within EhRho1, revealed by the crystal structure models, were observed to contribute a moderating influence on fast spontaneous nucleotide exchange. Collectively, these observations indicate that EhRho1 is a bona fide member of the Rho GTPase family, albeit with unique structural and functional aspects compared with mammalian Rho GTPases.

A proteomic and cellular analysis of uropods in the pathogen Entamoeba histolytica

Exposure of Entamoeba histolytica to specific ligands induces cell polarization via the activation of signalling pathways and cytoskeletal elements. The process leads to formation of a protruding pseudopod at the front of the cell and a retracting uropod at the rear. In the present study, we show that the uropod forms during the exposure of trophozoites to serum isolated from humans suffering of amoebiasis. To investigate uropod assembly, we used LC-MS/MS technology to identify protein components in isolated uropod fractions. The galactose/N-acetylgalactosamine lectin, the immunodominant antigen M17 (which is specifically recognized by serum from amoeba-infected persons) and a few other cells adhesion-related molecules were primarily involved. Actin-rich cytoskeleton components, GTPases from the Rac and Rab families, filamin, α-actinin and a newly identified ezrin-moesin-radixin protein were the main factors found to potentially interact with capped receptors. A set of specific cysteine proteases and a serine protease were enriched in isolated uropod fractions. However, biological assays indicated that cysteine proteases are not involved in uropod formation in E. histolytica, a fact in contrast to the situation in human motile immune cells. The surface proteins identified here are testable biomarkers which may be either recognized by the immune system and/or released into the circulation during amoebiasis.

A Sequential Model of Host Cell Killing and Phagocytosis by Entamoeba histolytica

The protozoan parasite Entamoeba histolytica is responsible for invasive intestinal and extraintestinal amebiasis. The virulence of Entamoeba histolytica is strongly correlated with the parasite's capacity to effectively kill and phagocytose host cells. The process by which host cells are killed and phagocytosed follows a sequential model of adherence, cell killing, initiation of phagocytosis, and engulfment. This paper presents recent advances in the cytolytic and phagocytic processes of Entamoeba histolytica in context of the sequential model.

Acetaldehyde/alcohol dehydrogenase-2 (EhADH2) and clathrin are involved in internalization of human transferrin by Entamoeba histolytica

Transferrin (Tf) is a host glycoprotein capable of binding two ferric-iron ions to become holotransferrin (holoTf), which transports iron in to all cells. Entamoeba histolytica is a parasitic protozoan able to use holoTf as a sole iron source in vitro. The mechanism by which this parasite scavenges iron from holoTf is unknown. An E. histolytica holoTf-binding protein (EhTfbp) was purified by using an anti-human transferrin receptor (TfR) monoclonal antibody. EhTfbp was identified by MS/MS analysis and database searches as E. histolytica acetaldehyde/alcohol dehydrogenase-2 (EhADH2), an iron-dependent enzyme. Both EhTfbp and EhADH2 bound holoTf and were recognized by the anti-human TfR antibody, indicating that they correspond to the same protein. It was found that the amoebae internalized holoTf through clathrin-coated pits, suggesting that holoTf endocytosis could be important for the parasite during colonization and invasion of the intestinal mucosa and liver.

The antiretroviral lectin cyanovirin-N targets well-known and novel targets on the surface of Entamoeba histolytica trophozoites

Entamoeba histolytica, the protist that causes amebic dysentery and liver abscess, has a truncated Asn-linked glycan (N-glycan) precursor composed of seven sugars (Man(5)GlcNAc(2)). Here, we show that glycoproteins with unmodified N-glycans are aggregated and capped on the surface of E. histolytica trophozoites by the antiretroviral lectin cyanovirin-N and then replenished from large intracellular pools. Cyanovirin-N cocaps the Gal/GalNAc adherence lectin, as well as glycoproteins containing O-phosphodiester-linked glycans recognized by an anti-proteophosphoglycan monoclonal antibody. Cyanovirin-N inhibits phagocytosis by E. histolytica trophozoites of mucin-coated beads, a surrogate assay for amebic virulence. For technical reasons, we used the plant lectin concanavalin A rather than cyanovirin-N to enrich secreted and membrane proteins for mass spectrometric identification. E. histolytica glycoproteins with occupied N-glycan sites include Gal/GalNAc lectins, proteases, and 17 previously hypothetical proteins. The latter glycoproteins, as well as 50 previously hypothetical proteins enriched by concanavalin A, may be vaccine targets as they are abundant and unique. In summary, the antiretroviral lectin cyanovirin-N binds to well-known and novel targets on the surface of E. histolytica that are rapidly replenished from large intracellular pools.

Localization of phosphatidylinositol (3,4,5)-trisphosphate to phagosomes in entamoeba histolytica achieved using glutathione S-transferase- and green fluorescent protein-tagged lipid biosensors

Entamoeba histolytica is an intestinal protozoan parasite that causes amoebic dysentery and liver abscess. Phagocytosis by the parasite is a critical virulence process, since it is a prerequisite for tissue invasion and establishment of chronic infection. While the roles of many of the proteins that regulate phagocytosis-related signaling events in E. histolytica have been characterized, the functions of lipids in this cellular process remain largely unknown in this parasite. In other systems, phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), a major product of phosphoinositide 3 kinase (PI3-kinase) activity, is essential for phagocytosis. Pleckstrin homology (PH) domains are protein domains that specifically bind to PIP(3). In this study, we utilized glutathione S-transferase (GST)- and green fluorescent protein (GFP)-labeled PH domains as lipid biosensors to characterize the spatiotemporal aspects of PIP(3) distribution during various endocytic processes in E. histolytica. PIP(3)-specific biosensors accumulated at extending pseudopodia and in phagosomal cups in trophozoites exposed to erythrocytes but did not localize to pinocytic compartments during the uptake of a fluid-phase marker, dextran. Our results suggest that PIP(3) is involved in the early stages of phagosome formation in E. histolytica. In addition, we demonstrated that PIP(3) exists at high steady-state levels in the plasma membrane of E. histolytica and that these levels, unlike those in mammalian cells, are not abolished by serum withdrawal. Finally, expression of a PH domain in trophozoites inhibited erythrophagocytosis and enhanced motility, providing genetic evidence supporting the role of PI3-kinase signaling in these processes in E. histolytica.

Entamoeba histolyticaEhGEF1 structure and mutational analysis: New specific residues critical for function

This paper reports the EhGEF1-EhRacG and EhGEF1-EhRho1 molecular complexes from Entamoeba histolytica. The not conserved amino acids Gln201,Tyr299, Gln302, Lys312, Asn313, Phe314 and Ile324 were localized, by means of an in silico computational analysis, at the interface of the exposed face from the DH domain of EhGEF1, which are important to establish the contact with its target GTPases EhRacG and EhRho1. Functional studies of nucleotide exchange of Phe314Ala mutant showed a decrement of 80% on EhRacG GTPase; in contrast the Ile324Ala mutant exhibited a reduction of 77%, specifically on EhRho1; meanwhile the Gln302Ala mutant showed a reduction of approximately 50% on the exchange activity for both GTPases. Moreover, the functional studies of the protein EhGEF1 mutants in the conserved residues Thr194Ala, Asn366Ala and Glu367Ala indicated that contrary to what has been reported for other systems, the mutation of these residues did not alter considerably its catalytic activity.

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.

C1q- and collectin-dependent phagocytosis of apoptotic host cells by the intestinal protozoan Entamoeba histolytica

BACKGROUND

Entamoeba histolytica, the cause of invasive amebiasis, phagocytoses apoptotic host cells during tissue invasion. In mammals, collectin family members (e.g., mannose-binding lectin [MBL]) and the structurally related protein C1q bind to apoptotic cells and stimulate macrophage phagocytosis via a conserved collagenous tail domain. The collectins also bind to bacteria, the usual source of nutrients for E. histolytica.

METHODS

To test the possibility that the collectins are ligands that stimulate E. histolytica phagocytosis, we used a flow cytometry-based assay for amebic phagocytosis, a method for making single-ligand particles to delineate a given ligand's ability to initiate phagocytosis, and purified human C1q, MBL, and collagenous collectin tails.

RESULTS

Apoptotic lymphocytes opsonized with serum or human C1q were phagocytosed more efficiently than control cells, an effect that was dependent on ligand density. C1q and the collectins alone were adequate to trigger amebic phagocytosis, because single-ligand particles coated with C1q, MBL, or purified collectin tails were phagocytosed more efficiently than control particles. Furthermore, C1q, MBL, and the tail domain of C1q were all chemoattractants for E. histolytica.

CONCLUSIONS

C1q and MBL can serve as opsonins on apoptotic cells that stimulate E. histolytica phagocytosis, an effect mediated at least in part by the collagenous collectin tail domain.

Entamoeba histolytica encodes unique formins, a subset of which regulates DNA content and cell division

The formin family of proteins mediates dynamic changes in actin assembly in eukaryotes, and therefore it is important to understand the function of these proteins in Entamoeba histolytica, where actin forms the major cytoskeletal network. In this study we have identified the formin homologs encoded in the E. histolytica genome based on sequence analysis. Using multiple tools, we have analyzed the primary sequences of the eight E. histolytica formins and discovered three subsets: (i) E. histolytica formin-1 to -3 (Ehformin-1 to -3), (ii) Ehformin-4, and (iii) Ehformin-5 to -8. Two of these subsets (Ehformin-1 to -3 and Ehformin-4) showed significant sequence differences from their closest homologs, while Ehformin-5 to -8 were unique among all known formins. Since Ehformin-1 to -3 showed important sequence differences from Diaphanous-related formins (DRFs), we have studied the functions of Ehformin-1 and -2 in E. histolytica transformants. Like other DRFs, Ehformin-1 and -2 associated with F-actin in response to serum factors, in pseudopodia, in pinocytic and phagocytic vesicles, and at cell division sites. Ehformin-1 and -2 also localized with the microtubular assembly in the nucleus, indicating their involvement in genome segregation. While increased expression of Ehformin-1 and -2 did not affect phagocytosis or motility, it clearly showed an increase in the number of binucleated cells, the number of nuclei in multinucleated cells, and the average DNA content of each nucleus, suggesting that these proteins regulate both mitosis and cytokinesis in E. histolytica.

Genome re-duplication and irregular segregation occur during the cell cycle of Entamoeba histolytica

Heterogeneity of genome content is commonly observed in axenic cultures of Entamoeba histolytica. Cells with multiple nuclei and nuclei with heterogenous genome contents suggest that regulatory mechanisms that ensure alternation of DNA synthesis and mitosis are absent in this organism. Therefore, several endo-reduplicative cycles may occur without mitosis. The data also shows that unlike other endo-reduplicating organisms, E.histolytica does not undergo a precise number of endo-reduplicative cycles. We propose that irregular endo-reduplication and genome partitioning lead to heterogeneity in the genome content of E.histolytica trophozoites in their proliferative phase. The goal of future studies should be aimed at understanding the mechanisms that are involved in (a) accumulation of multiple genome contents in a single nucleus; (b) genome segregation in nuclei that contain multiple genome contents and (c) maintenance of genome fidelity in E. histolytica.