Role of signalling and cytoskeletal rearrangements in the pathogenesis of Entamoeba histolytica

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.

The evolution and diversity of actin-dependent cell migration

Many eukaryotic cells, including animal cells and unicellular amoebae, use dynamic-actin networks to crawl across solid surfaces. Recent discoveries of actin-dependent crawling in additional lineages have sparked interest in understanding how and when this type of motility evolved. Tracing the evolution of cell crawling requires understanding the molecular mechanisms underlying motility. Here we outline what is known about the diversity and evolution of the molecular mechanisms that drive cell motility, with a focus on actin-dependent crawling. Classic studies and recent work have revealed a surprising number of distinct mechanical modes of actin-dependent crawling used by different cell types and species to navigate different environments. The overlap in actin network regulators driving multiple types of actin-dependent crawling, along with cortical-actin networks that support the plasma membrane in these cells, suggest that actin motility and cortical actin networks might have a common evolutionary origin. The rapid development of additional evolutionarily diverse model systems, advanced imaging technologies, and CRISPR-based genetic tools, is opening the door to testing these and other new ideas about the evolution of actin-dependent cell crawling.

Molecular Characterization of the Interplay between Fasciola hepatica Juveniles and Laminin as a Mechanism to Adhere to and Break through the Host Intestinal Wall

Fasciola hepatica is the main causative agent of fasciolosis, a zoonotic parasitic disease of growing public health concern. F. hepatica metacercariae are ingested by the host and excyst in the intestine, thereby releasing the newly excysted juveniles (FhNEJ), which traverse the gut wall and migrate towards the biliary ducts. Since blocking F. hepatica development is challenging after crossing of the intestinal wall, targeting this first step of migration might result in increased therapeutic success. The intestinal extracellular matrix (ECM) is constituted by a network of structural proteins, including laminin (LM) and fibronectin (FN), that provide mechanical support while acting as physical barrier against intestinal pathogens. Here, we employed ELISA and immunofluorescent assays to test for the presence of LM- and FN-binding proteins on a tegument-enriched antigenic fraction of FhNEJ, and further determined their identity by two-dimensional electrophoresis coupled to mass spectrometry. Additionally, we performed enzymatic assays that revealed for the first time the capability of the juvenile-specific cathepsin L3 to degrade LM, and that LM degradation by FhNEJ proteins is further potentiated in the presence of host plasminogen. Finally, a proteomic analysis showed that the interaction with LM triggers protein changes in FhNEJ that may be relevant for parasite growth and adaptation inside the mammalian host. Altogether, our study provides valuable insights into the molecular interplay between FhNEJ and the intestinal ECM, which may lead to the identification of targetable candidates for the development of more effective control strategies against fasciolosis.

Identification of differential protein recognition pattern between Naegleria fowleri and Naegleria lovaniensis

Many pathogenicity factors are involved in the development of primary amoebic meningoencephalitis (PAM) caused by N fowleri. However, most of them are not exclusive for N fowleri and they have not even been described in other nonpathogenic Naegleria species. Therefore, the objective of this work was to identify differential proteins and protein pattern recognition between Naegleria fowleri and Naegleria lovaniensis using antibodies anti-N fowleri as strategy to find vaccine candidates against meningoencephalitis. Electrophoresis and Western blots conventional and 2-DE were performed for the identification of antigenic proteins, and these were analysed by the mass spectrometry technique. The results obtained in 2-DE gels and Western blot showed very notable differences in spot intensity between these two species, specifically those with relative molecular weight of 100, 75, 50 and 19 kDa. Some spots corresponding to these molecular weights were identified as actin fragment, myosin II, heat shock protein, membrane protein Mp2CL5 among others, with differences in theoretical post-translational modifications. In this work, we found differences in antigenic proteins between both species, proteins that could be used for a further development of vaccines against N fowleri infection.

Adherens junctions and desmosomes are damaged by Entamoeba histolytica: Participation of EhCPADH complex and EhCP112 protease

Entamoeba histolytica trophozoites adhere to epithelium at the cell-cell contact and perturb tight junctions disturbing the transepithelial electrical resistance. Behind tight junctions are the adherens junctions (AJs) that reinforce them and the desmosomes (DSMs) that maintain the epithelium integrity. The damage produced to AJs and DMSs by this parasite is unknown. Here, we studied the effect of the trophozoites, the EhCPADH complex, and the EhCP112 recombinant enzyme (rEhCP112) on AJ and DSM proteins. We found that trophozoites degraded β-cat, E-cad, Dsp l/ll, and Dsg-2 with the participation of EhCPADH and EhCP112. After contact of epithelial cells with trophozoites, immunofluorescence and transmission electron microscopy assays revealed EhCPADH and rEhCP112 at the intercellular space where they colocalised with β-cat, E-cad, Dsp l/ll, and Dsg-2. Moreover, our results suggested that rEhCP112 could be internalised by caveolae and clathrin-coated vesicles. Immunoprecipitation assays showed the interaction of EhCPADH with β-cat and Dsp l/ll. Besides, in vivo assays demonstrated that rEhCP112 concentrates at the cellular borders of the mouse intestine degrading E-cad and Dsp I/II. Our research gives the first clues on the trophozoite attack to AJs and DSMs and point out the role of the EhCPADH and EhCP112 in the multifactorial event of trophozoites virulence.

Amoeboid movement in protozoan pathogens

Entamoeba histolytica, the causative agent of amoebiasis, is a protozoan parasite characterised by its amoeboid motility, which is essential to its survival and invasion of the human host. Elucidating the molecular mechanisms leading to invasion of human tissues by E. histolytica requires a quantitative understanding of how its cytoskeleton deforms and tailors its mode of migration to the local microenvironment. Here we review the wide range of methods available to extract biophysical information from amoeboid cells, from interventional techniques to computational modelling approaches, and discuss how recent developments in bioimaging and bioimage informatics can complement our understanding of cellular morphodynamics at the intracellular level.

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.

In Entamoeba histolytica, a BspA family protein is required for chemotaxis toward tumour necrosis factor

BACKGROUND

Entamoeba histolytica cell migration is essential for the development of human amoebiasis (an infectious disease characterized by tissue invasion and destruction). The tissue inflammation associated with tumour necrosis factor (TNF) secretion by host cells is a well-documented feature of amoebiasis. Tumour necrosis factor is a chemoattractant for E. histolytica, and the parasite may have a TNF receptor at its cell surface.

METHODS

confocal microscopy, RNA Sequencing, bioinformatics, RNA antisense techniques and histological analysis of human colon explants were used to characterize the interplay between TNF and E. histolytica.

RESULTS

an antibody against human TNF receptor 1 (TNFR1) stained the E. histolytica trophozoite surface and (on immunoblots) binds to a 150-kDa protein. Proteome screening with the TNFR1 sequence revealed a BspA family protein in E. histolytica that carries a TNFR signature domain and six leucine-rich repeats (named here as "cell surface protein", CSP, in view of its cellular location). Cell surface protein shares structural homologies with Toll-Like receptors, colocalizes with TNF and is internalized in TNF-containing vesicles. Reduction of cellular CSP levels abolished chemotaxis toward TNF and blocked parasite invasion of human colon.

CONCLUSIONS

there is a clear link between TNF chemotaxis, CSP and pathogenesis.

Entamoeba histolytica rhomboid protease 1 has a role in migration and motility as validated by two independent genetic approaches

Rhomboid proteins represent a recently discovered family of intramembrane proteases present in a broad range of organisms and with increasing links to human diseases. The enteric parasite Entamoeba histolytica has evolved multiple mechanisms to adapt to the human host environment and establish infection. Our recent studies identified EhROM1 as a functional E. histolytica rhomboid protease with roles in adhesion to and phagocytosis of host cells. Since those studies were performed in a non-virulent strain, roles in parasite virulence could not be assessed. We focused this study on the comparison and validation of two genetic manipulation techniques: overexpression of a dominant-negative catalytic mutant of EhROM1 and knock down of EhROM1 using a RNAi-based silencing approach followed by functional studies of phenotypic analyses in virulent parasites. Both the EhROM1 catalytic mutant and parasites with EhROM1 downregulation were reduced in cytotoxicity, hemolytic activity, and directional and non-directional transwell migration. Importantly, the role for EhROM1 in cell migration mimics similar roles for rhomboid proteases from mammalian and apicomplexan systems. However, the EhROM1 catalytic mutant and EhROM1 downregulation parasites had different phenotypes for erythrophagocytosis, while complement resistance was not affected in either strain. In summary, in this study we genetically manipulated E. histolytica rhomboid protease EhROM1 by two different approaches and identified similarly attenuated phenotypes by both approaches, suggesting a novel role for EhROM1 in amebic motility.

Amoebic PI3K and PKC is required for Jurkat T cell death induced by Entamoeba histolytica

The enteric protozoan parasite Entamoeba histolytica is the causative agent of human amebiasis. During infection, adherence of E. histolytica through Gal/GalNAc lectin on the surface of the amoeba can induce caspase-3-dependent or -independent host cell death. Phosphorylinositol 3-kinase (PI3K) and protein kinase C (PKC) in E. histolytica play an important function in the adhesion, killing, or phagocytosis of target cells. In this study, we examined the role of amoebic PI3K and PKC in amoeba-induced apoptotic cell death in Jurkat T cells. When Jurkat T cells were incubated with E. histolytica trophozoites, phosphatidylserine (PS) externalization and DNA fragmentation in Jurkat cells were markedly increased compared to those of cells incubated with medium alone. However, when amoebae were pretreated with a PI3K inhibitor, wortmannin before being incubated with E. histolytica, E. histolytica-induced PS externalization and DNA fragmentation in Jurkat cells were significantly reduced compared to results for amoebae pretreated with DMSO. In addition, pretreatment of amoebae with a PKC inhibitor, staurosporine strongly inhibited Jurkat T cell death. However, E. histolytica-induced cleavage of caspase-3, -6, and -7 were not inhibited by pretreatment of amoebae with wortmannin or staurosporin. In addition, we found that amoebic PI3K and PKC have an important role on amoeba adhesion to host compartment. These results suggest that amebic PI3K and PKC activation may play an important role in caspase-independent cell death in Entamoeba-induced apoptosis.

Localization of phosphatidylinositol 4,5-bisphosphate to lipid rafts and uroids in the human protozoan parasite Entamoeba histolytica

Entamoeba histolytica is an intestinal protozoan parasite and is the causative agent of amoebiasis. During invasive infection, highly motile amoebae destroy the colonic epithelium, enter the blood circulation, and disseminate to other organs such as liver, causing liver abscess. Motility is a key factor in E. histolytica pathogenesis, and this process relies on a dynamic actomyosin cytoskeleton. In other systems, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is known to regulate a wide variety of cellular functions, including signal transduction, actin remodeling, and cell motility. Little is known about the role of PI(4,5)P2 in E. histolytica pathogenicity. In this study, we demonstrate that PI(4,5)P2 is localized to cholesterol-rich microdomains, lipid rafts, and the actin-rich fractions of the E. histolytica membrane. Microscopy revealed that the trailing edge of polarized trophozoites, uroids, are highly enriched in lipid rafts and their constituent lipid, PI(4,5)P2. Polarization and enrichment of uroids and rafts with PI(4,5)P2 were enhanced upon treatment of E. histolytica cells with cholesterol. Exposure to cholesterol also increased intracellular calcium, which is a downstream effector of PI(4,5)P2, with a concomitant increase in motility. Together, our data suggest that in E. histolytica, PI(4,5)P2 may signal from lipid rafts and cholesterol may play a role in triggering PI(4,5)P2-mediated signaling to enhance the motility of this pathogen.

Structural determinants of RGS-RhoGEF signaling critical to Entamoeba histolytica pathogenesis

G protein signaling pathways, as key components of physiologic responsiveness and timing, are frequent targets for pharmacologic intervention. Here, we identify an effector for heterotrimeric G protein α subunit (EhGα1) signaling from Entamoeba histolytica, the causative agent of amoebic colitis. EhGα1 interacts with this effector and guanosine triphosphatase-accelerating protein, EhRGS-RhoGEF, in a nucleotide state-selective fashion. Coexpression of EhRGS-RhoGEF with constitutively active EhGα1 and EhRacC leads to Rac-dependent spreading in Drosophila S2 cells. EhRGS-RhoGEF overexpression in E. histolytica trophozoites leads to reduced migration toward serum and lower cysteine protease activity, as well as reduced attachment to, and killing of, host cells. A 2.3 Å crystal structure of the full-length EhRGS-RhoGEF reveals a putative inhibitory helix engaging the Dbl homology domain Rho-binding surface and the pleckstrin homology domain. Mutational analysis of the EhGα1/EhRGS-RhoGEF interface confirms a canonical "regulator of G protein signaling" domain rather than a RhoGEF-RGS ("rgRGS") domain, suggesting a convergent evolution toward heterotrimeric and small G protein cross-talk.

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.

A functional study of nucleocytoplasmic transport signals of the EhNCABP166 protein from Entamoeba histolytica

EhNCABP166 is an Entamoeba histolytica actin-binding protein that localizes to the nucleus and cytoplasm. Bioinformatic analysis of the EhNCABP166 amino acid sequence shows the presence of 3 bipartite nuclear localization signals (NLS) and a nuclear export signal (NES). The present study aimed to investigate the functionality of these signals in 3 ways. First, we fused each potential NLS to a cytoplasmic domain of ehFLN to determine whether the localization of this domain could be altered by the presence of the NLSs. Furthermore, the localization of each domain of EhNCABP166 was determined. Similarly, we generated mutations in the first block of bipartite signals from the domains that contained these signals. Additionally, we added an NES to 2 constructs that were then evaluated. We confirmed the intranuclear localization of EhNCABP166 using transmission electron microscopy. Fusion of each NLS resulted in shuttling of the cytoplasmic domain to the nucleus. With the exception of 2 domains, all of the evaluated domains localized within the nucleus. A mutation in the first block of bipartite signals affected the localization of the domains containing an NLS. The addition of an NES shifted the localization of these domains to the cytoplasm. The results presented here establish EhNCABP166 as a protein containing functional nuclear localization signals and a nuclear export signal.

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.

The domain structure of Entamoeba α-actinin2

Entamoeba histolytica, a major agent of human amoebiasis, expresses two distinct forms of α-actinin, a ubiquitous actin-binding protein that is present in most eukaryotic organisms. In contrast to all metazoan α-actinins, in both isoforms the intervening rod domain that connects the N-terminal actin-binding domain with the C-terminal EF-hands is much shorter. It is suggested that these α-actinins may be involved in amoeboid motility and phagocytosis, so we cloned and characterised each domain of one of these α-actinins to better understand their functional role. The results clearly showed that the domains have properties very similar to those of conventional α-actinins.

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.

Identification and molecular characterization of numerous Histomonas meleagridis proteins using a cDNA library

SUMMARYHistomonas meleagridis is a protozoan parasite of various galliform birds causing a type of enterohepatitis termed histomonosis or 'blackhead disease'. Due to the ban of chemotherapeutic substances and an increase in free-range poultry production, histomonosis is currently a re-emerging disease. So far limited molecular knowledge is available. In the present work, mRNAs coding for antigenic proteins of H. meleagridis were identified. For this purpose, a cDNA expression library was constructed from a mono-eukaryotic culture of H. meleagridis. The library was screened with polyclonal rabbit serum raised against purified H. meleagridis trophozoites. Polyclonal rabbit serum specifically recognized the same major H. meleagridis antigens as chicken and turkey sera originating from animal trials, but displayed a significantly lower bacteria-dependent background signal. After 2 rounds of screening, a total of 95 positive clones were sequenced. Bioinformatics analyses were performed on nucleotide and deduced amino acid sequences, identifying 37 unique clones. Based on the homology to other protozoan parasites, mostly Trichomonas vaginalis, the clones were grouped according to functional aspects: structural proteins, possible surface proteins, oxygen reducing proteins, ribosomal proteins, protein kinases and various other intracellular proteins.

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.

EhLimA, a novel LIM protein, localizes to the plasma membrane in Entamoeba histolytica

The parasitic protozoan Entamoeba histolytica relies on a very dynamic cytoskeleton in order to invade and survive in host tissues. Identification of cytoskeletal elements is key to understanding these processes. Here we present the characterization of EhLimA, the first LIM protein of E. histolytica. EhLimA consists of a single LIM domain at its N terminus and exhibits the highest degree of homology with DdLimE from Dictyostelium discoideum. Immunofluorescence localization of EhLimA using anti-EhLimA antibodies revealed that EhLimA is highly concentrated at the plasma membrane of cells. Silencing or overexpression of the EhLimA gene did not have a significant effect on the growth or morphology of the parasite. EhLimA associates with the cytoskeleton as demonstrated by the enrichment of the protein in cytoskeleton fractions as well as in pull-down assays that revealed that cytoskeleton association involves interaction with actin. EhLimA binding to actin was shown to be dependent on the N-terminal LIM domain of EhLimA, as removal of even half of the LIM domain resulted in almost complete inhibition of the binding to actin. We also found that a portion of EhLimA floats to the lower-density regions of a sucrose gradient together with portions of the Gal-lectin light subunit and actin. Treatment of cells with the cholesterol-sequestering agent digitonin resulted in increased solubility of EhLimA. These results indicate that in addition to cytoskeletal association, EhLimA may also associate with lipid rafts in the parasite plasma membrane and suggest that EhLimA may be part of the molecular system connecting the actin cytoskeleton to membrane rafts.

Initiation of inflammation and cell death during liver abscess formation by Entamoeba histolytica depends on activity of the galactose/N-acetyl-d-galactosamine lectin

The parasite Entamoeba histolytica colonizes the human intestine causing amoebic colitis and disseminates through the vascular route to form liver abscesses. The Gal/GalNAc lectin is an adhesion protein complex which sustains tissue invasion by E. histolytica. Disruption of the Gal/GalNAc lectin function in engineered parasites (HGL-2 trophozoites) changed the pathophysiology of hamster liver abscess formation. HGL-2 trophozoites produced numerous small inflammatory foci located in the vicinity of blood vessels. The low penetration of HGL-2 trophozoites into hepatic tissue was shown to be associated with weak attraction of neutrophils and macrophages to the infiltrated areas and absence of pro-inflammatory tumour necrosis factor, in contrast to wild type or control vector infections. The low host inflammatory response in HGL-2 infections correlated with a delay in apoptosis of hepatic cells, whereas apoptosis of endothelial cells was not detected. Triggering of apoptosis in both host cell types most likely has a central role in modulating inflammation, a major landmark in hepatic amoebiasis. These data highlight the key role of the Gal/GalNAc lectin in initiation of E. histolytica hepatic infection.

Signaling through the phosphatidylinositol 3-kinase regulates mechanotaxis induced by local low magnetic forces in Entamoeba histolytica

In micro-organisms, as well as in metazoan cells, cellular polarization and directed migration are finely regulated by external stimuli, including mechanical stresses. The mechanisms sustaining the transduction of such external stresses into intracellular biochemical signals remain mainly unknown. Using an external magnetic tip, we generated a magnetic field gradient that allows migration analysis of cells submitted to local low-intensity magnetic forces (50 pN). We applied our system to the amoeba Entamoeba histolytica. Indeed, motility and chemotaxis are key activities that allow this parasite to invade and destroy the human tissues during amoebiasis. The magnetic force was applied either inside the cytoplasm or externally at the rear pole of the amoeba. We observed that the application of an intracellular force did not affect cell polarization and migration, whereas the application of the force at the rear pole of the cell induced a persistent polarization and strongly directional motion, almost directly opposed to the magnetic force. This phenomenon was completely abolished when phosphatidylinositol 3-kinase activity was inhibited by wortmanin. This result demonstrated that the applied mechanical stimulus was transduced and amplified into an intracellular biochemical signal, a process that allows such low-intensity force to strongly modify the migration behavior of the cell.

Entamoeba histolytica cell movement: a central role for self-generated chemokines and chemorepellents

Entamoeba histolytica cells, the cause of amoebic dysentery, are highly motile, and this motility is an essential feature of the pathogenesis and morbidity of amoebiasis. However, the control of E. histolytica motility within the gut and during invasion is poorly understood. We have used an improved chemotaxis assay to identify the key extracellular signals mediating Entamoeba chemotaxis. The dominant responses we observe are caused by factors generated by E. histolytica cells themselves. Medium that has been conditioned by E. histolytica growth causes both chemokinesis and negative chemotaxis. The speed of random movement is more than doubled in conditioned compared with fresh medium, and cells move efficiently away from conditioned medium by negative chemotaxis. Ethanol, the product of Entamoeba glucose metabolism, is the principal component of the chemokinetic response. The closely related but nonpathogenic Entamoeba dispar shows no change in motility in response to conditioned medium implying that these responses are central to E. histolytica pathogenesis.

The interaction between the amoebaBalamuthia mandrillarisand extracellular matrix glycoproteinsin vitro

Balamuthia mandrillaris, a soil amoeba, is the causative agent ofBalamuthiagranulomatous amoebic encephalitis, a life-threatening brain infection. This amoeba is acquired from contaminated soil and may enter the host through cutaneous lesions or through nasal passages, migrating to the lungs or brain. During invasion,B. mandrillarishas access to components of the extracellular matrix (ECM) of the host. Therefore, we investigated the interaction ofB. mandrillariswith 3 ECM glycoproteins (collagen-I, fibronectin and laminin-1) that are encountered in host connective tissues and at the basal lamina. Using optical microscopy, amoeba association on ECM-coated surfaces was examined. Binding of amoebae on laminin was greater than that on collagen or fibronectin. Laminin-adheredB. mandrillarisexhibited elongated and spread forms, distinctive from those observed for amoebae on a plastic surface. Collagen and fibronectin-adheredB. mandrillarispresented elongated shapes with cellular expansions. Binding to collagen, fibronectin, or laminin was inhibited when amoebae were pre-treated with sialic acid. Treatment with galactose resulted in diminished binding of amoebae on laminin, while mannose increased binding in all coating conditions tested. Dependence of divalent cations on amoeba binding was demonstrated for laminin-amoeba interaction. Collectively, the results indicate thatB. mandrillarisrecognizes specific glycoproteins of the mammalian extracellular matrix.

Host tissue invasion by Entamoeba histolytica is powered by motility and phagocytosis

During amebiasis, E. histolytica motility is a key factor to achieve its progression across tissues. The pathogenicity of E. histolytica includes its capacity to phagocyte human cells. Motility requires polarization of E. histolytica that involves protrusion of a pseudopod containing actin and associated proteins [myosin IB, ABP-120 and a p21-activated kinase (PAK)] and whole-cell propulsion after contraction of the rear of the cell, where myosin II and F-actin are concentrated. An interesting characteristic of this parasite is the presence of two unique myosins (myosin II and unconventional myosin IB), in contrast to several actin genes. Little is known about the regulation of the actin-myosin cytoskeleton dynamics of E. histolytica, and a better understanding of signaling pathways that stimulate and coordinate regulators protein and cytoskeleton elements will provide new insight into the cell biology of the parasite and in amebiasis. Here we summarize the pleiotropic functions described for myosin II and PAK in E. histolytica. We propose that survival and pathogenicity of E. histolytica require an active actin-myosin cytoskeleton to cap surface receptors, to adhere to host components, to migrate through tissues, to phagocyte human cells and to form liver abscesses.

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.

Activation of MAPK kinase pathway by Gal/GalNAc adherence lectin of E. histolytica: gateway to host response

Amoebiasis caused by the protozoan parasite Entamoeba histolytica is one of the leading parasitic causes of morbidity and mortality in the developing countries. Among the variety of virulence factors, an adherence lectin (Gal/GalNAc, 260 kDa) has been known to mediate colonization and subsequent host responses. It is a major cell surface antigen which is universally recognized by the immune sera of patients with amoebic liver abscess (ALA). The role of this lectin in cytolysis and phagocytosis of human colonic mucin glycoproteins has also been established. The objective of the present study was to elucidate the signal transduction events induced in response to Entamoeba histolytica derived Gal/GalNAc lectin in the target epithelial cells. We have attempted to define a pathway in target cells that could link this immunodominant antigen to a known biological pathway for target cell activation and triggering of subsequent disease pathology/parasite survival. Lectin stimulated cells showed immediate rise in (Ca2+)i concentration corresponding to 1517.31+/-16.3 nM (approximately) at 0-2 min. The intracellular calcium also extruded from the cells as was measured by increase in calcium green-1 fluorescence. Expression of several protein kinases was checked by western blotting to delineate the signaling pathway. Results showed that the expression of PLA2, PI3K, Ras p21, Ras GAP, ERK-MAPK, p38MAPK and PKC was significantly increased. Expression of Raf-1 and MEK-1 was also found to be significant, as determined by intensity analysis. Overall, it indicated activation of MAPKinase pathway which is implicated in a variety of cellular functions. On the basis of our observations it can be stated that there is a calcium mediated activation of PKC in target cells, by lectin, which inturn activates cyclic nucleotides and other protein kinases. These protein kinases further phosphorylated downstream signals in a sequential manner, thus leading to the activation of MAPKinase cascade. Activation of MAPK cascade, in our studies, is implicated in a variety of physiological cellular functions including apoptosis, proliferation, cytoskeleton rearrangements and permeability changes. However, future screening of the genes responsible for the transcription and translation of new proteins and their biological functions in response to lectin stimulation will prove useful in understanding this host-parasite relationship.

Roles of cell adhesion and cytoskeleton activity in Entamoeba histolytica pathogenesis: a delicate balance

The protozoan parasite Entamoeba histolytica colonizes the human large bowel. Invasion of the intestinal epithelium causes amoebic colitis and opens the route for amoebic liver abscesses. The parasite relies on its dynamic actomyosin cytoskeleton and on surface adhesion molecules for dissemination in the human tissues. Here we show that the galactose/N-acetylgalactosamine (Gal/GalNAc) lectin clusters in focal structures localized in the region of E. histolytica that contacts monolayers of enterocytes. Disruption of myosin II activity impairs the formation of these structures and renders the trophozoites avirulent for liver abscess development. Production of the cytoplasmic domain of the E. histolytica Gal/GalNAc lectin in engineered trophozoites causes reduced adhesion to enterocytes. Intraportal delivery of these parasites to the liver leads to the formation of a large number of small abscesses with disorganized morphology that are localized in the vicinity of blood vessels. The data support a model for invasion in which parasite motility is essential for establishment of infectious foci, while the adhesion to host cells modulates the distribution of trophozoites in the liver and their capacity to migrate in the hepatic tissue.

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.

Myosin II and the Gal-GalNAc lectin play a crucial role in tissue invasion by Entamoeba histolytica

Entamoeba histolytica is the human parasite responsible of amoebiasis, during which highly motile trophozoites invade the intestinal epithelium leading to amoebic colitis, and disseminate via the blood circulation causing liver abscesses. The invasive process, central to the pathogenesis, is known to be driven by parasites motility. To investigate molecules responsible for in vivo motion, we performed a high resolution dynamic imaging analysis using two-photon laser scanning microscopy. Image analysis of the parasites during invasion of Caco-2 cell monolayers, an enterocyte-like model, and hamster liver shows that E. histolytica undergoes non-Brownian motion. However, studies of movements of parasite strains dominant negative for myosin II, a central component of the cytoskeleton, and for Gal-GalNAc lectin, a major adhesion molecule, indicate that myosin II is essential for E. histolytica intercellular motility through intestinal cell monolayers and for its motility in liver. In contrast, the Gal-GalNAc lectin exclusively triggers invasion of the liver. These observations are in agreement with emerging studies that highlight marked differences in the way that cells migrate in vitro in two dimensions versus in vivo in three dimensions. The approach that we have developed should be powerful to identify adhesive complexes required for in vivo cell migration in normal and pathogenic situations and may, thereby, lead to new therapeutic drug, for pathologies based on cell motility and adhesion.

Signal transduction through the Gal–GalNAc lectin of Entamoeba histolytica involves a spectrin-like protein

Capping followed by uroid formation in Entamoeba histolytica has been implicated in resistance against the host immune response during development of amoebiasis. The amebic actomyosin cytoskeleton is essential for such a process. A protein from the spectrin family co-localizes with the Gal-GalNAc lectin during capping of this surface protein complex. Co-localization is not observed when capping of the Gal-GalNAc lectin is specifically inhibited by production of the carboxyl-terminal region of its heavy chain that includes the lectin cytoplasmic tail. A peptide encompassing the lectin last 77 amino acids fused to glutathione-S-transferase interacts in vitro with purified spectrin. The spectrin-binding site was narrowed down to a stretch of 21 amino acids within the lectin cytoplasmic domain. This is the first report identifying an amino acid sequence involved in the interaction between the Gal-GalNAc lectin and cytoskeletal spectrin.

EhPAK, a member of the p21-activated kinase family, is involved in the control of Entamoeba histolytica migration and phagocytosis

Entamoeba histolytica migration is essential for the development of amoebiasis, a human disease characterised by invasion and destruction of tissues. Amoebic motility requires both polarisation of the cell and formation of a predominant pseudopod. As p21-activated kinases PAKs are known to regulate eukaryotic cell motility and morphology, we investigated the role of PAK in E. histolytica. We showed that the C-terminal domain of EhPAK comprised a constitutive kinase activity in vitro and that overproduction of this fragment, in E. histolytica, caused a significant reduction in amoeboid migration, as measured by dynamic image analysis, indicating an involvement of EhPAK in this process. A dramatic loss of polarity, as indicated by the increased number of membrane extensions all around E. histolytica, was also observed, suggesting that the N-terminal domain of EhPAK was necessary for maintenance of cell polarity. To support this view, we showed that despite the absence of the consensus motif to bind to Rac and Cdc42, the N-terminal domain of EhPAK bound to Rac1, suggesting that the N-terminal region was a regulatory domain. In addition, we also found an increased rate of human red blood cell phagocytosis, suggesting for the first time an active role for a PAK protein in this process. Taking together, the results suggest strongly that EhPAK is a key regulatory element in polarity, motility and phagocytosis of E. histolytica.

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

Previously, we characterized a 140-kDa protein from Entamoeba histolytica as a beta1-integrin-like molecule that binds fibronectin. In this work we present data showing that the amoebic receptor is associated with another surface molecule, the 220-kDa lectin, and with protein tyrosine kinase activity. By immunoprecipitation with the alphabeta1Eh antibody, we demonstrated by immune complex assays for tyrosine protein kinases that the amoebic fibronectin receptor was associated with two phosphorylated proteins of 50 and 70 kDa when internal membranes were used as the source of antigen. When cells were stimulated with fibronectin, two proteins of 55 and 90 kDa were tyrosine phosphorylated, as shown by Western blot with alphaPY20, its phosphorylation being time dependent after fibronectin stimulation. However, when the actin cytoskeleton of fibronectin-stimulated trophozoites was stabilized with phalloidin, the level and the pattern of phosphorylated proteins were different. In this case, a high-molecular-weight component, heavily phosphorylated, was present, which may include the 220-kDa lectin. We also present data confirming that the signaling pathway that is activated by fibronectin is specific. This was demonstrated by comparing the pattern of phosphoproteins obtained in immune complexes prepared with alphabeta1Eh, alphaL220, and alphaPY20 from total extracts obtained in the presence of phalloidin, from cells that had been exposed to fibronectin, soluble concanavalin A, or concanavalin-A-coated substrate. The presence of tyrosine kinases associated with the beta1-integrin-like amoebic molecule was confirmed by immunoprecipitation assays along with the combined use of a tyrosine kinase-specific substrate, the peptide RR-SRC, and a tyrosine kinase inhibitor, genistein.

Cell polarization and adhesion in a motile pathogenic protozoan: role and fate of the Entamoeba histolytica Gal/GalNAc lectin

The human pathogenic protozoan Entamoeba histolytica is a motile cell polarized into a front pseudopod and a rear uroid. The amoebic Gal/GalNAc surface lectin is a major adhesion molecule composed of an immunodominant 170-kDa heavy subunit, mostly extracellular except for a short cytoplasmic tail, and of an extracellular light subunit. The binding of multivalent ligands triggers lectin capping and recruitment to the uroid. The properties of the Gal/GalNAc lectin and its role in amoeba adhesion and uroid polarization are reviewed in the context of the molecular mechanisms underlying cell polarization and locomotion.

Myosin IB from Entamoeba histolytica is involved in phagocytosis of human erythrocytes

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery in humans. The disease is prevalent worldwide. Infection with E. histolytica results in invasion of the intestine by the parasite, followed by tissue damage and inflammation. During this invasive process, parasites kill and phagocytose human epithelial cells, immune cells and erythrocytes. Expression of amoebic pathogenicity requires a dynamic cytoskeleton that allows movement, tissue penetration and changes in parasite morphology. Myosin IB is a member of the myosin I family of motor proteins. Studies conducted both with Dictyostelium discoideum, a non-pathogenic amoeba, and with the yeast Saccharomyces cerevisiae indicate the involvement of myosin IB in cellular processes including movement, phagocytosis and endocytosis. Recently, we isolated the gene encoding myosin IB from E. histolytica. Thus, we decided to analyze the role of myosin IB in pathogenesis of amoeba. Using a specific anti-myosin IB antibody, this protein was localized in cell regions including the pseudopod, vesicles and underneath the plasma membrane. When E. histolytica was activated for erythrophagocytosis, myosin IB was markedly recruited to both the phagocytic cup and around internalized phagosomes. To analyze the role of myosin IB in phagocytosis, a strain overexpressing the myosin IB gene was constructed. This strain synthesizes threefold more myosin IB than the wild-type strain. Challenge of the transfected cell line with erythrocytes showed that these amoebae were deficient in erythrophagocytosis mainly in the uptake step, suggesting a role for myosin IB in the pathogenic activity of a human parasite.

Cloning and molecular characterization of a cDNA clone coding for Trichomonas vaginalis alpha-actinin and intracellular localization of the protein

We have identified and sequenced a cDNA clone coding for Trichomonas vaginalis alpha-actinin. Analysis of the obtained sequence revealed that the 2,857-nucleotide-long cDNA contained an open reading frame encoding 849 amino acids which showed consistent homology with alpha-actinins of different species. Such homology was particularly significant in regions which have been reported to represent the actin-binding and Ca2+-binding domains in other alpha-actinins. The deduced protein was also characterized by the presence of a divergent central region thought to play a role in its high immunogenicity. A study of protein localization performed by immunofluorescence revealed that the protein is diffusely distributed throughout the T. vaginalis cytoplasm when the cell is pear shaped. When parasites adhere and transform into the amoeboid morphology, the protein is located only in areas close to the cytoplasmic membrane and colocalizes with actin. Concomitantly with transformation into the amoeboid morphology, alpha-actinin mRNA expression is upregulated.

Host-pathogen interaction in amebiasis and progress in vaccine development

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

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

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