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

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.

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.

EhRho6 mediated actin degradation in Entamoeba histolytica is associated with compromised pathogenicity

Entamoeba histolytica causes amoebiasis which is a major health concern in developing countries. E. histolytica pathogenicity has been implicated to a large repertoire of small GTPases which switch between the inactive GDP bound state and the active GTP bound state with the help of guanine nucleotide exchange factors (GEFs) and GTPase activating protein (GAPs). Rho family of small GTPases are well known to modulate the actin cytoskeletal dynamics which plays a major role in E. histolytica pathogenicity. Here we report an atypical amoebic RhoGEF, and its preferred substrate EhRho6, which, upon overexpression abrogated the pathogenic behavior of the amoeba such as adhesion to host cell, monolayer destruction, erythrophagocytosis, and formation of actin dots. A causative immunoblot analysis revealed actin degradation in the EhRho6 overexpressing trophozoites that could be inhibited by blocking the amoebic proteasomal pathway. A careful analysis of the results from a previously published transcriptomics study, in conjunction with our observations, led to the identification of a clade of Rho GTPases in this pathogenic amoeba which we hypothesize to have implications during the amoebic encystation.

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.

Multinucleation and Polykaryon Formation is Promoted by the EhPC4 Transcription Factor in Entamoeba histolytica

Entamoeba histolytica is the intestinal parasite responsible for human amoebiasis that is a leading cause of death in developing countries. In this protozoan, heterogeneity in DNA content, polyploidy and genome plasticity have been associated to alterations in mechanisms controlling DNA replication and cell division. Studying the function of the transcription factor EhPC4, we unexpectedly found that it is functionally related to DNA replication, and multinucleation. Site-directed mutagenesis on the FRFPKG motif revealed that the K127 residue is required for efficient EhPC4 DNA-binding activity. Remarkably, overexpression of EhPC4 significantly increased cell proliferation, DNA replication and DNA content of trophozoites. A dramatically increase in cell size resulting in the formation of giant multinucleated trophozoites (polykaryon) was also found. Multinucleation event was associated to cytokinesis failure leading to abortion of ongoing cell division. Consistently, genome-wide profiling of EhPC4 overexpressing trophozoites revealed the up-regulation of genes involved in carbohydrates and nucleic acids metabolism, chromosome segregation and cytokinesis. Forced overexpression of one of these genes, EhNUDC (nuclear movement protein), led to alterations in cytokinesis and partially recapitulated the multinucleation phenotype. These data indicate for the first time that EhPC4 is associated with events related to polyploidy and genome stability in E. histolytica.

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.

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.

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.

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.

RNA interference in Entamoeba histolytica: implications for parasite biology and gene silencing

Entamoeba histolytica is a major health threat to people in developing countries, where it causes invasive diarrhea and liver abscesses. The study of this important human pathogen has been hindered by a lack of tools for genetic manipulation. Recently, a number of genetic approaches based on variations of the RNAi method have been successfully developed and cloning of endogenous small-interfering RNAs from E. histolytica revealed an abundant population of small RNAs with an unusual 5´-polyphosphate structure. However, little is known about the implications of these findings to amebic biology or the mechanisms of gene silencing in this organism. In this article we review the literature relevant to RNAi in E. histolytica, discuss its implications for advances in gene silencing in this organism and outline potential future directions towards understanding the repertoire of RNAi and its impact on the biology of this deep-branching eukaryotic parasite.

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.

Inter-cellular variation in DNA content of Entamoeba histolytica originates from temporal and spatial uncoupling of cytokinesis from the nuclear cycle

Accumulation of multiple copies of the genome in a single nucleus and several nuclei in a single cell has previously been noted in Entamoeba histolytica, contributing to the genetic heterogeneity of this unicellular eukaryote. In this study, we demonstrate that this genetic heterogeneity is an inherent feature of the cell cycle of this organism. Chromosome segregation occurs on a variety of novel microtubular assemblies including multi-polar spindles. Cytokinesis in E. histolytica is completed by the mechanical severing of a thin cytoplasmic bridge, either independently or with the help of neighboring cells. Importantly, cytokinesis is uncoupled from the nuclear division cycle, both temporally and spatially, leading to the formation of unequal daughter cells. Sorting of euploid and polyploid cells showed that each of these sub-populations acquired heterogeneous DNA content upon further growth. Our study conclusively demonstrates that genetic heterogeneity originates from the unique mode of cell division events in this protist.

Proliferating eukaryotic cells regulate their DNA synthesis, chromosome segregation, and cell division with great precision so that daughter cells are genetically identical. Our study demonstrates that in proliferating cells of the protist pathogen Entamoeba histolytica re-duplication of DNA followed by segregation on atypical and diverse microtubular structures is frequently observed. In this parasite, cell division is erratic, so that each daughter cell may contain one or more nuclei and sometimes no nuclei. This uncoupling of cell cycle events and survival of daughter cells with unequal DNA contents leads to genetic heterogeneity in E. histolytica. Our study highlights the inherent plasticity of the Entamoeba genome and the ability of this protist to survive in the absence of strict regulatory mechanisms that are a hallmark of the eukaryotic cell cycle.

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 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.

The endoplasmic reticulum chaperone calreticulin is recruited to the uropod during capping of surface receptors in Entamoeba histolytica

Calreticulin (CRT), an intracellular chaperone protein, is crucial for proper folding and transport of proteins through the endoplasmic reticulum (ER). It has recently been identified as a critical regulator of some several different cellular functions such as migration, phagocytosis of apoptotic cells and cytotoxic T lymphocyte- or natural killer cell-mediated lysis. Characterization of CRT isolated from parasites may thus help to decipher the contribution of this protein in the parasites' biology and host-parasite interactions. Here, we report descriptive data on the localization of Entamoeba histolytica's CRT at rest and following cap formation by Concanavalin A. As expected, CRT from E. histolytica localizes in the ER. However, the protein was surprisingly found to localize to the parasite surface and, furthermore, to concentrate in the uropod following activation of surface receptors by capping with Concanavalin A.

Molecular and functional characterization of EhPAK3, a p21 activated kinase from Entamoeba histolytica

p21-activated kinases (PAKs) are a family of serine/threonine kinases whose activity is regulated by the binding of the small Rho family GTPases as well as by RhoGTPase independent mechanisms. PAKs have wide-ranging functions which include cytoskeletal organisation, cell motility, cell proliferation and survival. We have identified a PAK from Entamoeba histolytica - EhPAK3 that is distributed in the cytoplasm of unstimulated cells and localizes to the caps after induction of capping with Concanavalin A. EhPAK3 contains a GTPase interacting (CRIB) domain, an N-terminal pleckstrin homology (PH) domain and a C-terminal kinase domain. Among the PAKs of E. histolytica studied so far, EhPAK3 bears the maximum similarity to Dictyostelium discoideum PAKC (DdPAKC). Phylogenetic analysis showed that EhPAK3 was closely related to DdPAKC and forms a group with DdPAKA, Dd Myosin I heavy chain kinase (DdMIHCK), and a PAK reported earlier from E. histolytica EhPAK2. Recombinant full-length EhPAK3 undergoes auotophosphorylation and phosphorylates histone H1 in vitro in the absence of any small GTPase. This is the first comprehensive characterization of a PAK protein from E. histolytica, which has constitutive activity and has demonstrated a strong involvement in receptor capping.

EhGEF3, a novel Dbl family member, regulates EhRacA activation during chemotaxis and capping inEntamoeba histolytica

Rho GTPases are critical elements involved in the regulation of signal transduction cascades from extracellular stimuli to cytoskeleton. The Rho guanine nucleotide exchange factors (RhoGEFs) have been implicated in direct activation of these GTPases. Here, we describe a novel RhoGEF, denominated EhGEF3 from the parasite Entamoeba histolytica, which encodes a 110 kDa protein containing the domain arrangement of a Dbl homology domain in tandem with a pleckstrin homology domain, the DH domain of EhGEF3 is closely related with the one of the Vav3 protein. Biochemical analysis revealed that EhGEF3 is capable of stimulating nucleotide exchange on the E. histolytica EhRacA and EhRho1 GTPases in vitro, however only a partial GEF activity toward Cdc42 was observed. Conserved residue analysis showed that the N816 and L817 residues are critical for EhGEF3 activity. Cellular studies revealed that EhGEF3 colocalises with EhRacA in the rear of migrating cells, probably regulating the retraction of the uroid and promoting the activation of these GTPase during the chemotactic response toward fibronectin, and that EhGEF3 also regulates EhRacA activation during the capping of cell receptors. These results suggest that EhGEF3 should have a direct role in activating EhRacA, and in bringing the activated GTPase to specific target sites such as the uroid.

EhGEF2, a Dbl-RhoGEF from Entamoeba histolytica has atypical biochemical properties and participates in essential cellular processes

Dbl proteins are a family of factors that exchange the guanine nucleotide which promote the activation of Rho small GTPases. This paper reports the molecular, structural, biochemical and functional characterization of EhGEF2, a new member of the Dbl family. EhGEF2 is the second GEF studied in parasites and in the protozoan Entamoeba histolytica, and it is also the first member of the Dbl family that was found to have Arm repeats. The catalytic domain (DH) of EhGEF2 has the conserved residues T421, N590 and E591, which are important for the activation of the GTPases. Biochemical studies on EhGEF2 showed that it could activate in vitro the amoebic GTPases EhRacA, EhRacB, EhRacC, EhRacD, EhRacG, EhRacH and EhCdc42, being EhRacG its main target. It was found that the DH domain binds specifically phosphatidic acid (PA); docking and lipid dot blot studies indicated that this binding does not interfere with the contact surface of EhRacG. Functional studies showed that both the Arm repeats and the catalytic domain of EhGEF2 participate in its localization at the amoebic membrane. Expression of a negative dominant version of EhGEF2 protein in E. histolytica provoked a 30% decrease in its ability to phagocyte human erythrocytes as well as severe effects on both the proliferation and the cellular chemotaxis which suggest that EhGEF2 participates in these cellular processes.

EhRho1, a RhoA-like GTPase of Entamoeba histolytica, is modified by clostridial glucosylating cytotoxins

Clostridial glucosylating cytotoxins inactivate mammalian Rho GTPases by mono-O glucosylation of a conserved threonine residue located in the switch 1 region of the target protein. Here we report that EhRho1, a RhoA-like GTPase from the protozoan parasite Entamoeba histolytica, is glucosylated by clostridial cytotoxins. Recombinant glutathione S-transferase-EhRho1 and EhRho1 from cell lysate of Entamoeba histolytica were glucosylated by Clostridium difficile toxin B and Clostridium novyi alpha-toxin. In contrast, Clostridium difficile toxin A, which shares the same mammalian protein substrates with toxin B, did not modify EhRho1. Change of threonine 52 of EhRho1 to alanine prevented glucosylation by toxin B from Clostridium difficile and by alpha-toxin from Clostridium novyi, which suggests that the equivalent threonine residues are glucosylated in mammalian and Entamoeba Rho GTPases. Lethal toxin from Clostridium sordellii did not glucosylate EhRho1 but labeled several other substrate proteins in lysates from Entamoeba histolytica in the presence of UDP-[14C]glucose.

EhPAK2, a novel p21-activated kinase, is required for collagen invasion and capping in Entamoeba histolytica

p21-activated kinases (PAKs) are a highly conserved family of enzymes that are activated by Rho GTPases. All PAKs contain an N-terminal Cdc42/Rac interacting binding (CRIB) domain, which confers binding to these GTPases, and a C-terminal kinase domain. In addition, some PAKs such as Cla4p, Skm1p and Pak2p contain an N-terminal pleckstrin homology (PH) domain and form a distinct group of PAK proteins involved in cell morphology, cell-cycle and gene transcription. Here, we describe a novel p21-activated kinase, denominated EhPAK2, on the parasitic protozoan Entamoeba histolytica. This is the first reported Entamoeba PAK member that contains a N-terminal PH domain and a highly conserved CRIB domain. EhPAK2 CRIB domain shares 29% of amino acid identity and 53% of amino acid homology with these of DdPAKC from Dictyostelium discoideum and Cla4p from Saccharomyces cerevisiae and binds in vitro and in vivo to EhRacA GTPase. This domain also possesses the conserved residues His123, Phe134 and Trp141, which are important for the interaction with the effector loop and strand beta2 of the GTPase; and the residues Met121 and Phe145, which are specific for the interaction of EhPAK2 with EhRacA. Functional studies of EhPAK2 showed that its C-terminal kinase domain had activity toward myelin basic protein. Cellular studies showed that Entamoeba trophozoites transfected with the vector pExEhNeo/kinase-myc, had a 90% decrease in the ability to invade a collagen matrix as well as severe defects in capping, suggesting the involvement of EhPAK2 in these cellular processes.

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.

Pleiotropic phenotype in Entamoeba histolytica overexpressing DNA methyltransferase (Ehmeth)

The protozoan parasite Entamoeba histolytica expresses a cytosine-5 DNA methyltransferase (Ehmeth) that belongs to the Dnmt2 proteins family. The biological function of members of the Dnmt2 family is unknown. Constitutive overexpression of Ehmeth resulted in a pleiotropic phenotype that includes accumulation of multinucleated cells, upregulation of Heat shock protein 70 (HSP70) expression and resistance to oxidative stress. This pleiotropic phenotype suggests that Ehmeth plays an important role in the control of key cellular processes in the parasite.

Characterization of protein geranylgeranyltransferase I from the enteric protist Entamoeba histolytica

Entamoeba histolytica is a unique protozoan parasite possessing both protein farnesyltransferase and geranylgeranyltrasferase I (GGT-I) for isoprenylation of small GTPases. In this study, we demonstrated unique enzymological properties of the amebic GGT-I (EhGGT-I), including substrate specificity and insensitivity to known mammalian inhibitors. Some of important residues of the catalytic beta subunit implicated in the specificity for GTPase acceptors and prenyl donors are substituted in EhGGT-I. Recombinant alpha and beta subunits of EhGGT-I, co-expressed in Escherichia coli, showed activity to transfer geranylgeranyl to both human wild-type (CVLS) and mutant (CVLL) H-Ras, while the mammalian GGT-I geranylgeranylated, but not farnesylated, only mutant H-Ras. All the representative amebic Ras and Rho/Rac small GTPases with phenylalanine, leucine, methionine, or alanine terminus were preferentially geranylgeranylated by EhGGT-I. This indicates that the acceptor specificity of the amebic GGT-I is remarkably broader than that of its mammalian counterpart. In contrast to EhFT, which farnesylates but not geranylgeranlylates solely EhRas4-CVVA, EhGGT-I also showed significant farnesyltransferase activity against Ras GTPase acceptors. EhGGT-I showed remarkable resistance to peptidomimetics known to inhibit mammalian GGT-I. Together with our previous observation that this parasite does not appear to depend on farnesylation for a majority of Ras and Rho/Rac, these data indicate that biological and biochemical advantages leading to the evolutional selection of this isoprenyl modification must exist uniquely in this parasitic protist. Finally, remarkable biochemical differences in binding to substrates and inhibitors between amebic and mammalian GGT-I highlight this enzyme as an attractive target for the development of new chemotherapeutics against amebiasis.

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.

Proteomic analysis of phagocytosis in the enteric protozoan parasite Entamoeba histolytica

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

A unique Rab GTPase, EhRabA, is involved in motility and polarization of Entamoeba histolytica cells

Entamoeba histolytica, an enteric protozoan parasite, infects 10% of the world's population leading to 50 million cases of invasive amoebiasis annually. Motility, which requires cell polarization, is important to the virulence of this pathogen, as it may result in destruction of host tissues and invasion. To gain insight into these processes in Entamoeba, a unique Rab GTPase, EhRabA, which localizes to the leading edge of cells, was characterized. Cell lines expressing a dominant negative version of EhRabA (EhRabA-DN) were generated. These mutant cells exhibited alterations in cell shape, polarity, and motility, supporting a role for this Rab in the regulation of these processes. Consistent with the notion that a dynamic actin cytoskeleton is crucial to cell polarity and motility, these mutants also exhibited alterations in the actin cytoskeleton. Cells expressing EhRabA-DN also displayed defects in several virulence functions including the ability to adhere to host cells, destroy host cells, and release cysteine proteases. Mislocalization of a prominent adhesion molecule, the galactose/N-acetylgalactosamine (Gal/GalNAc) adherence lectin and reorganization of ordered lipid domains, known as lipid rafts, also accompanied expression of EhRabA-DN. Interestingly, several endocytic processes were unaffected by expression of EhRabA-DN. Together, these data suggest that EhRabA may be involved in the regulation of polarization, motility and actin cytoskeletal dynamics: functions that participate in the pathogenicity of Entamoeba.

Entamoeba histolytica: inhibition of cellular functions by overexpression of EhGEF1, a novel Rho/Rac guanine nucleotide exchange factor

The molecular, biochemical, and cellular characterization of EhGEF1 protein is described. Complete cDNA sequence of 1890 bp revealed an open reading frame that encodes a protein of 69 kDa. EhGEF1 is constituted of Dbl homology domain, pleckstrin homology domain, and several putative regulation sites. Studies of guanine nucleotide exchange activity of EhGEF1 on several GTPases from Entamoeba histolytica and Homo sapiens showed preferential activation on EhRacG, suggesting that EhGEF1 protein could be involved in mechanisms related to actin cytoskeleton activation, cytokinesis, capping, and uroid formation in trophozoite. Confocal microscopy studies of pExEhNeo/HSV-tagged-EhGEF1-transfected cells showed that trophozoites stimulated with ConA, EhGEF1, and EhRacG were localized at plasma membrane. Cellular studies showed that F-actin content of pExEhNeo/HSV-tagged-EhGEF1-transfected trophozoites as well as cellular migration and cell damage capacity were significantly altered. The observations suggest that EhRacG was the principal target of EhGEF1 and that EhGEF1 may provide a link between F-actin dynamics and EhRacG signaling.

Rab5-associated Vacuoles Play a Unique Role in Phagocytosis of the Enteric Protozoan Parasite Entamoeba histolytica

In mammals, Rab5 and Rab7 play a specific and coordinated role in a sequential process during phagosome maturation. Here, we report that Rab5 and Rab7 in the enteric protozoan parasite Entamoeba histolytica, EhRab5 and EhRab7A, are involved in steps that are distinct from those known for mammals. EhRab5 and EhRab7A were localized to independent small vesicular structures at steady state. Priming with red blood cells induced the formation of large vacuoles associated with both EhRab5 and EhRab7A ("prephagosomal vacuoles (PPV)") in the amoeba within an incubation period of 5-10 min. PPV emerged de novo physically and distinct from phagosomes. PPV were gradually acidified and matured by fusion with lysosomes containing a digestive hydrolase, cysteine proteinase, and a membrane-permeabilizing peptide amoebapore. After EhRab5 dissociated from PPV, 5-10 min later, the EhRab7A-PPV fused with phagosomes, and EhRab7A finally dissociated from the phagosomes. Immunoelectron and light micrographs showed that PPV contained small vesicle-like structures containing fluid-phase markers and amoebapores, which were not evenly distributed within PPV, suggesting that the mechanism was similar to multivesicular body formation in PPV generation. In contrast to Rab5 from other organisms, EhRab5 was involved exclusively in phagocytosis, but not in endocytosis. Overexpression of wild-type EhRab5 enhanced phagocytosis and the transport of amoebapore to phagosomes. Conversely, expression of an EhRab5Q67L GTP form mutant impaired the formation of PPV and phagocytosis. Altogether, we propose that the amoebic Rab5 plays an important role in the formation of unique vacuoles, which is essential for engulfment of erythrocytes and important for packaging of lysosomal hydrolases, prior to the targeting to phagosomes.

Inhibition of gene expression with double strand RNA interference in Entamoeba histolytica

In order to inhibit gene expression in Entamoeba histolytica, we have developed a method based on expressing double strand RNA interference constructs in stable transformants. The 5' end of Eh Dia was cloned head to head with an intervening non-specific stuffer fragment in the E. histolytica expression vector pJST4. This construct was transformed in E. histolytica HM1:IMSS trophozoites and stable transformants were selected with 20microg/ml G418. Our results show that expression of Eh Dia was completely inhibited in these transformants. These stable transformants could be maintained indefinitely without expression of Eh Dia. This method therefore provides an effective tool to study the phenotypic changes, which occur due to inhibition of gene expression in the absence of mutants and other microbiological manipulations in this protozoan parasite.

Molecular Cloning and Characterization of a Protein Farnesyltransferase from the Enteric Protozoan Parasite Entamoeba histolytica

Genes encoding alpha- and beta-subunits of a putative protein farnesyltransferase (FT) from the enteric protozoan parasite Entamoeba histolytica were obtained and their biochemical properties were characterized. Deduced amino acid sequences of the alpha- and beta-subunit of E. histolytica FT (EhFT) were 298- and 375-residues long with a molecular mass of 35.6 and 42.6 kDa, and a pI of 5.43 and 5.65, respectively. They showed 24% to 36% identity to and shared common signature domains and repeats with those from other organisms. Recombinant alpha- and beta-subunits, co-expressed in Escherichia coli, formed a heterodimer and showed activity to transfer farnesyl using farnesylpyrophosphate as a donor to human H-Ras possessing a C-terminal CVLS, but not a mutant H-Ras possessing CVLL. Among a number of small GTPases that belong to the Ras superfamily from this parasite, we identified EhRas4, which possesses CVVA at the C terminus, as a sole farnesyl acceptor for EhFT. This is in contrast to mammalian FT, which utilizes a variety of small GTPases that possess a C-terminal CaaX motif, where X is serine, methionine, glutamine, cysteine, or alanine. EhFT also showed remarkable resistance against a variety of known inhibitors of mammalian FT. These results suggest that remarkable biochemical differences in binding to substrates and inhibitors exist between amebic and mammalian FTs, which highlights this enzyme as a novel target for the development of new chemotherapeutics against amebiasis.

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.

Phagocytosis of microbes: complexity in action

The phagocytic response of innate immune cells such as macrophages is defined by the activation of complex signaling networks that are stimulated by microbial contact. Many individual proteins have been demonstrated to participate in phagocytosis, and the application of high-throughput tools has indicated that many more remain to be described. In this review, we examine this complexity and describe how during recognition, multiple receptors are simultaneously engaged to mediate internalization, activate microbial killing, and induce the production of inflammatory cytokines and chemokines. Many signaling molecules perform multiple functions during phagocytosis, and these molecules are likely to be key regulators of the process. Indeed, pathogenic microorganisms target many of these molecules in their attempts to evade destruction.

MgcRacGAP is involved in cytokinesis through associating with mitotic spindle and midbody

We have recently cloned a cDNA for a full-length form of MgcRacGAP. Here we show using anti-MgcRacGAP antibodies that, unlike other known GAPs for Rho family, MgcRacGAP localized to the nucleus in interphase, accumulated to the mitotic spindle in metaphase, and was condensed in the midbody during cytokinesis. Overexpression of an N-terminal deletion mutant resulted in the production of multinucleated cells in HeLa cells. This mutant lost the ability to localize in the mitotic spindle and midbody. MgcRacGAP was also found to bind alpha-, beta-, and gamma-tubulins through its N-terminal myosin-like domain. These results indicate that MgcRacGAP dynamically moves during cell cycle progression probably through binding to tubulins and plays critical roles in cytokinesis. Furthermore, using a GAP-inactive mutant, we have shown that the GAP activity of MgcRacGAP is required for cytokinesis, suggesting that inactivation of the Rho family of GTPases may be required for normal progression of cytokinesis.

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.