Cell killing by the human parasite Entamoeba histolytica is inhibited by the rho-inactivating C3 exoenzyme

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.

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.

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.

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.

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.

Identification, structure, and phylogenetic relationships of a mitogen-activated protein kinase homologue from the parasitic protist Entamoeba histolytica

A gene encoding mitogen-activated protein kinase (MAPK) from the human enteric parasite, Entamoeba histolytica has been identified. Sequence analyses of the polymerase chain reaction (PCR) and reverse transcription PCR (RT-PCR) products reveal that the EhMAPK gene is intronless and encodes a protein of 352 amino acids. EhMAPK shows significant homology with other MAPKs and contains the 11 subdomains including the invariant residues characteristic of serine/threonine protein kinases. The MAPK signature residues and motifs are also present in EhMAPK. The atomic model of EhMAPK built with rat ERK2 as template exhibits the conservation of all major secondary structural features. However, a deletion in close proximity to the dual phosphorylation/activation site is of particular interest as it may have functional implications. Phylogenetic analysis indicates that EhMAPK is tightly clustered with Giardia intestinalis ERK2 and Dictyostelium discoideum ERK2. Detailed sequence analysis and phylogenetic study aided us to postulate that EhMAPK belongs to the extracellular signal-regulated kinase (ERK) family. Although EhMAPK bears good homology and phylogenetic closeness with human ERK8 and rat ERK7, sequence analysis indicates that they may be functionally different. The significant differences such as the deletions in the vicinity of the phosphorylation lip, variations in the P+1 specificity pocket, presence of additional acidic amino acids in the common docking domain provide a ground for postulations that activators and substrates for EhMAPK may be to some extent divergent from that of the ERKs of the mammalian host. Although functional characterization of EhMAPK remains to be done, this is the first study of any member of the MAPK signaling system in this organism.

The bittersweet interface of parasite and host: lectin-carbohydrate interactions during human invasion by the parasite Entamoeba histolytica

Entamoeba histolytica, as its name suggests, is an enteric parasite with a remarkable ability to lyse host tissues. However, the interaction of the parasite with the host is more complex than solely destruction and invasion. It is at the host-parasite interface that cell-signaling events commit the parasite to (a) commensal, noninvasive infection, (b) developmental change from trophozoite to cyst, or (c) invasion and potential death of the human host. The molecule central to these processes is an amebic cell surface protein that recognizes the sugars galactose (Gal) and N-acetylgalactosamine (GalNAc) on the surface of host cells. Engagement of the Gal/GalNAc lectin to the host results in cytoskeletal reorganization in the parasite. The parasite cytoskeleton regulates the extracellular adhesive activity of the lectin and recruits to the host-parasite interface factors required for parasite survival within its host. If the parasite lectin attaches to the host mucin glycoproteins lining the intestine, the result is commensal infection. In contrast, attachment of the lectin to a host cell surface glycoprotein leads to lectin-induced host cell calcium transients, caspase activation, and destruction via apoptosis. Finally, trophozoite quorum sensing via the lectin initiates the developmental pathway resulting in encystment. The structure and function of the lectin that controls these divergent cell biologic processes are the subject of this review.

An extracellular monoADP-ribosyl transferase activity in Entamoeba histolytica trophozoites

Due to the important role of monoADP-ribosyl transferases in physiological and pathological events, we investigated whether the protozoan parasite Entamoeba histolytica had monoADP-ribosyl transferase activity. Reactions were initiated using ameba-free medium as the source of both enzyme and ADP-ribosylation substrate(s) and [32P]NAD+ as source of ADP-ribose. Proteins were analyzed by electrophoresis, and [32P]-labeled proteins were detected by autoradiography. Using the crude extracellular medium, a major labeled product of Mr 37.000 was observed. The yield of this product was reduced markedly using medium from Brefeldin A-treated trophozoites, indicating that the extracellular monoADP-ribosyl transferase and/or its substrate depended on vesicular transport. The labeling of the 37-kDa substrate was dependent on reaction time, temperature, pH, and the ratio of unlabeled NAD+ to [32P]NAD+. After two purification steps, several new substrates were observed, perhaps due to their enrichment. The reaction measured ADP-ribosylation since [14C-carbonyl]NAD+ was not incorporated into ameba substrates and a 75-fold molar excess of ADP-ribose caused no detectable inhibition of the monoADP-ribosyl transferase reaction. On the basis of sensitivity to NH2OH, the extracellular monoADP-ribosyl transferase of E. histolytica may be an arginine-specific enzyme. These results demonstrate the existence in E. histolytica of at least one extracellular monoADP-ribosyl transferase, whose localization depends upon a secretion process.

A Rho-like small GTPase of Entamoeba histolytica contains an unusual amino acid residue in a conserved GDP-stabilization region and is not a substrate for C3 exoenzyme

The Entamoeba histolytica small GTP-binding protein EhRho1 has an unusual amino acid residue at a conserved site found in all known Ras superfamily proteins. EhRho1 has an isoleucine at position 45, which corresponds to position 28 of human Ras and Rac and position 30 of human Rho and Cdc42. All other known small GTPases have an aromatic residue (typically phenylalanine) at this position, and mutation to a leucine renders other Ras proteins constitutively active by reason of diminished affinity for GDP. It was determined that the EhRho1 protein has a half-time of GDP dissociation similar to that of a human Rho protein, HsRhoA, and therefore an isoleucine at this site in EhRho1 is not likely to render EhRho1 constitutively active. It was also found that EhRho1 is not a substrate for the Rho-specific C3 exoenzyme. Thus EhRho1 appears to be an unusual member of the Ras family.

Intermediate subunit of the Gal/GalNAc lectin of Entamoeba histolytica is a member of a gene family containing multiple CXXC sequence motifs

Killing by Entamoeba histolytica requires parasite adherence to host galactose- and N-acetyl-D-galactosamine (Gal/GalNAc)-containing cell surface receptors. A 260-kDa heterodimeric E. histolytica Gal/GalNAc lectin composed of heavy (Hgl) and light (Lgl) subunits has been previously described. Here we present the cloning and characterization of Igl, a 150-kDa intermediate subunit of the Gal/GalNAc lectin. Igl, Hgl, and Lgl colocalized on the surface membrane of trophozoites. Two unlinked copies of genes encoding Igl shared 81% amino acid sequence identity (GenBank accession no. AF337950 and AF337951). They encoded cysteine-rich proteins with amino- and carboxy-terminal hydrophobic signal sequences characteristic of glycosylphosphatidylinositol (GPI)-anchored membrane proteins. The igl genes lacked carbohydrate recognition domains but were members of a large family of amebic genes containing CXXC and CXC motifs. These data indicate that Igl is part of the parasite's multimolecular Gal/GalNAc adhesin required for host interaction.