Characterization of the Ehrab8 gene, a marker of the late stages of the secretory pathway of Entamoeba histolytica

Probing the Peculiarity of EhRabX10, a pseudoRab GTPase, from the Enteric Parasite Entamoeba histolytica through In Silico Modeling and Docking Studies

Entamoeba histolytica (Eh) is a pathogenic eukaryote that often resides silently in humans under asymptomatic stages. Upon indeterminate stimulus, it develops into fulminant amoebiasis that causes severe hepatic abscesses with 50% mortality. This neglected tropical pathogen relies massively on membrane modulation to flourish and cause disease; these modulations range from the phagocytic mode for food acquisition to a complex trogocytosis mechanism for tissue invasion. Rab GTPases form the largest branch of the Ras-like small GTPases, with a diverse set of roles across the eukaryotic kingdom. Rab GTPases are vital for the orchestration of membrane transport and the secretory pathway responsible for transporting the pathogenic effectors, such as cysteine proteases (EhCPs) which help in tissue invasion. Rab GTPases thus play a crucial role in executing the cytolytic effect of E. histolytica. First, they interact with Gal/Nac lectins required for adhering to the host cells, and then, they assist in the secretion of EhCPs. Additionally, amoebic Rab GTPases are vital for encystation because substantial vesicular trafficking is required to create dormant amoebic cysts. These cysts are the infective agent and help to spread the disease. The absence of a "bonafide" vesicular transport machinery in Eh and the existence of a diverse repertoire of amoebic Rab GTPases (EhRab) hint at their contribution in supporting this atypical machinery. Here, we provide insights into a pseudoRab GTPase, EhRabX10, by performing physicochemical analysis, predictive 3D structure modeling, protein-protein interaction studies, and in silico molecular docking. Our group is the first one to classify EhRabX10 as a pseudoRab GTPase with four nonconserved G-motifs. It possesses the basic fold of the P-loop containing nucleotide hydrolases. Through this in silico study, we provide an introduction to the characterization of the atypical EhRabX10 and set the stage for future explorations into the mechanisms of nucleotide recognition, binding, and hydrolysis employed by the pseudoEhRab GTPase family.

A comparative in silico analysis of Rab5 proteins from pathogenic species to find its role in the pathogenesis

The enteric protozoan parasite, Entamoeba histolytica (Eh), is the causative agent of amoebic dysentery and liver abscess in humans. It infects around 50 million people worldwide, which is a third general cause of death from parasitic diseases after malaria and schistosomiasis. The other prevalent form of the disease is Visceral leishmaniasis caused by Leishmania donovani which is a human blood parasite. On the other hand, the Toxoplasma gondii is an obligate intracellular protozoan parasite; it causes serious opportunistic infections in HIV-positive persons. The biological processes in all living organisms are mostly mediated by the proteins, and recognizing new target proteins and finding their function in pathogenesis will help in choosing better diagnostic markers. In eukaryotes, Rab protein plays a major role in pathogenesis. Rabs represent the largest branch in the Ras superfamily of GTPases. Among them, the Rab5 is important in the endocytosis and thus involved in pathogenesis. In this paper, we discussed the physiochemical profiling, modelling, and docking of the Rab5 protein from pathogenic species that is Entamoeba histolytica, Leishmania donovani, and Toxoplasma gondii. The modeled structures from this study and the key residues identified would give a better understanding of the three-dimensional structure and functional insights into these proteins and help in developing new drug targets.

Rab GTPases take centre stage in understanding Entamoeba histolytica biology

Rab GTPases constitute the largest subgroup in the Ras superfamily of GTPases. It is well established that different Rab GTPases are localized in discrete subcellular localization and regulate the membrane trafficking in nearly all eukaryotic cells. Rab GTPase diversity is often regarded as an expression of vesicular trafficking complexity. The pathogenic amoeba Entamoeba histolytica harbours 91 Rab GTPases which is the highest among the currently available genome sequences from the eukaryotic kingdom. Here, we review the current status of amoebic Rab GTPases diversity, unique biochemical and structural features and summarise their predicted regulators. We discuss how amoebic Rab GTPases are involved in cellular processes such as endocytosis, phagocytosis, and invasion of host cellular components, which are essential for parasite survival and virulence.

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.

Role of EhRab7A in phagocytosis of type 1 fimbriated E. coli by Entamoeba histolytica

Entamoeba histolytica, the causative agent of amoebic colitis and liver abscess in human, ingests the intestinal bacteria and variety of host cells. Phagocytosis of bacteria by the amebic trophozoite has been reported to be important for the virulence of the parasite. Here, we set out to characterize different stages of phagocytosis of type 1 E. coli and investigated the role of a set of amoebic Rab GTPases in the process. The localizations of the Rab GTPases during different stages of the phagocytosis were investigated using laser scanning confocal microscopy and their functional relevance were determined using fluorescence activated cell sorter based assay as well as colony forming unit assay. Our results demonstrate that EhRab7A is localized on the phagosomes and involved in both early and late stages of type 1 E. coli phagocytosis. We further showed that the E. coli or RBC containing phagosomes are distinct from the large endocytic vacuoles in the parasite which are exclusively used to transport human holotransferrin and low density lipoprotein. Remarkably, type 1 E. coli uptake was found to be insensitive to cytochalasin D treatment, suggesting that the initial stage of E. coli phagocytosis is independent of the formation of actin filaments.

Endoplasmic reticulum-resident Rab8A GTPase is involved in phagocytosis in the protozoan parasite Entamoeba histolytica

Phagocytosis is indispensable for the pathogenesis of the intestinal protozoan parasite Entamoeba histolytica. Here, we showed that in E. histolytica Rab8A, which is generally involved in trafficking from the trans‐Golgi network to the plasma membrane in other organisms but was previously identified in phagosomes of the amoeba in the proteomic analysis, primarily resides in the endoplasmic reticulum (ER) and participates in phagocytosis. We demonstrated that down‐regulation of EhRab8A by small antisense RNA‐mediated transcriptional gene silencing remarkably reduced adherence and phagocytosis of erythrocytes, bacteria and carboxylated latex beads. Surface biotinylation followed by SDS‐PAGE analysis revealed that the surface expression of several proteins presumably involved in target recognition was reduced in the EhRab8A gene‐silenced strain. Further, overexpression of wild‐type EhRab8A augmented phagocytosis, whereas expression of the dominant‐negative form of EhRab8A resulted in reduced phagocytosis. These results indicated that EhRab8A regulates transport of surface receptor(s) for the prey from the ER to the plasma membrane. To our knowledge, this is the first report that the ER‐resident Rab GTPase is involved in phagocytosis through the regulation of trafficking of a surface receptor, supporting a premise of direct involvement of the ER in phagocytosis.

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

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

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.

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

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

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

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

Conservation and function of Rab small GTPases in Entamoeba: annotation of E. invadens Rab and its use for the understanding of Entamoeba biology

Entamoeba invadens is a reptilian enteric protozoan parasite closely related to the human pathogen Entamoeba histolytica and a good model organism of encystation. To understand the molecular mechanism of vesicular trafficking involved in the encystation of Entamoeba, we examined the conservation of Rab small GTPases between the two species. E. invadens has over 100 Rab genes, similar to E. histolytica. Most of the Rab subfamilies are conserved between the two species, while a number of species-specific Rabs are also present. We annotated all E. invadens Rabs according to the previous nomenclature [Saito-Nakano, Y., Loftus, B.J., Hall, N., Nozaki, T., 2005. The diversity of Rab GTPases in Entamoeba histolytica. Experimental Parasitology 110, 244-252]. Comparative genomic analysis suggested that the fundamental vesicular traffic machinery is well conserved, while there are species-specific protein transport mechanisms. We also reviewed the function of Rabs in Entamoeba, and proposed the use of the annotation of E. invadens Rab genes to understand the ubiquitous importance of Rab-mediated membrane trafficking during important biological processes including differentiation in Entamoeba.

Structure and content of the Entamoeba histolytica genome

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

Entamoeba histolytica: biochemical and molecular insights into the activities within microsomal fractions

One of the most fascinating aspects of the Entamoeba histolytica trophozoite ultrastructure is the lack of a typical secretory pathway, particularly of rough endoplasmic reticulum and Golgi system, in a cell with such a high secretory activity. Here, we describe the isolation of amoeba cell structures containing ER-typical activities. Following isopycnic centrifugation of plasma membrane-free extracts, microsomes enriched in enzymatic activities such as dolichol-P-mannose synthase (DPMS; EC 2.4.1.83), UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (NAGPT; EC 2.7.8.15), and UDP-D-GlcNAc:dolichol-PP GlcNAc (NAGT; EC 2.4.1.141) were resolved from phagolysosomal fractions. Sec61alpha-subunit, an ER-marker involved in the translocation of nascent proteins to the ER, was found to co-fractionate with DPMS activity indicating that they are contained in microsomes with a similar density. Further, we optimized conditions for trophozoite homogenization and differential centrifugation that resulted in the separation of a 57,000 g-sedimenting microsomal fraction containing EhSec61alpha-subunit, EhDPMS, and EhPDI (protein disulfide isomerase, a soluble marker of the lumen of the ER). A relevant observation was the lack of ER markers associated to the nuclear fraction. Large macromolecular structures such as Ehproteasome were sedimented at a higher speed. Our knowledge of the molecular machinery involved in the biosynthesis of dolichol-linked oligosaccharide was enriched with the identification of putative genes related to the stepwise assembly of the dolichol-PP-GlcNAc(2)Man(5) core. No evidence of genes supporting further assembly steps was obtained at this time.

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.

The diversity of Rab GTPases in Entamoeba histolytica

Rab proteins are ubiquitous small GTP-binding proteins that form a highly conserved family and regulate vesicular trafficking. Recent completion of the genome of the enteric protozoan parasite Entamoeba histolytica enabled us to identify an extremely large number (>90) of putative Rab genes. Multiple alignment and phylogenic analysis of amebic, human, and yeast Rab showed that only 22 amebic Rab proteins including EhRab1, EhRab2, EhRab5, EhRab7, EhRab8, EhRab11, and EhRab21 showed significant similarity to Rab from other organisms. The 69 remaining amebic Rab proteins showed only moderate similarity (<40% identity) to Rab proteins from other organisms. Approximately one-third of Rab proteins including Rab7, Rab11, and RabC form 15 subfamilies, which contain up to nine isoforms. Approximately 70% of amebic Rab genes contain single or multiple introns, and this proportion is significantly higher than that of common genes in this organism. Twenty-five Rabs possess an atypical carboxyl terminus such as CXXX, XCXX, XXCX, XXXC, and no cysteine. We propose annotation of amebic Rab genes and discuss biological significance of this extraordinary diversity of EhRab proteins in this organism.

Entamoeba histolytica: intracellular distribution of the sec61alpha subunit of the secretory pathway and down-regulation by antisense peptide nucleic acids

The Sec61alpha protein is defined as a highly conserved essential integral component of the endoplasmic reticulum in eukaryotic cells. We report a detailed immunolocalization of the Entamoeba histolytica homologue of the Sec61alpha subunit (EhSec61alpha), which shows an irregular pattern throughout the cell and is also found on the cell surface, its effective down-regulation by means of antisense peptide nucleic acids and its effects on cell proliferation, subcellular distribution of two virulence factors, and the ability of the trophozoites to cause liver abscess in hamsters. Although Sec61alpha levels are specifically decreased in antisense PNA-treated trophozoites, which proliferate more slowly than the controls, mobilization of the cysteine protease 5 and amoebapore to the cell surface is not significantly impeded and the capacity to induce liver abscess in hamsters is largely unaffected. The implications of these findings are discussed in the context of the peculiar cell biology of E. histolytica.

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.

A unique Rab GTPase, EhRabA, of Entamoeba histolytica, localizes to the leading edge of motile cells

Entamoeba histolytica, an enteric protozoan parasite, infects 10% of the world's population leading to 50 million cases of invasive amoebiasis annually. Parasite vesicle trafficking and motility, which relies on vesicle trafficking to deliver membrane and membrane components to the leading edge, are important for virulence however little is known about the molecular mechanisms regulating these functions. Since Rab GTPases are known modulators of vesicle trafficking we have characterized a Rab GTPase of Entamoeba, EhRabA. Sequence analysis revealed that EhRabA shared limited homology with any known Rab suggesting that it is a novel member of this protein family. Immunofluorescence microscopy using EhRabA-specific antibodies demonstrated that EhRabA did not colocalize with markers for the Golgi apparatus, endoplasmic reticulum, pinosomes, or phagosomes. These data suggest that this Rab may not play a role in vesicle trafficking between these organelles. In quiescent Entamoeba cells, EhRabA localized to vesicles throughout the cytoplasm consistent with a role in vesicle trafficking, however, in motile cells this protein localized to small vesicles in the leading edge. In addition, when E. histolytica trophozoites were exposed to an N-formyl peptide (N-formylmethionylleucylphenylalanine) cell polarization, the formation of membrane extensions, and the translocation of EhRabA to these membrane extensions was observed. Taken together, these results suggest that EhRabA may function in the formation of membrane extensions perhaps by regulating the delivery of membrane and/or cell surface molecules to the plasma membrane.

Entamoeba histolytica: intracellular distribution of the proteasome

We have studied the intracellular distribution of proteasome subunits, corresponding to the catalytic (20S) core and the regulatory (19S) cap, in the extracellular protozoan parasite Entamoeba histolytica. Contrary to all cell types described to date, notably mammalian and yeast, in which the proteasome is found in the nucleus and actively imported into it, microscopic analysis and subcellular fractionation of E. histolytica trophozoites show that the proteasome is absent from the nucleus of these cells. We speculate that, given the relative abundance of mono- and multinucleated trophozoites in culture, a relationship may exist between this unusual distribution of the proteasome and the frequent lack of synchrony between karyo- and cytokinesis in this primitive eukaryote.

Pathogenesis of amebiasis

It is an exciting time in the study of Entamoeba histolytica. Over the past two years, the natural history and burden of disease in humans has been redefined, mucosal immune responses associated with protection identified, and the developmental regulation of encystation outlined. The number of genes sequenced has increased from a few hundred to a few thousand, and study of the genome structure is revealing unusual repetitive elements and plasticity. DNA microarrays promise the first ability to examine global patterns of mRNA abundance. The mechanism of transcriptional control via histone modifications and sequence-specific DNA-binding proteins are to be delineated. Advances in cell biology are providing new insights into invasion through the intestinal epithelium.

Characterization of a Rab7-like GTPase, EhRab7: a marker for the early stages of endocytosis in Entamoeba histolytica

We have identified a 667 base pair Rab7-like cDNA (EhRab7) from Entamoeba histolytica. The EhRab7 cDNA predicts a polypeptide of at least 206 amino acids with a molecular mass of at least 24.5 kDa. Alignment of EhRab7 with other Rab proteins demonstrated that EhRab7 shared significant homology at the amino acid level with Rab7-like proteins from a number of other eukaryotes, suggesting that EhRab7 is a Rab7 homolog for E. histolytica. Using immunofluorescence microscopy, EhRab7 was demonstrated to be associated with early fluid-phase endosomes (<30 min) and secretory vesicles. The association of EhRab7 with early endosomes disappeared 1 h after their formation. Immunofluorescence microscopy also revealed that this GTPase did not colocalize significantly with phagosomes nor with markers for other organelles including the endoplasmic reticulum, Golgi and late endosomes. These results, together with the known function for Rab7 in other systems, suggest that EhRab7 is bound to vesicles, and that it may participate in vesicle docking and fusion in secretory events, and in the early stages of fluid-phase endocytosis in E. histolytica.