Entamoeba histolytica: intracellular distribution of the proteasome

In silico analysis and molecular identification of an anaphase-promoting complex homologue from human pathogen Entamoeba histolytica

BACKGROUND

Amoebiasis, being endemic worldwide, is the second leading cause of parasite-associated morbidity and mortality after malaria. The human parasite Entamoeba histolytica is responsible for the disease. Metronidazole is considered as the gold standard for the treatment of amoebiasis, but this antibiotic is carcinogenic and the development of antibiotic resistance against E. histolytica is a major health concern. Chromosome segregation is irregular in this parasite due to the absence of a few cell cycle checkpoint proteins. Anaphase-promoting complex (APC/C or cyclosome) is an E3 ubiquitin ligase that synchronizes chromosome segregation and anaphase progression via the ubiquitin-proteasome system. Proteasome is considered to be an attractive drug target for protozoan parasites. For the present study, EhApc11 from E. histolytica, a homologue of Apc11 in humans, is selected for elucidating its structural and functional aspects by detailed in silico analysis and molecular methods. Its physicochemical characteristics, identification of probable interactors, 3D model and quality analysis are done using standard bioinformatics tools. cDNA sequence of EhAPC11 has been further cloned for molecular characterization.

RESULT

Conserved domain analysis revealed that EhApc11 belongs to the RING (really interesting new gene) superfamily and has ligand binding capacity. Expression study in Escherichia coli BL21 (DE3) revealed that the molecular weight of glutathione S-transferase (GST)-tagged protein is ~ 36 kDa.

CONCLUSION

EhApc11 is a hydrophilic, thermostable, extracellular protein with potent antigenicity. The study will serve as a groundwork for future in-depth analysis regarding the validation of protein-protein interaction of EhApc11 with its substrates identified by STRING analysis and the potential of EhApc11 to serve as an anti-amoebic drug target.

Naegleria fowleri and Naegleria gruberi 20S proteasome: identification and characterization

The Naegleria are ubiquitous free-living amoebae and are characterized by the presence of three phases in their biological cycle: trophozoite, cyst and flagellate. Of this genus, only Naegleria fowleri has been reported as pathogenic to humans. The proteasome is a multi-catalytic complex and is considered to be the most important structure responsible for the degradation of intracellular proteins. This structure is related to the maintenance of cellular homeostasis and, in pathogenic microorganisms, to the modulation of their virulence. Until now, the proteasome and its function have not been described for the Naegleria genus. In the current study, using bioinformatic analysis, protein sequences homologous to those reported for the subunits of the 20S proteasome in other organisms were found, and virtual modelling was used to determine their three-dimensional structure. The presence of structural and catalytic subunits of the 20S proteasome was detected by Western and dot blot assays. Its localization was observed by immunofluorescence microscopy to be mainly in the cytoplasm, and a leading role of the chymotrypsin-like catalytic activity was determined using fluorogenic peptidase assays and specific proteasome inhibitors. Finally, the role of the 20S proteasome in the proliferation and differentiation of Naegleria genus trophozoites was demonstrated.

Characterisation of 20S Proteasome in Tritrichomonas foetus and Its Role during the Cell Cycle and Transformation into Endoflagellar Form

Proteasomes are intracellular complexes that control selective protein degradation in organisms ranging from Archaea to higher eukaryotes. These structures have multiple proteolytic activities that are required for cell differentiation, replication and maintaining cellular homeostasis. Here, we document the presence of the 20S proteasome in the protist parasite Tritrichomonas foetus. Complementary techniques, such as a combination of whole genome sequencing technologies, bioinformatics algorithms, cell fractionation and biochemistry and microscopy approaches were used to characterise the 20S proteasome of T. foetus. The 14 homologues of the typical eukaryotic proteasome subunits were identified in the T. foetus genome. Alignment analyses showed that the main regulatory and catalytic domains of the proteasome were conserved in the predicted amino acid sequences from T. foetus-proteasome subunits. Immunofluorescence assays using an anti-proteasome antibody revealed a labelling distributed throughout the cytosol as punctate cytoplasmic structures and in the perinuclear region. Electron microscopy of a T. foetus-proteasome-enriched fraction confirmed the presence of particles that resembled the typical eukaryotic 20S proteasome. Fluorogenic assays using specific peptidyl substrates detected presence of the three typical peptidase activities of eukaryotic proteasomes in T. foetus. As expected, these peptidase activities were inhibited by lactacystin, a well-known specific proteasome inhibitor, and were not affected by inhibitors of serine or cysteine proteases. During the transformation of T. foetus to endoflagellar form (EFF), also known as pseudocyst, we observed correlations between the EFF formation rates, increases in the proteasome activities and reduced levels of ubiquitin-protein conjugates. The growth, cell cycle and EFF transformation of T. foetus were inhibited after treatment with lactacystin in a dose-dependent manner. Lactacystin treatment also resulted in an accumulation of ubiquitinated proteins and caused increase in the amount of endoplasmic reticulum membranes in the parasite. Taken together, our results suggest that the ubiquitin-proteasome pathway is required for cell cycle and EFF transformation in T. foetus.

Role of the Ubiquitin-Proteasome Systems in the Biology and Virulence of Protozoan Parasites

In eukaryotic cells, proteasomes perform crucial roles in many cellular pathways by degrading proteins to enforce quality control and regulate many cellular processes such as cell cycle progression, signal transduction, cell death, immune responses, metabolism, protein-quality control, and development. The catalytic heart of these complexes, the 20S proteasome, is highly conserved in bacteria, yeast, and humans. However, until a few years ago, the role of proteasomes in parasite biology was completely unknown. Here, we summarize findings about the role of proteasomes in protozoan parasites biology and virulence. Several reports have confirmed the role of proteasomes in parasite biological processes such as cell differentiation, cell cycle, proliferation, and encystation. Proliferation and cell differentiation are key steps in host colonization. Considering the importance of proteasomes in both processes in many different parasites such as Trypanosoma, Leishmania, Toxoplasma, and Entamoeba, parasite proteasomes might serve as virulence factors. Several pieces of evidence strongly suggest that the ubiquitin-proteasome pathway is also a viable parasitic therapeutic target. Research in recent years has shown that the proteasome is a valid drug target for sleeping sickness and malaria. Then, proteasomes are a key organelle in parasite biology and virulence and appear to be an attractive new chemotherapeutic target.

The cysteine protease inhibitors EhICP1 and EhICP2 perform different tasks in the regulation of endogenous protease activity in trophozoites of Entamoeba histolytica

Trophozoites of E. histolytica are equipped with two chagasin-like cysteine protease inhibitors, EhICP1 and EhICP2, also known as amoebiasin 1 and 2. Expression studies using E. invadens as model organism showed that corresponding mRNAs were detectable in both life stages of the parasite, cyst and trophozoite state. Unlike EhICP1 known to act in the cytosol, EhICP2 co-localized with cysteine protease EhCP-A1 in lysosome-like vesicles, as demonstrated by immunofluorescence microscopy. Silencing or overexpressing of the two inhibitors did not show any effect on morphology and viability of the trophozoites. Overexpression of the EhICPs, however, although dramatically dampening the proteolytic activity of cell extracts from the corresponding cell lines, did not influence expression rate or localization of the major amoebic cysteine proteases as well as phagocytosis and digestion of erythrocytes. Activity gels of cell extracts from strains overexpressing ehicp1 showed a drastically reduced activity of EhCP-A1 suggesting a high affinity of EhICP1 towards this protease. From these data, we propose that EhCP-A1 accidentally released into the cytosol is the main target of EhICP1, whereas EhICP2, beside its role in house-keeping processes, may control the proteolytic processing of other hydrolases or fulfils other tasks different from protease inhibition.

Entamoeba histolytica: Fibrilar aggregates in dividing trophozoites

Entamoeba histolytica trophozoite cytokinesis is dependent upon cytoskeletal elements such as filamentous actin and myosin. Here we present confocal and transmission electron microscopy studies of this process. A sequence in the formation of the contractile ring was shown with rhodamine-phalloidine staining. Ultrastructural analysis revealed the presence of fibrilar aggregates in the cytoplasm of dividing trophozoites. Among them two filaments of different diameter were identified. These aggregates presented repeating assemblies of thin and thick filaments that in cross section revealed a muscle-like appearance. Our results suggest that these aggregates constitute the contractile ring responsible for the separation of daughter cells.

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.