Nuclear transport in Entamoeba histolytica: knowledge gap and therapeutic potential

Multiple types of nuclear localization signals in Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that belongs to the Amoebozoa supergroup whose study related to the nucleocytoplasmic transport of proteins through the nucleus is poorly studied. In this work, we have performed in silico predictions of the potential nuclear localization signals (NLS) corresponding to the proteome of 8201 proteins from Entamoeba histolytica annotated in the AmoebaDB database. We have found the presence of monopartite nuclear localization signals (MNLSs), bipartite nuclear localization signals (BNLSs), and non-canonical monopartite NLSs with lengths exceeding 20 amino acid residues. Additionally, we detected a new type of NLS consisting of multiple juxtaposed bipartite NLSs (JNLSs) that have not been described in any eukaryotic organism. Also, we have generated consensus sequences for the nuclear import of proteins with the NLSs obtained. Docking experiments between EhImportin α and an MNLS, BNLS, and JNLS outlined the interacting residues between the Importin and cargo proteins, emphasizing their putative roles in nuclear import. By transfecting HA-tagged protein constructs, we assessed the nuclear localization of MNLS (U1A and U2AF1), JMNLS (U2AF2), and non-canonical NLS (N-terminus of Pol ll) in vivo. Our data provide the basis for understanding the nuclear transport process in E. histolytica.

Proteomic and Functional Analysis of the Effects of Quinoxaline Derivatives on Entamoeba histolytica

Quinoxalines are heterocyclic compounds that contain a benzene ring and a pyrazine ring. The oxidation of both nitrogen of the pyrazine ring results in quinoxaline derivatives (QdNO), which exhibit a variety of biological properties, including antiparasitic activity. However, its activity against Entamoeba histolytica, the protozoan that causes human amebiasis, is poorly understood. Recently, our group reported that various QdNOs produce morphological changes in E. histolytica trophozoites, increase reactive oxygen species, and inhibit thioredoxin reductase activity. Notably, T-001 and T-017 derivatives were among the QdNOs with the best activity. In order to contribute to the characterization of the antiamebic effect of QdNOs, in this work we analyzed the proteomic profile of E. histolytica trophozoites treated with the QdNOs T-001 and T-017, and the results were correlated with functional assays. A total number of 163 deregulated proteins were found in trophozoites treated with T-001, and 131 in those treated with T-017. A set of 21 overexpressed and 24 under-expressed proteins was identified, which were mainly related to cytoskeleton and intracellular traffic, nucleic acid transcription, translation and binding, and redox homeostasis. Furthermore, T-001 and T-017 modified the virulence of trophozoites, since they altered their erythrophagocytosis, migration, adhesion and cytolytic capacity. Our results show that in addition to alter reactive oxygen species, and thioredoxin reductase activity, T-001 and T-017 affect essential functions related to the actin cytoskeleton, which eventually affects E. histolytica virulence and survival.

Drug repositioning: antiprotozoal activity of terfenadine against Entamoeba histolytica trophozoites

The infection caused by Entamoeba histolytica is still a serious public health problem, especially in developing countries. The goal of this study was to evaluate the effect of terfenadine against Entamoeba histolytica. The trophozoites were exposed to 1, 2, 3, and 4 μM of terfenadine, for 24 and 48 h. Consequently, the viability of cells was determined by trypan blue exclusion test. The effect of terfenadine on adhesion of Entamoeba histolytica was evaluated in Caco-2 cells. In addition, the effect of terfenadine on the erythrophagocytic capacity of the parasite was investigated. The results show that terfenadine affects the growth and cell viability in a time- and dose-dependent manner. The higher inhibitory effects were observed with 4 µM at 48 h; 91.6% of growth inhibition and only 22.5% of trophozoites were viable. Additionally, we demonstrate that terfenadine is highly selective for the parasite and has low toxicity on Caco-2 cells. Furthermore, adhesion to Caco-2 cells and erythrophagocytic capacity were significantly inhibited. These findings demonstrate that terfenadine exerts significant effects on the virulence of Entamoeba histolytica. This is the first study demonstrating the amoebicidal activity of terfenadine and the results suggest it may be effective in the treatment of amoebiasis.

Entamoeba histolytica protein CaBP3 uses a calcium dependent nuclear localisation pathway in mammalian cells

Entamoeba histolytica is a major cause of dysentery that leads to a high level of morbidity and mortality, especially in developing countries. Calmodulin-like calcium binding protein EhCaBP3 of E. histolytica is directly involved in disease mechanisms with roles in cytoskeleton dynamics and scission during erythrophagocytosis in a calcium dependent fashion. Interestingly, EhCaBP3 is also present in the nucleus of E. histolytica. We have used a transfected cell system to show that EhCaBP3 is capable of calcium dependent nucleocytoplasmic trafficking. Our data confirms and extends recent findings suggesting presence of a calcium dependent nuclear transport pathway in E. histolytica.

Evidence of nuclear transport mechanisms in the protozoan parasite Giardia lamblia

Nuclear-cytoplasmic trafficking of proteins is a highly regulated process that modulates multiple biological processes in eukaryotic cells. In Giardia lamblia, shuttling has been described from the cytoplasm to nuclei of proteins during the biological cell cycle of the parasite. This suggests that a mechanism of nucleocytoplasmic transport is present and functional in G. lamblia. By means of computational biology analyses, we found that there are only two genes for nuclear transport in this parasite, named Importin α and Importin β. When these transporters were overexpressed, both localized close to the nuclear envelope, and no change was observed in trophozoite growth rate. However, during the encystation process, both transporters induced an increase in the number of cysts produced. Importazole and Ivermectin, two known specific inhibitors of importins, separately influenced the encysting process by inducing an arrest in the trophozoite stage that prevents the production of cysts. This effect was more noticeable when Ivermectin, an anti-parasitic drug, was used. Finally, we tested whether the enzyme arginine deiminase, which shuttles from the cytoplasm to the nuclei during encystation, was influenced by these transporters. We found that treatment with each of the inhibitors abrogates arginine deiminase nuclear translocation and favors perinuclear localization. This suggests that Importin α and Importin β are key transporters during the encystation process and are involved, at least, in the transport of arginine deiminase into the nuclei. Considering the effect produced by Ivermectin during growth and encystation, we postulate that this drug could be used to treat giardiasis.

Functional Characterization of Entamoeba histolytica Argonaute Proteins Reveals a Repetitive DR-Rich Motif Region That Controls Nuclear Localization

The RNA interference (RNAi) pathway regulates gene expression in many eukaryotic organisms. Argonaute (Ago) proteins, together with bound small RNAs (sRNAs), are key effectors that mediate gene silencing function. However, there is limited knowledge of Ago proteins and their functions in nonmodel systems. In the protozoan parasite Entamoeba histolytica, RNAi is a robust means for stable gene silencing mediated via large populations of antisense sRNAs. Here, we report functional characterization of three Ago proteins in E. histolytica (EhAgo2-1, EhAgo2-2, and EhAgo2-3). Our data show that each EhAgo protein has a distinct subcellular localization and binds 27-nucleotide (nt) sRNAs and that the localization of EhAgo proteins is altered in response to stress conditions. Via mutagenesis analyses, we demonstrated that the Ago PAZ (Piwi/Argonaute/Zwille) domain in all three EhAgos is essential for sRNA binding. With mutation of the PAZ domain in EhAgo2-2, there was no effect on the nuclear localization of the protein but a strong phenotype and a growth defect. We further show that EhAgo2-2 contains an unusual repetitive DR-rich (aspartic acid, arginine-rich) motif region which functions as a nuclear localization signal (NLS) and is both necessary and sufficient to mediate nuclear localization. Overall, our data delineate the localization and sRNA binding features of the three E. histolytica Ago proteins and demonstrate that the PAZ domain is necessary for sRNA binding. The repetitive DR-rich motif region in EhAgo2-2 has not previously been defined in other systems, which adds to the novel observations that can be made when studies of the RNAi pathway are extended to nonmodel systems.IMPORTANCE The protozoan parasite Entamoeba histolytica, which causes amebiasis and affects over 50 million people worldwide, contains an important RNAi pathway for gene silencing. Gene silencing via the RNAi pathway is mediated by the Argonaute (Ago) proteins. However, we lack knowledge on Ago function(s) in this nonmodel system. In this paper, we discovered that three E. histolytica Ago proteins (EhAgo2-1, EhAgo2-2, and EhAgo2-3) all bind 27-nt small RNAs and have distinct subcellular localizations, which change in response to stress conditions. The EhAgos bind small RNA populations via their PAZ domains. An unusual repetitive DR-rich motif region is identified in EhAgo2-2 that functions as a nuclear localization signal. Our results show for the first time an active nuclear transport process of the EhAgo2-2 RNA-induced silencing complex (RISC) in this parasite. These data add to the novel observations that can be made when studies of the RNAi pathway are extended to nonmodel systems.