Flow cytometric characterization of encystation in Entamoeba invadens

Hypothetical proteins play a role in stage conversion, virulence, and the stress response in the Entamoeba species

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery and amoebic liver abscess in humans, affecting millions of people worldwide. This pathogen possesses a two-stage life cycle consisting of an environmentally stable cyst and a pathogenic amoeboid trophozoite. As cysts can be ingested from contaminated food and water, this parasite is prevalent in underdeveloped countries and poses a significant health burden. Until recently there was no reliable method for inducing stage conversion in E. histolytica in vitro. As such, the reptilian pathogen, Entamoeba invadens, has long-served as a surrogate. Much remains unclear about stage conversion in these parasites and current treatments for amoebiasis are lacking, as they cause severe side effects. Therefore, new therapeutic strategies are needed. The genomes of these parasites remain enigmatic as approximately 54% of E. histolytica genes and 66% of E. invadens genes are annotated as hypothetical proteins. In this study, we characterized two hypothetical proteins in the Entamoeba species, EIN_059080, in E. invadens, and its homolog, EHI_056700, in the human pathogen, E. histolytica. EHI_056700 has no homolog in the human host. We used an RNAi-based silencing system to reduce expression of these genes in E. invadens and E. histolytica trophozoites. Loss of EIN_059080 resulted in a decreased rate of encystation and an increased rate of erythrophagocytosis, an important virulence function. Additionally, mutant parasites were more susceptible to oxidative stress. Similarly, loss of EHI_056700 in E. histolytica trophozoites resulted in increased susceptibility to oxidative stress and glucose deprivation, but not to nitrosative stress. Unlike the E. invadens mutants, E. histolytica parasites with decreased reduced expression of EHI_056700 exhibited a decreased rate of erythrophagocytosis of and adhesion to host cells. Taken together, these data suggest that these hypothetical proteins play a role in stage conversion, virulence, and the response to stress in the Entamoebae. Since parasites with reduced expression of EHI_056700 show decreased virulence functions and increased susceptibility to physiologically relevant stressors, EHI_056700 may represent a possible therapeutic target for the treatment of amoebiasis.

Stress-responsive AMP Kinase like protein regulates encystation of Entamoeba invadens

Starvation is always accompanied by an increase in the ratio of AMP/ATP followed by activation of AMPK. It is one of the sensors for cellular energy status and is highly conserved across various species. Its role in the stage differentiation process of protozoan species like Giardia, Plasmodium, Trypanosome, and Toxoplasma has been reported. Since Entamoeba undergoes encystation in glucose-starved conditions; it intrigued us to investigate the existence and role of AMPK during the differentiation of trophozoites to the cyst. By employing in silico approaches, we have identified an AMPK homologue which is denominated here as EiAMPK (AMPK-like protein in Entamoeba invadens). Sequence and structural analysis indicate that EiAMPK is sequentially and structurally similar to the AMPK alpha subunit of other organisms. The recombinant form of EiAMPK was functionally active and in accordance, its activity was inhibited by an AMPK-specific inhibitor (eg. Compound C). The increased expression of EiAMPK during different stresses indicated that EiAMPK is a stress-responsive gene. To further investigate, whether EiAMPK has any role in encystation, we employed RNAi-mediated gene silencing that demonstrated its active involvement in encystation. It is known that Entamoeba maintains a flow of glucose from the glycolytic pathway to chitin synthesis for cyst wall formation during encystation. It is conceivable that EiAMPK might have a command over such glucose metabolism. As anticipated, the chitin synthesis was found greatly inhibited in both EiAMPK knockdown and Compound C treated cells, indicating that EiAMPK regulates the cyst wall chitin synthesis.

Eukaryotic Initiation Factor 2α Kinases Regulate Virulence Functions, Stage Conversion, and the Stress Response in Entamoeba invadens

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery and liver abscess. This pathogen possesses a two-stage life cycle consisting of an environmentally stable cyst and a pathogenic amoeboid trophozoite. Since infection is acquired by ingestion of cysts from contaminated food and water, this parasite is prevalent in underdeveloped countries. A reptilian pathogen, Entamoeba invadens, which can encyst in culture, has long served as a surrogate to study stage conversion. In the host, Entamoeba species must manage stress, including nutrient deprivation and host immune pressure. In many systems, the stress response is characterized by downregulation of translation, which is initiated by the phosphorylation of eukaryotic initiation factor-2 alpha (eIF2α). In mammalian cells, this phosphorylation is carried out by a family of eIF2α kinases. A canonical eIF2α translational control system exists in Entamoeba species; however, no eIF2α kinases have been characterized. In this study, we identified two eIF2α kinases in E. invadens, EiIF2K-A and EiIF2K-B. Their identity as eIF2α kinases was validated using a heterologous yeast system. We used an RNA interference (RNAi) trigger-mediated silencing system to reduce expression of EiIF2K-A, which also reduced expression of EiIF2K-B. Parasites with decreased kinase expression exhibited decreased phosphorylation of eIF2α and increased sensitivity to oxidative stress. Diminished kinase expression also correlated with an increased rate of encystation, a decreased rate of excystation, and an increase in several virulence functions, erythrophagocytosis and adhesion to host cells. Taken together, these data suggest that EiIF2K-A and EiIF2K-B are authentic eIF2α kinases that may regulate the Entamoeba stress response.

IMPORTANCEEntamoeba histolytica is a human pathogen that causes dysentery and affects millions of people worldwide. This parasite possesses a two-stage life cycle: an environmentally stable cyst and the pathogenic trophozoite. Cysts are ingested from contaminated food and water; thus, this parasite in prevalent in underdeveloped countries. Current therapies commonly cause adverse side effects; therefore, new treatments are needed. In the host, Entamoeba experiences stress brought on, in part, by the host immune system. Understanding stage conversion and the stress response of this pathogen may lead to new drug therapies. Using the model organism E. invadens, we identified two kinases similar to those involved in stress and stage conversion in other systems. We determined that these kinases may regulate the oxidative stress response, stage conversion, and virulence. This work is significant, as it will inform future studies on the life cycle and pathogenicity of Entamoeba species.

Encystation of Entamoeba histolytica in Axenic Culture

Entamoeba histolytica is a parasitic protozoan that causes amoebic dysentery, which affects approximately 90 million people each year worldwide. E. histolytica is transmitted through ingestion of food and water contaminated with the cyst form, which undergoes excystation in the small intestine to the trophozoite form that colonizes the large intestine. The reptile pathogen Entamoeba invadens has served as a model for studying stage conversion between the trophozoite and cyst form due to lack of reproducible encystation of E. histolytica in the laboratory. Although much has been learned about encystation and excystation using E. invadens, the findings do not fully translate to E. histolytica due to the extensive genetic and host differences between these species. Here, we present the first reproducible encystation of E. histolytica in vitro. The cysts produced were viable and displayed the four characteristic hallmarks: round shape, chitinous cell wall, tetranucleation, and detergent resistance. Using flow cytometry analysis, glucose limitation and high cell density were key for encystation, as for E. invadens. Entry into encystation was enhanced by the short-chain fatty acids acetate and propionate, unlike for E. invadens. This new model will now allow the further study of E. histolytica stage conversion, transmission, and treatment.

Nucleotide sugar transporters of Entamoeba histolytica and Entamoeba invadens involved in chitin synthesis

Chitin, a homopolymer of β-(1,4) linked N-acetylglucosamine (GlcNAc), is a major component of cyst wall in the protozoan parasites Entamoeba histolytica (Eh) and Entamoeba invadens (Ei). The Entamoeba chitin synthase makes chitin at the vesicular membrane rather than the plasma membrane in fungi, even though the chemistry of chitin synthesis is most likely the same. However, the role of nucleotide sugar transporter(s) (NSTs) that are involved in chitin synthesis in Entamoeba are not yet established. In this study, we have identified the putative UDP-GlcNAc transporter (EiNst5) of Ei by BLASTP analysis using the amino acid sequence of EhNst3, the UDP-GlcNAc transporter of Eh. Heterologous expression of both EhNst3 and EiNst5 was found to complement the function of Yea4p (UDP-GlcNAc transporter of S. cerevisiae) in YEA4 null mutant and increased the cell wall chitin content. Like Yea4p in S. cerevisiae, Myc-epitope tagged EhNst3 and EiNst5 were localized to the endoplasmic reticulum in Δyea4 cells. The EiNST5 transcript was up-regulated during the in vitro encystation and oxidative stress in E. invadens. Similar up-regulation was also seen for EhNST3 under oxidative stress in E. histolytica. Down-regulation of EiNst5 expression using gene-specific dsRNA significantly reduced cyst formation during in vitro encystation in E. invadens. Our observations suggest for the first time the involvement of EhNst3 and EiNst5 in chitin synthesis and so in encystation of Entamoeba.

A Flow Cytometry Method for Dissecting the Cell Differentiation Process of Entamoeba Encystation

Amoebiasis is caused by Entamoeba histolytica infection, a protozoan parasite belonging to the phylum Amoebozoa. This parasite undergoes a fundamental cell differentiation process from proliferative trophozoite to dormant cyst, termed “encystation.” The cysts formed by encystation are solely responsible for the transmission of amoebiasis; therefore, Entamoeba encystation is an important subject from both biological and medical perspectives. Here, we have established a flow cytometry strategy for not only determining the percentage of formed cysts but also for monitoring changes in cell populations during encystation. This strategy together with fluorescence microscopy enables visualization of the cell differentiation process of Entamoeba encystation. We also standardized another flow cytometry protocol for counting live trophozoites. These two different flow cytometry techniques could be integrated into 96-well plate-based bioassays for monitoring the processes of cyst formation and trophozoite proliferation, which are crucial to maintain the Entamoeba life cycle. The combined two systems enabled us to screen a chemical library, the Pathogen Box of the Medicine for Malaria Venture, to obtain compounds that inhibit either the formation of cysts or the proliferation of trophozoites, or both. This is a prerequisite for the development of new drugs against amoebiasis, a global public health problem. Collectively, the two different 96-well plate-based Entamoeba bioassay and flow cytometry analysis systems (cyst formation and trophozoite proliferation) provide a methodology that can not only overcome the limitations of standard microscopic counting but also is effective in applied as well as basic Entamoeba biology.