Entamoeba invadens: restriction of ploidy by colonic short chain fatty acids

Entamoeba stage conversion: progress and new insights

Entamoeba histolytica, an anaerobic protozoan, is an important global health problem. This parasite has a biphasic life cycle consisting of a dormant cyst stage which is environmentally resistant and transmits the infection, and the proliferative trophozoite stage which is motile and causes invasive disease. The stage conversion process remains poorly understood despite being central to amoebic biology. In this review, we will highlight recent progress in our understanding of Entamoeba stage conversion including dissecting transcriptome analysis in development, characterization of transcriptional networks, demonstration of epigenetic regulation, and role of small molecules that regulate Entamoeba development.

Cellular Events of Multinucleated Giant Cells Formation During the Encystation of Entamoeba invadens

Entamoeba histolytica, the causative agent of amoebiasis, does not form cysts in vitro, so reptilian pathogen Entamoeba invadens is used as an Entamoeba encystation model. During the in vitro encystation of E. invadens, a few multinucleated giant cells (MGC) were also appeared in the culture along with cysts. Like the cyst, these MGC's were also formed in the multicellular aggregates found in the encystation culture. Time-lapse live cell imaging revealed that MGC's were the result of repeated cellular fusion with fusion-competent trophozoites as a starting point. The early MGC were non-adherent, and they moved slowly and randomly in the media, but under confinement, MGC became highly motile and directionally persistent. The increased motility resulted in rapid cytoplasmic fissions, which indicated the possibility of continuous cell fusion and division taking place inside the compact multicellular aggregates. Following cell fusion, each nucleus obtained from the fusion-competent trophozoites gave rise to four nuclei with half genomic content. All the haploid nuclei in MGC later aggregated and fused to form a polyploid nucleus. These observations have important implications on Entamoeba biology as they point toward the possibility of E. invadens undergoing sexual or parasexual reproduction.

Entamoeba invadens: dynamics of DNA synthesis during differentiation from trophozoite to cyst

The DNA dynamics which mediate conversion of uni-nucleate trophozoite into quadrinucleate cyst in Entamoeba histolytica is not well understood. Here, we have addressed this question in Entamoeba invadens (a model system for encystation) through a detailed time course study of the differentiation process. We combined flow cytometric analysis with the change in rate of thymidine incorporation and the number of nuclei per cell. Our data shows that during encystment the cell population passes through three phases: (1) Early phase (0-8h); of rapid DNA synthesis which may correspond to completion of ongoing DNA replication. Bi-nucleated cells increase with concomitant drop in uni-nucleated cells. (2) Commitment phase (8-24h); in which DNA synthesis rate slows down. Possibly new rounds of replication are initiated which proceed slowly, followed by mitosis at 20 h. After this the number of bi- and uni-nucleated cells gradually decline and the tri- and tetra-nucleated cells begin to increase. (3) Consolidation phase (24-72 h); in which the rate of DNA synthesis shows a small increase till 32 h and then begins to decline. The G2/M peak reappears at 48 h, showing that more rounds of DNA replication may be getting completed, followed by nuclear division. By 72 h the encystment is virtually complete. The bi-nucleated stage could be an intermediate both in the conversion of trophozoite to cyst and back. Our study provides a comprehensive view of DNA dynamics during encystation and excystation of E. invadens.

Entamoeba shows reversible variation in ploidy under different growth conditions and between life cycle phases

Under axenic growth conditions, trophozoites of Entamoeba histolytica contain heterogenous amounts of DNA due to the presence of both multiple nuclei and different amounts of DNA in individual nuclei. In order to establish if the DNA content and the observed heterogeneity is maintained during different growth conditions, we have compared E. histolytica cells growing in xenic and axenic cultures. Our results show that the nuclear DNA content of E. histolytica trophozoites growing in axenic cultures is at least 10 fold higher than in xenic cultures. Re-association of axenic cultures with their bacterial flora led to a reduction of DNA content to the original xenic values. Thus switching between xenic and axenic growth conditions was accompanied by significant changes in the nuclear DNA content of this parasite. Changes in DNA content during encystation-excystation were studied in the related reptilian parasite E. invadens. During excystation of E. invadens cysts, it was observed that the nuclear DNA content increased approximately 40 fold following emergence of trophozoites in axenic cultures. Based on the observed large changes in nuclear size and DNA content, and the minor differences in relative abundance of representative protein coding sequences, rDNA and tRNA sequences, it appears that gain or loss of whole genome copies may be occurring during changes in the growth conditions. Our studies demonstrate the inherent plasticity and dynamic nature of the Entamoeba genome in at least two species.

In contrast to the perception that DNA content of an organism is stable and maintained during different conditions and life cycle stages, new evidence shows that many organisms display changes in their DNA content at different stages of their life cycle. We have earlier identified intra-cellular and inter-cellular differences in DNA content of the protist pathogen Entamoeba histolytica and established that this organism can tolerate large variations in DNA content during axenic culture. In this study we have made an important advancement in the understanding of amoeba biology where we have shown that changes in growth conditions and life cycle stages are accompanied by large differences in DNA content involving gain or loss of whole genome copies. This property may well regulate the outcome of infection and subsequently the disease.

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.

Entamoeba histolytica: cyst-like structures in vitro induction

The cyst of Entamoeba histolytica is responsible for amebiasis infection. However, no axenic in vitro system exists that promotes mass encystation for studying this process of this human-infecting parasite. Cyst-like structures of E. histolytica obtained in this work were induced using TYI-S-33 media in combination with enterobacterias Escherichia coli and Enterococcus faecalis conditioned media, high CO2 tension and histamine. Cyst-like structures showed the same characteristics of a typical E. histolytica cyst: aggregation, resistance to 0.15% sarcosyl for 10 min, high signal of fluorescence under UV light when stained with 10% calcofluor M2r and the surface topology showed a wrinkled wall. In addition these structures are multinucleated with condensed chromatin attached to nuclear membrane, contain big vacuoles and ribonucleoproteic helices in the cytoplasm and also present a thin cell wall. Last all characteristics are all the same as a typical of E. histolytica cyst.