Canonical histone H2Ba and H2A.X dimerize in an opposite genomic localization to H2A.Z/H2B.Z dimers in Toxoplasma gondii

Unravelling the sexual developmental biology of Cystoisospora suis, a model for comparative coccidian parasite studies

Introduction

The apicomplexan parasite Cystoisospora suis has global significance as an enteropathogen of suckling piglets. Its intricate life cycle entails a transition from an asexual phase to sexual development, ultimately leading to the formation of transmissible oocysts.

Methods

To advance our understanding of the parasite's cellular development, we complemented previous transcriptome studies by delving into the proteome profiles at five distinct time points of in vitro cultivation through LC/MS-MS analysis.

Results

A total of 1,324 proteins were identified in the in vitro developmental stages of C. suis, and 1,082 proteins were identified as significantly differentially expressed. Data are available via ProteomeXchange with identifier PXD045050. We performed BLAST, GO enrichment, and KEGG pathway analyses on the up- and downregulated proteins to elucidate correlated events in the C. suis life cycle. Our analyses revealed intriguing metabolic patterns in macromolecule metabolism, DNA- and RNA-related processes, proteins associated with sexual stages, and those involved in cell invasion, reflecting the adaptation of sexual stages to a nutrient-poor and potentially stressful extracellular environment, with a focus on enzymes involved in metabolism and energy production.

Discussion

These findings have important implications for understanding the developmental biology of C. suis as well as other, related coccidian parasites, such as Eimeria spp. and Toxoplasma gondii. They also support the role of C. suis as a new model for the comparative biology of coccidian tissue cyst stages.

Previously Unidentified Histone H1-Like Protein Is Involved in Cell Division and Ribosome Biosynthesis in Toxoplasma gondii

Chromatin dynamics can regulate all DNA-dependent processes. Access to DNA within chromatin is orchestrated mainly by histones and their posttranslational modifications (PTMs). Like other eukaryotes, the apicomplexan parasite Toxoplasma gondii encodes four canonical histones and five histone variants. In contrast, the linker histone (H1) has never been identified in apicomplexan parasites. In other eukaryotes, histone H1 compacts the chromatin by linking the nucleosome and increasing the DNA compaction. H1 is a multifunctional protein and can be involved in different steps of DNA metabolism or associated with protein complexes related to distinct biological processes. We have identified a novel protein in T. gondii ("TgH1-like") that, although lacking the globular domain of mammalian H1, is remarkably like the H1-like proteins of bacteria and trypanosomatids. Our results demonstrate that TgH1-like is a nuclear protein associated with chromatin and other histones. Curiously, TgH1-like is also in the nucleolus and associated with ribosomal proteins, indicating a versatile function in this parasite. Although knockout of the tgh1-like gene does not affect the cell cycle, it causes endopolygeny and asynchronous division. Interestingly, mutation of posttranslationally modified amino acids results in defects in cell division like those in the Δtgh1-like mutant, showing that these sites are important for protein function. Furthermore, in the bradyzoite stage, this protein is expressed only in dividing parasites, reinforcing its importance in cell division. Indeed, the absence of TgH1-like decreases compaction of peripheral chromatin, confirming its role in the chromatin modulation in T. gondii. IMPORTANCE Histone H1, or linker histone, is an important protein that binds to the nucleosome, aiding chromatin compaction. Here, we characterize for the first time a linker histone in T. gondii, named TgH1-like. It is a small and basic protein that corresponds only to the C-terminal portion of the human H1 but is similar to histone H1 from trypanosomatids and bacteria. TgH1-like is located in the nucleus, interacts with nucleosome histones, and acts in chromatin structure and cell division. Our findings show for the first time the presence of a histone H1 protein in an apicomplexan parasite and will provide new insights into cell division and chromatin dynamics in T. gondii and related parasites.

Separate To Operate: the Centriole-Free Inner Core of the Centrosome Regulates the Assembly of the Intranuclear Spindle in Toxoplasma gondii

Centrosomes are the main microtubule-organizing center of the cell. They are normally formed by two centrioles, embedded in a cloud of proteins known as pericentriolar material (PCM). The PCM ascribes centrioles with their microtubule nucleation capacity. Toxoplasma gondii, the causative agent of toxoplasmosis, divides by endodyogeny. Successful cell division is critical for pathogenesis. The centrosome, one of the microtubule organizing centers of the cell, plays central roles in orchestrating the temporal and physical coordination of major organelle segregation and daughter cell formation during endodyogeny. The Toxoplasma centrosome is constituted by multiple domains: an outer core, distal from the nucleus; a middle core; and an inner core, proximal to the nucleus. This modular organization has been proposed to underlie T. gondii's cell division plasticity. However, the role of the inner core remains undeciphered. Here, we focus on understanding the function of the inner core by finely studying the localization and role of its only known molecular marker; TgCep250L1. We show that upon conditional degradation of TgCep250L1 parasites are unable to survive. Mutants exhibit severe nuclear segregation defects. In addition, the rest of the centrosome, defined by the position of the centrioles, disconnects from the nucleus. We explore the structural defects underlying these phenotypes by ultrastructure expansion microscopy. We show that TgCep250L1's location changes with respect to other markers, and these changes encompass the formation of the mitotic spindle. Moreover, we show that in the absence of TgCep250L1, the microtubule binding protein TgEB1, fails to localize at the mitotic spindle, while unsegregated nuclei accumulate at the residual body. Overall, our data support a model in which the inner core of the T. gondii centrosome critically participates in cell division by directly impacting the formation or stability of the mitotic spindle. IMPORTANCE Toxoplasma gondii parasites cause toxoplasmosis, arguably the most widespread and prevalent parasitosis of humans and animals. During the clinically relevant stage of its life cycle, the parasites divide by endodyogeny. In this mode of division, the nucleus, containing loosely packed chromatin and a virtually intact nuclear envelope, parcels into two daughter cells generated within a common mother cell cytoplasm. The centrosome is a microtubule-organizing center critical for orchestrating the multiple simultaneously occurring events of endodyogeny. It is organized in two distinct domains: the outer and inner cores. We demonstrate here that the inner core protein TgCEP250L1 is required for replication of T. gondii. Lack of TgCEP250L1 renders parasites able to form daughter cells, while unable to segregate their nuclei. We determine that, in the absence of TgCEP250L1, the mitotic spindle, which is responsible for karyokinesis, does not assemble. Our results support a role for the inner core in nucleation or stabilization of the mitotic spindle in T. gondii.

Genome-wide localization of histone variants in Toxoplasma gondii implicates variant exchange in stage-specific gene expression

BACKGROUND

Toxoplasma gondii is a protozoan parasite that differentiates from acute tachyzoite stages to latent bradyzoite forms in response to environmental cues that modify the epigenome. We studied the distribution of the histone variants CenH3, H3.3, H2A.X, H2A.Z and H2B.Z, by genome-wide chromatin immunoprecipitation to understand the role of variant histones in developmental transitions of T. gondii parasites.

RESULTS

H3.3 and H2A.X were detected in telomere and telomere associated sequences, whereas H3.3, H2A.X and CenH3 were enriched in centromeres. Histones H2A.Z and H2B.Z colocalize with the transcriptional activation mark H3K4me3 in promoter regions surrounding the nucleosome-free region upstream of the transcription start site. The H2B.Z/H2A.Z histone pair also localizes to the gene bodies of genes that are silent but poised for activation, including bradyzoite stage-specific genes. The majority of H2A.X and H2A.Z/H2B.Z loci do not overlap, consistent with variant histones demarcating specific functional regions of chromatin. The extent of enrichment of H2A.Z/H2B.Z (and H3.3 and H2A.X) within the entire gene (5'UTR and gene body) reflects the timing of gene expression during the cell cycle, suggesting that dynamic turnover of H2B.Z/H2A.Z occurs during the tachyzoite cell cycle. Thus, the distribution of the variant histone H2A.Z/H2B.Z dimer defines active and developmentally silenced regions of the T. gondii epigenome including genes that are poised for expression.

CONCLUSIONS

Histone variants mark functional regions of parasite genomes with the dynamic placement of the H2A.Z/H2B.Z dimer implicated as an evolutionarily conserved regulator of parasite and eukaryotic differentiation.

Study on Circulating Antigens in Serum of Mice With Experimental Acute Toxoplasmosis

Toxoplasma gondii is a ubiquitous apicomplexan protozoan parasite that can infect all warm-blooded animals, causing toxoplasmosis. Thus, efficient diagnosis methods for acute T. gondii infection are essential for its management. Circulating antigens (CAgs) are reliable diagnostic indicators of acute infection. In this study, we established a mouse model of acute T. gondii infection and explored new potential diagnostic factors. CAgs levels peaked 60 h after T. gondii inoculation and 31 CAgs were identified by immunoprecipitation-liquid chromatography-tandem mass spectrometry, among which RuvB-like helicase (TgRuvBL1), ribonuclease (TgRNaseH1), and ribosomal protein RPS2 (TgRPS2) were selected for prokaryotic expression. Polyclonal antibodies against these three proteins were prepared. Results from indirect enzyme-linked immunosorbent assay indicated that anti-rTgRuvBL1, anti-rTgRNase H1, and anti-rTgRPS2 mouse sera were recognized by natural excretory-secretory antigens from T. gondii tachyzoites. Moreover, immunofluorescence assays revealed that TgRuvBL1 was localized in the nucleus, while TgRNase H1 and TgRPS2 were in the apical end. Western blotting data confirmed the presence of the three proteins in the sera of the infected mice. Moreover, mice immunized with rTgRuvBL1 (10.0 ± 0.30 days), TgRNaseH1 (9.67 ± 0.14 days), or rTgRPS2 (11.5 ± 0.34 days) had slightly longer lifespan when challenged with a virulent T. gondii RH strain. Altogether, these findings indicate that these three proteins can potentially be diagnostic candidates for acute toxoplasmosis. However, they hold poor protective potential against highly virulent T. gondii infection.

Resveratrol induces H3 and H4K16 deacetylation and H2A.X phosphorylation in Toxoplasma gondii

Objective

Resveratrol (RSV) is a multitarget drug that has demonstrated activity against Toxoplasma gondii in macrophage and human foreskin fibroblast (HFF) cell line infection models. However, the mechanism of action of RSV has not yet been determined. Thus, with the aim of identifying a possible mechanism of the anti-T. gondii activity of this compound, we analyzed the effects of RSV on histones H3 and H4 lysine 16 acetylation (H4K16). We also analyzed RSV-induced DNA damage to intracellular tachyzoites by using the DNA damage marker phosphorylated histone H2A.X (γH2AX).

Results

RSV inhibited intracellular T. gondii tachyzoite growth at concentrations below the toxic threshold for host cells. The IC₅₀ value after 24 h of treatment was 53 μM. RSV induced a reduction in H4K16 acetylation (H4K16ac), a marker associated with transcription, DNA replication and homologous recombination repair. A similar deacetylation effect was observed on histone H3. RSV also increased T. gondii H2A.X phosphorylation at the SQE motif (termed γH2A.X), which is a DNA damage-associated posttranslational modification. Our findings suggest a possible link between RSV and DNA damage or repair processes that is possibly associated with DNA replication stress.

Emerging Therapeutic Targets Against Toxoplasma gondii: Update on DNA Repair Response Inhibitors and Genotoxic Drugs

Toxoplasma gondii is the causative agent of toxoplasmosis in animals and humans. This infection is transmitted to humans through oocysts released in the feces of the felines into the environment or by ingestion of undercooked meat. This implies that toxoplasmosis is a zoonotic disease and T. gondii is a foodborne pathogen. In addition, chronic toxoplasmosis in goats and sheep is the cause of recurrent abortions with economic losses in the sector. It is also a health problem in pets such as cats and dogs. Although there are therapies against this infection in its acute stage, they are not able to permanently eliminate the parasite and sometimes they are not well tolerated. To develop better, safer drugs, we need to elucidate key aspects of the biology of T. gondii. In this review, we will discuss the importance of the homologous recombination repair (HRR) pathway in the parasite's lytic cycle and how components of these processes can be potential molecular targets for new drug development programs. In that sense, the effect of different DNA damage agents or HHR inhibitors on the growth and replication of T. gondii will be described. Multitarget drugs that were either associated with other targets or were part of general screenings are included in the list, providing a thorough revision of the drugs that can be tested in other scenarios.

Identification of Novel Dense-Granule Proteins in Toxoplasma gondii by Two Proximity-Based Biotinylation Approaches

Toxoplasma gondii is an opportunistic pathogen infecting humans and a variety of vertebrate animals. Secretory dense-granule proteins (GRAs) play diverse roles in the mediation of host-parasite interactions and facilitate parasitism, but many of them still remain to be identified. Here, we used two proximity-based protein labeling techniques to identify novel GRA proteins. Taking GRA1 as bait, transgenic strains expressing GRA1-BirA* or GRA1-APEX were constructed to biotinylate GRAs. Using these methods, a total of 46 proteins were identified, 20 of which were known GRA proteins. Among these 46, 17 were identified by both strategies, and 14 out of the 17 were known GRAs. The other three were all confirmed to localize to dense granules. Nonetheless a significant portion of the proteins were only identified by either APEX or BirA*, indicating that there are differences between these methods. Of the 26 novel GRAs, 5 were validated as bona fide GRAs by localization studies. The majority of these novel GRAs are only present in coccidian parasites and are likely dispensable for parasite growth in vitro; they may play roles during animal infections. The identification of novel GRAs laid the foundation for further studies investigating the mechanisms underlying parasite-host interactions.