N-linked glycosylation of proteins in the protozoan parasite Toxoplasma gondii

Neospora caninum as delivery vehicle for anti-PD-L1 scFv-Fc: A novel approach for cancer immunotherapy

Neospora caninum, a potential anticancer agent able to reactivate the immune response within the tumor microenvironment (TME), has recently shown enhanced immunomodulatory properties in different tumor models when armed with the cytokine, Il-15. In the current area of combination immunotherapy strategies designed to overcome treatment resistance, we engineered for the first time the protozoan Neospora caninum to vectorize and secrete a single-chain variable fragment fused to fragment crystallizable region (scFv-Fc) targeting human programmed cell death ligand 1 (PD-L1). Following validation of its secretion through the micronemes (protozoa secretory organelles), we demonstrated that the scFv-Fc could bind PD-L1 on mouse and human tumor cells, block the programmed cell death protein 1 (PD-1)/PD-L1 pathway leading to potentiate the T cell lymphocyte activity. Additionally, the scFv-Fc induced antibody-dependent cellular phagocytosis (ADCP) and antibody-dependent cellular cytotoxicity (ADCC). Those data demonstrate the feasibility of vectoring and secreting a functional antibody fragment by N. caninum, opening promising avenues for future research.

N-acetylglucosamine supplementation fails to bypass the critical acetylation of glucosamine-6-phosphate required for Toxoplasma gondii replication and invasion

The cell surface of Toxoplasma gondii is rich in glycoconjugates which hold diverse and vital functions in the lytic cycle of this obligate intracellular parasite. Additionally, the cyst wall of bradyzoites, that shields the persistent form responsible for chronic infection from the immune system, is heavily glycosylated. Formation of glycoconjugates relies on activated sugar nucleotides, such as uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). The glucosamine-phosphate-N-acetyltransferase (GNA1) generates N-acetylglucosamine-6-phosphate critical to produce UDP-GlcNAc. Here, we demonstrate that downregulation of T. gondii GNA1 results in a severe reduction of UDP-GlcNAc and a concomitant drop in glycosylphosphatidylinositols (GPIs), leading to impairment of the parasite's ability to invade and replicate in the host cell. Surprisingly, attempts to rescue this defect through exogenous GlcNAc supplementation fail to completely restore these vital functions. In depth metabolomic analyses elucidate diverse causes underlying the failed rescue: utilization of GlcNAc is inefficient under glucose-replete conditions and fails to restore UDP-GlcNAc levels in GNA1-depleted parasites. In contrast, GlcNAc-supplementation under glucose-deplete conditions fully restores UDP-GlcNAc levels but fails to rescue the defects associated with GNA1 depletion. Our results underscore the importance of glucosamine-6-phosphate acetylation in governing T. gondii replication and invasion and highlight the potential of the evolutionary divergent GNA1 in Apicomplexa as a target for the development of much-needed new therapeutic strategies.

Essential role of the conserved oligomeric Golgi complex in Toxoplasma gondii

IMPORTANCE

The Golgi is an essential eukaryotic organelle and a major place for protein sorting and glycosylation. Among apicomplexan parasites, Toxoplasma gondii retains the most developed Golgi structure and produces many glycosylated factors necessary for parasite survival. Despite its importance, Golgi function received little attention in the past. In the current study, we identified and characterized the conserved oligomeric Golgi complex and its novel partners critical for protein transport in T. gondii tachyzoites. Our results suggest that T. gondii broadened the role of the conserved elements and reinvented the missing components of the trafficking machinery to accommodate the specific needs of the opportunistic parasite T. gondii.

Shedding light on a mysterious link between Toxoplasma Gondii and cancer: A review

The ongoing conflict regarding the affiliation of Toxoplasma gondii to cancer; whether an inducer or a suppressor needs to be resolved. Human epidemiological studies oscillate without attaining a firm ground. Some studies confirmed the detection of high seroprevalence of anti-Toxoplasma antibodies in different cancer patients without further justification whether being causation, co-incidences, or part of opportunistic infections. Others reported a state of resistance to cancer accompanying low titer of anti-Toxoplasma antibody. Worthwhile, preclinical experimental work confirmed the antineoplastic potency of Toxoplasma. Thus, further investigational research is essential to validate the potential application of Toxoplasma as a promising cancer immunotherapeutic vaccine candidate. In this paper, we present a review of this issue by examining epidemiological and preclinical experimental studies that explored the linkage between Toxoplasma gondii and cancer<i.></i> We consider this review an important step towards shedding a light on this mysterious link and a stepping-stone for potential research work addressing Toxoplasma as a cancer suppressor rather than a cancer inducer.

Prophylactic antineoplastic activity of Toxoplasma gondii RH derived antigen against ehrlich solid carcinoma with evidence of shared antigens by comparative immunoblotting

BACKGROUND

With cancer cases escalation, an urgent request to develop novel combating strategies arise. Pathogen-based cancer-immunotherapy is getting more consideration. Autoclaved parasitic antigens seem promising candidates, taking steadily their first steps. Our aim was to examine the prophylactic antineoplastic activity of autoclaved Toxoplasma vaccine (ATV) and to test for the shared antigen theory between Toxoplasma gondii and cancer cells.

METHODS

Mice were immunized with ATV followed by Ehrlich solid carcinoma (ESC) inoculation. Tumor weight, volume, histopathology, and immunohistochemistry for CD8⁺ T cells, Treg cells and VEGF were assessed. In addition, the proposed shared antigen theory between parasites and cancer was also verified using SDS-PAGE and immunoblotting.

RESULTS

Results revealed powerful prophylactic activity of ATV with 13.3% inhibition of ESC incidence, significant reduction in tumor weight and volume in ATV vaccinated mice. Immunologically, significantly higher CD8⁺T cells and lower FOXP3⁺ Treg cells surrounded and infiltrated ESC in ATV immunized mice with higher CD8⁺T/Treg cells ratio and significant antiangiogenic effect. Moreover, SDS-PAGE and immunoblotting showed four shared bands between Ehrlich carcinoma and ATV of approximate molecular weights 60, 26, 22 and 12.5 KDa.

CONCLUSION

Exclusively, we demonstrated a prophylactic antineoplastic activity of autoclaved Toxoplasma vaccine against ESC. Moreover, to the best of our knowledge this is the first report highlighting the existence of cross-reactive antigens between Toxoplasma gondi parasite and cancer cells of Ehrlich carcinoma.

Toxoplasma gondii-derived antigen modifies tumor microenvironment of Ehrlich solid carcinoma murine model and enhances immunotherapeutic activity of cyclophosphamide

Pathogen-based cancer vaccine is a promising immunotherapeutic weapon to stimulate cancer immunosuppressive state. Toxoplasma gondii is a potent immunostimulant, and low-dose infection was linked to cancer resistance. Our goal was to evaluate the therapeutic antineoplastic activity of autoclaved Toxoplasma vaccine (ATV) against Ehrlich solid carcinoma (ESC) in mice in reference to and in combination with low-dose cyclophosphamide (CP), a cancer immunomodulator. Mice inoculation with ESC was followed by applying different treatment modalities including ATV, CP, and CP/ATV. We evaluated the impact of the different treatments on liver enzymes and pathology, tumor weight, volume, and histopathological changes. Using immunohistochemistry, we evaluated CD8⁺ T cell, FOXP3⁺ Treg, CD8⁺/Treg outside and inside ESC, and angiogenesis. Results showed significant tumor weights and volumes reduction with all treatments with 13.3% inhibition of tumor development upon combined CP/ATV use. Significant necrosis and fibrosis were noted in ESC by all treatments with improved hepatic functions versus non-treated control. Although ATV was almost equivalent to CP in tumor gross and histopathology, it promoted an immunostimulatory activity with significant Treg cells depletion outside ESC and CD8⁺ T cells infiltration inside ESC with higher CD8⁺ T/Treg ratio inside ESC superior to CP. Combined with CP, ATV exhibited significant synergistic immunotherapeutic and antiangiogenic action compared to either treatment alone with significant Kupffer cells hyperplasia and hypertrophy. Exclusively, therapeutic antineoplastic and antiangiogenic activity of ATV against ESC was verified that boosted CP immunomodulatory action which highlights a novel biological cancer immunotherapeutic vaccine candidate.

Design of a stabilized non-glycosylated Pfs48/45 antigen enables a potent malaria transmission-blocking nanoparticle vaccine

A malaria vaccine that blocks parasite transmission from human to mosquito would be a powerful method of disrupting the parasite lifecycle and reducing the incidence of disease in humans. Pfs48/45 is a promising antigen in development as a transmission blocking vaccine (TBV) against the deadliest malaria parasite Plasmodium falciparum. The third domain of Pfs48/45 (D3) is an established TBV candidate, but production challenges have hampered development. For example, to date, a non-native N-glycan is required to stabilize the domain when produced in eukaryotic systems. Here, we implement a SPEEDesign computational design and in vitro screening pipeline that retains the potent transmission blocking epitope in Pfs48/45 while creating a stabilized non-glycosylated Pfs48/45 D3 antigen with improved characteristics for vaccine manufacture. This antigen can be genetically fused to a self-assembling single-component nanoparticle, resulting in a vaccine that elicits potent transmission-reducing activity in rodents at low doses. The enhanced Pfs48/45 antigen enables many new and powerful approaches to TBV development, and this antigen design method can be broadly applied towards the design of other vaccine antigens and therapeutics without interfering glycans.

Glycosylation Analysis of Feline Small Intestine Following Toxoplasma gondii Infection

Simple Summary

Toxoplasma gondii has a serious impact on public health and the economic development of animal husbandry. Glycosylation, especially N-glycosylation, the pattern modification of proteins, is closely related to the biological functions of proteins, and our study used it to analyze glycosylation alterations in the small intestine of cats infected with T. gondii. The results of the present study showed that 56 glycosylated peptides were upregulated and 37 glycosylated peptides were downregulated. Additionally, we also identified eight N-glycosylated proteins of T. gondii including eight N-glycopeptides and eight N-glycosylation sites. Moreover, the protein eEF2 and its corresponding peptide sequence were identified, with GO terms (i.e., cellular process and metabolic process, cell and cell part, and catalytic activity) that were significantly enriched in the T. gondii MAPK pathway. In addition, the Clusters of Orthologous Groups of proteins (COG) function prediction results showed that posttranslational modification, protein turnover, and chaperones (11%) had the highest enrichment for T. gondii. The host proteins ICAM-1 and PPT1 and the endoplasmic reticulum stress pathway may play an important role in the glycosylation of T. gondii-infected hosts. Our study may provide a new target for T. gondii detection to prevent the spread of T. gondii oocysts in the future.

Abstract

Toxoplasma gondii (T. gondii) is responsible for severe human and livestock diseases, huge economic losses, and adversely affects the health of the public and the development of animal husbandry. Glycosylation is a common posttranslational modification of proteins in eukaryotes, and N-glycosylation is closely related to the biological functions of proteins. However, glycosylation alterations in the feline small intestine following T. gondii infection have not been reported. In this study, the experimental group was intragastrically challenged with 600 brain cysts of the Prugniuad (Pru) strain that were collected from infected mice. The cats’ intestinal epithelial tissues were harvested at 10 days post-infection and then sent for protein glycosylation analysis. High-performance liquid chromatography coupled to tandem mass spectrometry was used to analyze the glycosylation alterations in the small intestine of cats infected with T. gondii. The results of the present study showed that 56 glycosylated peptides were upregulated and 37 glycosylated peptides were downregulated in the feline small intestine infected by T. gondii. Additionally, we also identified eight N-glycosylated proteins of T. gondii including eight N-glycopeptides and eight N-glycosylation sites. The protein A0A086JND6_TOXGO (eEF2) and its corresponding peptide sequence were identified in T. gondii infection. Some special GO terms (i.e., cellular process and metabolic process, cell and cell part, and catalytic activity) were significantly enriched, and the Clusters of Orthologous Groups of proteins (COG) function prediction results showed that posttranslational modification, protein turnover, and chaperones (11%) had the highest enrichment for T. gondii. Interestingly, eEF2, a protein of T. gondii, is also involved in the significantly enriched T. gondii MAPK pathway. The host proteins ICAM-1 and PPT1 and the endoplasmic reticulum stress pathway may play an important role in the glycosylation of Toxoplasma-infected hosts. This is the first report showing that T. gondii oocysts can undergo N-glycosylation in the definitive host and that eEF2 is involved, which may provide a new target for T. gondii detection to prevent the spread of T. gondii oocysts in the future.

Disrupting the plastidic iron-sulfur cluster biogenesis pathway in Toxoplasma gondii has pleiotropic effects irreversibly impacting parasite viability

Like many other apicomplexan parasites, Toxoplasma gondii contains a plastid harboring key metabolic pathways, including the sulfur utilization factor (SUF) pathway that is involved in the biosynthesis of iron-sulfur clusters. These cofactors are crucial for a variety of proteins involved in important metabolic reactions, potentially including plastidic pathways for the synthesis of isoprenoid and fatty acids. It was shown previously that impairing the NFS2 cysteine desulfurase, involved in the first step of the SUF pathway, leads to an irreversible killing of intracellular parasites. However, the metabolic impact of disrupting the pathway remained unexplored. Here, we generated another mutant of this pathway, deficient in the SUFC ATPase, and investigated in details the phenotypic consequences of TgNFS2 and TgSUFC depletion on the parasites. Our analysis confirms that Toxoplasma SUF mutants are severely and irreversibly impacted in division and membrane homeostasis, and suggests a defect in apicoplast-generated fatty acids. However, we show that increased scavenging from the host or supplementation with exogenous fatty acids do not fully restore parasite growth, suggesting that this is not the primary cause for the demise of the parasites and that other important cellular functions were affected. For instance, we also show that the SUF pathway is key for generating the isoprenoid-derived precursors necessary for the proper targeting of GPI-anchored proteins and for parasite motility. Thus, we conclude plastid-generated iron-sulfur clusters support the functions of proteins involved in several vital downstream cellular pathways, which implies the SUF machinery may be explored for new potential anti-Toxoplasma targets.

Towards understanding the extensive diversity of protein N-glycan structures in eukaryotes

N-glycosylation is an important post-translational modification of proteins that has been highly conserved during evolution and is found in Eukaryota, Bacteria and Archaea. In eukaryotes, N-glycan processing is sequential, involving multiple specific steps within the secretory pathway as proteins travel through the endoplasmic reticulum and the Golgi apparatus. In this review, we first summarize the different steps of the N-glycan processing and further describe recent findings regarding the diversity of N-glycan structures in eukaryotic clades. This comparison allows us to explore the different regulation mechanisms of N-glycan processing among eukaryotic clades. Recent findings regarding the regulation of protein N-glycosylation are highlighted, especially the regulation of the biosynthesis of complex-type N-glycans through manganese and calcium homeostasis and the specific role of transmembrane protein 165 (TMEM165) for which homologous sequences have been identified in several eukaryotic clades. Further research will be required to characterize the function of TMEM165 homologous sequences in different eukaryotic clades.

N-Glycosylation in Piroplasmids: Diversity within Simplicity

N-glycosylation has remained mostly unexplored in Piroplasmida, an order of tick-transmitted pathogens of veterinary and medical relevance. Analysis of 11 piroplasmid genomes revealed three distinct scenarios regarding N-glycosylation: Babesia sensu stricto (s.s.) species add one or two N-acetylglucosamine (NAcGlc) molecules to proteins; Theileria equi and Cytauxzoon felis add (NAcGlc)2-mannose, while B. microti and Theileria s.s. synthesize dolichol-P-P-NAcGlc and dolichol-P-P-(NAcGlc)2 without subsequent transfer to proteins. All piroplasmids possess the gene complement needed for the synthesis of the N-glycosylation substrates, dolichol-P and sugar nucleotides. The oligosaccharyl transferase of Babesia species, T. equi and C. felis, is predicted to be composed of only two subunits, STT3 and Ost1. Occurrence of short N-glycans in B. bovis merozoites was experimentally demonstrated by fluorescence microscopy using a NAcGlc-specific lectin. In vitro growth of B. bovis was significantly impaired by tunicamycin, an inhibitor of N-glycosylation, indicating a relevant role for N-glycosylation in this pathogen. Finally, genes coding for N-glycosylation enzymes and substrate biosynthesis are transcribed in B. bovis blood and tick stages, suggesting that this pathway is biologically relevant throughout the parasite life cycle. Elucidation of the role/s exerted by N-glycans will increase our understanding of these successful parasites, for which improved control measures are needed.

Development of a hexavalent recombinant protein vaccine adjuvanted with Montanide ISA 50 V and determination of its protective efficacy against acute toxoplasmosis

Background

Toxoplasma gondii is an obligate intracellular parasite that can infect almost all warm-blooded animals, avian species and humans. Toxoplasmosis is asymptomatic in healthy individuals, whereas it may lead to death in immune suppressed or deficient patients. A vaccine against T. gondii is required to prevent consequences of the infection. The aim of this study is to generate a multivalent recombinant protein vaccine against T. gondii.

Methods

49 previously discovered antigenic proteins of T gondii were evaluated by their expression level in E. coli and by comprehensive bioinformatics analyses to determine antigenic epitopes. Based on these analyses, six vaccine candidate proteins were selected to generate a hexavalent recombinant protein vaccine adjuvanted with Montanide ISA 50 V. Humoral and cellular immune responses were determined by flow cytometry and ELISA. Vaccinated mice were challenged with T. gondii Ankara strain tachyzoites.

Results

In mice vaccinated with hexavalent vaccine, strong total IgG (P < 0.0001) and IgG2a (P < 0.001) responses were induced compared to controls, the ratio of CD4⁺ and CD8⁺ T lymphocytes secreting IFN-γ increased, and significantly higher extracellular IFN-γ secretion was achieved compared to the controls (P < 0.001). The survival time of the vaccinated mice increased to 8.38 ± 2.13 days which was significantly higher than controls (P < 0.01).

Conclusions

Altogether, these results show that the hexavalent vaccine which is developed for the first time against T. gondii induced strong and balanced Th1 and Th2 immune responses as well as conferred significant protection against challenge with lethal toxoplasmosis in murine model.

Re-annotation of the Theileria parva genome refines 53% of the proteome and uncovers essential components of N-glycosylation, a conserved pathway in many organisms

BACKGROUND

The apicomplexan parasite Theileria parva causes a livestock disease called East coast fever (ECF), with millions of animals at risk in sub-Saharan East and Southern Africa, the geographic distribution of T. parva. Over a million bovines die each year of ECF, with a tremendous economic burden to pastoralists in endemic countries. Comprehensive, accurate parasite genome annotation can facilitate the discovery of novel chemotherapeutic targets for disease treatment, as well as elucidate the biology of the parasite. However, genome annotation remains a significant challenge because of limitations in the quality and quantity of the data being used to inform the location and function of protein-coding genes and, when RNA data are used, the underlying biological complexity of the processes involved in gene expression. Here, we apply our recently published RNAseq dataset derived from the schizont life-cycle stage of T. parva to update structural and functional gene annotations across the entire nuclear genome.

RESULTS

The re-annotation effort lead to evidence-supported updates in over half of all protein-coding sequence (CDS) predictions, including exon changes, gene merges and gene splitting, an increase in average CDS length of approximately 50 base pairs, and the identification of 128 new genes. Among the new genes identified were those involved in N-glycosylation, a process previously thought not to exist in this organism and a potentially new chemotherapeutic target pathway for treating ECF. Alternatively-spliced genes were identified, and antisense and multi-gene family transcription were extensively characterized.

CONCLUSIONS

The process of re-annotation led to novel insights into the organization and expression profiles of protein-coding sequences in this parasite, and uncovered a minimal N-glycosylation pathway that changes our current understanding of the evolution of this post-translational modification in apicomplexan parasites.

Characterization of a novel glycosylated glutathione transferase of Onchocerca ochengi, closest relative of the human river blindness parasite

Filarial nematodes possess glutathione transferases (GSTs), ubiquitous enzymes with the potential to detoxify xenobiotic and endogenous substrates, and modulate the host immune system, which may aid worm infection establishment, maintenance and survival in the host. Here we have identified and characterized a σ class glycosylated GST (OoGST1), from the cattle-infective filarial nematode Onchocerca ochengi, which is homologous (99% amino acid identity) with an immunodominant GST and potential vaccine candidate from the human parasite, O. volvulus, (OvGST1b). Onchocerca ochengi native GSTs were purified using a two-step affinity chromatography approach, resolved by 2D and 1D SDS-PAGE and subjected to enzymic deglycosylation revealing the existence of at least four glycoforms. A combination of lectin-blotting and mass spectrometry (MS) analyses of the released N-glycans indicated that OoGST1 contained mainly oligomannose Man₅GlcNAc₂ structure, but also hybrid- and larger oligommanose-type glycans in a lower proportion. Furthermore, purified OoGST1 showed prostaglandin synthase activity as confirmed by Liquid Chromatography (LC)/MS following a coupled-enzyme assay. This is only the second reported and characterized glycosylated GST and our study highlights its potential role in host-parasite interactions and use in the study of human onchocerciasis.

The Methods Employed in Mass Spectrometric Analysis of Posttranslational Modifications (PTMs) and Protein-Protein Interactions (PPIs)

Mass Spectrometry (MS) has revolutionized the way we study biomolecules, especially proteins, their interactions and posttranslational modifications (PTM). As such MS has established itself as the leading tool for the analysis of PTMs mainly because this approach is highly sensitive, amenable to high throughput and is capable of assigning PTMs to specific sites in the amino acid sequence of proteins and peptides. Along with the advances in MS methodology there have been improvements in biochemical, genetic and cell biological approaches to mapping the interactome which are discussed with consideration for both the practical and technical considerations of these techniques. The interactome of a species is generally understood to represent the sum of all potential protein-protein interactions. There are still a number of barriers to the elucidation of the human interactome or any other species as physical contact between protein pairs that occur by selective molecular docking in a particular spatiotemporal biological context are not easily captured and measured.PTMs massively increase the complexity of organismal proteomes and play a role in almost all aspects of cell biology, allowing for fine-tuning of protein structure, function and localization. There are an estimated 300 PTMS with a predicted 5% of the eukaryotic genome coding for enzymes involved in protein modification, however we have not yet been able to reliably map PTM proteomes due to limitations in sample preparation, analytical techniques, data analysis, and the substoichiometric and transient nature of some PTMs. Improvements in proteomic and mass spectrometry methods, as well as sample preparation, have been exploited in a large number of proteome-wide surveys of PTMs in many different organisms. Here we focus on previously published global PTM proteome studies in the Apicomplexan parasites T. gondii and P. falciparum which offer numerous insights into the abundance and function of each of the studied PTM in the Apicomplexa. Integration of these datasets provide a more complete picture of the relative importance of PTM and crosstalk between them and how together PTM globally change the cellular biology of the Apicomplexan protozoa. A multitude of techniques used to investigate PTMs, mostly techniques in MS-based proteomics, are discussed for their ability to uncover relevant biological function.

CRISPR/Cas9 and glycomics tools for Toxoplasma glycobiology

Infection with the protozoan parasite Toxoplasma gondii is a major health risk owing to birth defects, its chronic nature, ability to reactivate to cause blindness and encephalitis, and high prevalence in human populations. Unlike most eukaryotes, Toxoplasma propagates in intracellular parasitophorous vacuoles, but like nearly all other eukaryotes, Toxoplasma glycosylates many cellular proteins and lipids and assembles polysaccharides. Toxoplasma glycans resemble those of other eukaryotes, but species-specific variations have prohibited deeper investigations into their roles in parasite biology and virulence. The Toxoplasma genome encodes a suite of likely glycogenes expected to assemble N-glycans, O-glycans, a C-glycan, GPI-anchors, and polysaccharides, along with their precursors and membrane transporters. To investigate the roles of specific glycans in Toxoplasma, here we coupled genetic and glycomics approaches to map the connections between 67 glycogenes, their enzyme products, the glycans to which they contribute, and cellular functions. We applied a double-CRISPR/Cas9 strategy, in which two guide RNAs promote replacement of a candidate gene with a resistance gene; adapted MS-based glycomics workflows to test for effects on glycan formation; and infected fibroblast monolayers to assess cellular effects. By editing 17 glycogenes, we discovered novel Glc_0-2-Man₆-GlcNAc₂-type N-glycans, a novel HexNAc-GalNAc-mucin-type O-glycan, and Tn-antigen; identified the glycosyltransferases for assembling novel nuclear O-Fuc-type and cell surface Glc-Fuc-type O-glycans; and showed that they are important for in vitro growth. The guide sequences, editing constructs, and mutant strains are freely available to researchers to investigate the roles of glycans in their favorite biological processes.

Characterization of a SRS13: a new cyst wall mucin-like domain containing protein

Toxoplasma gondii, an obligate intracellular apicomplexan, causes latent infection in about one third of the human population. During latent infection, T. gondii bradyzoites are found within cysts, a modified parasitophorous vacuole. This parasite has a large family of SRS (surface antigen-1 related sequence) proteins which are reported to be involved in attachment of these organisms to their mammalian host cells and in immune subversion during latent infection. We have identified a novel mucin domain containing SRS protein, using a glycoepitope-specific antibody, which recognizes the cyst wall. SRS13 has two SRS domains and between these domains is a threonine-rich tandem repeat mucin-like domain that is similar to the mucin-like domain seen in another cyst wall specific SRS protein CST1 (SRS44). SRS13 is upregulated in bradyzoites and O-GalNAc glycosylated by ppGalNAc-T2 and T3. Similar to the cyst wall protein CST1, SRS13 localizes to the cyst wall, but unlike CST1, SRS13 is dispensable for normal cyst wall formation. Together, these findings elucidate the role of a second SRS mucin domain protein, SRS13, in bradyzoite biology and expands the previously reported functions of the SRS protein family.

Post-translational modifications as key regulators of apicomplexan biology: insights from proteome-wide studies

Parasites of the Apicomplexa phylum, such as Plasmodium spp. and Toxoplasma gondii, undergo complex life cycles involving multiple stages with distinct biology and morphologies. Post-translational modifications (PTMs), such as phosphorylation, acetylation and glycosylation, regulate numerous cellular processes, playing a role in every aspect of cell biology. PTMs can occur on proteins at any time in their lifespan and through alterations of target protein activity, localization, protein-protein interactions, among other functions, dramatically increase proteome diversity and complexity. In addition, PTMs can be induced or removed on changes in cellular environment and state. Thus, PTMs are likely to be key regulators of developmental transitions, biology and pathogenesis of apicomplexan parasites. In this review we examine the roles of PTMs in both parasite-specific and conserved eukaryotic processes, and the potential crosstalk between PTMs, that together regulate the intricate lives of these protozoa.

Heterologous expression of the N-acetylglucosaminyltransferase I dictates a reinvestigation of the N-glycosylation pathway in Chlamydomonas reinhardtii

Eukaryotic N-glycosylation pathways are dependent of N-acetylglucosaminyltransferase I (GnTI), a key glycosyltransferase opening the door to the formation of complex-type N-glycans by transferring a N-acetylglucosamine residue onto the Man₅GlcNAc₂ intermediate. In contrast, glycans N-linked to Chlamydomonas reinhardtii proteins arise from a GnTI-independent Golgi processing of oligomannosides giving rise to Man₅GlcNAc₂ substituted eventually with one or two xylose(s). Here, complementation of C. reinhardtii with heterologous GnTI was investigated by expression of GnTI cDNAs originated from Arabidopsis and the diatom Phaeodactylum tricornutum. No modification of the N-glycans was observed in the GnTI transformed cells. Consequently, the structure of the Man₅GlcNAc₂ synthesized by C. reinhardtii was reinvestigated. Mass spectrometry analyses combined with enzyme sequencing showed that C. reinhardtii proteins carry linear Man₅GlcNAc₂ instead of the branched structure usually found in eukaryotes. Moreover, characterization of the lipid-linked oligosaccharide precursor demonstrated that C. reinhardtii exhibit a Glc₃Man₅GlcNAc₂ dolichol pyrophosphate precursor. We propose that this precursor is then trimmed into a linear Man₅GlcNAc₂ that is not substrate for GnTI. Furthermore, cells expressing GnTI exhibited an altered phenotype with large vacuoles, increase of ROS production and accumulation of starch granules, suggesting the activation of stress responses likely due to the perturbation of the Golgi apparatus.

A Systematic Review of In vitro and In vivo Activities of Anti-Toxoplasma Drugs and Compounds (2006-2016)

The currently available anti-Toxoplasma agents have serious limitations. This systematic review was performed to evaluate drugs and new compounds used for the treatment of toxoplasmosis. Data was systematically collected from published papers on the efficacy of drugs/compounds used against Toxoplasma gondii (T. gondii) globally during 2006-2016. The searched databases were PubMed, Google Scholar, Science Direct, ISI Web of Science, EBSCO, and Scopus. One hundred and eighteen papers were eligible for inclusion in this systematic review, which were both in vitro and in vivo studies. Within this review, 80 clinically available drugs and a large number of new compounds with more than 39 mechanisms of action were evaluated. Interestingly, many of the drugs/compounds evaluated against T. gondii act on the apicoplast. Therefore, the apicoplast represents as a potential drug target for new chemotherapy. Based on the current findings, 49 drugs/compounds demonstrated in vitro half-maximal inhibitory concentration (IC50) values of below 1 μM, but most of them were not evaluated further for in vivo effectiveness. However, the derivatives of the ciprofloxacin, endochin-like quinolones and 1-[4-(4-nitrophenoxy) phenyl] propane-1-one (NPPP) were significantly active against T. gondii tachyzoites both in vitro and in vivo. Thus, these compounds are promising candidates for future studies. Also, compound 32 (T. gondii calcium-dependent protein kinase 1 inhibitor), endochin-like quinolones, miltefosine, rolipram abolish, and guanabenz can be repurposed into an effective anti-parasitic with a unique ability to reduce brain tissue cysts (88.7, 88, 78, 74, and 69%, respectively). Additionally, no promising drugs are available for congenital toxoplasmosis. In conclusion, as current chemotherapy against toxoplasmosis is still not satisfactory, development of well-tolerated and safe specific immunoprophylaxis in relaxing the need of dependence on chemotherapeutics is a highly valuable goal for global disease control. However, with the increasing number of high-risk individuals, and absence of a proper vaccine, continued efforts are necessary for the development of novel treatment options against T. gondii. Some of the novel compounds reviewed here may represent good starting points for the discovery of effective new drugs. In further, bioinformatic and in silico studies are needed in order to identify new potential toxoplasmicidal drugs.

Cryptosporidium hominis gene catalog: a resource for the selection of novel Cryptosporidium vaccine candidates

Human cryptosporidiosis, caused primarily by Cryptosporidium hominis and a subset of Cryptosporidium parvum, is a major cause of moderate-to-severe diarrhea in children under 5 years of age in developing countries and can lead to nutritional stunting and death. Cryptosporidiosis is particularly severe and potentially lethal in immunocompromised hosts. Biological and technical challenges have impeded traditional vaccinology approaches to identify novel targets for the development of vaccines against C. hominis, the predominant species associated with human disease. We deemed that the existence of genomic resources for multiple species in the genus, including a much-improved genome assembly and annotation for C. hominis, makes a reverse vaccinology approach feasible. To this end, we sought to generate a searchable online resource, termed C. hominis gene catalog, which registers all C. hominis genes and their properties relevant for the identification and prioritization of candidate vaccine antigens, including physical attributes, properties related to antigenic potential and expression data. Using bioinformatic approaches, we identified ∼400 C. hominis genes containing properties typical of surface-exposed antigens, such as predicted glycosylphosphatidylinositol (GPI)-anchor motifs, multiple transmembrane motifs and/or signal peptides targeting the encoded protein to the secretory pathway. This set can be narrowed further, e.g. by focusing on potential GPI-anchored proteins lacking homologs in the human genome, but with homologs in the other Cryptosporidium species for which genomic data are available, and with low amino acid polymorphism. Additional selection criteria related to recombinant expression and purification include minimizing predicted post-translation modifications and potential disulfide bonds. Forty proteins satisfying these criteria were selected from 3745 proteins in the updated C. hominis annotation. The immunogenic potential of a few of these is currently being tested.

Database URL:http://cryptogc.igs.umaryland.edu

Identification of Novel O-Linked Glycosylated Toxoplasma Proteins by Vicia villosa Lectin Chromatography

Toxoplasma gondii maintains its intracellular life cycle using an extraordinary arsenal of parasite-specific organelles including the inner membrane complex (IMC), rhoptries, micronemes, and dense granules. While these unique compartments play critical roles in pathogenesis, many of their protein constituents have yet to be identified. We exploited the Vicia villosa lectin (VVL) to identify new glycosylated proteins that are present in these organelles. Purification of VVL-binding proteins by lectin affinity chromatography yielded a number of novel proteins that were subjected to further study, resulting in the identification of proteins from the dense granules, micronemes, rhoptries and IMC. We then chose to focus on three proteins identified by this approach, the SAG1 repeat containing protein SRS44, the rhoptry neck protein RON11 as well as a novel IMC protein we named IMC25. To assess function, we disrupted their genes by homologous recombination or CRISPR/Cas9. The knockouts were all successful, demonstrating that these proteins are not essential for invasion or intracellular survival. We also show that IMC25 undergoes substantial proteolytic processing that separates the C-terminal domain from the predicted glycosylation site. Together, we have demonstrated that lectin affinity chromatography is an efficient method of identifying new glycosylated parasite-specific proteins.

Extracellular Toxoplasma gondii tachyzoites metabolize and incorporate unnatural sugars into cellular proteins

Toxoplasma gondii is an obligate intracellular parasite that infects all nucleated cell types in diverse warm-blooded organisms. Many of the surface antigens and effector molecules secreted by the parasite during invasion and intracellular growth are modified by glycans. Glycosylated proteins in the nucleus and cytoplasm have also been reported. Despite their prevalence, the complete inventory and biological significance of glycosylated proteins in Toxoplasma remain unknown. In this study, we aimed to globally profile parasite glycoproteins using a bioorthogonal chemical reporter strategy. This strategy involves the metabolic incorporation of unnatural functional groups (i.e., "chemical reporters") into Toxoplasma glycans, followed by covalent labeling with visual probes or affinity tags. The two-step approach enables the visualization and identification of newly biosynthesized glycoconjugates in the parasite. Using a buffer that mimics intracellular conditions, extracellular Toxoplasma tachyzoites were found to metabolize and incorporate unnatural sugars (equipped with bioorthogonal functional groups) into diverse proteins. Covalent chemistries were used to visualize and retrieve these labeled structures. Subsequent mass spectrometry analysis revealed 89 unique proteins. This survey identified novel proteins as well as previously characterized proteins from lectin affinity analyses.

Isoprenoid metabolism in apicomplexan parasites

Apicomplexan parasites include some of the most prevalent and deadly human pathogens. Novel antiparasitic drugs are urgently needed. Synthesis and metabolism of isoprenoids may present multiple targets for therapeutic intervention. The apicoplast-localized methylerythritol phosphate (MEP) pathway for isoprenoid precursor biosynthesis is distinct from the mevalonate (MVA) pathway used by the mammalian host, and this pathway is apparently essential in most Apicomplexa. In this review, we discuss the current field of research on production and metabolic fates of isoprenoids in apicomplexan parasites, including the acquisition of host isoprenoid precursors and downstream products. We describe recent work identifying the first MEP pathway regulator in apicomplexan parasites, and introduce several promising areas for ongoing research into this well-validated antiparasitic target.

The effect of tunicamycin on the glucose uptake, growth, and cellular adhesion in the protozoan parasite Crithidia fasciculata

Crithidia fasciculata represents a very interesting model organism to study biochemical, cellular, and genetic processes unique to members of the family of the Trypanosomatidae. Thus, C. fasciculata parasitizes several species of insects and has been widely used to test new therapeutic strategies against parasitic infections. By using tunicamycin, a potent inhibitor of glycosylation in asparaginyl residues of glycoproteins (N-glycosylation), we demonstrate that N-glycosylation in C. fasciculata cells is involved in modulating glucose uptake, dramatically impacting growth, and cell adhesion. C. fasciculata treated with tunicamycin was severely affected in their ability to replicate and to adhere to polystyrene substrates and losing their ability to aggregate into small and large groups. Moreover, under tunicamycin treatment, the parasites were considerably shorter and rounder and displayed alterations in cytoplasmic vesicles formation. Furthermore, glucose uptake was significantly impaired in a tunicamycin dose-dependent manner; however, no cytotoxic effect was observed. Interestingly, this effect was reversible. Thus, when tunicamycin was removed from the culture media, the parasites recovered its growth rate, cell adhesion properties, and glucose uptake. Collectively, these results suggest that changes in the tunicamycin-dependent glycosylation levels can influence glucose uptake, cell growth, and adhesion in the protozoan parasite C. fasciculata.

The unfolded protein response in the protozoan parasite Toxoplasma gondii features translational and transcriptional control

The unfolded protein response (UPR) is an important regulatory network that responds to perturbations in protein homeostasis in the endoplasmic reticulum (ER). In mammalian cells, the UPR features translational and transcriptional mechanisms of gene expression aimed at restoring proteostatic control. A central feature of the UPR is phosphorylation of the α subunit of eukaryotic initiation factor-2 (eIF2) by PERK (EIF2AK3/PEK), which reduces the influx of nascent proteins into the ER by lowering global protein synthesis, coincident with preferential translation of key transcription activators of genes that function to expand the processing capacity of this secretory organelle. Upon ER stress, the apicomplexan parasite Toxoplasma gondii is known to induce phosphorylation of Toxoplasma eIF2α and lower translation initiation. To characterize the nature of the ensuing UPR in this parasite, we carried out microarray analyses to measure the changes in the transcriptome and in translational control during ER stress. We determined that a collection of transcripts linked with the secretory process are induced in response to ER stress, supporting the idea that a transcriptional induction phase of the UPR occurs in Toxoplasma. Furthermore, we determined that about 500 gene transcripts showed enhanced association with translating ribosomes during ER stress. Many of these target genes are suggested to be involved in gene expression, including JmjC5, which continues to be actively translated during ER stress. This study indicates that Toxoplasma triggers a UPR during ER stress that features both translational and transcriptional regulatory mechanisms, which is likely to be important for parasite invasion and development.

The role of sialyl glycan recognition in host tissue tropism of the avian parasite Eimeria tenella

Eimeria spp. are a highly successful group of intracellular protozoan parasites that develop within intestinal epithelial cells of poultry, causing coccidiosis. As a result of resistance against anticoccidial drugs and the expense of manufacturing live vaccines, it is necessary to understand the relationship between Eimeria and its host more deeply, with a view to developing recombinant vaccines. Eimeria possesses a family of microneme lectins (MICs) that contain microneme adhesive repeat regions (MARR). We show that the major MARR protein from Eimeria tenella, EtMIC3, is deployed at the parasite-host interface during the early stages of invasion. EtMIC3 consists of seven tandem MAR1-type domains, which possess a high specificity for sialylated glycans as shown by cell-based assays and carbohydrate microarray analyses. The restricted tissue staining pattern observed for EtMIC3 in the chicken caecal epithelium indicates that EtMIC3 contributes to guiding the parasite to the site of invasion in the chicken gut. The microarray analyses also reveal a lack of recognition of glycan sequences terminating in the N-glycolyl form of sialic acid by EtMIC3. Thus the parasite is well adapted to the avian host which lacks N-glycolyl neuraminic acid. We provide new structural insight into the MAR1 family of domains and reveal the atomic resolution basis for the sialic acid-based carbohydrate recognition. Finally, a preliminary chicken immunization trial provides evidence that recombinant EtMIC3 protein and EtMIC3 DNA are effective vaccine candidates.

Eimeria spp. are highly successful protozoan parasites of the intestine of birds and one of the most important diseases in modern poultry farming. The economic impact is significant causing billion dollar losses to the industry and as a result there is pressing need for new therapeutic approaches. Anticoccidial drugs are thwarted by resistance, live vaccines are expensive to manufacture and few recombinant vaccine antigens have been characterized in detail. We show that the microneme protein, MIC3 from Eimeria tenella, is deployed at the parasite-host interface during the early stages of invasion. We provide new atomic resolution insight into its predilection for sialic acid-bearing glycans and demonstrate its role in invasion. We also provide evidence that EtMIC3-based vaccines induce protection in preliminary immunization studies.

Analysis of the glycoproteome of Toxoplasma gondii using lectin affinity chromatography and tandem mass spectrometry

Glycoproteins are involved in many important molecular recognition processes including invasion, adhesion, differentiation, and development. To identify the glycoproteins of Toxoplasma gondii, a proteomic analysis was undertaken. T. gondii proteins were prepared and fractioned using lectin affinity chromatography. The proteins in each fraction were then separated using SDS-PAGE and identified by tryptic in gel digestion followed by tandem mass spectrometry. Utilizing these methods 132 proteins were identified. Among the identified proteins were 17 surface proteins, 9 microneme proteins, 15 rhoptry proteins, 11 heat shock proteins (HSP), and 32 hypothetical proteins. Several proteins had 1-5 transmembrane domains (TMD) with some being as large as 608.3 kDa. Both lectin-fluorescence labeling and lectin blotting were employed to confirm the presence of carbohydrates on the surface or cytoplasm of T. gondii parasites. PCR demonstrated that selected hypothetical proteins were expressed in T. gondii tachyzoites. This data provides a large-scale analysis of the T. gondii glycoproteome. Studies of the function of glycosylation of these proteins may help elucidate mechanism(s) involved in invasion improving drug therapy as well as identify glycoproteins that may prove to be useful as vaccine candidates.

Unusual N-glycan structures required for trafficking Toxoplasma gondii GAP50 to the inner membrane complex regulate host cell entry through parasite motility

Toxoplasma gondii motility, which is essential for host cell entry, migration through host tissues, and invasion, is a unique form of actin-dependent gliding. It is powered by a motor complex mainly composed of myosin heavy chain A, myosin light chain 1, gliding associated proteins GAP45, and GAP50, the only integral membrane anchor so far described. In the present study, we have combined glycomic and proteomic approaches to demonstrate that all three potential N-glycosylated sites of GAP50 are occupied by unusual N-glycan structures that are rarely found on mature mammalian glycoproteins. Using site-directed mutagenesis, we show that N-glycosylation is a prerequisite for GAP50 transport from the endoplasmic reticulum to the Golgi apparatus and for its subsequent delivery into the inner complex membrane. Assembly of key partners into the gliding complex, and parasite motility are severely impaired in the unglycosylated GAP50 mutants. Furthermore, comparative affinity purification using N-glycosylated and unglycosylated GAP50 as bait identified three novel hypothetical proteins including the recently described gliding associated protein GAP40, and we demonstrate that N-glycans are required for efficient binding to gliding partners. Collectively, these results provide the first detailed analyses of T. gondii N-glycosylation functions that are vital for parasite motility and host cell entry.

Differential effects of three canonical Toxoplasma strains on gene expression in human neuroepithelial cells

Strain type is one of the key factors suspected to play a role in determining the outcome of Toxoplasma infection. In this study, we examined the transcriptional profile of human neuroepithelioma cells in response to representative strains of Toxoplasma by using microarray analysis to characterize the strain-specific host cell response. The study of neural cells is of interest in light of the ability of Toxoplasma to infect the brain and to establish persistent infection within the central nervous system. We found that the extents of the expression changes varied considerably among the three strains. Neuroepithelial cells infected with Toxoplasma type I exhibited the highest level of differential gene expression, whereas type II-infected cells had a substantially smaller number of genes which were differentially expressed. Cells infected with type III exhibited intermediate effects on gene expression. The three strains also differed in the individual genes and gene pathways which were altered following cellular infection. For example, gene ontology (GO) analysis indicated that type I infection largely affects genes related to the central nervous system, while type III infection largely alters genes which affect nucleotide metabolism; type II infection does not alter the expression of a clearly defined set of genes. Moreover, Ingenuity Pathways Analysis (IPA) suggests that the three lineages differ in the ability to manipulate their host; e.g., they employ different strategies to avoid, deflect, or subvert host defense mechanisms. These observed differences may explain some of the variation in the neurobiological effects of different strains of Toxoplasma on infected individuals.

Direct evidence of O-GlcNAcylation in the apicomplexan Toxoplasma gondii: a biochemical and bioinformatic study

Toxoplasma gondii and Plasmodium falciparum are apicomplexan parasites responsible for serious diseases in humans. Many studies have focused on the post-translational modifications (PTMs) found in the two protists including phosphorylation, acetylation or SUMOylation but only a few of these are concerned with the nuclear and cytosolic-specific glycosylation O-GlcNAcylation. O-GlcNAcylation is a highly dynamic PTM-regulated by the ON and OFF enzymes: O-GlcNAc transferase and O-GlcNAcase-that can compete with phosphorylation but its function remains unclear. In this work, we directly prove the O-GlcNAcylation in T. gondii using antibodies specifically directed against the modification and we strongly suggest its occurrence in P. falciparum. We found that the inducible 70 kDa-Heat Shock Protein is O-GlcNAcylated, or associated with an O-GlcNAc-partner, in T. gondii. Using anti-OGT antibodies we were able to detect the expression of the glycosyltransferase in T. gondii cultured both in human foreskin fibroblast and in Vero cells and report its putative sequence. For the first time the presence of O-GlcNAcylation is unequivocally shown in T. gondii and suspected in P. falciparum. Since the O-GlcNAcylation is implicated in many biological fundamental processes this study opens a new research track in the knowledge of apicomplexans' life cycle and pathogenic potential.

Immunological reactions in response to apicomplexan glycosylphosphatidylinositols

Apicomplexan protozoa are a phylum of parasites that includes pathogens such as Plasmodium, the causative agent of the most severe form of malaria responsible for almost 1 million deaths per year and Toxoplasma gondii causing toxoplasmosis, a disease leading to cerebral meningitis in immunocompromised individuals or to abortion in farm animals or in women that are infected for the first time during pregnancy. The initial immune reactions developed by the host are similar in response to an infection with Plasmodium and Toxoplasma in the sense that the same cells of the innate immune system are stimulated to produce inflammatory cytokines. The glycosylphosphatidylinositol (GPI) anchor is the major carbohydrate modification in parasite proteins and the GPIs are essential for parasite survival. Two immediate GPI precursors with the structures ethanolamine phosphate-6(Manalpha1-2)Manalpha1-2Manalpha1-6Manalpha1-4GlcN-PI and ethanolamine phosphate-6Manalpha1-2Manalpha1-6Man-alpha1-4-GlcN-PI are synthesized by P. falciparum. Two main structures are synthesized by T. gondii: ethanolamine phosphate-6Manalpha1-2Manalpha1-6(GalNAcbeta1-4)Manalpha1-4GlcN-PI and ethanolamine phosphate-6Manalpha1-2Manalpha1-6(Glcalpha1-4GalNAcbeta1-4)Manalpha1-4GlcN-PI. This review describes the biosynthesis of the apicomplexan GPIs and their role in the activation of the host immune system.

Production of recombinant proteins from protozoan parasites

Although the past decade has witnessed sequencing from an increasing number of parasites, modern high-throughput DNA sequencing technologies have the potential to generate complete genome sequences at even higher rates. Along with the discovery of genes that might constitute potential targets for chemotherapy or vaccination, the need for novel protein expression platforms has become a pressing matter. In addition to reviewing the advantages and limitations of the currently available and emerging expression systems, we discuss novel approaches that could overcome current limitations, including the 'pseudoparasite' concept, an expression platform in which the choice of the surrogate organism is based on its phylogenetic affinity to the target parasite, while taking advantage of the whole engineered organism as a vaccination adjuvant.

Suggestive evidence for Darwinian Selection against asparagine-linked glycans of Plasmodium falciparum and Toxoplasma gondii

We are interested in asparagine-linked glycans (N-glycans) of Plasmodium falciparum and Toxoplasma gondii, because their N-glycan structures have been controversial and because we hypothesize that there might be selection against N-glycans in nucleus-encoded proteins that must pass through the endoplasmic reticulum (ER) prior to threading into the apicoplast. In support of our hypothesis, we observed the following. First, in protists with apicoplasts, there is extensive secondary loss of Alg enzymes that make lipid-linked precursors to N-glycans. Theileria makes no N-glycans, and Plasmodium makes a severely truncated N-glycan precursor composed of one or two GlcNAc residues. Second, secreted proteins of Toxoplasma, which uses its own 10-sugar precursor (Glc(3)Man(5)GlcNAc(2)) and the host 14-sugar precursor (Glc(3)Man(9)GlcNAc(2)) to make N-glycans, have very few sites for N glycosylation, and there is additional selection against N-glycan sites in its apicoplast-targeted proteins. Third, while the GlcNAc-binding Griffonia simplicifolia lectin II labels ER, rhoptries, and surface of plasmodia, there is no apicoplast labeling. Similarly, the antiretroviral lectin cyanovirin-N, which binds to N-glycans of Toxoplasma, labels ER and rhoptries, but there is no apicoplast labeling. We conclude that possible selection against N-glycans in protists with apicoplasts occurs by eliminating N-glycans (Theileria), reducing their length (Plasmodium), or reducing the number of N-glycan sites (Toxoplasma). In addition, occupation of N-glycan sites is markedly reduced in apicoplast proteins versus some secretory proteins in both Plasmodium and Toxoplasma.

Molecular characterization and expression analysis of a P-glycoprotein homologue in Toxoplasma gondii

ATP-binding cassette (ABC) transporters represent an important family of membrane proteins involved in drug resistance and other biological activities. The present study reports on the characterization of a P-glycoprotein (Pgp), TgABC.B1, in the protozoan parasite Toxoplasma gondii. The protein encoded by the TgABC.B1 gene displays the typical (TMD-NBD)2 structural organization of the "full" ABC transporter and shows significant identity and similarity with two apicomplexan Pgps; Pgh1 in Plasmodium falciparum and CpABC3 in Cryptosporidium parvum. The TgABC.B1 gene is a single copy gene transcribed into a full-length mRNA of 4.3kb and expressed as a protein of approximately 150kDa, which cellular localization revealed a membrane-associated labelling in tachyzoites. The TgABC.B1 gene is constitutively expressed in the three major T. gondii genotypes but demonstrated a higher expression in virulent type I, at both transcriptional and translational levels. Further characterization of this Pgp-like protein will increase our knowledge of the membrane transport system in this parasite and could result in the identification of a new therapeutic target in Toxoplasma.

Toxoplasma: the next 100years

It has been 100 years since Toxoplasma gondii was initially described in Tunis by Nicolle and Manceaux (1908) in the tissues of the gundi (Ctenodoactylus gundi) and in Brazil by Splendore (1908) in the tissues of a rabbit. T. gondii is a ubiquitous, Apicomplexan parasite of warm-blooded animals that can cause several clinical syndromes including encephalitis, chorioretinitis and congenital infection. Due to the extensive repertoire of applicable experimental techniques available for this pathogen it has become a model organism for the study of intracellular pathogens. Data obtained from genome-wide expression studies, including ChIP on chip and proteomics surveys, are refining our understanding of the genetic networks involved in the developmental biology of this pathogen as well as the interactions of the parasite with its host. This review addresses recent advances in our understanding of the developmental biology and host-pathogen relationships of T. gondii.

Translation regulation by eukaryotic initiation factor-2 kinases in the development of latent cysts in Toxoplasma gondii

A key problem in the treatment of numerous pathogenic eukaryotes centers on their development into latent forms during stress. For example, the opportunistic protist Toxoplasma gondii converts to latent cysts (bradyzoites) responsible for recrudescence of disease. We report that Toxoplasma eukaryotic initiation factor-2alpha (TgIF2alpha) is phosphorylated during stress and establish that protozoan parasites utilize translation control to modulate gene expression during development. Importantly, TgIF2alpha remains phosphorylated in bradyzoites, explaining how these cells maintain their quiescent state. Furthermore, we have characterized novel eIF2 kinases; one in the endoplasmic reticulum and a likely regulator of the unfolded protein response (TgIF2K-A) and another that is a probable responder to cytoplasmic stresses (TgIF2K-B). Significantly, our data suggest that 1) the regulation of protein translation through eIF2 kinases is associated with development, 2) eIF2alpha phosphorylation is employed by cells to maintain a latent state, and 3) endoplasmic reticulum and cytoplasmic stress responses evolved in eukaryotic cells before the early diverging Apicomplexa. Given its importance to pathogenesis, eIF2 kinase-mediated stress responses may provide opportunities for novel therapeutics.