The opportunistic pathogen Toxoplasma gondii deploys a diverse legion of invasion and survival proteins

Proteomics Applications in Toxoplasma gondii: Unveiling the Host-Parasite Interactions and Therapeutic Target Discovery

Toxoplasma gondii, a protozoan parasite with the ability to infect various warm-blooded vertebrates, including humans, is the causative agent of toxoplasmosis. This infection poses significant risks, leading to severe complications in immunocompromised individuals and potentially affecting the fetus through congenital transmission. A comprehensive understanding of the intricate molecular interactions between T. gondii and its host is pivotal for the development of effective therapeutic strategies. This review emphasizes the crucial role of proteomics in T. gondii research, with a specific focus on host-parasite interactions, post-translational modifications (PTMs), PTM crosstalk, and ongoing efforts in drug discovery. Additionally, we provide an overview of recent advancements in proteomics techniques, encompassing interactome sample preparation methods such as BioID (BirA*-mediated proximity-dependent biotin identification), APEX (ascorbate peroxidase-mediated proximity labeling), and Y2H (yeast two hybrid), as well as various proteomics approaches, including single-cell analysis, DIA (data-independent acquisition), targeted, top-down, and plasma proteomics. Furthermore, we discuss bioinformatics and the integration of proteomics with other omics technologies, highlighting its potential in unraveling the intricate mechanisms of T. gondii pathogenesis and identifying novel therapeutic targets.

Subversion strategies of lysosomal killing by intracellular pathogens

Many pathogenic organisms need to reach either an intracellular compartment or the cytoplasm of a target cell for their survival, replication or immune system evasion. Intracellular pathogens frequently penetrate into the cell through the endocytic and phagocytic pathways (clathrin-mediated endocytosis, phagocytosis and macropinocytosis) that culminates in fusion with lysosomes. However, several mechanisms are triggered by pathogenic microorganisms - protozoan, bacteria, virus and fungus - to avoid destruction by lysosome fusion, such as rupture of the phagosome and thereby release into the cytoplasm, avoidance of autophagy, delaying in both phagolysosome biogenesis and phagosomal maturation and survival/replication inside the phagolysosome. Here we reviewed the main data dealing with phagosome maturation and evasion from lysosomal killing by different bacteria, protozoa, fungi and virus.

A Role for Basigin in Toxoplasma gondii Infection

The role of specific host cell surface receptors during Toxoplasma gondii invasion of host cells is poorly defined. Here, we interrogated the role of the well-known malarial invasion receptor, basigin, in T. gondii infection of astrocytes. We found that primary astrocytes express two members of the BASIGIN (BSG) immunoglobulin family, basigin and embigin, but did not express neuroplastin. Antibody blockade of either basigin or embigin caused a significant reduction of parasite infectivity in astrocytes. The specific role of basigin during T. gondii invasion was further examined using a mouse astrocytic cell line (C8-D30), which exclusively expresses basigin. CRISPR-mediated deletion of basigin in C8-D30 cells resulted in decreased T. gondii infectivity. T. gondii replication and invasion efficiency were not altered by basigin deficiency, but parasite attachment to astrocytes was markedly reduced. We also conducted a proteomic screen to identify T. gondii proteins that interact with basigin. Toxoplasma-encoded cyclophilins, the protein 14-3-3, and protein disulfide isomerase (TgPDI) were among the putative basigin-ligands identified. Recombinant TgPDI produced in E. coli bound to basigin and pretreatment of tachyzoites with a PDI inhibitor decreased parasite attachment to host cells. Finally, mutagenesis of the active site cysteines of TgPDI abolished enzyme binding to basigin. Thus, basigin and its related immunoglobulin family members may represent host receptors that mediate attachment of T. gondii to diverse cell types.

Unraveling Toxoplasma gondii GT1 Strain Virulence and New Protein-Coding Genes with Proteogenomic Analyses

Toxoplasma gondii is one of the most widespread parasites of great relevance to planetary health. It infects approximately one-third of the world population. T. gondii establishes itself in warm-blooded animals and causes adverse health outcomes, particularly in immunocompromised patients. T. gondii is also widely used as a model organism to study other related apicomplexan parasites, which requires a deeper understanding of its molecular biology. Type I strains (GT1 and RH) of T. gondii are considered the most virulent forms. The whole-genome sequencing of T. gondii annotated 8460 predicted gene models in the parasite. To this end, the proteogenomics technology allows harnessing of mass spectrometry (MS)-derived proteomic data to unravel new protein-coding genes, not to mention validation and correction of the existing gene models. In this study using the proteogenomic approach, we report the identification of 31 novel protein-coding genes while reannotating 88 existing gene models. Notably, the genome annotations were corrected for genes, such as SAG5C, GRA6, ROP4, ROP5, and ROP26. The associated proteins are known to play important roles in host-parasite interactions, particularly in relation to parasite virulence, suppression of host immune response, and distinctively pertinent for the survival of the parasite inside the host system. These new findings offer new insights, informing planetary health broadly and the knowledge base on T. gondii virulence specifically. The proteogenomics approach also provides a concrete example to study related apicomplexan organisms of relevance to planetary health, and so as to develop new diagnostics and therapeutics against toxoplasmosis and related diseases.

Toxoplasma gondii Toxolysin 4 Contributes to Efficient Parasite Egress from Host Cells

Egress from host cells is an essential step in the lytic cycle of T. gondii and other apicomplexan parasites; however, only a few parasite secretory proteins are known to affect this process. The putative metalloproteinase toxolysin 4 (TLN4) was previously shown to be an extensively processed microneme protein, but further characterization was impeded by the inability to genetically ablate TLN4. Here, we show that TLN4 has the structural properties of an M16 family metalloproteinase, that it possesses proteolytic activity on a model substrate, and that genetic disruption of TLN4 reduces the efficiency of egress from host cells. Complementation of the knockout strain with the TLN4 coding sequence significantly restored egress competency, affirming that the phenotype of the Δtln4 parasite was due to the absence of TLN4. This work identifies TLN4 as the first metalloproteinase and the second microneme protein to function in T. gondii egress. The study also lays a foundation for future mechanistic studies defining the precise role of TLN4 in parasite exit from host cells. IMPORTANCE After replicating within infected host cells, the single-celled parasite Toxoplasma gondii must rupture out of such cells in a process termed egress. Although it is known that T. gondii egress is an active event that involves disruption of host-derived membranes surrounding the parasite, very few proteins that are released by the parasite are known to facilitate egress. In this study, we identify a parasite secretory protease that is necessary for efficient and timely egress, laying the foundation for understanding precisely how this protease facilitates T. gondii exit from host cells.

A Family of Toxoplasma gondii Genes Related to GRA12 Regulate Cyst Burdens and Cyst Reactivation

Toxoplasma gondii causes a chronic infection that renders the immunocompromised human host susceptible to toxoplasmic encephalitis triggered by cyst reactivation in the central nervous system. The dense granule protein GRA12 is a major parasite virulence factor required for parasite survival during acute infection. Here, we characterized the role of four GRA12-related genes in acute and chronic stages of infection. While GRA12A, GRA12B, and GRA12D were highly expressed in asexual stage tachyzoites and bradyzoites, expression of GRA12C appeared to be restricted to the sexual stages. In contrast to deletion of GRA12 (Δgra12), no major defects in acute virulence were observed in Δgra12A, Δgra12B, or Δgra12D parasites, though Δgra12B parasites exhibited an increased tachyzoite replication rate. Bradyzoites secreted GRA12A, GRA12B, and GRA12D and incorporated these molecules into the developing cyst wall, as well as the cyst matrix in distinct patterns. Similar to GRA12, GRA12A, GRA12B, and GRA12D colocalized with the dense granules in extracellular tachyzoites, with GRA2 and the intravacuolar network in the tachyzoite stage parasitophorous vacuole and with GRA2 in the cyst matrix and cyst wall. Chronic stage cyst burdens were decreased in mice infected with Δgra12A parasites and were increased in mice infected with Δgra12B parasites. However, Δgra12B cysts were not efficiently maintained in vivo Δgra12A, Δgra12B, and Δgra12D in vitro cysts displayed a reduced reactivation efficiency, and reactivation of Δgra12A cysts was delayed. Collectively, our results suggest that a family of genes related to GRA12 play significant roles in the formation, maintenance, and reactivation of chronic stage cysts.IMPORTANCE If host immunity weakens, Toxoplasma gondii cysts recrudesce in the central nervous system and cause a severe toxoplasmic encephalitis. Current therapies target acute stage infection but do not eliminate chronic cysts. Parasite molecules that mediate the development and persistence of chronic infection are poorly characterized. Dense granule (GRA) proteins such as GRA12 are key virulence factors during acute infection. Here, we investigated four GRA12-related genes. GRA12-related genes were not major virulence factors during acute infection. Instead, GRA12-related proteins localized at the cyst wall and cyst matrix and played significant roles in cyst development, persistence, and reactivation during chronic infection. Similar to GRA12, the GRA12-related proteins selectively associated with the intravacuolar network of membranes inside the vacuole. Collectively, our results support the hypothesis that GRA12 proteins associated with the intravacuolar membrane system support parasite virulence during acute infection and cyst development, persistence, and reactivation during chronic infection.

Ferlins and TgDOC2 in Toxoplasma Microneme, Rhoptry and Dense Granule Secretion

The host cell invasion process of apicomplexan parasites like Toxoplasma gondii is facilitated by sequential exocytosis of the microneme, rhoptry and dense granule organelles. Exocytosis is facilitated by a double C2 domain (DOC2) protein family. This class of C2 domains is derived from an ancestral calcium (Ca2+) binding archetype, although this feature is optional in extant C2 domains. DOC2 domains provide combinatorial power to the C2 domain, which is further enhanced in ferlins that harbor 5-7 C2 domains. Ca2+ conditionally engages the C2 domain with lipids, membranes, and/or proteins to facilitating vesicular trafficking and membrane fusion. The widely conserved T. gondii ferlins 1 (FER1) and 2 (FER2) are responsible for microneme and rhoptry exocytosis, respectively, whereas an unconventional TgDOC2 is essential for microneme exocytosis. The general role of ferlins in endolysosmal pathways is consistent with the repurposed apicomplexan endosomal pathways in lineage specific secretory organelles. Ferlins can facilitate membrane fusion without SNAREs, again pertinent to the Apicomplexa. How temporal raises in Ca2+ combined with spatiotemporally available membrane lipids and post-translational modifications mesh to facilitate sequential exocytosis events is discussed. In addition, new data on cross-talk between secretion events together with the identification of a new microneme protein, MIC21, is presented.

Detection of toxoplasmic encephalitis in HIV positive patients in urine with hydrogel nanoparticles

BACKGROUND

Diagnosis of toxoplasmic encephalitis (TE) is challenging under the best clinical circumstances. The poor clinical sensitivity of quantitative polymerase chain reaction (qPCR) for Toxoplasma in blood and CSF and the limited availability of molecular diagnostics and imaging technology leaves clinicians in resource-limited settings with few options other than empiric treatment.

METHOLOGY/PRINCIPLE FINDINGS

Here we describe proof of concept for a novel urine diagnostics for TE using Poly-N-Isopropylacrylamide nanoparticles dyed with Reactive Blue-221 to concentrate antigens, substantially increasing the limit of detection. After nanoparticle-concentration, a standard western blotting technique with a monoclonal antibody was used for antigen detection. Limit of detection was 7.8pg/ml and 31.3pg/ml of T. gondii antigens GRA1 and SAG1, respectively. To characterize this diagnostic approach, 164 hospitalized HIV-infected patients with neurological symptoms compatible with TE were tested for 1) T. gondii serology (121/147, positive samples/total samples tested), 2) qPCR in cerebrospinal fluid (11/41), 3) qPCR in blood (10/112), and 4) urinary GRA1 (30/164) and SAG1 (12/164). GRA1 appears to be superior to SAG1 for detection of TE antigens in urine. Fifty-one HIV-infected, T. gondii seropositive but asymptomatic persons all tested negative by nanoparticle western blot and blood qPCR, suggesting the test has good specificity for TE for both GRA1 and SAG1. In a subgroup of 44 patients, urine samples were assayed with mass spectrometry parallel-reaction-monitoring (PRM) for the presence of T. gondii antigens. PRM identified antigens in 8 samples, 6 of which were concordant with the urine diagnostic.

CONCLUSION/SIGNIFICANCES

Our results demonstrate nanoparticle technology's potential for a noninvasive diagnostic test for TE. Moving forward, GRA1 is a promising target for antigen based diagnostics for TE.

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.

Proteomic Insights into the Biology of the Most Important Foodborne Parasites in Europe

Foodborne parasitoses compared with bacterial and viral-caused diseases seem to be neglected, and their unrecognition is a serious issue. Parasitic diseases transmitted by food are currently becoming more common. Constantly changing eating habits, new culinary trends, and easier access to food make foodborne parasites' transmission effortless, and the increase in the diagnosis of foodborne parasitic diseases in noted worldwide. This work presents the applications of numerous proteomic methods into the studies on foodborne parasites and their possible use in targeted diagnostics. Potential directions for the future are also provided.

Toxoplasmosis and Psychiatric and Neurological Disorders: A Step toward Understanding Parasite Pathogenesis

Toxoplasmosis, a disease that disrupts fetal brain development and severely affects the host's brain, has been linked to many behavioral and neurological disorders. There is growing interest in how a single-celled neurotropic parasite, Toxoplasma gondii, can control or change the behavior of the host as well as how it dominates the host's neurons. Secrets beyond these could be answered by decoding the Toxoplasma gondii genome, unravelling the function of genomic sequences, and exploring epigenetics and mRNAs alterations, as well as the postulated mechanisms contributing to various neurological and psychiatric symptoms caused by this parasite. Substantial efforts have been made to elucidate the action of T. gondii on host immunity and the biology of its infection. However, the available studies on the molecular aspects of toxoplasmosis that affect central nervous system (CNS) circuits remain limited, and much research is still needed on this interesting topic. In my opinion, this parasite is a gift for studying the biology of the nervous system and related diseases. We should utilize the unique features of Toxoplasma, such as its abilities to modulate brain physiology, for neurological studies or as a possible tool or approach to cure neurological disease.

Head-to-head comparisons of Toxoplasma gondii and its near relative Hammondia hammondi reveal dramatic differences in the host response and effectors with species-specific functions

Toxoplasma gondii and Hammondia hammondi are closely-related coccidian intracellular parasites that differ in their ability to cause disease in animal and (likely) humans. The role of the host response in these phenotypic differences is not known and to address this we performed a transcriptomic analysis of a monocyte cell line (THP-1) infected with these two parasite species. The pathways altered by infection were shared between species ~95% the time, but the magnitude of the host response to H. hammondi was significantly higher compared to T. gondii. Accompanying this divergent host response was an equally divergent impact on the cell cycle of the host cell. In contrast to T. gondii, H. hammondi infection induces cell cycle arrest via pathways linked to DNA-damage responses and cellular senescence and robust secretion of multiple chemokines that are known to be a part of the senescence associated secretory phenotype (SASP). Remarkably, prior T. gondii infection or treatment with T. gondii-conditioned media suppressed responses to H. hammondi infection, and promoted the replication of H. hammondi in recipient cells. Suppression of inflammatory responses to H. hammondi was found to be mediated by the T. gondii effector IST, and this finding was consistent with reduced functionality of the H. hammondi IST ortholog compared to its T. gondii counterpart. Taken together our data suggest that T. gondii manipulation of the host cell is capable of suppressing previously unknown stress and/or DNA-damage induced responses that occur during infection with H. hammondi, and that one important impact of this T. gondii mediated suppression is to promote parasite replication.

The unicellular eukaryotic parasite Toxoplasma gondii hijacks the migration machinery of mononuclear phagocytes to promote its dissemination

Toxoplasma gondii is an obligate intracellular protozoan with the ability to infect virtually any type of nucleated cell in warm-blooded vertebrates including humans. Toxoplasma gondii invades immune cells, which the parasite employs as shuttles for dissemination by a Trojan horse mechanism. Recent findings are starting to unveil how this parasite orchestrates the subversion of the migratory functions of parasitised mononuclear phagocytes, especially dendritic cells (DCs) and monocytes. Here, we focus on how T. gondii impacts host cell signalling that regulates leukocyte motility and systemic migration in tissues. Shortly after active parasite invasion, DCs undergo mesenchymal-to-amoeboid transition and adopt a high-speed amoeboid mode of motility. To trigger migratory activation - termed hypermigratory phenotype - T. gondii induces GABAergic signalling, which results in calcium fluxes mediated by voltage-gated calcium channels in parasitised DCs and brain microglia. Additionally, a TIMP-1-CD63-ITGB1-FAK signalling axis and signalling via the receptor tyrosine kinase MET promotes sustained hypermigration of parasitised DCs. Recent reports show that the activated signalling pathways converge on the small GTPase Ras to activate the MAPK Erk signalling cascade, a central regulator of cell motility. To date, three T. gondii-derived putative effector molecules have been linked to hypermigration: Tg14-3-3, TgWIP and ROP17. Here, we discuss their impact on the hypermigratory phenotype of phagocytes. Altogether, the emerging concept suggests that T. gondii induces metastasis-like migratory properties in parasitised mononuclear phagocytes to promote infection-related dissemination.

What a Difference 30 Years Makes! A Perspective on Changes in Research Methodologies Used to Study Toxoplasma gondii

Toxoplasma gondii is a remarkable species with a rich cell, developmental, and population biology. It is also sometimes responsible for serious disease in animals and humans and the stages responsible for such disease are relatively easy to study in vitro or in laboratory animal models. As a result of all this, Toxoplasma has become the subject of intense investigation over the last several decades, becoming a model organism for the study of the phylum of which it is a member, Apicomplexa. This has led to an ever-growing number of investigators applying an ever-expanding set of techniques to dissecting how Toxoplasma "ticks" and how it interacts with its many hosts. In this perspective piece I first wind back the clock 30 years and then trace the extraordinary pace of methodologies that have propelled the field forward to where we are today. In keeping with the theme of this collection, I focus almost exclusively on the parasite, rather than host side of the equation. I finish with a few thoughts about where the field might be headed-though if we have learned anything, the only sure prediction is that the pace of technological advance will surely continue to accelerate and the future will give us still undreamed of methods for taking apart (and then putting back together) this amazing organism with all its intricate biology. We have so far surely just scratched the surface.

ToxoDB: Functional Genomics Resource for Toxoplasma and Related Organisms

ToxoDB is a free online resource that provides access to genomic and functional genomic data. All data is made available through an intuitive queryable interface that enables scientists to build in silico experiments and develop testable hypothesis. The resource contains 32 fully sequenced and annotated genomes, with genomic sequence from multiple strains available for variant detection and copy number variation analysis. In addition to genomic sequence data, ToxoDB contains numerous functional genomic datasets including microarray, RNAseq, proteomics, ChIP-seq, and phenotypic data. In addition, results from a number of whole-genome analyses are incorporated including mapping to orthology clusters which allows users to leverage phylogenetic relationships in their analyses. Integration of primary data is made possible through a private galaxy interface and custom export tools that allow users to interrogate their own results in the context of all other data in the database.

Toxoplasma gondii RPL40 is a circulating antigen with immune protection effect

Screening and identification of protective antigens are essential for the prevention of infections with Toxoplasma gondii (Nicolle et Manceaux, 1908). In our previous study, T. gondii ribosomal-ubiquitin protein L40 (TgRPL40) was identified as a circulating antigen. However, the function and protective value of TgRPL40 was unknown. In the current study, recombinant TgRPL40 was expressed in Escherichia coli BL21 and antibody was prepared. Western blotting analysis indicated that TgRPL40 was present in circulating antigens and excretory/secretary antigens (ESA). Immunofluorescence and immunoelectron microscopy analysis revealed that TgRPL40 protein is widely distributed in the tachyzoites. Immunisation with recombinant TgRPL40 prolonged the survival of mice infected with tachyzoites. Quantitative real-time polymerase chain reaction analysis showed that immunisation with recombinant TgRPL40 reduced the parasite burden in blood, liver, spleen and brain of mice infected with tachyzoites. These observations indicate that TgRPL40 is a circulating antigen and is an effector of immune protection against acute T. gondii infection.

A systematic review of Toxoplasma gondii antigens to find the best vaccine candidates for immunization

At present, there is not any available accepted vaccine for prevention of Toxoplasma gondii (T. gondii) in human and animals. We conducted literature search through English (Google Scholar, PubMed, Science Direct, Scopus, EBSCO, ISI Web of Science) scientific paper databases to find the best vaccine candidates against toxoplasmosis among T. gondii antigens. Articles with information on infective stage, pathogenicity, immunogenicity and characterization of antigens were selected. We considered that the ideal and significant vaccines should include different antigens and been expressed in all infective stages of the parasite with a high pathogenicity and immunogenicity. Evaluation within this systematic review indicates that MIC 3, 4, 13, ROP 2, RON 5, GRA 1, 6, 8, 14 are expressed in all three infective stages and have pathogenicity and immunogenicity. MIC 5, ROM 4, GRA 2, 4, 15, ROP 5, 16, 17, 38, RON 4, MIC 1, GRA 10, 12, 16, SAG 3 are expressed in only tachyzoites and bradyzoites stages of T. gondii with pathogenicity/immunogenicity. Some antigens appeared to be expressed in a single stage (tachyzoites) but have high pathogenicity and induce immune response. They include enolase2 (ENO2), SAG 1, SAG5D, HSP 70, ROM 1, ROM 5, AMA 1, ROP 18, RON2 and GRA 24. In conclusion, current vaccination against T. gondii infection is not satisfactory, and with the increasing number of high-risk individuals, the development of an effective and safe specific vaccine is greatly valuable for toxoplasmosis prevention. This systematic review reveals prepare candidates for immunization studies.

An in silico pipeline to filter the Toxoplasma gondii proteome for proteins that could traffic to the host cell nucleus and influence host cell epigenetic regulation

Toxoplasma gondii uses epigenetic mechanisms to regulate both endogenous and host cell gene expression. To identify genes with putative epigenetic functions, we developed an in silico pipeline to interrogate the T. gondii proteome of 8313 proteins. Step 1 employs PredictNLS and NucPred to identify genes predicted to target eukaryotic nuclei. Step 2 uses GOLink to identify proteins of epigenetic function based on Gene Ontology terms. This resulted in 611 putative nuclear localised proteins with predicted epigenetic functions. Step 3 filtered for secretory proteins using SignalP, SecretomeP, and experimental data. This identified 57 of the 611 putative epigenetic proteins as likely to be secreted. The pipeline is freely available online, uses open access tools and software with user-friendly Perl scripts to automate and manage the results, and is readily adaptable to undertake any such in silico search for genes contributing to particular functions.

Toxoplasma gondii dense granule protein 15 induces apoptosis in choriocarcinoma JEG-3 cells through endoplasmic reticulum stress

Background

Toxoplasma gondii, a single-celled parasite commonly found in mammals, has been shown to induce trophoblast cell apoptosis and subsequently cause fetal damage and abortion. Although dense granule protein 15 (GRA15) has been identified as a key component in innate immunity to T. gondii infection and its pathogenesis, its role in host cell apoptosis remains unclarified.

Methods

Type II GRA15 (GRA15_II) cDNA was inserted into a plasmid encoding enhanced green fluorescent protein (pEGFP). Choriocarcinoma JEG-3 cells were transfected with either pEGFP or pEGFP-GRA15_II and cultured for 24 h. Cell apoptosis and endoplasmic reticulum stress (ERS) responses were assessed. Inhibitors targeting inositol-requiring kinase 1α (IRE1α; 4μ8C, 100 nM) or c-Jun N-terminal kinase (JNK; SP6000125, 20 μM) were added 12 h after plasmid transfection, followed by testing the effect of GRA15_II on ERS.

Results

When compared to pEGFP, pEGFP-GRA15_II transfection facilitated cell apoptosis (P < 0.05), increased mRNA expression of caspase-3, caspase-4, 78-kDa glucose-regulated protein (GRP78), C/EBP homologous protein (CHOP) and X-box binding protein-1 (XBP1) (all P < 0.05), and promoted protein expression of cleaved caspase-3, cleaved poly(ADP-ribose) polymerase, Bax, CHOP, GRP78, phospho-JNK, and phospho-IRE1α (all P < 0.05). The 4μ8C and SP6000125 decreased apoptosis and protein expression of XBP1s, CHOP, TNF receptor-associated factor 2 (TRAF2), phosphorylated apoptosis signal-regulating kinase 1 (ASK1), cleaved caspase-3, phospho-JNK, and Bax (all P < 0.05) in pEGFP-GRA15_II transfected cells.

Conclusions

Toxoplasma GRA15_II induced ERS and subsequently caused apoptosis of choriocarcinoma JEG-3 cells.

Toward detection of toxoplasmosis from urine in mice using hydro-gel nanoparticles concentration and parallel reaction monitoring mass spectrometry

Diagnosis of clinical toxoplasmosis remains a challenge, thus limiting the availability of human clinical samples. Though murine models are an approximation of human response, their definitive infection status and tissue availability make them critical to the diagnostic development process. Hydrogel mesh nanoparticles were used to concentrate antigen to detectable levels for mass spectrometry. Seven Toxoplasma gondii isolates were used to develop a panel of potential peptide sequences for detection by parallel reaction monitoring (PRM) mass spectrometry. Nanoparticles were incubated with decreasing concentrations of tachyzoite lysate to explore the limits of detection of PRM. Mice whose toxoplasmosis infection status was confirmed by quantitative real-time PCR had urine tested by PRM after hydrogel mesh concentration for known T. gondii peptides. Peptides from GRA1, GRA12, ROP4, ROP5, SAG1, and SAG2A proteins were detected by PRM after nanoparticle concentration of urine, confirming detection of T. gondii antigen in the urine of an infected mouse.

Identification of a TNF-α inducer MIC3 originating from the microneme of non-cystogenic, virulent Toxoplasma gondii

Toxoplasma gondii is an opportunistic parasite with avirulent cystogenic and highly virulent non-cystogenic isolates. Although non-cystogenic strains are considered the most virulent, there are also marked genetic and virulence differences among these strains. Excretory-secretory antigens (ESAs) of T. gondii are critical for the invasion process and the immune response of the host. To better understand the differences in virulence between non-cystogenic T. gondii isolates, we studied ESAs of the RH strain (Type I), and the very prevalent in China, but less virulent TgCtwh3 strain (Chinese 1). ESAs of RH and TgCtwh3 triggered different levels of TNF-α production and macrophage M1 polarization. Using iTRAQ analysis, 27 differentially expressed proteins originating from secretory organelles and surface were quantified. Of these proteins, 11 microneme-associated proteins (MICs), 6 rhoptry proteins, 2 dense granule proteins and 5 surface proteins were more abundant in RH than in TgCtwh3. The protein-protein correlation network was employed to identify the important functional node protein MIC3, which was upregulated 5-fold in RH compared with TgCtwh3. MIC3 was experimentally confirmed to evoke a TNF-α secretory response, and it also induced macrophage M1 polarization. This result suggests that MIC3 is a potentially useful immunomodulator that induces TNF-α secretion and macrophage M1 polarization.

Mechanisms and pathways of Toxoplasma gondii transepithelial migration

Toxoplasma gondii is a ubiquitous parasite and a prevalent food-borne parasitic pathogen. Infection of the host occurs principally through oral consumption of contaminated food and water with the gastrointestinal tract being the primary route for entry into the host. To promote infection, T. gondii has evolved highly specialized strategies for rapid traversal of the single cell thick intestinal epithelial barrier. Parasite transmigration via the paracellular pathway between adjacent cells enables parasite dissemination to secondary sites of infection where chronic infection of muscle and brain tissue is established. It has recently been proposed that parasite interactions with the integral tight junction (TJ) protein occludin influences parasite transmigration of the intestinal epithelium. We review here the emerging mechanisms of T. gondii transmigration of the small intestinal epithelium alongside the developing role played in modulating the wider TJ-associated proteome to rewire host cell regulatory systems for the benefit of the parasite.

Identification and characterization of an immunogenic antigen, enolase 2, among excretory/secretory antigens (ESA) of Toxoplasma gondii

An immunogenic protein, enolase 2, was identified among the secreted excretory/secretory antigens (ESAs) from Toxoplasma gondii strain RH using immunoproteomics based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Enolase 2 was cloned, sequenced, and heterologously expressed. BLAST analysis revealed 75-96% similarity with enolases from other parasites. Immunoblotting demonstrated good immunoreactivity of recombinant T. gondii enolase (Tg-enolase 2) to T. gondii-infected animal serum. Purified Tg-enolase 2 was found to catalyze dehydration of 2-phospho-d-glycerate to phosphoenolpyruvate. In vitro studies revealed maximal activity at pH 7.5 and 37 °C, and activity was inhibited by K(+), Ni(2+), Al(3+), Na(+), Cu(2+) and Cr(3+). A monoclonal antibody against Tg-enolase 2 was prepared, 1D6, with the isotype IgG2a/κ. Western blotting revealed that 1D6 reacts with Tg-enolase 2 and native enolase 2, present among T. gondii ESAs. The indirect immunofluorescence assays showed that enolase 2 could be specifically detected on the growing T. gondii tachyzoites. Immunoelectron microscopy revealed the surface and intracellular locations of enolase 2 on T. gondii cells. In conclusion, our results clearly show that the enzymatic activity of T. gondii enolase 2 is ion dependent and that it could be influenced by environmental factors. We also provide evidence that enolase 2 is an important immunogenic protein of ESAs from T. gondii and that it is a surface-exposed protein with strong antigenicity and immunogenicity. Our findings indicate that enolase 2 could play important roles in metabolism, immunogenicity and pathogenicity and that it may serve as a novel drug target and candidate vaccine against T. gondii infection.

Twenty-six circulating antigens and two novel diagnostic candidate molecules identified in the serum of canines with experimental acute toxoplasmosis

BACKGROUND

The protozoan Toxoplasma gondii is a pathogen that causes severe opportunistic disease in a wide range of hosts. Efficient methods to diagnose acute T. gondii infection are essential for the administration of appropriate treatments and to reduce economic losses. In animals with acute infections, circulating antigens (CAgs) were detected as early as two days post-infection; these CAgs were reliable diagnostic indicators of acute infection. However, only a limited number of CAgs have been identified to date. The objective of this study was to identify a broader spectrum of CAgs and to explore novel diagnostic candidates in serum.

METHODS

A canine model of acute toxoplasmiosis was established. For this purpose, six dogs were infected by intraperitoneal inoculation of tachyzoites. The CAgs spectrum in the serum was identified with the immunoprecipitation-shotgun approach. Two CAgs with low homology to other species, coronin protein (TgCOR) and ELMO protein (TgELMO), were heterologously expressed in Escherichia coli. Polyclonal antibodies against these two proteins were prepared, and the presence of these proteins in the serum was verified by Western blotting. The two CAgs were detected and evaluated by indirect ELISA methods.

RESULTS

The CAgs levels peaked between two and five days after inoculation, and twenty-six CAgs were identified. Western blotting showed the presence of the two proteins in the serum during acute infection. Based on ELISA tests, the two CAgs were detected during acute infection.

CONCLUSIONS

We identified twenty-six CAgs in the serum of canines with experimental acute toxoplasmosis and discovered two novel diagnostic candidates. We also provide new insights into the diagnosis of acute toxoplasmosis.

Migratory activation of parasitized dendritic cells by the protozoan Toxoplasma gondii 14-3-3 protein

The obligate intracellular parasite Toxoplasma gondii exploits cells of the immune system to disseminate. Upon infection, parasitized dendritic cells (DCs) and microglia exhibit a hypermigratory phenotype in vitro that has been associated with enhancing parasite dissemination in vivo in mice. One unresolved question is how parasites commandeer parasitized cells to achieve systemic dissemination by a 'Trojan-horse' mechanism. By chromatography and mass spectrometry analyses, we identified an orthologue of the 14-3-3 protein family, T. gondii 14-3-3 (Tg14-3-3), as mediator of DC hypermotility. We demonstrate that parasite-derived polypeptide fractions enriched for Tg14-3-3 or recombinant Tg14-3-3 are sufficient to induce the hypermotile phenotype when introduced by protein transfection into murine DCs, human DCs or microglia. Further, gene transfer of Tg14-3-3 by lentiviral transduction induced hypermotility in primary human DCs. In parasites expressing Tg14-3-3 in a ligand-regulatable fashion, overexpression of Tg14-3-3 was correlated with induction of hypermotility in parasitized DCs. Localization studies in infected DCs identified Tg14-3-3 within the parasitophorous vacuolar space and a rapid recruitment of host cell 14-3-3 to the parasitophorous vacuole membrane. The present work identifies a determinant role for Tg14-3-3 in the induction of the migratory activation of immune cells by T. gondii. Collectively, the findings reveal Tg14-3-3 as a novel target for an intracellular pathogen that acts by hijacking the host cell's migratory properties to disseminate.

A Toxoplasma gondii Ortholog of Plasmodium GAMA Contributes to Parasite Attachment and Cell Invasion

Toxoplasma gondii is a successful human pathogen in the same phylum as malaria-causing Plasmodium parasites. Invasion of a host cell is an essential process that begins with secretion of adhesive proteins onto the parasite surface for attachment and subsequent penetration of the host cell. Conserved invasion proteins likely play roles that were maintained through the divergence of these parasites. Here, we identify a new conserved invasion protein called glycosylphosphatidylinositol-anchored micronemal antigen (GAMA). Tachyzoites lacking TgGAMA were partially impaired in parasite attachment and invasion of host cells, yielding the first genetic evidence of a specific role in parasite entry into host cells. These findings widen our appreciation of the repertoire of conserved proteins that apicomplexan parasites employ for cell invasion.

Toxoplasma gondii and its Plasmodium kin share a well-conserved invasion process, including sequential secretion of adhesive molecules for host cell attachment and invasion. However, only a few orthologs have been shown to be important for efficient invasion by both genera. Bioinformatic screening to uncover potential new players in invasion identified a previously unrecognized T. gondii ortholog of Plasmodium glycosylphosphatidylinositol-anchored micronemal antigen (TgGAMA). We show that TgGAMA localizes to the micronemes and is processed into several proteolytic products within the parasite prior to secretion onto the parasite surface during invasion. TgGAMA from parasite lysate bound to several different host cell types in vitro, suggesting a role in parasite attachment. Consistent with this function, tetracycline-regulatable TgGAMA and TgGAMA knockout strains showed significant reductions in host cell invasion at the attachment step, with no defects in any of the other stages of the parasite lytic cycle. Together, the results of this work reveal a new conserved component of the adhesive repertoire of apicomplexan parasites.

IMPORTANCEToxoplasma gondii is a successful human pathogen in the same phylum as malaria-causing Plasmodium parasites. Invasion of a host cell is an essential process that begins with secretion of adhesive proteins onto the parasite surface for attachment and subsequent penetration of the host cell. Conserved invasion proteins likely play roles that were maintained through the divergence of these parasites. Here, we identify a new conserved invasion protein called glycosylphosphatidylinositol-anchored micronemal antigen (GAMA). Tachyzoites lacking TgGAMA were partially impaired in parasite attachment and invasion of host cells, yielding the first genetic evidence of a specific role in parasite entry into host cells. These findings widen our appreciation of the repertoire of conserved proteins that apicomplexan parasites employ for cell invasion.

Phosphatidylthreonine and Lipid-Mediated Control of Parasite Virulence

The major membrane phospholipid classes, described thus far, include phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylserine (PtdSer), and phosphatidylinositol (PtdIns). Here, we demonstrate the natural occurrence and genetic origin of an exclusive and rather abundant lipid, phosphatidylthreonine (PtdThr), in a common eukaryotic model parasite, Toxoplasma gondii. The parasite expresses a novel enzyme PtdThr synthase (TgPTS) to produce this lipid in its endoplasmic reticulum. Genetic disruption of TgPTS abrogates de novo synthesis of PtdThr and impairs the lytic cycle and virulence of T. gondii. The observed phenotype is caused by a reduced gliding motility, which blights the parasite egress and ensuing host cell invasion. Notably, the PTS mutant can prevent acute as well as yet-incurable chronic toxoplasmosis in a mouse model, which endorses its potential clinical utility as a metabolically attenuated vaccine. Together, the work also illustrates the functional speciation of two evolutionarily related membrane phospholipids, i.e., PtdThr and PtdSer.

An exclusive membrane lipid, phosphatidylthreonine, is revealed to be naturally abundant in the widespread protist parasite Toxoplasma gondii, where it has evolved adaptively and is essential for parasite virulence.

Lipids are essential constituents of biological membranes, and most organisms across the tree of life use a relatively limited repertoire of lipids in their membranes. This work reveals the natural and abundant presence of an exclusive lipid phosphatidylthreonine (PtdThr) in Toxoplasma gondii, a ubiquitous protozoan parasite of humans and animals. PtdThr is made by a novel parasite-specific enzyme, PtdThr synthase, which has evolved from the widespread enzyme phosphatidylserine synthase. The study shows that PtdThr is required for asexual reproduction and virulence of the parasite in vivo, and a metabolically attenuated mutant strain of Toxoplasma lacking PtdThr can protect vaccinated mice against acute and currently incurable chronic infection. This discovery demonstrates adaptive “speciation” of PtdThr from an otherwise near-universal membrane lipid phosphatidylserine and reveals de novo PtdThr synthesis in T. gondii as a potential drug target.

Immunoproteomic technology offers an extraordinary diagnostic approach for Toxoplasma gondii infection

Immunoproteomic technology offers an exceptional tool to fill the blanks that still exist in diagnosis of Toxoplasma gondii infection despite its annotated sequence. The pitfalls of serological assays and current immunoproteomic approaches are accentuated, and new approaches are presented to improve the signal and to eliminate the noise produced by blocking-specific background. This review also highlights examples where immunoproteomic studies have contributed to broaden our understanding of toxoplasmosis diagnosis. Further promising solutions, which immunoproteomic technology can grant for toxoplasmosis diagnosis are part of an intense discussion.

A large-scale proteogenomics study of apicomplexan pathogens-Toxoplasma gondii and Neospora caninum

Proteomics data can supplement genome annotation efforts, for example being used to confirm gene models or correct gene annotation errors. Here, we present a large-scale proteogenomics study of two important apicomplexan pathogens: Toxoplasma gondii and Neospora caninum. We queried proteomics data against a panel of official and alternate gene models generated directly from RNASeq data, using several newly generated and some previously published MS datasets for this meta-analysis. We identified a total of 201 996 and 39 953 peptide-spectrum matches for T. gondii and N. caninum, respectively, at a 1% peptide FDR threshold. This equated to the identification of 30 494 distinct peptide sequences and 2921 proteins (matches to official gene models) for T. gondii, and 8911 peptides/1273 proteins for N. caninum following stringent protein-level thresholding. We have also identified 289 and 140 loci for T. gondii and N. caninum, respectively, which mapped to RNA-Seq-derived gene models used in our analysis and apparently absent from the official annotation (release 10 from EuPathDB) of these species. We present several examples in our study where the RNA-Seq evidence can help in correction of the current gene model and can help in discovery of potential new genes. The findings of this study have been integrated into the EuPathDB. The data have been deposited to the ProteomeXchange with identifiers PXD000297and PXD000298.

Secreted effectors in Toxoplasma gondii and related species: determinants of host range and pathogenesis?

Recent years have witnessed the discovery of a number of secreted proteins in Toxoplasma gondii that play important roles in host-pathogen interactions and parasite virulence, particularly in the mouse model. However, the role that these proteins play in driving the unique features of T. gondii compared to some of its nearest apicomplexan relatives (Hammondia hammondi and Neospora caninum) is unknown. These unique features include distinct dissemination characteristics in vivo and a vast host range. In this review we comprehensively survey what is known about disease outcome, the host response and host range for T. gondii, H. hammondi, and N. caninum. We then review what is presently known about recently identified secreted virulence effectors in these three genetically related, but phenotypically distinct, species. Finally we exploit the existence of genome sequences for these three organisms and discuss what is known about the presence, and functionality, of key T. gondii effectors in these three species.

Secreted Toxoplasma gondii molecules interfere with expression of MHC-II in interferon gamma-activated macrophages

The obligate intracellular protozoan parasite Toxoplasma gondii interferes with major histocompatibility complex class II antigen presentation to dampen host CD4(+) T cell responses. While it is known that T. gondii inhibits major histocompatibility complex class II gene transcription and expression in infected host cells, the mechanism of this host manipulation is unknown. Here, we show that soluble parasite proteins inhibit IFNγ-induced expression of major histocompatibility complex class II on the surface of the infected cell in a dose-dependent response that was abolished by protease treatment. Subcellular fractionation of T. gondii tachyzoites revealed that the major histocompatibility complex class II inhibitory activity co-partitioned with rhoptries and/or dense granules. However, parasite mutants deleted for single rhoptries or dense granules genes (ROP1, 4/7, 14, 16 and 18 or GRA 2-9 and 12 knock-out strains) retained the ability to inhibit expression of major histocompatibility complex class II. In addition, excreted/secreted antigens released by extracellular tachyzoites displayed immunomodulatory activity characterized by an inhibition of major histocompatibility complex class II expression, and reduced expression and release of TNFα by macrophages. Tandem MS analysis of parasite excreted/secreted antigens generated a list of T. gondii secreted proteins that may participate in major histocompatibility complex class II inhibition and the modulation of host immune functions.

Toxoplasma gondii prevalent in China induce weaker apoptosis of neural stem cells C17.2 via endoplasmic reticulum stress (ERS) signaling pathways

Background

Toxoplasma gondii, an obligate intracellular pathogen, has a strong affinity for the nervous system. TgCtwh3, a representative Chinese 1 Toxoplasma strain prevalent in China, has the polymorphic features of the effectors ROP16_I/III with type I and GRA15_II with type II Toxoplasma strains. The interaction of this atypical strain with host cells remains extremely elusive.

Methods

Using a transwell system, neural stem cells C17.2 were co-cultured with the tachyzoites of TgCtwh3 or standard type I RH strain. The apoptosis levels of C17.2 cells and the expression levels of related proteins in the endoplasmic reticulum stress (ERS)-mediated pathway were detected by flow cytometry and Western blotting.

Results

The apoptosis level of C17.2 cells co-cultured with TgCtwh3 had a significant increase compared to the negative control group; however, the apoptosis level in the TgCtwh3 group was significantly lower than that in the RH co-culture group. Western blotting analyses reveal that, after the C17.2 cells were co-cultured with TgCtwh3 and RH tachyzoites, the expression levels of caspase-12, CHOP and p-JNK in the cells increased significantly when compared to the control groups. After the pretreatment of Z-ATAD-FMK, an inhibitor of caspase-12, the apoptosis level of the C17.2 cells co-cultured with TgCtwh3 or RH tachyzoites had an apparent decline, and correspondingly, the expression levels of those related proteins were notably decreased.

Conclusions

Our findings suggest that TgCtwh3 may induce the apoptosis of the C17.2 cells by up-regulation of caspase-12, CHOP, and p-JNK, which are associated with ERS signaling pathways. This work contributes to better understanding the possible mechanism of brain pathology induced by T. gondii Chinese 1 isolates prevalent in China, and also reveals the potential value of ERS inhibitors to treat such related diseases in the future.

The calcium signaling toolkit of the Apicomplexan parasites Toxoplasma gondii and Plasmodium spp

Apicomplexan parasites have complex life cycles, frequently split between different hosts and reliant on rapid responses as the parasites react to changing environmental conditions. Calcium ion (Ca(2+)) signaling is consequently essential for the cellular and developmental changes that support Apicomplexan parasitism. Apicomplexan genomes reveal a rich repertoire of genes involved in calcium signaling, although many of the genes responsible for observed physiological changes remain unknown. There is evidence, for example, for the presence of a nifedipine-sensitive calcium entry mechanism in Toxoplasma, but the molecular components involved in Ca(2+) entry in both Toxoplasma and Plasmodium, have not been identified. The major calcium stores are the endoplasmic reticulum (ER), the acidocalcisomes, and the plant-like vacuole in Toxoplasma, or the food vacuole in Plasmodium spp. Pharmacological evidence suggests that Ca(2+) release from intracellular stores may be mediated by inositol 1,4,5-trisphosphate (IP3) or cyclic ADP ribose (cADPR) although there is no molecular evidence for the presence of receptors for these second messengers in the parasites. Several Ca(2+)-ATPases are present in Apicomplexans and a putative mitochondrial Ca(2+)/H(+) exchanger has been identified. Apicomplexan genomes contain numerous genes encoding Ca(2+)-binding proteins, with the notable expansion of calcium-dependent protein kinases (CDPKs), whose study has revealed roles in gliding motility, microneme secretion, host cell invasion and egress, and parasite differentiation. Microneme secretion has also been shown to depend on the C2 domain containing protein DOC2 in both Plasmodium spp. and Toxoplasma, providing further evidence for the complex transduction of Ca(2+) signals in these organisms. The characterization of these pathways could lead to the discovery of novel drug targets and to a better understanding of the role of Ca(2+) in these parasites.

Comparative studies on the proteomic expression patterns in the third- and fifth-stage larvae of Angiostrongylus cantonensis

Angiostrongylus cantonensis is an important zoonotic parasite causing eosinophilic meningitis and eosinophilic meningoencephalitis in humans. In this study, the protein expression profiles of the infective third- and pathogenic fifth-stage larvae (L3 and L5) of this parasite were compared by proteomic techniques. Isolated protein samples were separated by two-dimensional gel electrophoresis (2-DE), stained with silver nitrate, and analyzed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS). Proteins from L5 were mainly at pH 5-7 and with molecular weight (MW) 40-100 kDa, whereas those from L3 were at pH 5-6 and with 5-35 kDa. Of 100 protein spots identified, 33 were from L3 whereas 67 from L5 and 63 had known identities, whereas 37 were hypothetical proteins. There were 15 spots of stress proteins, and HSP60 was the most frequently found heat stress proteins in L5. More binding and protein transport-related proteins were found in L5 including peptidylprolyl isomerase (cyclophilin)-like 2, serum albumin, preproalbumin precursor, and dilute class unconventional myosin. L3 had a higher expression of cytoskeleton and membrane proteins than L5. In addition, four protein spots were identified in the sera of the rat host by Western blot analysis. The present proteomic study revealed different protein expression profiles in L3 and L5 of A. cantonensis. These changes may reflect the development of L3 from the poikilothermic snails to L5 in the homoeothemic rats. This information may be useful for the finding of stage-specific proteins and biomarker for diagnosis of angiostrongyliasis.

Toxoplasma effector MAF1 mediates recruitment of host mitochondria and impacts the host response

The intracellular human protozoan parasite Toxoplasma gondii uses a novel secreted protein to recruit host mitochondria and alter the host's response to infection.

Recent information has revealed the functional diversity and importance of mitochondria in many cellular processes including orchestrating the innate immune response. Intriguingly, several infectious agents, such as Toxoplasma, Legionella, and Chlamydia, have been reported to grow within vacuoles surrounded by host mitochondria. Although many hypotheses have been proposed for the existence of host mitochondrial association (HMA), the causes and biological consequences of HMA have remained unanswered. Here we show that HMA is present in type I and III strains of Toxoplasma but missing in type II strains, both in vitro and in vivo. Analysis of F1 progeny from a type II×III cross revealed that HMA is a Mendelian trait that we could map. We use bioinformatics to select potential candidates and experimentally identify the polymorphic parasite protein involved, mitochondrial association factor 1 (MAF1). We show that introducing the type I (HMA⁺) MAF1 allele into type II (HMA⁻) parasites results in conversion to HMA⁺ and deletion of MAF1 in type I parasites results in a loss of HMA. We observe that the loss and gain of HMA are associated with alterations in the transcription of host cell immune genes and the in vivo cytokine response during murine infection. Lastly, we use exogenous expression of MAF1 to show that it binds host mitochondria and thus MAF1 is the parasite protein directly responsible for HMA. Our findings suggest that association with host mitochondria may represent a novel means by which Toxoplasma tachyzoites manipulate the host. The existence of naturally occurring HMA⁺ and HMA⁻ strains of Toxoplasma, Legionella, and Chlamydia indicates the existence of evolutionary niches where HMA is either advantageous or disadvantageous, likely reflecting tradeoffs in metabolism, immune regulation, and other functions of mitochondria.

Recent discoveries have revealed the remarkable functional diversity of mitochondria in roles other than energy production, including an integral role for mitochondria and their dynamics in the regulation of the innate immune response. Interestingly, host mitochondria are recruited to the membranes that surround certain intracellular bacteria and parasites during infection. To date, how and why this phenomenon occurs has been a mystery, although it has been proposed to provide a metabolic benefit to the microbes. Here we identify mitochondrial association factor 1 (MAF1) as the parasite protein that mediates the association between the protozoan pathogen Toxoplasma and host mitochondria during infection. We show that MAF1 is needed to recruit host mitochondria to the Toxoplasma-containing vacuole and that this process is associated with changes in the immune response in infected cells and animals. These findings show that recruitment and association with host mitochondria is an important means by which intracellular pathogens interface with their host. We also find that the cost–benefit outcome of altering mitochondrial function might differ between strains depending on the precise niche in which they evolved; for infectious agents, these differences likely reflect different host organisms.

Natural killer cell intrinsic toll-like receptor MyD88 signaling contributes to IL-12-dependent IFN-γ production by mice during infection with Toxoplasma gondii

Myeloid differentiation factor 88 (MyD88)-dependent IL-12 secretion by dendritic cells is critical for natural killer cell-mediated IFN-γ production and innate resistance to Toxoplasma gondii. Although MyD88(-/-) mice challenged with T. gondii have defective IL-12 responses and succumb to infection, administration of IL-12 to MyD88(-/-) mice fails to prevent acute mortality, suggesting that MyD88 may mediate signals within natural killer cells important for IL-12-dependent IFN-γ production and innate resistance to this parasite. In this study, we found that T. gondii antigens and IL-12 could synergistically trigger IFN-γ secretion by natural killer cells, which was dependent on toll-like receptor-MyD88 signaling. Further analysis showed that p38 mitogen-activated protein kinase, extracellular signal-regulated kinase, c-Jun N-terminal kinase and NF-κB multiple pathways downstream of MyD88 contributed to IFN-γ production by natural killer cells. Moreover, the well-established toll-like receptor agonists, T. gondii profilin (Tgprofilin) and T. gondii heat shock protein 70 (TgHSP70) could evoke a similar IFN-γ secretory response in natural killer cells to that evoked by T. gondii antigens. In vivo adoptive transfer experiments showed that, upon challenge with T. gondii, NOD/SCID-β2 microglobulin null (NOD/SCID-β2m(-/-)) mice injected i.v. with MyD88(-/-) natural killer cells had reduced serum IFN-γ levels and increased splenic tachyzoite burdens compared with those injected i.v. with wild-type natural killer cells. Taken together, these findings demonstrate a critical role for natural killer cell intrinsic toll-like receptor-MyD88 signaling in IL-12-dependent early IFN-γ production and innate resistance to T. gondii.

Unravelling the Neospora caninum secretome through the secreted fraction (ESA) and quantification of the discharged tachyzoite using high-resolution mass spectrometry-based proteomics

BACKGROUND

The apicomplexan parasite Neospora caninum causes neosporosis, a disease that leads to abortion or stillbirth in cattle, generating an economic impact on the dairy and beef cattle trade. As an obligatory intracellular parasite, N. caninum needs to invade the host cell in an active manner to survive. The increase in parasite cytosolic Ca2+ upon contact with the host cell mediates critical events, including the exocytosis of phylum-specific secretory organelles and the activation of the parasite invasion motor. Because invasion is considered a requirement for pathogen survival and replication within the host, the identification of secreted proteins (secretome) involved in invasion may be useful to reveal interesting targets for therapeutic intervention.

METHODS

To chart the currently missing N. caninum secretome, we employed mass spectrometry-based proteomics to identify proteins present in the N. caninum tachyzoite using two different approaches. The first approach was identifying the proteins present in the tachyzoite-secreted fraction (ESA). The second approach was determining the relative quantification through peptide stable isotope labelling of the tachyzoites submitted to an ethanol secretion stimulus (discharged tachyzoite), expecting to identify the secreted proteins among the down-regulated group.

RESULTS

As a result, 615 proteins were identified at ESA and 2,011 proteins quantified at the discharged tachyzoite. We have analysed the connection between the secreted and the down-regulated proteins and searched for putative regulators of the secretion process among the up-regulated proteins. An interaction network was built by computational prediction involving the up- and down-regulated proteins. The mass spectrometry proteomics data have been deposited to the ProteomeXchange with identifier PXD000424.

CONCLUSIONS

The comparison between the protein abundances in ESA and their measure in the discharged tachyzoite allowed for a more precise identification of the most likely secreted proteins. Information from the network interaction and up-regulated proteins was important to recognise key proteins potentially involved in the metabolic regulation of secretion. Our results may be helpful to guide the selection of targets to be investigated against Neospora caninum and other Apicomplexan organisms.

Electron tomography of Plasmodium falciparum merozoites reveals core cellular events that underpin erythrocyte invasion

Erythrocyte invasion by merozoites forms of the malaria parasite is a key step in the establishment of human malaria disease. To date, efforts to understand cellular events underpinning entry have been limited to insights from non-human parasites, with no studies at sub-micrometer resolution undertaken using the most virulent human malaria parasite, Plasmodium falciparum. This leaves our understanding of the dynamics of merozoite sub-cellular compartments during infectionincomplete, in particular that of the secretory organelles. Using advances in P. falciparum merozoite isolation and new imaging techniques we present a three-dimensional study of invasion using electron microscopy, cryo-electron tomography and cryo-X-ray tomography. We describe the core architectural features of invasion and identify fusion between rhoptries at the commencement of invasion as a hitherto overlooked event that likely provides a critical step that initiates entry. Given the centrality of merozoite organelle proteins to vaccine development, these insights provide a mechanistic framework to understand therapeutic strategies targeted towards the cellular events of invasion.

Global proteomic analysis of the oocyst/sporozoite of Toxoplasma gondii reveals commitment to a host-independent lifestyle

Background

Toxoplasmosis is caused by the apicomplexan parasite Toxoplasma gondii and can be acquired either congenitally or via the oral route. In the latter case, transmission is mediated by two distinct invasive stages, i.e., bradyzoites residing in tissue cysts or sporozoites contained in environmentally resistant oocysts shed by felids in their feces. The oocyst plays a central epidemiological role, yet this stage has been scarcely investigated at the molecular level and the knowledge of its expressed proteome is very limited.

Results

Using one-dimensional gel electrophoresis coupled to liquid chromatography-linked tandem mass spectrometry, we analysed total or fractionated protein extracts of partially sporulated T. gondii oocysts, producing a dataset of 1304 non reduntant proteins (~18% of the total predicted proteome), ~59% of which were classified according to the MIPS functional catalogue database. Notably, the comparison of the oocyst dataset with the extensively covered proteome of T. gondii tachyzoite, the invasive stage responsible for the clinical signs of toxoplasmosis, identified 154 putative oocyst/sporozoite-specific proteins, some of which were validated by Western blot. The analysis of this protein subset showed that, compared to tachyzoites, oocysts have a greater capability of de novo amino acid biosynthesis and are well equipped to fuel the Krebs cycle with the acetyl-CoA generated through fatty acid β-oxidation and the degradation of branched amino acids.

Conclusions

The study reported herein significantly expanded our knowledge of the proteome expressed by the oocyst/sporozoite of T. gondii, shedding light on a stage-specifc subset of proteins whose functional profile is consistent with the adaptation of T. gondii oocysts to the nutrient-poor and stressing extracellular environment.

The obligate intracellular parasite Toxoplasma gondii secretes a soluble phosphatidylserine decarboxylase

Toxoplasma gondii is an obligate intracellular parasite capable of causing fatal infections in immunocompromised individuals and neonates. Examination of the phosphatidylserine (PtdSer) metabolism of T. gondii reveals that the parasite secretes a soluble form of PtdSer decarboxylase (TgPSD1), which preferentially decarboxylates liposomal PtdSer with an apparent K(m) of 67 μM. The specific enzyme activity increases by 3-fold during the replication of T. gondii, and soluble phosphatidylserine decarboxylase (PSD) accounts for ∼20% of the total PSD, prior to the parasite egress from the host cells. Extracellular T. gondii secreted ∼20% of its total PSD activity at 37 °C, and the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) inhibited the process by 50%. Cycloheximide, brefeldin A, ionic composition of the medium, and exogenous PtdSer did not modulate the enzyme secretion, which suggests a constitutive discharge of a presynthesized pool of PSD in axenic T. gondii. TgPSD1 consists of 968 amino acids with a 26-amino acid hydrophobic peptide at the N terminus and no predicted membrane domains. Parasites overexpressing TgPSD1-HA secreted 10-fold more activity compared with the parental strain. Exposure of apoptotic Jurkat cells to transgenic parasites demonstrated interfacial catalysis by secreted TgPSD1 that reduced host cell surface exposure of PtdSer. Immunolocalization experiments revealed that TgPSD1 resides in the dense granules of T. gondii and is also found in the parasitophorous vacuole of replicating parasites. Together, these findings demonstrate novel features of the parasite enzyme because a secreted, soluble, and interfacially active form of PSD has not been previously described for any organism.

Proteomic analysis of fractionated Toxoplasma oocysts reveals clues to their environmental resistance

Toxoplasma gondii is an obligate intracellular parasite that is unique in its ability to infect a broad range of birds and mammals, including humans, leading to an extremely high worldwide prevalence and distribution. This work focuses on the environmentally resistant oocyst, which is the product of sexual replication in felids and an important source of human infection. Due to the difficulty in producing and working with oocysts, relatively little is known about how this stage is able to resist extreme environmental stresses and how they initiate a new infection, once ingested. To fill this gap, the proteome of the wall and sporocyst/sporozoite fractions of mature, sporulated oocysts were characterized using one-dimensional gel electrophoresis followed by LC-MS/MS on trypsin-digested peptides. A combined total of 1021 non-redundant T. gondii proteins were identified in the sporocyst/sporozoite fraction and 226 were identified in the oocyst wall fraction. Significantly, 172 of the identified proteins have not previously been identified in Toxoplasma proteomic studies. Among these are several of interest for their likely role in conferring environmental resistance including a family of small, tyrosine-rich proteins present in the oocyst wall fractions and late embryogenesis abundant domain-containing (LEA) proteins in the cytosolic fractions. The latter are known from other systems to be key to enabling survival against desiccation.

Characterization of Neospora caninum microneme protein 10 (NcMIC10) and its potential use as a diagnostic marker for neosporosis

Improvements in the serological diagnosis of neosporosis are needed to differentiate acute versus chronic Neospora caninum infections. In the present study, N. caninum microneme protein 10 (NcMIC10), similar to other microneme proteins, was shown to be released in a calcium-dependent manner. NcMIC10 may be discharged during active invasion of host cells by the parasite, and thus represent an excellent marker for the diagnosis of neosporosis. In order to test this hypothesis, recombinant NcMIC10 (rNcMIC10) was expressed in Escherichia coli, and polyclonal antibodies were generated against non-overlapping fragments of the protein. A capture ELISA was developed using these antibodies, and was found to be highly accurate and reproducible with a detection range of 10-10,000 pg/ml. The anti-rNcMIC10 antibodies used in this study did not cross-react with the Toxoplasma gondii antigens. NcMIC10 was detected by the ELISA in sera of 9 out of 10 goats (90%) experimentally infected with N. caninum tachyzoites. In general, goats infected with a lower dose (10(4)) of the parasite displayed a peak in NcMIC10 levels between weeks 4 and 5 post infection. Goats infected with a higher parasite dose (10(6)) displayed a more rapid increase in NcMIC10 levels. In most animals, NcMIC10 decreased to undetectable levels by week 6 post infection. This is the first circulating Neospora antigen-based assay which may complement the existing antibody-based assays for a rapid and cost-effective definitive diagnosis of neosporosis in livestock.

Toxoplasma gondii excretory secretory antigenic proteins of diagnostic potential

Infection with Toxoplasma gondii is widespread and important in humans, especially pregnant women and immunosuppressed patients. A panel of tests is usually required for diagnosis toxoplasmosis. Excretory secretory antigen (ESA) is highly immunogenic, and thus it is a good candidate for investigation into new infection markers. ESA was prepared from tachyzoites of RH strain of T. gondii by mice intraperitoneal infection. Sera were obtained from several categories of individuals who differed in their status of anti-Toxoplasma IgM, IgG and IgG avidity antibodies. The ESA was subjected to SDS-PAGE, two-dimensional gel electrophoresis and Western blot analysis. Antigenic bands of approximate molecular weights of 12, 20 and 30 kDa, when probed with anti-human IgM-HRP and IgA-HRP, showed good potential as infection markers. The highest sensitivity of the bands was 98.7% with combination of IgM and IgA blots with sera of patients with anti-Toxoplasma IgM+ IgG+. The specificities were 84% and 70% with sera from other infections and healthy controls in IgM blots and IgA blots respectively. By mass spectrometry, the 12 kDa protein was identified as thioredoxin. The two top proteins identified for 20 kDa molecule were microneme protein 10 and dense granule protein 7; whereas that for 30 kDa were phosphoglycerate mutase 1 and phosphoglycerate mutase.

Identification of novel proteins in Neospora caninum using an organelle purification and monoclonal antibody approach

Neospora caninum is an important veterinary pathogen that causes abortion in cattle and neuromuscular disease in dogs. Neospora has also generated substantial interest because it is an extremely close relative of the human pathogen Toxoplasma gondii, yet does not appear to infect humans. While for Toxoplasma there are a wide array of molecular tools and reagents available for experimental investigation, relatively few reagents exist for Neospora. To investigate the unique biological features of this parasite and exploit the recent sequencing of its genome, we have used an organelle isolation and monoclonal antibody approach to identify novel organellar proteins and develop a wide array of probes for subcellular localization. We raised a panel of forty-six monoclonal antibodies that detect proteins from the rhoptries, micronemes, dense granules, inner membrane complex, apicoplast, mitochondrion and parasite surface. A subset of the proteins was identified by immunoprecipitation and mass spectrometry and reveal that we have identified and localized many of the key proteins involved in invasion and host interaction in Neospora. In addition, we identified novel secretory proteins not previously studied in any apicomplexan parasite. Thus, this organellar monoclonal antibody approach not only greatly enhances the tools available for Neospora cell biology, but also identifies novel components of the unique biological characteristics of this important veterinary pathogen.

Toxoplasma gondii toxolysin 4 is an extensively processed putative metalloproteinase secreted from micronemes

Proteases play central roles in cell invasion by Toxoplasma gondii and other apicomplexan parasites. Herein we report the cloning and characterization of a novel secretory putative metalloproteinase, Toxolysin 4 (TLN4). T. gondii tachyzoites store TLN4 in the micronemes and secrete it in response to elevated calcium, suggesting a possible role in cell invasion. TLN4 is initially synthesized as a large (∼260 kDa) precursor, which is extensively processed into multiple proteolytic fragments within the parasite secretory system. At least some of these proteolytic fragments remain associated in a large molecular complex. Whereas precomplementation with the TLN4 cDNA allowed disruption of the TLN4 gene, multiple attempts to directly knockout TLN4 without precomplementation failed. TLN4 knockout parasites were detected by PCR in transfected populations but were lost from the cultures during drug selection and growth suggesting that TLN4 contributes to parasite fitness.