The toxoplasma micronemal protein MIC4 is an adhesin composed of six conserved apple domains

Functional Characterization of 11 Tentative Microneme Proteins in Type I RH Strain of Toxoplasma gondii Using the CRISPR-Cas9 System

Toxoplasma gondii, a pathogenic apicomplexan parasite, infects approximately one third of the world's population and poses a serious threat to global public health. Microneme proteins (MICs) secreted by the microneme, an apical secretory organelle of T. gondii, play important roles in the invasion, motility, and intracellular survival of T. gondii. In this study, we selected 11 genes of interest (GOIs) of T. gondii, tentative MICs predicted to be localized in micronemes, and we used the CRISPR-Cas9 system to construct epitope tagging strains and gene knockout strains to explore the localization and function of these 11 tentative MICs. Immunofluorescence assay showed that nine tentative MICs (TGME49_243930, TGME49_200270, TGME49_273320, TGME49_287040, TGME49_261710, TGME49_205680, TGME49_304490, TGME49_245485, and TGME49_224620) were localized or partially localized in the microneme, consistent with the prediction. However, TGME49_272380 and TGME49_243790 showed different localizations from the prediction, being localized in the endoplasmic reticulum and the dense granule, respectively. Further functional characterization of the 11 RHΔGOI strains revealed that deletion of these 11 GOIs had no significant effect on plaque formation, intracellular replication, egress, invasion ability, and virulence of T. gondii. Although these 11 GOIs are not essential genes for the growth and virulence of tachyzoites of type I RH strain, they may have potential roles in other developmental stages or other genotypes of T. gondii. Thus, further research should be performed to explore the possible role of the nine mics and the other two GOIs in other life cycle stages and other genotypes of T. gondii.

Host protein EPCAM interacting with EtMIC8-EGF is essential for attachment and invasion of Eimeria tenella in chickens

The five epidermal growth factor-like domains (EGF) of Eimeria tenella microneme protein 8 (EtMIC8) (EtMIC8-EGF) plays a vital role in host cell attachment and invasion. These processes require interactions between parasite proteins and receptors on the surface of host cells. In this study, five chicken membrane proteins potentially interacting with EtMIC8-EGF were identified using the GST pull-down assay and mass spectrometry analysis, and only chicken (Gallus gallus) epithelial cell adhesion molecule (EPCAM) could bind to EtMIC8-EGF. EPCAM-specific antibody and recombinant EPCAM protein (rEPCAM) inhibited the EtMIC8-EGF binding to host cells in a concentration-dependent manner. Furthermore, the rEPCAM protein showed a binding activity to sporozoites in vitro, and a significant reduction of E. tenella invasion in DF-1 cells was further observed after pre-incubation of sporozoites with rEPCAM. The specific anti-EPCAM antibody further significantly decreased weight loss, lesion score and oocyst output during E. tenella infection, displaying partial inhibition of E. tenella infection. These results indicate that chicken EPCAM is an important EtMIC8-interacting host protein involved in E. tenella-host cell adhesion and invasion. The findings will contribute to a better understanding of the role of adhesion-associated microneme proteins in E. tenella.

Proteomic Analysis of Fractionated Eimeria tenella Sporulated Oocysts Reveals Involvement in Oocyst Wall Formation

Eimeria tenella is the most pathogenic intracellular protozoan parasite of the Eimeria species. Eimeria oocyst wall biogenesis appears to play a central role in oocyst transmission. Proteome profiling offers insights into the mechanisms governing the molecular basis of oocyst wall formation and identifies targets for blocking parasite transmission. Tandem mass tags (TMT)-labeled quantitative proteomics was used to analyze the oocyst wall and sporocysts of E. tenella. A combined total of 2865 E. tenella proteins were identified in the oocyst wall and sporocyst fractions; among these, 401 DEPs were identified, of which 211 were upregulated and 190 were downregulated. The 211 up-regulated DEPs were involved in various biological processes, including DNA replication, fatty acid metabolism and biosynthesis, glutathione metabolism, and propanoate metabolism. Among these proteins, several are of interest for their likely role in oocyst wall formation, including two tyrosine-rich gametocyte proteins (EtGAM56, EtSWP1) and two cysteine-rich proteins (EtOWP2, EtOWP6). Concurrently, 96 uncharacterized proteins may also participate in oocyst wall formation. The present study significantly expands our knowledge of the proteome of the oocyst wall of E. tenella, thereby providing a theoretical basis for further understanding of the biosynthesis and resilience of the E. tenella oocyst wall.

Construction of a Lactobacillus plantarum-based claudin-3 targeting delivery system for the development of vaccines against Eimeria tenella

Avian coccidiosis causes huge economic losses to the poultry industry worldwide and currently lacks effective live vector vaccines. Achieving efficient antigen delivery to mucosa-associated lymphoid tissue (MALT) is critical for improving the effectiveness of vaccines. Here, chicken claudin-3 (CLDN3), a tight junction protein expressed in MALT, was identified as a target, and the C-terminal region of Clostridium perfringens enterotoxin (C-CPE) was proven to bind to chicken CLDN3. Then, a CLDN3-targeting Lactobacillus plantarum NC8-expressing C-CPE surface display system (NC8/GFP-C-CPE) was constructed to successfully express the heterologous protein on the surface of L. plantarum. The colonization level of NC8/GFP-C-CPE was significantly increased compared to the non-targeting strain and could persist in the intestine for at least 72 h. An oral vaccine strain expressing five EGF domains of Eimeria tenella microneme protein 8 (EtMIC8-EGF) (NC8/EtMIC8-EGF-C-CPE) was constructed to evaluate the protective efficacy against E. tenella infection. The results revealed that CLDN3-targeting L. plantarum induced stronger mucosal immunity in gut-associated lymphoid tissues (GALT) as well as humoral responses and conferred better protection in terms of parasite replication and pathology than the non-targeting strain. Overall, we successfully constructed a CLDN3-targeting L. plantarum NC8 surface display system characterized by MALT-targeting, which is an efficient antigen delivery system to confer enhanced protective efficacy in chickens against E. tenella infection.

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.

Profile of the in silico secretome of the palm dieback pathogen, Fusarium oxysporum f. sp. albedinis, a fungus that puts natural oases at risk

Understanding biotic changes that occur alongside climate change constitute a research priority of global significance. Here, we address a plant pathogen that poses a serious threat to life on natural oases, where climate change is already taking a toll and severely impacting human subsistence. Fusarium oxysporum f. sp. albedinis is a pathogen that causes dieback disease on date palms, a tree that provides several critical ecosystem services in natural oases; and consequently, of major importance in this vulnerable habitat. Here, we assess the current state of global pathogen spread, we annotate the genome of a sequenced pathogen strain isolated from the native range and we analyse its in silico secretome. The palm dieback pathogen secretes a large arsenal of effector candidates including a variety of toxins, a distinguished profile of secreted in xylem proteins (SIX) as well as an expanded protein family with an N-terminal conserved motif [SG]PC[KR]P that could be involved in interactions with host membranes. Using agrobiodiversity as a strategy to decrease pathogen infectivity, while providing short term resilient solutions, seems to be widely overcome by the pathogen. Hence, the urgent need for future mechanistic research on the palm dieback disease and a better understanding of pathogen genetic diversity.

Ricin B lectin-like proteins of the microsporidian Encephalitozoon cuniculi and Anncaliia algerae are involved in host-cell invasion

Microsporidia are obligate intracellular pathogens capable of infecting a wide variety of hosts ranging from invertebrates to vertebrates. The infection process requires a step of prior adherence of Microsporidia to the surface of host cells. A few studies demonstrated the involvement of proteins containing a ricin-B lectin (RBL) domain in parasite infection. In this study Anncalia algerae and Encephalitozoon cuniculi genomes were screened by bioinformatic analysis to identify proteins with an extracellular prediction and possessing RBL-type carbohydrate-binding domains, being both potentially relevant factors contributing to host cell adherence. Three proteins named AaRBLL-1 and AaRBLL-2 from A. algerae and EcRBLL-1 from E. cuniculi, were selected and comparative analysis of sequences suggested their belonging to a multigenic family, with a conserved structural RBL domain despite a significant amino acid sequence divergence. The production of recombinant proteins and antibodies against the three proteins allowed their subcellular localization on the spore wall and/or the polar tube. Adherence inhibition assays based on pre-treatments with recombinant proteins or antibodies highlighted the significant decrease of the proliferation of both E. cuniculi and A. algerae, strongly suggesting that these proteins are involved in the infection process.

Diverse Roles of TgMIC1/4/6 in the Toxoplasma Infection

Toxoplasma gondii microneme is a specialized secretory organelle that discharges its contents at the apical tip of this apicomplexan parasite in a sequential and regulated manner. Increasing number of studies on microneme proteins (MICs) have shown them as a predominant and important role in host cell attachment, invasion, motility and pathogenesis. In this review, we summarize the research advances in one of the most important MICs complexes, TgMIC1/4/6, which will contribute to improve the understanding of the molecular mechanism of T. gondii infection and provide a theoretical basis for the effective control against T. gondii.

Molecular evolution of kalliklectin in teleost and identification of the novel type with eight apple domains in channel catfish, Ictalurus punctatus

Kalliklectin is a unique fish-specific lectin that demonstrates sequence similarity to mammalian plasma kallikrein and coagulation factor XI, which are not lectins but proteases. Reported fish kalliklectins and these mammalian proteases comprise four characteristic "apple domains" (APDs). Bioinformatics analysis revealed that Siluriformes species possess anomalous kalliklectins comprising 6 to 16 APDs. Complementary DNA cloning showed that the full-length nucleotide sequence of Ictalurus punctatus consists of 2240 bp that encode 720 amino acid residues to produce a mature protein with a putative 18 amino acid N-terminus peptide sequence. This protein has a predicted molecular mass of 83,417.23 Da. Reverse transcription-polymerase chain reaction (RT-PCR) showed that this lectin gene expresses in the liver but not in any other tissues, including the mucosal tissues. This differential expression pattern makes this lectin unique compared to other lectins described in previous studies. We successfully detected an 85-kDa protein in the serum using western blotting analysis, suggesting that this lectin protein is produced by the liver and secreted into the bloodstream. We characterized a novel cDNA sequence encoding a new type of kalliklectin with eight APDs isolated from channel catfish, I. punctatus. Based on phylogenetic analysis, we speculated that there was a duplication of the third and fourth APD set in a common Siluriformes ancestor at some point after its separation from the common teleost ancestor and that these duplications then underwent independent repeats in different lineages resulting in the generation of the [APD1]-[APD2]-{[APD3]-[APD-4]} × n structure in modern catfishes.

Multi-epitope vaccine expressed in Leishmania tarentolae confers protective immunity to Toxoplasma gondii in BALB/c mice

Current study deals with a novel multi-epitope vaccine designed in silico and its confirmation experiments for potential efficacy in BALB/c mice. Major histocompatibility complex (MHC)-binding and B-cell binding epitopes of five Toxoplasma antigens (SAG1, ROP16, GRA12, MIC4 and M2AP) were predicted. Selected epitopes were fused together using SAPGTP linker, and antigenicity, allergenicity, physico-chemical features, secondary and tertiary structures and validations were all performed via bioinformatics servers. Then, vaccine construct was cloned into pLEXSY-neo 2.1 vector. After Leishmania tarentolae transfection, live recombinant and wild parasites were subcutaneously injected into 6-8 week female BALB/c mice and immune responses were measured. Results showed that the peptide possessed 282 amino acid residues with average molecular weight of 28.06 kDa. About 90% of the peptide residues were incorporated in favored and allowed regions of the Ramachandran plot. Vaccinated mice showed remarkably elevated levels of specific antibodies (P < 0.05) with predominance of IgG2a production. Also, a Th₁ immune response with production of IFN-γ and relatively increased survival rate against intraperitoneal challenge with RH strain was demonstrated in immunized mice than control groups (P < 0.05). Also, very low levels of IL-4 were demonstrated, which showed statistically significant association with controls (P < 0.05). The findings clarified that multi-epitope vaccine expressed in Leishmania tarentolae induced significant immune responses against acute toxoplasmosis.

Microneme Proteins 1 and 4 From Toxoplasma gondii Induce IL-10 Production by Macrophages Through TLR4 Endocytosis

The protozoan parasite Toxoplasma gondii modulates host cell responses to favor its success in the early stage of infections by secreting proteins from its apical organelles. Some of these proteins, including microneme proteins (MICs) 1 and 4, trigger pro-inflammatory host cell responses. The lectins MIC1 and MIC4 interact with N-linked glycans on TLR2 and TLR4, activating NF-κB and producing IL-12, TNF-α, and IL-6. Interestingly, MIC1 and MIC4 also trigger secretion of the anti-inflammatory cytokine IL-10 through mechanisms as yet unknown. Herein, we show that the ability of these MICs to induce macrophages to produce IL-10 depends on TLR4 internalization from the cell surface. Macrophages subjected to blockade of endocytosis by Dynasore continued to release TNF-α, but failed to produce IL-10, in response to MIC1 or MIC4 exposure. Similarly, IL-10 was not produced by Dynasore-conditioned T. gondii-infected macrophages. Furthermore, MIC1- or MIC4-stimulated macrophages gained transient tolerance to LPS. We report a previously undiscovered mechanism by which well-defined T. gondii components inhibit a host inflammatory response.

Immunoinformatic analysis of immunogenic B- and T-cell epitopes of MIC4 protein to designing a vaccine candidate against Toxoplasma gondii through an in-silico approach

Purpose

Toxoplasmosis, transmitted by Toxoplasma gondii, is a worldwide parasitic disease that affects approximately one-third of the world's inhabitants. Today, there are no appropriate drugs to deter tissue cysts from developing in infected hosts. So, developing an effective vaccine would be valuable to avoid from toxoplasmosis. Considering the role of microneme antigens such as microneme protein 4 (MIC4) in T. gondii pathogenesis, it can be used as potential candidates for vaccine against T. gondii.

Materials and Methods

In this study several bioinformatics methods were used to assess the different aspects of MIC4 protein such as secondary and tertiary structure, physicochemical characteristics, the transmembrane domains, subcellular localization, B-cell, helper-T lymphocyte, cytotoxic-T lymphocyte epitopes, and other notable characteristic of this protein design a suitable vaccine against T. gondii.

Results

The studies revealed that MIC4 protein includes 59 potential post-translational modification sites without any transmembrane domains. Moreover, several probable epitopes of B- and T-cells were detected for MIC4. The secondary structure comprised 55.69% random coil, 5.86% beta-turn, 19.31% extended strand, and 19.14% alpha helix. According to the Ramachandran plot results, 87.42% of the amino acid residues were located in the favored, 9.44% in allowed, and 3.14% in outlier regions. The protein allergenicity and antigenicity revealed that it was non-allergenic and antigenic.

Conclusion

This study gives vital basic on MIC4 protein for further research and also established an effective vaccine with different techniques against acute and chronic toxoplasmosis.

Glutaredoxin 1 Deficiency Leads to Microneme Protein-Mediated Growth Defects in Neospora caninum

Neospora caninum is an obligate intracellular protozoan parasite that infects a wide range of mammalian species and causes spontaneous abortion in cattle. N. caninum is exposed to oxidative stress during its life cycle. Oxidoreductase is crucial for parasite response to the environmental stresses. Glutaredoxins (Grxs) are small oxidoreductases of the thioredoxin family proteins that catalyze thiol-disulfide exchange reactions by utilizing electrons from the tripeptide glutathione (γGlu-Cys-Gly; GSH). Grxs are key elements in redox signaling and cell signal transduction. However, Grxs are an unexplored set of oxidoreductases in N. caninum. Here, we identified two cytoplasm located glutaredoxin domain-containing proteins (NcGrx1 and NcGrx3) in N. caninum. To better understand the functions of these Grx proteins, we generated NcGrx1 and NcGrx3 deficiency and overexpression strains. The deletion or overexpression of NcGrx3 had no significant effect on the growth of N. caninum in vitro and in vivo. NcGrx1 knockout parasites displayed a significant growth defect, which was due to the influence on invasion and egress abilities. Moreover, NcGrx1 deficiency decreased the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) (GSH/GSSG ratio), caused a significant accumulation of hydroxyl radical in parasites, and an increase in apoptotic cells under oxidative stress (H₂O₂) condition. To determine the cause of growth defects in ΔNcGrx1, we examined the transcription levels of various invasion-egress related genes as measured by qPCR. We found a significant decrease in MIC1, MIC4, and MIC6 genes. Further investigation found that the secretion of MIC1, MIC4, and MIC6 proteins was significantly affected. Collectively, Ncgrx1 is important for microneme protein-mediated parasite growth, and maybe a potential intervention target for the N. caninum.

Cell invasion by intracellular parasites - the many roads to infection

Intracellular parasites from the genera Toxoplasma, Plasmodium, Trypanosoma, Leishmania and from the phylum Microsporidia are, respectively, the causative agents of toxoplasmosis, malaria, Chagas disease, leishmaniasis and microsporidiosis, illnesses that kill millions of people around the globe. Crossing the host cell plasma membrane (PM) is an obstacle these parasites must overcome to establish themselves intracellularly and so cause diseases. The mechanisms of cell invasion are quite diverse and include (1) formation of moving junctions that drive parasites into host cells, as for the protozoans Toxoplasma gondii and Plasmodium spp., (2) subversion of endocytic pathways used by the host cell to repair PM, as for Trypanosoma cruzi and Leishmania, (3) induction of phagocytosis as for Leishmania or (4) endocytosis of parasites induced by specialized structures, such as the polar tubes present in microsporidian species. Understanding the early steps of cell entry is essential for the development of vaccines and drugs for the prevention or treatment of these diseases, and thus enormous research efforts have been made to unveil their underlying biological mechanisms. This Review will focus on these mechanisms and the factors involved, with an emphasis on the recent insights into the cell biology of invasion by these pathogens.

Receptor Heterodimerization and Co-Receptor Engagement in TLR2 Activation Induced by MIC1 and MIC4 from Toxoplasma gondii

The microneme organelles of Toxoplasma gondii tachyzoites release protein complexes (MICs), including one composed of the transmembrane protein MIC6 plus MIC1 and MIC4. In this complex, carbohydrate recognition domains of MIC1 and MIC4 are exposed and interact with terminal sialic acid and galactose residues, respectively, of host cell glycans. Recently, we demonstrated that MIC1 and MIC4 binding to the N-glycans of Toll-like receptor (TLR) 2 and TLR4 on phagocytes triggers cell activation and pro-inflammatory cytokine production. Herein, we investigated the requirement for TLR2 heterodimerization and co-receptors in MIC-induced responses, as well as the signaling molecules involved. We used MICs to stimulate macrophages and HEK293T cells transfected with TLR2 and TLR1 or TLR6, both with or without the co-receptors CD14 and CD36. Then, the cell responses were analyzed, including nuclear factor-kappa B (NF-κB) activation and cytokine production, which showed that (1) only TLR2, among the studied factors, is crucial for MIC-induced cell activation; (2) TLR2 heterodimerization augments, but is not critical for, activation; (3) CD14 and CD36 enhance the response to MIC stimulus; and (4) MICs activate cells through a transforming growth factor beta-activated kinase 1 (TAK1)-, mammalian p38 mitogen-activated protein kinase (p38)-, and NF-κB-dependent pathway. Remarkably, among the studied factors, the interaction of MIC1 and MIC4 with TLR2 N-glycans is sufficient to induce cell activation, which promotes host protection against T. gondii infection.

The lectin-specific activity of Toxoplasma gondii microneme proteins 1 and 4 binds Toll-like receptor 2 and 4 N-glycans to regulate innate immune priming

Infection of host cells by Toxoplasma gondii is an active process, which is regulated by secretion of microneme (MICs) and rhoptry proteins (ROPs and RONs) from specialized organelles in the apical pole of the parasite. MIC1, MIC4 and MIC6 assemble into an adhesin complex secreted on the parasite surface that functions to promote infection competency. MIC1 and MIC4 are known to bind terminal sialic acid residues and galactose residues, respectively and to induce IL-12 production from splenocytes. Here we show that rMIC1- and rMIC4-stimulated dendritic cells and macrophages produce proinflammatory cytokines, and they do so by engaging TLR2 and TLR4. This process depends on sugar recognition, since point mutations in the carbohydrate-recognition domains (CRD) of rMIC1 and rMIC4 inhibit innate immune cells activation. HEK cells transfected with TLR2 glycomutants were selectively unresponsive to MICs. Following in vitro infection, parasites lacking MIC1 or MIC4, as well as expressing MIC proteins with point mutations in their CRD, failed to induce wild-type (WT) levels of IL-12 secretion by innate immune cells. However, only MIC1 was shown to impact systemic levels of IL-12 and IFN-γ in vivo. Together, our data show that MIC1 and MIC4 interact physically with TLR2 and TLR4 N-glycans to trigger IL-12 responses, and MIC1 is playing a significant role in vivo by altering T. gondii infection competency and murine pathogenesis.

Toxoplasmosis is caused by the protozoan Toxoplasma gondii, belonging to the Apicomplexa phylum. This phylum comprises important parasites able to infect a broad diversity of animals, including humans. A particularity of T. gondii is its ability to invade virtually any nucleated cell of all warm-blooded animals through an active process, which depends on the secretion of adhesin proteins. These proteins are discharged by specialized organelles localized in the parasite apical region, and termed micronemes and rhoptries. We show in this study that two microneme proteins from T. gondii utilize their adhesion activity to stimulate innate immunity. These microneme proteins, denoted MIC1 and MIC4, recognize specific sugars on receptors expressed on the surface of mammalian immune cells. This binding activates these innate immune cells to secrete cytokines, which promotes efficient host defense mechanisms against the parasite and regulate their pathogenesis. This activity promotes a chronic infection by controlling parasite replication during acute infection.

A glycoform of the secreted purple acid phosphatase AtPAP26 co-purifies with a mannose-binding lectin (AtGAL1) upregulated by phosphate-starved Arabidopsis

The purple acid phosphatase AtPAP26 plays a central role in Pi-scavenging by Pi-starved (-Pi) Arabidopsis. Mass spectrometry (MS) of AtPAP26-S1 and AtPAP26-S2 glycoforms secreted by -Pi suspension cells demonstrated that N-glycans at Asn³⁶⁵ and Asn⁴²² were modified in AtPAP26-S2 to form high-mannose glycans. A 55-kDa protein that co-purified with AtPAP26-S2 was identified as a Galanthus nivalis agglutinin-related and apple domain lectin-1 (AtGAL1; At1g78850). MS revealed that AtGAL1 was bisphosphorylated at Tyr³⁸ and Thr³⁹ and glycosylated at four conserved Asn residues. When AtGAL was incubated in the presence of a thiol-reducing reagent prior to immunoblotting, its cross-reactivity with anti-AtGAL1-IgG was markedly attenuated (consistent with three predicted disulfide bonds in AtGAL1's apple domain). Secreted AtGAL1 polypeptides were upregulated to a far greater extent than AtGAL1 transcripts during Pi deprivation, indicating posttranscriptional control of AtGAL1 expression. Growth of a -Pi atgal1 mutant was unaffected, possibly due to compensation by AtGAL1's closest paralog, AtGAL2 (At1g78860). Nevertheless, AtGAL1's induction by numerous stresses combined with the broad distribution of AtGAL1-like lectins in diverse species implies an important function for AtGAL1 orthologs within the plant kingdom. We hypothesize that binding of AtPAP26-S2's high-mannose glycans by AtGAL1 enhances AtPAP26 function to facilitate Pi-scavenging by -Pi Arabidopsis.

Micronemal protein 13 contributes to the optimal growth of Toxoplasma gondii under stress conditions

Toxoplasma gondii is a ubiquitous parasitic protozoan infecting humans and a wide variety of animals. Fast-replicating tachyzoites during acute infection and slowly growing bradyzoites during chronic infection are the two basic forms of T. gondii in intermediate hosts. Interconversion between the two contributes to the transmission and pathogenesis of this parasite. Secretory micronemal proteins are thought to mediate interactions with host cells and facilitate parasite invasion, therefore the majority of them are highly expressed in tachyzoites. Micronemal protein 13 (MIC13) is unique in that its expression is low in tachyzoites and is upregulated under bradyzoite-inducing conditions. Previous attempts to disrupt this gene were not successful, implying that it may play critical roles during parasite growth. However, in this study, MIC13 was successfully disrupted in type 1 strain RH and type 2 strain ME49 using CRISPR/Cas9-mediated gene disruption techniques. Consistent with its low expression in tachyzoites and increased expression under stress or bradyzoite-inducing conditions, MIC13-inactivated mutants displayed normal growth, host cell invasion, intracellular replication, and egress, as well as acute virulence at the tachyzoite stage. However, under stress conditions, such as high pH or oxygen limitation, MIC13-disrupted parasites showed significantly slower growth rates compared to the parental strains, suggesting that it is required for optimal parasite growth under bradyzoite-inducing or stress conditions. This is the first micronemal protein reported to have such expression pattern and function modes, which expands our understanding of the diverse functions of micronemal proteins.

TgTKL1 Is a Unique Plant-Like Nuclear Kinase That Plays an Essential Role in Acute Toxoplasmosis

In the protozoan parasite Toxoplasma gondii, protein kinases have been shown to play key roles in regulating parasite motility, invasion, replication, egress, and survival within the host. The tyrosine kinase-like (TKL) family of proteins are an unexplored set of kinases in Toxoplasma Of the eight annotated TKLs in the Toxoplasma genome, a recent genome-wide loss-of-function screen showed that six are important for tachyzoite fitness. By utilizing an endogenous tagging approach, we showed that these six T. gondii TKLs (TgTKLs) localize to various subcellular compartments, including the nucleus, the cytosol, the inner membrane complex, and the Golgi apparatus. To gain insight into the function of TKLs in Toxoplasma, we first characterized TgTKL1, which contains the plant-like enhanced disease resistance 1 (EDR1) domain and localizes to the nucleus. TgTKL1 knockout parasites displayed significant defects in progression through the lytic cycle; we show that the defects were due to specific impairment of host cell attachment. Transcriptomics analysis identified over 200 genes of diverse functions that were differentially expressed in TgTKL1 knockout parasites. Importantly, numerous genes implicated in host cell attachment and invasion were among those most significantly downregulated, resulting in defects in microneme secretion and processing. Significantly, all of the mice inoculated intraperitoneally with TgTKL1 knockout parasites survived the infection, suggesting that TgTKL1 plays an essential role in acute toxoplasmosis. Together, these findings suggest that TgTKL1 mediates a signaling pathway that regulates the expression of multiple factors required for parasite virulence, underscoring the potential of this kinase as a novel therapeutic target.IMPORTANCEToxoplasma gondii is a protozoan parasite that can cause chronic and life-threatening disease in mammals; new drugs are greatly needed for treatment. One attractive group of drug targets consists of parasite kinases containing unique features that distinguish them from host proteins. In this report, we identify and characterize a previously unstudied kinase, TgTKL1, that localizes to the nucleus and contains a domain architecture unique to plants and protozoa. By disrupting TgTKL1, we showed that this kinase is required for the proper expression of hundreds of genes, including many that are required for the parasite to gain entry into the host cell. Specifically, parasites lacking TgTKL1 have defects in host cell attachment, resulting in impaired growth in vitro and a complete loss of virulence in mice. This report provides insight into the importance of the parasite tyrosine kinase-like kinases and establishes TgTKL1 as a novel and essential virulence factor in Toxoplasma.

Research advances in interactions related to Toxoplasma gondii microneme proteins

Toxoplasma gondii microneme proteins (TgMICs), secreted by micronemes upon contact with host cells, are reported to play important roles in multiple stages of the T. gondii life cycle, including parasite motility, invasion, intracellular survival, and egress from host cells. Meanwhile, during these processes, TgMICs participate in many protein-protein and protein-carbohydrate interactions, such as undergoing proteolytic maturation, binding to aldolase, engaging the host cell receptors and forming the moving junction (MJ), relying on different types of ectodomains, transmembrane (TM) domains and cytoplasmic domains (CDs). In this review, we summarize the research advances in protein-protein and protein-carbohydrate interactions related to TgMICs, and their intimate associations with corresponding biological processes during T. gondii infection, which will contribute to an improved understanding of the molecular pathogenesis of T. gondii infection, and provide a basis for developing effective control strategies against T. gondii.

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.

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.

The Conoid Associated Motor MyoH Is Indispensable for Toxoplasma gondii Entry and Exit from Host Cells

Many members of the phylum of Apicomplexa have adopted an obligate intracellular life style and critically depend on active invasion and egress from the infected cells to complete their lytic cycle. Toxoplasma gondii belongs to the coccidian subgroup of the Apicomplexa, and as such, the invasive tachyzoite contains an organelle termed the conoid at its extreme apex. This motile organelle consists of a unique polymer of tubulin fibres and protrudes in both gliding and invading parasites. The class XIV myosin A, which is conserved across the Apicomplexa phylum, is known to critically contribute to motility, invasion and egress from infected cells. The MyoA-glideosome is anchored to the inner membrane complex (IMC) and is assumed to translocate the components of the circular junction secreted by the micronemes and rhoptries, to the rear of the parasite. Here we comprehensively characterise the class XIV myosin H (MyoH) and its associated light chains. We show that the 3 alpha-tubulin suppressor domains, located in MyoH tail, are necessary to anchor this motor to the conoid. Despite the presence of an intact MyoA-glideosome, conditional disruption of TgMyoH severely compromises parasite motility, invasion and egress from infected cells. We demonstrate that MyoH is necessary for the translocation of the circular junction from the tip of the parasite, where secretory organelles exocytosis occurs, to the apical position where the IMC starts. This study attributes for the first time a direct function of the conoid in motility and invasion, and establishes the indispensable role of MyoH in initiating the first step of motility along this unique organelle, which is subsequently relayed by MyoA to enact effective gliding and invasion.

The Apicomplexa phylum groups important pathogens that infect humans and animals. Host cell invasion and egress from infected cells are key events in the lytic cycle of these obligate intracellular parasites. Host cell entry is powered by gliding motility and initiated by the discharge of apical secretory organelles at the site of contact with the host cell. Anchored to the parasite pellicle, the glideosome composed of myosin A and the gliding associated proteins is the molecular machine which translocates the secreted adhesins from the apical to the posterior pole of the parasite and hence propels the parasite into the host cell. Toxoplasma gondii exhibits a helical form of gliding motility and as member of the coccidian-subgroup of Apicomplexa possesses an apical organelle called the conoid, which protrudes during invasion and egress and consists in helically organized polymer of tubulin fibers. We have deciphered here the function of a novel myosin associated to the microtubules composing the conoid. Myosin H is essential and prerequisite for motility, invasion and egress from infected cells. This unusual motor links actin- and tubulin-based cytoskeletons and uncovers a direct role of the conoid in motility and invasion.

Structural and functional dissection of Toxoplasma gondii armadillo repeats only protein (TgARO)

Rhoptries are club-shaped, regulated secretory organelles that cluster at the apical pole of apicomplexan parasites. Their discharge is essential for invasion and the establishment of an intracellular lifestyle. Little is known about rhoptry biogenesis and recycling during parasite division. In Toxoplasma gondii, positioning of rhoptries involves the armadillo repeats only protein (TgARO) and myosin F (TgMyoF). Here, we show that two TgARO partners, ARO interacting protein (TgAIP) and adenylate cyclase β (TgACβ) localize to a rhoptry subcompartment. In absence of TgAIP, TgACβ disappears from the rhoptries. By assessing the contribution of each TgARO armadillo (ARM) repeat, we provide evidence that TgARO is multifunctional, participating not only in positioning but also in clustering of rhoptries. Structural analyses show that TgARO resembles the myosin-binding domain of the myosin chaperone UNC-45. A conserved patch of aromatic and acidic residues denotes the putative TgMyoF-binding site, and the overall arrangement of the ARM repeats explains the dramatic consequences of deleting each of them. Lastly, Plasmodium falciparum ARO functionally complements TgARO depletion and interacts with the same partners, highlighting the conservation of rhoptry biogenesis in Apicomplexa.

Evidence for Conformational Mechanism on the Binding of TgMIC4 with β-Galactose-Containing Carbohydrate Ligand

A deeper understanding of the role of sialic/desialylated groups during TgMIC4-glycoproteins interactions has importance to better clarify the odd process of host cell invasion by members of the apicomplexan phylum. Within this context, we evaluated the interaction established by recombinant TgMIC4 (the whole molecule) with sialylated (bovine fetuin) and desialylated (asialofetuin) glycoproteins by using functionalized quartz crystal microbalance with dissipation monitoring (QCM-D). A suitable receptive surface containing recombinant TgMIC4 for monitoring β-galactose-containing carbohydrate ligand (limit of quantification ∼ 40 μM) was designed and used as biomolecular recognition platform to study the binding and conformational mechanisms of TgMIC4 during the interaction with glycoprotein containing (fetuin), or not, terminal sialic group (asialofetuin). It was inferred that the binding/interaction monitoring depends on the presence/absence of sialic groups in target protein and is possible to be differentiated through a slower binding kinetic step using QCM-D approach (which we are inferring to be thus associated with β-galactose ligand). This slower binding/interaction step is likely supposed (from mechanical energetic analysis obtained in QCM-D measurements) to be involved with Toxoplasma gondii (the causative agent of toxoplasmosis) parasitic invasion accompanied by ligand (galactose) induced binding conformational change (i.e., cell internalization process can be additionally dependent on structural conformational changes, controlled by the absence of sialic groups and to the specific binding with galactose), in addition to TgMIC4-glycoprotein solely recognition binding process.

Probing the Functions of Carbohydrate Binding Modules in the CBEL Protein from the Oomycete Phytophthora parasitica

Oomycetes are microorganisms that are distantly related to true fungi and many members of this phylum are major plant pathogens. Oomycetes express proteins that are able to interact with plant cell wall polysaccharides, such as cellulose. This interaction is thought to be mediated by carbohydrate-binding modules that are classified into CBM family 1 in the CAZy database. In this study, the two CBMs (1–1 and 1–2) that form part of the cell wall glycoprotein, CBEL, from Phytophthora parasitica have been submitted to detailed characterization, first to better quantify their interaction with cellulose and second to determine whether these CBMs can be useful for biotechnological applications, such as biomass hydrolysis. A variety of biophysical techniques were used to study the interaction of the CBMs with various substrates and the data obtained indicate that CBEL’s CBM1-1 exhibits much greater cellulose binding ability than CBM1-2. Engineering of the family 11 xylanase from Talaromyces versatilis (TvXynB), an enzyme that naturally bears a fungal family 1 CBM, has produced two variants. The first one lacks its native CBM, whereas the second contains the CBEL CBM1-1. The study of these enzymes has revealed that wild type TvXynB binds to cellulose, via its CBM1, and that the substitution of its CBM by oomycetal CBM1-1 does not affect its activity on wheat straw. However, intriguingly the addition of CBEL during the hydrolysis of wheat straw actually potentiates the action of TvXynB variant lacking a CBM1. This suggests that the potentiating effect of CBM1-1 might not require the formation of a covalent linkage to TvXynB.

Research advances in microneme protein 3 of Toxoplasma gondii

Toxoplasma gondii (T. gondii) is an obligate intracellular protozoan parasite. It has extensive host populations and is prevalent globally; T. gondii infection can cause a zoonotic parasitic disease. Microneme protein 3 (MIC3) is a secreted protein that is expressed in all stages of the T. gondii life cycle. It has strong immunoreactivity and plays an important role in the recognition, adhesion and invasion of host cells by T. gondii. This article reviews the molecular structure of MIC3, its role in the invasion of host cells by parasites, its relationship with parasite virulence, and its induction of immune protection to lay a solid foundation for an in-depth study of potential diagnostic agents and vaccines for preventing toxoplasmosis.

Distinct contribution of Toxoplasma gondii rhomboid proteases 4 and 5 to micronemal protein protease 1 activity during invasion

Host cell entry by the Apicomplexa is associated with the sequential secretion of invasion factors from specialized apical organelles. Secretion of micronemal proteins (MICs) complexes by Toxoplasma gondii facilitates parasite gliding motility, host cell attachment and entry, as well as egress from infected cells. The shedding of MICs during these steps is mediated by micronemal protein proteases MPP1, MPP2 and MPP3. The constitutive activity of MPP1 leads to the cleavage of transmembrane MICs and is linked to the surface rhomboid protease 4 (ROM4) and possibly to rhomboid protease 5 (ROM5). To determine their importance and respective contribution to MPP1 activity, in this study ROM4 and ROM5 genes were abrogated using Cre-recombinase and CRISPR-Cas9 nuclease, respectively, and shown to be dispensable for parasite survival. Parasites lacking ROM4 predominantly engage in twirling motility and exhibit enhanced attachment and impaired invasion, whereas intracellular growth and egress is not affected. The substrates MIC2 and MIC6 are not cleaved in rom4-ko parasites, in contrast, intramembrane cleavage of AMA1 is reduced but not completely abolished. Shedding of MICs and invasion are not altered in the absence of ROM5; however, this protease responsible for the residual cleavage of AMA1 is able to cleave other AMA family members and exhibits a detectable contribution to invasion in the absence of ROM4.

Imaging host-Leishmania interactions: significance in visceral leishmaniasis

Leishmaniasis is a neglected disease that is associated with a spectrum of clinical manifestations ranging from self-healing cutaneous lesions to fatal visceral infections, which primarily depends on the parasite species. In visceral leishmaniasis (VL), as opposed to cutaneous leishmaniasis (CL), parasites that infect host cells at the sand fly bite site have the striking ability to disseminate to visceral organs where they proliferate and persist for long periods of time. Imaging the dynamics of the host-Leishmania interaction in VL provides a powerful approach to understanding the mechanisms underlying host cell invasion, Leishmania dissemination and persistence within visceral organs and, to dissecting the immune responses to infection. Therefore, by allowing the visualization of the critical steps involved in the pathogenesis of VL, state-of-the-art microscopy technologies have the great potential to aid in the identification of better intervention strategies for this devastating disease. In this review, we emphasize the current knowledge and the potential significance of imaging technologies in understanding the infection process of visceralizing Leishmania species. Then, we discuss how application of innovative microscopy technologies to the study of VL will provide rich opportunities for investigating host-parasite interactions at a previously unexplored level and elucidating visceral disease-promoting mechanisms.

Characterization of a serine hydrolase targeted by acyl-protein thioesterase inhibitors in Toxoplasma gondii

In eukaryotic organisms, cysteine palmitoylation is an important reversible modification that impacts protein targeting, folding, stability, and interactions with partners. Evidence suggests that protein palmitoylation contributes to key biological processes in Apicomplexa with the recent palmitome of the malaria parasite Plasmodium falciparum reporting over 400 substrates that are modified with palmitate by a broad range of protein S-acyl transferases. Dynamic palmitoylation cycles require the action of an acyl-protein thioesterase (APT) that cleaves palmitate from substrates and conveys reversibility to this posttranslational modification. In this work, we identified candidates for APT activity in Toxoplasma gondii. Treatment of parasites with low micromolar concentrations of β-lactone- or triazole urea-based inhibitors that target human APT1 showed varied detrimental effects at multiple steps of the parasite lytic cycle. The use of an activity-based probe in combination with these inhibitors revealed the existence of several serine hydrolases that are targeted by APT1 inhibitors. The active serine hydrolase, TgASH1, identified as the homologue closest to human APT1 and APT2, was characterized further. Biochemical analysis of TgASH1 indicated that this enzyme cleaves substrates with a specificity similar to APTs, and homology modeling points toward an APT-like enzyme. TgASH1 is dispensable for parasite survival, which indicates that the severe effects observed with the β-lactone inhibitors are caused by the inhibition of non-TgASH1 targets. Other ASH candidates for APT activity were functionally characterized, and one of them was found to be resistant to gene disruption due to the potential essential nature of the protein.

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.

In planta stage-specific fungal gene profiling elucidates the molecular strategies of Fusarium graminearum growing inside wheat coleoptiles

The ascomycete Fusarium graminearum is a destructive fungal pathogen of wheat (Triticum aestivum). To better understand how this pathogen proliferates within the host plant, we tracked pathogen growth inside wheat coleoptiles and then examined pathogen gene expression inside wheat coleoptiles at 16, 40, and 64 h after inoculation (HAI) using laser capture microdissection and microarray analysis. We identified 344 genes that were preferentially expressed during invasive growth in planta. Gene expression profiles for 134 putative plant cell wall-degrading enzyme genes suggest that there was limited cell wall degradation at 16 HAI and extensive degradation at 64 HAI. Expression profiles for genes encoding reactive oxygen species (ROS)-related enzymes suggest that F. graminearum primarily scavenges extracellular ROS before a later burst of extracellular ROS is produced by F. graminearum enzymes. Expression patterns of genes involved in primary metabolic pathways suggest that F. graminearum relies on the glyoxylate cycle at an early stage of plant infection. A secondary metabolite biosynthesis gene cluster was specifically induced at 64 HAI and was required for virulence. Our results indicate that F. graminearum initiates infection of coleoptiles using covert penetration strategies and switches to overt cellular destruction of tissues at an advanced stage of infection.

The protozoan pathogen Toxoplasma gondii targets the paracellular pathway to invade the intestinal epithelium

Abstract  Toxoplasma gondii is a ubiquitous parasite found within all mammals and birds worldwide that can cause fatal infections in immunocompromised persons and fetuses. The parasite causes chronic infections by residing in long-living tissues of the muscle and brain. T. gondii infects the host through contaminated meat and water consumption with the gastrointestinal tract (GI tract) being the first point of contact with the host. The mechanisms by which the parasite invades the host through the GI tract are unknown, although it has been suggested that the paracellular pathway is important for parasite dissemination. Studies indicate that epithelial tight junction-associated proteins are affected by T. gondii, although which junctional proteins are affected and the nature of host protein-parasite interactions have not been established. We have uncovered evidence that T. gondii influences the cellular distribution of occludin to transmigrate the intestinal epithelium and suggest how candidate binding partners can be identified.

Microneme protein 5 regulates the activity of Toxoplasma subtilisin 1 by mimicking a subtilisin prodomain

Toxoplasma gondii is the model parasite of the phylum Apicomplexa, which contains obligate intracellular parasites of medical and veterinary importance. Apicomplexans invade host cells by a multistep process involving the secretion of adhesive microneme protein (MIC) complexes. The subtilisin protease TgSUB1 trims several MICs on the parasite surface to activate gliding motility and host invasion. Although a previous study showed that expression of the secretory protein TgMIC5 suppresses TgSUB1 activity, the mechanism was unknown. Here, we solve the three-dimensional structure of TgMIC5 by nuclear magnetic resonance (NMR), revealing that it mimics a subtilisin prodomain including a flexible C-terminal peptide that may insert into the subtilisin active site. We show that TgMIC5 is an almost 50-fold more potent inhibitor of TgSUB1 activity than the small molecule inhibitor N-[N-(N-acetyl-L-leucyl)-L-leucyl]-L-norleucine (ALLN). Moreover, we demonstrate that TgMIC5 is retained on the parasite plasma membrane via its physical interaction with the membrane-anchored TgSUB1.

Ss-Sl2, a novel cell wall protein with PAN modules, is essential for sclerotial development and cellular integrity of Sclerotinia sclerotiorum

The sclerotium is an important dormant body for many plant fungal pathogens. Here, we reported that a protein, named Ss-Sl2, is involved in sclerotial development of Sclerotinia sclerotiorum. Ss-Sl2 does not show significant homology with any protein of known function. Ss-Sl2 contains two putative PAN modules which were found in other proteins with diverse adhesion functions. Ss-Sl2 is a secreted protein, during the initial stage of sclerotial development, copious amounts of Ss-Sl2 are secreted and accumulated on the cell walls. The ability to maintain the cellular integrity of RNAi-mediated Ss-Sl2 silenced strains was reduced, but the hyphal growth and virulence of Ss-Sl2 silenced strains were not significantly different from the wild strain. Ss-Sl2 silenced strains could form interwoven hyphal masses at the initial stage of sclerotial development, but the interwoven hyphae could not consolidate and melanize. Hyphae in these interwoven bodies were thin-walled, and arranged loosely. Co-immunoprecipitation and yeast two-hybrid experiments showed that glyceraldehyde-3-phosphate dehydrogenase (GAPDH), Woronin body major protein (Hex1) and elongation factor 1-alpha interact with Ss-Sl2. GAPDH-knockdown strains showed a similar phenotype in sclerotial development as Ss-Sl2 silenced strains. Hex1-knockdown strains showed similar impairment in maintenance of hyphal integrity as Ss-Sl2 silenced strains. The results suggested that Ss-Sl2 functions in both sclerotial development and cellular integrity of S. sclerotiorum.

Galactose recognition by the apicomplexan parasite Toxoplasma gondii

Toxosplasma gondii is the model parasite of the phylum Apicomplexa, which contains numerous obligate intracellular parasites of medical and veterinary importance, including Eimeria, Sarcocystis, Cryptosporidium, Cyclospora, and Plasmodium species. Members of this phylum actively enter host cells by a multistep process with the help of microneme protein (MIC) complexes that play important roles in motility, host cell attachment, moving junction formation, and invasion. T. gondii (Tg)MIC1-4-6 complex is the most extensively investigated microneme complex, which contributes to host cell recognition and attachment via the action of TgMIC1, a sialic acid-binding adhesin. Here, we report the structure of TgMIC4 and reveal its carbohydrate-binding specificity to a variety of galactose-containing carbohydrate ligands. The lectin is composed of six apple domains in which the fifth domain displays a potent galactose-binding activity, and which is cleaved from the complex during parasite invasion. We propose that galactose recognition by TgMIC4 may compromise host protection from galectin-mediated activation of the host immune system.

A novel PAN/apple domain-containing protein from Toxoplasma gondii: characterization and receptor identification

Toxoplasma gondii is an intracellular parasite that invades nucleated cells, causing toxoplasmosis in humans and animals worldwide. The extremely wide range of hosts susceptible to T. gondii is thought to be the result of interactions between T. gondii ligands and receptors on its target cells. In this study, a host cell-binding protein from T. gondii was characterized, and one of its receptors was identified. P104 (GenBank Access. No. CAJ20677) is 991 amino acids in length, containing a putative 26 amino acid signal peptide and 10 PAN/apple domains, and shows low homology to other identified PAN/apple domain-containing molecules. A 104-kDa host cell-binding protein was detected in the T. gondii lysate. Immunofluorescence assays detected P104 at the apical end of extracellular T. gondii. An Fc-fusion protein of the P104 N-terminus, which contains two PAN/apple domains, showed strong affinity for the mammalian and insect cells evaluated. This binding was not related to protein-protein or protein-lipid interactions, but to a protein-glycosaminoglycan (GAG) interaction. Chondroitin sulfate (CS), a kind of GAG, was shown to be involved in adhesion of the Fc-P104 N-terminus fusion protein to host cells. These results suggest that P104, expressed at the apical end of the extracellular parasite, may function as a ligand in the attachment of T. gondii to CS or other receptors on the host cell, facilitating invasion by the parasite.

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.

Expressed sequence tags reveal genetic diversity and putative virulence factors of the pathogenic oomycete Pythium insidiosum

Oomycetes are unique eukaryotic microorganisms that share a mycelial morphology with fungi. Many oomycetes are pathogenic to plants, and a more limited number are pathogenic to animals. Pythium insidiosum is the only oomycete that is capable of infecting both humans and animals, and causes a life-threatening infectious disease, called "pythiosis". In the majority of pythiosis patients life-long handicaps result from the inevitable radical excision of infected organs, and many die from advanced infection. Better understanding P. insidiosum pathogenesis at molecular levels could lead to new forms of treatment. Genetic and genomic information is lacking for P. insidiosum, so we have undertaken an expressed sequence tag (EST) study, and report on the first dataset of 486 ESTs, assembled into 217 unigenes. Of these, 144 had significant sequence similarity with known genes, including 47 with ribosomal protein homology. Potential virulence factors included genes involved in antioxidation, thermal adaptation, immunomodulation, and iron and sterol binding. Effectors resembling pathogenicity factors of plant-pathogenic oomycetes were also discovered, such as, a CBEL-like protein (possible involvement in host cell adhesion and hemagglutination), a putative RXLR effector (possibly involved in host cell modulation) and elicitin-like (ELL) proteins. Phylogenetic analysis mapped P. insidiosum ELLs to several novel clades of oomycete elicitins (ELIs), and homology modeling predicted that P. insidiosum ELLs should bind sterols. Most of the P. insidiosum ESTs showed homology to sequences in the genome or EST databases of other oomycetes, but one putative gene, with unknown function, was found to be unique to P. insidiosum. The EST dataset reported here represents the first steps in identifying genes of P. insidiosum and beginning transcriptome analysis. This genetic information will facilitate understanding of pathogenic mechanisms of this devastating pathogen.

Toxoplasma gondii protease TgSUB1 is required for cell surface processing of micronemal adhesive complexes and efficient adhesion of tachyzoites

Host cell invasion by Toxoplasma gondii is critically dependent upon adhesive proteins secreted from the micronemes. Proteolytic trimming of microneme contents occurs rapidly after their secretion onto the parasite surface and is proposed to regulate adhesive complex activation to enhance binding to host cell receptors. However, the proteases responsible and their exact function are still unknown. In this report, we show that T. gondii tachyzoites lacking the microneme subtilisin protease TgSUB1 have a profound defect in surface processing of secreted microneme proteins. Notably parasites lack protease activity responsible for proteolytic trimming of MIC2, MIC4 and M2AP after release onto the parasite surface. Although complementation with full-length TgSUB1 restores processing, complementation of Δsub1 parasites with TgSUB1 lacking the GPI anchor (Δsub1::ΔGPISUB1) only partially restores microneme protein processing. Loss of TgSUB1 decreases cell attachment and in vitro gliding efficiency leading to lower initial rates of invasion. Δsub1 and Δsub1::ΔGPISUB1 parasites are also less virulent in mice. Thus TgSUB1 is involved in micronemal protein processing and regulation of adhesive properties of macromolecular adhesive complexes involved in host cell invasion.

Toxoplasma gondii transmembrane microneme proteins and their modular design

Host cell invasion by the Apicomplexa critically relies on regulated secretion of transmembrane micronemal proteins (TM-MICs). Toxoplasma gondii possesses functionally non-redundant MIC complexes that participate in gliding motility, host cell attachment, moving junction formation, rhoptry secretion and invasion. The TM-MICs are released onto the parasite's surface as complexes capable of interacting with host cell receptors. Additionally, TgMIC2 simultaneously connects to the actomyosin system via binding to aldolase. During invasion these adhesive complexes are shed from the surface notably via intramembrane cleavage of the TM-MICs by a rhomboid protease. Some TM-MICs act as escorters and assure trafficking of the complexes to the micronemes. We have investigated the properties of TgMIC6, TgMIC8, TgMIC8.2, TgAMA1 and the new micronemal protein TgMIC16 with respect to interaction with aldolase, susceptibility to rhomboid cleavage and presence of trafficking signals. We conclude that several TM-MICs lack targeting information within their C-terminal domains, indicating that trafficking depends on yet unidentified proteins interacting with their ectodomains. Most TM-MICs serve as substrates for a rhomboid protease and some of them are able to bind to aldolase. We also show that the residues responsible for binding to aldolase are essential for TgAMA1 but dispensable for TgMIC6 function during invasion.

High-resolution characterization of Toxoplasma gondii transcriptome with a massive parallel sequencing method

For the last couple of years, a method that permits the collection of precise positional information of transcriptional start sites (TSSs) together with digital information of the gene-expression levels in a high-throughput manner was established. We applied this novel method, ‘tss-seq’, to elucidate the transcriptome of tachyzoites of the Toxoplasma gondii, which resulted in the identification of 124 000 TSSs, and they were clustered into 10 000 transcription regions (TRs) with a statistics-based analysis. The TRs and annotated ORFs were paired, resulting in the identification of 30% of the TRs and 40% of the ORFs without their counterparts, which predicted undiscovered genes and stage-specific transcriptions, respectively. The massive data for TSSs make it possible to execute the first systematic analysis of the T. gondii core promoter structure, and the information showed that T. gondii utilized an initiator-like motif for their transcription in the major and novel motif, the downstream thymidine cluster, which was similar to the Y patch observed in plants. This encyclopaedic analysis also suggested that the TATA box, and the other well-known core promoter elements were hardly utilized.

Sialic acids: key determinants for invasion by the Apicomplexa

Sialic acids are ubiquitously found on the surface of all vertebrate cells at the extremities of glycan chains and widely exploited by viruses and bacteria to enter host cells. Carbohydrate-bearing receptors are equally important for host cell invasion by the obligate intracellular protozoan parasites of the phylum Apicomplexa. Host cell entry is an active process relying crucially on proteins that engage with receptors on the host cell surface and promote adhesion and internalisation. Assembly into complexes, proteolytic processing and oligomerization are important requirements for the functionality of these adhesins. The combination of adhesive proteins with varying stringency in specificity confers some flexibility to the parasite in face of receptor heterogeneity and immune pressure. Sialic acids are now recognised to critically contribute to selective host cell recognition by various species of the phylum.

Members of a novel protein family containing microneme adhesive repeat domains act as sialic acid-binding lectins during host cell invasion by apicomplexan parasites

Numerous intracellular pathogens exploit cell surface glycoconjugates for host cell recognition and entry. Unlike bacteria and viruses, Toxoplasma gondii and other parasites of the phylum Apicomplexa actively invade host cells, and this process critically depends on adhesins (microneme proteins) released onto the parasite surface from intracellular organelles called micronemes (MIC). The microneme adhesive repeat (MAR) domain of T. gondii MIC1 (TgMIC1) recognizes sialic acid (Sia), a key determinant on the host cell surface for invasion by this pathogen. By complementation and invasion assays, we demonstrate that TgMIC1 is one important player in Sia-dependent invasion and that another novel Sia-binding lectin, designated TgMIC13, is also involved. Using BLAST searches, we identify a family of MAR-containing proteins in enteroparasitic coccidians, a subclass of apicomplexans, including T. gondii, suggesting that all these parasites exploit sialylated glycoconjugates on host cells as determinants for enteric invasion. Furthermore, this protein family might provide a basis for the broad host cell range observed for coccidians that form tissue cysts during chronic infection. Carbohydrate microarray analyses, corroborated by structural considerations, show that TgMIC13, TgMIC1, and its homologue Neospora caninum MIC1 (NcMIC1) share a preference for alpha2-3- over alpha2-6-linked sialyl-N-acetyllactosamine sequences. However, the three lectins also display differences in binding preferences. Intense binding of TgMIC13 to alpha2-9-linked disialyl sequence reported on embryonal cells and relatively strong binding to 4-O-acetylated-Sia found on gut epithelium and binding of NcMIC1 to 6'sulfo-sialyl Lewis(x) might have implications for tissue tropism.