Differential membrane targeting of the secretory proteins GRA4 and GRA6 within the parasitophorous vacuole formed by Toxoplasma gondii

The Toxoplasma Effector GRA4 Hijacks Host TBK1 to Oppositely Regulate Anti-T. Gondii Immunity and Tumor Immunotherapy

Toxoplasma gondii (T. gondii)-associated polymorphic effector proteins are crucial in parasite development and regulating host anti-T. gondii immune responses. However, the mechanism remains obscure. Here, it is shown that Toxoplasma effector dense granules 4 (GRA4) restricts host IFN-I activation. Infection with Δgra4 mutant T. gondii strain induces stronger IFN-I responses and poses a severe threat to host health. Mechanistically, GRA4 binds to phosphorylated TBK1 to promote TRIM27-catalyzed K48-ubiquitination at Lys251/Lys372 residues, which enhances its recognition by autophagy receptor p62, ultimately leading to TBK1 autophagic degradation. Furthermore, an avirulent Δgra4 strain (ME49Δompdc/gra4) is constructed for tumor immunotherapy due to its ability to enhance IFN-I production. Earlier vaccination with ME49Δompdc/gra4 confers complete host resistance to the tumor compared with the classical ME49Δompdc treatment. Notably, ME49Δompdc/gra4 vaccination induces a specific CD64+MAR-1+CD11b+ dendritic cell subset, thereby enhancing T cell anti-tumor responses. Overall, these findings identify the negative role of T. gondii GRA4 in modulating host IFN-I signaling and suggest that GRA4 can be a potential target for the development of T. gondii vaccines and tumor immunotherapy.

ToxoNet: A high confidence map of protein-protein interactions in Toxoplasma gondii

The apicomplexan intracellular parasite Toxoplasma gondii is a major food borne pathogen that is highly prevalent in the global population. The majority of the T. gondii proteome remains uncharacterized and the organization of proteins into complexes is unclear. To overcome this knowledge gap, we used a biochemical fractionation strategy to predict interactions by correlation profiling. To overcome the deficit of high-quality training data in non-model organisms, we complemented a supervised machine learning strategy, with an unsupervised approach, based on similarity network fusion. The resulting combined high confidence network, ToxoNet, comprises 2,063 interactions connecting 652 proteins. Clustering identifies 93 protein complexes. We identified clusters enriched in mitochondrial machinery that include previously uncharacterized proteins that likely represent novel adaptations to oxidative phosphorylation. Furthermore, complexes enriched in proteins localized to secretory organelles and the inner membrane complex, predict additional novel components representing novel targets for detailed functional characterization. We present ToxoNet as a publicly available resource with the expectation that it will help drive future hypotheses within the research community.

Toxoplasma sortilin interacts with secretory proteins and it is critical for parasite proliferation

BACKGROUND

The human sortilin protein is an important drug target and detection marker for cancer research. The sortilin from Toxoplasma gondii transports proteins associated with the apical organelles of the parasite. In this study, we aimed to determine the intracellular localization and structural domains of T. gondii sortilin, which may mediate protein transportation. Approaches to the functional inhibition of sortilin to establish novel treatments for T. gondii infections were explored.

METHODS

A gene encoding the sortilin protein was identified in the T. gondii genome. Immunoprecipitation and mass spectrometry were performed to identify the protein species transported by T. gondii sortilin. The interaction of each structural domain of sortilin with the transported proteins was investigated using bio-layer interferometry. The binding regions of the transported proteins in sortilin were identified. The effect of the sortilin inhibitor AF38469 on the infectivity of T. gondii was investigated. The binding site of AF38469 on sortilin was determined.

RESULTS

The subdomains Vps10, sortilin-C, and sortilin-M of the sortilin were identified as the binding regions for intracellular transportation of the target proteins. The sortilin inhibitor AF38469 bound to the Vps10 structural domain of T. gondii sortilin, which inhibited parasite invasion, replication, and intracellular growth in vitro and was therapeutic in mice infected with T. gondii.

CONCLUSION

The Vps10, sortilin-C, and sortilin-M subdomains of T. gondii sortilin were identified as functional regions for intracellular protein transport. The binding region for the sortilin inhibitor AF38469 was also identified as the Vps10 subdomain. This study establishes sortilin as a promising drug target against T. gondii and provides a valuable reference for the development of anti-T. gondii drug-target studies.

Insight into the current Toxoplasma gondii DNA vaccine: a review article

INTRODUCTION

Toxoplasma gondii (T.gondii) is a widespread protozoan with significant economic losses and public health importance. But so far, the protective effect of reported DNA-based vaccines fluctuates widely, and no study has demonstrated complete protection.

AREAS COVERED

This review provides an inclusive summary of T. gondii DNA vaccine antigens, adjuvants, and some other parameters. A total of 140 articles from 2000 to 2021 were collected from five databases. By contrasting the outcomes of acute and chronic challenges, we aimed to investigate and identify viable immunological strategies for optimum protection. Furthermore, we evaluated and discussed the impact of several parameters on challenge outcomes in the hopes of developing some recommendations to assist better future horizontal comparisons among research.

EXPERT OPINION

In the coming five years of research, the exploration of vaccine cocktails combining invasion antigens and metabolic antigens with genetic adjuvants or novel DNA delivery methods may offer us desirable protection against this multiple stage of life parasite. In addition to finding a better immune strategy, developing better in silico prediction methods, solving problems posed by variables in practical applications, and gaining a more profound knowledge of T.gondii-host molecular interaction is also crucial towards a successful vaccine.

Toxoplasma Shelph, a Phosphatase Located in the Parasite Endoplasmic Reticulum, Is Required for Parasite Virulence

Toxoplasma gondii is a single-celled parasitic eukaryote that evolved to successfully propagate in any nucleated cell. As with any other eukaryote, its life cycle is regulated by signaling pathways controlled by kinases and phosphatases. T. gondii encodes an atypical bacterial-like phosphatase absent from mammalian genomes, named Shelph, after its first identification in the psychrophilic bacterium Schewanella sp. Here, we demonstrate that Toxoplasma Shelph is an active phosphatase localized in the parasite endoplasmic reticulum. The phenotyping of a shelph knockout (KO) line showed a minor impairment in invasion on human fibroblasts, while the other steps of the parasite lytic cycle were not affected. In contrast with Plasmodium ortholog Shelph1, this invasion deficiency was not correlated with any default in the biogenesis of secretory organelles. However, Shelph-KO parasites displayed a much-pronounced defect in virulence in vivo. These phenotypes could be rescued by genetic complementation, thus supporting an important function for Shelph in the context of a natural infection. IMPORTANCE Toxoplasma gondii belongs to the Apicomplexa phylum, which comprises more than 5,000 species, among which is Plasmodium falciparum, the notorious agent of human malaria. Intriguingly, the Apicomplexa genomes encode at least one phosphatase closely related to the bacterial Schewanella phosphatase, or Shelph. To better understand the importance of these atypical bacterial enzymes in eukaryotic parasites, we undertook the functional characterization of T. gondii Shelph. Our results uncovered its subcellular localization and its enzymatic activity, revealed its subtle involvement during the tachyzoite invasion step of the lytic cycle, and more importantly, highlighted a critical requirement of this phosphatase for parasite propagation in mice. Overall, this study revealed an unexpected role for T. gondii Shelph in the maintenance of parasite virulence in vivo.

Tomato Xylem Sap Hydrophobins Vdh4 and Vdh5 Are Important for Late Stages of Verticillium dahliae Plant Infection

Verticillium dahliae causes economic losses to a wide range of crops as a vascular fungal pathogen. This filamentous ascomycete spends long periods of its life cycle in the plant xylem, a unique environment that requires adaptive processes. Specifically, fungal proteins produced in the xylem sap of the plant host may play important roles in colonizing the plant vasculature and in inducing disease symptoms. RNA sequencing revealed over 1500 fungal transcripts that are significantly more abundant in cells grown in tomato xylem sap compared with pectin-rich medium. Of the 85 genes that are strongly induced in the xylem sap, four genes encode the hydrophobins Vdh1, Vdh2, Vdh4 and Vdh5. Vdh4 and Vhd5 are structurally distinct from each other and from the three other hydrophobins (Vdh1-3) annotated in V. dahliae JR2. Their functions in the life cycle and virulence of V. dahliae were explored using genetics, cell biology and plant infection experiments. Our data revealed that Vdh4 and Vdh5 are dispensable for V. dahliae development and stress response, while both contribute to full disease development in tomato plants by acting at later colonization stages. We conclude that Vdh4 and Vdh5 are functionally specialized fungal hydrophobins that support pathogenicity against plants.

Dense granule biogenesis, secretion, and function in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite and the causative agent of Toxoplasmosis. A key to understanding and treating the disease lies with determining how the parasite can survive and replicate within cells of its host. Proteins released from specialized secretory vesicles, named the dense granules (DGs), have diverse functions that are critical for adapting the intracellular environment, and are thus key to survival and pathogenicity. In this review, we describe the current understanding and outstanding questions regarding dense granule biogenesis, trafficking, and regulation of secretion. In addition, we provide an overview of dense granule protein ("GRA") function upon secretion, with a focus on proteins that have recently been identified.

Toxoplasma gondii's Basal Complex: The Other Apicomplexan Business End Is Multifunctional

The Apicomplexa are famously named for their apical complex, a constellation of organelles at their apical end dedicated to invasion of their host cells. In contrast, at the other end of the cell, the basal complex (BC) has been overshadowed since it is much less prominent and specific functions were not immediately obvious. However, in the past decade a staggering array of functions have been associated with the BC and strides have been made in understanding its structure. Here, these collective insights are supplemented with new data to provide an overview of the understanding of the BC in Toxoplasma gondii. The emerging picture is that the BC is a dynamic and multifunctional complex, with a series of (putative) functions. The BC has multiple roles in cell division: it is the site where building blocks are added to the cytoskeleton scaffold; it exerts a two-step stretch and constriction mechanism as contractile ring; and it is key in organelle division. Furthermore, the BC has numerous putative roles in 'import', such as the recycling of mother cell remnants, the acquisition of host-derived vesicles, possibly the uptake of lipids derived from the extracellular medium, and the endocytosis of micronemal proteins. The latter process ties the BC to motility, whereas an additional role in motility is conferred by Myosin C. Furthermore, the BC acts on the assembly and/or function of the intravacuolar network, which may directly or indirectly contribute to the establishment of chronic tissue cysts. Here we provide experimental support for molecules acting in several of these processes and identify several new BC proteins critical to maintaining the cytoplasmic bridge between divided parasites. However, the dispensable nature of many BC components leaves many questions unanswered regarding its function. In conclusion, the BC in T. gondii is a dynamic and multifunctional structure at the posterior end of the parasite.

Proximity-Labeling Reveals Novel Host and Parasite Proteins at the Toxoplasma Parasitophorous Vacuole Membrane

Toxoplasma gondii is a ubiquitous, intracellular parasite that envelops its parasitophorous vacuole with a protein-laden membrane (PVM). The PVM is critical for interactions with the infected host cell, such as nutrient transport and immune defense. Only a few parasite and host proteins have so far been identified on the host-cytosolic side of the Toxoplasma PVM. We report here the use of human foreskin fibroblasts expressing the proximity-labeling enzyme miniTurbo, fused to a domain that targets it to this face of the PVM, in combination with quantitative proteomics to specifically identify proteins present at this interface. Out of numerous human and parasite proteins with candidate PVM localization, we validate three parasite proteins (TGGT1_269950 [GRA61], TGGT1_215360 [GRA62], and TGGT1_217530 [GRA63]) and four new host proteins (PDCD6IP/ALIX, PDCD6, CC2D1A, and MOSPD2) as localized to the PVM in infected human cells through immunofluorescence microscopy. These results significantly expand our knowledge of proteins present at the Toxoplasma PVM and, given that three of the validated host proteins are components of the ESCRT (endosomal sorting complexes required for transport) machinery, they further suggest that novel biology is operating at this crucial host-pathogen interface.

Do All Coccidia Follow the Same Trafficking Rules?

The Coccidia are a subclass of the Apicomplexa and include several genera of protozoan parasites that cause important diseases in humans and animals, with Toxoplasma gondii becoming the ‘model organism’ for research into the coccidian molecular and cellular processes. The amenability to the cultivation of T. gondii tachyzoites and the wide availability of molecular tools for this parasite have revealed many mechanisms related to their cellular trafficking and roles of parasite secretory organelles, which are critical in parasite-host interaction. Nevertheless, the extrapolation of the T. gondii mechanisms described in tachyzoites to other coccidian parasites should be done carefully. In this review, we considered published data from Eimeria parasites, a coccidian genus comprising thousands of species whose infections have important consequences in livestock and poultry. These studies suggest that the Coccidia possess both shared and diversified mechanisms of protein trafficking and secretion potentially linked to their lifecycles. Whereas trafficking and secretion appear to be well conversed prior to and during host-cell invasion, important differences emerge once endogenous development commences. Therefore, further studies to validate the mechanisms described in T. gondii tachyzoites should be performed across a broader range of coccidians (including T. gondii sporozoites). In addition, further genus-specific research regarding important disease-causing Coccidia is needed to unveil the individual molecular mechanisms of pathogenesis related to their specific lifecycles and hosts.

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.

Emerging Mechanisms of Endocytosis in Toxoplasma gondii

Eukaryotes critically rely on endocytosis of autologous and heterologous material to maintain homeostasis and to proliferate. Although mechanisms of endocytosis have been extensively identified in mammalian and plant systems along with model systems including budding yeast, relatively little is known about endocytosis in protozoan parasites including those belonging to the phylum Apicomplexa. Whereas it has been long established that the apicomplexan agents of malaria (Plasmodium spp.) internalize and degrade hemoglobin from infected red blood cells to acquire amino acids for growth, that the related and pervasive parasite Toxoplasma gondii has a functional and active endocytic system was only recently discovered. Here we discuss emerging and hypothesized mechanisms of endocytosis in Toxoplasma gondii with reference to model systems and malaria parasites. Establishing a framework for potential mechanisms of endocytosis in Toxoplasma gondii will help guide future research aimed at defining the molecular basis and biological relevance of endocytosis in this tractable and versatile parasite.

Genome-wide screens identify Toxoplasma gondii determinants of parasite fitness in IFNγ-activated murine macrophages

Macrophages play an essential role in the early immune response against Toxoplasma and are the cell type preferentially infected by the parasite in vivo. Interferon gamma (IFNγ) elicits a variety of anti-Toxoplasma activities in macrophages. Using a genome-wide CRISPR screen we identify 353 Toxoplasma genes that determine parasite fitness in naїve or IFNγ-activated murine macrophages, seven of which are further confirmed. We show that one of these genes encodes dense granule protein GRA45, which has a chaperone-like domain, is critical for correct localization of GRAs into the PVM and secretion of GRA effectors into the host cytoplasm. Parasites lacking GRA45 are more susceptible to IFNγ-mediated growth inhibition and have reduced virulence in mice. Together, we identify and characterize an important chaperone-like GRA in Toxoplasma and provide a resource for the community to further explore the function of Toxoplasma genes that determine fitness in IFNγ-activated macrophages.

Rab11A regulates dense granule transport and secretion during Toxoplasma gondii invasion of host cells and parasite replication

Toxoplasma gondii possesses an armada of secreted virulent factors that enable parasite invasion and survival into host cells. These factors are contained in specific secretory organelles, the rhoptries, micronemes and dense granules that release their content upon host cell recognition. Dense granules are secreted in a constitutive manner during parasite replication and play a crucial role in modulating host metabolic and immune responses. While the molecular mechanisms triggering rhoptry and microneme release upon host cell adhesion have been well studied, constitutive secretion remains a poorly explored aspect of T. gondii vesicular trafficking. Here, we investigated the role of the small GTPase Rab11A, a known regulator of exocytosis in eukaryotic cells. Our data revealed an essential role of Rab11A in promoting the cytoskeleton driven transport of dense granules and the release of their content into the vacuolar space. Rab11A also regulates transmembrane protein trafficking and localization during parasite replication, indicating a broader role of Rab11A in cargo exocytosis at the plasma membrane. Moreover, we found that Rab11A also regulates extracellular parasite motility and adhesion to host cells. In line with these findings, MIC2 secretion was altered in Rab11A-defective parasites, which also exhibited severe morphological defects. Strikingly, by live imaging we observed a polarized accumulation of Rab11A-positive vesicles and dense granules at the apical pole of extracellular motile and invading parasites suggesting that apically polarized Rab11A-dependent delivery of cargo regulates early secretory events during parasite entry into host cells.

Toxoplasma gondii (T. gondii) is a highly prevalent parasite infecting a wide range of animals as well as humans. T. gondii secretes numerous virulent factors contained in specific organelles, termed the rhoptries, micronemes and dense granules. These factors are released upon host cell recognition and enable parasite invasion and subsequent development into an intracellular vacuole. In particular, dense granules contain critical effectors that modulate intrinsic defenses of infected host cells ensuring parasite survival and dissemination. The mechanisms regulating dense granule secretion have not been elucidated. In this study, we unraveled a novel role for the T. gondii GTPase Rab11A in promoting dense granule transport along the parasite cytoskeleton and their content release into the vacuolar space during parasite replication. We also found that T. gondii Rab11A regulates extracellular parasite motility and adhesion to host cells suggesting a broader role in distinct secretory pathways essential for parasite virulence.

Cell type- and species-specific host responses to Toxoplasma gondii and its near relatives

Toxoplasma gondii is remarkably unique in its ability to successfully infect vertebrate hosts from multiple phyla and can successfully infect most cells within these organisms. The infection outcome in each of these species is determined by the complex interaction between parasite and host genotype. As techniques to quantify global changes in cell function become more readily available and precise, new data are coming to light about how (i) different host cell types respond to parasitic infection and (ii) different parasite species impact the host. Here we focus on recent studies comparing the response to intracellular parasitism by different cell types and insights into understanding host-parasite interactions from comparative studies on T. gondii and its close extant relatives.

Succinylated Wheat Germ Agglutinin Colocalizes with the Toxoplasma gondii Cyst Wall Glycoprotein CST1

The glycosylated mucin domain of the Toxoplasma gondii cyst wall glycoprotein CST1 is heavily stained by Dolichos biflorus agglutinin, a lectin that binds to N-acetylgalactosamine. The cyst wall is also heavily stained by the chitin binding lectin succinylated wheat germ agglutinin (s-WGA), which selectively binds to N-acetylglucosamine-decorated structures. Here, we tracked the localization of N-acetylglucosamine-decorated structures that bind to s-WGA in immature and mature in vitro cysts. s-WGA localization was observed at the cyst periphery 6 h after the differentiation of the tachyzoite-stage parasitophorous vacuole. By day 1 and at all later times after differentiation, s-WGA was localized in a continuous staining pattern at the cyst wall. Coinciding with the maturation of the cyst matrix by day 3 of cyst development, s-WGA also localized in a continuous matrix pattern inside the cyst. s-WGA localized in both the outer and inner layer regions of the cyst wall and in a continuous matrix pattern inside mature 7- and 10-day-old cysts. In addition, s-WGA colocalized in the cyst wall with CST1, suggesting that N-acetylglucosamine- and N-acetylgalactosamine-decorated molecules colocalized in the cyst wall. In contrast to CST1, GRA4, and GRA6, the relative accumulation of the molecules that bind s-WGA in the cyst wall was not dependent on the expression of GRA2. Our results suggest that GRA2-dependent and GRA2-independent mechanisms regulate the trafficking and accumulation of glycosylated molecules that colocalize in the cyst wall.IMPORTANCE Chronic Toxoplasma gondii infection is maintained in the central nervous system by thick-walled cysts. If host immunity wanes, cysts recrudesce and cause severe and often lethal toxoplasmic encephalitis. Currently, there are no therapies to eliminate cysts, and little biological information is available regarding cyst structure(s). Here, we investigated cyst wall molecules recognized by succinylated wheat germ agglutinin (s-WGA), a lectin that specifically binds to N-acetylglucosamine-decorated structures. N-Acetylglucosamine regulates cell signaling and plays structural roles at the cell surface in many organisms. The cyst wall and cyst matrix were heavily stained by s-WGA in mature cysts and were differentially stained during cyst development. The relative accumulation of molecules that bind to s-WGA in the cyst wall was not dependent on the expression of GRA2. Our findings suggest that glycosylated cyst wall molecules gain access to the cyst wall via GRA2-dependent and GRA2-independent mechanisms and colocalize in the cyst wall.

Novel roles of dense granule protein 12 (GRA12) in Toxoplasma gondii infection

Dense granule protein 12 (GRA12) is implicated in a range of processes related to the establishment of Toxoplasma gondii infection, such as the formation of the intravacuolar network (IVN) within the parasitophorous vacuole (PV). This protein is also thought to be important for T. gondii-host interaction, pathogenesis, and immune evasion, but their exact roles remain unknown. In this study, the contributions of GRA12 to the molecular pathogenesis of T. gondii infection were examined in vitro and in vivo. Deletion of GRA12 in type I RH and type II Pru T. gondii strains did not affect the parasite growth and replication in vitro, however, it caused a significant reduction in the parasite virulence and tissue cyst burden in vivo. T. gondii Δgra12 mutants were more vulnerable to be eliminated by host immunity, without the accumulation of immunity-related GTPase a6 (Irga6) onto the PV membrane. The ultrastructure of IVN in Δgra12 mutants appeared normal, suggesting that GRA12 is not required for biogenesis of the IVN. Combined deletion of GRA12 and ROP18 induced more severe attenuation of virulence compared to single Δgra12 or Δrop18 mutant strains. These data suggest a functional association between GRA12 and ROP18 that is revealed by the severe attenuation of virulence in a double mutant relative to the single individual mutations. Future studies are needed to define the molecular basis of this putative association. Collectively these findings indicate that although GRA12 is not essential for the parasite growth and replication in vitro, it contributes to the virulence and growth of T. gondii in mice.

Toxoplasma gondii Intravacuolar-Network-Associated Dense Granule Proteins Regulate Maturation of the Cyst Matrix and Cyst Wall

Little is known regarding how the chronic Toxoplasma gondii cyst develops. Here, we investigated intravacuolar-network-associated dense granule (GRA) proteins GRA1, GRA2, GRA4, GRA6, GRA9, and GRA12 during cyst development in vitro after differentiation of the tachyzoite-stage parasitophorous vacuole. By day 1 postdifferentiation, GRA1, GRA4, GRA6, GRA9, and GRA12 colocalized with Dolichos biflorus agglutinin stain at the cyst periphery. In contrast, GRA2 remained in the cyst matrix. By day 2 postdifferentiation, coinciding with localization of GRA2 to the cyst periphery, GRA1, GRA4, GRA6, and GRA9 established a continuous matrix pattern in the cyst. In contrast, GRA2 and GRA12 were colocalized in prominent cyst matrix puncta throughout cyst development. While GRA2, GRA6, and GRA12 localized in outer and inner layers of the cyst wall, GRA1, GRA4, and GRA9 localized predominantly in the inner layers of the cyst wall. GRA2 and GRA12 were colocalized in the cyst wall by day 7 postdifferentiation. However, by day 10 postdifferentiation, GRA12 was relocalized from the cyst wall to puncta in the cyst matrix. Differentiation of Δgra2 parasites revealed a defect in the ability to establish a normal cyst matrix. In addition, the deletion of any intravacuolar-network-associated GRA protein, except GRA1, reduced the rate of accumulation of cyst wall proteins at the cyst periphery relative to the cyst interior. Our findings reveal dynamic patterns of GRA protein localization during cyst development and suggest that intravacuolar-network-associated GRA proteins regulate the formation and maturation of the cyst matrix and cyst wall structures.IMPORTANCEToxoplasma gondii establishes chronic infection in humans by forming thick-walled cysts that persist in the brain. If host immunity wanes, cysts reactivate to cause severe, and often lethal, toxoplasmic encephalitis. There is no available therapy to eliminate cysts or to prevent their reactivation. Moreover, how the vital and characteristic cyst matrix and cyst wall structures develop is poorly understood. Here, we visualized and tracked the localization of Toxoplasma intravacuolar-network-associated dense granule (GRA) proteins during cyst development in vitro Intravacuolar-network GRAs were present within the cyst matrix and at the cyst wall in developing cysts, and genetic deletion of intravacuolar-network-associated GRAs reduced the rate of accumulation of cyst wall material at the cyst periphery. Our results show that intravacuolar-network-associated GRAs, particularly GRA2 and GRA12, play dynamic and essential roles in the development and maturation of the cyst matrix and the cyst wall structures.

A CRISPR platform for targeted in vivo screens identifies Toxoplasma gondii virulence factors in mice

Genome-wide CRISPR screening is a powerful tool to identify genes required under selective conditions. However, the inherent scale of genome-wide libraries can limit their application in experimental settings where cell numbers are restricted, such as in vivo infections or single cell analysis. The use of small scale CRISPR libraries targeting gene subsets circumvents this problem. Here we develop a method for rapid generation of custom guide RNA (gRNA) libraries using arrayed single-stranded oligonucleotides for reproducible pooled cloning of CRISPR/Cas9 libraries. We use this system to generate mutant pools of different sizes in the protozoan parasite Toxoplasma gondi and describe optimised analysis methods for small scale libraries. An in vivo genetic screen in the murine host identifies novel and known virulence factors and we confirm results using cloned knock-out parasites. Our study also reveals a potential trans-rescue of individual knock-out parasites in pools of mutants compared to homogenous knock-out lines of the key virulence factor MYR1.

Toxoplasma gondii Parasitophorous Vacuole Membrane-Associated Dense Granule Proteins Orchestrate Chronic Infection and GRA12 Underpins Resistance to Host Gamma Interferon

Toxoplasma gondii evades host immunity to establish a chronic infection. Here, we assessed the role of parasitophorous vacuole (PV) membrane (PVM)- and intravacuolar network (IVN) membrane-localized dense granule (GRA) proteins in the development of acute and chronic Toxoplasma infection. Deletion of PVM-associated GRA3, GRA7, GRA8, and GRA14 or IVN membrane-associated GRA2, GRA9, and GRA12 in the low-virulence type II Prugniaud (Pru) strain induced severe defects in the development of chronic-stage cysts in vivo without affecting the parasite growth rate or the ability to differentiate into cysts in vitro Acute virulence of the PruΔgra2, PruΔgra3, and PruΔgra4 mutants was reduced but not abolished. In contrast, the PruΔgra12 mutant was avirulent in mice and PruΔgra12 parasites failed to establish a chronic infection. High-virulence type I strain RHΔgra12 parasites also exhibited a major defect in acute virulence. In gamma interferon (IFN-γ)-activated macrophages, type I RHΔgra12 and type II PruΔgra12 parasites resisted the coating of the PVM with host immunity-related GTPases as effectively as the parental type I RHΔku80 and type II PruΔku80 strains, respectively. Despite this resistance, Δgra12 PVs ultimately succumbed to IFN-γ-activated host cell innate immunity. Our findings uncover a key role for GRA12 in mediating resistance to host IFN-γ and reveal that many other IVN membrane-associated GRA proteins, as well as PVM-localized GRA proteins, play important roles in establishing chronic infection.IMPORTANCEToxoplasma gondii cysts reactivate during immune deficiency and cause fatal encephalitis. Parasite molecules that coordinate the development of acute and chronic infection are poorly characterized. Here, we show that many intravacuolar network membrane and parasitophorous vacuole membrane-associated dense granule (GRA) proteins orchestrate the development of chronic cysts in vivo A subset of these GRA proteins also modulate acute virulence, and one protein that associates with the intravacuolar network membranes, namely GRA12, was identified as a major virulence factor required for parasite resistance to host gamma interferon (IFN-γ). Our results revealed that many parasitophorous vacuole membrane and intravacuolar network membrane-associated GRA proteins are essential for successful chronic infection.

Divergent kinase regulates membrane ultrastructure of the Toxoplasma parasitophorous vacuole

Apicomplexan parasites replicate within a protective organelle, called the parasitophorous vacuole (PV). The Toxoplasma gondii PV is filled with a network of tubulated membranes, which are thought to facilitate trafficking of effectors and nutrients. Despite being critical to parasite virulence, there is scant mechanistic understanding of the network's functions. Here, we identify the parasite-secreted kinase WNG1 (With-No-Gly-loop) as a critical regulator of tubular membrane biogenesis. WNG1 family members adopt an atypical protein kinase fold lacking the glycine rich ATP-binding loop that is required for catalysis in canonical kinases. Unexpectedly, we find that WNG1 is an active protein kinase that localizes to the PV lumen and phosphorylates PV-resident proteins, several of which are essential for the formation of a functional intravacuolar network. Moreover, we show that WNG1-dependent phosphorylation of these proteins is required for their membrane association, and thus their ability to tubulate membranes. Consequently, WNG1 knockout parasites have an aberrant PV membrane ultrastructure. Collectively, our results describe a unique family of Toxoplasma kinases and implicate phosphorylation of secreted proteins as a mechanism of regulating PV development during parasite infection.

Characterization of strain-specific phenotypes associated with knockout of dense granule protein 9 in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that can invade any nucleated cell of mammals and cause toxoplasmosis. Dense granule proteins play major structural functions within the parasitophorous vacuole (PV) and the cyst wall of T. gondii. Moreover, their particular location within the PV allows them to be involved in various interactions between parasites and the host cells. Dense granule protein 9 (GRA9) gene has been identified in T. gondii, although its role in the lytic cycle remains unclear. In the current study, the function of GRA9 in type I and type II Toxoplasma parasites was characterized. T. gondii GRA9 sequence and its expression were analyzed and derivatives of T. gondii RH and PLK strains with a null mutation in GRA9 were generated using CRISPR/Cas9 system. The phenotypes of GRA9 in wild types, knockout and complemented strains were analyzed in vitro and in vivo using Vero cells and BALB/c mice, respectively. Alignment of the amino acid sequence indicated that RH strain GRA9 contained one amino acid substitution when compared with PLK strain. Western blot analysis revealed that PLK strain had a higher expression level of GRA9 than RH strain. The phenotype analysis revealed that knockout of GRA9 in PLK parasites inhibited the plaque formation and egress from PV. Both the plaque formation and egress ability of PLKΔGRA9 strain were restored by complementation with a synonymous allele of PLK strain GRA9. Mouse experiments demonstrated that loss of GRA9 in PLK strain significantly reduced the pathogenicity of T. gondii. However, there was no phenotypic diferences between RH and RHΔGRA9 strains except the defect in host cell invasion. Overall, T. gondii GRA9 knockout only influenced the growth and virulence of PLK strain. These results indicate that GRA9 may be involved in parasite egress and virulence in mice in a strain-specific manner.

Calcium negatively regulates secretion from dense granules in Toxoplasma gondii

Apicomplexan parasites including Toxoplasma gondii and Plasmodium spp. manufacture a complex arsenal of secreted proteins used to interact with and manipulate their host environment. These proteins are organised into three principle exocytotic compartment types according to their functions: micronemes for extracellular attachment and motility, rhoptries for host cell penetration, and dense granules for subsequent manipulation of the host intracellular environment. The order and timing of these events during the parasite's invasion cycle dictates when exocytosis from each compartment occurs. Tight control of compartment secretion is, therefore, an integral part of apicomplexan biology. Control of microneme exocytosis is best understood, where cytosolic intermediate molecular messengers cGMP and Ca²⁺ act as positive signals. The mechanisms for controlling secretion from rhoptries and dense granules, however, are virtually unknown. Here, we present evidence that dense granule exocytosis is negatively regulated by cytosolic Ca²⁺, and we show that this Ca²⁺‐mediated response is contingent on the function of calcium‐dependent protein kinases TgCDPK1 and TgCDPK3. Reciprocal control of micronemes and dense granules provides an elegant solution to the mutually exclusive functions of these exocytotic compartments in parasite invasion cycles and further demonstrates the central role that Ca²⁺ signalling plays in the invasion biology of apicomplexan parasites.

Two Phosphoglucomutase Paralogs Facilitate Ionophore-Triggered Secretion of the Toxoplasma Micronemes

Ca²⁺-dependent exocytosis is essential for the life cycle of apicomplexan parasites. Toxoplasma gondii harbors a phosphoglucomutase (PGM) ortholog, PRP1, previously associated with Ca²⁺-dependent microneme secretion. Here it is shown that genetic deletion of either PRP1, its PGM2 ortholog, or both genes is dispensable for the parasite’s lytic cycle, including host cell egress and invasion. Depletion of the proteins abrogated high Ca²⁺-mediated microneme secretion induced by the ionophore A23187; however, the constitutive and phosphatidic acid-mediated release remained unaffected. Secretion mediated by the former pathway is not essential for tachyzoite survival or acute in vivo infection in the mice.

Paralogs of the widely prevalent phosphoglucomutase (PGM) protein called parafusin function in calcium (Ca²⁺)-mediated exocytosis across eukaryotes. In Toxoplasma gondii, the parafusin-related protein 1 (PRP1) has been associated with Ca²⁺-dependent microneme organelle secretion required for essential processes like host cell invasion and egress. Using reverse genetics, we observed PRP1 to be dispensable for completion of the lytic cycle, including host cell invasion and egress by the parasite. However, the absence of the gene affected increased microneme release triggered by A23187, a Ca²⁺ ionophore used to raise the cytoplasmic Ca²⁺ concentration mimicking the physiological role of Ca²⁺ during invasion and egress. The basal levels of constitutive microneme release in extracellular parasites and phosphatidic acid-triggered microneme secretion were unaffected in the mutant. The phenotype of the deletion mutant of the second PGM-encoding gene in Toxoplasma, PGM2, was similar to the phenotype of the PRP1 deletion mutant. Furthermore, the ability of the tachyzoites to induce acute infection in the mice remained normal in the absence of both PGM paralogs. Our data thus reveal that the microneme secretion upon high Ca²⁺ flux is facilitated by the Toxoplasma PGM paralogs, PRP1 and PGM2. However, this protein-mediated release is neither essential for lytic cycle completion nor for acute virulence of the parasite.

IMPORTANCE Ca²⁺-dependent exocytosis is essential for the life cycle of apicomplexan parasites. Toxoplasma gondii harbors a phosphoglucomutase (PGM) ortholog, PRP1, previously associated with Ca²⁺-dependent microneme secretion. Here it is shown that genetic deletion of either PRP1, its PGM2 ortholog, or both genes is dispensable for the parasite’s lytic cycle, including host cell egress and invasion. Depletion of the proteins abrogated high Ca²⁺-mediated microneme secretion induced by the ionophore A23187; however, the constitutive and phosphatidic acid-mediated release remained unaffected. Secretion mediated by the former pathway is not essential for tachyzoite survival or acute in vivo infection in the mice.

MHC I presentation of Toxoplasma gondii immunodominant antigen does not require Sec22b and is regulated by antigen orientation at the vacuole membrane

The intracellular Toxoplasma gondii parasite replicates within a parasitophorous vacuole (PV). T. gondii secretes proteins that remain soluble in the PV space, are inserted into PV membranes or are exported beyond the PV boundary. In addition to supporting T. gondii growth, these proteins can be processed and presented by MHC I for CD8⁺ T-cell recognition. Yet it is unclear whether membrane binding influences the processing pathways employed and if topology of membrane antigens impacts their MHC I presentation. Here we report that the MHC I pathways of soluble and membrane-bound antigens differ in their requirement for host ER recruitment. In contrast to the soluble SAG1-OVA model antigen, we find that presentation of the membrane-bound GRA6 is independent from the SNARE Sec22b, a key molecule for transfer of host endoplasmic reticulum components onto the PV. Using parasites modified to secrete a transmembrane antigen with opposite orientations, we further show that MHC I presentation is highly favored when the C-terminal epitope is exposed to the host cell cytosol, which corresponds to GRA6 natural orientation. Our data suggest that the biochemical properties of antigens released by intracellular pathogens critically guide their processing pathway and are valuable parameters to consider for vaccination strategies.

Identification and characterization of GRA6/GRA7 of Neospora caninum in MDBK cells

Neospora caninum, an apicomplexan parasite, is recognized as a major bovine abortifacient. Dense granule antigens (GRAs) play important roles in the formation and modification of parasitophorous vacuoles (PVs) in Toxoplasma gondii. However, a few studies investigating GRAs have been reported in N. caninum. The aim of the present study was to characterize the dense GRA6/GRA7 of N. caninum in PVs using MDBK cells as a host cell model. Neospora caninum was inoculated into MDBK cells, and changes were observed using a transmission electron microscope (TEM). Neospora caninum GRA6/GRA7 were identified and characterized using bioinformatics, cell fractionation, and immunofluorescence. The TEM results revealed that integrated PVs were present in MDBK cells after N. caninum infection. Bioinformatics analysis showed that NcGRA6/NcGRA7 shared 28.76% and 29.66% homology with T. gondii GRA6/GRA7 (TgGRA6/TgGRA7) but had similar signal peptides, transmembrane domains, and motifs. Cell fractionation and subcellular localization analyses both showed that NcGRA6 was distributed in the lumen and intravacuolar network in soluble and transmembrane forms. The transmembrane form of NcGRA7 was observed in the PV membrane. These data lay a foundation for further study on bovine neosporosis and NcGRA6/NcGRA7 function during PV formation.

Phenotypes Associated with Knockouts of Eight Dense Granule Gene Loci (GRA2-9) in Virulent Toxoplasma gondii

Toxoplasma gondii actively invades host cells and establishes a parasitophorous vacuole (PV) that accumulates many proteins secreted by the dense granules (GRA proteins). To date, at least 23 GRA proteins have been reported, though the function(s) of most of these proteins still remains unknown. We targeted gene knockouts at ten GRA gene loci (GRA1-10) to investigate the cellular roles and essentiality of these classical GRA proteins during acute infection in the virulent type I RH strain. While eight of these genes (GRA2-9) were successfully knocked out, targeted knockouts at the GRA1 and GRA10 loci were not obtained, suggesting these GRA proteins may be essential. As expected, the Δgra2 and Δgra6 knockouts failed to form an intravacuolar network (IVN). Surprisingly, Δgra7 exhibited hyper-formation of the IVN in both normal and lipid-free growth conditions. No morphological alterations were identified in parasite or PV structures in the Δgra3, Δgra4, Δgra5, Δgra8, or Δgra9 knockouts. With the exception of the Δgra3 and Δgra8 knockouts, all of the GRA knockouts exhibited defects in their infection rate in vitro. While the single GRA knockouts did not exhibit reduced replication rates in vitro, replication rate defects were observed in three double GRA knockout strains (Δgra4Δgra6, Δgra3Δgra5 and Δgra3Δgra7). However, the virulence of single or double GRA knockout strains in CD1 mice was not affected. Collectively, our results suggest that while the eight individual GRA proteins investigated in this study (GRA2-9) are not essential, several GRA proteins may provide redundant and potentially important functions during acute infection.

The tandemly repeated NTPase (NTPDase) from Neospora caninum is a canonical dense granule protein whose RNA expression, protein secretion and phosphorylation coincides with the tachyzoite egress

Background

NTPases (also NTPDases) are enzymes with apyrase activity. They are widely distributed among eukaryotes, and also among members of the family Sarcocystidae. In Toxoplasma gondii, the TgNTPase accumulates in the dense granules, and has been commonly associated with the strain virulence. In the closely related Neospora caninum, the NcNTPase lacks nucleoside diphosphate hydrolase activity and appears to be more abundant in virulent isolates, indicating that it may contribute to the pathogenicity of neosporosis. However, so far no additional information on NcNTPase has been provided.

Methods

Herein, the NcNTPase coding sequences were analysed by different in silico and de novo sequencing approaches. A comparative analysis of NcNTPase and NcGRA7 in terms of protein dynamics, secretion, phosphorylation, and mRNA expression profiles during the tachyzoite lytic cycle was also carried out. Moreover, NcNTPase immunolocalization was analysed by confocal microscopy techniques over a set number of time-points.

Results

We describe the presence of three different loci containing three copies of the NcNTPase within the Nc-Liv genome, and report the existence of up to four different NcNTPase alleles in Nc-Liv. We also provide evidence for the occurrence of diverse protein species of the NcNTPase by two-dimensional gel electrophoresis. Both NcNTPase and NcGRA7 were similarly up-regulated and secreted during the egress and/or early invasion phases, and were phosphorylated. However, its secretion was not affected by the addition of calcium modulators such as A23187 and ethanol. NcNTPase and NcGRA7 localized in dense granules and parasitophorous vacuole membrane throughout the lytic cycle, although differed in their inmunolocalization during early invasion and egress.

Conclusions

The present study reveals the complexity of the NcNTPase loci in N. caninum. We hypothesize that the expression of different isoforms of the NcNTPase protein could contribute to parasite virulence. Our findings showed regulation of expression, secretion and phosphorylation of NcNTPase suggesting a potential role for progression through the tachyzoites lytic cycle.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1620-4) contains supplementary material, which is available to authorized users.

Intravacuolar Membranes Regulate CD8 T Cell Recognition of Membrane-Bound Toxoplasma gondii Protective Antigen

Apicomplexa parasites such as Toxoplasma gondii target effectors to and across the boundary of their parasitophorous vacuole (PV), resulting in host cell subversion and potential presentation by MHC class I molecules for CD8 T cell recognition. The host-parasite interface comprises the PV limiting membrane and a highly curved, membranous intravacuolar network (IVN) of uncertain function. Here, using a cell-free minimal system, we dissect how membrane tubules are shaped by the parasite effectors GRA2 and GRA6. We show that membrane association regulates access of the GRA6 protective antigen to the MHC I pathway in infected cells. Although insertion of GRA6 in the PV membrane is key for immunogenicity, association of GRA6 with the IVN limits presentation and curtails GRA6-specific CD8 responses in mice. Thus, membrane deformations of the PV regulate access of antigens to the MHC class I pathway, and the IVN may play a role in immune modulation.

Characterization of the Neospora caninum NcROP40 and NcROP2Fam-1 rhoptry proteins during the tachyzoite lytic cycle

Virulence factors from the ROP2-family have been extensively studied in Toxoplasma gondii, but in the closely related Neospora caninum only NcROP2Fam-1 has been partially characterized to date. NcROP40 is a member of this family and was found to be more abundantly expressed in virulent isolates. Both NcROP2Fam-1 and NcROP40 were evaluated as vaccine candidates and exerted a synergistic effect in terms of protection against vertical transmission in mouse models, which suggests that they may be relevant for parasite pathogenicity. NcROP40 is localized in the rhoptry bulbs of tachyzoites and bradyzoites, but in contrast to NcROP2Fam-1, the protein does not associate with the parasitophorous vacuole membrane due to the lack of arginine-rich amphipathic helix in its sequence. Similarly to NcROP2Fam-1, NcROP40 mRNA levels are highly increased during tachyzoite egress and invasion. However, NcROP40 up-regulation does not appear to be linked to the mechanisms triggering egress. In contrast to NcROP2Fam-1, phosphorylation of NcROP40 was not observed during egress. Besides, NcROP40 secretion into the host cell was not successfully detected by immunofluorescence techniques. These findings indicate that NcROP40 and NcROP2Fam-1 carry out different functions, and highlight the need to elucidate the role of NcROP40 within the lytic cycle and to explain its relative abundance in tachyzoites.

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: biochemical and biophysical characterization of recombinant soluble dense granule proteins GRA2 and GRA6

The most prominent structural feature of the parasitophorous vacuole (PV) in which the intracellular parasite Toxoplasma gondii proliferates is a membranous nanotubular network (MNN), which interconnects the parasites and the PV membrane. The MNN function remains unclear. The GRA2 and GRA6 proteins secreted from the parasite dense granules into the PV have been implicated in the MNN biogenesis. Amphipathic alpha-helices (AAHs) predicted in GRA2 and an alpha-helical hydrophobic domain predicted in GRA6 have been proposed to be responsible for their membrane association, thereby potentially molding the MMN in its structure. Here we report an analysis of the recombinant proteins (expressed in detergent-free conditions) by circular dichroism, which showed that full length GRA2 displays an alpha-helical secondary structure while recombinant GRA6 and GRA2 truncated of its AAHs are mainly random coiled. Dynamic light scattering and transmission electron microscopy showed that recombinant GRA6 and truncated GRA2 constitute a homogenous population of small particles (6-8 nm in diameter) while recombinant GRA2 corresponds to 2 populations of particles (∼8-15 nm and up to 40 nm in diameter, respectively). The unusual properties of GRA2 due to its AAHs are discussed.

Toxoplasma secretory granules: one population or more?

In Toxoplasma gondii, dense granules are known as the storage secretory organelles of the so-called GRA proteins (for dense granule proteins), which are destined to the parasitophorous vacuole (PV) and the PV-derived cyst wall. Recently, newly annotated GRA proteins targeted to the host cell nucleus have enlarged this view. Here we provide an update on the latest developments on the Toxoplasma secreted proteins, which to date have been mainly studied at both the tachyzoite and bradyzoite stages, and we point out that recent discoveries could open the issue of a possible, yet uncharacterized, distinct secretory pathway in Toxoplasma.

Selective and strain-specific NFAT4 activation by the Toxoplasma gondii polymorphic dense granule protein GRA6

Ma et al. show that the Toxoplasma gondii polymorphic dense granule protein GRA6 triggers the activation of the host transcription factor NFAT4, thus affecting the host immune response and maximizing parasite virulence.

Toxoplasma gondii infection results in co-option and subversion of host cellular signaling pathways. This process involves discharge of T. gondii effector molecules from parasite secretory organelles such as rhoptries and dense granules. We report that the T. gondii polymorphic dense granule protein GRA6 regulates activation of the host transcription factor nuclear factor of activated T cells 4 (NFAT4). GRA6 overexpression robustly and selectively activated NFAT4 via calcium modulating ligand (CAMLG). Infection with wild-type (WT) but not GRA6-deficient parasites induced NFAT4 activation. Moreover, GRA6-deficient parasites failed to exhibit full virulence in local infection, and the treatment of WT mice with an NFAT inhibitor mitigated virulence of WT parasites. Notably, NFAT4-deficient mice displayed prolonged survival, decreased recruitment of CD11b⁺ Ly6G⁺ cells to the site of infection, and impaired expression of chemokines such as Cxcl2 and Ccl2. In addition, infection with type I parasites culminated in significantly higher NFAT4 activation than type II parasites due to a polymorphism in the C terminus of GRA6. Collectively, our data suggest that GRA6-dependent NFAT4 activation is required for T. gondii manipulation of host immune responses to maximize the parasite virulence in a strain-dependent manner.

Toxoplasma gondii ingests and digests host cytosolic proteins

The protozoan parasite Toxoplasma gondii resides within a nonfusogenic vacuole during intracellular replication. Although the limiting membrane of this vacuole provides a protective barrier to acidification and degradation by lysosomal hydrolases, it also physically segregates the parasite from the host cytosol. Accordingly, it has been suggested that T. gondii acquires material from the host via membrane channels or transporters. The ability of the parasite to internalize macromolecules via endocytosis during intracellular replication has not been tested. Here, we show that Toxoplasma ingests host cytosolic proteins and digests them using cathepsin L and other proteases within its endolysosomal system. Ingestion was reduced in mutant parasites lacking an intravacuolar network of tubular membranes, implicating this apparatus as a possible conduit for trafficking to the parasite. Genetic ablation of proteins involved in the pathway is associated with diminished parasite replication and virulence attenuation. We show that both virulent type I and avirulent type II strain parasites ingest and digest host-derived protein, indicating that the pathway is not restricted to highly virulent strains. The findings provide the first definitive evidence that T. gondii internalizes proteins from the host during intracellular residence and suggest that protein digestion within the endolysosomal system of the parasite contributes to toxoplasmosis.

Toxoplasma gondii causes significant disease in individuals with weak immune systems. Treatment options for this infection have drawbacks, creating a need to understand how this parasite survives within the cells it infects as a prelude to interrupting its survival strategies. This study reveals that T. gondii internalizes proteins from the cytoplasm of the cells it infects and degrades such proteins within a digestive compartment within the parasite. Disruption of proteins involved in the pathway reduced parasite replication and lessened disease severity. The identification of a novel parasite ingestion pathway opens opportunities to interfere with this process and improve the outcome of infection.

Location of the CD8 T cell epitope within the antigenic precursor determines immunogenicity and protection against the Toxoplasma gondii parasite

CD8 T cells protect the host from disease caused by intracellular pathogens, such as the Toxoplasma gondii (T. gondii) protozoan parasite. Despite the complexity of the T. gondii proteome, CD8 T cell responses are restricted to only a small number of peptide epitopes derived from a limited set of antigenic precursors. This phenomenon is known as immunodominance and is key to effective vaccine design. However, the mechanisms that determine the immunogenicity and immunodominance hierarchy of parasite antigens are not well understood. Here, using genetically modified parasites, we show that parasite burden is controlled by the immunodominant GRA6-specific CD8 T cell response but not by responses to the subdominant GRA4- and ROP7-derived epitopes. Remarkably, optimal processing and immunodominance were determined by the location of the peptide epitope at the C-terminus of the GRA6 antigenic precursor. In contrast, immunodominance could not be explained by the peptide affinity for the MHC I molecule or the frequency of T cell precursors in the naive animals. Our results reveal the molecular requirements for optimal presentation of an intracellular parasite antigen and for eliciting protective CD8 T cells.

Evaluation of Toxoplasma gondii-recombinant dense granular protein (GRA2) for serodiagnosis by western blot

Toxoplasma gondii infects all warm-blooded animals including humans, causing serious public health problems and great economic loss in the food industry. Commonly used serological tests involve preparation of whole Toxoplasma lysate antigens from tachyzoites which are costly and hazardous. An alternative method for better antigen production involving the prokaryotic expression system was therefore used in this study. Recombinant dense granular protein, GRA2, was successfully cloned, expressed, and purified in Escherichia coli, BL21 (DE3) pLysS. The potential of this purified antigen for diagnosis of human infections was evaluated through western blot analysis against 100 human serum samples. Results showed that the rGRA2 protein has 100 and 61.5 % sensitivity towards acute and chronic infection, respectively, in T. gondii-infected humans, indicating that this protein is useful in differentiating present and past infections. Therefore, it is suitable to be used as a sensitive and specific molecular marker for the serodiagnosis of Toxoplasma infection in both humans and animals.

Contribution of the residual body in the spatial organization of Toxoplasma gondii tachyzoites within the parasitophorous vacuole

Toxoplasma gondii proliferates and organizes within a parasitophorous vacuole in rosettes around a residual body and is surrounded by a membranous nanotubular network whose function remains unclear. Here, we characterized structure and function of the residual body in intracellular tachyzoites of the RH strain. Our data showed the residual body as a body limited by a membrane formed during proliferation of tachyzoites probably through the secretion of components and a pinching event of the membrane at the posterior end. It contributes in the intravacuolar parasite organization by the membrane connection between the tachyzoites posterior end and the residual body membrane to give place to the rosette conformation. Radial distribution of parasites in rosettes favors an efficient exteriorization. Absence of the network and presence of atypical residual bodies in a ΔGRA2-HXGPRT knock-out mutant affected the intravacuolar organization of tachyzoites and their exteriorization.

Evidence for host cells as the major contributor of lipids in the intravacuolar network of Toxoplasma-infected cells

The intracellular parasite Toxoplasma gondii develops inside a parasitophorous vacuole (PV) that derives from the host cell plasma membrane during invasion. Previous electron micrograph images have shown that the membrane of this vacuole undergoes an extraordinary remodeling with an extensive network of thin tubules and vesicles, the intravacuolar network (IVN), which fills the lumen of the PV. While dense granule proteins, secreted during and after invasion, are the main factors for the organization and tubulation of the network, little is known about the source of lipids used for this remodeling. By selectively labeling host cell or parasite membranes, we uncovered evidence that strongly supports the host cell as the primary, if not exclusive, source of lipids for parasite IVN remodeling. Fluorescence recovery after photobleaching (FRAP) microscopy experiments revealed that lipids are surprisingly dynamic within the parasitophorous vacuole and are continuously exchanged or replenished by the host cell. The results presented here suggest a new model for development of the parasitophorous vacuole whereby the host provides a continuous stream of lipids to support the growth and maturation of the PVM and IVN.

Type II Toxoplasma gondii KU80 knockout strains enable functional analysis of genes required for cyst development and latent infection

Type II Toxoplasma gondii KU80 knockouts (Δku80) deficient in nonhomologous end joining were developed to delete the dominant pathway mediating random integration of targeting episomes. Gene targeting frequency in the type II Δku80 Δhxgprt strain measured at the orotate (OPRT) and the uracil (UPRT) phosphoribosyltransferase loci was highly efficient. To assess the potential of the type II Δku80 Δhxgprt strain to examine gene function affecting cyst biology and latent stages of infection, we targeted the deletion of four parasite antigen genes (GRA4, GRA6, ROP7, and tgd057) that encode characterized CD8(+) T cell epitopes that elicit corresponding antigen-specific CD8(+) T cell populations associated with control of infection. Cyst development in these type II mutant strains was not found to be strictly dependent on antigen-specific CD8(+) T cell host responses. In contrast, a significant biological role was revealed for the dense granule proteins GRA4 and GRA6 in cyst development since brain tissue cyst burdens were drastically reduced specifically in mutant strains with GRA4 and/or GRA6 deleted. Complementation of the Δgra4 and Δgra6 mutant strains using a functional allele of the deleted GRA coding region placed under the control of the endogenous UPRT locus was found to significantly restore brain cyst burdens. These results reveal that GRA proteins play a functional role in establishing cyst burdens and latent infection. Collectively, our results suggest that a type II Δku80 Δhxgprt genetic background enables a higher-throughput functional analysis of the parasite genome to reveal fundamental aspects of parasite biology controlling virulence, pathogenesis, and transmission.

Post-translational membrane sorting of the Toxoplasma gondii GRA6 protein into the parasite-containing vacuole is driven by its N-terminal domain

How eukaryotic pathogens export and sort membrane-bound proteins destined for host-cell compartments is still poorly understood. The dense granules of the intracellular protozoan Toxoplasma gondii constitute an unusual secretory pathway that allows soluble export of the GRA proteins which become membrane-associated within the parasite replicative vacuole. This process relies on both the segregation of the proteins routed to the dense granules from those destined to the parasite plasma membrane and on the sorting of the secreted GRA proteins to their proper final membranous system. Here, we provide evidence that the soluble trafficking of GRA6 to the dense granules relies on the N-terminal domain of the protein, which is sufficient to prevent GRA6 targeting to the parasite plasma membrane. We also show that the GRA6 N-terminal domain, possibly by interacting with negatively charged lipids, is fundamental for proper GRA6 association with the vacuolar membranous network of nanotubes. These results support our emerging model: sorting of transmembrane GRA proteins to the host cell vacuole is mainly driven by the dual role of their N-terminal hydrophilic domain and is compartmentally regulated.

GRA proteins of Toxoplasma gondii: maintenance of host-parasite interactions across the parasitophorous vacuolar membrane

The dense granule of Toxoplasma gondii is a secretory vesicular organelle of which the proteins participate in the modification of the parasitophorous vacuole (PV) and PV membrane for the maintenance of intracellular parasitism in almost all nucleated host cells. In this review, the archives on the research of GRA proteins are reviewed on the foci of finding GRA proteins, characterizing molecular aspects, usefulness in diagnostic antigen, and vaccine trials in addition to some functions in host-parasite interactions.

Dynamics of T cell, antigen-presenting cell, and pathogen interactions during recall responses in the lymph node

Memory T cells circulate through lymph nodes where they are poised to respond rapidly upon re-exposure to a pathogen; however, the dynamics of memory T cell, antigen-presenting cell, and pathogen interactions during recall responses are largely unknown. We used a mouse model of infection with the intracellular protozoan parasite, Toxoplasma gondii, in conjunction with two-photon microscopy, to address this question. After challenge, memory T cells migrated more rapidly than naive T cells, relocalized toward the subcapsular sinus (SCS) near invaded macrophages, and engaged in prolonged interactions with infected cells. Parasite invasion of T cells occurred by direct transfer of the parasite from the target cell into the T cell and corresponded to an antigen-specific increase in the rate of T cell invasion. Our results provide insight into cellular interactions during recall responses and suggest a mechanism of pathogen subversion of the immune response.

Selection of polymorphic peptides from GRA6 and GRA7 sequences of Toxoplasma gondii strains to be used in serotyping

The evaluation of Toxoplasma gondii isolates obtained from geographical environments other than Europe and North America revealed the existence of atypical strains that are not included in the three archetypal clonal lineages (lineages I, II, and III). GRA6 and GRA7 are polymorphic genes that have been used for the genotyping of Toxoplasma. The coding regions of GRA6 and GRA7 from 49 nonarchetypal strains were sequenced and compared with the sequences of type I, II, and III reference strains. Eighteen and 10 different amino acid sequences were found for GRA6 and GRA7, respectively. The polymorphisms found between the different sequences were analyzed, with the objective of defining peptides to be used for the serotyping of Toxoplasma infections. Two peptides specific for clonal lineages I and III (peptides GRA7I and GRA7III, respectively) were selected from the GRA7 locus. Three peptides specific for some atypical strains (peptides Am6, Af6, and Am7) were selected from both the GRA6 and the GRA7 loci. Serum samples from humans infected with Toxoplasma strains of known genotypes were serotyped with the selected peptides. Peptide GRA7III seems to be a good candidate for the serotyping of infections caused by type III strains. Peptide GRA7I had a very low sensitivity. Peptides Am6 and Af6 had low specificities, since they reacted with serum samples from patients infected with strains belonging to the three archetypal lineages. Although peptide Am7 was specific, it had low sensitivity.

Biogenesis of and activities at the Toxoplasma gondii parasitophorous vacuole membrane

Apicomplexan parasites like Toxoplasma gondii are distinctive in their utilization of para site encoded motor systems to invade cells. Invasion results in the establishment of the parasitophorous vacuole (PV) within the infected cell. Most apicomplexans complete their intracellular tenure within the infected cell in the PV that is demarcated from the host cytoplasm by the parasitophorous vacuole membrane (PVM). In this chapter I focus on the events surrounding the formation of the PVM and selected activities attributed to it. Its central role as the interface between the parasite and its immediate environment, the host cytoplasm, is validated by the diversity of functions attributed to it. While functions in structural organization, nutrient acquisitions and signaling have been defined their molecular bases remain largely unknown. Several recent studies and the decoding of the Toxoplasma genome have set the stage for a rapid expansion in our understanding of the role of the PVM in parasite biology. Toxoplasma gondii, like all apicomplexan parasites are obligate intracellular pathogens. This family of parasites utilize their own actin-myosin based motor systems to gain entry into susceptible cells establishing themselves, in some cases transiently (e.g., Theileria spp) in specialized vacuolar compartment, the parasitophorous vacuole (PV). The T. gondii PV is highly dynamic compartment defining the replication permissive niche for the parasite. The delimiting membrane defining the parasitophorous vacuole, the parasitophorous vacuole membrane or PVM is increasingly being recognized as a specialized "organelle" that in the context of the infected cell is extracorporeal to the parent organism, the parasite. A systematic study of this enigmatic organelle has been severely limited by several issues. Primary among these is the fact that it is formed only in the context of the infected cell thereby limiting the amount of material. Secondly, unlike other cellular organelles that can often be purified by conventional approaches, the PVM, cannot be purified away from host cell organelles (see below). In spite of these significant obstacles considerable progress has been made in recent years toward understanding the biogenesis of the PVM, identification of its protein complement and the characterization of activities within it. These studies demonstrate that the PVM, on its own and by virtue of its interactions with cellular components, plays critical functions in the structural integrity of the vacuole, nutrient acquisition and the manipulation of cellular functions. In addition it appears that the repertoire of activities at the PVM is likely to be plastic reflecting temporal changes associated with the replicative phase of parasite growth. Finally, the PVM likely forms the foundation for the cyst wall as the parasite differentiates in the establishment of latent infection. As the critical border crossing between the parasite and invaded cell the study of the PVM provides a fertile area for new investigation aided by the recent decoding of the Toxoplasma genome (available at wwww.ToxoDB.org) and the application of proteomic analyses to basic questions in parasite biology.