ROP18 is a key factor responsible for virulence difference between Toxoplasma gondii and Neospora caninum

A candidate virulence factor of Eimeria tenella (EtROP30) predicted by virulence enhancement of transgenic Toxoplasma gondii

Difficulties of in vitro culture and genetic manipulation of Eimeria tenella have hindered the screening of virulence factors in this parasite. In this study, the E. tenella rhoptry protein 30 (EtROP30) was expressed in Toxoplasma gondii (RH∆Ku80-EtROP30), and its effect on the proliferation and virulence of parasites was investigated. The results revealed that the expression of EtROP30 had no impact on the invasion and egress processes. However, the RH∆Ku80-EtROP30 strain formed larger plaques compared to the RH∆Ku80, indicating that the EtROP30 expression promotes T. gondii proliferation. Furthermore, the RH∆Ku80-EtROP30 strain exhibited greater pathogenicity, resulting in earlier mortality and shorter overall survival time compared to RH∆Ku80. These results imply that EtROP30 expression facilitates parasite intracellular proliferation and virulence in mice, suggesting that EtROP30 might be a candidate virulence factor of E. tenella.

Toxoplasma gondii Suppresses Th2-Induced by Trichinella spiralis Infection and Downregulates Serine Protease Genes Expression: A Critical Role in Vaccine Development

Background

Toxoplasma gondii coinfection can modify host immune responses and the severity and spread of other parasites. We investigated how T. gondii and Trichinella spiralis infections counter-regulate each other's immune responses.

Methods

The parasite burden, the expression of T. gondii rhoptry kinase ROP18 and T. spiralis putative serine protease (TsSP), the IgG1 and IgG2a responses, besides histopathological and immunohistochemical staining with iNOS and arginase were used to evaluate the dynamics of coinfection.

Results

Through their effects on host immune responsiveness, coinfection with T. gondii modified the virulence of T. spiralis infection. Coinfected animals with high and low doses of T. gondii demonstrated significant reductions in the T. spiralis burden of 75.2% and 68.2%, respectively. TsSP expression was downregulated in both groups by 96.2% and 86.7%, whereasROP18 expression was downregulated by only 6% and10.6%, respectively. In coinfected mice, elevated levels of T. gondii-specific IgG2a antibodies were detected. Th1 induced by T. gondii inhibits the Th2 response to T. spiralis in coinfected animals with high iNOS expression andlow-arginine1 expression.

Conclusion

T. gondii infection induces a shift toward a Th1-type immune response while suppressing a helminth-specific Th2 immune response, paving the way for developing novel vaccines and more efficient control strategies.

Efficient Gene Knockout and Knockdown Systems in Neospora caninum Enable Rapid Discovery and Functional Assessment of Novel Proteins

The development of molecular genetics has greatly enhanced the study of the biology and pathology associated with parasites of the phylum Apicomplexa. While the molecular tools are highly developed for the apicomplexan Toxoplasma gondii, the closely related parasite Neospora caninum lacks efficient tools for genetic manipulation. To enable efficient homologous recombination in N. caninum, we targeted the Ku heterodimer DNA repair mechanism in the genomic reference strain, Nc-Liverpool (NcLiv), and show that deletion of Ku80 results in a destabilization and loss of its partner Ku70. Disruption of Ku80 generated parasites in which genes are efficiently epitope tagged and only short homology regions are required for gene knockouts. We used this improved strain to target novel nonessential genes encoding dense granule proteins that are unique to N. caninum or conserved in T. gondii. To expand the utility of this strain for essential genes, we developed the auxin-inducible degron system for N. caninum using parasite-specific promoters. As a proof of concept, we knocked down a novel nuclear factor in both N. caninum and T. gondii and showed that it is essential for survival of both parasites. Together, these efficient knockout and knockdown technologies will enable the field to unravel specific gene functions in N. caninum, which is likely to aid in the identification of targets responsible for the phenotypic differences observed between these two closely related apicomplexan parasites.

IMPORTANCENeospora caninum is a parasite with veterinary relevance, inducing severe disease in dogs and reproductive disorders in ruminants, especially cattle, leading to major losses. The close phylogenetic relationship to Toxoplasma gondii and the lack of pathogenicity in humans drives an interest of the scientific community toward using N. caninum as a model to study the pathogenicity of T. gondii. To enable this comparison, it is important to develop efficient molecular tools for N. caninum, to gain accuracy and save time in genetic manipulation protocols. Here, we have developed base strains and protocols using the genomic reference strain of N. caninum to enable efficient knockout and knockdown assays in this model. We demonstrate that these tools are effective in targeting known and previously unexplored genes. Thus, these tools will greatly improve the study of this protozoan, as well as enhance its ability to serve as a model to understand other apicomplexan parasites.

Toxoplasma gondii virulence prediction using hierarchical cluster analysis based on coding sequences (CDS) of sag1, gra7 and rop18

Toxoplasma gondii consists of three genotypes, namely genotype I, II and III. Based on its virulence, T. gondii can be divided into virulent and avirulent strains. This study intends to evaluate an alternative method for predicting T. gondii virulence using hierarchical cluster analysis based on complete coding sequences (CDS) of sag1, gra7 and rop18 genes. Dendrogram was constructed using UPGMA with a Kimura 80 nucleotide distance measurement. The results showed that the prediction errors of T. gondii virulence using sag1, gra7 and rop18 were 7.41%, 6.89% and 9.1%, respectively. Analysis based on CDS of gra7 and rop18 was able to differentiate avirulent strains into genotypes II and III, whereas sag1 failed to differentiate.

Biotinylation of the Neospora caninum parasitophorous vacuole reveals novel dense granule proteins

Background

Neospora caninum is an obligate intracellular parasite that invades host cells and replicates within the parasitophorous vacuole (PV), which resists fusion with host cell lysosomal compartments. To modify the PV, the parasite secretes an array of proteins, including dense granule proteins (GRAs). The vital role of GRAs in the Neospora life cycle cannot be overestimated. Despite this important role, only a subset of these proteins have been identified, and most of their functions have not been elucidated. Our previous study demonstrated that NcGRA17 is specifically targeted to the delimiting membrane of the parasitophorous vacuole membrane (PVM). In this study, we utilize proximity-dependent biotin identification (BioID) to identify novel components of the dense granules.

Methods

NcGRA17 was BirA* epitope-tagged in the Nc1 strain utilizing the CRISPR/Cas9 system to create a fusion of NcGRA17 with the biotin ligase BirA*. The biotinylated proteins were affinity-purified for mass spectrometric analysis, and the candidate GRA proteins from BioID data set were identified by gene tagging. To verify the biological role of novel identified GRA proteins, we constructed the NcGRA23 and NcGRA11 (a–e) knockout strains using the CRISPR/Cas9 system and analyzed the phenotypes of these mutants.

Results

Using NcGRA17-BirA* fusion protein as bait, we have identified some known GRAs and verified localization of 11 novel GRA proteins by gene endogenous tagging or overexpression in the Nc1 strain. We proceeded to functionally characterize NcGRA23 and NcGRA11 (a–e) by gene knockout. The lack of NcGRA23 or NcGRA11 (a–e) did not affect the parasite propagation in vitro and virulence in vivo.

Conclusions

In summary, our findings reveal that BioID is effective in discovering novel constituents of N. caninum dense granules. The exact biological functions of the novel GRA proteins are yet unknown, but this could be explored in future studies.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05023-7.

The Host Autophagy During Toxoplasma Infection

Autophagy is an important homeostatic mechanism, in which lysosomes degrade and recycle cytosolic components. As a key defense mechanism against infections, autophagy is involved in the capture and elimination of intracellular parasites. However, intracellular parasites, such as Toxoplasma gondii, have developed several evasion mechanisms to manipulate the host cell autophagy for their growth and establish a chronic infection. This review provides an insight into the autophagy mechanism used by the host cells in the control of T. gondii and the host exploitation by the parasite. First, we summarize the mechanism of autophagy, xenophagy, and LC3-associated phagocytosis. Then, we illustrate the process of autophagy proteins-mediated T. gondii clearance. Furthermore, we discuss how the parasite blocks and exploits this process for its survival.

Function of Neospora caninum dense granule protein 7 in innate immunity in mice

The intracellular parasite Neospora caninum can parasitize all nucleated cells of the host. Dense granule proteins (GRAs) secreted by dense granules are an important material involved in the formation of parasitophorous vacuoles (PVs), which facilitate parasite survival and replication in host cells. Due to the secretory and immune properties of NcGRA7, it is considered to be a promising serodiagnosis marker and an effective neosporosis vaccine candidate. However, the intracellular regulatory mechanisms involved in NcGRA7-induced host responses have rarely been examined. Here, we used the CRISPR/Cas9 genome editing system to obtain a NcGRA7 knockout strain (ΔNcGRA7) and a NcGRA7 complementary strain (iΔNcGRA7) to study their function. We found that ΔNcGRA7 exhibited slower growth in vitro and weakened virulence in mice compared with Nc1 and iΔNcGRA7. All parasite strains can stimulate host immune cells to produce IFN-γ, and the amount of IFN-γ production stimulated by Nc1 was significantly higher than that stimulated by ΔNcGRA7. The transcription levels of the cellular immune factors GBP1, GBP2, IRGa6, and IRGb6 were significantly higher after stimulation with ΔNcGRA7 parasites than after stimulation with Nc1. Furthermore, ΔNcGRA7 infection resulted in greater IRGa6 recruitment to the PVM than Nc1 infection. ΔNcGRA7 parasites were more easily cleared by macrophages than Nc1 parasites. Collectively, these results showed that NcGRA7 plays an important role in regulating the immune factors of mice and the aggregation of IRGa6 at the PVM, which affects the pathogenicity of N. caninum.

From Signaling Pathways to Distinct Immune Responses: Key Factors for Establishing or Combating Neospora caninum Infection in Different Susceptible Hosts

: Neospora caninum is an intracellular protozoan parasite affecting numerous animal species. It induces significant economic losses because of abortion and neonatal abnormalities in cattle. In case of infection, the parasite secretes numerous arsenals to establish a successful infection in the host cell. In the same context but for a different purpose, the host resorts to different strategies to eliminate the invading parasite. During this battle, numerous key factors from both parasite and host sides are produced and interact for the maintaining and vanishing of the infection, respectively. Although several reviews have highlighted the role of different compartments of the immune system against N. caninum infection, each one of them has mostly targeted specific points related to the immune component and animal host. Thus, in the current review, we will focus on effector molecules derived from the host cell or the parasite using a comprehensive survey method from previous reports. According to our knowledge, this is the first review that highlights and discusses immune response at the host cell-parasite molecular interface against N. caninum infection in different susceptible hosts.

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.

Detecting sequence variants in clinically important protozoan parasites

Second and third generation sequencing methods are crucial for population genetic studies, and variant detection is a popular approach for exploiting this sequence data. While mini- and microsatellites are historically useful markers for studying important Protozoa such as Toxoplasma and Plasmodium spp., detecting non-repetitive variants such as those found in genes can be fundamental to investigating a pathogen's biology. These variants, namely single nucleotide polymorphisms and insertions and deletions, can help elucidate the genetic basis of an organism's pathogenicity, identify selective pressures, and resolve phylogenetic relationships. They also have the added benefit of possessing a comparatively low mutation rate, which contributes to their stability. However, there is a plethora of variant analysis tools with nuanced pipelines and conflicting recommendations for best practise, which can be confounding. This lack of standardisation means that variant analysis requires careful parameter optimisation, an understanding of its limitations, and the availability of high quality data. This review explores the value of variant detection when applied to non-model organisms such as clinically important protozoan pathogens. The limitations of current methods are discussed, including special considerations that require the end-users' attention to ensure that the results generated are reproducible, and the biological conclusions drawn are valid.

Identification and characterization of stearoyl-CoA desaturase in Toxoplasma gondii

Few information of the function of stearoyl-coenzyme A (CoA) desaturase (SCD) in apicomplaxan parasite has been obtained. In this study, we retrieved a putative fatty acyl-CoA desaturase (TGGT1_238950) by a protein alignment with Plasmodium falciparum SCD in ToxoDB. A typical Δ9-desaturase domain was revealed in this protein. The putative desaturase was tagged with HA endogenously in Toxoplasma gondii, and the endoplasmic reticulum localization of the putative desaturase was revealed, which was consistent with the fatty acid desaturases in other organisms. Therefore, the TGGT1_238950 was designated T. gondii SCD. Based on CRISPR/Cas9 gene editing technology, SCD conditional knockout mutants in the T. gondii TATi strain were obtained. The growth in vitro and pathogenicity in mice of the mutants suggested that SCD might be dispensable for tachyzoite growth and proliferation. The SCD-overexpressing line was constructed to further explore SCD function. The portion of palmitoleic acid and oleic acid were increased in SCD-overexpressing parasites, compared with the RH parental strain, indicating that T. gondii indeed is competent for unsaturated fatty acid synthesis. The SCD-overexpressing tachyzoites propagated slower than the parental strain, with a decreased invasion capability and weaker pathogenicity in mice. The TgIF2α phosphorylation and the expression changes of several genes demonstrated that ER stress was triggered in the SCD-overexpressing parasites, which were more apt toward autophagy and apoptosis. The function of unsaturated fatty acid synthesis of TgSCD was consistent with our hypothesis. On the other hand, SCD might also be involved in tachyzoite autophagy and apoptosis.

Genome Wide Identification of Mutational Hotspots in the Apicomplexan Parasite Neospora caninum and the Implications for Virulence

Neospora caninum is an apicomplexan parasite responsible for neosporosis, a disease causing hind limb paralysis in dogs and abortion in cattle, resulting in substantial economic losses to beef and dairy industries. Marked differences in pathogenicity exist between N. caninum strains suggesting that intrinsic genetic differences exist between them. These differences likely exist in genes expressed during the tachyzoite lifecycle stage which is responsible for the pathogenesis of neosporosis. An improved understanding of these genetic differences is essential to understanding N. caninum virulence, though such knowledge is scarce. Using a variant detection workflow we compared the tachyzoite transcriptomes of two N. caninum strains with different virulence properties: NC-Liverpool (virulent) and NC-Nowra (avirulent). This workflow identified 3130 SNPs and 6123 indels between the strains, and nine markers capturing 30 variants were Sanger sequenced for both strains. Sequencing of these loci was extended to an additional eight strains and subsequent phylogenetic analysis supported a genetic population structure comprised of two major clades with no geographical segregation. Sequence polymorphisms within coding regions of tachyzoite-associated genes were concentrated on chromosomes XI and XII, with 19 distinct tachyzoite-associated SNP hotspot regions identified within coding regions of the N. caninum nuclear genome. The variants were predominantly located in loci associated with protein binding, protein-protein interactions, transcription, and translation. Furthermore, 468 nonsynonymous SNPs identified within protein-coding genes were associated with protein kinase activity, protein binding, protein phosphorylation, and proteolysis. This work may implicate these processes and the specific proteins involved as novel effectors of N. caninum tachyzoite virulence.

NcGRA17 is an important regulator of parasitophorous vacuole morphology and pathogenicity of Neospora caninum

Neospora caninum is an obligate intracellular protozoan parasite that infects a wide range of mammalian species, and particularly causes the reproductive loss in cattle. We identified a novel dense granule protein, N. caninum granule protein 17 (NcGRA17) using the CRISPR/cas9 genome editing system and studied its function. We generated the NcGRA17 knockout strain (ΔNcGRA17) and NcGRA17 complementary strain (iΔNcGRA17). Plaque assays and intracellular proliferation tests showed that the ΔNcGRA17 strain formed smaller plaques and had slower intracellular growth. Mouse virulence assay showed loss of virulence for the ΔNcGRA17 strain. We observed that the parasitophorous vacuoles (PVs) of NcGRA17-deficient parasites have aberrant morphology. To investigate the contribution of NcGRA17 α-helices to aberrant morphology of PVs, we transfected four truncated forms of NcGRA17 into NcGRA17 knockout strain and the phenotypes of these mutants were analysed. Lack of the N-terminal region (NT) failed to target the protein to dense granules, while NcGRA17 (Δα1)-HA, NcGRA17 (Δα2-4)-HA and NcGRA17 (Δα5-8)-HA were targeted to dense granules, but failed to rescue the aberrant PV morphology. Our results indicate that NcGRA17 as a dense granule protein determines PV morphology and pathogenicity, and α-helices of NcGRA17 may be responsible for the aberrant morphology of N. caninum PVs.

Characterization of novel microneme adhesive repeats (MAR) in Eimeria tenella

Background

The phylum Apicomplexa comprises a wide variety of parasites of significant medical and economic relevance. These parasites have extremely different host and tissue tropisms; for example Toxoplasma gondii can invade virtually any nucleated cell and infect almost all warm-blooded vertebrates, whereas Eimeria tenella infects only chickens and is restricted in its growth to epithelial cells of the caecum. Proteins released from the microneme secretory organelles (MICs) are critical for apicomplexan invasion of host cells and allow parasites to bind a diverse range of host cell oligosaccharide epitopes. MICs bear modular arrangements of sequences with adhesive proteins and interestingly the sialic-acid binding MAR (microneme adhesive repeat) domain containing proteins (MCPs) are suggested to make significant contributions to the different host and tissue tropisms of T. gondii and E. tenella.

Results

In this study, we evaluated the binding capacity of Type I MAR domains from novel E. tenella MCPs. Variants of the previously described HxT motif were analysed showing that HxT and VxT variants bind, whereas HxS and YxE variants did not. One of these MCP containing a single MAR (EtMCP2) showed an apical localization when expressed as a fusion with the fluorescent reporter mCherry in transgenic populations and a similar pattern of transcripts per zoite during endogenous development in vitro as the well-characterised microneme protein EtMIC2.

Conclusions

Variation in the binding properties of the MAR of different EtMCPs was confirmed and their ability to bind a wider range of sialic acids and terminal linkages should be studied. In addition, transgenesis technology has been used for first time in Eimeria parasites as a rapid tool for the study of endogenous protein localization by fusion with a fluorescent reporter.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2454-4) contains supplementary material, which is available to authorized users.

New molecular tools in Neospora caninum for studying apicomplexan parasite proteins

The development of molecular genetics has greatly enhanced the study of the biology and pathology associated with parasites of the phylum Apicomplexa. We have established a system specifically designed for Neospora caninum, and used this system as a heterologous platform for the expression of foreign genes. Plasmid constructs containing fluorescent proteins or targeted genes of Toxoplasma gondii, driven by N. caninum promoters, have yielded robust expression and correct trafficking of target gene products as assessed by immunofluorescence assays and Western blot analyses. Using this approach, we here demonstrated that N. caninum expressing T. gondii’s GRA15 and ROP16 kinase are biologically active and induced immunological phenotypes consistent with T. gondii strains. N. caninum expressing TgGRA15 differentially disturbed the NF-κB pathway, inducing an increased IL-12 production. On the other hand, N. caninum expressing TgROP16 induced host STAT3 phosphorylation and consequent reduction of IL-12 synthesis. These results indicate that heterologous gene expression in N. caninum is a useful tool for the study of specific gene functions and may allow the identification of antigenic targets responsible for the phenotypic differences observed between these two closely related apicomplexan parasites. Additionally, these observations may prove to be useful for the development of vaccine protocols to control toxoplasmosis and/or neosporosis.

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.

Rhoptry protein 5 (ROP5) Is a Key Virulence Factor in Neospora caninum

Neospora caninum, of the Apicomplexa phylum, is a common cause of abortions in cattle and nervous system dysfunction in dogs. Rhoptry proteins of Apicomplexa play an important role in virulence. The objectives of this study were to study functions of NcROP5 in N. caninum by deleting the NcROP5 gene from the wild Nc-1 strain. We selected NcROP5 in ToxoDB and successfully constructed an NcROP5 gene-deleted vector, pTCR-NcROP5-CD KO. Then we screened the NcROP5 knockout strains (ΔNcROP5) at the gene, protein and transcription levels. Plaque assay, host cell invasion assay and intracellular proliferation test showed that the ΔNcROP5 strain had less plaque space, weakened invasion capacity and slower intracellular growth. Animal testing showed significantly lower cerebral load of ΔNcROP5 than the load of the Nc-1 strain, as well as a loss of virulence for the ΔNcROP5 strains. Phenotypic analyses using the label-free LC-MS/MS assay-based proteomic method and KEGG pathway enrichment analysis showed a reduction of NcGRA7 transcription and altered expression of multiple proteins including the apicomplexan family of binding proteins. The present study indicated that ROP5 is a key virulence factor in N. caninum in mice. The proteomic profiling of Nc-1 and ΔNcROP5 provided some data on differential proteins. These data provide a foundation for future research of protein functions in N. caninum.

A new microneme protein of Neospora caninum, NcMIC8 is involved in host cell invasion

Microneme proteins play an important role in the invasion process of Apicomplexan parasites through adhesion to host cells. We discovered a new N. caninum protein, NcMIC8, which is highly identical to TgMIC8. The NcMIC8 sequence has 2049 bp and no intron in the open reading fragment. It has a molecular weight of 73.8 kDa and contains a signal peptide, a transmembrane region, a low complexity region and 10 epidermal growth factor (EGF) domains. Immuno-fluorescence assay showed that NcMIC8 is located in the microneme. NcMIC8 was secreted to culture medium under stimulation of 1% ethanol, and cleaved to form the mature body of 40 kDa before transporting to microneme or during secretion. Blocking NcMIC8 using anti-NcMIC8 serum effectively inhibited host cell invasion by tachyzoites in vitro. NcMIC8 in the form of mature body interacts with NcMIC3, and the two microneme proteins form a complex probably during transportation. NcMIC8 is a new microneme protein of N. caninum and could be an attractive target for the control of neosporosis.

Neospora caninum Activates p38 MAPK as an Evasion Mechanism against Innate Immunity

Due to the high prevalence and economic impact of neosporosis, the development of safe and effective vaccines and therapies against this parasite has been a priority in the field and is crucial to limit horizontal and vertical transmission in natural hosts. Limited data is available regarding factors that regulate the immune response against this parasite and such knowledge is essential in order to understand Neospora caninum induced pathogenesis. Mitogen-activated protein kinases (MAPKs) govern diverse cellular processes, including growth, differentiation, apoptosis, and immune-mediated responses. In that sense, our goal was to understand the role of MAPKs during the infection by N. caninum. We found that p38 phosphorylation was quickly triggered in macrophages stimulated by live tachyzoites and antigen extracts, while its chemical inhibition resulted in upregulation of IL-12p40 production and augmented B7/MHC expression. In vivo blockade of p38 resulted in an amplified production of cytokines, which preceded a reduction in latent parasite burden and enhanced survival against the infection. Additionally, the experiments indicate that the p38 activation is induced by a mechanism that depends on GPCR, PI3K and AKT signaling pathways, and that the phenomena here observed is distinct that those induced by Toxoplasma gondii’s GRA24 protein. Altogether, these results showed that N. caninum manipulates p38 phosphorylation in its favor, in order to downregulate the host’s innate immune responses. Additionally, those results infer that active interference in this signaling pathway may be useful for the development of a new therapeutic strategy against neosporosis.

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.

T. gondii rhoptry protein ROP18 induces apoptosis of neural cells via endoplasmic reticulum stress pathway

Background

The neurotropic parasite T. gondii is widespread among mammalian hosts including humans. During the course of T. gondii infection, the central nervous system is the most commonly damaged of all invasive organs. The polymorphic rhoptry protein ROP18 has been identified as a key factor in the pathogenesis of T. gondii; however, the molecular mechanism by which this protein exerts neuropathogenesis remains elusive.

Methods

Immunofluorescence staining was performed to detect neuropathogenesis of the mouse brain tissues. The apoptosis of neural cells and the expressions of related proteins in the endoplasmic reticulum stress (ER Stress)-mediated apoptosis pathway were detected by flow cytometry and Western blotting.

Results

Immunofluorescence staining reveals induction of the propidium iodide (PI) - positive neural cells in mouse cerebral cortex and hippocampus infected with ROP18 over-expressing transgenic tachyzoites. Western blotting analyses reveal that ROP18 increases the expressions of cleaved caspase-12, CHOP and cleaved caspase-3 when compared to the control groups. After the pretreatment of Z-ATAD-FMK (a specific caspase-12 inhibitor), the apoptotic level of neural cells had an apparent decline, and correspondingly, the expressions of those related proteins were notably decreased.

Conclusions

Our findings here highlight that the virulence factor ROP18 in T. gondii may contribute to neuronal apoptosis through the ER stress-mediated apoptosis pathway, which may be a potential molecular mechanism responsible for neurological disorders of toxoplasmosis.

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

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

Neospora caninum Recruits Host Cell Structures to Its Parasitophorous Vacuole and Salvages Lipids from Organelles

Toxoplasma gondii and Neospora caninum, which cause the diseases toxoplasmosis and neosporosis, respectively, are two closely related apicomplexan parasites. They have similar heteroxenous life cycles and conserved genomes and share many metabolic features. Despite these similarities, T. gondii and N. caninum differ in their transmission strategies and zoonotic potential. Comparative analyses of the two parasites are important to identify the unique biological features that underlie the basis of host preference and pathogenicity. T. gondii and N. caninum are obligate intravacuolar parasites; in contrast to T. gondii, events that occur during N. caninum infection remain largely uncharacterized. We examined the capability of N. caninum (Liverpool isolate) to interact with host organelles and scavenge nutrients in comparison to that of T. gondii (RH strain). N. caninum reorganizes the host microtubular cytoskeleton and attracts endoplasmic reticulum (ER), mitochondria, lysosomes, multivesicular bodies, and Golgi vesicles to its vacuole though with some notable differences from T. gondii. For example, the host ER gathers around the N. caninum parasitophorous vacuole (PV) but does not physically associate with the vacuolar membrane; the host Golgi apparatus surrounds the N. caninum PV but does not fragment into ministacks. N. caninum relies on plasma lipoproteins and scavenges cholesterol from NPC1-containing endocytic organelles. This parasite salvages sphingolipids from host Golgi Rab14 vesicles that it sequesters into its vacuole. Our data highlight a remarkable degree of conservation in the intracellular infection program of N. caninum and T. gondii. The minor differences between the two parasites related to the recruitment and rearrangement of host organelles around their vacuoles likely reflect divergent evolutionary paths.