Transcriptomic analysis of toxoplasma development reveals many novel functions and structures specific to sporozoites and oocysts

Genome-wide expression patterns of calcium-dependent protein kinases in Toxoplasma gondii

Background

Calcium-dependent protein kinases (CDPKs) are found in plants and some Apicomplexan parasites but not in animals or fungi. CDPKs have been shown to play important roles in various calcium-signaling pathways such as host cell invasion, egress and protein secretion in Toxoplasma gondii. The objectives of the present study were to examine the T. gondii CDPK genes expression patterns during different development stages and stress responses.

Methods

We carried out a comprehensive expression analysis of CDPK genes based on previously published microarray datasets, and we also used real time quantitative RT-PCR to study ten T. gondii CDPK genes expression patterns under acid, alkali, high temperature and low temperature conditions.

Results

Microarrays analysis indicated that some TgCDPK genes exhibited different expression levels in IFN-γ stimuli conditions or at different developmental stages, suggesting that CDPK genes may play different roles in these processes. Expression profiles under low temperature, high temperature, acid and alkaline indicated that most CDPK may be involved in regulating high temperature, acid and alkaline signaling pathways.

Conclusions

We present a genome-wide expression analysis of CDPK genes in T. gondii for the first time, and the mRNA levels change with abiotic and biotic stresses, suggesting their functional roles in these processes. These results will provide a solid basis for future functional studies of the CDPK gene family in T. gondii.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0917-z) contains supplementary material, which is available to authorized users.

RNA Seq analysis of the Eimeria tenella gametocyte transcriptome reveals clues about the molecular basis for sexual reproduction and oocyst biogenesis

Background

The protozoan Eimeria tenella is a common parasite of chickens, causing avian coccidiosis, a disease of on-going concern to agricultural industries. The high prevalence of E. tenella can be attributed to the resilient oocyst stage, which is transmitted between hosts in the environment. As in related Coccidia, development of the eimerian oocyst appears to be dependent on completion of the parasite’s sexual cycle. RNA Seq transcriptome profiling offers insights into the mechanisms governing the biology of E. tenella sexual stages (gametocytes) and the potential to identify targets for blocking parasite transmission.

Results

Comparisons between the sequenced transcriptomes of E. tenella gametocytes and two asexual developmental stages, merozoites and sporozoites, revealed upregulated gametocyte transcription of 863 genes. Many of these genes code for proteins involved in coccidian sexual biology, such as oocyst wall biosynthesis and fertilisation, and some of these were characterised in more depth. Thus, macrogametocyte-specific expression and localisation was confirmed for two proteins destined for incorporation into the oocyst wall, as well as for a subtilisin protease and an oxidoreductase. Homologues of an oocyst wall protein and oxidoreductase were found in the related coccidian, Toxoplasma gondii, and shown to be macrogametocyte-specific. In addition, a microgametocyte gamete fusion protein, EtHAP2, was discovered.

Conclusions

The need for novel vaccine candidates capable of controlling coccidiosis is rising and this panel of gametocyte targets represents an invaluable resource for development of future strategies to interrupt parasite transmission, not just in Eimeria but in other Coccidia, including Toxoplasma, where transmission blocking is a relatively unexplored strategy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1298-6) contains supplementary material, which is available to authorized users.

Asexual expansion of Toxoplasma gondii merozoites is distinct from tachyzoites and entails expression of non-overlapping gene families to attach, invade, and replicate within feline enterocytes

Background

The apicomplexan parasite Toxoplasma gondii is cosmopolitan in nature, largely as a result of its highly flexible life cycle. Felids are its only definitive hosts and a wide range of mammals and birds serve as intermediate hosts. The latent bradyzoite stage is orally infectious in all warm-blooded vertebrates and establishes chronic, transmissible infections. When bradyzoites are ingested by felids, they transform into merozoites in enterocytes and expand asexually as part of their coccidian life cycle. In all other intermediate hosts, however, bradyzoites differentiate exclusively to tachyzoites, and disseminate extraintestinally to many cell types. Both merozoites and tachyzoites undergo rapid asexual population expansion, yet possess different effector fates with respect to the cells and tissues they develop in and the subsequent stages they differentiate into.

Results

To determine whether merozoites utilize distinct suites of genes to attach, invade, and replicate within feline enterocytes, we performed comparative transcriptional profiling on purified tachyzoites and merozoites. We used high-throughput RNA-Seq to compare the merozoite and tachyzoite transcriptomes. 8323 genes were annotated with sequence reads across the two asexually replicating stages of the parasite life cycle. Metabolism was similar between the two replicating stages. However, significant stage-specific expression differences were measured, with 312 transcripts exclusive to merozoites versus 453 exclusive to tachyzoites. Genes coding for 177 predicted secreted proteins and 64 membrane- associated proteins were annotated as merozoite-specific. The vast majority of known dense-granule (GRA), microneme (MIC), and rhoptry (ROP) genes were not expressed in merozoites. In contrast, a large set of surface proteins (SRS) was expressed exclusively in merozoites.

Conclusions

The distinct expression profiles of merozoites and tachyzoites reveal significant additional complexity within the T. gondii life cycle, demonstrating that merozoites are distinct asexual dividing stages which are uniquely adapted to their niche and biological purpose.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1225-x) contains supplementary material, which is available to authorized users.

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.

Reassessment of the role of aromatic amino acid hydroxylases and the effect of infection by Toxoplasma gondii on host dopamine

Toxoplasma gondii infection has been described previously to cause infected mice to lose their fear of cat urine. This behavioral manipulation has been proposed to involve alterations of host dopamine pathways due to parasite-encoded aromatic amino acid hydroxylases. Here, we report successful knockout and complementation of the aromatic amino acid hydroxylase AAH2 gene, with no observable phenotype in parasite growth or differentiation in vitro and in vivo. Additionally, expression levels of the two aromatic amino acid hydroxylases were negligible both in tachyzoites and in bradyzoites. Finally, we were unable to confirm previously described effects of parasite infection on host dopamine either in vitro or in vivo, even when AAH2 was overexpressed using the BAG1 promoter. Together, these data indicate that AAH enzymes in the parasite do not cause global or regional alterations of dopamine in the host brain, although they may affect this pathway locally. Additionally, our findings suggest alternative roles for the AHH enzymes in T. gondii, since AAH1 is essential for growth in nondopaminergic cells.

Hammondia hammondi harbors functional orthologs of the host-modulating effectors GRA15 and ROP16 but is distinguished from Toxoplasma gondii by a unique transcriptional profile

Toxoplasma gondii and its nearest extant relative, Hammondia hammondi, are phenotypically distinct despite their remarkable similarity in gene content, synteny, and functionality. To begin to identify genetic differences that might drive distinct infection phenotypes of T. gondii and H. hammondi, in the present study we (i) determined whether two known host-interacting proteins, dense granule protein 15 (GRA15) and rhoptry protein 16 (ROP16), were functionally conserved in H. hammondi and (ii) performed the first comparative transcriptional analysis of H. hammondi and T. gondii sporulated oocysts. We found that GRA15 and ROP16 from H. hammondi (HhGRA15 and HhROP16) modulate the host NF-κB and STAT6 pathways, respectively, when expressed heterologously in T. gondii. We also found the transcriptomes of H. hammondi and T. gondii to be highly distinct. Consistent with the spontaneous conversion of H. hammondi tachyzoites into bradyzoites both in vitro and in vivo, H. hammondi high-abundance transcripts are enriched for genes that are of greater abundance in T. gondii bradyzoites. We also identified genes that are of high transcript abundance in H. hammondi but are poorly expressed in multiple T. gondii life stages, suggesting that these genes are uniquely expressed in H. hammondi. Taken together, these data confirm the functional conservation of known T. gondii virulence effectors in H. hammondi and point to transcriptional differences as a potential source of the phenotypic differences between these species.

Diagnostic value of a Rec-ELISA using Toxoplasma gondii recombinant SporoSAG, BAG1, and GRA1 proteins in murine models infected orally with tissue cysts and oocysts

Toxoplasma gondii causes congenital toxoplasmosis in newborns resulting with fetal anomalies. Determining the initiation time of infection is very important for pregnant women and current serological assays have drawbacks in distinguishing the recently acute toxoplasmosis. Diagnosis of recently acute infection may be improved by using stage specific antigens in serological assays. In the present study, the diagnostic value of sporozoite specific SporoSAG, bradyzoite specific BAG1 proteins and GRA1 protein expressed by all forms of the parasite have been evaluated ELISA using sera systematically collected from mice administered orally with tissue cyst and oocysts. The anti-SporoSAG IgM antibodies in sera obtained from mice infected with oocysts peaked significantly at days 1, 10, and 15 (P<0.01). The anti-BAG1 IgM antibodies in sera obtained from mice infected with tissue cysts peaked significantly at days 15, 40, and 120 (P<0.05). The anti-GRA1 IgM antibodies in sera obtained from mice infected with oocysts peaked significantly at days 2, 10, and 40 (P<0.01). The anti-GRA1 IgM antibodies in sera obtained from mice infected with tissue cysts peaked significantly only at day 40 (P<0.05). The anti-SporoSAG, anti-BAG1, and anti-GRA1 IgG titers of mice showed significant increases at day 40 (P<0.05) and decrement started for only anti-GRA1 IgG at day 120. The presence of anti-SporoSAG IgM and IgG antibodies can be interpreted as recently acute infection between days 10-40 because IgM decreases at day 40. Similarly, presence of anti-BAG1 IgM and absence of IgG can be evaluated as a recently acute infection that occurred 40 days before because IgG peaks at day 40. A peak in anti-GRA1 antibody level at first testing and reduction in consecutive sample can be considered as an infection approximately around day 40 or prior. Overall, recombinant SporoSAG, BAG1 and GRA1 proteins can be accepted as valuable diagnostic markers of recently acute toxoplasmosis.

Dual transcriptional profiling of mice and Toxoplasma gondii during acute and chronic infection

BACKGROUND

The obligate intracellular parasite Toxoplasma gondii establishes a life-long chronic infection within any warm-blooded host. After ingestion of an encysted parasite, T. gondii disseminates throughout the body as a rapidly replicating form during acute infection. Over time and after stimulation of the host immune response, T. gondii differentiates into a slow growing, cyst form that is the hallmark of chronic infection. Global transcriptome analysis of both host and parasite during the establishment of chronic T. gondii infection has not yet been performed. Here, we conducted a dual RNA-seq analysis of T. gondii and its rodent host to better understand host and parasite responses during acute and chronic infection.

RESULTS

We obtained nearly one billion paired-end RNA sequences from the forebrains of uninfected, acutely and chronically infected mice, then aligned them to the genomic reference files of both T. gondii and Mus musculus. Gene ontology (GO) analysis of the 100 most highly expressed T. gondii genes showed less than half were shared between acute and chronic infection. The majority of the highly expressed genes common in both acute and chronic infection were involved in transcription and translation, underscoring that parasites in both stages are actively synthesizing proteins. Similarly, most of the T. gondii genes highly expressed during chronic infection were involved in metabolic processes, again highlighting the activity of the cyst stage at 28 days post-infection. Comparative analyses of host genes using uninfected forebrain revealed over twice as many immune regulatory genes were more abundant during chronic infection compared to acute. This demonstrates the influence of parasite development on host gene transcription as well as the influence of the host environment on parasite gene transcription.

CONCLUSIONS

RNA-seq is a valuable tool to simultaneously analyze host and microbe transcriptomes. Our data shows that T. gondii is metabolically active and synthesizing proteins at 28 days post-infection and that a distinct subset of host genes associated with the immune response are more abundant specifically during chronic infection. These data suggest host and pathogen interplay is still present during chronic infection and provides novel T. gondii targets for future drug and vaccine development.

Gene Set Enrichment Analysis (GSEA) of Toxoplasma gondii expression datasets links cell cycle progression and the bradyzoite developmental program

Background

Large amounts of microarray expression data have been generated for the Apicomplexan parasite Toxoplasma gondii in an effort to identify genes critical for virulence or developmental transitions. However, researchers’ ability to analyze this data is limited by the large number of unannotated genes, including many that appear to be conserved hypothetical proteins restricted to Apicomplexa. Further, differential expression of individual genes is not always informative and often relies on investigators to draw big-picture inferences without the benefit of context. We hypothesized that customization of gene set enrichment analysis (GSEA) to T. gondii would enable us to rigorously test whether groups of genes serving a common biological function are co-regulated during the developmental transition to the latent bradyzoite form.

Results

Using publicly available T. gondii expression microarray data, we created Toxoplasma gene sets related to bradyzoite differentiation, oocyst sporulation, and the cell cycle. We supplemented these with lists of genes derived from community annotation efforts that identified contents of the parasite-specific organelles, rhoptries, micronemes, dense granules, and the apicoplast. Finally, we created gene sets based on metabolic pathways annotated in the KEGG database and Gene Ontology terms associated with gene annotations available at http://www.toxodb.org. These gene sets were used to perform GSEA analysis using two sets of published T. gondii expression data that characterized T. gondii stress response and differentiation to the latent bradyzoite form.

Conclusions

GSEA provides evidence that cell cycle regulation and bradyzoite differentiation are coupled. Δgcn5A mutants unable to induce bradyzoite-associated genes in response to alkaline stress have different patterns of cell cycle and bradyzoite gene expression from stressed wild-type parasites. Extracellular tachyzoites resemble a transitional state that differs in gene expression from both replicating intracellular tachyzoites and in vitro bradyzoites by expressing genes that are enriched in bradyzoites as well as genes that are associated with the G1 phase of the cell cycle. The gene sets we have created are readily modified to reflect ongoing research and will aid researchers’ ability to use a knowledge-based approach to data analysis facilitating the development of new insights into the intricate biology of Toxoplasma gondii.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-515) contains supplementary material, which is available to authorized users.

Transcript maturation in apicomplexan parasites

The complex life cycles of apicomplexan parasites are associated with dynamic changes of protein repertoire. In Toxoplasma gondii, global analysis of gene expression demonstrates that dynamic changes in mRNA levels unfold in a serial cascade during asexual replication and up to 50% of encoded genes are unequally expressed in development. Recent studies indicate transcription and mRNA processing have important roles in fulfilling the 'just-in-time' delivery of proteins to parasite growth and development. The prominence of post-transcriptional mechanisms in the Apicomplexa was demonstrated by mechanistic studies of the critical RNA-binding proteins and regulatory kinases. However, it is still early in our understanding of how transcription and post-transcriptional mechanisms are balanced to produce adequate numbers of specialized forms that is required to complete the parasite life cycle.

Toxoplasma gondii merozoite gene expression analysis with comparison to the life cycle discloses a unique expression state during enteric development

BACKGROUND

Considerable work has been carried out to understand the biology of tachyzoites and bradyzoites of Toxoplasma gondii in large part due to in vitro culture methods for these stages. However, culturing methods for stages that normally develop in the gut of the definitive felid host, including the merozoite and sexual stages, have not been developed hindering the ability to study a large portion of the parasite's life cycle. Here, we begin to unravel the molecular aspects of enteric stages by providing new data on merozoite stage gene expression.

RESULTS

To profile gene expression differences in enteric stages we harvested merozoites from the intestine of infected cats and hybridized mRNA to the Affymetrix Toxoplasma GeneChip. We analyzed the merozoite data in context of the life cycle by comparing it to previously published data for the oocyst, tachyzoite, and bradyzoite stages. Principal component analysis highlighted the unique profile of merozoites, placing them approximately half-way on a continuum between the tachyzoite/bradyzoite and oocyst samples. Prior studies have shown that antibodies to surface antigen one (SAG1) and many dense granule proteins do not label merozoites: our microarray data confirms that these genes were not expressed at this stage. Also, the expression for many rhoptry and microneme proteins was drastically reduced while the expression for many surface antigens was increased at the merozoite stage. Gene Ontology and KEGG analysis revealed that genes involved in transcription/translation and many metabolic pathways were upregulated at the merozoite stage, highlighting unique growth requirements of this stage. To functionally test these predictions, we demonstrated that an upstream promoter region of a merozoite specific gene was sufficient to control expression in merozoites in vivo.

CONCLUSIONS

Merozoites are the first developmental stage in the coccidian cycle that takes place within the gut of the definitive host. The data presented here describe the global gene expression profile of the merozoite stage and the creation of transgenic parasite strains that show stage-specific expression of reporter genes in the cat intestine. These data and reagents will be useful in unlocking how the parasite senses and responds to the felid gut environment to initiate enteric development.

GRA25 is a novel virulence factor of Toxoplasma gondii and influences the host immune response

The obligate intracellular parasite Toxoplasma gondii is able to infect a broad range of hosts and cell types due, in part, to the diverse arsenal of effectors it secretes into the host cell. Here, using genetic crosses between type II and type III Toxoplasma strains and quantitative trait locus (QTL) mapping of the changes they induce in macrophage gene expression, we identify a novel dense granule protein, GRA25. Encoded on chromosome IX, GRA25 is a phosphoprotein that is secreted outside the parasites and is found within the parasitophorous vacuole. In vitro experiments with a type II Δgra25 strain showed that macrophages infected with this strain secrete lower levels of CCL2 and CXCL1 than those infected with the wild-type or complemented control parasites. In vivo experiments showed that mice infected with a type II Δgra25 strain are able to survive an otherwise lethal dose of Toxoplasma tachyzoites and that complementation of the mutant with an ectopic copy of GRA25 largely rescues this phenotype. Interestingly, the type II and type III versions of GRA25 differ in endogenous expression levels; however, both are able to promote parasite expansion in vivo when expressed in a type II Δgra25 strain. These data establish GRA25 as a novel virulence factor and immune modulator.

A selective review of advances in coccidiosis research

Coccidiosis is a widespread and economically significant disease of livestock caused by protozoan parasites of the genus Eimeria. This disease is worldwide in occurrence and costs the animal agricultural industry many millions of dollars to control. In recent years, the modern tools of molecular biology, biochemistry, cell biology and immunology have been used to expand greatly our knowledge of these parasites and the disease they cause. Such studies are essential if we are to develop new means for the control of coccidiosis. In this chapter, selective aspects of the biology of these organisms, with emphasis on recent research in poultry, are reviewed. Topics considered include taxonomy, systematics, genetics, genomics, transcriptomics, proteomics, transfection, oocyst biogenesis, host cell invasion, immunobiology, diagnostics and control.

Differential locus expansion distinguishes Toxoplasmatinae species and closely related strains of Toxoplasma gondii

Toxoplasma gondii is a human obligate intracellular parasite that has infected over 20% of the world population and has a vast intermediate host range compared to those of its nearest relatives Neospora caninum and Hammondia hammondi. While these 3 species have highly syntenic genomes (80 to 99%), in this study we examined and compared species-specific structural variations, specifically at loci that have undergone local (i.e., tandem) duplication and expansion. To do so, we used genomic sequence coverage analysis to identify and curate T. gondii and N. caninum loci that have undergone duplication and expansion (expanded loci [ELs]). The 53 T. gondii ELs are significantly enriched for genes with predicted signal sequences and single-exon genes and genes that are developmentally regulated at the transcriptional level. We validated 24 T. gondii ELs using comparative genomic hybridization; these data suggested significant copy number variation at these loci. High-molecular-weight Southern blotting for 3 T. gondii ELs revealed that copy number varies across T. gondii lineages and also between members of the same clonal lineage. Using similar methods, we identified 64 N. caninum ELs which were significantly enriched genes belonging to the SAG-related surface (SRS) antigen family. Moreover, there is significantly less overlap (30%) between the expanded gene sets in T. gondii and N. caninum than would be predicted by overall genomic synteny (81%). Consistent with this finding, only 59% of queried T. gondii ELs are similarly duplicated/expanded in H. hammondi despite over 99% genomic synteny between these species.

Gene duplication, expansion, and diversification are a basis for phenotypic differences both within and between species. This study represents the first characterization of both the extent and degree of overlap in gene duplication and locus expansion across multiple apicomplexan parasite species. The most important finding of this study is that the locus duplications/expansions are quantitatively and qualitatively distinct, despite the high degree of genetic relatedness between the species. Given that these differential expansions are prominent species-specific genetic differences, they may also contribute to some of the more striking phenotypic differences between these species. More broadly, this work is important in providing further support for the idea that postspeciation selection events may have a dramatic impact on locus structure and copy number that overshadows selection on single-copy genes.

Toxoplasma gondii merozoite gene expression analysis with comparison to the life cycle discloses a unique expression state during enteric development

Background

Considerable work has been carried out to understand the biology of tachyzoites and bradyzoites of Toxoplasma gondii in large part due to in vitro culture methods for these stages. However, culturing methods for stages that normally develop in the gut of the definitive felid host, including the merozoite and sexual stages, have not been developed hindering the ability to study a large portion of the parasite’s life cycle. Here, we begin to unravel the molecular aspects of enteric stages by providing new data on merozoite stage gene expression.

Results

To profile gene expression differences in enteric stages we harvested merozoites from the intestine of infected cats and hybridized mRNA to the Affymetrix Toxoplasma GeneChip. We analyzed the merozoite data in context of the life cycle by comparing it to previously published data for the oocyst, tachyzoite, and bradyzoite stages. Principal component analysis highlighted the unique profile of merozoites, placing them approximately half-way on a continuum between the tachyzoite/bradyzoite and oocyst samples. Prior studies have shown that antibodies to surface antigen one (SAG1) and many dense granule proteins do not label merozoites: our microarray data confirms that these genes were not expressed at this stage. Also, the expression for many rhoptry and microneme proteins was drastically reduced while the expression for many surface antigens was increased at the merozoite stage. Gene Ontology and KEGG analysis revealed that genes involved in transcription/translation and many metabolic pathways were upregulated at the merozoite stage, highlighting unique growth requirements of this stage. To functionally test these predictions, we demonstrated that an upstream promoter region of a merozoite specific gene was sufficient to control expression in merozoites in vivo.

Conclusions

Merozoites are the first developmental stage in the coccidian cycle that takes place within the gut of the definitive host. The data presented here describe the global gene expression profile of the merozoite stage and the creation of transgenic parasite strains that show stage-specific expression of reporter genes in the cat intestine. These data and reagents will be useful in unlocking how the parasite senses and responds to the felid gut environment to initiate enteric development.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-350) contains supplementary material, which is available to authorized users.

Identification of three novel Toxoplasma gondii rhoptry proteins

The rhoptries are key secretory organelles from apicomplexan parasites that contain proteins involved in invasion and modulation of the host cell. Some rhoptry proteins are restricted to the posterior bulb (ROPs) and others to the anterior neck (RONs). As many rhoptry proteins have been shown to be key players in Toxoplasma invasion and virulence, it is important to identify, understand and characterise the biological function of the components of the rhoptries. In this report, we identified putative novel rhoptry genes by identifying Toxoplasma genes with similar cyclical expression profiles as known rhoptry protein encoding genes. Using this approach we identified two new rhoptry bulb (ROP47 and ROP48) and one new rhoptry neck protein (RON12). ROP47 is secreted and traffics to the host cell nucleus, RON12 was not detected at the moving junction during invasion. Deletion of ROP47 or ROP48 in a type II strain did not show major influence in in vitro growth or virulence in mice.

Sex andEimeria: a molecular perspective

Eimeriais a common genus of apicomplexan parasites that infect diverse vertebrates, most notably poultry, causing serious disease and economic loss. Like all apicomplexans, eimerians have a complex life cycle characterized by asexual divisions that amplify the parasite population in preparation for sexual reproduction. This can be divided into three events: gametocytogenesis, producing gametocytes from merozoites; gametogenesis, producing microgametes and macrogametes from gametocytes; and fertilization of macrogametes by microgametes, producing diploid zygotes with ensuing meiosis completing the sexual phase. Sexual development inEimeriadepends on the differential expression of stage-specific genes, rather than presence or absence of sex chromosomes. Thus, it involves the generation of specific structures and, implicitly, storage of proteins and regulation of protein expression in macrogametes, in preparation for fertilization. InEimeria, the formation of a unique, resilient structure, the oocyst wall, is essential for completion of the sexual phase and parasite transmission. In this review, we piece together the molecular events that underpin sexual reproduction inEimeriaand use additional details from analogous events inPlasmodiumto fill current knowledge gaps. The mechanisms governing sexual stage formation and subsequent fertilization may represent targets for counteracting parasite transmission.

Toxoplasma gondii sporozoites invade host cells using two novel paralogues of RON2 and AMA1

Toxoplasma gondii is an obligate intracellular parasite of the phylum Apicomplexa. The interaction of two well-studied proteins, Apical Membrane Antigen 1 (AMA1) and Rhoptry Neck protein 2 (RON2), has been shown to be critical for invasion by the asexual tachyzoite stage. Recently, two paralogues of these proteins, dubbed sporoAMA1 and sporoRON2 (or RON2L2), respectively, have been identified but not further characterized in proteomic and transcriptomic analyses of Toxoplasma sporozoites. Here, we show that sporoAMA1 and sporoRON2 localize to the apical region of sporozoites and that, in vitro, they interact specifically and exclusively, with no detectable interaction of sporoAMA1 with generic RON2 or sporoRON2 with generic AMA1. Structural studies of the interacting domains of sporoRON2 and sporoAMA1 indicate a novel pairing that is similar in overall form but distinct in detail from the previously described interaction of the generic pairing. Most notably, binding of sporoRON2 domain 3 to domains I/II of sporoAMA1 results in major alterations in the latter protein at the site of binding and allosterically in the membrane-proximal domain III of sporoAMA1 suggesting a possible role in signaling. Lastly, pretreatment of sporozoites with domain 3 of sporoRON2 substantially impedes their invasion into host cells while having no effect on tachyzoites, and vice versa for domain 3 of generic RON2 (which inhibits tachyzoite but not sporozoite invasion). These data indicate that sporozoites and tachyzoites each use a distinct pair of paralogous AMA1 and RON2 proteins for invasion into host cells, possibly due to the very different environment in which they each must function.

Mechanics of the Toxoplasma gondii oocyst wall

The ability of microorganisms to survive under extreme conditions is closely related to the physicochemical properties of their wall. In the ubiquitous protozoan parasite Toxoplasma gondii, the oocyst stage possesses a bilayered wall that protects the dormant but potentially infective parasites from harsh environmental conditions until their ingestion by the host. None of the common disinfectants are effective in killing the parasite because the oocyst wall acts as a primary barrier to physical and chemical attacks. Here, we address the structure and chemistry of the wall of the T. gondii oocyst by combining wall surface treatments, fluorescence imaging, EM, and measurements of its mechanical characteristics by using atomic force microscopy. Elasticity and indentation measurements indicated that the oocyst wall resembles common plastic materials, based on the Young moduli, E, evaluated by atomic force microscopy. Our study demonstrates that the inner layer is as robust as the bilayered wall itself. Besides wall mechanics, our results suggest important differences regarding the nonspecific adhesive properties of each layer. All together, these findings suggest a key biological role for the oocyst wall mechanics in maintaining the integrity of the T. gondii oocysts in the environment or after exposure to disinfectants, and therefore their potential infectivity to humans and animals.

High-throughput RNA sequencing profiles and transcriptional evidence of aerobic respiratory enzymes in sporulating oocysts and sporozoites of Eimeria tenella

Seven species of Eimeria are responsible for coccidiosis in chickens. Eimeria tenella is one of the most pathogenic parasites since it is associated with high mortality and great economic impact. The life cycle of the parasite includes development in the environment and in the intestinal tract. We conducted RNA sequencing using a next generation sequencer to obtain transcriptome information from the sporulating oocysts, and sporozoites. We collected 2.8 million 75 bp reads of a short-tag sequence, and 25,880 contigs were generated by the Oases assembler. A Blastx search of GenBank databases revealed that 7780 contigs (30.1%) had significant homology with deposited sequence data (E-value <1e-6); among these contigs, 6051 contigs were similar to those of Toxoplasma gondii while only 513 contigs (6.6%) were similar to those of E. tenella. After an orthological analysis conducted with the UniProt database of T. gondii, 6661 contigs were distributed within the categories of cellular components (1528 gene categories), biological processes (861 gene categories), and molecular functions (241 gene categories). The significantly matched contigs contained high numbers of enzymes associated with glycolysis, TCA, and the pentose-phosphate pathway. Most of the enzymes, measured by quantitative reverse transcription-PCR, were up-regulated in sporulating stage. These results suggest that the intracellular carbohydrate amylopectin could be used as an energy source for ATP production including glycolysis and the pentose-phosphate pathway, which generates NADPH and pentoses. Our data also suggest that Eimeria might possess a partial or similar pathway to the TCA cycle essential for aerobic respiration. Furthermore, the newly annotated and non-annotated contigs might contain E. tenella-specific or novel sequences.

Molecules to modeling: Toxoplasma gondii oocysts at the human-animal-environment interface

Environmental transmission of extremely resistant Toxoplasma gondii oocysts has resulted in infection of diverse species around the world, leading to severe disease and deaths in human and animal populations. This review explores T. gondii oocyst shedding, survival, and transmission, emphasizing the importance of linking laboratory and landscape from molecular characterization of oocysts to watershed-level models of oocyst loading and transport in terrestrial and aquatic systems. Building on discipline-specific studies, a One Health approach incorporating tools and perspectives from diverse fields and stakeholders has contributed to an advanced understanding of T. gondii and is addressing transmission at the rapidly changing human-animal-environment interface.

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

Background

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

Results

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

Conclusions

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

Bradyzoite pseudokinase 1 is crucial for efficient oral infectivity of the Toxoplasma gondii tissue cyst

The tissue cyst formed by the bradyzoite stage of Toxoplasma gondii is essential for persistent infection of the host and oral transmission. Bradyzoite pseudokinase 1 (BPK1) is a component of the cyst wall, but nothing has previously been known about its function. Here, we show that immunoprecipitation of BPK1 from in vitro bradyzoite cultures, 4 days postinfection, identifies at least four associating proteins: MAG1, MCP4, GRA8, and GRA9. To determine the role of BPK1, a strain of Toxoplasma was generated with the bpk1 locus deleted. This BPK1 knockout strain (Δbpk1) was investigated in vitro and in vivo. No defect was found in terms of in vitro cyst formation and no difference in pathogenesis or cyst burden 4 weeks postinfection (wpi) was detected after intraperitoneal (i.p.) infection with Δbpk1 tachyzoites, although the Δbpk1 cysts were significantly smaller than parental or BPK1-complemented strains at 8 wpi. Pepsin-acid treatment of 4 wpi in vivo cysts revealed that Δbpk1 parasites are significantly more sensitive to this treatment than the parental and complemented strains. Consistent with this, 4 wpi Δbpk1 cysts showed reduced ability to cause oral infection compared to the parental and complemented strains. Together, these data reveal that BPK1 plays a crucial role in the in vivo development and infectivity of Toxoplasma cysts.

Transcriptome analysis reveals unique metabolic features in the Cryptosporidium parvum Oocysts associated with environmental survival and stresses

Background

Cryptosporidium parvum is a globally distributed zoonotic parasite and an important opportunistic pathogen in immunocompromised patients. Little is known on the metabolic dynamics of the parasite, and study is hampered by the lack of molecular and genetic tools. Here we report the development of the first Agilent microarray for C. parvum (CpArray15K) that covers all predicted ORFs in the parasite genome. Global transcriptome analysis using CpArray15K coupled with real-time qRT-PCR uncovered a number of unique metabolic features in oocysts, the infectious and environmental stage of the parasite.

Results

Oocyst stage parasites were found to be highly active in protein synthesis, based on the high transcript levels of genes associated with ribosome biogenesis, transcription and translation. The proteasome and ubiquitin associated components were also highly active, implying that oocysts might employ protein degradation pathways to recycle amino acids in order to overcome the inability to synthesize amino acids de novo. Energy metabolism in oocysts was featured by the highest level of expression of lactate dehydrogenase (LDH) gene. We also studied parasite responses to UV-irradiation, and observed complex and dynamic regulations of gene expression. Notable changes included increased transcript levels of genes involved in DNA repair and intracellular trafficking. Among the stress-related genes, TCP-1 family members and some thioredoxin-associated genes appear to play more important roles in the recovery of UV-induced damages in the oocysts. Our observations also suggest that UV irradiation of oocysts results in increased activities in cytoskeletal rearrangement and intracellular membrane trafficking.

Conclusions

CpArray15K is the first microarray chip developed for C. parvum, which provides the Cryptosporidium research community a needed tool to study the parasite transcriptome and functional genomics. CpArray15K has been successfully used in profiling the gene expressions in the parasite oocysts as well as their responses to UV-irradiation. These observations shed light on how the parasite oocysts might adapt and respond to the hostile external environment and associated stress such as UV irradiation.

Identification of Toxoplasma gondii genes responsive to the host immune response during in vivo infection

Toxoplasma gondii is an obligate intracellular protozoa parasite that causes the disease toxoplasmosis. It resides within host cells in a parasitophorous vacuole distinct from the host cell endocytic system. T. gondii was used as a model to investigate how obligate intracellular parasites alter their gene expression in response to the host immune response during infection compared to growth in host cells in vitro. While bacterial pathogens clearly alter gene expression to adapt to the host environment during infection, the degree to which the external environment affects gene expression by obligate intracellular pathogens sequestered within host cells is less clear. The global transcriptome of T. gondii was analyzed in vivo in the presence and absence of the IFN-γ-dependent host innate immune response. The parasites' in vivo transcriptome was also compared to its transcriptome in vitro in fibroblast cells. Our results indicate that the parasite transcriptome is significantly altered during in vivo infection in the presence, but not absence, of IFN–γ-dependent immunity compared with fibroblasts infected in vitro. Many of the parasite genes increased in vivo appear to be common to an early general stress response by the parasite; surprisingly putative oocyst stage specific genes were also disproportionately increased during infection.

In vitro culture systems for the study of apicomplexan parasites in farm animals

In vitro culture systems represent powerful tools for the study of apicomplexan parasites such as Cryptosporidium, Eimeria, Sarcocystis, Neospora, Toxoplasma, Besnoitia, Babesia and Theileria, all with high relevance for farm animals. Proliferative stages of these parasites have been cultured in vitro employing a large variety of cell culture and explant approaches. For some, such as Cryptosporidium and Eimeria, the sexual development has been reproduced in cell cultures, while for others, animal experimentation is required to fulfill the life cycle. In vitro cultures have paved the way to exploit the basic biology of these organisms, and had a major impact on the development of tools for diagnostic purposes. With the aid of in vitro cultivation, studies on host-parasite interactions, on factors involved in innate resistance, stage conversion and differentiation, genetics and transfection technology, vaccine candidates and drug effectiveness could be carried out. The use of transgenic parasites has facilitated high-throughput screening of anti-microbial compounds that are active against the proliferative stages. Here, we review the basic features of cell culture-based in vitro systems for apicomplexan parasites that are relevant for farm animals, and discuss their applications with a focus on drug identification and studies of stage differentiation.