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

Transcriptomic Insights into the Developmental Dynamics of Eimeria acervulina: A Comparative Study of a Precocious Line and the Wild Type

Coccidiosis, a parasitic disease caused by single or multiple Eimeria species, leads to significant economic losses in the poultry industry. The Eimeria life cycle includes schizogony, gametogony, and sporogony. To investigate the dynamics of gene expression and regulatory networks during the development of Eimeria acervulina, we employed time-course transcriptomics to rigorously compare the gene expression patterns between a precocious line (PL) and the wild type (WT) of E. acervulina. The results revealed that the PL enters into gametogony 12 h earlier than the WT, and both the PL and WT exhibited distinct clustering patterns during the development phase. A weighted gene co-expression network analysis (WGCNA) identified genes specifically expressed at four distinct developmental stages, schizogony, gametogony, sporulated oocysts, and unsporulated oocysts, clarifying the key biological processes at each stage. This study used global transcriptome profiling to elucidate molecular variations throughout the E. acervulina life cycle, providing critical insights into molecular characterization and valuable resources for investigating other apicomplexan parasites of public health importance.

AP2XII-1 and AP2XI-2 Suppress Schizogony Gene Expression in Toxoplasma gondii

Toxoplasma gondii is an intracellular parasite that is important in medicine and veterinary science and undergoes distinct developmental transitions in its intermediate and definitive hosts. The switch between stages of T. gondii is meticulously regulated by a variety of factors. Previous studies have explored the role of the microrchidia (MORC) protein complex as a transcriptional suppressor of sexual commitment. By utilizing immunoprecipitation and mass spectrometry, constituents of this protein complex have been identified, including MORC, Histone Deacetylase 3 (HDAC3), and several ApiAP2 transcription factors. Conditional knockout of MORC or inhibition of HDAC3 results in upregulation of a set of genes associated with schizogony and sexual stages in T. gondii tachyzoites. Here, our focus extends to two primary ApiAP2s (AP2XII-1 and AP2XI-2), demonstrating their significant impact on the fitness of asexual tachyzoites and their target genes. Notably, the targeted disruption of AP2XII-1 and AP2XI-2 resulted in a profound alteration in merozoite-specific genes targeted by the MORC-HDAC3 complex. Additionally, considerable overlap was observed in downstream gene profiles between AP2XII-1 and AP2XI-2, with AP2XII-1 specifically binding to a subset of ApiAP2 transcription factors, including AP2XI-2. These findings reveal an intricate cascade of ApiAP2 regulatory networks involved in T. gondii schizogony development, orchestrated by AP2XII-1 and AP2XI-2. This study provides valuable insights into the transcriptional regulation of T. gondii growth and development, shedding light on the intricate life cycle of this parasitic pathogen.

The Toxoplasma gondii F-Box Protein L2 Functions as a Repressor of Stage Specific Gene Expression

Toxoplasma gondii is a foodborne pathogen that can cause severe and life-threatening infections in fetuses and immunocompromised patients. Felids are its only definitive hosts, and a wide range of animals, including humans, serve as intermediate hosts. When the transmissible bradyzoite stage is orally ingested by felids, they transform into merozoites that expand asexually, ultimately generating millions of gametes for the parasite sexual cycle. However, bradyzoites in intermediate hosts differentiate exclusively to disease-causing tachyzoites, which rapidly disseminate throughout the host. Though tachyzoites are well-studied, the molecular mechanisms governing transitioning between developmental stages are poorly understood. Each parasite stage can be distinguished by a characteristic transcriptional signature, with one signature being repressed during the other stages. Switching between stages require substantial changes in the proteome, which is achieved in part by ubiquitination. F-box proteins mediate protein poly-ubiquitination by recruiting substrates to SKP1, Cullin-1, F-Box protein E3 ubiquitin ligase (SCF-E3) complexes. We have identified an F-box protein named Toxoplasma gondii F-Box Protein L2 (TgFBXL2), which localizes to distinct perinucleolar sites. TgFBXL2 is stably engaged in an SCF-E3 complex that is surprisingly also associated with a COP9 signalosome complex that negatively regulates SCF-E3 function. At the cellular level, TgFBXL2-depleted parasites are severely defective in centrosome replication and daughter cell development. Most remarkable, RNAseq data show that TgFBXL2 conditional depletion induces the expression of stage-specific genes including a large cohort of genes necessary for sexual commitment. Together, these data suggest that TgFBXL2 is a latent guardian of stage specific gene expression in Toxoplasma and poised to remove conflicting proteins in response to an unknown trigger of development.

A newly characterized dense granule protein (GRA76) is important for the growth and virulence of Toxoplasma gondii

Pathogenicity of the zoonotic pathogen Toxoplasma gondii largely depends on the secretion of effector proteins into the extracellular milieu and host cell cytosol, including the dense granule proteins (GRAs). The protein-encoding gene TGME49_299780 was previously identified as a contributor to parasite fitness. However, its involvement in parasite growth, virulence and infectivity in vitro and in vivo remains unknown. Here, we comprehensively examined the role of this new protein, termed GRA76, in parasite pathogenicity. Subcellular localization revealed high expression of GRA76 in tachyzoites inside the parasitophorous vacuole (PV). However, its expression was significantly decreased in bradyzoites. A CRISPR-Cas9 approach was used to knock out the gra76 gene in the T. gondii type I RH strain and type II Pru strain. The in vitro plaque assays and intracellular replication showed the involvement of GRA76 in replication of RH and Pru strains. Deletion of the gra76 gene significantly decreased parasite virulence, and reduced the brain cyst burden in mice. Using RNA sequencing, we detected a significant increase in the expression of bradyzoite-associated genes such as BAG1 and LDH2 in the PruΔgra76 strain compared with the wild-type Pru strain. Using an in vitro bradyzoite differentiation assay, we showed that loss of GRA76 significantly increased the propensity for parasites to form bradyzoites. Immunization with PruΔgra76 conferred partial protection against acute and chronic infection in mice. These findings show the important role of GRA76 in the pathogenesis of T. gondii and highlight the potential of PruΔgra76 as a candidate for a live-attenuated vaccine.

From TgO/GABA-AT, GABA, and T-263 Mutant to Conception of Toxoplasma

Toxoplasma gondii causes morbidity, mortality, and disseminates widely via cat sexual stages. Here, we find T. gondii ornithine aminotransferase (OAT) is conserved across phyla. We solve TgO/GABA-AT structures with bound inactivators at 1.55 Å and identify an inactivator selective for TgO/GABA-AT over human OAT and GABA-AT. However, abrogating TgO/GABA-AT genetically does not diminish replication, virulence, cyst-formation, or eliminate cat's oocyst shedding. Increased sporozoite/merozoite TgO/GABA-AT expression led to our study of a mutagenized clone with oocyst formation blocked, arresting after forming male and female gametes, with "Rosetta stone"-like mutations in genes expressed in merozoites. Mutations are similar to those in organisms from plants to mammals, causing defects in conception and zygote formation, affecting merozoite capacitation, pH/ionicity/sodium-GABA concentrations, drawing attention to cyclic AMP/PKA, and genes enhancing energy or substrate formation in TgO/GABA-AT-related-pathways. These candidates potentially influence merozoite's capacity to make gametes that fuse to become zygotes, thereby contaminating environments and causing disease.

Toxoplasma protein export and effector function

Toxoplasma gondii is a single-celled eukaryotic parasite with a considerable host range that must invade the cells of warm-blooded hosts to survive and replicate. The challenges and opportunities that such a strategy represent have been met by the evolution of effectors that are delivered into host cells, counter host defences and co-opt host cell functions for their own purposes. These effectors are delivered in two waves using distinct machinery for each. In this Review, we focus on understanding the architecture of these protein-export systems and how their protein cargo is recognized and selected. We discuss the recent findings on the role that host manipulation has in latent Toxoplasma infections. We also discuss how these recent findings compare to protein export in the related Plasmodium spp. (the causative agent of malaria) and how this can inform our understanding of host manipulation in the larger Apicomplexa phylum and its evolution.

Toxoplasma gondii F-Box Protein L2 Silences Feline-Restricted Genes Necessary for Sexual Commitment

Toxoplasma gondii is a foodborne pathogen that can cause severe and life-threatening infections in fetuses and immunocompromised patients. Felids are its only definitive hosts, and a wide range of animals, including humans, serve as intermediate hosts. When the transmissible bradyzoite stage is orally ingested by felids, they transform into merozoites that expand asexually, ultimately generating millions of gametes for the parasite sexual cycle. However, bradyzoites in intermediate hosts differentiate exclusively to disease-causing tachyzoites, which rapidly disseminate throughout the host. Though tachyzoites are well-studied, the molecular mechanisms governing transitioning between developmental stages are poorly understood. Each parasite stage can be distinguished by a characteristic transcriptional signature, with one signature being repressed during the other stages. Switching between stages requires substantial changes in the proteome, which is achieved in part by ubiquitination. F-box proteins mediate protein poly-ubiquitination by recruiting substrates to SKP1, Cullin-1, F-Box protein E3 ubiquitin ligase (SCF-E3) complexes. We have identified an F-box protein named Toxoplasma gondii F-Box Protein L2 (TgFBXL2), which localizes to distinct nuclear sites. TgFBXL2 is stably engaged in an SCF-E3 complex that is surprisingly also associated with a COP9 signalosome complex that negatively regulates SCF-E3 function. At the cellular level, TgFBXL2-depleted parasites are severely defective in centrosome replication and daughter cell development. Most remarkable, RNA seq data show that TgFBXL2 conditional depletion induces the expression of genes necessary for sexual commitment. We suggest that TgFBXL2 is a latent guardian of sexual stage development in Toxoplasma and poised to remove conflicting proteins in response to an unknown trigger of sexual development.

Toxoplasma ceramide synthases: Gene duplication, functional divergence, and roles in parasite fitness

Toxoplasma gondii is an obligate, intracellular apicomplexan protozoan parasite of both humans and animals that can cause fetal damage and abortion and severe disease in the immunosuppressed. Sphingolipids have indispensable functions as signaling molecules and are essential and ubiquitous components of eukaryotic membranes that are both synthesized and scavenged by the Apicomplexa. Ceramide is the precursor for all sphingolipids, and here we report the identification, localization and analyses of the Toxoplasma ceramide synthases TgCerS1 and TgCerS2. Interestingly, we observed that while TgCerS1 was a fully functional orthologue of the yeast ceramide synthase (Lag1p) capable of catalyzing the conversion of sphinganine to ceramide, in contrast TgCerS2 was catalytically inactive. Furthermore, genomic deletion of TgCerS1 using CRISPR/Cas-9 led to viable but slow-growing parasites indicating its importance but not indispensability. In contrast, genomic knock out of TgCerS2 was only accessible utilizing the rapamycin-inducible Cre recombinase system. Surprisingly, the results demonstrated that this "pseudo" ceramide synthase, TgCerS2, has a considerably greater role in parasite fitness than its catalytically active orthologue (TgCerS1). Phylogenetic analyses indicated that, as in humans and plants, the ceramide synthase isoforms found in Toxoplasma and other Apicomplexa may have arisen through gene duplication. However, in the Apicomplexa the duplicated copy is hypothesized to have subsequently evolved into a non-functional "pseudo" ceramide synthase. This arrangement is unique to the Apicomplexa and further illustrates the unusual biology that characterize these protozoan parasites.

AP2XII-1 is a negative regulator of merogony and presexual commitment in Toxoplasma gondii

IMPORTANCE

Sexual development is vital for the transmission, genetic hybridization, and population evolution of apicomplexan pathogens, which include several clinically relevant parasites, such as Plasmodium, Eimeria, and Toxoplasma gondii. Previous studies have demonstrated different morphological characteristics and division patterns between asexual and sexual stages of the parasites. However, the primary regulation is poorly understood. A transition from the asexual to the sexual stage is supposedly triggered/accompanied by rewiring of gene expression and controlled by transcription factors and chromatin modulators. Herein, we discovered a tachyzoite-specific transcriptional factor AP2XII-1, which represses the presexual development in the asexual tachyzoite stage of T. gondii. Conditional knockdown of AP2XII-1 perturbs tachyzoite proliferation by endodyogeny and drives a transition to a morphologically and transcriptionally distinct merozoite stage. The results also suggest a hierarchical transcriptional regulation of sexual development by AP2 factors and provide a path to culturing merozoites and controlling inter-host transmission of T. gondii.

Unravelling the sexual developmental biology of Cystoisospora suis, a model for comparative coccidian parasite studies

Introduction

The apicomplexan parasite Cystoisospora suis has global significance as an enteropathogen of suckling piglets. Its intricate life cycle entails a transition from an asexual phase to sexual development, ultimately leading to the formation of transmissible oocysts.

Methods

To advance our understanding of the parasite's cellular development, we complemented previous transcriptome studies by delving into the proteome profiles at five distinct time points of in vitro cultivation through LC/MS-MS analysis.

Results

A total of 1,324 proteins were identified in the in vitro developmental stages of C. suis, and 1,082 proteins were identified as significantly differentially expressed. Data are available via ProteomeXchange with identifier PXD045050. We performed BLAST, GO enrichment, and KEGG pathway analyses on the up- and downregulated proteins to elucidate correlated events in the C. suis life cycle. Our analyses revealed intriguing metabolic patterns in macromolecule metabolism, DNA- and RNA-related processes, proteins associated with sexual stages, and those involved in cell invasion, reflecting the adaptation of sexual stages to a nutrient-poor and potentially stressful extracellular environment, with a focus on enzymes involved in metabolism and energy production.

Discussion

These findings have important implications for understanding the developmental biology of C. suis as well as other, related coccidian parasites, such as Eimeria spp. and Toxoplasma gondii. They also support the role of C. suis as a new model for the comparative biology of coccidian tissue cyst stages.

Role of 6-phosphogluconate dehydrogenase enzyme 1 in growth and virulence of Toxoplasma gondii and development of attenuated live vaccine

Toxoplasma gondii is a ubiquitous pathogen that infects all warm-blooded animals, including humans, causing substantial socioeconomic and healthcare burdens. However, there is no ideal vaccine for toxoplasmosis. As metabolism is important in the growth and virulence of Toxoplasma, some key pathways are promising antiparasitic targets. Here, we identified 6-phosphogluconate dehydrogenase 1 (Tg6PGDH1) in the oxidative pentose phosphate pathway as a cytoplasmic protein that is dispensable for tachyzoite growth of T. gondii in vitro but critical for virulence and cyst formation in vivo. The depletion of Tg6PGDH1 causes decreased gene transcription involved in signal transduction, transcriptional regulation and virulence. Furthermore, we analysed the protective effect of the ME49Δ6pgdh1 mutant as an attenuated vaccine and found that ME49Δ6pgdh1 immunization stimulated strong protective immunity against lethal challenges and blocked cyst formation caused by reinfection. Furthermore, we showed that ME49Δ6pgdh1 immunization stimulated increased levels of interferon-gamma, tumour necrosis factor-alpha and Toxoplasma-specific IgG antibodies. These data highlight the role of Tg6PGDH1 in the growth and virulence of T. gondii and its potential as a target for the development of a live-attenuated vaccine.

TgLaforin, a glucan phosphatase, reveals the dynamic role of storage polysaccharides in Toxoplasma gondii tachyzoites and bradyzoites

The asexual stages of Toxoplasma gondii are defined by the rapidly growing tachyzoite during the acute infection and by the slow growing bradyzoite housed within tissue cysts during the chronic infection. These stages represent unique physiological states, each with distinct glucans reflecting differing metabolic needs. A defining feature of T. gondii bradyzoites is the presence of insoluble storage glucans known as amylopectin granules (AGs) that are believed to play a role in reactivation, but their functions during the chronic infection remain largely unexplored. More recently, the presence of storage glucans has been recognized in tachyzoites where their precise function and architecture have yet to be fully defined. Importantly, the T. gondii genome encodes activities needed for glucan turnover: a glucan phosphatase (TgLaforin; TGME49_205290) and a glucan kinase (TgGWD; TGME49_214260) that catalyze a cycle of reversible glucan phosphorylation required for glucan degradation by amylases. The expression of these enzymes in tachyzoites supports the existence of a storage glucan, evidence that is corroborated by specific labeling with the anti-glycogen antibody IV58B6. Disruption of reversible glucan phosphorylation via a CRISPR/Cas9 knockout (KO) of TgLaforin revealed no growth defects under nutrient-replete conditions in tachyzoites. However, the growth of TgLaforin-KO tachyzoites was severely stunted when starved of glutamine, even under glucose replete conditions. The loss of TgLaforin also resulted in the attenuation of acute virulence in mice accompanied by a lower cyst burden. Defective cyst formation due to profound changes in AG morphology was also observed in TgLaforin-KO parasites, both in vitro and in vivo. Together, these data demonstrate the importance of glucan turnover across the T. gondii asexual cycle. These findings, alongside our previously identified class of small molecules that inhibit TgLaforin, implicate reversible glucan phosphorylation as a legitimate target for the development of new drugs against chronic T. gondii infections.

Mitochondrial ADP/ATP Carrier 1 Is Important for the Growth of Toxoplasma Tachyzoites

Metabolism associated with energy production is highly compartmentalized in eukaryotic cells. During this process, transporters that move metabolites across organelle membranes play pivotal roles. The highly conserved ADP/ATP carrier (AAC) involved in ATP and ADP exchange between the mitochondria and cytoplasm is key to linking the metabolic activities in these 2 compartments. The ATP produced in mitochondria can be exchanged with cytoplasmic ADP by AAC, thus satisfying the energy needs in the cytoplasm. Toxoplasma gondii is an obligate intracellular parasite with a wide range of hosts. Previous studies have shown that mitochondrial metabolism helps Toxoplasma to parasitize diverse host cells. Here, we identified 2 putative mitochondria ADP/ATP carriers in Toxoplasma with significant sequence similarity to known AACs from other eukaryotes. We examined the ATP transport function of TgAACs by expressing them in Escherichia coli cells and found that only TgAAC1 had ATP transport activity. Moreover, knockdown of TgAAC1 caused severe growth defects of parasites and heterologous expression of mouse ANT2 in the TgAAC1 depletion mutant restored its growth, revealing its importance for parasite growth. These results verified that TgAAC1 functions as the mitochondrial ADP/ATP carrier in T. gondii and the functional studies demonstrated the importance of TgAAC1 for tachyzoites growth. IMPORTANCE T. gondii has an efficient and flexible energy metabolism system to meet different growth needs. ATP is an energy-carrying molecule and needs to be exchanged between organelles with the assistance of transporters. However, the function of TgAACs has yet to be characterized. Here, we identified 2 putative AACs of T. gondii and verified that only TgAAC1 had ATP transport activity with expression in the intact E. coli cells. Detailed analyses found that TgAAC1 is critical for the growth of tachyzoites and TgAAC2 is dispensable. Moreover, complementation with mouse ANT2 restored the growth speed of iTgAAC1, further suggesting TgAAC1 functions as a mitochondrial ADP/ATP carrier. Our research demonstrated the importance of TgAAC1 for tachyzoites growth.

Arsinothricin Inhibits Plasmodium falciparum Proliferation in Blood and Blocks Parasite Transmission to Mosquitoes

Malaria, caused by Plasmodium protozoal parasites, remains a leading cause of morbidity and mortality. The Plasmodium parasite has a complex life cycle, with asexual and sexual forms in humans and Anopheles mosquitoes. Most antimalarials target only the symptomatic asexual blood stage. However, to ensure malaria eradication, new drugs with efficacy at multiple stages of the life cycle are necessary. We previously demonstrated that arsinothricin (AST), a newly discovered organoarsenical natural product, is a potent broad-spectrum antibiotic that inhibits the growth of various prokaryotic pathogens. Here, we report that AST is an effective multi-stage antimalarial. AST is a nonproteinogenic amino acid analog of glutamate that inhibits prokaryotic glutamine synthetase (GS). Phylogenetic analysis shows that Plasmodium GS, which is expressed throughout all stages of the parasite life cycle, is more closely related to prokaryotic GS than eukaryotic GS. AST potently inhibits Plasmodium GS, while it is less effective on human GS. Notably, AST effectively inhibits both Plasmodium erythrocytic proliferation and parasite transmission to mosquitoes. In contrast, AST is relatively nontoxic to a number of human cell lines, suggesting that AST is selective against malaria pathogens, with little negative effect on the human host. We propose that AST is a promising lead compound for developing a new class of multi-stage antimalarials.

CRISPR Screens Identify Toxoplasma Genes That Determine Parasite Fitness in Interferon Gamma-Stimulated Human Cells

Toxoplasma virulence depends on its ability to evade or survive the toxoplasmacidal mechanisms induced by interferon gamma (IFNγ). While many Toxoplasma genes involved in the evasion of the murine IFNγ response have been identified, genes required to survive the human IFNγ response are largely unknown. In this study, we used a genome-wide loss-of-function screen to identify Toxoplasma genes important for parasite fitness in IFNγ-stimulated primary human fibroblasts. We generated gene knockouts for the top six hits from the screen and confirmed their importance for parasite growth in IFNγ-stimulated human fibroblasts. Of these six genes, three have homology to GRA32, localize to dense granules, and coimmunoprecipitate with each other and GRA32, suggesting they might form a complex. Deletion of individual members of this complex leads to early parasite egress in IFNγ-stimulated cells. Thus, prevention of early egress is an important Toxoplasma fitness determinant in IFNγ-stimulated human cells. IMPORTANCE Toxoplasma infection causes serious complications in immunocompromised individuals and in the developing fetus. During infection, certain immune cells release a protein called interferon gamma that activates cells to destroy the parasite or inhibit its growth. While most Toxoplasma parasites are cleared by this immune response, some can survive by blocking or evading the IFNγ-induced restrictive environment. Many Toxoplasma genes that determine parasite survival in IFNγ-activated murine cells are known but parasite genes conferring fitness in IFNγ-activated human cells are largely unknown. Using a Toxoplasma adapted genome-wide loss-of-function screen, we identified many Toxoplasma genes that determine parasite fitness in IFNγ-activated human cells. The gene products of four top hits play a role in preventing early parasite egress in IFNγ-stimulated human cells. Understanding how IFNγ-stimulated human cells inhibit Toxoplasma growth and how Toxoplasma counteracts this, could lead to the development of novel therapeutics.

Toxoplasma IWS1 Determines Fitness in Interferon-γ-Activated Host Cells and Mice by Indirectly Regulating ROP18 mRNA Expression

Toxoplasma gondii secretes various virulence effector molecules into host cells to disrupt host interferon-γ (IFN-γ)-dependent immunity. Among these effectors, ROP18 directly phosphorylates and inactivates IFN-inducible GTPases, such as immunity-related GTPases (IRGs) and guanylate-binding proteins (GBPs), leading to the subversion of IFN-inducible GTPase-induced cell-autonomous immunity. The modes of action of ROP18 have been studied extensively; however, little is known about the molecular mechanisms by which ROP18 is produced in the parasite itself. Here, we report the role of T. gondii transcription factor IWS1 in ROP18 mRNA expression in the parasite. Compared with wild-type virulent type I T. gondii, IWS1-deficient parasites showed dramatically increased loading of IRGs and GBPs onto the parasitophorous vacuole membrane (PVM). Moreover, IWS1-deficient parasites displayed decreased virulence in wild-type mice but retained normal virulence in mice lacking the IFN-γ receptor. Furthermore, IWS1-deficient parasites showed severely decreased ROP18 mRNA expression; however, tagged IWS1 did not directly bind with genomic regions of the ROP18 locus. Ectopic expression of ROP18 in IWS1-deficient parasites restored the decreased loading of effectors onto the PVM and in vivo virulence in wild-type mice. Taken together, these data demonstrate that T. gondii IWS1 indirectly regulates ROP18 mRNA expression to determine fitness in IFN-γ-activated host cells and mice. IMPORTANCE The parasite Toxoplasma gondii has a counterdefense system against interferon-γ (IFN-γ)-dependent host immunity which relies on the secretion of parasite effector proteins. ROP18 is one of the effector, which is released into host cells to inactivate IFN-γ-dependent anti-Toxoplasma host proteins. The mechanism by which Toxoplasma ROP18 subverts host immunity has been extensively analyzed, but how Toxoplasma produces this virulence factor remains unclear. Here, we show that Toxoplasma transcription factor IWS1 is important for ROP18 mRNA expression in the parasite. Loss of IWS1 from virulent Toxoplasma leads to dramatically decreased ROP18 mRNA expression, resulting in profoundly decreased virulence due to greater activity of IFN-γ-dependent host immune responses. Thus, Toxoplasma prepares the critical virulence factor ROP18 via an IWS1-dependent system to negate IFN-γ-dependent antiparasitic immunity and thus survive in the host.

Late Embryogenesis Abundant Proteins Contribute to the Resistance of Toxoplasma gondii Oocysts against Environmental Stresses

Toxoplasma gondii oocysts, which are shed in large quantities in the feces from infected felines, are very stable in the environment, resistant to most inactivation procedures, and highly infectious. The oocyst wall provides an important physical barrier for sporozoites contained inside oocysts, protecting them from many chemical and physical stressors, including most inactivation procedures. Furthermore, sporozoites can withstand large temperature changes, even freeze-thawing, as well as desiccation, high salinity, and other environmental insults; however, the genetic basis for this environmental resistance is unknown. Here, we show that a cluster of four genes encoding Late Embryogenesis Abundant (LEA)-related proteins are required to provide Toxoplasma sporozoites resistance to environmental stresses. Toxoplasma LEA-like genes (TgLEAs) exhibit the characteristic features of intrinsically disordered proteins, explaining some of their properties. Our in vitro biochemical experiments using recombinant TgLEA proteins show that they have cryoprotective effects on the oocyst-resident lactate dehydrogenase enzyme and that induced expression in E. coli of two of them leads to better survival after cold stress. Oocysts from a strain in which the four LEA genes were knocked out en bloc were significantly more susceptible to high salinity, freezing, and desiccation compared to wild-type oocysts. We discuss the evolutionary acquisition of LEA-like genes in Toxoplasma and other oocyst-producing apicomplexan parasites of the Sarcocystidae family and discuss how this has likely contributed to the ability of sporozoites within oocysts to survive outside the host for extended periods. Collectively, our data provide a first molecular detailed view on a mechanism that contributes to the remarkable resilience of oocysts against environmental stresses. IMPORTANCE Toxoplasma gondii oocysts are highly infectious and may survive in the environment for years. Their resistance against disinfectants and irradiation has been attributed to the oocyst and sporocyst walls by acting as physical and permeability barriers. However, the genetic basis for their resistance against stressors like changes in temperature, salinity, or humidity, is unknown. We show that a cluster of four genes encoding Toxoplasma Late Embryogenesis Abundant (TgLEA)-related proteins are important for this resistance to environmental stresses. TgLEAs have features of intrinsically disordered proteins, explaining some of their properties. Recombinant TgLEA proteins show cryoprotective effects on the parasite's lactate dehydrogenase, an abundant enzyme in oocysts, and expression in E. coli of two TgLEAs has a beneficial effect on growth after cold stress. Moreover, oocysts from a strain lacking all four TgLEA genes were more susceptible to high salinity, freezing, and desiccation compared to wild-type oocysts, highlighting the importance of the four TgLEAs for oocyst resilience.

Functional Characterization of 15 Novel Dense Granule Proteins in Toxoplasma gondii Using the CRISPR-Cas9 System

The analysis of the subcellular localization and function of dense granule proteins (GRAs) is of central importance for the understanding of host-parasite interaction and pathogenesis of Toxoplasma gondii infection. Here, we identified 15 novel GRAs and used C-terminal endogenous gene tagging to determine their localization at the intravacuolar network (IVN), parasitophorous vacuole (PV), or PV membrane (PVM) in the tachyzoites and at the periphery of the bradyzoites-containing cysts. The functions of the 15 gra genes were examined in type I RH strain and 5 of these gra genes were also evaluated in the cyst-forming type II Pru strain. The 15 novel gra genes were successfully disrupted by using CRISPR-Cas9 mediated homologous recombination and the results showed that 13 gra genes were not individually essential for T. gondii replication in vitro or virulence in mice during acute and chronic infection. Intriguingly, deletion of TGME49_266410 and TGME49_315910 in both RH and Pru strains decreased the parasite replication in vitro and attenuated its virulence, and also reduced the cyst-forming ability of the Pru strain in mice during chronic infection. Comparison of the transcriptomic profiles of the 15 gra genes suggests that they may play roles in other life cycle stages and genotypes of T. gondii. Taken together, our findings improve the understanding of T. gondii pathogenesis and demonstrate the involvement of two novel GRAs, TGME49_266410 and TGME49_315910, in the parasite replication and virulence. IMPORTANCE Dense granule proteins (GRAs) play important roles in Toxoplasma gondii pathogenicity. However, the functions of many putative GRAs have not been elucidated. Here, we found that 15 novel GRAs are secreted into intravacuolar network (IVN), parasitophorous vacuole (PV), or PV membrane (PVM) in tachyzoites and are located at the periphery of the bradyzoite-containing cysts. TGME49_266410 and TGME49_315910 were crucial to the growth of RH and Pru strains in vitro. Deletion of TGME49_266410 and TGME49_315910 attenuated the parasite virulence in mice. However, disruption of other 13 gra genes did not have a significant impact on the proliferation and pathogenicity of T. gondii in vitro or in vivo. The marked effects of the two novel GRAs (TGME49_266410 and TGME49_315910) on the in vitro growth and virulence of T. gondii are notable and warrant further elucidation of the temporal and spatial dynamics of translocation of these two novel GRAs and how do they interfere with host cell functions.

Toxoplasma gondii AP2XII-2 Contributes to Transcriptional Repression for Sexual Commitment

Toxoplasma gondii is a widespread protozoan parasite that has a significant impact on human and veterinary health. The parasite undergoes a complex life cycle involving multiple hosts and developmental stages. How Toxoplasma transitions between life cycle stages is poorly understood yet central to controlling transmission. Of particular neglect are the factors that contribute to its sexual development, which takes place exclusively in feline intestines. While epigenetic repressors have been shown to play an important role in silencing the spurious gene expression of sexually committed parasites, the specific factors that recruit this generalized machinery to the appropriate genes remain largely unexplored. Here, we establish that a member of the AP2 transcription factor family, AP2XII-2, is targeted to genomic loci associated with sexually committed parasites along with epigenetic regulators of transcriptional silencing, HDAC3 and MORC. Despite its widespread association with gene promoters, AP2XII-2 is required for the silencing of relatively few genes. Using the CUT&Tag (cleavage under targets and tagmentation) methodology, we identify two major genes associated with sexual development downstream of AP2XII-2 control, AP2X-10 and the amino acid hydroxylase AAH1. Our findings show that AP2XII-2 is a key contributor to the gene regulatory pathways modulating Toxoplasma sexual development. IMPORTANCE Toxoplasma gondii is a parasite that undergoes its sexual stage exclusively in feline intestines, making cats a major source of transmission. A better understanding of the proteins controlling the parasite's life cycle stage transitions is needed for the development of new therapies aimed at treating toxoplasmosis and the transmission of the infection. Genes that regulate the sexual stages need to be turned on and off at the appropriate times, activities that are mediated by specific transcription factors that recruit general machinery to silence or activate gene expression. In this study, we identify a transcription factor called AP2XII-2 as being important for the repression of a subset of sexual stage genes, including a sexual stage-specific AP2 factor (AP2X-10) and a protein (AAH1) required to construct the infectious oocysts expelled from infected cats.

Effect of deleting four Toxoplasma gondii calcium-binding EGF domain-containing proteins on parasite replication and virulence

Several calcium-binding proteins including calcium-dependent protein kinases play important roles in several facets of the intracellular infection cycle of the apicomplexan protozoan parasite Toxoplasma gondii. However, the role of the calcium-binding epidermal growth factor (EGF) domain-containing proteins (CBDPs) remains poorly understood. In this study, we examined the functions of four CBDP genes in T. gondii RH strain of type I by generating knock-out strains using CRISPR-Cas9 system. We investigated the ability of mutant strains deficient in CBDP1, CBDP2, CBDP3, or CBDP4 to form plaques, replicate intracellularly, and egress from the host cells. The results showed that no definite differences between any of these four CBDP mutant strains and the wild-type strain in terms of their ability to form plaques, intracellular replication, and egress. Additionally, CBDP mutants did not exhibit any significant attenuated virulence compared to the wild-type strain in mice. The expression profiles of CBDP2-4 genes were conserved among T. gondii strains of different genotypes, life cycle stages, and developmental forms. Whether other CBDP genes play any roles in the pathogenicity of T. gondii strains of different genotypes remains to be elucidated.

Dissection of Besnoitia besnoiti intermediate host life cycle stages: From morphology to gene expression

Cyst-forming Apicomplexa (CFA) of the Sarcocystidae have a ubiquitous presence as pathogens of humans and farm animals transmitted through the food chain between hosts with few notable exceptions. The defining hallmark of this family of obligate intracellular protists consists of their ability to remain for very long periods as infectious tissue cysts in chronically infected intermediate hosts. Nevertheless, each closely related species has evolved unique strategies to maintain distinct reservoirs on global scales and ensuring efficient transmission to definitive hosts as well as between intermediate hosts. Here, we present an in-depth comparative mRNA expression analysis of the tachyzoite and bradyzoite stages of Besnoitia besnoiti strain Lisbon14 isolated from an infected farm animal based on its annotated genome sequence. The B. besnoiti genome is highly syntenic with that of other CFA and also retains the capacity to encode a large majority of known and inferred factors essential for completing a sexual cycle in a yet unknown definitive host. This work introduces Besnoitia besnoiti as a new model for comparative biology of coccidian tissue cysts which can be readily obtained in high purity. This model provides a framework for addressing fundamental questions about the evolution of tissue cysts and the biology of this pharmacologically intractable infectious parasite stage.

Metabolic flexibilities and vulnerabilities in the pentose phosphate pathway of the zoonotic pathogen Toxoplasma gondii

Metabolic pathways underpin the growth and virulence of intracellular parasites and are therefore promising antiparasitic targets. The pentose phosphate pathway (PPP) is vital in most organisms, providing a reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) and ribose sugar for nucleotide synthesis; however, it has not yet been studied in Toxoplasma gondii, a widespread intracellular pathogen and a model protozoan organism. Herein, we show that T. gondii has a functional PPP distributed in the cytoplasm and nucleus of its acutely-infectious tachyzoite stage. We produced eight parasite mutants disrupting seven enzymes of the PPP in T. gondii. Our data show that of the seven PPP proteins, the two glucose-6-phosphate dehydrogenases (TgG6PDH1, TgG6PDH2), one of the two 6-phosphogluconate dehydrogenases (Tg6PGDH1), ribulose-5-phosphate epimerase (TgRuPE) and transaldolase (TgTAL) are dispensable in vitro as well as in vivo, disclosing substantial metabolic plasticity in T. gondii. Among these, TgG6PDH2 plays a vital role in defense against oxidative stress by the pathogen. Further, we show that Tg6PGDH2 and ribulose-5-phosphate isomerase (TgRPI) are critical for tachyzoite growth. The depletion of TgRPI impairs the flux of glucose in central carbon pathways, and causes decreased expression of ribosomal, microneme and rhoptry proteins. In summary, our results demonstrate the physiological need of the PPP in T. gondii while unraveling metabolic flexibility and antiparasitic targets.

Metabolic pathways are intimately associated with the survival and replication of parasitic Toxoplasma gondii and thus represent potential targets for antiparasitic strategies. Herein, we focused on the pentose phosphate pathway (PPP) in T. gondii and examined its roles in supporting the growth of this ubiquitous pathogen. We found that TgG6PDH1 and TgG6PDH2 were needed to defend oxidative stress but not for pentose synthesis. We revealed that inactivation of the Tg6PGDH2 and TgRPI severely impaired the asexual reproduction of tachyzoites. We also highlighted the remarkable metabolic plasticity in tachyzoites that enables them to acquire some of the PPP intermediates from multiple routes. This study provides significant insights into the carbon metabolism properties of Toxoplasma parasites, opening avenues for targeting this pathway to develop therapeutic interventions against toxoplasmosis.

Nuclear Factor AP2X-4 Governs the Expression of Cell Cycle- and Life Stage-Regulated Genes and is Critical for Toxoplasma Growth

Toxoplasma gondii is a ubiquitous pathogen infecting one third of the world's population and diverse animals. It has a complex life cycle alternating among different developmental stages, which contributes to its transmission and pathogenesis. The parasite has a sophisticated gene regulation network that enables timely expression of genes at designated stages. However, little is known about the underlying regulatory mechanisms. Here, we identified an AP2 family transcription factor named TgAP2X-4, which was crucial for parasite growth during the acute infection stage. TgAP2X-4 deletion leads to reduced expression of many genes that are normally upregulated during the M phase of the cell cycle. These include genes that encode rhoptry neck proteins that are key for parasite invasion. As a result, the Δap2X-4 mutant displayed significantly decreased efficiency of host cell invasion. Transcriptomic analyses suggested that TgAP2X-4 also regulates a large group of genes that are typically induced during chronic infection, such as BAG1 and LDH2. Given the diverse impacts on gene expression, TgAP2X-4 inactivation results in severely impaired parasite growth, as well as drastic attenuation of parasite virulence and complete inability to form chronic infection. Therefore, TgAP2X-4 represents a candidate for antitoxoplasmic drug and vaccine designs. IMPORTANCE Toxoplasma gondii has a complicated gene regulation network that allows "just in time" expression of genes to cope with the physiological needs at each stage during the complex life cycle. However, how such regulation is achieved is largely unknown. Here, we identified a transcription factor named TgAP2X-4 that is critical for the growth and life cycle progression of the parasite. Detailed analyses found that TgAP2X-4 regulated the expression of many cell cycle-regulated genes, including a subset of rhoptry genes that were essential for the parasites to enter host cells. It also regulated the expression of many genes involved in the development of chronic infection. Because of the diverse impacts on gene expression, TgAP2X-4 inactivation caused reduced parasite growth in vitro and attenuated virulence in vivo. Therefore, it is a potential target for drug or vaccine designs against Toxoplasma infections.

Serological survey of Toxoplasma gondii infection in cats in Khon Kaen, Northeast Thailand

Background and Aim: Toxoplasmosis is a zoonosis caused by Toxoplasma gondii. Cats are known to be the definitive hosts that can excrete these environmentally resistant oocysts. Other mammals, avians, and even humans can serve as the intermediate host. T. gondii infection is often asymptomatic in healthy individuals; however, it could result in serious health problems in immunocompromised and pregnant individuals. This study investigated the occurrence of T. gondii infection in cats in Khao Suan Kwang and Mueang Khon Kaen. Materials and Methods: In total, 100 serum samples from cats, that is, 62 owned cats (31 males and 31 females) and 38 adopted stray cats (21 males and 17 females), were examined for antibodies against T. gondii through rapid immunochromatographic tests (ICT). Owners were asked to sign a consent form and answer the questionnaires before sample collection. Demographic information about the cats and their owners was also recorded. Results: The overall seroprevalence of cats positive for T. gondii antibodies was found to be 5%. Notably, the Toxoplasma antibody prevalence was significantly higher in the adopted stray cats (10.53% [4/38]) that roamed the zoo than in the owned cats (1.61% [1/62]) (p > 0.05). No significant difference was observed between male (8.33%) and female (1.92%) cats. The cat owners' questionnaire revealed that more than half had never heard of toxoplasmosis before (67.7%), whereas 30.6% knew nothing about the disease transmission routes. Conclusion: This study presented a low seroprevalence of antibodies to T. gondii in owned cats from the Mueang Khon Kaen District, whereas high seroprevalence was detected in the adopted stray cats from Khao Suan Kwang. Adopted stray cats can have a higher potential for T. gondii infection; thus, they could be a source of toxoplasmosis transmission to humans. Therefore, it is essential to control the number of stray cats, and a screening test for antitoxoplasmosis could be recommended before adoption. Although the total seroprevalence was noted to be low, the zoonotic disease was present. Therefore, raising the community's awareness and knowledge might reduce the disease transmission from animals to humans.

New Avenues to Design Toxoplasma Vaccines Based on Oocysts and Cysts

Toxoplasmosis is a worldwide disease affecting all warm-blooded animals, including humans. Vaccination strategies aimed at inducing an efficient immune response while preventing transmission have been attempted in the past. While many different approaches can partially protect immunized animals against subsequent infections, full and lasting protection is rarely attained and only with live-attenuated vaccines. In addition, vaccines based on mutant strains that are deficient in forming the chronic phase of the parasite (such as Toxovax™) cannot be extensively used due to their zoonotic potential and the possibility of reversion to virulent phenotypes. An increasing number of studies using emerging genetic-engineering tools have been conducted to design novel vaccines based on recombinant proteins, DNA or delivery systems such as nanoparticles. However, these are usually less efficient due to their antigenic simplicity. In this perspective article we discuss potential target genes and novel strategies to generate live-attenuated long-lasting vaccines based on tissue cysts and oocysts, which are the environmentally resistant chronic forms of Toxoplasma. By selectively disrupting genes important for parasite dissemination, cyst formation and/or sporozoite invasion, alone or in combination, a vaccine based on a live-attenuated strain that elicits a protective immune response while preventing the transmission of Toxoplasma could be created. Finally, further improvements of protocols to generate Toxoplasma sexual stages in vitro might lead to the production of oocysts from such a strain without the need for using mice or cats.

Paving the Way: Contributions of Big Data to Apicomplexan and Kinetoplastid Research

In the age of big data an important question is how to ensure we make the most out of the resources we generate. In this review, we discuss the major methods used in Apicomplexan and Kinetoplastid research to produce big datasets and advance our understanding of Plasmodium, Toxoplasma, Cryptosporidium, Trypanosoma and Leishmania biology. We debate the benefits and limitations of the current technologies, and propose future advancements that may be key to improving our use of these techniques. Finally, we consider the difficulties the field faces when trying to make the most of the abundance of data that has already been, and will continue to be, generated.

The transcriptome from asexual to sexual in vitro development of Cystoisospora suis (Apicomplexa: Coccidia)

The apicomplexan parasite Cystoisospora suis is an enteropathogen of suckling piglets with woldwide distribution. As with all coccidian parasites, its lifecycle is characterized by asexual multiplication followed by sexual development with two morphologically distinct cell types that presumably fuse to form a zygote from which the oocyst arises. However, knowledge of the sexual development of C. suis is still limited. To complement previous in vitro studies, we analysed transcriptional profiles at three different time points of development (corresponding to asexual, immature and mature sexual stages) in vitro via RNASeq. Overall, transcription of genes encoding proteins with important roles in gametes biology, oocyst wall biosynthesis, DNA replication and axonema formation as well as proteins with important roles in merozoite biology was identified. A homologue of an oocyst wall tyrosine rich protein of Toxoplasma gondii was expressed in macrogametes and oocysts of C. suis. We evaluated inhibition of sexual development in a host-free culture for C. suis by antiserum specific to this protein to evaluate whether it could be exploited as a candidate for control strategies against C. suis. Based on these data, targets can be defined for future strategies to interrupt parasite transmission during sexual development.

TSS-seq of Toxoplasma gondii sporozoites revealed a novel motif in stage-specific promoters

Toxoplasma gondii is one of the most common zoonotic protozoan parasites. It has three major infectious stages: rapidly multiplying tachyzoites (Tz), slowly replicating bradyzoites (Bz) and a resting/free-living stage, sporozoites (Sz). The regulatory mechanisms governing stage-specific gene expression are not fully understood. Few transcriptional start sites (TSS) are known for Sz. In this study, we obtained TSS of Sz using an oligo-capping method and RNA-seq analysis. We identified 1,043,503 TSS in the Sz transcriptome. These defined 38,973 TSS clusters, of which, 11,925 were expressed in Sz and 1535 TSS differentially expressed in Sz. Based on these data, we defined promoter regions and novel sporozoite stage-specific motifs using MEME. TGTANNTACA was distributed around -55 to -75 regions from each TSS. Interestingly, the same motif was reported in another apicomplexan, Plasmodium berghei, as a cis-element of female-specific gametocyte genes, implying the presence of common regulatory machinery. Further comparative analysis should better define the distribution and function of these elements in other members of this important parasitic phylum.

Transcriptional signatures of clonally derived Toxoplasma tachyzoites reveal novel insights into the expression of a family of surface proteins

Toxoplasma gondii has numerous, large, paralogous gene families that are likely critical for supporting its unparalleled host range: nearly any nucleated cell in almost any warm-blooded animal. The SRS (SAG1-related sequence) gene family encodes over 100 proteins, the most abundant of which are thought to be involved in parasite attachment and, based on their stage-specific expression, evading the host immune response. For most SRS proteins, however, little is understood about their function and expression profile. Single-parasite RNA-sequencing previously demonstrated that across an entire population of lab-grown tachyzoites, transcripts for over 70 SRS genes were detected in at least one parasite. In any one parasite, however, transcripts for an average of only 7 SRS genes were detected, two of which, SAG1 and SAG2A, were extremely abundant and detected in virtually all. These data do not address whether this pattern of sporadic SRS gene expression is consistently inherited among the progeny of a given parasite or arises independently of lineage. We hypothesized that if SRS expression signatures are stably inherited by progeny, subclones isolated from a cloned parent would be more alike in their expression signatures than they are to the offspring of another clone. In this report, we compare transcriptomes of clonally derived parasites to determine the degree to which expression of the SRS family is stably inherited in individual parasites. Our data indicate that in RH tachyzoites, SRS genes are variably expressed even between parasite samples subcloned from the same parent within approximately 10 parasite divisions (72 hours). This suggests that the pattern of sporadically expressed SRS genes is highly variable and not driven by inheritance mechanisms, at least under our conditions.

Functional Characterization of 17 Protein Serine/Threonine Phosphatases in Toxoplasma gondii Using CRISPR-Cas9 System

Protein serine/threonine phosphatases (PSPs), found in various plants and protozoa, are involved in the regulation of various biological processes. However, very little is known about the role of PSPs in the pathogenicity of the apicomplexan protozoan Toxoplasma gondii. Herein, the subcellular localization of 17 PSPs (PP5, PP7, EFPP, SLP, PPM3F, PPM4, PPM5A, PPM5B, PPM6, PPM8, PPM9, PPM12, PPM14, PPM18, CTD1, CTD2, and CTD3) was examined by 6× HA tagging of endogenous genes in C-terminal. The PSPs were detected in the cytoplasm (PP5, EFPP, PPM8, and CTD2), dense granules (SLP), nucleus (PPM4 and PPM9), inner membrane complex (PPM12), basal complex (CTD3), and apical pole (PP7). The remaining PSPs exhibited low or undetectable level of expression. To characterize the contribution of these genes to the infectivity of T. gondii, knock-out (KO) strains of type I RH strain deficient in the 17 psp genes and KO type II Pru strain deficient in pp7 and slp genes were constructed. The pathogenicity of individual RHΔpsp mutants was characterized in vitro using plaque, egress, and intracellular replication assays, and mouse infection, while pathogenicity of PruΔpp7 and PruΔslp mutant strains was evaluated by examining the parasite lytic cycle in vitro and assessment of brain cyst burden in mice. No significant differences were observed between 16 RHΔpsp strains and wild-type (WT) RH strain. However, RHΔpp7 exhibited significantly lower invasion efficiency and parasitophorous vacuole formation in vitro, and less virulence in mice compared with other RHΔpsp and WT strains. In addition, PruΔpp7 exhibited marked attenuation of virulence and significant reduction in the brain cyst burden in mice compared with PruΔslp and WT strains, suggesting the key role of PP7 in the virulence of T. gondii. Comparative transcriptomic profiling of the 17 psp genes showed that they may play different roles in the pathogenesis of different genotypes or life cycle stages of T. gondii. These findings provide new insight into the role of PSPs in the pathogenesis of T. gondii.

The Extracellular Milieu of Toxoplasma's Lytic Cycle Drives Lab Adaptation, Primarily by Transcriptional Reprogramming

Evolve and resequencing (E&R) was applied to lab adaptation of Toxoplasma gondii for over 1,500 generations with the goal of mapping host-independent in vitro virulence traits. Phenotypic assessments of steps across the lytic cycle revealed that only traits needed in the extracellular milieu evolved. Nonsynonymous single-nucleotide polymorphisms (SNPs) in only one gene, a P4 flippase, fixated across two different evolving populations, whereas dramatic changes in the transcriptional signature of extracellular parasites were identified. Newly developed computational tools correlated phenotypes evolving at different rates with specific transcriptomic changes. A set of 300 phenotype-associated genes was mapped, of which nearly 50% is annotated as hypothetical. Validation of a select number of genes by knockouts confirmed their role in lab adaptation and highlights novel mechanisms underlying in vitro virulence traits. Further analyses of differentially expressed genes revealed the development of a “pro-tachyzoite” profile as well as the upregulation of the fatty acid biosynthesis (FASII) pathway. The latter aligned with the P4 flippase SNP and aligned with a low abundance of medium-chain fatty acids at low passage, indicating this is a limiting factor in extracellular parasites. In addition, partial overlap with the bradyzoite differentiation transcriptome in extracellular parasites indicated that stress pathways are involved in both situations. This was reflected in the partial overlap between the assembled ApiAP2 and Myb transcription factor network underlying the adapting extracellular state with the bradyzoite differentiation program. Overall, E&R is a new genomic tool successfully applied to map the development of polygenic traits underlying in vitro virulence of T. gondii.

IMPORTANCE It has been well established that prolonged in vitro cultivation of Toxoplasma gondii augments progression of the lytic cycle. This lab adaptation results in increased capacities to divide, migrate, and survive outside a host cell, all of which are considered host-independent virulence factors. However, the mechanistic basis underlying these enhanced virulence features is unknown. Here, E&R was utilized to empirically characterize the phenotypic, genomic, and transcriptomic changes in the non-lab-adapted strain, GT1, during 2.5 years of lab adaptation. This identified the shutdown of stage differentiation and upregulation of lipid biosynthetic pathways as the key processes being modulated. Furthermore, lab adaptation was primarily driven by transcriptional reprogramming, which rejected the starting hypothesis that genetic mutations would drive lab adaptation. Overall, the work empirically shows that lab adaptation augments T. gondii’s in vitro virulence by transcriptional reprogramming and that E&R is a powerful new tool to map multigenic traits.

The pathogenicity and virulence of Toxoplasma gondii

Toxoplasma gondii is a parasitic protist infecting a wide group of warm-blooded animals, ranging from birds to humans. While this infection is usually asymptomatic in healthy individuals, it can also lead to severe ocular or neurological outcomes in immunocompromised individuals or in developing fetuses. This obligate intracellular parasite has the ability to infect a considerable range of nucleated cells and can propagate in the intermediate host. Yet, under the pressure of the immune system it transforms into an encysted persistent form residing primarily in the brain and muscle tissues. Encysted parasites, which are resistant to current medication, may reactivate and give rise to an acute infection. The clinical outcome of toxoplasmosis depends on a complex balance between the host immune response and parasite virulence factors. Susceptibility to the disease is thus determined by both parasite strains and host species. Recent advances on our understanding of host cell-parasite interactions and parasite virulence have brought new insights into the pathophysiology of T. gondii infection and are summarized here.

Characterization of a MOB1 Homolog in the Apicomplexan Parasite Toxoplasma gondii

Simple Summary

Monopolar spindle One Binder1 (MOB1) proteins regulate key cellular functions, namely cell multiplication and cell division. The unicellular parasite Toxoplasma gondii transitions between several morphological stages, with the need to control the number of parasites in its cellular environment. We hypothesized that MOB1 proteins could participate in the regulation of the T. gondii life cycle, having identified one MOB1 protein (TgMOB1) coded in its genome. However, this study shows that TgMOB1 presents divergent features. While in organisms studied to date the lack of MOB1 has led to cell division defects, this did not occur in T. gondii in vitro cultures where mob1 was not an essential gene. Additionally, the identification of TgMOB1 proximity interacting partners detected novel MOB1 interactors. Still, TgMOB1 localizes to the region between the new-forming nuclei during cell division, and T. gondii parasites multiply slower with TgMOB1 overexpression and faster when there is a lack of TgMOB1, indicating an intricate role for TgMOB1 in T. gondii. This study uncovers new features of the T. gondii biology, a zoonotic parasite and model organism for the phylum Apicomplexa, and highlights the complex roles MOB1 proteins may assume, with possible implications for disease processes.

Abstract

Monopolar spindle One Binder1 (MOB1) proteins are conserved components of the tumor-suppressing Hippo pathway, regulating cellular processes such as cytokinesis. Apicomplexan parasites present a life cycle that relies on the parasites’ ability to differentiate between stages and regulate their proliferation; thus, Hippo signaling pathways could play an important role in the regulation of the apicomplexan life cycle. Here, we report the identification of one MOB1 protein in the apicomplexan Toxoplasma gondii. To characterize the function of MOB1, we generated gain-of-function transgenic lines with a ligand-controlled destabilization domain, and loss-of-function clonal lines obtained through CRISPR/Cas9 technology. Contrary to what has been characterized in other eukaryotes, MOB1 is not essential for cytokinesis in T. gondii. However, this picture is complex since we found MOB1 localized between the newly individualized daughter nuclei at the end of mitosis. Moreover, we detected a significant delay in the replication of overexpressing tachyzoites, contrasting with increased replication rates in knockout tachyzoites. Finally, using the proximity-biotinylation method, BioID, we identified novel members of the MOB1 interactome, a probable consequence of the observed lack of conservation of some key amino acid residues. Altogether, the results point to a complex evolutionary history of MOB1 roles in apicomplexans, sharing properties with other eukaryotes but also with divergent features, possibly associated with their complex life cycle.

Identification of Oocyst-Driven Toxoplasma gondii Infections in Humans and Animals through Stage-Specific Serology-Current Status and Future Perspectives

The apicomplexan zoonotic parasite Toxoplasma gondii has three infective stages: sporozoites in sporulated oocysts, which are shed in unsporulated form into the environment by infected felids; tissue cysts containing bradyzoites, and fast replicating tachyzoites that are responsible for acute toxoplasmosis. The contribution of oocysts to infections in both humans and animals is understudied despite being highly relevant. Only a few diagnostic antigens have been described to be capable of discriminating which parasite stage has caused an infection. Here we provide an extensive overview of the antigens and serological assays used to detect oocyst-driven infections in humans and animals according to the literature. In addition, we critically discuss the possibility to exploit the increasing knowledge of the T. gondii genome and the various 'omics datasets available, by applying predictive algorithms, for the identification of new oocyst-specific proteins for diagnostic purposes. Finally, we propose a workflow for how such antigens and assays based on them should be evaluated to ensure reproducible and robust results.

Toxoplasma gondii Tyrosine-Rich Oocyst Wall Protein: A Closer Look through an In Silico Prism

Toxoplasmosis is a global threat with significant zoonotic concern. The present in silico study was aimed at determination of bioinformatics features and immunogenic epitopes of a tyrosine-rich oocyst wall protein (TrOWP) of Toxoplasma gondii. After retrieving the amino acid sequence from UniProt database, several parameters were predicted including antigenicity, allergenicity, solubility and physico-chemical features, signal peptide, transmembrane domain, and posttranslational modifications. Following secondary and tertiary structure prediction, the 3D model was refined, and immunogenic epitopes were forecasted. It was a 25.57 kDa hydrophilic molecule with 236 residues, a signal peptide, and significant antigenicity scores. Moreover, several linear and conformational B-cell epitopes were present. Also, potential mouse and human cytotoxic T-lymphocyte (CTL) and helper T-lymphocyte (HTL) epitopes were predicted in the sequence. The findings of the present in silico study are promising as they render beneficial characteristics of TrOWP to be included in future vaccination experiments.

Sexual Development in Non-Human Parasitic Apicomplexa: Just Biology or Targets for Control?

Simple Summary

Cellular reproduction is a key part of the apicomplexan life cycle, and both mitotic (asexual) and meiotic (sexual) cell divisions produce new individual cells. Sexual reproduction in most eukaryotic taxa indicates that it has had considerable success during evolution, and it must confer profound benefits, considering its significant costs. The phylum Apicomplexa consists of almost exclusively parasitic single-celled eukaryotic organisms that can affect a wide host range of animals from invertebrates to mammals. Their development is characterized by complex steps in which asexual and sexual replication alternate and the fertilization of a macrogamete by a microgamete results in the formation of a zygote that undergoes meiosis, thus forming a new generation of asexual stages. In apicomplexans, sex is assumed to be induced by the (stressful) condition of having to leave the host, and either gametes or zygotes (or stages arising from it) are transmitted to a new host. Therefore, sex and meiosis are linked to parasite transmission, and consequently dissemination, which are key to the parasitic lifestyle. We hypothesize that improved knowledge of the sexual biology of the Apicomplexa will be essential to design and implement effective transmission-blocking strategies for the control of the major parasites of this group.

Abstract

The phylum Apicomplexa is a major group of protozoan parasites including gregarines, coccidia, haemogregarines, haemosporidia and piroplasms, with more than 6000 named species. Three of these subgroups, the coccidia, hemosporidia, and piroplasms, contain parasites that cause important diseases of humans and animals worldwide. All of them have complex life cycles involving a switch between asexual and sexual reproduction, which is key to their development. Fertilization (i.e., fusion of female and male cells) results in the formation of a zygote that undergoes meiosis, forming a new generation of asexual stages. In eukaryotes, sexual reproduction is the predominant mode of recombination and segregation of DNA. Sex is well documented in many protist groups, and together with meiosis, is frequently linked with transmission to new hosts. Apicomplexan sexual stages constitute a bottleneck in the life cycle of these parasites, as they are obligatory for the development of new transmissible stages. Consequently, the sexual stages represent attractive targets for vaccination. Detailed understanding of apicomplexan sexual biology will pave the way for the design and implementation of effective transmission-blocking strategies for parasite control. This article reviews the current knowledge on the sexual development of Apicomplexa and the progress in transmission-blocking vaccines for their control, their advantages and limitations and outstanding questions for the future.

Dynamically expressed genes provide candidate viability biomarkers in a model coccidian

Eimeria parasites cause enteric disease in livestock and the closely related Cyclosporacayetanensis causes human disease. Oocysts of these coccidian parasites undergo maturation (sporulation) before becoming infectious. Here, we assessed transcription in maturing oocysts of Eimeria acervulina, a widespread chicken parasite, predicted gene functions, and determined which of these genes also occur in C. cayetanensis. RNA-Sequencing yielded ~2 billion paired-end reads, 92% of which mapped to the E. acervulina genome. The ~6,900 annotated genes underwent temporally-coordinated patterns of gene expression. Fifty-three genes each contributed >1,000 transcripts per million (TPM) throughout the study interval, including cation-transporting ATPases, an oocyst wall protein, a palmitoyltransferase, membrane proteins, and hypothetical proteins. These genes were enriched for 285 gene ontology (GO) terms and 13 genes were ascribed to 17 KEGG pathways, defining housekeeping processes and functions important throughout sporulation. Expression differed in mature and immature oocysts for 40% (2,928) of all genes; of these, nearly two-thirds (1,843) increased their expression over time. Eight genes expressed most in immature oocysts, encoding proteins promoting oocyst maturation and development, were assigned to 37 GO terms and 5 KEGG pathways. Fifty-six genes underwent significant upregulation in mature oocysts, each contributing at least 1,000 TPM. Of these, 40 were annotated by 215 GO assignments and 9 were associated with 18 KEGG pathways, encoding products involved in respiration, carbon fixation, energy utilization, invasion, motility, and stress and detoxification responses. Sporulation orchestrates coordinated changes in the expression of many genes, most especially those governing metabolic activity. Establishing the long-term fate of these transcripts in sporulated oocysts and in senescent and deceased oocysts will further elucidate the biology of coccidian development, and may provide tools to assay infectiousness of parasite cohorts. Moreover, because many of these genes have homologues in C. cayetanensis, they may prove useful as biomarkers for risk.

The developmental trajectories of Toxoplasma stem from an elaborate epigenetic rewiring

Toxoplasma gondii is considered to be one of the most successful parasitic pathogens. It owes this success to its flexibility in responding to signals emanating from the different environments it encounters during its multihost life cycle. The adaptability of this unicellular organism relies on highly coordinated and evolutionarily optimized developmental abilities that allow it to adopt the forms best suited to the requirements of each environment. Here we discuss recent outstanding studies that have uncovered how master regulators epigenetically regulate the cryptic process of sexual development and the transition to chronicity. We also highlight the molecular and technical advances that allow the field to embark on a new journey of epigenetic reprogramming of T. gondii development.

Impact of Toxoplasma gondii infection on TM3 Leydig cells: Alterations in testosterone and cytokines levels

Leydig cells play pivotal roles in eliciting male characteristics by producing testosterone and any damage to these cells can compromise male fertility Toxoplasma gondii (T. gondii) is an intracellular parasite capable to invade any nucleated cell, including cells from male reproductive system. Herein, we evaluated the capacity of RH strain of T. gondii to infect TM3 Leydig cells and the impact of this infection on testosterone and inflammatory mediators production. We first, by performing adherence, infection, and intracellular proliferation assays, we found a significant increase in the number of infected Leydig cells, peaking 48 h after the infection with T. gondii. Supernatants of TM3 infected cells exhibited, in a time-dependent manner, increased levels of testosterone as well as monocyte chemoattractant protein-1 (MCP-1) and interferon-γ (IFN-γ), which is correlated with the robust T. gondii infection. In conclusion, our study provides new insights regarding the harmful effects of T. gondii infection on male reproductive system.

Recent achievements and doors opened for coccidian parasite research and development through transcriptomics of enteric sexual stages

The Coccidia is the largest group of parasites within the Apicomplexa, a phylum of unicellular, obligate parasites characterized by the possession of an apical complex of organelles and structures in the asexual stages of their life cycles, as well as by a sexual reproductive phase that occurs enterically in host animals. Coccidian sexual reproduction involves morphologically distinct microgametes and macrogametes that combine to form a diploid zygote and, ultimately, following meiosis and mitosis, haploid, infectious sporozoites, inside sporocysts within an oocyst. Recent transcriptomic analyses have identified genes involved in coccidian sexual stage development and reproduction, including genes encoding for microgamete- and macrogamete-specific proteins with roles in gamete motility, fusion and fertilization, and in the formation of the resilient oocyst wall that allows coccidians to persist for long periods in the environment. Transcriptomics has also provided important clues about the regulation of gene expression in the transformation of parasites from one developmental stage to the next, a complex sequence of events that may involve transcription factors such as the apicomplexan Apetala2 (ApiAP2) family, alternative splicing, regulatory RNAs and MORC (a microrchida homologue and regulator of sexual stage development in Toxoplasma gondii). The molecular dissection of coccidian sexual development and reproduction by transcriptomic analyses may lead to the development of novel transmission-blocking strategies.

Protein kinases in Toxoplasma gondii

Toxoplasma gondii is an obligatory intracellular pathogen that causes life threatening illness in immunodeficient individuals, miscarriage in pregnant woman, and blindness in newborn children. Similar to any other eukaryotic cell, protein kinases play critical and essential roles in the Toxoplasma life cycle. Accordingly, many studies have focused on identifying and defining the mechanism of function of these signalling proteins with a long-term goal to develop anti-Toxoplasma therapeutics. In this review, we briefly discuss classification and key components of the catalytic domain which are critical for functioning of kinases, with a focus on domains, families, and groups of kinases within Toxoplasma. More importantly, this article provides a comprehensive, current overview of research on kinase groups in Toxoplasma including the established eukaryotic AGC, CAMK, CK1, CMGC, STE, TKL families and the apicomplexan-specific FIKK, ROPK and WNG family of kinases. This work provides an overview and discusses current knowledge on Toxoplasma kinases including their localization, function, signalling network and role in acute and chronic pathogenesis, with a view towards the future in probing kinases as viable drug targets.

Insulinase-like Protease 1 Contributes to Macrogamont Formation in Cryptosporidium parvum

The apicomplexan parasite Cryptosporidium parvum contains an expanded family of 22 insulinase-like proteases (INS), a feature that contrasts with their otherwise streamlined genome. Here, we examined the function of INS1, which is most similar to the human insulinase protease that cleaves a variety of small peptide substrates. INS1 is an M16A clan member and contains a signal peptide, an N-terminal domain with the HXXEH active site, followed by three inactive domains. Unlike previously studied C. parvum INS proteins that are expressed in sporozoites and during merogony, INS1 was expressed exclusively in macrogamonts, where it was localized in small cytoplasmic vesicles. Although INS1 did not colocalize with the oocyst wall protein recognized by the antibody OW50, immune-electron microscopy indicated that INS1 resides in small vesicles in the secretory system. Notably, these small INS1-positive vesicles were often in close proximity to large OW50-positive vacuoles resembling wall-forming bodies, which contain precursors for oocyst wall formation. Genetic deletion of INS1, or replacement with an active-site mutant, resulted in lower formation of macrogamonts in vitro and reduced oocyst shedding in vivo Our findings reveal that INS1 functions in the formation or maturation of macrogamonts and that its loss results in attenuated virulence in immunocompromised mice.IMPORTANCE Cryptosporidiosis is a debilitating diarrheal disease in young children in developing countries. The absence of effective treatments or vaccines makes this infection very difficult to manage in susceptible populations. Although the oral dose of oocysts needed to cause infection is low, infected individuals shed very high numbers of oocysts, readily contaminating the environment. Our studies demonstrate that the protease INS1 is important for formation of female sexual stages and that in its absence, parasites produce fewer oocysts and are attenuated in immunocompromised mice. These findings suggest that mutants lacking INS1, or related proteases, are useful for further characterizing the pathway that leads to macrogamont maturation and oocyst wall formation.

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

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

High-Resolution Mapping of Transcription Initiation in the Asexual Stages of Toxoplasma gondii

Toxoplasma gondii is a common parasite of humans and animals, causing life-threatening disease in the immunocompromized, fetal abnormalities when contracted during gestation, and recurrent ocular lesions in some patients. Central to the prevalence and pathogenicity of this protozoan is its ability to adapt to a broad range of environments, and to differentiate between acute and chronic stages. These processes are underpinned by a major rewiring of gene expression, yet the mechanisms that regulate transcription in this parasite are only partially characterized. Deciphering these mechanisms requires a precise and comprehensive map of transcription start sites (TSSs); however, Toxoplasma TSSs have remained incompletely defined. To address this challenge, we used 5'-end RNA sequencing to genomically assess transcription initiation in both acute and chronic stages of Toxoplasma. Here, we report an in-depth analysis of transcription initiation at promoters, and provide empirically-defined TSSs for 7603 (91%) protein-coding genes, of which only 1840 concur with existing gene models. Comparing data from acute and chronic stages, we identified instances of stage-specific alternative TSSs that putatively generate mRNA isoforms with distinct 5' termini. Analysis of the nucleotide content and nucleosome occupancy around TSSs allowed us to examine the determinants of TSS choice, and outline features of Toxoplasma promoter architecture. We also found pervasive divergent transcription at Toxoplasma promoters, clustered within the nucleosomes of highly-symmetrical phased arrays, underscoring chromatin contributions to transcription initiation. Corroborating previous observations, we asserted that Toxoplasma 5' leaders are among the longest of any eukaryote studied thus far, displaying a median length of approximately 800 nucleotides. Further highlighting the utility of a precise TSS map, we pinpointed motifs associated with transcription initiation, including the binding sites of the master regulator of chronic-stage differentiation, BFD1, and a novel motif with a similar positional arrangement present at 44% of Toxoplasma promoters. This work provides a critical resource for functional genomics in Toxoplasma, and lays down a foundation to study the interactions between genomic sequences and the regulatory factors that control transcription in this parasite.

Key Limitations and New Insights Into the Toxoplasma gondii Parasite Stage Switching for Future Vaccine Development in Human, Livestock, and Cats

Toxoplasmosis is a parasitic disease affecting human, livestock and cat. Prophylactic strategies would be ideal to prevent infection. In a One Health vaccination approach, the objectives would be the prevention of congenital disease in both women and livestock, prevention/reduction of T. gondii tissue cysts in food-producing animals; and oocyst shedding in cats. Over the last few years, an explosion of strategies for vaccine development, especially due to the development of genetic-engineering technologies has emerged. The field of vaccinology has been exploring safer vaccines by the generation of recombinant immunogenic proteins, naked DNA vaccines, and viral/bacterial recombinants vectors. These strategies based on single- or few antigens, are less efficacious than recombinant live-attenuated, mostly tachyzoite T. gondii vaccine candidates. Reflections on the development of an anti-Toxoplasma vaccine must focus not only on the appropriate route of administration, capable of inducing efficient immune response, but also on the choice of the antigen (s) of interest and the associated delivery systems. To answer these questions, the choice of the animal model is essential. If mice helped in understanding the protection mechanisms, the data obtained cannot be directly transposed to humans, livestock and cats. Moreover, effectiveness vaccines should elicit strong and protective humoral and cellular immune responses at both local and systemic levels against the different stages of the parasite. Finally, challenge protocols should use the oral route, major natural route of infection, either by feeding tissue cysts or oocysts from different T. gondii strains. Effective Toxoplasma vaccines depend on our understanding of the (1) protective host immune response during T. gondii invasion and infection in the different hosts, (2) manipulation and modulation of host immune response to ensure survival of the parasites able to evade and subvert host immunity, (3) molecular mechanisms that define specific stage development. This review presents an overview of the key limitations for the development of an effective vaccine and highlights the contributions made by recent studies on the mechanisms behind stage switching to offer interesting perspectives for vaccine development.

MOB: Pivotal Conserved Proteins in Cytokinesis, Cell Architecture and Tissue Homeostasis

The MOB family proteins are constituted by highly conserved eukaryote kinase signal adaptors that are often essential both for cell and organism survival. Historically, MOB family proteins have been described as kinase activators participating in Hippo and Mitotic Exit Network/ Septation Initiation Network (MEN/SIN) signaling pathways that have central roles in regulating cytokinesis, cell polarity, cell proliferation and cell fate to control organ growth and regeneration. In metazoans, MOB proteins act as central signal adaptors of the core kinase module MST1/2, LATS1/2, and NDR1/2 kinases that phosphorylate the YAP/TAZ transcriptional co-activators, effectors of the Hippo signaling pathway. More recently, MOBs have been shown to also have non-kinase partners and to be involved in cilia biology, indicating that its activity and regulation is more diverse than expected. In this review, we explore the possible ancestral role of MEN/SIN pathways on the built-in nature of a more complex and functionally expanded Hippo pathway, by focusing on the most conserved components of these pathways, the MOB proteins. We discuss the current knowledge of MOBs-regulated signaling, with emphasis on its evolutionary history and role in morphogenesis, cytokinesis, and cell polarity from unicellular to multicellular organisms.

Host sensing and signal transduction during Toxoplasma stage conversion

The intracellular parasite Toxoplasma gondii infects nucleated cells in virtually all warm-blooded vertebrates, including one-third of the human population. While immunocompetent hosts do not typically show symptoms of acute infection, parasites are retained in latent tissue cysts that can be reactivated upon immune suppression, potentially damaging key organ systems. Toxoplasma has a multistage life cycle that is intimately linked to environmental stresses and host signals. As this protozoan pathogen is transmitted between multiple hosts and tissues, it evaluates these external signals to appropriately differentiate into distinct life cycle stages, such as the transition from its replicative stage (tachyzoite) to the latent stage (bradyzoite) that persists as tissue cysts. Additionally, in the gut of its definitive host, felines, Toxoplasma converts into gametocytes that produce infectious oocysts (sporozoites) that are expelled into the environment. In this review, we highlight recent advances that have illuminated the interfaces between Toxoplasma and host and how these interactions control parasite stage conversion. Mechanisms underlying these stage transitions are important targets for therapeutic intervention aimed at thwarting parasite transmission and pathogenesis.

X-ray crystal structure and specificity of the Toxoplasma gondii ME49 TgAPN2

Toxoplasmosis is a parasitic disease caused by infection with Toxoplasma gondii that currently has few therapeutic options. The M1 aminopeptidase enzymes have been shown to be attractive targets for anti-parasitic agents and/or vaccine candidates, suggesting potential to re-purpose inhibitors between parasite M1 aminopeptidase targets. The M1 aminopeptidase TgAPN2 has been suggested to be a potential new drug target for toxoplasmosis. Here we investigate the structure and function of TgAPN2, a homologue of the antimalarial drug target PfA-M1, and evaluate the capacity to use inhibitors that target PfA-M1 against TgAPN2. The results show that despite a similar overall fold, the TgAPN2 has a unique substrate specificity and inhibition profile. Sequence and structure differences are investigated and show how comparative structure-activity relationships may provide a route to obtaining potent inhibitors of TgAPN2.

Transcriptional ups and downs: patterns of gene expression in the life cycle of Toxoplasma gondii

Toxoplasma gondii reproduces sexually in felines and asexually in virtually all warm-blooded animals, including humans. This obligate intracellular parasite alternates between biologically distinct developmental stages throughout its complex life cycle. Stage conversion is crucial for T. gondii transmission, persistence, and the maintenance of genetic diversity within the species. Genome-wide comparative transcriptomic studies have contributed invaluable insights into the regulatory gene networks underlying T. gondii development.