Identification of tissue cyst wall components by transcriptome analysis of in vivo and in vitro Toxoplasma gondii bradyzoites

Could metformin modulate the outcome of chronic murine toxoplasmosis?

Toxoplasmosis is a pervasive parasitic infection possessing a chief impact on both public health and veterinary medicine. Unfortunately, the commercially-available anti-Toxoplasma agents have either serious side effects or diminished efficiency, specifically on the Toxoplasma tissue cysts. In the present study, metformin (The first-line treatment for type 2 diabetes mellitus) was investigated for the first time against chronic cerebral toxoplasmosis in mice model experimentally-infected with ME49 strain versus spiramycin. Two metformin regimens were applied; starting one week before the infection and four weeks PI. Parasitological, ultrastructural, histopathological, immunohistochemical, immunological, and biochemical assessments were performed. The anti-parasitic effect of metformin was granted by the statistically-significant reduction in tissue-cyst burden in both treatment regimens. This was accompanied by markedly-mutilated ultrastructure and profound amelioration of the cerebral histopathology with remarkable decline in the brain CD4+ and CD8+ T cell count. Besides, diminution of anti-Toxoplasma IgG and brain GSH levels was evident. Ultimately, the present findings highlighted the powerful promising therapeutic role of metformin in the management of chronic toxoplasmosis on a basis of anti-parasitic, anti-inflammatory, and anti-oxidant possessions.

Enzymatically enhanced ultrastructure expansion microscopy unlocks expansion of in vitro Toxoplasma gondii cysts

Expansion microscopy (ExM) is an innovative approach to achieve super-resolution images without using super-resolution microscopes, based on the physical expansion of the sample. The advent of ExM has unlocked the detail of super-resolution images for a broader scientific circle, lowering the cost and entry skill requirements for the field. One of its branches, ultrastructure expansion microscopy (U-ExM), has become popular among research groups studying apicomplexan parasites, including the acute stage of Toxoplasma gondii infection. Here, we show that the chronic cyst-forming stage of Toxoplasma, however, resists U-ExM expansion, impeding precise protein localization. We then solve the in vitro cyst's resistance to denaturation required for successful U-ExM. As the cyst's main structural protein CST1 contains a mucin domain, we added an enzymatic digestion step using the pan-mucinase StcE prior to the expansion protocol. This allowed full expansion of the cysts in fibroblasts and primary neuronal cell culture without disrupting immunofluorescence analysis of parasite proteins. Using StcE-enhanced U-ExM, we clarified the localization of the GRA2 protein, which is important for establishing a normal cyst, observing GRA2 granules spanning across the CST1 cyst wall. The StcE-U-ExM protocol allows accurate pinpointing of proteins in the bradyzoite cyst, which will greatly facilitate investigation of the underlying biology of cyst formation and its vulnerabilities.

IMPORTANCE

Toxoplasma gondii is an intracellular parasite capable of establishing long-term chronic infection in nearly all warm-blooded animals. During the chronic stage, parasites encapsulate to form cysts predominantly in neurons and skeletal muscle. Current anti-Toxoplasma drugs do not eradicate chronic parasites, leaving a reservoir of infection. The cyst is critical for disease transmission and pathology, yet it is harder to study, with the function of many chronic-stage proteins still unknown. Ultrastructure expansion microscopy, a new method to overcome the light microscopy's diffraction limit by physically expanding the sample, allowed in-depth studies of acute Toxoplasma infection. We show that Toxoplasma cysts resist expansion using standard protocol, but an additional enzymatic digestion with the mucinase StcE allows full expansion. This protocol offers new avenues for examining the chronic stage, including precise spatial organization of cyst-specific proteins, linking these locations to morphological structures, and detailed investigations of components of the durable cyst wall.

Constitutive upregulation of transcription factors underlies permissive bradyzoite differentiation in a natural isolate of Toxoplasma gondii

Toxoplasma gondii bradyzoites play a critical role in pathology due to their long-term persistence in intermediate hosts and their potential to reactivate, resulting in severe diseases in immunocompromised individuals. Currently, there is no effective treatment for eliminating bradyzoites. Hence, better in vitro models of T. gondii bradyzoite development would facilitate identification of therapeutic targets for bradyzoites. Herein, we characterized a natural isolate of T. gondii, called Tg68, which showed slower in vitro replication of tachyzoites, and permissive bradyzoite development under stress conditions in vitro. Transcriptional analysis revealed constitutive expression in Tg68 tachyzoites of the key regulators of bradyzoite development including BFD1, BFD2, and several AP2 factors. Consistent with this finding, Tg68 tachyzoites expressed high levels of bradyzoite-specific genes including BAG1, ENO1, and LDH2. Moreover, after stress-induced differentiation, Tg68 bradyzoites exhibited gene expression profiles of mature bradyzoites, even at early time points. These data suggest that Tg68 tachyzoites exist in a pre-bradyzoite stage primed to readily develop into mature bradyzoites under stress conditions in vitro. Tg68 presents a novel model for differentiation in vitro that will serve as a useful tool for the investigation of bradyzoite biology and the development of therapeutics.

IMPORTANCE

Toxoplasma gondii is a widespread protozoan that chronically infects ~30% of the world's population. T. gondii can differentiate between the fast-growing life stage that causes acute infection and the slow-growing stage that persists in the host for extended periods of time. The slow-growing stage cannot be eliminated by the host immune response or currently known antiparasitic drugs. Studies on the slow-growing stage have been limited due to the limitations of in vivo experiments and the challenges of in vitro manipulation. Here, we characterize a natural isolate of T. gondii, which constitutively expresses factors that drive development and that is permissive to convert to the slow-growing stage under stress conditions in vitro. The strain presents a novel in vitro model for studying the chronic phase of toxoplasmosis and identifying new therapeutic treatments for chronic infections.

Translation initiation factor eIF1.2 promotes Toxoplasma stage conversion by regulating levels of key differentiation factors

The parasite Toxoplasma gondii persists in its hosts by converting from replicating tachyzoites to latent bradyzoites housed in tissue cysts. The molecular mechanisms that mediate T. gondii differentiation remain poorly understood. Through a mutagenesis screen, we identified translation initiation factor eIF1.2 as a critical factor for T. gondii differentiation. A F97L mutation in eIF1.2 or the genetic ablation of eIF1.2 (∆eif1.2) markedly impeded bradyzoite cyst formation in vitro and in vivo. We demonstrated, at single-molecule level, that the eIF1.2 F97L mutation impacts the scanning process of the ribosome preinitiation complex on a model mRNA. RNA sequencing and ribosome profiling experiments unveiled that ∆eif1.2 parasites are defective in upregulating bradyzoite induction factors BFD1 and BFD2 during stress-induced differentiation. Forced expression of BFD1 or BFD2 significantly restored differentiation in ∆eif1.2 parasites. Together, our findings suggest that eIF1.2 functions by regulating the translation of key differentiation factors necessary to establish chronic toxoplasmosis.

Toxoplasma gondii chitinase-like protein TgCLP1 regulates the parasite cyst burden

Toxoplasma, an important intracellular parasite of humans and animals, causes life-threatening toxoplasmosis in immunocompromised individuals. Although Toxoplasma secretory proteins during acute infection (tachyzoite, which divides rapidly and causes inflammation) have been extensively characterized, those involved in chronic infection (bradyzoite, which divides slowly and is surrounded by a cyst wall) remain uncertain. Regulation of the cyst wall is essential to the parasite life cycle, and polysaccharides, such as chitin, in the cyst wall are necessary to sustain latent infection. Toxoplasma secretory proteins during the bradyzoite stage may have important roles in regulating the cyst wall via polysaccharides. Here, we focused on characterizing the hypothetical T. gondii chitinase, chitinase-like protein 1 (TgCLP1). We found that the chitinase-like domain containing TgCLP1 is partially present in the bradyzoite microneme and confirmed, albeit partially, its previous identification in the tachyzoite microneme. Furthermore, although parasites lacking TgCLP1 could convert from tachyzoites to bradyzoites and make an intact cyst wall, they failed to convert from bradyzoites to tachyzoites, indicating that TgCLP1 is necessary for bradyzoite reactivation. Taken together, our findings deepen our understanding of the molecular basis of recrudescence and could contribute to the development of novel strategies for the control of toxoplasmosis.

Translation initiation factor eIF1.2 promotes Toxoplasma stage conversion by regulating levels of key differentiation factors

The parasite Toxoplasma gondii persists in its hosts by converting from replicating tachyzoites to latent bradyzoites housed in tissue cysts. The molecular mechanisms that mediate T. gondii differentiation remain poorly understood. Through a mutagenesis screen, we identified translation initiation factor eIF1.2 as a critical factor for T. gondii differentiation. A F97L mutation in eIF1.2 or the genetic ablation of eIF1.2 (Δ eif1.2 ) markedly impeded bradyzoite cyst formation in vitro and in vivo . We demonstrated, at single-molecule level, that the eIF1.2 F97L mutation impacts the scanning process of the ribosome preinitiation complex on a model mRNA. RNA sequencing and ribosome profiling experiments unveiled that Δ eif1.2 parasites are defective in upregulating bradyzoite induction factors BFD1 and BFD2 during stress-induced differentiation. Forced expression of BFD1 or BFD2 significantly restored differentiation in Δ eif1.2 parasites. Together, our findings suggest that eIF1.2 functions by regulating the translation of key differentiation factors necessary to establish chronic toxoplasmosis.

Constitutive upregulation of transcription factors underlies permissive bradyzoite differentiation in a natural isolate of Toxoplasma gondii

Toxoplasma gondii bradyzoites play a critical role in pathology due to their long-term persistence in intermediate hosts and their potential to reactivate, resulting in severe diseases in immunocompromised individuals. Currently there is no effective treatment for eliminating bradyzoites. Hence, better in vitro models of T. gondii cyst development would facilitate identification of therapeutic targets for bradyzoites. Herein we characterized a natural isolate of T. gondii, called Tg68, which showed slower in vitro replication of tachyzoites, and permissive bradyzoite development under stress conditions in vitro. Transcriptional analysis revealed constitutive expression in Tg68 tachyzoites of the key regulators of bradyzoite development including BFD1, BFD2, and several AP2 factors. Consistent with this finding, Tg68 tachyzoites expressed high levels of bradyzoite-specific genes including BAG1, ENO1, and LDH2. Moreover, after stress induced differentiation, Tg68 bradyzoites exhibited gene expression profiles of mature bradyzoites, even at early time points. These data suggest that Tg68 tachyzoites exist in a pre-bradyzoite stage primed to readily develop into mature bradyzoites under stress conditions in vitro. Tg68 presents a novel model for differentiation in vitro that will serve as a useful tool for investigation of bradyzoite biology and development of therapeutics.

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.

In vitro screening technologies for the discovery and development of novel drugs against Toxoplasma gondii

INTRODUCTION

Toxoplasmosis constitutes a challenge for public health, animal production and welfare. Since more than 60 years, only a limited panel of drugs has been in use for clinical applications.

AREAS COVERED

Herein, the authors describe the methodology and the results of library screening approaches to identify inhibitors of Toxoplasma gondii and related strains. The authors then provide the reader with their expert perspectives for the future.

EXPERT OPINION

Various library screening projects, in particular those using reporter strains, have led to the identification of numerous compounds with good efficacy and specificity in vitro. However, only few compounds are effective in suitable animal models such as rodents. Whereas no novel compound has cleared the hurdle to applications in humans, the few compounds with known indication and application profiles in human patients are of interest for further investigations. Taken together, drug repurposing as well as high-throughput screening of novel compound libraries may shorten the way to novel drugs against toxoplasmosis.

An ex vivo model of Toxoplasma recrudescence reveals developmental plasticity of the bradyzoite stage

IMPORTANCE

The classical depiction of the Toxoplasma lifecycle is bradyzoite excystation conversion to tachyzoites, cell lysis, and immune control, followed by the reestablishment of bradyzoites and cysts. In contrast, we show that tachyzoite growth slows independent of the host immune response at a predictable time point following excystation. Furthermore, we demonstrate a host cell-dependent pathway of continuous amplification of the cyst-forming bradyzoite population. The developmental plasticity of the excysted bradyzoites further underlines the critical role the cyst plays in the flexibility of the lifecycle of this ubiquitous parasite. This revised model of Toxoplasma recrudescence uncovers previously unknown complexity in the clinically important bradyzoite stage of the parasite, which opens the door to further study these novel developmental features of the Toxoplasma intermediate life cycle.

A transcriptional network required for bradyzoite development in Toxoplasma gondii is dispensable for recrudescent disease

Identification of regulators of Toxoplasma gondii bradyzoite development and cyst formation is the most direct way to address the importance of parasite development in long-term persistence and reactivation of this parasite. Here we show that a T. gondii gene (named Regulator of Cystogenesis 1; ROCY1) is sufficient for T. gondii bradyzoite formation in vitro and in vivo. ROCY1 encodes an RNA binding protein that has a preference for 3' regulatory regions of hundreds of T. gondii transcripts, and its RNA-binding domains are required to mediate bradyzoite development. Female mice infected with ΔROCY1 parasites have reduced (>90%) cyst burden. While viable parasites can be cultivated from brain tissue for up to 6 months post-infection, chronic brain-resident ΔROCY1 parasites have reduced oral infectivity compared to wild type. Despite clear defects in bradyzoite formation and oral infectivity, ΔROCY1 parasites were able to reactivate with similar timing and magnitude as wild type parasites for up to 5 months post-infection. Therefore while ROCY1 is a critical regulator of the bradyzoite developmental pathway, it is not required for parasite reactivation, raising new questions about the persisting life stage responsible for causing recrudescent disease.

The apicoplast is important for the viability and persistence of Toxoplasma gondii bradyzoites

Toxoplasma gondii is responsible for toxoplasmosis, a disease that can be serious when contracted during pregnancy, but can also be a threat for immunocompromised individuals. Acute infection is associated with the tachyzoite form that spreads rapidly within the host. However, under stress conditions, some parasites can differentiate into cyst-forming bradyzoites, residing mainly in the central nervous system, retina and muscle. Because this latent form of the parasite is resistant to all currently available treatments, and is central to persistence and transmission of the parasite, specific therapeutic strategies targeting this developmental stage need to be found. T. gondii contains a plastid of endosymbiotic origin called the apicoplast, which is an appealing drug target because it is essential for tachyzoite viability and contains several key metabolic pathways that are largely absent from the mammalian host. Its function in bradyzoites, however, is unknown. Our objective was thus to study the contribution of the apicoplast to the viability and persistence of bradyzoites during chronic toxoplasmosis. We have used complementary strategies based on stage-specific promoters to generate conditional bradyzoite mutants of essential apicoplast genes. Our results show that specifically targeting the apicoplast in both in vitro or in vivo-differentiated bradyzoites leads to a loss of long-term bradyzoite viability, highlighting the importance of this organelle for this developmental stage. This validates the apicoplast as a potential area to look for therapeutic targets in bradyzoites, with the aim to interfere with this currently incurable parasite stage.

A positive feedback loop controls Toxoplasma chronic differentiation

Successful infection strategies must balance pathogen amplification and persistence. In the obligate intracellular parasite Toxoplasma gondii this is accomplished through differentiation into dedicated cyst-forming chronic stages that avoid clearance by the host immune system. The transcription factor BFD1 is both necessary and sufficient for stage conversion; however, its regulation is not understood. In this study we examine five factors that are transcriptionally activated by BFD1. One of these is a cytosolic RNA-binding protein of the CCCH-type zinc-finger family, which we name bradyzoite formation deficient 2 (BFD2). Parasites lacking BFD2 fail to induce BFD1 and are consequently unable to fully differentiate in culture or in mice. BFD2 interacts with the BFD1 transcript under stress, and deletion of BFD2 reduces BFD1 protein levels but not messenger RNA abundance. The reciprocal effects on BFD2 transcription and BFD1 translation outline a positive feedback loop that enforces the chronic-stage gene-expression programme. Thus, our findings help explain how parasites both initiate and commit to chronic differentiation. This work provides new mechanistic insight into the regulation of T. gondii persistence, and can be exploited in the design of strategies to prevent and treat these key reservoirs of human infection.

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.

Advances towards the complete in vitro life cycle of Toxoplasma gondii

The full life cycle of Toxoplasma gondii cannot be recapitulated in vitro, and access to certain stages, such as mature tissue cysts (bradyzoites) and oocysts (sporozoites), traditionally requires animal experiments. This has greatly hindered the study of the biology of these morphologically and metabolically distinct stages, which are essential for the infection of humans and animals. However, several breakthrough advances have been made in recent years towards obtaining these life stages in vitro, such as the discovery of several molecular factors that induce differentiation and commitment to the sexual cycle, and different culture methods that use, for example, myotubes and intestinal organoids to obtain mature bradyzoites and different sexual stages of the parasite. We review these novel tools and approaches, highlight their limitations and challenges, and discuss what research questions can already be answered with these models. We finally identify future routes for recapitulating the entire sexual cycle in vitro.

The protein phosphatase 2A holoenzyme is a key regulator of starch metabolism and bradyzoite differentiation in Toxoplasma gondii

Phenotypic switching between tachyzoite and bradyzoite is the fundamental mechanism underpinning the pathogenicity and adaptability of the protozoan parasite Toxoplasma gondii. Although accumulation of cytoplasmic starch granules is a hallmark of the quiescent bradyzoite stage, the regulatory factors and mechanisms contributing to amylopectin storage in bradyzoites are incompletely known. Here, we show that T. gondii protein phosphatase 2A (PP2A) holoenzyme is composed of a catalytic subunit PP2A-C, a scaffold subunit PP2A-A and a regulatory subunit PP2A-B. Disruption of any of these subunits increased starch accumulation and blocked the tachyzoite-to-bradyzoite differentiation. PP2A contributes to the regulation of amylopectin metabolism via dephosphorylation of calcium-dependent protein kinase 2 at S679. Phosphoproteomics identified several putative PP2A holoenzyme substrates that are involved in bradyzoite differentiation. Our findings provide novel insight into the role of PP2A as a key regulator of starch metabolism and bradyzoite differentiation in T. gondii.

Importance of aspartyl protease 5 in the establishment of the intracellular niche during acute and chronic infection of Toxoplasma gondii

Virulence and persistence of the obligate intracellular parasite Toxoplasma gondii involve the secretion of effector proteins belonging to the family of dense granule proteins (GRAs) that act notably as modulators of the host defense mechanisms and participate in cyst wall formation. The subset of GRAs residing in the parasitophorous vacuole (PV) or exported into the host cell, undergo proteolytic cleavage in the Golgi upon the action of the aspartyl protease 5 (ASP5). In tachyzoites, ASP5 substrates play central roles in the morphology of the PV and the export of effectors across the translocon complex MYR1/2/3. Here, we used N-terminal amine isotopic labeling of substrates to identify novel ASP5 cleavage products by comparing the N-terminome of wild-type and Δasp5 lines in tachyzoites and bradyzoites. Validated substrates reside within the PV or PVM in an ASP5-dependent manner. Remarkably, Δasp5 bradyzoites are impaired in the formation of the cyst wall in vitro and exhibit a considerably reduced cyst burden in chronically infected animals. More specifically two-photon serial tomography of infected mouse brains revealed a comparatively reduced number and size of the cysts throughout the establishment of persistence in the absence of ASP5.

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.

The determinants regulating Toxoplasma gondii bradyzoite development

Toxoplasma gondii is an obligate intracellular zoonotic pathogen capable of infecting almost all cells of warm-blooded vertebrates. In intermediate hosts, this parasite reproduces asexually in two forms, the tachyzoite form during acute infection that proliferates rapidly and the bradyzoite form during chronic infection that grows slowly. Depending on the growth condition, the two forms can interconvert. The conversion of tachyzoites to bradyzoites is critical for T. gondii transmission, and the reactivation of persistent bradyzoites in intermediate hosts may lead to symptomatic toxoplasmosis. However, the mechanisms that control bradyzoite differentiation have not been well studied. Here, we review recent advances in the study of bradyzoite biology and stage conversion, aiming to highlight the determinants associated with bradyzoite development and provide insights to design better strategies for controlling toxoplasmosis.

Dense granule biogenesis, secretion, and function in Toxoplasma gondii

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

Immunomagnetic separation of Toxoplasma gondii and Hammondia spp. tissue cysts generated in cell culture

Toxoplasma gondii is commonly transmitted among animals and humans by ingestion of infected animal tissues or by consumption of food and water contaminated with environmentally-resistant oocysts excreted by cats. Tissue cysts and oocysts have different walls, whose structures and compositions are poorly known. Herein, we describe an immunomagnetic separation (IMS) method that was successfully used for purification of T. gondii tissue cysts generated in cell culture. We used an IgG monoclonal antibody (mAb) that reacts against antigens in tissue cyst walls. Many in vitro produced cysts were obtained by this IMS; >2,000 T. gondii cysts were isolated from a single culture flask of 25 cm². Tissue cysts from two Hammondia spp., H. hammondi, and H. heydorni, produced in cell culture were also separated using this method. As a reference, purification of tissue cysts by Percoll gradients was used. Percoll was able to separate T. gondii tissue cysts produced in mice but was not suitable for purifying T. gondii tissue cysts produced in vitro. The IMS described here should favor proteomic studies involving tissue cysts of T. gondii.

Starch Branching Enzyme 1 Is Important for Amylopectin Synthesis and Cyst Reactivation in Toxoplasma gondii

Toxoplasma gondii (T. gondii) bradyzoites facilitate chronic infections that evade host immune response. Furthermore, reactivation in immunocompromised individuals causes severe toxoplasmosis. The presence of abundant granules containing the branched starch amylopectin is major characteristic of bradyzoites that is nearly absent from tachyzoites that drive acute disease. T. gondii genome encodes to potential Starch branching enzyme 1 (SBE1) that creates branching during amylopectin biosynthesis. However, the physiological function of the amylopectin in T. gondii remains unclear. In this study, we generated a SBE1 knockout parasites and revealed that deletion of SBE1 caused amylopectin synthesis defects while having no significant impact on the growth of tachyzoites under normal culture conditions in vitro as well as virulence and brain cyst formation. Nevertheless, SBE1 knockout decreased the influx of exogenous glucose and reduced tachyzoites proliferation in nutrition-deficient conditions. Deletion of SBE1 together with the α-amylase (α-AMY), responsible for starch digestion, abolished amylopectin production and attenuated virulence while restoring brain cyst formation. In addition, cysts with defective amylopectin metabolism showed abnormal morphology and were avirulent to mice. In conclusion, SBE1 is essential for the synthesis of amylopectin, which serves as energy storage during the development and reactivation of bradyzoites. IMPORTANCE Toxoplasmosis has become a global, serious public health problem due to the extensiveness of the host. There are great differences in the energy metabolism in the different stages of infection. The most typical difference is the abundant accumulation of amylopectin granules in bradyzoites, which is almost absent in tachyzoites. Until now, the physiological functions of amylopectin have not been clearly elucidated. We focused on starch branching enzyme 1 (SBE1) in the synthesis pathway to reveal the exact physiological significance of amylopectin. Our study clarified the role of SBE1 in the synthesis pathway and amylopectin in tachyzoites and bradyzoites, and demonstrated that amylopectin, as an important carbon source, was critical to parasites growth under an unfavorable environment and the reactivation of bradyzoites to tachyzoites. The findings obtained from our study provides a new avenue for the development of Toxoplasma vaccines and anti-chronic toxoplasmosis drugs.

A functionally divergent transcription elongation factor 1-like protein in Toxoplasma gondii

Zinc ribbons, one of the largest fold groups among zinc fingers, often include proteins involved in the transcription machinery. Here, we identify and characterize one such zinc ribbon-bearing protein in the apicomplexan parasite Toxoplasma gondii, annotated as putative transcription elongation factor 1 (ELF1), with predicted functions in transcription and chromatin maintenance. We show that this ELF1 homolog, referred to as T. gondii ELF1-like divergent (TgELD), is expressed in both tachyzoite and bradyzoite developmental stages. TgELD associates with the cytoskeleton in the tachyzoites, while it transiently becomes a part of the cyst wall in the early bradyzoites, followed by a cytosolic and peripheral localization in late bradyzoites. TgELD is phosphorylated by a casein kinase 2-like protein, which has potential implications for its localization and function in the parasite.

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.

Primary brain cell infection by Toxoplasma gondii reveals the extent and dynamics of parasite differentiation and its impact on neuron biology

Toxoplasma gondii is a eukaryotic parasite that forms latent cysts in the brain of immunocompetent individuals. The latent parasite infection of the immune-privileged central nervous system is linked to most complications. With no drug currently available to eliminate the latent cysts in the brain of infected hosts, the consequences of neurons' long-term infection are unknown. It has long been known that T. gondii specifically differentiates into a latent form (bradyzoite) in neurons, but how the infected neuron responds to the infection remains to be elucidated. We have established a new in vitro model resulting in the production of mature bradyzoite cysts in brain cells. Using dual, host and parasite RNA-seq, we characterized the dynamics of differentiation of the parasite, revealing the involvement of key pathways in this process. Moreover, we identified how the infected brain cells responded to the parasite infection revealing the drastic changes that take place. We showed that neuronal-specific pathways are strongly affected, with synapse signalling being particularly affected, especially glutamatergic synapse signalling. The establishment of this new in vitro model allows investigating both the dynamics of parasite differentiation and the specific response of neurons to long-term infection by this parasite.

The Toxoplasma Polymorphic Effector GRA15 Mediates Seizure Induction by Modulating Interleukin-1 Signaling in the Brain

Toxoplasmic encephalitis can develop in individuals infected with the protozoan parasite Toxoplasma gondii and is typified by parasite replication and inflammation within the brain. Patients often present with seizures, but the parasite genes and host pathways involved in seizure development and/or propagation are unknown. We previously reported that seizure induction in Toxoplasma-infected mice is parasite strain dependent. Using quantitative trait locus mapping, we identify four loci in the Toxoplasma genome that potentially correlate with seizure development. In one locus, we identify the polymorphic virulence factor, GRA15, as a Toxoplasma gene associated with onset of seizures. GRA15 was previously shown to regulate host NF-κB-dependent gene expression during acute infections, and we demonstrate a similar role for GRA15 in brains of toxoplasmic encephalitic mice. GRA15 is important for increased expression of interleukin 1 beta (IL-1β) and other IL-1 pathway host genes, which is significant since IL-1 signaling is involved in onset of seizures. Inhibiting IL-1 receptor signaling reduced seizure severity in Toxoplasma-infected mice. These data reveal one mechanism by which seizures are induced during toxoplasmic encephalitis.

Role of amylopectin synthesis in Toxoplasma gondii and its implication in vaccine development against toxoplasmosis

Toxoplasma gondii is a ubiquitous pathogen infecting one-third of the global population. A significant fraction of toxoplasmosis cases is caused by reactivation of existing chronic infections. The encysted bradyzoites during chronic infection accumulate high levels of amylopectin that is barely present in fast-replicating tachyzoites. However, the physiological significance of amylopectin is not fully understood. Here, we identified a starch synthase (SS) that is required for amylopectin synthesis in T. gondii. Genetic ablation of SS abolished amylopectin production, reduced tachyzoite proliferation, and impaired the recrudescence of bradyzoites to tachyzoites. Disruption of the parasite Ca²⁺-dependent protein kinase 2 (CDPK2) was previously shown to cause massive amylopectin accumulation and bradyzoite death. Therefore, the Δcdpk2 mutant is thought to be a vaccine candidate. Notably, deleting SS in a Δcdpk2 mutant completely abolished starch accrual and restored cyst formation as well as virulence in mice. Together these results suggest that regulated amylopectin production is critical for the optimal growth, development and virulence of Toxoplasma. Not least, our data underscore a potential drawback of the Δcdpk2 mutant as a vaccine candidate as it may regain full virulence by mutating amylopectin synthesis genes like SS.

Toxoplasma gondii PPM3C, a secreted protein phosphatase, affects parasitophorous vacuole effector export

The intracellular parasite Toxoplasma gondii infects a large proportion of humans worldwide and can cause adverse complications in the settings of immune-compromise and pregnancy. T. gondii thrives within many different cell types due in part to its residence within a specialized and heavily modified compartment in which the parasite divides, termed the parasitophorous vacuole. Within this vacuole, numerous proteins optimize intracellular survival following their secretion by the parasite. We investigated the contribution of one of these proteins, TgPPM3C, predicted to contain a PP2C-class serine/threonine phosphatase domain and previously shown to interact with the protein MYR1, an essential component of a putative vacuolar translocon that mediates effector export into the host cell. Parasites lacking the TgPPM3C gene exhibit a minor growth defect in vitro, are avirulent during acute infection in mice, and form fewer cysts in mouse brain during chronic infection. Phosphoproteomic assessment of TgPPM3C deleted parasite cultures demonstrated alterations in the phosphorylation status of many secreted vacuolar proteins including two exported effector proteins, GRA16 and GRA28, as well as MYR1. Parasites lacking TgPPM3C are defective in GRA16 and GRA28 export, but not in the export of other MYR1-dependant effectors. Phosphomimetic mutation of two GRA16 serine residues results in export defects, suggesting that de-phosphorylation is a critical step in the process of GRA16 export. These findings provide another example of the emerging role of phosphatases in regulating the complex environment of the T. gondii parasitophorous vacuole and influencing the export of specific effector proteins from the vacuolar lumen into the host cell.

The flexible life cycle of the intracellular parasite Toxoplasma gondii allows it to infect many different types of warm-blooded hosts, as well as diverse cell types once inside the host organism. This formidable achievement is partly mediated by the establishment of a unique compartment following host cell invasion, termed the parasitophorous vacuole. While advancements have been made in cataloguing Toxoplasma secreted proteins that reside within this vacuole, the specific functions and contributions of many of these secreted parasite “tools” remain elusive. Here, we assessed the contribution of a parasite vacuolar protein called TgPPM3C, predicted to function as an enzyme that dephosphorylates other proteins. We found that deleting the TgPPM3C gene in the parasite results in a profound virulence defect during infection in mice, likely due to the dysregulated phosphorylation status of many vacuolar proteins detected by phosphoproteomic analysis of TgPPM3C-deleted parasites. We found that the phosphorylation status of one such protein, GRA16, influences its ability to cross the parasitophorous vacuole membrane and enter the host cell, where it is known to induce host transcriptional changes that benefit parasite growth. These findings illustrate the emerging role of Toxoplasma vacuolar phosphatases in regulating host-parasite interactions during infection.

Proximity biotinylation reveals novel secreted dense granule proteins of Toxoplasma gondii bradyzoites

Toxoplasma gondii is an obligate intracellular parasite which is capable of establishing life-long chronic infection in any mammalian host. During the intracellular life cycle, the parasite secretes an array of proteins into the parasitophorous vacuole (PV) where it resides. Specialized organelles called the dense granules secrete GRA proteins that are known to participate in nutrient acquisition, immune evasion, and host cell-cycle manipulation. Although many GRAs have been discovered which are expressed during the acute infection mediated by tachyzoites, little is known about those that participate in the chronic infection mediated by the bradyzoite form of the parasite. In this study, we sought to uncover novel bradyzoite-upregulated GRA proteins using proximity biotinylation, which we previously used to examine the secreted proteome of the tachyzoites. Using a fusion of the bradyzoite upregulated protein MAG1 to BirA* as bait and a strain with improved switch efficiency, we identified a number of novel GRA proteins which are expressed in bradyzoites. After using the CRISPR/Cas9 system to characterize these proteins by gene knockout, we focused on one of these GRAs (GRA55) and found it was important for the establishment or maintenance of cysts in the mouse brain. These findings highlight new components of the GRA proteome of the tissue-cyst life stage of T. gondii and identify potential targets that are important for maintenance of parasite persistence in vivo.

The Bradyzoite: A Key Developmental Stage for the Persistence and Pathogenesis of Toxoplasmosis

Toxoplasma gondii is a ubiquitous parasitic protist found in a wide variety of hosts, including a large proportion of the human population. Beyond an acute phase which is generally self-limited in immunocompetent individuals, the ability of the parasite to persist as a dormant stage, called bradyzoite, is an important aspect of toxoplasmosis. Not only is this stage not eliminated by current treatments, but it can also reactivate in immunocompromised hosts, leading to a potentially fatal outcome. Yet, despite its critical role in the pathology, the bradyzoite stage is relatively understudied. One main explanation is that it is a considerably challenging model, which essentially has to be derived from in vivo sources. However, recent progress on genetic manipulation and in vitro differentiation models now offers interesting perspectives for tackling key biological questions related to this particularly important developmental stage.

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

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

MAG2, a Toxoplasma gondii Bradyzoite Stage-Specific Cyst Matrix Protein

Toxoplasma gondii causes a chronic infection that affects a significant portion of the world's population, and this latent infection is the source of reactivation of toxoplasmosis. An attribute of the slowly growing bradyzoite stage of the parasite is the formation of a cyst within infected cells, allowing the parasite to escape the host's immune response. In this study, a new bradyzoite cyst matrix antigen (MAG) was identified through a hybridoma library screen. This cyst matrix antigen, matrix antigen 2 (MAG2), contains 14 tandem repeats consisting of acidic, basic, and proline residues. Immunoblotting revealed that MAG2 migrates at a level higher than its predicted molecular weight, and computational analysis showed that the structure of MAG2 is highly disordered. Cell fractionation studies indicated that MAG2 was associated with both insoluble and soluble cyst matrix material, suggesting that it interacts with the intracyst network (ICN). Examination of the kinetics of MAG2 within the cyst matrix using fluorescence recovery after photobleaching (FRAP) demonstrated that MAG2 does not readily diffuse within the cyst matrix. Kinetic studies of MAG1 demonstrated that this protein has different diffusion kinetics in tachyzoite and bradyzoite vacuoles and that its mobility is not altered in the absence of MAG2. In addition, deletion of MAG2 does not influence growth, cystogenesis, or cyst morphology.IMPORTANCE This report expands on the list of characterized Toxoplasma gondii cyst matrix proteins. Using fluorescence recovery after photobleaching (FRAP), we have shown that matrix proteins within the cyst matrix are not mainly in a mobile state, providing further evidence of how proteins behave within the cyst matrix. Understanding the proteins expressed during the bradyzoite stage of the parasite reveals how the parasite functions during chronic infection.

A single-parasite transcriptional atlas of Toxoplasma Gondii reveals novel control of antigen expression

Toxoplasma gondii, a protozoan parasite, undergoes a complex and poorly understood developmental process that is critical for establishing a chronic infection in its intermediate hosts. Here, we applied single-cell RNA-sequencing (scRNA-seq) on >5,400 Toxoplasma in both tachyzoite and bradyzoite stages using three widely studied strains to construct a comprehensive atlas of cell-cycle and asexual development, revealing hidden states and transcriptional factors associated with each developmental stage. Analysis of SAG1-related sequence (SRS) antigenic repertoire reveals a highly heterogeneous, sporadic expression pattern unexplained by measurement noise, cell cycle, or asexual development. Furthermore, we identified AP2IX-1 as a transcription factor that controls the switching from the ubiquitous SAG1 to rare surface antigens not previously observed in tachyzoites. In addition, comparative analysis between Toxoplasma and Plasmodium scRNA-seq results reveals concerted expression of gene sets, despite fundamental differences in cell division. Lastly, we built an interactive data-browser for visualization of our atlas resource.

The Toxoplasma gondii Cyst Wall Interactome

A characteristic of the latent cyst stage of Toxoplasma gondii is a thick cyst wall that forms underneath the membrane of the bradyzoite vacuole. Previously, our laboratory group published a proteomic analysis of purified in vitro cyst wall fragments that identified an inventory of cyst wall components. To further refine our understanding of the composition of the cyst wall, several cyst wall proteins were tagged with a promiscuous biotin ligase (BirA*), and their interacting partners were screened by streptavidin affinity purification. Within the cyst wall pulldowns, previously described cyst wall proteins, dense granule proteins, and uncharacterized hypothetical proteins were identified. Several of the newly identified hypothetical proteins were validated to be novel components of the cyst wall and tagged with BirA* to expand the model of the cyst wall interactome. Community detection of the cyst wall interactome model revealed three distinct clusters: a dense granule, a cyst matrix, and a cyst wall cluster. Characterization of several of the identified cyst wall proteins using genetic strategies revealed that MCP3 affects in vivo cyst sizes. This study provides a model of the potential protein interactions within the cyst wall and the groundwork to understand cyst wall formation.IMPORTANCE A model of the cyst wall interactome was constructed using proteins identified through BioID. The proteins within this cyst wall interactome model encompass several proteins identified in a prior characterization of the cyst wall proteome. This model provides a more comprehensive understanding of the composition of the cyst wall and may lead to insights on how the cyst wall is formed.

Identification of a Master Regulator of Differentiation in Toxoplasma

Toxoplasma gondii chronically infects a quarter of the world's population, and its recrudescence can cause life-threatening disease in immunocompromised individuals and recurrent ocular lesions in the immunocompetent. Acute-stage tachyzoites differentiate into chronic-stage bradyzoites, which form intracellular cysts resistant to immune clearance and existing therapies. The molecular basis of this differentiation is unknown, despite being efficiently triggered by stresses in culture. Through Cas9-mediated screening and single-cell profiling, we identify a Myb-like transcription factor (BFD1) necessary for differentiation in cell culture and in mice. BFD1 accumulates during stress and its synthetic expression is sufficient to drive differentiation. Consistent with its function as a transcription factor, BFD1 binds the promoters of many stage-specific genes and represents a counterpoint to the ApiAP2 factors that dominate our current view of parasite gene regulation. BFD1 provides a genetic switch to study and control Toxoplasma differentiation and will inform prevention and treatment of chronic infections.

What a Difference 30 Years Makes! A Perspective on Changes in Research Methodologies Used to Study Toxoplasma gondii

Toxoplasma gondii is a remarkable species with a rich cell, developmental, and population biology. It is also sometimes responsible for serious disease in animals and humans and the stages responsible for such disease are relatively easy to study in vitro or in laboratory animal models. As a result of all this, Toxoplasma has become the subject of intense investigation over the last several decades, becoming a model organism for the study of the phylum of which it is a member, Apicomplexa. This has led to an ever-growing number of investigators applying an ever-expanding set of techniques to dissecting how Toxoplasma "ticks" and how it interacts with its many hosts. In this perspective piece I first wind back the clock 30 years and then trace the extraordinary pace of methodologies that have propelled the field forward to where we are today. In keeping with the theme of this collection, I focus almost exclusively on the parasite, rather than host side of the equation. I finish with a few thoughts about where the field might be headed-though if we have learned anything, the only sure prediction is that the pace of technological advance will surely continue to accelerate and the future will give us still undreamed of methods for taking apart (and then putting back together) this amazing organism with all its intricate biology. We have so far surely just scratched the surface.

Proteomic and transcriptomic analyses of early and late-chronic Toxoplasma gondii infection shows novel and stage specific transcripts

Background

The protozoan pathogen Toxoplasma gondii has the unique ability to develop a chronic infection in the brain of its host by transitioning from the fast growing tachyzoite morphology to latent bradyzoite morphology. A hallmark of the bradyzoite is the development of neuronal cysts that are resilient against host immune response and current therapeutics. The bradyzoite parasites within the cyst have a carbohydrate and protein-rich wall and a slow-replication cycle, allowing them to remain hidden from the host. The intracellular, encysted lifestyle of T. gondii has made them recalcitrant to molecular analysis in vivo.

Results

Here, we detail the results from transcriptional and proteomic analyses of bradyzoite-enriched fractions isolated from mouse brains infected with T. gondii over a time course of 21 to 150 days. The enrichment procedure afforded consistent identification of over 2000 parasitic peptides from the mixed-organism sample, representing 366 T. gondii proteins at 28, 90, and 120 day timepoints. Deep sequencing of transcripts expressed during these three timepoints revealed that a subpopulation of genes that are transcriptionally expressed at a high level. Approximately one-third of these transcripts are more enriched during bradyzoite conditions compared to tachyzoites and approximately half are expressed at similar levels during each phase. The T. gondii transcript which increased the most over the course of chronic infection, sporoAMA1, shows stage specific isoform expression of the gene.

Conclusions

We have expanded the transcriptional profile of in vivo bradyzoites to 120 days post-infection and provided the first in vivo proteomic profile of T. gondii bradyzoites. The RNA sequencing depth of in vivo bradyzoite T. gondii was over 250-fold greater than previous reports and allowed us to identify low level transcripts and a novel bradyzoite-specific isoform of sporoAMA1.

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

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

Enrichment and Proteomic Characterization of the Cyst Wall from In Vitro Toxoplasma gondii Cysts

The tissue cyst of Toxoplasma gondii, found in latent infection, serves a critical role in both transmission and reactivation of this organism. Within infected cells, slowly replicating parasites (bradyzoites) are surrounded by a cyst matrix, cyst wall, and cyst membrane. The cyst wall is clearly delineated by ultrastructural analysis; however, the composition and function of this layer in host-parasite interactions are not fully understood. In order to understand the composition of the cyst wall, a proteomic analysis of purified cyst wall fragments, that were enriched with Percoll gradients and subsequently immunoprecipitated with CST1 antibody, was performed. Known cyst wall proteins, such as CST1, BPK1, MCP4, MAG1, GRA2, GRA3, and GRA5, were identified in this preparation by liquid chromatography-tandem mass spectrometry (LC-MS/MS). In addition, dense granule proteins (GRAs) not previously shown to associate with the cyst wall, as well as uncharacterized hypothetical proteins, were identified in this cyst wall preparation. Several of these hypothetical cyst wall (CST) proteins were epitope tagged, and immunofluorescence assays confirmed their localization as novel cyst matrix and cyst wall proteins. Expression of two of these newly identified cyst wall proteins was eliminated by gene knockout (CST2-KO and CST3-KO). CST2-KO parasites were highly attenuated in virulence and did not establish detectable cyst burdens. This targeted proteomic approach allowed the identification of new components of the cyst wall that probably have roles in the parasite/host interface.IMPORTANCEToxoplasma gondii is a highly prevalent parasite worldwide that presents life-threatening risks to immunocompromised and pregnant individuals. Whereas the life stage responsible for acute infection can be treated, the life stage responsible for chronic infection is refractory to currently available therapeutics. Little is known about the protein composition of the cyst wall, an amorphous structure formed by parasites that is suspected to facilitate persistence within muscle and nervous tissue during chronic (latent) infection. By implementing a refined approach to selectively purify cyst wall fragments, we identified several known and novel cyst wall proteins from our sample preparations. We confirmed the localizations of several proteins from this data set and identified one that is involved in parasite virulence. These data will propel further studies on cyst wall structure and function, leading to therapeutic strategies that can eliminate the chronic infection stage.

ApiAP2 Transcription Factors in Apicomplexan Parasites

Apicomplexan parasites are protozoan organisms that are characterised by complex life cycles and they include medically important species, such as the malaria parasite Plasmodium and the causative agents of toxoplasmosis (Toxoplasma gondii) and cryptosporidiosis (Cryptosporidium spp.). Apicomplexan parasites can infect one or more hosts, in which they differentiate into several morphologically and metabolically distinct life cycle stages. These developmental transitions rely on changes in gene expression. In the last few years, the important roles of different members of the ApiAP2 transcription factor family in regulating life cycle transitions and other aspects of parasite biology have become apparent. Here, we review recent progress in our understanding of the different members of the ApiAP2 transcription factor family in apicomplexan parasites.

Impact of Engineered Expression of Mitochondrial Association Factor 1b on Toxoplasma gondii Infection and the Host Response in a Mouse Model

The parasite Toxoplasma gondii currently infects approximately one-third of the world’s population and causes life-threatening toxoplasmosis in individuals with undeveloped or weakened immune systems. Current treatments are unable to cure T. gondii infection, leaving infected individuals with slow-growing tissue cysts for the remainder of their lives. Previous work has shown that expression of the parasite protein mitochondrial association factor 1 (MAF1b) is responsible for the association of T. gondii parasites with host mitochondria and provides a selective advantage during acute infection. Here we examine the impact of MAF1b expression during chronic T. gondii infection. We find that mice infected with MAF1b-expressing parasites have higher cyst burden and cytokine levels than their wild-type counterparts. A better understanding of the genes involved in establishing and maintaining chronic infection will aid in discovering effective therapeutics for chronically infected individuals.

The opportunistic intracellular parasite Toxoplasma gondii causes a lifelong chronic infection capable of reactivating in immunocompromised individuals, which can lead to life-threatening complications. Following invasion of the host cell, host mitochondria associate with the parasitophorous vacuole membrane. This phenotype is T. gondii strain specific and is mediated by expression of a host mitochondrial association-competent (HMA⁺) paralog of the parasite protein mitochondrial association factor 1 (MAF1b). Previous work demonstrated that expression of MAF1b in strains that do not normally associate with host mitochondria increases their fitness during acute infection in vivo. However, the impact of MAF1b expression during chronic T. gondii infection is unclear. In this study, we assess the impact of MAF1b expression on cyst formation and cytokine production in mice. Despite generally low numbers of cysts generated by the in vitro culture-adapted strains used in this study, we find that parasites expressing MAF1b have higher numbers of cysts in the brains of chronically infected mice and that MAF1b⁺ cyst burden significantly increases during the course of chronic infection. Consistent with this, mice infected with MAF1b⁺ parasites have higher levels of the serum cytokines RANTES and VEGF (vascular endothelial growth factor) at day 57 postinfection, although this could be due to higher parasite burden at this time point rather than direct manipulation of these cytokines by MAF1b. Overall these data indicate that MAF1b expression may also be important in determining infection outcome during the chronic phase, either by directly altering the cytokine/signaling environment or by increasing proliferation during the acute and/or chronic phase.

IMPORTANCE The parasite Toxoplasma gondii currently infects approximately one-third of the world’s population and causes life-threatening toxoplasmosis in individuals with undeveloped or weakened immune systems. Current treatments are unable to cure T. gondii infection, leaving infected individuals with slow-growing tissue cysts for the remainder of their lives. Previous work has shown that expression of the parasite protein mitochondrial association factor 1 (MAF1b) is responsible for the association of T. gondii parasites with host mitochondria and provides a selective advantage during acute infection. Here we examine the impact of MAF1b expression during chronic T. gondii infection. We find that mice infected with MAF1b-expressing parasites have higher cyst burden and cytokine levels than their wild-type counterparts. A better understanding of the genes involved in establishing and maintaining chronic infection will aid in discovering effective therapeutics for chronically infected individuals.

Observations on bradyzoite biology

Tachyzoites of the Apicomplexan Toxoplasma gondii cause acute infection, disseminate widely in their host, and eventually differentiate into a latent encysted form called bradyzoites that are found within tissue cysts. During latent infection, whenever transformation to tachyzoites occurs, any tachyzoites that develop are removed by the immune system. In contrast, cysts containing bradyzoites are sequestered from the immune system. In the absence of an effective immune response released organisms that differentiate into tachyzoites cause acute infection. Tissue cysts, therefore, serve as a reservoir for the reactivation of toxoplasmosis when the host becomes immunocompromised by conditions such as HIV infection, organ transplantation, or due to the impaired immune response that occurs when pathogens are acquired in utero. While tachyzoites and bradyzoites are well defined morphologically, there is no clear consensus on how interconversion occurs or what exact signal(s) mediate this transformation. Advances in research methods have facilitated studies on T. gondii bradyzoites providing important new insights into the biology of latent infection.

Functional Characterization of Dense Granule Proteins in Toxoplasma gondii RH Strain Using CRISPR-Cas9 System

Infection with the apicomplexan protozoan parasite Toxoplasma gondii is an ongoing public health problem. The parasite's ability to invade and replicate within the host cell is dependent on many effectors, such as dense granule proteins (GRAs) released from the specialized organelle dense granules, into host cells. GRAs have emerged as important determinants of T. gondii pathogenesis. However, the functions of some GRAs remain undefined. In this study, we used CRISPR-Cas9 technique to disrupt 17 GRA genes (GRA11, GRA12 bis, GRA13, GRA14, GRA20, GRA21, GRA28-31, GRA33-38, and GRA40) in the virulent T. gondii RH strain. The CRISPR-Cas9 constructs abolished the expression of the 17 GRA genes. Functional characterization of single ΔGRA mutants was achieved in vitro using cell-based plaque assay and egress assay, and in vivo in BALB/c mice. Targeted deletion of these 17 GRA genes had no significant effect neither on the in vitro growth and egress of the mutant strains from the host cells nor on the parasite virulence in the mouse model of infection. Comparative analysis of the transcriptomics data of the 17 GRA genes suggest that GRAs may serve different functions in different genotypes and life cycle stages of the parasite. In sum, although these 17 GRAs might not be essential for RH strain growth in vitro or virulence in mice, they may have roles in other strains or parasite stages, which warrants further investigations.

A latent ability to persist: differentiation in Toxoplasma gondii

A critical factor in the transmission and pathogenesis of Toxoplasma gondii is the ability to convert from an acute disease-causing, proliferative stage (tachyzoite), to a chronic, dormant stage (bradyzoite). The conversion of the tachyzoite-containing parasitophorous vacuole membrane into the less permeable bradyzoite cyst wall allows the parasite to persist for years within the host to maximize transmissibility to both primary (felids) and secondary (virtually all other warm-blooded vertebrates) hosts. This review presents our current understanding of the latent stage, including the factors that are important in bradyzoite induction and maintenance. Also discussed are the recent studies that have begun to unravel the mechanisms behind stage switching.

Transcriptional repression by ApiAP2 factors is central to chronic toxoplasmosis

Tachyzoite to bradyzoite development in Toxoplasma is marked by major changes in gene expression resulting in a parasite that expresses a new repertoire of surface antigens hidden inside a modified parasitophorous vacuole called the tissue cyst. The factors that control this important life cycle transition are not well understood. Here we describe an important transcriptional repressor mechanism controlling bradyzoite differentiation that operates in the tachyzoite stage. The ApiAP2 factor, AP2IV-4, is a nuclear factor dynamically expressed in late S phase through mitosis/cytokinesis of the tachyzoite cell cycle. Remarkably, deletion of the AP2IV-4 locus resulted in the expression of a subset of bradyzoite-specific proteins in replicating tachyzoites that included tissue cyst wall components BPK1, MCP4, CST1 and the surface antigen SRS9. In the murine animal model, the mis-timing of bradyzoite antigens in tachyzoites lacking AP2IV-4 caused a potent inflammatory monocyte immune response that effectively eliminated this parasite and prevented tissue cyst formation in mouse brain tissue. Altogether, these results indicate that suppression of bradyzoite antigens by AP2IV-4 during acute infection is required for Toxoplasma to successfully establish a chronic infection in the immune-competent host.

The Toxoplasma biology that underlies the establishment of a chronic infection is developmental conversion of the acute tachyzoite stage into the latent bradyzoite-tissue cyst stage. Despite the important clinical consequences of this developmental pathway, the molecular basis of the switch mechanisms that control formation of the tissue cyst is still poorly understood. A fundamental feature of tissue cyst formation is the expression of bradyzoite-specific genes. Here we show the transcription factor AP2IV-4 directly silences bradyzoite mRNA and protein expression in the acute tachyzoite stage demonstrating that developmental control of tissue cyst formation is as much about when not to express bradyzoite genes as it is about when to activate them. Losing the suppression of bradyzoite gene expression in the acute tachyzoite stage caused by deleting AP2IV-4 blocked the establishment of chronic disease in healthy animals via increased protective immunity suggesting a possible strategy for preventing chronic Toxoplasma infections.

Molecular cloning, expression, and characterization of UDP N-acetyl-α-d-galactosamine: Polypeptide N-acetylgalactosaminyltransferase 4 from Cryptosporidium parvum

Cryptosporidium spp. are the causative agents of diarrheal disease worldwide, but effective treatments are lacking. Cryptosporidium employs mucin-like glycoproteins with O-glycans to attach to and infect host intestinal epithelial cells. The Tn antigen (GalNAcα1-Ser/Thr) is an O-glycan essential for these processes, as Tn-specific lectins and a Tn-specific monoclonal antibody block attachment to and infection of host cells in vitro. The enzymes in Cryptosporidium catalyzing their synthesis, however, have not been studied. Previously, we identified four genes encoding putative UDP N-acetyl-α-d-galactosamine:polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts) in the genomes of three Cryptosporidium spp. Here we report the in silico analysis, cloning, expression, purification, and characterization of one of the four enzymes Cryptosporidium parvum (Cp)-ppGalNAc-T4. This enzyme contains the characteristic domains and motifs conserved in ppGalNAc-Ts and is expressed at multiple time points during in vitro infection. Recombinant soluble Cp-ppGalNAc-T4 was enzymatically active against an unmodified EA2 peptide suggesting that it may function as an "initiating" ppGalNAc-T. Cp-ppGalNAc-T4 also exhibited a strong preference for UDP-GalNAc over other nucleotide sugar donors and was active against unmodified and O-glycosylated versions of the C. parvum gp40-derived peptide, with a preference for the former, suggesting it may play a role in modifying this glycoprotein in vivo. Given the importance of mucin-type O-glycosylation in Cryptosporidium spp., the enzymes that catalyze their synthesis may serve as potential therapeutic targets.

Integrative transcriptome and proteome analyses define marked differences between Neospora caninum isolates throughout the tachyzoite lytic cycle

Neospora caninum is one of the main causes of transmissible abortion in cattle. Intraspecific variations in virulence have been widely shown among N. caninum isolates. However, the molecular basis governing such variability have not been elucidated to date. In this study label free LC-MS/MS was used to investigate proteome differences between the high virulence isolate Nc-Spain7 and the low virulence isolate Nc-Spain1H throughout the tachyzoite lytic cycle. The results showed greater differences in the abundance of proteins at invasion and egress with 77 and 62 proteins, respectively. During parasite replication, only 19 proteins were differentially abundant between isolates. The microneme protein repertoire involved in parasite invasion and egress was more abundant in the Nc-Spain1H isolate, which displays a lower invasion rate. Rhoptry and dense granule proteins, proteins related to metabolism and stress responses also showed differential abundances between isolates. Comparative RNA-Seq analyses during tachyzoite egress were also performed, revealing an expression profile of genes associated with the bradyzoite stage in the low virulence Nc-Spain1H isolate. The differences in proteome and RNA expression profiles between these two isolates reveal interesting insights into likely mechanisms involved in specific phenotypic traits and virulence in N. caninum.

SIGNIFICANCE

The molecular basis that governs biological variability in N. caninum and the pathogenesis of neosporosis has not been well-established yet. This is the first study in which high throughput technology of LC-MS/MS and RNA-Seq is used to investigate differences in the proteome and transcriptome between two well-characterized isolates. Both isolates displayed different proteomes throughout the lytic cycle and the transcriptomes also showed marked variations but were inconsistent with the proteome results. However, both datasets identified a pre-bradyzoite status of the low virulence isolate Nc-Spain1H. This study reveals interesting insights into likely mechanisms involved in virulence in N. caninum and shed light on a subset of proteins that are potentially involved in the pathogenesis of this parasite.