The Toxoplasma gondii rhoptry protein ROP18 is an Irga6-specific kinase and regulated by the dense granule protein GRA7

Trx4, a novel thioredoxin protein, is important for Toxoplasma gondii fitness

BACKGROUND

To successfully replicate within the host cell, Toxoplasma gondii employs several mechanisms to overcome the host cell defenses and mitigate the harmful effects of the free radicals resulting from its own metabolic processes using effectors such as thioredoxin proteins. In this study, we characterize the location and functions of a newly identified thioredoxin in T. gondii, which was named Trx4.

METHODS

We characterized the functional role of Trx4 in T. gondii Type I RH and Type II Pru strains by gene knockout and studied its subcellular localization by endogenous protein HA tagging using CRISPR-Cas9 gene editing. The enzyme-catalyzed proximity labeling technique, the TurboID system, was employed to identify the proteins in proximity to Trx4.

RESULTS

Trx4 was identified as a dense granule protein of T. gondii predominantly expressed in the parasitophorous vacuole (PV) and was partially co-localized with GRA1 and GRA5. Functional analysis showed that deletion of trx4 markedly influenced the parasite lytic cycle, resulting in impaired host cell invasion capacity in both RH and Pru strains. Mutation of Trx domains in Trx4 in RH strain revealed that two Trx domains were important for the parasite invasion. By utilizing the TurboID system to biotinylate proteins in proximity to Trx4, we identified a substantial number of proteins, some of which are novel, and others are previously characterized, predominantly distributed in the dense granules. In addition, we uncovered three novel proteins co-localized with Trx4. Intriguingly, deletion of trx4 did not affect the localization of these three proteins. Finally, a virulence assay demonstrated that knockout of trx4 resulted in a significant attenuation of virulence and a significant reduction in brain cyst loads in mice.

CONCLUSIONS

Trx4 plays an important role in T. gondii invasion and virulence in Type I RH strain and Type II Pru strain. Combining the TurboID system with CRISPR-Cas9 technique revealed many PV-localized proximity proteins associated with Trx4. These findings suggest a versatile role of Trx4 in mediating the processes that occur in this distinctive intracellular membrane-bound vacuolar compartment.

IFNs in host defence and parasite immune evasion during Toxoplasma gondii infections

Interferons (IFNs) are a family of cytokines with diverse functions in host resistance to pathogens and in immune regulation. Type II IFN, i.e. IFN-γ, is widely recognized as a major mediator of resistance to intracellular pathogens, including the protozoan Toxoplasma gondii. More recently, IFN-α/β, i.e. type I IFNs, and IFN-λ (type III IFN) have been identified to also play important roles during T. gondii infections. This parasite is a widespread pathogen of humans and animals, and it is a model organism to study cell-mediated immune responses to intracellular infection. Its success depends, among other factors, on the ability to counteract the IFN system, both at the level of IFN-mediated gene expression and at the level of IFN-regulated effector molecules. Here, I review recent advances in our understanding of the molecular mechanisms underlying IFN-mediated host resistance and immune regulation during T. gondii infections. I also discuss those mechanisms that T. gondii has evolved to efficiently evade IFN-mediated immunity. Knowledge of these fascinating host-parasite interactions and their underlying signalling machineries is crucial for a deeper understanding of the pathogenesis of toxoplasmosis, and it might also identify potential targets of parasite-directed or host-directed supportive therapies to combat the parasite more effectively.

Genetic diversity of Toxoplasma gondii in South America: occurrence, immunity, and fate of infection

Toxoplasma gondii is an intracellular parasite with a worldwide distribution. Toxoplasma gondii infections are of great concern for public health, and their impact is usually most severe in pregnant women and their foetuses, and in immunocompromised individuals. Displaying considerable genetic diversity, T. gondii strains differ widely according to geographical location, with archetypal strains predominantly found in the Northern Hemisphere and non-archetypal (atypical) strains, with highly diverse genotypes, found mainly in South America. In this review, we present an overview of the identification and distribution of non-archetypal strains of T. gondii. Special attention is paid to the strains that have been isolated in Brazil, their interaction with the host immunological response, and their impact on disease outcomes. The genetic differences among the strains are pivotal to the distinct immunological responses that they elicit. These differences arise from polymorphisms of key proteins released by the parasite, which represent important virulence factors. Infection with divergent non-archetypal strains can lead to unusual manifestations of the disease, even in immunocompetent individuals.

Toxoplasma rhoptry proteins that affect encephalitis outcome

Toxoplasma gondii, a widespread obligate intracellular parasite, can infect almost all warm-blooded animals, including humans. The cellular barrier of the central nervous system (CNS) is generally able to protect the brain parenchyma from infectious damage. However, T. gondii typically causes latent brain infections in humans and other vertebrates. Here, we discuss how T. gondii rhoptry proteins (ROPs) affect signaling pathways in host cells and speculate how this might affect the outcome of Toxoplasma encephalitis.

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.

IFN-λ is protective against lethal oral Toxoplasma gondii infection

Interferons are essential for innate and adaptive immune responses against a wide variety of pathogens. Interferon lambda (IFN-λ) protects mucosal barriers during pathogen exposure. The intestinal epithelium is the first contact site for Toxoplasma gondii (T. gondii) with its hosts and the first defense line that limits parasite infection. Knowledge of very early T. gondii infection events in the gut tissue is limited and a possible contribution of IFN-λ has not been investigated so far. Here, we demonstrate with systemic interferon lambda receptor (IFNLR1) and conditional (Villin-Cre) knockout mouse models and bone marrow chimeras of oral T. gondii infection and mouse intestinal organoids a significant impact of IFN-λ signaling in intestinal epithelial cells and neutrophils to T. gondii control in the gastrointestinal tract. Our results expand the repertoire of interferons that contribute to the control of T. gondii and may lead to novel therapeutic approaches against this world-wide zoonotic pathogen.

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.

ROP39 is an Irgb10-specific parasite effector that modulates acute Toxoplasma gondii virulence

Toxoplasma gondii (T. gondii) is a zoonotic apicomplexan parasite that is an important cause of clinical disability in humans. On a global scale, one third of the human population is infected with T. gondii. Mice and other small rodents are believed to be responsible for transmission of T. gondii to the domestic cat, its definitive host. Interferon-inducible Immunity-Related GTPases (IRG proteins) are important for control of murine T. gondii infections. Virulence differences between T. gondii strains are linked to polymorphic rhoptry proteins (ROPs) that cooperate to inactivate individual IRG family members. In particular, the pseudokinase ROP5 isoform B is critically important in laboratory strains of mice. We identified T. gondii ROP39 in complex with ROP5B and demonstrate its contribution to acute T. gondii virulence. ROP39 directly targets Irgb10 and inhibits homodimer formation of the GTPase leading to an overall reduction of IRG protein loading onto the parasitophorous vacuolar membrane (PVM). Maintenance of PVM integrity rescues the parasite from IRG protein-mediated clearance in vitro and in vivo. This study identifies a novel T. gondii effector that is important for specific inactivation of the IRG resistance system. Our data reveal that yet unknown T. gondii effectors can emerge from identification of direct interaction partners of ROP5B.

NcGRA7 and NcROP40 Play a Role in the Virulence of Neospora caninum in a Pregnant Mouse Model

The intraspecific variability among Neospora caninum isolates in their in vitro behaviour and in vivo virulence has been widely studied. In particular, transcriptomic and proteomic analyses have shown a higher expression/abundance of specific genes/proteins in high-virulence isolates. Consequently, the dense granule protein NcGRA7 and the rhoptry protein NcROP40 were proposed as potential virulence factors. The objective of this study was to characterize the role of these proteins using CRISPR/Cas9 knockout (KO) parasites in a well-established pregnant BALB/c mouse model of N. caninum infection at midgestation. The deletion of NcGRA7 and NcROP40 was associated with a reduction of virulence, as infected dams displayed milder clinical signs, lower parasite burdens in the brain, and reduced mortality rates compared to those infected with the wild-type parasite (Nc-Spain7). Specifically, those infected with the NcGRA7 KO parasites displayed significantly milder clinical signs and a lower brain parasite burden. The median survival time of the pups from dams infected with the two KO parasites was significantly increased, but differences in neonatal mortality rates were not detected. Overall, the present study indicates that the disruption of NcGRA7 considerably impairs virulence in mice, while the impact of NcROP40 deletion was more modest. Further research is needed to understand the role of these virulence factors during N. caninum infection.

Modulation of autophagy as a therapeutic strategy for Toxoplasma gondii infection

Toxoplasma gondii infection is a severe health threat that endangers billions of people worldwide. T. gondii utilizes the host cell membrane to form a parasitophorous vacuole (PV), thereby fully isolating itself from the host cell cytoplasm and making intracellular clearance difficult. PV can be targeted and destroyed by autophagy. Autophagic targeting results in T. gondii killing via the fusion of autophagosomes and lysosomes. However, T. gondii has developed many strategies to suppress autophagic targeting. Accordingly, the interplay between host cell autophagy and T. gondii is an emerging area with important practical implications. By promoting the canonical autophagy pathway or attenuating the suppression of autophagic targeting, autophagy can be effectively utilized in the development of novel therapeutic strategies against T gondii. Here, we have illustrated the complex interplay between host cell mediated autophagy and T. gondii. Different strategies to promote autophagy in order to target the parasite have been elucidated. Besides, we have analyzed some potential new drug molecules from the DrugBank database using bioinformatics tools, which can modulate autophagy. Various challenges and opportunities focusing autophagy mediated T. gondii clearance have been discussed, which will provide new insights for the development of novel drugs against the parasite.

IFN-γ stimulated murine and human neurons mount anti-parasitic defenses against the intracellular parasite Toxoplasma gondii

Dogma holds that Toxoplasma gondii persists in neurons because neurons cannot clear intracellular parasites, even with IFN-γ stimulation. As several recent studies questioned this idea, here we use primary murine neuronal cultures from wild type and transgenic mice in combination with IFN-γ stimulation and parental and transgenic parasites to reassess IFN-γ dependent neuronal clearance of intracellular parasites. We find that neurons respond to IFN-γ and that a subset of neurons clear intracellular parasites via immunity regulated GTPases. Whole neuron reconstructions from mice infected with parasites that trigger neuron GFP expression only after full invasion reveal that ~50% of these T. gondii-invaded neurons no longer harbor parasites. Finally, IFN-γ stimulated human pluripotent stem cell derived neurons show an ~50% decrease in parasite infection rate when compared to unstimulated cultures. This work highlights the capability of human and murine neurons to mount cytokine-dependent anti-T. gondii defense mechanisms in vitro and in vivo.

Host cell proteins modulated upon Toxoplasma infection identified using proteomic approaches: a molecular rationale

Toxoplasma gondii is a pathogenic protozoan parasite belonging to the apicomplexan phylum that infects the nucleated cells of warm-blooded hosts leading to an infectious disease known as toxoplasmosis. Apicomplexan parasites such as T. gondii can display different mechanisms to control or manipulate host cells signaling at different levels altering the host subcellular genome and proteome. Indeed, Toxoplasma is able to modulate host cell responses (especially immune responses) during infection to its advantage through both structural and functional changes in the proteome of different infected cells. Consequently, parasites can transform the invaded cells into a suitable environment for its own replication and the induction of infection. Proteomics as an applicable tool can identify such critical proteins involved in pathogen (Toxoplasma)-host cell interactions and consequently clarify the cellular mechanisms that facilitate the entry of pathogens into host cells, and their replication and transmission, as well as the central mechanisms of host defense against pathogens. Accordingly, the current paper reviews several proteins (identified using proteomic approaches) differentially expressed in the proteome of Toxoplasma-infected host cells (macrophages and human foreskin fibroblasts) and tissues (brain and liver) and highlights their plausible functions in the cellular biology of the infected cells. The identification of such modulated proteins and their related cell impact (cell responses/signaling) can provide further information regarding parasite pathogenesis and biology that might lead to a better understanding of therapeutic strategies and novel drug targets.

Extracellular matrix in cancer progression and therapy

The tumor ecosystem with heterogeneous cellular compositions and the tumor microenvironment has increasingly become the focus of cancer research in recent years. The extracellular matrix (ECM), the major component of the tumor microenvironment, and its interactions with the tumor cells and stromal cells have also enjoyed tremendously increased attention. Like the other components of the tumor microenvironment, the ECM in solid tumors differs significantly from that in normal organs and tissues. We review recent studies of the complex roles the tumor ECM plays in cancer progression, from tumor initiation, growth to angiogenesis and invasion. We highlight that the biomolecular, biophysical, and mechanochemical interactions between the ECM and cells not only regulate the steps of cancer progression, but also affect the efficacy of systemic cancer treatment. We further discuss the strategies to target and modify the tumor ECM to improve cancer therapy.

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.

A Newly Discovered Dense Granule Protein 3 in Neospora caninum

INTRODUCTION

Neospora caninum, an obligate intracellular parasite of the phylum Apicomplexa, typically causes an illness known as neosporosis. Dense granule proteins (GRAs) are secreted by apicomplexan and constitute the parasitophorous vacuoles (PVs) structure where tachyzoites proliferate after invasion into host cells. In Toxoplasma gondii, TgGRA3 proteins are strongly associated with PVs membrane and enhance its virulence in vivo, however, research on NcGRA3 has not been reported.

METHODS

Here, a novel NcGRA3 protein in N. caninum was discovered using bioinformatics analysis; the location of NcGRA3 was determined in the extracellular Nc-1 tachyzoites and intracellular PVs after invasion using immunofluorescence assays; the NcGRA3 protein existing form in the PVs membranes was analyzed using western blotting.

RESULTS

NcGRA3 shared 41.67% nucleotide homology and 22.17% amino acid homology with TgGRA3. Amino acid sequences ranging from 1 to 25 were signal peptide regions and 135-157 were transmembrane domains. The immunofluorescence assays showed that NcGRA3 was an apical organ secreted dense granule protein and expressed at the posterior end of tachyzoites; the partial co-localization with NcGRA6 in PVs demonstrated that NcGRA3 were expressed in the intravacuolar network structure and PVs membrane. The western blotting assays showed that NcGRA3 protein behaved as an integrated transmembrane protein in PVs. Overall, characterization of the newly discovered NcGRA3 protein will lay a foundation for its function research in the further.

Nanos gigantium humeris insidentes: old papers informing new research into Toxoplasma gondii

Since Nicolle, Manceaux and Splendore first described Toxoplasma gondii as a parasite of rodents and rabbits in the early 20th century, a diverse and vigorous research community has been built around studying this fascinating intracellular parasite. In addition to its importance as a pathogen of humans, livestock and wildlife, modern researchers are attracted to T. gondii as a facile experimental system to study many aspects of evolutionary biology, cellular biology, host-microbe interactions, and host immunity. For new researchers entering the field, the extensive literature describing the biology of the parasite, and the interactions with its host, can be daunting. In this review, we examine four foundational studies that describe various aspects of T. gondii biology, presenting a 'journal club'-style analysis of each. We have chosen a paper that established the beguiling life cycle of the parasite (Hutchison et al., 1971), a paper that described key features of its cellular biology that the parasite shares with related organisms (Gustafson et al., 1954), a paper that characterised the origin of the unique compartment in which the parasite resides within host cells (Jones and Hirsch, 1972), and a paper that established a key mechanism in the host immune response to parasite infection (Pfefferkorn, 1984). These interesting and far-reaching studies set the stage for subsequent research into numerous facets of parasite biology. As well as providing new researchers with an entry point into the literature surrounding the parasite, revisiting these studies can remind us of the roots of our discipline, how far we have come, and the new directions in which we might head.

Lessons from Toxoplasma: Host responses that mediate parasite control and the microbial effectors that subvert them

The intracellular parasite Toxoplasma gondii has long provided a tractable experimental system to investigate how the immune system deals with intracellular infections. This review highlights the advances in defining how this organism was first detected and the studies with T. gondii that contribute to our understanding of how the cytokine IFN-γ promotes control of vacuolar pathogens. In addition, the genetic tractability of this eukaryote organism has provided the foundation for studies into the diverse strategies that pathogens use to evade antimicrobial responses and now provides the opportunity to study the basis for latency. Thus, T. gondii remains a clinically relevant organism whose evolving interactions with the host immune system continue to teach lessons broadly relevant to host–pathogen interactions.

Secreted Effectors Modulating Immune Responses to Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that chronically infects a third of humans. It can cause life-threatening encephalitis in immune-compromised individuals. Congenital infection also results in blindness and intellectual disabilities. In the intracellular milieu, parasites encounter various immunological effectors that have been shaped to limit parasite infection. Parasites not only have to suppress these anti-parasitic inflammatory responses but also ensure the host organism's survival until their subsequent transmission. Recent advancements in T. gondii research have revealed a plethora of parasite-secreted proteins that suppress as well as activate immune responses. This mini-review will comprehensively examine each secreted immunomodulatory effector based on the location of their actions. The first section is focused on secreted effectors that localize to the parasitophorous vacuole membrane, the interface between the parasites and the host cytoplasm. Murine hosts are equipped with potent IFNγ-induced immune-related GTPases, and various parasite effectors subvert these to prevent parasite elimination. The second section examines several cytoplasmic and ER effectors, including a recently described function for matrix antigen 1 (MAG1) as a secreted effector. The third section covers the repertoire of nuclear effectors that hijack transcription factors and epigenetic repressors that alter gene expression. The last section focuses on the translocation of dense-granule effectors and effectors in the setting of T. gondii tissue cysts (the bradyzoite parasitophorous vacuole).

Toxoplasma gondii ROP17 promotes autophagy via the Bcl-2-Beclin 1 pathway

The apicomplexan Toxoplasma gondii (Nicolle et Manceaux, 1908) secretes a group of serine/threonine kinases from rhoptries, which play vital roles in boosting intracellular infection. Toxoplasma gondii rhoptry organelle protein 17 (ROP17) is one of these important kinase proteins. Nevertheless, its function remains unclear. Here, we showed that ROP17 induced autophagy in vitro and in vivo. The autophagy of small intestine tissues of T. gondii tachyzoite (RH strain)-infected mice was detected by the immunohistochemistry staining of LC3B, Beclin 1 and P62. ROP17 overexpression augmented starvation-induced autophagy in HEK 293T cells as measured by MDC staining, transmission electron microscopy (TEM), fluorescence microscopy and Western blot analysis. Moreover, the interaction of ROP17 and Bcl-2 was confirmed using co-immunoprecipitation analysis, and the data demonstrated that ROP17 had an autophagic role dependent on the Beclin 1-Bcl-2 pathway, which was also revealed in an in vivo model through immunohistochemical staining. Pearson coefficient analysis showed that there existed strong positive correlations between the expression of ROP17 and LC3B, Beclin 1 and phosphorylation of Bcl-2, while strong negative correlations between the expression of ROP17 and p62 and Bcl-2. Collectively, our findings indicate that ROP17 plays a pivotal role in maintaining T. gondii proliferation in host cells via the promotion of autophagy-dependent survival.

Genome-Wide CRISPR/Cas9 Screen Identifies New Genes Critical for Defense Against Oxidant Stress in Toxoplasma gondii

Toxoplasma gondii is one of the most widespread apicomplexans and can cause serious infections in humans and animals. Its antioxidant system plays an important role in defending against oxidant stress imposed by the host. Some genes encoding the antioxidant enzymes of T. gondii have been identified; however, critical genes that function in response to reactive oxygen species (ROS) stress are still poorly understood. Here, we performed genome-wide CRISPR/Cas9 loss-of-function screening in the T. gondii RH strain to identify potential genes contributing to the ROS stress response. Under hydrogen peroxide treatment, 30 single guide RNAs targeting high-confidence genes were identified, including some known important antioxidant genes such as catalase and peroxiredoxin PRX3. In addition, several previously uncharacterized genes were identified, among which five hypothetical protein-coding genes, namely, HP1–HP5, were selected for further functional characterization. Targeted deletion of HP1 in T. gondii RH led to significant sensitivity to H₂O₂, suggesting that HP1 is critical for oxidative stress management. Furthermore, loss of HP1 led to decreased antioxidant capacity, invasion efficiency, and proliferation in vitro. In vivo results also revealed that the survival time of mice infected with the HP1-KO strain was significantly prolonged relative to that of mice infected with the wild-type strain. Altogether, these findings demonstrate that the CRISPR/Cas9 system can be used to identify potential genes critical for oxidative stress management. Furthermore, HP1 may confer protection against oxidative damage and contributes to T. gondii virulence in mice.

Toxoplasma gondii manipulates host cell signaling pathways via its secreted effector molecules

The obligate intracellular parasite Toxoplasma gondii secretes a vast variety of effector molecules from organelles known as rhoptries (ROPs) and dense granules (GRAs). ROP proteins are released into the cytosol of the host cell where they are directed to the cell nucleus or to the parasitophorous vacuole (PV) membrane. ROPs secrete proteins that enable host cell penetration and vacuole formation by the parasites, as well as hijacking host-immune responses. After invading host cells, T. gondii multiplies within a PV that is maintained by the parasite proteins secreted from GRAs. Most GRA proteins remain within the PV, but some are known to access the host cytosol across the PV membrane, and a few are able to traffic into the host-cell nucleus. These effectors bind to host cell proteins and affect host cell signaling pathways to favor the parasite. Studies on host-pathogen interactions have identified many infection-altered host signal transductions. Notably, the relationship between individual parasite effector molecules and the specific targeting of host-signaling pathways is being elucidated through the advent of forward and reverse genetic strategies. Understanding the complex nature of the host-pathogen interactions underlying how the host-signaling pathway is manipulated by parasite effectors may lead to new molecular biological knowledge and novel therapeutic methods for toxoplasmosis. In this review, we discuss how T. gondii modulates cell signaling pathways in the host to favor its survival.

The Host Autophagy During Toxoplasma Infection

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

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

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

Toxoplasma gondii GRA60 is an effector protein that modulates host cell autonomous immunity and contributes to virulence

Toxoplasma gondii infects virtually any nucleated cell and resides inside a non-phagocytic vacuole surrounded by a parasitophorous vacuolar membrane (PVM). Pivotal to the restriction of T. gondii dissemination upon infection in murine cells is the recruitment of immunity regulated GTPases (IRGs) and guanylate binding proteins (GBPs) to the PVM that leads to pathogen elimination. The virulent T. gondii type I RH strain secretes a handful of effectors including the dense granule protein GRA7, the serine-threonine kinases ROP17 and ROP18, and a pseudo-kinase ROP5, that synergistically inhibit the recruitment of IRGs to the PVM. Here, we characterise GRA60, a novel dense granule effector, which localises to the vacuolar space and PVM and contributes to virulence of RH in mice, suggesting a role in the subversion of host cell defence mechanisms. Members of the host cell IRG defence system Irgb10 and Irga6 are recruited to the PVM of RH parasites lacking GRA60 as observed previously for the avirulent RHΔrop5 mutant, with RH preventing such recruitment. Deletion of GRA60 in RHΔrop5 leads to a recruitment of IRGs comparable to the single knockouts. GRA60 therefore represents a novel parasite effector conferring resistance to IRGs in type I parasites, and found associated to ROP18, a member of the virulence complex.

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

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

Toxoplasma gondii Dense Granule Proteins 7, 14, and 15 Are Involved in Modification and Control of the Immune Response Mediated via NF-κB Pathway

Toxoplasma gondii infects almost all warm-blooded animals, including humans, leading to both cellular and humoral immune responses in the host. The virulence of T. gondii is strain specific and is defined by secreted effector proteins that disturb host immunity. Here, we focus on nuclear factor-kappa B (NFκB) signaling, which regulates the induction of T-helper type 1 immunity. A luciferase assay for screening effector proteins, including ROPs and GRAs that have biological activity against an NFκB-dependent reporter plasmid, found that overexpression of GRA7, 14, and 15 of a type II strain resulted in a strong activity. Thus, our study was aimed at understanding the involvement of NFκB in the pathogenesis of toxoplasmosis through a comparative analysis of these three molecules. We found that GRA7 and GRA14 were partially involved in the activation of NFκB, whereas GRA15 was essential for NFκB activation. The deletion of GRA7, GRA14, and GRA15 in the type II Prugniaud (Pru) strain resulted in a defect in the nuclear translocation of RelA. Cells infected with the PruΔgra15 parasite showed reduced phosphorylation of inhibitor-κBα. GRA7, GRA14, and GRA15 deficiency decreased the levels of interleukin-6 in RAW246.7 cells, and RNA-seq analysis revealed that GRA7, GRA14, and GRA15 deficiency predominantly resulted in downregulation of gene expression mediated by NFκB. The virulence of all mutant strains increased, but PruΔgra14 only showed a slight increase in virulence. However, the intra-footpad injection of the highly-virulent type I RHΔgra14 parasites in mice resulted in increased virulence. This study shows that GRA7, 14, and 15-induced host immunity via NFκB limits parasite expansion.

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

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

The Secreted Acid Phosphatase Domain-Containing GRA44 from Toxoplasma gondii Is Required for c-Myc Induction in Infected Cells

During host cell invasion, the eukaryotic pathogen Toxoplasma gondii forms a parasitophorous vacuole to safely reside within the cell, while it is partitioned from host cell defense mechanisms. From within this safe niche, parasites sabotage multiple host cell systems, including gene expression, apoptosis, and intracellular immune recognition, by secreting a large arsenal of effector proteins. Many parasite proteins studied for active host cell manipulative interactions have been kinases. The translocation of effectors from the parasitophorous vacuole into the host cell is mediated by a putative translocon complex, which includes the proteins MYR1, MYR2, and MYR3. Whether other proteins are involved in the structure or regulation of this putative translocon is not known. We have discovered that the secreted protein GRA44, which contains a putative acid phosphatase domain, interacts with members of this complex and is required for host cell effects downstream of effector secretion. We have determined that GRA44 is processed in a region with homology to sequences targeted by protozoan proteases of the secretory pathway and that both major cleavage fragments are secreted into the parasitophorous vacuole. Immunoprecipitation experiments showed that GRA44 interacts with a large number of secreted proteins, including MYR1. Importantly, conditional knockdown of GRA44 resulted in a lack of host cell c-Myc upregulation, which mimics the phenotype seen when members of the translocon complex are genetically disrupted. Thus, the putative acid phosphatase GRA44 is crucial for host cell alterations during Toxoplasma infection and is associated with the translocon complex which Toxoplasma relies upon for success as an intracellular pathogen.IMPORTANCE Approximately one-third of humans are infected with the parasite Toxoplasma gondiiToxoplasma infections can lead to severe disease in those with a compromised or suppressed immune system. Additionally, infections during pregnancy present a significant health risk to the developing fetus. Drugs that target this parasite are limited, have significant side effects, and do not target all disease stages. Thus, a thorough understanding of how the parasite propagates within a host is critical in the discovery of novel therapeutic targets. Toxoplasma replication requires that it enter the cells of the infected organism. In order to survive the environment inside a cell, Toxoplasma secretes a large repertoire of proteins, which hijack a number of important cellular functions. How these Toxoplasma proteins move from the parasite into the host cell is not well understood. Our work shows that the putative phosphatase GRA44 is part of a protein complex responsible for this process.

Toxoplasma gondii Parasitophorous Vacuole Membrane-Associated Dense Granule Proteins Regulate Maturation of the Cyst Wall

After differentiation is triggered, the tachyzoite-stage Toxoplasma gondii parasitophorous vacuole membrane (PVM) has been hypothesized to transition into the cyst membrane that surrounds the cyst wall and encloses bradyzoites. Here, we tracked the localization of two PVM dense granule (GRA) proteins (GRA5 and GRA7) after in vitro differentiation of the tachyzoite stage parasitophorous vacuole into the mature cyst. GRA5 and GRA7 were visible at the cyst periphery at 6 h and at all later times after differentiation, suggesting that the PVM remained intact as it transitioned into the cyst membrane. By day 3 postdifferentiation, GRA5 and GRA7 were visible in a continuous pattern at the cyst periphery. In mature 7- and 10-day-old cysts permeabilized with a saponin pulse, GRA5 and GRA7 were localized to the cyst membrane and the cyst wall regions. Cysts at different stages of cyst development exhibited differential susceptibility to saponin permeabilization, and, correspondingly, saponin selectively removed GRA5 from the cyst membrane and cyst wall region in 10-day-old cysts. GRA5 and GRA7 were localized at the cyst membrane and cyst wall region at all times after differentiation of the parasitophorous vacuole, which supports a previous model proposing that the PVM develops into the cyst membrane. In addition, evaluation of Δgra3, Δgra5, Δgra7, Δgra8, and Δgra14 mutants revealed that PVM-localized GRAs were crucial to support the normal rate of accumulation of cyst wall proteins at the cyst periphery.IMPORTANCEToxoplasma gondii establishes chronic infection in humans by forming thick-walled cysts that persist in the brain. Once 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. Furthermore, how the cyst membrane and cyst wall structures develop is poorly understood. Here, we visualized and tracked the localization of Toxoplasma parasitophorous vacuole membrane (PVM) dense granules (GRA) proteins during cyst development in vitro. PVM-localized GRA5 and GRA7 were found at the cyst membrane and cyst wall region throughout cyst development, suggesting that the PVM remains intact and develops into the cyst membrane. In addition, our results show that genetic deletion of PVM GRAs reduced the rate of accumulation of cyst wall cargo at the cyst periphery and suggest that PVM-localized GRAs mediate the development and maturation of the cyst wall and cyst membrane.

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

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

Rhoptry and Dense Granule Secreted Effectors Regulate CD8+ T Cell Recognition of Toxoplasma gondii Infected Host Cells

Toxoplasma gondii secretes rhoptry (ROP) and dense granule (GRA) effector proteins to evade host immune clearance mediated by interferon gamma (IFN-γ), immunity-related GTPase (IRG) effectors, and CD8⁺ T cells. Here, we investigated the role of parasite-secreted effectors in regulating host access to parasitophorous vacuole (PV) localized parasite antigens and their presentation to CD8⁺ T cells by the major histocompatibility class I (MHC-I) pathway. Antigen presentation of PV localized parasite antigens by MHC-I was significantly increased in macrophages and/or dendritic cells infected with mutant parasites that lacked expression of secreted GRA (GRA2, GRA3, GRA4, GRA5, GRA7, GRA12) or ROP (ROP5, ROP18) effectors. The ability of various secreted GRA or ROP effectors to suppress antigen presentation by MHC-I was dependent on cell type, expression of IFN-γ, or host IRG effectors. The suppression of antigen presentation by ROP5, ROP18, and GRA7 correlated with a role for these molecules in preventing PV disruption by IFN-γ-activated host IRG effectors. However, GRA2 mediated suppression of antigen presentation was not correlated with PV disruption. In addition, the GRA2 antigen presentation phenotypes were strictly co-dependent on the expression of the GRA6 protein. These results show that MHC-I antigen presentation of PV localized parasite antigens was controlled by mechanisms that were dependent or independent of IRG effector mediated PV disruption. Our findings suggest that the GRA6 protein underpins an important mechanism that enhances CD8⁺ T cell recognition of parasite-infected cells with damaged or ruptured PV membranes. However, in intact PVs, parasite secreted effector proteins that associate with the PV membrane or the intravacuolar network membranes play important roles to actively suppress antigen presentation by MHC-I to reduce CD8⁺ T cell recognition and clearance of Toxoplasma gondii infected host cells.

Toxoplasma GRA15 Activates the NF-κB Pathway through Interactions with TNF Receptor-Associated Factors

The parasite Toxoplasma can cause birth defects and severe disease in immunosuppressed patients. Strain differences in pathogenicity exist, and these differences are due to polymorphic effector proteins that Toxoplasma secretes into the host cell to coopt host cell functions. The effector protein GRA15 of some Toxoplasma strains activates the nuclear factor kappa B (NF-κB) pathway, which plays an important role in cell death, innate immunity, and inflammation. We show that GRA15 interacts with TNF receptor-associated factors (TRAFs), which are adaptor proteins functioning upstream of the NF-κB transcription factor. Deletion of TRAF-binding sites in GRA15 greatly reduces its ability to activate the NF-κB pathway, and TRAF2 knockout cells have impaired GRA15-mediated NF-κB activation. Thus, we determined the mechanism for GRA15-dependent NF-κB activation.

The protozoan parasite Toxoplasma gondii secretes proteins from specialized organelles, the rhoptries, and dense granules, which are involved in the modulation of host cell processes. Dense granule protein GRA15 activates the nuclear factor kappa B (NF-κB) pathway, which plays an important role in cell death, innate immunity, and inflammation. Exactly how GRA15 activates the NF-κB pathway is unknown. Here we show that GRA15 interacts with tumor necrosis factor receptor-associated factors (TRAFs), which are adaptor proteins functioning upstream of the NF-κB transcription factor. We identified several TRAF binding sites in the GRA15 amino acid sequence and showed that these are involved in NF-κB activation. Furthermore, a TRAF2 knockout cell line has impaired GRA15-mediated NF-κB activation. Thus, we determined the mechanism for GRA15-dependent NF-κB activation.

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

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

Time-Frequency Approach Applied to Finding Interaction Regions in Pathogenic Proteins

Protein-protein interactions govern all molecular processes for living organisms, even those involved in pathogen infection. Pathogens such as virus, bacteria, and parasites contain proteins that help the pathogen to attach, penetrate, and settle inside the target cell. Thus, it is necessary to know the regions in pathogenic proteins that interact with host cell receptors. Currently, powerful pathogen databases are available and many pathogenic proteins have been recognized, but many pathogenic proteins have not been characterized. This work developed a program in MATLAB environment based on the time-frequency analysis to recognize important sites in proteins. Our program highlights the highest energy patches in proteins from their time-frequency distribution and matches the corresponding frequency. We sought to know if this approach is able to recognize stretches residues related to interaction. Our approach was applied to five study cases from pathogenic co-crystallized structures that have been well characterized. We searched the frequencies that characterize interaction regions in pathogenic proteins and with this information tried to identify new interaction patches in either paralogs or orthologs. We found that our program generates a well-interpretable graphic under several descriptors that can show important regions in proteins even those related to interaction. We propose that this MATLAB program could be used as a tool to explore outstanding regions in uncharacterized proteins.

Interplay Between Toxoplasma gondii, Autophagy, and Autophagy Proteins

Survival of Toxoplasma gondii within host cells depends on its ability of reside in a vacuole that avoids lysosomal degradation and enables parasite replication. The interplay between immune-mediated responses that lead to either autophagy-driven lysosomal degradation or disruption of the vacuole and the strategies used by the parasite to avoid these responses are major determinants of the outcome of infection. This article provides an overview of this interplay with an emphasis on autophagy.

Strategies Developed by Toxoplasma gondii to Survive in the Host

One of the most successful intracellular parasites, Toxoplasma gondii has developed several strategies to avoid destruction by the host. These include approaches such as rapid and efficient cell invasion to avoid phagocytic engulfment, negative regulation of the canonical CD40-CD40L-mediated autophagy pathway, impairment of the noncanonical IFN-γ-dependent autophagy pathway, and modulation of host cell survival and death to obtain lifelong parasite survival. Different virulent strains have even evolved different ways to cope with and evade destruction by the host. This review aims to illustrate every aspect of the game between the host and Toxoplasma during the process of infection. A better understanding of all aspects of the battle between Toxoplasma and its hosts will be useful for the development of better strategies and drugs to control the parasite.

A non-death function of the mitochondrial apoptosis apparatus in immunity

Apoptosis is a frequent form of programmed cell death, but the apoptotic signaling pathway can also be engaged at a low level, in the absence of cell death. We here report that such sub-lethal engagement of mitochondrial apoptosis signaling causes the secretion of cytokines from human epithelial cells in a process controlled by the Bcl-2 family of proteins. We further show that sub-lethal signaling of the mitochondrial apoptosis pathway is initiated by infections with all tested viral, bacterial, and protozoan pathogens and causes damage to the genomic DNA. Epithelial cells infected with these pathogens secreted cytokines, and this cytokine secretion upon microbial infection was substantially reduced if mitochondrial sub-lethal apoptosis signaling was blocked. In the absence of mitochondrial pro-apoptotic signaling, the ability of epithelial cells to restrict intracellular bacterial growth was impaired. Triggering of the mitochondrial apoptosis apparatus thus not only causes apoptosis but also has an independent role in immune defense.

The Toxoplasma gondii dense granule protein TgGRA3 interacts with host Golgi and dysregulates anterograde transport

After entry into the host cell, the intracellular parasite Toxoplasma gondii resides within a membrane-bound compartment, the parasitophorous vacuole (PV). The PV defines an intracellular, parasite-specific niche surrounded by host organelles, including the Golgi apparatus. The mechanism by which T. gondii hijacks the host Golgi and subverts its functions remains unknown. Here, we present evidence that the dense granule protein TgGRA3 interacts with host Golgi, leading to the formation of tubules and the entry of host Golgi material into the PV. Targeted disruption of the TgGRA3 gene delays this engulfment of host Golgi. We also demonstrate that TgGRA3 oligomerizes and binds directly to host Golgi membranes. In addition, we show that TgGRA3 dysregulates anterograde transport in the host cell, thereby revealing one of the mechanisms employed by T. gondii to recruit host organelles and divert their functions.

This article has an associated First Person interview with the first author of the paper.

Summary : Toxoplasma gondii recruits various host organelles to enable parasite intracellular development. We describe a new role for TgGRA3 in modulating the host anterograde transport by binding to the Golgi apparatus.

Molecular mechanism for the control of virulent Toxoplasma gondii infections in wild-derived mice

Some strains of the protozoan parasite Toxoplasma gondii (such as RH) are virulent in laboratory mice because they are not restricted by the Immunity-Related GTPase (IRG) resistance system in these mouse strains. In some wild-derived Eurasian mice (such as CIM) on the other hand, polymorphic IRG proteins inhibit the replication of such virulent T. gondii strains. Here we show that this resistance is due to direct binding of the IRG protein Irgb2-b1_CIM to the T. gondii virulence effector ROP5 isoform B. The Irgb2-b1 interface of this interaction is highly polymorphic and under positive selection. South American T. gondii strains are virulent even in wild-derived Eurasian mice. We were able to demonstrate that this difference in virulence is due to polymorphic ROP5 isoforms that are not targeted by Irgb2-b1_CIM, indicating co-adaptation of host cell resistance GTPases and T. gondii virulence effectors.

Toxoplasma gondii virulence in wild-derived mice is restricted by Immunity-Related GTPases (IRG). Here, the authors show specific binding of the IRG tandem protein Irgb2-b1 with the virulence effector ROP5, and provide insights into how different ROP5 isoforms and IRG alleles shape virulence among T. gondii strains.

Prediction of Toxoplasma gondii virulence factor ROP18 competitive inhibitors by virtual screening

Background

Rhoptry protein 18 (ROP18) is a key virulence factor of Toxoplasma gondii. The host’s immune responses mediated by immune-related GTPases (IRGs) could be blocked by ROP18’s kinase activity. ROP18 also interacts with various substrates, such as activating transcription factor 6 beta (ATF6β) and affects multiple physiological functions within host cells, thereby inducing intense virulence. In this study, competitive inhibitors targeted to ROP18 were subjected to virtual screening based on the principle of structure-based drug design (SBDD).

Methods

The preparation of the ROP18 structure was conducted using the “Structure Prepare” function of Molecular Operating Environment (MOE) software. The ATP-binding pocket was selected as the starting point for virtual screening. Construction of the pharmacophore model used Extended Hückel Theory (EHT) half-quantitative measurement and construction, as well as the characteristics of Type I kinase inhibitors. The pharmacophore model of ROP18 was imported into the Specs database for small molecule similarity screening using EHT pharmacophore measurement. Hit compounds were selected using the functions of London dG and generalized-born volume integral/weighted surface area (GBVI/WSA) scoring. The top 100 hits were analyzed by molecular docking and structure activity relationships (SAR) analysis.

Results

The final pharmacophore comprised three typical characteristics: three hydrogen bond acceptors/donors, two ring aromatic features occupying the hydrophobic core, and one cation group feature targeted to the terminus of ATP. A total of 1314 hit compounds analogous to ROP18 pharmacophore were passed through the Specs. After two rounds of docking, 25 out of 100 hits were identified as belonging to two main scaffold types: phthalimide ring structure, thiazole ring and styrene structure. Additionally, the screen also identified 13 inhibitors with distinct scaffold types. The docking models and SAR analysis demonstrated that these hits could engage in multiple hydrogen bonds, salt bridges halogen bonds, and hydrophobic interactions with ROP18, and para-position halo substituents on the benzene ring may enhance their affinity scoring.

Conclusions

A pharmacophore against the ROP18 ATP-binding pocket was successfully constructed, and 25 representative inhibitors from 15 scaffold clusters were screened using the Specs database. Our results provide useful scaffold types for the chemical inhibition of ROP18 or alternative conformations to develop new anti-toxoplasmosis drug leads.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3341-y) contains supplementary material, which is available to authorized users.

Protein targets of thiazolidinone derivatives in Toxoplasma gondii and insights into their binding to ROP18

Background

Thiazolidinone derivatives show inhibitory activity (IC₅₀) against the Toxoplasma gondii parasite, as well as high selectivity with high therapeutic index. To disclose the target proteins of the thiazolidinone core in this parasite, we explored in silico the active sites of different T. gondii proteins and estimated the binding-free energy of reported thiazolidinone molecules with inhibitory effect on invasion and replication of the parasite inside host cells. This enabled us to describe some of the most suitable structural characteristics to design a compound derived from the thiazolidinone core.

Results

The best binding affinity was observed in the active site of kinase proteins, we selected the active site of the T. gondii ROP18 kinase, because it is an important factor for the virulence and survival of the parasite. We present the possible effect of a derivative of thiazolidinone core in the active site of T. gondii ROP18 and described some characteristics of substituent groups that could improve the affinity and specificity of compounds derived from the thiazolidinone core against T. gondii.

Conclusions

The results of our study suggest that compounds derived from the thiazolidinone core have a preference for protein kinases of T. gondii, being promising compounds for the development of new drugs with potential anti-toxoplasmosis activity. Our findings highlight the importance of use computational studies for the understanding of the action mechanism of compounds with biological activity.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5223-7) contains supplementary material, which is available to authorized users.

Identification of Novel Dense-Granule Proteins in Toxoplasma gondii by Two Proximity-Based Biotinylation Approaches

Toxoplasma gondii is an opportunistic pathogen infecting humans and a variety of vertebrate animals. Secretory dense-granule proteins (GRAs) play diverse roles in the mediation of host-parasite interactions and facilitate parasitism, but many of them still remain to be identified. Here, we used two proximity-based protein labeling techniques to identify novel GRA proteins. Taking GRA1 as bait, transgenic strains expressing GRA1-BirA* or GRA1-APEX were constructed to biotinylate GRAs. Using these methods, a total of 46 proteins were identified, 20 of which were known GRA proteins. Among these 46, 17 were identified by both strategies, and 14 out of the 17 were known GRAs. The other three were all confirmed to localize to dense granules. Nonetheless a significant portion of the proteins were only identified by either APEX or BirA*, indicating that there are differences between these methods. Of the 26 novel GRAs, 5 were validated as bona fide GRAs by localization studies. The majority of these novel GRAs are only present in coccidian parasites and are likely dispensable for parasite growth in vitro; they may play roles during animal infections. The identification of novel GRAs laid the foundation for further studies investigating the mechanisms underlying parasite-host interactions.

The role of host autophagy machinery in controlling Toxoplasma infection

Toxoplasma gondii is an obligate intracellular parasitic protist that infects a wide range of warm-blooded vertebrates. Although this parasite can cause serious complications, infections are often asymptomatic, allowing T. gondii to persist in its host and possibly enhancing the chances of its transmission. T. gondii has thus evolved multiple mechanisms of host manipulation to establish chronic infection. This persistence involves a balance between host immunity and parasite evasion of this immune response. This review highlights recent investigations that have demonstrated the important role played by the autophagy machinery in this balance, both in parasite control by the host, and in host exploitation by the parasite.

The Protozoan Parasite Toxoplasma gondii Selectively Reprograms the Host Cell Translatome

The intracellular parasite Toxoplasma gondii promotes infection by targeting multiple host cell processes; however, whether it modulates mRNA translation is currently unknown. Here, we show that infection of primary murine macrophages with type I or II T. gondii strains causes a profound perturbation of the host cell translatome. Notably, translation of transcripts encoding proteins involved in metabolic activity and components of the translation machinery was activated upon infection. In contrast, the translational efficiency of mRNAs related to immune cell activation and cytoskeleton/cytoplasm organization was largely suppressed. Mechanistically, T. gondii bolstered mechanistic target of rapamycin (mTOR) signaling to selectively activate the translation of mTOR-sensitive mRNAs, including those with a 5'-terminal oligopyrimidine (5' TOP) motif and those encoding mitochondrion-related proteins. Consistent with parasite modulation of host mTOR-sensitive translation to promote infection, inhibition of mTOR activity suppressed T. gondii replication. Thus, selective reprogramming of host mRNA translation represents an important subversion strategy during T. gondii infection.

The Expressed MicroRNA—mRNA Interactions of Toxoplasma gondii

MicroRNAs (miRNAs) are involved in post-transcriptional modulation of gene expression and thereby have a large influence on the resulting phenotype. We have previously shown that miRNAs may be involved in the communication between Toxoplasma gondii and its hosts and further confirmed a number of proposed specific miRNAs. Yet, little is known about the internal regulation via miRNAs in T. gondii. Therefore, we predicted pre-miRNAs directly from the type II ME49 genome and filtered them. For the confident hairpins, we predicted the location of the mature miRNAs and established their target genes. To add further confidence, we evaluated whether the hairpins and their targets were co-expressed. Such co-expressed miRNA and target pairs define a functional interaction. We extracted all such functional interactions and analyzed their differential expression among strains of all three clonal lineages (RH, PLK, and CTG) and between the two stages present in the intermediate host (tachyzoites and bradyzoites). Overall, we found ~65,000 expressed interactions of which ~5,500 are differentially expressed among strains but none are significantly differentially expressed between developmental stages. Since miRNAs and target decoys can be used as therapeutics we believe that the list of interactions we provide will lead to novel approaches in the treatment of toxoplasmosis.

Regulation of innate immune functions by guanylate-binding proteins

Guanylate-binding proteins (GBP) are a family of dynamin-related large GTPases which are expressed in response to interferons and other pro-inflammatory cytokines. GBPs mediate a broad spectrum of innate immune functions against intracellular pathogens ranging from viruses to bacteria and protozoa. Several binding partners for individual GBPs have been identified and several different mechanisms of action have been proposed depending on the organisms, the cell type and the pathogen used. Many of these anti-pathogenic functions of GBPs involve the recruitment to and the subsequent destruction of pathogen containing vacuolar compartments, the assembly of large oligomeric innate immune complexes such as the inflammasome, or the induction of autophagy. Furthermore, GBPs often cooperate with immunity-related GTPases (IRGs), another family of dynamin-related GTPases, to exert their anti-pathogenic function, but since most IRGs have been lost in the evolution of higher primates, the anti-pathogenic function of human GBPs seems to be IRG-independent. GBPs and IRGs share biochemical and structural properties with the other members of the dynamin superfamily such as low nucleotide affinity and a high intrinsic GTPase activity which can be further enhanced by oligomerisation. Furthermore, GBPs and IRGs can interact with lipid membranes. In the case of three human and murine GBP isoforms this interaction is mediated by C-terminal isoprenylation. Based on cell biological studies, and in analogy to the function of other dynamins in membrane scission events, it has been postulated that both GBPs and IRGs might actively disrupt the outer membrane of pathogen-containing vacuole leading to the detection and destruction of the pathogen by the cytosolic innate immune system of the host. Recent evidence, however, indicates that GBPs might rather function by mediating membrane tethering events similar to the dynamin-related atlastin and mitofusin proteins, which mediate fusion of the ER and mitochondria, respectively. The aim of this review is to highlight the current knowledge on the function of GBPs in innate immunity and to combine it with the recent progress in the biochemical characterisation of this protein family.

Host-Toxoplasma gondii Coadaptation Leads to Fine Tuning of the Immune Response

Toxoplasma gondii has successfully developed strategies to evade host's immune response and reach immune privileged sites, which remains in a controlled environment inside quiescent tissue cysts. In this review, we will approach several known mechanisms used by the parasite to modulate mainly the murine immune system at its favor. In what follows, we review recent findings revealing interference of host's cell autonomous immunity and cell signaling, gene expression, apoptosis, and production of microbicide molecules such as nitric oxide and oxygen reactive species during parasite infection. Modulation of host's metalloproteinases of extracellular matrix is also discussed. These immune evasion strategies are determinant to parasite dissemination throughout the host taking advantage of cells from the immune system to reach brain and retina, crossing crucial hosts' barriers.

Host Cell Vimentin Restrains Toxoplasma gondii Invasion and Phosphorylation of Vimentin is Partially Regulated by Interaction with TgROP18

The obligate intracellular parasite, Toxoplasma gondii, manipulates the cytoskeleton of its host cells to facilitate infection. A significant rearrangement of host cell vimentin around Toxoplasma parasitophorous vacuoles is observed during the course of infection. ROP18 (TgROP18) is a serine-threonine kinase secreted by T. gondii rhoptry and a major virulence factor; however, the mechanisms by which this kinase modulates host factors remain poorly understood. Different and dynamic patterns of vimentin solubility, phosphorylation, and expression levels were observed in host cells infected with T. gondii strain RH and RH Δrop18 strains, suggesting that TgROP18 contributes to the regulation of these dynamic patterns. Additionally, host cell vimentin was demonstrated to interact with and be phosphorylated by TgROP18. A significant increase in T. gondii infection rate was observed in vimentin knockout human brain microvessel endothelial cells (HBMEC), while vimentin knockout or knock down in host cells had no impact on parasite proliferation and egress. These results indicate that host cell vimentin can inhibit T. gondii invasion. Interestingly, western blotting of different mouse tissues indicated that the lowest vimentin expression level was present in the brain, which may explain the mechanism underlying the nervous system tropism of T. gondii, and the phenomenon of huge cyst burdens developing in the mouse brain during chronic infection.