Analysis of the virulence determination mechanisms in a local Toxoplasma strain (T.gHB1) isolated from central China

Essential Functions of Calmodulin and Identification of Its Proximal Interacting Proteins in Tachyzoite-Stage Toxoplasma gondii via BioID Technology

Toxoplasma gondii (T. gondii) is a pathogen belonging to the apicomplexan phylum, and it threatens human and animal health. Calcium ions, a critical second messenger in cells, can regulate important biological processes, including parasite invasion and egress. Calmodulin (CaM) is a small, highly conserved, Ca²⁺-binding protein found in all eukaryotic cells. After binding to Ca²⁺, CaM can be activated to interact with various proteins. However, little is known about CaM's function and its interacting proteins in T. gondii. In this study, we successfully knocked down CaM in the T. gondii parent strain TATI using a tetracycline-off system with the Toxoplasma CaM promoter. The results indicated that CaM was required for tachyzoite proliferation, invasion, and egress and that CaM depletion resulted in apicoplast loss, thus threatening parasite survival in the next lytic cycle. In the tachyzoite stage, CaM loss caused significant anomalies in the parasite's basal constriction, motility, and parasite rosette-like arrangement in the parasitophorous vacuole (PV). These phenotypic defects caused by CaM depletion indicate the importance of CaM in T. gondii. Therefore, it is important to identify the CaM-interacting proteins in T. gondii. Applying BioID technology, more than 300 CaM's proximal interacting proteins were identified from T. gondii. These CaM partners were broadly distributed throughout the parasite. Furthermore, the protein interactome and transcriptome analyses indicated the potential role of CaM in ion binding, cation binding, metal ion binding, calcium ion binding, and oxidation-reduction. Our findings shed light on the CaM function and CaM-interactome in T. gondii and other eukaryotes. IMPORTANCE Toxoplasma gondii is an intracellular pathogen that threatens human and animal health. This unicellular parasite is active in many biological processes, such as egress and invasion. The implementation efficiency of T. gondii biological processes is dependent on signal transmission. Ca²⁺, as a second messenger, is essential for the parasite's life cycle. Calmodulin, a ubiquitous Ca²⁺ receptor protein, is highly conserved and mediates numerous Ca²⁺-dependent events in eukaryotes. Few CaM functions or regulated partners have been characterized in T. gondii tachyzoites. Here, we reported the essential functions of calmodulin in T. gondii tachyzoite and the identification of its interacting partners using BioID technology, shedding light on the CaM function and CaM-interactome in Toxoplasma gondii and other eukaryotes.

Activation of NF-κB signaling by the dense granule protein GRA15 of a newly isolated type 1 Toxoplasma gondii strain

Background

It has been reported that the NF-κB pathway, an important component of host defense system against pathogens infections, can be differentially modulated by different Toxoplasma gondii strains, depending on the polymorphism of the GRA15 protein. The recently isolated Toxoplasma strain T.gHB1 is a type 1 (ToxoDB#10) strain but shows different virulence determination mechanisms compared to the classic type 1 strains like RH (ToxoDB#10). Therefore, it is worth investigating whether the T.gHB1 strain (ToxoDB#10) affects the host NF-κB signaling pathway.

Methods

The effects of T.gHB1 (ToxoDB#10) on host NF-κB pathway were investigated in HEK293T cells. The GRA15 gene product was analyzed by bioinformatics, and its effect on NF-κB activation was examined by Western blotting and nuclear translocation of p65. Different truncations of T.gHB1 GRA15 were constructed to map the critical domains for NF-κB activation.

Results

We demonstrated that the NF-κB pathway signaling pathway could be activated by the newly identified type 1 T.gHB1 strain (ToxoDB#10) of Toxoplasma, while the classic type 1 strain RH (ToxoDB#10) did not. T.gHB1 GRA15 possesses only one transmembrane region with an extended C terminal region, which is distinct from that of classic type 1 (ToxoDB#10) and type 2 (ToxoDB#1) strains. T.gHB1 GRA15 could clearly induce IκBα phosphorylation and p65 nuclear translocation. Dual luciferase assays in HEK293T cells revealed a requirement for 194–518 aa of T.gHB1 GRA15 to effectively activate NF-κB.

Conclusions

The overall results indicated that the newly isolated type 1 isolate T.gHB1 (ToxoDB#10) had a unique GRA15, which could activate the host NF-κB signaling through inducing IκBα phosphorylation and p65 nuclear translocation. These results provide new insights for our understanding of the interaction between Toxoplasma parasites and its hosts.

Graphical Abstract

A Comparison of Transcriptional Diversity of Swine Macrophages Infected With TgHB1 Strain of Toxoplasma gondii Isolated in China

Toxoplasma gondii is an apicomplexan parasite infecting human and animals, causing huge public health concerns and economic losses. Swine alveolar macrophage plays an important role in controlling T. gondii infection. However, the mechanism by which macrophages infected with T. gondii function in the immunity to the infection is unclear, especially for local isolates such as TgHB1 isolated in China. RNA-seq as a valuable tool was applied to simultaneously analyze transcriptional changes of pig alveolar macrophages infected with TgRH (typeI), TgME49 (typeII) or TgHB1 at different time points post infection (6, 12, and 24 h). Paired-end clean reads were aligned to the Sscrofa10.2 pig genome and T. gondii ME49 genome. The differentially expressed genes of macrophages and T. gondii were enriched through Gene Ontology and Kyoto Encyclopedia of Genes and Genomes, respectively. Compared to the TgRH and TgME49 infection groups, 307 down-regulated macrophage genes (mainly enriched for development and metabolism) and 419 up-regulated genes (mainly enriched for immune pathways) were uniquely expressed in the TgHB1 infection group. Additionally, 557 down-regulated and 674 up-regulated T. gondii genes (mainly enriched in metabolism and biosynthesis) were uniquely expressed in the TgHB1 infection group. For validation purposes, some of the differentially expressed genes of macrophages involved in immune-related signaling pathways were used for further analysis via real time quantitative reverse-transcription polymerase-chain reaction (qRT-PCR). This work provides important insights into the temporal immune responses of swine alveolar macrophages to infection by the strain TgHB1 isolated from China, and is helpful for better understanding of the T. gondii genotype-associated activation of macrophages during early phase of the infection.

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.

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.

Association of ROP18 and ROP5 was efficient as a marker of virulence in atypical isolates of Toxoplasma gondii obtained from pigs and goats in Piauí, Brazil

Recent studies indicate that proteins GRA15, ROP5, ROP16, ROP17, and ROP18 of Toxoplasma gondii are involved in the process of interaction, cellular invasion, and immune response of the host. Among these proteins, alleles of the polymorphic ROP18 and ROP5 seem to be directly associated with T. gondii virulence in mice. The purpose of this work was to isolate and genotype T. gondii from pig, goat, and sheep slaughtered for human consumption in the state of Piauí, Northeastern Brazil and relate the variability of genes that express virulence proteins of the parasite to virulence in mice. T. gondii was isolated from 16 pigs and 9 goats. The parasite was not isolated from sheep samples. Eleven different genotypes were identified using PCR-RFLP. A unique genotype not yet described in any other host and or anywhere else was common to three pig isolates. Eighteen isolates (72%) were characterized as avirulent, four (16%) as intermediate virulence and three (12%) as virulent to mice. The combined analysis of ROP18 and ROP5 in the isolates studied in Piauí, showed four different allele associations: 4/3 (virulent strains), 3/3, 3/1, and 2/3 (avirulent strains). The association 2/3 was not previously described in the literature. Our results indicated that GRA15, ROP16, and ROP17 alleles were not associated with T. gondii virulence in mice. Pigs and goats raised and slaughtered for human consumption in the state of Piauí are infected with isolates of T. gondii presenting different genotypes. We concluded that the virulence protein ROP18, analyzed alone or in combination with ROP5, was effective in determining virulence for mice for the new isolates of T. gondii.

QTL Mapping and CRISPR/Cas9 Editing to Identify a Drug Resistance Gene in Toxoplasma gondii

Scientific knowledge is intrinsically linked to available technologies and methods. This article will present two methods that allowed for the identification and verification of a drug resistance gene in the Apicomplexan parasite Toxoplasma gondii, the method of Quantitative Trait Locus (QTL) mapping using a Whole Genome Sequence (WGS) -based genetic map and the method of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 -based gene editing. The approach of QTL mapping allows one to test if there is a correlation between a genomic region(s) and a phenotype. Two datasets are required to run a QTL scan, a genetic map based on the progeny of a recombinant cross and a quantifiable phenotype assessed in each of the progeny of that cross. These datasets are then formatted to be compatible with R/qtl software that generates a QTL scan to identify significant loci correlated with the phenotype. Although this can greatly narrow the search window of possible candidates, QTLs span regions containing a number of genes from which the causal gene needs to be identified. Having WGS of the progeny was critical to identify the causal drug resistance mutation at the gene level. Once identified, the candidate mutation can be verified by genetic manipulation of drug sensitive parasites. The most facile and efficient method to genetically modify T. gondii is the CRISPR/Cas9 system. This system comprised of just 2 components both encoded on a single plasmid, a single guide RNA (gRNA) containing a 20 bp sequence complementary to the genomic target and the Cas9 endonuclease that generates a double-strand DNA break (DSB) at the target, repair of which allows for insertion or deletion of sequences around the break site. This article provides detailed protocols to use CRISPR/Cas9 based genome editing tools to verify the gene responsible for sinefungin resistance and to construct transgenic parasites.

Functional Characterization of Rhoptry Kinome in the Virulent Toxoplasma gondii RH Strain

Toxoplasma gondii is an obligatory intracellular apicomplexan protozoan which can infect any warm-blooded animal and causes severe diseases in immunocompromised individuals or infants infected in utero. The survival and success of this parasite require that it colonizes the host cell, avoids host immune defenses, replicates within an appropriate niche, and exits the infected host cell to spread to neighboring non-infected cells. All of these processes depend on the parasite ability to synthesis and export secreted proteins. Amongst the secreted proteins, rhoptry organelle proteins (ROPs) are essential for the parasite invasion and host cell manipulation. Even though the functions of most ROPs have been elucidated in the less virulent T. gondii (type II), the roles of ROPs in the highly virulent type I strain remain largely un-characterized. Herein, we investigated the contributions of 15 ROPs (ROP10, ROP11, ROP15, ROP20, ROP23, ROP31, ROP32, ROP33, ROP34, ROP35, ROP36, ROP40, ROP41, ROP46, and ROP47) to the infectivity of the high virulent type I T. gondii (RH strain). Using CRISPR-Cas9, these 15 ROPs genes were successfully disrupted and the effects of gene knockout on the parasite's ability to infect cells in vitro and BALB/c mice in vivo were investigated. These results showed that deletions of these ROPs did not interfere with the parasite ability to grow in cultured human foreskin fibroblast cells and did not significantly alter parasite pathogenicity for BALB/c mice. Although these ROPs did not seem to be essential for the acute infectious stage of type I T. gondii in the mouse model, they might have different functions in other intermediate hosts or play different roles in other life cycle forms of this parasite due to the different expression patterns; this warrants further investigations.