Selection based on the expression of antisense hypoxanthine-xanthine-guanine-phosphoribosyltransferase RNA in Toxoplasma gondii

m6A RNA methylation facilitates pre-mRNA 3'-end formation and is essential for viability of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that can cause serious opportunistic disease in the immunocompromised or through congenital infection. To progress through its life cycle, Toxoplasma relies on multiple layers of gene regulation that includes an array of transcription and epigenetic factors. Over the last decade, the modification of mRNA has emerged as another important layer of gene regulation called epitranscriptomics. Here, we report that epitranscriptomics machinery exists in Toxoplasma, namely the methylation of adenosines (m6A) in mRNA transcripts. We identified novel components of the m6A methyltransferase complex and determined the distribution of m6A marks within the parasite transcriptome. m6A mapping revealed the modification to be preferentially located near the 3’-boundary of mRNAs. Knockdown of the m6A writer components METTL3 and WTAP resulted in diminished m6A marks and a complete arrest of parasite replication. Furthermore, we examined the two proteins in Toxoplasma that possess YTH domains, which bind m6A marks, and showed them to be integral members of the cleavage and polyadenylation machinery that catalyzes the 3’-end processing of pre-mRNAs. Loss of METTL3, WTAP, or YTH1 led to a defect in transcript 3’-end formation. Together, these findings establish that the m6A epitranscriptome is essential for parasite viability by contributing to the processing of mRNA 3’-ends.

Toxoplasma gondii is a parasite of medical importance that causes disease upon immuno-suppression. Uncovering essential pathways that the parasite uses for its basic biological processes may reveal opportunities for new anti-parasitic drug therapies. Here, we describe the machinery that Toxoplasma uses to modify specific adenosine residues within its messenger RNAs (mRNA) by N6-adenosine methylation (m6A). We discovered that m6A mRNA methylation is prevalent in multiple stages of the parasite life cycle and is required for parasite replication. We also establish that m6A plays a major role in the proper maturation of mRNA. Two proteins that bind m6A modifications on mRNA associate with factors responsible for the cleavage and final processing steps of mRNA maturation. Since all of the machinery is conserved from plants to Toxoplasma and other related parasites, we propose that this system operates similarly in these organisms.

Antisense technologies in the studying of Toxoplasma gondii

This review covers a brief history of antisense RNAs and its applications, and summarizes the current stage of antisense technologies used in Toxoplasma gondii, a fascinating model organism with a unique characteristic blend of genetic regulatory systems normally found in plants or animals. Based on the current knowledge of regulatory RNAs and non-coding RNA (ncRNA), the antisense technologies are reviewed according to the classification of ncRNAs, which are roughly categorized into small, ranging from ~20-200 nucleotides in length, and long >200 nucleotides. Techniques utilizing small regulatory RNAs such as siRNA, miRNA, antagomirs, ribozymes and morpholino oligomers are discussed along with long non-coding RNA (lncRNA) including antisense and double stranded. These antisense technologies can be used in forward and reverse genetics studies. The future of technologies is limitless, particularly by combining these technologies with conventional methods, and should allow for ever greater understanding of gene regulation of the organism and related pathogenic microorganisms.

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

The transcription machinery and the molecular toolbox to control gene expression in Toxoplasma gondii and other protozoan parasites

The phylum of Apicomplexa groups a large variety of obligate intracellular protozoan parasites that exhibit complicated life cycles, involving transmission and differentiation within and between different hosts. Little is known about the level of regulation and the nature of the factors controlling gene expression throughout their life stages. Unravelling the mechanisms that govern gene regulation is critical for the development of adequate tools to manipulate these parasites and modulate gene expression, in order to study their function in molecular terms in vivo. A comparative analysis of the transcriptional machinery of several apicomplexan genomes and other protozoan parasites has revealed the existence of a primitive eukaryotic transcription apparatus consisting only of a subset of the general transcription factors found in higher eukaryotes. These findings have some direct implications on development of tools.

Toxopain-1 is critical for infection in a novel chicken embryo model of congenital toxoplasmosis

We tested the hypothesis that cathepsins and specifically toxopain-1, a cathepsin B, play a critical role in the pathogenesis of toxoplasmosis. We found that inhibiting the expression of toxopain-1-specific mRNA and protein by >60% significantly decreased the capacity of the parasites to multiply and invade in vitro. To relate these in vitro results to the role of toxopain-1 in pathogenesis in vivo, we developed a novel chicken embryo model of congenital toxoplasmosis. Inhibiting either toxopain-1 expression or specific cysteine proteinase activity significantly reduced congenital infection of chicken embryos, as determined by histopathology and by the number of parasites quantified by real-time PCR. Our new model provides key in vivo validation for the hypothesis that toxopain-1 is a potential drug target in Toxoplasma gondii and also provides a new animal model for rapid, inexpensive screening of antiparasitic compounds.

cDNA cloning and expression of UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase T1 from Toxoplasma gondii

We report the cloning, expression, and characterization of the first UDP-GalNAc:polypetide N-acetylgalactosaminyltransferase (ppGalNAc-T) from the human disease-causing parasite, Toxoplasma gondii. This enzyme is also the first characterized ppGalNAc-T of protozoan origin. This type of enzyme catalyzes the initial step of mucin-type O-glycosylation, that is, the transfer of GalNAc in O-glycosidic linkage to serine and threonine residues in polypeptides. We used polymerase chain reaction amplification with degenerate primers and hybridization screening of a T. gondii cDNA library to identify this enzyme. The resulting 84-kDa type II membrane protein contains a 49-amino acid N-terminal cytoplasmic domain, a 22-amino acid hydrophobic transmembrane domain, and a 680-amino acid C-terminal lumenal domain. Conceptual translation of the cDNA sequence reveals a relatively long (i.e. 135 amino acids) stem region and the presence of several important sequence motifs. The latter include a glycosyltransferase 1 (GT1) motif containing a DXH sequence, a Gal/GalNAc-T motif, and a region homologous to ricin lectin. Northern blot analysis identified a single 5.5-kb ppGalNAc-T transcript. Comparison of the cDNA and genomic DNA sequences reveals that this transferase is encoded by 10 exons in a 10 kb region. When the recombinant construct was expressed in stably transfected Drosophila melanogaster S2 cells, the purified protein exhibited transferase activity in vitro. The identification of this enzyme in T. gondii demonstrates that this human parasite has its own enzymatic machinery for the O-glycosylation of toxoplasmal proteins.

Pleiotropic effect due to targeted depletion of secretory rhoptry protein ROP2 in Toxoplasma gondii

Long after their discovery, the function and biogenesis of rhoptries remain enigmatic. In Apicomplexan parasites, these organelles discharge and their contents are exocytosed at the time of host cell invasion, and are thus proposed to play an essential role in establishing the parasitophorous vacuole. In Toxoplasma gondii, ROP2 is suspected to serve as the molecular link between host cell mitochondria and parasitophorous vacuole membrane. In this study we addressed the function of ROP2. Targeted depletion of ROP2 using a ribozyme-modified antisense RNA strategy resulted in multiple effects on parasite morphology because of a disruption in the formation of mature rhoptries, and an arrest in cytokinesis. The association of host cell mitochondria with the parasitophorous vacuole membrane was abolished and the ROP2-deficient parasites had a reduced uptake of sterol from the host cell. Furthermore, these parasites invaded human fibroblasts poorly and had markedly attenuated virulence in mice. We conclude that rhoptry discharge, and in particular release of ROP2, are essential for parasite invasion, replication and host cell-parasite interaction.

Double-stranded RNA can mediate the suppression of uracil phosphoribosyltransferase expression in Toxoplasma gondii

Double-stranded RNA (dsRNA) homologous to the Toxoplasma gondii uracil phosphoribosyltransferase (TgUPRT) gene is able to modulate the UPRT gene expression in T. gondii. The dsRNA, which was produced either from a constructed plasmid or from an in vitro transcription reaction, was capable of down-regulating the expression of TgUPRT. Stably transformed T. gondii expressing the dsRNA, which was capable of growing in the presence of the prodrug 5-fluoro-2(')-deoxyuridine (FDUR), appeared to maintain the engineered plasmid as an extra-chromosomal DNA. When cultured in the absence of the selection pressure, the FDUR resistant parasites slowly reverted to the FDUR sensitive phenotype. The level of the dsRNA necessary to confer FDUR resistance was estimated at 2-8 copies per parasite. More importantly the introduction of the in vitro synthesized dsRNA homologous to the TgUPRT gene into T. gondii can also induce the specific mRNA degradation, resulting in a lowered UPRT activity.

Comparative analysis of antisense RNA, double-stranded RNA, and delta ribozyme-mediated gene regulation in Toxoplasma gondii

RNA tools, namely, antisense RNA, double-stranded RNA (dsRNA), and delta ribozyme, were comparatively analyzed for the development of effective RNA-based gene modulators. The gene encoding uracil phosphoribosyltransferase (UPRT) of Toxoplasma gondii was used as a target and a negative selectable marker. Using plasmid transformation and drug selection assays, we obtained T. gondii transformants resistant to 5-fluoro-2'-deoxyuridine (FDUR), the cytotoxic prodrug and substrate of UPRT, when the plasmids expressing dsRNA and active delta ribozyme were used. No resistant transformants were detected when the plasmids carrying the antisense RNA, the inactive delta ribozyme, or the chloramphenicol acetyltransferase (CAT) genes were used. Parasites generated using the plasmids expressing dsRNA and the delta ribozyme become resistant to FDUR with an LD50 of 50 +/- 5 microM and 25 +/- 8 microM, respectively. These values are approximately 25-fold and 12-fold higher than that of the RH parental parasite strain, indicating that UPRT activity of the transformed parasites was drastically inhibited. Using Northern and Southern blot analysis, we demonstrated that dsRNA and the delta ribozyme interrupt the expression of UPRT. These two RNA tools should, thus, be very useful for the study of gene expression.