Protein O-fucosyltransferase 2-mediated O-glycosylation of the adhesin MIC2 is dispensable for Toxoplasma gondii tachyzoite infection

N-Glycoproteomics of the Apicomplexan Parasite Toxoplasma gondii

Protein N-glycosylation influences protein folding, stability, and trafficking, and has prominent functions in cell-cell adhesion and recognition. For the parasite Toxoplasma gondii, N-glycosylation of proteins is crucial for initial adhesion to host cells, parasite motility, and consequently, its ability to invade host cells. However, the glycoproteome of T. gondii remains largely unknown. In this study, we used the α-mannose-specific Burkholderia cenocepacia lectin A (BC2L-A) to enrich glycopeptides from T. gondii tachyzoites and analysed them by tandem mass spectrometry. The data enable the identification of over 100 N-glycoproteins with the glycosylation site(s) and the composition of the N-glycans at each site. T. gondii glycoproteins include known virulence factors, vaccine candidates as well as numerous uncharacterised proteins. These data provide ground knowledge to deepen our understanding of the role of glycoproteins in invasion and assist the rational design of vaccines.

Novel antibodies detect nucleocytoplasmic O-fucose in protist pathogens, cellular slime molds, and plants

Cellular adaptations to change often involve post-translational modifications of nuclear and cytoplasmic proteins. An example found in protists and plants is the modification of serine and threonine residues of dozens to hundreds of nucleocytoplasmic proteins with a single fucose (O-fucose). A nucleocytoplasmic O-fucosyltransferase occurs in the pathogen Toxoplasma gondii, the social amoeba Dictyostelium, and higher plants, where it is called Spy because mutants have a spindly appearance. O-fucosylation, which is required for optimal proliferation of Toxoplasma and Dictyostelium, is paralogous to the O-GlcNAcylation of nucleocytoplasmic proteins of plants and animals that are involved in stress and nutritional responses. O-fucose was first discovered in Toxoplasma using Aleuria aurantia lectin, but its broad specificity for terminal fucose residues on N- and O-linked glycans in the secretory pathway limits its use. Here we present affinity-purified rabbit antisera that are selective for the detection and enrichment of proteins bearing fucose-O-Ser or fucose-O-Thr. These antibodies detect numerous nucleocytoplasmic proteins in Toxoplasma, Dictyostelium, and Arabidopsis, as well as O-fucose occurring on secretory proteins of Dictyostelium and mammalian cells except when blocked by further glycosylation. The antibodies label Toxoplasma, Acanthamoeba, and Dictyostelium in a pattern reminiscent of O-GlcNAc in animal cells including nuclear pores. The O-fucome of Dictyostelium is partially conserved with that of Toxoplasma and is highly induced during starvation-induced development. These antisera demonstrate the unique antigenicity of O-fucose, document the conservation of the O-fucome among unrelated protists, and enable the study of the O-fucomes of other organisms possessing O-fucosyltransferase-like genes.IMPORTANCEO-fucose (O-Fuc), a form of mono-glycosylation on serine and threonine residues of nuclear and cytoplasmic proteins of some parasites, other unicellular eukaryotes, and plants, is understudied because it is difficult to detect owing to its neutral charge and lability during mass spectrometry. Yet, the O-fucosyltransferase enzyme (OFT) is required for optimal growth of the agent for toxoplasmosis, Toxoplasma gondii, and an unrelated protist, the social amoeba Dictyostelium discoideum. Furthermore, O-fucosylation is closely related to the analogous process of O-GlcNAcylation of thousands of proteins of animal cells, where it plays a central role in stress and nutritional responses. O-Fuc is currently best detected using Aleuria aurantia lectin (AAL), but in most organisms, AAL also recognizes a multitude of proteins in the secretory pathway that are modified with fucose in different ways. By establishing the potential to induce highly specific rabbit antisera that discriminate O-Fuc from all other forms of protein fucosylation, this study expands knowledge about the protist O-fucome and opens a gateway to explore the potential occurrence and roles of this intriguing posttranslational modification in bacteria and other protist pathogens such as Acanthamoeba castellanii.

Novel antibodies detect nucleocytoplasmic O-fucose in protist pathogens, cellular slime molds, and plants

Cellular adaptations to change often involve post-translational modifications of nuclear and cytoplasmic proteins. An example found in protists and plants is the modification of serine and threonine residues of dozens to hundreds of nucleocytoplasmic proteins with a single fucose (O-Fuc). A nucleocytoplasmic O-fucosyltransferase (OFT) occurs in the pathogen Toxoplasma gondii, the social amoeba Dictyostelium, and higher plants, where it is called Spy because mutants have a spindly appearance. O-fucosylation, which is required for optimal proliferation of Toxoplasma and Dictyostelium, is paralogous to the O-GlcNAcylation of nucleocytoplasmic proteins of plants and animals that is involved in stress and nutritional responses. O-Fuc was first discovered in Toxoplasma using Aleuria aurantia lectin, but its broad specificity for terminal fucose residues on N- and O-linked glycans in the secretory pathway limits its use. Here we present affinity purified rabbit antisera that are selective for the detection and enrichment of proteins bearing fucose-O-Ser or fucose-O-Thr. These antibodies detect numerous nucleocytoplasmic proteins in Toxoplasma, Dictyostelium, and Arabidopsis, as well as O-Fuc occurring on secretory proteins of Dictyostelium and mammalian cells, although the latter are frequently blocked by further glycosylation. The antibodies label Toxoplasma, Acanthamoeba, and Dictyostelium in a pattern reminiscent of O-GlcNAc in animal cells including nuclear pores. The O-fucome of Dictyostelium is partially conserved with that of Toxoplasma and is highly induced during starvation-induced development. These antisera demonstrate the unique antigenicity of O-Fuc, document conservation of the O-fucome among unrelated protists, and will enable the study of the O-fucomes of other organisms possessing OFT-like genes.

Protein disulfide isomerase PDI8 is indispensable for parasite growth and associated with secretory protein processing in Toxoplasma gondii

Protein disulfide isomerase, containing thioredoxin (Trx) domains, serves as a vital enzyme responsible for oxidative protein folding (the formation, reduction, and isomerization of disulfide bonds in newly synthesized proteins) in the endoplasmic reticulum (ER). However, the role of ER-localized PDI proteins in parasite growth and their interaction with secretory proteins remain poorly understood. In this study, we identified two ER-localized PDI proteins, TgPDI8 and TgPDI6, in Toxoplasma gondii. Conditional knockdown of TgPDI8 resulted in a significant reduction in intracellular proliferation and invasion abilities, leading to a complete block in plaque formation on human foreskin fibroblast monolayers, whereas parasites lacking TgPDI6 did not exhibit any apparent fitness defects. The complementation of TgPDI8 with mutant variants highlighted the critical role of the CXXC active site cysteines within its Trx domains for its enzymatic activity. By utilizing TurboID-based proximity labeling, we uncovered a close association between PDI proteins and canonical secretory proteins. Furthermore, parasites lacking TgPDI8 showed a significant reduction in the expression of secretory proteins, especially those from micronemes and dense granules. In summary, our study elucidates the roles of TgPDI8 and sets the stage for future drug discovery studies.

IMPORTANCE

Apicomplexans, a phylum of intracellular parasites, encompass various zoonotic pathogens, including Plasmodium, Cryptosporidium, Toxoplasma, and Babesia, causing a significant economic burden on human populations. These parasites exhibit hypersensitivity to disruptions in endoplasmic reticulum (ER) redox homeostasis, necessitating the presence of ER-localized thioredoxin (Trx) superfamily proteins, particularly protein disulfide isomerase (PDI), for proper oxidative folding. However, the functional characteristics of ER-localized PDI proteins in Toxoplasma gondii remain largely unexplored. In this study, we identified two ER-localized proteins, namely, TgPDI8 and TgPDI6, and demonstrated the indispensable role of TgPDI8 in parasite survival. Through a comprehensive multi-omics analysis, we elucidated the crucial role of TgPDI8 in the processing of secretory proteins in T. gondii. Additionally, we introduced a novel ER-anchored TurboID method to label and identify canonical secretory proteins in T. gondii. This research opens up new avenues for understanding oxidative folding and the secretory pathway in apicomplexan parasites, laying the groundwork for future advancements in antiparasitic drug development.

Protein O-Fucosyltransferase Is Required for the Efficient Invasion of Hepatocytes by Plasmodium berghei Sporozoites

The O-fucosylation of the thrombospondin type I repeat (TSR) domain is important for TSR-containing proteins' optimal folding and stability. However, the importance of Plasmodium O-fucosyltransferase 2 (POFut2) remains unclear due to two different reports. Here, we disrupted the POFut2 gene in Plasmodium berghei and demonstrated that POFut2 KO parasites develop normally in blood and mosquito stages but show reduced infectivity in mice. We found that the reduced infectivity of POFut2 KO sporozoites was due to a diminished level of TRAP that affected the parasite gliding motility and hepatocyte infectivity. Using all-atom MD simulation, we also hypothesize that O-fucosylation impacts the TSR domain's stability more than its heparin binding capacity.

Conservation, abundance, glycosylation profile, and localization of the TSP protein family in Cryptosporidium parvum

Cryptosporidium parvum is a zoonotic apicomplexan parasite and a common cause of diarrheal disease worldwide. The development of vaccines to prevent or limit infection remains an important goal for tackling cryptosporidiosis. At present, the only approved vaccine against any apicomplexan parasite targets a conserved adhesin possessing a thrombospondin repeat domain. C. parvum possesses 12 orthologous thrombospondin repeat domain-containing proteins known as CpTSP1-12, though little is known about these potentially important antigens. Here, we explore the architecture and conservation of the CpTSP protein family, as well as their abundance at the protein level within the sporozoite stage of the life cycle. We examine the glycosylation states of these proteins using a combination of glycopeptide enrichment techniques to demonstrate that these proteins are modified with C-, O-, and N-linked glycans. Using expansion microscopy, and an antibody against the C-linked mannose that is unique to the CpTSP protein family within C. parvum, we show that these proteins are found both on the cell surface and in structures that resemble the secretory pathway of C. parvum sporozoites. Finally, we generated a polyclonal antibody against CpTSP1 to show that it is found at the cell surface and within micronemes, in a pattern reminiscent of other apicomplexan motility-associated adhesins, and is present both in sporozoites and meronts. This work sheds new light on an understudied family of C. parvum proteins that are likely to be important to both parasite biology and the development of vaccines against cryptosporidiosis.

Glycopeptide-Centric Approaches for the Characterization of Microbial Glycoproteomes

Protein glycosylation is increasingly recognized as a common class of modifications within microbial species that can shape protein functions and the proteome at large. Due to this, there is an increasing need for robust analytical methods, which allow for the identification and characterization of microbial glycopeptides from proteome samples in a high-throughput manner. Using affinity-based enrichment (either hydrophilicity or antibody-based approaches) glycopeptides can easily be separated from non-glycosylated peptides and analyzed using mass spectrometry. By utilizing multiple mass spectrometry fragmentation approaches and open searching-based bioinformatic techniques, novel glycopeptides can be identified and characterized without prior knowledge of the glycans used for glycosylation. Using these approaches, glycopeptides within samples can rapidly be identified as well as quantified to understand how glycosylation changes in response to stimuli or how changes in glycosylation systems impact the glycoproteome. This chapter outlines a set of robust protocols for the initial preparation, enrichment, and analysis of microbial glycopeptides for both qualitative and quantitative glycoproteomic studies. Using these approaches, glycosylation events can be easily identified by researchers without the need for extensive manual analysis of proteomic datasets.

C-Mannosylation of Toxoplasma gondii proteins promotes attachment to host cells and parasite virulence

C-Mannosylation is a common modification of thrombospondin type 1 repeats present in metazoans and recently identified also in apicomplexan parasites. This glycosylation is mediated by enzymes of the DPY19 family that transfer α-mannoses to tryptophan residues in the sequence WX ₂WX ₂C, which is part of the structurally essential tryptophan ladder. Here, deletion of the dpy19 gene in the parasite Toxoplasma gondii abolished C-mannosyltransferase activity and reduced levels of the micronemal protein MIC2. The loss of C-mannosyltransferase activity was associated with weakened parasite adhesion to host cells and with reduced parasite motility, host cell invasion, and parasite egress. Interestingly, the C-mannosyltransferase-deficient Δdpy19 parasites were strongly attenuated in virulence and induced protective immunity in mice. This parasite attenuation could not simply be explained by the decreased MIC2 level and strongly suggests that absence of C-mannosyltransferase activity leads to an insufficient level of additional proteins. In summary, our results indicate that T. gondii C-mannosyltransferase DPY19 is not essential for parasite survival, but is important for adhesion, motility, and virulence.

Protein O- and C-Glycosylation pathways in Toxoplasma gondii and Plasmodium falciparum

Apicomplexan parasites are amongst the most prevalent and morbidity-causing pathogens worldwide. They are responsible for severe diseases in humans and livestock and are thus of great public health and economic importance. Until the sequencing of apicomplexan genomes at the beginning of this century, the occurrence of N- and O-glycoproteins in these parasites was much debated. The synthesis of rudimentary and divergent N-glycans due to lineage-specific gene loss is now well established and has been recently reviewed. Here, we will focus on recent studies that clarified classical O-glycosylation pathways and described new nucleocytosolic glycosylations in Toxoplasma gondii, the causative agents of toxoplasmosis. We will also review the glycosylation of proteins containing thrombospondin type 1 repeats by O-fucosylation and C-mannosylation, newly discovered in Toxoplasma and the malaria parasite Plasmodium falciparum. The functional significance of these post-translational modifications has only started to emerge, but the evidence points towards roles for these protein glycosylation pathways in tissue cyst wall rigidity and persistence in the host, oxygen sensing, and stability of proteins involved in host invasion.

Isolation and characterization of glycosylated neuropeptides

Glycosylation is one of the most ubiquitous and complex post-translational modifications (PTMs). It plays pivotal roles in various biological processes. Studies at the glycopeptide level are typically considered as a downstream work resulting from enzymatic digested glycoproteins. Less attention has been focused on glycosylated endogenous signaling peptides due to their low abundance, structural heterogeneity and the lack of enabling analytical tools. Here, protocols are presented to isolate and characterize glycosylated neuropeptides utilizing nanoflow liquid chromatography coupled with mass spectrometry (LC-MS). We first demonstrate how to extract neuropeptides from raw tissues and perform further separation/cleanup before MS analysis. Then we describe hybrid MS methods for glycosylated neuropeptide profiling and site-specific analysis. We also include recommendations for data analysis to identify glycosylated neuropeptides in crustaceans where a complete neuropeptide database is still lacking. Other strategies and future directions are discussed to provide readers with alternative approaches and further unravel biological complexity rendered by glycosylation.

Protein O-Fucosyltransferase 2 Is Not Essential for Plasmodium berghei Development

Thrombospondin type I repeat (TSR) domains are commonly O-fucosylated by protein O-fucosyltransferase 2 (PoFUT2), and this modification is required for optimal folding and secretion of TSR-containing proteins. The human malaria parasite Plasmodium falciparum expresses proteins containing TSR domains, such as the thrombospondin-related anonymous protein (TRAP) and circumsporozoite surface protein (CSP), which are O-fucosylated. TRAP and CSP are present on the surface of sporozoites and play essential roles in mosquito and human host invasion processes during the transmission stages. Here, we have generated PoFUT2 null-mutant P. falciparum and Plasmodium berghei (rodent) malaria parasites and, by phenotyping them throughout their complete life cycle, we show that PoFUT2 disruption does not affect the growth through the mosquito stages for both species. However, contrary to what has been described previously by others, P. berghei PoFUT2 null mutant sporozoites showed no deleterious motility phenotypes and successfully established blood stage infection in mice. This unexpected result indicates that the importance of O-fucosylation of TSR domains may differ between human and RODENT malaria parasites; complicating our understanding of glycosylation modifications in malaria biology.

Sweet and CRISP(R)y parasite engineering

Toxoplasma gondii is an intracellular parasite that is highly prevalent within human populations. Its genome encodes a range of enzymes involved in glycan biosynthesis and metabolism. A new study presents a library of CRISPR/Cas9-based glyco-relevant gene knockouts and their examination in glycomic and functional assays. This new resource can pave the way for a better understanding of the role of carbohydrates in infection and immunomodulation by this significant protozoan parasite.

Protein O-fucosylation: structure and function

Fucose is a common terminal modification on protein and lipid glycans. Fucose can also be directly linked to protein via an O-linkage to Serine or Threonine residues located within consensus sequences contained in Epidermal Growth Factor-like (EGF) repeats and Thrombospondin Type 1 Repeats (TSRs). In this context, fucose is added exclusively to properly folded EGF repeats and TSRs by Protein O-fucosyltransferases 1 and 2, respectively. In both cases, the O-linked fucose can also be elongated with other sugars. Here, we describe the biological importance of these O-fucose glycans and molecular mechanisms by which they affect the function of the proteins they modify. O-Fucosylation of EGF repeats modulates the Notch signaling pathway, while O-fucosylation of TSRs is predicted to influence secretion of targets including several extracellular proteases. Recent data show O-fucose glycans mediate their effects by participating in both intermolecular and intramolecular interactions.

O-Fucosylation of thrombospondin-like repeats is required for processing of microneme protein 2 and for efficient host cell invasion by Toxoplasma gondii tachyzoites

Toxoplasma gondii is an intracellular parasite that causes disseminated infections that can produce neurological damage in fetuses and immunocompromised individuals. Microneme protein 2 (MIC2), a member of the thrombospondin-related anonymous protein (TRAP) family, is a secreted protein important for T. gondii motility, host cell attachment, invasion, and egress. MIC2 contains six thrombospondin type I repeats (TSRs) that are modified by C-mannose and O-fucose in Plasmodium spp. and mammals. Here, using MS analysis, we found that the four TSRs in T. gondii MIC2 with protein O-fucosyltransferase 2 (POFUT2) acceptor sites are modified by a dHexHex disaccharide, whereas Trp residues within three TSRs are also modified with C-mannose. Disruption of genes encoding either POFUT2 or the putative GDP-fucose transporter (NST2) resulted in loss of MIC2 O-fucosylation, as detected by an antibody against the GlcFuc disaccharide, and in markedly reduced cellular levels of MIC2. Furthermore, in 10-15% of the Δpofut2 or Δnst2 vacuoles, MIC2 accumulated earlier in the secretory pathway rather than localizing to micronemes. Dissemination of tachyzoites in human foreskin fibroblasts was reduced for these knockouts, which both exhibited defects in attachment to and invasion of host cells comparable with the Δmic2 phenotype. These results, indicating that O-fucosylation of TSRs is required for efficient processing of MIC2 and for normal parasite invasion, are consistent with the recent demonstration that Plasmodium falciparum Δpofut2 strain has decreased virulence and also support a conserved role for this glycosylation pathway in quality control of TSR-containing proteins in eukaryotes.

CRISPR/Cas9 and glycomics tools for Toxoplasma glycobiology

Infection with the protozoan parasite Toxoplasma gondii is a major health risk owing to birth defects, its chronic nature, ability to reactivate to cause blindness and encephalitis, and high prevalence in human populations. Unlike most eukaryotes, Toxoplasma propagates in intracellular parasitophorous vacuoles, but like nearly all other eukaryotes, Toxoplasma glycosylates many cellular proteins and lipids and assembles polysaccharides. Toxoplasma glycans resemble those of other eukaryotes, but species-specific variations have prohibited deeper investigations into their roles in parasite biology and virulence. The Toxoplasma genome encodes a suite of likely glycogenes expected to assemble N-glycans, O-glycans, a C-glycan, GPI-anchors, and polysaccharides, along with their precursors and membrane transporters. To investigate the roles of specific glycans in Toxoplasma, here we coupled genetic and glycomics approaches to map the connections between 67 glycogenes, their enzyme products, the glycans to which they contribute, and cellular functions. We applied a double-CRISPR/Cas9 strategy, in which two guide RNAs promote replacement of a candidate gene with a resistance gene; adapted MS-based glycomics workflows to test for effects on glycan formation; and infected fibroblast monolayers to assess cellular effects. By editing 17 glycogenes, we discovered novel Glc_0-2-Man₆-GlcNAc₂-type N-glycans, a novel HexNAc-GalNAc-mucin-type O-glycan, and Tn-antigen; identified the glycosyltransferases for assembling novel nuclear O-Fuc-type and cell surface Glc-Fuc-type O-glycans; and showed that they are important for in vitro growth. The guide sequences, editing constructs, and mutant strains are freely available to researchers to investigate the roles of glycans in their favorite biological processes.