Toxoplasma gondii grown in human cells uses GalNAc-containing glycosylphosphatidylinositol precursors to anchor surface antigens while the immunogenic Glc–GalNAc-containing precursors remain free at the parasite cell surface

N-acetylglucosamine supplementation fails to bypass the critical acetylation of glucosamine-6-phosphate required for Toxoplasma gondii replication and invasion

The cell surface of Toxoplasma gondii is rich in glycoconjugates which hold diverse and vital functions in the lytic cycle of this obligate intracellular parasite. Additionally, the cyst wall of bradyzoites, that shields the persistent form responsible for chronic infection from the immune system, is heavily glycosylated. Formation of glycoconjugates relies on activated sugar nucleotides, such as uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). The glucosamine-phosphate-N-acetyltransferase (GNA1) generates N-acetylglucosamine-6-phosphate critical to produce UDP-GlcNAc. Here, we demonstrate that downregulation of T. gondii GNA1 results in a severe reduction of UDP-GlcNAc and a concomitant drop in glycosylphosphatidylinositols (GPIs), leading to impairment of the parasite's ability to invade and replicate in the host cell. Surprisingly, attempts to rescue this defect through exogenous GlcNAc supplementation fail to completely restore these vital functions. In depth metabolomic analyses elucidate diverse causes underlying the failed rescue: utilization of GlcNAc is inefficient under glucose-replete conditions and fails to restore UDP-GlcNAc levels in GNA1-depleted parasites. In contrast, GlcNAc-supplementation under glucose-deplete conditions fully restores UDP-GlcNAc levels but fails to rescue the defects associated with GNA1 depletion. Our results underscore the importance of glucosamine-6-phosphate acetylation in governing T. gondii replication and invasion and highlight the potential of the evolutionary divergent GNA1 in Apicomplexa as a target for the development of much-needed new therapeutic strategies.

The importance of side branches of glycosylphosphatidylinositol anchors: a molecular dynamics perspective

Many proteins are anchored to the cell surface of eukaryotes using a unique family of glycolipids called glycosylphosphatidylinositol (GPI) anchors. These glycolipids also exist without a covalently bound protein, in particular on the cell surfaces of protozoan parasites where they are densely populated. GPIs and GPI-anchored proteins participate in multiple cellular processes such as signal transduction, cell adhesion, protein trafficking and pathogenesis of Malaria, Toxoplasmosis, Trypanosomiasis and prion diseases, among others. All GPIs share a common conserved glycan core modified in a cell-dependent manner with additional side glycans or phosphoethanolamine residues. Here, we use atomistic molecular dynamic simulations and perform a systematic study to evaluate the structural properties of GPIs with different side chains inserted in lipid bilayers. Our results show a flop-down orientation of GPIs with respect to the membrane surface and the presentation of the side chain residues to the solvent. This finding agrees well with experiments showing the role of the side residues as active epitopes for recognition of GPIs by macrophages and induction of GPI-glycan-specific immune responses. Protein-GPI interactions were investigated by attaching parasitic GPIs to Green Fluorescent Protein. GPIs are observed to recline on the membrane surface and pull down the attached protein close to the membrane facilitating mutual contacts between protein, GPI and the lipid bilayer. This model is efficient in evaluating the interaction of GPIs and GPI-anchored proteins with membranes and can be extended to study other parasitic GPIs and proteins and develop GPI-based immunoprophylaxis to treat infectious diseases.

N-Acetylglucosamine Sensing and Metabolic Engineering for Attenuating Human and Plant Pathogens

During evolution, both human and plant pathogens have evolved to utilize a diverse range of carbon sources. N-acetylglucosamine (GlcNAc), an amino sugar, is one of the major carbon sources utilized by several human and phytopathogens. GlcNAc regulates the expression of many virulence genes of pathogens. In fact, GlcNAc catabolism is also involved in the regulation of virulence and pathogenesis of various human pathogens, including Candida albicans, Vibrio cholerae, Leishmania donovani, Mycobacterium, and phytopathogens such as Magnaporthe oryzae. Moreover, GlcNAc is also a well-known structural component of many bacterial and fungal pathogen cell walls, suggesting its possible role in cell signaling. Over the last few decades, many studies have been performed to study GlcNAc sensing, signaling, and metabolism to better understand the GlcNAc roles in pathogenesis in order to identify new drug targets. In this review, we provide recent insights into GlcNAc-mediated cell signaling and pathogenesis. Further, we describe how the GlcNAc metabolic pathway can be targeted to reduce the pathogens’ virulence in order to control the disease prevalence and crop productivity.

Phosphoglycan-sensitized platform for specific detection of anti-glycan IgG and IgM antibodies in serum

Glycosylphosphatidylinositol anchored proteins (GPI-APs) are natural conjugates in the plasma membrane of eukaryotic cells that result from the attachment of a glycolipid to the C-terminus of many proteins. GPI-APs play a crucial role in cell signaling and adhesion and have implications in health and diseases. GPI-APs and GPIs without protein (free GPIs) are found in abundance on the surface of the protozoan parasite Toxoplasma gondii. The detection of anti-GPI IgG and IgM antibodies allows differentiation between toxoplasmosis patients and healthy individuals using serological assays. However, these methods are limited by their poor efficiency, cross-reactivity and need for sophisticated laboratory equipment and qualified personnel. Here, we established a label-free electrochemical glycobiosensor for the detection of anti-GPI IgG and IgM antibodies in serum from toxoplasmosis seropositive patients. This biosensor uses a synthetic GPI phosphoglycan bioreceptor immobilized on screen-printed gold electrodes through a linear alkane thiol phosphodiester. The antigen-antibody interaction was detected and quantified by electrochemical impedance spectroscopy (EIS). The resultant device showed a linear dynamic range of anti-GPI antibodies in serum ranging from 1.0 to 10.0 IU mL^-1, with a limit of detection of 0.31 IU mL^-1. This method also holds great potential for the detection of IgG antibodies related to other multiple medical conditions characterized by overexpression of antibodies.

Babesia divergens glycosylphosphatidylinositols modulate blood coagulation and induce Th2-biased cytokine profiles in antigen presenting cells

Glycosylphosphatidylinositols (GPIs) are glycolipids described as toxins of protozoan parasites due to their inflammatory properties in mammalian hosts characterized by the production of interleukin (IL)-1, IL-12 and tumor necrosis factor (TNF)-α. In the present work, we studied the cytokines produced by antigen presenting cells in response to ten different GPI species extracted from Babesia divergens, responsible for babesiosis. Interestingly, B. divergens GPIs induced the production of anti-inflammatory cytokines (IL-2, IL-5) and of the regulatory cytokine IL-10 by macrophages and dendritic cells. In contrast to all protozoan GPIs studied until now, GPIs from B. divergens did not stimulate the production of TNF-α and IL-12, leading to a unique Th1/Th2 profile. Analysis of the carbohydrate composition of the B. divergens GPIs indicated that the di-mannose structure was different from the evolutionary conserved tri-mannose structure, which might explain the particular cytokine profile they induce. Expression of major histocompatibility complex (MHC) molecules on dendritic cells and apoptosis of mouse peritoneal cells were also analysed. B. divergens GPIs did not change expression of MHC class I, but decreased expression of MHC class II at the cell surface, while GPIs slightly increased the percentages of apoptotic cells. During pathogenesis of babesiosis, the inflammation-coagulation auto-amplification loop can lead to thrombosis and the effect of GPIs on coagulation parameters was investigated. Incubation of B. divergens GPIs with rat plasma ex vivo led to increase of fibrinogen levels and to prolonged activated partial thromboplastin time, suggesting a direct modulation of the extrinsic coagulation pathway by GPIs.

Detection of Anti-Toxoplasma gondii Antibodies in Human Sera Using Synthetic Glycosylphosphatidylinositol Glycans on a Bead-Based Multiplex Assay

Toxoplasmosis, while often an asymptomatic parasitic disease in healthy individuals, can cause severe complications in immunocompromised persons and during pregnancy. The most common method to diagnose Toxoplasma gondii infections is the serological determination of antibodies directed against parasite protein antigens. Here we report the use of a bead-based multiplex assay containing a synthetic phosphoglycan portion of the Toxoplasma gondii glycosylphosphatidylinositol (GPI1) for the detection of GPI1-specific antibodies in human sera. The glycan was conjugated to beads at the lipid site to retain its natural orientation and its immunogenic groups. We compared the response against GPI1 with that against the protein antigen SAG1, a common component of commercial serological assays, via the detection of parasite-specific human IgG and IgM antibodies, respectively. The GPI1-based test is in excellent agreement with the results for the commercial ELISA, as the ROC analysis of the GPI1 test shows 97% specificity and 98% sensitivity for the assay. GPI1 was a more reliable predictor for a parasite-specific IgM response compared to SAG1, indicating that a bead-based multiplex assay using GPI1 in combination with SAG1 may strengthen Toxoplasma gondii serology, in particular in seroepidemiological studies.

Mutations in PIGU Impair the Function of the GPI Transamidase Complex, Causing Severe Intellectual Disability, Epilepsy, and Brain Anomalies

The glycosylphosphatidylinositol (GPI) anchor links over 150 proteins to the cell surface and is present on every cell type. Many of these proteins play crucial roles in neuronal development and function. Mutations in 18 of the 29 genes implicated in the biosynthesis of the GPI anchor have been identified as the cause of GPI biosynthesis deficiencies (GPIBDs) in humans. GPIBDs are associated with intellectual disability and seizures as their cardinal features. An essential component of the GPI transamidase complex is PIGU, along with PIGK, PIGS, PIGT, and GPAA1, all of which link GPI-anchored proteins (GPI-APs) onto the GPI anchor in the endoplasmic reticulum (ER). Here, we report two homozygous missense mutations (c.209T>A [p.Ile70Lys] and c.1149C>A [p.Asn383Lys]) in five individuals from three unrelated families. All individuals presented with global developmental delay, severe-to-profound intellectual disability, muscular hypotonia, seizures, brain anomalies, scoliosis, and mild facial dysmorphism. Using multicolor flow cytometry, we determined a characteristic profile for GPI transamidase deficiency. On granulocytes this profile consisted of reduced cell-surface expression of fluorescein-labeled proaerolysin (FLAER), CD16, and CD24, but not of CD55 and CD59; additionally, B cells showed an increased expression of free GPI anchors determined by T5 antibody. Moreover, computer-assisted facial analysis of different GPIBDs revealed a characteristic facial gestalt shared among individuals with mutations in PIGU and GPAA1. Our findings improve our understanding of the role of the GPI transamidase complex in the development of nervous and skeletal systems and expand the clinical spectrum of disorders belonging to the group of inherited GPI-anchor deficiencies.

In vitro cellular responses to Neospora caninum glycosylphosphatidylinositols depend on the host origin of antigen presenting cells

Neosporosis due to Neospora caninum causes abortions in farm animals such as cattle. No treatment and vaccine exist to fight this disease, responsible for considerable economic losses. It is thus important to better understand the immune responses occurring during the pathogenesis to control them in a global strategy against the parasite. In this context, we studied the roles of N. caninum glycosylphosphatidylinositols (GPIs), glycolipids defined as toxins in the related parasite Plasmodium falciparum. We demonstrated for the first time that GPIs could be excreted in the supernatant of N. caninum culture and trigger cell signalling through the Toll-like receptors 2 and 4. In addition, antibodies specific to N. caninum GPIs were detected in the serum of infected mice. As shown for other protozoan diseases, they could play a role in neutralizing GPIs. N. caninum GPIs were able to induce the production of tumour necrosis factor-α, interleukin(IL)-1β and IL-12 cytokines by murine macrophages and dendritic cells. Furthermore, GPIs significantly reduced expression of major histocompatibility complex (MHC) molecules of class I on murine dendritic cells. In contrast to murine cells, bovine blood mononuclear cells produced increased levels of IFN-γ and IL-10, but reduced levels of IL-12p40 in response to GPIs. On these bovine cells, GPI had the tendency to up-regulate MHC class I, but to down-regulate MHC class II. Altogether, these results suggest that N. caninum GPIs might differentially participate in the responses of antigen presenting cells induced by the whole parasite in mouse models of neosporosis and in the natural cattle host.

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.

Investigation of the protective properties of glycosylphosphatidylinositol-based vaccine candidates in a Toxoplasma gondii mouse challenge model

Vaccination against the ubiquitous parasite Toxoplasma gondii would provide the most efficient prevention against toxoplasmosis-related congenital, brain and eye diseases in humans. We investigated the immune response elicited by pathogen-specific glycosylphosphatidylinositol (GPI) glycoconjugates using carbohydrate microarrays in a BALB/c mouse model. We further examined the protective properties of the glycoconjugates in a lethal challenge model using the virulent T. gondii RH strain. Upon immunization, mice raised antibodies that bind to the respective GPIs on carbohydrate microarrays, but were mainly directed against an unspecific GPI epitope including the linker. The observed immune response, though robust, was unable to provide protection in mice when challenged with a lethal dose of viable tachyzoites. We demonstrate that anti-GPI antibodies raised against the here described semi-synthetic glycoconjugates do not confer protective immunity against T. gondii in BALB/c mice.

Diagnosis of toxoplasmosis using a synthetic glycosylphosphatidylinositol glycan

Around 2 billion people worldwide are infected with the apicomplexan parasite Toxoplasma gondii which induces a variety of medical conditions. For example, primary infection during pregnancy can result in fetal death or mental retardation of the child. Diagnosis of acute infections in pregnant women is challenging but crucially important as the drugs used to treat T. gondii infections are potentially harmful to the unborn child. Better, faster, more reliable, and cheaper means of diagnosis by using defined antigens for accurate serological tests are highly desirable. Synthetic pathogen-specific glycosylphosphatidylinositol (GPI) glycan antigens are diagnostic markers and have been used to distinguish between toxoplasmosis disease states using human sera.

Virulent and avirulent strains of Toxoplasma gondii which differ in their glycosylphosphatidylinositol content induce similar biological functions in macrophages

Glycosylphosphatidylinositols (GPIs) from several protozoan parasites are thought to elicit a detrimental stimulation of the host innate immune system aside their main function to anchor surface proteins. Here we analyzed the GPI biosynthesis of an avirulent Toxoplasma gondii type 2 strain (PTG) by metabolic radioactive labeling. We determined the biological function of individual GPI species in the PTG strain in comparison with previously characterized GPI-anchors of a virulent strain (RH). The GPI intermediates of both strains were structurally similar, however the abundance of two of six GPI intermediates was significantly reduced in the PTG strain. The side-by-side comparison of GPI-anchor content revealed that the PTG strain had only ∼34% of the protein-free GPIs as well as ∼70% of the GPI-anchored proteins with significantly lower rates of protein N-glycosylation compared to the RH strain. All mature GPIs from both strains induced comparable secretion levels of TNF-α and IL-12p40, and initiated TLR4/MyD88-dependent NF-κBp65 activation in macrophages. Taken together, these results demonstrate that PTG and RH strains differ in their GPI biosynthesis and possess significantly different GPI-anchor content, while individual GPI species of both strains induce similar biological functions in macrophages.

In vivo incorporation of an azide-labeled sugar analog to detect mammalian glycosylphosphatidylinositol molecules isolated from the cell surface

N-Acetylgalactosamine (GalNAc) linked to the first mannose of glycosylphosphatidylinositol (GPI) core has been previously reported to be heterogeneously present on some mammalian GPI-anchored proteins. Here we present a method for profiling GalNAc-containing GPI-anchored proteins in mammalian cells by metabolic labeling with tetraacetylated N-azidoacetylgalactosamine (GalNAz) followed by biotinylation of the incorporated sugar analog. We have labeled both endogenous and recombinant GPI-anchored proteins with GalNAz, and demonstrated that the azide-activated sugar gets incorporated into the GPI glycan, likely as an unsubstituted side branch of the core structure. GalNAz was detected only on GPI molecules attached to proteins, and not on GPI precursors, indicating that GalNAc modification takes place after the GPI anchor is transferred to protein. We have highlighted the utility of this cell labeling approach by demonstrating the ability to examine specific GalNAc-containing GPI-anchored proteins isolated non-destructively from separate membrane domains (apical and basolateral) in polarized epithelial cells. This study represents the first demonstration of site-specific in vivo labeling of a GPI moiety with a synthetic sugar analog.

A general method for synthesis of GPI anchors illustrated by the total synthesis of the low-molecular-weight antigen from Toxoplasma gondii

Building blocks: a new, general synthetic strategy, which allows the construction of branched glycosylphosphatidylinositols (GPIs), enables the synthesis of parasitic glycolipid 1 from Toxoplasma gondii. In addition, the structure is further confirmed by recognition of monoclonal antibodies.

Immunological reactions in response to apicomplexan glycosylphosphatidylinositols

Apicomplexan protozoa are a phylum of parasites that includes pathogens such as Plasmodium, the causative agent of the most severe form of malaria responsible for almost 1 million deaths per year and Toxoplasma gondii causing toxoplasmosis, a disease leading to cerebral meningitis in immunocompromised individuals or to abortion in farm animals or in women that are infected for the first time during pregnancy. The initial immune reactions developed by the host are similar in response to an infection with Plasmodium and Toxoplasma in the sense that the same cells of the innate immune system are stimulated to produce inflammatory cytokines. The glycosylphosphatidylinositol (GPI) anchor is the major carbohydrate modification in parasite proteins and the GPIs are essential for parasite survival. Two immediate GPI precursors with the structures ethanolamine phosphate-6(Manalpha1-2)Manalpha1-2Manalpha1-6Manalpha1-4GlcN-PI and ethanolamine phosphate-6Manalpha1-2Manalpha1-6Man-alpha1-4-GlcN-PI are synthesized by P. falciparum. Two main structures are synthesized by T. gondii: ethanolamine phosphate-6Manalpha1-2Manalpha1-6(GalNAcbeta1-4)Manalpha1-4GlcN-PI and ethanolamine phosphate-6Manalpha1-2Manalpha1-6(Glcalpha1-4GalNAcbeta1-4)Manalpha1-4GlcN-PI. This review describes the biosynthesis of the apicomplexan GPIs and their role in the activation of the host immune system.

Subcellular localization and dynamics of a digalactolipid-like epitope in Toxoplasma gondii

Toxoplasma gondii is a unicellular parasite characterized by unique extracellular and intracellular membrane compartments. The lipid composition of subcellular membranes has not been determined, limiting our understanding of lipid homeostasis, control, and trafficking, a series of processes involved in pathogenesis. In addition to a mitochondrion, Toxoplasma contains a plastid called the apicoplast. The occurrence of a plastid raised the question of the presence of chloroplast galactolipids. Using three independent rabbit and rat antibodies against digalactosyldiacylglycerol (DGDG) from plant chloroplasts, we detected a class of Toxoplasma lipids harboring a digalactolipid-like epitope (DGLE). Immunolabeling characterization supports the notion that the DGLE polar head is similar to that of DGDG. Mass spectrometry analyses indicated that dihexosyl lipids having various hydrophobic moieties (ceramide, diacylglycerol, and acylalkylglycerol) might react with anti-DGDG, but we cannot exclude the possibility that more complex dihexosyl-terminated lipids might also be immunolabeled. DGLE localization was analyzed by immunofluorescence and immunoelectron microscopy and confirmed by subcellular fractionation. No immunolabeling of the apicoplast could be observed. DGLE was scattered in pellicle membrane domains in extracellular tachyzoites and was relocalized to the anterior tip of the cell upon invasion in an actin-dependent manner, providing insights on a possible role in pathogenetic processes. DGLE was detected in other Apicomplexa (i.e., Neospora, Plasmodium, Babesia, and Cryptosporidium).

The role of inositol acylation and inositol deacylation in the Toxoplasma gondii glycosylphosphatidylinositol biosynthetic pathway

Toxoplasma gondii is a ubiquitous parasitic protozoan that invades nucleated cells in a process thought to be in part due to several surface glycosylphosphatidylinositol (GPI)-anchored proteins, like the major surface antigen SAG1 (P30), which dominates the plasma membrane. The serine protease inhibitors phenylmethylsulfonyl fluoride and diisopropyl fluoride were found to have a profound effect on the T. gondii GPI biosynthetic pathway, leading to the observation and characterization of novel inositol-acylated mannosylated GPI intermediates. This inositol acylation is acyl-CoA-dependent and takes place before mannosylation, but uniquely for this class of inositol-acyltransferase, it is inhibited by phenylmethylsulfonyl fluoride. The subsequent inositol deacylation of fully mannosylated GPI intermediates is inhibited by both phenylmethylsulfonyl fluoride and diisopropyl fluoride. The use of these serine protease inhibitors allows observations as to the timing of inositol acylation and subsequent inositol deacylation of the GPI intermediates. Inositol acylation of the non-mannosylated GPI intermediate D-GlcNalpha1-6-D-myo-inositol-1-HPO4-sn-lipid precedes mannosylation. Inositol deacylation of the fully mannosylated GPI intermediate allows further processing, i.e. addition of GalNAc side chain to the first mannose. Characterization of the phosphatidylinositol moieties present on both free GPIs and GPI-anchored proteins shows the presence of a diacylglycerol lipid, whose sn-2 position contains almost exclusively an C18:1 acyl chain. The data presented here identify key novel inositol-acylated mannosylated intermediates, allowing the formulation of an updated T. gondii GPI biosynthetic pathway along with identification of the putative genes involved.

Toxoplasma gondii glycosylphosphatidylinositols are not involved in T. gondii-induced host cell survival

Toxoplasma gondii is an intracellular parasite able to both promote and inhibit apoptosis. T. gondii renders infected cells resistant to programmed cell death induced by multiple apoptotic triggers. On the other hand, increased apoptosis of immune cells after in vivo infection with T. gondii may suppress the immune response to the parasite. Glycosylphosphatidylinositol (GPI)-anchored proteins dominate the surface of T. gondii tachyzoites and GPIs are involved in the pathogenicity of protozoan parasites. In this report, we determine if GPIs are responsible for inhibition or induction of host cell apoptosis. We show here that T. gondii GPIs fail to block apoptosis that was triggered in human-derived cells via extrinsic or intrinsic apoptotic pathways. Furthermore, characteristics of apoptosis, e.g. caspase-3/7 activity, phosphatidylserine exposition at the cell surface or DNA strand breaks, were not observed in the presence of T. gondii GPIs. These results indicate that T. gondii GPIs are not involved in survival or in apoptosis of host cells. This absence of effect on apoptosis could be a feature common to GPIs of other parasites.