Biosynthesis of glycolipid precursors for glycosylphosphatidylinositol membrane anchors in a Toxoplasma gondii cell-free system

Extracellular Toxoplasma gondii tachyzoites metabolize and incorporate unnatural sugars into cellular proteins

Toxoplasma gondii is an obligate intracellular parasite that infects all nucleated cell types in diverse warm-blooded organisms. Many of the surface antigens and effector molecules secreted by the parasite during invasion and intracellular growth are modified by glycans. Glycosylated proteins in the nucleus and cytoplasm have also been reported. Despite their prevalence, the complete inventory and biological significance of glycosylated proteins in Toxoplasma remain unknown. In this study, we aimed to globally profile parasite glycoproteins using a bioorthogonal chemical reporter strategy. This strategy involves the metabolic incorporation of unnatural functional groups (i.e., "chemical reporters") into Toxoplasma glycans, followed by covalent labeling with visual probes or affinity tags. The two-step approach enables the visualization and identification of newly biosynthesized glycoconjugates in the parasite. Using a buffer that mimics intracellular conditions, extracellular Toxoplasma tachyzoites were found to metabolize and incorporate unnatural sugars (equipped with bioorthogonal functional groups) into diverse proteins. Covalent chemistries were used to visualize and retrieve these labeled structures. Subsequent mass spectrometry analysis revealed 89 unique proteins. This survey identified novel proteins as well as previously characterized proteins from lectin affinity analyses.

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.

Membrane topology and transient acylation of Toxoplasma gondii glycosylphosphatidylinositols

Using hypotonically permeabilized Toxoplasma gondii tachyzoites, we investigated the topology of the free glycosylphosphatidylinositols (GPIs) within the endoplasmic reticulum (ER) membrane. The morphology and permeability of parasites were checked by electron microscopy and release of a cytosolic protein. The membrane integrity of organelles (ER and rhoptries) was checked by protease protection assays. In initial experiments, GPI biosynthetic intermediates were labeled with UDP-[6-(3)H]GlcNAc in permeabilized parasites, and the transmembrane distribution of the radiolabeled lipids was probed with phosphatidylinositol-specific phospholipase C (PI-PLC). A new early intermediate with an acyl modification on the inositol was identified, indicating that inositol acylation also occurs in T. gondii. A significant portion of the early GPI intermediates (GlcN-PI and GlcNAc-PI) could be hydrolyzed following PI-PLC treatment, indicating that these glycolipids are predominantly present in the cytoplasmic leaflet of the ER. Permeabilized T. gondii parasites labeled with either GDP-[2-(3)H]mannose or UDP-[6-(3)H]glucose showed that the more mannosylated and side chain (Glc-GalNAc)-modified GPI intermediates are also preferentially localized in the cytoplasmic leaflet of the ER.

Evolutionary genomics of the HAD superfamily: understanding the structural adaptations and catalytic diversity in a superfamily of phosphoesterases and allied enzymes

The HAD (haloacid dehalogenase) superfamily includes phosphoesterases, ATPases, phosphonatases, dehalogenases, and sugar phosphomutases acting on a remarkably diverse set of substrates. The availability of numerous crystal structures of representatives belonging to diverse branches of the HAD superfamily provides us with a unique opportunity to reconstruct their evolutionary history and uncover the principal determinants that led to their diversification of structure and function. To this end we present a comprehensive analysis of the HAD superfamily that identifies their unique structural features and provides a detailed classification of the entire superfamily. We show that at the highest level the HAD superfamily is unified with several other superfamilies, namely the DHH, receiver (CheY-like), von Willebrand A, TOPRIM, classical histone deacetylases and PIN/FLAP nuclease domains, all of which contain a specific form of the Rossmannoid fold. These Rossmannoid folds are distinguished from others by the presence of equivalently placed acidic catalytic residues, including one at the end of the first core beta-strand of the central sheet. The HAD domain is distinguished from these related Rossmannoid folds by two key structural signatures, a "squiggle" (a single helical turn) and a "flap" (a beta hairpin motif) located immediately downstream of the first beta-strand of their core Rossmanoid fold. The squiggle and the flap motifs are predicted to provide the necessary mobility to these enzymes for them to alternate between the "open" and "closed" conformations. In addition, most members of the HAD superfamily contains inserts, termed caps, occurring at either of two positions in the core Rossmannoid fold. We show that the cap modules have been independently inserted into these two stereotypic positions on multiple occasions in evolution and display extensive evolutionary diversification independent of the core catalytic domain. The first group of caps, the C1 caps, is directly inserted into the flap motif and regulates access of reactants to the active site. The second group, the C2 caps, forms a roof over the active site, and access to their internal cavities might be in part regulated by the movement of the flap. The diversification of the cap module was a major factor in the exploration of a vast substrate space in the course of the evolution of this superfamily. We show that the HAD superfamily contains 33 major families distributed across the three superkingdoms of life. Analysis of the phyletic patterns suggests that at least five distinct HAD proteins are traceable to the last universal common ancestor (LUCA) of all extant organisms. While these prototypes diverged prior to the emergence of the LUCA, the major diversification in terms of both substrate specificity and reaction types occurred after the radiation of the three superkingdoms of life, primarily in bacteria. Most major diversification events appear to correlate with the acquisition of new metabolic capabilities, especially related to the elaboration of carbohydrate metabolism in the bacteria. The newly identified relationships and functional predictions provided here are likely to aid the future exploration of the numerous poorly understood members of this large superfamily of enzymes.

Contribution of host lipids to Toxoplasma pathogenesis

As an actively dividing organism, the intracellular parasite Toxoplasma gondii must adjust the size and composition of its membranes in order to accommodate changes due to housekeeping activities, to commit division and in fine to produce new viable progenies. Lipid inventory of T. gondii reveals that the biological membranes of this parasite are composed of a complex mixture of neutral and polar lipids. After examination of the origin of T. gondii membrane lipids, three categories of lipids can be described: (i) lipids scavenged by T. gondii from the host cell; (ii) lipids synthesized in large amounts by the parasite, independently from its host cell; and (iii) lipids produced de novo by the parasite, but whose synthesis does not come close to satisfying the entire parasite's needs. These latter must be adeptly acquired from the host environment. To this end, T. gondii diverts a large variety of lipid precursors from host cytoplasm and efficiently manufacture them into complex lipids. This rather remarkable reliance on host lipid resources for parasite survival opens new avenues to restrict parasite growth. Indeed, parasite starvation can be induced upon deprivation from essential host lipids. Lipid analogues with anti-proliferative properties are voraciously taken up by the parasites, which results in parasite membrane defects, and ultimately death.

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

Toxoplasma gondii is a ubiquitous parasite that infects nearly all warm-blooded animals. Developmental switching in T. gondii, from the virulent tachyzoite to the relatively quiescent bradyzoite stage, is responsible for the disease propagation after alteration of the immune status of the carrier. The redifferentiation event is characterized by an over expression of a tachyzoite specific set of glycosylphosphatidylinositol anchored surface antigens and free GPIs. T. gondii grown in animal cells uses two glycosylphosphatidylinositol precursors to anchor the parasite surface proteins. The first form has an N-acetylgalactosamine residue bound to a conserved three-mannosyl core glycan, while the second structure contains an additional terminal glucose linked to the N-acetylgalactosamine side branch. Sera from persons infected with T. gondii reacted only with the glucose-N-acetylgalactosamine-containing structure. Here we report that T. gondii cultured in human cells uses predominantly the N-acetylgalactosamine-containing structure to anchor the parasite surface antigens. On the other hand, glycosylphosphatidylinositol structures having an additional terminal glucose are found exclusively on the parasite cell surface as free glycolipids participating in the production of cytokines that are implicated in the pathogenesis of T. gondii. We also provide evidence that such free glycosylphosphatidylinositols are restricted mainly to the lipid microdomains in the parasite cell surface membrane and mostly associated with proteins involved in the parasite motility as well as invasion of the host cell.

Removal of phospholipid contaminants through precipitation of glycosylphosphatidylinositols

Parasitic glycosylphosphatidylinositols (GPIs) are thought to be involved in induced cell signaling that leads to proinflammatory responses. Increasing interest in elucidation of the mechanisms involved in signaling pathways drives the finding of rapid and reliable methods to purify GPIs. GPIs are usually extracted using mixtures of chloroform/methanol/water, followed by a phase partition between water and water-saturated n-butanol. GPIs recovered in the butanol phase are separated by thin-layer chromatography, scraped, eluted from the silica, and used for studying the structure-function relationship. The presence of phospholipid contaminants or other hydrophobic components in the samples cannot be excluded. Furthermore, the standard procedures to purify GPIs harbor several drawbacks, including the need to handle large amounts of culture, poor yields, time-consuming, and interfering contaminants. Here we report on the development of a simple and reliable method to isolate and purify both free and bound GPIs from one cell pellet. We exploited the low solubility of GPIs in water-saturated n-butanol to remove the phospholipid contaminants completely. After delipidation, GPI proteins were solubilized from the pellet using a mixture of organic solvent containing ethanol and water.

Inhibitors of glycosyl-phosphatidylinositol anchor biosynthesis

Glycosyl-phosphatidylinositol (GPI) is a complex glycolipid structure that acts as a membrane anchor for many cell-surface proteins of eukaryotes. GPI-anchored proteins are particularly abundant in protozoa such as Trypanosoma brucei, Leishmania major, Plasmodium falciparum and Toxoplasma gondii, and represent the major carbohydrate modification of many cell-surface parasite proteins. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. Therefore, the characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. In vitro and in vivo studies using sugars and substrate analogues as well as natural compounds have shown that it is possible to interfere with GPI biosynthesis at different steps in a species-specific manner. Here we review the recent and promising progress in the field of GPI inhibition.

Phospholipids in parasitic protozoa

Parasitic protozoa are surrounded by membrane structures that have a different lipid and protein composition relative to membranes of the host. The parasite membranes are essential structurally and also for parasite specific processes, like host cell invasion, nutrient acquisition or protection against the host immune system. Furthermore, intracellular parasites can modulate membranes of their host, and trafficking of membrane components occurs between host membranes and those of the intracellular parasite. Phospholipids are major membrane components and, although many parasites scavenge these phospholipids from their host, most parasites also synthesise phospholipids de novo, or modify a large part of the scavenged phospholipids. It was recently shown that some parasites like Plasmodium have unique phospholipid metabolic pathways. This review will focus on new developments in research on phospholipid metabolism of parasitic protozoa in relation to parasite-specific membrane structures and function, as well as on several targets for interference with the parasite phospholipid metabolism with a view to developing new anti-parasitic drugs.

Clostridium septicum alpha-toxin is active against the parasitic protozoan Toxoplasma gondii and targets members of the SAG family of glycosylphosphatidylinositol-anchored surface proteins

As is the case with many other protozoan parasites, glycosylphosphatidylinositol (GPI)-anchored proteins dominate the surface of Toxoplasma gondii tachyzoites. The mechanisms by which T. gondii GPI-anchored proteins are synthesized and transported through the unusual triple-membrane structure of the parasite pellicle to the plasma membrane remain largely unknown. As a first step in developing tools to study these processes, we show here that Clostridium septicum alpha-toxin, a pore-forming toxin that targets GPI-anchored protein receptors on the surface of mammalian cells, is active against T. gondii tachyzoites (50% effective concentration, 0.2 nM). Ultrastructural studies reveal that a tight physical connection between the plasma membrane and the underlying membranes of the inner membrane complex is locally disrupted by toxin treatment, resulting in a massive outward extension of the plasma membrane and ultimately lysis of the parasite. Toxin treatment also causes swelling of the parasite endoplasmic reticulum, providing the first direct evidence that alpha-toxin is a vacuolating toxin. Alpha-toxin binds to several parasite GPI-anchored proteins, including surface antigen 3 (SAG3) and SAG1. Interestingly, differences in the toxin-binding profiles between the virulent RH and avirulent P strain were observed. Alpha-toxin may prove to be a powerful experimental tool for molecular genetic analysis of GPI anchor biosynthesis and GPI-anchored protein trafficking in T. gondii and other susceptible protozoa.

Glycoconjugate structures of parasitic protozoa

Glycoconjugates are abundant and ubiquitious on the surface of many protozoan parasites. Their tremendous diversity has implicated their critical importance in the life cycle of these organisms. This review highlights our current knowledge of the major glycoconjugates, with particular emphasis on their structures, of representative protozoan parasites, including Leishmania, Trypanosoma, Giardia, Plasmodia, and others.

The structure, biosynthesis and functions of glycosylphosphatidylinositol anchors, and the contributions of trypanosome research

The discovery of glycosylphosphatidylinositol (GPI) membrane anchors has had a significant impact on several areas of eukaryote cell biology. Studies of the African trypanosome, which expresses a dense surface coat of GPI-anchored variant surface glycoprotein, have played important roles in establishing the general structure of GPI membrane anchors and in delineating the pathway of GPI biosynthesis. The major cell-surface molecules of related parasites are also rich in GPI-anchored glycoproteins and/or GPI-related glycophospholipids, and differences in substrate specificity between enzymes of trypanosomal and mammalian GPI biosynthesis may have potential for the development of anti-parasite therapies. Apart from providing stable membrane anchorage, GPI anchors have been implicated in the sequestration of GPI-anchored proteins into specialised membrane microdomains, known as lipid rafts, and in signal transduction events.

Delineation of three pathways of glycosylphosphatidylinositol biosynthesis in Leishmania mexicana. Precursors from different pathways are assembled on distinct pools of phosphatidylinositol and undergo fatty acid remodeling

Glycosylphosphatidylinositol (GPI) glycolipids are major cell surface constituents in the Leishmania parasites. Distinct classes of GPI are present as membrane anchors for several surface glycoproteins and an abundant lipophosphoglycan as well as being the major glycolipids (GIPLs) in the plasma membrane. In this study we have identified putative precursors for the protein and lipophosphoglycan anchors and delineated the complete pathway for GIPL biosynthesis in Leishmania mexicana promastigotes. Based on the structural analyses of these GPI intermediates and their kinetics of labeling in vivo and in cell-free systems, we provide evidence that the GIPLs are the products of an independent biosynthetic pathway rather than being excess precursors of the anchor pathways. First, we show that the similar glycan head groups of the GIPL and protein/lipophosphoglycan anchor precursors are assembled on two distinct pools of PI corresponding to 1-O-(C18:0)alkyl-2-stearoyl-PI and 1-O-(C24:0/C26:0)-2-stearoyl-PI, respectively. These PI species account for 20 and 1% of the total PI pool, respectively, indicating a remarkable specificity in their selection. Second, analysis of the flux of intermediates through these pathways in vivo and in a cell-free system suggests that the GIPL and anchor pathways are independently regulated. We also show that GIPL biosynthesis requires fatty acid remodeling, in which the sn-2 stearoyl chains are replaced with myristoyl or lauroyl chains. Fatty acid remodeling is dependent on CoA and ATP and occurs on pre-existing but not on de novo synthesized GIPLs. We suggest that the compartmentalization of different GPI pathways may be important in regulating the species and stage-specific expression of different GPI structures in these parasites.

1L-myo-inositol-1-phosphate synthase

1L-myo-Inositol-1-phosphate synthase catalyzes the conversion of D-glucose 6-phosphate to 1L-myo-inositol-1-phosphate, the first committed step in the production of all inositol-containing compounds, including phospholipids, either directly or by salvage. The enzyme exists in a cytoplasmic form in a wide range of plants, animals, and fungi. It has also been detected in several bacteria and a chloroplast form is observed in alga and higher plants. The enzyme has been purified from a wide range of organisms and its active form is a multimer of identical subunits ranging in molecular weight from 58,000 to 67,000. The activity of the synthase is stimulated by NH4Cl and inhibited by glucitol 6-phosphate and 2-deoxyglucose 6-phosphate. Structural genes (INO1) encoding the 1L-myo-inositol-1-phosphate synthase subunit have been isolated from several eukaryotic microorganisms and higher plants. In baker's yeast, Saccharomyces cerevisiae, the transcriptional regulation of the INO1 gene has been studied in detail and its expression is sensitive to the availability of phospholipid precursors as well as growth phase. The regulation of the structural gene encoding 1L-myo-inositol-1-phosphate synthase has also been analyzed at the transcriptional level in the aquatic angiosperm, Spirodela polyrrhiza and the halophyte, Mesembryanthemum crystallinum.

Molecular structure of the "low molecular weight antigen" of Toxoplasma gondii: a glucose alpha 1-4 N-acetylgalactosamine makes free glycosyl-phosphatidylinositols highly immunogenic

Toxoplasma gondii is a ubiquitous parasitic protozoan causing congenital infection and severe encephalitis in the course of the acquired immunodeficiency syndrome. Glycosyl-phosphatidylinositols of T. gondii have been shown to be identical with the low molecular weight antigen which elicits an early immunoglobulin M immune response in humans. A detailed study of the structures of these glycolipid antigens was performed. Radiolabelled glycolipids were extensively analysed by chemical and exoglycosidase treatments in combination with high pH anion-exchange chromatography, gel-filtration and lectin affinity chromatography. In addition, carbohydrate fragments prepared and purified from bulk preparations of unlabelled glycolipids by high performance liquid chromatography were subjected to two-dimensional 1H nuclear magnetic resonance spectroscopy, fast-atom bombardment-mass spectrometry, and methylation linkage analysis in order to elucidate the structure of T. gondii GPIs. The following structures were identified: (ethanolamine-PO4)-Man alpha 1-2Man alpha 1-6(GalNAc beta 1-4)Man alpha 1-4GlcN alpha-inositol-PO4-lipid and the novel structure (ethanolamine-PO4)-Man alpha 1-2Man alpha 1-6(Glc alpha 1-4GalNAc beta 1-4)Man alpha 1-4 GlcN alpha-inositol-PO4-lipid both with and without terminal ethanolamine phosphate. Evidence is provided, that only T. gondii GPIs bearing the unique glucose-N-acetylgalactosamine side branch are immunogenic in humans and that this structure is widely distributed among T. gondii isolates. Monoclonal antibodies have been characterized to recognize structures with different degrees of side-chain modification. We suggest that these reagents in combination with recently devised techniques for insertional mutagenesis in T. gondii should greatly facilitate the cloning of genes essential for GPI side-chain modification.

Substrate specificity of the dolichol phosphate mannose: glucosaminyl phosphatidylinositol alpha1-4-mannosyltransferase of the glycosylphosphatidylinositol biosynthetic pathway of African trypanosomes

The biosynthesis of glycosylphosphatidylinositol (GPI) precursors in Trypanosoma brucei involves the D-mannosylation of D-GlcN alpha 1-6-D-myo-inositol-1-PO4-sn-1,2-diacylglycerol (GlcN-PI). An assay for the first mannosyltransferase of the pathway, Dol-P-Man:GlcN-PI alpha 1-4-mannosyltransferase, is described. Analysis of the acceptor specificity revealed (a) that the enzyme requires the myo-inositol residue of the GlcN-PI substrate have the D configuration; (b) that the enzyme requires the presence of the NH2 group of the D-GlcN residue; (c) that GlcNAc-PI is more efficiently presented to the enzyme than GlcN-PI, suggesting a degree of substrate channelling via the preceding GlcNAc-PI de-N-acetylase enzyme; (d) that the fatty acid and phosphoglycerol components of the phosphatidyl moiety are important for enhancing substrate presentation and substrate recognition, respectively; and (e) that D-GlcN alpha 1-6-D-myo-inositol is the minimum structure that can support detectable acceptor activity. Analysis of the donor specificity revealed that short chain (C5 and C15) analogues of dolichol phosphate can act as substrates for the trypanosomal dolichol-phosphomannose synthetase, whereas the corresponding mannopyranosides cannot act as donors for the Dol-P-Man:GlcN-PI alpha 1-4-mannosyltransferase.