Regulation of host cell function by glycosylphosphatidylinositols of the parasitic protozoa

Plasmodium falciparum Malaria Vaccines and Vaccine Adjuvants

Malaria-a parasite vector-borne disease-is a global health problem, and Plasmodium falciparum has proven to be the deadliest among Plasmodium spp., which causes malaria in humans. Symptoms of the disease range from mild fever and shivering to hemolytic anemia and neurological dysfunctions. The spread of drug resistance and the absence of effective vaccines has made malaria disease an ever-emerging problem. Although progress has been made in understanding the host response to the parasite, various aspects of its biology in its mammalian host are still unclear. In this context, there is a pressing demand for the development of effective preventive and therapeutic strategies, including new drugs and novel adjuvanted vaccines that elicit protective immunity. The present article provides an overview of the current knowledge of anti-malarial immunity against P. falciparum and different options of vaccine candidates in development. A special emphasis has been made on the mechanism of action of clinically used vaccine adjuvants.

Metabolic Labeling and Structural Analysis of Glycosylphosphatidylinositols from Parasitic Protozoa

Glycosylphosphatidylinositol (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 and represent the major carbohydrate modification of many cell-surface parasite proteins. A minimal GPI-anchor precursor consists of core glycan (ethanolamine-PO₄-Manα1-2Manα1-6Manα1-4GlcNH₂) linked to the 6-position of the D-myo-inositol ring of phosphatidylinositol. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. The preassembled GPI-anchor precursor is post-translationally transferred to a variety of membrane proteins in the lumen of the endoplasmic reticulum in a transamidase-like reaction during which a C-terminal GPI attachment signal is released. Increasing evidence shows that a significant proportion of the synthesized GPIs are not used for protein anchoring, particularly in protozoa in which a large amount of free GPIs are being displayed at the cell surface. The characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. Especially this pathway, at least for Trypanosoma brucei, has been validated as a drug target. Furthermore, thanks to an increase of new innovative strategies to produce pure synthetic carbohydrates, a novel era in the use of GPIs in diagnostic, anti-GPI antibody production, as well as parasitic protozoa GPI-based vaccine approach is developing fast.

Isolation and purification of glycosylphosphatidylinositols (GPIs) in the schizont stage of Theileria annulata and determination of antibody response to GPI anchors in vaccinated and infected animals

BACKGROUND

Tropical theileriosis is widely distributed from North Africa to East Asia. It is a tick-borne disease caused by Theileria annulata, an obligate two-host intracellular protozoan parasite of cattle. Theileria annulata use leukocytes and red blood cells for completion of the life-cycle in mammalian hosts. The stage of Theileria annulata in monocytes and B lymphocytes of cattle is an important step in pathogenicity and diagnosis of the disease. Glycosylphosphatidylinositols (GPIs) are a distinct class of glycolipid structures found in eukaryotic cells and are implicated in several biological functions. GPIs are particularly abundant in protozoan parasites, where they are found as free glycolipids or attached to proteins in the plasma membrane.

RESULTS

In this study we first isolated and purified schizonts of Theileria annulata from infected leukocytes in Theileria annulata vaccine cell line (S15) by aerolysin-percoll technique. Then, the free GPIs of schizont stage and isolated GPI from cell membrane glycoproteins were purified by high performance liquid chromatography (HPLC) and confirmed by gas chromatography-mass spectrometry (GC-MS). Furthermore, enzyme linked immunosorbent assay (ELISA) on the serum samples obtained from naturally infected, as well as Theileria annulata-vaccinated animals, confirmed a significant (P < 0.01) high level of anti-GPI antibody in their serum.

CONCLUSIONS

The results presented in this study show, to our knowledge for the first time, the isolation of GPI from the schizont stage of Theileria annulata and demonstrate the presence of anti-GPI antibody in the serum of naturally infected as well as vaccinated animals. This finding is likely to be valuable in studies aimed at the evaluation of chemically structures of GPIs in the schizont stage of Theileria annulata and also for pathogenicity and immunogenicity studies with the aim to develop GPI-based therapies or vaccines.

High yield purification of Plasmodium falciparum merozoites for use in opsonizing antibody assays

Plasmodium falciparum merozoite antigens are under development as potential malaria vaccines. One aspect of immunity against malaria is the removal of free merozoites from the blood by phagocytic cells. However assessing the functional efficacy of merozoite specific opsonizing antibodies is challenging due to the short half-life of merozoites and the variability of primary phagocytic cells. Described in detail herein is a method for generating viable merozoites using the E64 protease inhibitor, and an assay of merozoite opsonin-dependent phagocytosis using the pro-monocytic cell line THP-1. E64 prevents schizont rupture while allowing the development of merozoites which are released by filtration of treated schizonts.  Ethidium bromide labelled merozoites are opsonized with human plasma samples and added to THP-1 cells. Phagocytosis is assessed by a standardized high throughput protocol. Viable merozoites are a valuable resource for assessing numerous aspects of P. falciparum biology, including assessment of immune function. Antibody levels measured by this assay are associated with clinical immunity to malaria in naturally exposed individuals. The assay may also be of use for assessing vaccine induced antibodies.  

Efficient measurement of opsonising antibodies to Plasmodium falciparum merozoites

Background

Antibodies targeting merozoites are important in protection from malaria. Therefore, merozoite surface proteins are attractive vaccine candidates. There is a need for robust functional assays to investigate mechanisms of acquired immunity and vaccine efficacy. To date, the study of merozoite phagocytosis has been confounded by the complexity and variability of in vitro assays.

Methodology/Principal findings

We have developed a new flow cytometry-based merozoite phagocytosis assay. An optimized merozoite preparation technique produced high yields of merozoites separated from haemozoin. Phagocytosis by the undifferentiated THP-1 monocytic cell line was mediated only by Fc Receptors, and was therefore ideal for studying opsonising antibody responses. The assay showed robust phagocytosis with highly diluted immune sera and strong inter-assay correlation. The assay effectively measured differences in opsonisation-dependent phagocytosis among individuals.

Conclusions/Significance

This highly reproducible assay has potential applications in assessing the role of opsonic phagocytosis in naturally acquired immunity and vaccine trials.

CD36 modulates proinflammatory cytokine responses to Plasmodium falciparum glycosylphosphatidylinositols and merozoites by dendritic cells

Studies have shown that glycosylphosphatidylinositols (GPIs) of Plasmodium falciparum activate macrophages mainly through Toll-like receptor 2 (TLR2)-mediated signalling and to certain extent through TLR4-mediated signalling to induce proinflammatory cytokine production. However, the ability of parasite GPIs to activate dendritic cells (DCs) has not been reported. Here, we show that parasite GPIs efficiently activate DCs through TLR2-mediated signalling mechanism and induce the production of TNF-α and IL-12. We also studied the role of scavenger receptor CD36 in P. falciparum GPI- and merozoite-induced cytokine responses by DCs. The results indicate that CD36 modulates the cytokine-inducing activity of the parasite GPIs by collaborating with TLR2 in DCs. Furthermore, our data reveal that CD36 modulates the activity of P. falciparum merozoites, likely by the contribution of phagocytosis-coupled CD36-mediated signalling to the signalling induced by merozoites. Altogether, these results contribute towards understanding of signalling mechanisms in malaria parasite-induced activation of the innate immune system.

Turbot resistance to Philasterides dicentrarchi is more dependent on humoral than on cellular immune responses

Philasterides dicentrarchi is a ciliate that causes high mortalities in cultured turbot, Psetta maxima (L.). This pathogen displays high phagocytic activity and after entering the body it multiplies and feeds on host cells and tissue components. In previous studies, we found that complement, activated through the classical pathway, is a potent killer of P. dicentrarchi. Here, we compared the killing activity of turbot leucocytes and humoral factors against two virulent isolates of P. dicentrarchi, in order to determine the importance of leucocytes in the defence against this pathogen. Components of P. dicentrarchi (ciliary and membrane) stimulated turbot leucocytes, and increased the respiratory burst, degranulation and the expression of pro-inflammatory cytokines. We tested the susceptibility of ciliates to reactive oxygen and nitrogen species, by incubating them with different oxidative systems (H(2)O(2), Fe/ascorbate, which induces lipid peroxidation, an O(2)(-) donor (XOD/HX), an NO donor (SNAP) and an ONOO(-) donor (SIN-1)), for 24h. Both isolates were susceptible to high concentrations of H(2)O(2,) Fe/ascorbate, XOD/HX, and SIN-1 but were resistant to incubation with SNAP. Leucocytes became strongly activated when they were in contact with or were phagocytosed by the ciliate. Incubation of P. dicentrarchi with a combination of fresh serum and specific antibodies killed most of the ciliates, but the addition of leucocytes to ciliate cultures did not increase the toxicity to the ciliates. On the contrary, the number of ciliates increased when leucocytes were added to the culture because the ciliates fed on them. Despite being activated, leucocytes did not produce sufficiently high concentrations of toxic substances to kill the parasite. The most virulent isolate was that which induced greatest activation of leucocytes but was least susceptible to complement. We concluded that humoral factors such as complement (activated through the classical pathway) are critical for fish defence against P. dicentrarchi and that cellular responses appear to play a minor role, if any, in defence against this ciliate.

Natural regulatory T cells in malaria: host or parasite allies?

Plasmodium falciparum malaria causes 500 million clinical cases with approximately one million deaths each year. After many years of exposure, individuals living in endemic areas develop a form of clinical immunity to disease known as premunition, which is characterised by low parasite burdens rather than sterilising immunity. The reason why malaria parasites persist under a state of premunition is unknown but it has been suggested that suppression of protective immunity might be a mechanism leading to parasite persistence. Although acquired immunity limits the clinical impact of infection and provides protection against parasite replication, experimental evidence indicates that cell-mediated immune responses also result in detrimental inflammation and contribute to the aetiology of severe disease. Thus, an appropriate regulatory balance between protective immune responses and immune-mediated pathology is required for a favourable outcome of infection. As natural regulatory T (T_reg) cells are identified as an immunosuppressive lineage able to modulate the magnitude of effector responses, several studies have investigated whether this cell population plays a role in balancing protective immunity and pathogenesis during malaria. The main findings to date are summarised in this review and the implication for the induction of pathogenesis and immunity to malaria is discussed.

Processing and presentation of variant surface glycoprotein molecules to T cells in African trypanosomiasis

Th1 cell responses to the variant surface glycoprotein (VSG) of African trypanosomes play a critical role in controlling infection through the production of IFN-gamma, but the role of APCs in the induction and regulation of T cell-mediated protection is poorly understood. In this study, we have investigated the Ag presentation capabilities of dendritic cells (DCs) and macrophages during early trypanosome infection in relatively resistant responder and susceptible nonresponder mouse strains. Splenic DCs appeared to be the primary cell responsible for activating naive VSG-specific Th cell responses in resistant responder animals through the coordinated up-regulation of costimulatory molecules, secretion of IL-12, and presentation of VSG peptides to T cells in vivo. Splenic DC depletion and the down-regulation of costimulatory markers on splenic macrophages were observed in susceptible animals and may be associated with the inability of these animals to elicit a significant VSG-specific T cell response. In contrast to splenic APCs, peritoneal macrophages secreted NO, failed to activate naive Th cells in vitro, and presented relatively low levels of VSG peptides to T cells in vivo. Thus, VSG-specific Th1 cell responses may be determined by tissue- and cell-specific differences in Ag presentation. Additionally, all APCs from resistant and susceptible strains displayed a reduced ability to process and present newly encountered exogenous Ag, including new VSG molecules, during high parasitemia. Thus, initial uptake of VSG (or other trypanosome factors) may interfere with Ag presentation and have dramatic consequences for subsequent T cell responses to other proteins.

Metabolic labeling and structural analysis of glycosylphosphatidylinositols from parasitic protozoa

Glycosylphosphatidylinositol (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 and represent the major carbohydrate modification of many cell-surface parasite proteins. A minimal GPI-anchor precursor consists of core glycan (ethanolamine-P-Manalpha1-2Manalpha1-6Manalpha1-4GlcNH2) linked to the 6-position of the D-myo-inositol ring of phos-phatidylinositol. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. The preassembled GPI-anchor precursor is post-translationally transferred to a variety of membrane proteins in the lumen of the endoplasmic reticulum in a transamidase-like reaction during which a C-terminal GPI attachment signal is released. Increasing evidence show that a significant proportion of the synthesized GPIs are not used for protein anchoring, particularly in protozoa in which a large amount of free GPIs are being displayed at the cell surface. The characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. Especially as this pathway, at least for Trypanosoma brucei, has been validated as a drug target.

Recent developments in the molecular, biochemical and functional characterization of GPI8 and the GPI-anchoring mechanism [review]

Glycoconjugates are utilized by eukaryotic organisms ranging from yeast to humans for the cell surface expression of a wide variety of proteins and lipids. These glycoconjugates are expressed as enzymes or receptors and serve a diversity of functions, including cell signaling and cell survival. In parasitic protozoans, glycoconjugates play roles in infectivity, survival, virulence and immune evasion. Among the alternate glycoconjugate structures that have been identified, glycosylphosphatidylinositols (GPIs) represent a universal structure for the anchorage of proteins, lipids, and phosphosaccharides to cellular membranes. Biosynthesis of the GPI is a multi-step process that culminates in the attachment of the assembled GPI to a precursor protein. This final step in the transfer of the GPI to a protein is catalyzed by GPI8 of the putative transamidase complex (TAM). GPI8 functions dually to perform the proteolytic cleavage of the C-terminal signal sequence of the precursor protein, followed by the formation of an amide bond between the protein and the ethanolamine phosphate of the GPI. This review summarizes the current aggregate of biochemical, gene-disruption and active site mutagenesis studies, which provide evidence that GPI8 is responsible for the protein-GPI anchoring reaction. We describe recently published studies that have identified other potential components of the TAM complex and that have elucidated their likely role in protein-GPI attachment. Further, we discuss the biochemical, molecular and functional differences between protozoan and mammalian GPI8 and the protein-GPI anchoring machinery. Finally, we will present the implications of these studies for the development of anti-parasite drug therapies.

Regulation of innate and acquired immunity in African trypanosomiasis

African trypanosomes are well known for their ability to avoid immune elimination by switching the immunodominant variant surface glycoprotein (VSG) coat during infection. However, antigenic variation is only one of several means by which trypanosomes manipulate the immune system of their hosts. In this article, the role of parasite factors such as GPI anchor residues of the shed VSG molecule and the release of CpG DNA, in addition to host factors such as IFN-gamma, in regulating key aspects of innate and acquired immunity during infection is examined. The biological relevance of these immunoregulatory events is discussed in the context of host and parasite survival.

Glycosylinositolphosphate soluble variant surface glycoprotein inhibits IFN-gamma-induced nitric oxide production via reduction in STAT1 phosphorylation in African trypanosomiasis

Macrophages are centrally involved in the host immune response to infection with Trypanosoma brucei rhodesiense, a protozoan parasite responsible for human sleeping sickness in Africa. During trypanosome infections, the host is exposed to parasite-derived molecules that mediate macrophage activation, specifically GPI anchor substituents associated with the shed variant surface glycoprotein (VSG), plus the host-activating agent IFN-gamma, which is derived from activated T cells and is essential for resistance to trypanosomes. In this study, we demonstrate that the level and timing of exposure of macrophages to IFN-gamma vs GPI ultimately determine the macrophage response at the level of induced gene expression. Treatment of macrophages with IFN-gamma followed by GIP-sVSG (the soluble form of VSG containing the glycosylinositolphosphate substituent that is released by parasites) stimulated the induction of gene expression, including transcription of TNF-alpha, IL-6, GM-CSF, and IL-12p40. In contrast, treatment of macrophages with GIP-sVSG before IFN-gamma stimulation resulted in a marked reduction of IFN-gamma-induced responses, including transcription of inducible NO synthase and secretion of NO. Additional experiments revealed that the inhibitory activity of GIP-sVSG was associated with reduction in the level of STAT1 phosphorylation, an event required for IFN-gamma-induced macrophage activation. These results suggest that modulation of specific aspects of the IFN-gamma response may be one mechanism by which trypanosomes overcome host resistance during African trypanosomiasis.

Genomics and its impact on parasitology and the potential for development of new parasite control methods

Parasitic organisms remain the scourge of the developed and underdeveloped worlds. Malaria, schistosomiasis, leishmaniasis, and trypanosomiasis, for example, still result in a large number of human deaths each year worldwide, while drug resistance among nematodes still poses a major problem to the livestock industries. Genome projects involving parasitic organisms are now abundant, and technologies for the investigations of the parasite transcriptome and proteome are well established. There is no doubt the era of the "omics" is with parasitology, and current trends in the discipline are addressing fundamental biological questions that can make best use of the new technologies, as well as the vast amount of new data being generated. Will this become the "golden age of molecular parasitology," leading to the control of parasitic diseases that have plagued mankind for hundreds of years? The primary aim of this paper is to review advances in the general area of parasite genomics, and to outline where the application of "omics" technologies can and have impacted on the development of new control methods for parasitic organisms.

Glycosylphosphatidyl-inositols in murine malaria: Plasmodium yoelii yoelii

Glycosylphosphatidyl-inositols (GPIs) are vital major glycoconjugates in intraerythrocytic stages of Plasmodium. Here, we report on the biosynthesis and the characterization of GPIs synthesized by the murine malarial parasite P. yoelii yoelii YM. Parasitized erythrocytes were labeled in vivo and in vitro with either radioactive nucleotide sugar precursors, ethanolamine or glucosamine. The pathway leading to the formation of GPI precursors was found to resemble that described for P. falciparum; however, in P. yoelii, the formation of an additional hydrophilic precursor containing an acid-labile modification was detected. The data suggest that this modification is linked to the fourth mannose attached to the trimannosyl backbone in an alpha1-2 linkage. The modification was susceptible to hydrofluoric acid (HF), but not to nitrous acid (HNO(2)). Data obtained from size-exclusion chromatography on Bio-Gel P4, and Mono Q analysis of the fragments generated by HNO(2) deamination suggest that the modification is due to the presence of an additional ethanolamine linked to the fourth mannose via a phosphodiester bond.

Candida albicans phospholipomannan, a new member of the fungal mannose inositol phosphoceramide family

The pathogenic yeast Candida albicans has the ability to synthesize unique sequences of beta-1,2-oligomannosides that act as adhesins, induce cytokine production, and generate protective antibodies. Depending on the growth conditions, beta-1,2-oligomannosides are associated with different carrier molecules in the cell wall. Structural evidence has been obtained for the presence of these residues in the polysaccharide moiety of the glycolipid, phospholipomannan (PLM). In this study, the refinement of purification techniques led to large quantities of PLM being extracted from Candida albicans cells. A combination of methanolysis, gas chromatography, mass spectrometry, and nuclear magnetic resonance analyses allowed the complete structure of PLM to be deduced. The lipid moiety was shown to consist of a phytoceramide associating a C(18)/C(20) phytosphingosine and C(25), C(26), or mainly C(24) hydroxy fatty acids. The spacer linking the glycan part was identified as a unique structure: -Man-P-Man-Ins-P-. Therefore, in contrast to the major class of membranous glycosphingolipids represented by mannose diinositol phosphoceramide, which is derived from mannose inositol phosphoceramide by the addition of inositol phosphate, PLM seems to be derived from mannose inositol phosphoceramide by the addition of mannose phosphate. In relation to a previous study of the glycan part of the molecule, the assignment of the second phosphorus position leads to the definition of PLM beta-1,2-oligomannosides as unbranched linear structures that may reach up to 19 residues in length. Therefore, PLM appears to be a new type of glycosphingolipid, which is glycosylated extensively through a unique spacer. The conferred hydrophilic properties allow PLM to diffuse into the cell wall in which together with mannan it presents C. albicans beta-1,2-oligomannosides to host cells.

Genes for glycosylphosphatidylinositol toxin biosynthesis in Plasmodium falciparum

About 2.5 million people die of Plasmodium falciparum malaria every year. Fatalities are associated with systemic and organ-specific inflammation initiated by a parasite toxin. Recent studies show that glycosylphosphatidylinositol (GPI) functions as the dominant parasite toxin in the context of infection. GPIs also serve as membrane anchors for several of the most important surface antigens of parasite invasive stages. GPI anchoring is a complex posttranslational modification produced through the coordinated action of a multicomponent biosynthetic pathway. Here we present eight new genes of P. falciparum selected for encoding homologs of proteins essential for GPI synthesis: PIG-A, PIG-B, PIG-M, PIG-O, GPI1, GPI8, GAA-1, and DPM1. We describe the experimentally verified mRNA and predicted amino acid sequences and in situ localization of the gene products to the parasite endoplasmic reticulum. Moreover, we show preliminary evidence for the PIG-L and PIG-C genes. The biosynthetic pathway of the malaria parasite GPI offers potential targets for drug development and may be useful for studying parasite cell biology and the molecular basis for the pathophysiology of parasitic diseases.

The immune response to Plasmodium falciparum malaria

Malaria is still a major cause of severe disease which is responsible for millions of deaths, mostly in children under 5 years old, in tropical countries, especially sub-Saharan Africa. Complications of severe anaemia and cerebral malaria are thought to be the major cause of morbidity and mortality but recent evidence suggests that the host's immunological response could also contribute to the pathophysiology of the disease in human beings. Intensive studies of the immune response to malaria parasites in human beings have provided a wealth of information about the cells and cytokines implicated in the pathophysiology of survival and fatal outcome in severe infections. This review focuses on the pivotal role of macrophages and other important cellular effectors, molecules, and cytokines involved in the activation of the immune response at the different stages of human falciparum malaria. Our understanding of the putative mechanisms by which cytokines may mediate beneficial and harmful effects, through activation of phagocytic cells, could help to develop new treatment strategies, regardless of the emergence of parasite multidrug resistance.

Contribution of phospholipomannan to the surface expression of beta-1,2-oligomannosides in Candida albicans and its presence in cell wall extracts

beta-1,2-Oligomannosides (beta-1,2-Man) derived from Candida albicans mannan have been shown to act as adhesins and to induce protective antibodies. We used monoclonal antibodies specific for beta-1,2-Man in electron, confocal, and fluorescence microscopy to study the surface expression of beta-1,2-Man epitopes. These monoclonal antibodies were also used for Western blotting of cell surface extracts to study the nature of the molecules expressing the beta-Man epitopes. Evidence was obtained for the contribution of a glycolipid, phospholipomannan (PLM), to the complex expression of beta-1,2-Man epitopes at the cell wall surfaces of yeasts grown on solid media. PLM was present in intercellular matrixes of colonies grown on agar and was detected as a contaminant in mannan batches prepared by conventional methods.

Inhibition of glycosyl-phosphatidylinositol biosynthesis in Plasmodium falciparum by C-2 substituted mannose analogues

Mannose analogues (2-deoxy-D-glucose, 2-deoxy-2-fluoro-D-glucose and 2-amino-2-deoxy-D-mannose) have been used to study glycosylphosphatidylinositol (GPtdIns) biosynthesis and GPtdIns protein anchoring in protozoal and mammalian systems. The effects of these analogues on GPtdIns biosynthesis and GPtdIns-protein anchoring of the human malaria parasite Plasmodium falciparum were evaluated in this study. At lower concentrations of 2-deoxy-D-glucose and 2-deoxy-2-fluoro-D glucose (0.2 and 0.1 mm, respectively), GPtdIns biosynthesis is inhibited without significant effects on total protein biosynthesis. At higher concentrations of 2-deoxy-D-glucose and 2-deoxy-2-fluoro-D-glucose (1.5 and 0.8 mm, respectively), the incorporation of [3H]glucosamine into glycolipids was inhibited by 90%, and the attachment of GPtdIns anchor to merozoite surface protein-1 (MSP-1) was prevented. However, at these concentrations, both sugar analogues inhibit MSP-1 synthesis and total protein biosynthesis. In contrast to 2-deoxy-2-fluoro-D-glucose and 2-amino-2-deoxy-D-mannose (mannosamine), the formation of new glycolipids was observed only in the presence of tritiated or nonradiolabelled 2-deoxy-D-glucose. Mannosamine inhibits GPtdIns biosynthesis at a concentration of 5 mm, but neither an accumulation of aberrant intermediates nor significant inhibition of total protein biosynthesis was observed in the presence of this analogue. Furthermore, the [3H]mannosamine-labelled glycolipid spectrum resembled the one described for [3H]glucosamine labelling. Total hydrolysis of mannosamine labelled glycolipids showed that half of the tritiated mannosamine incorporated into glycolipids was converted to glucosamine. This high rate of conversion led us to suggest that no actual inhibition from GPtdIns biosynthesis is achieved with the treatment with mannosamine, which is different to what has been observed for mammalian cells and other parasitic protozoa.

Synthesis of the fully phosphorylated GPI anchor pseudohexasaccharide of Toxoplasma gondii

Retrosynthesis of the fully phosphorylated glycosylphosphatidyl inositol (GPI) anchor pseudohexasaccharide 1a led to building blocks 2-6, of which 5 and 6 are known. The formation of pseudodisaccharide building block 2 is based on readily available building block 7, which gave, via derivative 11 and its glycosylation with known donor 12, the desired compound 2. Building block 3, with the required access to all hydroxy groups being permitted, was prepared from mannose in five steps. From a readily available precursor, building block 4 was obtained, which on reaction with 3 gave disaccharide 23. The synthesis of the decisive pseudohexasaccharide intermediate 32 was based on the reaction of 23 with 5, then with 6, and finally with 2. To obtain high stereoselectivity and good yields in the glycosylation reactions, anchimeric assistance was employed. To enable regioselective attachment of the two different phosphorus esters, the 6f-O-silyl group of 32 was first removed and the aminoethyl phosphate residue was attached. Then the MPM group was oxidatively removed, and the second phosphate residue was introduced. Unprotected 1a was then liberated in two steps: treatment with sodium methanolate removed the acetyl protecting groups, and finally, catalytic hydrogenation afforded the desired target molecule, which could be fully structurally assigned.

Regulation of Paramecium primaurelia glycosylphosphatidyl-inositol biosynthesis via dolichol phosphate mannose synthesis

A set of glycosylinositol-phosphoceramides, belonging to a family of glycosylphosphatidyl-inositols (GPIs) synthesized in a cell-free system prepared from the free-living protozoan Paramecium primaurelia has been described. The final GPI precursor was identified and structurally characterized as: ethanolamine-phosphate-6Man alpha 1-2Man alpha 1-6(mannosylphosphate) Man alpha 1-4glucosamine-inositol-phospho-ceramide. During our investigations on the biosynthesis of the acid-labile modification, the additional mannosyl phosphate substitution, we observed that the use of the nucleotide triphosphate analogue GTP gamma S (guanosine 5-O-(thiotriphosphate)) blocks the biosynthesis of the mannosylated GPI glycolipids. We show that GTP gamma S inhibits the synthesis of dolichol-phosphate-mannose, which is the donor of the mannose residues for GPI biosynthesis. Therefore, we investigated the role of GTP binding regulatory 'G' proteins using cholera and pertussis toxins and an intracellular second messenger cAMP analogue, 8-bromo-cAMP. All the data obtained suggest the involvement of classical heterotrimeric G proteins in the regulation of GPI-anchor biosynthesis through dolichol-phosphate-mannose synthesis via the activation of adenylyl cyclase and protein phosphorylation. Furthermore, our data suggest that GTP gamma S interferes with synthesis of dolichol monophosphate, indicating that the dolichol kinase is regulated by the heterotrimeric G proteins.

Biosynthesis of glycosphingolipids de-novo by the human malaria parasite Plasmodium falciparum

Glycolipids are important components of cellular membranes involved in various biological functions. In this report we describe the identification of the de-novo synthesis of glycosphingolipids by intraerythrocytic, asexual stages of the malaria parasite, Plasmodium falciparum. Parasite-specific glycolipids were identified in organic solvent extracts of parasites metabolically labeled with tritiated serine and glucosamine and characterised as sphingolipids based on their insensitivity towards alkaline treatment. While the de-novo synthesis of parasite glycosphingolipids was affected by fumonisin B1, threo-PPMP, cyclo-serine and myriocin, these well established inhibitors of de-novo ceramide biosynthesis were unable to arrest the intraerythrocytic development of the parasites in culture.

The Schizosaccharomyces pombe GPI8 gene complements a Saccharomyces cerevisiae GPI8 anchoring mutant

The final step in glycosylphosphatidylinositol (GPI) anchoring of cell surface proteins consists of a transamidation reaction, in which preassembled GPI donors are substituted for C-terminal signal sequences in nascent polypeptides. The Saccharomyces cerevisiae GPI8 gene (ScGPI8) encodes a protein which is involved in the GPI transamidation reaction. We have cloned and isolated the Schizosaccharomyces pombe GPI8 homologous gene (SpGPI8). The SpGPI8 gene encodes a protein of 411 amino acids with a calculated molecular weight of about 47 kDa. It shows 53.5% identity with the ScGPI8 and complements a S. cerevisiae GPI8 anchoring mutant.

Transient N-acetylgalactosaminylation of mannosyl phosphate side chain in Paramecium primaurelia glycosylphosphatidylinositols

The surface antigens of the free-living protozoan Paramecium primaurelia belong to the family of glycosylphosphatidylinositol (GPtdIns)-anchored proteins. Using a cell-free system prepared from P. primaurelia, we have described the structure and biosynthetic pathway for GPtdIns glycolipids. The core glycans of the polar glycolipids are modified by a mannosyl phosphate side chain. The data suggest that the mannosyl phosphate side chain is added onto the core glycan in two steps. The first step involves the phosphorylation of the GPtdIns trimannosyl conserved core glycan via an ATP-dependent kinase, prior to the addition of the mannose linked to the phosphate group. We show that dolichol phosphate mannose is the donor of all mannose residues including the mannose linked to phosphate. Furthermore, we were able to identify in vitro a hydrophilic intermediate containing an additional N-acetylgalactosamine linked to the mannosyl phosphate side chain. The addition of this purified hydrophilic radiolabelled intermediate into the cell-free system leads to a loss of the GalNAc residue and its conversion to the penultimate intermediate having only mannosyl phosphate as a side chain. Together the data indicate that the GalNAc-containing intermediate is a transitional intermediate. We suggest that the GalNAc-containing intermediate is essential for biosynthesis and maturation of GPtdIns precursors. It is hypothesized that this oligosaccharide processing in the course of GPtdIns biosynthesis is required for the translocation of GPtdIns from the cytoplasmic side of the endoplasmic reticulum to the luminal side.

Specificity in signal transduction among glycosylphosphatidylinositols of Plasmodium falciparum, Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp

Glycosylphosphatidylinositols (GPIs) and related glycoconjugates of parasite origin have been shown to regulate both the innate and acquired immune systems of the host. This is achieved through the activation of novel GPI-dependent signalling pathways in macrophages, lymphocytes and other cell types. Parasite GPIs impart at least two distinct signals to host cells through the structurally distinct inositolphosphoglycan (IPG) and fatty acid domains. Binding of IPG to as yet uncharacterized cell surface receptor(s) leads to activation of src-family protein tyrosine kinases: depending upon structure, GPI-derived fatty acids can either activate or antagonize protein kinase C, and may enter the sphingomyelinase pathway. The degree of fatty acid saturation may also contribute to signalling activity. Thus, variation in structure of parasite GPIs imparts different properties of signal transduction upon this class of glycolipid. The divergent activities of GPIs from various protozoal taxa reflect global aspects of the host/parasite relationship, suggesting that GPI signalling is a central determinant of disease in malaria, leishmaniasis and both American and African trypanosomiases.

The Candida albicans phospholipomannan is a family of glycolipids presenting phosphoinositolmannosides with long linear chains of beta-1,2-linked mannose residues

In a series of studies, we have shown that Candida albicans synthesizes a glycolipid, phospholipomannan (PLM), which reacted with antibodies specific for beta-1,2-oligomannosides and was biosynthetically labeled by [(3)H]mannose, [(3)H]palmitic acid, and [(32)P]phosphorus. PLM has also been shown to be released from the C. albicans cell wall and to bind to and stimulate macrophage cells. In this study, we show by thin layer chromatography scanning of metabolically radiolabeled extracts that the C. albicans PLM corresponds to a family of mannose and inositol co-labeled glycolipids. We describe the purification process of the molecule and the release of its glycan fraction through alkaline hydrolysis. Analysis of this glycan fraction by radiolabeling and methylation-methanolysis confirmed the presence of inositol and of 1, 2-linked mannose units. NMR studies evidenced linear chains of beta-1,2-oligomannose as the major PLM components. Mass spectrometry analysis revealed that these chains were present in phosphoinositolmannosides with degrees of polymerization varying from 8 to 18 sugar residues. The PLM appears as a new type of eukaryotic inositol-tagged glycolipid in relationship to both the absence of glucosamine and the organization of its glycan chains. This first structural evidence for the presence of beta-1, 2-oligomannosides in a glycoconjugate other than the C. albicans phosphopeptidomannan may have some pathophysiological relevance to the adhesive, protective epitope, and signaling properties thus far established for these residues.

Regulation of Entamoeba invadens encystation and gene expression with galactose and N-acetylglucosamine

Encystation of Entamoeba invadens parasites is prevented by the presence of free galactose or N-acetylglucosamine in the encystation medium. Galactose prevents the formation of amoeba cellular aggregates which develop during the early phase of encystation, suggesting the presence of functional cell surface galactose-binding molecules, whereas N-acetylglucosamine allows aggregation to occur and prevents cyst formation at a later point. While studying sugar inhibition of amoeba encystation, it was found that high efficiency encystation required the inclusion in encystation medium of precise amounts of polyvalent galactose-terminated molecules, and these molecules could be supplied by serum or by defined glycoconjugates, including mucin. Addition of free galactose to encystation medium prevented the accumulation of three transcripts which are normally upregulated during encystation, and N-acetylglucosamine prevented accumulation of one of the transcripts. These results suggest the presence of distinct sugar-sensitive pathways that regulate differentiation of the amoeba trophozoite into infectious cysts.

Biosynthesis of glycosylphosphatidylinositols in mammals and unicellular microbes

Membrane anchoring of cell surface proteins via glycosylphosphatidylinositol (GPI) occurs in all eukaryotic organisms. In addition, GPI-related glycophospholipids are important constituents of the glycan coat of certain protozoa. Defects in GPI biosynthesis can retard, if not abolish growth of these organisms. In humans, a defect in GPI biosynthesis can cause paroxysmal nocturnal hemoglobinuria (PNH), a severe acquired bone marrow disorder. Here, we review advances in the characterization of GPI biosynthesis in parasitic protozoa, yeast and mammalian cells. The GPI core structure as well as the major steps in its biosynthesis are conserved throughout evolution. However, there are significant biosynthetic differences between mammals and microbes. First indications are that these differences could be exploited as targets in the design of novel pharmacotherapeutics that selectively inhibit GPI biosynthesis in unicellular microbes.

Glycosylphosphatidylinositol-specific phospholipase D in blood serum: is the liver the only source of the enzyme?

In cases of systemic inflammatory response syndrome, sepsis, and septic shock, the activity of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) in serum amounts to 20 to 25% of the activity found in a healthy control group. The activity of serum GPI-PLD is positively correlated with inflammatory markers and counts of monocytes and stab cells (bands) and negatively correlated with polymorphonuclear neutrophils and lymphocytes in severe diseases. This indicates a yet unknown involvement of the inflammatory system in GPI-PLD liberation and suggests that the liver is not the only source of the plasma enzyme. Plasma was shown to contain an effective inhibitor of GPI-PLD which is soluble in organic solvents. Its concentration in capillary plasma is 20-fold higher than in venous plasma. To find possible other sources of plasma GPI-PLD besides the liver, the GPI-degrading activity was measured in different organs of the rat. Product formation was analysed using [125I]TID-labeled GPI-AP.

Oligopeptidase B-dependent signaling mediates host cell invasion by Trypanosoma cruzi

Mammalian cell invasion by the intracellular protozoan parasite Trypanosoma cruzi is mediated by recruitment and fusion of host cell lysosomes, an unusual process that has been proposed to be dependent on the ability of parasites to trigger intracellular free calcium concentration ([Ca2+]i) transients in host cells. Previous work implicated the T.cruzi serine hydrolase oligopeptidase B in the generation of Ca2+-signaling activity in parasite extracts. Here we show that deletion of the gene encoding oligopeptidase B results in a marked defect in host cell invasion and in the establishment of infections in mice. The invasion defect is associated with the inability of oligopeptidase B null mutant trypomastigotes to mobilize Ca2+ from thapsigargin-sensitive stores in mammalian cells. Exogenous recombinant oligopeptidase B reconstitutes the oligopeptidase B-dependent Ca2+ signaling activity in null mutant parasite extracts, demonstrating that this enzyme is responsible for the generation of a signaling agonist for mammalian cells.

Plasmodium falciparum: asexual erythrocytic stages synthesize two structurally distinct free and protein-bound glycosylphosphatidylinositols in a maturation-dependent manner

Glycosylphosphatidylinositols represent the predominant class of glycolipids synthesized by the asexual, intraerythrocytic stages of Plasmodium falciparum. These glycolipids have been implicated as malarial toxins involved in parasite-induced release of cytokines, such as tumor necrosis factor-alpha and interleukin-1. Two potential glycosylphosphatidylinositol membrane-anchor precursors with the structures ethanolamine phosphate (mannose-alpha 1,2)mannose-alpha 1,2-mannose-alpha 1,6-mannose-alpha 1,4-glucosamine-inositol(acyl)phosphate diacylglycerol (P.f.alpha) and ethanolamine-phosphate-mannose-alpha 1,2-mannose-alpha 1,6-mannose-alpha 1,4-glucosamine-inositol(acyl)phosphate diacylglycerol (P.f.beta) have been described in P. falciparum. Only one (P.f.alpha) has been demonstrated to serve as an anchor for merozoite surface protein-1 and merozoite surface protein-2. In this report we present data showing that asexual, intraerythrocytic stages of P. falciparum use both glycosylphosphatidylinositols to anchor proteins. The synthesis of the two glycosylphosphatidylinositol membrane anchor precursors and the protein-bound glycosylphosphatidylinositol anchors is tightly regulated and varies throughout the intraerythrocytic development of the asexual stages of P. falciparum. The glycosylphosphatidylinositol membrane-anchor precursor P.f.beta is synthesized and transferred to protein predominantly in trophozoite stages (about 30 h).

Antigenic variation and immune evasion in Plasmodium falciparum malaria

Plasmodium falciparum malaria is responsible for 2 million deaths each year. Even in endemic regions, immunity to malaria builds slowly and is rarely complete. Strategies such as antigenic variation and antigenic diversity are critical to a parasite's ability to evade the host immune response and infect previously exposed individuals. In this short review, the phenomenon of antigenic variation is discussed in relation to immune evasion and its impact on parasite pathogenesis. Recent advances in the understanding of the underlying molecular mechanisms of antigenic variation are examined and questions posed for future research.