Evidence for N-linked glycosylation in Toxoplasma gondii

Single Cell Expression Systems for the Production of Recombinant Proteins for Immunodiagnosis and Immunoprophylaxis of Toxoplasmosis

Toxoplasmosis represents a significant public health and veterinary concern due to its widespread distribution, zoonotic transmission, and potential for severe health impacts in susceptible individuals and animal populations. The ability to design and produce recombinant proteins with precise antigenic properties is fundamental, as they serve as tools for accurate disease detection and effective immunization strategies, contributing to improved healthcare outcomes and disease control. Most commonly, a prokaryotic expression system is employed for the production of both single antigens and multi-epitope chimeric proteins; however, the cloning strategies, bacterial strain, vector, and expression conditions vary. Moreover, literature reports show the use of alternative microbial systems such as yeast or Leishmania tarentolae. This review provides an overview of the methods and strategies employed for the production of recombinant Toxoplasma gondii antigenic proteins for the serological detection of T. gondii infection and vaccine development.

Effect of alginate nanoparticles on the immunogenicity of excretory-secretory antigens against acute toxoplasmosis in murine model

Efficacy of alginate nanoparticles (Alg-NPs) as vaccine delivery for the excretory-secretory antigens (ESAs) against the virulent strain of Toxoplasma gondii was evaluated. Swiss albino mice were intraperitoneally immunized with three doses of either in vivo and in vitro-prepared ESA vaccines, 20 µg each, at 2-week intervals, then were challenged with 2500 tachyzoites of the RH HXGPRT (-) Toxoplasma gondii strain, four weeks later. Mice mortality, tachyzoite number in both peritoneal fluid and impression smear, and viability, ultrastructural tachyzoite changes, measuring immunological markers, and histopathological changes of both liver and spleen were studied, in comparison to alum adjuvanted ESAs and infected control subgroups. The in vivo-prepared Alg-NPs loaded ESAs vaccinated subgroups induced significant reduction in mice mortality, tachyzoite count in both peritoneal fluid and impression smears and viability. Scanning electron microscopy revealed tachyzoites deformities with multiple irregularities, while transmission electron microscopy showed tachyzoites distortion, disrupted plasma membranes, loss of nuclear integrities, and absence of dense granules with extensive vacuolations. A statistically significant increase in the level of both IFN-γ and anti-Toxoplasma IgG was noted. Histopathological results recorded amelioration of the pathological changes induced by Toxoplasma infection in both liver and spleen, with scanty parasites. Therefore, Alg-NPs proved its effectiveness in enhancing the ESAs antigencity, and recommended to test its potentiality as drugs carrier for anti-Toxoplasma agents to enhance their therapeutic efficacy.

Toxoplasma gondii Recruits Factor H and C4b-Binding Protein to Mediate Resistance to Serum Killing and Promote Parasite Persistence in vivo

Regulating complement is an important step in the establishment of infection by microbial pathogens. Toxoplasma gondii actively resists complement-mediated killing in non-immune human serum (NHS) by inactivating C3b, however the precise molecular basis is unknown. Here, a flow cytometry-based C3b binding assay demonstrated that Type II strains had significantly higher levels of surface-bound C3b than Type I strains. However, both strains efficiently inactivated C3b and were equally resistant to serum killing, suggesting that resistance is not strain-dependent. Toxoplasma activated both the lectin (LP) and alternative (AP) pathways, and the deposition of C3b was both strain and lectin-dependent. A flow cytometry-based lectin binding assay identified strain-specific differences in the level and heterogeneity of surface glycans detected. Specifically, increased lectin-binding by Type II strains correlated with higher levels of the LP recognition receptor mannose binding lectin (MBL). Western blot analyses demonstrated that Toxoplasma recruits both classical pathway (CP) and LP regulator C4b-binding proteins (C4BP) and AP regulator Factor H (FH) to the parasite surface to inactivate bound C3b-iC3b and C3dg and limit formation of the C5b-9 attack complex. Blocking FH and C4BP contributed to increased C5b-9 formation in vitro. However, parasite susceptibility in vitro was only impacted when FH was blocked, indicating that down regulation of the alternative pathway by FH may be more critical for parasite resistance. Infection of C3 deficient mice led to uncontrolled parasite growth, acute mortality, and reduced antibody production, indicating that both the presence of C3, and the ability of the parasite to inactivate C3, was protective. Taken together, our results establish that Toxoplasma regulation of the complement system renders mice resistant to acute infection by limiting parasite proliferation in vivo, but susceptible to chronic infection, with all mice developing transmissible cysts to maintain its life cycle.

Reexamining Chronic Toxoplasma gondii Infection: Surprising Activity for a "Dormant" Parasite

PURPOSE OF REVIEW

Despite over a third of the world's population being chronically infected with Toxoplasma gondii, little is known about this largely asymptomatic phase of infection. This stage is mediated in vivo by bradyzoites within tissue cysts. The absence of overt symptoms has been attributed to the dormancy of bradyzoites. In this review, we reexamine the conventional view of chronic toxoplasmosis in light of emerging evidence challenging both the nature of dormancy and the consequences of infection in the CNS.

RECENT FINDINGS

New and emerging data reveal a previously unrecognized level of physiological and replicative capacity of bradyzoites within tissue cysts. These findings have emerged in the context of a reexamination of the chronic infection in the brain that correlates with changes in neuronal architecture, neurochemistry, and behavior that suggest that the chronic infection is not without consequence.

SUMMARY

The emerging data driven by the development of new approaches to study the progression of chronic toxoplasma infection reveals significant physiological and replicative capacity for what has been viewed as a dormant state. The emergence of bradyzoite and tissue cyst biology from what was viewed as a physiological "black box" offers exciting new areas for investigation with direct implications on the approaches to drug development targeting this drug-refractory state. In addition, new insights from studies on the neurobiology on chronic infection reveal a complex and dynamic interplay between the parasite, brain microenvironment, and the immune response that results in the detente that promotes the life-long persistence of the parasite in the host.

Cryptosporidium hominis gene catalog: a resource for the selection of novel Cryptosporidium vaccine candidates

Human cryptosporidiosis, caused primarily by Cryptosporidium hominis and a subset of Cryptosporidium parvum, is a major cause of moderate-to-severe diarrhea in children under 5 years of age in developing countries and can lead to nutritional stunting and death. Cryptosporidiosis is particularly severe and potentially lethal in immunocompromised hosts. Biological and technical challenges have impeded traditional vaccinology approaches to identify novel targets for the development of vaccines against C. hominis, the predominant species associated with human disease. We deemed that the existence of genomic resources for multiple species in the genus, including a much-improved genome assembly and annotation for C. hominis, makes a reverse vaccinology approach feasible. To this end, we sought to generate a searchable online resource, termed C. hominis gene catalog, which registers all C. hominis genes and their properties relevant for the identification and prioritization of candidate vaccine antigens, including physical attributes, properties related to antigenic potential and expression data. Using bioinformatic approaches, we identified ∼400 C. hominis genes containing properties typical of surface-exposed antigens, such as predicted glycosylphosphatidylinositol (GPI)-anchor motifs, multiple transmembrane motifs and/or signal peptides targeting the encoded protein to the secretory pathway. This set can be narrowed further, e.g. by focusing on potential GPI-anchored proteins lacking homologs in the human genome, but with homologs in the other Cryptosporidium species for which genomic data are available, and with low amino acid polymorphism. Additional selection criteria related to recombinant expression and purification include minimizing predicted post-translation modifications and potential disulfide bonds. Forty proteins satisfying these criteria were selected from 3745 proteins in the updated C. hominis annotation. The immunogenic potential of a few of these is currently being tested.

Database URL:http://cryptogc.igs.umaryland.edu

Levels of Transforming Growth Factor-Beta After Immunization of Mice With in vivo prepared Toxoplasma gondii Excretory/Secretory Proteins

BACKGROUND

Zoonotic parasite Toxoplasma gondii has a high prevalence in human populations. A suitable vaccine for animals can stop the transmission of infection between animal and human.

OBJECTIVES

The aim of this study was to evaluate in vivo prepared excretory/secretory antigens (E/SA) as a potential candidate for immunization against the parasite and its effect on the production of transforming growth factor-beta (TGF-β).

MATERIALS AND METHODS

Toxoplasma gondii tachyzoites were inoculated in the peritoneal cavity of mice and E/SA was harvested and used in animal immunization with and without adjuvant. Serum levels of anti-E/SA antibodies and TGF-β were measured in days 0, 3, 7, 14, 28 and 56 after immunization using ELISA technique. The measurements were statistically analyzed.

RESULTS

Our results showed that the serum levels of anti-E/SA immunoglobulins significantly increased in all of the immunized groups. The differences of the serum levels of TGF-β between the groups were statistically significant at days 28 and 56 after immunization with E/SA.

CONCLUSIONS

Based on our study, in vivo prepared E/SA may be considered as a good candidate for animal immunization.

Interleukin-10 Serum Levels after Vaccination with In Vivo Prepared Toxoplasma gondii Excreted/Secreted Antigens

OBJECTIVES

Toxoplasma gondii is a worldwide prevalent zoonotic parasite which causes toxoplasmosis. An appropriate vaccine for animals could interrupt the circle between animals and humans. Our previous study showed that excreted/secreted antigens (E/SA), derived from the peritoneum of mice infected with T. gondii tachyzoites could be considered as a good candidate for animal vaccination. Interleukin-10 (IL-10) inhibits proliferation of B and T lymphocytes and induces homeostasis in immune system responses. However, since IL-10 has also been shown to suppress the killing of T. gondii by human macrophages, the aim of this study was to evaluate IL-10 serum levels after vaccination with T. gondii E/SA prepared in vivo.

METHODS

T. gondii tachyzoites were inoculated in the peritoneum of mice and harvested E/SA were used as a vaccine, with and without adjuvant, in T. gondii infected and un-infected mice. IL-10 serum levels were evaluated using the ELISA technique.

RESULTS

The data showed that although serum levels of IL-10 were not changed at the early phases, they were elevated at the end phases of vaccination with T. gondii E/SA.

CONCLUSION

Based on these and our previous results, it can be concluded that in vivo prepared T. gondii E/SA could be considered as a good candidate for animal vaccination.

Analysis of the glycoproteome of Toxoplasma gondii using lectin affinity chromatography and tandem mass spectrometry

Glycoproteins are involved in many important molecular recognition processes including invasion, adhesion, differentiation, and development. To identify the glycoproteins of Toxoplasma gondii, a proteomic analysis was undertaken. T. gondii proteins were prepared and fractioned using lectin affinity chromatography. The proteins in each fraction were then separated using SDS-PAGE and identified by tryptic in gel digestion followed by tandem mass spectrometry. Utilizing these methods 132 proteins were identified. Among the identified proteins were 17 surface proteins, 9 microneme proteins, 15 rhoptry proteins, 11 heat shock proteins (HSP), and 32 hypothetical proteins. Several proteins had 1-5 transmembrane domains (TMD) with some being as large as 608.3 kDa. Both lectin-fluorescence labeling and lectin blotting were employed to confirm the presence of carbohydrates on the surface or cytoplasm of T. gondii parasites. PCR demonstrated that selected hypothetical proteins were expressed in T. gondii tachyzoites. This data provides a large-scale analysis of the T. gondii glycoproteome. Studies of the function of glycosylation of these proteins may help elucidate mechanism(s) involved in invasion improving drug therapy as well as identify glycoproteins that may prove to be useful as vaccine candidates.

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.

N-linked glycosylation of proteins in the protozoan parasite Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite of animal cells. Infection of humans is common and may result in devastating disease, especially in immunocompromised individuals. Despite previous reports that N-glycosylation of proteins may be a rare post-translational modification in this and related organisms, we demonstrate that it is actually quite prevalent in Toxoplasma. N-Glycosylation is completely inhibited by treatment of parasites with tunicamycin, but this does not appear to exert its major effect on the parasites until they have egressed from their host cells. Although the tunicamycin-treated parasites appear structurally normal at this time they are not motile and mostly incapable of invading new host cells. The few tunicamycin-treated parasites that do invade are severely affected in their ability to replicate and accumulate with a distended endoplasmic reticulum, deformed nuclei, and without recognizable late secretory organelles. We provide experimental evidence that indicate that Toxoplasma N-glycans differ structurally from those in other eukaryotes.

Proteolysis and Toxoplasma invasion

Apicomplexan parasites including Toxoplasma gondii cause widespread human and animal diseases, often with the most severe manifestations involving the central nervous system. The need for new therapeutic agents along with the fascinating biology of these parasites has fueled a keen interest in understanding how key steps in the life cycle are regulated. Proteolysis is intimately associated with cell and tissue invasion by these obligate intracellular parasites and recent studies have begun to identify the proteases involved in these processes. Based on clues from inhibitor experiments and cleavage site mapping studies, several groups are using emerging genome information, chemical proteomics and molecular genetics to identify and validate proteases that regulate secretory organelle biogenesis and invasion protein activity. These studies are revealing roles for an assortment of proteases including cathepsins, subtilases and rhomboids in cell and tissue invasion. The identification of highly selective inhibitors for these proteases has the potential to not only further dissect their roles in infection but also to ameliorate disease.

Dense granules: are they key organelles to help understand the parasitophorous vacuole of all apicomplexa parasites?

Together with micronemes and rhoptries, dense granules are specialised secretory organelles of Apicomplexa parasites. Among Apicomplexa, Plasmodium represents a model of parasites propagated by way of an insect vector, whereas Toxoplasma is a model of food borne protozoa forming cysts. Through comparison of both models, this review summarises data accumulated over recent years on alternative strategies chosen by these parasites to develop within a parasitophorous vacuole and explores the role of dense granules in this process. One of the characteristics of the Plasmodium erythrocyte stages is to export numerous parasite proteins into both the host cell cytoplasm and/or plasma membrane via the vacuole used as a step trafficking compartment. Whether this feature can be correlated to few storage granules and a restricted number of dense granule proteins, is not yet clear. By contrast, the Toxoplasma developing vacuole is decorated by abundantly expressed dense granule proteins and is characterised by a network of membranous nanotubes. Although the exact function of most of these proteins remains currently unknown, recent data suggest that some of these dense granule proteins could be involved in building the intravacuolar membranous network. Conserved expression of the Toxoplasma dense granule proteins throughout most of the parasite stages suggests that they could also be key elements of the cyst formation.

High-level expression of the Toxoplasma gondii STT3 gene is required for suppression of the yeast STT3 gene mutation

N-linked glycosylation is the most frequent modification of secretory proteins. The central reaction of this process in eukaryotic cells is catalyzed by the hetero-oligomeric protein complex oligosaccharyltransferase (OST). The gene STT3 gene encodes a protein, which is the most conserved among the components of the OST. In this report, we describe the isolation and functional characterization of a STT3 homologue from Toxoplasma gondii. The topology of the TgStt3p is similar to that of the yeast Stt3p with 47% identity. We demonstrate that high level expression of the homologues gene is required to completely suppress the defect caused by a stt3 mutation in yeast, suggesting that homologous Stt3 proteins can serve analogous functions in distantly related eukaryotic cells regardless of their degree of conservation.

Comparative analysis of apicomplexa and genomic diversity in eukaryotes

The apicomplexans Plasmodium and Cryptosporidium have developed distinctive adaptations via lineage-specific gene loss and gene innovation in the process of diverging from a common parasitic ancestor. The two lineages have acquired distinct but overlapping sets of surface protein adhesion domains typical of animal proteins, but in no case do they share multidomain architectures identical to animals. Cryptosporidium, but not Plasmodium, possesses an animal-type O-linked glycosylation pathway, along with >30 predicted surface proteins having mucin-like segments. The two parasites have notable qualitative differences in conserved protein architectures associated with chromatin dynamics and transcription. Cryptosporidium shows considerable reduction in the number of introns and a concomitant loss of spliceosomal machinery components. We also describe additional molecular characteristics distinguishing Apicomplexa from other eukaryotes for which complete genome sequences are available.

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.

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.

Golgi biogenesis in Toxoplasma gondii

Two models have been put forward to explain the growth of new Golgi during the cell cycle. The first suggests that a new Golgi grows out of the endoplasmic reticulum by de novo synthesis. The second suggests that a pre-existing Golgi is needed for the growth of a new one, that is, the Golgi is an autonomously replicating organelle. To resolve this issue, we have exploited the simplicity of the apicomplexan parasite Toxoplasma gondii, which has only a single Golgi stack. Here we show, by using video fluorescence microscopy and three-dimensional reconstructions of serial thin sections, that the Golgi grows by a process of lateral extension followed by medial fission. Further fission leads to the inheritance by each daughter of a pair of Golgi structures, which then coalesce to re-form a single Golgi. Our results indicate that new Golgi grow by autonomous duplication and raise the possibility that the Golgi is a paired structure that is analogous to centrioles.

Toxoplasma gondii: conserved protein machinery in an unusual secretory pathway?

The protozoan parasite Toxoplasma gondii has a very specialized secretory apparatus. In this review we will discuss how different methodological approaches, including morphological studies, biochemistry and genetics, are revealing a novel secretory organization, whose function is performed, mainly, by a highly conserved protein machinery found in mammalian cells.

Detection of surface-associated and intracellular glycoconjugates and glycoproteins in Neospora caninum tachyzoites

The surface-associated molecules of the invasive stages of apicomplexan parasites such as Neospora caninum and Toxoplasma gondii are most likely crucially involved in mediating the interaction between the parasite and its host cell. In N. caninum, several antigens have recently been identified which could participate in host cell adhesion and/or invasion. These are antigens which are either constitutively expressed on the outer plasma membrane, or antigens which are only transiently localised on the surface as they are expulsed from the secretory vesicles either prior, or after host cell invasion. Some of these proteins have been characterised at the molecular level, and it has been shown that they are, with respect to protein sequences, closely related to homologous counterparts in T. gondii. Nevertheless, there is only a low degree of cross-antigenicity between the two species. In microbial interactions it has been shown that carbohydrates could also play a crucial role in host cell recognition and immunological host parasite interactions. In this study we present data which strongly suggest that the surface of N. caninum tachyzoites is glycosylated. In SDS-PAGE, glycoproteins comigrated largely with glycosylphosphatidylinositol-anchored proteins which were identified using in vivo [3H]ethanolamine labelling followed by autoradiography. The lectin Con A reacted strongly with the surface of these parasites, binding of which is indicative for the presence of N-glycans. Additional surface binding was observed, although only in a subpopulation of all tachyzoites, for wheat germ agglutinin and Jacalin. Intracellular binding sites for Con A were mainly associated with the parasite dense granules. By lectin labelling of Western blots of N. caninum protein extracts, glycoproteins were identified which reacted specifically with the lectins Con A, wheat germ agglutinin, Jacalin and soy bean agglutinin.

Theileria parva 104 kDa microneme--rhoptry protein is membrane-anchored by a non-cleaved amino-terminal signal sequence for entry into the endoplasmic reticulum

The 104 kDa microneme-rhoptry protein (p104) is the only known apical complex organelle-specific protein of Theileria parva. p104 exhibits striking structural similarities to circumsporozoite protein and sporozoite surface protein 2 of Plasmodium yoelii. Their primary sequences contain two hydrophobic segments, located at the amino-and the carboxy-terminus. The p104 amino-terminal hydrophobic region was suggested to be a signal peptide for entry into the endoplasmic reticulum and the extreme carboxy-terminal region to function as a membrane anchor. We have studied the biogenesis of p104 in a cell-free expression system and found that p104 is co-translationally transported into membranes derived from endoplasmic reticulum. The amino-terminal signal peptide is not cleaved off and anchors the protein in the membrane with the carboxy-terminal portion translocated into the lumen. We suggest that in vivo p104 is co-translationally integrated into the membrane of the endoplasmic reticulum, from where it is further transported to the microneme-rhoptry complex. Thus, p104 appears to be a suitable marker to study the development of micronemes and rhoptries in T. parva.

Glucosylation of glycosylphosphatidylinositol membrane anchors: identification of uridine diphosphate-glucose as the direct donor for side chain modification in Toxoplasma gondii using carbohydrate analogues

Toxoplasma gondii is an obligate intracellular parasite of the phylum apicomplexa and a common and often life-threatening opportunistic infection associated with AIDS. A family of parasite-specific glycosylphosphatidylinositols containing a novel glucosylated side chain has been shown to be highly immunogenic in humans (Striepen et al. (1997) J. Mol. Biol. 266, 797-813). In contrast to trypanosomes in T. gondii side chain modification takes place before addition to protein in the endoplasmic reticulum. The biosynthesis of these modifications was studied in an in vitro system prepared from hypotonically lysed T. gondii parasites. Radiolabeled glucose-containing glycosylphosphatidylinositol precursors were synthesized by T. gondii membrane preparations upon incubation with uridine diphosphate-[3H]glucose. Synthesis of glucosylated glycolipids took place only in the presence of exogenous uridine diphosphate-glucose and was stimulated by unlabeled uridine diphosphate-glucose in a dose-dependent manner. In contrast to glycosylphosphatidylinositol mannosylation, glucosylation was shown to be insensitive to amphomycin treatment. In addition, the glucose analogue 2-deoxy-D-glucose was used to trace the glycosylphosphatidylinositol glucosylation pathway. Detailed analysis of glycolipids synthesized in vitro in the presence of UDP and GDP derivatives of D-glucose and 2-deoxy-D-glucose ruled out an involvement of dolichol phosphate-glucose and demonstrates direct transfer of glucose from uridine diphosphate-glucose.

The conformation of purified Toxoplasma gondii SAG1 antigen, secreted from engineered Pichia pastoris, is adequate for serorecognition and cell proliferation

A truncated form of SAG1, the immunodominant surface antigen of Toxoplasma gondii, has been produced in the methylotrophic yeast, Pichia pastoris. By construction, the recombinant protein lacks C-terminal residues 308-336 which, in native SAG1, encompass the glycosylphosphatidylinositol anchorage site. Secretion of anchor-less SAG1 proceeded via the yeast prepro alpha-mating factor signal peptide and yielded two immunoreactive protein species having apparent molecular masses of 31.5 and 34.5 kDa, respectively, and differing only by N-glycosylation of the single Asn-X-Ser site present in the molecule. Purification of the anchor-less SAG1 was achieved by a combination of ion-exchange and size-exclusion chromatographies. N-terminal amino acid sequencing of the products indicated the presence of additional residues glutamic acid--alanine at the N-terminal end of the products. Despite incomplete processing and unnatural glycosylation, anchor-less SAG1 proteins apparently adopted a suitable conformation recognized by monoclonal and human serum-derived antibodies, specific for the native SAG1. In addition, the recombinant anchor-less SAG1 proved competent for inducing proliferation, in vitro, of mononuclear cells from seropositive individuals. Finally, properly adjuvanted anchor-less SAG1 was able to induce protection of mice against a lethal challenge with T. gondii tachyzoites.

The surface of Toxoplasma: more and less

As for any intracellular parasite, the surface of the Apicomplexan parasite Toxoplasma gondii must fulfil many functions including a role in attachment, signalling, invasion, transport and interaction with the immune response of the host. In this review, we describe the current state of knowledge on the molecules that are found on the surface of the different developmental stages of this parasite and speculate as to how at least some of these multiple functions are fulfilled. Special emphasis is given to the growing family of surface antigens that are related to the tachyzoite-specific surface antigen 1. We conclude that the surface (of tachyzoites, at least) is both more and less complex than previously thought: there are more proteins present but their sequences suggest that the majority may share a similar overall structure typified by surface antigen 1.

Analysis of Toxoplasma gondii stably transfected with a transmembrane variant of its major surface protein, SAG1

We have genetically engineered Toxoplasma gondii so that its major surface antigen SAG1 is anchored by a human transmembrane domain (SAG1-TM) instead of its natural GPI anchor (SAG1-GPI) in order to initiate studies to address the function of this protein anchor in parasitic protozoa as well as to get insights into the functional role of SAG1. Our results show that SAG1-TM is correctly folded (at least as judged by the presence of conformationally dependent epitopes) and targeted to the surface of the parasite, indicating that the GPI anchor does not determine its localization nor overall three-dimensional structure. No significant difference was seen in any aspect of the growth of the SAG1-TM mutant. However, compared to the natural SAG1-GPI, SAG1-TM does not form strong associations with itself and/or other molecules in high molecular weight complexes suggesting that allowing such complexes to form may be one role of the GPI anchor. The in vitro half-life of SAG1-TM of extracellular parasites is significantly lower than that of SAG1-GPI suggesting a stabilizing function of the glycolipid anchor against degradation and/or membrane release. Antibodies to SAG1 are shed from SAG1-TM parasites as they invade, just as they are stripped from SAG1-GPI bearing parasites. The stripping, therefore, is unlikely to be driven by the action of lipases.

Targeting the secretory pathway of Toxoplasma gondii

Little is known about the extent of conservation in the organization of the secretory pathway in organisms as different as prokaryotes, eukaryotes, and humans. The protozoan parasite Toxoplasma gondii allows easy genetic manipulations, and numerous vectors for selection of transgenic parasites have been developed. One approach to study the molecular mechanism of protein sorting and trafficking is the expression of foreign proteins. Here we describe the design and application of a vector that targets proteins to the secretory pathway of T. gondii and yields high-level expression of Escherichia coli reporter proteins. The general strategies and potential problems in expressing foreign proteins in T. gondii are discussed.

Structural analysis of the glycosyl-phosphatidylinositol membrane anchor of the merozoite surface proteins-1 and -2 of Plasmodium falciparum

Plasmodium falciparum accumulates the two merozoite surface proteins-1 and -2 during schizogony. Both proteins are proposed to be anchored in membranes by glycosyl-phosphatidylinositol membrane anchors. In this report the identity of these GPI-anchors is confirmed by labelling with tritiated precursors and additionally by specific enzymatic and chemical treatments. Detailed structural analysis of the core-glycans showed that the GPI-anchors of both proteins possess an extra alpha 1-2 linked mannose at the conserved trimannosyl-core-glycan. MSP-1 and MSP-2 labelled with tritiated myristic acid possess primarily radioactive myristic acid at inositol rings in both GPI-anchors. Additionally the hydrophobic fragments released from [3H]myristic acid labelled GPI-anchors were identified as diacyl-glycerols, carrying preferentially [3H]palmitic acid in an ester-linkage.

Immunolocalization and characterization of the low molecular weight antigen (4-5 kDa) of Toxoplasma gondii that elicits an early IgM response upon primary infection

A striking feature of toxoplasmic seroconversion is the prominent and early IgM response to a low molecular weight antigen of 4-5 kDa. Two different monoclonal antibodies directed against the 4-5 kDa antigen have been generated and used to characterize this molecule. Using these monoclonal antibodies, we could demonstrate the surface localization of the low M(r) antigen by immunofluorescence and immuno-electron microscopy assays. By immunoblotting, we observed that one of the monoclonal antibodies was unable to recognize the 4-5 kDa antigen in tachyzoites propagated in cell culture, indicating an epitope variability between Toxoplasma gondii tachyzoites grown in vivo and in vitro. We discuss the implications of this latter finding in the design of diagnostic reagents.

Glycosylation reactions in Plasmodium falciparum, Toxoplasma gondii, and Trypanosoma brucei brucei probed by the use of synthetic peptides

Synthetic peptides were used to probe O- and N-glycosylation reactions in cell-free systems of the parasitic protozoa Plasmodium falciparum, Toxoplasma gondii, and Trypanosoma brucei brucei. O-Glycosylation of the peptide Pro-Tyr-Thr-Val-Val was observed with lysates from all organisms. However, the spectrum of sugars transferred from their respective nucleotide or dolichol-phosphate derivatives to the peptide varied greatly according to the parasite. N-glycosylation of the peptides N-Bz-Asn-Gly-ThrNH2 and DNP-Arg-Asn-Ala-Thr-Ala-ValNH2 by exogenous radioactive dolichol-pyrophosphate linked oligosaccharide donors was observed only when lysates of T. gondii or T. b. brucei were used, but not in P. falciparum. To assay for endogenous N-glycosylation donors, the radiolabeled tripeptide [3H]Ac-Asn-Gly-ThrNHMe was used as acceptor. The peptide was N-glycosylated only by T. gondii and T. b. brucei preparations. Only in these latter two parasites dolichol-cycle mannosyltransferase activity was demonstrated by the elongation of exogenous radiolabeled dolichol-PP-chitobiose. The data substantiate the occurrence of protein O-glycosylation in parasitic protozoa and the exceptional absence of protein N-glycosylation in the asexual intraerythrocytic stage of the malaria parasite, P. falciparum.

Conformationally appropriate expression of the Toxoplasma antigen SAG1 (p30) in CHO cells

The Toxoplasma gondii major surface antigen, called SAG1 or p30, is a highly immunogenic protein which has generated great interest as a diagnostic reagent, as a potential subunit vaccine, and for its role in invasion. Unfortunately, bacterial recombinant protein is grossly misfolded so that, for example, it is not effectively recognized by antibodies to native SAG1. To overcome this, we have turned to expression in CHO cells, using cotransfection of the SAG1 gene and the mouse dihydrofolate reductase (DHFR) gene into CHO cells that are DHFR-. SAG1 expression was amplified by methotrexate coselection of CHO cells in combination with fluorescence-activated cell sorting for SAG1 expression. The resulting population expressed recombinant SAG1 that is recognized by antiserum specific for natural, nonreduced SAG1, indicating that, unlike in bacteria, expression in CHO cells results in proper folding. Processing was at least partially correct in that, like natural SAG1, recombinant SAG1 was attached to the plasma membrane via a glycolipid anchor, although tunicamycin treatment was necessary to prevent N-glycosylation (SAG1 is not glycosylated in the parasite but does have a consensus N-linked site). Finally, purified recombinant SAG1 was recognized by human sera known to be reactive to toxoplasma proteins, indicating that this material has potential as a diagnostic reagent and possibly as a component of a subunit vaccine.