Glycosylphosphatidylinositol anchors represent the major carbohydrate modification in proteins of intraerythrocytic stage Plasmodium falciparum

Apicoplast-derived isoprenoids are essential for biosynthesis of GPI protein anchors, and consequently for egress and invasion in Plasmodium falciparum

Glycophosphatidylinositol (GPI) anchors are the predominant glycoconjugate in Plasmodium parasites, enabling modified proteins to associate with biological membranes. GPI biosynthesis commences with donation of a mannose residue held by dolichol-phosphate at the endoplasmic reticulum membrane. In Plasmodium dolichols are derived from isoprenoid precursors synthesised in the Plasmodium apicoplast, a relict plastid organelle of prokaryotic origin. We found that treatment of Plasmodium parasites with apicoplast inhibitors decreases the synthesis of isoprenoid and GPI intermediates resulting in GPI-anchored proteins becoming untethered from their normal membrane association. Even when other isoprenoids were chemically rescued, GPI depletion led to an arrest in schizont stage parasites, which had defects in segmentation and egress. In those daughter parasites (merozoites) that did form, proteins that would normally be GPI-anchored were mislocalised, and when these merozoites were artificially released they were able to attach to but not invade new red blood cells. Our data provides further evidence for the importance of GPI biosynthesis during the asexual cycle of P. falciparum, and indicates that GPI biosynthesis, and by extension egress and invasion, is dependent on isoprenoids synthesised in the apicoplast.

The major surface protein of malaria sporozoites is GPI-anchored to the plasma membrane

Glycosylphosphatidylinositol (GPI) anchor protein modification in Plasmodium species is well known and represents the principal form of glycosylation in these organisms. The structure and biosynthesis of GPI anchors of Plasmodium spp. has been primarily studied in the asexual blood stage of Plasmodium falciparum and is known to contain the typical conserved GPI structure of EtN-P-Man3GlcN-PI. Here, we have investigated the circumsporozoite protein (CSP) for the presence of a GPI anchor. CSP is the major surface protein of Plasmodium sporozoites, the infective stage of the malaria parasite. While it is widely assumed that CSP is a GPI-anchored cell surface protein, compelling biochemical evidence for this supposition is absent. Here, we employed metabolic labeling and mass-spectrometry-based approaches to confirm the presence of a GPI anchor in CSP. Biosynthetic radiolabeling of CSP with [3H]-palmitic acid and [3H]-ethanolamine, with the former being base-labile and therefore ester-linked, provided strong evidence for the presence of a GPI anchor on CSP, but these data alone were not definitive. To provide further evidence, immunoprecipitated CSP was analyzed for the presence of myo-inositol (a characteristic component of GPI anchor) using strong acid hydrolysis and GC-MS for highly sensitive and quantitative detection. The single ion monitoring (SIM) method for GC-MS analysis confirmed the presence of the myo-inositol component in CSP. Taken together, these data provide confidence that the long-assumed presence of a GPI anchor on this important parasite protein is correct.

The major surface protein of malaria sporozoites is GPI-anchored to the plasma membrane

Glycosylphosphatidylinositol (GPI) anchor protein modification in Plasmodium species is well known and represents the principal form of glycosylation in these organisms. The structure and biosynthesis of GPI anchors of Plasmodium spp. has been primarily studied in the asexual blood stage of P. falciparum and is known to contain the typical conserved GPI structure of EtN-P-Man3GlcN-PI. Here, we have investigated the circumsporozoite protein (CSP) for the presence of a GPI-anchor. CSP is the major surface protein of Plasmodium sporozoites, the infective stage of the malaria parasite. While it is widely assumed that CSP is a GPI-anchored cell surface protein, compelling biochemical evidence for this supposition is absent. Here, we employed metabolic labeling and mass-spectrometry based approaches to confirm the presence of a GPI anchor in CSP. Biosynthetic radiolabeling of CSP with [ 3 H]-palmitic acid and [ 3 H]-ethanolamine, with the former being base-labile and therefore ester-linked, provided strong evidence for the presence of a GPI anchor on CSP, but these data alone were not definitive. To provide further evidence, immunoprecipitated CSP was analyzed for presence of myo -inositol (a characteristic component of GPI anchor) using strong acid hydrolysis and GC-MS for a highly sensitive and quantitative detection. The single ion monitoring (SIM) method for GC-MS analysis confirmed the presence of the myo -inositol component in CSP. Taken together, these data provide confidence that the long-assumed presence of a GPI anchor on this important parasite protein is correct.

Glycosylation in malaria parasites: what do we know?

In malaria parasites, although post-translational modification of proteins with N-. O-, and C-glycosidic bond-linked glycans is limited, it is confined to relatively fewer proteins in which the glycans are present at significant levels and may have important functions. Furthermore, several proteins are modified with glycosylphosphatidylinositols (GPIs) which represent the predominant glycan synthesized by parasites. Modification of proteins with GPIs is obligatory for parasite survival as GPI-anchored proteins (GPI-APs) play essential roles in all life cycle stages of the parasites, including development, egress, gametogenesis, motility, and host cell adhesion and invasion. Here, we discuss the current knowledge on the structures and potential functions of the glycan moieties of parasite proteins. The knowledge has important implications for the development of drugs and vaccines for malaria.

The glycobiology of plasmodium falciparum: New approaches and recent advances

Like any other microorganism, pathogenic protozoan parasites rely heavily on glycoconjugates and glycan binding proteins to protect themselves from the environment and to interact with their diverse hosts. A thorough comprehension of how glycobiology contributes to the survival and virulence of these organisms may reveal unknown aspects of their biology and may open much needed avenues for the design of new strategies against them. In the case of Plasmodium falciparum, which causes the vast majority of malaria cases and deaths, the restricted variety and the simplicity of its glycans seemed to confer limited significance to the role played by glycoconjugates in the parasite. Nonetheless, the last 10 to 15 years of research are revealing a clearer and more defined picture. Thus, the use of new experimental techniques and the results obtained provide new avenues for understanding the biology of the parasite, as well as opportunities for the development of much required new tools against malaria.

Engineering of a plant-produced virus-like particle to improve the display of the Plasmodium falciparum Pfs25 antigen and transmission-blocking activity of the vaccine candidate

Malaria kills around 409,000 people a year, mostly children under the age of five. Malaria transmission-blocking vaccines work to reduce malaria prevalence in a community and have the potential to be part of a multifaceted approach required to eliminate the parasites causing the disease. Pfs25 is a leading malaria transmission-blocking antigen and has been successfully produced in a plant expression system as both a subunit vaccine and as a virus-like particle. This study demonstrates an improved version of the virus-like particle antigen display molecule by eliminating known protease sites from the prior A85 variant. This re-engineered molecule, termed B29, displays three times the number of Pfs25 antigens per virus-like particle compared to the original Pfs25 virus-like particle. An improved purification scheme was also developed, resulting in a substantially higher yield and improved purity. The molecule was evaluated in a mouse model and found to induce improved transmission-blocking activity at lower doses and longer durations than the original molecule.

Glycosylphosphatidylinositol Mannosyltransferase Ⅰ Protects Chinese Giant Salamander, Andrias davidianus, against Iridovirus

Glycosylphosphatidylinositol mannosyltransferase I (GPI-MT-I) is an essential glycosyltransferase of glycosylphosphatidylinositol-anchor proteins (GPI-APs) that transfers the first of the four mannoses in GPI-AP precursors, which have multiple functions, including immune response and signal transduction. In this study, the GPI-MT-I gene that regulates GPI-AP biosynthesis in Andrias davidianus (AdGPI-MT-I) was characterized for the first time. The open reading frame (ORF) of AdGPI-MT-I is 1293 bp and encodes a protein of 430 amino acids that contains a conserved PMT2 superfamily domain. AdGPI-MT-I mRNA was widely expressed in the tissues of the Chinese giant salamander. The mRNA expression level of AdGPI-MT-I in the spleen, kidney, and muscle cell line (GSM cells) was significantly upregulated post Chinese giant salamander iridovirus (GSIV) infection. The mRNA expression of the virus major capsid protein (MCP) in AdGPI-MT-I-overexpressed cells was significantly reduced. Moreover, a lower level of virus MCP synthesis and gene copying in AdGPI-MT-I-overexpressed cells was confirmed by western blot and ddPCR. These results collectively suggest that GSIV replication in GSM cells was significantly reduced by the overexpression of the AdGPI-MT-I protein, which may contribute to a better understanding of the antiviral mechanism against iridovirus infection.

Enzymatic and structural characterization of HAD5, an essential phosphomannomutase of malaria-causing parasites

The malaria-causing parasite Plasmodium falciparum is responsible for over 200 million infections and 400,000 deaths per year. At multiple stages during its complex life cycle, P. falciparum expresses several essential proteins tethered to its surface by glycosylphosphatidylinositol (GPI) anchors, which are critical for biological processes such as parasite egress and reinvasion of host red blood cells. Targeting this pathway therapeutically has the potential to broadly impact parasite development across several life stages. Here, we characterize an upstream component of parasite GPI anchor biosynthesis, the putative phosphomannomutase (PMM) (EC 5.4.2.8), HAD5 (PF3D7_1017400). We confirmed the PMM and phosphoglucomutase activities of purified recombinant HAD5 by developing novel linked enzyme biochemical assays. By regulating the expression of HAD5 in transgenic parasites with a TetR-DOZI-inducible knockdown system, we demonstrated that HAD5 is required for malaria parasite egress and erythrocyte reinvasion, and we assessed the role of HAD5 in GPI anchor synthesis by autoradiography of radiolabeled glucosamine and thin layer chromatography. Finally, we determined the three-dimensional X-ray crystal structure of HAD5 and identified a substrate analog that specifically inhibits HAD5 compared to orthologous human PMMs in a time-dependent manner. These findings demonstrate that the GPI anchor biosynthesis pathway is exceptionally sensitive to inhibition in parasites and that HAD5 has potential as a specific, multistage antimalarial target.

Lee L, Alonso-Padilla J, Izquierdo L. Compounds targeting GPI biosynthesis or N-glycosylation are active against Plasmodium falciparum

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Compounds targeting key steps in GPI biosynthesis abrogate P. falciparum growth.

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N-glycosylation disruption halts parasite development and induces delayed death.

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Tunicamycin-induced delayed death is not linked with the synthesis of isoprenoids.

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In summary, two metabolic pathways are outlined for further drug target exploration.

The emergence of resistance to first-line antimalarials, including artemisinin, the last effective malaria therapy in some regions, stresses the urgent need to develop new effective treatments against this disease. The identification and validation of metabolic pathways that could be targeted for drug development may strongly contribute to accelerate this process. In this study, we use fully characterized specific inhibitors targeting glycan biosynthetic pathways as research tools to analyze their effects on the growth of the malaria parasite Plasmodium falciparum and to validate these metabolic routes as feasible chemotherapeutic targets. Through docking simulations using models predicted by AlphaFold, we also shed new light into the modes of action of some of these inhibitors. Molecules inhibiting N-acetylglucosaminyl-phosphatidylinositol de-N-acetylase (GlcNAc-PI de-N-acetylase, PIGL/GPI12) or the inositol acyltransferase (GWT1), central for glycosylphosphatidylinositol (GPI) biosynthesis, halt the growth of intraerythrocytic asexual parasites during the trophozoite stages of the intraerythrocytic developmental cycle (IDC). Remarkably, the nucleoside antibiotic tunicamycin, which targets UDP-N-acetylglucosamine:dolichyl-phosphate N-acetylglucosaminephosphotransferase (ALG7) and N-glycosylation in other organisms, induces a delayed-death effect and inhibits parasite growth during the second IDC after treatment. Our data indicate that tunicamycin induces a specific inhibitory effect, hinting to a more substantial role of the N-glycosylation pathway in P. falciparum intraerythrocytic asexual stages than previously thought. To sum up, our results place GPI biosynthesis and N-glycosylation pathways as metabolic routes with potential to yield much-needed therapeutic targets against the parasite.

Sialylated N-glycans mediate monocyte uptake of extracellular vesicles secreted from Plasmodium falciparum-infected red blood cells

Glycoconjugates on extracellular vesicles (EVs) play a vital role in internalization and mediate interaction as well as regulation of the host immune system by viruses, bacteria, and parasites. During their intraerythrocytic life-cycle stages, malaria parasites, Plasmodium falciparum (Pf) mediate the secretion of EVs by infected red blood cells (RBCs) that carry a diverse range of parasitic and host-derived molecules. These molecules facilitate parasite-parasite and parasite-host interactions to ensure parasite survival. To date, the number of identified Pf genes associated with glycan synthesis and the repertoire of expressed glycoconjugates is relatively low. Moreover, the role of Pf glycans in pathogenesis is mostly unclear and poorly understood. As a result, the expression of glycoconjugates on Pf-derived EVs or their involvement in the parasite life-cycle has yet to be reported. Herein, we show that EVs secreted by Pf-infected RBCs carry significantly higher sialylated complex N-glycans than EVs derived from healthy RBCs. Furthermore, we reveal that EV uptake by host monocytes depends on N-glycoproteins and demonstrate that terminal sialic acid on the N-glycans is essential for uptake by human monocytes. Our results provide the first evidence that Pf exploits host sialylated N-glycans to mediate EV uptake by the human immune system cells.

Unveiling the Sugary Secrets of Plasmodium Parasites

Plasmodium parasites cause malaria disease, one of the leading global health burdens for humanity, infecting hundreds of millions of people each year. Different glycans on the parasite and the host cell surface play significant roles in both malaria pathogenesis and host defense mechanisms. So far, only small, truncated N- and O-glycans have been identified in Plasmodium species. In contrast, complex glycosylphosphatidylinositol (GPI) glycolipids are highly abundant on the parasite’s cell membrane and are essential for its survival. Moreover, the parasites express lectins that bind and exploit the host cell surface glycans for different aspects of the parasite life cycle, such as adherence, invasion, and evasion of the host immune system. In parallel, the host cell glycocalyx and lectin expression serve as the first line of defense against Plasmodium parasites and directly dictate susceptibility to Plasmodium infection. This review provides an overview of the glycobiology involved in Plasmodium-host interactions and its contribution to malaria pathogenesis. Recent findings are presented and evaluated in the context of potential therapeutic exploitation.

Immunological Evaluation of Synthetic Glycosylphosphatidylinositol Glycoconjugates as Vaccine Candidates against Malaria

Glycosylphosphatidylinositols (GPIs) are complex glycolipids present on the surfaces of Plasmodium parasites that may act as toxins during the progression of malaria. GPIs can activate the immune system during infection and induce the formation of anti-GPI antibodies that neutralize their activity. Therefore, an antitoxic vaccine based on GPI glycoconjugates may prevent malaria pathogenesis. To evaluate the role of three key modifications on Plasmodium GPI glycan in the activity of these glycolipids, we synthesized and investigated six structurally distinct GPI fragments from Plasmodium falciparum. The synthetic glycans were conjugated to the CRM197 carrier protein and were tested for immunogenicity and efficacy as antimalarial vaccine candidates in an experimental cerebral malaria model using C57BL/6JRj mice. Protection may be dependent on both the antibody and the cellular immune response to GPIs, and the elicited immune response depends on the orientation of the glycan, the number of mannoses in the structure, and the presence of the phosphoethanolamine and inositol units. This study provides insights into the epitopes in GPIs and contributes to the development of GPI-based antitoxin vaccine candidates against cerebral malaria.

Can Plasmodium's tricks for enhancing its transmission be turned against the parasite? New hopes for vector control

Approximately 120 years ago the link between mosquito and the malaria transmission was discovered. However, even today it remains an open question whether the parasite is able to direct the blood-seeking and feeding behavior of its mosquito vector to maximize the probability of transmission. If the parasite has this ability, could it occur only through the alteration of the vertebrate host's volatile organic compounds (VOCs) and/or the parasite alteration of the behavior of the infected vector in a manner that favors its transmission? Although some recent empirical evidence supports the hypothesis regarding the parasite ability in alteration of the vertebrate host's VOCs, the role of parasite alteration and behavioral differences between infected and uninfected female mosquitoes toward infected and uninfected hosts has not yet been considered in the implementation of control measures. This review will discuss the current evidence, which shows 1. Plasmodium can direct uninfected mosquito blood-seeking and feeding behavior via alteration of vertebrate-host odor profiles and production of phagostimulants and 2. Plasmodium also manipulates its vector during the sporogony cycle to increase transmission. Briefly, we also consider the next generation of methods for moving the empirical laboratory evidence to potential application in future integrated malaria control programs.

Innate immunity to malaria-The role of monocytes

Monocytes are innate immune cells essential for host protection against malaria. Upon activation, monocytes function to help reduce parasite burden through phagocytosis, cytokine production, and antigen presentation. However, monocytes have also been implicated in the pathogenesis of severe disease through production of damaging inflammatory cytokines, resulting in systemic inflammation and vascular dysfunction. Understanding the molecular pathways influencing the balance between protection and pathology is critical. In this review, we discuss recent data regarding the role of monocytes in human malaria, including studies of innate sensing of the parasite, immunometabolism, and innate immune training. Knowledge gained from these studies may guide rational development of novel antimalarial therapies and inform vaccine development.

Delayed Death by Plastid Inhibition in Apicomplexan Parasites

The discovery of a plastid in apicomplexan parasites was hoped to be a watershed moment in the treatment of parasitic diseases as it revealed drug targets that are implicitly divergent from host molecular processes. Indeed, this organelle, known as the apicoplast, has since been a productive therapeutic target for pharmaceutical interventions against infections by Plasmodium, Toxoplasma, Babesia, and Theileria. However, some inhibitors of the apicoplast are restricted in their treatment utility because of their slow-kill kinetics, and this characteristic is called the delayed death effect. Here we review the recent genetic and pharmacological experiments that interrogate the causes of delayed death and explore the foundation of this phenomenon in Plasmodium and Toxoplasma parasites.

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

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

Parasite Recognition and Signaling Mechanisms in Innate Immune Responses to Malaria

Malaria caused by the Plasmodium family of parasites, especially P.falciparum and P. vivax, is a major health problem in many countries in the tropical and subtropical regions of the world. The disease presents a wide array of systemic clinical conditions and several life-threatening organ pathologies, including the dreaded cerebral malaria. Like many other infectious diseases, malaria is an inflammatory response-driven disease, and positive outcomes to infection depend on finely tuned regulation of immune responses that efficiently clear parasites and allow protective immunity to develop. Immune responses initiated by the innate immune system in response to parasites play key roles both in protective immunity development and pathogenesis. Initial pro-inflammatory responses are essential for clearing infection by promoting appropriate cell-mediated and humoral immunity. However, elevated and prolonged pro-inflammatory responses owing to inappropriate cellular programming contribute to disease conditions. A comprehensive knowledge of the molecular and cellular mechanisms that initiate immune responses and how these responses contribute to protective immunity development or pathogenesis is important for developing effective therapeutics and/or a vaccine. Historically, in efforts to develop a vaccine, immunity to malaria was extensively studied in the context of identifying protective humoral responses, targeting proteins involved in parasite invasion or clearance. The innate immune response was thought to be non-specific. However, during the past two decades, there has been a significant progress in understanding the molecular and cellular mechanisms of host-parasite interactions and the associated signaling in immune responses to malaria. Malaria infection occurs at two stages, initially in the liver through the bite of a mosquito, carrying sporozoites, and subsequently, in the blood through the invasion of red blood cells by merozoites released from the infected hepatocytes. Soon after infection, both the liver and blood stage parasites are sensed by various receptors of the host innate immune system resulting in the activation of signaling pathways and production of cytokines and chemokines. These immune responses play crucial roles in clearing parasites and regulating adaptive immunity. Here, we summarize the knowledge on molecular mechanisms that underlie the innate immune responses to malaria infection.

Potential targets for next generation antimicrobial glycoconjugate vaccines

Cell surface carbohydrates have been proven optimal targets for vaccine development. Conjugation of polysaccharides to a carrier protein triggers a T-cell-dependent immune response to the glycan moiety. Licensed glycoconjugate vaccines are produced by chemical conjugation of capsular polysaccharides to prevent meningitis caused by meningococcus, pneumococcus and Haemophilus influenzae type b. However, other classes of carbohydrates (O-antigens, exopolysaccharides, wall/teichoic acids) represent attractive targets for developing vaccines. Recent analysis from WHO/CHO underpins alarming concern toward antibiotic-resistant bacteria, such as the so called ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) and additional pathogens such as Clostridium difficile and Group A Streptococcus. Fungal infections are also becoming increasingly invasive for immunocompromised patients or hospitalized individuals. Other emergencies could derive from bacteria which spread during environmental calamities (Vibrio cholerae) or with potential as bioterrorism weapons (Burkholderia pseudomallei and mallei, Francisella tularensis). Vaccination could aid reducing the use of broad-spectrum antibiotics and provide protection by herd immunity also to individuals who are not vaccinated.

This review analyzes structural and functional differences of the polysaccharides exposed on the surface of emerging pathogenic bacteria, combined with medical need and technological feasibility of corresponding glycoconjugate vaccines.

Parasite Carbohydrate Vaccines

Vaccination is an efficient means of combating infectious disease burden globally. However, routine vaccines for the world's major human parasitic diseases do not yet exist. Vaccines based on carbohydrate antigens are a viable option for parasite vaccine development, given the proven success of carbohydrate vaccines to combat bacterial infections. We will review the key components of carbohydrate vaccines that have remained largely consistent since their inception, and the success of bacterial carbohydrate vaccines. We will then explore the latest developments for both traditional and non-traditional carbohydrate vaccine approaches for three of the world's major protozoan parasitic diseases-malaria, toxoplasmosis, and leishmaniasis. The traditional prophylactic carbohydrate vaccine strategy is being explored for malaria. However, given that parasite disease biology is complex and often arises from host immune responses to parasite antigens, carbohydrate vaccines against deleterious immune responses in host-parasite interactions are also being explored. In particular, the highly abundant glycosylphosphatidylinositol molecules specific for Plasmodium, Toxoplasma, and Leishmania spp. are considered exploitable antigens for this non-traditional vaccine approach. Discussion will revolve around the application of these protozoan carbohydrate antigens for vaccines currently in preclinical development.

The disruption of GDP-fucose de novo biosynthesis suggests the presence of a novel fucose-containing glycoconjugate in Plasmodium asexual blood stages

Glycosylation is an important posttranslational protein modification in all eukaryotes. Besides glycosylphosphatidylinositol (GPI) anchors and N-glycosylation, O-fucosylation has been recently reported in key sporozoite proteins of the malaria parasite. Previous analyses showed the presence of GDP-fucose (GDP-Fuc), the precursor for all fucosylation reactions, in the blood stages of Plasmodium falciparum. The GDP-Fuc de novo pathway, which requires the action of GDP-mannose 4,6-dehydratase (GMD) and GDP-L-fucose synthase (FS), is conserved in the parasite genome, but the importance of fucose metabolism for the parasite is unknown. To functionally characterize the pathway we generated a PfGMD mutant and analyzed its phenotype. Although the labelling by the fucose-binding Ulex europaeus agglutinin I (UEA-I) was completely abrogated, GDP-Fuc was still detected in the mutant. This unexpected result suggests the presence of an alternative mechanism for maintaining GDP-Fuc in the parasite. Furthermore, PfGMD null mutant exhibited normal growth and invasion rates, revealing that the GDP-Fuc de novo metabolic pathway is not essential for the development in culture of the malaria parasite during the asexual blood stages. Nonetheless, the function of this metabolic route and the GDP-Fuc pool that is generated during this stage may be important for gametocytogenesis and sporogonic development in the mosquito.

Interrogating the Plasmodium Sporozoite Surface: Identification of Surface-Exposed Proteins and Demonstration of Glycosylation on CSP and TRAP by Mass Spectrometry-Based Proteomics

Malaria parasite infection is initiated by the mosquito-transmitted sporozoite stage, a highly motile invasive cell that targets hepatocytes in the liver for infection. A promising approach to developing a malaria vaccine is the use of proteins located on the sporozoite surface as antigens to elicit humoral immune responses that prevent the establishment of infection. Very little of the P. falciparum genome has been considered as potential vaccine targets, and candidate vaccines have been almost exclusively based on single antigens, generating the need for novel target identification. The most advanced malaria vaccine to date, RTS,S, a subunit vaccine consisting of a portion of the major surface protein circumsporozoite protein (CSP), conferred limited protection in Phase III trials, falling short of community-established vaccine efficacy goals. In striking contrast to the limited protection seen in current vaccine trials, sterilizing immunity can be achieved by immunization with radiation-attenuated sporozoites, suggesting that more potent protection may be achievable with a multivalent protein vaccine. Here, we provide the most comprehensive analysis to date of proteins located on the surface of or secreted by Plasmodium falciparum salivary gland sporozoites. We used chemical labeling to isolate surface-exposed proteins on sporozoites and identified these proteins by mass spectrometry. We validated several of these targets and also provide evidence that components of the inner membrane complex are in fact surface-exposed and accessible to antibodies in live sporozoites. Finally, our mass spectrometry data provide the first direct evidence that the Plasmodium surface proteins CSP and TRAP are glycosylated in sporozoites, a finding that could impact the selection of vaccine antigens.

Malaria remains one of the most important infectious diseases in the world, responsible for an estimated 500 million new cases and 600,000 deaths annually. The etiologic agents of the disease are protozoan parasites of the genus Plasmodium that have a complex cycle between mosquito and mammalian hosts. Though all clinical symptoms are attributable to the blood stages, it is only by attacking the transmission stages that we can make an impact on the economic and health burdens of malaria. Infection is initiated when mosquitoes inoculate sporozoites into the skin as they probe for blood. Sporozoites must locate blood vessels and enter the circulation to reach the liver where they invade and grow in hepatocytes. The inoculum is low and these early stages of infection are asymptomatic. Though the small amounts of material available for study has made large scale -omics studies difficult, killing the parasite at this stage would prevent infection and block downstream transmission to mosquitoes, thus preventing spread of disease. Here we use state-of-the-art biochemistry tools to identify the proteins on the sporozoite surface and find that two of the most studied proteins, CSP and TRAP, have post-translational modifications. These studies will aid investigations into the novel biology of sporozoites and importantly, significantly expand the pool of potential vaccine candidates.

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.

Sugar activation and glycosylation in Plasmodium

Glycoconjugates are important mediators of host-pathogen interactions and are usually very abundant in the surface of many protozoan parasites. However, in the particular case of Plasmodium species, previous works show that glycosylphosphatidylinositol anchor modifications, and to an unknown extent, a severely truncated N-glycosylation are the only glycosylation processes taking place in the parasite. Nevertheless, a detailed analysis of the parasite genome and the recent identification of the sugar nucleotide precursors biosynthesized by Plasmodium falciparum support a picture in which several overlooked, albeit not very prominent glycosylations may be occurring during the parasite life cycle. In this work, the authors review recent developments in the characterization of the biosynthesis of glycosylation precursors in the parasite, focusing on the outline of the possible fates of these precursors.

The apicomplexan parasite Babesia divergens internalizes band 3, glycophorin A and spectrin during invasion of human red blood cells

Plasmodium falciparum invades human red blood cells (RBC), while Babesia divergens infects bovine and, occasionally, human RBC. The mammalian RBC is normally unable to endocytose or phagocytose and the events leading to invasion are incompletely understood. Initially, both parasites are surrounded by the RBC plasma membrane-derived parasitophorous vacuolar membrane (PVM) that is formed during invasion. In P. falciparum-infected RBC, the PVM persists at least until parasite replication is completed whereas it has been proposed that the B. divergens PVM is disintegrated soon upon invasion. Here, we have used a B. divergens strain adapted to human RBC to investigate the formation and fate of the PVM. Using ultrastructural analysis and whole-mount or on-section immunofluorescence and immunogold labelling, we demonstrate that the initial vacuolar membrane is formed from protein and lipid components of the RBC plasma membrane. Integral membrane proteins band 3 and glycophorin A and the cytoskeletal protein spectrin are associated with the PVM of the B. divergens, but are absent from the PVM of P. falciparum at the ring or the trophozoite stage. Our results provide evidence that the biophysical properties of the RBC cytoskeleton per se do not preclude the internalization of cytoskeletal proteins by invading parasites.

Isoprenoid metabolism in apicomplexan parasites

Apicomplexan parasites include some of the most prevalent and deadly human pathogens. Novel antiparasitic drugs are urgently needed. Synthesis and metabolism of isoprenoids may present multiple targets for therapeutic intervention. The apicoplast-localized methylerythritol phosphate (MEP) pathway for isoprenoid precursor biosynthesis is distinct from the mevalonate (MVA) pathway used by the mammalian host, and this pathway is apparently essential in most Apicomplexa. In this review, we discuss the current field of research on production and metabolic fates of isoprenoids in apicomplexan parasites, including the acquisition of host isoprenoid precursors and downstream products. We describe recent work identifying the first MEP pathway regulator in apicomplexan parasites, and introduce several promising areas for ongoing research into this well-validated antiparasitic target.

Isoprenoid biosynthesis in Plasmodium falciparum

Malaria kills nearly 1 million people each year, and the protozoan parasite Plasmodium falciparum has become increasingly resistant to current therapies. Isoprenoid synthesis via the methylerythritol phosphate (MEP) pathway represents an attractive target for the development of new antimalarials. The phosphonic acid antibiotic fosmidomycin is a specific inhibitor of isoprenoid synthesis and has been a helpful tool to outline the essential functions of isoprenoid biosynthesis in P. falciparum. Isoprenoids are a large, diverse class of hydrocarbons that function in a variety of essential cellular processes in eukaryotes. In P. falciparum, isoprenoids are used for tRNA isopentenylation and protein prenylation, as well as the synthesis of vitamin E, carotenoids, ubiquinone, and dolichols. Recently, isoprenoid synthesis in P. falciparum has been shown to be regulated by a sugar phosphatase. We outline what is known about isoprenoid function and the regulation of isoprenoid synthesis in P. falciparum, in order to identify valuable directions for future research.

A novel malaria vaccine candidate antigen expressed in Tetrahymena thermophila

Development of effective malaria vaccines is hampered by the problem of producing correctly folded Plasmodium proteins for use as vaccine components. We have investigated the use of a novel ciliate expression system, Tetrahymena thermophila, as a P. falciparum vaccine antigen platform. A synthetic vaccine antigen composed of N-terminal and C-terminal regions of merozoite surface protein-1 (MSP-1) was expressed in Tetrahymena thermophila. The recombinant antigen was secreted into the culture medium and purified by monoclonal antibody (mAb) affinity chromatography. The vaccine was immunogenic in MF1 mice, eliciting high antibody titers against both N- and C-terminal components. Sera from immunized animals reacted strongly with P. falciparum parasites from three antigenically different strains by immunofluorescence assays, confirming that the antibodies produced are able to recognize parasite antigens in their native form. Epitope mapping of serum reactivity with a peptide library derived from all three MSP-1 Block 2 serotypes confirmed that the MSP-1 Block 2 hybrid component of the vaccine had effectively targeted all three serotypes of this polymorphic region of MSP-1. This study has successfully demonstrated the use of Tetrahymena thermophila as a recombinant protein expression platform for the production of malaria vaccine antigens.

Biosynthesis of GDP-fucose and other sugar nucleotides in the blood stages of Plasmodium falciparum

Carbohydrate structures play important roles in many biological processes, including cell adhesion, cell-cell communication, and host-pathogen interactions. Sugar nucleotides are activated forms of sugars used by the cell as donors for most glycosylation reactions. Using a liquid chromatography-tandem mass spectrometry-based method, we identified and quantified the pools of UDP-glucose, UDP-galactose, UDP-N-acetylglucosamine, GDP-mannose, and GDP-fucose in Plasmodium falciparum intraerythrocytic life stages. We assembled these data with the in silico functional reconstruction of the parasite metabolic pathways obtained from the P. falciparum annotated genome, exposing new active biosynthetic routes crucial for further glycosylation reactions. Fucose is a sugar present in glycoconjugates often associated with recognition and adhesion events. Thus, the GDP-fucose precursor is essential in a wide variety of organisms. P. falciparum presents homologues of GDP-mannose 4,6-dehydratase and GDP-L-fucose synthase enzymes that are active in vitro, indicating that most GDP-fucose is formed by a de novo pathway that involves the bioconversion of GDP-mannose. Homologues for enzymes involved in a fucose salvage pathway are apparently absent in the P. falciparum genome. This is in agreement with in vivo metabolic labeling experiments showing that fucose is not significantly incorporated by the parasite. Fluorescence microscopy of epitope-tagged versions of P. falciparum GDP-mannose 4,6-dehydratase and GDP-L-fucose synthase expressed in transgenic 3D7 parasites shows that these enzymes localize in the cytoplasm of P. falciparum during the intraerythrocytic developmental cycle. Although the function of fucose in the parasite is not known, the presence of GDP-fucose suggests that the metabolite may be used for further fucosylation reactions.

Terminal galactosylation of glycoconjugates in Plasmodium falciparum asexual blood stages and Trypanosoma brucei bloodstream trypomastigotes

There is definitive biochemical evidence for the presence of terminal α-galactosyl residues (α-gal) in the N-linked oligosaccharides and glycophosphatidylinositol anchors (GPI anchors) of the variant surface glycoprotein of Trypanosoma brucei bloodstream trypomastigotes. Indirect evidence also exists for α-gal in Plasmodium falciparum asexual blood stage glycoproteins and glycolipids. The occurrence of α-gal in glycoproteins and glycolipids of T. brucei bloodstream trypomastigotes and P. falciparum late asexual blood stages was investigated by the binding of α-gal-specific Bandeirea simplicifolia B4 lectin 1 (BSB4), incorporation of [(3)H]galactose from UDP-[(3)H]galactose into glycoproteins and glycolipids in microsomes in vitro, and bioinformatic searches for galactosyl-transferase coding sequences. The findings confirm the presence of α-gal in a spectrum of T. brucei bloodstream trypomastigote glycoproteins and glycolipids and indicate its relative absence from P. falciparum asexual blood stage glycoconjugates.

Metabolic oligosaccharide engineering of Plasmodium falciparum intraerythrocytic stages allows direct glycolipid analysis by mass spectrometry

A recent addition to the arsenal of tools for glycome analysis is the use of metabolic labels that allow covalent tagging of glycans with imaging probes. In this work we show that N-azidoglucosamine was successfully incorporated into glycolipidic structures of Plasmodium falciparum intraerythrocytic stages. The ability to tag glycoconjugates selectively with a fluorescent reporter group permits TLC detection of the glycolipids providing a new method to quantify dynamic changes in the glycosylation pattern and facilitating direct mass spectrometry analyses. Presence of glycosylphosphatidylinositol and glycosphingolipid structures was determined in the different extracts. Furthermore, the fluorescent tag was used as internal matrix for the MALDI experiment making even easier the analysis.

A constitutive pan-hexose permease for the Plasmodium life cycle and transgenic models for screening of antimalarial sugar analogs

Glucose is considered essential for erythrocytic stages of the malaria parasite, Plasmodium falciparum. Importance of sugar and its permease for hepatic and sexual stages of Plasmodium, however, remains elusive. Moreover, increasing global resistance to current antimalarials necessitates the search for novel drugs. Here, we reveal that hexose transporter 1 (HT1) of Plasmodium berghei can transport glucose (K(m)~87 μM), mannose (K(i)~93 μM), fructose (K(i)~0.54 mM), and galactose (K(i)~5 mM) in Leishmania mexicana mutant and Xenopus laevis; and, therefore, is functionally equivalent to HT1 of P. falciparum (Glc, K(m)~175 μM; Man, K(i)~276 μM; Fru, K(i)~1.25 mM; Gal, K(i)~5.86 mM). Notably, a glucose analog, C3361, attenuated hepatic (IC(50)~15 μM) and ookinete development of P. berghei. The PbHT1 could be ablated during intraerythrocytic stages only by concurrent complementation with PbHT1-HA or PfHT1. Together; these results signify that PbHT1 and glucose are required for the entire life cycle of P. berghei. Accordingly, PbHT1 is expressed in the plasma membrane during all parasite stages. To permit a high-throughput screening of PfHT1 inhibitors and their subsequent in vivo assessment, we have generated Saccharomyces cerevisiae mutant expressing codon-optimized PfHT1, and a PfHT1-dependent Δpbht1 parasite strain. This work provides a platform to facilitate the development of drugs against malaria, and it suggests a disease-control aspect by reducing parasite transmission.

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.

Suggestive evidence for Darwinian Selection against asparagine-linked glycans of Plasmodium falciparum and Toxoplasma gondii

We are interested in asparagine-linked glycans (N-glycans) of Plasmodium falciparum and Toxoplasma gondii, because their N-glycan structures have been controversial and because we hypothesize that there might be selection against N-glycans in nucleus-encoded proteins that must pass through the endoplasmic reticulum (ER) prior to threading into the apicoplast. In support of our hypothesis, we observed the following. First, in protists with apicoplasts, there is extensive secondary loss of Alg enzymes that make lipid-linked precursors to N-glycans. Theileria makes no N-glycans, and Plasmodium makes a severely truncated N-glycan precursor composed of one or two GlcNAc residues. Second, secreted proteins of Toxoplasma, which uses its own 10-sugar precursor (Glc(3)Man(5)GlcNAc(2)) and the host 14-sugar precursor (Glc(3)Man(9)GlcNAc(2)) to make N-glycans, have very few sites for N glycosylation, and there is additional selection against N-glycan sites in its apicoplast-targeted proteins. Third, while the GlcNAc-binding Griffonia simplicifolia lectin II labels ER, rhoptries, and surface of plasmodia, there is no apicoplast labeling. Similarly, the antiretroviral lectin cyanovirin-N, which binds to N-glycans of Toxoplasma, labels ER and rhoptries, but there is no apicoplast labeling. We conclude that possible selection against N-glycans in protists with apicoplasts occurs by eliminating N-glycans (Theileria), reducing their length (Plasmodium), or reducing the number of N-glycan sites (Toxoplasma). In addition, occupation of N-glycan sites is markedly reduced in apicoplast proteins versus some secretory proteins in both Plasmodium and Toxoplasma.

Babesia bovis contains an abundant parasite-specific protein-free glycerophosphatidylinositol and the genes predicted for its assembly

Autonomous glycosylphosphatidylinositol (GPI) molecules (also protein-free GPIs or free GPIs) have been reported to be particularly abundant in some parasitic protozoa and mediate strong immunomodulatory effects on the host immune system. In the work at hand we have investigated the existence of free GPIs in Babesia bovis. Comparative thin layer chromatographic analysis of the protein-free glycolipid fraction of in vitro cultured B. bovis merozoites and erythrocyte membranes demonstrated the presence of an abundant parasite-specific band. Its chemical analysis revealed a GPI species containing a chain of two mannose residues, N-glucosamine and non-acylated inositol. The lipid moiety linked to inositol was diacylglycerol. The total fatty acid composition showed predominantly long-carbon chain molecules (12% of C(22:0) and 45% of C(24:0)). The potential of B. bovis to assemble the presented free GPI species was verified by the existence of seven genes in its genome that putatively encode the following GPI biosynthetic enzymes: PI N-acetyl-GlcN-transferase (PIG-A and GPI-1), N-acetyl-GlcN-PI-de-N-acetylase (PIG-L), acyltransferase (PIG-W), dolichyl-phosphate mannosyl transferase (DPM-1), GPI mannosyltransferase I (PIG-M), and GPI mannosyltransferase II (PIG-V). GPI biosynthesis is vital for the intraerythrocytic parasite stage as mannosamine, an inhibitor of GPI biosynthesis, impaired in vitro growth of B. bovis merozoites. Absence of the vast majority of N-glycan metabolism encoding genes in the B. bovis genome underscores that the growth inhibitory effect of mannosamine is attributable to its interference with GPI biosynthesis and not with assembly of N-linked oligosaccharides, as has been described for higher eukaryotes. Elucidation of the structure and biosynthesis of GPI may allow to facilitate the development of future immune interventions against bovine babesiosis.

Post-translational modifications in Plasmodium: more than you think!

Recent evidences indicate that transcription in Plasmodium may be hard-wired and rigid, deviating from the classical model of transcriptional gene regulation. Thus, it is important that other regulatory pathways be investigated as a comprehensive effort to curb the deadly malarial parasite. Research in post-translational modifications in Plasmodium is an emerging field that may provide new venues for drug discovery and potential new insights into how parasitic protozoans regulate their life cycle. Here, we discuss the recent findings of post-translational modifications in Plasmodium.

Quantitative protein expression profiling reveals extensive post-transcriptional regulation and post-translational modifications in schizont-stage malaria parasites

BACKGROUND

Malaria is a one of the most important infectious diseases and is caused by parasitic protozoa of the genus Plasmodium. Previously, quantitative characterization of the P. falciparum transcriptome demonstrated that the strictly controlled progression of these parasites through their intra-erythrocytic developmental cycle is accompanied by a continuous cascade of gene expression. Although such analyses have proven immensely useful, the correlations between abundance of transcripts and their cognate proteins remain poorly characterized.

RESULTS

Here, we present a quantitative time-course analysis of relative protein abundance for schizont-stage parasites (34 to 46 hours after invasion) based on two-dimensional differential gel electrophoresis of protein samples labeled with fluorescent dyes. For this purpose we analyzed parasite samples taken at 4-hour intervals from a tightly synchronized culture and established more than 500 individual protein abundance profiles with high temporal resolution and quantitative reproducibility. Approximately half of all profiles exhibit a significant change in abundance and 12% display an expression peak during the observed 12-hour time interval. Intriguingly, identification of 54 protein spots by mass spectrometry revealed that 58% of the corresponding proteins--including actin-I, enolase, eukaryotic initiation factor (eIF)4A, eIF5A, and several heat shock proteins--are represented by more than one isoform, presumably caused by post-translational modifications, with the various isoforms of a given protein frequently showing different expression patterns. Furthermore, comparisons with transcriptome data generated from the same parasite samples reveal evidence of significant post-transcriptional gene expression regulation.

CONCLUSIONS

Together, our data indicate that both post-transcriptional and post-translational events are widespread and of presumably great biological significance during the intra-erythrocytic development of P. falciparum.

Systems analysis of chaperone networks in the malarial parasite Plasmodium falciparum

Molecular chaperones participate in the maintenance of cellular protein homeostasis, cell growth and differentiation, signal transduction, and development. Although a vast body of information is available regarding individual chaperones, few studies have attempted a systems level analysis of chaperone function. In this paper, we have constructed a chaperone interaction network for the malarial parasite, Plasmodium falciparum. P. falciparum is responsible for several million deaths every year, and understanding the biology of the parasite is a top priority. The parasite regularly experiences heat shock as part of its life cycle, and chaperones have often been implicated in parasite survival and growth. To better understand the participation of chaperones in cellular processes, we created a parasite chaperone network by combining experimental interactome data with in silico analysis. We used interolog mapping to predict protein-protein interactions for parasite chaperones based on the interactions of corresponding human chaperones. This data was then combined with information derived from existing high-throughput yeast two-hybrid assays. Analysis of the network reveals the broad range of functions regulated by chaperones. The network predicts involvement of chaperones in chromatin remodeling, protein trafficking, and cytoadherence. Importantly, it allows us to make predictions regarding the functions of hypothetical proteins based on their interactions. It allows us to make specific predictions about Hsp70-Hsp40 interactions in the parasite and assign functions to members of the Hsp90 and Hsp100 families. Analysis of the network provides a rational basis for the anti-malarial activity of geldanamycin, a well-known Hsp90 inhibitor. Finally, analysis of the network provides a theoretical basis for further experiments designed toward understanding the involvement of this important class of molecules in parasite biology.

Plasmodium falciparum GPI mannosyltransferase-III has novel signature sequence and is functional

The glycosylphosphatidylinositol (GPI) anchors of Plasmodium falciparum are indispensable for parasite survival since merozoite surface proteins-1, -2, -4, -5, and -10, crucial for erythrocyte invasion, are GPI-anchored. Therefore, the GPI biosynthetic pathway can offer potential targets for novel anti-malarial drugs. Here, we characterized the putative P. falciparum PIG-B gene (PfPIGB) that encodes mannosyltransferase-III of GPI biosynthesis. PfPIGB mRNA is transcribed in a developmental stage specific manner. A protein corresponding to the expected size of PfPIG-B is expressed by the parasite and is localized in the endoplasmic reticulum. Treatment of parasites with PfPIG-B specific siRNA caused reduction in GPI synthesis, affecting the PIG-B specific GPI intermediate. These data demonstrate that PfPIG-B is functional and encodes mannosyltransferase-III of the parasite GPI biosynthesis. The parasite PfPIG-B is novel in that its signature sequence HKEHKI is unique and is only partially conserved as compared to HKEXRF signature motif of mammalian PIG-B enzymes.

The Princeton Protein Orthology Database (P-POD): a comparative genomics analysis tool for biologists

Many biological databases that provide comparative genomics information and tools are now available on the internet. While certainly quite useful, to our knowledge none of the existing databases combine results from multiple comparative genomics methods with manually curated information from the literature. Here we describe the Princeton Protein Orthology Database (P-POD, http://ortholog.princeton.edu), a user-friendly database system that allows users to find and visualize the phylogenetic relationships among predicted orthologs (based on the OrthoMCL method) to a query gene from any of eight eukaryotic organisms, and to see the orthologs in a wider evolutionary context (based on the Jaccard clustering method). In addition to the phylogenetic information, the database contains experimental results manually collected from the literature that can be compared to the computational analyses, as well as links to relevant human disease and gene information via the OMIM, model organism, and sequence databases. Our aim is for the P-POD resource to be extremely useful to typical experimental biologists wanting to learn more about the evolutionary context of their favorite genes. P-POD is based on the commonly used Generic Model Organism Database (GMOD) schema and can be downloaded in its entirety for installation on one's own system. Thus, bioinformaticians and software developers may also find P-POD useful because they can use the P-POD database infrastructure when developing their own comparative genomics resources and database tools.

Plasmodium falciparum biosynthesizes sulfoglycosphingolipids

Sulfated glycosphingolipids are present on the surface of a variety of cells. They are active participants in adhesion processes in many systems and appear to be involved in the regulation of cell proliferation, differentiation and other developmental cellular events. However, the body of knowledge about synthesis, structure, and function of glycolipids in parasitic protozoa is very limited so far. In this work, we show by metabolic incorporation of [(14)C]palmitic acid, [(14)C]glucose and Na(2)(35)SO(4) that sulfoglycosphingolipids are biosynthesized in the three intraerythrocytic stages of Plasmodium falciparum. After saponification, purification of the labelled acidic components was achieved and two components named SPf1 and SPf2 were characterized. Chemical degradations and TLC analysis pointed out to sulfolipidic structures. Analysis by UV-MALDI-TOF mass spectrometry in the negative ion mode using nor-harmane as matrix showed for SPf2 a structure consisting in a disulfated hexose linked to a 20:1 sphingosine acylated with C18:0 fatty acid. Interestingly, parasite treatment with low concentrations of d,l-threo-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) caused an arrest on parasite development associated to the inhibition of sulfoglycolipid biosynthesis. Taking into account that sulfoglycolipidic structures are currently involved in adhesion processes, our findings open the possibility to study the participation of this type of structures in the described aggregation phenomena in severe malaria and may contribute to clarify the pathogenesis of the disease. This report shows for the first time the synthesis of sulfoglycolipids in Apicomplexa.

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

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

Adhesion molecules and other secreted host-interaction determinants in Apicomplexa: insights from comparative genomics

Apicomplexa have developed distinctive adaptations for invading and surviving within animal cells. Here a synthetic overview of the diversity and evolutionary history of cell membrane-associated, -secreted, and -exported proteins related to apicomplexan parasitism is presented. A notable feature in this regard was the early acquisition of adhesion protein domains and glycosylation systems through lateral transfer from animals. These were utilized in multiple contexts, including invasion of host cells and parasite-specific developmental processes. Apicomplexans possess a specialized version of the ancestral alveolate extrusion machinery, the rhoptries and micronemes, which are deployed in invasion and delivery of proteins into host cells. Each apicomplexan lineage has evolved a unique spectrum of extruded proteins that modify host molecules in diverse ways. Hematozoans, in particular, appear to have evolved novel systems for export of proteins into the host organelles and cell membrane during intracellular development. These exported proteins are an important aspect of the pathogenesis of Plasmodium and Theileria, being involved in response to fever and in leukocyte proliferation respectively. The complement of apicomplexan surface proteins has primarily diversified via massive lineage-specific expansions of certain protein families, which are often coded by subtelomeric gene arrays. Many of these families have been found to be central to immune evasion. Domain shuffling and accretion have resulted in adhesins with new domain architectures. In terms of individual genes, constant selective pressures from the host immune response has resulted in extensive protein polymorphisms and gene losses. Apicomplexans have also evolved complex regulatory mechanisms controlling expression and maturation of surface proteins at the chromatin, transcriptional, posttranscriptional, and posttranslational levels. Evolutionary reconstruction suggests that the ancestral apicomplexan had thrombospondin and EGF domain adhesins, which were linked to the parasite cytoskeleton, and played a central role in invasion through formation of the moving junction. It also suggests that the ancestral parasite had O-linked glycosylation of surface proteins which was partially or entirely lost in hematozoan lineages.