Critical roles of glycosylphosphatidylinositol for Trypanosoma brucei

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

Glycosylphosphatidylinositol anchor lipid remodeling directs proteins to the plasma membrane and governs cell wall mechanics

Glycosylphosphatidylinositol (GPI) anchoring is a common protein modification that targets proteins to the plasma membrane (PM). Knowledge about the GPI lipid tail, which guides the secretion of GPI-anchored proteins (GPI-APs), is limited in plants. Here, we report that rice (Oryza sativa) BRITTLE CULM16 (BC16), a membrane-bound O-acyltransferase (MBOAT) remodels GPI lipid tails and governs cell wall biomechanics. The bc16 mutant exhibits fragile internodes, resulting from reduced cell wall thickness and cellulose content. BC16 is the only MBOAT in rice and is located in the endoplasmic reticulum and Golgi apparatus. Yeast gup1Δ mutant restoring assay and GPI lipid composition analysis demonstrated BC16 as a GPI lipid remodelase. Loss of BC16 alters GPI lipid structure and disturbs the targeting of BC1, a GPI-AP for cellulose biosynthesis, to the PM lipid nanodomains. Atomic force microscopy revealed compromised deposition of cellulosic nanofibers in bc16, leading to an increased Young's modulus and abnormal mechanical properties. Therefore, BC16-mediated lipid remodeling directs the GPI-APs, such as BC1, to the cell surface to fulfill multiple functions, including cellulose organization. Our work unravels a mechanism by which GPI lipids are remodeled in plants and provides insights into the control of cell wall biomechanics, offering a tool for breeding elite crops with improved support strength.

Fatty acid uptake in Trypanosoma brucei: Host resources and possible mechanisms

Trypanosoma brucei spp. causes African Sleeping Sickness in humans and nagana, a wasting disease, in cattle. As T. brucei goes through its life cycle in its mammalian and insect vector hosts, it is exposed to distinct environments that differ in their nutrient resources. One such nutrient resource is fatty acids, which T. brucei uses to build complex lipids or as a potential carbon source for oxidative metabolism. Of note, fatty acids are the membrane anchoring moiety of the glycosylphosphatidylinositol (GPI)-anchors of the major surface proteins, Variant Surface Glycoprotein (VSG) and the Procyclins, which are implicated in parasite survival in the host. While T. brucei can synthesize fatty acids de novo, it also readily acquires fatty acids from its surroundings. The relative contribution of parasite-derived vs. host-derived fatty acids to T. brucei growth and survival is not known, nor have the molecular mechanisms of fatty acid uptake been defined. To facilitate experimental inquiry into these important aspects of T. brucei biology, we addressed two questions in this review: (1) What is known about the availability of fatty acids in different host tissues where T. brucei can live? (2) What is known about the molecular mechanisms mediating fatty acid uptake in T. brucei? Finally, based on existing biochemical and genomic data, we suggest a model for T. brucei fatty acid uptake that proposes two major routes of fatty acid uptake: diffusion across membranes followed by intracellular trapping, and endocytosis of host lipoproteins.

Common and unique features of glycosylation and glycosyltransferases in African trypanosomes

Eukaryotic protein glycosylation is mediated by glycosyl- and oligosaccharyl-transferases. Here, we describe how African trypanosomes exhibit both evolutionary conservation and significant divergence compared with other eukaryotes in how they synthesise their glycoproteins. The kinetoplastid parasites have conserved components of the dolichol-cycle and oligosaccharyltransferases (OSTs) of protein N-glycosylation, and of glycosylphosphatidylinositol (GPI) anchor biosynthesis and transfer to protein. However, some components are missing, and they process and decorate their N-glycans and GPI anchors in unique ways. To do so, they appear to have evolved a distinct and functionally flexible glycosyltransferases (GT) family, the GT67 family, from an ancestral eukaryotic β3GT gene. The expansion and/or loss of GT67 genes appears to be dependent on parasite biology. Some appear to correlate with the obligate passage of parasites through an insect vector, suggesting they were acquired through GT67 gene expansion to assist insect vector (tsetse fly) colonisation. Others appear to have been lost in species that subsequently adopted contaminative transmission. We also highlight the recent discovery of a novel and essential GT11 family of kinetoplastid parasite fucosyltransferases that are uniquely localised to the mitochondria of Trypanosoma brucei and Leishmania major. The origins of these kinetoplastid FUT1 genes, and additional putative mitochondrial GT genes, are discussed.

A Soluble, Minimalistic Glycosylphosphatidylinositol Transamidase (GPI-T) Retains Transamidation Activity

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a eukaryotic, post-translational modification catalyzed by GPI transamidase (GPI-T). The Saccharomyces cerevisiae GPI-T is composed of five membrane-bound subunits: Gpi8, Gaa1, Gpi16, Gpi17, and Gab1. GPI-T has been recalcitrant to in vitro structure and function studies because of its complexity and membrane-solubility. Furthermore, a reliable, quantitative, in vitro assay for this important post-translational modification has remained elusive despite its discovery more than three decades ago.Three recent reports describe the structure of GPI-T from S. cerevisiae and humans, shedding critical light on this important enzyme and offering insight into the functions of its different subunits. Here, we present the purification and characterization of a truncated soluble GPI-T heterotrimer complex (Gpi8_23-306, Gaa1_50-343, and Gpi16_20-551) without transmembrane domains. Using this simplified heterotrimer, we report the first quantitative method to measure GPI-T activity in vitro and demonstrate that this soluble, minimalistic GPI-T retains transamidase activity. These results contribute to our understanding of how this enzyme is organized and functions, and provide a method to screen potential GPI-T inhibitors.

The Leishmania donovani Ortholog of the Glycosylphosphatidylinositol Anchor Biosynthesis Cofactor PBN1 Is Essential for Host Infection

Visceral leishmaniasis is a deadly infectious disease caused by Leishmania donovani, a kinetoplastid parasite for which no licensed vaccine is available. To identify potential vaccine candidates, we systematically identified genes encoding putative cell surface and secreted proteins essential for parasite viability and host infection. We identified a protein encoded by LdBPK_061160 which, when ablated, resulted in a remarkable increase in parasite adhesion to tissue culture flasks. Here, we show that this phenotype is caused by the loss of glycosylphosphatidylinositol (GPI)-anchored surface molecules and that LdBPK_061160 encodes a noncatalytic component of the L. donovani GPI-mannosyltransferase I (GPI-MT I) complex. GPI-anchored surface molecules were rescued in the LdBPK_061160 mutant by the ectopic expression of both human genes PIG-X and PIG-M, but neither gene could complement the phenotype alone. From further sequence comparisons, we conclude that LdBPK_061160 is the functional orthologue of yeast PBN1 and mammalian PIG-X, which encode the noncatalytic subunits of their respective GPI-MT I complexes, and we assign LdBPK_061160 as LdPBN1. The LdPBN1 mutants could not establish a visceral infection in mice, a phenotype that was rescued by constitutive expression of LdPBN1. Although mice infected with the null mutant did not develop an infection, exposure to these parasites provided significant protection against subsequent infection with a virulent strain. In summary, we have identified the orthologue of the PBN1/PIG-X noncatalytic subunit of GPI-MT I in trypanosomatids, shown that it is essential for infection in a murine model of visceral leishmaniasis, and demonstrated that the LdPBN1 mutant shows promise for the development of an attenuated live vaccine.

Persistence of Trypanosoma brucei as early procyclic forms and social motility are dependent on glycosylphosphatidylinositol transamidase

Glycosylphosphatidylinositol (GPI)‐linked molecules are surface‐exposed membrane components that influence the infectivity, virulence and transmission of many eukaryotic pathogens. Procyclic (insect midgut) forms of Trypanosoma brucei do not require GPI‐anchored proteins for growth in suspension culture. Deletion of TbGPI8, and inactivation of the GPI:protein transamidase complex, is tolerated by cultured procyclic forms. Using a conditional knockout, we show TbGPI8 is required for social motility (SoMo). This collective migration by cultured early procyclic forms has been linked to colonization of the tsetse fly digestive tract. The SoMo‐negative phenotype was observed after a lag phase with respect to loss of TbGPI8 and correlated with an unexpectedly slow loss of procyclins, the major GPI‐anchored proteins. Procyclins are not essential for SoMo, however, suggesting a requirement for at least one other GPI‐anchored protein. Loss of TbGPI8 initiates the transition from early to late procyclic forms; this effect was observed in a subpopulation in suspension culture, and was more pronounced when cells were cultured on SoMo plates. Our results indicate two, potentially interlinked, scenarios that may explain the previously reported failure of TbGPI8 deletion mutants to establish a midgut infection in the tsetse fly: interference with stage‐specific gene expression and absence of SoMo.

Identification of TbPBN1 in Trypanosoma brucei reveals a conserved heterodimeric architecture for glycosylphosphatidylinositol-mannosyltransferase-I

Glycosylphosphatidylinositol (GPI)‐anchored proteins are found in all eukaryotes and are especially abundant on the surface of protozoan parasites such as Trypanosoma brucei. GPI‐mannosyltransferase‐I (GPI‐MT‐I) catalyzes the addition of the first of three mannoses that make up the glycan core of GPI. Mammalian and yeast GPI‐MT‐I consist of two essential subunits, the catalytic subunit PIG‐M/Gpi14 and the accessory subunit PIG‐X/Pbn1(mammals/yeast). T. brucei GPI‐MT‐I has been highlighted as a potential antitrypanosome drug target but has not been fully characterized. Here, we show that T. brucei GPI‐MT‐I also has two subunits, TbGPI14 and TbPBN1. Using TbGPI14 deletion, and TbPBN1 RNAi‐mediated depletion, we show that both proteins are essential for the mannosyltransferase activity needed for GPI synthesis and surface expression of GPI‐anchored proteins. In addition, using native PAGE and co‐immunoprecipitation analyses, we demonstrate that TbGPI14 and TbPBN1 interact to form a higher‐order complex. Finally, we show that yeast Gpi14 does not restore GPI‐MT‐I function in TbGPI14 knockout trypanosomes, consistent with previously demonstrated species specificity within GPI‐MT‐I subunit associations. The identification of an essential trypanosome GPI‐MT‐I subcomponent indicates wide conservation of the heterodimeric architecture unusual for a glycosyltransferase, leaving open the question of the role of the noncatalytic TbPBN1 subunit in GPI‐MT‐I function.

The Glycosylphosphatidylinositol Anchor: A Linchpin for Cell Surface Versatility of Trypanosomatids

The use of glycosylphosphatidylinositol (GPI) to anchor proteins to the cell surface is widespread among eukaryotes. The GPI-anchor is covalently attached to the C-terminus of a protein and mediates the protein’s attachment to the outer leaflet of the lipid bilayer. GPI-anchored proteins have a wide range of functions, including acting as receptors, transporters, and adhesion molecules. In unicellular eukaryotic parasites, abundantly expressed GPI-anchored proteins are major virulence factors, which support infection and survival within distinct host environments. While, for example, the variant surface glycoprotein (VSG) is the major component of the cell surface of the bloodstream form of African trypanosomes, procyclin is the most abundant protein of the procyclic form which is found in the invertebrate host, the tsetse fly vector. Trypanosoma cruzi, on the other hand, expresses a variety of GPI-anchored molecules on their cell surface, such as mucins, that interact with their hosts. The latter is also true for Leishmania, which use GPI anchors to display, amongst others, lipophosphoglycans on their surface. Clearly, GPI-anchoring is a common feature in trypanosomatids and the fact that it has been maintained throughout eukaryote evolution indicates its adaptive value. Here, we explore and discuss GPI anchors as universal evolutionary building blocks that support the great variety of surface molecules of trypanosomatids.

An essential, kinetoplastid-specific GDP-Fuc: β-D-Gal α-1,2-fucosyltransferase is located in the mitochondrion of Trypanosoma brucei

Fucose is a common component of eukaryotic cell-surface glycoconjugates, generally added by Golgi-resident fucosyltransferases. Whereas fucosylated glycoconjugates are rare in kinetoplastids, the biosynthesis of the nucleotide sugar GDP-Fuc has been shown to be essential in Trypanosoma brucei. Here we show that the single identifiable T. brucei fucosyltransferase (TbFUT1) is a GDP-Fuc: β-D-galactose α-1,2-fucosyltransferase with an apparent preference for a Galβ1,3GlcNAcβ1-O-R acceptor motif. Conditional null mutants of TbFUT1 demonstrated that it is essential for both the mammalian-infective bloodstream form and the insect vector-dwelling procyclic form. Unexpectedly, TbFUT1 was localized in the mitochondrion of T. brucei and found to be required for mitochondrial function in bloodstream form trypanosomes. Finally, the TbFUT1 gene was able to complement a Leishmania major mutant lacking the homologous fucosyltransferase gene (Guo et al., 2021). Together these results suggest that kinetoplastids possess an unusual, conserved and essential mitochondrial fucosyltransferase activity that may have therapeutic potential across trypanosomatids.

Elimination of GPI2 suppresses glycosylphosphatidylinositol GlcNAc transferase activity and alters GPI glycan modification in Trypanosoma brucei

Many eukaryotic cell-surface proteins are post-translationally modified by a glycosylphosphatidylinositol (GPI) moiety that anchors them to the cell membrane. The biosynthesis of GPI anchors is initiated in the endoplasmic reticulum by transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol. This reaction is catalyzed by GPI GlcNAc transferase, a multisubunit complex comprising the catalytic subunit Gpi3/PIG-A as well as at least five other subunits, including the hydrophobic protein Gpi2, which is essential for the activity of the complex in yeast and mammals, but the function of which is not known. To investigate the role of Gpi2, we exploited Trypanosoma brucei (Tb), an early diverging eukaryote and important model organism that initially provided the first insights into GPI structure and biosynthesis. We generated insect-stage (procyclic) trypanosomes that lack TbGPI2 and found that in TbGPI2-null parasites, (i) GPI GlcNAc transferase activity is reduced, but not lost, in contrast with yeast and human cells, (ii) the GPI GlcNAc transferase complex persists, but its architecture is affected, with loss of at least the TbGPI1 subunit, and (iii) the GPI anchors of procyclins, the major surface proteins, are underglycosylated when compared with their WT counterparts, indicating the importance of TbGPI2 for reactions that occur in the Golgi apparatus. Immunofluorescence microscopy localized TbGPI2 not only to the endoplasmic reticulum but also to the Golgi apparatus, suggesting that in addition to its expected function as a subunit of the GPI GlcNAc transferase complex, TbGPI2 may have an enigmatic noncanonical role in Golgi-localized GPI anchor modification in trypanosomes.

Proteomic identification of the UDP-GlcNAc: PI α1-6 GlcNAc-transferase subunits of the glycosylphosphatidylinositol biosynthetic pathway of Trypanosoma brucei

The first step of glycosylphosphatidylinositol (GPI) anchor biosynthesis in all eukaryotes is the addition of N-acetylglucosamine (GlcNAc) to phosphatidylinositol (PI) which is catalysed by a UDP-GlcNAc: PI α1-6 GlcNAc-transferase, also known as GPI GnT. This enzyme has been shown to be a complex of seven subunits in mammalian cells and a similar complex of six homologous subunits has been postulated in yeast. Homologs of these mammalian and yeast subunits were identified in the Trypanosoma brucei predicted protein database. The putative catalytic subunit of the T. brucei complex, TbGPI3, was epitope tagged with three consecutive c-Myc sequences at its C-terminus. Immunoprecipitation of TbGPI3-3Myc followed by native polyacrylamide gel electrophoresis and anti-Myc Western blot showed that it is present in a ~240 kDa complex. Label-free quantitative proteomics were performed to compare anti-Myc pull-downs from lysates of TbGPI-3Myc expressing and wild type cell lines. TbGPI3-3Myc was the most highly enriched protein in the TbGPI3-3Myc lysate pull-down and the expected partner proteins TbGPI15, TbGPI19, TbGPI2, TbGPI1 and TbERI1 were also identified with significant enrichment. Our proteomics data also suggest that an Arv1-like protein (TbArv1) is a subunit of the T. brucei complex. Yeast and mammalian Arv1 have been previously implicated in GPI biosynthesis, but here we present the first experimental evidence for physical association of Arv1 with GPI biosynthetic machinery. A putative E2-ligase has also been tentatively identified as part of the T. brucei UDP-GlcNAc: PI α1-6 GlcNAc-transferase complex.

Complexity of the eukaryotic dolichol-linked oligosaccharide scramblase suggested by activity correlation profiling mass spectrometry

The oligosaccharide required for asparagine (N)-linked glycosylation of proteins in the endoplasmic reticulum (ER) is donated by the glycolipid Glc₃Man₉GlcNAc₂-PP-dolichol. Remarkably, whereas glycosylation occurs in the ER lumen, the initial steps of Glc₃Man₉GlcNAc₂-PP-dolichol synthesis generate the lipid intermediate Man₅GlcNAc₂-PP-dolichol (M5-DLO) on the cytoplasmic side of the ER. Glycolipid assembly is completed only after M5-DLO is translocated to the luminal side. The membrane protein (M5-DLO scramblase) that mediates M5-DLO translocation across the ER membrane has not been identified, despite its importance for N-glycosylation. Building on our ability to recapitulate scramblase activity in proteoliposomes reconstituted with a crude mixture of ER membrane proteins, we developed a mass spectrometry-based 'activity correlation profiling' approach to identify scramblase candidates in the yeast Saccharomyces cerevisiae. Data curation prioritized six polytopic ER membrane proteins as scramblase candidates, but reconstitution-based assays and gene disruption in the protist Trypanosoma brucei revealed, unexpectedly, that none of these proteins is necessary for M5-DLO scramblase activity. Our results instead strongly suggest that M5-DLO scramblase activity is due to a protein, or protein complex, whose activity is regulated at the level of quaternary structure.

Bridging the GAPs in plant reproduction: a comparison of plant and animal GPI-anchored proteins

Glycosylphosphatidylinositol (GPI)-anchored proteins (GAPs) are a unique type of membrane-associated proteins in eukaryotes. GPI and GAP biogenesis and function have been well studied in non-plant models and play an important role in the fertility of mouse sperm and egg. Although GPI and GAP biogenesis and function in plants are less known, they are critical for flowering plant reproduction because of their essential roles in the fertility of the male and female gametophytes. In Eukaryotes, GPI, a glycolipid molecule, can be post-translationally attached to proteins to serve as an anchor in the plasma membrane. GPI-anchoring, compared to other modes of membrane attachment and lipidation processes, localizes proteins to the extracellular portion of the plasma membrane and confers several unique attributes including specialized sorting during secretion, molecular painting onto membranes, and enzyme-mediated release of protein through anchor cleavage. While the biosynthesis, structure, and role of GPI are mostly studied in mammals, yeast and protists, the function of GPI and GAPs in plants is being discovered, particularly in gametophyte development and function. Here, we review GPI biosynthesis, protein attachment, and remodeling in plants with insights about this process in mammals. Additionally, we summarize the reproductive phenotypes of all loss of function mutations in Arabidopsis GPI biosynthesis and GAP genes and compare these to the reproductive phenotypes seen in mice to serve as a framework to identify gaps in our understanding of plant GPI and GAPs. In addition, we present an analysis on the gametophyte expression of all Arabidopsis GAPs to assist in further research on the role of GPI and GAPs in all aspects of the gametophyte generation in the life cycle of a plant.

CDP-diacylglycerol, a critical intermediate in lipid metabolism

The roles of lipids expand beyond the basic building blocks of biological membranes. In addition to forming complex and dynamic barriers, the thousands of different lipid species in the cell contribute to essentially all the processes of life. Specific lipids are increasingly identified in cellular processes, including signal transduction, membrane trafficking, metabolic control and protein regulation. Tight control of their synthesis and degradation is essential for homeostasis. Most of the lipid molecules in the cell originate from a small number of critical intermediates. Thus, regulating the synthesis of intermediates is essential for lipid homeostasis and optimal biological functions. Cytidine diphosphate diacylglycerol (CDP-DAG) is an intermediate which occupies a branch point in lipid metabolism. CDP-DAG is incorporated into different synthetic pathways to form distinct phospholipid end-products depending on its location of synthesis. Identification and characterization of CDP-DAG synthases which catalyze the synthesis of CDP-DAG has been hampered by difficulties extracting these membrane-bound enzymes for purification. Recent developments have clarified the cellular localization of the CDP-DAG synthases and identified a new unrelated CDP-DAG synthase enzyme. These findings have contributed to a deeper understanding of the extensive synthetic and signaling networks stemming from this key lipid intermediate.

Protozoan Parasites Glycosylphosphatidylinositol Anchors: Structures, Functions and Trends for Drug Discovery

BACKGROUND

Glycosylphosphatidylinositol (GPI) anchors are molecules located on cell membranes of all eukaryotic organisms. Proteins, enzymes, and other macromolecules which are anchored by GPIs are essential elements for interaction between cells, and are widely used by protozoan parasites when compared to higher eukaryotes.

METHODS

More than one hundred references were collected to obtain broad information about mammalian and protozoan parasites' GPI structures, biosynthetic pathways, functions and attempts to use these molecules as drug targets against parasitic diseases. Differences between GPI among species were compared and highlighted. Strategies for drug discovery and development against protozoan GPI anchors were discussed based on what has been reported on literature.

RESULTS

There are many evidences that GPI anchors are crucial for parasite's survival and interaction with hosts' cells. Despite all GPI anchors contain a conserved glycan core, they present variations regarding structural features and biosynthetic pathways between organisms, which could offer adequate selectivity to validate GPI anchors as drug targets. Discussion was developed with focus on the following parasites: Trypanosoma brucei, Trypanosoma cruzi, Leishmania, Plasmodium falciparum and Toxoplasma gondii, causative agents of tropical neglected diseases.

CONCLUSION

This review debates the main variances between parasitic and mammalian GPI anchor biosynthesis and structures, as well as clues for strategic development for new anti-parasitic therapies based on GPI anchors.

Trypanosoma cruzi Phosphomannomutase and Guanosine Diphosphate-Mannose Pyrophosphorylase Ligandability Assessment

Chagas’ disease, which is caused by the Trypanosoma cruzi parasite, has become a global health problem that is currently treated with poorly tolerated drugs that require prolonged dosing. Therefore, there is a clinical need for new therapeutic agents that can mitigate these issues. The phosphomannomutase (PMM) and GDP-mannose pyrophosphorylase (GDP-MP) enzymes form part of the de novo biosynthetic pathway to the nucleotide sugar GDP-mannose.

Chagas’ disease, which is caused by the Trypanosoma cruzi parasite, has become a global health problem that is currently treated with poorly tolerated drugs that require prolonged dosing. Therefore, there is a clinical need for new therapeutic agents that can mitigate these issues. The phosphomannomutase (PMM) and GDP-mannose pyrophosphorylase (GDP-MP) enzymes form part of the de novo biosynthetic pathway to the nucleotide sugar GDP-mannose. This nucleotide sugar is used either directly, or indirectly via the formation of dolichol-phosphomannose, for the assembly of all mannose-containing glycoconjugates. In T. cruzi, mannose-containing glycoconjugates include the cell-surface glycoinositol-phospholipids and the glycosylphosphatidylinositol-anchored mucin-like glycoproteins that dominate the cell surface architectures of all life cycle stages. This makes PMM and GDP-MP potentially attractive targets for a drug discovery program against Chagas’ disease. To assess the ligandability of these enzymes in T. cruzi, we have screened 18,117 structurally diverse compounds exploring drug-like chemical space and 16,845 small polar fragment compounds using an assay interrogating the activities of both PMM and GDP-MP enzymes simultaneously. This resulted in 48 small fragment hits, and on retesting 20 were found to be active against the enzymes. Deconvolution revealed that these were all inhibitors of T. cruzi GDP-MP, with compounds 2 and 3 acting as uncompetitive and competitive inhibitors, respectively. Based on these findings, the T. cruzi PMM and GDP-MP enzymes were deemed not ligandable and poorly ligandable, respectively, using small molecules from conventional drug discovery chemical space. This presents a significant hurdle to exploiting these enzymes as therapeutic targets for Chagas’ disease.

Nanoscale analysis reveals no domain formation of glycosylphosphatidylinositol-anchored protein SAG1 in the plasma membrane of living Toxoplasma gondii

Glycosylphosphatidylinositol (GPI)-anchored proteins typically localise to lipid rafts. GPI-anchored protein microdomains may be present in the plasma membrane; however, they have been studied using heterogeneously expressed GPI-anchored proteins, and the two-dimensional distributions of endogenous molecules in the plasma membrane are difficult to determine at the nanometre scale. Here, we used immunoelectron microscopy using a quick-freezing and freeze-fracture labelling (QF-FRL) method to examine the distribution of the endogenous GPI-anchored protein SAG1 in Toxoplasma gondii at the nanoscale. QF-FRL physically immobilised molecules in situ, minimising the possibility of artefactual perturbation. SAG1 labelling was observed in the exoplasmic, but not cytoplasmic, leaflets of T. gondii plasma membrane, whereas none was detected in any leaflet of the inner membrane complex. Point pattern analysis of SAG1 immunogold labelling revealed mostly random distribution in T. gondii plasma membrane. The present method obtains information on the molecular distribution of natively expressed GPI-anchored proteins and demonstrates that SAG1 in T. gondii does not form significant microdomains in the plasma membrane.

In-depth analysis of the genome of Trypanosoma evansi, an etiologic agent of surra

Trypanosoma evansi is the causative agent of the animal trypanosomiasis surra, a disease with serious economic burden worldwide. The availability of the genome of its closely related parasite Trypanosoma brucei allows us to compare their genetic and evolutionarily shared and distinct biological features. The complete genomic sequence of the T. evansi YNB strain was obtained using a combination of genomic and transcriptomic sequencing, de novo assembly, and bioinformatic analysis. The genome size of the T. evansi YNB strain was 35.2 Mb, showing 96.59% similarity in sequence and 88.97% in scaffold alignment with T. brucei. A total of 8,617 protein-coding genes, accounting for 31% of the genome, were predicted. Approximately 1,641 alternative splicing events of 820 genes were identified, with a majority mediated by intron retention, which represented a major difference in post-transcriptional regulation between T. evansi and T. brucei. Disparities in gene copy number of the variant surface glycoprotein, expression site-associated genes, microRNAs, and RNA-binding protein were clearly observed between the two parasites. The results revealed the genomic determinants of T. evansi, which encoded specific biological characteristics that distinguished them from other related trypanosome species.

The natural anticancer agent cantharidin alters GPI-anchored protein sorting by targeting Cdc1-mediated remodeling in endoplasmic reticulum

Cantharidin (CTD) is a potent anticancer small molecule produced by several species of blister beetle. It has been a traditional medicine for the management of warts and tumors for many decades. CTD suppresses tumor growth by inducing apoptosis, cell cycle arrest, and DNA damage and inhibits protein phosphatase 2 phosphatase activator (PP2A) and protein phosphatase 1 (PP1). CTD also alters lipid homeostasis, cell wall integrity, endocytosis, adhesion, and invasion in yeast cells. In this study, we identified additional molecular targets of CTD using a Saccharomyces cerevisiae strain that expresses a cantharidin resistance gene (CRG1), encoding a SAM-dependent methyltransferase that methylates and inactivates CTD. We found that CTD specifically affects phosphatidylethanolamine (PE)-associated functions that can be rescued by supplementing the growth media with ethanolamine (ETA). CTD also perturbed endoplasmic reticulum (ER) homeostasis and cell wall integrity by altering the sorting of glycosylphosphatidylinositol (GPI)-anchored proteins. A CTD-dependent genetic interaction profile of CRG1 revealed that the activity of the lipid phosphatase cell division control protein 1 (Cdc1) in GPI-anchor remodeling is the key target of CTD, independently of PP2A and PP1 activities. Moreover, experiments with human cells further suggested that CTD functions through a conserved mechanism in higher eukaryotes. Altogether, we conclude that CTD induces cytotoxicity by targeting Cdc1 activity in GPI-anchor remodeling in the ER.

Discovery and Genetic Validation of Chemotherapeutic Targets for Chagas' Disease

There is an urgent need to develop new treatments for Chagas' disease. To identify drug targets, it is important to understand the basic biology of Trypanosoma cruzi, in particular with respect to the biological pathways or proteins that are essential for its survival within the host. This review provides a streamlined approach for identifying drug targets using freely available chemogenetic databases and outlines the relevant characteristics of an ideal chemotherapeutic target. Among those are their essentiality, druggability, availability of structural information, and selectivity. At the moment only 16 genes have been found as essential by gene disruption in T. cruzi. At the TDR Targets database, a chemogenomics resource for neglected diseases, information about published structures for these genes was only found for three of these genes, and annotation of validated inhibitors was found in two. These inhibitors have activity against the parasitic stages present in the host. We then analyzed three of the pathways that are considered promising in the search for new targets: (1) Ergosterol biosynthesis, (2) Resistance to oxidative stress, (3) Synthesis of surface glycoconjugates. We have annotated all the genes that participate in them, identified those that are considered as druggable, and incorporated evidence from either Trypanosoma brucei, and Leishmania spp. that supports the hypothesis that these pathways are essential for T. cruzi survival.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

A biocatalytic approach towards the stereoselective synthesis of protected inositols

Immobilized TbINO1 produces >400 mg of chiral inositol 1-phosphate in a biocatalytic flow process.

A Mutation in the Catalytic Subunit of the Glycosylphosphatidylinositol Transamidase Disrupts Growth, Fertility, and Stomata Formation

GPI-anchored proteins (GPI-APs) are essential for plant growth and development; knockout mutations in enzymes responsible for anchor biosynthesis or attachment are gametophyte or embryo lethal. In a genetic screen targeted to identify genes regulating stomata formation, we discovered a missense mutation in the Arabidopsis (Arabidopsis thaliana) homolog of GPI8/PIG-K, a Cys protease that transfers an assembled GPI anchor to proteins. The Arabidopsis genome has a single copy of AtGPI8, and the atgpi8-1 mutation reduces the efficiency of this enzyme, leading to reduced accumulation of GPI-anchored proteins. While the atgpi8-1 mutation strongly disrupts plant growth, it is not lethal. Phenotypic analysis of atgpi8-1 mutants suggests that GPI-APs are important for root and shoot growth, stomata formation, apical dominance, transition to flowering, and male gametophyte viability. In addition, atgpi8-1 mutants accumulate higher levels of callose and have reduced plasmodesmata permeability. Genetic interactions of atgpi8-1 with mutations in ERECTA family (ERf) genes suggest the existence of a GPI-AP in a branch of the ERf signaling pathway that regulates stomata formation. Activation of the ERf signal transduction cascade by constitutively active YODA rescues stomata clustering in atgpi8-1, indicating that a GPI-AP functions upstream of the MAP kinase cascade. TOO MANY MOUTHS (TMM) is a receptor-like protein that is able to form heterodimers with ERfs. Our analysis demonstrates that tmm-1 is epistatic to atgpi8-1, indicating that either TMM is a GPI-AP or there is another GPI-AP regulating stomata development whose function is dependent upon TMM.

The Glycerol-3-Phosphate Acyltransferase TbGAT is Dispensable for Viability and the Synthesis of Glycerolipids in Trypanosoma brucei

Glycerolipids are the main constituents of biological membranes in Trypanosoma brucei, which causes sleeping sickness in humans. Importantly, they occur as a structural component of the glycosylphosphatidylinositol lipid anchor of the abundant cell surface glycoproteins procyclin in procyclic forms and variant surface glycoprotein in bloodstream form, that play crucial roles for the development of the parasite in the insect vector and the mammalian host, respectively. The present work reports the characterization of the glycerol-3-phosphate acyltransferase TbGAT that initiates the biosynthesis of ester glycerolipids. TbGAT restored glycerol-3-phosphate acyltransferase activity when expressed in a Leishmania major deletion strain lacking this activity and exhibited preference for medium length, unsaturated fatty acyl-CoAs. TbGAT localized to the endoplasmic reticulum membrane with its N-terminal domain facing the cytosol. Despite that a TbGAT null mutant in T. brucei procyclic forms lacked glycerol-3-phosphate acyltransferase activity, it remained viable and exhibited similar growth rate as the wild type. TbGAT was dispensable for the biosynthesis of phosphatidylcholine, phosphatidylinositol, phosphatidylserine, and GPI-anchored protein procyclin. However, the null mutant exhibited a slight decrease in phosphatidylethanolamine biosynthesis that was compensated with a modest increase in production of ether phosphatidylcholine. Our data suggest that an alternative initial acyltransferase takes over TbGAT's function in its absence.

Transcriptional Profiling the 150 kb Linear Megaplasmid of Borrelia turicatae Suggests a Role in Vector Colonization and Initiating Mammalian Infection

Adaptation is key for survival as vector-borne pathogens transmit between the arthropod and vertebrate, and temperature change is an environmental signal inducing alterations in gene expression of tick-borne spirochetes. While plasmids are often associated with adaptation, complex genomes of relapsing fever spirochetes have hindered progress in understanding the mechanisms of vector colonization and transmission. We utilized recent advances in genome sequencing to generate the most complete version of the Borrelia turicatae 150 kb linear megaplasmid (lp150). Additionally, a transcriptional analysis of open reading frames (ORFs) in lp150 was conducted and identified regions that were up-regulated during in vitro cultivation at tick-like growth temperatures (22°C), relative to bacteria grown at 35°C and infected murine blood. Evaluation of the 3’ end of lp150 identified a cluster of ORFs that code for putative surface lipoproteins. With a microbe’s surface proteome serving important roles in pathogenesis, we confirmed the ORFs expression in vitro and in the tick compared to spirochetes infecting murine blood. Transcriptional evaluation of lp150 indicates the plasmid likely has essential roles in vector colonization and/or initiating mammalian infection. These results also provide a much needed transcriptional framework to delineate the molecular mechanisms utilized by relapsing fever spirochetes during their enzootic cycle.

In silico drug re-purposing against African sleeping sickness using GlcNAc-PI de-N-acetylase as an experimental target

Trypanosoma brucei is a protozoan that causes African sleeping sickness in humans. Many glycoconjugate compounds are present on the entire cell surface of Trypanosoma brucei to control the infectivity and survival of this pathogen. These gycoconjugates are anchored to the plasma membrane with the help of glycosyl phosphatidyl inositol (GPI) anchors. This type of anchor is much more common in protozoans than in other eukaryotes. The second step of glycosyl phosphatidyl inositol (GPI) anchor biosynthesis is catalyzed by an enzyme, which is GlcNAc-PI de-N-acetylase. GlcNAc-PI de-N-acetylase has a conserved GPI domain, which is responsible for the functionality of this enzyme. In this study, the three-dimensional structure of the target is modelled by I-TASSER and the ligand is modelled by PRODRG server. It is found that the predicted active site residues of the GPI domain are ultra-conserved for the Trypanosomatidae family. The predicted active site residues are His41, Pro42, Asp43, Asp44, Met47, Phe48, Ser74, Arg80, His103, Val144, Ser145, His147 and His150. Two hydrogen bond acceptors and four hydrogen bond donors are found in the modelled pharmacophore. All compounds of the Drugbank database and twenty three known inhibitors have been considered for structure based virtual screening. This work is focused on approved drugs because they are already tested for safety and effectiveness in humans. After the structure-based virtual screening, seventeen approved drugs and two inhibitors are found, which interact with the ligand on the basis of the designed pharmacophore. The docking has been performed for the resultant seventeen approved drugs and two known inhibitors. Two approved drugs have negative binding energy and their pKa values are similar to the selected known inhibitors. The result of this study suggests that the approved drugs Ethambutol (DB00330) and Metaraminol (DB00610) may prove useful in the treatment of African sleeping sickness.

GPI-anchored protein organization and dynamics at the cell surface

The surface of eukaryotic cells is a multi-component fluid bilayer in which glycosylphosphatidylinositol (GPI)-anchored proteins are an abundant constituent. In this review, we discuss the complex nature of the organization and dynamics of GPI-anchored proteins at multiple spatial and temporal scales. Different biophysical techniques have been utilized for understanding this organization, including fluorescence correlation spectroscopy, fluorescence recovery after photobleaching, single particle tracking, and a number of super resolution methods. Major insights into the organization and dynamics have also come from exploring the short-range interactions of GPI-anchored proteins by fluorescence (or Förster) resonance energy transfer microscopy. Based on the nanometer to micron scale organization, at the microsecond to the second time scale dynamics, a picture of the membrane bilayer emerges where the lipid bilayer appears inextricably intertwined with the underlying dynamic cytoskeleton. These observations have prompted a revision of the current models of plasma membrane organization, and suggest an active actin-membrane composite.

Assembly of a Complex Branched Oligosaccharide by Combining Fluorous-Supported Synthesis and Stereoselective Glycosylations using Anomeric Sulfonium Ions

There is an urgent need to develop reliable strategies for the rapid assembly of complex oligosaccharides. This paper presents a set of strategically selected orthogonal protecting groups, glycosyl donors modified by a (S)-phenylthiomethylbenzyl ether at C-2, and a glycosyl acceptor containing a fluorous tag, which makes it possible to rapidly prepare complex branched oligosaccharides of biological importance. The C-2 auxiliary controlled the 1,2-cis anomeric selectivity of the various galactosylations. The orthogonal protecting groups, 2-naphthylmethyl ether (Nap) and levulinic ester (Lev), made it possible to generate glycosyl acceptors and allowed the installation of a crowded branching point. After the glycosylations, the chiral auxiliary could be removed using acidic conditions, which was compatible with the presence of the orthogonal protecting groups Lev and Nap, thereby allowing the efficient installation of 1,2-linked glycosides. The light fluorous tag made it possible to purify the compounds by a simple filtration method using silica gel modified by fluorocarbons. The set of building blocks was successfully employed for the preparation of the carbohydrate moiety of the GPI anchor of Trypanosoma brucei, which is a parasite that causes sleeping sickness in humans and similar diseases in domestic animals.

Evaluation of aegerolysins as novel tools to detect and visualize ceramide phosphoethanolamine, a major sphingolipid in invertebrates

Ceramide phosphoethanolamine (CPE), a sphingomyelin analog, is a major sphingolipid in invertebrates and parasites, whereas only trace amounts are present in mammalian cells. In this study, mushroom-derived proteins of the aegerolysin family—pleurotolysin A2 (PlyA2; K(D) = 12 nM), ostreolysin (Oly; K(D) = 1.3 nM), and erylysin A (EryA; K(D) = 1.3 nM)—strongly associated with CPE/cholesterol (Chol)-containing membranes, whereas their low affinity to sphingomyelin/Chol precluded establishment of the binding kinetics. Binding specificity was determined by multilamellar liposome binding assays, supported bilayer assays, and solid-phase studies against a series of neutral and negatively charged lipid classes mixed 1:1 with Chol or phosphatidylcholine. No cross-reactivity was detected with phosphatidylethanolamine. Only PlyA2 also associated with CPE, independent of Chol content (K(D) = 41 μM), rendering it a suitable tool for visualizing CPE in lipid-blotting experiments and biologic samples from sterol auxotrophic organisms. Visualization of CPE enrichment in the CNS of Drosophila larvae (by PlyA2) and in the bloodstream form of the parasite Trypanosoma brucei (by EryA) by fluorescence imaging demonstrated the versatility of aegerolysin family proteins as efficient tools for detecting and visualizing CPE.

The essential roles of cytidine diphosphate-diacylglycerol synthase in bloodstream form Trypanosoma brucei

Lipid metabolism in Trypanosoma brucei, the causative agent of African sleeping sickness, differs from its human host in several fundamental ways. This has lead to the validation of a plethora of novel drug targets, giving hope of novel chemical intervention against this neglected disease. Cytidine diphosphate diacylglycerol (CDP‐DAG) is a central lipid intermediate for several pathways in both prokaryotes and eukaryotes, being produced by CDP‐DAG synthase (CDS). However, nothing is known about the single T. brucei CDS gene (Tb927.7.220/EC 2.7.7.41) or its activity. In this study we show TbCDS is functional by complementation of a non‐viable yeast CDS null strain and that it is essential in the bloodstream form of the parasite via a conditional knockout. The TbCDS conditional knockout showed morphological changes including a cell‐cycle arrest due in part to kinetoplast segregation defects. Biochemical phenotyping of TbCDS conditional knockout showed drastically altered lipid metabolism where reducing levels of phosphatidylinositol detrimentally impacted on glycoylphosphatidylinositol biosynthesis. These studies also suggest that phosphatidylglycerol synthesized via the phosphatidylglycerol‐phosphate synthase is not synthesized from CDP‐DAG, as was previously thought. TbCDS was shown to localized the ER and Golgi, probably to provide CDP‐DAG for the phosphatidylinositol synthases.

Identification and functional characterization of a highly divergent N-acetylglucosaminyltransferase I (TbGnTI) in Trypanosoma brucei

Trypanosoma brucei expresses a diverse repertoire of N-glycans, ranging from oligomannose and paucimannose structures to exceptionally large complex N-glycans. Despite the presence of the latter, no obvious homologues of known β1–4-galactosyltransferase or β1–2- or β1–6-N-acetylglucosaminyltransferase genes have been found in the parasite genome. However, we previously reported a family of putative UDP-sugar-dependent glycosyltransferases with similarity to the mammalian β1–3-glycosyltransferase family. Here we characterize one of these genes, TbGT11, and show that it encodes a Golgi apparatus resident UDP-GlcNAc:α3-d-mannoside β1–2-N-acetylglucosaminyltransferase I activity (TbGnTI). The bloodstream-form TbGT11 null mutant exhibited significantly modified protein N-glycans but normal growth in vitro and infectivity to rodents. In contrast to multicellular organisms, where the GnTI reaction is essential for biosynthesis of both complex and hybrid N-glycans, T. brucei TbGT11 null mutants expressed atypical “pseudohybrid” glycans, indicating that TbGnTII activity is not dependent on prior TbGnTI action. Using a functional in vitro assay, we showed that TbGnTI transfers UDP-GlcNAc to biantennary Man₃GlcNAc₂, but not to triantennary Man₅GlcNAc₂, which is the preferred substrate for metazoan GnTIs. Sequence alignment reveals that the T. brucei enzyme is far removed from the metazoan GnTI family and suggests that the parasite has adapted the β3-glycosyltransferase family to catalyze β1–2 linkages.

Fragment screening reveals salicylic hydroxamic acid as an inhibitor of Trypanosoma brucei GPI GlcNAc-PI de-N-acetylase

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First non-substrate analogue inhibitor of the trypanosome GPI pathway.

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Active against recombinant enzyme and cell-free system.

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Low molecular weight and good ligand efficiency.

The zinc-metalloenzyme GlcNAc-PI de-N-acetylase is essential for the biosynthesis of mature GPI anchors and has been genetically validated in the bloodstream form of Trypanosoma brucei, which causes African sleeping sickness. We screened a focused library of zinc-binding fragments and identified salicylic hydroxamic acid as a GlcNAc-PI de-N-acetylase inhibitor with high ligand efficiency. This is the first small molecule inhibitor reported for the trypanosome GPI pathway. Investigating the structure activity relationship revealed that hydroxamic acid and 2-OH are essential for potency, and that substitution is tolerated at the 4- and 5-positions.

A novel allosteric inhibitor of the uridine diphosphate N-acetylglucosamine pyrophosphorylase from Trypanosoma brucei

Uridine diphosphate N-acetylglucosamine pyrophosphorylase (UAP) catalyzes the final reaction in the biosynthesis of UDP-GlcNAc, an essential metabolite in many organisms including Trypanosoma brucei, the etiological agent of Human African Trypanosomiasis. High-throughput screening of recombinant T. brucei UAP identified a UTP-competitive inhibitor with selectivity over the human counterpart despite the high level of conservation of active site residues. Biophysical characterization of the UAP enzyme kinetics revealed that the human and trypanosome enzymes both display a strictly ordered bi–bi mechanism, but with the order of substrate binding reversed. Structural characterization of the T. brucei UAP–inhibitor complex revealed that the inhibitor binds at an allosteric site absent in the human homologue that prevents the conformational rearrangement required to bind UTP. The identification of a selective inhibitory allosteric binding site in the parasite enzyme has therapeutic potential.

Identification and functional analysis of Trypanosoma cruzi genes that encode proteins of the glycosylphosphatidylinositol biosynthetic pathway

Background

Trypanosoma cruzi is a protist parasite that causes Chagas disease. Several proteins that are essential for parasite virulence and involved in host immune responses are anchored to the membrane through glycosylphosphatidylinositol (GPI) molecules. In addition, T. cruzi GPI anchors have immunostimulatory activities, including the ability to stimulate the synthesis of cytokines by innate immune cells. Therefore, T. cruzi genes related to GPI anchor biosynthesis constitute potential new targets for the development of better therapies against Chagas disease.

Methodology/Principal Findings

In silico analysis of the T. cruzi genome resulted in the identification of 18 genes encoding proteins of the GPI biosynthetic pathway as well as the inositolphosphorylceramide (IPC) synthase gene. Expression of GFP fusions of some of these proteins in T. cruzi epimastigotes showed that they localize in the endoplasmic reticulum (ER). Expression analyses of two genes indicated that they are constitutively expressed in all stages of the parasite life cycle. T. cruzi genes TcDPM1, TcGPI10 and TcGPI12 complement conditional yeast mutants in GPI biosynthesis. Attempts to generate T. cruzi knockouts for three genes were unsuccessful, suggesting that GPI may be an essential component of the parasite. Regarding TcGPI8, which encodes the catalytic subunit of the transamidase complex, although we were able to generate single allele knockout mutants, attempts to disrupt both alleles failed, resulting instead in parasites that have undergone genomic recombination and maintained at least one active copy of the gene.

Conclusions/Significance

Analyses of T. cruzi sequences encoding components of the GPI biosynthetic pathway indicated that they are essential genes involved in key aspects of host-parasite interactions. Complementation assays of yeast mutants with these T. cruzi genes resulted in yeast cell lines that can now be employed in high throughput screenings of drugs against this parasite.

Chagas disease, considered one of the most neglected tropical diseases, is caused by the blood-borne parasite Trypanosoma cruzi and currently affects about 8 million people in Latin America. T. cruzi can be transmitted by insect vectors, blood transfusion, organ transplantation and mother-to-baby as well as through ingestion of contaminated food. Although T. cruzi causes life-long infections that can result in serious damage to the heart, the two drugs currently available to treat Chagas disease, benznidazole and nifurtimox, which have been used for more than 40 years, have proven efficacy only during the acute phase of the disease. Thus, there is an urgent need to develop new drugs that are more targeted, less toxic, and more effective against this parasite. Here we described the characterization of T. cruzi genes involved in the biosynthesis of GPI anchors, a molecule responsible for holding different types of glycoproteins on the parasite membrane. Since GPI anchored proteins are essential molecules T. cruzi uses during infection, besides helping understand how this parasite interacts with its host, this work may contribute to the development of better therapies against Chagas disease.

Inhibition of nucleotide sugar transport in Trypanosoma brucei alters surface glycosylation

Nucleotide sugar transporters (NSTs) are indispensible for the biosynthesis of glycoproteins by providing the nucleotide sugars needed for glycosylation in the lumen of the Golgi apparatus. Mutations in NST genes cause human and cattle diseases and impaired cell walls of yeast and fungi. Information regarding their function in the protozoan parasite, Trypanosoma brucei, a causative agent of African trypanosomiasis, is unknown. Here, we characterized the substrate specificities of four NSTs, TbNST1-4, which are expressed in both the insect procyclic form (PCF) and mammalian bloodstream form (BSF) stages. TbNST1/2 transports UDP-Gal/UDP-GlcNAc, TbNST3 transports GDP-Man, and TbNST4 transports UDP-GlcNAc, UDP-GalNAc, and GDP-Man. TbNST4 is the first NST shown to transport both pyrimidine and purine nucleotide sugars and is demonstrated here to be localized at the Golgi apparatus. RNAi-mediated silencing of TbNST4 in the procyclic form caused underglycosylated surface glycoprotein EP-procyclin. Similarly, defective glycosylation of the variant surface glycoprotein (VSG221) as well as the lysosomal membrane protein p67 was observed in Δtbnst4 BSF T. brucei. Relative infectivity analysis showed that defects in glycosylation of the surface coat resulting from tbnst4 deletion were insufficient to impact the ability of this parasite to infect mice. Notably, the fact that inactivation of a single NST gene results in measurable defects in surface glycoproteins in different life cycle stages of the parasite highlights the essential role of NST(s) in glycosylation of T. brucei. Thus, results presented in this study provide a framework for conducting functional analyses of other NSTs identified in T. brucei.

Catalysis by N-acetyl-D-glucosaminylphosphatidylinositol de-N-acetylase (PIG-L) from Entamoeba histolytica: new roles for conserved residues

We showed previously that Entamoeba histolytica PIG-L exhibits a novel metal-independent albeit metal-stimulated activity. Using mutational and biochemical analysis, here we identify Asp-46 and His-140 of the enzyme as being important for catalysis. We show that these mutations neither affect the global conformational of the enzyme nor alter its metal binding affinity. The defect in catalysis, due to the mutations, is specifically due to an effect on V(max) and not due to altered substrate affinity (or K(m)). We propose a general acid-base pair mechanism to explain our results.

Chemical biology of glycosylphosphatidylinositol anchors

Glycosylphosphatidylinositols (GPIs) are complex glycolipids that are covalently linked to the C-terminus of proteins as a posttranslational modification. They anchor the attached protein to the cell membrane and are essential for normal functioning of eukaryotic cells. GPI-anchored proteins are structurally and functionally diverse. Many GPIs have been structurally characterized but comprehension of their biological functions, beyond the simple physical anchoring, remains largely speculative. Work on functional elucidation at a molecular level is still limited. This Review focuses on the roles of GPI unraveled by using synthetic molecules and summarizes the structural diversity of GPIs, as well as their biological and chemical syntheses.

Inhibitors incorporating zinc-binding groups target the GlcNAc-PI de-N-acetylase in Trypanosoma brucei, the causative agent of African sleeping sickness

Disruption of glycosylphosphatidylinositol biosynthesis is genetically and chemically validated as a drug target against the protozoan parasite Trypanosoma brucei, the causative agent of African sleeping sickness. The N-acetylglucosamine-phosphatidylinositol de-N-acetylase (deNAc) is a zinc metalloenzyme responsible for the second step of glycosylphosphatidylinositol biosynthesis. We recently reported the synthesis of eight deoxy-2-C-branched monosaccharides containing carboxylic acid, hydroxamic acid, or N-hydroxyurea substituents at the C2 position that may act as zinc-binding groups. Here, we describe the synthesis of a glucocyclitol-phospholipid incorporating a hydroxamic acid moiety and report the biochemical evaluation of the monosaccharides and the glucocyclitol-phospholipid as inhibitors of the trypanosome deNAc in the cell-free system and against recombinant enzyme. Monosaccharides with carboxylic acid or hydroxamic acid substituents were found to be the inhibitors of the trypanosome deNAc with IC₅₀ values 0.1–1.5 mm, and the glucocyclitol-phospholipid was found to be a dual inhibitor of the deNAc and the α1-4-mannose transferase with an apparent IC₅₀ = 19 ± 0.5 μm.

E1210, a new broad-spectrum antifungal, suppresses Candida albicans hyphal growth through inhibition of glycosylphosphatidylinositol biosynthesis

Continued research toward the development of new antifungals that act via inhibition of glycosylphosphatidylinositol (GPI) biosynthesis led to the design of E1210. In this study, we assessed the selectivity of the inhibitory activity of E1210 against Candida albicans GWT1 (Orf19.6884) protein, Aspergillus fumigatus GWT1 (AFUA_1G14870) protein, and human PIG-W protein, which can catalyze the inositol acylation of GPI early in the GPI biosynthesis pathway, and then we assessed the effects of E1210 on key C. albicans virulence factors. E1210 inhibited the inositol acylation activity of C. albicans Gwt1p and A. fumigatus Gwt1p with 50% inhibitory concentrations (IC(50)s) of 0.3 to 0.6 μM but had no inhibitory activity against human Pig-Wp even at concentrations as high as 100 μM. To confirm the inhibition of fungal GPI biosynthesis, expression of ALS1 protein, a GPI-anchored protein, on the surfaces of C. albicans cells treated with E1210 was studied and shown to be significantly lower than that on untreated cells. However, the ALS1 protein levels in the crude extract and the RHO1 protein levels on the cell surface were found to be almost the same. Furthermore, E1210 inhibited germ tube formation, adherence to polystyrene surfaces, and biofilm formation of C. albicans at concentrations above its MIC. These results suggested that E1210 selectively inhibited inositol acylation of fungus-specific GPI which would be catalyzed by Gwt1p, leading to the inhibition of GPI-anchored protein maturation, and also that E1210 suppressed the expression of some important virulence factors of C. albicans, through its GPI biosynthesis inhibition.

Identification of a second catalytically active trans-sialidase in Trypanosoma brucei

The procyclic stage of Trypanosoma brucei is covered by glycosylphosphatidylinositol (GPI)-anchored surface proteins called procyclins. The procyclin GPI anchor contains a side chain of N-acetyllactosamine repeats terminated by sialic acids. Sialic acid modification is mediated by trans-sialidases expressed on the parasite's cell surface. Previous studies suggested the presence of more than one active trans-sialidases, but only one has so far been reported. Here we cloned and examined enzyme activities of four additional trans-sialidase homologs, and show that one of them, Tb927.8.7350, encodes another active trans-sialidase, designated as TbSA C2. In an in vitro assay, TbSA C2 utilized α2-3 sialyllactose as a donor, and produced an α2-3-sialylated product, suggesting that it is an α2-3 trans-sialidase. We suggest that TbSA C2 plays a role in the sialic acid modification of the trypanosome cell surface.

Structural remodeling, trafficking and functions of glycosylphosphatidylinositol-anchored proteins

Glycosylphosphatidylinositol (GPI) is a glycolipid that is covalently attached to proteins as a post-translational modification. Such modification leads to the anchoring of the protein to the outer leaflet of the plasma membrane. Proteins that are decorated with GPIs have unique properties in terms of their physical nature. In particular, these proteins tend to accumulate in lipid rafts, which are critical for the functions and trafficking of GPI-anchored proteins (GPI-APs). Recent studies mainly using mutant cells revealed that various structural remodeling reactions occur to GPIs present in GPI-APs as they are transported from the endoplasmic reticulum to the cell surface. This review examines the recent progress describing the mechanisms of structural remodeling of mammalian GPI-anchors, such as inositol deacylation, glycan remodeling and fatty acid remodeling, with particular focus on their trafficking and functions, as well as the pathogenesis involving GPI-APs and their deficiency.

Screening the MayBridge Rule of 3 Fragment Library for Compounds That Interact with the Trypanosoma brucei myo-Inositol-3-Phosphate Synthase and/or Show Trypanocidal Activity

Inositol-3-phosphate synthase (INO1) has previously been genetically validated as a drug target against Trypanosoma brucei, the causative agent of African sleeping sickness. Chemical intervention of this essential enzyme could lead to new therapeutic agents. Unfortunately, no potent inhibitors of INO1 from any organism have been reported, so a screen for potential novel inhibitors of T. brucei INO1was undertaken. Detection of inhibition of T. brucei INO1 is problematic due to the nature of the reaction. Direct detection requires differentiation between glucose-6-phosphate and inositol-3-phosphate. Coupled enzyme assays could give false positives as potentially they could inhibit the coupling enzyme. Thus, an alternative approach of differential scanning fluorimetry to identify compounds that interact with T. brucei INO1 was employed to screen ~670 compounds from the MayBridge Rule of 3 Fragment Library. This approach identified 38 compounds, which significantly altered the T_m of TbINO1. Four compounds showed trypanocidal activity with ED50s in the tens of micromolar range, with 2 having a selectivity index in excess of 250. The trypanocidal and general cytotoxicity activities of all of the compounds in the library are also reported, with the best having ED50S of ~20 μM against T. brucei.

Synthesis of potential metal-binding group compounds to examine the zinc dependency of the GPI de-N-acetylase metalloenzyme in Trypanosoma brucei

A small zinc-binding group (ZBG) library of deoxy-2-C-branched-monosaccharides, for example, 1,5-anhydroglucitols, consisting of either monodentate ligand binding carboxylic acids or bidentate ligand binding hydroxamic acids, were prepared to assess the zinc affinity of the putative metalloenzyme 2-acetamido-2-deoxy-α-d-glucopyranosyl-(1→6)-phosphatidylinositol de-N-acetylase (EC 3.5.1.89) of glycosylphosphatidylinositol biosynthesis. The N-ureido thioglucoside was also synthesised and added to the ZBG library because a previous N-ureido analogue, synthesised by us, had inhibitory activity against the aforementioned de-N-acetylase, presumably via the N-ureido motif.

The Sphingolipid Biosynthetic Pathway Is a Potential Target for Chemotherapy against Chagas Disease

The protozoan parasite Trypanosoma cruzi is the causative agent of human Chagas disease, for which there currently is no cure. The life cycle of T. cruzi is complex, including an extracellular phase in the triatomine insect vector and an obligatory intracellular stage inside the vertebrate host. These phases depend on a variety of surface glycosylphosphatidylinositol-(GPI-) anchored glycoconjugates that are synthesized by the parasite. Therefore, the surface expression of GPI-anchored components and the biosynthetic pathways of GPI anchors are attractive targets for new therapies for Chagas disease. We identified new drug targets for chemotherapy by taking the available genome sequence information and searching for differences in the sphingolipid biosynthetic pathways (SBPs) of mammals and T. cruzi. In this paper, we discuss the major steps of the SBP in mammals, yeast and T. cruzi, focusing on the IPC synthase and ceramide remodeling of T. cruzi as potential therapeutic targets for Chagas disease.

The Candida albicans homologue of PIG-P, CaGpi19p: gene dosage and role in growth and filamentation

Glycosylphosphatidyl inositol (GPI)-anchored proteins in Candida albicans are responsible for a vast range of functions, and deletions in certain GPI-anchored proteins severely reduce adhesion and virulence of this organism. In addition, completely modified GPIs are necessary for virulence. GPI anchor biosynthesis is essential for viability and starts with the transfer of N-acetylglucosamine to phosphatidylinositol. This step is catalysed by a multi-subunit complex, GPI–N-acetylglucosaminyltransferase (GPI–GnT). In this, the first report to our knowledge on a subunit of the Candida GPI–GnT complex, we show that CaGpi19p is the functional equivalent of the Saccharomyces cerevisiae Gpi19p. An N-terminal truncation mutant of CaGpi19p functionally complements a conditionally lethal S. cerevisiae gpi19 mutant. Further, we constructed a conditional null mutant of CaGPI19 by disrupting one allele and placing the remaining copy under the control of the MET3 promoter. Repression leads to growth defects, cell wall biogenesis aberrations, azole sensitivity and hyperfilamention. In addition, there is a noticeable gene dosage effect, with the heterozygote also displaying intermediate degrees of most phenotypes. The mutants also displayed a reduced susceptibility to the antifungal agent amphotericin B. Collectively, the results suggest that CaGPI19 is required for normal morphology and cell wall architecture.