Inhibition of glycosylphosphatidylinositol anchor formation by mannosamine

Biological Role of the Intercellular Transfer of Glycosylphosphatidylinositol-Anchored Proteins: Stimulation of Lipid and Glycogen Synthesis

Glycosylphosphatidylinositol-anchored proteins (GPI-APs), which are anchored at the outer leaflet of plasma membranes (PM) only by a carboxy-terminal GPI glycolipid, are known to fulfill multiple enzymic and receptor functions at the cell surface. Previous studies revealed that full-length GPI-APs with the complete GPI anchor attached can be released from and inserted into PMs in vitro. Moreover, full-length GPI-APs were recovered from serum, dependent on the age and metabolic state of rats and humans. Here, the possibility of intercellular control of metabolism by the intercellular transfer of GPI-APs was studied. Mutant K562 erythroleukemia (EL) cells, mannosamine-treated human adipocytes and methyl-ß-cyclodextrin-treated rat adipocytes as acceptor cells for GPI-APs, based on their impaired PM expression of GPI-APs, were incubated with full-length GPI-APs, prepared from rat adipocytes and embedded in micelle-like complexes, or with EL cells and human adipocytes with normal expression of GPI-APs as donor cells in transwell co-cultures. Increases in the amounts of full-length GPI-APs at the PM of acceptor cells as a measure of their transfer was assayed by chip-based sensing. Both experimental setups supported both the transfer and upregulation of glycogen (EL cells) and lipid (adipocytes) synthesis. These were all diminished by serum, serum GPI-specific phospholipase D, albumin, active bacterial PI-specific phospholipase C or depletion of total GPI-APs from the culture medium. Serum inhibition of both transfer and glycogen/lipid synthesis was counteracted by synthetic phosphoinositolglycans (PIGs), which closely resemble the structure of the GPI glycan core and caused dissociation of GPI-APs from serum proteins. Finally, large, heavily lipid-loaded donor and small, slightly lipid-loaded acceptor adipocytes were most effective in stimulating transfer and lipid synthesis. In conclusion, full-length GPI-APs can be transferred between adipocytes or between blood cells as well as between these cell types. Transfer and the resulting stimulation of lipid and glycogen synthesis, respectively, are downregulated by serum proteins and upregulated by PIGs. These findings argue for the (patho)physiological relevance of the intercellular transfer of GPI-APs in general and its role in the paracrine vs. endocrine (dys)regulation of metabolism, in particular. Moreover, they raise the possibility of the use of full-length GPI-APs as therapeutics for metabolic diseases.

Biosynthesis of GPI-anchored proteins: special emphasis on GPI lipid remodeling

Glycosylphosphatidylinositols (GPIs) act as membrane anchors of many eukaryotic cell surface proteins. GPIs in various organisms have a common backbone consisting of ethanolamine phosphate (EtNP), three mannoses (Mans), one non-N-acetylated glucosamine, and inositol phospholipid, whose structure is EtNP-6Manα-2Manα-6Manα-4GlNα-6myoinositol-P-lipid. The lipid part is either phosphatidylinositol of diacyl or 1-alkyl-2-acyl form, or inositol phosphoceramide. GPIs are attached to proteins via an amide bond between the C-terminal carboxyl group and an amino group of EtNP. Fatty chains of inositol phospholipids are inserted into the outer leaflet of the plasma membrane. More than 150 different human proteins are GPI anchored, whose functions include enzymes, adhesion molecules, receptors, protease inhibitors, transcytotic transporters, and complement regulators. GPI modification imparts proteins with unique characteristics, such as association with membrane microdomains or rafts, transient homodimerization, release from the membrane by cleavage in the GPI moiety, and apical sorting in polarized cells. GPI anchoring is essential for mammalian embryogenesis, development, neurogenesis, fertilization, and immune system. Mutations in genes involved in remodeling of the GPI lipid moiety cause human diseases characterized by neurological abnormalities. Yeast Saccharomyces cerevisiae has >60 GPI-anchored proteins (GPI-APs). GPI is essential for growth of yeast. In this review, we discuss biosynthesis of GPI-APs in mammalian cells and yeast with emphasis on the lipid moiety.

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.

Detection of glycophospholipid anchors on proteins

Many eukaryotic proteins are tethered to the plasma membrane by glycosyl phosphatidylinositol (GPI) membrane anchors. This unit provides a general approach for detecting GPI-anchored proteins. First, the detergent-partitioning behavior of a protein of interest is examined for characteristics of GPI-linked species. The partitioning of total cellular and isolated proteins with Triton X-114 is described in this unit, and precondensation of Triton X-114, which is necessary to remove hydrophilic contaminants before partitioning, is outlined in a Support Protocol 1. The protein may also be subjected to specific enzymatic or chemical cleavages to release it from its GPI anchor. Phospholipase cleavage (starting with intact cells or membranes, or with isolated protein) is detailed, and chemical cleavage with nitrous acid is also described. If GPI-anchored proteins are radiolabeled with fatty acids, it facilitates the detection of the GPI protein products following the cleavage reactions. A protocol for separation of lipid moieties released from proteins is provided and base hydrolysis of proteins is also presented.

Ethanolamine phosphate linked to the first mannose residue of glycosylphosphatidylinositol (GPI) lipids is a major feature of the GPI structure that is recognized by human GPI transamidase

Glycosylphosphatidylinositol (GPI) anchoring of proteins is catalyzed by GPI transamidase (GPIT), a multisubunit, endoplasmic reticulum (ER)-localized enzyme. GPIT recognizes ER-translocated proteins that have a GPI-directing C-terminal signal sequence and replaces this sequence with a preassembled GPI anchor. Although the GPI signal sequence has been extensively characterized, little is known about the structural features of the GPI lipid substrate that enable its recognition by GPIT. In a previous study we showed that mature GPIs could be co-immunoprecipitated with GPIT complexes containing functional subunits (Vainauskas, S., and Menon, A. K. (2004) J. Biol. Chem. 279, 6540-6545). We now use this approach, as well as a method that reconstitutes the interaction between GPIs and GPIT, to define the basis of the interaction between GPI and human GPIT. We report that (i) human GPIT can interact with GPI biosynthetic intermediates, not just mature GPIs competent for transfer to protein, (ii) the ethanolamine phosphate group on the third mannose residue of the GPI glycan is not critical for GPI recognition by GPIT, (iii) the ethanolamine phosphate residue linked to the first mannose of the GPI structure is a major feature of GPIs that is recognized by human GPIT, and (iv) the simplest GPI recognized by human GPIT is EtN-P-2Manalpha1-4GlcN-(acyl)-phosphatidyl-inositol. These studies define the molecular characteristics of GPI that are recognized by GPIT and open the way to identifying GPIT subunits that are involved in this process.

Purification, characterization and molecular cloning of glycosylphosphatidylinositol-anchored arginine-specific ADP-ribosyltransferases from chicken

Mono-ADP-ribosylation is a post-translational modification that regulates the functions of target proteins or peptides by attaching an ADP-ribose moiety. Here we report the purification, molecular cloning, characterization and tissue-specific distribution of novel arginine-specific Arts (ADP-ribosyltransferases) from chicken. Arts were detected in various chicken tissues as GPI (glycosylphosphatidylinositol)-anchored forms, and purified from the lung membrane fraction. By molecular cloning based on the partial amino acid sequence using 5'- and 3'-RACE (rapid amplification of cDNA ends), two full-length cDNAs of chicken GPI-anchored Arts, cgArt1 (chicken GPI-anchored Art1) and cgArt2, were obtained. The cDNA of cgArt1 encoded a novel polypeptide of 298 amino acids which shows a high degree of identity with cgArt2 (82.9%), Art6.1 (50.2%) and rabbit Art1 (42.1%). In contrast, the nucleotide sequence of cgArt2 was identical with that of Art7 cloned previously from chicken erythroblasts. cgArt1 and cgArt2 proteins expressed in DT40 cells were shown to be GPI-anchored Arts with a molecular mass of 45 kDa, and these Arts showed different enzymatic properties from the soluble chicken Art, Art6.1. RNase protection assays and real-time quantitative PCR revealed distinct expression patterns of the two Arts; cgArt1 was expressed predominantly in the lung, spleen and bone marrow, followed by the heart, kidney and muscle, while cgArt2 was expressed only in the heart and skeletal muscle. Thus GPI-anchored Arts encoded by the genes cgArt1 and cgArt2 are expressed extensively in chicken tissues. It may be worthwhile determining the functional roles of ADP-ribosylation in each tissue.

Mannosamine can replace glucosamine in glycosylphosphatidylinositols of Plasmodium falciparum in vitro

Mannosamine (2-deoxy-2-amino-D-mannose) is unable to block GPI biosynthesis in Plasmodium falciparum: neither parasite development nor GPI biosynthesis were blocked by mannosamine treatment in P. falciparum cultures. Further, it was shown by metabolic labeling with [3H]mannosamine and subsequent monosaccharide analysis by high pH anion exchange chromatography that mannosamine is converted at a high rate into glucosamine. Both mannosamine and glucosamine are incorporated into P. falciparum glycolipids, but the characterization of mannosamine-labeled glycolipids synthesized in vivo proved difficult. Therefore, a cell-free system was developed to investigate the incorporation of [3H]mannosamine into glycolipids in P. falciparum. It was observed that mannosamine is incorporated in vitro into P. falciparum glycolipids, which possess a phosphate group. Chemical (nitrous acid deamination, mild acid hydrolysis and alkaline hydrolysis) and enzymatic (PI-PLC) treatments of [3H]mannosamine-labeled glycolipids synthesized in vitro showed the presence of GPIs. Further analyses by Bio-Gel P4 size-exclusion chromatography and HPAEC demonstrated the presence of a mannosamine-containing GPI-like structures, where mannosamine is incorporated instead of glucosamine, i.e. Man3-ManN-PI. This utilization of mannosamine is novel and not been described for any other cellular or parasitic system.

Effects of tunicamycin, mannosamine, and other inhibitors of glycoprotein processing on skeletal alkaline phosphatase in human osteoblast-like cells

Skeletal alkaline phosphatase (sALP) is a glycoprotein- approximately 20% carbohydrate by weight, with five presumptive sites for N-linked glycosylation, as well as a carboxy-terminal site for attachment of the glycolipid structure (glycosylphosphatidylinositol, GPI), which anchors sALP to the outer surface of osteoblasts. The current studies were intended to characterize the effects of inhibiting glycosylation and glycosyl-processing on the synthesis, plasma membrane attachment, cellular-extracellular distribution, and reaction kinetics of sALP in human osteosarcoma (SaOS-2) cells. sALP synthesis, glycosylation, and GPI-anchor attachment were assessed as total protein synthesis/immunospecific sALP synthesis, sialic acid content (i.e., wheat germ agglutinin precipitation), and insolubility (i.e., temperature-dependent phase-separation), respectively. sALP reaction kinetics were characterized by analysis of dose-dependent initial velocity data, with a phosphoryl substrate. The results of these studies revealed that the inhibition of either N-linked glycosylation or oligosaccharide synthesis for GPI-anchor addition could affect the synthesis and the distribution of sALP, but not the kinetics of the phosphatase reaction. Tunicamycin-which blocks N-linked glycosylation by inhibiting core oligosaccharide synthesis-decreased cell layer protein and the total amount of sALP in the cells, while increasing the relative level of sALP in the cell-conditioned culture medium (CM, i.e., the amount of sALP released). These effects were attributed to dose- and time-dependent decreases in sALP synthesis and N-linked glycosylation, and an increase in apoptotic cell death (P <0.001 for each). In contrast to the effects of tunicamycin on N-linked glycosylation, the effects of mannosamine, which inhibits GPI-anchor glycosylation/formation, included (1) an increase in cell layer protein; (2) decreases in sALP specific activity, in the cells and in the CM; and (3) increases in the percentages of both anchorless and wheat germ agglutinin (WGA)-soluble sALP in the medium, but not in the cells (P <0.005 for each). These effects of mannosamine were, presumably, a consequence of inhibiting the insertion/attachment of sALP to the outside of the plasma membrane surface. Neither mannosammine nor tunicamycin had any effect on the reaction kinetics of sALP or on the apparent affinity (the value of KM) for the phosphoryl substrate.

Effects of long-term exposure to glucosamine and mannosamine on aggrecan degradation in articular cartilage

OBJECTIVE

To investigate the effect of long-term exposure to glucosamine or mannosamine on the catabolism of aggrecan by explant cultures of bovine articular cartilage maintained in the presence of retinoic acid.

DESIGN

The kinetics of loss of 35S-labeled and total aggrecan from explant cultures of bovine articular cartilage maintained in the presence of 1 micro M retinoic acid and exposed to varying concentrations of glucosamine or mannosamine was investigated over a 9-day culture period. In other experiments, the reversibility of the inhibition of aggrecan catabolism by glucosamine or mannosamine was investigated in cultures exposed to these amino sugars for the first 5 days of a 15-day culture period. The metabolism of chondrocytes exposed to these amino sugars was evaluated by measurement of lactate production or 3H-serine and 35S-sulfate incorporation into protein and glycosaminoglycans, respectively. The direct effect of these amino sugars on soluble aggrecanase activity was determined from immunoblots of aggrecan digests.

RESULTS

Glucosamine at 5mM concentration and mannosamine at 2mM concentration inhibited degradation of radiolabeled and chemical levels of aggrecan. At concentrations of up to 10mM amino sugars, the metabolism of chondrocytes was not impaired, as determined by lactate production, protein synthesis and the incorporation of 35S-sulfate into proteoglycans. These amino sugars did not inhibit soluble aggrecanase activity. The exposure of articular cartilage explants to 5mM glucosamine or mannosamine for 5 days in culture in the presence or absence of retinoic acid did not provide long-term suppression of stimulated aggrecan loss.

CONCLUSIONS

This study indicates that continuous presence of amino sugars is required to protect cartilage from stimulated loss of aggrecan.

Inhibitors of glycosyl-phosphatidylinositol anchor biosynthesis

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

Glucosamine inhibits inositol acylation of the glycosylphosphatidylinositol anchors in intraerythrocytic Plasmodium falciparum

Glycosylphosphatidylinositol (GPI) anchors are crucial for the survival of the intraerythrocytic stage Plasmodium falciparum because of their role in membrane anchoring of merozoite surface proteins involved in parasite invasion of erythrocytes. Recently, we showed that mannosamine can prevent the growth of P. falciparum by inhibiting the GPI biosynthesis. Here, we investigated the effect of isomeric amino sugars glucosamine, galactosamine, and their N-acetyl derivatives on parasite growth and GPI biosynthesis. Glucosamine, but not galactosamine, N-acetylglucosamine, and N-acetylgalactosamine inhibited the growth of the parasite in a dose-dependent manner. Glucosamine specifically arrested the maturation of trophozoites, a stage at which the parasite synthesizes all of its GPI anchor pool and had no effect during the parasite growth from rings to early trophozoites and from late trophozoites to schizonts and merozoites. An analysis of GPI intermediates formed when parasites incubated with glucosamine indicated that the sugar interferes with the inositol acylation of glucosamine-phosphatidylinositol (GlcN-PI) to form GlcN-(acyl)PI. Consistent with the non-inhibitory effect on parasite growth, galactosamine, N-acetylglucosamine, and N-acetylgalactosamine had no significant effect on the parasite GPI biosynthesis. The results indicate that the enzyme that transfers the fatty acyl moiety to inositol residue of GlcN-PI discriminates the configuration at C-4 of hexosamines. An analysis of GPIs formed in a cell-free system in the presence and absence of glucosamine suggests that the effect of the sugar is because of direct inhibition of the enzyme activity and not gene repression. Because the fatty acid acylation of inositol is an obligatory step for the addition of the first mannosyl residue during the biosynthesis of GPIs, our results offer a strategy for the development of novel anti-malarial drugs. Furthermore, this is the first study to report the specific inhibition of GPI inositol acylation by glucosamine in eukaryotes.

A GPI-linked isoform of the IgD receptor regulates resting B cell activation

The induction of a humoral response depends upon efficient cross-linking by antigen of surface immunoglobulin on primary B lymphocytes. We demonstrate here the presence of a glycosylphosphatidylinositol-linked isoform of membrane IgD (mIgD) receptors on murine resting B cells. This subset was constitutively localized to cell membrane raft microdomains. Its stimulation resulted in the activation of cAMP-dependent signaling pathways, which integrated with signals derived from the transmembrane mIgD receptors. This, in turn, provided a mechanism by which the activation status of the target cells could be variably regulated. Thus, by partitioning receptor activity, preimmune B cells can moderate the extent to which they are activated, depending upon the strength of the antigenic stimulus.

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

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

Developmental stage-specific biosynthesis of glycosylphosphatidylinositol anchors in intraerythrocytic Plasmodium falciparum and its inhibition in a novel manner by mannosamine

Glycosylphosphatidylinositols (GPIs) are the major glycoconjugates in intraerythrocytic stage Plasmodium falciparum. Several functional proteins including merozoite surface protein 1 are anchored to the cell surface by GPI modification, and GPIs are vital to the parasite. Here, we studied the developmental stage-specific biosynthesis of GPIs by intraerythrocytic P. falciparum. The parasite synthesizes GPIs exclusively during the maturation of early trophozoites to late trophozoites but not during the development of rings to early trophozoites or late trophozoites to schizonts and merozoites. Mannosamine, an inhibitor of GPI biosynthesis, inhibits the growth of the parasite specifically at the trophozoite stage, preventing further development to schizonts and causing death. Mannosamine has no effect on the development of either rings to early trophozoites or late trophozoites to schizonts and merozoites. The analysis of GPIs and proteins synthesized by the parasite in the presence of mannosamine demonstrates that the effect is because of the inhibition of GPI biosynthesis. The data also show that mannosamine inhibits GPI biosynthesis by interfering with the addition of mannose to an inositol-acylated GlcN-phosphatidylinositol (PI) intermediate, which is distinctively different from the pattern seen in other organisms. In other systems, mannosamine inhibits GPI biosynthesis by interfering with either the transfer of a mannose residue to the Manalpha1-6Manalpha1-4GlcN-PI intermediate or the formation of ManN-Man-GlcN-PI, an aberrant GPI intermediate, which cannot be a substrate for further addition of mannose. Thus, the parasite GPI biosynthetic pathway could be a specific target for antimalarial drug development.

Identification of a GPI-anchored type HDL-binding protein on human macrophages

To identify the HDL3-binding proteins on human macrophages, we examined the involvement of GPI-anchored protein in the binding of HDL3, and tried to purify HDL3-binding protein. From membrane fractions of macrophages, we obtained 80- and 130-kDa HDL3-binding proteins by ligand blotting. Treatment of macrophages with phosphatidylinositol-specific phospholipase C (PI-PLC) significantly decreased the specific HDL3-binding in a dose-dependent manner. Furthermore, treatment with mannosamine, which blocks GPI-anchor formation, decreased specific HDL3-binding in a dose-dependent manner. PI-PLC treatment released from the cells the proteins with an M(r) of 80 kDa, which could also bind HDL3. PI-PLC as well as mannosamine treatment markedly reduced cholesterol efflux from macrophages in association with the decreased HDL-binding. Using HDL3-affinity chromatography, we purified 80-kDa GPI-anchored type HDL3-binding protein. In summary, we demonstrate the implication of 80-kDa GPI-anchored protein in the binding of HDL3 to human macrophages, which might have some role in reverse cholesterol transport.

Mannosamine inhibits aggrecanase-mediated changes in the physical properties and biochemical composition of articular cartilage

The enzymatic processes underlying the degradation of aggrecan in cartilage and the corresponding changes in the biomechanical properties of the tissue are an important part of the pathophysiology of osteoarthritis. Recent studies have demonstrated that the hexosamines glucosamine (GlcN) and mannosamine (ManN) can inhibit aggrecanase-mediated cleavage of aggrecan in IL-1-treated cartilage cultures. The term aggrecanase describes two or more members of the ADAMTS family of metalloproteinases whose glutamyl endopeptidase activity is known to be responsible for much of the aggrecan degradation seen in human arthritides. In this study we examined the effect of ManN and GlcN on aggrecanase-mediated degradation of aggrecan induced by IL-1alpha and the corresponding tissue mechanical properties in newborn bovine articular cartilage. After 6 days of culture in 10 ng/ml IL-1 plus ManN, mechanical testing of explants in confined compression demonstrated that ManN inhibited the IL-1alpha-induced degradation in tissue equilibrium modulus, dynamic stiffness, streaming potential, and hydraulic permeability, in a dose-dependent fashion, with peak inhibition ( approximately 75-100% inhibition) reached by a concentration of 1.35 mM. Aggrecan from explants cultured in IL-1 was found by Western analysis to be almost entirely processed down to the G1-NITEGE(373) end product. Addition of ManN or GlcN was found to produce 75-90% inhibition of this cleavage, but the proportion of aggrecan remaining in the tissue which was cleaved at aggrecanase sites in the chondroitin sulfate (CS)-rich region (Glu(1501) and Glu(1687)) was higher than with IL-1 alone. This result suggests that the preservation of mechanical properties by hexosamines in explants is primarily due to inhibition of cleavage at the Glu(373) site in the interglobular domain. While the precise mechanism by which hexosamines function in this system is unclear, the present analysis suggests that the mechanical properties examined may be predominantly a function of electrostatic repulsion due to the charged CS chains in the tightly packed repetitive sequences of the CS-1 region.

Role of exogenous inositol and phosphatidylinositol in glycosylphosphatidylinositol anchor synthesis of GP49 by Giardia lamblia

Although Giardia lamblia trophozoites are unable to carry out de novo phospholipid synthesis, they can assemble complex glycophospholipids from simple lipids and fatty acids acquired from the host. Previously, we have reported that G. lamblia synthesizes GP49, an invariant surface antigen with a glycosylphosphatidylinositol (GPI) anchor. It is therefore possible that myo-inositol (Ins), phosphatidylinositol (PI) and other GPI precursors are obtained from the dietary products of the human small intestine, where the trophozoites colonize. In this report, we have investigated the role of exogenous Ins and PI on GPI anchor synthesis by G. lamblia. The results demonstrate that [(3)H]Ins and PI internalized by trophozoites, metabolically transformed into GlcN(acyl)-PI and downstream GPI molecules. Further investigations suggest that G. lamblia expresses cytidine monophosphate (CMP)-dependent (Mg(2+)-stimulated) and independent (Mn(2+)-stimulated) inositol headgroup exchange enzymes, which are responsible for exchanging free Ins with cellular PI. We observed that 3-deoxy-3-fluoro-D-myo-inositol (3-F-Ins) and 1-deoxy-1-F-scyllo-Ins (1-F-scyllo-Ins), which are considered potent inhibitors of Mn(2+)-stimulated headgroup exchange enzyme, inhibited the incorporation of [(3)H]Ins into PI and GPI molecules significantly, suggesting that CMP-independent (Mn(2+)-stimulated) exchange enzyme may be important for these reactions. However, 3-F-Ins and 1-F-scyllo-Ins were not effective in blocking the incorporation of exogenously supplied [(3)H]PI into GPI glycolipids. Thus, it can be concluded that G. lamblia can use exogenously supplied [(3)H]PI and [(3)H]Ins to synthesize GPI glycolipids of GP49; while PI is directly incorporated into GPI molecules, free Ins is first converted into PI by headgroup exchange enzymes, and this newly formed PI participates in GPI anchor synthesis.

The GPI biosynthetic pathway as a therapeutic target for African sleeping sickness

African sleeping sickness is a debilitating and often fatal disease caused by tsetse fly transmitted African trypanosomes. These extracellular protozoan parasites survive in the human bloodstream by virtue of a dense cell surface coat made of variant surface glycoprotein. The parasites have a repertoire of several hundred immunologically distinct variant surface glycoproteins and they evade the host immune response by antigenic variation. All variant surface glycoproteins are anchored to the plasma membrane via glycosylphosphatidylinositol membrane anchors and compounds that inhibit the assembly or transfer of these anchors could have trypanocidal potential. This article compares glycosylphosphatidylinositol biosynthesis in African trypanosomes and mammalian cells and identifies several steps that could be targets for the development of parasite-specific therapeutic agents.

Chondrocyte-mediated catabolism of aggrecan: evidence for a glycosylphosphatidylinositol-linked protein in the aggrecanase response to interleukin-1 or retinoic acid

The control of chondrocyte-mediated degradation of aggrecan has been studied in rat chondrosarcoma cells and bovine cartilage explants treated with either IL-1 or retinoic acid. The capacity of glucosamine to inhibit the aggrecanase-mediated response (J. D. Sandy, D. Gamett, V. Thompson, and C. Verscharen (1998) Biochem. J. 335, 59-66) has been extended to an investigation of the effect of other hexosamines. Mannosamine inhibits the aggrecanase response to both IL-1 and RA at about one-tenth the concentration of glucosamine in both rat cell and bovine explant systems. This effect of mannosamine appears to be due to its capacity to inhibit the synthesis of glycosylphosphatidylinositol (GPI)-linked proteins by chondrocytes since the GPI synthesis inhibitor 2-deoxyfluoroglucose (2-DFG) also inhibited the aggrecanase response to IL-1b and RA in rat cells. Moreover, phosphatidylinositol-specific phospholipase C (PIPLC) treatment of rat cells markedly inhibited the aggrecanase response to IL-1b and RA. These inhibitory effects of mannosamine, 2-DFG, and PIPLC in rat cells did not appear to be due to an interference with general biosynthetic activity of the cells as measured by [3H]proline incorporation into secreted proteins. We suggest that the aggrecanase response by chondrocytes to IL-1 and RA is dependent on the activity of a GPI-anchored protein on the chondrocyte cell surface.

Intracellular transport, cell-surface exposure and release of recombinant Tamm-Horsfall glycoprotein

Human Tamm-Horsfall glycoprotein (T-H), first described as the major urinary glycoprotein, is a glycosylphosphatidyl-inositol (GPI)-anchored membrane protein which mainly resides at the luminal face of cells of the thick ascending limb of Henle's loop (TAL) and early distal convoluted tubules of nephron. Since no human renal cell-line producing T-H is available, T-H cDNA was transfected in HeLa cells and a cell line was selected in which 95% of the cells stably expressed T-H, in order to elucidate the biosynthesis, mechanisms regulating the transport of T-H along the exocytic pathway, exposure at the cell surface and release in soluble form. Treatment of cells with an exogenous reducing agent results in a drastic delay in the conversion from precursor to mature T-H. Since the accumulating T-H-precursor carries glycans not yet processed by Golgi-mannosidases, we propose that the formation of a correct set of intrachain disulphide bonds is required for T-H exit out the endoplasmic reticulum. Even the treatment of cells with an inhibitor of GPI-anchor biosynthesis results in an intracellular accumulation of T-H precursor, loss of T-H localization into Golgi apparatus and reduced surface exposure. These results indicate that the GPI-anchor addition is necessary for T-H delivery to the cell-surface. The release rate of new synthesized T-H shows an initial lag time very likely depending on the time required for T-H surface exposure. A portion of released T-H appears to contain ethanolamine, a component of GPI anchor, indicating that, at least in HeLa cells, a GPI-specific phospholipase contributes to the T-H release. Exposure of cells to monensin and brefeldin A results in a loss of accumulation of T-H in the Golgi perinuclear region and a reduced delivery to the cell surface. Under monensin treatment an intermediate T-H form non-exposed at the cell surface is released in the medium, indicating that a soluble T-H may be produced inside the cell under conditions that alter the Golgi apparatus. If such an event occurs in polarized kidney cells, a T-H release from the basolateral face may be postulated, inasmuch as the GPI-anchor is an apical sorting signal. Since T-H is a powerful autoantigen, the accumulation of soluble T-H in the interstitium of TAL may cause the formation of immunocomplexes.

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

The nature and extent of carbohydrate modification in intraerythrocytic stage Plasmodium falciparum proteins have been controversial. This study describes the characterization of the carbohydrates in intraerythrocytic P. falciparum proteins and provides an overall picture of the nature of carbohydrate modification in the parasite proteins. P. falciparum strains were metabolically labeled with radioactive sugar precursors and ethanolamine at different developmental stages. The individual parasite proteins separated on SDS-polyacrylamide gels and whole parasite cell lysates were analyzed for the carbohydrate moieties. The results established the following: 1) glycosylphosphatidylinositol (GPI) anchors represent the major carbohydrate modification in the intraerythrocytic stage P. falciparum proteins; 2) in contrast to previous reports, O-linked carbohydrates are either absent or present only at very low levels in the parasite; and 3) P. falciparum contains low levels of N-glycosylation capability. The amount of N-linked carbohydrates in whole parasite proteins is approximately 6% compared with the GPI anchors attached to proteins based on radioactive GlcN incorporated into the proteins. The glycan cores of multiple parasite protein GPI anchors are all similar, consisting of protein-ethanolamine-phosphate-(Manalpha1-2)6Manalpha1-2M analpha1-6Ma nalpha1- 4GlcN. The fourth Man residues distal to GlcN of the GPI anchor glycan cores contain unidentified substituents that are susceptible to conditions of nitrous acid deamination. This unusual structural feature may contribute to the reported pathogenic properties of the P. falciparum GPI anchors.

Glycosyl-phosphatidylinositol molecules of the parasite and the host

The glycosyl-phosphatidylinositol (GPI) protein-membrane anchors are ubiquitous among the eukaryotes. However, while mammalian cells typically express in the order of 100 thousand copies of GPI-anchor per cell, the parasitic protozoa, particularly the kinetoplastids, express up to 10-20 million copies of GPI-anchor and/or GPI-related glycolipids per cell. Thus GPI-family members dominate the cell surface molecular architecture of these organisms. In several cases, GPI-anchored proteins, such as the variant surface glycoprotein (VSG) of the African trypanosomes, or GPI-related glycolipids, such as the lipophosphoglycan (LPG) of the Leishmania, are known to be essential for parasite survival and infectivity. The highly elevated levels and specialised nature of GPI metabolism in the kinetoplastid parasites suggest that the GPI biosynthetic pathways might be good targets for the development of chemotherapeutic agents. This article introduces the range of GPI structures found in protozoan parasites, and their mammalian hosts, and discusses some aspects of GPI biosynthesis.

GPI-anchor synthesis

The glycosyl phosphatidylinositol (GPI) anchor of membrane proteins is widely distributed in eukaryotes and parasitic protozoa. The structure and biosynthetic pathway of its core have been elucidated and appear to be conserved in various species. Some of the genes involved in mammalian GPI-anchor biosynthesis have recently been isolated using GPI-anchor-deficient mutant cell lines and expression cloning methods. One of these genes proved to be responsible for a GPI-anchor deficiency known as paroxysmal nocturnal hemoglobinuria. Since the core of the GPI anchor is variously modified in different species and since there may be other differences between its biosynthetic pathway in parasites and their hosts, this pathway could be a target for chemotherapy. In this review, Taroh Kinoshita and Junji Takeda focus on the GPI-anchor biosynthetic pathway and the genes involved in it.