Evidence that free GPI glycolipids are essential for growth of Leishmania mexicana

Regulated degradation of an endoplasmic reticulum membrane protein in a tubular lysosome in Leishmania mexicana

The cell surface of the human parasite Leishmania mexicana is coated with glycosylphosphatidylinositol (GPI)-anchored macromolecules and free GPI glycolipids. We have investigated the intracellular trafficking of green fluorescent protein- and hemagglutinin-tagged forms of dolichol-phosphate-mannose synthase (DPMS), a key enzyme in GPI biosynthesis in L. mexicana promastigotes. These functionally active chimeras are found in the same subcompartment of the endoplasmic reticulum (ER) as endogenous DPMS but are degraded as logarithmically growing promastigotes reach stationary phase, coincident with the down-regulation of endogenous DPMS activity and GPI biosynthesis in these cells. We provide evidence that these chimeras are constitutively transported to and degraded in a novel multivesicular tubule (MVT) lysosome. This organelle is a terminal lysosome, which is labeled with the endocytic marker FM 4-64, contains lysosomal cysteine and serine proteases and is disrupted by lysomorphotropic agents. Electron microscopy and subcellular fractionation studies suggest that the DPMS chimeras are transported from the ER to the lumen of the MVT via the Golgi apparatus and a population of 200-nm multivesicular bodies. In contrast, soluble ER proteins are not detectably transported to the MVT lysosome in either log or stationary phase promastigotes. Finally, the increased degradation of the DPMS chimeras in stationary phase promastigotes coincides with an increase in the lytic capacity of the MVT lysosome and changes in the morphology of this organelle. We conclude that lysosomal degradation of DPMS may be important in regulating the cellular levels of this enzyme and the stage-dependent biosynthesis of the major surface glycolipids of these parasites.

Disruption of mannose activation in Leishmania mexicana: GDP-mannose pyrophosphorylase is required for virulence, but not for viability

In eukaryotes, the enzyme GDP-mannose pyrophosphorylase (GDPMP) is essential for the formation of GDP-mannose, the central activated mannose donor in glycosylation reactions. Deletion of its gene is lethal in fungi, most likely as a consequence of disrupted glycoconjugate biosynthesis. Furthermore, absence of GDPMP enzyme activity and the expected loss of all mannose-containing glycoconjugates have so far not been observed in any eukaryotic organism. In this study we have cloned and characterized the gene encoding GDPMP from the eukaryotic protozoan parasite Leishmania mexicana. We report the generation of GDPMP gene deletion mutants of this human pathogen that are devoid of detectable GDPMP activity and completely lack mannose-containing glycoproteins and glycolipids, such as lipophosphoglycan, proteophosphoglycans, glycosylphosphatidylinositol protein membrane anchors, glycoinositolphospholipids and N-glycans. The loss of GDPMP renders the parasites unable to infect macrophages or mice, while gene addback restores virulence. Our study demonstrates that GDP-mannose biosynthesis is not essential for Leishmania viability in culture, but constitutes a virulence pathway in these human pathogens.

Specificity of GlcNAc-PI de-N-acetylase of GPI biosynthesis and synthesis of parasite-specific suicide substrate inhibitors

The substrate specificities of Trypanosoma brucei and human (HeLa) GlcNAc-PI de-N-acetylases were determined using 24 substrate analogues. The results show the following. (i) The de-N-acetylases show little specificity for the lipid moiety of GlcNAc-PI. (ii) The 3'-OH group of the GlcNAc residue is essential for substrate recognition whereas the 6'-OH group is dispensable and the 4'-OH, while not required for recognition, cannot be epimerized or substituted. (iii) The parasite enzyme can act on analogues containing betaGlcNAc or aromatic N-acyl groups, whereas the human enzyme cannot. (iv) Three GlcNR-PI analogues are de-N-acetylase inhibitors, one of which is a suicide inhibitor. (v) The suicide inhibitor most likely forms a carbamate or thiocarbamate ester to an active site hydroxy-amino acid or Cys or residue such that inhibition is reversed by certain nucleophiles. These and previous results were used to design two potent (IC50 = 8 nM) parasite-specific suicide substrate inhibitors. These are potential lead compounds for the development of anti-protozoan parasite drugs.

Function and assembly of the Leishmania surface coat

Like many trypanosomatids, the cell surface coat of Leishmania spp. is responsible for mediating various host-parasite interactions as well as acting as a dense physical barrier. This confers protection to the parasites in the hostile environments of the sandfly midgut and the macrophage phagolysosome. The major components of the surface coat are tethered to the cell surface via glycosylphosphatidylinositol glycolipids, and the composition of this surface coat is exquisitely regulated during the course of the parasite life-cycle. In this paper, we review what is known about the composition, biosynthesis and function of these glycosylphosphatidylinositol-containing molecules found within the parasite surface coat.

The Essential Smp3 Protein Is Required for Addition of the Side-branching Fourth Mannose during Assembly of Yeast Glycosylphosphatidylinositols

The major glycosylphosphatidylinositols (GPIs) transferred to protein in mammals and trypanosomes contain three mannoses. In Saccharomyces cerevisiae, however, the GPI transferred to protein bears a fourth, alpha1,2-linked Man on the alpha1,2-Man that receives the phosphoethanolamine (EthN-P) moiety through which GPIs become linked to protein. We report that temperature-sensitive smp3 mutants accumulate a GPI containing three mannoses and that smp3 is epistatic to the gpi11, gpi13, and gaa1 mutations, which normally result in the accumulation of Man(4)-GPIs, including the presumed substrate for the yeast GPI transamidase. The Smp3 protein, which is encoded by an essential gene, is therefore required for addition of the fourth Man to yeast GPI precursors. The finding that smp3 prevents the formation of the Man(4)-GPI that accumulates when addition of EthN-P to Man-3 is blocked in a gpi13 mutant suggests that the presence of the fourth Man is important for transfer of EthN-P to Man-3 of yeast GPIs. The Man(3)-GPI that accumulates in smp3 is a mixture of two dominant isoforms, one bearing a single EthN-P side branch on Man-1, the other with EthN-P on Man-2, and these isoforms can be placed in separate arms of a branched GPI assembly pathway. Smp3-related proteins are encoded in the genomes of Schizosaccharomyces pombe, Candida albicans, Drosophila melanogaster, and Homo sapiens and form a subgroup of a family of proteins, the other groups of which are defined by the Pig-B(Gpi10) protein, which adds the third GPI mannose, and by the Alg9 and Alg12 proteins, which act in the dolichol pathway for N-glycosylation. Because Man(4)-containing GPI precursors are normally formed in yeast and Plasmodium falciparum, whereas addition of a fourth Man during assembly of mammalian GPIs is rare and not required for GPI transfer to protein, Smp3p-dependent addition of a fourth Man represents a target for antifungal and antimalarial drugs.

Glycoconjugate structures of parasitic protozoa

Glycoconjugates are abundant and ubiquitious on the surface of many protozoan parasites. Their tremendous diversity has implicated their critical importance in the life cycle of these organisms. This review highlights our current knowledge of the major glycoconjugates, with particular emphasis on their structures, of representative protozoan parasites, including Leishmania, Trypanosoma, Giardia, Plasmodia, and others.

The role of phosphomannose isomerase in Leishmania mexicana glycoconjugate synthesis and virulence

Phosphomannose isomerase (PMI) catalyzes the reversible interconversion of fructose 6-phosphate and mannose 6-phosphate, which is the first step in the biosynthesis of activated mannose donors required for the biosynthesis of various glycoconjugates. Leishmania species synthesize copious amounts of mannose-containing glycolipids and glycoproteins, which are involved in virulence of these parasitic protozoa. To investigate the role of PMI for parasite glycoconjugate synthesis, we have cloned the PMI gene (lmexpmi) from Leishmania mexicana, generated gene deletion mutants (Delta lmexpmi), and analyzed their phenotype. Delta lmexpmi mutants lack completely the high PMI activity found in wild type parasites, but are, in contrast to fungi, able to grow in media deficient for free mannose. The mutants are unable to synthesize phosphoglycan repeats [-6-Gal beta 1-4Man alpha 1-PO(4)-] and mannose-containing glycoinositolphospholipids, and the surface expression of the glycosylphosphatidylinositol-anchored dominant surface glycoprotein leishmanolysin is strongly decreased, unless the parasite growth medium is supplemented with mannose. The Delta lmexpmi mutant is attenuated in infections of macrophages in vitro and of mice, suggesting that PMI may be a target for anti-Leishmania drug development. L. mexicana Delta lmexpmi provides the first conditional mannose-controlled system for parasite glycoconjugate assembly with potential applications for the investigation of their biosynthesis, intracellular sorting, and function.

GPI-anchored proteins and glycoconjugates segregate into lipid rafts in Kinetoplastida

The plasma membranes of the divergent eukaryotic parasites, Leishmania and Trypanosoma, are highly specialised, with a thick coat of glycoconjugates and glycoproteins playing a central role in virulence. Unusually, the majority of these surface macro-molecules are attached to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. In mammalian cells and yeast, many GPI-anchored molecules associate with sphingolipid and cholesterol-rich detergent-resistant membranes, known as lipid rafts. Here we show that GPI-anchored parasite macro-molecules (but not the dual acylated Leishmania surface protein (hydrophilic acylated surface protein) or a subset of the GPI-anchored glycoinositol phospholipid glycolipids) are enriched in a sphingolipid/sterol-rich fraction resistant to cold detergent extraction. This observation is consistent with the presence of functional lipid rafts in these ancient, highly polarised organisms.

Phosphoglycan repeat-deficient Leishmania mexicana parasites remain infectious to macrophages and mice

The human pathogen Leishmania synthesizes phosphoglycans (PGs) formed by variably modified phosphodisaccharide [6-Galbeta1-4Manalpha1-PO(4)] repeats and mannooligosaccharide phosphate [(Manalpha1-2)(0-5)Manalpha1-PO(4)] caps that occur lipid-bound on lipophosphoglycan, protein-bound on proteophosphoglycans, and as an unlinked form. PG repeat synthesis has been described as essential for survival and development of Leishmania throughout their life cycle, including for virulence to the mammalian host. In this study, this proposal was investigated in Leishmania mexicana using a spontaneous mutant that was fortuitously isolated from an infected mouse, and by generating a lmexlpg2 gene deletion mutant (Deltalmexlpg2), that lacks a Golgi GDP-Man transporter. The spontaneous mutant lacks PG repeats but synthesizes normal levels of mannooligosaccharide phosphate caps, whereas the Deltalmexlpg2 mutant is deficient in PG repeat synthesis and down-regulates cap expression. In contrast to expectations, both L. mexicana mutants not only retain their ability to bind to macrophages, but are also indistinguishable from wild type parasites with respect to colonization of and multiplication within host cells. Moreover, in mouse infection studies, the spontaneous L. mexicana repeat-deficient mutant and the Deltalmexlpg2 mutant showed no significant difference to a wild type strain with respect to the severity of disease caused by these parasites. Therefore, at least in Leishmania mexicana, PG repeat synthesis is not an absolute requirement for virulence.

Dissecting and manipulating the pathway for glycosylphos-phatidylinositol-anchor biosynthesis

The pathway for glycosylphosphatidylinositol-anchor biosynthesis consists of at least 10 reaction steps. Many of the genes encoding the enzymes and regulators involved in this pathway have been recently cloned and their products characterised. These studies have revealed the common and also different characteristics of glycosylphosphatidyl-inositol biosynthesis enzymes in different organisms, leading to the development of species-specific inhibitors of the pathway.

Dolichol phosphate mannose synthase from the filamentous fungus Trichoderma reesei belongs to the human and Schizosaccharomyces pombe class of the enzyme

Dolichol phosphate mannose (DPM) synthase activity, which is required in N:-glycosylation, O-mannosylation, and glycosylphosphatidylinositol membrane anchoring of protein, has been postulated to regulate the Trichoderma reesei secretory pathway. We have cloned a T.reesei cDNA that encodes a 243 amino acid protein whose amino acid sequence shows 67% and 65% identity, respectively, to the Schizosaccharomyces pombe and human DPM synthases, and which lacks the COOH-terminal hydrophobic domain characteristic of the Saccharomyces cerevisiae class of synthase. The Trichoderma dpm1 (Trdpm1) gene complements a lethal null mutation in the S.pombe dpm1(+) gene, but neither restores viability of a S.cerevisiae dpm1-disruptant nor complements the temperature-sensitivity of the S. cerevisiae dpm1-6 mutant. The T.reesei DPM synthase is therefore a member of the "human" class of enzyme. Overexpression of Trdpm1 in a dpm1(+)::his7/dpm1(+) S.pombe diploid resulted in a 4-fold increase in specific DPM synthase activity. However, neither the wild type T. reesei DPM synthase, nor a chimera consisting of this protein and the hydrophobic COOH terminus of the S.cerevisiae DPM synthase, complemented an S.cerevisiae dpm1 null mutant or gave active enzyme when expressed in E.coli. The level of the Trdpm1 mRNA in T.reesei QM9414 strain was dependent on the composition of the culture medium. Expression levels of Trdpm1 were directly correlated with the protein secretory capacity of the fungus.

Exploring the outcome of genetic modifications of glycosylation in cultured cell lines by concurrent isolation of the major classes of vertebrate glycans

In the preceding article (Manzi,A.E., Norgard-Sumnicht,K., Argade,S., Marth,J.D., van Halbeek,H. and Varki.A. [2000] GLYCOBIOLOGY:, 10, 669-688), we reported a comprehensive approach for the extraction, fractionation, and isolation of all of the major classes of sugar chains (glycans) from vertebrate tissues. Here we apply this "Glycan Isolation Protocol" to a variety of cultured mammalian cell lines, including two wild-type Chinese hamster ovary (CHO) cell lines and some of their genetically modified variants that were predicted or known to have defined abnormalities in the biosynthesis of one or more classes of glycans. We also use this approach to characterize clone 489, a new derivative of the GAG-deficient CHO clone pgsA-745, in which sulfation has been restored by transfection of a wild-type CHO cDNA library. By metabolically labeling the cell lines with [6-(3)H]glucosamine we were able to monitor the recovery of all major classes of glycans. The results allow us to reach several conclusions: first, the protocol described in the preceding paper is further validated by finding good recovery of total radioactivity and appropriate distribution of label in the correct glycan classes in the fractions from a variety of cell lines; second, the amount of radioactivity recovered in free glycosylphosphatidylinositol (GPI) lipids is remarkably high when compared to that found in GPI anchors, with the former being the dominant form in some cells; third, cells with known genetic mutations in specific glycosylation pathways are shown to have the expected changes in the distribution of recovered radioactivity in the appropriate fractions; fourth, the N- and O- glycans recovered via the protocol are of adequate quality to demonstrate marked differences in their structural profiles and/or content; fifth, the protocol can pick up unexpected differences of glycan classes not predicted to be affected by the primary defect; finally, the reappearance of sulfation in the novel clone 489 is not due to restoration of GAG sulfation, but rather due to the new expression of sulfation in the fraction enriched in N- and O-linked glycopeptides. These results demonstrate the power of this comprehensive approach for the concurrent exploration and profiling of the different major classes of glycans in cells.

Human dolichol-phosphate-mannose synthase consists of three subunits, DPM1, DPM2 and DPM3

Dolichol-phosphate-mannose (DPM) synthase generates mannosyl donors for glycosylphosphatidylinositols, N-glycan and protein O- and C-mannosylation. In Saccharomyces cerevisiae, this enzyme is encoded by DPM1. We reported previously that mammalian DPM synthase contains catalytic DPM1 and regulatory DPM2 subunits, and that DPM1 requires DPM2 for its stable expression in the endoplasmic reticulum. Here we report that human DPM synthase consists of three subunits. The third subunit, DPM3, comprises 92 amino acids associated with DPM1 via its C-terminal domain and with DPM2 via its N-terminal portion. The stability of DPM3 was dependent upon DPM2. However, overexpression of DPM3 in Lec15 cells, a null mutant of DPM2, restored the biosynthesis of DPM with an increase in DPM1, indicating that DPM3 directly stabilized DPM1. Therefore, DPM2 stabilizes DPM3 and DPM3 stabilizes DPM1. DPM synthase activity was 10 times higher in the presence of DPM2, indicating that DPM2 also plays a role in the enzymatic reaction. Schizosaccharomyces pombe has proteins that resemble three human subunits; S.pombe DPM3 restored biosynthesis of DPM in Lec15 cells, indicating its orthologous relationship to human DPM3.

Glycosylphosphatidylinositol biosynthesis defects in Gpi11p- and Gpi13p-deficient yeast suggest a branched pathway and implicate gpi13p in phosphoethanolamine transfer to the third mannose

Glycosylphosphatidylinositols (GPIs) are critical for membrane anchoring and intracellular transport of certain secretory proteins. GPIs have a conserved trimannosyl core bearing a phosphoethanolamine (EthN-P) moiety on the third mannose (Man-3) through which the glycolipid is linked to protein, but diverse GPI precursors with EthN-Ps on Man-1 and Man-2 have also been described. We report on two essential yeast genes whose products are required late in GPI assembly. GPI11 (YDR302w) encodes a homologue of human Pig-Fp, a protein implicated in the addition of EthN-P to Man-3. PIG-F complements the gpi11 deletion, but the rescued haploids are temperature sensitive. Abolition of Gpi11p or Pig-Fp function in GPI11 disruptants blocks GPI anchoring and formation of complete GPI precursors and leads to accumulation of two GPIs whose glycan head groups contain four mannoses but differ in the positioning and number of side chains, probably EthN-Ps. The less polar GPI bears EthN-P on Man-2, whereas the more polar lipid has EthN-P on Man-3. The latter finding indicates that Gpi11p is not required for adding EthN-P to Man-3. Gpi13p (YLL031cp), a member of a family of phosphoryltransferases, is a candidate for the enzyme responsible for adding EthN-P to Man-3. Depletion of Gpi13p in a Gpi11p-defective strain prevents formation of the GPI bearing EthN-P on Man-3, and Gpi13p-deficient strains accumulate a Man(4)-GPI isoform that bears EthN-P on Man-1. We further show that the lipid accumulation phenotype of Gpi11p-deficient cells resembles that of cells lacking Gpi7p, a sequence homologue of Gpi13p known to add EthN-P to Man-2 of a late-stage GPI precursor. This result suggests that in yeast a Gpi11p-deficiency can affect EthN-P addition to Man-2 by Gpi7p, in contrast to the Pig-Fp defect in mammalian cells, which prevents EthN-P addition to Man-3. Because Gpi11p and Pig-Fp affect EthN-P transfer to Man-2 and Man-3, respectively, these proteins may act in partnership with the GPI-EthN-P transferases, although their involvement in a given EthN-P transfer reaction varies between species. Possible roles for Gpi11p in the supply of the EthN-P donor are discussed. Because Gpi11p- and Gpi13p-deficient cells accumulate isoforms of Man(4)-GPIs with EthN-P on Man-2 and on Man-1, respectively, and because the GPIs that accumulate in Gpi11p-defective strains are likely to have been generated independently of one another, we propose that the yeast GPI assembly pathway is branched.

Leishmania mexicana mutants lacking glycosylphosphatidylinositol (GPI):protein transamidase provide insights into the biosynthesis and functions of GPI-anchored proteins

The major surface proteins of the parasitic protozoon Leishmania mexicana are anchored to the plasma membrane by glycosylphosphatidylinositol (GPI) anchors. We have cloned the L. mexicana GPI8 gene that encodes the catalytic component of the GPI:protein transamidase complex that adds GPI anchors to nascent cell surface proteins in the endoplasmic reticulum. Mutants lacking GPI8 (DeltaGPI8) do not express detectable levels of GPI-anchored proteins and accumulate two putative protein-anchor precursors. However, the synthesis and cellular levels of other non-protein-linked GPIs, including lipophosphoglycan and a major class of free GPIs, are not affected in the DeltaGPI8 mutant. Significantly, the DeltaGPI8 mutant displays normal growth in liquid culture, is capable of differentiating into replicating amastigotes within macrophages in vitro, and is infective to mice. These data suggest that GPI-anchored surface proteins are not essential to L. mexicana for its entry into and survival within mammalian host cells in vitro or in vivo and provide further support for the notion that free GPIs are essential for parasite growth.

Cell surface display and intracellular trafficking of free glycosylphosphatidylinositols in mammalian cells

In addition to serving as membrane anchors for cell surface proteins, glycosylphosphatidylinositols (GPIs) can be found abundantly as free glycolipids in mammalian cells. In this study we analyze the subcellular distribution and intracellular transport of metabolically radiolabeled GPIs in three different cell lines. We use a variety of membrane isolation techniques (subcellular fractionation, plasma membrane vesiculation to isolate pure plasma membrane fractions, and enveloped viruses to sample cellular membranes) to provide direct evidence that free GPIs are not confined to their site of synthesis, the endoplasmic reticulum, but can redistribute to populate other subcellular organelles. Over short labeling periods (2.5 h), radiolabeled GPIs were found at similar concentration in all subcellular fractions with the exception of a mitochondria-enriched fraction where GPI concentration was low. Pulse-chase experiments over extended chase periods showed that although the total amount of cellular radiolabeled GPIs decreased, the plasma membrane complement of labeled GPIs increased. GPIs at the plasma membrane were found to populate primarily the exoplasmic leaflet as detected using periodate oxidation of the cell surface. Transport of GPIs to the cell surface was inhibited by Brefeldin A and blocked at 15 degrees C, suggesting that GPIs are transported to the plasma membrane via a vesicular mechanism. The rate of transport of radiolabeled GPIs to the cell surface was found to be comparable with the rate of secretion of newly synthesized soluble proteins destined for the extracellular space.

Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review)

Glycosylphosphatidylinositols (GPIs) represent an abundant and ubiquitous class of eukaryotic glycolipids. Although these structures were originally discovered in the form of GPI-anchored cell surface glycoproteins, it is becoming increasingly clear that a significant proportion of the GPI synthetic output of a cell is not directed to protein anchoring. Indeed, pools of non-protein-linked GPIs can approach 10(7) molecules per cell in some cell types, especially the protozoa, with a large proportion of these molecules being displayed at the cell surface. Recent studies which form the subject of this review indicate that there is (a) considerable diversity in the range of structural modifications found on GPI glycolipids within and between species and cell types, (b) complexity in the topological arrangement of the GPI biosynthetic pathway in the endoplasmic reticulum, and (c) spatial restriction of the biosynthetic pathway within the endoplasmic reticulum. Furthermore, consistent with additional functional roles for these lipids beyond serving as protein anchor precursors, products of the GPI biosynthetic pathway appear to be widely distributed in the cellular endomembrane system. These studies indicate that there is still much to learn about the organization of glycolipid biosynthetic pathways in eukaryotic cells, the nature and subcellular distribution of the lipid products of these pathways, and the function of these lipids within cells.