Characterization of a phosphorylated pentasaccharide isolated from Hansenula holstii NRRL Y-2448 phosphomannan

Theoretical and NMR-based Conformational Analysis of Phosphodiester-linked Disaccharides

The conformational behaviour of three phosphate-bridged dimannosides was studied by means of NMR and computational molecular modelling. First, the conformations of the phosphodiester linker were determined by quantum chemistry methods using dimethyl phosphate as a model. Then, a series of conformations was constructed for each of the studied molecules. Preliminary molecular dynamics (MD) simulations revealed that the inclusion of a cation had a drastic influence on the obtained results. Additionally, triethylammonium had the same effect as sodium as the counter-ion. After that, another series of MD simulations was run. The resulting MD trajectories were used to define the conformations responsible for the observed nuclear Overhauser effects and inter-nuclear coupling.

Identification of the linkage between A-polysaccharide and the core in the A-lipopolysaccharide of Porphyromonas gingivalis W50

Porphyromonas gingivalis synthesizes two lipopolysaccharides (LPSs), O-LPS and A-LPS. The structure of the core oligosaccharide (OS) of O-LPS and the attachment site of the O-polysaccharide (O-PS) repeating unit [ → 3)-α-D-Galp-(1 → 6)-α-D-Glcp-(1 → 4)-α-L-Rhap-(1 → 3)-β-D-GalNAcp-(1 → ] to the core have been elucidated using the ΔPG1051 (WaaL, O-antigen ligase) and ΔPG1142 (Wzy, O-antigen polymerase) mutant strains, respectively. The core OS occurs as an "uncapped" glycoform devoid of O-PS and a "capped" glycoform that contains the attachment site of O-PS via β-d-GalNAc at position O-3 of the terminal α-(1 → 3)-linked mannose (Man) residue. In this study, the attachment site of A-PS to the core OS was determined based on structural analysis of SR-type LPS (O-LPS and A-LPS) isolated from a P. gingivalis ΔPG1142 mutant strain by extraction with aqueous hot phenol to minimize the destruction of A-LPS. Application of one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy in combination with methylation analysis showed that the A-PS repeating unit is linked to a nonterminal α-(1 → 3)-linked Man of the "capped core" glycoform of outer core OS at position O-4 via a → 6)-[α-D-Man-α-(1 → 2)-α-D-Man-1-phosphate → 2]-α-D-Man-(1 → motif. In order to verify that O-PS and A-PS are attached to almost identical core glycoforms, we identified a putative α-mannosyltransferase (PG0129) in P. gingivalis W50 that may be involved in the formation of core OS. Inactivation of PG0129 led to the synthesis of deep-R-type LPS with a truncated core that lacks α-(1 → 3)-linked mannoses and is devoid of either O-PS or A-PS. This indicated that PG0129 is an α-1,3-mannosyltransferase required for synthesis of the outer core regions of both O-LPS and A-LPS in P. gingivalis.

IMPORTANCE

Porphyromonas gingivalis, a Gram-negative anaerobe, is considered to be an important etiologic agent in periodontal disease, and among the virulence factors produced by the organism are two lipopolysaccharides (LPSs), O-LPS and A-LPS. The structures of the O-PS and A-PS repeating units, the core oligosaccharide (OS), and the linkage of the O-PS repeating unit to the core OS in O-LPS have been elucidated by our group. It is important to establish whether the attachment site of the A-PS repeating unit to the core OS in A-LPS is similar to or differs from that of the O-PS repeating unit in O-LPS. As part of understanding the biosynthetic pathway of the two LPSs in P. gingivalis, PG0129 was identified as an α-mannosyltransferase that is involved in the synthesis of the outer core regions of both O-LPS and A-LPS.

Identification of phosphorylation sites in N-linked glycans by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Glycan phosphorylation is a significant feature of complex carbohydrate chemistry and glycobiology. For example, N-linked glycans containing mannose-6-phosphate (Man-6-P) residues play a key role as targeting signals for the transport of proteins from the Golgi apparatus to lysosomes. Structural information on Man-6-P glycans involved in transport of proteins is usually obtained using nuclear magnetic resonance (NMR) spectroscopy. However, an alternative and simple method with comparable accuracy is desirable because large amounts of samples and special techniques are required for structural analysis using NMR. Recently, postsource decay (PSD) fragment spectra obtained by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) have provided critical information on complex carbohydrates. Since few Man-6-P-containing glycans are commercially available, very little information has been collected on the mass spectrometry of phosphorylated glycans. In this report, four kinds of phosphorylated glycans labeled with 2-aminopyridine (PA) were purified from yeast mannan, and their PSD spectra were measured in the positive ion mode. The phospho-6-O-mannose monoester linkages (PO3H-Man) in glycans are stable, although cleavage of the mannose-1-phosphate linkage (Man-alpha-1-PO3H) occurs readily. Fragment ions indicated the presence of the alpha-1,3-branching chain of an N-linked high-mannose-type glycan, and characteristic fragmentation patterns were observed for phosphorylated glycans. On the basis of the MALDI-PSD spectra, we deduced fragmentation rules for phosphorylated N-glycans that will be valuable for distinguishing the position of phosphorylation.

Natural phosphoglycans containing glycosyl phosphate units: structural diversity and chemical synthesis

An anomeric phosphodiester linkage formed by a glycosyl phosphate unit and a hydroxyl group of another monosaccharide is found in many glycopolymers of the outer membrane in bacteria (e.g., capsular polysaccharides and lipopolysaccharides), yeasts and protozoa. The polymers (phosphoglycans) composed of glycosyl phosphate (or oligoglycosyl phosphate) repeating units could be chemically classified as poly(glycosyl phosphates). Their importance as immunologically active components of the cell wall and/or capsule of numerous microorganisms upholds the need to develop routes for the chemical preparation of these biopolymers. In this paper, we (1) present a review of the primary structures (known to date) of natural phosphoglycans from various sources, which contain glycosyl phosphate units, and (2) discuss different approaches and recent achievements in the synthesis of glycosyl phosphosaccharides and poly(glycosyl phosphates).

Structural analysis of a novel anionic polysaccharide fromPorphyromonas gingivalisstrain W50 related to Arg-gingipain glycans

The Arg-gingipains (RgpsA and B) of Porphyromonas gingivalis are a family of extracellular cysteine proteases and are important virulence determinants of this periodontal bacterium. A monoclonal antibody, MAb1B5, which recognizes an epitope on glycosylated monomeric RgpAs also cross-reacts with a cell-surface polysaccharide of P. gingivalis W50 suggesting that the maturation pathway of the Arg-gingipains may be linked to the biosynthesis of a surface carbohydrate. We report the purification and structural characterization of the cross-reacting anionic polysaccharide (APS), which is distinct from both the lipopolysaccharide and serotype capsule polysaccharide of P. gingivalis W50. The structure of APS was determined by 1D and 2D NMR spectroscopy and methylation analysis, which showed it to be a phosphorylated branched mannan. The backbone is built up of alpha-1,6-linked mannose residues and the side-chains contain alpha-1,2-linked mannose oligosaccharides of different lengths (one to two sugar residues) attached to the backbone via 1,2-linkage. One of the side-chains in the repeating unit contains Manalpha1-2Manalpha1-phosphate linked via phosphorus to a backbone mannose at position 2. De-O-phosphorylation of APS abolished cross-reactivity suggesting that Manalpha1-2Manalpha1-phosphate fragment forms part of the epitope recognized by MAb1B5. This phosphorylated branched mannan represents a novel polysaccharide that is immunologically related to the post-translational additions of Arg-gingipains.

Synthesis of a 6(V)-sulfated mannopentasaccharide analogue related to PI-88

An efficient and convergent synthesis of a regioselectively 6(V)-sulfated mannopentasaccharide derivative 1c, octyl 6-O-sulfo-alpha-D-mannopyranosyl-(1-->3)-alpha-D-mannopyranosyl-(1-->3)-alpha-d-mannopyranosyl-(1-->3)-alpha-D-mannopyranosyl-(1-->2)-alpha-D-mannopyranoside, was achieved by a '3 + 2' strategy. The target was designed to mimic the promising anticancer agent PI-88 and was obtained from the building blocks, octyl 3,4,6-tri-O-benzoyl-alpha-D-mannopyranoside, allyl 2,4,6-tri-O-benzoyl-3-O-(4-methoxybenzyl)-alpha-D-mannopyranoside, and 6-O-acetyl-2,3,4-tri-O-benzoyl-alpha-D-mannopyranosyl trichloroacetimidate (11), under TMSOTf-catalyzed glycosylation conditions. Compound 1c displays a mild anti-angiogenic activity based on a chorioallantoic membrane (CAM) model study.

Conjugation of Mannose 6-Phosphate-containing Oligosaccharides to Acid α-Glucosidase Improves the Clearance of Glycogen in Pompe Mice

Clinical studies of enzyme replacement therapy for Pompe disease have indicated that relatively high doses of recombinant human acid alpha-glucosidase (rhGAA) may be required to reduce the abnormal glycogen storage in cardiac and skeletal muscles. This may be because of inefficient cation-independent mannose 6-phosphate receptor (CI-MPR)-mediated endocytosis of the enzyme by the affected target cells. To address this possibility, we examined whether the addition of a high affinity ligand to rhGAA would improve its delivery to these cells. Chemical conjugation of high mannose oligosaccharides harboring mono- and bisphosphorylated mannose 6-phosphates onto rhGAA (neo-rhGAA) significantly improved its uptake characteristics by muscle cells in vitro. Infusion of neo-rhGAA into Pompe mice also resulted in greater delivery of the enzyme to muscle tissues when compared with the unmodified enzyme. Importantly, this increase in enzyme levels was associated with significantly improved clearance of glycogen ( approximately 5-fold) from the affected tissues. These results suggest that CI-MPR-mediated endocytosis of rhGAA is an important pathway by which the enzyme is delivered to the affected lysosomes of Pompe muscle cells. Hence, the generation of rhGAA containing high affinity ligands for the CI-MPR represents a strategy by which the potency of rhGAA and therefore the clinical efficacy of enzyme replacement therapy for Pompe disease may be improved.

Determination of the composition of the oligosaccharide phosphate fraction of Pichia (Hansenula) holstii NRRL Y-2448 phosphomannan by capillary electrophoresis and HPLC

The promising new anticancer agent, PI-88, is prepared by the sulfonation of the oligosaccharide phosphate fraction of the extracellular phosphomannan produced by the yeast Pichia (Hansenula) holstii NRRL Y-2448. The composition of the oligosaccharide phosphate fraction was determined by capillary electrophoresis (CE) with indirect UV detection using 6 mM potassium sorbate at pH 10.3 as the background electrolyte. Further confirmation of the composition was obtained by HPLC analysis of a sample dephosphorylated by treatment with alkaline phosphatase. The structure of the hexasaccharide component has been determined by isolation and NMR spectroscopic analysis of its dephosphorylated derivative. Additionally, the structure of a second, previously undetected tetrasaccharide component (a hexosamine) has been determined by isolation and NMR spectroscopic analysis of the acetate of its dephosphorylated derivative. It is demonstrated that CE is an ideal method for the quality control of the oligosaccharide phosphate fraction for use in the production of PI-88.

Novel drug development opportunities for heparin

The glycosaminoglycan heparin has been used in the clinic as an anticoagulant for more than 50 years. A fully characterized sequence in native heparin is known to be responsible for this activity. However, heparin is a complex polysaccharide, which has an array of properties that are unrelated to its anticoagulant activity. Recent research has provided us with an increased understanding of the specific structural requirements for the various actions of heparin, indicating that it might be possible to create 'tailor-made' sequences based on the heparin template to isolate specific therapeutic activities. This research should provide the basis for novel drug treatments for a range of diseases, including cancer and various inflammatory diseases.

Characterisation of the anticoagulant properties of a range of structurally diverse sulfated oligosaccharides

In this study, 17 sulfated oligosaccharides were assessed by the activated partial thromboplastin time (APTT) test for their anticoagulant activity and nine were found to exhibit significant activity. Chain length, monosaccharide makeup, and linkage all appear to be critical factors in determining anticoagulant activity, with the most active compounds being five- to sixfold less potent than unfractionated heparin (UFH). Phosphomannopentaose sulfate (PI-88), one of the most active sulfated oligosaccharides and a promising anticancer drug, was selected for further study. PI-88 gave a more linear APTT dose-response curve and displayed less patient-to-patient variation than UFH, with its activity being neutralised by protamine sulfate. However, PI-88 showed considerable species-to-species variation in its anticoagulant effect. It was found that PI-88 acted as an anticoagulant by enhancing the ability of heparin cofactor II (HCII) to inhibit thrombin, and did not act via antithrombin III (AT-III) in either inhibiting Factor Xa or thrombin. PI-88 also mildly prolonged the prothrombin time (PT), whilst it had no platelet pro-aggregatory activity, nor did it demonstrate direct fibrinolytic activity. Thus, PI-88 represents a potential antithrombotic agent deserving further study.

Large-scale preparation of the oligosaccharide phosphate fraction of Pichia holstii NRRL Y-2448 phosphomannan for use in the manufacture of PI-88

Mild acid-catalysed hydrolysis of the extracellular phosphomannan of the yeast Pichia holstii NRRL Y-2448 produces a high-molecular-weight phosphomannan core, a low-molecular-weight oligosaccharide phosphate fraction, and a neutral oligosaccharide fraction. A method was developed for the large-scale preparation of the oligosaccharide phosphate fraction, consisting predominantly of the pentasaccharide phosphate, 6-O-PO3H2-alpha-D-Man-(1 --> 3)-alpha-D-Man-(1 --> 3)-alpha-D-Man-(1 C 3)-alpha-D-Man-(1 --> 2)-D-Man, for use in the manufacture of the promising new anti-cancer agent, PI-88. Further insights were also gained into the structure of the phosphomannan by HPLC analysis of the time course of the hydrolysis reaction.

Structural basis for recognition of phosphorylated high mannose oligosaccharides by the cation-dependent mannose 6-phosphate receptor

Mannose 6-phosphate receptors (MPRs) play an important role in the targeting of newly synthesized soluble acid hydrolases to the lysosome in higher eukaryotic cells. These acid hydrolases carry mannose 6-phosphate recognition markers on their N-linked oligosaccharides that are recognized by two distinct MPRs: the cation-dependent mannose 6-phosphate receptor and the insulin-like growth factor II/cation-independent mannose 6-phosphate receptor. Although much has been learned about the MPRs, it is unclear how these receptors interact with the highly diverse population of lysosomal enzymes. It is known that the terminal mannose 6-phosphate is essential for receptor binding. However, the results from several studies using synthetic oligosaccharides indicate that the binding site encompasses at least two sugars of the oligosaccharide. We now report the structure of the soluble extracytoplasmic domain of a glycosylation-deficient form of the bovine cation-dependent mannose 6-phosphate receptor complexed to pentamannosyl phosphate. This construct consists of the amino-terminal 154 amino acids (excluding the signal sequence) with glutamine substituted for asparagine at positions 31, 57, 68, and 87. The binding site of the receptor encompasses the phosphate group plus three of the five mannose rings of pentamannosyl phosphate. Receptor specificity for mannose arises from protein contacts with the 2-hydroxyl on the terminal mannose ring adjacent to the phosphate group. Glycosidic linkage preference originates from the minimization of unfavorable interactions between the ligand and receptor.

The kangaroo cation-independent mannose 6-phosphate receptor binds insulin-like growth factor II with low affinity

The mammalian cation-independent mannose 6-phosphate receptor (CI-MPR) binds mannose 6-phosphate-bearing glycoproteins and insulin-like growth factor (IGF)-II. However, the CI-MPR from the opossum has been reported to bind bovine IGF-II with low affinity (Dahms, N. M., Brzycki-Wessell, M. A., Ramanujam, K. S., and Seetharam, B. (1993) Endocrinology 133, 440-446). This may reflect the use of a heterologous ligand, or it may represent the intrinsic binding affinity of this receptor. To examine the binding of IGF-II to a marsupial CI-MPR in a homologous system, we have previously purified kangaroo IGF-II (Yandell, C. A., Francis, G. L., Wheldrake, J. F., and Upton, Z. (1998) J. Endocrinol. 156, 195-204), and we now report the purification and characterization of the CI-MPR from kangaroo liver. The interaction of the kangaroo CI-MPR with IGF-II has been examined by ligand blotting, radioreceptor assay, and real-time biomolecular interaction analysis. Using both a heterologous and homologous approach, we have demonstrated that the kangaroo CI-MPR has a lower binding affinity for IGF-II than its eutherian (placental mammal) counterparts. Furthermore, real-time biomolecular interaction analysis revealed that the kangaroo CI-MPR has a higher affinity for kangaroo IGF-II than for human IGF-II. The cDNA sequence of the kangaroo CI-MPR indicates that there is considerable divergence in the area corresponding to the IGF-II binding site of the eutherian receptor. Thus, the acquisition of a high-affinity binding site for regulating IGF-II appears to be a recent event specific to the eutherian lineage.

Mannose 6-phosphate receptors (MPR 300 and MPR 46) from a teleostean fish (trout)

Mannose 6-phosphate receptors (MPRs) are known to occur in mammals, birds, reptiles and amphibians. Here we provide evidence for the presence of two MPRs in fish, the earliest vertebrates. Using phosphomannan-Sepharose affinity chromatography, MPR 300 was purified from liver membrane extract of trout. The purified trout liver MPR 300 showed similar electrophoretic mobility as the goat liver receptor and a pH optimum of 7.0 for binding to phosphomannan. The presence of MPR 46 in fish was shown by metabolically labelling embryonic fish cells (Xiphophorus) and immunoprecipitation with an antibody against the cytoplasmic tail of human MPR 46 (anti-MSC1). This antibody had recently been shown to immunoprecipitate MPR 46 also from reptiles and amphibians.

The extracellular polysaccharide of Pichia (Hansenula) holstii NRRL Y-2448: the phosphorylated side chains

The exopolysaccharide produced by Pichia (Hansenula) holstii NRRL Y-2448 is composed of a phosphomannan core to which oligosaccharide diester phosphate side chains are appended. The oligosaccharides of the side chains were released as oligosaccharide phosphates and neutral oligosaccharides by mild hydrolysis with aqueous acetic acid and aqueous hydrogen fluoride, respectively. The liberated oligosaccharide phosphates were studied by NMR spectroscopy and by electrospray and fast atom bombardment mass spectrometry. The structures of the neutral oligosaccharides were determined by 1D and 2D NMR spectroscopic experiments. Further insight into the length of the side chains was obtained from a matrix assisted laser desorption ionisation-time of flight mass spectrometric study of high and low molecular weight fragments obtained from partial acid hydrolysis of the native polysaccharide.

Mannose-6-phosphate/insulin-like growth factor-II receptor is a receptor for retinoic acid

Retinoic acid (RA) exerts diverse biological effects in the control of cell growth in embryogenesis and oncogenesis. These effects of RA are thought to be mediated by the nuclear retinoid receptors. Mannose-6-phosphate (M6P)/insulin-like growth factor-II (IGF-II) receptor is a multifunctional membrane glycoprotein that is known to bind both M6P and IGF-II and function primarily in the binding and trafficking of lysosomal enzymes, the activation of transforming growth factor-beta, and the degradation of IGF-II. M6P/IGF-II receptor has recently been implicated in fetal development and carcinogenesis. Despite the functional similarities between RA and the M6P/IGF-II receptor, no direct biochemical link has been established. Here, we show that the M6P/IGF-II receptor also binds RA with high affinity at a site that is distinct from those for M6P and IGF-II, as identified by a photoaffinity labeling technique. We also show that the binding of RA to the M6P/IGF-II receptor enhances the primary functions of this receptor. The biological consequence of the interaction appears to be the suppression of cell proliferation and/or induction of apoptosis. These findings suggest that the M6P/IGF-II receptor mediates a RA response pathway that is important in cell growth regulation. This discovery of the interaction of RA with the M6P/IGF-II receptor may have important implications for our understanding of the roles of RA and the M6P/IGF-II receptor in development, carcinogenesis, and lysosomal enzyme-related diseases.

The extracellular polysaccharide of Pichia (Hansenula) holstii NRRL Y-2448: the structure of the phosphomannan backbone

The phosphomannan core of the exopolysaccharide of Pichia (Hansenula) holstii NRRL Y-2448 was isolated after hydrolytic removal of the oligosaccharide phosphate side-chains. The core polysaccharide and its dephosphorylated derivative were subjected to extensive 1D and 2D NMR spectroscopy which yielded information on the linkage sites and on the sequence of the mannosyl residues in the major oligosaccharide repeating unit. The most probable structure for the repeating unit was -[6-O-PO3H2-alpha-D-Man-(1-->3)-alpha-D-Man-(1-->2)-alpha-D-Man-(1 -->2)]-alpha-D-Man-(1-->6)-[alpha-D-Man-(1-->2)]-alpha-D-Man-(1-->6)-. A semiquantitative conformational analysis was performed by Monte Carlo simulations and the result was confirmed by comparison with the experimentally determined NMR data. The distance distribution for the phosphate groups was determined from the modeling and was found to cover the expected range of distances for phosphorylated high-mannose oligosaccharides.

An affinity method for the purification of mannose 6-phosphate receptor proteins (MPR 215) from rat tissues and goat liver

An affinity matrix (Sepharose 6B-divinyl sulfone-pentaphosphomannan) has been developed which can be efficiently used for the purification of the MPR 215 from different tissues of rat as well as from goat liver. The matrix developed is relatively easy to prepare compared with the available procedures, and can be used for the purification of similar receptor proteins from other sources.

Characterization of the mannose 6-phosphate-dependent pathway of lysosomal enzyme routing in an invertebrate

The mannose 6-phosphate (Man6P)-dependent pathway for routing lysosomal enzymes was characterized in the hepatopancreas of the estuary crab Chasmagnatus granulata: (a) an acid alpha-L-fucosidase was purified to homogeneity from the above-mentioned organ and was shown to contain mannose-linked phosphate residues; (b) high-mannose-type oligosaccharides isolated from a protein fraction enriched in acid hydrolases were found to contain acid-labile N-acetylglucosamine (GlcNAc) residues; (c) a membrane-bound UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase was detected that phosphorylated the estuary-crab alpha-L-fucosidase and bovine uteroferrin but not bovine pancreas ribonuclease B; (d) a GlcNAc-1-phosphodiester alpha-N-acetylglucosaminidase that released GlcNAc units from GlcNAc alpha 1-P6Man alpha 1-methyl was detected in microsomal membranes of the hepatopancreas; (e) two detergent-solubilized microsomal proteins having molecular masses of 205 and 215 kDa that were retained by a Man6P-rich mannan-Sepharose column, from where they were eluted with Man6P but not with glucose 6-phosphate, were recognized by antisera raised against bovine large (215 kDa) and small (46 kDa) Man6P receptors. This is the first description of all the components of the Man6P-dependent mechanism for routing lysosomal enzymes in an invertebrate.

Glycamine formation via reductive amination of oligosaccharides with benzylamine: efficient coupling of oligosaccharides to protein

The conventional reagents for the reductive amination of sugars, ammonium salts or ammonia, require relatively harsh conditions such as high temperatures or high concentrations. In addition, they give substantial amounts of dimeric byproducts. We have developed a method of using benzylamine as a donor to achieve near quantitative amination of reducing oligosaccharides. Benzylamine reacts with reducing oligosaccharides faster and yields less dimeric byproduct than ammonium ion, rendering it especially advantageous for preparative operation. In combination with a heterobifunctional reagent, 5-[N-(2,2-dimethoxyethyl)carbamoyl]pentanoyl azide, [Lee et al. Biochemistry, 28 (1989) 1856-1861], we could couple a nearly maximal number of phosphorylated mannopentaose molecules to ribonuclease A via its primary amino groups.

Biotinylated diaminopyridine: an approach to tagging oligosaccharides and exploring their biology

Fluorescent tagging of free oligosaccharides by reductive amination permits sensitive detection and fractionation of these molecules. To expand the scope of this approach, we have synthesized a fluorescent reagent, 2-amino-(6-amidobiotinyl)pyridine. This reagent can tag oligosaccharides under nondegradative conditions with high efficiency. The resulting adducts show excellent fractionation by reverse-phase HPLC with sensitive detection in the low picomole range. When combined with sequential exoglycosidase digestion, stepwise sequencing of the sugar chains is possible. The biotinyl group can also be used to recover the sugar chain from reaction mixtures. The high-affinity interaction of the biotinyl group with multivalent avidin or streptavidin can be used to create the functional equivalent of neoglycoproteins carrying multiple copies of oligosaccharides of defined structure. These complexes allow the production of IgG antibodies directed against the oligosaccharide chain. They can also harness the power of (strept)avidin-biotin technology for the detection and isolation of oligosaccharide-specific receptors from native sources of recombinant libraries.

Site-directed removal of N-glycosylation sites in the bovine cation-dependent mannose 6-phosphate receptor: effects on ligand binding, intracellular targetting and association with binding immunoglobulin protein

The bovine cation-dependent mannose 6-phosphate receptor (CD-MPR) contains five potential N-linked glycosylation sites, four of which are utilized. To evaluate the function of these oligosaccharides, site-directed mutagenesis was used to generate glycosylation-deficient CD-MPR mutants lacking various potential glycosylation sites. The mutants were constructed in both a full-length and a soluble truncated (STOP155 construct) form of the receptor and their properties were examined in transfected COS-1 cells. The results showed that the presence of a single oligosaccharide chain, particularly at position 87, on the CD-MPR significantly enhanced its mannose 6-phosphate (Man-6-P)-binding ability when compared with non-glycosylated receptors. In addition, the presence of a single oligosaccharide chain at position 87, and to a lesser degree at position 31 or 81, promoted the secretion of the STOP155 CD-MPR. Pulse-labelling of transfected COS-1 cells followed by immunoprecipitation with binding immunoglobulin protein (BiP)-specific and CD-MPR-specific antibodies indicated that BiP associated with the non-glycosylated forms of the receptor but not with the wild-type CD-MPR. Furthermore, the association of the various glycosylation-deficient forms of the CD-MPR with BiP correlated inversely with their ability to bind Man-6-P. From these results we conclude that N-glycosylation of the bovine CD-MPR facilities the folding of the nascent polypeptide chain into a conformation that is conductive for intracellular transport and ligand binding.

High affinity binding of the leucocyte adhesion molecule L-selectin to 3'-sulphated-Le(a) and -Le(x) oligosaccharides and the predominance of sulphate in this interaction demonstrated by binding studies with a series of lipid-linked oligosaccharides

The binding of the leucocyte adhesion molecule L-selectin has been investigated toward several structurally defined lipid-linked oligosaccharides immobilized on silica gel chromatograms or plastic wells. In both assay systems the 3'-sulphated Le(a)/Le(x) type tetrasaccharides [formula: see text] were more strongly bound than 3'-sialyl analogues. A considerable binding was observed to the 3'-sulphated oligosaccharide backbone in the absence of fucose but not to a 3'-sialyl analogue or fuco-oligosaccharide analogues lacking sulphate or sialic acid. Affinity for other sulphated saccharides: 3'-sulphoglucuronyl neolactotetraosyl ceramide and glycolipids with sulphate 3'-linked to terminal or sub-terminal galactose or N-acetylgalactosamine was detected in the chromatogram assay only. These studies, together with earlier reports that L-selectin binding to endothelium is inhibited by sulphatide, highlight the relative importance of sulphate in the adhesive specificity of this protein.

Novel sulfated ligands for the cell adhesion molecule E-selectin revealed by the neoglycolipid technology among O-linked oligosaccharides on an ovarian cystadenoma glycoprotein

E-selectin is the inducible adhesion protein on the surface of endothelial cells which has a crucial role in the initial stages of recruitment of leucocytes to sites of inflammation. In addition, it is almost certainly involved in tumor cell adhesion and metastasis. This report is concerned with identification of a new class of oligosaccharide ligand--sulfate-containing--for the human E-selectin molecule from among oligosaccharides on an ovarian cystadenoma glycoprotein. This has been achieved by application of the neoglycolipid technology to oligosaccharides released from the glycoprotein by mild alkaline beta-elimination. Oligosaccharides were conjugated to lipid, resolved by thin-layer chromatography, and tested for binding by Chinese hamster ovary cells which had been transfected to express the full-length E-selectin molecule. Several components with strong E-selectin binding activity were revealed among acidic oligosaccharides. The smallest among these was identified by liquid secondary ion mass spectrometric analysis of the neoglycolipid, in conjunction with methylation analysis of the purified oligosaccharide preparation as an equimolar mixture of the Le(a)- and Le(x)/SSEA-1-type fucotetrasaccharides sulfated at position 3 of outer galactose: [formula: see text] To our knowledge this is the first report of a sulfofucooligosaccharide ligand for E-selectin. The binding activity is substantially greater than those of lipid-linked Le(a) and Le(x)/SSEA-1 sequences and is at least equal to that of the 3'-sialyl-Le(x)/SSEA-1 glycolipid analogue.

Binding specificity of D-mannose 6-phosphate receptor of rabbit alveolar macrophages

The existence of terminal D-mannosyl 6-phosphate groups (Man-6-P) was required (for an inhibitor) to exert a strong inhibitory potency against the binding of bovine serum albumin (BSA) conjugated with 17 molecules of penta-D-mannose 6-phosphate [(M5P)17-BSA] to the Man-6-P receptor in rabbit alveolar macrophages. In addition, the underlying oligosaccharide structures, such as linkage mode between the nonreducing sugar group and the penultimate sugar residue, and the length of sugar chain also affected the inhibitory potency in this system. In general, the oligosaccharides with an alpha-(1----2)-linked Man-6-P unit gave stronger inhibitory potencies than those with an alpha-(1----3)- or alpha-(1----6)-linked Man-6-P unit. Trisaccharides containing a terminal Man-6-P group were more potent inhibitors than disaccharides. A synthetic, branched, and divalent ligand, which does not have a penultimate sugar residue, gave about the same level of inhibitory potency as Man-6-P itself. The "cluster effect" was observed in this system, i.e., as the number of Man-6-P units conjugated to BSA [(Man-6-P)5,5,8, and 46-BSA] increased, the stronger inhibitory potencies were observed with decreasing I50 values of 1.93, 1.36, and 0.0345 microM, respectively. Synthetic divalent oligosaccharides also showed higher inhibitory potencies than the corresponding monovalent oligosaccharides.

Preparation and application of a pentamannosyl monophosphate-bovine serum albumin conjugate

Pentamannosyl monophosphate, derived from Hansenula holstii O-phosphomannan, was conjugated to bovine serum albumin by reductive amination. The conjugate inhibited the binding of the porcine testis mannose 6-phosphate receptor to the insoluble phosphomannan core. A mannose 6-phosphate receptor with a molecular weight of 200,000 was purified from porcine liver membranes, using an affinity matrix of the conjugate attached to Sepharose 4B. Rabbits were immunised with the conjugate, and the antisera were purified on a phosphomannan core-Sepharose 4B column in order to give an antibody which was specific for the 6-phosphate group and the equatorial HO-4 of D-mannose 6-phosphate. On Western blot analysis using the purified antibodies, ovalbumin, which contained a typical high-mannose type of oligosaccharide, was not recognised. However, a testicular glycoprotein fraction formed an immunostaining band. These results indicate the effectiveness of the conjugate as a ligand for mannose 6-phosphate receptors. The antibodies highly specific for mannose 6-phosphate may be used to detect or purify lysosomal enzymes.

Isolation and characterization of the 36-kDa D-mannose 6-phosphate receptor from porcine testis

A D-mannose 6-phosphate receptor was isolated from total membranes of porcine testis, and its interaction with ligands was examined. The receptor was a glycoprotein comprised of several 36-kDa sub-units with an isoelectric point (pI) of 6.1. The binding of the receptor to the insoluble phosphomannan core occurred in the absence of divalent cations, but was selectively stimulated by MnCl2 and effectively inhibited by D-mannose 6-phosphate, D-fructose 1-phosphate, and pentamannosyl monophosphate. The phosphate group and HO-2 of D-mannose 6-phosphate are important in the receptor-ligand interaction, HO-4 probably contributes to a lesser extent, and HO-1 seems to have no interaction.

Mannose 6-phosphate/insulin like growth factor II receptor: the two types of ligands bind simultaneously to one receptor at different sites

Pentamannose 6-phosphate/trilysine substituted aprotinin (PMP-lys-aprotinin) and insulin like growth factor II (IGF II) were used as affinity ligands for the mannose 6-phosphate (M6P) and IGF II binding sites of the M6P/IGF II receptor. Both ligands were cross linked to intact receptor and tryptic fragments of the receptor. The pattern of receptor fragments with M6P and IGF II binding sites differed indicating that the two binding sites are located on different segments of the receptor. The receptor was incubated with [125I]IGF II and pentamannose 6-phosphate substituted bovine serum albumin (PMP-BSA). From these mixtures [125I]IGF II receptor complexes could be precipitated with antibodies against the PMP-BSA indicating that the M6P/IGF II receptor can bind simultaneously IGF II and M6P-containing ligands.

46-kDa mannose 6-phosphate-specific receptor: purification, subunit composition, chemical modification

A cation-dependent mannose 6-phosphate-specific receptor has recently been isolated from murine P388D1 macrophages and bovine liver (B. Hoflack & S. Kornfeld, (1985) J. Biol. Chem. 260, 12008-12014). The receptor purified from human liver has a subunit molecular size of 43 kDa, is rich in hydrophobic and charged amino acids and contains threonine at the N-terminus. The receptors from human and rat liver are antigenically related. Both are immunologically distinct from the cation-independent 215-kDa mannose 6-phosphate-specific receptor from human liver. Cross-linking experiments indicate that the cation-dependent receptor exists in solution as a tetramer. Modification of arginine and histidine residues, reduced drastically the binding of the receptor to immobilized ligands. Presence of mannose 6-phosphate during modification of arginine residues protected the binding properties of the receptor, suggesting that arginine is a constituent of the mannose 6-phosphate binding site of the receptor. The significance of the inability of histidine-modified receptors to bind ligands remains to be established.

Multiple carbohydrate receptors on lymphocytes revealed by adhesion to immobilized polysaccharides

Phosphomannan polysaccharides and fucoidan, a polymer of fucose 4-sulfate, have been demonstrated to inhibit adhesion of lymphocytes to tissue sections that contain high endothelial venules (Stoolman, L. M., T. S. Tenforde, and S. D. Rosen, 1984, J. Cell Biol., 99:1535-1540). We have investigated the potential cell surface carbohydrate receptors involved by quantitating adhesion of rat cervical lymph node lymphocytes to purified polysaccharides immobilized on otherwise inert polyacrylamide gels. One-sixth of the lymphocytes adhered specifically to surfaces derivatized with PPME (a phosphomannan polysaccharide prepared from Hansenula holstii yeast), whereas up to half of the cells adhered to surfaces derivatized with fucoidan. Several lines of evidence demonstrated that two distinct receptors were involved. Adhesion to PPME-derivatized gels was labile at 37 degrees C (decreasing to background levels within 120 min) whereas adhesion to fucoidan-derivatized gels was stable. Soluble PPME and other phosphomannans blocked adhesion only to PPME-derivatized gels; fucoidan and a structurally related fucan blocked adhesion to fucoidan-derivatized gels. Other highly charged anionic polysaccharides, such as heparin, did not block adhesion to either polysaccharide-derivatized gel. Adhesion to PPME-derivatized gels was dependent on divalent cations, whereas that to fucoidan-derivatized gels was not. The PPME-adherent lymphocytes were shown to be a subpopulation of the fucoidan-adhesive lymphocytes which contained both saccharide receptors. These data reveal that at least two distinct carbohydrate receptors can be found on peripheral lymphocytes.

Is movement of mannose 6-phosphate-specific receptor triggered by binding of lysosomal enzymes?

Mannose 6-phosphate-specific receptors with an apparent molecular mass of 215,000 are present in fibroblasts at the cell surface and in intracellular membranes. The cell surface receptors mediate endocytosis of exogenous lysosomal enzymes and exchange with the intracellular receptors, which function in the sorting of endogenous lysosomal enzymes. In the present study, several methods independent of receptor ligands were designed in order to examine the exchange of receptors under conditions where receptor-ligand complexes do not dissociate (weak bases and monensin) or where receptor-ligand complexes are not formed due to absence of endogenous ligands as a result of inhibition of protein synthesis. Weak bases and monensin reduce the concentration of receptors at the cell surface by 20-30% and free cell surface receptors were replaced by occupied receptors. The latter continued to be exchanged with internal ligand-occupied receptors and the rates of the exchange were similar to the control values. The exchange of receptors between the cell surface and internal membranes was also not affected when the receptor ligands were depleted from the transport compartments by treating the cells with cycloheximide for up to 10 h. We conclude from these results that movement of mannose 6-phosphate-specific receptors along the endocytosis and sorting pathways is constitutive and not triggered by binding or dissociation of ligands.

The carbohydrate specificity of conglutinin and its homology to proteins in the hepatic lectin family

Inhibition experiments with D-mannose oligosaccharides establish that conglutinin recognises terminal alpha 1----2 mannobiosyl units present in the glycopeptide of the alpha-chain of the complement component C3b. On the basis of its three domain structure and the homology of its N-terminal amino-acid sequence to that of the dog pulmonary surfactant protein, it is proposed that conglutinin is a member of the hepatic lectin family.

Heterozygosity for phosphodiester glycosidase deficiency: a novel human mutation of lysosomal enzyme processing

We have carried out studies on the fibroblasts of III-3, a clinically normal Lebanese individual previously reported to have abnormally high plasma lysosomal enzyme levels. Mannose-6-phosphate (man-6-P) receptors in III-3 fibroblasts were found to be functioning normally, but the cells had only half normal levels of phosphodiester glycosidase activity. Pinocytosis of III-3 fibroblast secreted beta-hexosaminidase B (hex B) into Sandhoff disease fibroblasts was 18% of control, and the apparent KD for binding of III-3 hex B to man-6-P receptors was 3.7 X 10(-9) M compared to 1.25 X 10(-9) M for control enzyme. Hex B secreted by III-3 fibroblasts included an enzyme pool less electro-negative than control enzyme which had a very low affinity for man-6-P receptors and which did not bind to DEAE-Sephadex. Treatment of this abnormal hex B with exogenous placental phosphodiester glycosidase increased its binding to man-6-P receptors three-fold. Secretion rates of seven lysosomal enzymes from III-3 fibroblasts were, on average, twice as great as rates measured for two I-cell disease heterozygote fibroblast lines. The results suggest that III-3 fibroblasts are heterozygous for phosphodiester glycosidase deficiency. The possibility that an individual homozygous for this enzyme deficiency would develop I-cell disease is discussed.

Lysosomal enzyme binding to mouse P388D1 macrophage membranes lacking the 215-kDa mannose 6-phosphate receptor: evidence for the existence of a second mannose 6-phosphate receptor

Mouse P388D1 macrophages target newly synthesized acid hydrolases to lysosomes in spite of their lack of the 215-kDa mannose 6-phosphate (Man-6-P) receptor. We now report that these cells contain a membrane-associated Man-6-P receptor that is distinct from the previously described receptor. The new receptor binds lysosomal enzymes containing phosphomannosyl residues. This binding is inhibited by Man-6-P or by pretreatment of the lysosomal enzymes with alkaline phosphatase. Lysosomal enzyme binding occurs at neutral pH and dissociation of the bound ligand occurs at low pH values comparable to those found within endosomes or lysosomes. The new receptor differs from the 215-kDa Man-6-P receptor in two ways. It has an absolute requirement for divalent cations and is unable to bind Dictyostelium discoideum lysosomal enzymes, which contain methylphosphomannosyl residues rather than the usual phosphomannosyl monoesters. Based on the difference in cation requirement, we suggest that the 215-kDa receptor be referred to as Man-6-P receptor CI (cation independent) and the new receptor as Man-6-P receptor CD (cation dependent). We conclude that the Man-6-P receptor CD functions in the targeting of newly synthesized acid hydrolases to lysosomes in P388D1 macrophages.