Poly-N-acetyllactosamine glycans of cellular glycoproteins: predominance of linear chains in mouse neuroblastoma and rat pheochromocytoma cell lines

Nucleoplasmic and cytoplasmic glycoproteins

We have described a new form of protein glycosylation in which N-acetylglucosamine is glycosidically linked to the hydroxyl of serine or threonine (O-GlcNAc). Unlike most other forms of protein glycosylation, O-GlcNAc is predominantly localized in the nuclear and cytoplasmic compartments of cells, where it occurs on important nuclear pore glycoproteins, well-characterized cytoskeletal proteins, as well as on many chromatin proteins, including factors that regulate gene transcription. Gas-phase protein sequencing of three O-GlcNAc-modified proteins has identified a common structural feature at sites of O-GlcNAc addition. An assay for UDP-GlcNAc:polypeptide O-GlcNAc transferase has been developed. The enzyme appears to be membrane-associated, its active site is cytoplasmic, and it has an absolute requirement for Mn2+. We are now purifying this glycosyltransferase, characterizing its substrate specificity, and determining the extent of elongation of attached saccharide moieties. The functions of O-GlcNAc remain largely unknown, but it may be important in blocking phosphorylation sites, it may be required for the assembly of specific multiprotein complexes, it might serve as a nuclear transport signal, or it may be directly involved in the active transport of macromolecules across nuclear pores.

Structural and biological properties of the carbohydrate units of nervous tissue glycoproteins

We have identified structures in nervous tissue glycoproteins that are novel for glycoproteins in general or enriched in nervous tissue or cells of neural origin. These include: (alpha 2-8)-linked polysialic acid units, the linear form of poly-N-acetyllactosamine glycans, the sialylated X antigen determinant NeuAc(alpha 2-3)-Gal(beta 1-4) [Fuc(alpha 1-3)]GlcNAc, a series of Man-O-Ser(Thr)-linked glycans, and the O-glycosidically linked disaccharide unit Gal(alpha 1-3)GalNAc. The polysialic and poly-N-acetyllactosamine glycans are also developmentally regulated. The polysialic acid units in the cell adhesion molecule N-CAM. The poly-N-acetyllactosamine units occur in the adhesion molecule NILE (which is immunologically similar to Ng-CAM and L1) and in some other components revealed by a cell surface-labelling method specific for these glycans. The mannose-linked glycans occur in a chondroitin sulphate proteoglycan involved in neuron-glia interactions. Other biological interactions of the carbohydrates include their serving as bacterial receptors in meningitis, their serving as models for molecular mimicry by the capsules of meningitis-causing bacteria, and the role of some structures as antigens in autoimmune conditions. At the molecular level, two types of mechanisms are suggested for the glycans in molecular interactions: they may function either as mediators of interactions by serving as specific recognition ligands, or as modulators of the interactions determined by polypeptides or other molecules.

Activity of lysosomal exoglycosidases in human gliomas

There is a lot of data suggesting that modifications of cell glycoconjugates may be important in progression of cancer. In the present work we studied activities of lysosomal exoglycosidases: beta-hexosaminidase and its isoenzymes A and B, beta-galactosidase and alpha-mannosidase, in human gliomas. Enzyme activity was determined spectrophotometrically based on the release of p-nitrophenol from p-nitrophenyl-derivative of appropriate sugars. The activities of the exoglycosidases tested were significantly higher in malignant glial tumors than in control tissue (normal brain tissue) and non-glial tumors. The highest activities of exoglycosidases were observed in high-grade gliomas, and a positive correlation of enzyme activities and degree of malignancy was noted. Our results suggest that lysosomal exoglycosidases may participate in the progression and dynamical development of glial tumors.

Occurrence of poly-N-acetyllactosamine synthesis in Sf-9 cells upon transfection of individual human beta-1,4-galactosyltransferase I, II, III, IV, V and VI cDNAs

Lectin blot analysis of membrane glycoprotein samples from Sf-9 cells upon transfection of individual human beta-1,4-galactosyltransferase (beta-1,4-GalT) I, II, III, IV, V et VI cDNAs showed that the endogenous N-linked oligosaccharides are galactosylated (Guo et al., Glycobiology (2001), in press). Further analysis revealed that membrane glycoprotein samples from all the gene-transfected cells are also reactive to Lycopersicon esculentum agglutinin (LEA) et Datura stramonium agglutinin (DSA), both of which bind to oligosaccharides with poly-N-acetyllactosamine chains while no lectin reactive protein bands are detected when blots are pretreated with a mixture of diplococcal beta-1,4-galactosidase et jack bean beta-N-acetylhexosaminidase or N-glycanase. Analysis of endo-beta-galactosidase-digestion products revealed the presence of the Gal1-->GlcNAc1-->Gal and/or GlcNAc1-->Gal structures in the gene-transfected cells. When the homogenates of the gene-transfected cells were used as enzyme sources towards oligosaccharides with the GlcNAc beta 1-->(3Gal beta 1-->4GlcNAc)(1-3) structures, human recombinant beta-1,4-GalTs I et II galactosylated these oligosaccharides more effectively than other beta-1,4-GalTs. These results indicate that beta-1,4-GalTs I-VI can synthesize poly-N-acetyllactosamine chains with beta-1,3-N-acetylglucosaminyltransferase.

Biogenesis of multilamellar bodies via autophagy

Transfection of Mv1Lu mink lung type II alveolar cells with beta1-6-N-acetylglucosaminyl transferase V is associated with the expression of large lysosomal vacuoles, which are immunofluorescently labeled for the lysosomal glycoprotein lysosomal-associated membrane protein-2 and the beta1-6-branched N-glycan-specific lectin phaseolis vulgaris leucoagglutinin. By electron microscopy, the vacuoles present the morphology of multilamellar bodies (MLBs). Treatment of the cells with the lysosomal protease inhibitor leupeptin results in the progressive transformation of the MLBs into electron-dense autophagic vacuoles and eventual disappearance of MLBs after 4 d of treatment. Heterologous structures containing both membrane lamellae and peripheral electron-dense regions appear 15 h after leupeptin addition and are indicative of ongoing lysosome-MLB fusion. Leupeptin washout is associated with the formation after 24 and 48 h of single or multiple foci of lamellae within the autophagic vacuoles, which give rise to MLBs after 72 h. Treatment with 3-methyladenine, an inhibitor of autophagic sequestration, results in the significantly reduced expression of multilamellar bodies and the accumulation of inclusion bodies resembling nascent or immature autophagic vacuoles. Scrape-loaded cytoplasmic FITC-dextran is incorporated into lysosomal-associated membrane protein-2-positive MLBs, and this process is inhibited by 3-methyladenine, demonstrating that active autophagy is involved in MLB formation. Our results indicate that selective resistance to lysosomal degradation within the autophagic vacuole results in the formation of a microenvironment propicious for the formation of membrane lamella.

Identification of a major poly-N-acetyllactosamine-containing cell-surface glycoprotein of mouse teratocarcinoma cells. Appearance on cells induced to primitive endoderm but not parietal endoderm differentiation

Mouse teratocarcinoma F9 cells were induced to primitive endoderm differentiation with retinoic acid, and poly-N-acetyllactosamine-containing surface glycoproteins were identified by radiolabelling endo-beta-galactosidase-cleavable glycans with galactosyltransferase and radiolabelled UDP-galactose. One major radiolabelled band with an apparent size of 250-500 kDa was identified which differed from the known poly-N-acetyllactosamine-containing glycoproteins laminin, fibronectin, lysosome-associated membrane protein (LAMP)-1 and LAMP-2. This acidic glycoprotein, resistant to glycosaminoglycan-degrading enzymes and proteases, was purified by extraction and phase partition with Triton X-114, octyl Sepharose and Helix pomatia lectin chromatography. The purified glycoprotein could be digested by endo-beta-galactosidase and glycopeptide N-glycosidase F to an apparent size of 160-240 kDa. During retinoic-acid-induced differentiation into primitive endoderm cells, the glycoprotein showed a several-fold increase and a broadening to an apparent size of 200- > 700 kDa. The glycoprotein was no longer detected in retinoic-acid and dibutyryl-cAMP-treated cells which had undergone further differentiation to parietal endoderm cells, nor in the permanently differentiated parietal endoderm line F9-AC. The results suggest that the glycoprotein is a major carrier of poly-N-acetyllactosamine chains on differentiating teratocarcinoma F9 cells, and that its expression as revealed by the poly-N-acetyllactosamine labelling method is regulated by the stage of cellular differentiation.

Immunocytochemical analysis of a novel carbohydrate differentiation antigen (CDA-3C2) associated with olfactory and otic systems during embryogenesis in the rat

Carbohydrate differentiation antigens are known to display specific patterns of expression during mammalian development and are thought to participate in significant morphogenetic events. In the present study, two monoclonal antibodies that react with a novel carbohydrate differentiation antigen (CDA-3C2) were used to analyze, by light microscopy, the spatiotemporal distribution of this unique high molecular weight antigen during embryogenesis in the rat. Correlative analysis of the development of peripheral neural structures, in which CDA-3C2 was expressed, was carried out with an anti-neurofilament antibody. Enzymatic digestion, combined with Western blots, reveal that the CDA-3C2 epitope is a carbohydrate which is carried on a high molecular weight glycoprotein with a mass of greater than 1 million Daltons. Characteristic of carbohydrate antigens, immunoreactivity was found in several distinct cellular patterns: only along the apical border of cells, along lateral and basal membranes of cells, and extracellular-like staining in the mesenchyme. During neurulation, CDA-3C2 showed differential staining in the ectoderm, distinguishing lateral from neural regions. Following closure of the neural tube, there was a striking specificity of expression of CDA-3C2 in the periphery, found almost exclusively in olfactory and otic epithelial structures. While CDA-3C2 is found in placode-derived tissues that subserve sensory transduction, it appears to be primarily associated with the supportive cells (and their secretions) in both otic and olfactory regions and less so with the sensory cells. The data suggest that a unique carbohydrate antigen on a large macromolecule may play a role in neurulation and/or morphogenesis of the placode-derived otic and olfactory structures.

Soluble galactoside-binding vertebrate lectins: a protein family with common properties

1. Soluble galactoside-binding lectins could play a key role in vertebrates by specifical binding to complementary glycoconjugates. 2. Their expression and localization are developmentally regulated. 3. They constitute a large family of structurally related proteins which contain a series of conserved aminoacids. 4. Their functional role could vary from an organ to another, and the same lectin may probably mediate several biological activities.

Lactose- and heparin-inhibitable agglutinin from human fetal brain

An agglutinin activity which was sensitive to lactose and heparin was estimated during prenatal brain development. The agglutinin showed higher specific activities in cerebral cortex and midbrain. There was an increase in lectin specific activity in all the brain regions with development. In addition to brain, other fetal organs also showed the presence of developmentally regulated agglutinin. Cerebral cortical agglutinin was purified by Sepharose CL-6B gel filtration and asialofetuin- and heparin-Sepharose affinity chromatography. Purified agglutinin was strongly inhibited by lactose, asialofetuin, and heparin. It showed no requirement for divalent cations and was maximally active at pH 8.0. Electrophoretic characterization showed the aggregate nature of the agglutinin, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave subunit molecular weights of 58,000, 45,000, and 24,000.

Poly(N-acetyllactosaminyl) oligosaccharides of chromaffin granule membrane glycoproteins

Poly(N-acetyllactosaminyl) oligosaccharides have been identified, on the basis of their susceptibility to endo-beta-galactosidase, in a large-molecular-size glycopeptide fraction derived from chromaffin granule membrane glycoproteins. The glycoproteins containing poly(N-acetyl-lactosaminyl) oligosaccharides were selectively labeled by treatment of chromaffin granule membranes with endo-beta-galactosidase to expose N-acetylglucosamine residues, followed by incubation with galactosyltransferase and UDP-[14C]galactose. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and fluorography demonstrated specific labeling in the 41-47 kilodalton (kD) region and in a distinct band at 90 kDa. Two-dimensional SDS-PAGE revealed that the poly(N-acetyllactosaminyl) oligosaccharides are predominantly present in glycoprotein IV, together with lesser labeling of glycoproteins II and III, whereas they are absent from dopamine beta-hydroxylase and carboxypeptidase H, which are the major glycoproteins of chromaffin granule membranes.