Structural analysis by fast-atom-bombardment mass spectrometry of the mixture of alditols derived from the O-linked oligosaccharides of murine glycophorins

Molecular characterisation of transport mechanisms at the developing mouse blood-CSF interface: a transcriptome approach

Exchange mechanisms across the blood-cerebrospinal fluid (CSF) barrier in the choroid plexuses within the cerebral ventricles control access of molecules to the central nervous system, especially in early development when the brain is poorly vascularised. However, little is known about their molecular or developmental characteristics. We examined the transcriptome of lateral ventricular choroid plexus in embryonic day 15 (E15) and adult mice. Numerous genes identified in the adult were expressed at similar levels at E15, indicating substantial plexus maturity early in development. Some genes coding for key functions (intercellular/tight junctions, influx/efflux transporters) changed expression during development and their expression patterns are discussed in the context of available physiological/permeability results in the developing brain. Three genes: Secreted protein acidic and rich in cysteine (Sparc), Glycophorin A (Gypa) and C (Gypc), were identified as those whose gene products are candidates to target plasma proteins to choroid plexus cells. These were investigated using quantitative- and single-cell-PCR on plexus epithelial cells that were albumin- or total plasma protein-immunopositive. Results showed a significant degree of concordance between plasma protein/albumin immunoreactivity and expression of the putative transporters. Immunohistochemistry identified SPARC and GYPA in choroid plexus epithelial cells in the embryo with a subcellular distribution that was consistent with transport of albumin from blood to cerebrospinal fluid. In adult plexus this pattern of immunostaining was absent. We propose a model of the cellular mechanism in which SPARC and GYPA, together with identified vesicle-associated membrane proteins (VAMPs) may act as receptors/transporters in developmentally regulated transfer of plasma proteins at the blood-CSF interface.

Isolation and characterization of glycophorin from nucleated (chicken) erythrocytes

A sialoglycoprotein fraction was isolated from chicken erythrocytes by two methods based on the phenol extraction or chloroform/2-propanol extraction of differently prepared erythrocyte membranes. Both preparations gave in SDS-PAGE two major PAS-stained bands (GP2 and GP3), which migrated as 60- and 33-kDa species, respectively, compared to reference proteins, or as 44- and 23-kDa molecules, compared to human glycophorins. Some less abundant slower migrating PAS-stained components, antigenically related to GP2 and GP3, also were detected. No evidence for the presence of antigenically distinct glycoproteins of leukosialin type was obtained. Interconversion in SDS-PAGE, similar carbohydrate composition, and similar antigenic properties of GP2 and GP3 indicated that they are a dimer and monomer, respectively, of the same glycoprotein which shows properties that allow it to be classified as a glycophorin. Lectin binding studies and methylation analysis of beta-elimination products of chicken glycophorin preparation showed the presence of O-glycans and N-glycans. The major O-glycans include sialylated Galbeta1-3GalNAc units and more complex GlcNAc-containing chains. Among the N-glycans, there are complex-type biantennary structures with a bisecting GlcNAc residue, accompanied by chains with additional antennas linked to alpha-mannose residues. A characteristic feature of the chicken glycophorin is a relatively high proportion of N-glycans to O-glycans, compared to the glycophorin A from human erythrocytes.

Impaired galactosylation of core 2 O-glycans in erythrocytes of beta1,4-galactosyltransferase knockout mice

O- and N-glycans included in erythrocyte membrane glycoproteins from beta1,4-galactosyltransferase I (GalT-I) knockout mice were analyzed to examine how this enzyme deficiency affects glycosylation of proteins in erythroid cells. The results indicated that greater than 80% of core 2 O-glycans from GalT-I-/- mice are not galactosylated by beta1,4 linkage, resulting in the expression of Neu5Acalpha2 --> 3Galbeta1 --> 3(GlcNAcbeta1 --> 6)GalNAc, while core 2 O-glycans from GalT-I+/+ mice are fully galactosylated and occur as Neu5Acalpha2 --> 3Galbeta1 --> 3(Neu5Acalpha2 --> 3Galbeta1 --> 4GlcNAcbeta1 --> 6)GalNAc. On the other hand, beta1, 4-galactosylation of N-glycans of the mutant was approximately 60% that of the wild type. Thus, it is suggested that GalT-I is predominantly responsible for beta1,4-galactosylation of the core 2 O-glycan branch in erythroid cells.

Analysis of protein glycosylation by mass spectrometry

There is a growing pharmaceutical market for protein-based drugs for use in therapy and diagnosis. The rapid developments in molecular and cell biology have resulted in production of expression systems for manufacturing of recombinant proteins and monoclonal antibodies. These proteins are glycosylated when expressed in cell systems with glycosylation ability. For glycoproteins intended for therapeutic administration it is important to have knowledge about the structure of the carbohydrate side chains to avoid cell systems that produce structures, which in humans can cause undesired reactions, e.g., immunological and unfavorable serum clearance rate. Structural analysis of glycoprotein oligosaccharides requires sophisticated instruments like mass spectrometers and nuclear magnetic resonance spectrometers. However, before the structural analysis can be conducted, the carbohydrate chains have to be released from the protein and purified to homogeneity, and this is often the most time-consuming step. Mass spectrometry has played and still plays an important role in analysis of protein glycosylation. The superior sensitivity compared to other spectroscopic methods is its main asset. Structural analysis of carbohydrates faces several problems, however, due to the chemical nature of the constituent monosaccharide residues. For oligosaccharides or glycoconjugates, the structural information from mass spectrometry is essentially limited to monosaccharide sequence, molecular weight, an only in exceptional cases glycosidic linkage positions can be obtained. In order to completely establish an oligosaccharide structure, several other structural parameters have to be determined, e.g., linkage positions, anomeric configuration and identification of the monosaccharide building blocks. One way to address some of these problems is to work on chemical pretreatment of the glycoconjugate, to specifically modify the carbohydrate chain. In order to introduce specific modifications, we have used periodate oxidation and trifluoroacetolysis with the objective of determining glycosidic linkage positions by mass spectrometry.

Determination of linkage positions in peracetylated (methyl) xylo-oligosaccharides with fast atom bombardment mass spectrometry

Distinction between the linkage types 1-->2, 1-->3 and 1-->4 of xylobioses can be achieved on the basis of the unimolecular decomposition spectra of the oxonium ions of the per-O-acetylated methyl glycosides. The spectra of the oxonium ions of various unbranched xylotri-, tetra- and pentaoses allow determination of the linkage position between the xylose residues. This indicates that in unbranched peracetylated xylo-oligosaccharides the linkage between the xylose residues at the non-reducing end can be determined.

Endoplasmic reticulum-through-Golgi transport assay based on O-glycosylation of native glycophorin in permeabilized erythroleukemia cells: role for Gi3

An assay for endoplasmic reticulum (ER)-through-Golgi transport has been developed in streptolysin O-permeabilized murine erythroleukemia (MEL) cells. The reporter proteins are metabolically labeled native murine glycophorins, which display a distinctive shift in electrophoretic mobility after acquisition of O-linked oligosaccharides. The O-linked sugars are acquired at a site distal to a brefeldin A block, presumably in a cis Golgi compartment, and sialylation occurs in middle and/or trans Golgi compartments. In permeabilized cells supplemented with cytosolic proteins and an ATP-generating system, 20-50% of the radiolabeled precursor glycophorins can be converted to the mature, sialylated form. This maturation process is ATP- and cytosol-dependent and is blocked by guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]). Electron microscopy of permeabilized MEL cells shows retention of ER elements, stacked Golgi cisternae, free polysomes, and other subcellular components. In the presence of GTP[gamma S], dilated vesicles accumulate around the Golgi stacks. Antisera to the carboxyl terminus of the Golgi resident alpha subunit of Gi3 inhibit maturation of glycophorin. To our knowledge, a transport assay utilizing O-glycosylation of an endogenous protein as a monitor of ER-through-Golgi traffic in permeabilized cells has not been reported previously. Furthermore, the data provide evidence for heterotrimeric GTP-binding protein involvement in Golgi function.

An improved approach to the analysis of the structure of small oligosaccharides of glycoproteins: application to the O-linked oligosaccharides from human glycophorin A

Treatment of purified human glycophorin A with alkaline borohydride cleaved the oligosaccharide side chains to yield alditol derivatives that were separated by gel filtration into three mixtures of low molecular weight compounds. Each mixture was oxidised with periodate, and the products were reduced with borohydride and analysed after acetylation or methylation by GLC-MS and FABMS. The resulting data allowed the monosaccharide sequence and linkage positions to be assigned to each component of the mixtures. The anomeric configuration was determined by 1H NMR spectroscopy of the intact fractions. The structures of a desialylated tetrasaccharide, two monosialylated trisaccharides, and five other minor products were defined.

Mass spectrometry of high-mannose oligosaccharides after trifluoroacetolysis and periodate oxidation

High-mannose oligosaccharides were treated with a mixture of trifluoroacetic acid and trifluoroacetic anhydride under conditions where reducing terminal N-acetylglucosamine residues are specifically degraded. The resulting mannose-containing products were, after further chemical modifications, analyzed by mass spectrometry in fast atom bombardment and electron ionization modes. Binding positions between monosaccharide residues were deduced from mass spectra of peracetylated compounds, which, prior to the derivatization, had been subjected to periodate oxidation and borodeuteride reduction.

A procedure for the analysis by mass spectrometry of the structure of oligosaccharides from high-mannose glycoproteins

A strategy, based on mass spectrometry, for analysis of the structure of oligosaccharides obtained from high-mannose glycoproteins, is described. The oligosaccharides are analysed first by f.a.b.-m.s. as the acetylated alditols, then O-deacetylated, oxidised with periodate, and reduced with borodeuteride. The products are analysed further by f.a.b.-m.s. after acetylation and subsequently by both f.a.b.- and e.i.-m.s. after methylation. The entire procedure is carried out on the same sample and the data allow assignment of the positions of all of the glycosidic linkages, including those of the branched residues. Individual components in mixtures of isomeric compounds can often be identified from f.a.b.-m.s. by their molecular weights after periodate oxidation.

Structural analysis of the N-linked oligosaccharides from murine glycophorin

Glycophorins, isolated from BALB/c mouse erythrocytes, were degraded under mild and strong reductive alkaline conditions and the N-linked oligosaccharides were isolated as alditols. The oligosaccharide alditols were fractionated and purified using gel filtration, concanavalin A-Sepharose affinity chromatography, and high-performance ion-exchange chromatography. Structural analysis was carried out by chemical analyses, periodate oxidation in combination with fast atom bombardment mass spectrometry, and 500-MHz 1H NMR spectroscopy. The results revealed the presence of sialylated biantennary, triantennary, and tetraantennary complex type oligosaccharides, all fucosylated at the innermost N-acetylglucosamine residue. The tri- and tetraantennary oligosaccharide-containing fractions also contained species elongated by one and/or two N-acetyllactosamine (-3Gal beta 1-4GlcNAc beta 1-) sequences. The N-linked oligosaccharides were shown to be combined only with one (the low molecular weight) of the two mouse glycophorins.

Analysis of glycoprotein oligosaccharides by fast atom bombardment mass spectrometry

A strategy is outlined for isolation and structural analysis of oligosaccharides derived from glycoproteins. Oligosaccharides, N- and O-linked, are released by chemical and enzymatic methods and separated using gel filtration, concanavalin A affinity chromatography and high-performance ion-exchange chromatography. Structural studies are carried out by fast atom bombardment mass spectrometry. The structural information from the latter can be extended to include determination of binding positions between monosaccharide residues. Prior to the analysis, samples are subjected to periodate oxidation, reduction and permethylation. The positions of glycosidic linkages are deduced from the spectrum by the primary and secondary sequence ions. Structural information can be obtained from mixtures of isomeric compounds.