Secretion of unphosphorylated and phosphorylated xyloside-induced glycosaminoglycan chains

Saccharide Primers Comprising Xylosyl-Serine Primed Phosphorylated Oligosaccharides Act as Intermediates in Glycosaminoglycan Biosynthesis

β-Xylosides have been used as an artificial initiator of glycosaminoglycan (GAG) biosynthesis to investigate its mechanism and to obtain these oligosaccharides. In GAG biosynthesis, phosphorylation on the xylose residue is a crucial step. However, little attention has been paid to phosphorylated oligosaccharides obtained from β-xylosides. In a previous study, we demonstrated that a novel β-xyloside, N-lauryl-O-β-xyloyranosyl-serinamide (Xyl-Ser-C12), had excellent GAG-type oligosaccharide priming ability, whereas phosphorylated oligosaccharides were not found in the primed oligosaccharides. This study examines the potential of Xyl-Ser-C12 and three of its derivatives for use as a probe to investigate the GAG biosynthesis mechanism. Glycosylated products were obtained by incubation of the β-xylosides in normal human dermal fibroblast cells and compared by liquid chromatography-electrospray ionization-mass spectrometry. By the optimized method to detect phosphorylated products, Xyl-Ser-C12 was demonstrated to prime not only GAG-type oligosaccharides but also a variety of xylose-phosphorylated products. Among the synthesized β-xylosides, those consisting of xylosyl-serine primed large amounts of phosphorylated and GAG-type oligosaccharides, whereas the others primed sialyloligosaccharides mainly. The majority of the phosphorylated products were considered to be GAG intermediates, which are less observed in nature. To our best knowledge, this is the first report showing that the amino acid residues around the Xyl attachment position strongly affect the phosphorylation efficiency and GAG chain-priming ability of β-xylosides. This study leads to the possibility of the use of β-xyloside as a probe to observe the Xyl phosphorylation process during GAG biosynthesis and investigate comparative glycosaminoglycomics between different cells.

Synthesis and sorting of proteoglycans

Proteoglycans are widely expressed in animal cells. Interactions between negatively charged glycosaminoglycan chains and molecules such as growth factors are essential for differentiation of cells during development and maintenance of tissue organisation. We propose that glycosaminoglycan chains play a role in targeting of proteoglycans to their proper cellular or extracellular location. The variability seen in glycosaminoglycan chain structure from cell type to cell type, which is acquired by use of particular Ser-Gly sites in the protein core, might therefore be important for post-synthesis sorting. This links regulation of glycosaminoglycan synthesis to the post-Golgi fate of proteoglycans.

Chinese hamster ovary cell mutants defective in glycosaminoglycan assembly and glucuronosyltransferase I

The proteoglycans of animal cells typically contain one or more heparan sulfate or chondroitin sulfate chains. These glycosaminoglycans assemble on a tetrasaccharide primer, -GlcAbeta1, 3Galbeta1,3Galbeta1,4Xylbeta-O-, attached to specific serine residues in the core protein. Studies of Chinese hamster ovary cell mutants defective in the first or second enzymes of the pathway (xylosyltransferase and galactosyltransferase I) show that the assembly of the primer occurs by sequential transfer of single monosaccharide residues from the corresponding high energy nucleotide sugar donor to the non-reducing end of the growing chain. In order to study the other reactions involved in linkage tetrasaccharide assembly, we have devised a powerful selection method based on induced resistance to a mitotoxin composed of basic fibroblast growth factor-saporin. One class of mutants does not incorporate 35SO4 and [6-3H]GlcN into glycosaminoglycan chains. Incubation of these cells with naphthol-beta-D-xyloside (Xylbeta-O-Np) resulted in accumulation of linkage region intermediates containing 1 or 2 mol of galactose (Galbeta1, 4Xylbeta-O-Np and Galbeta1, 3Galbeta1, 4Xylbeta-O-Np) and sialic acid (Siaalpha2,3Galbeta1, 3Galbeta1, 4Xylbeta-O-Np) but not any GlcA-containing oligosaccharides. Extracts of the mutants completely lacked UDP-glucuronic acid:Galbeta1,3Gal-R glucuronosyltransferase (GlcAT-I) activity, as measured by the transfer of GlcA from UDP-GlcA to Galbeta1,3Galbeta-O-naphthalenemethanol (<0.2 versus 3.6 pmol/min/mg). The mutation most likely lies in the structural gene encoding GlcAT-I since transfection of the mutant with a cDNA for GlcAT-I completely restored enzyme activity and glycosaminoglycan synthesis. These findings suggest that a single GlcAT effects the biosynthesis of common linkage region of both heparan sulfate and chondroitin sulfate in Chinese hamster ovary cells.

Initiation of galactosaminoglycan biosynthesis. Separate galactosylation and dephosphorylation pathways for phosphoxylosylated decorin protein and exogenous xyloside

By using various radiolabelled precursors, glycosylation and phosphorylation of decorin in a rat fibroblast cell line was investigated in the presence of increasing concentrations of p-nitrophenyl-O-beta-d-xylopyranoside. Decorin core protein glycanation was suppressed to approximately 25% of the normal level in the presence of 2 mm and 3 mm xyloside. Glycans/saccharides were released from the core protein and size-separated by gel chromatography. The intracellular decorin obtained from cells treated with 2 mm xyloside was substituted with Xyl and also with Gal-Xyl and Gal-Gal-Xyl, but not with longer saccharides. Only the trisaccharide contained an almost fully phosphorylated Xyl. We conclude that galactosylation of endogenous, xylosylated decorin and exogenous xyloside probably follow separate pathways or that xylosides and early decorin glycoforms are kept separated. At the addition of the first glucuronic acid the two pathways seem to merge and dephosphorylation of decorin takes place. Xyloside-primed and secreted galactosaminoglycan chains produced simultanously retained phosphorylated Xyl. Inadequate dephosphorylation could be due to excess substrate or to a short transit.time. As shown previously [Moses, J., Oldberg, A., Eklund, E. & Fransson, L.-A. (1997) Eur. J. Biochem. 248, 767-774], brefeldin A-arrested decorin is substituted with the linkage-region extended with an undersulphated and incomplete galactosaminoglycan chain. In cells treated with this drug, xylosides were unable to prime galactosaminoglycan synthesis and unable to inhibit glycosylation and phosporylation of decorin.

Influence of monosaccharide derivatives on liver cell glycosaminoglycan synthesis: 3-deoxy-D-xylo-hexose (3-deoxy-D-galactose) and methyl (methyl 4-chloro-4-deoxy-beta-D-galactopyranosid) uronate

An improved, convenient synthesis of 3-deoxy-D-xylo-hexose (3-deoxy-D-galactose) has been developed, and the chemical synthesis of a novel monosaccharide derivative, methyl (methyl 4-chloro-4-deoxy-beta-D-galactopyranosid)uronate (compound 10), is described. Using primary hepatocytes in culture, each was used to explore its effect on glycosaminoglycan (GAG) synthesis. In the absence of analogues hepatocytes synthesize primarily (92-95%) heparan sulphate. At 1 mM, 3-deoxy-D-galactose had little observable effect on either liver cell GAG or protein synthesis. At 10 mM and 20 mM, 3-deoxy-D-galactose reduced [3H]glucosamine and 35SO4 incorporation into hepatocyte cellular GAGs to, respectively, 75% and 60% of the control cells. This inhibition of GAG synthesis occurred without any effect on hepatocyte protein synthesis, indicating that 3-deoxy-D-galactose's effect on GAG synthesis is not mediated through an inhibition of proteoglycan core protein synthesis. Furthermore, GAGs in the presence of 20 mM of the analogue were significantly reduced in size, 17 kDa vs. 66 kDa in untreated cells. These results reflect either impaired cellular GAG chain elongation, and/or altered GAG chain degradation. Compound 10 exhibited a concentration-dependent inhibition of both hepatocyte cellular GAG and protein synthesis. At concentrations of 5, 10 and 20 mM, compound 10 inhibited GAG and protein synthesis by 20, 65 and 90%, respectively. Exogenous uridine was able to restore partially the inhibition of protein synthesis, but was unable to reverse the effect of compound 10 on GAG synthesis. These results show that part of the effect of compound 10 on GAG synthesis is not mediated by an inhibition of proteoglycan core protein synthesis. GAGs in the presence of compound 10 are half as large as those in the absence of this compound (33 and 66 kDa, respectively). These results again may reflect either impaired cellular GAG chain elongation and/or altered GAG chain degradation. Potential metabolic routes for each analogue's effect are presented.

A method for determination of galactosyltransferase I activity synthesizing the proteoglycan linkage region

An assay method was devised for measuring the activity of galactosyltransferase I (UDP-D-galactose:D-xylose galactosyltransferase), which is one of the enzymes synthesizing the linkage region between the core protein and glycosaminoglycan chains of proteoglycan. For this method, the reaction mixture contained a fluorescent substrate, 4-methylumbelliferyl-beta-D-xyloside as an acceptor, UDP-galactose as a donor and D-galactal as a competitive inhibitor of endogenous beta-galactosidase in the enzyme solution. The reaction mixture was incubated at 37 degrees C with enzyme solution prepared from an extract of cultured cells, and galactosyl-xylosyl-4-methylumbelliferone was produced as a reaction product. Measurement of galactosyltransferase I activity was performed by separation and quantitative analysis of this reaction product using high-performance liquid chromatography. Utilizing this method, easier and more sensitive detection of galactosyltransferase I activity in a cell-free system became possible. Application of the method revealed that cultured human skin fibroblasts contained galactosyltransferase I activity.

Sequence analysis of p-hydroxyphenyl-O-beta-D-xyloside initiated and radio-iodinated dermatan sulfate from skin fibroblasts

To generate xyloside-primed dermatan sulfate suitable for sequence analysis, skin fibroblasts were incubated with p-hydroxyphenyl-beta-D-xylopyranoside and [3H]galactose, and free [3H]glycosaminoglycan chains were isolated from the culture medium by ion exchange and gel chromatography. After 125I labelling of their reducing-terminal hydroxyphenyl groups, chains were subjected to various chemical and enzymatic degradations, both partial and complete, followed by gradient polyacrylamide gel electrophoresis and autoradiographic identification of fragments extending from the labelled reducing-end to the point of cleavage. Results of periodate oxidation-alkaline scission indicated that the xylose moiety remained unsubstituted at C-2/C-3; exhaustive treatment with chondroitin AC-I lyase afforded the fragment delta HexA-Gal-Gal-Xyl-R (R = radio-iodinated hydroxyphenyl group), and complete degradations with chondroitin ABC lyase as well as testicular hyaluronidase yielded the fragments delta HexA/HexA-GalNAc-GlcA-Gal-Gal-Xyl-R with or without sulfate on the N-acetylgalactosamine. Partial digestions with testicular hyaluronidase or chondroitin B lyase indicated that glucuronic acid was common in the first three repeats after the linkage region and that iduronic acid could occupy any position thereafter. Hence, there were no indications of a repeated, periodic appearance of the clustered GlcA-GalNAc repeats which was previously observed in proteoglycan derived dermatan sulfate [Fransson L-A, Havsmark B, Silverberg I (1990) Biochem J 269:381-8], suggesting a role for the protein part in controlling the formation of particular copolymeric features during glycosaminoglycan assembly.