Assessment of glycosaminoglycan-protein linkage tetrasaccharides as acceptors for GalNAc- and GlcNAc-transferases from mouse mastocytoma

Histories of Dermatan Sulfate Epimerase and Dermatan 4-O-Sulfotransferase from Discovery of Their Enzymes and Genes to Musculocontractural Ehlers-Danlos Syndrome

Dermatan sulfate (DS) and its proteoglycans are essential for the assembly of the extracellular matrix and cell signaling. Various transporters and biosynthetic enzymes for nucleotide sugars, glycosyltransferases, epimerase, and sulfotransferases, are involved in the biosynthesis of DS. Among these enzymes, dermatan sulfate epimerase (DSE) and dermatan 4-O-sulfotranserase (D4ST) are rate-limiting factors of DS biosynthesis. Pathogenic variants in human genes encoding DSE and D4ST cause the musculocontractural type of Ehlers-Danlos syndrome, characterized by tissue fragility, joint hypermobility, and skin hyperextensibility. DS-deficient mice exhibit perinatal lethality, myopathy-related phenotypes, thoracic kyphosis, vascular abnormalities, and skin fragility. These findings indicate that DS is essential for tissue development as well as homeostasis. This review focuses on the histories of DSE as well as D4ST, and their knockout mice as well as human congenital disorders.

Molecular cloning and expression of a second chondroitin N-acetylgalactosaminyltransferase involved in the initiation and elongation of chondroitin/dermatan sulfate

We identified a novel human chondroitin N-acetylgalactosaminyltransferase, designated chondroitin GalNAcT-2 after a BLAST analysis of the GenBank(TM) data base using the sequence of a previously described human chondroitin N-acetylgalactosaminyltransferase (chondroitin GalNAcT-1) as a probe. The new cDNA sequence contained an open reading frame encoding a protein of 542 amino acids with a type II transmembrane protein topology. The amino acid sequence displayed 60% identity to that of human chondroitin GalNAcT-1. Like chondroitin GalNAcT-1, the expression of a soluble form of the protein in COS-1 cells produced an active enzyme, which not only transferred beta1,4-N-acetylgalactosamine (GalNAc) from UDP-[(3)H]GalNAc to a polymer chondroitin representing growing chondroitin chains (beta-GalNAc transferase II activity) but also to GlcUA beta 1-3Gal beta 1-O-C(2)H(4)NHCbz, a synthetic substrate for beta-GalNAc transferase I that transfers the first GalNAc to the core tetrasaccharide in the protein-linkage region of chondroitin sulfate. In contrast, the tetrasaccharide serine (GlcUA beta 1-3Gal beta 1-3Gal beta 1-4Xyl beta 1-O-Ser) derived from the linkage region, which is an inert acceptor substrate for chondroitin GalNAcT-1, served as an acceptor substrate. The coding region of this enzyme was divided into seven discrete exons, which is similar to the genomic organization of the chondroitin GalNAcT-1 gene, and was localized to chromosome 10q11.22. Northern blot analysis revealed that the chondroitin GalNAcT-2 gene exhibited a ubiquitous but differing expression in human tissues, and the expression pattern differed from that of chondroitin GalNAcT-1. Thus, we demonstrated redundancy in the chondroitin GalNAc transferases involved in the biosynthetic initiation and elongation of chondroitin sulfate, which is important for understanding the biosynthetic mechanisms leading to the selective chain assembly of chondroitin/dermatan sulfate on the linkage region tetrasaccharide common to various proteoglycans containing chondroitin/dermatan sulfate and heparin/heparan sulfate chains.

Hereditary multiple exostoses and heparan sulfate polymerization

Hereditary multiple exostoses (HME, OMIM 133700, 133701) results from mutations in EXT1 and EXT2, genes encoding the copolymerase responsible for heparan sulfate (HS) biosynthesis. Members of this multigene family share the ability to transfer N-acetylglucosamine to a variety of oligosaccharide acceptors. EXT1 and EXT2 encode the copolymerase, whereas the roles of the other EXT family members (EXTL1, L2, and L3) are less clearly defined. Here, we provide an overview of HME, the EXT family of proteins, and possible models for the relationship of altered HS biosynthesis to the ectopic bone growth characteristic of the disease.

[Biosynthetic mechanism of the bioactive sulfated glycosaminoglycans]

Sulfated glycosaminoglycans including heparin/heparan sulfate and chondroitin/dermatan sulfate have been implicated in numerous pathophysiological phenomena in vertebrates and invertebrates. The critical roles of glycosaminoglycans, especially heparan sulfate, in developmental processes involving the signaling of morphogens such as Wingless and Hedgehog proteins, as well as of fibroblast growth factor, in Drosophila have recently become evident. In biosynthesis, the tetrasaccharide sequence (GlcA-Gal-Gal-Xyl-), designated the protein linkage region, is first built on a specific Ser residue at the glycosaminoglycan attachment site of a core protein. A heparin/heparan sulfate chain is then polymerized on this fragment by alternate additions of N-acetylglucosamine and glucuronic acid (GlcA) through the actions of glycosyltransferases with overlapping specificity encoded by the tumor suppressor EXT family genes. In contrast, a chondroitin/dermatan sulfate chain is synthesized on the linkage region by alternate additions of N-acetylgalactosamine and GlcA through the actions of glycosyltransferases, designated chondroitin synthases. Recent studies have achieved purification of a few and molecular cloning of all of the glycosyltransferases responsible for these reactions and have revealed the bifunctional nature of a few of these enzymes. The availability of the cDNA probes has provided several important clues to help solve the molecular mechanisms of the biosynthetic sorting of heparin/heparan sulfate and chondroitin/dermatan sulfate chains, as well as of the chain elongation and polymerization of these glycosaminoglycans.

Molecular cloning and expression of human chondroitin N-acetylgalactosaminyltransferase: the key enzyme for chain initiation and elongation of chondroitin/dermatan sulfate on the protein linkage region tetrasaccharide shared by heparin/heparan sulfate

Based on sequence homology with the recently cloned human chondroitin synthase, we identified a novel beta1,4-N-acetylgalactosaminyltransferase, which consisted of 532 amino acids with a type II transmembrane protein topology. The amino acid sequence displayed 27% identity to that of human chondroitin synthase. The expression of a soluble form of the protein in COS-1 cells produced an active enzyme, which transferred beta1,4-N-acetylgalactosamine (GalNAc) from UDP-[(3)H]GalNAc not only to a polymer chondroitin representing growing chondroitin chains (beta-GalNAc transferase II activity) but also to GlcUAbeta1--3Galbeta1-O-C(2)H(4)NH-benzyloxycarbonyl, a synthetic substrate for beta-GalNAc transferase I that transfers the first GalNAc to the core tetrasaccharide in the protein linkage region of chondroitin sulfate. Hence, the enzyme is involved in the biosynthetic initiation and elongation of chondroitin sulfate and is the key enzyme responsible for the selective chain assembly of chondroitin/dermatan sulfate on the linkage region tetrasaccharide common to various proteoglycans containing chondroitin/dermatan sulfate or heparin/heparan sulfate chains. The coding region of this enzyme was divided into seven discrete exons and localized to chromosome 8. Northern blot analysis revealed that the chondroitin GalNAc transferase gene exhibited a ubiquitous but markedly differential expression in human tissues and that the expression pattern was similar to that of chondroitin synthase. Thus, more than two distinct enzymes forming the novel gene family are required for chain initiation and elongation in chondroitin/dermatan sulfate as in the biosynthesis of heparin/heparan sulfate.

Recent advances in the study of the biosynthesis and functions of sulfated glycosaminoglycans

Recent cDNA cloning of the glycosyltransferases involved in the synthesis of the sulfated glycosaminoglycan sidechains of proteoglycans has provided important clues to answering long-standing questions concerning the mechanisms of both chain polymerization and the biosynthetic sorting of glucosaminoglycans (heparin/heparan sulfate) and galactosaminoglycans (chondroitin/dermatan sulfate). These biosynthetic mechanisms are crucial to the expression and regulation of the biological functions of glycosaminoglycans in development and pathophysiology.

Characterization of mammalian UDP-GalNAc:glucuronide alpha 1-4-N-acetylgalactosaminyltransferase

We previously reported that cultured cells incubated with beta-xylosides synthesized alpha-GalNAc-capped GAG-related xylosides, GalNAc alpha GlcA beta Gal beta Gal beta Xyl beta-R and GalNAc alpha GlcA beta GalNAc beta GlcA beta Gal beta Gal beta Xyl beta-R, where R is 4-methylumbelliferyl or p-nitrophenyl (Manzi et al., 1995; Miura and Freeze, 1998). In this study, we characterized an alpha-N-acetylgalactosaminyltransferase (alpha-GalNAc-T) that probably adds the alpha-GalNAc residue to the above xylosides. Microsomes from several animal cells and mouse brain contained the enzyme activity which requires divalent cations, and has a relatively broad pH optimal range around neutral. The apparent K(m) values were in the submillimolar range for the acceptors tested, and 19 microM for UDP-GalNAc. 1H-NMR analysis of the GlcA-beta-MU acceptor product showed the GalNAc residue is transferred in alpha 1,4-linkage to the glucuronide, which is consistent with previous results reported on alpha-GalNAc-capped Xyl-MU (Manzi et al., 1995). Various artificial glucuronides were tested as acceptors to assess the influence of the aglycone. Glucuronides with a bicyclic aromatic ring, such as 4-methylumbelliferyl beta-D-glucuronide (GlcA-beta-MU) and alpha-naphthyl beta-D-glucuronide, were the best acceptors. Interestingly, a synthetic acceptor that resembles the HNK-1 carbohydrate epitope but lacking the sulfate group, GlcA beta 1,3Gal beta 1,4GlcNAc beta-O-octyl (delta SHNK-C8), was a better acceptor for alpha-GalNAc-T than the glycosaminoglycan-protein linkage region tetrasaccharyl xyloside, GlcA beta 1,3Gal beta 1,3Gal beta 1,4Xyl beta-MU. GlcA-beta-MU and delta SHNK-C8 competed for the alpha-GalNAc-T activity, suggesting that the same activity catalyzes the transfer of the GalNAc residue to both acceptors. Taken together, the results show that the alpha-GalNAc-T described here is not restricted to GAG-type oligosaccharide acceptors, but rather is a UDP-GalNAc:glucuronide alpha 1-4-N-acetylgalactosaminyltransferase.

The tumor suppressor EXT-like gene EXTL2 encodes an alpha1, 4-N-acetylhexosaminyltransferase that transfers N-acetylgalactosamine and N-acetylglucosamine to the common glycosaminoglycan-protein linkage region. The key enzyme for the chain initiation of heparan sulfate

We previously demonstrated a unique alpha-N-acetylgalactosaminyltransferase that transferred N-acetylgalactosamine (GalNAc) to the tetrasaccharide-serine, GlcAbeta1-3Galbeta1-3Galbeta1-4Xylbeta1-O-Ser (GlcA represents glucuronic acid), derived from the common glycosaminoglycan-protein linkage region, through an alpha1,4-linkage. In this study, we purified the enzyme from the serum-free culture medium of a human sarcoma cell line. Peptide sequence analysis of the purified enzyme revealed 100% identity to the multiple exostoses-like gene EXTL2/EXTR2, a member of the hereditary multiple exostoses (EXT) gene family of tumor suppressors. The expression of a soluble recombinant form of the protein produced an active enzyme, which transferred alpha-GalNAc from UDP-[3H]GalNAc to various acceptor substrates including GlcAbeta1-3Galbeta1-3Galbeta1-4Xylbeta1-O-Ser. Interestingly, the enzyme also catalyzed the transfer of N-acetylglucosamine (GlcNAc) from UDP-[3H]GlcNAc to GlcAbeta1-3Galbeta1-O-naphthalenemethanol, which was the acceptor substrate for the previously described GlcNAc transferase I involved in the biosynthetic initiation of heparan sulfate. The GlcNAc transferase reaction product was sensitive to the action of heparitinase I, establishing the identity of the enzyme to be alpha1, 4-GlcNAc transferase. These results altogether indicate that EXTL2/EXTR2 encodes the alpha1,4-N-acetylhexosaminyltransferase that transfers GalNAc/GlcNAc to the tetrasaccharide representing the common glycosaminoglycan-protein linkage region and that is most likely the critical enzyme that determines and initiates the heparin/heparan sulfate synthesis, separating it from the chondroitin sulfate/dermatan sulfate synthesis.

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.

Demonstration of the immature glycosaminoglycan tetrasaccharide sequence GlcAbeta1-3Galbeta1-3Galbeta1-4Xyl on recombinant soluble human alpha-thrombomodulin. An oligosaccharide structure on a "part-time" proteoglycan

Thrombomodulin (TM), a cell surface glycoprotein, is a critical mediator of endothelial anticoagulant defenses occurring both as a chondroitin sulfate proteoglycan (beta-TM) and a protein (alpha-TM) unsubstituted by chondroitin sulfate (CS), hence its description as a "part-time" proteoglycan (PG) (Fransson, L. A. (1987) Trends Biochem. Sci. 12, 406-411). Sugar analysis was performed on alpha-TM to investigate a possible biosynthetic mechanism for part-time PGs. Recombinant human alpha-TM, which was expressed in CHO-K1 cells, separated by anion-exchange chromatography from beta-TM, and purified by immunoaffinity chromatography (Nawa, K., Sakano, K., Fujiwara, H., Sato, Y., Sugiyama, N., Teruuchi, T., Iwamoto, M., and Marumoto, Y. (1990) Biochem. Biophys. Res. Commun. 171, 729-737), was used for analysis. Preliminary sugar composition analysis after acid hydrolysis showed Xyl in addition to Gal, GalNAc, GlcNAc, Man, Fuc, and Glc. O-Glycosidically-linked oligosaccharides were liberated by mild alkaline treatment and purified. The isolated oligosaccharide fraction was derivatized with a fluorophore 2-aminobenzamide (2AB), resulting in two fluorescent components, a 2AB-oligosaccharide and a putative 2AB-Glc. Based on structural analysis by a combination of sequential exoglycosidase digestion and 500-MHz 1H NMR spectroscopy of the 2AB-oligosaccharide, the structure of the oligosaccharide was elucidated as GlcAbeta1-3Galbeta1-3Galbeta1-4Xyl, which turned out to represent a glycosaminoglycan (GAG)-protein linkage region tetrasaccharide common to various PGs and was considered to be a biosynthetic intermediate of an immature GAG chain. The results may indicate that at least one class of the so-called part-time PGs bear the linkage tetrasaccharide at the GAG attachment sites and that the critical determining step or the rate-limiting step for PG biosynthesis is the transfer of the fifth sugar residue, the first hexosamine, rather than xylose.