Analysis of the DXD motifs in human xylosyltransferase I required for enzyme activity

Controlled processivity in glycosyltransferases: A way to expand the enzymatic toolbox

Glycosyltransferases (GT) catalyse the biosynthesis of complex carbohydrates which are the most abundant group of molecules in nature. They are involved in several key mechanisms such as cell signalling, biofilm formation, host immune system invasion or cell structure and this in both prokaryotic and eukaryotic cells. As a result, research towards complete enzyme mechanisms is valuable to understand and elucidate specific structure-function relationships in this group of molecules. In a next step this knowledge could be used in GT protein engineering, not only for rational drug design but also for multiple biotechnological production processes, such as the biosynthesis of hyaluronan, cellooligosaccharides or chitooligosaccharides. Generation of these poly- and/or oligosaccharides is possible due to a common feature of several of these GTs: processivity. Enzymatic processivity has the ability to hold on to the growing polymer chain and some of these GTs can even control the number of glycosyl transfers. In a first part, recent advances in understanding the mechanism of various processive enzymes are discussed. To this end, an overview is given of possible engineering strategies for the purpose of new industrial and fundamental applications. In the second part of this review, we focused on specific chain length-controlling mechanisms, i.e., key residues or conserved regions, and this for both eukaryotic and prokaryotic enzymes.

Recent advances on glycosyltransferases involved in the biosynthesis of the proteoglycan linkage region

Proteoglycans (PGs) are an essential family of glycoproteins, which can play roles in many important biological events including cell proliferation, cancer development, and pathogen infections. Proteoglycans consist of a core protein with one or multiple glycosaminoglycan (GAG) chains, which are covalently attached to serine residues of serine-glycine dipeptide within the core protein through a common tetrasaccharide linkage. In the past three decades, four key glycosyl transferases involved in the biosynthesis of PG linkage have been discovered and investigated. This review aims to provide an overview on progress made on these four enzymes, with foci on enzyme expression/purification, substrate specificity, activity determination, product characterization, and structure-activity relationship analysis.

Systems identification and characterization of β-glucuronosyltransferase genes involved in arabinogalactan-protein biosynthesis in plant genomes

Utilizing plant biomass for bioethanol production requires an understanding of the molecular mechanisms involved in plant cell wall assembly. Arabinogalactan-proteins (AGPs) are glycoproteins that interact with other cell wall polymers to influence plant growth and developmental processes. Glucuronic acid, which is transferred to the AGP glycan by β-glucuronosyltransferases (GLCATs), is the only acidic sugar in AGPs with the ability to bind calcium. We carried out a comprehensive genome-wide analysis of a putative GLCAT gene family involved in AGP biosynthesis by examining its sequence diversity, genetic architecture, phylogenetic and motif characteristics, selection pressure and gene expression in plants. We report the identification of 161 putative GLCAT genes distributed across 14 plant genomes and a widely conserved GLCAT catalytic domain. We discovered a phylogenetic clade shared between bryophytes and higher land plants of monocot grass and dicot lineages and identified positively selected sites that do not result in functional divergence of GLCATs. RNA-seq and microarray data analyses of the putative GLCAT genes revealed gene expression signatures that likely influence the assembly of plant cell wall polymers which is critical to the overall growth and development of edible and bioenergy crops.

Biallelic variants in KYNU cause a multisystemic syndrome with hand hyperphalangism

Catel-Manzke syndrome is characterized by the combination of Pierre Robin sequence and radial deviation, shortening as well as clinodactyly of the index fingers, due to an accessory ossification center. Mutations in TGDS have been identified as one cause of Catel-Manzke syndrome, but cannot be found as causative in every patient with the clinical diagnosis. We performed a chromosome microarray and/or exome sequencing in three patients with hand hyperphalangism, heart defect, short stature, and mild to severe developmental delay, all of whom were initially given a clinical diagnosis of Catel-Manzke syndrome. In one patient, we detected a large deletion of exons 1-8 and the missense variant c.1282C > T (p.Arg428Trp) in KYNU (NM_003937.2), whereas homozygous missense variants in KYNU were found in the other two patients (c.989G > A (p.Arg330Gln) and c.326G > C (p.Trp109Ser)). Plasma and urine metabolomic analysis of two patients indicated a block along the tryptophan catabolic pathway and urine organic acid analysis showed excretion of xanthurenic acid. Biallelic loss-of-function mutations in KYNU were recently described as a cause of NAD deficiency with vertebral, cardiac, renal and limb defects; however, no hand hyperphalangism was described in those patients, and Catel-Manzke syndrome was not discussed as a differential diagnosis. In conclusion, we present unrelated patients identified with biallelic variants in KYNU leading to kynureninase deficiency and xanthurenic aciduria as a very likely cause of their hyperphalangism, heart defect, short stature, and developmental delay. We suggest performance of urine organic acid analysis in patients with suspected Catel-Manzke syndrome, particularly in those with cardiac or vertebral defects or without mutations in TGDS.

Xylosyltransferase 1 and the GAG Attachment Site

Xylosyltransferase initiates glycosaminoglycan synthesis on the proteoglycan core protein. In this issue of Structure, Briggs and Hohenester (2018) determined the crystal structure of xylosyltransferase 1 and its structure in ternary complex with UDP-xylose donor and peptide acceptors, providing a mechanistic insight into the role of xylosyltransferase for glycosaminoglycan site selection.

XYLT1 mutations in Desbuquois dysplasia type 2

Desbuquois dysplasia (DBQD) is a severe condition characterized by short stature, joint laxity, and advanced carpal ossification. Based on the presence of additional hand anomalies, we have previously distinguished DBQD type 1 and identified CANT1 (calcium activated nucleotidase 1) mutations as responsible for DBQD type 1. We report here the identification of five distinct homozygous xylosyltransferase 1 (XYLT1) mutations in seven DBQD type 2 subjects from six consanguineous families. Among the five mutations, four were expected to result in loss of function and a drastic reduction of XYLT1 cDNA level was demonstrated in two cultured individual fibroblasts. Because xylosyltransferase 1 (XT-I) catalyzes the very first step in proteoglycan (PG) biosynthesis, we further demonstrated in the two individual fibroblasts a significant reduction of cellular PG content. Our findings of XYLT1 mutations in DBQD type 2 further support a common physiological basis involving PG synthesis in the multiple dislocation group of disorders. This observation sheds light on the key role of the XT-I during the ossification process.

Synthesis and assessment of glycosaminoglycan priming activity of cluster-xylosides for potential use as proteoglycan mimetics

One of the distinct structural features of many proteoglycans (PGs) is the presence of two or more glycosaminoglycan (GAG) side chains covalently linked to a core protein. Previous studies have shown that the synergistic biological activity of multiple GAG chains, as found in the majority of PGs, cannot be accomplished by the sum of the activities of individual GAG chains. To delineate the biological significance of GAG valency, a number of cluster-xylosides carrying two, three, or four xylose residues on the same scaffold were synthesized using click chemistry. Assessment of cluster-xylosides for their GAG chain priming activity in a cellular system revealed that these cluster-xylosides prime multiple GAG chains per scaffold. Multivalent GAG chains, produced by cluster-xylosides, can better mimic PGs as they carry two or more GAG chains attached to a core protein and therefore can be used as molecular probes to examine the biological significance of GAG multivalency in model organisms.

Mutations in fam20b and xylt1 reveal that cartilage matrix controls timing of endochondral ossification by inhibiting chondrocyte maturation

Differentiating cells interact with their extracellular environment over time. Chondrocytes embed themselves in a proteoglycan (PG)-rich matrix, then undergo a developmental transition, termed "maturation," when they express ihh to induce bone in the overlying tissue, the perichondrium. Here, we ask whether PGs regulate interactions between chondrocytes and perichondrium, using zebrafish mutants to reveal that cartilage PGs inhibit chondrocyte maturation, which ultimately dictates the timing of perichondral bone development. In a mutagenesis screen, we isolated a class of mutants with decreased cartilage matrix and increased perichondral bone. Positional cloning identified lesions in two genes, fam20b and xylosyltransferase1 (xylt1), both of which encode PG synthesis enzymes. Mutants failed to produce wild-type levels of chondroitin sulfate PGs, which are normally abundant in cartilage matrix, and initiated perichondral bone formation earlier than their wild-type siblings. Primary chondrocyte defects might induce the bone phenotype secondarily, because mutant chondrocytes precociously initiated maturation, showing increased and early expression of such markers as runx2b, collagen type 10a1, and ihh co-orthologs, and ihha mutation suppressed early perichondral bone in PG mutants. Ultrastructural analyses demonstrated aberrant matrix organization and also early cellular features of chondrocyte hypertrophy in mutants. Refining previous in vitro reports, which demonstrated that fam20b and xylt1 were involved in PG synthesis, our in vivo analyses reveal that these genes function in cartilage matrix production and ultimately regulate the timing of skeletal development.

Genetic diversity of capsular polysaccharide biosynthesis in Klebsiella pneumoniae clinical isolates

Klebsiella pneumoniae is an enteric pathogen causing community-acquired and hospital-acquired infections in humans. Epidemiological studies have revealed significant diversity in capsular polysaccharide (CPS) type and clinical manifestation of K. pneumoniae infection in different geographical areas of the world. We have sequenced the capsular polysaccharide synthesis (cps) region of seven clinical isolates and compared the sequences with the publicly available cps sequence data of five strains: NTUH-K2044 (K1 serotype), Chedid (K2 serotype), MGH78578 (K52 serotype), A1142 (K57 serotype) and A1517. Among all strains, six genes at the 5' end of the cps clusters that encode proteins for CPS transportation and processing at the bacterial surface are highly similar to each other. The central region of the cps gene clusters, which encodes proteins for polymerization and assembly of the CPS subunits, is highly divergent. Based on the collected sequence, we found that either the wbaP gene or the wcaJ gene exists in a given K. pneumoniae strain, suggesting that there is a major difference in the CPS biosynthesis pathway and that the K. pneumoniae strains can be classified into at least two distinct groups. All isolates contain gnd, encoding gluconate-6-phosphate dehydrogenase, at the 3' end of the cps gene clusters. The rmlBADC genes were found in CPS K9-positive, K14-positive and K52-positive strains, while manC and manB were found in K1, K2, K5, K14, K62 and two undefined strains. Our data indicate that, while overall genomic organization is similar between different pathogenic K. pneumoniae strains, the genetic variation of the sugar moiety and polysaccharide linkage generate the diversity in CPS molecules that could help evade host immune attack.

Characterization of CalS9 in the biosynthesis of UDP-xylose and the production of xylosyl-attached hybrid compound

The gene cluster of calicheamicin contains calS9, which encodes UDP-GlcA decarboxylase that converts UDP-GlcA to UDP-xylose. calS9 was cloned in pET32a(+) and expressed in Escherichia coli BL21 (DE3) to characterize its putative function. The reaction product was analyzed by high-performance liquid chromatography (HPLC) and electrospray ionization-mass spectrometry. The deoxysugar biosynthesis of Streptomyces sp. KCTC 0041BP was inactivated by gene replacement to generate Streptomyces sp. GerSM2 mutant, which was unable to produce dihydrochalcomycin. calS7, calS8, and calS9 UDP-xylose biosynthetic genes were cloned in an integrative plasmid pSET152 to generate pBPDS, which was heterologously expressed in Streptomyces sp. GerSM2. Finally, novel glycosylated product, 5-O-xylosyl-chalconolide derivative, in the conjugal transformants was isolated and analyzed by HPLC and liquid chromatography-mass spectrometry.

The chondroitin polymerase K4CP and the molecular mechanism of selective bindings of donor substrates to two active sites

Bacterial chondroitin polymerase K4CP is a multifunctional enzyme with two active sites. K4CP catalyzes alternative transfers of glucoronic acid (GlcA) and N-acetylgalactosamine (GalNAc) to elongate a chain consisting of the repeated disaccharide sequence GlcAbeta1-3GalNAcbeta1-4. Unlike the polymerization reactions of DNA and RNA and polypeptide synthesis, which depend upon templates, the monosaccharide polymerization by K4CP does not. To investigate the catalytic mechanism of this reaction, we have used isothermal titration calorimetry to determine the binding of the donor substrates UDP-GlcA and UDP-GalNAc to purified K4CP protein and its mutants. Only one donor molecule bound to one molecule of K4CP at a time. UDP-GlcA bound only to the C-terminal active site at a high affinity (K(d)=6.81 microm), thus initiating the polymerization reaction. UDP-GalNAc could bind to either the N-terminal or C-terminal active sites at a low affinity (K(d)=266-283 microm) but not to both sites at the same time. The binding affinity of UDP-GalNAc to a K4CP N-terminal fragment (residues 58-357) was profoundly decreased, yielding the average K(d) value of 23.77 microm, closer to the previously reported K(m) value for the UDP-GalNAc transfer reaction that takes place at the N-terminal active site. Thus, the first step of the reaction appears to be the binding of UDP-GlcA to the C-terminal active site, whereas the second step involves the C-terminal region of the K4CP molecule regulating the binding of UDP-GalNAc to only the N-terminal active site. Alternation of these two specific bindings advances the polymerization reaction by K4CP.

Transforming growth factor beta1-regulated xylosyltransferase I activity in human cardiac fibroblasts and its impact for myocardial remodeling

In cardiac fibrosis remodeling of the failing myocardium is associated with a complex reorganization of the extracellular matrix (ECM). Xylosyltransferase I and Xylosyltransferase II (XT-I and XT-II) are the key enzymes in proteoglycan biosynthesis, which are an important fraction of the ECM. XT-I was shown to be a measure for the proteoglycan biosynthesis rate and a biochemical fibrosis marker. Here, we investigated the XT-I and XT-II expression in cardiac fibroblasts and in patients with dilated cardiomyopathy and compared our findings with nonfailing donor hearts. We analyzed XT-I and XT-II expression and the glycosaminoglycan (GAG) content in human cardiac fibroblasts incubated with transforming growth factor (TGF)-beta(1) or exposed to cyclic mechanical stretch. In vitro and in vivo no significant changes in the XT-II expression were detected. For XT-I we found an increased expression in parallel with an elevated chondroitin sulfate-GAG content after incubation with TGF-beta(1) and after mechanical stretch. XT-I expression and subsequently increased levels of GAGs could be reduced with neutralizing anti-TGF-beta(1) antibodies or by specific inhibition of the activin receptor-like kinase 5 or the p38 mitogen-activated protein kinase pathway. Usage of XT-I small interfering RNA could specifically block the increased XT-I expression under mechanical stress and resulted in a significantly reduced chondroitin sulfate-GAG content. In the left and right ventricular samples of dilated cardiomyopathy patients, our data show increased amounts of XT-I mRNA compared with nonfailing controls. Patients had raised levels of XT-I enzyme activity and an elevated proteoglycan content. Myocardial remodeling is characterized by increased XT-I expression and enhanced proteoglycan deposition. TGF-beta(1) and mechanical stress induce XT-I expression in cardiac fibroblasts and have impact for ECM remodeling in the dilated heart. Specific blocking of XT-I expression confirmed that XT-I catalyzes a rate-limiting step during fibrotic GAG biosynthesis.

Golgi complex organization in skeletal muscle: a role for Golgi-mediated glycosylation in muscular dystrophies?

The Golgi complex (GC) is the central organelle of the classical secretory pathway, and it receives, modifies and packages proteins and lipids en route to their intracellular or extracellular destinations. Recent studies of congenital muscular dystrophies in skeletal muscle suggest an exciting new role for an old and well-established function of the GC: glycosylation. Glycosylation is the exquisitely regulated enzymatic addition of nucleotide sugars to proteins and lipids mediated by glycosyltransferases (GTs). Mutations in putative Golgi-resident GTs, fukutin, fukutin-related protein and large1 cause these progressive muscle-wasting diseases. The appropriate localization of GTs to specific subcompartments of the Golgi is critical for the correct assembly line-like addition of glycan groups to proteins and lipids as they pass through the GC. Consequently, these studies of congenital muscular dystrophies have focused attention on the organization and function of the GC in skeletal muscle. In contrast to other cells and tissues, the GC in skeletal muscle has received relatively little attention; however, in recent years, several studies have shown that GC distribution in muscle is highly dynamic or plastic and adopts different distributions in muscle cells undergoing myogenesis, denervation, regeneration and maturation. Here, we review the current understanding of the dynamic regulation of GC organization in skeletal muscle and focus on the targeting of fukutin, fukutin-related protein and large1 to the GC in muscle cells.

XT-II, the second isoform of human peptide-O-xylosyltransferase, displays enzymatic activity

Peptide O-xylosyltransferase (EC 2.4.2.26) is the first enzyme required for the generation of chondroitin and heparan sulfate glycosaminoglycan chains of proteoglycans. Cloning of cDNAs has previously shown that, whereas invertebrates generally have a single xylosyltransferase gene, vertebrate genomes encode two similar proteins, xylosyltransferase I and II (XT-I and XT-II). To date, enzymatic activity has only been demonstrated for the human XT-I, Caenorhabditis SQV-6, and Drosophila OXT isoforms. In the present study, we demonstrate that a soluble form of human XT-II expressed in the xylosyltransferase-deficient pgsA-745 (S745) Chinese hamster ovary cell line is indeed capable of catalyzing the transfer of xylose to a variety of peptide substrates; its enzyme activity was also proven using a Pichia-expressed form of XT-II. Its pH, temperature, and cation dependences are similar to those of XT-I expressed in either mammalian cells or yeast. Our data suggest that XT-I and XT-II are, at least in vitro, functionally identical.

Structural Effects of Naturally Occurring Human Blood Group B Galactosyltransferase Mutations Adjacent to the DXD Motif

Human blood group A and B antigens are produced by two closely related glycosyltransferase enzymes. An N-acetylgalactosaminyltransferase (GTA) utilizes UDP-GalNAc to extend H antigen acceptors (Fuc alpha(1-2)Gal beta-OR) producing A antigens, whereas a galactosyltransferase (GTB) utilizes UDP-Gal as a donor to extend H structures producing B antigens. GTA and GTB have a characteristic (211)DVD(213) motif that coordinates to a Mn(2+) ion shown to be critical in donor binding and catalysis. Three GTB mutants, M214V, M214T, and M214R, with alterations adjacent to the (211)DVD(213) motif have been identified in blood banking laboratories. From serological phenotyping, individuals with the M214R mutation show the B(el) variant expressing very low levels of B antigens, whereas those with M214T and M214V mutations give rise to A(weak)B phenotypes. Kinetic analysis of recombinant mutant GTB enzymes revealed that M214R has a 1200-fold decrease in k(cat) compared with wild type GTB. The crystal structure of M214R showed that DVD motif coordination to Mn(2+) was disrupted by Arg-214 causing displacement of the metal by a water molecule. Kinetic characterizations of the M214T and M214V mutants revealed they both had GTA and GTB activity consistent with the serology. The crystal structure of the M214T mutant showed no change in DVD coordination to Mn(2+). Instead a critical residue, Met-266, which is responsible for determining donor specificity, had adopted alternate conformations. The conformation with the highest occupancy opens up the active site to accommodate the larger A-specific donor, UDP-GalNAc, accounting for the dual specificity.

Human xylosyltransferase II is involved in the biosynthesis of the uniform tetrasaccharide linkage region in chondroitin sulfate and heparan sulfate proteoglycans

Human xylosyltransferase I (XT-I) initiates the biosynthesis of the glycosaminoglycan (GAG) linkage tetrasaccharide in proteoglycans. Xylosyltransferase II (XT-II) is a protein homologous to XT-I but with hitherto unknown activity or physiological function. Here, we report the enzymatic activity of XT-II and provide evidence that XT-II initiates the biosynthesis of both heparan sulfate and chondroitin sulfate GAGs. Transfection of the xylosyltransferase-deficient Chinese hamster ovary mutant pgsA-745 with XT-I or XT-II coding cDNA completely restored GAG biosynthesis. GAG disaccharide analysis revealed that XT-I- and XT-II-transfected pgsA-745 cells produced similar amounts of chondroitin sulfate and heparan sulfate. Furthermore, a high xylosyltransferase activity was measured after transfection with cDNAs encoding either isozyme. Analysis of the enzyme activity revealed that XT-II catalyzes the transfer of xylose to similar peptide acceptors as XT-I but with different efficiency. The optimal XT-II acceptor was observed using a bikunin-related peptide (K(m) 5.2 microM). Analysis of XT-I and XT-II mRNA expression in murine tissues showed a differential expression pattern for both enzymes. In particular, XT-II is highly expressed in liver tissue, where XT-I transcripts were not detected. This is the first report on the enzyme activity of XT-II and its involvement in chondroitin sulfate and heparan sulfate biosynthesis.

The two N-glycans present on bovine Pofut1 are differently involved in its solubility and activity

O-Fucosylation is a post-translational glycosylation in which an O-fucose is covalently attached to the hydroxyl group of a specific serine or threonine residue. This modification occurs within the consensus sequence C2X(4-5)(S/T)C3 present on epidermal growth factor-like repeats of several proteins, including the Notch receptors and their ligands. The enzyme responsible for the addition of O-fucose to epidermal growth factor-like repeats is protein O-fucosyltransferase 1. Protein O-fucosyltransferase 1-mediated O-fucosylation is essential in Notch signaling, folding and targeting to the cell surface. Here, we studied the expression pattern of protein O-fucosyltransferase 1 in cattle and showed that the active enzyme is present in all tissues examined from embryo and adult as a glycoprotein with two N-glycans. By comparing protein O-fucosyltransferase 1 sequences available in databases, we observed that mammalian protein O-fucosyltransferase 1 enzymes possess two putative N-glycosylation sites, and that only the first is conserved among bilaterians. To gain more insight regarding the significance of N-glycans on protein O-fucosyltransferase 1, we substituted, by site-directed mutagenesis, bovine protein O-fucosyltransferase 1 N65, N163 or both, with L or Q. We demonstrated that the loss of N-glycan on N163 caused a slight decrease in protein O-fucosyltransferase 1 activity. In contrast, glycosylation of N65 was crucial for protein O-fucosyltransferase 1 functionality. Loss of glycosylation at N65 resulted in aggregation of protein O-fucosyltransferase 1, suggesting that N-glycosylation at this site is essential for proper folding of the enzyme.

Fold-recognition and comparative modeling of human beta3GalT I, II, IV, V and VI and beta3GalNAcT I: prediction of residues conferring acceptor substrate specificity

beta3GalTs are type II transmembrane proteins that transfer galactose from UDP-Gal donor substrate to acceptor GlcNAc, GalNAc or Gal in beta1-->3-linkage. beta1-->3-linked galactose have been found to be a part of many glycans like glycosphingolipids, core tetrasaccharide of proteoglycans, type 1 chains. The 3-D structure of none of the beta3GalTs is known to date. In this study, the 3-D structures of human beta3GalT I, II, IV, V, VI and beta3GalNAcT I have been modeled using fold-recognition and comparative modeling methods. Residues that constitute the UDP-Gal binding site have been predicted. The models are able to qualitatively rationalize data from the site-directed mutagenesis experiments reported in the literature. Residues likely to be involved in conferring differential acceptor substrate specificity have been predicted by a combination of specificity determining positions prediction (SDPs) and subsequent mapping on the generated 3-D models.

Comparative characterisation of recombinant invertebrate and vertebrate peptide O-Xylosyltransferases

Chondroitin and heparan sulphates have key functions in animal development and their synthesis is initiated by the action of UDP-alpha-D-xylose:proteoglycan core protein beta-D-xylosyltransferase (EC 2.4.2.26). cDNAs encoding this enzyme have been previously cloned from mammalian species; this in turn facilitated identification of corresponding Caenorhabditis elegans (sqv-6) and Drosophila melanogaster (oxt) genes. In the present study, we report the expression in Pichia pastoris and subsequent assay using either MALDI-TOF MS or RP-HPLC of recombinant forms of the Caenorhabditis xylosyltransferase SQV-6 and the human xylosyltransferase I, in addition to extending our previous studies on the xylosyltransferase from Drosophila. The enzyme activities were tested with a number of peptide substrates based on portions of the human bikunin, human perlecan and Drosophila syndecan core peptides. Whereas a variant of the latter, containing two Ser-Gly motifs was only modified on one of these motifs, the perlecan peptide with three Ser-Gly motifs could be multiply modified in vitro. Using this substrate, we could for the first time follow, by mass spectrometry, the xylosylation of a peptide with multiple xylosyltransferase acceptor motifs.

The formation of extracellular matrix during chondrogenic differentiation of mesenchymal stem cells correlates with increased levels of xylosyltransferase I

In vitro differentiation of mesenchymal stem cells (MSCs) into chondrogenic cells and their transplantation is promising as a technique for the treatment of cartilaginous defects. But the regulation of extracellular matrix (ECM) formation remains elusive. Therefore, the objective of this study was to analyze the regulation of proteoglycan (PG) biosynthesis during the chondrogenic differentiation of MSCs. In different stages of chondrogenic differentiation, we analyzed mRNA and protein expression of key enzymes and PG core proteins involved in ECM development. For xylosyltransferase I (XT-I), we found maximum mRNA levels 48 hours after chondrogenic induction with a 5.04 +/- 0.58 (mean +/- SD)-fold increase. This result correlates with significantly elevated levels of enzymatic XT-I activity (0.49 +/- 0.03 muU/1 x 10(6) cells) at this time point. Immunohistochemical staining of XT-I revealed a predominant upregulation in early chondrogenic stages. The highly homologous protein XT-II showed 4.7-fold (SD 0.6) increased mRNA levels on day 7. To determine the differential expression of heparan sulfate (HS), chondroitin sulfate (CS), and dermatan sulfate (DS) chains, we analyzed the mRNA expression of EXTL2 (alpha-4-N-acetylhexosaminyltransferase), GalNAcT (beta-1,4-N-acetylgalactosaminyltransferase), and GlcAC5E (glucuronyl C5 epimerase). All key enzymes showed a similar regulation with temporarily downregulated mRNA levels (up to -87-fold) after chondrogenic induction. In accordance to previous studies, we observed a similar increase in the expression of PG core proteins. In conclusion, we could show that key enzymes for CS, DS, and HS synthesis, especially XT-I, are useful markers for the developmental stages of chondrogenic differentiation.

Cloning and recombinant expression of active full-length xylosyltransferase I (XT-I) and characterization of subcellular localization of XT-I and XT-II

Xylosyltransferase I (XT-I) catalyzes the transfer of xylose from UDP-xylose to serine residues in proteoglycan core proteins. This is the first and apparently rate-limiting step in the biosynthesis of the tetrasaccharide linkage region in glycosaminoglycan-containing proteoglycans. The XYLT-II gene codes for a highly homologous protein, but its physiological function is not yet known. Here we present for the first time the construction of a vector encoding the full-length GFP-tagged human XT-I and the recombinant expression of the active enzyme in mammalian cells. We expressed XT-I-GFP and various GFP-tagged XT-I and XT-II mutants with C-terminal truncations and deletions in HEK-293 and SaOS-2 cells in order to investigate the intracellular localization of XT-I and XT-II. Immunofluorescence analysis showed a distinct perinuclear pattern of XT-I-GFP and XT-II-GFP similar to that of alpha-mannosidase II, which is a known enzyme of the Golgi cisternae. Furthermore, a co-localization of native human XT-I and alpha-mannosidase II could also be demonstrated in untransfected cells. Using brefeldin A, we could also show that both xylosyltransferases are resident in the early cisternae of the Golgi apparatus. For its complete Golgi retention, XT-I requires the N-terminal 214 amino acids. Unlike XT-I, for XT-II, the first 45 amino acids are sufficient to target and retain the GFP reporter in the Golgi compartment. Here we show evidence that the stem regions were indispensable for Golgi localization of XT-I and XT-II.

Mutational and functional analyses of xylosyltransferases and their implication in osteoarthritis

OBJECTIVE

The hallmark in osteoarthritis (OA) is the loss of proteoglycans (PGs) in articular cartilage (AC). Xylosyltransferase I (XT-I) catalyzes the transfer of xylose to serine residues in the core protein and initiates the biosynthesis of PGs in AC. The XYLT-II gene encodes a highly homologous protein but its biological function is not yet known. Here we investigate for the first time genetic variations in the XYLT-genes and serum XT-I activities and their implication in OA.

METHODS

Denaturing high-performance liquid chromatography (DHPLC) was used for the screening of the XYLT-genes in 49 OA patients. For a detailed characterization of XT-I amino acid exchanges we performed recombinant expression of XT-I mutants in insect cells. Furthermore, the XT activity was measured in the patients' serum.

RESULTS

The variation c.1569C>T (XYLT-II) occurs with a significantly higher frequency in younger OA patients in comparison with the older ones (P<0.001) and the controls (P<0.02). Furthermore, significantly higher serum XT activities were found in patients with a long disease duration of OA (P<0.04). The recombinant XT-I mutants p.P385L and p.I552S had reduced enzymatic activity (85% and 74%) compared with the wildtype (wt).

CONCLUSIONS

Our findings indicate a correlation of the c.1569 T-allele in XYLT-II with an earlier manifestation of OA and that the serum XT activity is a potential biochemical marker for staging and monitoring the progression of AC damage in OA. Comparison of XT-I activity in mutant enzymes in vivo and in vitro revealed that heterozygous mutations are not involved in OA.

Human xylosyltransferase I and N-terminal truncated forms: functional characterization of the core enzyme

Human XT-I (xylosyltransferase I; EC 2.4.2.26) initiates the biosynthesis of the glycosaminoglycan linkage region and is a diagnostic marker of an enhanced proteoglycan biosynthesis. In the present study, we have investigated mutant enzymes of human XT-I and assessed the impact of the N-terminal region on the enzymatic activity. Soluble mutant enzymes of human XT-I with deletions at the N-terminal domain were expressed in insect cells and analysed for catalytic activity. As many as 260 amino acids could be truncated at the N-terminal region of the enzyme without affecting its catalytic activity. However, truncation of 266, 272 and 273 amino acids resulted in a 70, 90 and >98% loss in catalytic activity. Interestingly, deletion of the single 12 amino acid motif G261KEAISALSRAK272 leads to a loss-of-function XT-I mutant. This is in agreement with our findings analysing the importance of the Cys residues where we have shown that C276A mutation resulted in a nearly inactive XT-I enzyme. Moreover, we investigated the location of the heparin-binding site of human XT-I using the truncated mutants. Heparin binding was observed to be slightly altered in mutants lacking 289 or 568 amino acids, but deletion of the potential heparin-binding motif P721KKVFKI727 did not lead to a loss of heparin binding capacity. The effect of heparin or UDP on the XT-I activity of all mutants was not significantly different from that of the wild-type. Our study demonstrates that over 80% of the nucleotide sequence of the XT-I-cDNA is necessary for expressing a recombinant enzyme with full catalytic activity.

Elevated xylosyltransferase I activities in pseudoxanthoma elasticum (PXE) patients as a marker of stimulated proteoglycan biosynthesis

Pseudoxanthoma elasticum (PXE) is a hereditary disorder of the connective tissue characterized by extracellular matrix alterations with elastin fragmentation and excessive proteoglycan deposition. Xylosyltransferase I (XT-I, E.C. 2.4.2.26) is the initial enzyme in the biosynthesis of the glycosaminoglycan chains in proteoglycans and has been shown to be a marker of tissue remodeling processes. Here, we investigated for the first time serum XT-I activities in a large cohort of German PXE patients and their unaffected relatives. XT-I activities were measured in serum samples from 113 Caucasian patients with PXE and 103 unaffected first-degree family members. The occurrence of the frequent ABCC6 gene mutation c.3421C>T (R1141X) and the hypertension-associated genetic variants T174M and M235T in the angiotensinogen (AGT) gene were determined. Serum XT-I activities in male and female PXE patients were significantly increased compared to unaffected family members (male patients, mean value 0.96 mU/l, SD 0.37; male relatives, 0.78 mU/l, SD 0.29; female patients, 0.91 mU/l, SD 0.31; female relatives, 0.76 mU/l, SD 0.34; p<0.05). The mean XT-I activities in PXE patients with hypertension were 24% higher than in patients without increased blood pressure (p<0.05). The AGT T174M and M235T frequencies were not different in hypertensive PXE patients, normotensive PXE patients, family members or blood donors. Our data show that the altered proteoglycan biosynthesis in PXE patients is closely related to an increased XT-I activity in blood. Serum XT-I, the novel fibrosis marker, may be useful for the assessment of extracellular matrix alterations and disease activity in PXE.