Novel roles for cooperating collagen receptor families in fibrotic niches

Recent data indicate that integrin and non-integrin collagen receptors cooperate in the fibrosis-specific microenvironment (i.e., the fibrotic niche). In certain tumor types, DDR1 can regulate the interaction with collagen III to regulate dormancy and metastasis, whereas in other tumor types, DDR1 can be shed and used to reorganize collagen. DDR1 expressed on tumor cells, together with DDR2 and α11β1 integrin expressed on cancer-associated fibroblasts, can increase tumor tissue stiffness. Integrin α1β1 and α2β1 are present on immune cells where they together with the immunosuppressive collagen receptor LAIR-1 can mediate binding to intratumor collagens. In summary, collagen-binding integrins together with DDRs, can create fibrillar collagen niches that act as traps to hinder immune cell trafficking into the tumor cell mass. Binding of collagens via LAIR-1 on immune cells in turn results in CD8+T-cell exhaustion. Continued studies of these complex interactions are needed for successful new stroma-based therapeutic interventions. In the current review, we will summarize recent data on collagen receptors with a special focus on their potential role in tumor fibrosis and highlight their collaborative roles in tumor fibrotic niches.

Integrin α11β1 in tumor fibrosis: more than just another cancer-associated fibroblast biomarker?

There is currently an increased interest in understanding the role of the tumor microenvironment (TME) in tumor growth and progression. In this context the role of integrins in cancer-associated fibroblasts (CAFs) will need to be carefully re-evaluated. Fibroblast-derived cells are not only in the focus in tumors, but also in tissue fibrosis as well as in inflammatory conditions. The recent transcriptional profiling of what has been called "the pan-fibroblast cell lineage" in mouse and human tissues has identified novel transcriptional biomarker mRNAs encoding the secreted ECM proteins dermatopontin and collagen XV as well as the phosphatidylinositol-anchored membrane protein Pi16. Some of the genes identified in these fibroblasts scRNA-seq datasets will be useful for rigorous comparative characterizations of fibroblast-derived cell subpopulations. At the same time, it will be a challenge in the coming years to validate these transcriptional mRNA datasets at the protein-(expression) and at tissue-(distribution) levels and to find useful protein biomarker reagents that will facilitate fibroblast profiling at the cell level. In the current review we will focus on the role of the collagen-binding integrin α11β1 in CAFs, summarizing our own work as well as published datasets with information on α11 mRNA expression in selected tumors. Our experimental data suggest that α11β1 is more than just another biomarker and that it as a functional collagen receptor in the TME is playing a central role in regulating collagen assembly and matrix remodeling, which in turn impact tumor growth and metastasis.

Heparan sulfate dependent binding of plasmatic von Willebrand factor to blood circulating melanoma cells attenuates metastasis

Heparan sulfate (HS), a highly negatively charged glycosaminoglycan, is ubiquitously present in all tissues and also exposed on the surface of mammalian cells. A plethora of molecules such as growth factors, cytokines or coagulation factors bear HS binding sites. Accordingly, HS controls the communication of cells with their environment and therefore numerous physiological and pathophysiological processes such as cell adhesion, migration, and cancer cell metastasis. In the present work, we found that HS exposed by blood circulating melanoma cells recruited considerable amounts of plasmatic von Willebrand factor (vWF) to the cellular surface. Analyses assisted by super-resolution microscopy indicated that HS and vWF formed a tight molecular complex. Enzymatic removal of HS or genetic engineering of the HS biosynthesis showed that a reduced length of the HS chains or complete lack of HS was associated with significantly reduced vWF encapsulation. In microfluidic experiments, mimicking a tumor-activated vascular system, we found that vWF-HS complexes prevented vascular adhesion. In line with this, single molecular force spectroscopy suggested that the vWF-HS complex promoted the repulsion of circulating cancer cells from the blood vessel wall to counteract metastasis. Experiments in wild type and vWF knockout mice confirmed that the HS-vWF complex at the melanoma cell surface attenuated hematogenous metastasis, whereas melanoma cells lacking HS evade the anti-metastatic recognition by vWF. Analysis of tissue samples obtained from melanoma patients validated that metastatic melanoma cells produce less HS. Transcriptome data further suggest that attenuated expression of HS-related genes correlate with metastases and reduced patients' survival. In conclusion, we showed that HS-mediated binding of plasmatic vWF to the cellular surface can reduce the hematogenous spread of melanoma. Cancer cells with low HS levels evade vWF recognition and are thus prone to form metastases. Therefore, therapeutic expansion of the cancer cell exposed HS may prevent tumor progression.

OUP accepted manuscript

NDST1 (glucosaminyl N-deacetylase/N-sulfotransferase) is a key enzyme in heparan sulfate (HS) biosynthesis, where it is responsible for HS N-deacetylation and N-sulfation. In addition to the full length human enzyme of 882 amino acids, here designated NDST1A, a shorter form containing 825 amino acids (NDST1B) is synthesized after alternative splicing of the NDST1 mRNA. NDST1B is mostly expressed at a low level, but increased amounts are seen in several types of cancer where it is associated with shorter survival. In this study, we aimed at characterizing the enzymatic properties of NDST1B and its effect on HS biosynthesis. Purified recombinant NDST1B lacked both N-deacetylase and N-sulfotransferase activities. Interestingly, HEK293 cells overexpressing NDST1B synthesized HS with reduced sulfation and altered domain structure. Fluorescence resonance energy transfer-microscopy demonstrated that both NDST1A and NDST1B had the capacity to interact with the HS copolymerase subunits EXT1 and EXT2 and also to form NDST1A/NDST1B dimers. Since lysates from cells overexpressing NDST1B contained less NDST enzyme activity than control cells, we suggest that NDST1B works in a dominant negative manner, tentatively by replacing the active endogenous NDST1 in the enzyme complexes taking part in biosynthesis.

Heparan sulfate in chronic kidney diseases: Exploring the role of 3-O-sulfation

One of the main feature of chronic kidney disease is the development of renal fibrosis. Heparan Sulfate (HS) is involved in disease development by modifying the function of growth factors and cytokines and creating chemokine gradients. In this context, we aimed to understand the function of HS sulfation in renal fibrosis. Using a mouse model of renal fibrosis, we found that total HS 2-O-sulfation was increased in damaged kidneys, whilst, tubular staining of HS 3-O-sulfation was decreased. The expression of HS modifying enzymes significantly correlated with the development of fibrosis with HS3ST1 demonstrating the strongest correlation. The pro-fibrotic factors TGFβ1 and TGFβ2/IL1β significantly downregulated HS3ST1 expression in both renal epithelial cells and renal fibroblasts. To determine the implication of HS3ST1 in growth factor binding and signalling, we generated an in vitro model of renal epithelial cells overexpressing HS3ST1 (HKC8-HS3ST1). Heparin Binding EGF like growth factor (HB-EGF) induced rapid, transient STAT3 phosphorylation in control HKC8 cells. In contrast, a prolonged response was demonstrated in HKC8-HS3ST1 cells. Finally, we showed that both HS 3-O-sulfation and HB-EGF tubular staining were decreased with the development of fibrosis. Taken together, these data suggest that HS 3-O-sulfation is modified in fibrosis and highlight HS3ST1 as an attractive biomarker of fibrosis progression with a potential role in HB-EGF signalling.

Heparan Sulfate: Biosynthesis, Structure, and Function

Heparan sulfate (HS) proteoglycans (PGs) are ubiquitously expressed on cell surfaces and in the extracellular matrix of most animal tissues, having essential functions in development and homeostasis, as well as playing various roles in disease processes. The functions of HSPGs are mainly dependent on interactions between the HS-side chains with a variety of proteins including cytokines, growth factors, and their receptors. In a given HS polysaccharide, negatively charged sulfate and carboxylate groups are arranged in various types of domains, generated through strictly regulated biosynthetic reactions and with enormous potential for structural variability. The mode of HS-protein interactions is assessed through binding experiments using saccharides of defined composition in vitro, signaling assays in cell models where HS structures are manipulated, and targeted disruption of genes for biosynthetic enzymes in animals (mouse, zebrafish, Drosophila, and Caenorhabditis elegans) followed by phenotype analysis. Whereas some protein ligands appear to require strictly defined HS structure, others bind to variable saccharide domains without apparent dependence on distinct saccharide sequence. These findings raise intriguing questions concerning the functional significance of regulation in HS biosynthesis and the potential for development of therapeutics targeting HS-protein interactions.

Reduced Expression of EXTL2, a Member of the Exostosin (EXT) Family of Glycosyltransferases, in Human Embryonic Kidney 293 Cells Results in Longer Heparan Sulfate Chains

Heparan sulfate proteoglycans are ubiquitously located on cell surfaces and in the extracellular matrices. The negatively charged heparan sulfate chains interact with a multitude of different proteins, thereby influencing a variety of cellular and developmental processes, for example cell adhesion, migration, tissue morphogenesis, and differentiation. The human exostosin (EXT) family of genes contains five members: the heparan sulfate polymerizing enzymes, EXT1 and EXT2, and three EXT-like genes, EXTL1, EXTL2, and EXTL3. EXTL2 has been ascribed activities related to the initiation and termination of heparan sulfate chains. Here we further investigated the role of EXTL2 in heparan sulfate chain elongation by gene silencing and overexpression strategies. We found that siRNA-mediated knockdown of EXTL2 in human embryonic kidney 293 cells resulted in increased chain length, whereas overexpression of EXTL2 in the same cell line had little or no effect on heparan sulfate chain length. To study in more detail the role of EXTL2 in heparan sulfate chain elongation, we tested the ability of the overexpressed protein to catalyze the in vitro incorporation of N-acetylglucosamine and N-acetylgalactosamine to oligosaccharide acceptors resembling unmodified heparan sulfate and chondroitin sulfate precursor molecules. Analysis of the generated products revealed that recombinant EXTL2 showed weak ability to transfer N-acetylgalactosamine to heparan sulfate precursor molecules but also, that EXTL2 exhibited much stronger in vitro N-acetylglucosamine-transferase activity related to elongation of heparan sulfate chains.

Role of 6-O-sulfated heparan sulfate in chronic renal fibrosis

Heparan sulfate (HS) plays a crucial role in the fibrosis associated with chronic allograft dysfunction by binding and presenting cytokines and growth factors to their receptors. These interactions critically depend on the distribution of 6-O-sulfated glucosamine residues, which is generated by glucosaminyl-6-O-sulfotransferases (HS6STs) and selectively removed by cell surface HS-6-O-endosulfatases (SULFs). Using human renal allografts we found increased expression of 6-O-sulfated HS domains in tubular epithelial cells during chronic rejection as compared with the controls. Stimulation of renal epithelial cells with TGF-β induced SULF2 expression. To examine the role of 6-O-sulfated HS in the development of fibrosis, we generated stable HS6ST1 and SULF2 overexpressing renal epithelial cells. Compared with mock transfectants, the HS6ST1 transfectants showed significantly increased binding of FGF2 (p = 0.0086) and pERK activation. HS6ST1 transfectants displayed a relative increase in mono-6-O-sulfated disaccharides accompanied by a decrease in iduronic acid 2-O-sulfated disaccharide structures. In contrast, SULF2 transfectants showed significantly reduced FGF2 binding and phosphorylation of ERK. Structural analysis of HS showed about 40% down-regulation in 6-O-sulfation with a parallel increase in iduronic acid mono-2-O-sulfated disaccharides. To assess the relevance of these data in vivo we established a murine model of fibrosis (unilateral ureteric obstruction (UUO)). HS-specific phage display antibodies (HS3A8 and RB4EA12) showed significant increase in 6-O-sulfation in fibrotic kidney compared with the control. These results suggest an important role of 6-O-sulfation in the pathogenesis of fibrosis associated with chronic rejection.

Fibroblast EXT1-levels influence tumor cell proliferation and migration in composite spheroids

Background

Stromal fibroblasts are important determinants of tumor cell behavior. They act to condition the tumor microenvironment, influence tumor growth, support tumor angiogenesis and affect tumor metastasis. Heparan sulfate proteoglycans, present both on tumor and stromal cells, interact with a large number of ligands including growth factors, their receptors, and structural components of the extracellular matrix. Being ubiquitously expressed in the tumor microenvironment heparan sulfate proteoglycans are candidates for playing central roles in tumor-stroma interactions. The objective of this work was to investigate the role of heparan sulfate expressed by stromal fibroblasts in modulating the growth of tumor cells and in controlling the interstitial fluid pressure in a 3-D model.

Methodology/Principal Findings

We generated spheroids composed of fibroblasts alone, or composite spheroids, composed of fibroblasts and tumor cells. Here we show that stromal fibroblasts with a mutation in the heparan sulfate elongating enzyme Ext1 and thus a low heparan sulfate content, formed composite fibroblast/tumor cell spheroids with a significant lower interstitial fluid pressure than corresponding wild-type fibroblast/tumor cell composite spheroids. Furthermore, immunohistochemistry of composite spheroids revealed that the cells segregated, so that after 6 days in culture, the wild-type fibroblasts formed an inner core and the tumor cells an outer layer of cells. For composite spheroids containing Ext1-mutated fibroblasts this segregation was less obvious, indicating impaired cell migration. Analysis of tumor cells expressing the firefly luciferase gene revealed that the changes in tumor cell migration in mutant fibroblast/tumor cell composite spheroids coincided with a lower proliferation rate.

Conclusions/Significance

This is the first demonstration that stromal Ext1-levels modulate tumor cell proliferation and affect the interstitial fluid pressure in a 3-D spheroid model. Learning how structural changes in stromal heparan sulfate influence tumor cells is essential for our understanding how non-malignant cells of the tumor microenvironment influence tumor cell progression.

Drosophila heparan sulfate, a novel design

Heparan sulfate (HS) proteoglycans play critical roles in a wide variety of biological processes such as growth factor signaling, cell adhesion, wound healing, and tumor metastasis. Functionally important interactions between HS and a variety of proteins depend on specific structural features within the HS chains. The fruit fly (Drosophila melanogaster) is frequently applied as a model organism to study HS function in development. Previous structural studies of Drosophila HS have been restricted to disaccharide composition, without regard to the arrangement of saccharide domains typically found in vertebrate HS. Here, we biochemically characterized Drosophila HS by selective depolymerization with nitrous acid. Analysis of the generated saccharide products revealed a novel HS design, involving a peripheral, extended, presumably single, N-sulfated domain linked to an N-acetylated sequence contiguous with the linkage to core protein. The N-sulfated domain may be envisaged as a heparin structure of unusually low O-sulfate content.

Mutation in the heparan sulfate biosynthesis enzyme EXT1 influences growth factor signaling and fibroblast interactions with the extracellular matrix

Heparan sulfate (HS) chains bind and modulate the signaling efficiency of many ligands, including members of the fibroblast growth factor (FGF) and platelet-derived growth factor families. We previously reported the structure of HS synthesized by embryonic fibroblasts from mice with a gene trap mutation of Ext1 that encodes a glycosyltransferase involved in HS chain elongation. The gene trap mutation results in low expression of Ext1, and, as a consequence, HS chain length is substantially reduced. In the present study, Ext1 mutant and wild-type mouse embryonic fibroblasts were analyzed for the functional consequences of the Ext1 mutation for growth factor signaling and interaction with the extracellular matrix. Here, we show that the phosphorylation of ERK1/2 in response to FGF2 stimulation was markedly decreased in the Ext1 mutant fibroblasts, whereas neither PDGF-BB nor FGF10 signaling was significantly affected. Furthermore, Ext1 mutants displayed reduced ability to attach to collagen I and to contract collagen lattices, even though no differences in the expression of collagen-binding integrins were observed. Reintroduction of Ext1in the Ext1 mutant fibroblasts rescued HS chain length, FGF2 signaling, and the ability of the fibroblasts to contract collagen. These data suggest that the length of the HS chains is a critical determinant of HS-protein interactions and emphasize the essential role of EXT1 in providing specific binding sites for growth factors and extracellular matrix proteins.

SULF1 and SULF2 regulate heparan sulfate-mediated GDNF signaling for esophageal innervation

Heparan sulfate (HS) plays an essential role in extracellular signaling during development. Biochemical studies have established that HS binding to ligands and receptors is regulated by the fine 6-O-sulfated structure of HS; however, mechanisms that control sulfated HS structure and associated signaling functions in vivo are not known. Extracellular HS 6-O-endosulfatases, SULF1 and SULF2, are candidate enzymatic regulators of HS 6-O-sulfated structure and modulate HS-dependent signaling. To investigate Sulf regulation of developmental signaling, we have disrupted Sulf genes in mouse and identified redundant functions of Sulfs in GDNF-dependent neural innervation and enteric glial formation in the esophagus, resulting in esophageal contractile malfunction in Sulf1(-/-);Sulf2(-/-) mice. SULF1 is expressed in GDNF-expressing esophageal muscle and SULF2 in innervating neurons, establishing their direct functions in esophageal innervation. Biochemical and cell signaling studies show that Sulfs are the major regulators of HS 6-O-desulfation, acting to reduce GDNF binding to HS and to enhance GDNF signaling and neurite sprouting in the embryonic esophagus. The functional specificity of Sulfs in GDNF signaling during esophageal innervation was established by showing that the neurite sprouting is selectively dependent on GDNF, but not on neurotrophins or other signaling ligands. These findings provide the first in vivo evidence that Sulfs are essential developmental regulators of cellular HS 6-O-sulfation for matrix transmission and reception of GDNF signal from muscle to innervating neurons.

Syndecans promote integrin-mediated adhesion of mesenchymal cells in two distinct pathways

Syndecans are transmembrane proteoglycans that support integrin-mediated adhesion. Well documented is the contribution of syndecan-4 that interacts through its heparan sulphate chains to promote focal adhesion formation in response to fibronectin domains. This process has requirements for integrin and signaling through the cytoplasmic domain of syndecan-4. Here an alternate pathway mediated by the extracellular domains of syndecans-2 and -4 is characterized that is independent of both heparan sulphate and syndecan signaling. This pathway is restricted to mesenchymal cells and was not seen in any epithelial cell line tested, apart from vascular endothelia. The syndecan ectodomains coated as substrates promoted integrin-dependent attachment, spreading and focal adhesion formation. Syndecan-4 null cells were competent, as were fibroblasts compromised in heparan sulphate synthesis that were unable to form focal adhesions in response to fibronectin. Consistent with actin cytoskeleton organization, the process required Rho-GTP and Rho kinase. While syndecan-2 and -4 ectodomains could both promote integrin-mediated adhesion, their pathways were distinct, as shown by competition assays. Evidence for an indirect interaction of beta1 integrin with both syndecan ectodomains was obtained, all of which suggests a distinct mechanism of integrin-mediated adhesion.

Contribution of EXT1, EXT2, and EXTL3 to heparan sulfate chain elongation

The exostosin (EXT) family of genes encodes glycosyltransferases involved in heparan sulfate biosynthesis. Five human members of this family have been cloned to date: EXT1, EXT2, EXTL1, EXTL2, and EXTL3. EXT1 and EXT2 are believed to form a Golgi-located hetero-oligomeric complex that catalyzes the chain elongation step in heparan sulfate biosynthesis, whereas the EXTL proteins exhibit overlapping glycosyl-transferase activities in vitro, so that it is not apparent what reactions they catalyze in vivo. We used gene-silencing strategies to investigate the roles of EXT1, EXT2, and EXTL3 in heparan sulfate chain elongation. Small interfering RNAs (siRNAs) directed against the human EXT1, EXT2, or EXTL3 mRNAs were introduced into human embryonic kidney 293 cells. Compared with cells transfected with control siRNA, those transfected with EXT1 or EXT2 siRNA synthesized shorter heparan sulfate chains, and those transfected with EXTL3 siRNA synthesized longer chains. We also generated human cell lines overexpressing the EXT proteins. Overexpression of EXT1 resulted in increased HS chain length, which was even more pronounced in cells coexpressing EXT2, whereas overexpression of EXT2 alone had no detectable effect on heparan sulfate chain elongation. Mutations in either EXT1 or EXT2 are associated with hereditary multiple exostoses, a human disorder characterized by the formation of cartilage-capped bony outgrowths at the epiphyseal growth plates. To further investigate the role of EXT2, we generated human cell lines overexpressing mutant EXT2. One of the mutations, EXT2-Y419X, resulted in a truncated protein. Interestingly, the capacity of wild type EXT2 to enhance HS chain length together with EXT1 was not shared by the EXT2-Y419X mutant.

Enzymatically active N-deacetylase/N-sulfotransferase-2 is present in liver but does not contribute to heparan sulfate N-sulfation

Heparan sulfate (HS) proteoglycans influence embryonic development through interactions with growth factors and morphogens. The interactions depend on HS structure, which is largely determined during biosynthesis by Golgi enzymes. NDST (glucosaminyl N-deacetylase/N-sulfotransferase), responsible for HS N-sulfation, is a key enzyme directing further modifications including O-sulfation. To elucidate the roles of the different NDST isoforms in HS biosynthesis, we took advantage of mice with targeted mutations in NDST1 and NDST2 and used liver as our model organ. Of the four NDST isoforms, only NDST1 and NDST2 transcripts were shown to be expressed in control liver. The absence of NDST1 or NDST2 in the knock-out mice did not affect transcript levels of other NDST isoforms or other HS modification enzymes. Although the sulfation level of HS synthesized in NDST1-/- mice was drastically lowered, liver HS from wild-type mice, from NDST1+/-, NDST2-/-, and NDST1+/- / NDST2-/- mice all had the same structure despite greatly reduced NDST enzyme activity (30% of control levels in NDST1+/- / NDST2-/- embryonic day 18.5 embryos). Enzymatically active NDST2 was shown to be present in similar amounts in wild-type, NDST1-/-, and NDST1+/- embryonic day 18.5 liver. Despite the substantial contribution of NDST2 to total NDST enzyme activity in embryonic day 18.5 liver (approximately 40%), its presence did not appear to affect HS structure as long as NDST1 was also present. In NDST1-/- embryonic day 18.5 liver, in contrast, NDST2 was responsible for N-sulfation of the low sulfated HS. A tentative model to explain these results is presented.

Purification of a 75 kDa protein from the organelle matrix of human neutrophils and identification as N-acetylglucosamine-6-sulphatase

A 75 kDa protein was purified to homogeneity from granule extracts of normal human granulocytes using Sephadex G-75 chromatography, Mono-S cation exchange chromatography and chromatofocusing. The protein consisted of one chain with a molecular mass of 75 kDa, as determined by SDS/PAGE. Tryptic peptide analysis by MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS and sequence analysis by MS/MS identified the protein to be N-acetylglucosamine-6-sulphatase (EC 3.1.6.14). The identity of the protein was confirmed by demostrating enzymatic activity towards the substrate N-acetylglucosamine 6-sulphate. The enzyme was active over a broad pH range with an optimum of pH 7.0, and showed a K(m) value of 13.0 mM and a V(max) value of approximately 1.8 microM/min per mg. The enzyme also showed O-desulphation activity towards heparan sulphate-derived saccharides. Subcellular fractionation of neutrophil organelles showed the presence of enzymatic activity mainly in the same fractions as primary granules. Furthermore, PMA treatment of the neutrophils induced release of the enzyme, indicating its matrix protein nature. The presence of N-acetylglucosamine-6-sulphatase in human neutrophils implies that neutrophils may play a role in the modulation of cell surface molecules and extracellular matrix by O-desulphation.

Substrate specificity and domain functions of extracellular heparan sulfate 6-O-endosulfatases, QSulf1 and QSulf2

The extracellular sulfatases (Sulfs) are an evolutionally conserved family of heparan sulfate (HS)-specific 6-O-endosulfatases. These enzymes remodel the 6-O-sulfation of cell surface HS chains to promote Wnt signaling and inhibit growth factor signaling for embryonic tissue patterning and control of tumor growth. In this study we demonstrate that the avian HS endosulfatases, QSulf1 and QSulf2, exhibit the same substrate specificity toward a subset of trisulfated disaccharides internal to HS chains. Further, we show that both QSulfs associate exclusively with cell membrane and are enzymatically active on the cell surface to desulfate both cell surface and cell matrix HS. Mutagenesis studies reveal that conserved amino acid regions in the hydrophilic domains of QSulf1 and QSulf2 have multiple functions, to anchor Sulf to the cell surface, bind to HS substrates, and to mediate HS 6-O-endosulfatase enzymatic activity. Results of our current studies establish the hydrophilic domain (HD) of Sulf enzymes as an essential multifunctional domain for their unique endosulfatase activities and also demonstrate the extracellular activity of Sulfs for desulfation of cell surface and cell matrix HS in the control of extracellular signaling for embryonic development and tumor progression.

Overexpression of heparan sulfate 6-O-sulfotransferases in human embryonic kidney 293 cells results in increased N-acetylglucosaminyl 6-O-sulfation

Heparan sulfate (HS) interacts with a variety of proteins and thus mediates numerous complex biological processes. These interactions critically depend on the patterns of O-sulfate groups within the HS chains that determine binding sites for proteins. In particular the distribution of 6-O-sulfated glucosamine residues influences binding and activity of HS-dependent signaling molecules. The protein binding domains of HS show large structural variability, potentially because of differential expression patterns of HS biosynthetic enzymes along with differences in substrate specificity. To investigate whether different isoforms of HS glucosaminyl 6-O-sulfotransferase (6-OST) give rise to differently sulfated domains, we have introduced mouse 6-OST1, 6-OST2, and 6-OST3 in human embryonic kidney 293 cells and compared the effects of overexpression on HS structure. High expression of any one of the 6-OST enzymes resulted in appreciably increased 6-O-sulfation of N-sulfated as well as N-acetylated glucosamine units. The increased 6-O-sulfation was accompanied by a decrease in nonsulfated as well as in iduronic acid 2-O-sulfated disaccharide structures. Furthermore, overexpression led to an altered HS domain structure, the most striking effect was the formation of extended 6-O-sulfated predominantly N-acetylated HS domains. Although the effect was most noticeable in 6-OST3-expressing cells, these results were largely independent of the particular 6-OST isoform expressed and mainly influenced by the level of overexpression.

Biosynthesis of Hyaluronan

Hyaluronan (HA), a functionally essential glycosaminoglycan in vertebrate tissues and a putative virulence factor in certain pathogenic bacteria, is an extended linear polymer composed of alternating units of glucuronic acid (GlcUA) and N-acetylglucosamine (GlcNAc). Uncertainty regarding the mechanism of HA biosynthesis has included the directionality of chain elongation, i.e. whether addition of monosaccharide units occurs at the reducing or non-reducing terminus of nascent chains. We have investigated this problem using yeast-derived recombinant HA synthases from Xenopus laevis (xlHAS1) and from Streptococcus pyogenes (spHAS). The enzymes were incubated with UDP-[3H]GlcUA and UDP-[14C]GlcNAc, under experimental conditions designed to yield HA chains with differentially labeled reducing-terminal and non-reducing terminal domains. Digestion of the products with a mixture of beta-glucuronidase and beta-N-acetylglucosaminidase exoenzymes resulted in truncation of the HA chain strictly from the non-reducing end and release of labeled monosaccharides. The change in 3H/14C ratio of the monosaccharide fraction, during the course of exoglycosidase digestion, was interpreted to indicate whether sugar units had been added at the reducing or non-reducing end. The results demonstrate that the vertebrate xlHAS1 and the bacterial spHAS extend HA in opposite directions. Chain elongation catalyzed by xlHAS1 occurs at the non-reducing end of the HA chain, whereas elongation catalyzed by spHAS occurs at the reducing end. The spHAS is the first glycosyltransferase that has been unanimously demonstrated to function at the reducing end of a growing glycosaminoglycan chain.

Generation of “Neoheparin” fromE.coliK5 Capsular Polysaccharide

Heparin remains a major drug in prevention of thromboembolic disease. Concerns related to its animal source have prompted search for heparin analogues. The anticoagulant activity of heparin depends on a specific pentasaccharide sequence that binds antithrombin. We report the generation of a product with antithrombin-binding, anticoagulant, and antithrombotic properties similar to those of heparin, through combined chemical and enzymatic modification of a bacterial (E. coli K5) polysaccharide. The process is readily applicable to large-scale production.

Heparan sulfate synthesized by mouse embryonic stem cells deficient in NDST1 and NDST2 is 6-O-sulfated but contains no N-sulfate groups

Heparan sulfate structure differs significantly between various cell types and during different developmental stages. The diversity is created during biosynthesis by sulfotransferases, which add sulfate groups to the growing chain, and a C5-epimerase, which converts selected glucuronic acid residues to iduronic acid. All these modifications are believed to depend on initial glucosamine N-sulfation carried out by the enzyme glucosaminyl N-deacetylase/N-sulfotransferase (NDST). Here we report that heparan sulfate synthesized by mouse embryonic stem cells deficient in NDST1 and NDST2 completely lacks N-sulfation but still contains 6-O-sulfate groups, demonstrating that 6-O-sulfation can occur without prior N-sulfation. Reverse transcriptase-PCR analysis indicates that all three identified 6-O-sulfotransferases are expressed by the cells, 6-O-sulfotransferase-1 being the dominating form. The 6-O-sulfated polysaccharide lacking N-sulfate groups also contains N-unsubstituted glucosamine units, raising questions about how these units are generated.

Embryonic Fibroblasts with a Gene Trap Mutation in Ext1 Produce Short Heparan Sulfate Chains

Mutational defects in either EXT1 or EXT2 genes cause multiple exostoses, an autosomal hereditary human disorder. The EXT1 and EXT2 genes encode glycosyltransferases that play an essential role in heparan sulfate chain elongation. In this study, we have analyzed heparan sulfate synthesized by primary fibroblast cell cultures established from mice with a gene trap mutation in Ext1. The gene trap mutation results in embryonic lethality, and homozygous mice die around embryonic day 14. Metabolic labeling and immunohistochemistry revealed that Ext1 mutant fibroblasts still produced small amounts of heparan sulfate. The domain structure of the mutant heparan sulfate was conserved, and the disaccharide composition was similar to that of wild type heparan sulfate. However, a dramatic difference was seen in the polysaccharide chain length. The average molecular sizes of the heparan sulfate chains from wild type and Ext1 mutant embryonic fibroblasts were estimated to be around 70 and 20 kDa, respectively. These data suggest that not only the sulfation pattern but also the length of the heparan sulfate chains is a critical determinant of normal mouse development.

In vitro polymerization of heparan sulfate backbone by the EXT proteins

Multiple exosotoses is a dominantly inherited bone disorder caused by defects in EXT1 and EXT2, genes encoding glycosyltransferases involved in heparan sulfate chain elongation. Heparan sulfate polymerization occurs by the alternating addition of glucuronic acid and N-acetylglucosamine units to the nonreducing end of the polysaccharide. EXT1 and EXT2 are suggested to be dual glucuronyl/N-acetylglucosaminyltransferases, and a heterooligomeric complex of EXT1 and EXT2 (EXT1/2) is considered to be the biological functional polymerization unit. Here, we have investigated the in vitro polymerization capacities of recombinant soluble EXT1, EXT2, and EXT1/2 complex on exogenous oligosaccharide acceptors derived from Escherichia coli K5 capsular polysaccharide. Incubations of recombinant EXT1 or EXT1/2 complex with 3H-labeled oligosaccharide acceptors and the appropriate nucleotide sugars resulted in conversion of the acceptors to higher molecular weight compounds but with different efficacies for EXT1 and EXT1/2. In contrast, incubations with recombinant EXT2 resulted in the addition of a single glucuronic acid but no further polymerization. These results indicate that EXT1 alone and the EXT1/2 heterocomplex can act as heparan sulfate polymerases in vitro without the addition of additional auxiliary proteins.

Sulfotransferases in glycosaminoglycan biosynthesis

Most of the sulfotransferases participating in glycosaminoglycan biosynthesis have now been identified. Their essential role in generating binding sites for proteins interacting with glycosaminoglycans is apparent. These interactions may influence important biological processes such as growth control, signal transduction, cell adhesion and lipid metabolism. Gene targeting in mice as well as studies in Drosophila melanogaster and Caenorhabditis elegans have shown that dysfunction or lack of glycosaminoglycan sulfotransferases may result in severely disturbed embryonic development.

Overexpression of UDP-glucose dehydrogenase in Escherichia coli results in decreased biosynthesis of K5 polysaccharide

The Escherichia coli K5 capsular polysaccharide (glycosaminoglycan) chains are composed of the repeated disaccharide structure: -GlcAbeta1,4-GlcNAcalpha1,4-(where GlcA is glucuronic acid and GlcNAc is N-acetyl-D-glucosamine). The GlcA, present in most glycosaminoglycans, is donated from UDP-GlcA, which, in turn, is generated from UDP-glucose by the enzyme UDP-glucose dehydrogenase (UDPGDH). The formation of UDP-GlcA is critical for the biosynthesis of glycosaminoglycans. To investigate the role of UDPGDH in glycosaminoglycan biosynthesis, we used K5 polysaccharide biosynthesis as a model. E. coli was transformed with the complete gene cluster for K5 polysaccharide production. Additional transformation with an extra copy of UDPGDH resulted in an approx. 15-fold increase in the in vitro UDPGDH enzyme activity compared with the strain lacking extra UDPGDH. UDP-GlcA levels were increased 3-fold in overexpressing strains. However, metabolic labelling with [14C]glucose showed, unexpectedly, that overexpression of UDPGDH lead to decreased formation of K5 polysaccharide. No significant difference in the K5 polysaccharide chain length was observed between control and overexpressing strains, indicating that the decrease in K5-polysaccharide production most probably was due to synthesis of fewer chains. Our results suggest that K5-polysaccharide biosynthesis is strictly regulated such that increasing the amount of available UDP-GlcA results in diminished K5-polysaccharide production.

QSulf1 remodels the 6-O sulfation states of cell surface heparan sulfate proteoglycans to promote Wnt signaling

The 6-O sulfation states of cell surface heparan sulfate proteoglycans (HSPGs) are dynamically regulated to control the growth and specification of embryonic progenitor lineages. However, mechanisms for regulation of HSPG sulfation have been unknown. Here, we report on the biochemical and Wnt signaling activities of QSulf1, a novel cell surface sulfatase. Biochemical studies establish that QSulf1 is a heparan sulfate (HS) 6-O endosulfatase with preference, in particular, toward trisulfated IdoA2S-GlcNS6S disaccharide units within HS chains. In cells, QSulf1 can function cell autonomously to remodel the sulfation of cell surface HS and promote Wnt signaling when localized either on the cell surface or in the Golgi apparatus. QSulf1 6-O desulfation reduces XWnt binding to heparin and HS chains of Glypican1, whereas heparin binds with high affinity to XWnt8 and inhibits Wnt signaling. CHO cells mutant for HS biosynthesis are defective in Wnt-dependent Frizzled receptor activation, establishing that HS is required for Frizzled receptor function. Together, these findings suggest a two-state "catch or present" model for QSulf1 regulation of Wnt signaling in which QSulf1 removes 6-O sulfates from HS chains to promote the formation of low affinity HS-Wnt complexes that can functionally interact with Frizzled receptors to initiate Wnt signal transduction.

Oligosaccharide library-based assessment of heparan sulfate 6-O-sulfotransferase substrate specificity

Heparan sulfate mediates numerous complex biological processes. Its action critically depends on the amount and the positions of O-sulfate groups (iduronyl 2-O-sulfates, glucosaminyl 6-O- and 3-O-sulfates) that form binding sites for proteins. The structures and distribution of these protein-binding domains are influenced by the expression and substrate specificity of heparan sulfate biosynthetic enzymes. We describe a general approach to assess substrate specificities of enzymes involved in glycosaminoglycan metabolism, here applied to 6-O-sulfotransferases involved in heparan sulfate biosynthesis. To understand how 2-O-sulfation affects subsequent 6-O-sulfation reactions, the substrate specificity of 6-O-sulfotransferase 3 was probed using substrates from a heparin-based octasaccharide library. Purified 3H-labeled N-sulfated octasaccharides from a library designed to sample 2-O-sulfated motifs were used as sulfate acceptors, 3'-phosphoadenosine 5'-phosphosulfate as sulfate donor, and cell extract from 6-O-sulfotransferase 3-overexpressing 293 cells as enzyme source in the 6-O-sulfotransferase-catalyzed reactions. The first 6-O-sulfate group was preferentially incorporated at the internal glucosamine unit of the octasaccharide substrate. As the reaction proceeded, the octasaccharides acquired three 6-O-sulfate groups. The specificities toward competing octasaccharide substrates, for 6-O-sulfotransferase 2 and 6-O-sulfotransferase 3, were determined using overexpressing 293 cell extracts and purified octasaccharides. Both 6-O-sulfotransferases showed a preference for 2-O-sulfated substrates. The specificity toward substrates with two to three 2-O-sulfate groups was three to five times higher as compared with octasaccharides with no or one 2-O-sulfate group.

Substrate specificities of mouse heparan sulphate glucosaminyl 6-O-sulphotransferases

Glycosaminoglycan heparan sulphate interacts with a variety of proteins, such as growth factors, cytokines, enzymes and inhibitors and, thus, influences cellular functions, including adhesion, motility, differentiation and morphogenesis. The interactions generally involve saccharide domains in heparan sulphate chains, with precisely located O-sulphate groups. The 6-O-sulphate groups on glucosamine units, supposed to be involved in various interactions of functional importance, occur in different structural contexts. Three isoforms of the glucosaminyl 6-O-sulphotransferase (6-OST) have been cloned and characterized [H. Habuchi, M. Tanaka, O. Habuchi, K. Yoshida, H. Suzuki, K. Ban and K. Kimata (2000) J. Biol. Chem. 275, 2859-2868]. We have studied the substrate specificities of the recombinant enzymes using various O-desulphated poly- and oligo-saccharides as substrates, and using adenosine 3'-phosphate 5'-phospho[(35)S]sulphate as sulphate donor. All three enzymes catalyse 6-O-sulphation of both -GlcA-GlcNS- and -IdoA-GlcNS- (where GlcA represents D-glucuronic acid, NS the N-sulphate group and IdoA the L-iduronic acid) sequences, with preference for IdoA-containing targets, with or without 2-O-sulphate substituents. 6-OST1 showed relatively higher activity towards target sequences lacking 2-O-sulphate, e.g. the -GlcA-GlcNS- disaccharide unit. Sulphation of such non-O-sulphated acceptor sequences was generally favoured at low acceptor polysaccharide concentrations. Experiments using partially O-desulphated antithrombin-binding oligosaccharide as the acceptor revealed 6-O-sulphation of N-acetylated as well as 3-O-sulphated glucosamine residues with each of the three 6-OSTs. We conclude that the three 6-OSTs have qualitatively similar substrate specificities, with minor differences in target preference.

Biosynthesis of heparan sulphate with diverse structures and functions: two alternatively spliced forms of human heparan sulphate 6-O-sulphotransferase-2 having different expression patterns and properties

Heparan sulphate 6- O -sulphotransferase (HS6ST) catalyses the transfer of sulphate from adenosine 3'-phosphate, 5'-phosphosulphate to the 6th position of the N -sulphoglucosamine residue in HS. We previously described the occurrence of three isoforms of mouse HS6ST, mHS6ST-1, -2, and -3 [Habuchi, Tanaka, Habuchi, Yoshida, Suzuki, Ban and Kimata (2000) J. Biol. Chem. 275, 2859-2868]. In the present study, we have characterized HS6ST-2 and HS6ST-1 human isologues, including their chromosomal localizations. In the process of their cDNA cloning, we found two forms of HS6ST-2: the original (hHS6ST-2) and a short form (hHS6ST-2S) with 40 amino acids deleted. Both hHS6ST-2 and hHS6ST-2S catalysed the same sulphation reaction, but their preferences for sulphation sites in HS substrates were different. Dot-blot analysis of the two forms showed that the original form was exclusively expressed in adult and foetal brain tissues, whereas the short form was expressed preferentially in ovary, placenta and foetal kidney, suggesting that the expression of two forms of hHS6ST-2 is strictly regulated to yield tissue-dependent differences in the fine structure of HS. A refined analysis of their reaction products has led us to another finding, that HS6STs could also transfer sulphate to N -sulphoglucosamine residues located at the non-reducing terminal of HS with high affinity.

Biosynthetic oligosaccharide libraries for identification of protein-binding heparan sulfate motifs. Exploring the structural diversity by screening for fibroblast growth factor (FGF)1 and FGF2 binding

Heparan sulfate is crucial for vital reactions in the body because of its ability to bind various proteins. The identification of protein-binding heparan sulfate sequences is essential to our understanding of heparan sulfate biology and raises the possibility to develop drugs against diseases such as cancer and inflammatory conditions. We present proof-of-principle that in vitro generated heparan sulfate oligosaccharide libraries can be used to explore interactions between heparan sulfate and proteins, and that the libraries expand the available heparan sulfate sequence space. Oligosaccharide libraries mimicking highly 6-O-sulfated domains of heparan sulfate were constructed by enzymatic O-sulfation of O-desulfated, end-group (3)H-labeled heparin octasaccharides. Acceptor oligosaccharides that were 6-O-desulfated but only partially 2-O-desulfated yielded oligosaccharide arrays with increased ratio of iduronyl 2-O-sulfate/glucosaminyl 6-O-sulfate. The products were probed by affinity chromatography on immobilized growth factors, fibroblast growth factor-1 (FGF1) and FGF2, followed by sequence analysis of trapped oligosaccharides. An N-sulfated octasaccharide, devoid of 2-O-sulfate but with three 6-O-sulfate groups, was unexpectedly found to bind FGF1 as well as FGF2 at physiological ionic strength. However, a single 2-O-sulfate group in the absence of 6-O-sulfation gave higher affinity for FGF2. FGF1 binding was also augmented by 2-O-sulfation, preferentially in combination with an adjacent upstream 6-O-sulfate group. These results demonstrate the potential of the enzymatically generated oligosaccharide libraries.

Demonstration of a novel gene DEXT3 of Drosophila melanogaster as the essential N-acetylglucosamine transferase in the heparan sulfate biosynthesis: chain initiation and elongation

Hereditary multiple exostoses gene (EXT) family members encode glycosyltransferases required for heparan sulfate (HS) biosynthesis in humans as well as in Drosophila. In the present study, we identified a novel Drosophila EXT protein with a type II transmembrane topology and demonstrated its glycosyltransferase activities. The truncated soluble form of this new homolog designated DEXT3 transferred N-acetylglucosamine (GlcNAc) through an alpha1,4-linkage not only to N-acetylheparosan oligosaccharides that represent growing HS chains (alpha-GlcNAc transferase II activity) but also to GlcUAbeta1-3Galbeta1-O-C(2)H(4)NHCbz, a synthetic substrate for alpha-GlcNAc transferase I that determines and initiates HS biosynthesis. The results suggest that DEXT3 is the ortholog of human EXTL3 and Caenorhabditis elegans rib-2. Semiquantitative reverse transcriptase-PCR analysis revealed ubiquitous expression of the DEXT3 mRNA. Based on the findings of the present study and those of a recent study where a fly mutant, deficient in the botv gene identical to DEXT3, affected HS proteoglycan-mediated developmental signalings, it is suggested that DEXT3 with the revealed glycosyltransferase activities is critically involved in HS formation in Drosophila. These results suggest the essential roles of DEXT3, its human ortholog EXTL3, and the C. elegans ortholog rib-2 in the biosynthesis of heparan sulfate and heparin, if present, in the respective organisms.

Human tumor suppressor EXT gene family members EXTL1 and EXTL3 encode alpha 1,4- N-acetylglucosaminyltransferases that likely are involved in heparan sulfate/ heparin biosynthesis

The tumor suppressors EXT1 and EXT2 are associated with hereditary multiple exostoses and encode bifunctional glycosyltransferases essential for chain polymerization of heparan sulfate (HS) and its analog, heparin (Hep). Three highly homologous EXT-like genes, EXTL1-EXTL3, have been cloned, and EXTL2 is an alpha1,4-GlcNAc transferase I, the key enzyme that initiates the HS/Hep synthesis. In the present study, truncated forms of EXTL1 and EXTL3, lacking the putative NH2-terminal transmembrane and cytoplasmic domains, were transiently expressed in COS-1 cells and found to harbor alpha-GlcNAc transferase activity. EXTL3 used not only N-acetylheparosan oligosaccharides that represent growing HS chains but also GlcAbeta1-3Galbeta1-O-C2H4NH-benzyloxycarbonyl (Cbz), a synthetic substrate for alpha-GlcNAc transferase I that determines and initiates HS/Hep synthesis. In contrast, EXTL1 used only the former acceptor. Neither EXTL1 nor EXTL3 showed any glucuronyltransferase activity as examined with N-acetylheparosan oligosaccharides. Heparitinase I digestion of each transferase-reaction product showed that GlcNAc had been transferred exclusively through an alpha1,4-configuration. Hence, EXTL3 most likely is involved in both chain initiation and elongation, whereas EXTL1 possibly is involved only in the chain elongation of HS and, maybe, Hep as well. Thus, their acceptor specificities of the five family members are overlapping but distinct from each other, except for EXT1 and EXT2 with the same specificity. It now has been clarified that all of the five cloned human EXT gene family proteins harbor glycosyltransferase activities, which probably contribute to the synthesis of HS and Hep.

Substrate specificity of the heparan sulfate hexuronic acid 2-O-sulfotransferase

The interaction of heparan sulfate with different ligand proteins depends on the precise location of O-sulfate groups in the polysaccharide chain. We have previously shown that overexpression in human kidney 293 cells of a mouse mastocytoma 2-O-sulfotransferase (2-OST), previously thought to catalyze the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to C2 of L-iduronyl residues, preferentially increases the level of 2-O-sulfation of D-glucuronyl units [Rong, J., Habuchi, H., Kimata, K., Lindahl, U., and Kusche-Gullberg, M. (2000) Biochem. J. 346, 463-468]. In the study presented here, we further investigated the substrate specificity of the mouse mastocytoma 2-OST. Different polysaccharide acceptor substrates were incubated with cell extracts from 2-OST-transfected 293 cells together with the sulfate donor 3'-phosphoadenosine 5'-phospho[(35)S]sulfate. Incubations with O-desulfated heparin, predominantly composed of [(4)alphaIdoA(1)-(4)alphaGlcNSO(3)(1)-](n)(), resulted in 2-O-sulfation of iduronic acid. When, on the other hand, an N-sulfated capsular polysaccharide from Escherichia coli K5, with the structure [(4)betaGlcA(1)-(4)alphaGlcNSO(3)(1)-](n)(), was used as an acceptor, sulfate was transferred almost exclusively to C2 of glucuronic acid. Substrates containing both iduronic and glucuronic acid residues in about equal proportions strongly favored sulfation of iduronic acid. In agreement with these results, the 2-OST was found to have a approximately 5-fold higher affinity for iduronic acid-containing substrate disaccharide units (K(m) approximately 3.7 microM) than for glucuronic acid-containing substrate disaccharide units (K(m) approximately 19.3 microM).

rib-2, a Caenorhabditis elegans homolog of the human tumor suppressor EXT genes encodes a novel alpha1,4-N-acetylglucosaminyltransferase involved in the biosynthetic initiation and elongation of heparan sulfate

The proteins encoded by the EXT1, EXT2, and EXTL2 genes, members of the hereditary multiple exostoses gene family of tumor suppressors, are glycosyltransferases required for the heparan sulfate biosynthesis. Only two homologous genes, rib-1 and rib-2, of the mammalian EXT genes were identified in the Caenorhabditis elegans genome. Although heparan sulfate is found in C. elegans, the involvement of the rib-1 and rib-2 proteins in heparan sulfate biosynthesis remains unclear. In the present study, the substrate specificity of a soluble recombinant form of the rib-2 protein was determined and compared with those of the recombinant forms of the mammalian EXT1, EXT2, and EXTL2 proteins. The present findings revealed that the rib-2 protein was a unique alpha1,4-N-acetylglucosaminyltransferase involved in the biosynthetic initiation and elongation of heparan sulfate. In contrast, the findings confirmed the previous observations that both the EXT1 and EXT2 proteins were heparan sulfate copolymerases with both alpha1,4-N-acetylglucosaminyltransferase and beta1,4-glucuronyltransferase activities, which are involved only in the elongation step of the heparan sulfate chain, and that the EXTL2 protein was an alpha1,4-N-acetylglucosaminyltransferase involved only in the initiation of heparan sulfate synthesis. These findings suggest that the biosynthetic mechanism of heparan sulfate in C. elegans is distinct from that reported for the mammalian system.

Papilin in development; a pericellular protein with a homology to the ADAMTS metalloproteinases

Papilin is an extracellular matrix glycoprotein that we have found to be involved in, (1) thin matrix layers during gastrulation, (2) matrix associated with wandering, phagocytic hemocytes, (3) basement membranes and (4) space-filling matrix during Drosophila development. Determination of its cDNA sequence led to the identification of Caenorhabditis and mammalian papilins. A distinctly conserved ‘papilin cassette’ of domains at the amino-end of papilins is also the carboxyl-end of the ADAMTS subgroup of secreted, matrix-associated metalloproteinases; this cassette contains one thrombospondin type 1 (TSR) domain, a specific cysteine-rich domain and several partial TSR domains. In vitro, papilin non-competitively inhibits procollagen N-proteinase, an ADAMTS metalloproteinase. Inhibiting papilin synthesis in Drosophila or Caenorhabditis causes defective cell arrangements and embryonic death. Ectopic expression of papilin in Drosophila causes lethal abnormalities in muscle, Malpighian tubule and trachea formation. We suggest that papilin influences cell rearrangements and may modulate metalloproteinases during organogenesis.

The EXT1/EXT2 tumor suppressors: catalytic activities and role in heparan sulfate biosynthesis

The D-glucuronyltransferase and N-acetyl-D-glucosaminyltransferase reactions in heparan sulfate biosynthesis have been associated with two genes, EXT1 and EXT2, which are also implicated in the inherited bone disorder, multiple exostoses. Since the cell systems used to express recombinant EXT proteins synthesize endogenous heparan sulfate, and the EXT proteins tend to associate, it has not been possible to define the functional roles of the individual protein species. We therefore expressed EXT1 and EXT2 in yeast, which does not synthesize heparan sulfate. The recombinant EXT1 and EXT2 were both found to catalyze both glycosyltransferase reactions in vitro. Coexpression of the two proteins, but not mixing of separately expressed recombinant EXT1 and EXT2, yields hetero-oligomeric complexes in yeast and mammalian cells, with augmented glycosyltransferase activities. This stimulation does not depend on the membrane-bound state of the proteins.

Expression of heparan sulphate L-iduronyl 2-O-sulphotransferase in human kidney 293 cells results in increased D-glucuronyl 2-O-sulphation

Functionally important interactions between heparan sulphate and a variety of proteins depend on the precise location of O-sulphate groups. Such residues occur at C-2 of L-iduronic (IdoA) and D-glucuronic acid (GlcA) units, and at C-3 and C-6 of D-glucosamine (GlcN) units. Stable transfection of human embryonic kidney 293 cells with a cDNA encoding mouse mastocytoma IdoA 2-O-sulphotransferase resulted in an approx. 6-fold increase in O-sulphotransferase activity, compared with control cells, as determined using O-desulphated heparin as an acceptor. Structural analysis of endogenous heparan sulphate in the transfected cells, following metabolic labelling with either [(3)H]GlcN or [(35)S]sulphate, showed appreciable formation of -GlcA(2-OSO(3))-GlcNSO(3)- disaccharide units (6% of total disaccharide units; 17% of total O-sulphated disaccharide units) that were essentially absent from heparan sulphate from control cells. The increase in GlcA 2-O-sulphation was accompanied by a decrease in the amount of IdoA formed, whereas overall 2-O-sulphation or 6-O-sulphation remained largely unaffected. These findings indicate that 2-O-sulphation of IdoA and GlcA residues is catalysed by the same enzyme in heparan sulphate biosynthesis.

cDNA cloning and chromosomal localization of human alpha(11) integrin. A collagen-binding, I domain-containing, beta(1)-associated integrin alpha-chain present in muscle tissues

We previously identified a novel integrin alpha-chain in human fetal muscle cells (Gullberg, D., Velling, T., Sjöberg, G., and Sejersen, T. (1995) Dev. Dyn. 204, 57-65). We have now isolated the full-length cDNA for this integrin subunit, alpha(11). The open reading frame of the cDNA encodes a precursor of 1188 amino acids. The predicted mature protein of 1166 amino acids contains seven conserved FG-GAP repeats, an I domain with a metal ion-dependent adhesion site motif, a short transmembrane region, and a unique cytoplasmic domain of 24 amino acids containing the sequence GFFRS. alpha(11), like other I domain integrins, lacks a dibasic cleavage site for generation of a heavy chain and a light chain, and it contains three potential divalent cation binding sites in repeats 5-7. The presence of 22 inserted amino acids in the extracellular stalk portion (amino acids 804-826) distinguishes the alpha(11) integrin sequence from other integrin alpha-chains. Amino acid sequence comparisons reveal the highest identity of 42% with the alpha(10) integrin chain. Immunoprecipitation with antibodies to alpha(11) integrin captures a 145-kDa protein distinctly larger than the 140-kDa alpha(2) integrin chain when analyzed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Fluorescence in situ hybridization maps the integrin alpha(11) gene to chromosome 15q23, in the vicinity of an identified locus for Bardet-Biedl syndrome. Based on Northern blotting, integrin alpha(11) mRNA levels are high in the adult human uterus and in the heart and intermediate in skeletal muscle and some other tissues tested. During in vitro myogenic differentiation, alpha(11) mRNA and protein are up-regulated. Studies of ligand binding properties show that alpha(11)beta(1) binds collagen type I-Sepharose, and cultured muscle cells localize alpha(11)beta(1) into focal contacts on collagen type I. Future studies will reveal the importance of alpha(11)beta(1) for muscle development and integrity in adult muscle and other tissues.

Abnormal mast cells in mice deficient in a heparin-synthesizing enzyme

Heparin is a sulphated polysaccharide, synthesized exclusively by connective-tissue-type mast cells and stored in the secretory granules in complex with histamine and various mast-cell proteases. Although heparin has long been used as an antithrombotic drug, endogenous heparin is not present in the blood, so it cannot have a physiological role in regulating blood coagulation. The biosynthesis of heparin involves a series of enzymatic reactions, including sulphation at various positions. The initial modification step, catalysed by the enzyme glucosaminyl N-deacetylase/N-sulphotransferase-2, NDST-2, is essential for the subsequent reactions. Here we report that mice carrying a targeted disruption of the gene encoding NDST-2 are unable to synthesize sulphated heparin. These NDST-2-deficient mice are viable and fertile but have fewer connective-tissue-type mast cells; these cells have an altered morphology and contain severely reduced amounts of histamine and mast-cell proteases. Our results indicate that one site of physiological action for heparin could be inside connective-tissue-type mast cells, where its absence results in severe defects in the secretory granules.

cDNA cloning and expression of UDP-glucose dehydrogenase from bovine kidney

We have isolated a cDNA encoding UDP-glucose dehydrogenase from a bovine kidney cDNA-library, the first mammalian cDNA clone published. [After submission of the manuscript, a study appeared describing the molecular cloning and characterization of the human and mouse UDP-glucose dehydrogenase genes (Spicer et al., 1998).] The enzyme catalyzes the conversion of UDP-glucose to UDP-glucuronic acid, an essential precursor in glycosaminoglycan biosynthesis. The cDNA has an open reading frame of 1482 nucleotides coding for a 55 kDa protein. Expression of the enzyme in COS-7 cells showed a 3-fold increase in UDP-glucose dehydrogenase activity; also, the C-terminal 23 amino acids was shown not to be necessary for enzyme activity. Northern blots from human and mouse tissues reveal high expression in liver and low in skeletal muscle. Human tissues have a major transcript size of 3.2 kilobases and a minor of 2.6 whereas mouse tissues have a single 2.6 kilobase transcript. We have also developed a sensitive and direct assay using UDP-[14C]Glc as a substrate for detection of small amounts of UDPGDH activity.

Identification and expression in mouse of two heparan sulfate glucosaminyl N-deacetylase/N-sulfotransferase genes

The biosynthesis of heparan sulfate/heparin is a complex process that requires the coordinate action of a number of different enzymes. In close connection with polymerization of the polysaccharide chain, the modification reactions are initiated by N-deacetylation followed by N-sulfation of N-acetylglucosamine units. These two reactions are carried out by a single protein. Proteins with such dual activities were first purified and cloned from rat liver and mouse mastocytoma. The mouse mastocytoma enzyme is encoded by an approximately 4-kilobase (kb) mRNA, whereas the rat liver transcript contains approximately 8 kb. In the present study, the primary structure of the enzyme encoded by the mouse 8-kb transcript is described. It is demonstrated that both the 4-and 8-kb transcripts have a wide tissue distribution and that they are encoded by separate genes. Characterization of the gene encoding the 4-kb transcript demonstrates that it spans a region of about 8 kb and that it contains at least 14 exons. The similarity of this gene and the previously characterized human gene for the 8-kb transcript is discussed.

Mouse mastocytoma cells synthesize undersulfated heparin and chondroitin sulfate in the presence of brefeldin A

In order to study the subcellular localization and organization of the enzymes involved in the glycosylation of the hybrid proteoglycan serglycin, mouse mastocytoma cells were metabolically labeled with [35S]sulfate or [3H]glucosamine in the absence or presence of brefeldin A. This drug is known to induce a disassembly of the proximal part of the Golgi complex, resulting in a redistribution of cis-, medial-, and trans-Golgi resident enzymes back to the endoplasmic reticulum, and to block the anterograde transport of proteins to the trans-Golgi network. Although the total incorporation of [3H]glucosamine into glycosaminoglycan chains was reduced to about 25% in brefeldin A-treated cells compared to control cells, both control cells and cells treated with brefeldin A synthesized heparin as well as chondroitin sulfate chains. Therefore, enzymes involved in the biosynthesis of both types of glycosaminoglycan chains seem to be present proximal to the trans-Golgi network in these cells. Chondroitin sulfate and heparin synthesized in cells exposed to brefeldin A were undersulfated, as demonstrated by ion-exchange chromatography, compositional analyses of disaccharides, as well as by a lower [35S]sulfate/[3H]glucosamine ratio compared to controls. In heparin biosynthesis, both N- and O-sulfation reactions were impaired, with a larger relative decrease in 2-O-sulfation than in 6-O-sulfation. Despite undersulfation, the heparin chains synthesized in the presence of brefeldin A were larger (30 kDa) than the heparin synthesized by control cells (20 kDa). The reduced [3H]glucosamine incorporation in brefeldin A-treated cells was partly due to decreased number of glycosaminoglycan chains synthesized, but also to the biosynthesis of chondroitin sulfate chains of smaller molecular size (8 versus 15 kDa in control cells). Brefeldin A had no effect on the glycosaminoglycan synthesis when used in a cell-free, microsomal fraction, indicating that brefeldin A does not interfere directly with the enzymes involved in the biosynthesis of glycosaminoglycans.

Stem cell factor-dependent human cord blood derived mast cells express alpha- and beta-tryptase, heparin and chondroitin sulphate

The present study sought to determine the expression of alpha- and beta-tryptase in in vitro differentiated human cord blood derived mast cells. We also analysed the glycosaminoglycan composition and the phenotype of the cells. The major protease in human mast cells is tryptase, and cDNAs for two different human tryptases have been characterized, the so-called alpha- and beta-tryptase. By reverse transcriptase-polymerase chain reaction (RT-PCR) we could show that stem cell factor (SCF)-dependent cord blood derived mast cells express both alpha- and beta-tryptase. Furthermore, the cells were stained with a monoclonal antibody (mAb) against tryptase, and the tryptase was enzymatically active cleaving the substrate Z-Gly-Pro-Arg- methoxy-2- naphthylamide (MNA). The majority of the cord blood derived mast cells could also be stained with mAbs against chymase, cathepsin G and CD68. They also expressed Kit/SCFR (CD117), CD13, CD29 and CD45 on the cell surface. The proteoglycan-derived polysaccharide composition of the cells was estimated to be 25-35% of heparin origin and 65-75% of chondroitin sulphate origin. Hence, the cord blood derived mast cells exhibit a phenotype in common with the so-called MCTC type of human mast cells.

Expression of the mouse mastocytoma glucosaminyl N-deacetylase/ N-sulfotransferase in human kidney 293 cells results in increased N-sulfation of heparan sulfate

The biosynthesis of heparin and heparan sulfate involves a series of polymer-modification reactions that is initiated by N-deacetylation and subsequent N-sulfation of N-acetylglucosamine residues. These reactions are catalysed by a combined N-deacetylase/N-sulfotransferase. Proteins expressing both activities have previously been purified from mouse mastocytoma, which generates heparin, and from rat liver, which produces heparan sulfate. In the present study, the mouse mastocytoma enzyme has been expressed in the human kidney cell line, 293, to investigate whether it could promote modification of the endogenous heparan sulfate precursor polysaccharide into a heparan-like molecule. The N-deacetylase activity of the stably transfected cell clones as approximately 8-fold higher, on a cell-protein basis, than that of control cells, while the N-sulfotransferase activity was increased approximately 2.5 fold. The amounts of glycosaminoglycans synthesized were the same in control and transfected cells, measured as incorporation of [3H]-glucosamine, whereas 35S-labeled glycosaminoglycans were approximately 50% increased in transfected cells, with an increased relative content of heparin sulfate. Structural analysis demonstrated the the glucosamine units of the "heparan sulfate" from transfected cells were almost exclusively N-sulfated, as expected for heparin, whereas more than half of the glucosamine units of the control polysaccharide remained N-acetylated. Notably, the increased N-sulfation was not accompanied by increased O-sulfation, not by C-5 epimerization of D-glucuronic to L-iduronic acid units. The implications of these findings are discussed with regard to the regulation of the biosynthetic process.

Presence of N-unsubstituted glucosamine units in native heparan sulfate revealed by a monoclonal antibody

Immunohistochemical application of antibodies against heparan sulfate proteoglycan core protein and heparitinase-digested heparan sulfate stubs showed the presence of heparan sulfate proteoglycan in all basement membranes of the rat kidney. However, a monoclonal antibody (JM-403) against native heparan sulfate (van den Born, J., van den Heuvel, L. P. W. J., Bakker, M. A. H., Veerkamp, J. H., Assmann, K. J. M., and Berden, J. H. M. (1992) Kidney Int. 41, 115-123) largely failed to stain tubular basement membranes, suggesting the presence of heparan sulfate chains lacking the specific JM-403 epitope. Heparan sulfate preparations from various sources differed markedly with regard to JM-403 binding, as demonstrated by liquid phase inhibition in enzyme-linked immunosorbent assay, the interaction decreasing with increasing sulfate contents of the polysaccharide. Mapping of the JM-403 epitope indicated that it was dominated by one or more N-unsubstituted glucosamine unit(s), since treatments that destroyed or altered the structure of such units in heparan sulfate preparations (cleavage at N-unsubstituted glucosamine units with HNO2 at pH 3.9 and N-acetylation with acetic anhydride, respectively), abolished antibody binding. Conversely, immunoreactivity could be induced in a (D-glucuronyl-1,4-N-acetyl-D-glucosaminyl-1,4) polysaccharide by the generation of N-unsubstituted glucosamine N-unsubstituted glucosamine in a JM-403-binding heparan sulfate (preparation HS-II from human aorta) was demonstrated by an approximately 3-fold reduction in molecular size following HNO2 (pH 3.9) treatment. Further characterization of the epitope recognized by JM-403, based on enzyme-linked immunosorbent assay inhibition tests with chemically/enzymatically modified polysaccharides, indicated that one or more N-sulfated glucosamine units are invariable present, whereas L-iduronic acid and O-sulfate residues appear to inhibit JM-403 reactivity. It is concluded that the epitope contains one or more N-unsubstituted glucosamine and D-glucuronic acid units and is located in a region of the heparan sulfate chain composed of mixed N-sulfated and N-acetylated disaccharide units.

Phenotypic characterization of the human mast-cell line HMC-1

The cell line HMC-1, derived from a patient with mast cell leukaemia, is the only established cell line exhibiting a phenotype similar to that of human mast cells. This paper reports on a detailed characterization of the expression of a panel of markers for various types of immature and mature haematopoietic cells in the HMC-1. We also studied the potential of HMC-1 to differentiate upon treatment with conditioned media from the human T-cell line Mo, retinoic acid or DMSO. HMC-1 was found to express several mast cell-related markers. A high expression of Kit, the receptor for stem-cell factor, was detected. The majority of the cells were stained with a MoAb against the mast cell-specific serine protease tryptase. Of particular interest was the finding that beta-tryptase mRNA, but not alpha-tryptase mRNA, was expressed in HMC-1. Using enzyme-histochemistry we were able to show that the beta-tryptase was enzymatically active, indicating that tryptase can form active homotetramers. Both heparin and chondroitin sulfate were found to be present in approximately equal amounts. HMC-1 lacked surface expression of the high-affinity IgE receptor, which was confirmed by the absence of mRNA of the alpha- and beta-chains of the IgE-receptor complex. However, a strong expression of the gamma-chain of the IgE-receptor complex was detected. A positive staining of the monocyte/macrophage marker CD68 was obtained, as well as a strong hybridization signal for the eosinophilic/basophilic-related differentiation marker the Charcot-Leyden crystal. Treatment of HMC-1 with conditioned media from the human T-cell line Mo, retinoic acid or DMSO induced only moderate changes in the surface or intracellular expression of the studied markers. The agents tested neither induced any of the monocyte/granulocyte markers examined, nor expression of the Fc epsilon RI alpha-chain.

Novel Drosophila laminin A chain reveals structural relationships between laminin subunits

Laminins are trimeric glycoproteins composed of A, B1, and B2 chains that play important roles in cell adhesion and differentiation and in the assembly of basement membranes. There is evidence that multiple independent gene products can be used to supply the A, B1, and B2-like chains, thereby generating diverse laminin molecules. The set of A chain-like polypeptides are the most divergent. The amino-terminal portion of each chain, called the short arm, is composed of alternating cysteine-rich and globular domains. The amino-terminal two-thirds of the newly characterized Drosophila laminin A chain appears to encode a novel structure, but the analysis presented here shows that substantial portions of its amino acid sequence are related to sequences found in other A, B1, and B2 chains. A portion of the Drosophila A chain is composed of sequences like those found in B2 chains. These sequences parallel a portion of the vertebrate A chains, suggesting a structure for a shared ancestral A chain. However, unlike vertebrate A chains, the Drosophila A chain also contains sequences typical of B1 chains, suggesting that its gene must have arisen by recombination of segments from different primordial laminin genes.