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

Combinatorial expression patterns of heparan sulfate sulfotransferases in zebrafish: II. The 6-O-sulfotransferase family

Heparan sulfate (HS) is an unbranched chain of repetitive disaccharides, which specifically binds ligands when attached to the cell surface or secreted extracellularly. HS chains contain sulfated domains termed the HS fine structure, which gives HS specific binding affinities for extracellular ligands. HS 6-O-sulfotransferases (6-OST) catalyze the transfer of sulfate groups to the 6-O position of glucosamine residues of HS. We report here the characterization and developmental expression analysis of the 6-OST gene family in the zebrafish. The zebrafish 6-OST gene family consists of four conserved vertebrate orthologues, including a gene duplication specific to zebrafish. We examined the mRNA expression patterns in several tissues/organs throughout early zebrafish development, including early cleavage stages, eyes, somites, brain, internal organ primordial, and pectoral fin development. Members of the 6-OST gene family have spatially and temporally distinct restricted expression, suggesting in vivo functional differences exist between members of this family.

Determination of Substrate Specificity of Sulfotransferases and Glycosyltransferases (Proteoglycans)

Proteoglycans have sulfated linear polysaccharide chains, that is, heparan sulfate, heparin, chondroitin sulfates, dermatan sulfate, and keratan sulfate. Many glycosyltransferases and sulfotransferases are involved in biosynthesis of the polysaccharides. Specificities of these enzymes have been mainly determined by evaluating their activities to various acceptor carbohydrates and by analyzing the structure of the products. For the latter purpose, enzymatic hydrolysis using heparitinases, heparinase, and chondroitinases or chemical degradation employing nitrous acid deamination has been effectively used in combination with high-performance liquid chromatography (HPLC) of the degraded products. As examples, we describe methods for assays and product characterization of sulfotransferases involved in biosynthesis of these polysaccharides, namely heparan sulfate 2-sulfotransferase, heparan sulfate 6-sulfotransferases, chondroitin 4-sulfotransferases, chondroitin 6-sulfotransferase, N-acetylgalactosamine 4-sulfate 6-sulfotransferase, and N-acetylglucosamine 6-sulfotransferases.

Thyroid hormone regulates heparan sulfate proteoglycan expression in the growth plate

Thyroid hormone is essential for normal skeletal development. Hypothyroidism is associated with growth arrest, failure of chondrocyte differentiation, and abnormal matrix synthesis. Thyroid hormone modulates the Indian hedgehog/PTHrP feedback loop and regulates fibroblast growth factor (FGF)/FGF receptor signaling. Because heparan sulfate (HS) proteoglycans (Prgs) (HSPGs) are absolutely required by these signaling pathways, we have investigated whether thyroid status affects HSPG expression within the growth plate. Tibial growth plate sections were obtained from 12-wk-old rats rendered euthyroid, thyrotoxic, or hypothyroid at 6 wk of age, 14-d-old congenitally hypothyroid Pax8-null mice, and TRalpha/TRbeta double-null mice lacking all thyroid hormone receptors. HS and chondroitin sulfate Prg expression was determined by immunohistochemistry using three monoclonal antibodies. There was increased HS staining in growth plates from hypothyroid animals predominantly within the extracellular matrix of reserve and proliferative zones. Cellular HS staining was also increased particularly in prehypertrophic chondrocytes. T3 regulation of HSPG core protein and HS synthetic and modification enzyme expression was studied in ATDC5 cells using semiquantitative RT-PCR. Thyroid hormone negatively regulated expression of the core protein Gpc6, the polymerase Ext1, and the modification enzyme Hs6st2. These studies demonstrate that the expression and distribution of growth plate Prgs are regulated by thyroid hormone, and the regulation of HSPG expression provides an important additional link between FGF and Indian hedgehog signaling and T3. These novel observations suggest that the cartilage matrix and especially HSPGs are critical mediators of the skeletal response to thyroid hormone.

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.

A unique role for 6-O sulfation modification in zebrafish vascular development

Heparan sulfate proteoglycans are important modulators of growth factor signaling in a variety of patterning processes. Secreted growth factors that play critical roles in angiogenesis bind to heparan sulfate, and this association is affected by 6-O-sulfation of the heparan sulfate chains. Addition of 6-O-sulfate is catalyzed by a family of sulfotransferases (HS6STs), and genetic manipulation of their function permits an assessment of their contribution to vascular assembly. We report on the biochemical activity and expression patterns of two zebrafish HS6ST genes. In situ hybridization reveals dynamic and distinct expression patterns of these two genes during development. Structural analysis of heparan sulfate from wild-type and morpholino antisense 'knockdown' embryos suggests that HS6ST-1 and HS6ST-2 have similar biochemical activity. HS6ST-2, but not HS6ST-1, morphants exhibit abnormalities in the branching morphogenesis of the caudal vein during embryonic development of the zebrafish. Our finding that HS6ST-2 is required for the branching morphogenesis of the caudal vein is the first in vivo evidence for an essential role of a gene encoding a heparan sulfate modifying enzyme in vertebrate angiogenesis. Our analysis of two zebrafish HS6ST genes suggests that a wide range of biological processes may be regulated by an array of sulfation-modifying enzymes in the vertebrate genome.

Identification of L-selectin Binding Heparan Sulfates Attached to Collagen Type XVIII

L-selectin is a C-type lectin expressed on leukocytes that is involved in both lymphocyte homing to the lymph node and leukocyte extravasation during inflammation. Known L-selectin ligands include sulfated Lewis-type carbohydrates, glycolipids, and proteoglycans. Previously, we have shown that in situ detection of different types of L-selectin ligands is highly dependent on the tissue fixation protocol used. Here we use this knowledge to specifically examine the expression of L-selectin binding proteoglycans in normal mouse tissues. We show that L-selectin binding chondroitin/dermatan sulfate proteoglycans are present in cartilage, whereas L-selectin binding heparan sulfate proteoglycans are present in spleen and kidney. Furthermore, we show that L-selectin only binds a subset of renal heparan sulfates, attached to a collagen type XVIII protein backbone and predominantly present in medullary tubular and vascular basement membranes. As L-selectin does not bind other renal heparan sulfate proteoglycans such as perlecan, agrin, and syndecan-4, and not all collagen type XVIII expressed in the kidney binds L-selectin, this indicates that there is a specific L-selectin binding domain on heparan sulfate glycosaminoglycan chains. Using an in vitro L-selectin binding assay, we studied the contribution of N-sulfation, O-sulfation, C5-epimerization, unsubstituted glucosamine residues, and chain length in L-selectin binding to heparan sulfate/heparin glycosaminoglycan chains. Based on our results and the accepted model of heparan sulfate domain organization, we propose a model for the interaction of L-selectin with heparan sulfate glycosaminoglycan chains. Interestingly, this opens the possibility of active regulation of L-selectin binding to heparan sulfate proteoglycans, e.g. under inflammatory conditions.

Differential expression of heparan sulfate 6-O-sulfotransferase isoforms in the mouse embryo suggests distinctive roles during organogenesis

Heparan sulfate (HS) interactions with secreted morphogens such as fibroblast growth factors, hedgehogs, and Wnts are essential for embryonic development. Formation of biologically relevant HS structures is a result of the coordinated action of various biosynthetic enzymes. HS 6-O-sulfotransferases (6OST) catalyze the transfer of sulfate groups to the 6-O position of glucosamine residues in HS. Three 6OST isoforms have been described in the mouse; however, little is known about their role in generating specific HS protein-binding sequences, expression pattern, and function in vivo. To gain insights into the distribution of these isoforms and their potential role in development, we mapped 6OST1-3 gene expression during mouse organogenesis. We report dynamic expression of these isoforms with striking differences in tissue distribution in many developing organs. We show that 6OST transcripts are differentially expressed in several sites where heparin-binding growth factors are critical for development. 6OST1 is predominantly transcribed in epithelial and neural-derived tissues, whereas 6OST2 is more mesenchymal. 6OST3 appears at later stages and in a more restricted manner. The patterns reported here strongly suggest that the HS structures modified by these enzymes have different roles in growth factor-induced developmental processes.

Sulfation pattern in glycosaminoglycan: does it have a code?

Heparan sulfate chains (HS) are initially synthesized on core proteins as linear polysaccharides composed of glucuronic acid--N-acetylglucosamine repeating units and subjected to marked structural modification by sulfation (N-, 2-O-, 6-O-, 3-O-sulfotransferases) and epimerization (C5-epimerase) at the Golgi lumen and further by desulfation (6-O- endosulfatase) at the cell surface, after which divergent fine structures are generated. The expression patterns and specificity of the modifying enzymes are, at least partly, responsible for the elaboration of these fine structures of heparan sulfate. HS interacts with many proteins including growth factors (GF) and morphogens through specific fine structures. Recent biochemical and genetic studies have presented evidence that HS plays important roles in cell behavior and organogenesis. In knock-down experiments of heparan sulfate 6-O-sulfotransferase, 6-O-sulfated units in HS have been shown to act as a stimulator or suppressor according to individual GF/morphogen signaling systems.

Stem domains of heparan sulfate 6-O-sulfotransferase are required for Golgi localization, oligomer formation and enzyme activity

Heparan sulfate O-sulfotransferases catalyze the O-sulfation of the glucosamine and uronic acid residues of heparan sulfate, thereby determining the binding sites for ligands necessary for important biological functions such as the formation of morphogen gradients and growth factor signaling. Here we investigated the localization of the three heparan sulfate 6-O-sulfotransferase (HS6ST) isoforms and the mechanism of their localization. All three GFP-tagged HS6STs localized in the Golgi apparatus. C-5 epimerase and HS2ST have been shown to form complexes that facilitate their localization in the Golgi but we found that the absence of HS2ST did not alter the localization of any of the HS6STs. Neither the forced expression of HS2ST in the rough endoplasmic reticulum (ER), the deletion of most of the lumenal domain nor increasing the length of the transmembrane domain had any effect on the localization of HS6STs. However, deletions in the stem region did affect the Golgi localization of the HS6STs and also reduced their sulfotransferase activity and oligomer formation. These findings suggest that the stem region of HS6ST plays an important role in normal functioning, including the transit of HS6ST to the Golgi apparatus and maintaining the active conformation essential for enzyme activity.

Heparan sulfates in skeletal muscle development and physiology

Recent years have seen an emerging interest in the composition of the skeletal muscle extracellular matrix (ECM) and in the developmental and physiological roles of its constituents. Many cell surface-associated and ECM-embedded molecules occur in highly organized spatiotemporal patterns, suggesting important roles in the development and functioning of skeletal muscle. Glycans are historically underrepresented in the study of skeletal muscle ECM, even though studies from up to 30 years ago have demonstrated specific carbohydrates and glycoproteins to be concentrated in neuromuscular junctions (NMJs). Changes in glycan profile and distribution during myogenesis and synaptogenesis hint at an active involvement of glycoconjugates in muscle development. A modest amount of literature involves glycoconjugates in muscle ion housekeeping, but a recent surge of evidence indicates that glycosylation defects are causal for many congenital (neuro)muscular disorders, rendering glycosylation essential for skeletal muscle integrity. In this review, we focus on a single class of ECM-resident glycans and their emerging roles in muscle development, physiology, and pathology: heparan sulfate proteoglycans (HSPGs), notably their heparan sulfate (HS) moiety.

Fgf9induces proliferation and nuclear localization of FGFR2 in Sertoli precursors during male sex determination

Recently, we demonstrated that loss of Fgf9 results in a block of testis development and a male to female sex-reversed phenotype; however, the function of Fgf9 in sex determination was unknown. We now show that Fgf9 is necessary for two steps of testis development just downstream of the male sex-determining gene, Sry: (1) for the proliferation of a population of cells that give rise to Sertoli progenitors; and (2) for the nuclear localization of an FGF receptor (FGFR2) in Sertoli cell precursors. The nuclear localization of FGFR2 coincides with the initiation of Sry expression and the nuclear localization of SOX9 during the early differentiation of Sertoli cells and the determination of male fate.

Heparan sulfate core proteins in cell-cell signaling

Heparan sulfate (HS) binds numerous extracellular ligands, including cell-cell signaling molecules and their signal-transducing receptors. Ligand binding sites in HS have specific sulfation patterns; and several observations suggest that the HS sulfation pattern is the same for every HS chain that a cell synthesizes, regardless of the core protein to which it is attached. Nonetheless, virtually every Drosophila, zebrafish, Xenopus, and mouse that lacks a specific HS core protein has a mutant phenotype, even though other HS core proteins are expressed in the affected cells. Genetic manipulation of HS core protein genes is beginning to indicate that HS core proteins have functional specificities that are required during distinct stages of development.

Distinctive Expression Patterns of Heparan Sulfate O-Sulfotransferases and Regional Differences in Heparan Sulfate Structure in Chick Limb Buds

The skeletal tissue development and patterning in chick limb buds are known to be under the spacio-temporal control of various heparin-binding cell growth factors such as fibroblast growth factors and bone morphogenetic proteins. Different structural regions on heparan sulfate (HS) chains of proteoglycans could be implicated in regional differences in the binding capacities of these cell growth factors, by which they could selectively interact with targeted cells and regulate their signaling in those processes. In this study we first demonstrated by cDNA cloning that one heparan sulfate 2-O-sulfotransferase (HS2ST) and two isoforms of heparan sulfate 6-O-sulfotransferase (HS6ST-1 and -2) occurred in chick embryos and had different substrate specificities each other. We next showed by whole mount in situ hybridization that the HS6ST-1 and HS6ST-2 transcripts were preferentially localized to the anterior proximal region and at the posterior proximal region of the limb bud, respectively, whereas the HS2ST transcript was distributed rather uniformly throughout the bud. Analyses of the structures of HS from different regions of the wing buds have shown variation in that 6-O-sulfated residues are more abundant in the proximal than distal region, whereas iduronosyl 6-O-sulfated residues are abundant in the anterior proximal region and glucuronosyl 6-O-sulfated residues in the posterior proximal region. These results suggest that HS with different sulfation patterns created with multiple sulfotransferase activities provides an appropriate extracellular environment for morphogenetic signal transduction.

Biosynthesis of 3-O-sulfated heparan sulfate: unique substrate specificity of heparan sulfate 3-O-sulfotransferase isoform 5

Heparan sulfate 3-O-sulfotransferase transfers sulfate to the 3-OH position of a glucosamine to generate 3-O-sulfated heparan sulfate (HS), which is a rare component in HS from natural sources. We previously reported that 3-O- sulfotransferase isoform 5 (3-OST-5) generates both an antithrombin-binding site to exhibit anticoagulant activity and a binding site for herpes simplex virus 1 glycoprotein D to serve as an entry receptor for herpes simplex virus. In this study, we characterize the substrate specificity of 3-OST-5 using the purified enzyme. The enzyme was expressed in insect cells using the baculovirus expression approach and was purified by using heparin-Sepharose and 3',5'-ADP- agarose chromatographies. As expected, the purified enzyme generates both an antithrombin binding site and a glycoprotein D binding site. We isolated IdoUA-AnMan3S and IdoUA-AnMan3S6S from nitrous acid-degraded 3-OST-5-modified HS (pH 1.5), suggesting that 3-OST-5 enzyme sulfates the glucosamine residue that is linked to an iduronic acid residue at the nonreducing end. We also isolated a disaccharide with a structure of DeltaUA2S-GlcNS3S and a tetrasaccharide with a structure of DeltaUA2S-GlcNS-IdoUA2S-GlcNH23S6S from heparin lyases-digested 3-OST-5-modified HS. Our results suggest that 3-OST-5 enzyme sulfates both N-sulfated glucosamine and N-unsubstituted glucosamine residues. Taken together, the results indicate that 3-OST-5 has broader substrate specificity than those of 3-OST-1 and 3-OST-3. The unique substrate specificity of 3-OST-5 serves as an additional tool to study the mechanism for the biosynthesis of biologically active HS.

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.