Localization of human heparan glucosaminyl N-deacetylase/N-sulphotransferase to the trans-Golgi network

ER-Golgi dynamics of HS-modifying enzymes via vesicular trafficking is a critical prerequisite for the delineation of HS biosynthesis

The cell surface and extracellular matrix polysaccharide, heparan sulfate (HS) conveys chemical information to control crucial biological processes. HS chains are synthesized in a non-template driven process mainly in the Golgi apparatus, involving a large number of enzymes capable of subtly modifying its substitution pattern, hence, its interactions and biological effects. Changes in the localization of HS-modifying enzymes throughout the Golgi were found to correlate with changes in the structure of HS, rather than protein expression levels. Following BFA treatment, the HS-modifying enzymes localized preferentially in COPII vesicles and at the trans-Golgi. Shortly after heparin treatment, the HS-modifying enzyme moved from cis to trans-Golgi, which coincided with increased HS sulfation. Finally, it was shown that COPI subunits and Sec24 gene expression changed. Collectively, these findings demonstrate that knowledge of the ER-Golgi dynamics of HS-modifying enzymes via vesicular trafficking is a critical prerequisite for the complete delineation of HS biosynthesis.

Natural Products Containing a Nitrogen-Sulfur Bond

Only about 100 natural products are known to contain a nitrogen-sulfur (N-S) bond. This review thoroughly categorizes N-S bond-containing compounds by structural class. Information on biological source, biological activity, and biosynthesis is included, if known. We also review the role of N-S bond functional groups as post-translational modifications of amino acids in proteins and peptides, emphasizing their role in the metabolism of the cell.

The function of heparan sulfate during branching morphogenesis

Branching morphogenesis is a fundamental process in the development of diverse epithelial organs such as the lung, kidney, liver, pancreas, prostate, salivary, lacrimal and mammary glands. A unifying theme during organogenesis is the importance of epithelial cell interactions with the extracellular matrix (ECM) and growth factors (GFs). The diverse developmental mechanisms giving rise to these epithelial organs involve many organ-specific GFs, but a unifying paradigm during organogenesis is the regulation of GF activity by heparan sulfates (HS) on the cell surface and in the ECM. This primarily involves the interactions of GFs with the sulfated side-chains of HS proteoglycans. HS is one of the most diverse biopolymers and modulates GF binding and signaling at the cell surface and in the ECM of all tissues. Here, we review what is known about how HS regulates branching morphogenesis of epithelial organs with emphasis on the developing salivary gland, which is a classic model to investigate epithelial-ECM interactions. We also address the structure, biosynthesis, turnover and function of HS during organogenesis. Understanding the regulatory mechanisms that control HS dynamics may aid in the development of therapeutic interventions for diseases and novel strategies for tissue engineering and regenerative medicine.

Secretome analysis identifies novel signal Peptide peptidase-like 3 (Sppl3) substrates and reveals a role of Sppl3 in multiple Golgi glycosylation pathways

Signal peptide peptidase-like 3 (Sppl3) is a Golgi-resident intramembrane-cleaving protease that is highly conserved among multicellular eukaryotes pointing to pivotal physiological functions in the Golgi network which are only beginning to emerge. Recently, Sppl3 was shown to control protein N-glycosylation, when the key branching enzyme N-acetylglucosaminyltransferase V (GnT-V) and other medial/trans Golgi glycosyltransferases were identified as first physiological Sppl3 substrates. Sppl3-mediated endoproteolysis releases the catalytic ectodomains of these enzymes from their type II membrane anchors. Protein glycosylation is a multistep process involving numerous type II membrane-bound enzymes, but it remains unclear whether only few of them are Sppl3 substrates or whether Sppl3 cleaves many of them and thereby controls protein glycosylation at multiple levels. Therefore, to systematically identify Sppl3 substrates we used Sppl3-deficient and Sppl3-overexpression cell culture models and analyzed them for changes in secreted membrane protein ectodomains using the proteomics "secretome protein enrichment with click sugars (SPECS)" method. SPECS analysis identified numerous additional new Sppl3 candidate glycoprotein substrates, several of which were biochemically validated as Sppl3 substrates. All novel Sppl3 substrates adopt a type II topology. The majority localizes to the Golgi network and is implicated in Golgi functions. Importantly, most of the novel Sppl3 substrates catalyze the modification of N-linked glycans. Others contribute to O-glycan and in particular glycosaminoglycan biosynthesis, suggesting that Sppl3 function is not restricted to N-glycosylation, but also functions in other forms of protein glycosylation. Hence, Sppl3 emerges as a crucial player of Golgi function and the newly identified Sppl3 substrates will be instrumental to investigate the molecular mechanisms underlying the physiological function of Sppl3 in the Golgi network and in vivo. Data are available via ProteomeXchange with identifier PXD001672.

Glycanogenomics: a qPCR-approach to investigate biological glycan function

As an indirect approach towards glycan structures, qRT-PCR analyses using the DeltaDeltaC(T) method were performed to investigate changes in expression levels of heparan sulfate-synthesising enzymes of stimulated and unstimulated HMVECs. We chose NDSTs as early enzymes initiating sulfation and 3OSTs which act late generating specific binding sites. Major changes in expression patterns were found for the NDST3 and 3OST1 isoforms. Both enzymes were down-regulated 7- and 6-fold, respectively, following TNF-alpha stimulation, and 3.5- and 7.6-fold following LPS-stimulation suggesting a common restructuring process of HS in inflammation leading to a less diverse sulfation pattern. Immunostaining of TNF-alpha-stimulated cells using a phage display-derived antibody specific for 3-O-sulfation and unsulfated regions of HS resulted in significant fluorescence changes between unstimulated and stimulated.

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.

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.

Disturbed Ca2+ kinetics in N-deacetylase/N-sulfotransferase-1 defective myotubes

The biosynthesis of heparan sulfate, present on the cell surface and in the basal lamina surrounding cells, is a multistep process in which each step is mediated by a specific enzyme. The initial modification of the precursor polysaccharide, N-deacetylation followed by N-sulfation of selected N-acetyl-D-glucosamine residues, is catalyzed by the enzyme glucosaminyl N-deacetylase/N-sulfotransferase (NDST). This event is a key step that regulates the overall sulfate content of the polysaccharide. Here, we report on the effects of NDST deficiency on Ca2+ kinetics in myotubes from NDST-1- and NDST-2-deficient mice, indicating a novel role for heparan sulfate in skeletal muscle physiology. Immunostaining for specific heparan sulfate epitopes showed major changes in the heparan sulfate composition in skeletal muscle tissue derived from NDST-1-/- mice and NDST-/- cultured myotubes. Biochemical analysis indicates a relative decrease in both N-sulfation and 2-O-sulfation of skeletal muscle heparan sulfate. The core protein of heparan sulfate proteoglycan perlecan was not affected, as judged by immunohistochemistry. Also, acetylcholine receptor clustering and the occurrence of other ion channels involved in excitation-contraction coupling were not altered. In NDST-2-/- mice and heterozygous mice no changes in heparan sulfate composition were observed. Using high-speed UV confocal laser scanning microscopy, aberrant Ca2+ kinetics were observed in NDST-1-/- myotubes, but not in NDST-2-/- or heterozygous myotubes. Electrically induced Ca2+ spikes had significantly lower amplitudes, and a reduced removal rate of cytosolic Ca2+, indicating the importance of heparan sulfate in muscle Ca2+ kinetics.

Heparan sulfate and development: differential roles of the N-acetylglucosamine N-deacetylase/N-sulfotransferase isozymes

Heparan sulfates (HSs) are N- and O-sulfated polysaccharide components of proteoglycans, which are important constituents of the cell surface as well as the extracellular matrix. Heparin, with extensive clinical application as an anticoagulant, is a highly sulfated form of HS present within the granules of connective tissue type mast cells. The diverse functions of HS, which include the modulation of growth factor/cytokine activity, interaction with matrix proteins and binding of enzymes to cell surfaces, depend greatly on the presence of specific, high affinity regions on the chains. N-acetylglucosamine N-deacetylase/N-sulfotransferases, NDSTs, are an important group of enzymes in HS biosynthesis, initiating the sulfation of the polysaccharide chains and thus determining the generation of the high affinity sites. Here, we review the role of the four vertebrate NDSTs in HS biosynthesis as well as their regulated expression. The main emphasis is the phenotypes of mice lacking one or more of the NDSTs.

Order out of chaos: assembly of ligand binding sites in heparan sulfate

Virtually every cell type in metazoan organisms produces heparan sulfate. These complex polysaccharides provide docking sites for numerous protein ligands and receptors involved in diverse biological processes, including growth control, signal transduction, cell adhesion, hemostasis, and lipid metabolism. The binding sites consist of relatively small tracts of variably sulfated glucosamine and uronic acid residues in specific arrangements. Their formation occurs in a tissue-specific fashion, generated by the action of a large family of enzymes involved in nucleotide sugar metabolism, polymer formation (glycosyltransferases), and chain processing (sulfotransferases and an epimerase). New insights into the specificity and organization of the biosynthetic apparatus have emerged from genetic studies of cultured cells, nematodes, fruit flies, zebrafish, rodents, and humans. This review covers recent developments in the field and provides a resource for investigators interested in the incredible diversity and specificity of this process.

A 96-well dot-blot assay for carbohydrate sulfotransferases

Here we describe an efficient dot-blot assay for high-throughput screening of two enzymes, heparan sulfate N-deacetylase/N-sulfotransferase (NDST-1) and high-endothelial cell GlcNAc-6-sulfotransferase (HEC-GlcNAc-6-ST). The assay proceeds by transfer of 35S-labeled sulfate from [35S]-3(')-phosphoadenosine-5(')-phosphosulfate (PAPS) to the free amino groups of de-N-sulfated heparin (NDST-1), or the 6-hydroxyl groups of N-acetylglucosamine residues linked to a polyacrylamide scaffold (HEC-GlcNAc-6-ST). The 35S-labeled products are then captured on an appropriate membrane, taking advantage of their polymeric architecture. In one step, 35S-labeled by-products are then eluted from the membrane, leaving spatially separated 35S-labeled product "dots" for subsequent quantification. This assay allows for direct product detection on the membrane, obviating excessive washing and elution steps endemic to other assays. The assay was validated by measuring K(M) values for PAPS and K(I) values for PAP, the product of sulfuryl transfer. The assay method should be useful for inhibitor screens for both enzymes. In addition, the general assay architecture should be readily applicable to high-throughput screens of other carbohydrate sulfotransferases.

S100A13 and S100A6 exhibit distinct translocation pathways in endothelial cells

S100 proteins have attracted great interest in recent years because of their cell- and tissue-specific expression and association with various human pathologies. Most S100 proteins are small acidic proteins with calcium-binding domains — the EF hands. It is thought that this group of proteins carry out their cellular functions by interacting with specific target proteins, an interaction that is mainly dependent on exposure of hydrophobic patches, which result from calcium binding. S100A13, one of the most recently identified members of the S100 family, is expressed in various tissues. Interestingly,hydrophobic exposure was not observed upon calcium binding to S100A13 even though the dimeric form displays two high- and two low- affinity sites for calcium. Here, we followed the translocation of S100A13 in response to an increase in intracellular calcium levels, as protein translocation has been implicated in assembly of signaling complexes and signaling cascades, and several other S100 proteins are involved in such events. Translocation of S100A13 was observed in endothelial cells in response to angiotensin II, and the process was dependent on the classic Golgi-ER pathway. By contrast, S100A6 translocation was found to be distinct and dependent on actin-stress fibers. These experiments suggest that different S100 proteins utilize distinct translocation pathways, which might lead them to certain subcellular compartments in order to perform their physiological tasks in the same cellular environment.

Effect of brefeldin A on heparan sulphate biosynthesis in Madin-Darby canine kidney cells

Brefeldin A (BFA) perturbs the organization of the Golgi apparatus, such that Golgi stack components are fused with the endoplasmic reticulum (ER) and separated from the trans-Golgi network. In many cell types, BFA blocks the secretion of macromolecules but still allows the action of Golgi enzymes in the ER. Treatment of cells with BFA has been reported to inhibit the secretion of heparan sulphate (HS) proteoglycans and alter the structure of their HS components, but the nature of such structural alterations has not been characterized in detail. We analysed the effect of BFA on HS biosynthesis in Madin-Darby canine kidney (MDCK) cells, in which the Golgi complex is more resistant towards BFA than in most other cell types. We found that MDCK cells were able to secrete HS proteoglycans in spite of BFA treatment. However, the secretion of HS was reduced and the secreted HS differed from that produced by untreated cells. In BFA-treated cells, two structurally distinct pools of HS were generated. One pool was similar to HS from control cells, with the exception that the 6-O-sulphation of glucosamine (GlcN) residues was reduced. In contrast, the other pool consisted of largely unmodified N-acetylheparosan polymers with a low (<20%) proportion of N-sulphated GlcN residues but a substantial proportion of N-unsubstituted GlcN units, indicating that it had been acted upon by N-deacetylases and partly by the N-sulphotransferases, but not by O-sulphotransferases. Together, these findings represent a previously unrecognized alteration in HS biosynthesis caused by BFA, and differ dramatically from our previous findings in MDCK cells pertaining to the undersulphation of HS caused by sodium chlorate treatment.

Expression of heparan sulphate N-deacetylase/N-sulphotransferase by vascular smooth muscle cells

Heparan sulphate is an important mediator in determining vascular smooth muscle cell (SMC) phenotype. The sulphation pattern of the heparan sulphate chains is critical to their function. We have examined the initial step in the biosynthesis of the sulphated domains mediated by the enzyme heparan sulphate N-deacetylase/N-sulphotransferase (NDST). Rabbit aortic SMC in primary culture exhibited NDST enzyme activity and expressed NDST-1 in their Golgi apparatus, with maximal expression in SMC 2 days after dispersal in primary culture confirmed by Western blot analysis. Endothelial cells, macrophages and fibroblasts expressed NDST-1 but had generally less intense staining than SMC, although SMC expression decreased with culture. The uninjured rat aorta also showed widespread expression of NDST-1. After balloon de-endothelialisation, NDST-1 could not be detected in SMC of the neointima in the early stages of neointimal formation, but was re-expressed at later time points (after 12 weeks). In human coronary arteries, SMC of the media and the diffuse intimal thickening expressed NDST-1, while SMC in the atherosclerotic plaque were negative for NDST-1. We conclude that SMC may regulate their heparan sulphate sulphation at the level of expression of the enzyme heparan sulphate NDST in a manner related to their phenotypic state.

Immunopurification of Golgi vesicles by magnetic sorting

We have designed a method that permits to isolate highly purified Golgi vesicles deprived of endoplasmic reticulum (ER), main contaminant of Golgi fractions. To this end, we prepared a rabbit polyclonal antibody against the cytosolic N-terminal oligopeptide of the enzyme heparan glucosaminyl N-deacetylase/N-sulphotransferase (HSST), a specific marker for Golgi apparatus. The Golgi localization of HSST was confirmed by indirect immunofluorescence microscopy. The antibody binding to Golgi vesicles was demonstrated by immunoelectronmicroscopy and allowed the immunopurification by magnetic sorting. Golgi vesicles subjected to purification by magnetic sorting showed the presence of HSST and p28, which is an integral membrane protein on the cis-Golgi also used as a specific Golgi marker. The purified material was devoid of calreticulin, a specific ER marker. This purification method will allow to improve studies requiring highly purified Golgi membranes such as identification of specific receptors and the electrophysiological characterization of Golgi membrane ion channels, which have been jeopardized up to now by ER membrane contamination.

Enzyme interactions in heparan sulfate biosynthesis: uronosyl 5-epimerase and 2-O-sulfotransferase interact in vivo

The formation of heparan sulfate occurs within the lumen of the endoplasmic reticulum-Golgi complex-trans-Golgi network by the concerted action of several glycosyltransferases, an epimerase, and multiple sulfotransferases. In this report, we have examined the location and interaction of tagged forms of five of the biosynthetic enzymes: galactosyltransferase I and glucuronosyltransferase I, required for the formation of the linkage region, and GlcNAc N-deacetylase/N-sulfotransferase 1, uronosyl 5-epimerase, and uronosyl 2-O-sulfotransferase, the first three enzymes involved in the modification of the chains. All of the enzymes colocalized with the medial-Golgi marker alpha-mannosidase II. To study whether any of these enzymes interacted with each other, they were relocated to the endoplasmic reticulum (ER) by replacing their cytoplasmic N-terminal tails with an ER retention signal derived from the cytoplasmic domain of human invariant chain (p33). Relocating either galactosyltransferase I or glucuronosyltransferase I had no effect on the other's location or activity. However, relocating the epimerase to the ER caused a parallel redistribution of the 2-O-sulfotransferase. Transfected epimerase was also located in the ER in a cell mutant lacking the 2-O-sulfotransferase, but moved to the Golgi when the cells were transfected with 2-O-sulfotransferase cDNA. Epimerase activity was depressed in the mutant, but increased upon restoration of 2-O-sulfotransferase, suggesting that their physical association was required for both epimerase stability and translocation to the Golgi. These findings provide in vivo evidence for the formation of complexes among enzymes involved in heparan sulfate biosynthesis. The functional significance of these complexes may relate to the rapidity of heparan sulfate formation.

Portable sulphotransferase domain determines sequence specificity of heparan sulphate 3-O-sulphotransferases

3-O-Sulphates are the rarest substituent of heparan sulphate and are therefore ideally suited to the selective regulation of biological activities. Individual isoforms of heparan sulphate D-glucosaminyl 3-O-sulphotransferase (3-OST) exhibit sequence-specific action, which creates heparan sulphate structures with distinct biological functions. For example, 3-OST-1 preferentially generates binding sites for anti-thrombin, whereas 3-OST-3 isoforms create binding sites for the gD envelope protein of herpes simplex virus 1 (HSV-1), which enables viral entry. 3-OST enzymes comprise a presumptive sulphotransferase domain and a divergent N-terminal region. To localize determinants of sequence specificity, we conducted domain swaps between cDNA species. The N-terminal region of 3-OST-1 was fused with the sulphotransferase domain of 3-OST-3(A) to generate N1-ST3(A). Similarly, the N-terminal region of 3-OST-3(A) was fused to the sulphotransferase domain of 3-OST-1 to generate N3(A)-ST1. Wild-type and chimaeric enzymes were transiently expressed in COS-7 cells and extracts were analysed for selective generation of binding sites for anti-thrombin. 3-OST-1 was 270-fold more efficient at forming anti-thrombin-binding sites than 3-OST-3(A), indicating its significantly greater selectivity for substrates that can be 3-O-sulphated to yield such sites. N3(A)-ST1 was as active as 3-OST-1, whereas the activity of N1-ST3(A) was as low as that of 3-OST-3(A). Analysis of Chinese hamster ovary cell transfectants revealed that only 3-OST-3(A) and N1-ST3(A) generated gD-binding sites and conveyed susceptibility to infection by HSV-1. Thus sequence-specific properties of 3-OSTs are defined by a self-contained sulphotransferase domain and are not directly influenced by the divergent N-terminal region.

Localization, purification and specificity of the full-length membrane-bound form of human recombinant alpha 1,3/4-fucosyltransferase from BHK-21B cells

Fucosyltransferase III [galactoside 3(4)-L-fucosyltransferase; EC 2.4.1.65] (FT3) is a Golgi type II membrane protein that catalyses the synthesis of fucosylated Lewis motifs that are associated with cell-adhesion events and are differentially expressed during cell differentiation. In the present work, the full-length membrane bound form of FT3 has been expressed in baby hamster kidney cells. The enzyme has been found in the trans-Golgi and trans-Golgi network (TGN) of the transfected cells, where it appeared as monomers and dimers, but not as oligomers with high molecular masses. Therefore oligomerization is not the basis for correct localization of FT3 in the Golgi. The enzyme has been purified, with a final yield of 2% and a total purification of 2900-fold, by DEAE-Sepharose, SP-Sepharose, GDP-Fractogel and Superdex 200 chromatography. The purified enzyme showed a clear preference for the Gal beta 3GlcNAc motif in oligosaccharides conjugated with the hydrophobic tail (CH(2))(3)-NHCO-(CH(2))(5)-NH-biotin. Substitution of galactose with alpha 2-linked fucose or alpha 2,3-linked N-acetylneuraminic acid yielded a 1.9-fold increase or a 43% decrease in activity respectively. The enzyme showed no activity towards asialofetuin, a glycoprotein containing the Gal beta 3GlcNAc acceptor motif. Therefore it has been concluded that the membrane-bound form of FT3 is present in the Golgi and the TGN in an equilibrium of monomers<-->dimers, which might fucosylate glycans from glycolipids, but not from glycoproteins.

Molecular cloning and expression of two distinct human chondroitin 4-O-sulfotransferases that belong to the HNK-1 sulfotransferase gene family

Using an expression cloning strategy, the cDNA encoding the human HNK-1 sulfotransferase (HNK-1ST) has been cloned. During this cloning we found that HNK-1ST and other Golgi-associated sulfotransferases cloned before share homologous sequences including the RDP motif (Ong, E., Yeh, J.-C., Ding, Y., Hindsgaul, O., and Fukuda, M. (1998) J. Biol. Chem. 223, 5190-5195). Using this conserved sequence in HNK-1ST as a probe, we identified two expressed sequence tags in EST data base which have 31.6 and 30.7% identity with HNK-1ST at the amino acid levels. Expression of these two full-length cDNAs failed to form HNK-1 glycan nor to add sulfate to CD34 or NCAM. Surprisingly, proteins expressed by these cDNAs transferred sulfate to the C-4 position of N-acetylgalactosamine in chondroitin and desulfated dermatan sulfate, thus we named these two enzymes, chondroitin 4-O-sulfotransferase 1 and -2 (C4ST-1 and C4ST-2). Both C4ST-1 and C4ST-2, however, did not form 4, 6-di-O-sulfated N-acetylgalactosamine when chondroitin sulfate C was used as an acceptor. Moreover, analysis of (35)S-labeled dermatan sulfate formed by C4ST-1 indicate that sulfation preferentially took place in GlcA-->GalNAc unit than in IdoA-->GalNAc unit, suggesting that 4-O-sulfation at N-acetylgalactosamine may precede epimerization of glucuronic acid to iduronic acid during dermatan sulfate biosynthesis. Northern analysis demonstrated that the transcript for C4ST-1 is predominantly expressed in peripheral leukocytes and hematopoietic tissues while the C4ST-2 transcript is more widely expressed in various tissues. These results indicate C4ST-1 and C4ST-2 play complementary roles in chondroitin and dermatan sulfate synthesis in different tissues.

Diversity and functions of glycosaminoglycan sulfotransferases

Sulfate residues attached to the specific position of the component sugar residues of glycosaminoglycans play important roles in the formation of functional domain structures. The introduction of a sulfate group is catalyzed by various sulfotransferases with strict substrate specificities. A rapid development achieved in the cloning of various glycosaminoglycan sulfotransferases has allowed us to study the biological functions of glycosaminoglycan sulfotransferases and their products, sulfated glycosaminoglycans.

Synthesis and sorting of proteoglycans

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

Selective effects of sodium chlorate treatment on the sulfation of heparan sulfate

We have analyzed the effect of sodium chlorate treatment of Madin-Darby canine kidney cells on the structure of heparan sulfate (HS), to assess how the various sulfation reactions during HS biosynthesis are affected by decreased availability of the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate. Metabolically [(3)H]glucosamine-labeled HS was isolated from chlorate-treated and untreated Madin-Darby canine kidney cells and subjected to low pH nitrous acid cleavage. Saccharides representing (i) the N-sulfated domains, (ii) the domains of alternating N-acetylated and N-sulfated disaccharide units, and (iii) the N-acetylated domains were recovered and subjected to compositional disaccharide analysis. Upon treatment with 50 mM chlorate, overall O-sulfation of HS was inhibited by approximately 70%, whereas N-sulfation remained essentially unchanged. Low chlorate concentrations (5 or 20 mM) selectively reduced the 6-O-sulfation of HS, whereas treatment with 50 mM chlorate reduced both 2-O- and 6-O-sulfation. Analysis of saccharides representing the different domain types indicated that 6-O-sulfation was preferentially inhibited in the alternating domains. These data suggest that reduced 3'-phosphoadenosine 5'-phosphosulfate availability has distinct effects on the N- and O-sulfation of HS and that O-sulfation is affected in a domain-specific fashion.

Heparin is essential for the storage of specific granule proteases in mast cells

All mammals produce heparin, a negatively charged glycosaminoglycan that is a major constituent of the secretory granules of mast cells which are found in the peritoneal cavity and most connective tissues. Although heparin is one of the most studied molecules in the body, its physiological function has yet to be determined. Here we describe transgenic mice, generated by disrupting the N-deacetylase/N-sulphotransferase-2 gene, that cannot express fully sulphated heparin. The mast cells in the skeletal muscle that normally contain heparin lacked metachromatic granules and failed to store appreciable amounts of mouse mast-cell protease (mMCP)-4, mMCP-5 and carboxypeptidase A (mMC-CPA), even though they contained substantial amounts of mMCP-7. We developed mast cells from the bone marrow of the transgenic mice. Although these cultured cells contained high levels of various protease transcripts and had substantial amounts of mMCP-6 protein in their granules, they also failed to express mMCP-5 and mMC-CPA. Our data show that heparin controls, through a post-translational mechanism, the levels of specific cassettes of positively charged proteases inside mast cells.

Multiple isoforms of heparan sulfate D-glucosaminyl 3-O-sulfotransferase. Isolation, characterization, and expression of human cdnas and identification of distinct genomic loci

3-O-Sulfated glucosaminyl residues are rare constituents of heparan sulfate and are essential for the activity of anticoagulant heparan sulfate. Cellular production of the critical active structure is controlled by the rate-limiting enzyme, heparan sulfate D-glucosaminyl 3-O-sulfotransferase-1 (3-OST-1) (EC 2.8.2.23). We have probed the expressed sequence tag data base with the carboxyl-terminal sulfotransferase domain of 3-OST-1 to reveal three novel, incomplete human cDNAs. These were utilized in library screens to isolate full-length cDNAs. Clones corresponding to predominant transcripts were obtained for the 367-, 406-, and 390-amino acid enzymes 3-OST-2, 3-OST-3A, and 3-OST-3B, respectively. These type II integral membrane proteins are comprised of a divergent amino-terminal region and a very homologous carboxyl-terminal sulfotransferase domain of approximately 260 residues. Also recovered were partial length clones for 3-OST-4. Expression of the full-length enzymes confirms the 3-O-sulfation of specific glucosaminyl residues within heparan sulfate (Liu, J., Shworak, N. W., Sinaÿ, P., Schwartz, J. J. Zhang, L., Fritze, L. M. S., and Rosenberg, R. D. (1999) J. Biol. Chem. 274, 5185-5192). Southern analyses suggest the human 3OST1, 3OST2, and 3OST4 genes, and the corresponding mouse isologs, are single copy. However, 3OST3A and 3OST3B genes are each duplicated in humans and show at least one copy each in mice. Intriguingly, the entire sulfotransferase domain sequence of the 3-OST-3B cDNA (774 base pairs) was 99.2% identical to the same region of 3-OST-3A. Together, these data argue that the structure of this functionally important region is actively maintained by gene conversion between 3OST3A and 3OST3B loci. Interspecific mouse back-cross analysis identified the loci for mouse 3Ost genes and syntenic assignments of corresponding human isologs were confirmed by the identification of mapped sequence-tagged site markers. Northern blot analyses indicate brain exclusive and brain predominant expression of 3-OST-4 and 3-OST-2 transcripts, respectively; whereas, 3-OST-3A and 3-OST-3B isoforms show widespread expression of multiple transcripts. The reiteration and conservation of the 3-OST sulfotransferase domain suggest that this structure is a self-contained functional unit. Moreover, the extensive number of 3OST genes with diverse expression patterns of multiple transcripts suggests that the novel 3-OST enzymes, like 3-OST-1, regulate important biologic properties of heparan sulfate proteoglycans.

Molecular cloning and expression of a third member of the heparan sulfate/heparin GlcNAc N-deacetylase/ N-sulfotransferase family

N-Deacetylation and N-sulfation of N-acetylglucosamine residues in heparan sulfate and heparin initiate a series of chemical modifications that ultimately lead to oligosaccharide sequences with specific ligand binding properties. These reactions are catalyzed by GlcNAc N-deacetylase/N-sulfotransferase (NDST), a monomeric enzyme with two catalytic activities. Two genes encoding NDST isozymes have been described, one from rat liver (NDST1) and another from murine mastocytoma (NDST2). Both isozymes are expressed in tissues in varying amounts, but their relative contribution to heparan sulfate formation in any one tissue is unknown. We now report the identification of a third member of the NDST family, designated NDST3. A full-length cDNA clone (3.2 kilobase pairs) encoding a 873-amino acid protein was obtained from a human fetal/infant brain cDNA library. Human NDST3 (hNDST3) has a nucleotide sequence homologous but not identical to hNDST1 and NDST2. The deduced amino acid sequence shows 70% and 65% amino acid identity to that of hNDST1 and NDST2, respectively. A soluble chimera of hNDST3 and protein A exhibited both N-deacetylase and N-sulfotransferase activity, confirming its enzymatic identity. Northern blot analysis of human fetal brain poly(A)+ RNA showed a single transcript of 6.4 kilobase pairs. Reverse transcription polymerase chain reaction analysis revealed more restricted tissue expression of hNDST3 than hNDST1 and NDST2, and high levels in brain, liver, and kidney. Analysis of Chinese hamster ovary cells revealed expression of NDST1 and NDST2, but not NDST3. In a Chinese hamster ovary cell mutant exhibiting reduced N-sulfotransferase activity and reduced sulfation of heparan sulfate (Bame, K. J., and Esko, J. D. (1989) J. Biol. Chem. 264, 8059-8065), expression of NDST1 was greatly reduced, but NDST2 was expressed normally, suggesting that both enzymes are involved in heparan sulfate assembly. The discovery of multiple NDST isozymes suggests that the assembly of heparan sulfate is much complicated than previously appreciated.

cDNA cloning, genomic organization and chromosomal localization of human heparan glucosaminyl N-deacetylase/N-sulphotransferase-2

The cDNA and gene encoding human heparan glucosaminyl N-deacetylase/N-sulphotransferase-2 have been cloned. The cDNA encoded a protein of 883 amino acids that was 94% similar to heparan N-sulphotransferase-2 from mouse mast cells. Comparison of the deduced amino acid sequences of human heparan N-sulphotransferase-1 and -2 showed that the enzymes were 70% similar; greater than 90% of the amino acids between residues 418 and 543 were identical. The least conserved amino acids were found in the N-terminus/putative transmembrane regions of the two enzymes. The human heparan N-sulphotransferase-2 gene was localized to chromosome arm 10q (band 10q22) by in situ fluorescent hybridization. The gene contains 13 exons spanning 6.5 kb, ranging in size from 88 bp (exon 2) to >1 kb (exon 1), and 12 introns, which were found to occur at similar sites within the coding sequence of the human heparan N-sulphotransferase-1 gene. The structure of the two genes differed in that the heparan N-sulphotransferase-1 gene contained one additional intron. The similarity of the heparan N-sulphotransferase-1 and -2 proteins and their similar exon-intron organization suggest that they derive from a common ancestral gene.