A new model for the domain structure of heparan sulfate based on the novel specificity of K5 lyase

Interactions of proteins with heparan sulfate

Heparan sulfate (HS) is a glycosaminoglycan, polysaccharides that are considered to have arisen in the last common unicellular ancestor of multicellular animals. In this light, the large interactome of HS and its myriad functions in relation to the regulation of cell communication are not surprising. The binding of proteins to HS determines their localisation and diffusion, essential for embryonic development and homeostasis. Following the biosynthesis of the initial heparosan polymer, the subsequent modifications comprise an established canonical pathway and a minor pathway. The more frequent former starts with N-deacetylation and N-sulfation of GlcNAc residues, the latter with C-5 epimerisation of a GlcA residue adjacent to a GlcNAc. The binding of proteins to HS is driven by ionic interactions. The multivalent effect arising from the many individual ionic bonds between a single protein and a polysaccharide chain results in a far stronger interaction than would be expected from an ion-exchange process. In many instances, upon binding, both parties undergo substantial conformational change, the resulting hydrogen and van der Waal bonds contributing significant free energy to the binding reaction. Nevertheless, ionic bonds dominate the protein-polysaccharide interaction kinetically. Together with the multivalent effect, this provides an explanation for the observed trapping of HS-binding proteins in extracellular matrix. Importantly, individual ionic bonds have been observed to be dynamic; breaking and reforming, while the protein remains bound to the polysaccharide. These considerations lead to a model for 1D diffusion of proteins in extracellular matrix on HS, involving mechanisms such as sliding, chain switching and rolling.

Enzymatic construction of a library of even- and odd-numbered heparosan oligosaccharides and their N-sulfonated derivatives

Low-molecular-weight heparins (LMWHs), especially the specific-sized heparin oligosaccharides, are attractive for the therapeutic applications, while their synthesis remains challenging. In the present study, unsaturated even-numbered heparosan oligosaccharides were firstly prepared by cleaving high-molecular-weight heparosan using recombinant heparinase III (HepIII). The conversion rates of the unsaturated disaccharides, tetrasaccharides, hexasaccharides, octasaccharides, and decasaccharides were 33.9 %, 47.9 %, 78.7 %, 71.8 %, and 53.4 %, respectively. After processing the aforementioned heparosan oligosaccharides with the Δ4,5 unsaturated glycuronidase, saturated odd-numbered heparosan trisaccharides, pentasaccharides, heptasaccharides, and nonasaccharides were produced. It was observed that among them, the pentasaccharides were the smallest units of saturated odd-numbered oligosaccharides recognized by HepIII. These oligosaccharides were further catalyzed with bifunctional heparan sulfate N-deacetylase/N-sulfotransferase (NDST) under optimized reaction conditions. It was found that the tetrasaccharide was defined as the smallest recognition unit for NDST, obtaining the N-sulfonated heparosan tetrasaccharides, pentasaccharides, and hexasaccharides with a single sulfonate group, as well as N-sulfonated heparosan heptasaccharides, octasaccharides, and nonasaccharides with multiple sulfonate groups. These results provide an easy pathway for constructing a library of specific-sized N-sulfonated heparosan oligosaccharides that can be used as the substrates for the enzymatic synthesis of LMWHs and heparin oligosaccharides, shedding new light on the substrate preference of NDST.

Extending the enzymatic toolbox for heparosan polymerization, depolymerization, and detection

Heparosan is an acidic polysaccharide expressed as a capsule polymer by pathogenic and commensal bacteria, e.g. by E. coli K5. As a precursor in the biosynthesis of heparan sulfate and heparin, heparosan has a high biocompatibility and is thus of interest for pharmaceutical applications. However, due to its low immunogenicity, developing antibodies against heparosan and detecting the polymer in biological samples has been challenging. In this study, we exploited the enzyme repertoire of E. coli K5 and the E. coli K5-specific bacteriophage ΦK5B for the controlled synthesis and depolymerization of heparosan. A fluorescently labeled heparosan nonamer was used as a priming acceptor to study the elongation mechanism of the E. coli K5 heparosan polymerases KfiA and KfiC. We could demonstrate that the enzymes act in a distributive manner, producing labeled heparosan of low dispersity. The enzymatically synthesized heparosan was a useful tool to identify the tailspike protein KflB of ΦK5B as heparosan lyase and to characterize its endolytic depolymerization mechanism. Most importantly, using site-directed mutagenesis and rational construct design, we generated an inactive version of KflB for the detection of heparosan in ELISA-based assays, on blots, and on bacterial and mammalian cells.

Glycosaminoglycans: What Remains To Be Deciphered?

Glycosaminoglycans (GAGs) are complex polysaccharides exhibiting a vast structural diversity and fulfilling various functions mediated by thousands of interactions in the extracellular matrix, at the cell surface, and within the cells where they have been detected in the nucleus. It is known that the chemical groups attached to GAGs and GAG conformations comprise "glycocodes" that are not yet fully deciphered. The molecular context also matters for GAG structures and functions, and the influence of the structure and functions of the proteoglycan core proteins on sulfated GAGs and vice versa warrants further investigation. The lack of dedicated bioinformatic tools for mining GAG data sets contributes to a partial characterization of the structural and functional landscape and interactions of GAGs. These pending issues will benefit from the development of new approaches reviewed here, namely (i) the synthesis of GAG oligosaccharides to build large and diverse GAG libraries, (ii) GAG analysis and sequencing by mass spectrometry (e.g., ion mobility-mass spectrometry), gas-phase infrared spectroscopy, recognition tunnelling nanopores, and molecular modeling to identify bioactive GAG sequences, biophysical methods to investigate binding interfaces, and to expand our knowledge and understanding of glycocodes governing GAG molecular recognition, and (iii) artificial intelligence for in-depth investigation of GAGomic data sets and their integration with proteomics.

Glypican-1 drives unconventional secretion of Fibroblast Growth Factor 2

Fibroblast Growth Factor 2 (FGF2) is a tumor cell survival factor that is transported into the extracellular space by an unconventional secretory mechanism. Cell surface heparan sulfate proteoglycans are known to play an essential role in this process. Unexpectedly, we found that among the diverse sub-classes consisting of syndecans, perlecans, glypicans and others, Glypican-1 (GPC1) is the principle and rate-limiting factor that drives unconventional secretion of FGF2. By contrast, we demonstrate GPC1 to be dispensable for FGF2 signaling into cells. We provide first insights into the structural basis for GPC1-dependent FGF2 secretion, identifying disaccharides with N-linked sulfate groups to be enriched in the heparan sulfate chains of GPC1 to which FGF2 binds with high affinity. Our findings have broad implications for the role of GPC1 as a key molecule in tumor progression.

Chromatographic Molecular Weight Measurements for Heparin , Its Fragments and Fractions, and Other Glycosaminoglycans

Glycosaminoglycan samples are usually polydisperse, consisting of molecules with differing length and differing sequence. Methods for measuring the molecular weight of heparin have been developed to assure the quality and consistency of heparin products for medicinal use, and these methods can be applied in other laboratory contexts. In the method described here, high-performance gel permeation chromatography is calibrated using appropriate heparin molecular weight markers or a single broad standard calibrant and used to characterize the molecular weight distribution of polydisperse samples or the peak molecular weight of monodisperse, or approximately monodisperse, heparin fractions. The same technology can be adapted for use with other glycosaminoglycans.

In Vitro Anti-Orthohantavirus Activity of the High-and Low-Molecular-Weight Fractions of Fucoidan from the Brown Alga Fucus evanescens

The Hantaan orthohantavirus (genovariant Amur-AMRV) is a rodent-borne zoonotic virus; it is the causative agent of haemorrhagic fever with renal syndrome in humans. The currently limited therapeutic options require the development of effective anti-orthohantavirus drugs. The ability of native fucoidan from Fucus evanescens (FeF) and its enzymatically prepared high-molecular-weight (FeHMP) and low-molecular-weight (FeLMP) fractions to inhibit different stages of AMRV infection in Vero cells was studied. The structures of derivatives obtained were determined using nuclear magnetic resonance (NMR) spectroscopy. We found that fucoidan and its derivatives exhibited significant antiviral activity by affecting the early stages of the AMRV lifecycle, notably virus attachment and penetration. The FeHMP and FeLMP fractions showed the highest anti-adsorption activity by inhibiting AMRV focus formation, with a selective index (SI) > 110; FeF had an SI of ~70. The FeLMP fraction showed a greater virucidal effect compared with FeF and the FeHMP fraction. It was shown by molecular docking that 2O-sulphated fucotetrasaccharide, a main component of the FeLMP fraction, is able to bind with the AMRV envelope glycoproteins Gn/Gc and with integrin β3 to prevent virus-cell interactions. The relatively small size of these sites of interactions explains the higher anti-AMRV activity of the FeLMP fraction.

Targeting of Mammalian Glycans Enhances Phage Predation in the Gastrointestinal Tract

The human gastrointestinal mucosal surface consists of a eukaryotic epithelium, a prokaryotic microbiota, and a carbohydrate-rich interface that separates them. In the gastrointestinal tract, the interaction of bacteriophages (phages) and their prokaryotic hosts influences the health of the mammalian host, especially colonization with invasive pathobionts. Antibiotics may be used, but they also kill protective commensals. Here, we report a novel phage whose lytic cycle is enhanced in intestinal environments. The tail fiber gene, whose protein product binds human heparan sulfated proteoglycans and localizes the phage to the epithelial cell surface, positions it near its bacterial host, a type of locational targeting mechanism. This finding offers the prospect of developing mucosal targeting phage to selectively remove invasive pathobiont species from mucosal surfaces.IMPORTANCE Invasive pathobionts or microbes capable of causing disease can reside deep within the mucosal epithelium of our gastrointestinal tract. Targeted effective antibacterial therapies are needed to combat these disease-causing organisms, many of which may be multidrug resistant. Here, we isolated a lytic bacteriophage (phage) that can localize to the epithelial surface by binding heparan sulfated glycans, positioning it near its host, Escherichia coli This targeted therapy can be used to selectively remove invasive pathobionts from the gastrointestinal tract, preventing the development of disease.

Heparan sulfates are critical regulators of the inhibitory megakaryocyte-platelet receptor G6b-B

The immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor G6b-B is critical for platelet production and activation. Loss of G6b-B results in severe macrothrombocytopenia, myelofibrosis and aberrant platelet function in mice and humans. Using a combination of immunohistochemistry, affinity chromatography and proteomics, we identified the extracellular matrix heparan sulfate (HS) proteoglycan perlecan as a G6b-B binding partner. Subsequent in vitro biochemical studies and a cell-based genetic screen demonstrated that the interaction is specifically mediated by the HS chains of perlecan. Biophysical analysis revealed that heparin forms a high-affinity complex with G6b-B and mediates dimerization. Using platelets from humans and genetically modified mice, we demonstrate that binding of G6b-B to HS and multivalent heparin inhibits platelet and megakaryocyte function by inducing downstream signaling via the tyrosine phosphatases Shp1 and Shp2. Our findings provide novel insights into how G6b-B is regulated and contribute to our understanding of the interaction of megakaryocytes and platelets with glycans.

The heparin-binding proteome in normal pancreas and murine experimental acute pancreatitis

Acute pancreatitis (AP) is acute inflammation of the pancreas, mainly caused by gallstones and alcohol, driven by changes in communication between cells. Heparin-binding proteins (HBPs) play a central role in health and diseases. Therefore, we used heparin affinity proteomics to identify extracellular HBPs in pancreas and plasma of normal mice and in a caerulein mouse model of AP. Many new extracellular HBPs (360) were discovered in the pancreas, taking the total number of HBPs known to 786. Extracellular pancreas HBPs form highly interconnected protein-protein interaction networks in both normal pancreas (NP) and AP. Thus, HBPs represent an important set of extracellular proteins with significant regulatory potential in the pancreas. HBPs in NP are associated with biological functions such as molecular transport and cellular movement that underlie pancreatic homeostasis. However, in AP HBPs are associated with additional inflammatory processes such as acute phase response signalling, complement activation and mitochondrial dysfunction, which has a central role in the development of AP. Plasma HBPs in AP included known AP biomarkers such as serum amyloid A, as well as emerging targets such as histone H2A. Other HBPs such as alpha 2-HS glycoprotein (AHSG) and histidine-rich glycoprotein (HRG) need further investigation for potential applications in the management of AP. Pancreas HBPs are extracellular and so easily accessible and are potential drug targets in AP, whereas plasma HBPs represent potential biomarkers for AP. Thus, their identification paves the way to determine which HBPs may have potential applications in the management of AP.

Heparin binding preference and structures in the fibroblast growth factor family parallel their evolutionary diversification

The interaction of a large number of extracellular proteins with heparan sulfate (HS) regulates their transport and effector functions, but the degree of molecular specificity underlying protein–polysaccharide binding is still debated. The 15 paracrine fibroblast growth factors (FGFs) are one of the paradigms for this interaction. Here, we measure the binding preferences of six FGFs (FGF3, FGF4, FGF6, FGF10, FGF17, FGF20) for a library of modified heparins, representing structures in HS, and model glycosaminoglycans, using differential scanning fluorimetry. This is complemented by the identification of the lysine residues in the primary and secondary binding sites of the FGFs by a selective labelling approach. Pooling these data with previous sets provides good coverage of the FGF phylogenetic tree, deduced from amino acid sequence alignment. This demonstrates that the selectivity of the FGFs for binding structures in sulfated polysaccharides and the pattern of secondary binding sites on the surface of FGFs follow the phylogenetic relationship of the FGFs, and so are likely to be the result of the natural selection pressures that led to the expansion of the FGF family in the course of the evolution of more complex animal body plans.

An integrated approach using orthogonal analytical techniques to characterize heparan sulfate structure

Heparan sulfate (HS), a glycosaminoglycan present on the surface of cells, has been postulated to have important roles in driving both normal and pathological physiologies. The chemical structure and sulfation pattern (domain structure) of HS is believed to determine its biological function, to vary across tissue types, and to be modified in the context of disease. Characterization of HS requires isolation and purification of cell surface HS as a complex mixture. This process may introduce additional chemical modification of the native residues. In this study, we describe an approach towards thorough characterization of bovine kidney heparan sulfate (BKHS) that utilizes a variety of orthogonal analytical techniques (e.g. NMR, IP-RPHPLC, LC-MS). These techniques are applied to characterize this mixture at various levels including composition, fragment level, and overall chain properties. The combination of these techniques in many instances provides orthogonal views into the fine structure of HS, and in other instances provides overlapping / confirmatory information from different perspectives. Specifically, this approach enables quantitative determination of natural and modified saccharide residues in the HS chains, and identifies unusual structures. Analysis of partially digested HS chains allows for a better understanding of the domain structures within this mixture, and yields specific insights into the non-reducing end and reducing end structures of the chains. This approach outlines a useful framework that can be applied to elucidate HS structure and thereby provides means to advance understanding of its biological role and potential involvement in disease progression. In addition, the techniques described here can be applied to characterization of heparin from different sources.

Pharmacology of Heparin and Related Drugs

Heparin has been recognized as a valuable anticoagulant and antithrombotic for several decades and is still widely used in clinical practice for a variety of indications. The anticoagulant activity of heparin is mainly attributable to the action of a specific pentasaccharide sequence that acts in concert with antithrombin, a plasma coagulation factor inhibitor. This observation has led to the development of synthetic heparin mimetics for clinical use. However, it is increasingly recognized that heparin has many other pharmacological properties, including but not limited to antiviral, anti-inflammatory, and antimetastatic actions. Many of these activities are independent of its anticoagulant activity, although the mechanisms of these other activities are currently less well defined. Nonetheless, heparin is being exploited for clinical uses beyond anticoagulation and developed for a wide range of clinical disorders. This article provides a "state of the art" review of our current understanding of the pharmacology of heparin and related drugs and an overview of the status of development of such drugs.

Combining measurements to estimate properties and characterization extent of complex biochemical mixtures; applications to Heparan Sulfate

Complex mixtures of molecular species, such as glycoproteins and glycosaminoglycans, have important biological and therapeutic functions. Characterization of these mixtures with analytical chemistry measurements is an important step when developing generic drugs such as biosimilars. Recent developments have focused on analytical methods and statistical approaches to test similarity between mixtures. The question of how much uncertainty on mixture composition is reduced by combining several measurements still remains mostly unexplored. Mathematical frameworks to combine measurements, estimate mixture properties, and quantify remaining uncertainty, i.e. a characterization extent, are introduced here. Constrained optimization and mathematical modeling are applied to a set of twenty-three experimental measurements on heparan sulfate, a mixture of linear chains of disaccharides having different levels of sulfation. While this mixture has potentially over two million molecular species, mathematical modeling and the small set of measurements establish the existence of nonhomogeneity of sulfate level along chains and the presence of abundant sulfate repeats. Constrained optimization yields not only estimations of sulfate repeats and sulfate level at each position in the chains but also bounds on these levels, thereby estimating the extent of characterization of the sulfation pattern which is achieved by the set of measurements.

Selectivity in glycosaminoglycan binding dictates the distribution and diffusion of fibroblast growth factors in the pericellular matrix

The range of biological outcomes generated by many signalling proteins in development and homeostasis is increased by their interactions with glycosaminoglycans, particularly heparan sulfate (HS). This interaction controls the localization and movement of these signalling proteins, but whether such control depends on the specificity of the interactions is not known. We used five fibroblast growth factors with an N-terminal HaloTag (Halo-FGFs) for fluorescent labelling, with well-characterized and distinct HS-binding properties, and measured their binding and diffusion in pericellular matrix of fixed rat mammary 27 fibroblasts. Halo-FGF1, Halo-FGF2 and Halo-FGF6 bound to HS, whereas Halo-FGF10 also interacted with chondroitin sulfate/dermatan sulfate, and FGF20 did not bind detectably. The distribution of bound FGFs in the pericellular matrix was not homogeneous, and for FGF10 exhibited striking clusters. Fluorescence recovery after photobleaching showed that FGF2 and FGF6 diffused faster, whereas FGF1 diffused more slowly, and FGF10 was immobile. The results demonstrate that the specificity of the interactions of proteins with glycosaminoglycans controls their binding and diffusion. Moreover, cells regulate the spatial distribution of different protein-binding sites in glycosaminoglycans independently of each other, implying that the extracellular matrix has long-range structure.

Bioengineered heparins and heparan sulfates

Heparin and heparan sulfates are closely related linear anionic polysaccharides, called glycosaminoglycans, which exhibit a number of important biological and pharmacological activities. These polysaccharides, having complex structures and polydispersity, are biosynthesized in the Golgi of animal cells. While heparan sulfate is a widely distributed membrane and extracellular glycosaminoglycan, heparin is found primarily intracellularly in the granules of mast cells. While heparin has historically received most of the scientific attention for its anticoagulant activity, interest has steadily grown in the multi-faceted role heparan sulfate plays in normal and pathophysiology. The chemical synthesis of these glycosaminoglycans is largely precluded by their structural complexity. Today, we depend on livestock animal tissues for the isolation and the annual commercial production of hundred ton quantities of heparin used in the manufacture of anticoagulant drugs and medical device coatings. The variability of animal-sourced heparin and heparan sulfates, their inherent impurities, the limited availability of source tissues, the poor control of these source materials and their manufacturing processes, suggest a need for new approaches for their production. Over the past decade there have been major efforts in the biotechnological production of these glycosaminoglycans, driven by both therapeutic applications and as probes to study their natural functions. This review focuses on the complex biology of these glycosaminoglycans in human health and disease, and the use of recombinant technology in the chemoenzymatic synthesis and metabolic engineering of heparin and heparan sulfates.

Fibroblast growth factors as tissue repair and regeneration therapeutics

Cell communication is central to the integration of cell function required for the development and homeostasis of multicellular animals. Proteins are an important currency of cell communication, acting locally (auto-, juxta-, or paracrine) or systemically (endocrine). The fibroblast growth factor (FGF) family contributes to the regulation of virtually all aspects of development and organogenesis, and after birth to tissue maintenance, as well as particular aspects of organism physiology. In the West, oncology has been the focus of translation of FGF research, whereas in China and to an extent Japan a major focus has been to use FGFs in repair and regeneration settings. These differences have their roots in research history and aims. The Chinese drive into biotechnology and the delivery of engineered clinical grade FGFs by a major Chinese research group were important enablers in this respect. The Chinese language clinical literature is not widely accessible. To put this into context, we provide the essential molecular and functional background to the FGF communication system covering FGF ligands, the heparan sulfate and Klotho co-receptors and FGF receptor (FGFR) tyrosine kinases. We then summarise a selection of clinical reports that demonstrate the efficacy of engineered recombinant FGF ligands in treating a wide range of conditions that require tissue repair/regeneration. Alongside, the functional reasons why application of exogenous FGF ligands does not lead to cancers are described. Together, this highlights that the FGF ligands represent a major opportunity for clinical translation that has been largely overlooked in the West.

Structural determinants of heparin-transforming growth factor-β1 interactions and their effects on signaling

Transforming growth factor-β1 (TGF-β1, Uniprot: P01137) is a heparin-binding protein that has been implicated in a number of physiological processes, including the initiation of chondrogenesis by human mesenchymal stem cells (hMSCs). Here, we identify the molecular features in the protein and in heparin required for binding and their effects on the potentiation of TGF-β1's activity on hMSCs. Using a proteomics "Protect and Label" approach, lysines K291, K304, K309, K315, K338, K373, K375 and K388 were identified as being directly involved in binding heparin (Data are available via ProteomeXchange with identifier PXD002772). Competition assays in an optical biosensor demonstrated that TGF-β1 does require N- and 6-O-sulfate groups for binding but that 2-O-sulfate groups are unlikely to underpin the interaction. Heparin-derived oligosaccharides as short as degree of polymerization (dp) 4 have a weak ability to compete for TGF-β1 binding to heparin, which increases with the length of the oligosaccharide to reach a maximum between dp18 and dp24. In cell-based assays, heparin, 2-O-, 6-O- and N-desulfated re-N-acetylated heparin and oligosaccharides 14-24 saccharides (dp14-24) in length all increased the phosphorylation of mothers against decapentaplegic homolog 2 (SMAD2) after 6 h of stimulation with TGF-β1. The results provide the structural basis for a model of heparin/heparan sulfate binding to TGF-β1 and demonstrate that the features in the polysaccharide required for binding are not identical to those required for sustaining the signaling by TGF-β1 in hMSCs.

Fell-Muir Lecture: Heparan sulphate and the art of cell regulation: a polymer chain conducts the protein orchestra

Heparan sulphate (HS) sits at the interface of the cell and the extracellular matrix. It is a member of the glycosaminoglycan family of anionic polysaccharides with unique structural features designed for protein interaction and regulation. Its client proteins include soluble effectors (e.g. growth factors, morphogens, chemokines), membrane receptors and cell adhesion proteins such as fibronectin, fibrillin and various types of collagen. The protein-binding properties of HS, together with its strategic positioning in the pericellular domain, are indicative of key roles in mediating the flow of regulatory signals between cells and their microenvironment. The control of transmembrane signalling is a fundamental element in the complex biology of HS. It seems likely that, in some way, HS orchestrates diverse signalling pathways to facilitate information processing inside the cell. A dictionary definition of an orchestra is 'a large group of musicians who play together on various instruments …' to paraphrase, the HS orchestra is 'a large group of proteins that play together on various receptors'. HS conducts this orchestra to ensure that proteins hit the right notes on their receptors but, in the manner of a true conductor, does it also set 'the musical pulse' and create rhythm and harmony attractive to the cell? This is too big a question to answer but fun to think about as you read this review.

Extracellular distribution of diffusible growth factors controlled by heparan sulfate proteoglycans during mammalian embryogenesis

During mouse embryogenesis, diffusible growth factors, i.e. fibroblast growth factors, Wnt, bone morphogenetic protein and Hedgehog family members, emanating from localized areas can travel through the extracellular space and reach their target cells to specify the cell fate and form tissue architectures in coordination. However, the mechanisms by which these growth factors travel great distances to their target cells and control the signalling activity as morphogens remain an enigma. Recent studies in mice and other model animals have revealed that heparan sulfate proteoglycans (HSPGs) located on the cell surface (e.g. syndecans and glypicans) and in the extracellular matrix (ECM; e.g. perlecan and agrin) play crucial roles in the extracellular distribution of growth factors. Principally, the function of HSPGs depends primarily on the fine features and localization of their heparan sulfate glycosaminoglycan chains. Cell-surface-tethered HSPGs retain growth factors as co-receptors and/or endocytosis mediators, and enzymatic release of HSPGs from the cell membrane allows HSPGs to transport or move multiple growth factors. By contrast, ECM-associated HSPGs function as a reservoir or barrier in a context-dependent manner. This review is focused on our current understanding of the extracellular distribution of multiple growth factors controlled by HSPGs in mammalian development.

Role of Deacetylase Activity of N-Deacetylase/N-Sulfotransferase 1 in Forming N-Sulfated Domain in Heparan Sulfate

Heparan sulfate (HS) is a highly sulfated polysaccharide that plays important physiological roles. The biosynthesis of HS involves a series of enzymes, including glycosyltransferases (or HS polymerase), epimerase, and sulfotransferases. N-Deacetylase/N-Sulfotransferase isoform 1 (NDST-1) is a critical enzyme in this pathway. NDST-1, a bifunctional enzyme, displays N-deacetylase and N-sulfotransferase activities to convert an N-acetylated glucosamine residue to an N-sulfo glucosamine residue. Here, we report the cooperative effects between N-deacetylase and N-sulfotransferase activities. Using baculovirus expression in insect cells, we obtained three recombinant proteins: full-length NDST-1 and the individual N-deacetylase and N-sulfotransferase domains. Structurally defined oligosaccharide substrates were synthesized to test the substrate specificities of the enzymes. We discovered that N-deacetylation is the limiting step and that interplay between the N-sulfotransferase and N-deacetylase accelerates the reaction. Furthermore, combining the individually expressed N-deacetylase and N-sulfotransferase domains produced different sulfation patterns when compared with that made by the NDST-1 enzyme. Our data demonstrate the essential role of domain cooperation within NDST-1 in producing HS with specific domain structures.

Chondroitin sulfate proteoglycans: Key modulators in the developing and pathologic central nervous system

Chondroitin Sulfate Proteoglycans (CSPGs) are a major component of the extracellular matrix in the central nervous system (CNS) and play critical role in the development and pathophysiology of the brain and spinal cord. Developmentally, CSPGs provide guidance cues for growth cones and contribute to the formation of neuronal boundaries in the developing CNS. Their presence in perineuronal nets plays a crucial role in the maturation of synapses and closure of critical periods by limiting synaptic plasticity. Following injury to the CNS, CSPGs are dramatically upregulated by reactive glia which form a glial scar around the lesion site. Increased level of CSPGs is a hallmark of all CNS injuries and has been shown to limit axonal plasticity, regeneration, remyelination, and conduction after injury. Additionally, CSPGs create a non-permissive milieu for cell replacement activities by limiting cell migration, survival and differentiation. Mounting evidence is currently shedding light on the potential benefits of manipulating CSPGs in combination with other therapeutic strategies to promote spinal cord repair and regeneration. Moreover, the recent discovery of multiple receptors for CSPGs provides new therapeutic targets for targeted interventions in blocking the inhibitory properties of CSPGs following injury. Here, we will provide an in depth discussion on the impact of CSPGs in normal and pathological CNS. We will also review the recent preclinical therapies that have been developed to target CSPGs in the injured CNS.

C5-epimerase and 2-O-sulfotransferase associate in vitro to generate contiguous epimerized and 2-O-sulfated heparan sulfate domains

Heparan sulfate (HS), a complex polysaccharide of the cell surface, is endowed with the remarkable ability to bind numerous proteins and, as such, regulates a large variety of biological processes. Protein binding depends on HS structure; however, in the absence of a template driving its biosynthesis, the mechanism by which protein binding sequences are assembled remains poorly known. Here, we developed a chemically defined 13C-labeled substrate and NMR based experiments to simultaneously follow in real time the activity of HS biosynthetic enzymes and characterize the reaction products. Using this new approach, we report that the association of C5-epimerase and 2-O-sulfotransferase, which catalyze the production of iduronic acid and its 2-O-sulfation, respectively, is necessary to processively generate extended sequences of contiguous IdoA2S-containing disaccharides, whereas modifications are randomly introduced when the enzymes are uncoupled. These data shed light on the mechanisms by which HS motifs are generated during biosynthesis. They support the view that HS structure assembly is controlled not only by the availability of the biosynthetic enzymes but also by their physical association, which in the case of the C5-epimerase and 2-O-sulfotransferase was characterized by an affinity of 80 nM as demonstrated by surface plasmon resonance experiments.

Proteoglycans and neuronal migration in the cerebral cortex during development and disease

Chondroitin sulfate proteoglycans and heparan sulfate proteoglycans are major constituents of the extracellular matrix and the cell surface in the brain. Proteoglycans bind with many proteins including growth factors, chemokines, axon guidance molecules, and cell adhesion molecules through both the glycosaminoglycan and the core protein portions. The functions of proteoglycans are flexibly regulated due to the structural variability of glycosaminoglycans, which are generated by multiple glycosaminoglycan synthesis and modifying enzymes. Neuronal cell surface proteoglycans such as PTPζ, neuroglycan C and syndecan-3 function as direct receptors for heparin-binding growth factors that induce neuronal migration. The lectican family, secreted chondroitin sulfate proteoglycans, forms large aggregates with hyaluronic acid and tenascins, in which many signaling molecules and enzymes including matrix proteases are preserved. In the developing cerebrum, secreted chondroitin sulfate proteoglycans such as neurocan, versican and phosphacan are richly expressed in the areas that are strategically important for neuronal migration such as the striatum, marginal zone, subplate and subventricular zone in the neocortex. These proteoglycans may anchor various attractive and/or repulsive cues, regulating the migration routes of inhibitory neurons. Recent studies demonstrated that the genes encoding proteoglycan core proteins and glycosaminoglycan synthesis and modifying enzymes are associated with various psychiatric and intellectual disorders, which may be related to the defects of neuronal migration.

Extracellular regulation of type IIa receptor protein tyrosine phosphatases: mechanistic insights from structural analyses

The receptor protein tyrosine phosphatases (RPTPs) exhibit a wide repertoire of cellular signalling functions. In particular, type IIa RPTP family members have recently been highlighted as hubs for extracellular interactions in neurons, regulating neuronal extension and guidance, as well as synaptic organisation. In this review, we will discuss the recent progress of structural biology investigations into the architecture of type IIa RPTP ectodomains and their interactions with extracellular ligands. Structural insights, in combination with biophysical and cellular studies, allow us to begin to piece together molecular mechanisms for the transduction and integration of type IIa RPTP signals and to propose hypotheses for future experimental validation.

Chromatographic molecular weight measurements for heparin, its fragments and fractions, and other glycosaminoglycans

Glycosaminoglycan samples are usually polydisperse, consisting of molecules with differing length and differing sequence. Methods for measuring the molecular weight of heparin have been developed to assure the quality and consistency of heparin products for medicinal use, and these methods can be applied in other laboratory contexts. In the method described here, high-performance gel permeation chromatography is calibrated using appropriate heparin molecular weight markers or a single broad standard calibrant, and used to characterize the molecular weight distribution of polydisperse samples or the peak molecular weight of monodisperse, or approximately monodisperse, heparin fractions. The same technology can be adapted for use with other glycosaminoglycans.

Proteoglycans of reactive rat cortical astrocyte cultures: abundance of N-unsubstituted glucosamine-enriched heparan sulfate

"Reactive" astrocytes and other glial cells in the injured CNS produce an altered extracellular matrix (ECM) that influences neuronal regeneration. We have profiled the glycosaminoglycan (GAG) component of proteoglycans (PGs) produced by reactive neonatal rat cortical astrocytes, and have quantified their neurite-outgrowth inhibitory activity. PGs extracted from cell layers and medium were fractionated on DEAE-Sephacel with a gradient of NaCl from 0.15 to 1.0 M. Monosaccharide analysis of the major peaks eluting at 0.6 M NaCl indicated an excess of GlcNH₂ to GalNH₂, suggesting an approximate HS/CS ratio of 6.2 in the cell layer and 4.2 in the medium. Chondroitinase ABC-generated disaccharide analysis of cell and medium PGs showed a >5-fold excess of chondroitin 4-sulfate over chondroitin 6-sulfate. Heparin lyase-generated disaccharides characteristic of the highly sulfated S-domain regions within HS were more abundant in cell layer than medium-derived PGs. Cell layer and medium HS disaccharides contained ~20% and ~40% N-unsubstituted glucosamine respectively, which is normally rare in HS isolated from most tissues. NGF-stimulated neurite outgrowth assays using NS-1 (PC12) neuronal cells on adsorbed substrata of PGs isolated from reactive astrocyte medium showed pronounced inhibition of neurite outgrowth, and aggregation of NS-1 cells. Cell layer PGs from DEAE-Sephacel pooled fractions having high charge density permitted greater NGF-stimulated outgrowth than PGs with lower charge density. Our results indicate the synthesis of both inhibitory and permissive PGs by activated astrocytes that may correlate with sulfation patterns and HS/CS ratios.

Characterisation of the interaction of neuropilin-1 with heparin and a heparan sulfate mimetic library of heparin-derived sugars

Background. Neuropilin-1 (NRP-1) is a multidomain membrane protein with soluble isoforms interacting with a complex network of other membrane receptors, their respective ligands and heparan sulfate (HS). It is involved in the development of vasculature, neural patterning, immunological responses and pathological angiogenesis. Methods. We have characterised the binding of a Fc fusion of rat NRP-1 (Fc rNRP-1) and of a soluble isoform, corresponding to the first four extracellular domains of human NRP-1, shNRP-1, using optical biosensor-based binding assays with a library of heparin derivatives. Selective labelling of lysines protected upon heparin binding allowed their identification by mass spectrometry. Results. Fc rNRP-1 bound to heparin with high affinity (2.5 nM) and fast ka (9.8 × 10(6) M(-1)s(-1)). Unusually, NRP-1 bound both highly sulfated and completely desulfated stretches of heparin and exhibited a complex pattern of preferences for chemically modified heparins possessing one or two sulfate groups, e.g., it bound heparin with just a 6-O sulfate group better than heparin with any two of N-sulfate, 6-O sulfate and 2-O sulfate. Mass-spectrometry based mapping identified that, in addition to the expected the b1 domain, the a1, and c domains and the L2 linker were also involved in the interaction. In contrast, shNRP-1 bound heparin far more weakly. This could only be shown by affinity chromatography and by differential scanning fluorimetry. Discussion. The results suggest that the interaction of NRP-1 with HS is more complex than anticipated and involving a far greater extent of the protein than just the b1-b2 domains. NRP-1's preference for binding long saccharide structures suggests it has the potential to bind large segments of HS chains and so organise their local structure. In contrast, the four domain soluble isoform, shNRP-1 binds heparin weakly and so would be expected to diffuse away rapidly from the source cell.

Demystifying heparan sulfate-protein interactions

Numerous proteins, including cytokines and chemokines, enzymes and enzyme inhibitors, extracellular matrix proteins, and membrane receptors, bind heparin. Although they are traditionally classified as heparin-binding proteins, under normal physiological conditions these proteins actually interact with the heparan sulfate chains of one or more membrane or extracellular proteoglycans. Thus, they are more appropriately classified as heparan sulfate-binding proteins (HSBPs). This review provides an overview of the various modes of interaction between heparan sulfate and HSBPs, emphasizing biochemical and structural insights that improve our understanding of the many biological functions of heparan sulfate.

Oligomannurarate sulfate inhibits CXCL12/SDF-1-mediated proliferation and invasion of human tumor cells in vitro

AIM

JG6 is a novel marine-derived oligosaccharide that has shown to inhibit angiogenesis and tumor metastasis. In this study, we sought to identify the potential target responsible for the anti-cancer activity of JG6.

METHODS

Human liver cancer cell line Bel-7402 and human cervical cancer cell line HeLa were examined. CXCL12-stimulated cell proliferation and migration were determined using a CCK-8 kit and a transwell assay, respectively. Western blotting was performed to examine the changes in CXCL12/CXCR4 axis. Molecular docking and surface plasmon resonance (SPR) were performed to characterize the possible interaction between JG6 and the CXCL12/CXCR4 axis.

RESULTS

Treatment with CXCL12 potently stimulated the proliferation and migration in both Bel-7402 and HeLa cells. Co-treatment of the cells with JG6 (10, 50 and 100 μg/mL) dose-dependently impeded the CXCL12-stimulated cell proliferation and migration. Furthermore, CXCL12 rapidly induced phosphorylation of AKT, ERK, FAK and Paxillin in Bel-7402 and HeLa cells, whereas pretreatment with JG6 dose-dependently inhibited the CXCL12-induced phosphorylation of these proteins. The SPR assay showed that JG6 bound to CXCL12 with a high affinity. In molecular docking study, JG6 appeared to interact with CXCL12 via multiple polar interactions, including 6 ionic bonds and 7 hydrogen bonds.

CONCLUSION

Inhibition of the CXCL12/CXCR4 axis by JG6 may account for its anticancer activity.

Roles of EXTL2, a member of the EXT family of tumour suppressors, in liver injury and regeneration processes

The gene products of two members of the EXT (exostosin) gene family, EXT1 and EXT2, function together as a polymerase in the biosynthesis of heparan sulfate. EXTL2 (EXT-like 2), one of the three EXTL genes in the human genome that are homologous to EXT1 and EXT2, encodes an N-acetylhexosaminyltransferase. We have demonstrated that EXTL2 terminates chain elongation of GAGs (glycosaminoglycans), and thereby regulates GAG biosynthesis. The abnormal GAG biosynthesis caused by loss of EXTL2 had no effect on normal development or normal adult homoeostasis. Therefore we examined the role of EXTL2 in CCl4 (carbon tetrachloride)-induced liver failure, a model of liver disease. On the fifth day after CCl4 administration, the liver/body weight ratio was significantly smaller for EXTL2-knockout mice than for wild-type mice. Consistent with this observation, hepatocyte proliferation following CCl4 treatment was lower in EXTL2-knockout mice than in wild-type mice. EXTL2-knockout mice experienced less HGF (hepatocyte growth factor)-mediated signalling than wild-type mice specifically because GAG synthesis was altered in these mutant mice. In addition, GAG synthesis in hepatic stellate cells was up-regulated during liver repair in EXTL2-knockout mice. Taken together, the results of the present study indicated that EXTL2-mediated regulation of GAG synthesis was important to the tissue regeneration processes that follow liver injury.

Podocyte-specific deletion of NDST1, a key enzyme in the sulfation of heparan sulfate glycosaminoglycans, leads to abnormalities in podocyte organization in vivo

Heparan sulfate proteoglycans have been shown to modulate podocyte adhesion to- and pedicel organization on- the glomerular basement membrane. Recent studies showed that foot process effacement developed in a mutant mouse model whose podocytes were unable to assemble heparan sulfate glycosaminoglycan chains. This study, a further refinement, explored the role of heparan N-sulfation on podocyte behavior. A novel mutant mouse (Ndst1^-/-) was developed, having podocyte-specific deletion of NDST1, the enzyme responsible for N-sulfation of heparan sulfate chains. Podocytes having this mutation had foot process effacement and abnormal adhesion to Bowman's capsule. Although glomerular hypertrophy did develop in the kidneys of mutant animals, mesangial expansion was not seen. The lack of heparan N-sulfation did not affect the expression of agrin or perlecan proteoglycan core proteins. Loss of N-sulfation did not result in significant proteinuria, but the increase in the albumin/creatinine ratio was coincident with the development of the enlarged lysosomes in the proximal tubules. Thus, although the renal phenotype of the Ndst1^-/- mouse is mild, the data show that heparan chain N-sulfation plays a key role in podocyte organization.

Affinity-selected heparan sulfate for bone repair

Bone morphogenetic protein (BMP)-2 is a potent bone healing compound produced at sites of bone trauma. Here we present a therapeutic strategy to harness the activity of endogenously produced BMP-2 by delivery of an affinity-matched heparan sulfate (HS) glycos aminoglycan biomaterial that increases the bioavailability, bioactivity and half-life of this growth factor. We have developed a robust, cost effective, peptide-based affinity platform to isolate a unique BMP-2 binding HS variant from commercially available preparations of HS, so removing the manufacturing bottleneck for their translation into the clinic. This affinity-matched HS enhanced BMP-2-induced osteogenesis through improved BMP-2 kinetics and receptor modulation, prolonged pSMAD signaling and reduced interactions with its antagonist noggin. When co-delivered with a collagen implant, the HS was as potent as exogenous BMP-2 for the healing of critical-sized bone defects in rabbits. This affinity platform can be readily tuned to isolate HS variants targeted ata range of clinically-relevant growth and adhesive factors.

Diversification of the structural determinants of fibroblast growth factor-heparin interactions: implications for binding specificity

The functions of a large number (>435) of extracellular regulatory proteins are controlled by their interactions with heparan sulfate (HS). In the case of fibroblast growth factors (FGFs), HS binding determines their transport between cells and is required for the assembly of high affinity signaling complexes with their cognate FGF receptor. However, the specificity of the interaction of FGFs with HS is still debated. Here, we use a panel of FGFs (FGF-1, FGF-2, FGF-7, FGF-9, FGF-18, and FGF-21) spanning five FGF subfamilies to probe their specificities for HS at different levels as follows: binding parameters, identification of heparin-binding sites (HBSs) in the FGFs, changes in their secondary structure caused by heparin binding and structures in the sugar required for binding. For interaction with heparin, the FGFs exhibit K(D) values varying between 38 nM (FGF-18) and 620 nM (FGF-9) and association rate constants spanning over 20-fold (FGF-1, 2,900,000 M(-1) s(-1) and FGF-9, 130,000 M(-1) s(-1)). The canonical HBS in FGF-1, FGF-2, FGF-7, FGF-9, and FGF-18 differs in its size, and these FGFs have a different complement of secondary HBS, ranging from none (FGF-9) to two (FGF-1). Differential scanning fluorimetry identified clear preferences in these FGFs for distinct structural features in the polysaccharide. These data suggest that the differences in heparin-binding sites in both the protein and the sugar are greatest between subfamilies and may be more restricted within a FGF subfamily in accord with the known conservation of function within FGF subfamilies.

Investigation of the substrate specificity of K5 lyase A from K5A bacteriophage

K5 lyase A (KflA) is a tailspike protein from the K5A phage that catalyzes the degradation of the capsule polysaccharide of K5 strains of Escherichia coli. The K5 E. coli capsule polysaccharide, also known as heparosan, is composed of the disaccharide repeating unit of [-4)-GlcA-β(1,4)-GlcNAc-α(1-] and therefore identical to the biological precursor of heparin and heparan sulfate (HS). KflA could supplement the heparin lyases for heparin and HS analysis. The first part of this study aimed to clarify ambiguity resulting from the revision of the KflA amino acid sequence in 2010 from that published in 2000. We found that only the expression of the updated sequence gave a soluble active enzyme, which produced heparosan degradation products similar to those of previous studies. Next, we examined the specificity of KflA toward heparosan oligosaccharides of varying sizes, all containing a single N-sulfated glucosamine (GlcNS) residue. The presence of GlcNS in an octasaccharide and a nonasaccharide chain directed cleavage by KflA to a single position at the reducing end of the substrate. However, an N-sulfated decasaccharide exhibited extensive cleavage at the nonreducing end of the chain, illustrating a distinct change in the cleavage pattern of KflA toward substrates of differing sizes. Because KflA is able to cleave a substrate containing isolated GlcNS residues, this enzyme could be used for the analysis of low-sulfate content HS domains.

Heparan sulfate biosynthesis: methods for investigation of the heparanosome

Heparan sulfate is perhaps the most complex polysaccharide known from animals. The basic repeating disaccharide is extensively modified by sulfation and uronic acid epimerization. Despite this, the fine structure of heparan sulfate is remarkably consistent with a particular cell type. This suggests that the synthesis of heparan sulfate is tightly controlled. Although genomics has identified the enzymes involved in glycosaminoglycan synthesis in a number of vertebrates and invertebrates, the regulation of the process is not understood. Moreover, the localization of the various enzymes in the Golgi apparatus has not been carried out in a detailed way using high-resolution microscopy. We have begun this process, using well-known markers for the various Golgi compartments, coupled with the use of characterized antibodies and cDNA expression. Laser scanning confocal microscopy coupled with line scanning provides high-quality resolution of the distribution of enzymes. The EXT2 protein, which when combined as heterodimers with EXT1 comprises the major polymerase in heparan sulfate synthesis, has been studied in depth. All the data are consistent with a cis-Golgi distribution and provide a starting point to establish whether all the enzymes are clustered in a multimolecular complex or are distributed through the various compartments of the Golgi apparatus.

Understanding the substrate specificity of the heparan sulfate sulfotransferases by an integrated biosynthetic and crystallographic approach

Heparan sulfates (HSs) have potential therapeutic value as anti-inflammatory and antimetastasis drugs, in addition to their current use as anticoagulants. Recent advances in chemoenzymatic synthesis of HS provide a way to conveniently produce homogenous HS with different biological properties. Crystal structures of sulfotransferases involved in this process are providing atomic detail of their substrate binding clefts and interactions with their HS substrates. In theory, the flexibility of this method can be increased by modifying the specificities of the sulfotransferases based on the structures, thereby producing a new array of products.

Transport of fibroblast growth factor 2 in the pericellular matrix is controlled by the spatial distribution of its binding sites in heparan sulfate

The heparan sulfate (HS) chains of proteoglycans are a key regulatory component of the extracellular matrices of animal cells, including the pericellular matrix around the plasma membrane. In these matrices they regulate transport, gradient formation, and effector functions of over 400 proteins central to cell communication. HS from different matrices differs in its selectivity for its protein partners. However, there has been no direct test of how HS in the matrix regulates the transport of its partner proteins. We address this issue by single molecule imaging and tracking in fibroblast pericellular matrix of fibroblast growth factor 2 (FGF2), stoichiometrically labelled with small gold nanoparticles. Transmission electron microscopy and photothermal heterodyne imaging (PHI) show that the spatial distribution of the HS-binding sites for FGF2 in the pericellular matrix is heterogeneous over length scales ranging from 22 nm to several µm. Tracking of individual FGF2 by PHI in the pericellular matrix of living cells demonstrates that they undergo five distinct types of motion. They spend much of their time in confined motion (∼110 nm diameter), but they are not trapped and can escape by simple diffusion, which may be slow, fast, or directed. These substantial translocations (µm) cover distances far greater than the length of a single HS chain. Similar molecular motion persists in fixed cells, where the movement of membrane PGs is impeded. We conclude that FGF2 moves within the pericellular matrix by translocating from one HS-binding site to another. The binding sites on HS chains form non-random, heterogeneous networks. These promote FGF2 confinement or substantial translocation depending on their spatial organisation. We propose that this spatial organisation, coupled to the relative selectivity and the availability of HS-binding sites, determines the transport of FGF2 in matrices. Similar mechanisms are likely to underpin the movement of many other HS-binding effectors.

Breast and ovarian cancers: a survey and possible roles for the cell surface heparan sulfate proteoglycans

Tumor markers are widely used in pathology not only for diagnostic purposes but also to assess the prognosis and to predict the treatment of the tumor. Because tumor marker levels may change over time, it is important to get a better understanding of the molecular changes during tumor progression. Occurrence of breast and ovarian cancer is high in older women. Common known risk factors of developing these cancers in addition to age are not having children or having children at a later age, the use of hormone replacement therapy, and mutations in certain genes. In addition, women with a history of breast cancer may also develop ovarian cancer. Here, the authors review the different tumor markers of breast and ovarian carcinoma and discuss the expression, mutations, and possible roles of cell surface heparan sulfate proteoglycans during tumorigenesis of these carcinomas. The focus is on two groups of proteoglycans, the transmembrane syndecans and the lipid-anchored glypicans. Both families of proteoglycans have been implicated in cellular responses to growth factors and morphogens, including many now associated with tumor progression.

Heparan sulphate: a heparin in miniature

Heparan sulphate (HS), discovered in 1948 in heparin by-products, only emerged slowly from the shadow of heparin. Its inauspicious beginning was followed by the gradual realisation that HS was a separate entity with distinctive features. Both HS and heparin follow a common biosynthetic route but while heparin reaches full maturity, HS holds on to some of its youthful traits. The novel design and complex patterning of sulphation in HS enable it fulfil key roles in many, diverse biological processes.

Molecular architecture of heparin and heparan sulfate: Recent developments in solution structural studies

The study of the relationship between the complex structures and numerous physiological functions of the glycosaminoglycans (GAGs) heparin and heparan sulfate (HS) has continued to thrive in the past decade. Though it is clear that the monosaccharide sequences of these polysaccharides must determine their ability to modulate the action of growth factors, morphogens, chemokines, cytokines, and many other extracellular proteins, the exact details of this dependence still prove elusive. Sequence determines the 3D structure of GAGs at more than one level; detailed sequences of highly sulfated regions may influence affinity for specific proteins in some cases, but in addition attention has been called to the importance of the length and spacing of these highly sulfated sequences, which are separated by unsulfated domains. Within the sulfated “S-domains”, the internal dynamics of the conformationally flexible iduronate pyranose ring have continued to interest NMR spectroscopists and molecular modelers. New studies of the relative degrees of flexibility of sulfated and unsulfated domains lead to an overall model of heparin/HS in which protein-binding, highly sulfated S-domains with well-defined conformations are separated by more flexible NA-domains.

Specific glycosaminoglycans modulate neural specification of mouse embryonic stem cells

Mouse embryonic stem (mES) cells express a low sulfated form of heparan sulfate (HS). HS chains displayed by ES cells and their progeny become more complex and more sulfated during progression from pluripotency to neuroectodermal precursors. Sulfated epitopes are important for recognition and binding of a variety of ligands including members of the fibroblast growth factor (FGF) family. We demonstrated previously that mES cells lacking HS cannot undergo neural specification but this activity can be recovered by adding soluble heparin, a highly sulfated glycosaminoglycan (GAG). Therefore, we hypothesized that soluble GAGs might be used to support neural differentiation of HS competent cells and that the mechanisms underlying this activity might provide useful information about the signaling pathways critical for loss of pluripotency and early lineage commitment. In this study, we demonstrate that specific HS/heparin polysaccharides support formation of Sox1(+) neural progenitor cells from wild-type ES cells. This effect is dependent on sulfation pattern, concentration, and length of saccharide. Using a selective inhibitor of FGF signal transduction, we show that heparin modulates signaling events regulating exit from pluripotency and commitment to primitive ectoderm and subsequently neuroectoderm. Interestingly, we were also able to demonstrate that multiple receptor tyrosine kinases were influenced by HS in this system. This suggests roles for additional factors, possibly in cell proliferation or protection from apoptosis, during the process of neural specification. Therefore, we conclude that soluble GAGs or synthetic mimics could be considered as suitable low-cost factors for addition to ES cell differentiation regimes.

The dominating role of N-deacetylase/N-sulfotransferase 1 in forming domain structures in heparan sulfate

Heparan sulfate (HS) is a highly sulfated polysaccharide participated in essential physiological functions from regulating cell growth to blood coagulation. HS contains sulfated domains known as N-S domains and low sulfate domains known as N-Ac domains. The distribution of the domain structures is likely governed by the action of glucosaminyl N-deacetylase/N-sulfotransferase (NDST). Here, we sought to determine the substrate specificity of NDST using model substrates and recombinant NDST protein. We discovered that NDST-1 carries out the modification in a highly ordered fashion. The enzyme sulfates the substrate from the nonreducing end toward the reducing end consecutively, leading to the product with a cluster of N-sulfo glucosamine residues. Furthermore, a preexisting N-sulfo glucosamine residue prevents the action of NDST-1 at the residues immediately located at the nonreducing end, allowing the formation of an N-Ac domain. Our results provide the long sought evidence for understanding the formation of sulfated versus nonsulfated domains in the HS isolated from cells and tissues. The study demonstrates the regulating role of NDST-1 in mapping the sulfation patterns of HS.