Structure of a heparan sulphate oligosaccharide that binds to basic fibroblast growth factor

Carbohydrate based biomaterials for neural interface applications

Neuroprosthetic devices that record and modulate neural activities have demonstrated immense potential for bypassing or restoring lost neurological functions due to neural injuries and disorders. However, implantable electrical devices interfacing with brain tissue are susceptible to a series of inflammatory tissue responses along with mechanical or electrical failures which can affect the device performance over time. Several biomaterial strategies have been implemented to improve device-tissue integration for high quality and stable performance. Ranging from developing smaller, softer, and more flexible electrode designs to introducing bioactive coatings and drug-eluting layers on the electrode surface, such strategies have shown different degrees of success but with limitations. With their hydrophilic properties and specific bioactivities, carbohydrates offer a potential solution for addressing some of the limitations of the existing biomolecular approaches. In this review, we summarize the role of polysaccharides in the central nervous system, with a primary focus on glycoproteins and proteoglycans, to shed light on their untapped potential as biomaterials for neural implants. Utilization of glycosaminoglycans for neural interface and tissue regeneration applications is comprehensively reviewed to provide the current state of carbohydrate-based biomaterials for neural implants. Finally, we will discuss the challenges and opportunities of applying carbohydrate-based biomaterials for neural tissue interfaces.

Abnormalities in reparative function of lung-derived mesenchymal stromal cells in emphysema

Mesenchymal stromal cells (MSCs) may provide crucial support in the regeneration of destructed alveolar tissue (emphysema) in chronic obstructive pulmonary disease (COPD). We hypothesized that lung-derived MSCs (LMSCs) from patients with emphysema are hampered in their repair capacity, either intrinsically or due to their interaction with the damaged microenvironment. LMSCs were isolated from the lung tissue of controls and patients with severe emphysema and characterized at baseline. In addition, LMSCs were seeded onto control and emphysematous decellularized lung tissue scaffolds and assessed for deposition of extracellular matrix (ECM). We observed no differences in surface markers, differentiation/proliferation potential, and expression of ECM genes between control- and COPD-derived LMSCs. Notably, COPD-derived LMSCs displayed lower expression of FGF10 and HGF messenger RNA (mRNA) and hepatocyte growth factor (HGF) and decorin protein. When seeded on control decellularized lung tissue scaffolds, control- and COPD-derived LMSCs showed no differences in engraftment, proliferation, or survival within 2  wk, with similar ability to deposit new matrix on the scaffolds. Moreover, LMSC numbers and the ability to deposit new matrix were not compromised on emphysematous scaffolds. Collectively, our data show that LMSCs from patients with COPD compared with controls show less expression of FGF10 mRNA, HGF mRNA and protein, and decorin protein, whereas other features including the mRNA expression of various ECM molecules are unaffected. Furthermore, COPD-derived LMSCs are capable of engraftment, proliferation, and functioning on native lung tissue scaffolds. The damaged, emphysematous microenvironment as such does not hamper the potential of LMSCs. Thus, specific intrinsic deficiencies in growth factor production by diseased LMSCs may contribute to impaired alveolar repair in emphysema.

Glypicans and Heparan Sulfate in Synaptic Development, Neural Plasticity, and Neurological Disorders

Heparan sulfate proteoglycans (HSPGs) are components of the cell surface and extracellular matrix, which bear long polysaccharides called heparan sulfate (HS) attached to the core proteins. HSPGs interact with a variety of ligand proteins through the HS chains, and mutations in HSPG-related genes influence many biological processes and cause various diseases. In particular, recent findings from vertebrate and invertebrate studies have raised the importance of glycosylphosphatidylinositol-anchored HSPGs, glypicans, as central players in the development and functions of synapses. Glypicans are important components of the synapse-organizing protein complexes and serve as ligands for leucine-rich repeat transmembrane neuronal proteins (LRRTMs), leukocyte common antigen-related (LAR) family receptor protein tyrosine phosphatases (RPTPs), and G-protein-coupled receptor 158 (GPR158), regulating synapse formation. Many of these interactions are mediated by the HS chains of glypicans. Neurexins (Nrxs) are also synthesized as HSPGs and bind to some ligands in common with glypicans through HS chains. Therefore, glypicans and Nrxs may act competitively at the synapses. Furthermore, glypicans regulate the postsynaptic expression levels of ionotropic glutamate receptors, controlling the electrophysiological properties and non-canonical BMP signaling of synapses. Dysfunctions of glypicans lead to failures in neuronal network formation, malfunction of synapses, and abnormal behaviors that are characteristic of neurodevelopmental disorders. Recent human genetics revealed that glypicans and HS are associated with autism spectrum disorder, neuroticism, and schizophrenia. In this review, we introduce the studies showing the roles of glypicans and HS in synapse formation, neural plasticity, and neurological disorders, especially focusing on the mouse and Drosophila as potential models for human diseases.

Glycosaminoglycan-based biomaterials for growth factor and cytokine delivery: Making the right choices

Controlled, localized drug delivery is a long-standing goal of medical research, realization of which could reduce the harmful side-effects of drugs and allow more effective treatment of wounds, cancers, organ damage and other diseases. This is particularly the case for protein "drugs" and other therapeutic biological cargoes, which can be challenging to deliver effectively by conventional systemic administration. However, developing biocompatible materials that can sequester large quantities of protein and release them in a sustained and controlled manner has proven challenging. Glycosaminoglycans (GAGs) represent a promising class of bio-derived materials that possess these key properties and can additionally potentially enhance the biological effects of the delivered protein. They are a diverse group of linear polysaccharides with varied functionalities and suitabilities for different cargoes. However, most investigations so far have focused on a relatively small subset of GAGs - particularly heparin, a readily available, promiscuously-binding GAG. There is emerging evidence that for many applications other GAGs are in fact more suitable for regulated and sustained delivery. In this review, we aim to illuminate the beneficial properties of various GAGs with reference to specific protein cargoes, and to provide guidelines for informed choice of GAGs for therapeutic applications.

Fully Synthetic Heparan Sulfate-Based Neural Tissue Construct That Maintains the Undifferentiated State of Neural Stem Cells

Heparin and heparan sulfate (HS) are attractive components for constructing biomaterials due to their ability to recruit and regulate the activity of growth factors. The structural and functional heterogeneity of naturally derived heparin and HS is, however, an impediment for the preparation of biomaterials for regenerative medicine. To address this problem, we have prepared hydrogels modified by well-defined synthetic HS-derived disaccharides. Human induced pluripotent cell-derived neural stem cells (HIP-NSCs) encapsulated in a polyethylene glycol-based hydrogel modified by a pendent HS disaccharide that is a known ligand for fibroblast growth factor-2 (FGF-2) exhibited a significant increase in proliferation and self-renewal. This observation is important because evidence is emerging that undifferentiated stems cells can yield significant therapeutic benefits via their paracrine signaling mechanisms. Our data indicate that the HS disaccharide protects FGF-2, which has a very short biological half-live, from degradation. It is anticipated that, by careful selection of a synthetic HS oligosaccharide, it will be possible to control retention and release of specific growth factor, which in turn will provide control over cell fate.

Osteogenic differentiation enhances the MC3T3-E1 secretion of glycosaminoglycans with an affinity for basic fibroblast growth factor and bone morphogenetic protein-2

Introduction

It is generally recognized that a wide variety of morphogens and growth factors bind to the glycosaminoglycans (GAG) of proteoglycans (PG) to affect their bioavailability to ligands. Many growth factors involving in osteogenic differentiation require the GAG side chains to facilitate their interaction to the cell surface receptors and the biosynthesis of osteogenic proteins. The objective of this study is to investigate the secretion of GAG from MC3T3-E1 pre-osteoblasts of a murine bone calvaria during the osteogenic differentiation.

Methods

When MC3T3-E1 cells were cultured in the differentiation medium (DM) containing bone morphogenetic protein (BMP)-2, the alkaline phosphatase activity, calcium content and the amount of basic fibroblast growth factor (bFGF)- or BMP-2-bound sulfated GAG were determined. Moreover, the disaccharide analysis of the GAG was performed.

Results

When MC3T3-E1 cells were cultured in the differentiation medium (DM) containing bone morphogenetic protein (BMP)-2, the alkaline phosphatase activity and calcium content were significantly enhanced compared with those of the BMP-2-free DM and normal medium with or without BMP-2. Significantly higher amount of GAG secreted was detected for cells cultured in the DM containing BMP-2, in contrast to other culture conditions. The GAG secreted had an affinity for BMP-2 and basic fibroblast growth factor (bFGF). The disaccharide analysis of GAG demonstrated that the percentage of ΔHexA α1,4GlcNSO₃ and ΔHexA (2-OSO₃) α1,4GlcNSO₃ increased, but that of ΔHexA α1,4GlcNSO₃(6-OSO₃) decreased (ΔHexA: unsaturated uronic acid residue, GlcNSO₃: N-sulfated glucosamine, ΔHexA (2-OSO₃): unsaturated uronic acid 2-sulfate residue, GlcNSO₃(6-OSO₃): N-sulfated glucosamine 6-sulfated).

Conclusion

It was found that the osteogenic differentiation allowed cells to enhance the secretion of GAG with an affinity for BMP-2 and bFGF.

Glycosaminoglycan Analysis by Cryogenic Messenger-Tagging IR Spectroscopy Combined with IMS-MS

We combine ion mobility spectrometry with cryogenic, messenger-tagging, infrared spectroscopy and mass spectrometry to identify different isomeric disaccharides of chondroitin sulfate (CS) and heparan sulfate (HS), which are representatives of two major subclasses of glycosaminoglycans. Our analysis shows that while CS and HS disaccharide isomers have similar drift times, they can be uniquely distinguished by their vibrational spectrum between ∼3200 and 3700 cm-1 due to their different OH hydrogen-bonding patterns. We suggest that this combination of techniques is well suited to identify and characterize glycan isomers directly, which presents tremendous challenges for existing methods.

The "in and out" of glucosamine 6-O-sulfation: the 6th sense of heparan sulfate

The biological properties of Heparan sulfate (HS) polysaccharides essentially rely on their ability to bind and modulate a multitude of protein ligands. These interactions involve internal oligosaccharide sequences defined by their sulfation patterns. Amongst these, the 6-O-sulfation of HS contributes significantly to the polysaccharide structural diversity and is critically involved in the binding of many proteins. HS 6-O-sulfation is catalyzed by 6-O-sulfotransferases (6OSTs) during biosynthesis, and it is further modified by the post-synthetic action of 6-O-endosulfatases (Sulfs), two enzyme families that remain poorly characterized. The aim of the present review is to summarize the contribution of 6-O-sulfates in HS structure/function relationships and to discuss the present knowledge on the complex mechanisms regulating HS 6-O-sulfation.

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.

Sugar glues for broken neurons

Proteoglycans (PGs) regulate diverse functions in the central nervous system (CNS) by interacting with a number of growth factors, matrix proteins, and cell surface molecules. Heparan sulfate (HS) and chondroitin sulfate (CS) are two major glycosaminoglycans present in the PGs of the CNS. The functionality of these PGs is to a large extent dictated by the fine sulfation patterns present on their glycosaminoglycan (GAG) chains. In the past 15 years, there has been a significant expansion in our knowledge on the role of HS and CS chains in various neurological processes, such as neuronal growth, regeneration, plasticity, and pathfinding. However, defining the relation between distinct sulfation patterns of the GAGs and their functionality has thus far been difficult. With the emergence of novel tools for the synthesis of defined GAG structures, and techniques for their characterization, we are now in a better position to explore the structure-function relation of GAGs in the context of their sulfation patterns. In this review, we discuss the importance of GAGs on CNS development, injury, and disorders with an emphasis on their sulfation patterns. Finally, we outline several GAG-based therapeutic strategies to exploit GAG chains for ameliorating various CNS disorders.

2-O Heparan Sulfate Sulfation by Hs2st Is Required for Erk/Mapk Signalling Activation at the Mid-Gestational Mouse Telencephalic Midline

Heparan sulfate (HS) is a linear carbohydrate composed of polymerized uronate-glucosamine disaccharide units that decorates cell surface and secreted glycoproteins in the extracellular matrix. In mammals HS is subjected to differential sulfation by fifteen different heparan sulfotransferase (HST) enzymes of which Hs2st uniquely catalyzes the sulfation of the 2-O position of the uronate in HS. HS sulfation is postulated to be important for regulation of signaling pathways by facilitating the interaction of HS with signaling proteins including those of the Fibroblast Growth Factor (Fgf) family which signal through phosphorylation of extracellular signal-regulated kinases Erk1/2. In the developing mouse telencephalon Fgf2 signaling regulates proliferation and neurogenesis. Loss of Hs2st function phenocopies the thinned cerebral cortex of mutant mice in which Fgf2 or Erk1/2 function are abrogated, suggesting the hypothesis that 2-O-sulfated HS structures play a specific role in Fgf2/Erk signaling pathway in this context in vivo. This study investigated the molecular role of 2-O sulfation in Fgf2/Erk signaling in the developing telencephalic midline midway through mouse embryogenesis at E12.5. We examined the expression of Hs2st, Fgf2, and Erk1/2 activity in wild-type and Hs2st-/- mice. We found that Hs2st is expressed at high levels at the midline correlating with high levels of Erk1/2 activation and Erk1/2 activation was drastically reduced in the Hs2st-/- mutant at the rostral telencephalic midline. We also found that 2-O sulfation is specifically required for the binding of Fgf2 protein to Fgfr1, its major cell-surface receptor at the rostral telencephalic midline. We conclude that 2-O sulfated HS structures generated by Hs2st are needed to form productive signaling complexes between HS, Fgf2 and Fgfr1 that activate Erk1/2 at the midline. Overall, our data suggest the interesting possibility that differential expression of Hs2st targets the rostral telencephalic midline for high levels of Erk signaling by increasing the sensitivity of cells to an Fgf2 signal that is rather more widespread.

Syntheses of chondroitin sulfate tetrasaccharide structures containing 4,6-disulfate patterns and analysis of their interaction with glycosaminoglycan-binding protein

Chondroitin sulfate tetrasaccharide ligand conjugates, namely GlcA-GalNAc6S-GlcA-GalNAc4S6S (CS-C+E) 1, GlcA2S-GalNAc6S-GlcA2S-GalNAc4S6S (CS-D+T) 2, GlcA-GalNAc4S6S-GlcA-GalNAc4S (CS-E+A) 3, GlcA-GalNAc4S6S-GlcA-GalNAc6S (CS-E+C) 4, and GlcA-GalNAc4S6S-GlcA-GalNAc4S6S (CS-E+E) 5, were systematically synthesized using a disaccharide building block 6. Synthesized CS tetrasaccharide structures were immobilized onto gold-coated chips to prepare array-type sugar chips, and the binding properties of protein were evaluated by surface plasmon resonance imaging biosensor. CS-D+T, CS-E+A, CS-E+C, and CS-E+E showed greater affinity for basic fibroblast growth factor than did other tetrasaccharides (CS-C+D, C+E, D+D).

Inhibitory effect of chondroitin sulfate oligosaccharides on bovine testicular hyaluronidase

Hyaluronan and chondroitin sulfates are prominent components of the extracellular matrices of animal tissues; however, their functions in relation to their oligosaccharide structures have not yet been fully elucidated. The oligosaccharides of hyaluronan and chondroitin sulfate were prepared and used to investigate their effects on the hydrolysis and transglycosylation reactions of bovine testicular hyaluronidase when hyaluronan was used as a substrate. Hydrolysis and transglycosylation activities were assessed in independent reaction systems by analyzing the products by HPLC. The hydrolysis and transglycosylation reactions of bovine testicular hyaluronidase were dose-dependently inhibited by chondroitin sulfate oligosaccharides, but not by hyaluronan or chondroitin oligosaccharides. A kinetic analysis of the hydrolysis reaction using hyaluronan octasaccharide revealed that the inhibition mode by chondroitin sulfate oligosaccharides was competitive.

Heparan sulfate in angiogenesis: a target for therapy

Heparan sulfate (HS), a long linear polysaccharide of alternating disaccharide residues, interacts with a wide variety of proteins, including many angiogenic factors. The involvement of HS in signaling of pro-angiogenic factors (e.g. vascular endothelial growth factor and fibroblast growth factor 2), as well as interaction with anti-angiogenic factors (e.g. endostatin), warrants its role as an important modifier of (tumor) angiogenesis. This review summarizes our current understanding of the role of HS in angiogenic growth factor signaling, and discusses therapeutic strategies to target HS and modulate angiogenesis.

Hyaluronan-chondroitin hybrid oligosaccharides as new life science research tools

Hyaluronan and chondroitin are glycosaminoglycans well-known as components of pharmaceutical agents and health foods. From these attractive molecules, using transglycosylation reaction of testicular hyaluronidase, we synthesized hybrid neo-oligosaccharides not found in nature. We also found a new site between the chondroitin disaccharide unit and hyaluronan disaccharide unit recognized by a hyaluronan lyase specific to hyaluronan using these hybrid oligosaccharides as substrates. We hope that these hybrid oligosaccharides will help to elucidate the involvement of hyaluronan, chondroitin, and chondroitin sulfates in the mechanisms of cell functions and diseases, based on the structures of these glycosaminoglycans.

Highly sulfated nonreducing end-derived heparan sulfate domains bind fibroblast growth factor-2 with high affinity and are enriched in biologically active fractions

Human fibroblast growth factor-2 (FGF2) regulates cellular processes including proliferation, adhesion, motility, and angiogenesis. FGF2 exerts its biological function by binding and dimerizing its receptor (FGFR), which activates signal transduction cascades. Effective binding of FGF2 to its receptor requires the presence of heparan sulfate (HS), a linear polysaccharide with N-sulfated domains (NS) localized at the cell surface and extracellular matrix. HS acts as a platform facilitating the formation of a functional FGF-FGFR-HS ternary complex. Crystal structures of the signaling ternary complex revealed two conflicting architectures. In the asymmetrical model, two FGFs and two FGFRs bind a single HS chain. In contrast, the symmetrical model postulates that one FGF and one FGFR bind to the free end of the HS chain and dimerization require these ends to join, bringing the two half-complexes together. In this study, we screened a hexasaccharide HS library for compositions that are able to bind FGF2. The library was composed primarily of NS domains internal to the HS chain with minor presence of non-reducing end (NRE) NS. The binders were categorized into low versus high affinity binders. The low affinity fraction contained primarily hexasaccharides with low degree of sulfation that were internal to the HS chains. In contrast, the high affinity bound fraction was enriched in NRE oligosaccharides that were considerably more sulfated and had the ability to promote FGFR-mediated cell proliferation. The results suggest a role of the NRE of HS in FGF2 signaling and favor the formation of the symmetrical architecture on short NS domains.

Functions of chondroitin sulfate and heparan sulfate in the developing brain

Chondroitin sulfate and heparan sulfate proteoglycans are major components of the cell surface and extracellular matrix in the brain. Both chondroitin sulfate and heparan sulfate are unbranched highly sulfated polysaccharides composed of repeating disaccharide units of glucuronic acid and N-acetylgalactosamine, and glucuronic acid and N-acetylglucosamine, respectively. During their biosynthesis in the Golgi apparatus, these glycosaminoglycans are highly modified by sulfation and C5 epimerization of glucuronic acid, leading to diverse heterogeneity in structure. Their structures are strictly regulated in a cell type-specific manner during development partly by the expression control of various glycosaminoglycan-modifying enzymes. It has been considered that specific combinations of glycosaminoglycan-modifying enzymes generate specific functional microdomains in the glycosaminoglycan chains, which bind selectively with various growth factors, morphogens, axon guidance molecules and extracellular matrix proteins. Recent studies have begun to reveal that the molecular interactions mediated by such glycosaminoglycan microdomains play critical roles in the various signaling pathways essential for the development of the brain.

Screening for anticoagulant heparan sulfate octasaccharides and fine structure characterization using tandem mass spectrometry

Heparan sulfate (HS) is a sulfated glycosaminoglycan located on the surface and extracellular matrix of mammalian cells. HS is constituted of highly N-sulfated domains (NS domains) interrupted by lower sulfation domains. The arrangement of these domains dictates the function of HS which is mainly involved in binding proteins and regulating their biological activities. Heparin, a heparan sulfate analogue present in mast cells, resembles the NS domains of HS but lacks the alternating high and low sulfation architecture. Because the NS domains that range up to hexadecasaccharide in size are the main protein binders, heparin has been used as a model for HS in protein binding studies. Heparan sulfate, however, is the more physiologically relevant modulator of growth factor-receptor interactions. In this work, liquid chromatography and mass spectrometry (LC-MS) were used to compare the compositions of affinity-purified heparin and HS octasaccharides with anticoagulant activities versus library octasaccharides. The fine structures of the biologically active HS compositions were then compared against those of library octasaccharides using low-energy collision-induced dissociation tandem mass spectrometry. This approach confirmed isomeric enrichment of these compositions and, most importantly, produces ions diagnostic of their biological activity.

Mice deficient in heparan sulfate 6-O-sulfotransferase-1

Heparan sulfate chains are initially synthesized on core proteins as linear polysaccharides composed of glucuronic acid-N-acetylglucosamine repeating units and subjected to marked structural modification by sulfation at various places and epimerization of hexuronic acid residues (C5-epimerase) at the Golgi lumen and further by 6-O-desulfation at the cell surface, which generates their characteristic divergent fine structures. This chapter focuses on the biological and physiological functions of 6-O-sulfation in HS and the characterization of the enzymes catalyzing 6-O-sulfation (HS6ST). HS6STs in mammals such as humans and mice comprise of three isoforms (HS6ST-1, -2, and -3) and one alternatively spliced form of HS6ST-2 (HS6ST-2S). Each of these isoforms has distinct substrate preferences, albeit overlapping each other. These HS6ST isoforms are expressed in a spatiotemporally regulated manner in most organs. HS6ST-1-deficient mice are lethal mostly at later embryonic stages and exhibit abnormal angiogenesis in labyrinthine zone of placenta and aberrant lung morphology similar to pulmonary emphysema. These knockout mice also exhibit retinal axon guidance abnormality at the optic chiasm. Other HS6ST-deficient animals reveal various malformations in muscle development and branching morphology of the caudal vein of zebrafish, in tracheal formation of Drosophila, and in axon guidance of ventral nerve cord interneurons of Caenorhabditis elegans. Mouse embryonic fibroblasts prepared from HS6ST-1/HS6ST-2 double knockout mice did produce HS lacking 6-O-sulfation and responded differently to various FGFs dependent signaling.

Multifunctionality of extracellular and cell surface heparan sulfate proteoglycans

Heparan sulfate proteoglycans are a remarkably diverse family of glycosaminoglycan-bearing protein cores that include the syndecans, the glypicans, perlecan, agrin, and collagen XVIII. Members of this protein class play key roles during normal processes that occur during development, tissue morphogenesis, and wound healing. As key components of basement membranes in organs and tissues, they also participate in selective filtration of biological fluids, in establishing cellular barriers, and in modulation of angiogenesis. The ability to perform these functions is provided both by the features of the protein cores as well as by the unique properties of heparan sulfate, which is assembled as a polymer of N-acetylglucosamine and glucuronic acid and modified by specific enzymes to generate specialized biologically active structures. This article discusses the structures and functions of this amazing family of proteoglycans and provides a platform for further study of the individual members.

Specific inhibition of FGF-2 signaling with 2-O-sulfated octasaccharides of heparan sulfate

In fibroblast growth factor (FGF)-2 signaling, the formation of a ternary complex of FGF-2, tyrosine-kinase fibroblast growth factor receptor (FGFR)-1, and cell surface heparan sulfate (HS) proteoglycan is known to be critical for the activation of FGFR-1 and downstream signal transduction. Exogenous heparin polymer and some octasaccharides inhibited FGF-2-induced phosphorylation both of FGFR-1 and of extracellular signal-regulated kinase (ERK1/2) in Chinese hamster ovary (CHO)-K1 cells transfected with FGFR-1, which present HS on their cell surface. The inhibitory effect of octasaccharide was dependent on the number of 2-O-sulfate groups within a molecule but independent of the number of 6-O-sulfate groups. Sulfation at the 2-O-position was a prerequisite not only for the binding of HS to FGF-2 but also for regulation of FGF-2 signaling and competitive inhibition with endogenous HS. Interestingly, FGF-4-induced phosphorylation was impeded only by specific octasaccharides containing both 2-O- and 6-O-sulfated groups, which were necessary for binding FGF-4. In CHO-677 cells deficient in HS biosynthesis, heparin enhanced FGF-2-induced phosphorylation of ERK1/2. On the other hand, an FGF-2-binding octasaccharide inhibited the phosphorylation. Our data suggest that the activity of particular heparin-binding factors can be inhibited by distinctive oligosaccharides that can bind the factors but cannot form functional signaling complexes irrespective of whether cells have a normal complement of HS or lack HS.

Dysplastic histogenesis of cartilage growth plate by alteration of sulphation pathway: a transgenic model

Mutations in the diastrophic dysplasia sulphate transporter (dtdst) gene causes different forms of chondrodysplasia in the human. The generation of a knock-in mouse strain with a mutation in dtdst gene provides the basis to study developmental dynamics in the epiphyseal growth plate and long bone growth after impairment of the sulphate pathway. Our microscopical and histochemical data demonstrate that dtdst gene impairment deeply affects tissue organization, matrix structure, and cell differentiation in the epiphyseal growth plate. In mutant animals, the height of the growth plate was significantly reduced, according to a concomitant decrease in cell density and proliferation. Although the pathway of chondrocyte differentiation seemed complete, alteration in cell morphology compared to normal counterparts was detected. In the extracellular matrix, it we observed a dramatic decrease in sulphated proteoglycans, alterations in the organization of type II and type X collagen fibers, and premature onset of mineralization. These data confirm the crucial role of sulphate pathway in proteoglycan biochemistry and suggest that a disarrangement of the extracellular matrix may be responsible for the development of dtdts cartilage dysplasia. Moreover, we corroborated the concept that proteoglycans not only are structural components of the cartilage architecture, but also play a dynamic role in the regulation of chondrocyte growth and differentiation.

6-O-sulfation of heparan sulfate differentially regulates various fibroblast growth factor-dependent signalings in culture

Heparan sulfate (HS) interacts with diverse heparin-binding growth factors and thereby regulates their bioactivities. These interactions depend on the structures characterized by the sulfation pattern and isomer of uronic acid residues. One of the biosynthetic modifications of HS, namely 6-O-sulfation, is catalyzed by three isoforms of HS6-O-sulfotransferase. We generated HS6ST-1- and/or HS6ST-2-deficient mice (6ST1-KO, 6ST2-KO, and double knock-out (dKO)) that exhibited different phenotypes. We examined the effects of HS 6-O-sulfation in heparin-binding growth factor signaling using fibroblasts derived from these mutant mice. Mouse embryonic fibroblasts (MEF) prepared from E14.5 dKO mice produced HS with little 6-O-sulfate, whereas 2-O-sulfation in HS from dKO-MEF (dKO-HS) was increased by 1.9-fold. HS6-O-sulfotransferase activity in the dKO-MEF was hardly detected, and HS2-O-sulfotransferase activity was 1.5-fold higher than that in wild type (WT)-MEFs. The response of dKO-MEFs to fibroblast growth factors (FGFs) was distinct from that of WT-MEFs; in dKO-MEFs, FGF-4- and FGF-2-dependent signalings were reduced to approximately 30 and 60% of WT-MEFs, respectively, and FGF-1-dependent signaling was moderately reduced compared with that of WT-MEFs but only at the lower FGF-1 concentrations. Analysis with a surface plasmon resonance biosensor demonstrated that the apparent affinity of dKO-HS for FGF-4 was markedly reduced and was also reduced for FGF-1. In contrast, the affinity of dKO-HS for FGF-2 was 2.5-fold higher than that of HS from WT-MEFs. Thus, 6-O-sulfate in HS may regulate the signalings of some of HB-GFs, including FGFs, by inducing different interactions between ligands and their receptors.

Diaphragm adaptations in patients with COPD

Inspiratory muscle weakness in patients with COPD is of major clinical relevance. For instance, maximum inspiratory pressure generation is an independent determinant of survival in severe COPD. Traditionally, inspiratory muscle weakness has been ascribed to hyperinflation-induced diaphragm shortening. However, more recently, invasive evaluation of diaphragm contractile function, structure, and biochemistry demonstrated that cellular and molecular alterations occur, of which several can be considered pathologic of nature. Whereas the fiber type shift towards oxidative type I fibers in COPD diaphragm is regarded beneficial, rendering the overloaded diaphragm more resistant to fatigue, the reduction of diaphragm fiber force generation in vitro likely contributes to diaphragm weakness. The reduced diaphragm force generation at single fiber level is associated with loss of myosin content in these fibers. Moreover, the diaphragm in COPD is exposed to oxidative stress and sarcomeric injury. This review postulates that the oxidative stress and sarcomeric injury activate proteolytic machinery, leading to contractile protein wasting and, consequently, loss of force generating capacity of diaphragm fibers in patients with COPD. Interestingly, several of these presumed pathologic alterations are already present early in the course of the disease (GOLD I/II), although these patients appear not limited in their daily life activities. Treatment of diaphragm dysfunction in COPD is complex since its etiology is unclear, but recent findings indicate the ubiquitin-proteasome pathway as a prime target to attenuate diaphragm wasting in COPD.

Essential role of heparan sulfate 2-O-sulfotransferase in chick limb bud patterning and development

The interactions of heparan sulfate (HS) with heparin-binding growth factors, such as fibroblast growth factors (FGFs), depend greatly on the chain structures. O-Sulfations at various positions on the chain are major factors determining HS structure; therefore, O-sulfation patterns may play a crucial role in controlling the developmental and morphogenetic processes of various tissues and organs by spatiotemporally regulating the activities of heparin-binding growth factors. In a previous study, we found that HS-2-O-sulfotransferase is strongly expressed throughout the mesoderm of chick limb buds during the early stages of development. Here we show that inhibition of HS-2-O-sulfotransferase in the prospective limb region by small inhibitory RNA resulted in the truncation of limb buds and reduced Fgf-8 expression in the apical ectodermal ridge. The treatment also reduced Fgf-10 expression in the mesenchyme. Moreover 2-O-sulfated HS, normally abundant in the basement membranes and mesoderm under ectoderm in limb buds, was significantly reduced in the treated buds. Phosphorylation levels of ERK and Akt were up-regulated in such truncated buds. Thus, we have shown for the first time that 2-O-sulfation of HS is essential for the FGF signaling required for limb bud development and outgrowth.

Regulation of Heparan Sulfate 6-O-Sulfation by β-Secretase Activity

The enzymes involved in glycosaminoglycan chain biosynthesis are mostly Golgi resident proteins, but some are secreted extracellularly. For example, the activities of heparan sulfate 6-O-sulfotransferase (HS6ST) and heparan sulfate 3-O-sulfotransferase are detected in the serum as well in the medium of cell lines. However, the biological significance of this is largely unknown. Here we have investigated by means of monitoring green fluorescent protein (GFP) fluorescence how C-terminally GFP-tagged HS6STs that are stably expressed in CHO-K1 cell lines are secreted/shed. Brefeldin A and monensin treatments revealed that the N-terminal hydrophobic domain of HS6ST3 is processed in the endoplasmic reticulum or cis/medial Golgi. Treatment of HS6ST3-GFP-expressing cells with various protease inhibitors revealed that the cell-permeable beta-secretase inhibitor N-benzyloxycarbonyl-Val-Leu-leucinal (Z-VLL-CHO) specifically inhibits HS6ST secretion, although this effect was specific for HS6ST3 but not for HS6ST1 and HS6ST2. However, Z-VLL-CHO treatment did not increase the molecular size of the HS6ST3-GFP that accumulated in the cell. Z-VLL-CHO treatment also induced the intracellular accumulation of SP-HS6ST3(-TMD)-GFP, a modified secretory form of HS6ST3 that has the preprotrypsin leader sequence as its N-terminal hydrophobic domain. Diminishment of beta-secretase activity by coexpressing the amyloid precursor protein of a Swedish mutant, a potent beta-secretase substrate, also induced intracellular HS6ST3-GFP accumulation. Moreover, Z-VLL-CHO treatment increased the 6-O-sulfate (6S) levels of HS, especially in the disaccharide unit of hexuronic acid-GlcNS(6S). Thus, the HS6ST3 enzyme in the Golgi apparatus and therefore the 6-O sulfation of heparan sulfates in the cell are at least partly regulated by beta-secretase via an indirect mechanism.

Novel heparan sulfate structures revealed by monoclonal antibodies

The sulfated glycosaminoglycan heparan sulfate (HS) is found ubiquitously on cell surfaces, in the extracellular matrix, and intracellularly as HS proteoglycans. Because of the structural heterogeneity of HS, tissue-derived HS preparations represent a mixture of HS chains originating from different cell types and tissue loci. Monoclonal anti-HS antibodies have been employed to detect the localization of specific HS epitopes in tissues, but limited information has been available on the saccharide structures recognized by the antibodies. We have studied the saccharide epitope structures of four anti-HS antibodies, HepSS1, JM13, JM403, and 10E4, which all recognize distinct HS species as demonstrated by different patterns of immunoreactivity upon staining of embryonic rat and adult human tissues. The epitopes recognized by JM13 and HepSS1 were found almost exclusively in basement membrane HS, whereas JM403 and 10E4 reacted also with cell-associated HS species. The binding of HepSS1, JM403, and 10E4 to HS was dependent on the GlcN N-substitution of the polysaccharide rather than O-sulfation. HepSS1 thus interacted with N-sulfated HS domains, JM403 binding was critically dependent on N-unsubstituted GlcN residues, and 10E4 bound to "mixed" HS domains containing both N-acetylated and N-sulfated disaccharide units. By contrast, JM13 binding seemed to require the presence of 2-O-sulfated glucuronic acid residues.

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.

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

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

Characterization of growth factor-binding structures in heparin/heparan sulfate using an octasaccharide library

Heparan sulfate (HS) chains interact with various growth and differentiation factors and morphogens, and the most interactions occur on the specific regions of the chains with certain monosaccharide sequences and sulfation patterns. Here we generated a library of octasaccharides by semienzymatic methods by using recombinant HS 2-O-sulfotransferase and HS 6-O-sulfotransferase, and we have made a systematic investigation of the specific binding structures for various heparin-binding growth factors. An octasaccharide (Octa-I, DeltaHexA-GlcNSO(3)-(HexA-GlcNSO(3))(3)) was prepared by partial heparitinase digestion from completely desulfated N-resulfated heparin. 2-O- and 6-O-sulfated Octa-I were prepared by enzymatically transferring one to three 2-O-sulfate groups and one to three 6-O-sulfate groups per molecule, respectively, to Octa-I. Another octasaccharide containing 3 units of HexA(2SO(4))-GlcNSO(3)(6SO(4)) was prepared also from heparin. This octasaccharide library was subjected to affinity chromatography for interactions with fibroblast growth factor (FGF)-2, -4, -7, -8, -10, and -18, hepatocyte growth factor, bone morphogenetic protein 6, and vascular endothelial growth factor, respectively. Based upon differences in the affinity to those octasaccharides, the growth factors could be classified roughly into five groups: group 1 needed 2-O-sulfate but not 6-O-sulfate (FGF-2); group 2 needed 6-O-sulfate but not 2-O-sulfate (FGF-10); group 3 had the affinity to both 2-O-sulfate and 6-O-sulfate but preferred 2-O-sulfate (FGF-18, hepatocyte growth factor); group 4 required both 2-O-sulfate and 6-O-sulfate (FGF-4, FGF-7); and group 5 hardly bound to any octasaccharides (FGF-8, bone morphogenetic protein 6, and vascular endothelial growth factor). The approach using the oligosaccharide library may be useful to define specific structures required for binding to various heparin-binding proteins. Octasaccharides with the high affinity to FGF-2 and FGF-10 had the activity to release them, respectively, from their complexes with HS. Thus, the library may provide new reagents to specifically regulate bindings of the growth factors to HS.

Characterization of a heparan sulfate 3-O-sulfotransferase-5, an enzyme synthesizing a tetrasulfated disaccharide

Heparan sulfate d-glucosaminyl 3-O-sulfotransferases (3-OSTs) catalyze the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to position 3 of the glucosamine residue of heparan sulfate and heparin. A sixth member of the human 3-OST family, named 3-OST-5, was recently reported (Xia, G., Chen, J., Tiwari, V., Ju, W., Li, J.-P., Malmstrom, A., Shukla, D., and Liu, J. (2002) J. Biol. Chem. 277, 37912-37919). In the present study, we cloned putative catalytic domain of the human 3-OST-5 and expressed it in insect cells as a soluble enzyme. Recombinant 3-OST-5 only exhibited sulfotransferase activity toward heparan sulfate and heparin. When incubated heparan sulfate with [35S]PAPS, the highest incorporation of35S was observed, and digestion of the product with a mixture of heparin lyases yielded two major35S-labeled disaccharides, which were determined as DeltaHexA-GlcN(NS,3S,6S) and DeltaHexA(2S)-GlcN(NS,3S) by further digestion with 2-sulfatase and degradation with mercuric acetate. However, when used heparin as acceptor, we identified a highly sulfated disaccharide unit as a major product. This had a structure of DeltaHexA(2S)-GlcN(NS,3S,6S). Quantitative real-time PCR analysis revealed that 3-OST-5 was highly expressed in fetal brain, followed by adult brain and spinal cord, and at very low or undetectable levels in the other tissues. Finally, we detected a tetrasulfated disaccharide unit in bovine intestinal heparan sulfate. To our knowledge, this is the first report to describe not only the natural occurrence of tetrasulfated disaccharide unit but also the enzymatic formation of this novel structure.

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

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

Heparan sulfate fine structure and specificity of proteoglycan functions

Heparan sulfate chains have markedly heterogeneous structures in which distinct patterns of sulfation determine the binding specificity for ligand proteins. These "fine structures" of heparan sulfate are mainly produced by the regulated introduction of sulfate groups at the N-, 2-O-, 6-O-, and 3-O-positions of the sugar chain. Recent biochemical, histochemical, and genetic studies have demonstrated that different fine structures mediate distinct molecular recognition events to regulate a variety of cellular functions. In this review, we focus on the molecular basis of growth factor control by the sulfation status of heparan sulfate.

Heparin/Heparan sulfate domains in binding and signaling of fibroblast growth factor 8b

The role of heparin and heparan sulfate in the binding and signaling of fibroblast growth factors (FGFs) has been subject to intense investigation, but the studies have largely been confined to two species (FGF1 and FGF2) of the family with approximately 20 members. We have investigated the structural requirements for heparin/heparan sulfate in binding and activation of FGF8 (splice variant b). We present evidence that the minimal FGF8b-binding saccharide domain encompasses 5-7 monosaccharide units. The N-, 2-O-, and 6-O-sulfate substituents of heparin/heparan sulfate (HS) are all involved in the interaction, preferentially in the form of trisulfated IdoUA(2-OSO(3))-GlcNSO(3)(6-OSO(3)) disaccharide constituents. These structural characteristics resemble those described earlier for FGF1. By contrast, the saccharide structures required for the biological activity of FGF8b differed significantly from those characteristic for FGF1 and FGF2. Experiments with cells lacking active HS indicated that extended >/=14-mer heparin domains were needed to enhance cell proliferation and Erk phosphorylation by FGF8b, whereas in cells stimulated with FGF1 or FGF2 the corresponding responses were achieved by much shorter, 6-8-mer, oligosaccharides. Furthermore, still longer domains were needed to activate FGF8b in cells with "non-optimal" FGF receptor expression. Collectively, our data suggest that the heparin/HS structures enhancing the biological activity of FGFs were influenced by the FGF species involved as well as by the cellular composition of FGF receptors.

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

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

Crystal structures of NK1-heparin complexes reveal the basis for NK1 activity and enable engineering of potent agonists of the MET receptor

NK1 is a splice variant of the polypeptide growth factor HGF/SF, which consists of the N-terminal (N) and first kringle (K) domain and requires heparan sulfate or soluble heparin for activity. We describe two X-ray crystal structures of NK1-heparin complexes that define a heparin-binding site in the N domain, in which a major role is played by R73, with further contributions from main chain atoms of T61, K63 and G79 and the side chains of K60, T61, R76, K62 and K58. Mutagenesis experiments demonstrate that heparin binding to this site is essential for dimerization in solution and biological activity of NK1. Heparin also comes into contact with a patch of positively charged residues (K132, R134, K170 and R181) in the K domain. Mutation of these residues yields NK1 variants with increased biological activity. Thus, we uncover a complex role for heparan sulfate in which binding to the primary site in the N domain is essential for biological activity whereas binding to the K domain reduces activity. We exploit the interaction between heparin and the K domain site in order to engineer NK1 as a potent receptor agonist and suggest that dual (positive and negative) control may be a general mechanism of heparan sulfate-dependent regulation of growth factor activity.

Regulation of renal proximal tubular epithelial cell fibroblast growth factor-2 generation by heparin

Progression of renal disease is closely correlated to the degree of renal interstitial fibrosis, and evidence is increasing that epithelial cells of the renal proximal tubule (PTCs) may contribute to its pathogenesis. Such cytokines as basic fibroblast growth factor (FGF-2) have been implicated in progressive renal injury, and we previously showed that PTCs are a source of this cytokine. FGF-2 is characterized by its high affinity for heparin, and numerous studies have suggested that heparin may modify the progression of renal disease. The current study examined how heparin influenced FGF-2 generation and bioactivity in the human renal epithelial PTC line, HK-2. Incubation of HK-2 cells with heparin led to a dose- and time-dependent increase in FGF-2 concentration in the culture supernatant that was not accompanied by alterations in FGF-2 messenger RNA expression, assessed by reverse-transcriptase polymerase chain reaction and Northern analysis. The heparin-induced increase in FGF-2 concentration was accompanied by a decrease in the amount of FGF-2 bound to the extracellular matrix, although this accounted for only a small proportion of the total FGF-2 generated. Induction of FGF-2 by 2-O-desulfated heparin, together with a reduction in total cell-associated FGF-2 and anti-FGF-2 antibody binding to fixed permeabilized cells after the addition of heparin, suggested that the FGF-2 released was mainly derived from a preformed intracellular source. That FGF-2 was predominantly derived from an intracellular pool was also confirmed by pulse chase experiments. The addition of heparin resulted in the generation of bioinactive FGF-2, judged by in vitro fibroblast proliferation. Conversely, heparitinase treatment of supernatant samples from heparin-treated cells and the addition of 2-O-desulfated heparin resulted in the generation of active FGF-2, suggesting that the generation of bioinactive FGF-2 was related to binding of FGF-2 by extracellular heparin after its release from cells. These data show that heparin depletes both the cell and surrounding matrix of FGF-2 and suggest that FGF-2 released from cells was mainly derived from a preformed intracellular source. Furthermore, FGF-2 released from epithelial PTCs after the application of heparin was bioinactive. This likely resulted from released FGF-2 binding to an excess of extracellular heparin. Results presented here therefore suggest a mechanism by which heparin, through its effect on depletion of matrix and cells of FGF-2 and its generation in an inactive form, may influence progressive renal interstitial fibrosis.

Sequence analysis of heparan sulfate epitopes with graded affinities for fibroblast growth factors 1 and 2

Proteins that belong to the fibroblast growth factor (FGF) family regulate proliferation, migration, and differentiation of many cell types. Several FGFs, including the prototype factors FGF-1 and FGF-2, depend on interactions with heparan sulfate (HS) proteoglycans for activity. We have assessed tissue-derived HS fragments for binding to FGF-1 and FGF-2 to identify the authentic saccharide motifs required for interactions. Sequence information on a range of N-sulfated HS octasaccharides spanning from low to high affinity for FGF-1 was obtained. All octasaccharides with high affinity for FGF-1 (> or =0.5 m NaCl required for elution) contained an internal IdoUA(2-OSO(3))-GlcNSO(3)(6-OSO(3))-IdoUA(2-OSO(3))-trisaccharide motif. Octasaccharides with a higher overall degree of sulfation but lacking the specific trisaccharide motif showed lower affinity for FGF-1. FGF-2 was shown to bind to a mono-O-sulfated HS 6-mer carrying a single internal IdoUA(2-OSO(3))-unit. However, a di-O-sulfated -IdoUA(2-OSO(3))-GlcNSO(3)-IdoUA(2-OSO(3))-trisaccharide sequence within a HS 8-mer gave stronger binding. These findings show that not only the number but also the positions of individual sulfate groups determine affinity of HS for FGFs. Our findings support the notion that FGF-dependent processes can be modulated in vivo by regulated expression of distinct HS sequences.

Drosophila heparan sulfate 6-O-sulfotransferase (dHS6ST) gene. Structure, expression, and function in the formation of the tracheal system

Heparan sulfate, one of the most abundant components of the cell surface and the extracellular matrix, is involved in a variety of biological processes such as growth factor signaling, cell adhesion, and enzymatic catalysis. The heparan sulfate chains have markedly heterogeneous structures in which distinct sequences of sulfate groups determine specific binding properties. Sulfation at each different position of heparan sulfate is catalyzed by distinct enzymes, sulfotransferases. In this study, we identified and characterized Drosophila heparan sulfate 6-O-sulfotransferase (dHS6ST). The deduced primary structure of dHS6ST exhibited several common features found in those of mammalian HS6STs. We confirmed that, when the protein encoded by the cDNA was expressed in COS-7 cells, it showed HS6ST activity. Whole mount in situ hybridization revealed highly specific expression of dHS6ST mRNA in embryonic tracheal cells. The spatial and temporal pattern of dHS6ST expression in these cells clearly resembles that of the Drosophila fibroblast growth factor (FGF) receptor, breathless (btl). RNA interference experiments demonstrated that reduced dHS6ST activity caused embryonic lethality and disruption of the primary branching of the tracheal system. These phenotypes were reminiscent of the defects observed in mutants of FGF signaling components. We also show that FGF-dependent mitogen-activated protein kinase activation is significantly reduced in dHS6ST double-stranded RNA-injected embryos. These findings indicate that dHS6ST is required for tracheal development in Drosophila and suggest the evolutionally conserved roles of 6-O-sulfated heparan sulfate in FGF signaling.

Mapping a heparin binding site on ErbB-3 epidermal growth factor receptor

Signaling via the ErbB-family of receptors plays an important role in mammalian development and oncogenesis. Here we show that the ErbB-3 receptor, but not other members of this receptor family, binds to immobilized heparin and can be dissociated only at a high ionic strength comparable to that required for fibroblast growth factor receptors. Competition-binding analysis suggests that this interaction is specific and requires highly sulfated species of heparan sulfate. Primary sequence analysis of ErbB-3 identified a basic amino acid cluster (466)KHNRPRR(472) localized to the proximal, cysteine-rich extracellular ligand binding domain of the receptor, with charge density and distribution compatible with, but different to, known linear heparin binding motifs. Site-directed mutagenesis, replacing this sequence with the corresponding residues from ErbB-1, resulted in complete loss of heparin binding activity of the chimeric receptor. Finally, antibodies directed to the putative heparin binding peptide, efficiently bind the native receptor suggesting a novel target for blocking heparin mediated ErbB-3 interactions.