Interactions between heparan sulfate and proteins: the concept of specificity

Proteoglycan (PG) coreceptors carry heparan sulfate (HS) chains that mediate interactions with growth factors, morphogens, and receptors. Thus, PGs modulate fundamental processes such as cell survival, division, adhesion, migration, and differentiation. This review summarizes recent biochemical and genetic information that sheds new light on the nature of HS–protein binding. Unexpectedly, many interactions appear to depend more on the overall organization of HS domains than on their fine structure.

Glycosaminoglycan sulfation requirements for respiratory syncytial virus infection

Glycosaminoglycans (GAGs) on the surface of cultured cells are important in the first step of efficient respiratory syncytial virus (RSV) infection. We evaluated the importance of sulfation, the major biosynthetic modification of GAGs, using an improved recombinant green fluorescent protein-expressing RSV (rgRSV) to assay infection. Pretreatment of HEp-2 cells with 50 mM sodium chlorate, a selective inhibitor of sulfation, for 48 h prior to inoculation reduced the efficiency of rgRSV infection to 40%. Infection of a CHO mutant cell line deficient in N-sulfation was three times less efficient than infection of the parental CHO cell line, indicating that N-sulfation is important. In contrast, infection of a cell line deficient in 2-O-sulfation was as efficient as infection of the parental cell line, indicating that 2-O-sulfation is not required for RSV infection. Incubating RSV with the purified soluble heparin, the prototype GAG, before inoculation had previously been shown to neutralize its infectivity. Here we tested chemically modified heparin chains that lack their N-, C6-O-, or C2-O-sulfate groups. Only heparin chains lacking the N-sulfate group lost the ability to neutralize infection, confirming that N-sulfation, but not C6-O- or C2-O-sulfation, is important for RSV infection. Analysis of heparin fragments identified the 10-saccharide chain as the minimum size that can neutralize RSV infectivity. Taken together, these results show that, while sulfate modification is important for the ability of GAGs to mediate RSV infection, only certain sulfate groups are required. This specificity indicates that the role of cell surface GAGs in RSV infection is not based on a simple charge interaction between the virus and sulfate groups but instead involves a specific GAG structural configuration that includes N-sulfate and a minimum of 10 saccharide subunits. These elements, in addition to iduronic acid demonstrated previously (L. K. Hallak, P. L. Collins, W. Knudson, and M. E. Peeples, Virology 271:264-275, 2000), partially define cell surface molecules important for RSV infection of cultured cells.

Defining the interleukin-8-binding domain of heparan sulfate

Interleukin-8, a member of the CXC chemokine family, has been shown to bind to glycosaminoglycans. It has been suggested that heparan sulfate on cell surfaces could provide specific ligand sites on endothelial cells to retain the highly diffusible inflammatory chemokine for presentation to leukocytes. By using selectively modified heparin and heparan sulfate fragments in a nitrocellulose filter trapping system, we have analyzed sequence requirements for interleukin-8 binding to heparin/heparan sulfate. We demonstrate that the affinity of a monomeric interleukin-8 molecule for heparin/heparan sulfate is too weak to allow binding at physiological ionic strength, whereas the dimeric form of the protein mediates binding to two sulfated domains of heparan sulfate. These domains, each an N-sulfated block of approximately 6 monosaccharide units, are contained within an approximately 22-24-mer sequence and are separated by a region of </=14 monosaccharide residues that may be fully N-acetylated. Binding to interleukin-8 correlates with the occurrence of the di-O-sulfated disaccharide unit -IdceA(2-OSO3)-GlcNSO3(6-OSO3)-. We suggest that the heparan sulfate sequence binds in horseshoe fashion over two antiparallel-oriented helical regions on the dimeric protein.

Heparan sulfate structure in mice with genetically modified heparan sulfate production

Using a high throughput heparan sulfate (HS) isolation and characterization protocol, we have analyzed HS structure in several tissues from mice/mouse embryos deficient in HS biosynthesis enzymes (N-deacetylase/N-sulfotransferase (NDST)-1, NDST-2, and C5-epimerase, respectively) and in mice lacking syndecan-1. The results have given us new information regarding HS biosynthesis with implications on the role of HS in embryonic development. Our main conclusions are as follows. 1) The HS content, disaccharide composition, and the overall degree of N- and O-sulfation as well as domain organization are characteristic for each individual mouse tissue. 2) Removal of a key biosynthesis enzyme (NDST-1 or C5-epimerase) results in similar structural alterations in all of the tissues analyzed. 3) Essentially no variation in HS tissue structure is detected when individuals of the same genotype are compared. 4) NDST-2, although generally expressed, does not contribute significantly to tissue-specific HS structures. 5) No change in HS structure could be detected in syndecan-1-deficient mice.

Differential roles for membrane-bound and soluble syndecan-1 (CD138) in breast cancer progression

The heparan sulfate proteoglycan syndecan-1 (Sdc1) modulates cell proliferation, adhesion, migration and angiogenesis. Proteinase-mediated shedding converts Sdc1 from a membrane-bound coreceptor into a soluble effector capable of binding the same ligands. In breast carcinomas, Sdc1 overexpression correlates with poor prognosis and an aggressive phenotype. To distinguish between the roles of membrane-bound and shed forms of Sdc1 in breast cancer progression, human MCF-7 breast cancer cells were stably transfected with plasmids overexpressing wild-type (WT), constitutively shed and uncleavable forms of Sdc1. Overexpression of WT Sdc1 increased cell proliferation, whereas overexpression of constitutively shed Sdc1 decreased proliferation. Fibroblast growth factor-2-mediated mitogen-activated protein kinase signaling was reduced following small-interfering RNA (siRNA)-mediated knockdown of Sdc1 expression. Constitutively, membrane-bound Sdc1 inhibited invasiveness, whereas soluble Sdc1 promoted invasion of MCF-7 cells into matrigel matrices. The latter effect was reversed by the matrix metalloproteinase inhibitors N-isobutyl-N-(4-methoxyphenylsufonyl) glycyl hydroxamic acid and tissue inhibitor of metalloproteinase (TIMP)-1. Affymetrix microarray analysis identified TIMP-1, Furin and urokinase-type plasminogen activator receptor as genes differentially regulated in soluble Sdc1-overexpressing cells. Endogenous TIMP-1 expression was reduced in cells overexpressing soluble Sdc1 and increased in those overexpressing the constitutively membrane-bound Sdc1. Moreover, E-cadherin protein expression was downregulated in cells overexpressing soluble Sdc1. Our results suggest that the soluble and membrane-bound forms of Sdc1 play different roles at different stages of breast cancer progression. Proteolytic conversion of Sdc1 from a membrane-bound into a soluble molecule marks a switch from a proliferative to an invasive phenotype, with implications for breast cancer diagnostics and potential glycosaminoglycan-based therapies.

Defective N-sulfation of heparan sulfate proteoglycans limits PDGF-BB binding and pericyte recruitment in vascular development

During vascular development, endothelial platelet-derived growth factor B (PDGF-B) is critical for pericyte recruitment. Deletion of the conserved C-terminal heparin-binding motif impairs PDGF-BB retention and pericyte recruitment in vivo, suggesting a potential role for heparan sulfate (HS) in PDGF-BB function during vascular development. We studied the participation of HS chains in pericyte recruitment using two mouse models with altered HS biosynthesis. Reduction of N-sulfation due to deficiency in N-deacetylase/N-sulfotransferase-1 attenuated PDGF-BB binding in vitro, and led to pericyte detachment and delayed pericyte migration in vivo. Reduced N-sulfation also impaired PDGF-BB signaling and directed cell migration, but not proliferation. In contrast, HS from glucuronyl C5-epimerase mutants, which is extensively N- and 6-O-sulfated, but lacks 2-O-sulfated L-iduronic acid residues, retained PDGF-BB in vitro, and pericyte recruitment in vivo was only transiently delayed. These observations were supported by in vitro characterization of the structural features in HS important for PDGF-BB binding. We conclude that pericyte recruitment requires HS with sufficiently extended and appropriately spaced N-sulfated domains to retain PDGF-BB and activate PDGF receptor beta (PDGFRbeta) signaling, whereas the detailed sequence of monosaccharide and sulfate residues does not appear to be important for this interaction.

Heparan sulfate: anchor for viral intruders?

Heparan sulfates (HS) are ubiquitous, polyanionic carbohydrate chains linked to core proteins in cell membranes and extracellular matrices of all eukaryotes. Due to the complex nature of the HS-biosynthesis, a wealth of different structures are produced. These seem to have a well defined distribution in different tissues and cells throughout development. Binding of endogenous proteins with different functional properties such as growth factors, adhesion molecules or enzymes, is one of the functions of HS. Besides interaction with endogenous factors, glycosaminoglycans (GAG) and especially HS have also been demonstrated to function as receptors for a number of different pathogens. What roles may HS play in the pathogenesis and tropism of different intruders like parasites or viruses? What implications does binding of viruses to HS have for the development of drugs or the application of viral vectors for gene targeting? In this review an attempt is made to collect our present knowledge on viral usage of HS and the implications that follow.

Heparan sulfate on endothelial cells mediates the binding of Plasmodium falciparum-infected erythrocytes via the DBL1alpha domain of PfEMP1

Plasmodium falciparum may cause severe forms of malaria when excessive sequestration of infected and uninfected erythrocytes occurs in vital organs. The capacity of wild-type isolates of P falciparum-infected erythrocytes (parasitized red blood cells [pRBCs]) to bind glycosaminoglycans (GAGs) such as heparin has been identified as a marker for severe disease. Here we report that pRBCs of the parasite FCR3S1.2 and wild-type clinical isolates from Uganda adhere to heparan sulfate (HS) on endothelial cells. Binding to human umbilical vein endothelial cells (HUVECs) and to human lung endothelial cells (HLECs) was found to be inhibited by HS/heparin or enzymes that remove HS from cell surfaces. (35)S-labeled HS extracted from HUVECs bound directly to the pRBCs' membrane. Using recombinant proteins corresponding to the different domains of P falciparum erythrocyte membrane protein 1 (PfEMP1), we identified Duffy-binding-like domain-1alpha (DBL1alpha) as the ligand for HS. DBL1alpha bound in an HS-dependent way to endothelial cells and blocked the adherence of pRBCs in a dose-dependent manner. (35)S-labeled HS bound to DBL1alpha-columns and eluted as a distinct peak at 0.4 mM NaCl. (35)S-labeled chondroitin sulfate (CS) of HUVECs did not bind to PfEMP1 or to the pRBCs' membrane. Adhesion of pRBCs of FCR3S1.2 to platelet endothelial cell adhesion molecule-1 (PECAM-1)/CD31, mediated by the cysteine-rich interdomain region 1alpha (CIDR1alpha), was found be operative with, but independent of, the binding to HS. HS and the previously identified HS-like GAG on uninfected erythrocytes may act as coreceptors in endothelial and erythrocyte binding of rosetting parasites, causing excessive sequestration of both pRBCs and RBCs.

Surface-exposed amino acid residues of HPV16 L1 protein mediating interaction with cell surface heparan sulfate

Efficient infection of cells by human papillomaviruses (HPVs) and pseudovirions requires primary interaction with cell surface proteoglycans with apparent preference for species carrying heparan sulfate (HS) side chains. To identify residues contributing to virus/cell interaction, we performed point mutational analysis of the HPV16 major capsid protein, L1, targeting surface-exposed amino acid residues. Replacement of lysine residues 278, 356, or 361 for alanine reduced cell binding and infectivity of pseudovirions. Various combinations of these amino acid exchanges further decreased cell attachment and infectivity with residual infectivity of less than 5% for the triple mutant, suggesting that these lysine residues cooperate in HS binding. Single, double, or triple exchanges for arginine did not impair infectivity, demonstrating that interaction is dependent on charge distribution rather than sequence-specific. The lysine residues are located within a pocket on the capsomere surface, which was previously proposed as the putative receptor binding site. Fab fragments of binding-neutralizing antibody H16.56E that recognize an epitope directly adjacent to lysine residues strongly reduced HS-mediated cell binding, further corroborating our findings. In contrast, mutation of basic surface residues located in the cleft between capsomeres outside this pocket did not significantly reduce interaction with HS or resulted in assembly-deficient proteins. Computer-simulated heparin docking suggested that all three lysine residues can form hydrogen bonds with 2-O-, 6-O-, and N-sulfate groups of a single HS molecule with a minimal saccharide domain length of eight monomer units. This prediction was experimentally confirmed in binding experiments using capsid protein, heparin molecules of defined length, and sulfate group modifications.

Structural requirement of heparan sulfate for interaction with herpes simplex virus type 1 virions and isolated glycoprotein C

Cell surface heparan sulfates mediate primary attachment of herpes simplex virus type 1, the first step in virus invasion of the cells. Removal of the host cell heparan sulfate results in a significantly diminished susceptibility of the cell to virus infection. On the virus envelope, glycoprotein C has been identified as the major binding site for heparan sulfate in the primary attachment of the virus to host cells. Using selectively desulfated heparins and metabolically labeled host cell heparan sulfate, we have analyzed the structural requirements of heparan sulfate to provide binding sites for glycoprotein C and the whole virus. Employing glycoprotein C affinity chromatography and a virus binding assay, we subfractionated oligosaccharides derived from heparan sulfate and partially desulfated heparin into selectively bound and unbound pools. These were chemically depolymerized and analyzed at the disaccharide level. The shortest glycoprotein C-binding fragment consisted of 10-12 monosaccharide units containing at least one 2-O- and one 6-O-sulfate group that have to be localized in a sequence-specific way, based on the finding that bound and unbound HS fragments do not differ in charge or composition. The binding sequence is found within N-sulfated blocks of heparan sulfate, although several N-acetyl groups can be tolerated within the minimal binding sequence. These minimal requirements for herpes simplex virus type 1 binding to heparan sulfate are clearly distinct from other identified protein binding sites.

Characterization of heparin and heparan sulfate domains binding to the long splice variant of platelet-derived growth factor A chain

Platelet-derived growth factors (PDGFs) are homo- or heterodimers of two related polypeptides, known as A and B chains. The A chain exists as two splice variants due to the alternative usage of exons 6 (PDGF-AL, longer) and 7 (PDGF-AS, shorter). Exon 6 encodes an 18-amino acid sequence rich in basic amino acid residues, which has been implicated as a cell retention signal. Several lines of evidence indicate that the retention is due to binding of PDGF-AL to glycosaminoglycans, especially to heparan sulfate. We have analyzed the saccharide domains of smooth muscle cell-derived heparan sulfate involved in this interaction. Furthermore, we have employed selectively modified heparin oligosaccharides to elucidate the dependence of the binding on different sulfate groups and on fragment length. The shortest PDGF-AL binding domain consists of 6-8 monosaccharide units. Studies using selectively desulfated heparins and heparin fragments suggest that N-, 2-O-, and 6-O-sulfate groups all contribute to the interaction. Structural comparison of heparan sulfate oligosaccharides separated by affinity chromatography on immobilized PDGF-AL showed that the bound pool was enriched in -IdceA(2-OSO3)-GlcNSO3(6-OSO3)- disaccharide units. Furthermore, analogous separation of a partially O-desulfated heparin decamer preparation, using a highly selective nitrocellulose filter-trapping system, yielded a PDGF-AL-bound fraction in which more than half of the disaccharide units had the structure -IdceA(2-OSO3)-GlcNSO3(6-OSO3)-. Our results suggest that the interaction between PDGF-AL and heparin/heparan sulfate is mediated via N-sulfated saccharide domains containing both 2-O- and 6-O-sulfate groups.

Heparin/heparan sulfate biosynthesis: processive formation of N-sulfated domains

Heparan sulfate (HS) proteoglycans influence embryonic development as well as adult physiology through interactions with various proteins, including growth factors/morphogens and their receptors. The interactions depend on HS structure, which is largely determined during biosynthesis by Golgi enzymes. A key step is the initial generation of N-sulfated domains, primary sites for further polymer modification and ultimately for functional interactions with protein ligands. Such domains, generated through action of a bifunctional GlcNAc N-deacetylase/N-sulfotransferase (NDST) on a [GlcUA-GlcNAc](n) substrate, are of variable size due to regulatory mechanisms that remain poorly understood. We have studied the action of recombinant NDSTs on the [GlcUA-GlcNAc](n) precursor in the presence and absence of the sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). In the absence of PAPS, NDST catalyzes limited and seemingly random N-deacetylation of GlcNAc residues. By contrast, access to PAPS shifts the NDST toward generation of extended N-sulfated domains that are formed through coupled N-deacetylation/N-sulfation in an apparent processive mode. Variations in N-substitution pattern could be obtained by varying PAPS concentration or by experimentally segregating the N-deacetylation and N-sulfation steps. We speculate that similar mechanisms may apply also to the regulation of HS biosynthesis in the living cell.

Multiple interactions of HIV-I Tat protein with size-defined heparin oligosaccharides

Tat protein, a transactivating factor of the human immunodeficiency virus type I, acts also as an extracellular molecule. Heparin affects the bioavailability and biological activity of extracellular Tat (Rusnati, M., Coltrini, D., Oreste, P., Zoppetti, G., Albini, A., Noonan, D., D'Adda di Fagagna, F., Giacca, M., and Presta, M. (1997) J. Biol. Chem. 272, 11313-11320). Here, a series of homogeneously sized, (3)H-labeled heparin fragments were evaluated for their capacity to bind to free glutathione S-transferase (GST)-Tat protein and to immobilized GST-Tat. Hexasaccharides represent the minimum sized heparin fragments able to interact with GST-Tat at physiological ionic strength. Also, the affinity of binding increases with increasing the molecular size of the oligosaccharides, with large fragments (>/=18 saccharides) approaching the affinity of full-size heparin. 6-Mer heparin binds GST-Tat with a dissociation constant (K(d)) equal to 0.7 +/- 0.4 microM and a molar oligosaccharide:GST-Tat ratio of about 1:1. Interaction of GST-Tat with 22-mer or full-size heparin is consistent instead with two-component binding. At subsaturating concentrations, a single molecule of heparin interacts with 4-6 molecules of GST-Tat with high affinity (K(d) values in the nanomolar range of concentration); at saturating concentrations, heparin binds GST-Tat with lower affinity (K(d) values in the micromolar range of concentration) and a molar oligosaccharide:GST-Tat ratio of about 1:1. In agreement with the binding data, a positive correlation exists between the size of heparin oligosaccharides and their capacity to inhibit cell internalization, long terminal repeat-transactivating activity of extracellular Tat in HL3T1 cells, and its mitogenic activity in murine adenocarcinoma T53 Tat-less cells. The data demonstrate that the modality of heparin-Tat interaction is strongly affected by the size of the saccharide chain. The possibility of establishing multiple interactions increases the affinity of large heparin fragments for Tat protein and the capacity of the glycosaminoglycan to modulate the biological activity of extracellular Tat.

3-O-sulfated oligosaccharide structures are recognized by anti-heparan sulfate antibody HS4C3

Antibodies against heparan sulfate (HS) are useful tools to study the structural diversity of HS. They demonstrate the large sequential variation within HS and show the distribution of HS oligosaccharide sequences within their natural environment. We analyzed the distribution and the structural characteristics of the oligosaccharide epitope recognized by anti-HS antibody HS4C3. Biosynthetic and synthetic heparin-related oligosaccharide libraries were used in affinity chromatography, immunoprecipitation, and enzyme-linked immunosorbent assay to identify this epitope as a 3-O-sulfated motif with antithrombin binding capacity. The antibody binds weakly to any N-sulfated, 2-O- and 6-O-sulfated hexa- to octasaccharide fragment but strongly to the corresponding oligosaccharide when there is a 3-O-sulfated glucosamine residue present in the sequence. This difference was highlighted by affinity interaction and immunohistochemistry at salt concentrations from 500 mm. At physiological salt conditions the antibody strongly recognized basal lamina of epithelia and endothelia. At 500 mm salt conditions, when 3-O sulfation is required for binding, antibody recognition was more restricted and selective. Antibody HS4C3 bound similar tissue structures as antithrombin in rat kidney. Furthermore, antithrombin and antibody HS4C3 could compete with one another for binding to heparin. Antibody HS4C3 was also able to inhibit the anti-coagulant activities of heparin and Arixtra as demonstrated using the activated partial thromboplastin time clotting and the anti-factor Xa assays. In summary, antibody HS4C3 selectively detects 3-O-sulfated HS structures and interferes with the coagulation activities of heparin by association with the anti-thrombin binding pentasaccharide sequence.

Characterization of a neutrophil cell surface glycosaminoglycan that mediates binding of platelet factor 4

Platelet factor 4 (PF-4) is a platelet-derived alpha-chemokine that binds to and activates human neutrophils to undergo specific functions like exocytosis or adhesion. PF-4 binding has been shown to be independent of interleukin-8 receptors and could be inhibited by soluble chondroitin sulfate type glycosaminoglycans or by pretreatment of cells with chondroitinase ABC. Here we present evidence that surface-expressed neutrophil glycosaminoglycans are of chondroitin sulfate type and that this species binds to the tetrameric form of PF-4. The glycosaminoglycans consist of a single type of chain with an average molecular mass of approximately 23 kDa and are composed of approximately 85-90% chondroitin 4-sulfate disaccharide units type CSA (-->4GlcAbeta1-->3GalNAc(4-O-sulfate)beta1-->) and of approximately 10-15% di-O-sulfated disaccharide units. A major part of these di-O-sulfated disaccharide units are CSE units (-->4GlcAbeta1-->3GalNAc(4,6-O-sulfate)beta1-->). Binding studies revealed that the interaction of chondroitin sulfate with PF-4 required at least 20 monosaccharide units for significant binding. The di-O-sulfated disaccharide units in neutrophil glycosaminoglycans clearly promoted the affinity to PF-4, which showed a Kd approximately 0.8 microM, as the affinities of bovine cartilage chondroitin sulfate A, porcine skin dermatan sulfate, or bovine cartilage chondroitin sulfate C, all consisting exclusively of monosulfated disaccharide units, were found to be 3-5-fold lower. Taken together, our data indicate that chondroitin sulfate chains function as physiologically relevant binding sites for PF-4 on neutrophils and that the affinity of these chains for PF-4 is controlled by their degree of sulfation.

Structural basis for the growth factor activity of human adenosine deaminase ADA2

Two distinct adenosine deaminases, ADA1 and ADA2, are found in humans. ADA1 has an important role in lymphocyte function and inherited mutations in ADA1 result in severe combined immunodeficiency. The recently isolated ADA2 belongs to the novel family of adenosine deaminase growth factors (ADGFs), which play an important role in tissue development. The crystal structures of ADA2 and ADA2 bound to a transition state analogue presented here reveal the structural basis of the catalytic/signaling activity of ADGF/ADA2 proteins. In addition to the catalytic domain, the structures discovered two ADGF/ADA2-specific domains of novel folds that mediate the protein dimerization and binding to the cell surface receptors. This complex architecture is in sharp contrast with that of monomeric single domain ADA1. An extensive glycosylation and the presence of a conserved disulfide bond and a signal peptide in ADA2 strongly suggest that ADA2, in contrast to ADA1, is specifically designed to act in the extracellular environment. The comparison of catalytic sites of ADA2 and ADA1 demonstrates large differences in the arrangement of the substrate-binding pockets. These structural differences explain the substrate and inhibitor specificity of adenosine deaminases and provide the basis for a rational design of ADA2-targeting drugs to modulate the immune system responses in pathophysiological conditions.

Release of sequestered malaria parasites upon injection of a glycosaminoglycan

Severe human malaria is attributable to an excessive sequestration of Plasmodium falciparum–infected and uninfected erythrocytes in vital organs. Strains of P. falciparum that form rosettes and employ heparan sulfate as a host receptor are associated with development of severe forms of malaria. Heparin, which is similar to heparan sulfate in that it is composed of the same building blocks, was previously used in the treatment of severe malaria, but it was discontinued due to the occurrence of serious side effects such as intracranial bleedings. Here we report to have depolymerized heparin by periodate treatment to generate novel glycans (dGAG) that lack anticoagulant-activity. The dGAGs disrupt rosettes, inhibit merozoite invasion of erythrocytes and endothelial binding of P. falciparum–infected erythrocytes in vitro, and reduce sequestration in in vivo models of severe malaria. An intravenous injection of dGAGs blocks up to 80% of infected erythrocytes from binding in the micro-vasculature of the rat and releases already sequestered parasites into circulation. P. falciparum–infected human erythrocytes that sequester in the non-human primate Macaca fascicularis were similarly found to be released in to the circulation upon a single injection of 500 μg of dGAG. We suggest dGAGs to be promising candidates for adjunct therapy in severe malaria.

Severe Plasmodium falciparum malaria is common and in part the result of an excessive binding of infected erythrocytes in the microvasculature. The parasite employs heparan sulfate during the adherence to the vascular endothelium and to erythrocytes. Heparin, which is related to heparan sulfate in that it is composed of the same building blocks, was here periodate-treated to generate depolymerized glycosaminoglycans (dGAGs) that possess no anticoagulant activity. The dGAGs disrupt erythrocyte and endothelial binding of P. falciparum–infected erythrocytes in vitro. An intravenous injection of dGAGs blocks infected erythrocytes from binding in the micro-vasculature of the rat and releases already sequestrated parasites into circulation both in the rat and in a non-human primate. If this approach is successfully translated to the clinical setting, it may offer help to patients whereby the injection of a dGAG releases already sequestered parasite-infected erythrocytes and re-establishes the micro-vascular blood flow. The authors suggest dGAGs to be promising candidates of adjunct therapy that may have an important impact on malaria mortality.

Structural basis of oligosaccharide receptor recognition by human papillomavirus

High risk human papillomavirus types 16 (HPV16) and 18 (HPV18) can cause cervical cancer. Efficient infection by HPV16 and HPV18 pseudovirions requires interactions of particles with cell-surface receptor heparan sulfate oligosaccharide. To understand the virus-receptor interactions for HPV infection, we determined the crystal structures of HPV16 and HPV18 capsids bound to the oligosaccharide receptor fragment using oligomeric heparin. The HPV-heparin structures revealed multiple binding sites for the highly negatively charged oligosaccharide fragment on the capsid surface, which is different from previously reported virus-receptor interactions in which a single type of binding pocket is present for a particular receptor. We performed structure-guided mutagenesis to generate mutant viruses, and cell binding and infectivity assays demonstrated the functional role of viral residues involved in heparin binding. These results provide a basis for understanding virus-heparan sulfate receptor interactions critical for HPV infection and for the potential development of inhibitors against HPV infection.

Role of heparan sulfate domain organization in endostatin inhibition of endothelial cell function

The anti-angiogenic activity of endostatin (ES) depends on interactions with heparan sulfate (HS). In the present study, intact HS chains of >/=15 kDa bound quantitatively to ES whereas N-sulfated HS decasaccharides, with affinity for several fibroblast growth factor (FGF) species, failed to bind. Instead, ES-binding oligosaccharides composed of mixed N-sulfated and N-acetylated disaccharide units were isolated from pig intestinal HS. A 10/12mer ES-binding epitope was identified, with two N-sulfated regions separated by at least one N-acetylated glucosamine unit (SAS-domain). Cleavage at the N-acetylation site disrupted ES binding. These findings point to interaction between discontinuous sulfated domains in HS and arginine clusters at the ES surface. The inhibitory effect of ES on vascular endothelial growth factor-induced endothelial cell migration was blocked by the ES-binding SAS-domains and by heparin oligosaccharides (12mers) similar in length to the ES-binding SAS-domains, but not by 6mers capable of FGF binding. We propose that SAS-domains modulate the biological activities of ES and other protein ligands with extended HS-binding sites. The results provide a rational explanation for the preferential interaction of ES with certain HS proteoglycan species.

Characterization of glycosaminoglycan (GAG) sulfatases from the human gut symbiont Bacteroides thetaiotaomicron reveals the first GAG-specific bacterial endosulfatase

Despite the importance of the microbiota in human physiology, the molecular bases that govern the interactions between these commensal bacteria and their host remain poorly understood. We recently reported that sulfatases play a key role in the adaptation of a major human commensal bacterium, Bacteroides thetaiotaomicron, to its host (Benjdia, A., Martens, E. C., Gordon, J. I., and Berteau, O. (2011) J. Biol. Chem. 286, 25973-25982). We hypothesized that sulfatases are instrumental for this bacterium, and related Bacteroides species, to metabolize highly sulfated glycans (i.e. mucins and glycosaminoglycans (GAGs)) and to colonize the intestinal mucosal layer. Based on our previous study, we investigated 10 sulfatase genes induced in the presence of host glycans. Biochemical characterization of these potential sulfatases allowed the identification of GAG-specific sulfatases selective for the type of saccharide residue and the attachment position of the sulfate group. Although some GAG-specific bacterial sulfatase activities have been described in the literature, we report here for the first time the identity and the biochemical characterization of four GAG-specific sulfatases. Furthermore, contrary to the current paradigm, we discovered that B. thetaiotaomicron possesses an authentic GAG endosulfatase that is active at the polymer level. This type of sulfatase is the first one to be identified in a bacterium. Our study thus demonstrates that bacteria have evolved more sophisticated and diverse GAG sulfatases than anticipated and establishes how B. thetaiotaomicron, and other major human commensal bacteria, can metabolize and potentially tailor complex host glycans.

Characterization of anti-heparan sulfate phage display antibodies AO4B08 and HS4E4

Heparan sulfates (HS) are linear carbohydrate chains, covalently attached to proteins, that occur on essentially all cell surfaces and in extracellular matrices. HS chains show extensive structural heterogeneity and are functionally important during embryogenesis and in homeostasis due to their interactions with various proteins. Phage display antibodies have been developed to probe HS structures, assess the availability of protein-binding sites, and monitor structural changes during development and disease. Here we have characterized two such antibodies, AO4B08 and HS4E4, previously noted for partly differential tissue staining. AO4B08 recognized both HS and heparin, and was found to interact with an ubiquitouys, N-, 2-O-, and 6-O-sulfated saccharide motif, including an internal 2-O-sulfate group. HS4E4 turned out to preferentially recognize low-sulfated HS motifs containing iduronic acid, and N-sulfated as well as N-acetylated glucosamine residues. Contrary to AO4B08, HS4E4 did not bind highly O-sulfated structures such as found in heparin.

Heparan sulfate-related oligosaccharides in ternary complex formation with fibroblast growth factors 1 and 2 and their receptors

Biosynthesis of heparan sulfate (HS) is strictly regulated to yield products with cell/tissue-specific composition. Interactions between HS and a variety of proteins, including growth factors and morphogens, are essential for embryonic development and for homeostasis in the adult. Fibroblast growth factors (FGFs) and their various receptors (FRs) form ternary complexes with HS, as required for receptor signaling. Libraries of HS-related, radiolabeled oligosaccharides were generated by chemo-enzymatic modification of heparin and tested for affinity to immobilized FR ectodomains in the presence of FGF1 or FGF2. Experiments were designed to enable assessment of N-sulfated 8- and 10-mers with defined numbers of iduronic acid 2-O-sulfate and glucosamine 6-O-sulfate groups. FGF1 and FGF2 were found to require similar oligosaccharides in complex formation with FR1c-3c, FGF2 affording somewhat more efficient oligosaccharide recruitment than FGF1. FR4, contrary to FR1c-3c, bound oligosaccharides at physiological ionic conditions even in the absence of FGFs, and this interaction was further promoted by FGF1 but not by FGF2. In all systems studied, the stability of FGF-oligosaccharide-FR complexes correlated with the overall level of saccharide O-sulfation rather than on the precise distribution of sulfate groups.