Calcium-dependent heparin-like ligands for L-selectin in nonlymphoid endothelial cells

Calcium-dependent oligonucleotide antagonists specific for L-selectin

The selectins are calcium-dependent C-type lectins that recognize complex anionic carbohydrate ligands, initiating many cell-cell interactions in the vascular system. Selectin blockade shows therapeutic promise in a variety of inflammatory and postischemic pathologies. However, the available oligosaccharide ligand mimetics have low affinities and show cross-reaction among the three selectins, precluding efficient and specific blockade. The SELEX (systematic evolution of ligands by exponential enrichment) process uses combinatorial chemistry and in vitro selection to yield high affinity oligonucleotides with unexpected binding specificities. Nuclease-stabilized randomized oligonucleotides subjected to SELEX against recombinant L-selectin yielded calcium-dependent antagonists with approximately 10(5) higher affinity than the conventional oligosaccharide ligand sialyl LewisX. Most of the isolated ligands shared a common consensus sequence. Unlike sialyl LewisX, these antagonists show little binding to E- or P-selectin. Moreover, they show calcium-dependent binding to native L-selectin on peripheral blood lymphocytes and block L-selectin-dependent interactions with the natural ligands on high endothelial venules.

13C-NMR relation study of heparin-disaccharide interactions with tripeptides GRG and GKG

Heparin is a polydisperse sulphated copolymer consisting mostly of 1-->4 linked glucosamine and uronic acid residues, i.e. 2-deoxy-2-sulphamido-D-glucopyranose 6-sulphate and L-idopyranosyluronic acid 2-sulphate. 13C NMR has been used to study the interactions of heparinase-derived and purified heparin disaccharide with N- and C-terminally-blocked tripeptides GRG and GKG. Titration of the disaccharide with peptide indicates that GRG binds the disaccharide more strongly than does GKG, with interactions in either case being stronger at uronate ring positions. In the presence of GRG, a carboxylate pKa depression suggests electrostatic interactions between the arginine guanidinium group and the uronate carboxylate group. 13C relaxation data have been acquired for all disaccharide and peptide carbons in the presence and absence of GRG and GKG. 13C relaxation rates for the disaccharide are significantly faster in the presence of peptide, especially with GRG. Analysis of these relaxation data has been done in terms of molecular diffusion constants, D [symbol: see text] and D parallel, and an angle alpha between D parallel and a molecular frame defined by the moment of inertia tensor calculated for an internally rigid disaccharide. Disaccharide conformational space in these calculations has been sampled for both uronate half-chair forms (2H1 and 1H2) and over a range of glycosidic bond angles defined by motional order parameters and inter-residue nuclear Overhauser effects (+/- 30 degree from the average). In the absence of peptide, the ratio D [symbol: see text] /D parallel falls between 0.4 and 0.7; therefore molecular diffusion occurs preferentially about D parallel, which runs through both disaccharide rings. In the presence of peptide, D [symbol: see text] /D parallel is decreased, indicating that GRG is oriented along D parallel and proximal to the uronic acid ring. A model for this is shown.

Anticoagulant Glycosaminoglycans Activate Respiratory Burst in Neutrophils and Monocytes

Using four different glycosaminoglycans (un fractionated heparin, low-molecular-weight heparin, pen tosan polysulfate, lactobionic acid) and the polypeptide hirudin, we investigated the influence of these anticoag ulant substances on both polymorphonuclear neutrophils and monocytes respiratory burst in vitro. Using real-time analysis, we demonstrated that both unfractionated and low-molecular-weight heparins, as well as pentosan polysulfate, trigger the respiratory burst in neutrophils and monocytes within 3 to 5 s at clinically relevant con centrations. These effects could be antagonized by pre- incubating the anticoagulant drugs with protamine. Scan ning electron microscopy confirmed an activated neutro phil phenotype after exposure to the anticoagulant compounds. Furthermore, we demonstrated that neither increased intracellular Ca2+ levels nor tyrosine phos phorylation of proteins, both mechanisms commonly in volved in the signal transduction in neutrophils, partici pate in the glycosaminoglycan-mediated activation of the respiratory burst. Our results indicate that defined anti coagulant substances can trigger the respiratory burst in vitro, which might also have clinical implications in cer tain diseases. Key Words: Glycosaminoglycans— Hirudin—Polymorphonuclear leukocytes—Monocytes— Respiratory burst—Flow cytometry.

Modulation of endothelial cell adhesion by hevin, an acidic protein associated with high endothelial venules

High endothelial venules (HEV) are specialized plump postcapillary venules in lymphoid tissues that support high levels of lymphocyte extravasation from the blood. We have recently identified a novel human transcript, expressed to high levels in HEV, that encodes a secreted, acidic protein closely related to the anti-adhesive extracellular matrix protein known as BM-40, osteonectin, and SPARC (secreted protein acidic and rich in cysteine). Here, we show that this protein, designated hevin, is associated with basal, lateral, and apical surfaces of HEV cells, and unlike MECA-79 antigen, is not expressed on the underlying basement membrane. In contrast to fibronectin or other adhesive extracellular matrix proteins, purified hevin does not support endothelial cell adhesion in vitro. Moreover, addition of soluble exogenous hevin inhibits attachment and spreading of endothelial cells on fibronectin substrates. Hevin-treated cells do not form focal adhesions and exhibit a rounded morphology. Together, these results suggest that hevin is an abundant extracellular protein that modulates high endothelial cell adhesion to the basement membrane.

Modulation of L-selectin ligand expression during an immune response accompanying tumorigenesis in transgenic mice

Immune surveillance depends on lymphocyte access to tissue. Lymphocytes emigrate from blood when adhesion receptors such as L-selectin and the alpha 4 beta 7 integrin on these cells bind to ligands expressed on venular endothelium. Among transgenic mouse lines expressing an oncoprotein (Tag) in islet beta cells, some recognize Tag as nonself. In these mice, Tag expression elicits both beta cell hyperplasia with subsequent progression to tumors and lymphocytic infiltration. Endothelial ligands for L-selectin and alpha 4 beta 7 were upregulated in infiltrated islets in these transgenic mice. These ligands were not expressed in tumors, which were devoid of lymphocytic infiltration. In contrast, the adhesion molecules PECAM-1, ICAM-1, and VCAM-1 were expressed on endothelium in both noninfiltrated tumors and infiltrated islets. Thus, upregulation of expression of endothelial ligands for L-selectin and alpha 4 beta 7 may contribute to autoimmune infiltration. Repression of expression of these same ligands may be involved in the failure of tumor immunity.

Presence of N-unsubstituted glucosamine units in native heparan sulfate revealed by a monoclonal antibody

Immunohistochemical application of antibodies against heparan sulfate proteoglycan core protein and heparitinase-digested heparan sulfate stubs showed the presence of heparan sulfate proteoglycan in all basement membranes of the rat kidney. However, a monoclonal antibody (JM-403) against native heparan sulfate (van den Born, J., van den Heuvel, L. P. W. J., Bakker, M. A. H., Veerkamp, J. H., Assmann, K. J. M., and Berden, J. H. M. (1992) Kidney Int. 41, 115-123) largely failed to stain tubular basement membranes, suggesting the presence of heparan sulfate chains lacking the specific JM-403 epitope. Heparan sulfate preparations from various sources differed markedly with regard to JM-403 binding, as demonstrated by liquid phase inhibition in enzyme-linked immunosorbent assay, the interaction decreasing with increasing sulfate contents of the polysaccharide. Mapping of the JM-403 epitope indicated that it was dominated by one or more N-unsubstituted glucosamine unit(s), since treatments that destroyed or altered the structure of such units in heparan sulfate preparations (cleavage at N-unsubstituted glucosamine units with HNO2 at pH 3.9 and N-acetylation with acetic anhydride, respectively), abolished antibody binding. Conversely, immunoreactivity could be induced in a (D-glucuronyl-1,4-N-acetyl-D-glucosaminyl-1,4) polysaccharide by the generation of N-unsubstituted glucosamine N-unsubstituted glucosamine in a JM-403-binding heparan sulfate (preparation HS-II from human aorta) was demonstrated by an approximately 3-fold reduction in molecular size following HNO2 (pH 3.9) treatment. Further characterization of the epitope recognized by JM-403, based on enzyme-linked immunosorbent assay inhibition tests with chemically/enzymatically modified polysaccharides, indicated that one or more N-sulfated glucosamine units are invariable present, whereas L-iduronic acid and O-sulfate residues appear to inhibit JM-403 reactivity. It is concluded that the epitope contains one or more N-unsubstituted glucosamine and D-glucuronic acid units and is located in a region of the heparan sulfate chain composed of mixed N-sulfated and N-acetylated disaccharide units.

PSGL-1 recognition of P-selectin is controlled by a tyrosine sulfation consensus at the PSGL-1 amino terminus

P-selectin binding to neutrophils requires a specific protein, P-selectin glycoprotein ligand 1 (PSGL-1), as well as sialyl-Lewis X (sLex) glycan determinants. We have found that a short segment near the amino terminus of PSGL-1 that contains a tyrosine sulfation consensus is essential for P-selectin adhesion and that addition of the amino-terminal segment to some but not all mucin-like molecules confers on those molecules the ability to bind P-selectin. PSGL-1 synthesized in the presence of sulfation inhibitors binds P-selectin weakly, and within the amino-terminal 20 residues, mutation of the tyrosines to phenylalanine abolishes binding. Rolling of HL-60 cells on P-selectin-coated coverslips is strongly attenuated by treatment of cells with an inhibitor of sulfation.

Identification of a homologous heparin binding peptide sequence present in fibronectin and the 70 kDa family of heat-shock proteins

This study was undertaken to characterize the potential heparin affinity of an amino-acid sequence within the 70 kDa heat-shock family of proteins (HSPs) that shares homology with a heparin-binding sequence present in the carboxy-terminus of fibronectin (FN), defined by the synthetic peptide, FN-C/H-II (KNNQKSEPLIGRKKT). To first define the heparin binding sequence within FN-C/H-II, solid phase binding assays were performed using overlapping, short (7 amino acids) synthetic peptides corresponding to the amino-acid sequence within FN-C/H-II. Only the sequence LIGRKKT bound [3H] heparin, and the LIGRKKT peptide blocked heparin binding to intact fibronectin by 47% (+/- 0.4, p < 0.001). A computer-generated homology search revealed that two members of the 70 kDa HSP family, HSP70 and HSC70, contain the sequences LIGRK and LIGRR, respectively. Examination of heparin binding using affinity chromatography indicated that while native HSC70 binds heparin, native HSP70 does not. Treatment of the heparin-unbound fraction with heat or urea led to enhanced HSP70 binding to heparin affinity columns. Soluble LIGRKKT peptide or anti-FN-C/H-II IgG also significantly inhibited heparin binding to HSC70 that had been purified by heparin affinity chromatography. Finally, Western blot analysis of HSC70 purified by heparin affinity chromatography demonstrated that polyclonal anti-FN-C/H-II IgG could recognize HSC70. These data demonstrate that LIGRK or LIGRR represent a a common heparin binding motif in fibronectin, HSP70, and HSC70, and are consistent with a proposed role for heparin or similar polyanionic structures in the function of the 70 kDa heat-shock proteins.

Biosynthesis of GlyCAM-1, a mucin-like ligand for L-selectin

L-selectin, a member of the selectin family of leukocyte-endothelial adhesion proteins, mediates the initial attachment of lymphocytes to lymph node high endothelial venules during lymphocyte recirculation. One of the endothelial-associated ligands for L-selectin is GlyCAM-1, a mucin-like glycoprotein, which presents novel sulfated, sialylated and fucosylated O-glycans. In order to understand the generation of these glycans, we have examined the biosynthesis of GlyCAM-1 in lymph node organ culture. Using peptide-specific antibodies, lectins, and recombinant L-selectin, we detected the following species of GlyCAM-1: unglycosylated (< 28 kDa); modified with GalNAc only (28-33 kDa); modified with sialic acid, fucose, and sulfate but lacking L-selectin reactivity (40-50 kDa); and mature (L-selectin-reactive) ligand (50-60 kDa). Pulse-chase labeling at 15 degrees C suggested that GalNAc is added in a pre-Golgi compartment. Treatment with brefeldin A almost completely blocked sulfation, indicating that this modification occurs in the trans-Golgi network. Two distinct sialylation events occurred in the presence of brefeldin A, while fucosylation was partially blocked. We conclude that sialylation precedes both fucosylation and sulfation during biosynthesis. This ordering will help to identify the critical acceptor structures recognized by lymph node glycosyltransferases and sulfotransferases.

Role of N-linked glycosylation in expression of E-selectin on human endothelial cells

E-selectin is a cytokine-inducible membrane glycoprotein capable of mediating adhesion of leukocytes to endothelial cells. It is highly glycosylated, containing 11 sites for N-linked glycosylation. N-Glycosylation of E-selectin was analyzed by endoglycosidase treatment. Analysis of immunoprecipitated E-selectin from human umbilical vein endothelial cells (HUVEC) by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate showed that E-selectin was completely resistant to endoglycosidase H, but sensitive to peptide N-glycanase F digestion. This suggested that all N-linked oligosaccharide chains were of the complex type. The role of N-linked glycosylation in surface expression and secretion of E-selectin was studied using interleukin-1-stimulated HUVEC, cultured in the presence of the soluble glycosylation inhibitors tunicamycin or castanospermine. Cell surface expression was analyzed by indirect flow cytometry. N-Glycosylation was blocked by tunicamycin, and resulted in a significantly reduced surface expression of E-selectin, whereas castanospermine only marginally reduced E-selectin expression. The deglycosylated forms of E-selectin were also found to be fully capable of mediating adhesion of HT-29 cells in vitro. In conclusion, these studies show that E-selectin is heavily glycosylated with complex type N-linked oligosaccharides and that N-glycosylation is important for expression of E-selectin on human endothelial cells.

Interleukin 1 beta up-regulates the expression of sulfoglucuronosyl paragloboside, a ligand for L-selectin, in brain microvascular endothelial cells

Treatment of cultured bovine brain microvascular endothelial cells (BMECs) with interleukin 1 beta (IL-1 beta), an inflammatory cytokine, was shown to induce the accumulation of sulfoglucuronosyl paragloboside (SGPG), a glycolipid bearing the HNK-1 epitope. This resulted in the attachment of a greater number of human lymphocytes to the treated than to the untreated BMEC monolayers. Attachment of human lymphocytes to the IL-1 beta-activated BMEC cells could be blocked either by incubation of the human lymphocytes with an anti-L-selectin antibody or by application of an anti-SGPG antibody to the BMECs. These results suggest that SGPG may act as an important ligand for L-selectin for the regulation of the attachment of activated lymphocytes and their subsequent invasion into the nervous system parenchyma in inflammatory disorders of the central and peripheral nervous systems.

Peripheral blood cell preparation influences the level of expression of leukocyte cell surface markers as assessed with quantitative multicolor flow cytometry

We have compared the influence of sample preparation upon the level of surface expression of T, B, and NK cell-related antigens as assessed by flow cytometry. Lysed whole blood (WBL), Ficoll-Paque separated peripheral blood lymphocyte (F-PBL), and frozen peripheral blood lymphocyte (Fr-PBL) were analyzed via single- and multicolor flow cytometry. The percentage of positive cells expressing the individual cell surface markers was not affected by the procedure for preparation of WBL, F-PBL, and Fr-PBL. In contrast, the fluorescence intensity level of individual cell surface markers varied depending on cell preparation. By using Quantum Simply Cellular (QSC) microbeads, the antibody binding capacity (ABC) of single-color stained cells was quantified and compared. The amount of monoclonal antibody (MAb) anti-CD3-FITC bound to Fr-PBL (mean ABC = 137,040) was significantly higher (P < 0.001) that the amounts bound to WBL (mean ABC = 112,410) and F-PBL (mean ABC = 107,738). In multicolor analysis, the fluorescence intensity of CD3-FITC and CD4-FITC was significantly higher on Fr-PBL than on WBL and F-PBL; CD8-PE and CD20-PerCP was significantly higher on WBL. Furthermore, the intensity of CD3 and CD4 was different on T-cell subsets. The intensity of CD3 staining in three-color analysis was lower than with single-color staining using the same fluorochrome. We conclude that particularly the method of cell preparation but also the selection of MAb combinations may influence the level of staining of certain lymphocyte antigens. This may be of relevance in the analysis of cellular activation and regulation of differentiation.

Therapeutic uses of heparin beyond its traditional role as an anticoagulant

A number of physiological effects have been ascribed to heparin since its discovery almost 80 years ago, many of which are independent from its first-described and best- characterized activity as an anticoagulant. Heparin and heparan sulphate are believed to possess many biological activities that include the ability to modulate embryonic development, neurite outgrowth, tissue homeostasis, wound healing, metastasis, cell differentation, cell proliferation and inflammation. In this review, David Tyrell, Stephen Kilfeather and Clive Page examine some of the activities of heparin (and heparin derivatives) beyond its effects as an anticoagulant, and discuss the therapeutic potential of this old, but certainly not antiquated, drug.

Endothelial heparan sulfate proteoglycans that bind to L-selectin have glucosamine residues with unsubstituted amino groups

We earlier reported calcium-dependent, heparin-like L-selectin ligands in cultured bovine endothelial cells (Norgard-Sumnicht, K. E., Varki, N. M., and Varki, A. (1993) Science 261,480-483). Here we show that these are heparan sulfate proteoglycans (HSPGs) associated either with the cultured cells or secreted into the medium and extracellular matrix. Activation of the endothelial cells with bacterial lipopolysaccharide (LPS) does not markedly alter the amount or distribution of this material. A major portion of the glycosaminoglycan (GAG) chains released from these HSPGs by alkaline beta-elimination rebinds to L-selectin in the presence of calcium, indicating that these saccharides alone can mediate the high affinity recognition. Heparin lyase digestions indicate that these GAG chains are enriched in heparan sulfate, not heparin sequences. Current understanding of the biosynthesis of heparan sulfate chains indicates that all glucosamine amino groups must be either N-acetylated or N-sulfated. However, nitrous acid deamination at pH 4.0 suggests the presence of some unsubstituted amino groups in these L-selectin-binding GAG chains from endothelial cell HSPGs. This is confirmed by chemical N-reacetylation and by reactivity with sulfo-N-hydroxysuccinimide-biotin. These unsubstituted amino groups are also found on HSPGs from human umbilical vein endothelial cells, but are not detected in those from Chinese hamster ovary cells. In both bovine and human endothelial cells, these novel groups are enriched for in the HS-GAG chains which bind to L-selectin. Despite this, studies with N-reacetylation and nitrous acid deamination do not show conclusive evidence for the direct involvement of the unsubstituted amino groups in L-selectin binding. This may be because the chemical reactions used to modify the amino groups do not go to completion. Alternatively, the unsubstituted amino groups may only be indirectly involved in generating binding, by dictating the biosynthesis of another critical group. Regardless, these studies shown that HSPGs from cultured endothelial cells which can bind to L-selectin are enriched with unsubstituted amino groups on their GAG chains. The possible biochemical mechanisms for generation of these novel groups are discussed.

Leukocyte trafficking mediated by selectin-carbohydrate interactions

The selectins have attracted intense interest because of their carbohydrate-recognition properties and their pivotal roles in leukocyte trafficking. Future studies will center on the mechanisms for regulating the expression of the selectins and their ligands, the molecular details of selectin binding to glycoprotein ligands and small carbohydrates, and the biophysical principles that selectins employ to mediate attachment and rolling of leukocytes under flow.

The proteasome pathway is required for cytokine-induced endothelial-leukocyte adhesion molecule expression

Multiple cell adhesion proteins are up-regulated in vascular endothelial cells in response to TNF alpha and other inflammatory cytokines. This increase in cell adhesion gene expression is thought to require the transcription factor NF-kappa B. Here, we show that peptide aldehyde inhibitors of the proteasome, a multicatalytic protease recently shown to be required for the activation of NF-kappa B, block TNF alpha induction of the leukocyte adhesion molecules E-selectin, VCAM-1, and ICAM-1. Striking functional consequences of this inhibition were observed in analyses of leukocyte-endothelial interactions under defined flow conditions. Lymphocyte attachment to TNF alpha-treated endothelial monolayers was totally blocked, while neutrophil attachment was partially reduced but transmigration was essentially prevented.

Heparan sulfate-mediated cell aggregation. Syndecans-1 and -4 mediate intercellular adhesion following their transfection into human B lymphoid cells

Because syndecans are present at sites of cell-cell contact in vivo it has been hypothesized that they play a role in mediating cell-cell adhesion. However, there has been no direct evidence to support this notion. To address this question, B lymphoid (ARH-77) cells were transfected with the cDNA for murine syndecan-1. Unlike the parental cells, the transfectants form large multicellular aggregates in suspension cultures and stain intensely for syndecan-1 at sites of cell-cell contact. Using rotation-mediated aggregation assays, we find that aggregation of syndecan-1-transfected cells is dependent on divalent cations and is inhibited by the following: (i) addition of heparin and heparin-like glycosaminoglycans, (ii) removal of heparan sulfate from the cell surface, or (iii) addition of exogenous purified syndecan-1. Mixing of syndecan-1-transfected and control-transfected cells results in aggregates containing both cell types indicating that aggregation occurs through a heterophilic adhesion mechanism in which heparan sulfate chains bind to a counter-receptor present on these cells. Importantly, syndecan-4-transfected cells also aggregate in a heparan sulfate-dependent manner, while in contrast, betaglycan-transfected cells aggregate poorly. Thus, syndecans may be important mediators of cell-cell adhesion, but this function may not be common to all transmembrane heparan sulfate-bearing proteoglycans.

P-selectin glycoprotein ligand-1 mediates rolling of human neutrophils on P-selectin

Neutrophils roll on P-selectin expressed by activated platelets or endothelial cells under the shear stresses in the microcirculation. P-selectin glycoprotein ligand-1 (PSGL-1) is a high affinity ligand for P-selectin on myeloid cells. However, it has not been demonstrated that PSGL-1 contributes to the rolling of neutrophils on P-selectin. We developed two IgG mAbs, PL1 and PL2, that appear to recognize protein-dependent epitopes on human PSGL-1. The mAbs bound to PSGL-1 on all leukocytes as well as on heterologous cells transfected with PSGL-1 cDNA. PL1, but not PL2, blocked binding of 125-I-PSGL-1 to immobilized P-selectin, binding of fluid-phase P-selectin to myeloid and lymphoid leukocytes, adhesion of neutrophils to immobilized P-selectin under static conditions, and rolling of neutrophils on P-selectin-expressing CHO cells under a range of shear stresses. PSGL-1 was localized to microvilli on neutrophils, a topography that may facilitate its adhesive function. These data indicate that (a) PSGL-1 accounts for the high affinity binding sites for P-selectin on leukocytes, and (b) PSGL-1 must interact with P-selectin in order for neutrophils to roll on P-selectin at physiological shear stresses.

Sequential contribution of L- and P-selectin to leukocyte rolling in vivo

Leukocyte recruitment into inflammatory sites is initiated by a reversible transient adhesive contact with the endothelium called leukocyte rolling, which is thought to be mediated by the selectin family of adhesion molecules. Selectin-mediated rolling precedes inflammatory cell emigration, which is significantly impaired in both P- and L-selectin gene-deficient mice. We report here that approximately 13% of all leukocytes passing venules of the cremaster muscle of wild-type mice roll along the endothelium at < 20 min after surgical dissection. Rolling leukocyte flux fraction reaches a maximum of 28% at 40-60 min and returns to 13% at 80-120 min. In P-selectin-deficient mice, rolling is absent initially and reaches 5% at 80-120 min. Rolling flux fraction in L-selectin-deficient mice is similar to wild type initially and declines to 5% at 80-120 min. In both wild-type and L-selectin-deficient mice, initial leukocyte rolling (0-60 min) is completely blocked by the P-selectin monoclonal antibody (mAb) RB40.34, but unaffected by L-selectin mAb MEL-14. Conversely, rolling at later time points (60-120 min) is inhibited by mAb MEL-14 but not by mAb RB40.34. After treatment with tumor necrosis factor (TNF)-alpha for 2 h, approximately 24% of all passing leukocytes roll in cremaster venules of wild-type and P-selectin gene-deficient mice. Rolling in TNF-alpha-treated mice is unaffected by P-selectin mAb or E-selectin mAb 10E9.6. By contrast, rolling in TNF-alpha-treated P-selectin-deficient mice is completely blocked by L-selectin mAb. These data show that P-selectin is important during the initial induction of leukocyte rolling after tissue trauma. At later time points and in TNF-alpha-treated preparations, rolling is largely L-selectin dependent. Under the conditions tested, we are unable to find evidence for involvement of E-selectin in leukocyte rolling in mice.

Enzymatic degradation of glycosaminoglycans

Glycosaminoglycans (GAGs) play an intricate role in the extracellular matrix (ECM), not only as soluble components and polyelectrolytes, but also by specific interactions with growth factors and other transient components of the ECM. Modifications of GAG chains, such as isomerization, sulfation, and acetylation, generate the chemical specificity of GAGs. GAGs can be depolymerized enzymatically either by eliminative cleavage with lyases (EC 4.2.2.-) or by hydrolytic cleavage with hydrolases (EC 3.2.1.-). Often, these enzymes are specific for residues in the polysaccharide chain with certain modifications. As such, the enzymes can serve as tools for studying the physiological effect of residue modifications and as models at the molecular level of protein-GAG recognition. This review examines the structure of the substrates, the properties of enzymatic degradation, and the enzyme substrate-interactions at a molecular level. The primary structure of several GAGs is organized macroscopically by segregation into alternating blocks of specific sulfation patterns and microscopically by formation of oligosaccharide sequences with specific binding functions. Among GAGs, considerable dermatan sulfate, heparin and heparan sulfate show conformational flexibility in solution. They elicit sequence-specific interactions with enzymes that degrade them, as well as with other proteins, however, the effect of conformational flexibility on protein-GAG interactions is not clear. Recent findings have established empirical rules of substrate specificity and elucidated molecular mechanisms of enzyme-substrate interactions for enzymes that degrade GAGs. Here we propose that local formation of polysaccharide secondary structure is determined by the immediate sequence environment within the GAG polymer, and that this secondary structure, in turn, governs the binding and catalytic interactions between proteins and GAGs.

Sulfation-dependent recognition of high endothelial venules (HEV)-ligands by L-selectin and MECA 79, and adhesion-blocking monoclonal antibody

L-selectin is a lectin-like receptor that mediates the attachment of lymphocytes to high endothelial venules (HEV) of lymph nodes during the process of lymphocyte recirculation. Two sulfated, mucin-like glycoproteins known as Sgp50/GlyCAM-1 and Sgp90/CD34 have previously been identified as HEV-associated ligands for L-selectin. These proteins were originally detected with an L-selectin/Ig chimera called LEC-IgG. GlyCAM-1 and CD34 are also recognized by an antiperipheral node addressin (PNAd) mAb called MECA 79, which blocks L-selectin-dependent adhesion and selectively stains lymph node HEV. The present study compares the requirements for the binding of MECA 79 and LEC-IgG to HEV-ligands. Whereas desialylation of GlyCAM-1 and CD34 drastically reduced binding to LEC-IgG, this treatment enhanced the binding of GlyCAM-1 to MECA 79. In contrast, the binding of both MECA 79 and LEC-IgG to GlyCAM-1 and CD34 was greatly decreased when the sulfation of these ligands was reduced with chlorate, a metabolic inhibitor of sulfation. Because MECA 79 stains HEV-like vessels at various sites of inflammation, recognition by L-selectin of ligands outside of secondary lymphoid organs may depend on sulfation. In addition to their reactivity with GlyCAM-1 and CD34, both MECA 79 and LEC-IgG recognize an independent molecule of approximately 200 kD in a sulfate-dependent manner. Thus, this molecule, which we designate Sgp200, is an additional ligand for L-selectin.

The selectins and their ligands

The selectins are a family of carbohydrate-binding proteins, or lectins, that have stimulated tremendous interest because of their involvement in a wide array of interactions between leukocytes and endothelial cells. Highlights of recent progress include an extension of the list of instances of selectin participation in inflammatory diseases, further definition of selectin carbohydrate specificities, and identification of their carbohydrate-based ligands.

Myelin localization of a central nervous system ligand for L-selectin

L-selectin is a lectin-like receptor involved in lymphocyte attachment to lymph node high endothelial venules (HEV). Previously, we showed that L-selectin also participates in the in vitro attachment of lymphocytes to central nervous system (CNS) white matter. Use of an L-selectin chimera demonstrated ligand sites within CNS white matter but not the peripheral nervous system (PNS). Now employing higher resolution mapping, including EM cytochemistry, we localize the ligands to the actual myelin sheaths of CNS neurons. In the shiverer mouse, which lacks compact myelin, ligands are greatly diminished. Comparison of the myelin-associated ligand with the previously characterized HEV-ligands demonstrates a number of differences.

Selectin ligands

The selectins initiate many critical interactions among blood cells. The volume of information and diversity of opinions on the nature of the biologically relevant ligands for selectins is remarkable. This review analyzes the matter and suggests the hypothesis that at least some of the specificity may involve recognition of "clustered saccharide patches."

Synthetic fibronectin peptides suppress arthritis in rats by interrupting leukocyte adhesion and recruitment

In an experimental model of arthritis, increased leukocyte adhesion is associated with the evolution of acute and chronic synovial inflammation. Whereas peripheral blood mononuclear cells (PBMC) from control animals bind minimally to fibronectin matrices, PBMC from animals receiving arthropathic doses of bacterial cell walls demonstrate increased integrin mRNA expression and enhanced adhesion. To determine whether this augmented adhesion was causal in the development of synovial pathology, peptides synthesized from several fibronectin domains which inhibited leukocyte adhesion in vitro were administered to arthritic animals either as free peptides or coupled to a carrier molecule. Not only were peptides containing either the RGD or CS-1 cell-binding domains inhibitory to chronic synovial pathology (articular index = 10.5 +/- 0.3 for untreated animals compared to 1.25 +/- 0.25 for RGD and 2.5 +/- 0.7 for CS-1), but three peptides synthesized from the carboxy-terminal 33-kD heparin-binding domain of fibronectin were also found to significantly inhibit leukocyte recruitment and the evolution of arthritis. Based on these data, which are the first to explore the therapeutic potential of heparin-binding fibronectin peptides in chronic inflammation, it appears that antagonism of cellular adhesion and recruitment by fibronectin peptides may provide an important mechanism for modulating the multi-step adhesion process and attenuating aberrant inflammatory responses.

A liquid-phase binding analysis for L-selectin. A strong dependency on highly clustered sulfate groups

L-selectin, together with E- and P-selectins, forms a newer group of cell adhesion molecules which are believed to interact with carbohydrate ligands [Lasky, L. A., Singer, M. S., Yednoch, T. A., Dowbenko, D., Fennie, C., Rodriguez, H., Nguyen, T., Stachel, S. & Rosen, S. D. (1989) Cell 56, 1045-1055]. Using radiolabeled fucoidan as a reference ligand, we have developed a new liquid-phase microcentrifugation assay where fine differences in binding affinity can be compared accurately. We found that glucan sulfates strongly inhibited the binding of fucoidan by murine L-selectin-IgG chimera. The efficacy of inhibition is extremely dependent on the size (up to 12 kDa) and the sulfate density (up to two sulfate groups/glucose molecule) of the glucan sulfates. The nature of the inter-glucose linkages is also important. These data suggest that the binding by L-selectin prefers certain clustering and proper spatial arrangement of the anionic groups.

Synthetic fibronectin peptides interrupt inflammatory cell infiltration in transforming growth factor beta 1 knockout mice

Pronounced mononuclear leukocyte (MNL) infiltration occurs in multiple organs of mice homozygous for a transforming growth factor beta 1 (TGF-beta 1) loss-of-function gene mutation [TGF-beta 1 (-/-)], followed by cachexia and eventually death. Consistent with the increased leukocyte adhesion and tissue infiltration, MNLs isolated from spleen, thymus, and peripheral blood of symptomatic TGF-beta 1 (-/-) mice, as compared to littermate controls, exhibited increased adhesion to extracellular matrix proteins and to endothelial cells in vitro. Incubation of TGF-beta 1 (-/-) MNLs with selected synthetic peptides corresponding to cell- and heparin-binding sequences of fibronectin (FN) significantly attenuated adhesion of these cells not only to FN but also to endothelial cells in vitro. Based on these observations, mice were treated with the FN peptides in an attempt to rescue them from tissue inflammation and cardiopulmonary failure. Daily injections of a combination of four synthetic FN peptides that interact with beta 1-integrins and/or cell surface proteoglycans blocked the massive infiltration of MNLs into the heart and lungs of TGF-beta 1 (-/-) mice. Peptide treatment initiated on day 8, coincident with the first evidence of increased leukocyte-endothelial cell interactions, not only blocked tissue infiltration but also moderated the lethal wasting syndrome.

Selectins

The selectins are three related receptors that initiate rolling of leukocytes on activated platelets or endothelium through Ca(2+)-dependent recognition of cell surface carbohydrates. Cell adhesion may be enhanced by a limited number of membrane glycoproteins that present high affinity carbohydrate ligands to specific selectins. The synthesis and surface display of the selectins is normally tightly controlled, but inappropriate expression may contribute to inflammatory disorders. Recent in vivo studies confirm the importance of the selectins in both physiological and pathological leukocyte recruitment.