The complement binding-like domains of the murine homing receptor facilitate lectin activity

Functional binding of E-selectin to its ligands is enhanced by structural features beyond its lectin domain

Selectins are key to mediating interactions involved in cellular adhesion and migration, underlying processes such as immune responses, metastasis, and transplantation. Selectins are composed of a lectin domain, an epidermal growth factor (EGF)-like domain, multiple short consensus repeats (SCRs), a transmembrane domain, and a cytoplasmic tail. It is well-established that the lectin and EGF domains are required to mediate interactions with ligands; however, the contributions of the other domains in mediating these interactions remain obscure. Using various E-selectin constructs produced in a newly developed silkworm-based expression system and several assays performed under both static and physiological flow conditions, including flow cytometry, glycan array analysis, surface plasmon resonance, and cell-rolling assays, we show here that a reduction in the number of SCR domains is correlated with a decline in functional E-selectin binding to hematopoietic cell E- and/or L-selectin ligand (HCELL) and P-selectin glycoprotein ligand-1 (PSGL-1). Moreover, the binding was significantly improved through E-selectin dimerization and by a substitution (A28H) that mimics an extended conformation of the lectin and EGF domains. Analyses of the association and dissociation rates indicated that the SCR domains, conformational extension, and dimerization collectively contribute to the association rate of E-selectin-ligand binding, whereas just the lectin and EGF domains contribute to the dissociation rate. These findings provide the first evidence of the critical role of the association rate in functional E-selectin-ligand interactions, and they highlight that the SCR domains have an important role that goes beyond the structural extension of the lectin and EGF domains.

E-selectin gene haplotypes are associated with the risk of myocardial infarction

INTRODUCTION

Endothelial dysfunction is one of the most important factors implicated in the pathogenesis of coronary artery disease (CAD). The aim of this study was to investigate the association of the E-selectin gene (SELE) with CAD and CAD-related traits using tagging polymorphisms.

MATERIAL AND METHODS

A total of 379 Polish patients who had undergone angiography were included: 261 patients with angiographically documented CAD, 202 CAD patients without myocardial infarction (CAD/MI(-) group) and 59 patients with myocardial infarction (CAD/MI(+) group) as well as 118 healthy control subjects (non-CAD). Eight tagging single nucleotide polymorphisms (SNPs) in the SELE gene were selected using genotype data from HapMap. Genotyping was performed using PCR-RFLP and PCR-DHPLC methods.

RESULTS

The most common SELE haplotype in this analysis ([C;G;T;C;G;T], 31.2%) showed a negative association with myocardial infarction (MI) (CAD/MI(+) vs. non-CAD) under the additive (p = 0.001), dominant (p = 0.006) and recessive (p = 0.012) model. Two other haplotypes ([C;G;C;C;A;C], [C;A;C;A;G;T], 5.73% and 18.1%, respectively) were also negatively associated with MI under the additive and dominant model. We also found two haplotypes ([T;G;T;C;G;T], [C;G;C;C;A;T], 1.52% and 6.71%, respectively) associated with the risk for MI (CAD/MI(+) vs. CAD/MI(-)), acting in both additive (p = 0.04, p = 0.007, respectively) and dominant (p = 0.04, p = 0.004, respectively) manner. There was no association with either CAD/MI(-) or with severity of CAD expressed as the number of vessels involved.

CONCLUSIONS

Our results suggest that SELE is one of the independent genetic factors modifying the risk of myocardial infarction.

Adhesion molecules, catecholamines and leucocyte redistribution during and following exercise

The circulating blood normally contains no more than 1-2% of the body's population of leucocytes. The numbers and phenotypes of circulating leucocyte subsets can change dramatically during and immediately following exercise. The surface expression of adhesion molecules makes an important contribution to such responses by changing patterns of cell trafficking. Alterations in the surface expression of adhesion molecules could reflect a shedding of molecules, selective apoptosis or differential trafficking of cells with a particular phenotype, effects from mechanical deformation of the cytoplasm, active biochemical processes involving cytokines, catecholamines, glucocorticoids or other hormones, or changes in the induction of adhesion molecules. The expression of adhesion molecules changes with maturation and activation of leucocytes. Typically, mature cells express lower densities of L-selectin (CD62L), the homing receptor for secondary lymphoid organs, and higher densities of LFA-1 (CD11a), the molecule associated with trafficking to non-lymphoid reservoir sites. The neutrophils and natural killer cells that are mobilised during exercise also express high levels of Mac-1 (CD11b), a marker associated with cellular activation. Possibly, exercise demarginates older cells that are awaiting destruction in the spleen. Plasma concentrations of catecholamines rise dramatically with exercise, and there is growing evidence that catecholamines, acting through a cyclic adenosine monophosphate second messenger system, play an important role in modifying the surface expression of adhesion molecules. Analogous changes can be induced by other forms of stress that release catecholamines or by catecholamine infusion, and responses are blocked by beta(2)-blocking agents. Catecholamines also modify adherence and expression of adhesion molecules in vitro. Cell trafficking is modified by genetic deficiencies in the expression of adhesion molecules, but leucocyte responses to exercise and catecholamines are generally unaffected by splenectomy. A number of clinical conditions including atherogenesis and metaplasia are marked by an altered expression of adhesion molecules. The effects of exercise on these molecules could thus have important health implications.

Design and synthesis of sialyl Lewis(x) mimics as E- and P-selectin inhibitors

The selectins are a family of cell-adhesion proteins that mediate the rolling of leukocytes on activated endothelial cells through the recognition of the carbohydrate epitope sialyl Lewis(x) (sLe(x)). Control of the leukocyte-endothelial cell adhesion process may prove useful in cases where excess recruitment of leukocytes can contribute to acute diseases such as stroke and reperfusion injury and chronic diseases such as psoriasis and rheumatoid arthritis. The development of molecules that block the interactions between sLe(x) and the selectins has become an active area of research. In this review, we will highlight the various approaches taken toward the development of sLe(x) mimetics as antagonists of E- and P-selectin, including the use of structural information about the selectins and their interactions with sLe(x) that have been revealed through the use of NMR, protein crystallography and molecular modeling.

A functional role for circulating mouse L-selectin in regulating leukocyte/endothelial cell interactions in vivo

L-selectin mediates the initial capture and subsequent rolling of leukocytes along inflamed vascular endothelium and mediates lymphocyte migration to peripheral lymphoid tissues. Leukocyte activation induces rapid endoproteolytic cleavage of L-selectin from the cell surface, generating soluble L-selectin (sL-selectin). Because human sL-selectin retains ligand-binding activity in vitro, mouse sL-selectin and its in vivo relevance were characterized. Comparable with humans, sL-selectin was present in adult C57BL/6 mouse sera at approximately 1.7 micro g/ml. Similar levels of sL-selectin were present in sera from multiple mouse strains, despite their pronounced differences in cell surface L-selectin expression levels. Adhesion molecule-deficient mice prone to spontaneous chronic inflammation and mice suffering from leukemia/lymphoma had 2.5- and 20-fold increased serum sL-selectin levels, respectively. By contrast, serum sL-selectin levels were reduced by 70% in Rag-deficient mice lacking mature lymphocytes. The majority of serum sL-selectin had a molecular mass of 65-75 kDa, consistent with its lymphocyte origin. Slow turnover may explain the relatively high levels of sL-selectin in vivo. The t(1/2) of sL-selectin, assessed by transferring sera from wild-type mice into L-selectin-deficient mice and monitoring serum sL-selectin levels by ELISA, was >20 h, and it remained detectable for longer than 1 wk. Short-term in vivo lymphocyte migration assays demonstrated that near physiologic levels ( approximately 0.9 micro g/ml) of sL-selectin decreased lymphocyte migration to peripheral lymph nodes by >30%, with dose-dependent inhibition occurring with increasing sL-selectin concentrations. These results suggest that sL-selectin influences lymphocyte migration in vivo and that the increased sL-selectin levels present in certain pathologic conditions may adversely affect leukocyte migration.

Mapping the epitope of a functional P-selectin monoclonal antibody (LYP20) to a short complement-like repeat (SCR 4) domain: use of human-mouse chimaera and homologue-replacement mutagenesis

P-selectin (CD62P), an adhesion molecule localized in platelet alpha-granules and endothelial cell Weibel-Palade bodies, is rapidly expressed on the surface of activated cells. This adhesion molecule, a member of the selectin family, mediates leucocyte interactions with activated platelets or endothelial cells. The aim of this study was to identify and characterize the epitope of a functional blocking P-selectin monoclonal antibody (mAb), LYP20. LYP20 recognizes human or rat, but not mouse, P-selectin. Human/mouse chimaeras and wild-type constructs, modified by homologue replacement mutagenesis, were expressed in COS cells. Blocking anti-(P-selectin) mAbs (G1, G3 or CLB-thromb/6) were observed, by flow cytometry, to bind to the lectin-like domain. In contrast, LYP20 was found to bind to one of the P-selectin short complement-like repeats (SCR domain 4). Homologue replacement mutagenesis of SCR domain 4 (region delineated by amino acid residues 359-457) identified three amino acids (Cys412-->Ser, Cys416-->Ser or Arg415-->Lys) as being implicated in the LYP20 epitope. Deleting the region bearing the LYP20 epitope, from a wild-type CD62P construct, showed a decrease in polymorphonuclear leucocyte (PMNL) binding to transfected COS cells. In addition, mutation of one of the three amino acids, implicated in the LYP20 epitope, markedly affected PMNL binding to transfected COS cells but did not affect the binding of mAbs G1 and CLB-thromb/6. These results are the first to indicate (1) that a functional blocking anti-P-selectin mAb binds to SCR 4, a site other than the lectin-like/epidermal growth factor-like domain, and (2) that SCR domain 4 has a functional role in P-selectin-leucocyte interactions.

Sialyl Lewisx (sLex) and an sLexMimetic, CGP69669A, Disrupt E-Selectin–Dependent Leukocyte Rolling In Vivo

Leukocyte rolling is the earliest observable event in their recruitment from the circulation to inflamed tissue. This rolling is mediated largely by interaction between the selectin family of adhesion molecules and their glycosylated ligands. Although the nature of these ligands and their interaction with the selectins is not fully understood, it is accepted that expression of fucosylated sialylated glycans such as sialyl Lewisx (sLex) is required for function. Despite findings that sLex inhibits binding of leukocytes to E-selectin in vitro, and has beneficial effects in inflammatory disease models, inhibition of E-selectin–dependent leukocyte rolling in vivo has not been described. Functional overlap between the selectins has been noted and reduction of rolling by E-selectin antibodies only occurs if P-selectin is absent or blocked. We demonstrate that leukocyte rolling velocity in tumor necrosis factor alpha (TNFα)-stimulated mouse cremaster is increased following treatment with either sLex or the sLex-mimetic CGP69669A and that rolling is dramatically reduced if CGP69669A is applied in the presence of anti–P-selectin antibody. These effects are characteristic of E-selectin antagonism. In contrast, surgically stimulated (L- or P-selectin–dependent) rolling is unaffected by either sLex or CGP69669A. Our data demonstrate that CGP69669A is an effective and selective antagonist of E-selectin in vivo.

Sialyl Lewisx (sLex) and an sLexMimetic, CGP69669A, Disrupt E-Selectin–Dependent Leukocyte Rolling In Vivo

Leukocyte rolling is the earliest observable event in their recruitment from the circulation to inflamed tissue. This rolling is mediated largely by interaction between the selectin family of adhesion molecules and their glycosylated ligands. Although the nature of these ligands and their interaction with the selectins is not fully understood, it is accepted that expression of fucosylated sialylated glycans such as sialyl Lewisx (sLex) is required for function. Despite findings that sLex inhibits binding of leukocytes to E-selectin in vitro, and has beneficial effects in inflammatory disease models, inhibition of E-selectin–dependent leukocyte rolling in vivo has not been described. Functional overlap between the selectins has been noted and reduction of rolling by E-selectin antibodies only occurs if P-selectin is absent or blocked. We demonstrate that leukocyte rolling velocity in tumor necrosis factor alpha (TNFα)-stimulated mouse cremaster is increased following treatment with either sLex or the sLex-mimetic CGP69669A and that rolling is dramatically reduced if CGP69669A is applied in the presence of anti–P-selectin antibody. These effects are characteristic of E-selectin antagonism. In contrast, surgically stimulated (L- or P-selectin–dependent) rolling is unaffected by either sLex or CGP69669A. Our data demonstrate that CGP69669A is an effective and selective antagonist of E-selectin in vivo.

Soluble Monomeric P-Selectin Containing Only the Lectin and Epidermal Growth Factor Domains Binds to P-Selectin Glycoprotein Ligand-1 on Leukocytes

Under shear stress, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to tether to and roll on P-selectin expressed on activated platelets or endothelial cells. P-selectin has an NH2-terminal lectin domain, an epidermal growth factor (EGF)-like motif, nine consensus repeats (CRs), a transmembrane domain, and a cytoplasmic tail. To determine whether the CRs are required for P-selectin to bind PSGL-1, we expressed a soluble protein (Lec-EGF) that contained only the lectin and EGF domains, plus a short C-terminal epitope tag. Electron microscopy and hydrodynamic analysis confirmed that Lec-EGF was monomeric, as previously shown for soluble P-selectin (sPS) that contained the lectin and EGF domains plus all nine CRs. Fluid-phase Lec-EGF or sPS inhibited binding of oligomeric125I-labeled membrane-derived P-selectin (mPS) to PSGL-1 on neutrophils and binding of 125I-PSGL-1 to immobilized mPS. The IC50 for inhibiting binding of mPS to neutrophils was fivefold greater for Lec-EGF than for sPS, whereas the IC50 for inhibiting binding of mPS to purified PSGL-1 was indistinguishable for Lec-EGF and sPS. Under static or shear conditions, neutrophils used PSGL-1 to tether to or roll on Lec-EGF that was captured by an immobilized monoclonal antibody to the C-terminal epitope. These data show that P-selectin requires only the lectin and EGF domains to bind to PSGL-1.

Soluble Monomeric P-Selectin Containing Only the Lectin and Epidermal Growth Factor Domains Binds to P-Selectin Glycoprotein Ligand-1 on Leukocytes

Under shear stress, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to tether to and roll on P-selectin expressed on activated platelets or endothelial cells. P-selectin has an NH2-terminal lectin domain, an epidermal growth factor (EGF)-like motif, nine consensus repeats (CRs), a transmembrane domain, and a cytoplasmic tail. To determine whether the CRs are required for P-selectin to bind PSGL-1, we expressed a soluble protein (Lec-EGF) that contained only the lectin and EGF domains, plus a short C-terminal epitope tag. Electron microscopy and hydrodynamic analysis confirmed that Lec-EGF was monomeric, as previously shown for soluble P-selectin (sPS) that contained the lectin and EGF domains plus all nine CRs. Fluid-phase Lec-EGF or sPS inhibited binding of oligomeric125I-labeled membrane-derived P-selectin (mPS) to PSGL-1 on neutrophils and binding of 125I-PSGL-1 to immobilized mPS. The IC50 for inhibiting binding of mPS to neutrophils was fivefold greater for Lec-EGF than for sPS, whereas the IC50 for inhibiting binding of mPS to purified PSGL-1 was indistinguishable for Lec-EGF and sPS. Under static or shear conditions, neutrophils used PSGL-1 to tether to or roll on Lec-EGF that was captured by an immobilized monoclonal antibody to the C-terminal epitope. These data show that P-selectin requires only the lectin and EGF domains to bind to PSGL-1.

Differential Effect of E-Selectin Antibodies on Neutrophil Rolling and Recruitment to Inflammatory Sites

The selectins are inducible adhesion molecules critically important for the inflammatory response. We investigate here the functional effects of three monoclonal antibodies (MoAbs) raised against murine E-selectin (9A9, 10E6, and 10E9.6) on neutrophil recruitment in vivo, leukocyte rolling and circulating leukocyte concentrations in vivo, and adhesion of myeloid cells to E-selectin transfectants and recombinant E-selectin–IgG fusion protein in vitro. MoAbs 9A9 and 10E6 map to the lectin and epidermal growth factor (EGF)-like domains of murine E-selectin, whereas 10E9.6 binds to the consensus repeat region. 10E9.6 blocked neutrophil recruitment in a model of thioglycollate-induced peritonitis in Balb/c mice by more than 90% but had no effect in C57BL/6 mice. 9A9 and 10E6 blocked neutrophil recruitment in this assay only when combined with a P-selectin antibody, 5H1. Neither 9A9 nor 10E9.6 alone blocked leukocyte rolling in tumor necrosis factor-α–treated venules of Balb/c mice, but 9A9 almost completely inhibited leukocyte rolling when combined with the function-blocking murine P-selectin MoAb, RB40.34. In contrast, 10E9.6 had no effect on leukocyte rolling in RB40.34-treated Balb/c or C57BL/6 mice. 10E9.6 did not affect adhesion of myeloid cells to E-selectin transfectants or attachment, rolling, and detachment of myeloid cells to murine E-selectin–IgG fusion protein. However, adhesion was completely blocked in the same assays by 9A9. Taken together, these results indicate that E-selectin serves a function, other than rolling, that appears to be critically important for neutrophil recruitment to inflammatory sites in Balb/c mice.

The role of selectins in Drosophila eye and bristle development

Mutations in the furrowed (fw) gene of Drosophila result in defects in the development of the eye and mechanosensory bristles. The eyes are reduced in size, have furrows or crevices in the retina, and show a disturbed patterning of ommatidia. In addition, the ommatidia have an altered morphology and often contain abnormal numbers of cells. The bristles show altered structure and polarity, and are occasionally duplicated or missing. These results suggest that the product of thefw gene is involved in cell determination events within sensory organs. Thefw gene has been cloned and shown to encode a protein homologous to vertebrate selectins. Like selectins, Fw contains a lectin-binding domain, ten complement binding repeats, and a transmembrane domain. The Fw protein is expressed in the larval imaginal discs where it might mediate carbohydrate-protein interactions necessary for proper development of a subset of adult sensory organs.

Estradiol induces E-cadherin degradation in mouse uterine epithelium during the estrous cycle and early pregnancy

Mouse uterine epithelium is a tissue that undergoes cyclic endocrine-regulated cell dissociation and regeneration. It shows a dramatic cell loss following normal estrus. If pregnancy ensues, cell loss is averted during the first 2.5-3.5 days. However, this is followed by a precipitous loss of basal-lateral cell adhesion and apoptosis in preparation for blastocyst invasion. By comparing epithelia isolated by protease treatment, we show that a reduction of lateral cell adhesion is a primary event in these instances of normal tissue loss. It was readily induced in ovariectomized adult and immature mice by injections of estradiol (E2), and to some extent also by progesterone (P4). The reduction of lateral adhesion induced by including ethylene glycol-bis (beta-aminoethyl ether) -N,N,N',N'-tetraacetic acid (EGTA) in the isolation medium mimicked and was additive to the effect of E2 injection. However, the E2 effect was different in not being prevented by adding Ca2+. The E2 effect also was mimicked by the action on isolated epithelium of monoclonal antibody against the calcium-dependent cell adhesion molecule, E-cadherin, suggesting that inactivation of E-cadherin was induced by E2. In detergent extracts of estrous and metestrous epithelium there was an increase in 80-kDa extracellular domain of E-cadherin relative to the intact 120-kDa molecule. The loss of adhesion between 3.5 and 4.5 days of pregnancy was associated with a loss of both intact membrane-associated 120-kDa E-cadherin and cleavage products. Cleavage of 80-kDa E-cadherin was uniquely induced by E2 in ovariectomized adult and immature mice; P4 was without effect. The cleavage of E-cadherin correlated with increased basal accumulation of E-cadherin antigen in estrous and E2-injected mice and a loss of both basal and lateral antigen at 4.5 days of pregnancy. Only the E-cadherin antigen within junctional complexes appeared unaffected. The data are consistent with the hypothesis that the cyclic and pregnancy-dependent disruption of uterine epithelial integrity are promoted by E2-dependent modification of E-cadherin, including its extracellular cleavage.

The carbohydrate-recognition domain of E-selectin is sufficient for ligand binding under both static and flow conditions

Selectins are a family of adhesion molecules with a well-defined domain structure comprised of a lectin or carbohydrate-recognition domain (CRD), an epidermal growth factor (EGF)-like motif, and a variable number of consensus repeats (CRs). While it is clear from various lines of evidence that the CRD plays a pivotal role in selectin-ligand interactions, little is known about the role of the non-lectin selectin domains. We expressed a series of soluble chimeric proteins with various domains switched between E- and L-selectin and measured binding of the resulting chimeras to sialyl Lewis(a) and sulfatide, two carbohydrate structures which are specific for the E- and L-CRDs, respectively. Both CRDs bind to their respective ligands with the same affinity regardless of the origin of the other domains they are attached to. The domain-switched chimeras were assayed for their ability to support static binding and rolling of various cell lines which bind specifically to E-selectin. In these assays, the E-CRD was indispensable for both static binding and rolling under physiological flow conditions. The E-CRD alone, when substituted into L-selectin, supported rolling without the requirement for additional ligand-recognition elements. We conclude that the EGF domain or the CRs of E- and L-selectin have no influence on the CRD's specificity to carbohydrates. Furthermore, at least in the case of E-selectin, they do not contribute to the specificity of binding to cell surface ligands.

Differential regulation of tissue-specific lymph node high endothelial venule cell adhesion molecules by tumour necrosis factor and transforming growth factor-beta 1

Lymphocytes migrate from blood into lymph nodes (LN) of rats specifically at segments of venules lined by high endothelium (HEV). We have previously shown that pretreatment of LN HEV cells with pro-inflammatory cytokines such as tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma), augments their adhesiveness for thoracic duct lymphocytes (TDL). Here we report that a mouse monoclonal antibody, 3C10, recognized tissue-specific endothelial determinants on rat LN HEV cells and blocked their adhesiveness for TDL and EL-4J cells transfected with rat L-selectin. In contrast, 3C10 antibody did not inhibit lymphocyte attachment to Peyer's patch (PP) frozen sections or cultured PP HEV cells. The antibody immunoprecipitated from LN HEV cells two proteins with apparent molecular weights of 90,000 and 50,000. The expression of 3C10 antigen on LN HEV cells was increased by incubation with TNF-alpha or IFN-gamma. Furthermore, pretreatment of cytokine-stimulated LN HEV cells with 3C10 antibody blocked TDL binding in a dose-dependent manner. In contrast, 3C10 antigen expression on LN HEV cells was significantly decreased following incubation of cells with transforming growth factor-beta 1 (TGF-beta 1). In addition, TGF-beta 1 also abrogated the adhesiveness of LN HEV cells stimulated with TNF-alpha, IFN-gamma or both cytokines. Together, these data suggest that endothelial determinants recognized by the 3C10 antibody are tissue-specific ligands for lymphocyte adhesion and cytokines such as TNF-alpha and TGF-beta differentially regulate their expression and function.

Cloning and expression of SEZ-6, a brain-specific and seizure-related cDNA

To clarify the molecular mechanism of neuronal bursting activity of seizures, we have constructed a cDNA library from mouse cerebrum cortex-derived cells treated with pentylentetrazole (PTZ), one of the convulsant drugs. Using a differential screening technique, several cDNA clones whose expressions change with PTZ-treatment were obtained. Among these clones, SEZ-6 was characterized by increased expression with PTZ. Detailed northern analysis showed that expression of SEZ-6 was limited to the brain and increased by the administration of PTZ not only in in vitro cultured cells but also in vivo. Analysis of SEZ-6 cDNA revealed multiple motifs, including typical signal sequence, threonine-rich domain, five copies of short consensus repeats (SCRs) or sushi domain (complement C3b/C4b binding site), two repeated sequences which were partially similar to the CUB domain or complement C1r/s-like repeat, one transmembrane domain and a short cytoplasmic segment in the C-terminal region. Although many proteins with multiple SCRs or CUB domains other than complement-related proteins have been found, this is the first report about a brain-specific cDNA which encodes membrane protein with both SCRs and CUB domain-like segments. Based on these findings, it is evident that SEZ-6 encodes a novel type of protein which may be related to seizure.

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."

Cell adhesion molecules in inflammation and immunity: relevance to periodontal diseases

Inflammatory and immune responses involve close contact between different populations of cells. These adhesive interactions mediate migration of cells to sites of inflammation and the effector functions of cells within the lesions. Recently, there has been significant progress in understanding the molecular basis of these intercellular contacts. Blocking interactions between cell adhesion molecules and their ligands has successfully suppressed inflammatory reactions in a variety of animal models in vivo. The role of the host response in periodontal disease is receiving renewed attention, but little is known of the function of cell adhesion molecules in these diseases. In this review we summarize the structure, distribution, and function of cell adhesion molecules involved in inflammatory/immune responses. The current knowledge of the distribution of cell adhesion molecules is described and the potential for modulation of cell adhesion molecule function is discussed.

Non-carbohydrate binding partners/domains of animal lectins

1. Protein-carbohydrate interactions are involved in a large number of biologically important recognition processes. 2. Among the participating classes of proteins lectins are defined as carbohydrate-binding proteins other than an antibody or an enzyme. 3. In addition to the essential carbohydrate-binding domain other functionally and/or structurally important sites, defined by sequence comparison or by experimental demonstration of protein-protein interactions, can be present within the lectin molecule and may be relevant for its physiological significance. 4. Sequence motifs of lectins for protein-protein interactions include amino acid structures designed for cell adhesion, growth regulatory biosignalling, intracellular routing and enzymatic activity. 5. Elucidation of the complete functional role(s) of a lectin requires accurate delineation of its carbohydrate and, if present, of its protein ligands. 6. Presence of more than one carbohydrate-binding domain in a single lectin, potential ligand properties of the glycopart of a lectin, regulatory interplay between different sites and possible interaction of complementarily shaped peptide sequences to the sugar-recognizing site should all be assessed in the quest to comprehensively explain the physiological role(s) of a lectin.

A role for the epidermal growth factor-like domain of P-selectin in ligand recognition and cell adhesion

The selectin family of adhesion molecules mediates the initial interactions of leukocytes with endothelium. The extracellular region of each selectin contains an amino-terminal C-type lectin domain, followed by an EGF-like domain and multiple short consensus repeat units (SCR). Previous studies have indirectly suggested a role for each of the extracellular domains of the selectins in cell adhesion. In this study, a panel of chimeric selectins created by exchange of domains between L- and P-selectin was used to directly examine the role of the extracellular domains in cell adhesion. Exchange of only the lectin domains between L- and P-selectin conferred the adhesive and ligand recognition functions of the lectin domain of the parent molecule. However, chimeric selectins which contained both the lectin domain of L-selectin and the EGF-like domain of P-selectin exhibited dual ligand-binding specificity. These chimeric proteins supported adhesion both to myeloid cells and to high endothelial venules (HEV) of lymph nodes and mesenteric venules in vivo. Exchange of the SCR domains had no detectable effect on receptor function or specificity. Thus, the EGF-like domain of P-selectin may play a direct role in ligand recognition and leukocyte adhesion mediated by P-selectin, with the lectin plus EGF-like domains collectively forming a functional ligand recognition unit.

Bovine L-selectin: a peripheral lymphocyte homing receptor

The cDNA clone encoding for the bovine peripheral lymph node homing receptor (L-selectin) was isolated and sequenced. The predicted amino acid sequence of bovine L-selectin showed an overall high identity with that of human and murine L-selectin. However, the cytoplasmic tail of bovine L-selectin showed little similarity to that of human and murine L-selectin. The monoclonal antibody DREG-56, which recognizes human L-selectin, blocked the binding of bovine peripheral blood lymphocytes to high-walled endothelial venules in murine peripheral lymph nodes. Surface expression of bovine L-selectin was high in lymphocytes isolated from peripheral lymph nodes and low in lymphocytes isolated from Peyer's patches. This evidence strongly suggests that bovine L-selectin is a peripheral lymphocyte homing receptor.

Interaction of P-selectin (CD62) and its cellular ligand: analysis of critical residues

P-Selectin (CD62, PADGEM, GMP140) is a membrane glycoprotein which is rapidly mobilized to the surface of activated platelets and endothelial cells where it mediates leukocyte-platelet and leukocyte-vascular endothelial cell adhesion, respectively. P-Selectin is a member of a family of adhesion molecules which includes the endothelial cell adhesion molecule E-selectin and the leukocyte adhesion molecule L-selectin. Selectins mediate cell-cell binding resulting from the interaction between the amino terminal lectin domains of the selectins and their respective carbohydrate ligands. Here we report on a three-dimensional model of the lectin domain of P-selectin which was derived on the basis of its structural homology to the rat mannose binding protein (MBP) whose crystal structure has recently been reported. On the basis of the model, a number of point mutants were prepared to identify the P-selectin binding site. The residues found to be important for binding are located in a shallow groove on the surface of the molecule composed of residues from the beta-2, -3, and -5 strands of the P-selectin lectin domain. A number of residues within this groove, which are conserved among all selectins, were found to be critical for P-selectin binding. They include Lys113, Tyr48, and Tyr94. The single substitutions Lys113Ala, Tyr48Ala, Tyr48Phe, Tyr94Ala, and Tyr94Phe abolished P-selectin binding to myeloid cells.

P- and E-selectin use common sites for carbohydrate ligand recognition and cell adhesion

The selectins are a family of three calcium-dependent lectins that mediate adhesive interactions between leukocytes and the endothelium during normal and abnormal inflammatory episodes. Previous work has implicated the carbohydrate sialyl Lewis(x) (sLe(x); sialic acid alpha 2-3 galactose beta 1-4 [Fucose alpha 1-3] N-acetyl glucosamine) as a component of the ligand recognized by E- and P-selectin. In the case of P-selectin, other components of the cell surface, including 2'6-linked sialic acid and sulfatide (galactose-4-sulfate ceramide), have also been proposed for adhesion mediated by this selectin. We have recently defined a region of the E-selectin lectin domain that appears to be directly involved with carbohydrate recognition and cell adhesion (Erbe, D. V., B. A. Wolitzky, L. G. Presta, C. R. Norton, R. J. Ramos, D. K. Burns, R. M. Rumberger, B. N. N. Rao, C. Foxall, B. K. Brandley, and L. A. Lasky. 1992. J. Cell Biol. 119:215-227). Here we describe a similar analysis of the P-selectin lectin domain which demonstrates that a homologous region of this glycoprotein's lectin motif is involved with carbohydrate recognition and cell binding. In addition, we present evidence that is inconsistent with a biological role for either 2'6-linked sialic acid or sulfatide in P-selectin-mediated adhesion. These results suggest that a common region of the E- and P-selectin lectin domains appears to mediate carbohydrate recognition and cell adhesion.

hikaru genki, a CNS-specific gene identified by abnormal locomotion in Drosophila, encodes a novel type of protein

We have identified a gene, hikaru genki (hig), whose mutant phenotype includes abnormal locomotor behavior. Mutant first instar larvae have uncoordinated movements, and both larvae and adults have reduced locomotion. Sequence analyses revealed that this gene encodes a novel type of protein with a signal sequence, but without transmembrane regions. One of its domains has similarities with immunoglobulin domains; three or four regions are similar to a complement-binding domain found in complement-related proteins and selectins. In situ hybridization to embryos revealed that accumulation of the hig transcripts is restricted to subsets of cells in the CNS. Our data suggest that hig has a role in the development of CNS functions involved in locomotor activity.

Selectins

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The selectin family of carbohydrate-binding proteins: structure and importance of carbohydrate ligands for cell adhesion

Protein-carbohydrate interactions have been found to be important in many steps in lymphocyte recirculation and inflammatory responses. A family of carbohydrate-binding proteins or lectins, termed selectins, has been discovered and shown to be involved directly in these processes. The three known selectins, termed L-, E- and P-selectins, have domains homologous to other Ca(2+)-dependent (C-type) lectins. L-selectin is expressed constitutively on lymphocytes, E-selectin is expressed by activated endothelial cells, and P-selectin is expressed by activated platelets and endothelial cells. Here, we review the nature of the carbohydrate determinants in tissues recognized by these selectins. The expression of specific sialylated, fucosylated and sulfated carbohydrates in activated endothelium and high endothelial venules promotes interactions with L-selectin on leukocyte surfaces. In contrast, E- and P-selectins recognize specific carbohydrate determinants related to sialyl Le(x) antigen on neutrophil and monocyte surfaces. The discovery of the selectins has generated excitement among glycoconjugate researchers that other carbohydrate-binding proteins and their cognate ligands will be found to function in regulating many types of cellular interactions.

The junction between cytokines and cell adhesion

Several aspects of the interactions between growth factors and cell adhesion are described. Recent advances in the field come from the identification of molecules resembling growth factors or growth factor receptors, which bear cell adhesion motifs as well as molecules participating in both cell growth control and adhesion.

Identification of an E-selectin region critical for carbohydrate recognition and cell adhesion

E-selectin elicits cell adhesion by binding to the cell surface carbohydrate, sialyl Lewis X (sLe(x)). We evaluated the effects of mutations in the E-selectin lectin domain on the binding of a panel of anti-E-selectin mAbs and on the recognition of immobilized sLe(x) glycolipid. Functional residues were then superimposed onto a three-dimensional model of the E-selectin lectin domain. This analysis demonstrated that the epitopes recognized by blocking mAbs map to a patch near the antiparallel beta sheet derived from the NH2 and COOH termini of the lectin domain and two adjacent loops. Mutations that affect sLe(x) binding map to this same region. These results thus define a small region of the E-selectin lectin domain that is critical for carbohydrate recognition.

Leukocyte-endothelial cell interactions

The movement of leukocytes from the blood circulation into organized lymphoid tissues or sites of inflammation requires cooperative interactions between signaling and adhesion molecules. Selectins mediate the initial rolling contacts of leukocytes with the endothelium. Following leukocyte activation, integrins strengthen adhesion and then direct migration beneath the endothelium. Unique combinations of signaling and adhesion molecules may regulate the subsets of leukocytes that are recruited into specific tissues.

Soluble L-selectin is present in human plasma at high levels and retains functional activity

L-selectin expressed by granulocytes, lymphocytes, and monocytes is responsible for initial leukocyte attachment to inflamed endothelium and high endothelial venules of peripheral lymph nodes. After leukocyte activation in vitro, L-selectin is rapidly shed from the cell surface. In this study, shed L-selectin (sL-selectin) from both lymphocytes and neutrophils was demonstrated to be present in high levels in human plasma by Western blot analysis and using a quantitative ELISA. In serum from normal human blood donors, a mean sL-selectin level of 1.6 +/- 0.8 micrograms/ml (n = 63) was found by ELISA. In addition, semipurified sL-selectin from plasma inhibited L-selectin-specific attachment of lymphocytes to cytokine-activated endothelium in a dose-dependent manner. L-selectin-dependent leukocyte attachment was completely inhibited at sL-selectin concentrations of 8-15 micrograms/ml, while physiological concentrations of sL-selectin caused a small but consistent inhibition of lymphocyte attachment. sL-selectin in plasma also inhibited anti-L-selectin mAb (2-5 micrograms/ml) binding to the surface of leukocytes. Interestingly, one epitope present within the EGF-like domain of L-selectin was lost in sL-selectin, suggesting a conformational change in the structure of the receptor after shedding. The presence of serum sL-selectin with functional activity indicates a potential role for sL-selectin in the regulation of leukocyte attachment to endothelium.

Complex carbohydrate-lectin interaction at the interface: a model for cellular adhesion. II. Reactivity of both the oligosaccharide chain and sugar-binding domain of a glycoprotein lectin

We describe studies of a new model cell adhesion system involving liposomes bearing lectins and the glycosphingolipid, asialomonosialoganglioside (asialoGM1). The model provides a simple analysis of experimental data to elucidate the mechanism of heterophilic cell-cell adhesion mediated by multiple protein-carbohydrate interactions. Phospholipid vesicles bearing the fatty acid conjugate of a glycoprotein lectin from Ricinus communis (RCAI vesicle) are shown to react with vesicles bearing the fatty acid conjugate of Concanavalin A (Con A) and asialoGM1 (Con A vesicle). The kinetics of aggregation and monosaccharide-induced disaggregation of the two types of vesicles were followed by monitoring the time-dependent change in turbidity. Depending on the surface density of the asialoGM1, 40-60% of the resulting precipitin complex was dissociable only in the presence of both RCAI-specific galactose and Con A-specific alpha-methyl-D-mannoside. Results indicate simultaneous participation of both the saccharide-binding domain and carbohydrate sequence of RCAI, a model cell adhesion molecule, to stabilize the encounter complex by two types of interactions. These findings support the possibility of stable cell-cell adhesion in vivo occurring via interactions between cell adhesion molecules on apposing cell-surface membranes.

Characterization of a functionally important and evolutionarily well-conserved epitope mapped to the short consensus repeats of E-selectin and L-selectin

Selectins represent a new family of adhesion molecules, expressed by leukocytes and endothelial cells, that are involved in the regulation of leukocyte traffic. Here we have characterized a new monoclonal antibody (mAb) (EL-246) that recognizes both human leukocyte L-selectin (previously called LAM-1, LECAM-1, or gp90MEL-14) and endothelial cell E-selectin (previously called ELAM-1). EL-246 recognized a 110-kD protein expressed on cells transfected with E-selectin cDNA and stained many postcapillary venules in inflamed human tonsil. EL-246 also stained human peripheral blood leukocytes and showed identity with anti-L-selectin mAb in two-color flow cytometric analysis. The expression of the leukocyte EL-246 antigen was regulated in the same manner as L-selectin and EL-246 recognized anti-L-selectin mAb affinity-purified antigen in SDS/PAGE Western blot analysis. Further, L-selectin cDNA transfectants were specifically stained by EL-246. EL-246 blocked greater than 95% of lymphocyte adhesion to peripheral lymph node high endothelial venules and greater than 90% of neutrophil adhesion to E-selectin transfectants. In addition to the EL-246 epitope being expressed on two different human selectins, it was detected on L-selectin from a variety of different animals. Interestingly, domain mapping studies localized the EL-246 epitope to the short consensus repeat (SCR) domains of L-selectin. EL-246 is the first mAb that recognizes two different selectins and potentially defines a functional epitope encoded by the SCR domains. Inhibitors of selectin function targeted to this region would be expected to have the added advantage of simultaneously blocking the activity of two distinct adhesion proteins involved in inflammation.

The three members of the selectin receptor family recognize a common carbohydrate epitope, the sialyl Lewis(x) oligosaccharide

The selectins (lectin-EGF-complement binding-cell adhesion molecules [LEC-CAMs]) are a family of mammalian receptors implicated in the initial interactions between leukocytes and vascular endothelia, leading to lymphocyte homing, platelet binding, and neutrophil extravasation. The three known selectins, L-selectin (leukocyte adhesion molecule-1 [LECAM-1]), E-selectin (endothelial-leukocyte adhesion molecule-1 [ELAM-1]), and P-selectin (GMP-140) share structural features that include a calcium-dependent lectin domain. The sialyl Lewis(x) carbohydrate epitope has been reported as a ligand for both E- and P-selectins. Although L-selectin has been demonstrated to bind to carbohydrates, structural features of potential mammalian carbohydrate ligand(s) have not been well defined. Using an ELISA developed with a sialyl Lewis(x)-containing glycolipid and an E-selectin-IgG chimera, we have demonstrated the direct binding of the L-selectin-IgG chimera to sialyl Lewis(x). This recognition was calcium dependent, and could be blocked by Mel-14 antibody but not by other antibodies. Recognition was confirmed by the ability of cells expressing the native L-selectin to adhere to immobilized sialyl Lewis(x). These data suggest that the sialyl Lewis(x) oligosaccharide may form the basis of a recognition domain common to all three selectins.

Structure and function of L-selectin

The selectins are a newly described family of carbohydrate-binding adhesion molecules involved in the regulation of leukocyte traffic. Selectins are composed of an N-terminal C-type lectin domain, a single EGF domain, a variable number of short consensus repeat (SCR) domains, a transmembrane region and a cytoplasmic tail. L-selectin (LAM-1/LECAM-1/LECCAM-1) is the only selectin expressed on leukocytes, and mediates a number of leukocyte-endothelial interactions, including the binding of lymphocytes to HEV of peripheral lymph node high endothelial venules (HEV), neutrophil rolling, and leukocyte attachment to cytokine-treated endothelium in vitro. Stable transfectants expressing a series of chimeric selectins and deletion mutants were functionally analyzed in order to determine the molecular basis of adhesion mediated by L-selectin. The specificity of adhesion was found to reside entirely within the lectin domain, suggesting that this domain is the only domain of the protein to interact with the carbohydrate ligand. These results make previous observations that certain mAbs which block function map to each of the extracellular domains difficult to interpret. In addition, deletion of the cytoplasmic tail of L-selectin abolished adhesion, without affecting ligand recognition. Thus, each domain of the selectins has an important, but distinct, role in cell adhesion.

Selectins and other mammalian sialic acid-binding lectins

Several recently discovered mammalian cell adhesion proteins recognize and bind to sialic acid-containing ligands. Reports concerning the molecular specificities of these interactions have been intriguing but somewhat confusing, partly because of pitfalls in methodology or interpretation. Nevertheless, these protein-carbohydrate recognition phenomena are important in the normal biology of blood cells and in the pathophysiology of many diseases.