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

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

SDA and IDA - Two aptamers to inhibit cancer cell adhesion

Aptamers which bind to proteins involved in cell-cell interactions could have significant value to directly affect cancer cell adhesion or for directed cargo delivery. Here, I discuss two aptamers: aptamer SDA which binds to E- and P-selectin, and aptamer IDA which binds to α6β4 integrin. Both aptamers (SDA 91 nt and IDA 77 nt) bind their target proteins with dissociation constants in the 100-150 nM range and substantially inhibit special cellular adhesion, possibly a first and pivotal step in transendothelial migration during metastasis formation. The aptamers' half-lives in cell culture media are between two and six hours. IDA is internalized by integrin presenting cells within minutes thus possibly serving as vehicle for directed cargo delivery.

E-selectin ligand complexes adopt an extended high-affinity conformation

E-selectin is a cell-adhesion molecule of the vascular endothelium that promotes essential leukocyte rolling in the early inflammatory response by binding to glycoproteins containing the tetrasaccharide sialyl Lewis(x) (sLe(x)). Efficient leukocyte recruitment under vascular flow conditions depends on an increased lifetime of E-selectin/ligand complexes under tensile force in a so-called catch-bond binding mode. Co-crystal structures of a representative fragment of the extracellular E-selectin region with sLe(x) and a glycomimetic antagonist thereof reveal an extended E-selectin conformation, which is identified as a high-affinity binding state of E-selectin by molecular dynamics simulations. Small-angle X-ray scattering experiments demonstrate a direct link between ligand binding and E-selectin conformational transition under static conditions in solution. This permits tracing a series of concerted structural changes connecting ligand binding to conformational stretching as the structural basis of E-selectin catch-bond-mediated leukocyte recruitment. The detailed molecular view of the binding site paves the way for the design of a new generation of selectin antagonists. This is of special interest, since their therapeutic potential was recently demonstrated with the pan-selectin antagonists GMI-1070 (Rivipansel).

Implications of the E-selectin S128R mutation for drug discovery

The C-type lectin E-selectin mediates the rolling of circulating leukocytes on vascular endothelial cells during the inflammatory process. In numerous studies, the S128R mutation of the E-selectin was associated with cardiovascular and autoimmune diseases. There is evidence that the S128R E-selectin mutation leads to a loss in ligand specificity, thus increasing leukocyte recruitment. Apart from the natural tetrasaccharide ligand sialyl Lewis(x) (sLe(x)), it has previously been proposed that non-fucosylated carbohydrates also bind to S128R E-selectin. To evaluate the therapeutic potential of the antagonism of the E-selectin mutant, ligand specificity was reinvestigated on a molecular basis. We determined the ligand specificity of wild-type and S128R E-selectin in a target-based competitive assay, a glycan array screen and cell-based binding assays under static and flow conditions. Regarding ligand-specificity, the binding properties of S128R E-selectin were identical to those of wt E-selectin, i.e., no mutant-specific binding of 3'-sialyl-N-acetyllactosamine, heparin, fetuin and K562 cells was observed. Additionally, the binding affinities of glycomimetic E-selectin antagonists were identical for wt and S128R E-selectin. Overall, the previous reports on carbohydrate ligand promiscuity of S128R E-selectin could not be confirmed.

Select human milk oligosaccharides directly modulate peripheral blood mononuclear cells isolated from 10-d-old pigs

Infant formulas lack the complex mixture of oligosaccharides found in human milk. These human milk oligosaccharides (HMO) may be pivotal to the development of the neonatal immune system. Few comprehensive analyses of the effects of HMO on immune cells from neonates have been undertaken. Herein, the direct effects of HMO on immune cells were analysed ex vivo. Peripheral blood mononuclear cells (PBMC) isolated from 10-d-old sow-reared (SR) or colostrum-deprived formula-fed (FF) pigs were stimulated for 72 h with single HMO, mixtures of single HMO or a complex mixture of HMO isolated from human milk (iHMO). T-cell phenotype, cytokine production and proliferation were measured by flow cytometry, immunoassay and [³H]thymidine incorporation, respectively. Stimulation with HMO had direct effects on PBMC. For instance, cells stimulated with iHMO produced more IL-10 than unstimulated cells, and cells stimulated with fucosylated HMO tended to proliferate less than unstimulated cells. Additionally, co-stimulation with HMO mixtures or single HMO altered PBMC responses to phytohaemagglutinin (PHA) or lipopolysaccharide (LPS) stimulation. Compared with PBMC stimulated with PHA alone, cells co-stimulated with iHMO and PHA proliferated more and had fewer detectable CD4⁺CD8⁺ T cells. Compared with PBMC stimulated by LPS alone, cells co-stimulated with a mixture of sialylated HMO and LPS proliferated more and tended to have fewer detectable CD4⁺ T cells. Differences in the baseline responses of PBMC isolated from the SR or FF pigs were observed. In summary, HMO directly affected PBMC populations and functions. Additionally, ex vivo measurements of PBMC phenotype, cytokine production and proliferation were influenced by the neonate's diet.

P-selectin glycoprotein ligand-1 forms dimeric interactions with E-selectin but monomeric interactions with L-selectin on cell surfaces

Interactions of selectins with cell surface glycoconjugates mediate the first step of the adhesion and signaling cascade that recruits circulating leukocytes to sites of infection or injury. P-selectin dimerizes on the surface of endothelial cells and forms dimeric bonds with P-selectin glycoprotein ligand-1 (PSGL-1), a homodimeric sialomucin on leukocytes. It is not known whether leukocyte L-selectin or endothelial cell E-selectin are monomeric or oligomeric. Here we used the micropipette technique to analyze two-dimensional binding of monomeric or dimeric L- and E-selectin with monomeric or dimeric PSGL-1. Adhesion frequency analysis demonstrated that E-selectin on human aortic endothelial cells supported dimeric interactions with dimeric PSGL-1 and monomeric interactions with monomeric PSGL-1. In contrast, L-selectin on human neutrophils supported monomeric interactions with dimeric or monomeric PSGL-1. Our work provides a new method to analyze oligomeric cross-junctional molecular binding at the interface of two interacting cells.

Sialyl Lewis(x): a "pre-organized water oligomer"?

Organized and released: Sialyl Lewis(x) (sLe(x)) represents a "pre-organized water oligomer", that is, a surrogate for clustered water molecules attached to a scaffold. The impetus for sLe(x) binding to E-selectin is shown to be the high degree of pre-organization allowing an array of directed hydrogen bonds, and the entropic benefit of the release of water molecules from the large binding interface to bulk water (see picture).

Chronic discrimination predicts higher circulating levels of E-selectin in a national sample: the MIDUS study

Chronic discrimination in both minority and non-minority populations is linked to adverse health outcomes, including increased risk of cardiovascular disease and increased mortality, but the biological processes through which discrimination affects health are unclear. The current study tested the hypothesis that discrimination in a sample of Caucasians would predict elevated serum levels of E-selectin, an indication of endothelial dysfunction which itself is associated with atherosclerosis and cardiovascular disease risk. Participants (N=804) in the biomarker sample from the Survey of Midlife in the United States (MIDUS) provided information about experiences of both major and everyday discrimination at two times separated by a 9-10 year interval. The discrimination measures were designed to assess perceived unfair treatment (e.g. being fired unfairly) independently of the perceived reasons for the unfair treatment (e.g. race, gender). Serum E-selectin was measured at the second wave of data collection. Women reported significantly more instances of major (P<0.05) and everyday P<0.001) discrimination than men. Analyses of Covariance (ANCOVA) showed that both greater lifetime exposure to major discrimination (P<0.05) and chronic exposure to everyday discrimination (P<0.05) predicted higher circulating levels of E-selectin, but only in men. These associations remained statistically significant after adjustments for potential confounding variables, including age, race, socioeconomic status, health status, and health behavior. These results highlight a potential biological mechanism by which exposure to unfair treatment may be related to health, particularly cardiovascular function. Moreover, they add to a growing literature suggesting that unfair treatment in general may predict adverse health outcomes.

Markers of endothelial dysfunction and low-grade inflammation are associated in the offspring of type 2 diabetic subjects

The offspring of type 2 diabetic patients are at elevated risk for type 2 diabetes and cardiovascular disease. The aim of our study was to characterize the role of various biomarkers of endothelial activation in a cohort of offspring of type 2 diabetic subjects and to assess the association of adhesion molecules with inflammatory markers and metabolic parameters. Cytokine and adhesion molecule levels were measured in 19 healthy subjects and in 129 offspring of patients with type 2 diabetes (109 with normal glucose tolerance and 20 with impaired glucose tolerance). Insulin sensitivity was determined with the hyperinsulinemic-euglycemic clamp, insulin secretion with the intravenous glucose tolerance test, and abdominal fat distribution with computed tomography. The levels of vascular cell adhesion molecule-1, intercellular adhesion molecule-1, E-Selectin and vascular adhesion protein-1 were not increased in offspring of type 2 diabetic subjects, but they correlated with inflammatory markers (C-reactive protein, tumor necrosis-alpha, interleukin-6, interleukin-1 beta, interleukin-1 receptor antagonist, interleukin-8, interleukin-10 and interleukin-18). In conclusion, the levels of adhesion molecules were not elevated in the prediabetic state. Inflammatory markers and adhesion molecules were correlated suggesting that low-grade inflammation may precede the elevation of levels of adhesion molecules.

Recombinant production and characterization of the carbohydrate recognition domain from Atlantic salmon C-type lectin receptor C (SCLRC)

The Atlantic salmon C-type lectin receptor C (SCLRC) locus encodes a potential oligomeric type II receptor. C-type lectins recognize carbohydrates in a Ca(2+)-dependent manner through structurally conserved, yet functionally diverse, C-type lectin-like domains (CTLDs). Many conserved amino acids in animal CTLDs are present in SCLRC, with the notable exception of an asparagine crucially involved in Ca(2+)- and carbohydrate-binding, which is tyrosine in SCLRC. SCLRC also contains six cysteines that form three disulfide bonds. Although SCLRC was originally identified as an up-regulated transcript responding to Aeromonas salmonicida infection, the biological role of this protein is still unknown. To study the structure and ligand binding properties of SCLRC, we created a homology model of the 17kDa CTLD and produced it as an affinity-tagged protein in the periplasm of Escherichia coli by co-expression of proteins that facilitate disulfide bond formation. The recombinant form of SCLRC was characterized by a protease protection assay, a solid-phase carbohydrate-binding assay, and frontal affinity chromatography. On the basis of this characterization, we classify SCLRC as a C-type lectin that binds to mannose and its derivatives.

Effect of rosiglitazone on factors related to endothelial dysfunction in patients with type 2 diabetes mellitus

The effect of the insulin sensitizer rosiglitazone (RSG) on biological markers of endothelial dysfunction in subjects with type 2 diabetes mellitus (T2DM) was investigated in a 12-week, multi-center, randomized, double-blind study. One hundred and thirty-six subjects aged 40-70 years, with FPG > or = 7.0 and < or = 15.0 mmol/l, previously treated with a single oral anti-diabetic agent or diet/exercise, were randomized to RSG 8 mg/day (n=65) or placebo (PBO, n=71). Results revealed that RSG significantly reduced soluble (s)E-selectin by -10.9% (P=0.004) compared with PBO, but did not significantly alter soluble vascular cell adhesion molecule-1 (+0.6%, P=NS). Compared with PBO, RSG also significantly reduced plasminogen activator inhibitor-1 (-36.9%, P<0.001), tissue plasminogen activator antigen (-22.7%, P<0.001), FPG (-2.8 mmol/l, P<0.001), fasting fructosamine (-42.0 mg/dl, P<0.001). Post-prandial AUC(0-4h) for free fatty acids (FFAs) reduced by -6.5 mg/dl*h from baseline (P=0.03), a change that positively and significantly correlated with changes in sE-selectin (r=0.22, P=0.05). The incidence of adverse events was similar in the two groups (RSG: 35.4%; PBO: 40.8%); the majority mild or moderate. These data support the hypothesis that, in patients with T2DM, rosiglitazone has beneficial effects on biological markers of endothelial dysfunction. Improvements in insulin sensitivity and decreases in FFAs may play a role in these effects.

Molecular evolution of the human SRPX2 gene that causes brain disorders of the Rolandic and Sylvian speech areas

Background

The X-linked SRPX2 gene encodes a Sushi Repeat-containing Protein of unknown function and is mutated in two disorders of the Rolandic/Sylvian speech areas. Since it is linked to defects in the functioning and the development of brain areas for speech production, SRPX2 may thus have participated in the adaptive organization of such brain regions. To address this issue, we have examined the recent molecular evolution of the SRPX2 gene.

Results

The complete coding region was sequenced in 24 human X chromosomes from worldwide populations and in six representative nonhuman primate species. One single, fixed amino acid change (R75K) has been specifically incorporated in human SRPX2 since the human-chimpanzee split. The R75K substitution occurred in the first sushi domain of SRPX2, only three amino acid residues away from a previously reported disease-causing mutation (Y72S). Three-dimensional structural modeling of the first sushi domain revealed that Y72 and K75 are both situated in the hypervariable loop that is usually implicated in protein-protein interactions. The side-chain of residue 75 is exposed, and is located within an unusual and SRPX-specific protruding extension to the hypervariable loop. The analysis of non-synonymous/synonymous substitution rate (Ka/Ks) ratio in primates was performed in order to test for positive selection during recent evolution. Using the branch models, the Ka/Ks ratio for the human branch was significantly different (p = 0.027) from that of the other branches. In contrast, the branch-site tests did not reach significance. Genetic analysis was also performed by sequencing 9,908 kilobases (kb) of intronic SRPX2 sequences. Despite low nucleotide diversity, neither the HKA (Hudson-Kreitman-Aguadé) test nor the Tajima's D test reached significance.

Conclusion

The R75K human-specific variation occurred in an important functional loop of the first sushi domain of SRPX2, indicating that this evolutionary mutation may have functional importance; however, positive selection for R75K could not be demonstrated. Nevertheless, our data contribute to the first understanding of molecular evolution of the human SPRX2 gene. Further experiments are now required in order to evaluate the possible consequences of R75K on SRPX2 interactions and functioning.

Factors responsible for the evolution of insulin resistance in Japanese type 2 diabetic patients: association with atherosclerosis

Type 2 diabetes is a heterogeneous syndrome characterized by defective insulin secretion and/or insulin resistance. In distinct from Caucasian populations, Japanese type 2 diabetic patients are divided into two categories: one with insulin resistance and the other with normal insulin sensitivity. This unique feature allows us to explore the factors responsible for the evolution of insulin resistance in Japanese type 2 diabetic patients. In this article, we describe the factors responsible for insulin resistance in Japanese type 2 diabetic patients and discuss the relationships between these factors and atherosclerosis. Japanese type 2 diabetic patients with insulin resistance had significantly higher concentrations of triglyceride, remnant-like particle cholesterol, subcutaneous and visceral abdominal fat areas, leptin, high sensitive C-reactive protein (hs-CRP), and soluble E-selectin and lower concentration of adiponectin when compared to those with normal insulin sensitivity. There were, however, no significant difference in tumor necrosis factor (TNF)-alpha and soluble TNF receptors between the two groups. Serum triglyceride was positively correlated to visceral abdominal fat area, while serum leptin was positively correlated with subcutaneous abdominal fat area. In contrast, serum adiponectin was negatively correlated to visceral abdominal fat area. High sensitive CRP was positively correlated to BMI, triglyceride, and leptin, but was negatively correlated to adiponectin. Tumor necrosis factor-alpha and soluble TNF receptors, however, were not associated with any of these factors. Thus, it may be hypothesized that Japanese type 2 diabetic patients are divided into two categories: one with normal insulin sensitivity and the other with insulin resistance. The former group has a low cardiovascular risk factor, whereas the latter one has a markedly increased cardiovascular disease risk factor. Furthermore, abdominal fat related insulin resistance seems to be associated with insulin resistance in Japanese type 2 diabetic patients. In this section, we would like to focus on the factors contributing to insulin resistance and discuss the association of these factors with atherosclerosis in Japanese type 2 diabetic patients.

Calcium is not required for immulectin-2 binding, but protects the protein from proteinase digestion

Mammalian C-type lectins are calcium-dependent carbohydrate-binding proteins. They serve as cell adhesion molecules in cell-cell interactions, or function as pattern-recognition receptors in innate immunity. Calcium is a direct ligand for carbohydrate binding in mammalian C-type lectins such as mannose-binding proteins and macrophage mannose receptor. In the tobacco hornworm Manduca sexta, a group of lectins named immulectins have been discovered. Each immulectin contains dual carbohydrate-recognition domains. Previously, we showed that immulectin-2 (IML-2) binds to a bacterial lipopolysaccharide, and agglutination of Escherichia coli cells by IML-2 is calcium dependent. In this study, we demonstrated that IML-2 bound to bacterial lipid A, smooth and rough mutants of lipopolysaccharide, lipoteichoic acid and peptidoglycan, as well as to fungal mannan and beta-1, 3-glucan (laminarin and curdlan). Binding of IML-2 to microbial components was calcium independent, and was increased by addition of spermine, a polyamine. In addition, plasma IML-2 bound to mannan-agarose independent of calcium. But trypsin digestion of IML-2 was inhibited in the presence of calcium. Our results suggest that calcium is not required for IML-2 binding but protects IML-2 from trypsin digestion.

Soluble E-selectin, leptin, triglycerides, and insulin resistance in nonobese Japanese type 2 diabetic patients

The aim of the present study was to investigate the relationships between insulin resistance and soluble E-selectin, body mass index (BMI), leptin, and serum lipid profile including triglycerides in nonobese Japanese type 2 diabetic patients. A total of 97 nonobese Japanese type 2 diabetic patients aged 43 to 84 years were examined. The duration of diabetes was 11.2 +/- 0.8 years. In conjunction with BMI and fasting concentrations of plasma glucose, serum lipids (triglycerides, total cholesterol, and high-density lipoprotein cholesterol) and serum insulin, soluble E-selectin, and leptin were also measured. The low-density lipoprotein (LDL) cholesterol level was calculated using the Friedewald formula. Insulin resistance was estimated by the homeostasis model assessment. The subjects were divided into 2 groups according to the value of insulin resistance estimated by the homeostasis model assessment. Values greater than 2.5 were indicative of the insulin-resistant state, and values less than 2.5 were indicative of the insulin-sensitive state. The insulin-resistant group had significantly higher levels of E-selectin, leptin, triglycerides, total and LDL cholesterol, and diastolic blood pressure as compared with the insulin-sensitive group. There was, however, no significant difference in age, sex, diabetes duration, BMI, systolic blood pressure, HbA1c, and high-density lipoprotein cholesterol between the 2 groups. Univariate regression analysis showed that insulin resistance was positively correlated to E-selectin (r = 0.305, P = .003), BMI (r = 0.283, P = .006), leptin (r = 0.296, P = .004), HbA1c (r = 0.241, P = .018), serum triglycerides (r = 0.385, P < .001), serum total (r = 0.240, P = .019) and LDL cholesterol (r = 0.254, P = .013) levels, and systolic (r = 0.247, P = .024) and diastolic (r = 0.305, P = .006) blood pressure. Multiple regression analyses showed that insulin resistance was independently predicted by serum E-selectin (F = 18.4), serum leptin (F = 14.0) and serum triglycerides (F = 20.0) levels, which explained 45.0% of the variability of insulin resistance. From these results, it can be concluded that in conjunction with serum triglycerides and serum leptin, serum E-selectin is another important independent factor associated with insulin resistance in nonobese Japanese type 2 diabetic patients.

Morphology and transfection study of human microvascular endothelial cell angiogenesis: an in vitro three-dimensional model

Microvascular endothelial cells from human neonatal foreskin were grown in vitro until a three-dimensional network of capillary-like structures was formed. All stages of the angiogenic cascade could be observed in this in vitro model, including the formation of an internal lumen. The microscopy focused on morphology, formation of an internal lumen, role of the extracellular matrix, polarity of the cells, and the time-course of the angiogenic cascade. Bright-field microscopy revealed cells arranged circularly side by side and the internal lumen of capillary-like structures was verified by electron microscopy. Immunolabeling revealed a peritubular localization of collagen IV. Reporter gene expression after the formation of capillary-like structures was marginally higher than control expression, but clearly lower than the expression of cells at the stage of proliferation. Highest transfection efficiencies were obtained using vectors with the CMV promoter and the long fragment of the Ets-1 promoter. This is a first study of transfection efficiencies mapped for stages of in vitro angiogenesis. We describe here the morphological features of a long-term in vitro model of angiogenesis of human microvascular endothelial cells that could be used for transfection studies, without the provision of an extracellular matrix substrate. The cells self-create their own extracellular matrix to proliferate and form a three-dimensional network of capillary-like structures with an internal lumen.

Quantifying the effects of molecular orientation and length on two-dimensional receptor-ligand binding kinetics

Surface presentation of adhesion receptors influences cell adhesion, although the mechanisms underlying these effects are not well understood. We used a micropipette adhesion frequency assay to quantify how the molecular orientation and length of adhesion receptors on the cell membrane affected two-dimensional kinetic rates of interactions with surface ligands. Interactions of P-selectin, E-selectin, and CD16A with their respective ligands or antibody were used to demonstrate such effects. Randomizing the orientation of the adhesion receptor or lowering its ligand- and antibody-binding domain above the cell membrane lowered two-dimensional affinities of the molecular interactions by reducing the forward rates but not the reverse rates. In contrast, the soluble antibody bound with similar three-dimensional affinities to cell-bound P-selectin constructs regardless of their orientation and length. These results demonstrate that the orientation and length of an adhesion receptor influences its rate of encountering and binding a surface ligand but does not subsequently affect the stability of binding.

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.

Minimal requirements for the binding of selectin ligands to a C-type carbohydrate-recognition domain

The C-type carbohydrate-recognition domains of E-selectin and rat serum mannose-binding protein have similar structures. Selectin/mannose-binding protein chimeras created by transfer of key sequences from E-selectin into mannose-binding protein have previously been shown to bind the selectin ligand sialyl-Lewis(X) through a Ca(2+)-dependent subsite, common to many C-type lectins, and an accessory site containing positively charged amino acid residues. Further characterization of these chimeras as well as analysis of novel constructs containing additional regions of E-selectin demonstrate that selectin-like interaction with sialyl-Lewis(X) can be faithfully reproduced even though structural evidence indicates that the mechanisms of binding to E-selectin and the chimeras are different. Selectin-like binding to the nonfucosylated sulfatide and sulfoglucuronyl glycolipids can also be reproduced with selectin/mannose-binding protein chimeras that contain the two subsites involved in sialyl-Lewis(X) binding. These results indicate that binding of structurally distinct anionic glycans to C-type carbohydrate-recognition domains can be mediated by the Ca(2+)-dependent subsite in combination with a positively charged region that forms an ionic strength-sensitive subsite.

Species-specific and conserved epitopes on mouse and human E-selectin important for leukocyte adhesion

Selectins are C-type, cell surface lectins that are key players in leukocyte adhesion to the blood vessel wall endothelium. We describe here epitopes for a series of novel monoclonal antibodies (moAbs), UZ4-UZ7, directed against mouse E-selectin. All four antibodies specifically bind to mouse E-selectin, but not to P- or L-selectin, and all inhibit the adhesion of granulocytes, peripheral blood lymphocytes, and promyelocytic HL-60 cells to cytokine-activated mouse endothelium. Three moAbs, UZ5, UZ7, and UZ6, specifically inhibit mouse E-selectin-mediated adhesion by binding to epitopes in domains CR1 or CR2. moAb UZ4 inhibits leukocyte adhesion to both human and murine endothelium activated with IL-1 or other proinflammatory stimuli. UZ4 is the first described moAb that detects an epitope in the lectin domain which is conserved in both murine and human E-selectin (CXKKKL), but is not present in the other members of the selectin family, P- and L-selectin. Interestingly, UZ5, UZ6, and UZ7 more efficiently interfere with lymphocyte than with granulocyte adhesion to cytokine-activated endothelium, while UZ4 completely blocks adhesion of PMN, lymphocytes, and HL-60 and U937 cell lines. The data suggest that E-selectin-ligand engagement differs between lymphocytes and PMN, and that these differences may be accentuated by the CR1 and CR2 domains in the E-selectin cell adhesion molecule.

Development of potent non-carbohydrate imidazole-based small molecule selectin inhibitors with antiinflammatory activity

A novel series of non-carbohydrate imidazole-based selectin inhibitors has been discovered via high-throughput screening using a P-selectin ELISA-based assay system. The initial lead 1 had an IC(50) of 17 microM in the P-selectin ELISA; this potency was significantly improved via an extensive SAR exploration. One of the current lead compounds (29) has an IC(50) of 300 nM in a P-selectin ELISA; it also has good activity in P- and E-selectin cell adhesion assays and shows efficacy in vivo. These compounds represent a novel series of sLe(X) mimetics with antiinflammatory activity. Their unique profile supports our interest in their further evaluation as drug candidates for the treatment of inflammation. Herein we describe the syntheses, optimization, and SAR of this series of novel potent selectin antagonists.

Novel mimics of sialyl Lewis X: design, synthesis and biological activity of a series of 2- and 3-malonate substituted galactoconjugates

A series of potent inhibitors of P-selectin as potential anti-inflammatory agents is reported. These compounds are derivatives of galactocerebrosides bearing a malonate side chain in positions 2 and 3 of the galactose moiety. Based on the binding mode of sialyl Lewis X, the two acidic groups of the malonate are designed to form ionic interactions with two important lysines in the active site of P-selectin, Lys113 and Lys111. On the other hand, the 4- and 6-hydroxy groups on the galactose ring are arranged to chelate the calcium ion in the P-selectin active site. The synthesis and the biological activity of this series of compounds are described. Lead compounds having a greater potency than sialyl Lewis X are identified.

Aberrant expression of adhesion molecules by Sézary cells: functional consequences under physiologic shear stress conditions

Although aberrations in adhesion molecule expression by lymphoma cells have been reported, the functional consequences of these changes are unclear. Herein, we report a patient with Sézary syndrome whose malignant peripheral blood T cells were TCRVbeta17+. Malignant T cell adhesion molecule abnormalities included an 80% downregulation of LFA-1 compared with normal controls and no detectable expression of alpha4 integrin. Under shear stress conditions, malignant T cells failed to arrest on recombinant ICAM-1 in the presence of chemokines and displayed an 80% decrease in the ability to arrest on TNF-alpha activated dermal microvascular endothelial cells compared with normal CD4+ memory T cells. Cutaneous lymphocyte-associated antigen expression was detected in approximately 25% of malignant T cells in the peripheral blood, but was substantially less than this in TCRVbeta17+ T cells in the dermis. By contrast, > 95% of malignant T cells in peripheral blood expressed L-selectin (CD62L), and L-selectin ligand was detected in dermal blood vessels at affected skin sites. Compared with normal CD4+, malignant T cells attached and rolled 6-fold more efficiently on L-selectin ligand (p < 0.0001). Thus, despite aberrant expression of LFA-1 and functional defects in the ability to arrest on activated endothelial cells, malignant T cells in this patient entered skin and produced significant clinical disease. We propose a mechanism by which the upregulated expression of L-selectin and L-selectin ligands may partially compensate for altered LFA-1 function.

Cutting edge: C-C chemokine receptor 6 is essential for arrest of a subset of memory T cells on activated dermal microvascular endothelial cells under physiologic flow conditions in vitro

Memory T cells (mTC) express multiple chemokine receptors (including CCR4 and CCR6) that may potentially be involved in their arrest on inflamed endothelia. Herein, we specifically addressed whether CCR6 is required for mTC to arrest on TNF-alpha-activated human dermal microvascular endothelial cells (HDMEC) in vitro under shear stress conditions. Recombinant liver and activation-regulated chemokine (LARC)/CCL20 (a CCR6 ligand) induced firm arrest of cutaneous lymphocyte Ag(+) mTC in a flow chamber system using purified substrates. Strikingly, desensitization of CCR6 with LARC, but not thymus and activation-regulated chemokine/CCL17 or secondary lymphoid tissue chemokine/CCL21, caused a 50-75% decrease (p < 0. 001) in arrest of mTC on HDMEC, which was indistinguishable from the reduction observed when total mTC were treated with pertussis toxin (p > 0.5). CCR6-depleted mTC also had a markedly reduced ability to arrest on HDMEC. Our results suggest that LARC production by activated endothelial cells and CCR6 expression by mTC may be critical components in the pertussis toxin-sensitive arrest of mTC on activated HDMEC.

Sweet Tooth, a novel receptor protein-tyrosine kinase with C-type lectin-like extracellular domains

A gene encoding a novel type of receptor protein-tyrosine kinase was identified in Hydra vulgaris. The extracellular portion of this receptor (which we have named Sweet Tooth) contains four C-type lectin-like domains (CTLDs). Comparison of the sequences of these domains with the sequences of the carbohydrate recognition domains of various vertebrate C-type lectins shows that Sweet Tooth CTLD1 and CTLD4 have amino acids in common with those shown to be involved in carbohydrate binding by the lectins. Comparison of sequences encoding CTLD1 from the Sweet Tooth genes from different species of Hydra shows variation in some of the conserved residues that participate in carbohydrate binding in C-type lectins. The Sweet Tooth gene is expressed widely in the Hydra polyp, and expression is particularly high in the endoderm of the tentacles. Treatment of polyps with peptides corresponding to sequences in the Sweet Tooth CTLDs results in the disintegration of the animal. These same peptides do not block adhesion or morphogenesis of Hydra cell aggregates.

Fatty acid modulation of endothelial activation

Dietary balance of long-chain fatty acids may influence processes involving leukocyte-endothelial interactions, such as atherogenesis and inflammation, that involve increased endothelial expression of leukocyte adhesion molecules, or endothelial activation. We compared the ability of various saturated, monounsaturated, and polyunsaturated fatty acids to modulate endothelial activation. Consumption of the n-3 fatty acid docosahexaenoic acid (DHA; 22:6n-3) reduced endothelial expression of vascular cell adhesion molecule 1 (VCAM-1), E-selectin, intercellular adhesion molecule 1 (ICAM-1), interleukin 6 (IL-6), and IL-8 in response to IL-1, IL-4, tumor necrosis factor, or bacterial endotoxin, with a half-maximal inhibitory concentration (IC(50)) of 1-25 micromol, ie, in the range of nutritionally achievable plasma concentrations. The magnitude of this effect paralleled its incorporation into cellular phospholipids. DHA also reduced the adhesion of human monocytes and monocytic U937 cells to cytokine-stimulated endothelial cells. These effects were accompanied by a reduction in VCAM-1 messenger RNA, indicating a pretranslational effect. To assess structural fatty acid determinants of VCAM-1 inhibitory activity, we compared various saturated, monounsaturated, and n-6 and n-3 polyunsaturated fatty acids for their VCAM-1 inhibitory activity. Saturated fatty acids did not inhibit cytokine-induced expression of adhesion molecules. However, a progressive increase in inhibitory activity was observed with dietary intake of fatty acids with the same chain length but increasing double bonds, ie, from monounsaturated to n-6 and, further, to n-3 fatty acids. Thus, the greater number of double bonds seems critical for the greater activity of n-3 compared with n-6 fatty acids in inhibiting endothelial activation. These properties are likely to be relevant to the antiatherogenic and antiinflammatory properties of n-3 fatty acids.

Functional characterization of atherosclerosis-associated Ser128Arg and Leu554Phe E-selectin mutations

The cellular adhesion molecule E-selectin is expressed on activated endothelial cells, and is involved in the process of adherence of blood cells to vessel endothelium in inflammatory events such as atherosclerosis. In a recent study we found a Ser128Arg mutation in the EGF domain as well as a Leu554Phe mutation in the membrane domain of E-selectin. We also established increased frequencies of both mutations among young patients with severe coronary atherosclerosis. In the present study we investigated the influence of these mutations on cell adhesion and on the release of soluble E-selectin. Mutants were created by site-directed mutagenesis and COS cells were transfected with E-selectin, either wild-type or mutant. Antibody-binding studies and cell-adhesion assays were performed on transfected COS cells and on interleukin-1 beta-stimulated HUVECs. Soluble E-selectin in supernatants of wild type and Leu554Phe mutant-transfected COS cells was measured by ELISA. We discovered significant differences in the strength of HL-60 cell adhesion for the Ser128Arg mutant: in comparison with the wild type, the strength of adhesion to the mutant was reduced on transfected COS cells (P < 0.01) as well as on stimulated HUVECs (P < 0.01). Significantly diminished release of soluble E-selectin was detected for the Leu554Phe membrane domain mutant, in comparison with the wild type. In summary, the mutations studied here influence the E-selectin function in vitro and may be considered as one of the risk factors involved in the complex pathogenesis of atherosclerosis.

Characterization of an apparently conserved epitope in E- and P-selectin identified by dual-specific monoclonal antibodies

E- and P-selectin recognize a wide and overlapping range of oligosaccharide ligands including sialyl-Lewis X (sLeX) through their highly homologous C-type lectin domains. We report that an epitope apparently conserved between E- and P-selectin is functionally involved in ligand recognition although distantly located from the conventional carbohydrate binding site. We found that a previously established anti-E-selectin monoclonal antibody (mAb), 1.2B6, is cross-reactive with P-selectin, and that the 1.2B6 epitope is in the C-type lectin domain and identical to or overlapping with an epitope recognized by other independently established anti-E- and P-selectin dual-specific mAb. The epitope has been mapped by others to a region distant from the previously identified carbohydrate binding site of E-selectin in its three-dimensional structure. Nevertheless, it is of note that all dual-specific mAb, including 1.2B6, inhibited E- or P-selectin-mediated cell adhesion and also binding to sLeX. Engagement of the apparently conserved epitope by the dual-specific mAb may lead to inhibition of the ligand binding ability of E- and P-selectin by a previously uncharacterized mechanism(s) rather than by direct inhibition of sLeX binding to the hitherto identified ligand binding site.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.

Mechanism of ligand binding to E- and P-selectin analyzed using selectin/mannose-binding protein chimeras

The mechanism of oligosaccharide binding to the selectin cell adhesion molecules has been analyzed by transferring regions of the carbohydrate-recognition domains of E- and P-selectin into corresponding sites in the homologous rat serum mannose-binding protein. Insertion of two basic regions and an adjacent glutamic acid residue leads to efficient binding of HL-60 cells and sialyl-Lewisx-conjugated serum albumin. Substitution of glycine for a histidine residue known to stabilize mannose in the binding site of wild type mannose-binding protein results in dramatic loss of affinity for mannose without decreasing binding to sialyl-Lewisx. The accumulated effect of these changes is to alter the ligand binding selectivity of the domain so that it resembles E- or P-selectin more closely than it resembles the parental mannose-binding domain. Affinity labeling using sialyl-Lewisx in which the sialic acid has been mildly oxidized has been used to verify this switch in specificity and to show that the sialic acid-containing portion of the ligand interacts near the sequence Lys-Lys-Lys corresponding to residues 111-113 of E-selectin. The binding of sialyl-Lewisx-serum albumin is inhibited dramatically at physiological and higher salt concentrations, consistent with a significant electrostatic component to the binding interaction. The binding characteristics of these gain-of-function chimeras suggest that they contain many of the selectin residues responsible for selective ligand binding.

Epitope mapping of mouse monoclonal antibody EP-5C7 which neutralizes both human E- and P-selectin

The epitope of mouse monoclonal antibody (mAb) EP-5C7, which binds to and blocks both human E- and P-selectin, was mapped onto the protein structure of E-selectin. Analyses with E- and L-selectin chimeric proteins and randomly mutagenized E-selectins demonstrated that the EP-5C7 epitope consists of the amino acid residues at positions 21, 22, 23, 119 and 120 of E-selectin. The binding of three neutralizing anti-E-selectin mAb's (E-1E4, H18/7 and CL2), whose epitopes were found to overlap with the E-selectin binding site for carbohydrate ligands, was not affected by the amino acid substitutions at these five positions. Inspection of the three-dimensional structure of E-selectin indicated that the EP-5C7 epitope is located near the junction between the lectin and EGF-like domains. The ligand binding site was distant from the EP-5C7 epitope, suggesting that the amino acid residues in the EP-5C7 epitope play an important role other than ligand binding in selectin-mediated cell adhesion.

Identification of CD44 residues important for hyaluronan binding and delineation of the binding site

CD44 is a widely distributed cell surface protein that plays a role in cell adhesion and migration. As a proteoglycan, CD44 is also implicated in growth factor and chemokine binding and presentation. The extracellular region of CD44 is variably spliced, giving rise to multiple CD44 isoforms. All isoforms contain an amino-terminal domain, which is homologous to cartilage link proteins. The cartilage link protein-like domain of CD44 is important for hyaluronan binding. The structure of the link protein domain of TSG-6 has been determined by NMR. Based on this structure, a molecular model of the link-homologous region of CD44 was constructed. This model was used to select residues for site-specific mutagenesis in an effort to identify residues important for ligand binding and to outline the hyaluronan binding site. Twenty-four point mutants were generated and characterized, and eight residues were identified as critical for binding or to support the interaction. In the model, these residues form a coherent surface the location of which approximately corresponds to the carbohydrate binding sites in two functionally unrelated calcium-dependent lectins, mannose-binding protein and E-selectin (CD62E).

Determining force dependence of two-dimensional receptor-ligand binding affinity by centrifugation

Analyses of receptor-ligand interactions are important to the understanding of cellular adhesion. Traditional methods of measuring the three-dimensional (3D) dissociation constant (Kd) require at least one of the molecular species in solution and hence cannot be directly applied to the case of cell adhesion. We describe a novel method of measuring 2D binding characteristics of receptors and ligands that are attached to surfaces and whose bonds are subjected to forces. The method utilizes a common centrifugation assay to quantify adhesion. A model for the experiment has been formulated, solved exactly, and tested carefully. The model is stochastically based and couples the bond force to the binding affinity. The method was applied to examine tumor cell adherence to recombinant E-selectin. Satisfactory agreement was found between predictions and data. The estimated zero-force 2D Kd for E-selectin/carbohydrate ligand binding was approximately 5 x 10(3) microm(-2), and the bond interaction range was subangstrom. Our results also suggest that the number of bonds mediating adhesion was small (<5).

Fine mapping of the epitopes of humanized anti-L-selectin monoclonal antibodies HuDREG-55 and HuDREG-200

Blocking the function of L-selectin with a monoclonal antibody (mAb) is a promising way to prevent neutrophils from causing tissue damage during inflammation. HuDREG-55 and HuDREG-200 are humanized mAb which bind to human L-selectin and block its function as an adhesion molecule. To understand the mechanism of the action of HuDREG-55 and HuDREG-200, we determined their epitopes on L-selectin at the amino acid level. The analysis of human E- and L-selectin chimeric proteins demonstrated that the lectin domain of L-selectin is necessary for the binding of HuDREG-55 and HuDREG-200. Mutational analysis of Escherichia coli-expressed L-selectin showed that HuDREG-55 binding is sensitive to amino acid changes at positions 11, 56, 87, 89, 105, 107 and 111 (counting from the amino-terminus of mature L-selectin) while HuDREG-200 binding is sensitive to amino acid changes at 45, 46 and 47. Both epitopes are located close to the predicted carbohydrate binding site, indicating that HuDREG-55 and HuDREG-200 block the function of L-selectin by directly inhibiting the binding to carbohydrate ligands.

Prediction of the tertiary structure of the complement control protein module

Complement control protein (CCP) modules, or short consensus repeats (SCR), exist in a wide variety of complement and adhesion proteins, principally the selectins. We have predicted the three-dimensional structure of a CCP module based upon secondary structural information derived by two-dimensional NMR [Barlow et al. (1991), Biochemistry 30, 997-1004]. Accordingly, the CCP is predicted to contain seven beta-strands with extensive hydrogen-bonding interactions, and shows a compact, globular structure. Comparison of this model to the X-ray structure of a kringle domain suggests that the CCP unit is more compact than a kringle structure, and that despite their similarities in size and disulfide bond format, the two are not homologous. Although the function of CCP domains is unknown, it is hoped that the structural model presented herein will facilitate further inquiry into how they contribute to so many systems of biological importance.

Role of E- and P-selectin in migration of monocytes and polymorphonuclear leucocytes to cytokine and chemoattractant-induced cutaneous inflammation in the rat

The contribution of E- and P-selectin in the rat to the migration of polymorphonuclear leucocytes (PMNL) and monocytes to acute dermal inflammation induced by a chemoattractant (C5ades Arg) or endothelial cell activating agents [lipopolysaccharide, tumour necrosis factor-alpha (TNF-alpha), alpha-thrombin and interferon-gamma] administered intradermally was investigated. Migration was quantitated using radiolabelled blood PMNL and monocytes and new, function-blocking monoclonal antibodies (mAbs) to rat E- and P-selectin were employed. Monocyte migration to inflamed skin was partially inhibited (40-75%) by P-selectin mAb with all stimuli tested, but not by anti-E-selectin. PMNL migration in response to all stimuli was also inhibited (50-75%) by blocking P-selectin, but in contrast to monocytes, PMNL accumulation was partially inhibited by mAb to E-selectin in alpha-thrombin and TNF-alpha lesions. When P-selectin was blocked by mAb, mAb to E-selectin significantly inhibited further (by 70-90%) both PMNL and monocyte accumulation in all lesions, indicating that both P- and E-selectin contribute to the migration of these leucocytes. Blocking L-selectin in addition to P- and E-selectin, had no effect on the remaining migration. Thus, optimal PMNL and monocyte migration to chemotactic factor- and cytokine-induced skin inflammation is P-selectin dependent. E-selectin becomes important, in most conditions used here, when P-selectin mediated recruitment is not operative. A selectin independent pathway likely mediates up to 20% of PMNL and monocyte migration to acute inflammation, at least in skin.

Cell-surface carbohydrate recognition by animal and viral lectins

Many animal and viral lectins are specific for monosaccharides found in particular glycosidic linkages, or for larger oligosaccharide structures. Recent crystal structures of complexes between these proteins and receptor fragments have provided insights into the recognition of linkage isomers and oligosaccharide conformation.

Generation and characterization of a novel adhesion function blocking monoclonal antibody recognizing both rat and mouse E-selectin

The prerequisite for the recruitment of circulating leukocytes to sites of inflammation is adhesion to vascular endothelial cells. Selectins play a significant role in the initiation of this multistep process by mediating "rolling" of the leukocytes on the endothelium. Investigation of selectin-dependent cell interactions using function blocking monoclonal antibodies (MAb) provides insights into the mechanisms involved in leukocyte migration into inflammation. Until now most studies in inflammation models in rats have relied on cross-reactive or polyclonal antibodies against rat E-selectin. In an E-selectin knockout mouse, we aimed to generate an adhesion function blocking MAb to rat E-selectin by immunization with rat E-selectin transfected Chinese hamster ovary cells (RESEC). An IgG1 kappa MAb was identified that reacts with RESEC but not with untransfected Chinese hamster ovary cells, as well as with recombinant mouse E-selectin protein as assessed by ELISA. This MAb is designated RME-1. It does not cross-react with rat L-selectin or rat P-selectin or E-selectin expressed on human umbilical vein endothelium. Adhesion of the HL-60 myeloid cells to immobilized mouse E-selectin was completely inhibited by MAb RME-1 under static conditions and adhesion of rat polymorphonuclear leukocytes to recombinant mouse E-selectin was blocked under rotation condition. This novel antibody thus recognizes a function-related epitope on rodent E-selectin.