Tyrosine sulfation enhances but is not required for PSGL-1 rolling adhesion on P-selectin

A physiologic roll for cell surface GlycoRNAs

Glycosylated RNA molecules that can be bound by lectins have been demonstrated on the surfaces of leukocytes, but their physiologic function(s) was not known. A recent study (PMID 38262409) demonstrates that at least 1 function is to promote capture and rolling of neutrophils in the vasculature. Of interest, the neutrophil glycosylated RNA molecules bind to P-selectin but not E-selectin.

Custom Workflow for the Confident Identification of Sulfotyrosine-Containing Peptides and Their Discrimination from Phosphopeptides

Protein tyrosine sulfation (sY) is a post-translational modification (PTM) catalyzed by Golgi-resident tyrosyl protein sulfo transferases (TPSTs). Information on sY in humans is currently limited to ∼50 proteins, with only a handful having verified sites of sulfation. As such, the contribution of sulfation to the regulation of biological processes remains poorly defined. Mass spectrometry (MS)-based proteomics is the method of choice for PTM analysis but has yet to be applied for systematic investigation of the "sulfome", primarily due to issues associated with discrimination of sY-containing from phosphotyrosine (pY)-containing peptides. In this study, we developed an MS-based workflow for sY-peptide characterization, incorporating optimized Zr4+ immobilized metal-ion affinity chromatography (IMAC) and TiO2 enrichment strategies. Extensive characterization of a panel of sY- and pY-peptides using an array of fragmentation regimes (CID, HCD, EThcD, ETciD, UVPD) highlighted differences in the generation of site-determining product ions and allowed us to develop a strategy for differentiating sulfated peptides from nominally isobaric phosphopeptides based on low collision energy-induced neutral loss. Application of our "sulfomics" workflow to a HEK-293 cell extracellular secretome facilitated identification of 21 new sulfotyrosine-containing proteins, several of which we validate enzymatically, and reveals new interplay between enzymes relevant to both protein and glycan sulfation.

Considerations for defining +80 Da mass shifts in mass spectrometry-based proteomics: phosphorylation and beyond

Post-translational modifications (PTMs) are ubiquitous and key to regulating protein function. Understanding the dynamics of individual PTMs and their biological roles requires robust characterisation. Mass spectrometry (MS) is the method of choice for the identification and quantification of protein modifications. This article focusses on the MS-based analysis of those covalent modifications that induce a mass shift of +80 Da, notably phosphorylation and sulfation, given the challenges associated with their discrimination and pinpointing the sites of modification on a polypeptide chain. Phosphorylation in particular is highly abundant, dynamic and can occur on numerous residues to invoke specific functions, hence robust characterisation is crucial to understanding biological relevance. Showcasing our work in the context of other developments in the field, we highlight approaches for enrichment and site localisation of phosphorylated (canonical and non-canonical) and sulfated peptides, as well as modification analysis in the context of intact proteins (top down proteomics) to explore combinatorial roles. Finally, we discuss the application of native ion-mobility MS to explore the effect of these PTMs on protein structure and ligand binding.

Computationally guided conversion of the specificity of E-selectin to mimic that of Siglec-8

Sulfated glycans have been found to be associated with various diseases and therefore have significant potential in molecular pathology as biomarkers. Although lectins are useful reagents for detecting glycans, there is a paucity of sulfate-recognizing lectins, and those that exist, such as from Maackia amurensis, display mixed specificities. Recombinant lectin engineering offers an emerging tool for creating novel glycan recognition by altering and/or enhancing endogenous specificities. The present study demonstrated the use of computational approaches in the engineering of a mutated form of E-selectin that displayed highly specific recognition of 6'-sulfo-sialyl Lewis X (6'-sulfo-sLe^x), with negligible binding to its endogenous nonsulfated ligand, sLe^x. This new specificity mimics that of the unrelated protein Siglec-8, for which 6'-sulfo-sLe^x is its preferred ligand. Molecular dynamics simulations and energy calculations predicted that two point mutations (E92A/E107A) would be required to stabilize binding to the sulfated oligosaccharide with E-selectin. In addition to eliminating putative repulsions between the negatively charged side chains and the sulfate moiety, the mutations also abolished favorable interactions with the endogenous ligand. Glycan microarray screening of the recombinantly expressed proteins confirmed the predicted specificity change but also identified the introduction of unexpected affinity for the unfucosylated form of 6'-sulfo-sLe^x (6'-sulfo-sLacNAc). Three key requirements were demonstrated in this case for engineering specificity for sulfated oligosaccharide: 1) removal of unfavorable interactions with the 6'-sulfate, 2) introduction of favorable interactions for the sulfate, and 3) removal of favorable interactions with the endogenous ligand.

Modulating tyrosine sulfation of recombinant antibodies in CHO cell culture by host selection and sodium chlorate supplementation

BACKGROUND

Tyrosine sulfation is a post-translational modification found on many surface receptors and plays an important role in cell-cell and cell-matrix interactions. However, tyrosine sulfation of therapeutic antibodies has only been reported very recently. Because of potential potency and immunogenicity concerns, tyrosine sulfation needs to be controlled during the manufacturing process.

METHODS AND RESULTS

In this study, we explored methods to modulate antibody tyrosine sulfation during cell line development and upstream production process. We found that tyrosine sulfation levels were significantly different in various Chinese hamster ovary (CHO) cell lines due to differential expression of genes in the sulfation pathway including tyrosylprotein sulfotransferase 2 (TPST2) and the sulfation substrate transporter SLC35B2. We also screened chemical inhibitors to reduce tyrosine sulfation in CHO culture and found that sodium chlorate could significantly inhibit tyrosine sulfation while having minimal impact on cell growth and antibody production. We further confirmed this finding in a standard fed-batch production assay. Sodium chlorate at 16 mM markedly inhibited tyrosine sulfation by more than 50% and had no significant impact on antibody titer or quality.

CONCLUSION

These data suggest that we can control tyrosine sulfation by selecting CHO cell lines based on the expression level of TPST2 and SLC35B2 or adding sodium chlorate in upstream production process.

The function of P-selectin glycoprotein ligand-1 is conserved from ancestral fishes to mammals

PSGL-1 is a mucin-like glycoprotein that supports, in mammals, leukocyte rolling on selectins. However, we have limited knowledge whether its function is conserved in non-mammals and how its structure adapted during evolution. To identify conserved amino acid sequences required for selectin binding, we performed multiple alignments of PSGL-1 sequences from 18 mammals, 4 birds, 3 reptiles, 1 amphibian, and 15 fishes. The amino-terminal T[D/E]PP[D/E] motif, which identifies in mammals a core-2 O-glycosylated threonine required for selectin-binding, is partially conserved in some fishes (e.g., T. rubripes) and birds (e.g., G. gallus), however, most non-mammals do not display it. The sulfated tyrosine residues of human PSGL-1, which bind L- and P-selectin, are not observed in non-mammals, suggesting that they are dispensable for selectin-binding or that other amino acids play their role. A mucin-like domain is present in all species. Interestingly, the alignment of cytoplasmic sequences of non-mammals reveals the conservation of ezrin/radixin/moesin binding site and two new motifs (M1 and M2). To examine the conservation of PSGL-1 function, we cloned PSGL-1 cDNA sequences of zebrafish and fugu, and established their cross-reactivity with human selectins under flow conditions. Importantly, deleting the well-conserved M1 motif strongly decreased PSGL-1 expression at leukocyte surface and induced retention of the precursor molecule in the endoplasmic reticulum, indicating that M1 motif provides a signal required to export PSGL-1 precursors to the Golgi complex. These data show for the first time the conservation of PSGL-1 function from fishes to mammals and reveal the function of a new motif.

Sialyl Lewisx-P-selectin cascade mediates tumor-mesothelial adhesion in ascitic fluid shear flow

Organ-specific colonization suggests that specific cell–cell recognition is essential. Yet, very little is known about this particular interaction. Moreover, tumor cell lodgement requires binding under shear stress, but not static, conditions. Here, we successfully isolate the metastatic populations of cancer stem/tumor-initiating cells (M-CSCs). We show that the M-CSCs tether more and roll slower than the non-metastatic (NM)-CSCs, thus resulting in the preferential binding to the peritoneal mesothelium under ascitic fluid shear stress. Mechanistically, this interaction is mediated by P-selectin expressed by the peritoneal mesothelium. Insulin-like growth factor receptor-1 carrying an uncommon non-sulfated sialyl-Lewis^x (sLe^x) epitope serves as a distinct P-selectin binding determinant. Several glycosyltransferases, particularly α1,3-fucosyltransferase with rate-limiting activity for sLe^x synthesis, are highly expressed in M-CSCs. Tumor xenografts and clinical samples corroborate the relevance of these findings. These data advance our understanding on the molecular regulation of peritoneal metastasis and support the therapeutic potential of targeting the sLe^x-P-selectin cascade.

Tumor cell in the peritoneum are often exposed to shear forces generated by ascitic flow during metastasis. Here, the authors show that metastatic cancer stem cells tether more and roll slower than the non-metastatic counterparts, and that sialyl-Lewis^x -P-selectin axis mediates peritoneal metastasis.

Structure-based design of nucleoside-derived analogues as sulfotransferase inhibitors

Regulated sulfation of biomolecules by sulfotransferases (STs) plays a role in many biological processes with implications for a number of disease areas. A structure-based approach and molecular docking were used to design a library of ST inhibitors.

Synthesis of Sulfotyrosine-Containing Peptides by Incorporating Fluorosulfated Tyrosine Using an Fmoc-Based Solid-Phase Strategy

Tyrosine O-sulfation is a common protein post-translational modification that regulates many biological processes, including leukocyte adhesion and chemotaxis. Many peptides with therapeutic potential contain one or more sulfotyrosine residues. We report a one-step synthesis for Fmoc-fluorosulfated tyrosine. An efficient Fmoc-based solid-phase peptide synthetic strategy is then introduced for incorporating the fluorosulfated tyrosine residue into peptides of interest. Standard simultaneous peptide-resin cleavage and removal of the acid-labile side-chain protecting groups affords the crude peptides containing fluorosulfated tyrosine. Basic ethylene glycol, serving both as solvent and reactant, transforms the fluorosulfated tyrosine peptides into sulfotyrosine peptides in high yield.

Preparation and Analysis of N-Terminal Chemokine Receptor Sulfopeptides Using Tyrosylprotein Sulfotransferase Enzymes

In most chemokine receptors, one or multiple tyrosine residues have been identified within the receptor N-terminal domain that are, at least partially, modified by posttranslational tyrosine sulfation. For example, tyrosine sulfation has been demonstrated for Tyr-3, -10, -14, and -15 of CCR5, for Tyr-3, -14, and -15 of CCR8, and for Tyr-7, -12, and -21 of CXCR4. While there is evidence for several chemokine receptors that tyrosine sulfation is required for optimal interaction with the chemokine ligands, the precise role of tyrosine sulfation for chemokine receptor function remains unclear. Furthermore, the function of the chemokine receptor N-terminal domain in chemokine binding and receptor activation is also not well understood. Sulfotyrosine peptides corresponding to the chemokine receptor N-termini are valuable tools to address these important questions both in structural and functional studies. However, due to the lability of the sulfotyrosine modification, these peptides are difficult to obtain using standard peptide chemistry methods. In this chapter, we provide methods to prepare sulfotyrosine peptides by enzymatic in vitro sulfation of peptides using purified recombinant tyrosylprotein sulfotransferase (TPST) enzymes. In addition, we also discuss alternative approaches for the generation of sulfotyrosine peptides and methods for sulfopeptide analysis.

Role of tyrosine-sulfated proteins in retinal structure and function

The extracellular matrix (ECM) plays a significant role in cellular and retinal health. The study of retinal tyrosine-sulfated proteins is an important first step toward understanding the role of ECM in retinal health and diseases. These secreted proteins are members of the retinal ECM. Tyrosine sulfation was shown to be necessary for the development of proper retinal structure and function. The importance of tyrosine sulfation is further demonstrated by the evolutionary presence of tyrosylprotein sulfotransferases, enzymes that catalyze proteins' tyrosine sulfation, and the compensatory abilities of these enzymes. Research has identified four tyrosine-sulfated retinal proteins: fibulin 2, vitronectin, complement factor H (CFH), and opticin. Vitronectin and CFH regulate the activation of the complement system and are involved in the etiology of some cases of age-related macular degeneration. Analysis of the role of tyrosine sulfation in fibulin function showed that sulfation influences the protein's ability to regulate growth and migration. Although opticin was recently shown to exhibit anti-angiogenic properties, it is not yet determined what role sulfation plays in that function. Future studies focusing on identifying all of the tyrosine-sulfated retinal proteins would be instrumental in determining the impact of sulfation on retinal protein function in retinal homeostasis and diseases.

Complement factor H, vitronectin, and opticin are tyrosine-sulfated proteins of the retinal pigment epithelium

Lack of tyrosine sulfation of ocular proteins results in disorganized photoreceptor structure and drastically reduced visual function, demonstrating the importance of this post-translational modification to vision. To understand the role that tyrosine sulfation plays in the function of ocular proteins, we identified some tyrosine-sulfated proteins in the retinal pigment epithelium using two independent methods, immuno-affinity column purification with an anti-sulfotyrosine specific antibody and computer-based sequence analysis of retinal pigment epithelium secretome by means of the prediction program Sulfinator. Radioactive labeling followed by thin layer electrophoresis revealed that three proteins, vitronectin, opticin, and complement factor H (CFH), were post-translationally modified by tyrosine sulfation. The identification of vitronectin and CFH as tyrosine-sulfated proteins is significant, since both are deposited in drusen in the eyes of patients with age-related macular degeneration (AMD). Furthermore, mutations in CFH have been determined to be a major risk factor in the development of AMD. Future studies that seek to understand the role of CFH in the development of AMD should take into account the role that tyrosine sulfation plays in the interaction of this protein with its partners, and examine whether modulating sulfation provides a potential therapeutic target.

Fibulin 2, a tyrosine O-sulfated protein, is up-regulated following retinal detachment

Retinal detachment is the physical separation of the retina from the retinal pigment epithelium. It occurs during aging, trauma, or during a variety of retinal disorders such as age-related macular degeneration, diabetic retinopathy, retinopathy of prematurity, or as a complication following cataract surgery. This report investigates the role of fibulin 2, an extracellular component, in retinal detachment. A major mechanism for detachment resolution is enhancement of cellular adhesion between the retina and the retinal pigment epithelium and prevention of its cellular migration. This report shows that fibulin 2 is mainly present in the retinal pigment epithelium, Bruch membrane, choriocapillary, and to a lesser degree in the retina. In vitro studies revealed the presence of two isoforms for fibulin 2. The small isoform is located inside the cell, and the large isoform is present inside and outside the cells. Furthermore, fibulin 2 is post-translationally modified by tyrosine sulfation, and the sulfated isoform is present outside the cell, whereas the unsulfated pool is internally located. Interestingly, sulfated fibulin 2 significantly reduced the rate of cellular growth and migration. Finally, levels of fibulin 2 dramatically increased in the retinal pigment epithelium following retinal detachment, suggesting a direct role for fibulin 2 in the re-attachment of the retina to the retinal pigment epithelium. Understanding the role of fibulin 2 in enhancing retinal attachment is likely to help improve the current therapies or allow the development of new strategies for the treatment of this sight-threatening condition.

Effects of microfluidic channel geometry on leukocyte rolling assays

Microfluidic cell adhesion assays have emerged as a means to increase throughput as well as reduce the amount of costly reagents. However as dimensions of the flow chamber are reduced and approach the diameter of a cell (D(c)), theoretical models have predicted that mechanical stress, force, and torque on a cell will be amplified. We fabricated a series of microfluidic devices that have a constant width:height ratio (10:1) but with varying heights. The smallest microfluidic device (200 μm ×20 μm) requires perfusion rates as low as 40 nL/min to generate wall shear stresses of 0.5 dynes/cm(2). When neutrophils were perfused through P-selectin coated chambers at equivalent wall shear stress, rolling velocities decreased by approximately 70 % as the ratio of cell diameter to chamber height (D(c)/H) increased from 0.08 (H = 100 μm) to 0.40 (H = 20 μm). Three-dimensional numerical simulations of neutrophil rolling in channels of different heights showed a similar trend. Complementary studies with PSGL-1 coated microspheres and paraformaldehyde-fixed neutrophils suggested that changes in rolling velocity were related to cell deformability. Using interference reflection microscopy, we observed increases in neutrophil contact area with increasing chamber height (9-33 %) and increasing wall shear stress (28-56 %). Our results suggest that rolling velocity is dependent not only on wall shear stress but also on the shear stress gradient experienced by the rolling cell. These results point to the D(c)/H ratio as an important design parameter of leukocyte microfluidic assays, and should be applicable to rolling assays that involve other cell types such as platelets or cancer cells.

Molecular surface chemistry in marine bioadhesion

This review covers the in situ molecular physicochemical characterisation of bioadhesives at solid/liquid interfaces, with the aim of elucidating the adhesion strategies that lie at the root of marine biofouling. It focuses on three major foulers: mussels, algae and barnacles. The dispersal of these organisms, their colonisation of surfaces, and ultimately their survival rely critically on the ability of the organisms' larvae or spores to locate a favourable settlement site and undergo metamorphosis, thus initiating their sessile existence. Differences in the composition of adhesive secretions and the strategies employed for their temporary or permanent implementation exists between the larval and adult life stages. To date, only a few adhesive secretions from marine fouling organisms have been adequately described in terms of their chemical composition, and a survey revealed the presence of certain recurrent functional groups, specifically catechol, carboxylate, monoester-sulphate and -phosphate. This review will describe the binding modes of such functionalities to wet mineral/metal oxides surfaces. Such functionalities will be ranked based on their ability to bind to hydrophilic surfaces replacing surface-bound water (Langmuir adsorption constant) as well as other adsorbates (competitive adsorption). A plausible explanation for the propensity of the reviewed adhesive functionalities to bind to hydrous metal oxide surfaces will be given on the basis of the Hard and Soft Acids and Bases principle, Hofmeister effects and entropic considerations. From the in situ analysis of marine organism bioadhesives and adsorption studies of functionalities relevant to the bioadhesion process, insights can be gleaned for a knowledge-based innovation of antifouling strategies and the synthesis of strong, durable adhesive materials, which are suitable for implementation in wet environments.

Tyrosine replacement of PSGL-1 reduces association kinetics with P- and L-selectin on the cell membrane

Binding of selectins to P-selectin glycoprotein ligand-1 (PSGL-1) mediates tethering and rolling of leukocytes on the endothelium during inflammation. Previous measurements obtained with a flow-chamber assay have shown that mutations of three tyrosines at the PSGL-1 N-terminus (Y46, Y48, and Y51) increase the reverse rates and their sensitivity to the force of bonds with P- and L-selectin. However, the effects of these mutations on the binding affinities and forward rates have not been studied. We quantified these effects by using an adhesion frequency assay to measure two-dimensional affinity and kinetic rates at zero force. Wild-type PSGL-1 has 2.2- to 8.5-fold higher binding affinities for P- and L-selectin than PSGL-1 mutants with two of three tyrosines substituted by phenylalanines, and 9.6- to 49-fold higher affinities than the PSGL-1 mutant with all three tyrosines replaced. In descending order, the affinity decreased from wild-type to Y48/51F, Y46/51F, Y46/48F, and Y46/48/51F. The affinity differences were attributed to major changes in the forward rate and minor changes in the reverse rate, suggesting that these tyrosines regulate the accessibility of PSGL-1 to P- and L-selectin via electrostatic interactions, which is supported by molecular-dynamics simulations. Our results provide insights into the structure-function relationship of receptor-ligand binding at a single-residue level.

Molecular cloning and expression analysis of P-selectin glycoprotein ligand-1 from zebrafish (Danio rerio)

To date, the best characterized glycoprotein ligand for P-selectin is P-selectin glycoprotein ligand-1 (PSGL-1). In this study, we cloned the full-length cDNA of PSGL-1 from zebrafish (Danio rerio). Zebrafish PSGl-1 cDNA is 1,594 bp and encodes a putative 284 amino acid protein with a theoretical molecular weight of 30.33 kDa and isoelectric point of 7.96. A signal peptide of 27 amino acids is predicted. The putative protein contains an extracellular mucin-like domain, a transmembrane domain and a cytoplasmic domain, with homology to mammalian PSGL-1. In the putative P-selectin binding region, there are 1 potential tyrosine sulfation site and 12 potential threonine O-glycosylation sites. A single extracellular cysteine, at the junction of the extracellular and transmembrane domains, suggests a disulfide-bonding pattern. The amino acid sequence of zebrafish PSGL-1 is 19-22% identical to that of mammalian PSGL-1. RT-PCR and whole-mount in situ hybridization analysis revealed that zebrafish PSGL-1 was expressed in early embryonic development, and the expression has an increased trend from 0.2 (1-cell stage) to 72 hpf. The results indicate that the general domain structure of PSGL-1 protein is conserved among species, and zebrafish PSGL-1 plays important roles in embryonic development and probably has similar biological function to that of mammalian PSGL-1.

Influence of Bifidobacterium longum CECT 7347 and gliadin peptides on intestinal epithelial cell proteome

Celiac disease is an enteropathy caused by an abnormal immune response to cereal gluten proteins (gliadin). To unravel the possible role of the interactions between gliadin peptides and specific intestinal bacteria, the response of intestinal epithelial (Caco-2) cells to gliadin subjected to gastrointestinal digestion in the presence or absence of Bifidobacterium longum CECT 7347 has been studied. Changes in the proteome of Caco-2 cells were determined by 2DE and MALDI-TOF. Gliadins digested without B. longum altered the expression of a higher number of proteins than in the presence of the bacterium (21 versus 9), and these proteins were involved in disorganization of cell cytoskeleton, inflammation, and apoptosis. Gliadins digested in the presence of the bacterium influenced the production of proteins involved in calcium homeostasis and cell survival and function. Therefore, B. longum CECT 7347 might ameliorate gliadin toxicity and modify the responses of intestinal epithelial cells to the gliadin challenge.

Protein tyrosine-O-sulfation in the retina

Tyrosine-O-sulfation, a post-translational modification, is catalyzed by two independent tyrosylprotein sulfotransferases (TPSTs). As an initial step towards understanding the role of TPSTs in retinal function, this study was undertaken to determine the extent to which tyrosine-O-sulfation of proteins is utilized in the retina. A previously characterized anti-sulfotyrosine antibody was used to determine the presence and localization of tyrosine-O-sulfated proteins (TOSPs) in the retina. Using Western blot, RT-PCR and immunohistochemical analyses, we detected TOSPs in the retinas from diverse species, including frog, fish, mouse and human. Some of the variability in the observed sizes of retinal TOSPs in the mouse, at least, may result from differential patterns of glycosylation; however, there seem to be species-specific sulfated retinal proteins as well. TOSPs were detected in most of the retinal layers as well as in the retinal pigment epithelium from human and mouse. Several retinal TOSPs were detected in the inter-photoreceptor matrix, which is consistent with the secreted nature of some sulfated proteins. Transcripts for both TPST-1 and TPST-2 were expressed in both the human and mouse retinas. These data show that retinal protein tyrosine-O-sulfation is highly conserved which suggest a functional significance of these proteins to retinal function and structure.

Action at a distance: lengthening adhesion bonds with poly(ethylene glycol) spacers enhances mechanically stressed affinity for improved vascular targeting of microparticles

Poly(ethylene glycol) (PEG) chains were used to decorate microparticles with long adhesion ligands to emulate the efficacy of selectin-mediated leukocyte homing mechanisms. Ligands for P-selectin, an endothelial cell inflammatory marker, were coupled to PEG spacers of two sizes (MW 3400 and 10,000 Da) to investigate the effects on adhesion kinetics to P-selectin substrates. Under shear flow 80 nm PEG spacers improved P-selectin-antibody adhesion frequency by up to 4.5-fold and bond lifetimes by 7-fold compared to microparticles bearing chemisorbed antibody. Presentation of the glycosulfopeptide P-selectin ligands (2-GSP-6) and its nonsulfated low affinity form (2-GP-6) by long PEG spacers led to improved lifetimes of stressed bonds formed with P-selectin in shear flow and the rolling fluxes. Thus, structural features far removed from the binding pocket of a receptor that increase molecular contour length may enhance affinity in mechanically stressed environments such as those existing within the confines of the blood vessel. Such features may be useful for improving the performance of vascular-targeted micro- and nanoparticles used for drug, gene, and image contrast delivery. Ligand presentation on molecularly extended stalks may also serve to enhance any particle-surface interaction that takes place in laminar shear flow.

Selectin catch-slip kinetics encode shear threshold adhesive behavior of rolling leukocytes

The selectin family of leukocyte adhesion receptors is principally recognized for mediating transient rolling interactions during the inflammatory response. Recent studies using ultrasensitive force probes to characterize the force-lifetime relationship between P- and L-selectin and their endogenous ligands have underscored the ability of increasing levels of force to initially extend the lifetime of these complexes before disrupting bond integrity. This so-called "catch-slip" transition has provided an appealing explanation for shear threshold phenomena in which increasing levels of shear stress stabilize leukocyte rolling under flow. We recently incorporated catch-slip kinetics into a mechanical model for cell adhesion and corroborated this hypothesis for neutrophils adhering via L-selectin. Here, using adhesive dynamics simulations, we demonstrate that biomembrane force probe measurements of various P- and L-selectin catch bonds faithfully predict differences in cell adhesion patterns that have been described extensively in vitro. Using phenomenological parameters to characterize the dominant features of molecular force spectra, we construct a generalized phase map that reveals that robust shear-threshold behavior is possible only when an applied force very efficiently stabilizes the bound receptor complex. This criteria explains why only a subset of selectin catch bonds exhibit a shear threshold and leads to a quantitative relationship that may be used to predict the magnitude of the shear threshold for families of catch-slip bonds directly from their force spectra. Collectively, our results extend the conceptual framework of adhesive dynamics as a means to translate complex single-molecule biophysics to macroscopic cell behavior.

Endoglycan, a member of the CD34 family of sialomucins, is a ligand for the vascular selectins

The interactions of the selectin family of adhesion molecules with their ligands are essential for the initial rolling stage of leukocyte trafficking. Under inflammatory conditions, the vascular selectins, E- and P-selectin, are expressed on activated vessels and interact with carbohydrate-based ligands on the leukocyte surface. While several ligands have been characterized on human T cells, monocytes and neutrophils, there is limited information concerning ligands on B cells. Endoglycan (EG) together with CD34 and podocalyxin comprise the CD34 family of sialomucins. We found that EG, previously implicated as an L-selectin ligand on endothelial cells, was present on human B cells, T cells and peripheral blood monocytes. Upon activation of B cells, EG increased with a concurrent decrease in PSGL-1. Expression of EG on T cells remained constant under the same conditions. We further found that native EG from several sources (a B cell line, a monocyte line and human tonsils) was reactive with HECA-452, a mAb that recognizes sialyl Lewis X and related structures. Moreover, immunopurified EG from these sources was able to bind to P-selectin and where tested E-selectin. This interaction was divalent cation-dependent and required sialylation of EG. Finally, an EG construct supported slow rolling of E- and P-selectin bearing cells in a sialic acid and fucose dependent manner, and the introduction of intact EG into a B cell line facilitated rolling interactions on a P-selectin substratum. These in vitro findings indicate that EG can function as a ligand for the vascular selectins.

Leuko-polymersomes

Polymersomes are vesicles whose membranes are comprised of self-assembled amphiphilic block co-polymers. Synthetic control of block co-polymer chemistry provides an advantageous diversity of polymersome functions, ranging from tunable materials strength, superior encaspulation of hydrophobic and hydrophilic drugs and optical dyes, and facile functionalization. We have exploited polymersome tunability to make leuko-polymersomes: polymersomes with the adhesive properties of leukocytes. By functionalizing the terminal groups on the outer shell of the vesicle with biotin, we have used modular avidin-biotin chemistry to attach adhesion ligands that mimic the two critical adhesion pathways that leukocytes utilize to achieve adhesion in the fast fluid flow of blood vessels--selectins and integrins. We demonstrate that adhesion is specific and is supported at hydrodynamic flow rates at which leukocytes adhere. We envision the use of such particles for monitoring or treating inflammation, cancer and cardiovascular disease.

Targeting selectins and selectin ligands in inflammation and cancer

Inflammation and cancer metastasis are associated with extravasation of leukocytes or tumor cells from blood into tissue. Such movement is believed to follow a coordinated and sequential molecular cascade initiated, in part, by the three members of the selectin family of carbohydrate-binding proteins: E-selectin (CD62E), L-selectin (CD62L) and P-selectin (CD62P). E-selectin is particularly noteworthy in disease by virtue of its expression on activated endothelium and on bone-skin microvascular linings and for its role in cell rolling, cell signaling and chemotaxis. E-selectin, along with L- or P-selectin, mediates cell tethering and rolling interactions through the recognition of sialo-fucosylated Lewis carbohydrates expressed on structurally diverse protein-lipid ligands on circulating leukocytes or tumor cells. Major advances in understanding the role of E-selectin in inflammation and cancer have been advanced by experiments assaying E-selectin-mediated rolling of leukocytes and tumor cells under hydrodynamic shear flow, by clinical models of E-selectin-dependent inflammation, by mice deficient in E-selectin and by mice deficient in glycosyltransferases that regulate the binding activity of E-selectin ligands. Here, the authors elaborate on how E-selectin and its ligands may facilitate leukocyte or tumor cell recruitment in inflammatory and metastatic settings. Antagonists that target cellular interactions with E-selectin and other members of the selectin family, including neutralizing monoclonal antibodies, competitive ligand inhibitors or metabolic carbohydrate mimetics, exemplify a growing arsenal of potentially effective therapeutics in controlling inflammation and the metastatic behavior of cancer.

Covalent immobilization of p-selectin enhances cell rolling

Cell rolling is an important physiological and pathological process that is used to recruit specific cells in the bloodstream to a target tissue. This process may be exploited for biomedical applications to capture and separate specific cell types. One of the most commonly studied proteins that regulate cell rolling is P-selectin. By coating surfaces with this protein, biofunctional surfaces that induce cell rolling can be prepared. Although most immobilization methods have relied on physisorption, chemical immobilization has obvious advantages, including longer functional stability and better control over ligand density and orientation. Here we describe chemical methods to immobilize P-selectin covalently on glass substrates. The chemistry was categorized on the basis of the functional groups on modified glass substrates: amine, aldehyde, and epoxy. The prepared surfaces were first tested in a flow chamber by flowing microspheres functionalized with a cell surface carbohydrate (sialyl Lewis(x)) that binds to P-selectin. Adhesion bonds between P-selectin and sialyl Lewis(x) dissociate readily under shear forces, leading to cell rolling. P-selectin immobilized on the epoxy glass surfaces exhibited enhanced long-term stability of the function and better homogeneity as compared to that for surfaces prepared by other methods and physisorbed controls. The microsphere rolling results were confirmed in vitro with isolated human neutrophils. This work is essential for the future development of devices for isolating specific cell types based on cell rolling, which may be useful for hematologic cancers and certain metastatic cancer cells that are responsive to immobilized selectins.

P-selectin glycoprotein ligand 1 and beta2-integrins cooperate in the adhesion of leukocytes to von Willebrand factor

Von Willebrand factor (VWF) is an essential component of hemostasis. However, animal studies using VWF-deficient mice suggest that VWF may also contribute to inflammation. In the present study, we demonstrate that VWF was able to interact with polymorphonuclear leukocytes (PMNs) and monocytes under static and flow conditions. Adhesion under flow was dominated by short-lasting contact with resting PMNs, whereas adhesion of phorbol-12-myristate-13-acetate (PMA)-stimulated PMNs was characterized by firm adhesion. Transient binding of PMNs to VWF appeared to be mediated by P-selectin glycoprotein ligand-1 (PSGL-1). Moreover, recombinant PSGL-1 protein and cell surface-expressed PSGL-1 directly interacted with VWF. As for stable adhesion by PMA-stimulated PMNs, we observed that static adhesion and adhesion under flow were strongly inhibited (greater than 75%) by neutrophil-inhibitory factor, an inhibitor of beta2-integrin function. In addition, the isolated I-domain of alphaMbeta2 bound to VWF, and cell lines expressing alphaLbeta2 or alphaXbeta2 adhered efficiently to VWF. Taken together, our data showed that VWF can function as an adhesive surface for various leukocyte subsets (monocytes, PMNs). Analogous to VWF-platelet interaction, VWF provided binding sites for leukocyte receptors involved in rolling (PSGL-1) and stable (beta2-integrins) adhesion. VWF is unique in its intrinsic capacity to combine the rolling and the stable adhesion step in the interaction with leukocytes.

Probing sialic acid binding Ig-like lectins (siglecs) with sulfated oligosaccharides

Soluble siglecs-1, -4, -5, -6, -7, -8, -9, and -10 were probed with polyacrylamide glycoconjugates in which: 1) the Neu5Ac residue was substituted by a sulfate group (Su); 2) glycoconjugates contained both Su and Neu5Ac; 3) sialoglycoconjugates contained a tyrosine-O-sulfate residue. It was shown that sulfate derivatives of LacNAc did not bind siglecs-1, -4, -5, -6, -7, -8, -9, and -10; binding of 6'-O-Su-LacNAc to siglec-8 was stronger than binding of 3'SiaLacNAc. The relative affinity of 3'-O-Su-TF binding to siglecs-1, -4, and -8 was similar to that of 3'SiaTF. 3'-O-Su-Le(c) displayed two-fold weaker binding to siglec-1 and siglec-4 than 3'SiaLe(c). The interaction of soluble siglecs with sulfated oligosaccharides containing sialic acid was also studied. It was shown that siglecs-1, -4, -5, -6, -7, -9, and -10 did not interact with these compounds; binding of 6-O-Su-3'SiaLacNAc and 6-O-Su-3'SiaTF to siglec-8 was weaker than that of the corresponding sulfate-free sialoside probes. Siglec-8 displayed affinity to 6'-O-Su-LacNAc and 6'-O-Su-SiaLe(x), and defucosylation of the latter compound led to an increase in the binding. Sialoside probes containing tyrosine-O-sulfate residue did not display increased affinity to siglecs-1 and -5 compared with glycoconjugates containing only sialoside. Cell-bound siglecs-1, -5, -7, and -9 did not interact with 6-O-Su-3'SiaLacNAc, whereas the sulfate-free probe 3'SiaLacNAc demonstrated binding. In contrast, the presence of sulfate in 6-O-Su-6'SiaLacNAc did not affect binding of the sialoside probe to siglecs. 6'-O-Su-SiaLe(x) displayed affinity to cell-bound siglecs-1 and -5; its isomer 6-O-Su-SiaLe(x) bound more strongly to siglecs-1, -5, and -9 than SiaLe(x).

Differential adhesion of microspheres mediated by DNA hybridization I: experiment

We have developed a novel method to study collective behavior of multiple hybridized DNA chains by measuring the adhesion of DNA-coated micron-scale beads under hydrodynamic flow. Beads coated with single-stranded DNA probes are linked to surfaces coated with single target strands through DNA hybridization, and hydrodynamic shear forces are used to discriminate between strongly and weakly bound beads. The adhesiveness of microspheres depends on the strength of interaction between DNA chains on the bead and substrate surfaces, which is a function of the degree of DNA chain overlap, the fidelity of the match between hybridizing pairs, and other factors that affect the hybridization energy, such as the salt concentration in the hybridization buffer. The force for bead detachment is linearly proportional to the degree of chain overlap. There is a detectable drop in adhesion strength when there is a single base mismatch in one of the hybridizing chains. The effect of single nucleotide mismatch was tested with two different strand chemistries, with mutations placed at several different locations. All mutations were detectable, but there was no comprehensive rule relating the drop in adhesive strength to the location of the defect. Since adhesiveness can be coupled to the strength of overlap, the method holds promise to be a novel methodology for oligonucleotide detection.

P-selectin glycoprotein ligand-1 (PSGL-1) is up-regulated on leucocytes from patients with chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is characterized by a dysregulated recruitment of circulating leucocytes into the lung which is associated with the onset and progress of the disease. P-selectin glycoprotein ligand-1 (PSGL-1) is expressed on leucocytes and plays an essential role in primary leucocyte-endothelial cell adhesive contacts. The present study investigated if PSGL-1 is up-regulated on leucocytes of COPD patients. Peripheral blood samples were collected from COPD patients as well as controls (smoking, nonsmoking volunteers) and subjected to analysis of PSGL-1 expression on leucocytes, i.e. neutrophils, eosinophils, monocytes and lymphocytes by flow cytometry. No significant difference was observed between healthy nonsmoking and healthy smoking control subjects. In contrast, PSGL-1 expression was found to be significantly increased on the surface of all four leucocyte populations in COPD patients compared to both control groups. The finding that PSGL-1 surface expression is up-regulated on leucocytes of COPD patients as compared to leucocytes of controls suggests PSGL-1 as a potential target for anti-inflammatory treatment.

Cobra cardiotoxin-induced cell death in fetal rat cardiomyocytes and cortical neurons: different pathway but similar cell surface target

Cobra cardiotoxins (CTXs) are basic polypeptides with diverse pharmacological functions that are cytotoxic to many different cell types through both necrotic and apoptotic cell death pathways. In this comparative study of the action of CTX A3 from the Taiwan cobra (Naja atra) on fetal rat cardiomyocytes and cortical neurons, it was shown that CTX A3 induced different patterns of elevation of intracellular Ca2+ concentration ([Ca2+]i), CTX internalization, caspase-3 activity and viability. Application of an anti-sulfatide monoclonal antibody, O4 specific for 3-sulfo-galactose lipid, but not in the control experiments using anti-GM3 monoclonal antibody, reduces CTX-induced [Ca2+]i elevation, CTX internalization and toxicity. Therefore, CTX may target similar sulfo-containing cell surface receptors in both fetal rat cardiomyocytes and cortical neurons, but induce cell death through different pathways specific to each cell type.

Leukocyte adhesion: what's the catch?

A recent study shows that the leukocyte adhesion molecules known as selectins form 'catch' bonds, the dissociation rate of which decreases with increasing applied force. The ability of selectins to switch between catch and slip bonds, where dissociation increases with force, can explain the shear threshold effect, in which leukocyte adhesion goes through a maximum with increasing shear rate.

Identification of equine P-selectin glycoprotein ligand-1 (CD162)

P-selectin glycoprotein ligand-1 (PSGL-1, CD162) is a dimeric, mucin-like, transmembrane glycoprotein constitutively expressed on leukocytes. A high baseline level of P-selectin expression in circulating equine platelets suggests a primed state toward inflammation and thrombosis via P-selectin/PSGL-1 adhesion. To investigate the potential role of equine P-selectin in these events, we first identified the cDNA sequence of equine PSGL-1 (ePSGL-1) using degenerate PCR and RACE-PCR and then compared the predicted sequence with that of human PSGL-1 (hPSGL-1). ePSGL-1 protein subunit is predicted to be 43 kDa and composed of 420 amino acids with a predicted 18-amino-acid signal sequence showing 78% homology to hPSGL-1. Previously published work has shown that binding of P-selectin requires sulfation of at least one of three tyrosines and O-glycosylation of one threonine in the N-terminus of human PSGL-1. However, the corresponding domain in ePSGL-1, spanning residues 19-43, contains only one tyrosine in the vicinity of two threonines at positions 25 and 41. ePSGL-1 contains 14 threonine/serine-rich decameric repeats as compared to hPSGL-1 which contains 14-16 threonine-rich decameric repeats. The transmembrane and cytoplasmic domains display 91% and 74% homology to corresponding human PSGL-1 domains, respectively. In summary, there is 71% homology in comparing the open reading frame (ORF) of ePSGL-1 with that of hPSGL-1. The greatest homologies between species exist in the transmembrane domain and cytoplasmic tail while substantial differences exist in the extracellular domain.

Endoglycan, a member of the CD34 family, functions as an L-selectin ligand through modification with tyrosine sulfation and sialyl Lewis x

During lymphocyte homing to secondary lymphoid organs and instances of inflammatory trafficking, the rolling of leukocytes on vascular endothelium is mediated by transient interactions between L-selectin on leukocytes and several carbohydrate-modified ligands on the endothelium. Most L-selectin ligands such as CD34 and podocalyxin present sulfated carbohydrate structures (6-sulfated sialyl Lewis x or 6-sulfo-sLex) as a recognition determinant within their heavily glycosylated mucin domains. We recently identified endoglycan as a new member of the CD34 family. We report here that endoglycan, like the two other members of this family (CD34 and podocalyxin) can function as a L-selectin ligand. However, endoglycan employs a different binding mechanism, interacting with L-selectin through sulfation on two tyrosine residues and O-linked sLex structures that are presented within its highly acidic amino-terminal region. Our analysis establishes striking parallels with PSGL-1, a leukocyte ligand that interacts with all three selectins, mediating leukocyte-endothelial, leukocyte-leukocyte, and platelet-leukocyte interactions. Since the distribution of endoglycan includes hematopoietic precursors and leukocyte subpopulations, in addition to endothelial cells, our findings suggest several potential settings for endoglycan-mediated adhesion events.

Molecular basis of leukocyte rolling on PSGL-1. Predominant role of core-2 O-glycans and of tyrosine sulfate residue 51

Interactions between the leukocyte adhesion receptor L-selectin and P-selectin glycoprotein ligand-1 play an important role in regulating the inflammatory response by mediating leukocyte tethering and rolling on adherent leukocytes. In this study, we have examined the effect of post-translational modifications of PSGL-1 including Tyr sulfation and presentation of sialylated and fucosylated O-glycans for L-selectin binding. The functional importance of these modifications was determined by analyzing soluble L-selectin binding and leukocyte rolling on CHO cells expressing various glycoforms of PSGL-1 or mutant PSGL-1 targeted at N-terminal Thr or Tyr residues. Simultaneous expression of core-2 beta1,6-N-acetylglucosaminyltransferase and fucosyltransferase VII was required for optimal L-selectin binding to PSGL-1. Substitution of Thr-57 by Ala but not of Thr-44, strongly decreased L-selectin binding and leukocyte rolling on PSGL-1. Substitution of Tyr by Phe revealed that PSGL-1 Tyr-51 plays a predominant role in mediating L-selectin binding and leukocyte rolling whereas Tyr-48 has a minor role, an observation that contrasts with the pattern seen for the interactions between PSGL-1 and P-selectin where Tyr-48 plays a key role. Molecular modeling analysis of L-selectin and P-selectin interactions with PSGL-1 further supported these observations. Additional experiments showed that core-2 O-glycans attached to Thr-57 were also of critical importance in regulating the velocity and stability of leukocyte rolling. These observations pinpoint the structural characteristics of PSGL-1 that are required for optimal interactions with L-selectin and may be responsible for the specific kinetic and mechanical bond properties of the L-selectin-PSGL-1 adhesion receptor-counterreceptor pair.

Distinct molecular and cellular contributions to stabilizing selectin-mediated rolling under flow

Leukocytes roll on selectins at nearly constant velocities over a wide range of wall shear stresses. Ligand-coupled microspheres roll faster on selectins and detach quickly as wall shear stress is increased. To examine whether the superior performance of leukocytes reflects molecular features of native ligands or cellular properties that favor selectin-mediated rolling, we coupled structurally defined selectin ligands to microspheres or K562 cells and compared their rolling on P-selectin. Microspheres bearing soluble P-selectin glycoprotein ligand (sPSGL)-1 or 2-glycosulfopeptide (GSP)-6, a GSP modeled after the NH2-terminal P-selectin-binding region of PSGL-1, rolled equivalently but unstably on P-selectin. K562 cells displaying randomly coupled 2-GSP-6 also rolled unstably. In contrast, K562 cells bearing randomly coupled sPSGL-1 or 2-GSP-6 targeted to a membrane-distal region of the presumed glycocalyx rolled more like leukocytes: rolling steps were more uniform and shear resistant, and rolling velocities tended to plateau as wall shear stress was increased. K562 cells treated with paraformaldehyde or methyl-beta-cyclodextrin before ligand coupling were less deformable and rolled unstably like microspheres. Cells treated with cytochalasin D were more deformable, further resisted detachment, and rolled slowly despite increases in wall shear stress. Thus, stable, shear-resistant rolling requires cellular properties that optimize selectin-ligand interactions.

Comparison of PSGL-1 microbead and neutrophil rolling: microvillus elongation stabilizes P-selectin bond clusters

A cell-scaled microbead system was used to analyze the force-dependent kinetics of P-selectin adhesive bonds independent of micromechanical properties of the neutrophil's surface microvilli, an elastic structure on which P-selectin ligand glycoprotein-1 (PSGL-1) is localized. Microvillus extension has been hypothesized in contributing to the dynamic range of leukocyte rolling observed in vivo during inflammatory processes. To evaluate PSGL-1/P-selectin bond kinetics of microbeads and neutrophils, rolling and tethering on P-selectin-coated substrates were compared in a parallel-plate flow chamber. The dissociation rates for PSGL-1 microbeads on P-selectin were briefer than those of neutrophils for any wall shear stress, and increased more rapidly with increasing flow. The microvillus length necessary to reconcile dissociation constants of PSGL-1 microbeads and neutrophils on P-selectin was 0.21 microm at 0.4 dyn/cm2, and increased to 1.58 microm at 2 dyn/cm2. The apparent elastic spring constant of the microvillus ranged from 1340 to 152 pN/microm at 0.4 and 2.0 dyn/cm2 wall shear stress. Scanning electron micrographs of neutrophils rolling on P-selectin confirmed the existence of micrometer-scaled tethers. Fixation of neutrophils to abrogate microvillus elasticity resulted in rolling behavior similar to PSGL-1 microbeads. Our results suggest that microvillus extension during transient PSGL-1/P-selectin bonding may enhance the robustness of neutrophil rolling interactions.