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

Selectins in Biology and Human Disease: Opportunity in E-selectin Antagonism

Selectins are cell adhesion proteins discovered in the 1980s. As C-type lectins, selectins contain an essential calcium ion in the ligand-binding pocket and recognize the isomeric tetrasaccharides sialyl Lewisx (sLex) and sialyl Lewisa (sLea). Three selectins, E-selectin, P-selectin, and L-selectin, play distinct, complementary roles in inflammation, hematopoiesis, and tumor biology. They have been implicated in the pathology of diverse inflammatory disorders, and several selectin antagonists have been tested clinically. E-selectin plays a unique role in leukocyte activation, making it an attractive target for intervention, for example, in sickle cell disease (SCD). This review summarizes selectin biology and pathology, structure and ligand binding, and selectin antagonists that have reached clinical testing with an emphasis on E-selectin.

Nanoscopic Characterization of Cell Migration under Flow Using Optical and Electron Microscopy

Hematopoietic stem/progenitor cell (HSPC) and leukemic cell homing is an important biological phenomenon that takes place through essential interactions with adhesion molecules on an endothelial cell layer. The homing process of HSPCs begins with the tethering and rolling of the cells on the endothelial layer, which is achieved by the interaction between selectins on the endothelium to the ligands on HSPC/leukemic cells under shear stress of the blood flow. Although many studies have been based on in vitro conditions of the cells rolling over recombinant proteins, significant challenges remain when imaging HSPC/leukemic cells on the endothelium, a necessity when considering characterizing cell-to-cell interaction and rolling dynamics during cell migration. Here, we report a new methodology that enables imaging of stem-cell-intrinsic spatiotemporal details during its migration on an endothelium-like cell monolayer. We developed optimized protocols that preserve transiently appearing structures on HSPCs/leukemic cells during its rolling under shear stress for fluorescence and scanning electron microscopy characterization. Our new experimental platform is closer to in vivo conditions and will contribute to indepth understanding of stem-cell behavior during its migration and cell-to-cell interaction during the process of homing.

A general numerical model of leukocyte adhesion in microchannels

Leukocyte adhesion on the vascular endothelium plays an important role in human immune system and reflects the physiological condition of a human body. In this paper, a generally implementable dynamic adhesion model based on the length limit of microvilli was developed to explore the behavior of a suspended leukocyte's adhesion process under microchannel shear flow. Simulations showed that the whole adhesion process can be divided into cell sedimentation, preliminary adhesion and stable dynamic adhesion stages. The cell tumbling kinetics, cell deformation, cell adhesion area and adhesion force were studied under the conditions of various bond strength, cell membrane surface tension, inlet flow velocity and cytoplasmic viscosity. Results showed that the bond strength affects the cell tumbling behaviors differently by changing the adhesion force. The cell with lower membrane surface tension induces a larger adhesion area, and eventually results in a greater adhesion and a lower cell tumbling velocity. The flow velocity changes cell velocity through the flow viscous force during the whole adhesion process. The cytoplasmic viscosity affects adhesion mainly in the preliminary adhesion stage by changing the cell deformation rate but has slight effect on the stabilized dynamic adhesion on cells. This study provides a simple theoretical basis to further clarify the mechanism of cell behaviors under stress and adhesion and becomes one of the prerequisites for study of tissue inflammation, wound healing, and disease treatments.

The N-terminal autoinhibitory module of the A1 domain in von Willebrand factor stabilizes the mechanosensor catch bond

The N-AIM of VWF-A1 forms a Rotini-like structure, therefore partially autoinhibit VWF-A1–GPIbα interaction. The N-AIM acts as a defending sword to protect and stabilize the VWF-A1 structure under harsh environments.

Distinct binding kinetics of E-, P- and L-selectins to CD44

Molecular-level selectin-cluster of differentiation 44 (CD44) interactions are far from clear because of the complexity and diversity of CD44 glycosylation and isoforms expressed on various types of cells. By combining experimental measurements and simulation predictions, the binding kinetics of three selectin members to the recombinant CD44 were quantified and the corresponding microstructural mechanisms were explored, respectively. Experimental results showed that the E-selectin-CD44 interactions mainly mediated the firm adhesion of microbeads under shear flow with the strongest rupture force. P- and L-selectins had similar interaction strength but different association and dissociation rates by mediating stable rolling and transient adhesions of microbeads, respectively. Molecular docking and molecular dynamics (MD) simulations predicted that the binding epitopes of CD44 to selectins are all located at the side face of each selectin, although the interfaces denoted as the hinge region are between lectin and epidermal growth factor domains of E-selectin, Lectin domain side of P-selectin and epidermal growth factor domain side of L-selectin, respectively. The lowest binding free energy, the largest rupture force and the longest lifetime for E-selectin, as well as the comparable values for P- and L-selectins, demonstrated in both equilibration and steered MD simulations, supported the above experimental results. These results offer basic data for understanding the functional differences of selectin-CD44 interactions.

The kinetics of E-selectin- and P-selectin-induced intermediate activation of integrin αLβ2 on neutrophils

Selectins and integrins are key players in the adhesion and signaling cascade that recruits leukocytes to inflamed tissues. Selectin binding induces β2 integrin binding to slow leukocyte rolling. Here, a micropipette was used to characterize neutrophil adhesion to E-selectin and intercellular adhesion molecule-1 (ICAM-1) at room temperature. The time-dependent adhesion frequency displayed two-stage kinetics, with an E-selectin-mediated fast increase to a low plateau followed by a slow increase to a high plateau mediated by intermediate-affinity binding of integrin αLβ2 to ICAM-1. The αLβ2 activation required more than 5 s contact to E-selectin and spleen tyrosine kinase (Syk) activity. A multi-zone channel was used to analyze αLβ2 activation by P-selectin in separate zones of receptors or antibodies, finding an inverse relationship between the rolling velocity on ICAM-1 and P-selectin dose, and a P-selectin dose-dependent change from bent to extended conformations with a closed headpiece that was faster at 37°C than at room temperature. Activation of αLβ2 exhibited different levels of cooperativity and persistent times depending on the strength and duration of selectin stimulation. These results define the precise timing and kinetics of intermediate activation of αLβ2 by E- and P-selectins.

Neutrophils lacking ERM proteins polarize and crawl directionally but have decreased adhesion strength

Ezrin/radixin/moesin (ERM) proteins are adaptors that link the actin cytoskeleton to the cytoplasmic domains of membrane proteins. Leukocytes express mostly moesin with lower levels of ezrin but no radixin. When leukocytes are activated, ERMs are postulated to redistribute membrane proteins from microvilli into uropods during polarization and to transduce signals that influence adhesion and other responses. However, these functions have not been tested in leukocytes lacking all ERMs. We used knockout (KO) mice with neutrophils lacking ezrin, moesin, or both proteins (double knockout [DKO]) to probe how ERMs modulate cell shape, adhesion, and signaling in vitro and in vivo. Surprisingly, chemokine-stimulated DKO neutrophils still polarized and redistributed ERM-binding proteins such as PSGL-1 and CD44 to the uropods. Selectin binding to PSGL-1 on moesin KO or DKO neutrophils activated kinases that enable integrin-dependent slow rolling but not those that generate neutrophil extracellular traps. Flowing neutrophils of all genotypes rolled normally on selectins and, upon chemokine stimulation, arrested on integrin ligands. However, moesin KO and DKO neutrophils exhibited defective integrin outside-in signaling and reduced adhesion strength. In vivo, DKO neutrophils displayed normal directional crawling toward a chemotactic gradient, but premature detachment markedly reduced migration from venules into inflamed tissues. Our results demonstrate that stimulated neutrophils do not require ERMs to polarize or to move membrane proteins into uropods. They also reveal an unexpected contribution of moesin to integrin outside-in signaling and adhesion strengthening.

Localization of Rolling and Firm-Adhesive Interactions Between Circulating Tumor Cells and the Microvasculature Wall

INTRODUCTION

The adhesion of tumor cells to vessel wall is a critical stage in cancer metastasis. Firm adhesion of cancer cells is usually followed by their extravasation through the endothelium. Despite previous studies identifying the influential parameters in the adhesive behavior of the cancer cell to a planer substrate, less is known about the interactions between the cancer cell and microvasculature wall and whether these interactions exhibit organ specificity. The objective of our study is to characterize sizes of microvasculature where a deformable circulating cell (DCC) would firmly adhere or roll over the wall, as well as to identify parameters that facilitate such firm adherence and underlying mechanisms driving adhesive interactions.

METHODS

A three-dimensional model of DCCs is applied to simulate the fluid-structure interaction between the DCC and surrounding fluid. A dynamic adhesion model, where an adhesion molecule is modeled as a spring, is employed to represent the stochastic receptor-ligand interactions using kinetic rate expressions.

RESULTS

Our results reveal that both the cell deformability and low shear rate of flow promote the firm adhesion of DCC in small vessels ( < 10 μ m ). Our findings suggest that ligand-receptor bonds of PSGL-1-P-selectin may lead to firm adherence of DCC in smaller vessels and rolling-adhesion of DCC in larger ones where cell velocity drops to facilitate the activation of integrin-ICAM-1 bonds.

CONCLUSIONS

Our study provides a framework to predict accurately where different DCC-types are likely to adhere firmly in microvasculature and to establish the criteria predisposing cancer cells to such firm adhesion.

Mechanical properties of P-selectin PSGL-1 bonds

The accurate determination of the mechanical properties of P-selectin and PSGL-1 is crucial for design and optimization of applications utilizing such bonds, e.g. biosensors and targeted drug delivery systems, as adhesion and mechanical interactions play a critical role in several key functions of biological cells. In current work, the spring constant and rupture force of a single P-selectin PSGL-1 ligand receptor bond and the Young's modulus of a layer made of these ligand receptors are reported. The work-of-adhesion of the P-selectin PSGL-1 interface is also characterized. In the reported experiments, PSGL-1 coated particles are deposited on a P-selectin coated substrate and their transient nanometer scale out-of-plane displacements are acquired employing a laser Doppler vibrometer as they are excited by an ultrasonic field. From the spectral response of a single particle, the resonance frequencies of its vibrational motion are identified, and with help of a particle adhesion model, the average rupture force and stiffness of a single P-selectin PSGL-1 ligand receptor are determined as F^rupt = 171 ± 56 pN and k^b = 0.56 ± 0.04 mN/m, respectively. Furthermore, the Young's modulus and work-of-adhesion of a layer of P-selectin PSGL-1 ligand receptors are extracted as E = 28.74 ± 3.96 MPa and W^A = 70.0 ± 8.0 mJ/m², respectively. Unlike Atomic Force Microscopy (AFM) and other probe-based techniques, the reported approach eliminates the need for direct contact with the sample, which could compromise the accuracy of the results by imposing unspecified additional contact interactions. Further, the current technique can be employed for measurements under various fluid flow conditions.

Effects of Transient Exposure to High Shear on Neutrophil Rolling Behavior

Introduction

Neutrophils display an array of behaviors ranging from rolling and migration to phagocytosis and granule secretion. Several of these behaviors are modulated by the local shear conditions. In the normal circulation, neutrophils experience shear rates from approximately 10-2,000 s^-1. However, neutrophils are also exposed to pathological shear levels in natural conditions such as severe stenosis and arteriosclerosis, as well as in blood-contacting devices such as ventricular assist devices (VADs) and hemodialysis machines. The effects of transiently (< 1 sec) exposing neutrophils to abnormally high shear rates (>3,000 s^-1) are not well understood.

Methods

We developed a set of microfluidic devices capable of exposing neutrophils to high shear rates for short durations (100-400 msec). Suspensions of isolated neutrophils were perfused through the devices and their rolling velocities on P-selectin were analyzed before and after shear exposure.

Results

We observed a significant increase in neutrophil rolling velocities on P-selectin coated regions following transient high shear exposure. The magnitude of the rolling velocity increase was dependent upon the duration of high shear exposure and became statistically significant for exposure times of 310 msec or longer. When polystyrene beads coated with a glycosulfopeptide that mimics the binding region of P-selectin glycoprotein ligand-1 (PSGL-1) were perfused through the devices, no change between the pre-shear and post-shear rolling velocities was observed.

Conclusions

These results suggest that high shear levels alter normal neutrophil rolling behavior and are important for understanding neutrophil biology in high shear conditions, as well as for improving medical device performance.

Computational study of cell adhesion and rolling in flow channel by meshfree method

Tethering and rolling of circulating leukocytes on the surface of endothelium are critical steps during an inflammatory response. A soft solid cell model was proposed to study monocytes tethering and rolling behaviors on substrate surface in shear flow. The interactions between monocytes and micro-channel surface were modeled by a coarse-grained molecular adhesive potential. The computational model was implemented in a Lagrange-type meshfree Galerkin formulation to investigate the monocyte tethering and rolling process with different flow rates. From the simulation results, it was found that the flow rate has profound effects on the rolling velocity, contact area and effective stress of monocytes. As the flow rate increased, the rolling velocity would increase linearly, whereas the contact area and average effective stress in monocyte showed nonlinear increase.

Cell adhesion during bullet motion in capillaries

A numerical analysis is presented of cell adhesion in capillaries whose diameter is comparable to or smaller than that of the cell. In contrast to a large number of previous efforts on leukocyte and tumor cell rolling, much is still unknown about cell motion in capillaries. The solid and fluid mechanics of a cell in flow was coupled with a slip bond model of ligand-receptor interactions. When the size of a capillary was reduced, the cell always transitioned to "bullet-like" motion, with a consequent decrease in the velocity of the cell. A state diagram was obtained for various values of capillary diameter and receptor density. We found that bullet motion enables firm adhesion of a cell to the capillary wall even for a weak ligand-receptor binding. We also quantified effects of various parameters, including the dissociation rate constant, the spring constant, and the reactive compliance on the characteristics of cell motion. Our results suggest that even under the interaction between P-selectin glycoprotein ligand-1 (PSGL-1) and P-selectin, which is mainly responsible for leukocyte rolling, a cell is able to show firm adhesion in a small capillary. These findings may help in understanding such phenomena as leukocyte plugging and cancer metastasis.

Surface deformation and shear flow in ligand mediated cell adhesion

We present a unified, multiscale model to study the attachment/detachment dynamics of two deforming, charged, near spherical cells, coated with binding ligands and subject to a slow, homogeneous shear flow in a viscous, ionic fluid medium. The binding ligands on the surface of the cells experience both attractive and repulsive forces in an ionic medium and exhibit finite resistance to rotation via bond tilting. The microscale drag forces and couples describing the fluid flow inside the small separation gap between the cells, are calculated using a combination of methods in lubrication theory and previously published numerical results. For a selected range of material and fluid parameters, a hysteretic transition of the sticking probability curves (i.e., the function [Formula: see text]) between the adhesion phase (when [Formula: see text]) and the fragmentation phase (when [Formula: see text]) is attributed to a nonlinear relation between the total nanoscale binding forces and the separation gap between the cells. We show that adhesion is favoured in highly ionic fluids, increased deformability of the cells, elastic binders and a higher fluid shear rate (until a critical threshold value of shear rate is reached). Within a selected range of critical shear rates, the continuation of the limit points (i.e., the turning points where the slope of [Formula: see text] changes sign) predict a bistable region, indicating an abrupt switching between the adhesion and the fragmentation regimes. Although, bistability in the adhesion-fragmentation phase diagram of two deformable, charged cells immersed in an ionic aqueous environment has been identified by some in vitro experiments, but until now, has not been quantified theoretically.

Catch bond interaction allows cells to attach to strongly hydrated interfaces

Hyaluronans are a class of glycosaminoglycans that are widespread in the mammalian body and serve a variety of functions. Their most striking characteristic is their pronounced hydrophilicity and their capability to inhibit unspecific adhesion when present at interfaces. Catch-bond interactions are used by the CD44 receptor to interact with this inert material and to roll on the surfaces coated with hyaluronans. In this minireview, the authors discuss the general properties of hyaluronans and the occurrence and relevance of the CD44 catch-bond interaction in the context of hematopoiesis, cancer development, and leukemia.

Endothelial Surface Protrusion by a Point Force

During leukocyte rolling on the endothelium, surface protrusion and membrane tether extraction occur consecutively on leukocytes. Both surface protrusion and tether extraction of leukocytes stabilize leukocyte rolling. Tethers can also be extracted from endothelial cells (ECs), but surface protrusion of ECs has never been confirmed to exist. In this study, we examined EC surface protrusion with the micropipette aspiration technique. We found that, like leukocytes, surface protrusion on an EC did exist when a point force was imposed. Both the protrusional stiffness and the crossover force of EC surface protrusion were dependent on the force loading rate and the cytoskeletal integrity, but neither of them was dependent on tumor necrosis factor α stimulation. Temperature (37°C) affected the protrusional stiffness only at small force loading rates. When a neutrophil was employed to directly impose the pulling force on the EC, simultaneous surface protrusion from both cells occurred, and it can be modeled as two springs connected in series, although the spring constants should be adjusted according to the force loading rate. Therefore, EC surface protrusion is an important aspect of leukocyte rolling, and it should not be ignored when leukocyte rolling stability is studied systematically.

Dual functions of Rap1 are crucial for T-cell homeostasis and prevention of spontaneous colitis

Rap1-GTP activates leukocyte function-associated antigen-1 (LFA-1) to induce arrest on the high endothelial venule (HEV). Here we show that Rap1-GDP restrains rolling behaviours of T cells on the peripheral lymph node addressin (PNAd), P-selectin and mucosal addressin cell adhesion molecule-1 (MadCAM-1) by inhibiting tether formation. Consequently, Rap1 deficiency impairs homing of naive T cells to peripheral lymph nodes, but accelerates homing of T_H17 and T_H1 cells to the colon, resulting in spontaneous colitis with tumours. Rap1-GDP associates with and activates lymphocyte-oriented kinase, which phosphorylates ERM (ezrin, radixin and moesin) in resting T cells. Phosphomimetic ezrin reduces the rolling of Rap1-deficient cells, and thereby decreases their homing into the colon. On the other hand, chemokines activate Rap1 at the plasma membrane within seconds, and Rap1-GTP binds to filamins, which diminishes its association with the β₂ chain of LFA-1 and results in LFA-1 activation. This Rap1-dependent regulation of T-cell circulation prevents the onset of colitis.

Rap1, a member of the Ras family of small guanine triphosphatases, mediates lymphocyte adhesion to high endothelial venules. Here the authors show that depending on its activation status Rap1 plays a dual role in T cell adhesion and by regulating T cell homeostasis is involved in the protection from colitis.

Regulation of L-selectin-dependent hydrodynamic shear thresholding by leukocyte deformability and shear dependent bond number

BACKGROUND

During inflammation leukocyte attachment to the blood vessel wall is augmented by capture of near-wall flowing leukocytes by previously adherent leukocytes. Adhesive interactions between flowing and adherent leukocytes are mediated by L-selectin and P-selectin Glycoprotein Ligand-1 (PSGL-1) co-expressed on the leukocyte surface and ultimately regulated by hydrodynamic shear thresholding.

OBJECTIVE

We hypothesized that leukocyte deformability is a significant contributory factor in shear thresholding and secondary capture.

METHODS

Cytochalasin D (CD) was used to increase neutrophil deformability and fixation was used to reduce deformability. Neutrophil rolling on PSGL-1 coated planar surfaces and collisions with PSGL-1 coated microbeads were analyzed using high-speed videomicroscopy (250 fps).

RESULTS

Increased deformability led to an increase in neutrophil rolling flux on PSGL-1 surfaces while fixation led to a decrease in rolling flux. Abrupt drops in flow below the shear threshold resulted in extended release times from the substrate for CD-treated neutrophils, suggesting increased bond number. In a cell-microbead collision assay lower flow rates were correlated with briefer adhesion lifetimes and smaller adhesive contact patches.

CONCLUSIONS

Leukocyte deformation may control selectin bond number at the flow rates associated with hydrodynamic shear thresholding. Model analysis supported a requirement for both L-selectin catch-slip bond properties and multiple bond formation for shear thresholding.

Multi-Inhibitory Effects of A2A Adenosine Receptor Signaling on Neutrophil Adhesion Under Flow

A2A adenosine receptor (A2AAR) signaling negatively regulates inflammatory responses in many disease models, but the detailed mechanisms remain unclear. We used the selective A2AAR agonist, ATL313, to examine how A2AAR signaling affects human and murine neutrophil adhesion under flow. Treating neutrophils with ATL313 inhibited selectin-induced, β2 integrin-dependent slow rolling and chemokine-induced, β2 integrin-dependent arrest on ICAM-1. ATL313 inhibited selectin-induced β2 integrin extension, which supports slow rolling, and chemokine-induced hybrid domain "swing-out," which supports arrest. Furthermore, ATL313 inhibited integrin outside-in signaling as revealed by reduced neutrophil superoxide production and spreading on immobilized anti-β2 integrin Ab. ATL313 suppressed selectin-triggered activation of Src family kinases (SFKs) and p38 MAPK, chemokine-triggered activation of Ras-related protein 1, and β2 integrin-triggered activation of SFKs and Vav cytoskeletal regulatory proteins. ATL313 activated protein kinase A and its substrate C-terminal Src kinase, an inhibitor of SFKs. Treating neutrophils with a protein kinase A inhibitor blocked the actions of ATL313. In vivo, ATL313-treated neutrophils rolled faster and arrested much less frequently in postcapillary venules of the murine cremaster muscle after TNF-α challenge. Furthermore, ATL313 markedly suppressed neutrophil migration into the peritoneum challenged with thioglycollate. ATL313 did not affect A2AAR-deficient neutrophils, confirming its specificity. Our findings provide new insights into the anti-inflammatory mechanisms of A2AAR signaling and the potential utility of A2AAR agonists in inflammatory diseases.

Analytical cell adhesion chromatography reveals impaired persistence of metastatic cell rolling adhesion to P-selectin

Selectins facilitate the recruitment of circulating cells from the bloodstream by mediating rolling adhesion, which initiates the cell-cell signaling that directs extravasation into surrounding tissues. To measure the relative efficiency of cell adhesion in shear flow for in vitro drug screening, we designed and implemented a microfluidic-based analytical cell adhesion chromatography system. The juxtaposition of instantaneous rolling velocities with elution times revealed that human metastatic cancer cells, but not human leukocytes, had a reduced capacity to sustain rolling adhesion with P-selectin. We define a new parameter, termed adhesion persistence, which is conceptually similar to migration persistence in the context of chemotaxis, but instead describes the capacity of cells to resist the influence of shear flow and sustain rolling interactions with an adhesive substrate that might modulate the probability of extravasation. Among cell types assayed, adhesion persistence to P-selectin was specifically reduced in metastatic but not leukocyte-like cells in response to a low dose of heparin. In conclusion, we demonstrate this as an effective methodology to identify selectin adhesion antagonist doses that modulate homing cell adhesion and engraftment in a cell-subtype-selective manner.

Von Willebrand factor-A1 domain binds platelet glycoprotein Ibα in multiple states with distinctive force-dependent dissociation kinetics

Circulating von Willebrand factor (VWF) adopts a closed conformation that shields the platelet glycoprotein Ibα (GPIbα) binding site in the VWF-A1 domain. Immobilized at sites of vascular injury, VWF is activated by its interaction with collagen and the exertion of increased hemodynamic forces. Studies on native VWF strings and isolated A1 domains suggest the existence of multiple A1 binding states in different biophysical contexts. In this single-molecule study, we have used a biomembrane force probe (BFP) and a flow chamber to identify and characterize a collagen binding induced conformation with a higher affinity to platelet GPIbα. As force increases, our results show that collagen binding increases the stability of GPIbα bond with both VWF and isolated A1 domain. However, the collagen 2D binding affinity for VWF-A3 domain is 10 times of that for A1 domain, suggesting the initial VWF capture is mediated by A3-collagen interaction while A1-collagen regulates the subsequent VWF activation. Our results reveal the molecular mechanism of collagen-regulated, A1-mediated platelet adhesion enhancement. Characterization of different A1 states provides insights into binding heterogeneity of VWF in different scenarios of inflammation and thrombosis.

Mechanical force effect on the two-state equilibrium of the hyaluronan-binding domain of CD44 in cell rolling

CD44 is the receptor for hyaluronan (HA) and mediates cell rolling under fluid shear stress. The HA-binding domain (HABD) of CD44 interconverts between a low-affinity, ordered (O) state and a high-affinity, partially disordered (PD) state, by the conformational change of the C-terminal region, which is connected to the plasma membrane. To examine the role of tensile force on CD44-mediated rolling, we used a cell-free rolling system, in which recombinant HABDs were attached to beads through a C-terminal or N-terminal tag. We found that the rolling behavior was stabilized only at high shear stress, when the HABD was attached through the C-terminal tag. In contrast, no difference was observed for the beads coated with HABD mutants that constitutively adopt either the O state or the PD state. Steered molecular dynamics simulations suggested that the force from the C terminus disrupts the interaction between the C-terminal region and the core of the domain, thus providing structural insights into how the mechanical force triggers the allosteric O-to-PD transition. Based on these results, we propose that the force applied from the C terminus enhances the HABD-HA interactions by inducing the conformational change to the high-affinity PD transition more rapidly, thereby enabling CD44 to mediate lymphocyte trafficking and hematopoietic progenitor cell homing under high-shear conditions.

Cytoskeletal regulation of CD44 membrane organization and interactions with E-selectin

Interactions of CD44 on neutrophils with E-selectin on activated endothelial cells mediate rolling under flow, a prerequisite for neutrophil arrest and migration into perivascular tissues. How CD44 functions as a rolling ligand despite its weak affinity for E-selectin is unknown. We examined the nanometer scale organization of CD44 on intact cells. CD44 on leukocytes and transfected K562 cells was cross-linked within a 1.14-nm spacer. Depolymerizing actin with latrunculin B reduced cross-linking. Fluorescence resonance energy transfer (FRET) revealed tight co-clustering between CD44 fused to yellow fluorescent protein (YFP) and CD44 fused to cyan fluorescent protein on K562 cells. Latrunculin B reduced FRET-reported co-clustering. Number and brightness analysis confirmed actin-dependent CD44-YFP clusters on living cells. CD44 lacking binding sites for ankyrin and for ezrin/radixin/moesin (ERM) proteins on its cytoplasmic domain (ΔANKΔERM) did not cluster. Unexpectedly, CD44 lacking only the ankyrin-binding site (ΔANK) formed larger but looser clusters. Fluorescence recovery after photobleaching demonstrated increased CD44 mobility by latrunculin B treatment or by deleting the cytoplasmic domain. ΔANKΔERM mobility increased only modestly, suggesting that the cytoplasmic domain engages the cytoskeleton by an additional mechanism. Ex vivo differentiated CD44-deficient neutrophils expressing exogenous CD44 rolled on E-selectin and activated Src kinases after binding anti-CD44 antibody. In contrast, differentiated neutrophils expressing ΔANK had impaired rolling and kinase activation. These data demonstrate that spectrin and actin networks regulate CD44 clustering and suggest that ankyrin enhances CD44-mediated neutrophil rolling and signaling.

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.

Quantitative dynamic footprinting microscopy

Quantitative dynamic footprinting (qDF) allows visualization of the footprints of live leukocytes rolling on a selectin-coated cover glass. qDF works on the principle of total internal reflection fluorescence, which involves fluorescence excitation in a thin slice (~200 nm) of the cell proximal to the cover glass while the rest of the cell remains dark. Dual color qDF (DqDF) is an advancement of qDF, which enables simultaneous visualization of two fluorochromes in the footprints of rolling leukocytes. When the fluorochrome is localized either in the cell cytoplasm or plasma membrane, the two-dimensional qDF image is used to create a three-dimensional rendition of the footprint topography. DqDF is a useful tool to study leukocyte adhesion under flow, and has recently been used to reveal mechanisms that enable neutrophils to roll at high shear stresses that prevail in venules during inflammation.

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.

Neutrophil rolling at high shear: flattening, catch bond behavior, tethers and slings

Neutrophil recruitment to sites of inflammation involves neutrophil rolling along the inflamed endothelium in the presence of shear stress imposed by blood flow. Neutrophil rolling in post-capillary venules in vivo is primarily mediated by P-selectin on the endothelium binding to P-selectin glycoprotein ligand-1 (PSGL-1) constitutively expressed on neutrophils. Blood flow exerts a hydrodynamic drag on the rolling neutrophil which is partially or fully balanced by the adhesive forces generated in the P-selectin-PSGL-1 bonds. Rolling is the result of rapid formation and dissociation of P-selectin-PSGL-1 bonds at the center and rear of the rolling cell, respectively. Neutrophils roll stably on P-selectin in post-capillary venules in vivo and flow chambers in vitro at wall shear stresses greater than 6 dyn cm(-2). However, the mechanisms that enable neutrophils to roll at such high shear stress are not completely understood. In vitro and in vivo studies have led to the discovery of four potential mechanisms, viz. cell flattening, catch bond behavior, membrane tethers, and slings. Rolling neutrophils undergo flattening at high shear stress, which not only increases the size of the cell footprint but also reduces the hydrodynamic drag experienced by the rolling cell. P-selectin-PSGL-1 bonds behave as catch bonds at small detachment forces and thus become stronger with increasing force. Neutrophils rolling at high shear stress form membrane tethers which can be longer than the cell diameter and promote the survival of P-selectin-PSGL-1 bonds. Finally, neutrophils rolling at high shear stress form 'slings', which act as cell autonomous adhesive substrates and support step-wise peeling. Tethers and slings act together and contribute to the forces balancing the hydrodynamic drag. How the synergy between the four mechanisms leads to stable rolling at high shear stress is an area that needs further investigation.

Leukocyte rolling on P-selectin: a three-dimensional numerical study of the effect of cytoplasmic viscosity

Rolling leukocytes deform and show a large area of contact with endothelium under physiological flow conditions. We studied the effect of cytoplasmic viscosity on leukocyte rolling using our three-dimensional numerical algorithm that treats leukocyte as a compound droplet in which the core phase (nucleus) and the shell phase (cytoplasm) are viscoelastic fluids. The algorithm includes the mechanical properties of the cell cortex by cortical tension and considers leukocyte microvilli that deform viscoelastically and form viscous tethers at supercritical force. Stochastic binding kinetics describes binding of adhesion molecules. The leukocyte cytoplasmic viscosity plays a critical role in leukocyte rolling on an adhesive substrate. High-viscosity cells are characterized by high mean rolling velocities, increased temporal fluctuations in the instantaneous velocity, and a high probability for detachment from the substrate. A decrease in the rolling velocity, drag, and torque with the formation of a large, flat contact area in low-viscosity cells leads to a dramatic decrease in the bond force and stable rolling. Using values of viscosity consistent with step aspiration studies of human neutrophils (5-30 Pa·s), our computational model predicts the velocities and shape changes of rolling leukocytes as observed in vitro and in vivo.

Force-dependent bond dissociation govern rolling of HL-60 cells through E-selectin

E-selectin-mediated rolling on vascular surface of circulating leukocyte on vascular surface is a key initial event during inflammatory response and lymphocyte homing. This event depends not only on the specific interactions of adhesive molecules but also on the hemodynamics of blood flow. Little is still understood about whether wall shear stress or shear rate regulates the rolling. With flow chamber techniques, we here measured the effects of transport, shear stress and cell deformation on rolling of both unfixed and fixed HL-60 cells on E-selectin either in the absence or in the presence of 3% Ficoll in medium at various wall shear stresses from 0.05 to 0.7 dyn/cm(2). The results demonstrated a triphasic force-dependent rolling, that is, as increasing of force, the rolling would be accelerated firstly, then followed a decelerating phase occurred at the initial shear threshold of about 0.1 dyn/cm(2), and lastly returned to an accelerating process starting at the optimal shear threshold of 0.35 dyn/cm(2) approximately. The catch bond regime was completely reflected to rolling behaviors, such as tether lifetime, cell stop time and rolling velocity, meaning that the dominant factor to govern rolling is force. The initial shear threshold might be the minimum level of wall shear stress to sustain a stationary rolling, and the optimal shear threshold would make rolling to the most stable and regular. These findings strongly elucidate the catch bond mechanism for flow-enhanced rolling through E-selectin since longer bond lifetimes led to slower and stabler rolling.

Factors affecting the endothelial retention of targeted microbubbles: influence of microbubble shell design and cell surface projection of the endothelial target molecule

BACKGROUND

In biologic systems, the arrest of circulating cells is mediated by adhesion molecules projecting their active binding domain above the cell surface to enhance bond formation and tether strength. Similarly, molecular spacers are used for ligands on particle-based molecular imaging agents. The aim of this study was to evaluate the influence of tether length for targeting ligands on ultrasound molecular imaging agents.

METHODS

Microbubbles bearing biotin at the end of variable-length polyethylene glycol spacer arms (MB(2000) and MB(3400)) were prepared. To assess in vivo attachment efficiency to endothelial counterligands that vary in their distance from the endothelial cell surface, contrast-enhanced ultrasound (CEU) molecular imaging of tumor necrosis factor-α-induced P-selectin (long distance) or intercellular adhesion molecule-2 (short distance) was performed with each agent in murine hind limbs. To assess the influence of the glycocalyx on microbubble attachment, CEU molecular imaging of intercellular adhesion molecule-2 was performed after degradation of the glycocalyx.

RESULTS

CEU molecular imaging targeted to P-selectin showed signal enhancement above control agent for MB(2000) and MB(3400), the degree of which was significantly higher for MB(3400) compared with MB(2000). CEU molecular imaging targeted to intercellular adhesion molecule-2 showed low overall signal for all agents and signal enhancement above control for MB(3400) only. Glycocalyx degradation increased signal for MB(3400) and MB(2000).

CONCLUSIONS

Microbubble targeting to endothelial ligands is influenced by (1) the tether length of the ligand, (2) the degree to which the endothelial target is projected from the cell surface, and (3) the status of the glycocalyx. These considerations are important for designing targeted imaging probes and understanding potential obstacles to molecular imaging.

Molecular dynamics simulation of shear- and stretch-induced dissociation of P-selectin/PSGL-1 complex

By mediating the tethering and rolling of leukocytes on vascular surfaces, the interactions between P-selectin and the P-selectin glycoprotein ligand 1 (PSGL-1) play crucial roles during inflammation cascade. Tensile stretch produced by rolling leukocytes and shear stress exerted by blood flow constitute the two types of mechanical forces that act on the P-selectin/PSGL-1 bond. These forces modulate not only dissociation kinetics of this bond, but also the leukocyte adhesion dynamics. However, the respective contribution of the two forces to bond dissociation and to the corresponding microstructural bases remains unclear. To mimic the mechanical microenvironment, we developed two molecular dynamics approaches; namely, an approach involving the shear flow field with a controlled velocity gradient, and the track dragging approach with a defined trajectory. With each approach or with both combined, we investigate the microstructural evolution and dissociation kinetics of the P-LE/SGP-3 construct, which is the smallest functional unit of the P-selectin/PSGL-1 complex. The results demonstrate that both shear flow and tensile stretch play important roles in the collapse of the construct and that, before bond dissociation, the former causes more destruction of domains within the construct than the latter. Dissociation of the P-LE/SGP-3 construct features intramolecular destruction of the epidermal-growth-factor (EGF) domain and the breaking of hydrogen-bond clusters at the P-selectin-lectin/EGF interface. Thus, to better understand how mechanics impacts the dissociation kinetics of the P-selectin/PSGL-1 complex, we propose herein two approaches to mimic its physiological mechanical environment.

Bistability of cell adhesion in shear flow

Cell adhesion plays a central role in multicellular organisms helping to maintain their integrity and homeostasis. This complex process involves many different types of adhesion proteins, and synergetic behavior of these proteins during cell adhesion is frequently observed in experiments. A well-known example is the cooperation of rolling and stationary adhesion proteins during the leukocytes extravasation. Despite the fact that such cooperation is vital for proper functioning of the immune system, its origin is not fully understood. In this study we constructed a simple analytic model of the interaction between a leukocyte and the blood vessel wall in shear flow. The model predicts existence of cell adhesion bistability, which results from a tug-of-war between two kinetic processes taking place in the cell-wall contact area-bond formation and rupture. Based on the model results, we suggest an interpretation of several cytoadhesion experiments and propose a simple explanation of the existing synergy between rolling and stationary adhesion proteins, which is vital for effective cell adherence to the blood vessel walls in living organisms.

Leukocyte ligands for endothelial selectins: specialized glycoconjugates that mediate rolling and signaling under flow

Reversible interactions of glycoconjugates on leukocytes with P- and E-selectin on endothelial cells mediate tethering and rolling of leukocytes in inflamed vascular beds, the first step in their recruitment to sites of injury. Although selectin ligands on hematopoietic precursors have been identified, here we review evidence that PSGL-1, CD44, and ESL-1 on mature leukocytes are physiologic glycoprotein ligands for endothelial selectins. Each ligand has specialized adhesive functions during tethering and rolling. Furthermore, PSGL-1 and CD44 induce signals that activate the β2 integrin LFA-1 and promote slow rolling, whereas ESL-1 induces signals that activate the β2 integrin Mac-1 in adherent neutrophils. We also review evidence for glycolipids, CD43, L-selectin, and other glycoconjugates as potential physiologic ligands for endothelial selectins on neutrophils or lymphocytes. Although the physiologic characterization of these ligands has been obtained in mice, we also note reported similarities and differences with human selectin ligands.

Biomechanics of leukocyte rolling

Leukocyte rolling on endothelial cells and other P-selectin substrates is mediated by P-selectin binding to P-selectin glycoprotein ligand-1 expressed on the tips of leukocyte microvilli. Leukocyte rolling is a result of rapid, yet balanced formation and dissociation of selectin-ligand bonds in the presence of hydrodynamic shear forces. The hydrodynamic forces acting on the bonds may either increase (catch bonds) or decrease (slip bonds) their lifetimes. The force-dependent 'catch-slip' bond kinetics are explained using the 'two pathway model' for bond dissociation. Both the 'sliding-rebinding' and the 'allosteric' mechanisms attribute 'catch-slip' bond behavior to the force-induced conformational changes in the lectin-EGF domain hinge of selectins. Below a threshold shear stress, selectins cannot mediate rolling. This 'shear-threshold' phenomenon is a consequence of shear-enhanced tethering and catch bond-enhanced rolling. Quantitative dynamic footprinting microscopy has revealed that leukocytes rolling at venular shear stresses (>0.6 Pa) undergo cellular deformation (large footprint) and form long tethers. The hydrodynamic shear force and torque acting on the rolling cell are thought to be synergistically balanced by the forces acting on tethers and stressed microvilli, however, their relative contribution remains to be determined. Thus, improvement beyond the current understanding requires in silico models that can predict both cellular and microvillus deformation and experiments that allow measurement of forces acting on individual microvilli and tethers.

Probing dynamic cell-substrate interactions using photochemically generated surface-immobilized gradients: application to selectin-mediated leukocyte rolling

Model substrates presenting biochemical cues immobilized in a controlled and well-defined manner are of great interest for their applications in biointerface studies that elucidate the molecular basis of cell receptor-ligand interactions. Herein, we describe a direct, photochemical method to generate surface-immobilized biomolecular gradients that are applied to the study of selectin-mediated leukocyte rolling. The technique employs benzophenone-modified glass substrates, which upon controlled exposure to UV light (350-365 nm) in the presence of protein-containing solutions facilitate the generation of covalently immobilized protein gradients. Conditions were optimized to generate gradient substrates presenting P-selectin and PSGL-1 (P-selectin glycoprotein ligand-1) immobilized at site densities over a 5- to 10-fold range (from as low as ∼200 molecules μm(-2) to as high as 6000 molecules μm(-2)). The resulting substrates were quantitatively characterized via fluorescence analysis and radioimmunoassays before their use in the leukocyte rolling assays. HL-60 promyelocytes and Jurkat T lymphocytes were assessed for their ability to tether to and roll on substrates presenting immobilized P-selectin and PSGL-1 under conditions of physiologically relevant shear stress. The results of these flow assays reveal the combined effect of immobilized protein site density and applied wall shear stress on cell rolling behavior. Two-component substrates presenting P-selectin and ICAM-1 (intercellular adhesion molecule-1) were also generated to assess the interplay between these two proteins and their effect on cell rolling and adhesion. These proof-of-principle studies verify that the described gradient generation approach yields well-defined gradient substrates that present immobilized proteins over a large range of site densities that are applicable for investigation of cell-materials interactions, including multi-parameter leukocyte flow studies. Future applications of this enabling methodology may lead to new insights into the biophysical phenomena and molecular mechanism underlying complex biological processes such as leukocyte recruitment and the inflammatory response.

Mimicking the inflammatory cell adhesion cascade by nucleic acid aptamer programmed cell-cell interactions

Nature has evolved effective cell adhesion mechanisms to deliver inflammatory cells to inflamed tissue; however, many culture-expanded therapeutic cells are incapable of targeting diseased tissues following systemic infusion, which represents a great challenge in cell therapy. Our aim was to develop simple approaches to program cell-cell interactions that would otherwise not exist toward cell targeting and understanding the complex biology of cell-cell interactions. We employed a chemistry approach to engineer P- or L-selectin binding nucleic acid aptamers onto mesenchymal stem cells (MSCs) to enable them to engage inflamed endothelial cells and leukocytes, respectively. We show for the first time that engineered cells with a single artificial adhesion ligand can recapitulate 3 critical cell interactions in the inflammatory cell adhesion cascade under dynamic flow conditions. Aptamer-engineered MSCs adhered on respective selectin surfaces under static conditions >10 times more efficiently than controls including scrambled-DNA modified MSCs. Significantly, engineered MSCs can be directly captured from the flow stream by selectin surfaces or selectin-expressing cells under flow conditions (≤2dyn/cm²). The simple chemistry approach and the versatility of aptamers permit the concept of engineered cell-cell interactions to be generically applicable for targeting cells to diseased tissues and elucidating the biology of cell-cell interactions.

A semianalytical model to study the effect of cortical tension on cell rolling

Cell rolling on the vascular endothelium plays an important role in trafficking of leukocytes, stem cells, and cancer cells. We describe a semianalytical model of cell rolling that focuses on the microvillus as the unit of cell-substrate interaction and integrates microvillus mechanics, receptor clustering, force-dependent receptor-ligand kinetics, and cortical tension that enables incorporation of cell body deformation. Using parameters obtained from independent experiments, the model showed excellent agreement with experimental studies of neutrophil rolling on P-selectin and predicted different regimes of cell rolling, including spreading of the cells on the substrate under high shear. The cortical tension affected the cell-surface contact area and influenced the rolling velocity, and modulated the dependence of rolling velocity on microvillus stiffness. Moreover, at the same shear stress, microvilli of cells with higher cortical tension carried a greater load compared to those with lower cortical tension. We also used the model to obtain a scaling dependence of the contact radius and cell rolling velocity under different conditions of shear stress, cortical tension, and ligand density. This model advances theoretical understanding of cell rolling by incorporating cortical tension and microvillus extension into a versatile, semianalytical framework.

Shear-stabilized rolling behavior of E. coli examined with simulations

Escherichia coli exhibit both shear-stabilized rolling and a transition to stationary adhesion while adhering in fluid flow. Understanding the mechanism by which this shear-enhanced adhesion occurs is an important step in understanding bacterial pathogenesis. In this work, simulations are used to investigate the relative contributions of fimbrial deformation and bond transitions to the rolling and stationary adhesion of E. coli. Each E. coli body is surrounded by many long, thin fimbriae terminating in a single FimH receptor that is capable of forming a catch bond with mannose. As simulated cells progress along a mannosylated surface under flow, the fimbriae bend and buckle as they interact with the surface, and FimH-mannose bonds form and break according to a two-state, allosteric catch-bond model. In simulations, shear-stabilized rolling resulted from an increase in the low-affinity bond number due to increased fimbrial deformation with shear. Catch-bond formation did not occur during cell rolling, but instead led to the transition to stationary adhesion. In contrast, in leukocyte and platelet systems, catch bonds appear to be involved in the stabilization of rolling, and integrin activation is required for stationary adhesion.

Molecular stiffness of selectins

During inflammation, selectin-ligand interactions provide forces for circulating leukocytes to adhere to vascular surfaces, which stretch the interacting molecules, suggesting that mechanical properties may be pertinent to their biological function. From mechanical measurements with atomic force microscopy, we analyzed the molecular characteristics of selectins complexed with ligands and antibodies. Respective stiffness of L-, E-, and P-selectins (4.2, 1.4, and 0.85 piconewton/nm) correlated inversely with the number (2, 6, and 9) of consensus repeats in the selectin structures that acted as springs in series to dominate their compliance. After reconstitution into a lipid bilayer, purified membrane P-selectin remained a dimer, capable of forming dimeric bonds with P-selectin glycoprotein ligand (PSGL)-1, endoglycan-Ig, and a dimeric form of a glycosulfopeptide modeled after the N terminus of PSGL-1. By comparison, purified membrane L- and E-selectin formed only monomeric bonds under identical conditions. Ligands and antibodies were much less stretchable than selectins. The length of endoglycan-Ig was found to be 51 ± 12 nm. These results provide a comprehensive characterization of the molecular stiffness of selectins and illustrate how mechanical measurements can be utilized for molecular analysis, e.g. evaluating the multimericity of selectins and determining the molecular length of endoglycan.

Examining the lateral displacement of HL60 cells rolling on asymmetric P-selectin patterns

The lateral displacement of cells orthogonal to a flow stream by rolling on asymmetrical receptor patterns presents a new opportunity for the label-free separation and analysis of cells. Understanding the nature of cell rolling trajectories on such substrates is necessary to the engineering of substrates and the design of devices for cell separation and analysis. Here, we investigate the statistical nature of cell rolling and the effect of pattern geometry and flow shear stress on cell rolling trajectories using micrometer-scale patterns of biomolecular receptors with well-defined edges. Leukemic myeloid HL60 cells expressing the PSGL-1 ligand were allowed to flow across a field of patterned lines fabricated using microcontact printing and functionalized with the P-selectin receptor, leveraging both the specific adhesion of this ligand-receptor pair and the asymmetry of the receptor pattern inclination angle with respect to the fluid shear flow direction (α = 5, 10, 15, and 20°). The effects of the fluid shear stress magnitude (τ = 0.5, 1, 1.5, and 2.0 dyn/cm(2)), α, and P-selectin incubation concentration were quantified in terms of the rolling velocity and edge tracking length. Rolling cells tracked along the inclined edges of the patterned lines before detaching and reattaching on another line. The detachment of rolling cells after tracking along the edge was consistent with a Poisson process of history-independent interactions. Increasing the edge inclination angle decreased the edge tracking length in an exponential manner, contrary to the shear stress magnitude and P-selectin incubation concentration, which did not have a significant effect. On the basis of these experimental data, we constructed an empirical model that predicted the occurrence of the maximum lateral displacement at an edge angle of 7.5°. We also used these findings to construct a Monte Carlo simulation for the prediction of rolling trajectories of HL60 cells on P-selectin-patterned substrates with a specified edge inclination angle. The prediction of lateral displacement in the range of 200 μm within a 1 cm separation length supports the feasibility of label-free cell separation via asymmetric receptor patterns in microfluidic devices.

Cell surface engineering of mesenchymal stem cells

By leveraging the capacity to promote regeneration, stem cell therapies offer enormous hope for solving some of the most tragic illnesses, diseases, and tissue defects world-wide. However, a significant barrier to the effective implementation of cell therapies is the inability to target a large quantity of viable cells with high efficiency to tissues of interest. Systemic infusion is desired as it minimizes the invasiveness of cell therapy, and maximizes practical aspects of repeated doses. However, cell types such as mesenchymal stem cells exhibit a poor homing capability or lose their capacity to home following culture expansion (i.e. FASEB J 21:3197-3207, 2007; Circulation 108:863-868, 2003; Stroke: A Journal of Cerebral Circulation 32:1005-1011; Blood 104:3581-3587, 2004). To address this challenge, we have developed a simple platform technology to chemically attach cell adhesion molecules to the cell surface to improve the homing efficiency to specific tissues. This chemical approach involves a stepwise process including (1) treatment of cells with sulfonated biotinyl-N-hydroxy-succinimide to introduce biotin groups on the cell surface, (2) addition of streptavidin that binds to the biotin on the cell surface and presents unoccupied binding sites, and (3) attachment of biotinylated targeting ligands that promote adhesive interactions with vascular endothelium. Specifically, in our model system, a biotinylated cell rolling ligand, sialyl Lewisx (SLeX), found on the surface of leukocytes (i.e., the active site of the P-selectin glycoprotein ligand (PSGL-1)), is conjugated on MSC surface. The SLeX engineered MSCs exhibit a rolling response on a P-selectin coated substrate under shear stress conditions. This indicates that this approach can be used to potentially target P-selectin expressing endothelium in the more marrow or at sites of inflammation. Importantly, the surface modification has no adverse impact on MSCs' native phenotype including their multilineage differentiation capacity, viability, proliferation, and adhesion kinetics. We anticipate that the present approach to covalently modify the cell surface and immobilize required ligands is not limited to MSCs or the SLeX ligand. Therefore, this technology should have broad implications on cell therapies that utilize systemic administration and require targeting of cells to specific tissues. The approach may also be useful to promote specific cell-cell interactions. In this protocol, we describe the conjugation of SLeX on MSC surface and methods to study cell rolling behaviors of SLeX-modified MSCs on a P-selectin coated substrate using an in vitro flow chamber assay. We also provide a brief description of cell characterization assays that can be used to examine the impact of the chemical modification regimen.

Triphasic force dependence of E-selectin/ligand dissociation governs cell rolling under flow

During inflammation, flowing leukocytes tether to and roll on vascular surfaces through the association and dissociation of selectin/ligand bonds. The interactions of P- and L- selectins with their respective ligands exhibit catch-slip bonds, such that increasing force initially prolongs and then shortens bond lifetimes. In addition, catch-slip bonds have been shown to govern L-selectin-mediated cell rolling. Using a flow chamber and biomembrane force probe, we show a triphasic force dependence of E-selectin/ligand dissociation that initially behaves as slip bonds, then transitions to catch bonds, and finally transitions again to slip bonds as the force increases. These transitions govern the velocities of neutrophils, HL-60 cells, and Colo-205 cells rolling on E-selectin, as evidenced by the fact that their velocities exhibited a triphasic force dependence that inversely matched the triphasic lifetime-force relationship. At low forces, slip bonds may also precede catch bonds for interactions of P- and L-selectin with their ligands.

Rolling cell adhesion

Rolling adhesion on vascular surfaces is the first step in recruiting circulating leukocytes, hematopoietic progenitors, or platelets to specific organs or to sites of infection or injury. Rolling requires the rapid yet balanced formation and dissociation of adhesive bonds in the challenging environment of blood flow. This review explores how structurally distinct adhesion receptors interact through mechanically regulated kinetics with their ligands to meet these challenges. Remarkably, increasing force applied to adhesive bonds first prolongs their lifetimes (catch bonds) and then shortens their lifetimes (slip bonds). Catch bonds mediate the counterintuitive phenomenon of flow-enhanced rolling adhesion. Force-regulated disruptions of receptor interdomain or intradomain interactions remote from the ligand-binding surface generate catch bonds. Adhesion receptor dimerization, clustering in membrane domains, and interactions with the cytoskeleton modulate the forces applied to bonds. Both inside-out and outside-in cell signals regulate these processes.

The constitutive equation for membrane tether extraction

Membrane tethers or nanotubes play a critical role in a variety of cellular and subcellular processes such as leukocyte rolling and intercellular mass transport. The current constitutive equations that describe the relationship between the pulling force and the tether velocity during tether extraction have serious limitations. In this article, we propose a new phenomenological constitutive equation that captures all known characteristics of nanotube formation, including nonlinearity, nonzero threshold force, and possible negative tether velocity. We used tether extraction from endothelial cells as a prototype to illustrate how to obtain the material constants in the constitutive equation. With the micropipette aspiration technique, we measured tether pulling forces at both positive and negative tether velocities. We also determined the threshold force of 55 pN experimentally for the first time. This new constitutive equation unites two established ones and provides us a unified platform to better understand not only the physiological role of tether extraction during leukocyte rolling and intercellular or intracellular transport, but also the physics of membrane tether growth or retraction.

Core 1-derived O-glycans are essential E-selectin ligands on neutrophils

Neutrophils roll on E-selectin in inflamed venules through interactions with cell-surface glycoconjugates. The identification of physiologic E-selectin ligands on neutrophils has been elusive. Current evidence suggests that P-selectin glycoprotein ligand-1 (PSGL-1), E-selectin ligand-1 (ESL-1), and CD44 encompass all glycoprotein ligands for E-selectin; that ESL-1 and CD44 use N-glycans to bind to E-selectin; and that neutrophils lacking core 2 O-glycans have partially defective interactions with E-selectin. These data imply that N-glycans on ESL-1 and CD44 and O-glycans on PSGL-1 constitute all E-selectin ligands, with neither glycan subset having a dominant role. The enzyme T-synthase transfers Gal to GalNAcalpha1-Ser/Thr to form the core 1 structure Galbeta1-3GalNAcalpha1-Ser/Thr, a precursor for core 2 and extended core 1 O-glycans that might serve as selectin ligands. Here, using mice lacking T-synthase in endothelial and hematopoietic cells, we found that E-selectin bound to CD44 and ESL-1 in lysates of T-synthase-deficient neutrophils. However, the cells exhibited markedly impaired rolling on E-selectin in vitro and in vivo, failed to activate beta2 integrins while rolling, and did not emigrate into inflamed tissues. These defects were more severe than those of neutrophils lacking PSGL-1, CD44, and the mucin CD43. Our results demonstrate that core 1-derived O-glycans are essential E-selectin ligands; that some of these O-glycans are on protein(s) other than PSGL-1, CD44, and CD43; and that PSGL-1, CD44, and ESL-1 do not constitute all glycoprotein ligands for E-selectin.

Profiles of carbohydrate ligands associated with adsorbed proteins on self-assembled monolayers of defined chemistries

Conserved protein-carbohydrate-lipid pathogen-associated molecular patterns (PAMPs) interact with cells of the innate immune system to mediate antigen recognition and internalization and activation of immune cells. We examined if analogous "biomaterial-associated molecular patterns" composed of proteins, specifically their carbohydrate modifications, existed on biomaterials, which can play a role in mediating the innate immune response to biomaterials. To probe for these carbohydrates in the adsorbed protein layer, as directed by the underlying biomaterial chemistry, self-assembled monolayers (SAMs) presenting -CH(3), -OH, -COOH, or -NH(2) were preincubated with serum/plasma, and the presence of carbohydrate ligands of C-type lectin receptors (CLRs) was investigated using lectin probes in an enzyme-linked lectin assay (ELLA). Presentation of CLR ligands was detected on control tissue culture polystyrene (TCPS). Absorbances of mannose or N-acetylglucosamine increased with decreasing incubating serum concentration, whereas absorbances of sialylated epitopes or fucose remained unchanged. Absorbances of alpha-galactose or N-acetylgalactosamine decreased with decreasing incubating serum concentration; beta-galactose was undetectable. Among SAM endgroups, preincubation with 10% serum resulted in differential presentation of CLR ligands: higher alpha-galactose on COOH SAMs than NH(2) or CH(3) SAMs, highest complex mannose on NH(2) SAMs, and higher complex mannose on OH SAMs than CH(3) SAMs. Least sialylated groups were detected on CH(3) SAMs. In summary, biomaterial chemistry may regulate protein adsorption and hence unique presentation of associated carbohydrates. The ultimate goal is to identify the effects of protein glycosylations associated with biomaterials in stimulating innate immune responses.

A congenital activating mutant of WASp causes altered plasma membrane topography and adhesion under flow in lymphocytes

Leukocytes rely on dynamic actin-dependent changes in cell shape to pass through blood vessels, which is fundamental to immune surveillance. Wiskott-Aldrich Syndrome protein (WASp) is a hematopoietic cell-restricted cytoskeletal regulator important for modulating cell shape through Arp2/3-mediated actin polymerization. A recently identified WASp(I294T) mutation was shown to render WASp constitutively active in vivo, causing increased filamentous (F)-actin polymerization, high podosome turnover in macrophages, and myelodysplasia. The aim of this study was to determine the effect of WASp(I294T) expression in lymphocytes. Here, we report that lymphocytes isolated from a patient with WASp(I294T), and in a cellular model of WASp(I294T), displayed abnormal microvillar architecture, associated with an increase in total cellular F-actin. Microvillus function was additionally altered as lymphocytes bearing the WASp(I294T) mutation failed to roll normally on L-selectin ligand under flow. This was not because of defects in L-selectin expression, shedding, cytoskeletal anchorage, or membranal positioning; however, under static conditions of adhesion, WASp(I294T)-expressing lymphocytes exhibited altered dynamic interaction with L-selectin ligand, with a significantly reduced rate of adhesion turnover. Together, our results demonstrate that WASp(I294T) significantly affects lymphocyte membrane topography and L-selectin-dependent adhesion, which may be linked to defective hematopoiesis and leukocyte function in affected patients.

E-selectin engages PSGL-1 and CD44 through a common signaling pathway to induce integrin alphaLbeta2-mediated slow leukocyte rolling

In inflamed venules, neutrophils rolling on E-selectin induce integrin alpha(L)beta(2)-dependent slow rolling on intercellular adhesion molecule-1 by activating Src family kinases (SFKs), DAP12 and Fc receptor-gamma (FcRgamma), spleen tyrosine kinase (Syk), and p38. E-selectin signaling cooperates with chemokine signaling to recruit neutrophils into tissues. Previous studies identified P-selectin glycoprotein ligand-1 (PSGL-1) as the essential E-selectin ligand and Fgr as the only SFK that initiate signaling to slow rolling. In contrast, we found that E-selectin engagement of PSGL-1 or CD44 triggered slow rolling through a common, lipid raft-dependent pathway that used the SFKs Hck and Lyn as well as Fgr. We identified the Tec kinase Bruton tyrosine kinase as a key signaling intermediate between Syk and p38. E-selectin engagement of PSGL-1 was dependent on its cytoplasmic domain to activate SFKs and slow rolling. Although recruiting phosphoinositide-3-kinase to the PSGL-1 cytoplasmic domain was reported to activate integrins, E-selectin-mediated slow rolling did not require phosphoinositide-3-kinase. Studies in mice confirmed the physiologic significance of these events for neutrophil slow rolling and recruitment during inflammation. Thus, E-selectin triggers common signals through distinct neutrophil glycoproteins to induce alpha(L)beta(2)-dependent slow rolling.

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.