Simulation of detachment of specifically bound particles from surfaces by shear flow

The role of adhesive receptor patterns on cell transport in complex microvessels

Cell transport is governed by the interaction of fluid dynamic forces and biochemical factors such as adhesion receptor expression and concentration. Although the effect of endothelial receptor density is well understood, it is not clear how the spacing and local spatial distribution of receptors affect cell adhesion in three-dimensional microvessels. To elucidate the effect of vessel shape on cell trajectory and the arrangement of endothelial receptors on cell adhesion, we employed a three-dimensional deformable cell model that incorporates microscale interactions between the cell and the endothelium. Computational cellular adhesion models are systematically altered to assess the influence of receptor spacing. We demonstrate that the patterns of receptors on the vessel walls are a key factor guiding cell movement. In straight microvessels, we show a relationship between cell velocity and the spatial distribution of adhesive endothelial receptors, with larger receptor patches producing lower translational velocities. The joint effect of the complex vessel topology seen in microvessel shapes such as curved and bifurcated vessels when compared to straight tubes is explored with results which showed the spatial distribution of receptors affecting cell trajectory. Our findings here represent demonstration of the previously undescribed relationship between receptor pattern and geometry that guides cellular movement in complex microenvironments.

Ultrafast Electrothermal Flow-Enhanced Magneto Biosensor for Highly Sensitive Protein Detection in Whole Blood

Current diagnostic tests for sensitive protein detection rely on immunological techniques, such as ELISA, which require sample purification, multiple washing steps and lengthy incubation, hindering their use for rapid testing. Here, we report a simple electrothermal flow-enhanced biosensor for ultrafast, high sensitivity measurements of protein biomarkers in whole blood. Magnetic nanobeads dually-labeled with a detection antibody and enzyme reporter are used to form immunocomplexes with the target protein, which are readily transported to the sensor via magnetic concentration. The incorporation of electrothermal flows enhances immunocomplex formation, allowing for rapid and sensitive detection without requiring blood purification or lengthy incubation. Proof of concept was carried out using Plasmodium falciparum histidine-rich protein 2 (PfHRP2), a malaria parasite biomarker, which could be detected at concentrations as low as 5.7 pg mL-1 (95 fM) in whole blood in 7 min. The speed, sensitivity and simplicity of this device make it attractive for rapid diagnostic testing.

Next-Generation Molecular Discovery: From Bottom-Up In Vivo and In Vitro Approaches to In Silico Top-Down Approaches for Therapeutics Neogenesis

Protein and drug engineering comprises a major part of the medical and research industries, and yet approaches to discovering and understanding therapeutic molecular interactions in biological systems rely on trial and error. The general approach to molecular discovery involves screening large libraries of compounds, proteins, or antibodies, or in vivo antibody generation, which could be considered "bottom-up" approaches to therapeutic discovery. In these bottom-up approaches, a minimal amount is known about the therapeutics at the start of the process, but through meticulous and exhaustive laboratory work, the molecule is characterised in detail. In contrast, the advent of "big data" and access to extensive online databases and machine learning technologies offers promising new avenues to understanding molecular interactions. Artificial intelligence (AI) now has the potential to predict protein structure at an unprecedented accuracy using only the genetic sequence. This predictive approach to characterising molecular structure-when accompanied by high-quality experimental data for model training-has the capacity to invert the process of molecular discovery and characterisation. The process has potential to be transformed into a top-down approach, where new molecules can be designed directly based on the structure of a target and the desired function, rather than performing screening of large libraries of molecular variants. This paper will provide a brief evaluation of bottom-up approaches to discovering and characterising biological molecules and will discuss recent advances towards developing top-down approaches and the prospects of this.

Shear-Resistant, Biological Tethering of Nanostimulators for Enhanced Therapeutic Cell Paracrine Factor Secretion

Mesenchymal stromal cells (MSCs) secreting multiple growth factors and immunomodulatory cytokines are promising for regenerative medicine. To further enhance their secretory activity, efforts have emerged to tether nanosized carriers of secretory stimuli, named nanostimulators, to the MSC surface by forming nonchemical bonds. Despite some successes, there is a great need to improve the retention of nanostimulators during transport through a syringe needle, where high shear stress exerted on the cell surface separates them. To this end, we hypothesize that poly(lactic-co-glycolic acid)-block-hyaluronic acid (PLGA-HA) conjugated with integrin-binding RGD peptides, denoted PLGA-HA-RGD, can form nanostimulators that remain on the cell surface stably during the injection. The resulting HA-CD44 and RGD-integrin bonds would synergistically increase the adhesion strength of nanostimulators. Interestingly, nanostimulators prepared with PLGA-HA-RGD show 3- to 6-fold higher retention than those made with PLGA-HA. Therefore, the PLGA-HA-RGD nanostimulators induced MSCs to secrete 1.5-fold higher vascular endothelial growth factors and a 1.2-fold higher tissue inhibitor of matrix metalloproteinase-1 as compared to PLGA-HA nanostimulators. Consequently, MSCs tethered with PLGA-HA-RGD nanostimulators served to stimulate endothelial cell activities to form a blood vessel-like endothelial lumen with increased length and number of junctions. The nanostimulator design strategy would also be broadly applicable to regulate, protect, and home a broad array of therapeutic or immune cells by tethering carriers with bioactive molecules of interest.

Adhesion and Particle Removal from Surface-Tethered Poly(N-Isopropylacrylamide) Coatings Using Hydrodynamic Shear Forces

Thermally responsive coatings of poly(N-isopropylacrylamide), or poly(NIPAAm), have a volume phase transition temperature (VPTT) near 32 °C. Below this temperature, the coating imbibes water and swells. Above this temperature, the coating rejects water and collapses. Herein, a spinning disk method is used to determine the hydrodynamic shear stress necessary to remove 10 μm polystyrene (PS) microspheres capped with either carboxylic acid (COOH) functionality or immunoglobulin (IgG) proteins from the coatings as a function of coating thickness and temperature. In the case of the PS-COOH, the hydrodynamic shear stress necessary to remove the microspheres was consistently larger below the VPTT than above the VPTT of the poly(NIPAAm) coating. In the case of PS-IgG, the trend was reversed, in which the hydrodynamic shear stress necessary to remove the microspheres was consistently smaller below the VPTT than above the VPTT. Simple scaling relationships were developed to explain the findings within the Johnson-Kendall-Roberts (JKR) model of contact mechanics, which illustrates the delicate interplay between the pull-off force and contact radius (as determined by the coating shear modulus) in governing particle removal from soft surfaces with hydrodynamic forces.

Gold Binding Peptide Identified from Microfluidic Biopanning: An Experimental and Molecular Dynamics Study

Biopanning refers to the processes of screening peptides with a high affinity to a target material. Microfluidic biopanning has advantages compared to conventional biopanning which requires large amounts of the target material and involves inefficient multiple pipetting steps to remove nonspecific or low-affinity peptides. Here, we fabricate a microfluidic biopanning system to identify a new gold-binding peptide (GBP). A polydimethylsiloxane microfluidic device is fabricated and bonded to a glass slide with a gold pattern that is deposited by electron-beam evaporation. The microfluidic biopanning system can provide high adjustability in the washing step during the biopanning process because the liquid flow rate and the resulting shear stress can be precisely controlled. The surface plasmon resonance analysis shows that the binding affinity of the identified GBP is comparable to previously reported GBPs. Moreover, molecular dynamics simulations are performed to understand its binding affinity against the gold surface in detail. Theoretical calculations suggest that the association and dissociation rates of the GBPs depend on their sequence-dependent conformations and interactions with the gold surface. These findings provide insight into designing efficient biopanning tools and peptides with a high affinity for various target materials.

On Stability of Specific Adhesion of Particles to Membranes in Simple Shear Flow

Adhesion of carrier particles to the luminal surface of endothelium under hemodynamic flow conditions is critical for successful vascular drug delivery. Endothelial cells line the inner surface of blood vessels. The effect of mechanical behavior of this compliant surface on the adhesion of blood-borne particles is unknown. In this contribution, we use a phase-plane method, first developed by Hammer and Lauffenburger [Biophysical Journal, 52, 475 (1987)], to analyze the stability of specific adhesion of a spherical particle to a compliant interface layer. We construct a phase diagram that predicts the state of particle adhesion, subjected to an incident simple shear flow, in terms of interfacial elasticity, shear rate, binding affinity of cell adhesive molecules, and their surface density. The main conclusion is that the local deformation of the flexible interface inhibits the stable adhesion of the particle. In comparison with adhesion to a rigid substrate, a greater ligand density is required to establish a stable adhesion between a particle and a compliant interface. The results can be used for the rational design of particles in vascular drug delivery.

Fundamentals of affinity cell separations

Cell separations using affinity methods continue to be an enabling science for a wide variety of applications. In this review, we discuss the fundamental aspects of affinity separation, including the competing forces for cell capture and elution, cell-surface interactions, and models for cell adhesion. Factors affecting separation performance such as bond affinity, contact area, and temperature are presented. We also discuss and demonstrate the effects of nonspecific binding on separation performance. Metrics for evaluating cell separations are presented, along with methods of comparing separation techniques for cell isolation using affinity capture.

Essential arterial hypertension patients present higher cell adhesion forces, contributing to fibrinogen-dependent cardiovascular risk

The increase of erythrocyte aggregation by high fibrinogen levels may be an indicator of cardiovascular risk. γ' fibrinogen variant has been considered as a possible player in enhancing aggregation. Here, we assessed, at the single-cell level, the influence of fibrinogen on erythrocyte aggregation in essential arterial hypertension. We also aimed at understanding how γ' fibrinogen is altered in this disease. Using atomic force microscopy (AFM), we show that the work and force necessary for erythrocyte-erythrocyte detachment is higher for patients than for healthy donors, with these parameters further increasing in both groups when higher fibrinogen concentrations are present. This can be associated with changes in blood flow, due to transient bridging of two erythrocytes by fibrinogen, representing an important cardiovascular risk factor. γ' fibrinogen can influence the increased risk in essential arterial hypertension, as we demonstrate that its levels are significantly increased in these patients' blood. Nevertheless, this cannot be the only cause for the changes observed in the AFM data.

A Molecular Communications Model for Drug Delivery

This paper considers the scenario of a targeted drug delivery system, which consists of deploying a number of biological nanomachines close to a biological target (e.g., a tumor), able to deliver drug molecules in the diseased area. Suitably located transmitters are designed to release a continuous flow of drug molecules in the surrounding environment, where they diffuse and reach the target. These molecules are received when they chemically react with compliant receptors deployed on the receiver surface. In these conditions, if the release rate is relatively high and the drug absorption time is significant, congestion may happen, essentially at the receiver site. This phenomenon limits the drug absorption rate and makes the signal transmission ineffective, with an undesired diffusion of drug molecules elsewhere in the body. The original contribution of this paper consists of a theoretical analysis of the causes of congestion in diffusion-based molecular communications. For this purpose, it is proposed a reception model consisting of a set of pure loss queuing systems. The proposed model exhibits an excellent agreement with the results of a simulation campaign made by using the Biological and Nano-Scale communication simulator version 2 (BiNS2), a well-known simulator for molecular communications, whose reliability has been assessed through in vitro experiments. The obtained results can be used in rate control algorithms to optimally determine the optimal release rate of molecules in drug delivery applications.

Plasma protein corona modulates the vascular wall interaction of drug carriers in a material and donor specific manner

The nanoscale plasma protein interaction with intravenously injected particulate carrier systems is known to modulate their organ distribution and clearance from the bloodstream. However, the role of this plasma protein interaction in prescribing the adhesion of carriers to the vascular wall remains relatively unknown. Here, we show that the adhesion of vascular-targeted poly(lactide-co-glycolic-acid) (PLGA) spheres to endothelial cells is significantly inhibited in human blood flow, with up to 90% reduction in adhesion observed relative to adhesion in simple buffer flow, depending on the particle size and the magnitude and pattern of blood flow. This reduced PLGA adhesion in blood flow is linked to the adsorption of certain high molecular weight plasma proteins on PLGA and is donor specific, where large reductions in particle adhesion in blood flow (>80% relative to buffer) is seen with ∼60% of unique donor bloods while others exhibit moderate to no reductions. The depletion of high molecular weight immunoglobulins from plasma is shown to successfully restore PLGA vascular wall adhesion. The observed plasma protein effect on PLGA is likely due to material characteristics since the effect is not replicated with polystyrene or silica spheres. These particles effectively adhere to the endothelium at a higher level in blood over buffer flow. Overall, understanding how distinct plasma proteins modulate the vascular wall interaction of vascular-targeted carriers of different material characteristics would allow for the design of highly functional delivery vehicles for the treatment of many serious human diseases.

A Bayesian hierarchical model for maximizing the vascular adhesion of nanoparticles

The complex vascular dynamics and wall deposition of systemically injected nanoparticles is regulated by their geometrical properties (size, shape) and biophysical parameters (ligand-receptor bond type and surface density, local shear rates). Although sophisticated computational models have been developed to capture the vascular behavior of nanoparticles, it is increasingly recognized that purely deterministic approaches, where the governing parameters are known a priori and conclusively describe behaviors based on physical characteristics, may be too restrictive to accurately reflect natural processes. Here, a novel computational framework is proposed by coupling the physics dictating the vascular adhesion of nanoparticles with a stochastic model. In particular, two governing parameters (i.e. the ligand-receptor bond length and the ligand surface density on the nanoparticle) are treated as two stochastic quantities, whose values are not fixed a priori but would rather range in defined intervals with a certain probability. This approach is used to predict the deposition of spherical nanoparticles with different radii, ranging from 750 to 6,000 nm, in a parallel plate flow chamber under different flow conditions, with a shear rate ranging from 50 to 90 sec^-1. It is demonstrated that the resulting stochastic model can predict the experimental data more accurately than the original deterministic model. This approach allows one to increase the predictive power of mathematical models of any natural process by accounting for the experimental and intrinsic biological uncertainties.

SPR bacterial pathogen biosensor: the importance of fluidic conditions and probing depth

The sensitivity of surface plasmon resonance (SPR) biosensor technology for detection of bacterial analytes is investigated as a function of (a) sample flow conditions and (b) depth of probing electromagnetic field. These parameters are extremely important as such analytes exhibit large (of around micrometer) size which significantly hinders their diffusion-driven transfer from a liquid sample to the sensor and their subsequent specific capture by attached recognition elements. This is due to small diffusion coefficient and strong shear stress that decreases the stability of bonds between the bacterium specific epitope and recognition elements immobilized at the sensor surface. The importance of accurate control of sample flow conditions and probing depth in order to maximize SPR sensor response is experimentally demonstrated and supported by an analytical theory. The tuning of the probing depth of surface plasmon evanescent field to match the size of the target analyte is pursued by using long range surface plasmons.

Using engineered single-chain antibodies to correlate molecular binding properties and nanoparticle adhesion dynamics

Elucidation of the relationship between targeting molecule binding properties and the adhesive behavior of therapeutic or diagnostic nanocarriers would aid in the design of optimized vectors and lead to improved efficacy. We measured the adhesion of 200-nm-diameter particles under fluid flow that was mediated by a diverse array of molecular interactions, including recombinant single-chain antibodies (scFvs), full antibodies, and the avidin/biotin interaction. Within the panel of scFvs, we used a family of mutants that display a spectrum of binding kinetics, allowing us to compare nanoparticle adhesion to bond chemistry. In addition, we explored the effect of molecular size by inserting a protein linker into the scFv fusion construct and by employing scFvs that are specific for targets with vastly different sizes. Using computational models, we extracted multivalent kinetic rate constants for particle attachment and detachment from the adhesion data and correlated the results to molecular binding properties. Our results indicate that the factors that increase encounter probability, such as adhesion molecule valency and size, directly enhance the rate of nanoparticle attachment. Bond kinetics had no influence on scFv-mediated nanoparticle attachment within the kinetic range tested, however, but did appear to affect antibody/antigen and avidin/biotin mediated adhesion. We attribute this finding to a combination of multivalent binding and differences in bond mechanical strength between recombinant scFvs and the other adhesion molecules. Nanoparticle detachment probability correlated directly with adhesion molecule valency and size, as well as the logarithm of the affinity for all molecules tested. On the basis of this work, scFvs can serve as viable targeting receptors for nanoparticles, but improvements to their bond mechanical strength would likely be required to fully exploit their tunable kinetic properties and maximize the adhesion efficiency of nanoparticles that bear them.

A biological breadboard platform for cell adhesion and detachment studies

The dynamic nature of cell adhesion and detachment, which plays a critical role in a variety of physiological and pathological phenomena, still remains unclear. This motivates the pursuit of controllable manipulation of cell adhesion and detachment for a better understanding of cellular dynamics. Here we present an addressable, multifunctional, and reusable platform, termed the biological breadboard (BBB), for spatiotemporal manipulation of cell adhesion and detachment at cellular and subcellular levels. The BBB, composed of multiple gold electrodes patterned on a Pyrex substrate, is surface-modified with arginine-glycine-aspartic acid terminated thiol (RTT) and polyethylene glycol (PEG) to achieve a cell-adhesive surface on the gold electrodes and a cell-resistive surface on the Pyrex substrate, respectively. Cell adhesion is regulated by the steric repulsion of PEG chains, while cell detachment is controlled by the reductive desorption of a gold-thiol self-assembled monolayer (SAM) at an activation potential of -0.90 to -1.65 V. Experimental characterizations using NIH 3T3 fibroblasts are presented to demonstrate the utility of our device.

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.

Adhesive dynamics simulation of G-protein-mediated chemokine-activated neutrophil adhesion

To reach sites of inflammation, a blood-borne neutrophil first rolls over the vessel wall, becoming firmly adherent on activation, and then transmigrates through the endothelium. In this study, we simulate the transition to firm adhesion via chemokine-induced integrin activation. To recreate the transition from rolling to firm adhesion, we use an integrated signaling adhesive dynamics simulation that includes selectin, integrin, and chemokine interactions between the cell and an adhesive substrate. Integrin bonds are of low affinity until activated by chemokine binding to G-protein coupled receptors on the model cell. The signal propagates within the cell through probabilistic diffusion and reaction of the signaling elements to induce the high-affinity integrins required for firm adhesion. This model showed that integrins become progressively active as cells roll and interact with chemokines, leading to a slight slowing before firm adhesion on a timescale similar to that observed in experiments. Increasing the density of chemokine resulted in decreases in the rolling time before stopping, consistent with experimental observations. However, a limit is reached where further increases in chemokine density do not increase adhesion. We found that the timescale for integrin activation correlated with the time to stop. Further, altering parameters within the intracellular signaling cascade that changed the speed of integrin activation, such as effector activation and dissociation rates, correspondingly affected the time to firm adhesion. For all conditions tested, the number of active integrin bonds at the point of firm adhesion was relatively constant. The model predicts that the time to stop would be relatively independent of selectin or integrin density, but strongly dependent on the shear rate because higher shear rates limit the intrinsic activation rate of integrins and require more integrins for adhesion.

Quantifying nanoparticle adhesion mediated by specific molecular interactions

Receptor-mediated targeting of nanometric contrast agents or drug carriers holds great potential for treating cardiovascular and vascular-associated diseases. However, predicting the ability of these vectors to adhere to diseased cells under dynamic conditions is complex due to the interplay of transport, hydrodynamic force, and multivalent bond formation dynamics. Therefore, we sought to determine the effects of adhesion molecule density and flow rate on adhesion of 210 nm particles, with the goal of identifying criteria to optimize binding efficiency and selectivity. Our system employed a physiologically relevant ligand, the vascular adhesion molecule ICAM-1, and an ICAM-1 specific antibody tethered to the nanoparticle using avidin-biotin chemistry. We measured binding and dissociation of these particles in a flow chamber as a function of antibody density, ligand density, and flow rate, and using a transport-reaction model we distilled overall kinetic rate constants for adhesion and detachment from the binding data. We demonstrate that both attachment and detachment of 210 nm particles can be correlated with receptor and ligand valency and are minimally affected by shear rate. Furthermore, we uncovered a time-dependent mechanism governing particle detachment, in which the rate of detachment decreases with contact time according to a power law. Finally, we use our results to illustrate how to engineer adhesion selectivity for specific molecular targeting applications. These results establish basic principles dictating nanoparticle adhesion and dissociation and can be used as a framework for the rational design of targeted nanoparticle therapeutics that possess optimum adhesive characteristics.

Microparticle adhesive dynamics and rolling mediated by selectin-specific antibodies under flow

In vitro studies were performed to characterize the relative performance of candidate receptors to target microparticles to inflammatory markers on vascular endothelium. To model the interactions of drug-bearing microparticles or imaging contrast agents with the vasculature, 6 micron polystyrene particles bearing antibodies, peptides, or carbohydrates were perfused over immobilized E- or P-selectin in a flow chamber. Microparticles conjugated with HuEP5C7.g2 (HuEP), a monoclonal antibody (mAb) specific to E- and P-selectin, supported leukocyte-like rolling and transient adhesion at venular shear rates. In contrast, microparticles conjugated with a higher affinity mAb specific for P-selectin (G1) were unable to form bonds at venular flow rates. When both HuEP and G1 were conjugated to the microparticle, HuEP supported binding to P-selectin in flow which allowed G1 to form bonds leading to stable adhesion. While the microparticle attachment and rolling performance was not as stable as that mediated by the natural ligands P-selectin Glycoprotein Ligand-1 or sialyl Lewis(x), HuEP performed significantly better than any previously characterized mAb in terms of mediating microparticle binding under flow conditions. HuEP may be a viable alternative to natural ligands to selectins for targeting particles to inflamed endothelium.

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.

Molecular resolution of cell adhesion forces

Recently, AFM-based force spectroscopy has been used to quantify single-molecule adhesion forces on living ameboid cells. Force spectroscopy was used to measure the rupture forces of single receptor-ligand bonds which can occur rapidly between the cell types used, a metastasising B16 melanoma cell and a vascular bEnd.3 endothelial cell. Parameters which influence the critical experimental conditions are discussed to discriminate between multiple bond ruptures and single bonds. Under physiological conditions of temperature and pH the force measurements show an average rupture force of 33 pN (SD = 12 pN) for single bonds. Single-molecule force spectroscopy will be very useful to study the regulation of cell adhesion on a molecular level in normal processes, such as leukocyte homing, and in major human disorders, including tumor metastasis, autoimmune diseases and atherosclerosis.

Computational modeling of cell adhesion and movement using a continuum-kinetics approach

Adhesion of leukocytes to substrate involves the coupling of disparate length and timescales between molecular mechanics and macroscopic transport, and existing models of cell adhesion do not use full cellular information. To address these challenges, a multiscale computational approach for studying the adhesion of a cell on a substrate is developed and assessed. The cellular level model consists of a continuum representation of the field equations and a moving boundary tracking capability to allow the cell to change its shape continuously. At the receptor-ligand level, a bond molecule is mechanically represented by a spring. Communication between the macro/micro- and nanoscale models is facilitated interactively during the computation. The computational model is assessed using an adherent cell, rolling and deforming along the vessel wall under imposed shear flows. Using this approach, we first confirm existing numerical and experimental results. In this study, the intracellular viscosity and interfacial tension are found to directly affect the rolling of a cell. Our results also show that the presence of a nucleus increases the bond lifetime, and decreases the cell rolling velocity. Furthermore, it is found that a cell with a larger diameter rolls faster, and decreases the bond lifetime. This study shows that cell rheological properties have significant effects on the adhesion process contrary to what has been hypothesized in most literature.

Inflammatory mediators promote strong sickle cell adherence to endothelium under venular flow conditions

Adherence of sickle erythrocytes to endothelium in venules is thought to initiate or propagate vaso-occlusive episodes. Because of blood shear forces with normal microvascular flow, adherence in post-capillary venules requires binding via high-affinity receptor-mediated pathways. Microvascular flow in sickle patients is episodic, even in asymptomatic patients, so adherence may also occur at low shear not requiring high-affinity binding. Sickle cell binding to endothelium was quantified under flow or static incubation with unusually large vWF, thrombospondin, alpha(4)beta(1)/VCAM-1 or alpha(4)beta(1)/fibronectin (FN). Adherence under flow at 0.5 dyne/cm(2) shear stress leads to the greatest number of adherent sickle cells. Adherence under flow at 1.0 dyne/cm(2) leads to the strongest adherence. Static incubation conditions promote weak adherence of low numbers of sickle cells to endothelium. Following attachment at 1.0 dyne/cm(2), adherence strength was 2.5 +/- 0.1 or 2.6 +/- 0.2 dynes/cm(2) for alpha(4)beta(1)/VCAM-1 or alpha(4)beta(1)/FN pathways, a level 50% greater than adherence strength mediated by thrombospondin or ULvWF (1.7 +/- 0.08 or 1.6 +/- 0.07 dynes/cm(2), respectively). Sickle cell adhesion promoted by simultaneous activation of alpha(4)beta(1)/VCAM-1 and alpha(4)beta(1)/FN pathways is the strongest at 6.2 +/- 0.2 dynes/cm(2) and adherent red cells resist detachment shear stresses up to 10 dynes/cm(2). These data demonstrate that sickle cell adhesion to endothelium is regulated both by receptor/ligand affinity and flow conditions. Thus, both microvascular flow conditions and receptor-ligand interactions may regulate sickle cell adherence in vivo.

Fluid shear contributions to bacteria cell detachment initiated by a monoclonal antibody

Receptor-mediated adhesion of bacteria to biological surfaces is a significant step leading to infection. Due to an increase in bacterial antibiotic resistance, novel methods to block and disrupt these specific interactions have gained considerable interest as possible therapeutic strategies. Recently, several monoclonal antibodies specific for the Staphylococcus aureus collagen receptor demonstrated specialized ability to displace attached cells from collagen in static assays. In this study, we experimentally examine the monoclonal antibody detachment functionality under physiological shear conditions to evaluate the role of this parameter in the detachment process. The detachment of staphylococci from collagen was quantified in real-time using a parallel plate flow chamber, phase contrast video-microscopy and digital image processing. The results demonstrate a unimodal dependence of detachment on fluid wall shear rate. The observed decrease in effective detachment rate with increasing force at the highest shear levels evaluated is counterintuitive and has not been previously demonstrated. Several possible mechanisms of this result are discussed.

Experimental Study of Fibrin/Fibrin-Specific Molecular Interactions Using a Sphere/Plane Adhesion Model

Fibrin, the biopolymer produced in the final step of the coagulation cascade, is involved in the resistance of arterial thrombi to fragmentation under shear flow. However, the nature and strength of specific interactions between fibrin monomers are unknown. Thus, the shear-induced detachment of spherical monodispersed fibrin-coated latex particles in adhesive contact with a plane fibrin-coated glass surface has been experimentally studied, using an especially designed shear stress flow chamber. A complete series of experiments for measuring the shear stress necessary to release individual particles under various conditions (various number of fibrin layers involved in the adhesive contact, absence or presence of plasmin, the main physiological fibrinolytic enzyme) has been performed. The nonspecific DLVO interactions have been shown to be negligible compared to the interactions between fibrin monomers. A simple adhesion model based on the balance of forces and torque on particles, assuming an elastic behavior of the fibrin polymer bonds, to analyze the experimental data in terms of elastic force at rupture of an elementary intermonomeric fibrin bond has been used. The results suggested that this force (of order 400 pN) is an intrinsic quantity, independent of the number of fibrin layers involved in the adhesive contact. Copyright 2001 Academic Press.

Molecular properties in cell adhesion: a physical and engineering perspective

The past several years have seen accelerating growth in research directed towards the understanding and control of cell adhesion processes, from a spectrum of disciplinary approaches including molecular cell biology, biochemistry, biophysics and bioengineering. Consequently, our understanding of the mechanisms involved in cell adhesion has increased substantially. Corresponding quantitative analysis and modeling of the key molecular properties governing their action in regulating dynamic cell attachment and detachment events is crucial for advancing conceptual insight along with technological applications.

The state diagram for cell adhesion under flow: leukocyte rolling and firm adhesion

Leukocyte adhesion under flow in the microvasculature is mediated by binding between cell surface receptors and complementary ligands expressed on the surface of the endothelium. Leukocytes adhere to endothelium in a two-step mechanism: rolling (primarily mediated by selectins) followed by firm adhesion (primarily mediated by integrins). Using a computational method called "Adhesive Dynamics," we have simulated the adhesion of a cell to a surface in flow, and elucidated the relationship between receptor-ligand functional properties and the dynamics of adhesion. We express this relationship in a state diagram, a one-to-one map between the biophysical properties of adhesion molecules and various adhesive behaviors. Behaviors that are observed in simulations include firm adhesion, transient adhesion (rolling), and no adhesion. We varied the dissociative properties, association rate, bond elasticity, and shear rate and found that the unstressed dissociation rate, k(r)(o), and the bond interaction length, gamma, are the most important molecular properties controlling the dynamics of adhesion. Experimental k(r)(o) and gamma values from the literature for molecules that are known to mediate rolling adhesion fall within the rolling region of the state diagram. We explain why L-selectin-mediated rolling, which has faster k(r)(o) than other selectins, is accompanied by a smaller value for gamma. We also show how changes in association rate, shear rate, and bond elasticity alter the dynamics of adhesion. The state diagram (which must be mapped for each receptor-ligand system) presents a concise and comprehensive means of understanding the relationship between bond functional properties and the dynamics of adhesion mediated by receptor-ligand bonds.

Adhesive dynamics simulations of sialyl-Lewis(x)/E-selectin-mediated rolling in a cell-free system

Selectin-mediated leukocyte rolling is crucial for the proper function of the immune response. Recently, selectin-mediated rolling was recreated in a cell-free system (Biophysical Journal 71:2902-2907 (1996)); it was shown that sialyl Lewis(x) (sLe(x))-coated microspheres roll over E-selectin-coated surfaces under hydrodynamic flow. The cell-free system removes many confounding cellular features, such as cell deformability and signaling, allowing us to focus on the role of carbohydrate/selectin physical chemistry in mediating rolling. In this paper, we use adhesive dynamics, a computational method that allows us to simulate adhesion, to analyze the experimental data produced in the cell-free system. We simulate the effects of shear rate, ligand density, and number of receptors per particle on rolling velocity and compare them with experimental results obtained with the cell-free system. If we assume the population of particles is homogeneous in receptor density, we predict that particle rolling velocity calculated in simulations is more sensitive to shear rate than found in experiments. Also, the calculated rolling velocity is more sensitive to the number of receptors on the microspheres than to the ligand density on the surface, again in contrast to experiment. We argue that heterogeneity in the distribution of receptors throughout the particle population causes these discrepancies. We improve the agreement between experiment and simulation by calculating the average rolling velocity of a population whose receptors follow a normal distribution, suggesting heterogeneity among particles significantly affects the experimental results. Further comparison between theory and experiment yields an estimate of the reactive compliance of sLe(x)/E-selectin interactions of 0.25 A, close to that reported in the literature for E-selectin and its natural ligand (0.3 A). We also provide an estimate of the value of the intrinsic association rate (between 10(4) and 10(5) s(-1)) for the formation of sLe(x)/E-selectin bonds.

Quantitative morphodynamics of endothelial cells within confluent cultures in response to fluid shear stress

To evaluate shear stress-induced effects on cultured cells we have extended the mechanical setup of a multichannel in vitro rheological system and developed software allowing entire processing control and image data analysis. The values of cell motility, degree of orientation (alignment), and cell elongation were correlated as a function of time (morphodynamics). Collective and individual endothelial cells within confluent cultures displayed a shear stress-dependent characteristic phase behavior of the following time course: resting conditions (phase I), change of motility (phase II), onset of alignment (phase III), and finally cell elongation (phase IV). Especially cell motility was characterized by a randomized zigzag movement around mean trajectories (fluctuations) together with mean cell locomotion. Onset of shear stress caused a down-regulation of fluctuations of 30% within <10 min and simultaneously increased locomotion velocities preferring the flow direction (phase II). After a lag period of 10 to 20 min cells orientated in the direction of flow (phase III) without significant cell elongation, which finally occurs within hours (phase IV). These data provide first evidence that cells within confluent endothelial monolayers respond to shear stress with a characteristic phase behavior.

Selectin-carbohydrate interactions in shear flow

Hydrodynamic shear creates mechanical stresses on selectin bonds, modulating affinity and kinetic parameters. Chemical modification of sialyl Lewis(x) increases the strength of L-selectin bonds without altering recognition, suggesting that mechanical and biorecognition characteristics are separable. L-selectin bond formation rates may be strongly influenced by sulfate orientation in sulfo sialyl Lewis(x).

A direct comparison of selectin-mediated transient, adhesive events using high temporal resolution

Leukocyte capture and rolling on the vascular endothelium is mediated principally by the selectin family of cell adhesion receptors. In a parallel plate flow chamber, neutrophil rolling on purified selectins or a selectin-ligand substrate was resolved by high speed videomicroscopy as a series of ratchet-like steps with a characteristic time constant (Kaplanski, G., C. Farnarier, O. Tissot, A. Pierres, A.-M. Benoliel, M. C. Alessi, S. Kaplanski, and P. Bongrand. 1993. Biophys. J. 64:1922-1933; Alon, R., D. A. Hammer, and T. A. Springer. 1995. Nature (Lond.). 374:539-542). Under shear, neutrophil arrests due to bond formation events were as brief as 4 ms. Pause time distributions for neutrophils tethering on P-, E-, L-selectin, or peripheral node addressin (PNAd) were compared at estimated single bond forces ranging from 37 to 250 pN. Distributions of selectin mediated pause times were fit to a first order exponential, resulting in a molecular dissociation constant (k(off)) for the respective selectin as a function of force. At estimated single bond forces of 125 pN and below, all three selectin dissociation constants fit the Bell and Hookean spring models of force-driven bond breakage equivalently. Unstressed k(off) values based on the Bell model were 2.4, 2.6, 2.8, 3.8 s(-1) for P-selectin, E-selectin, L-selectin, and PNAd, respectively. Bond separation distances (reactive compliance) were 0.39, 0.18, 1.11, 0.59 A for P-selectin, E-selectin, L-selectin, and PNAd, respectively. Dissociation constants for L-selectin and P-selectin at single bond forces above 125 pN were considerably lower than either Bell or Hookean spring model predictions, suggesting the existence of two regimes of reactive compliance. Additionally, interactions between L-selectin and its leukocyte ligand(s) were more labile in the presence of flow than the L-selectin endothelial ligand, PNAd, suggesting that L-selectin ligands may have different molecular and mechanical properties. Both types of L-selectin bonds had a higher reactive compliance than P-selectin or E-selectin bonds.

Numerical analysis of the deformation of an adherent drop under shear flow

The adhesion of leukocytes to substrates is an important biomedical problem and has drawn extensive research. In this study, employing both single and compound drop models, we investigate how hydrodynamics interacts with an adherent liquid drop in a shear flow. These liquid drop models have recently been used to describe the rheological behavior of leukocytes. Numerical simulation confirms that the drop becomes more elongated when either capillary number or initial contact angle increases. Our results show that there exists a thin region between the drop and the wall as the drop undergoes large stretching, which allows high pressure to build up and provides a lift force. In the literature, existing models regard the leukocyte as a rigid body to calculate the force and torque acting on the drop in order to characterize the binding between cell receptors and endothelial ligands. The present study indicates that such a rigid body model is inadequate and the force magnitude obtained from it is less than half of that obtained using the deformable drop models. Furthermore, because of its much higher viscosity, the cell nucleus introduces a hydrodynamic time scale orders of magnitude slower than the cytoplasm. Hence the single and compound drops experience different dynamics during stretching, but exhibit very comparable steady-state shapes. The present work offers a framework to facilitate the development of a comprehensive dynamic model for blood cells.

The juxtamembrane region of the cadherin cytoplasmic tail supports lateral clustering, adhesive strengthening, and interaction with p120ctn

Cadherin cell-cell adhesion molecules form membrane-spanning molecular complexes that couple homophilic binding by the cadherin ectodomain to the actin cytoskeleton. A fundamental issue in cadherin biology is how this complex converts the weak intrinsic binding activity of the ectodomain into strong adhesion. Recently we demonstrated that cellular cadherins cluster in a ligand-dependent fashion when cells attached to substrata coated with the adhesive ectodomain of Xenopus C-cadherin (CEC1-5). Moreover, forced clustering of the ectodomain alone significantly strengthened adhesiveness (Yap, A.S., W.M. Brieher, M. Pruschy, and B.M. Gumbiner. Curr. Biol. 7:308-315). In this study we sought to identify the determinants of the cadherin cytoplasmic tail responsible for clustering activity. A deletion mutant of C-cadherin (CT669) that retained the juxtamembrane 94-amino acid region of the cytoplasmic tail, but not the beta-catenin-binding domain, clustered upon attachment to substrata coated with CEC1-5. Like wild-type C-cadherin, this clustering was ligand dependent. In contrast, mutant molecules lacking either the complete cytoplasmic tail or just the juxtamembrane region did not cluster. The juxtamembrane region was itself sufficient to induce clustering when fused to a heterologous membrane-anchored protein, albeit in a ligand-independent fashion. The CT669 cadherin mutant also displayed significant adhesive activity when tested in laminar flow detachment assays and aggregation assays. Purification of proteins binding to the juxtamembrane region revealed that the major associated protein is p120(ctn). These findings identify the juxtamembrane region of the cadherin cytoplasmic tail as a functionally active region supporting cadherin clustering and adhesive strength and raise the possibility that p120(ctn) is involved in clustering and cell adhesion.