Kinetics of cell detachment: effect of ligand density

Characterization of fibronectin properties by integrated micro-fluidic experiments and fluid-structure interaction simulations

Fibronectin (Fn) has been observed to assemble in the extracellular matrix (ECM) of cell culture and stretch in response to the external force. The alteration of molecule domain functions generally follows the extension of Fn. Several researchers have investigated fibronectin extensively in molecular architecture and conformation structure. However, the bulk material behavior of the Fn in the ECM has not been fully depicted at the cell scale, and many studies have ignored physiological conditions. Conversely, microfluidic techniques that explore cellular properties based on cell deformation and adhesion have emerged as a powerful and effective platform to study cell rheological transformation in a physiological environment. However, directly quantifying properties from microfluidic measurements remains a challenge. Therefore, it is an efficient way to combine experimental measurements with a robust and reliable numerical framework to calibrate the mechanical stress distribution in the test sample. In this paper, we present a monolithic Lagrangian fluid-structure interaction (FSI) approach within the Optimal Transportation Meshfree (OTM) framework that enables the investigation of the adherent Red Blood Cell (RBC) interacting with fluid and overcomes the drawbacks of the traditional computational tools such as the mesh entanglement and interface tracking, etc. This study aims to assess the material properties of the RBC and Fn fiber by calibrating the numerical predictions to experimental measurements. Moreover, a physical-based constitutive model will be proposed to describe the bulk behavior of the Fn fiber inflow, and the rate-dependent deformation and separation of the Fn fiber will be discussed.

3D biofabrication strategies for tissue engineering and regenerative medicine

Over the past several decades, there has been an ever-increasing demand for organ transplants. However, there is a severe shortage of donor organs, and as a result of the increasing demand, the gap between supply and demand continues to widen. A potential solution to this problem is to grow or fabricate organs using biomaterial scaffolds and a person's own cells. Although the realization of this solution has been limited, the development of new biofabrication approaches has made it more realistic. This review provides an overview of natural and synthetic biomaterials that have been used for organ/tissue development. It then discusses past and current biofabrication techniques, with a brief explanation of the state of the art. Finally, the review highlights the need for combining vascularization strategies with current biofabrication techniques. Given the multitude of applications of biofabrication technologies, from organ/tissue development to drug discovery/screening to development of complex in vitro models of human diseases, these manufacturing technologies can have a significant impact on the future of medicine and health care.

EFFECT OF FLOW ACCELERATION ON DEFORMATION AND ADHESION DYNAMICS OF CAPTURED CELLS

Cell deformation and adhesion under shear flows play an important role in both cell migration in vivo and capture based microfluidic devices in vitro. Adhesion dynamics of captured cell (e.g., firm adhesion, cell rolling and cell detachment) under steady shear flows have been studied extensively. However, cell adhesion under accelerating flows is common both in vivo and in vitro, and dynamics of cell adhesion under accelerating flows remains unknown. As such, we used a mathematical model based on the front tracking method and investigated the effect of flow acceleration on deformation and adhesion dynamics of captured cells, including cell deformation index, cell shape evolution, the velocities of cell center, contact time and wall shear stress for cell rolling and detachment by using a series of parameter values for leukocyte. The results showed that the cell presented three dynamics states (i.e., firm adhesion, rolling and detachment) with increasing wall shear stress under uniform flows. Wall shear stresses were < 0.56 Pa and > 1.12 Pa for firm adhesion and detachment, respectively. The wall shear stresses were at the range 1.48–1.63 Pa (higher than 1.12 Pa) when cell left the bottom surface of the channel under flow accelerations (a = 0.975–1.625 m/s2). The minimum of deformation index under accelerating flow was smaller than that under uniform flow. In conclusion, the flow acceleration promotes the deformation and adhesion of captured cells. These findings could further the understanding of cell migration in vivo and promote the development of capture based microfluidic devices in vitro.

Isotopic tracing for calculating the surface density of arginine-glycine-aspartic acid-containing peptide on allogeneic bone

OBJECTIVE

To investigate the feasibility of determining the surface density of arginine-glycine-aspartic acid (RGD) peptides grafted onto allogeneic bone by an isotopic tracing method involving labeling these peptides with (125) I, evaluating the impact of the input concentration of RGD peptides on surface density and establishing the correlation between surface density and their input concentration.

METHODS

A synthetic RGD-containing polypeptide (EPRGDNYR) was labeled with (125) I and its specific radioactivity calculated. Reactive solutions of RGD peptide with radioactive (125) I-RGD as probe with input concentrations of 0.01 mg/mL, 0.10 mg/mL, 0.50 mg/mL, 1.00 mg/mL, 2.00 mg/mL and 4.00 mg/mL were prepared. Using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide as a cross-linking agent, reactions were induced by placing allogeneic bone fragments into reactive solutions of RGD peptide of different input concentrations. On completion of the reactions, the surface densities of RGD peptides grafted onto the allogeneic bone fragments were calculated by evaluating the radioactivity and surface areas of the bone fragments. The impact of input concentration of RGD peptides on surface density was measured and a curve constructed.

RESULTS

Measurements by a radiodensity γ-counter showed that the RGD peptides had been labeled successfully with (125) I. The allogeneic bone fragments were radioactive after the reaction, demonstrating that the RGD peptides had been successfully grafted onto their surfaces. It was also found that with increasing input concentration, the surface density increased.

CONCLUSION

It was concluded that the surface density of RGD peptides is quantitatively related to their input concentration. With increasing input concentration, the surface density gradually increases to saturation value.

Mechanical strength of specific bonds acting isolated or in pairs: a case study on engineered proteins

The dynamic strength of multiple specific bonds exposed to external mechanical force is of significant interest for the understanding of biological adhesion. Exploiting the well-established FLAG tag technology, we engineered model proteins exhibiting no, one, or two identical binding sites for a monoclonal antibody. Bonds between these engineered proteins and the antibody were studied with dynamic force spectroscopy. On single bonds between a FLAG-tag and the antibody, we observed two regimes corresponding to two different activated complexes, that is, two intermediate states along the reaction path for bond breakage. Dynamic force spectroscopy on double bonds showed the same two regimes. The actual yield forces of double bonds slightly exceeded those of single bonds. A simplified kinetic model with analytical solutions was developed and used to interpret the measured spectra.

Models of dynamic extraction of lipid tethers from cell membranes

When a ligand that is bound to an integral membrane receptor is pulled, the membrane and the underlying cytoskeleton can deform before either the membrane delaminates from the cytoskeleton or the ligand detaches from the receptor. If the membrane delaminates from the cytoskeleton, it may be further extruded and form a membrane tether. We develop a phenomenological model for this process by assuming that deformations obey Hooke's law up to a critical force at which the cell membrane locally detaches from the cytoskeleton and a membrane tether forms. We compute the probability of tether formation and show that tethers can be extruded only within an intermediate range of force loading rates and pulling velocities. The mean tether length that arises at the moment of ligand detachment is computed as are the force loading rates and pulling velocities that yield the longest tethers.

Cell adhesion to nanoligands: effects of ligand size and concentration in solution

Cells interact with both tethered and motile ligands in their extracellular environment to initiate and regulate signaling, adhesion, and migration. A quantitative and fundamental understanding of these receptor-ligand interactions is necessary for drug discovery, tissue engineering, and biomaterial fabrication. In this paper, we present a mean field approach to quantify the fundamental thermodynamics of interaction between the cell surface receptors and motile ligands in solvent. Our studies show that the free energy of interaction between the receptors and the nanosized ligands depends strongly on the ligand size and the effects at lower and higher concentrations show completely opposite trends that cannot be explained by simple scaling laws. In addition, we also observe various regimes of strong and weak adhesion as a function of ligand size and concentration. Our calculations provide insights into understanding cell-matrix interactions at a fundamental level as well as to identify potential avenues for fabrication of nanoligands for therapeutic and biotechnological purposes.

Modeling cell migration in 3D: Status and challenges

Cell migration is a multi-scale process that integrates signaling, mechanics and biochemical reaction kinetics. Various mathematical models accurately predict cell migration on 2D surfaces, but are unable to capture the complexities of 3D migration. Additionally, quantitative 3D cell migration models have been few and far between. In this review we look and characterize various mathematical models available in literature to predict cell migration in 3D matrices and analyze their strengths and possible changes to these models that could improve their predictive capabilities.

Affinity adsorption of cells to surfaces and strategies for cell detachment

The use of bio-specific interactions for the separation and recovery of bio-molecules is now widely established and in many cases the technique has successfully crossed the divide between bench and process scale operation. Although the major specificity advantage of affinity-based separations also applies to systems intended for cell fractionation, developments in this area have been slower. Many of the problems encountered result from attempts to take techniques developed for molecular systems and, with only minor modification to the conditions used, apply them for the separation of cells. This approach tends to ignore or at least trivialise the problems, which arise from the heterogeneous nature of a cell suspension and the multivalent nature of the cell/surface interaction. To develop viable separation processes on a larger scale, effective contacting strategies are required in separators that also allow detachment or recovery protocols that overcome the enhanced binding strength generated by multivalent interactions. The effects of interaction valency on interaction strength needs to be assessed and approaches developed to allow effective detachment and recovery of adsorbed cells without compromising cell viability. This article considers the influence of operating conditions on cell attachment and the extent to which multivalent interactions determine the strength of cell binding and subsequent detachment.

IL-8 Response of Cyclically Stretching Alveolar Epithelial Cells Exposed to Non-fibrous Particles

Using a cell stretcher device, we have previously shown that A549 cells exposed to asbestos fibers gave significantly increased cytokine responses (IL-8) when they were cyclically stretched [Tsuda, A., B. K. Stringer, S. M. Mijailovich, R. A. Rogers, K. Hamada, and M. L. Gray. Am. J. Respir. Cell Mol. Biol. 21(4):455-462, 1999]. In the present study, cell stretching experiments were performed using non-fibrous riebeckite particles, instead of fibrous particles. Riebeckite particles are ground asbestos fibers with the size of a few microns and non-fibrous shape, and are often used as "non-toxic" control particles in the studies of fibrous particle-induced pathogenesis. Although it is generally assumed that riebeckite particles do not elicit strong biological responses, in our studies in cyclically stretched cell cultures, the riebeckite particles coated with adhesion proteins induced significant IL-8 responses, but in static cell cultures the treatment with adhesion protein-coated riebeckite did not induce comparable cytokine responses. To interpret these data, we have developed a simple mathematical model of adhesive interactions between a cell layer and rigid fibrous/non-fibrous particles that were subjected to external tensile forces. The analysis showed that because of considerable dissimilarity in deformations (i.e., strain mismatch) between the cells and particles during breathing, the attachment of particles as small as 1 micro in size could induce significant mechanical forces on the cell surface receptors, which may trigger subsequent adverse cell response under dynamic stretching conditions.

Detachment of affinity-captured bioparticles by elastic deformation of a macroporous hydrogel

Adsorption of bioparticles to affinity surfaces involves polyvalent interactions, complicating greatly the recovery of the adsorbed material. A unique system for the efficient binding and release of different cells and particles is described. Affinity-bound bioparticles and synthetic particles are detached from the macroporous hydrogel matrix, a so-called cryogel, when the cryogel undergoes elastic deformation. The particle detachment upon elastic deformation is believed to be due to breaking of many of the multipoint attachments between the particles and the affinity matrix and the change in the distance between affinity ligands when the matrix is deformed. However, no release of affinity-bound protein occurred upon elastic deformation. The phenomenon of particle detachment upon elastic deformation is believed to be of a generic nature, because it was demonstrated for a variety of bioparticles of different sizes and for synthetic particles, for different ligand-receptor pairs (IgG-protein A, sugar-ConA, metal ion-chelating ligand), and when the deformation was caused by either external forces (mechanical deformation) or internal forces (the shrinkage of thermosensitive, macroporous hydrogel upon an increase in temperature). The elasticity of cryogel monoliths ensures high recovery of captured cells under mild conditions, with highly retained viability. This property, along with their continuous porous structure makes cryogel monoliths very attractive for applications in affinity cell separation.

Development and characterization of a high-throughput system for assessing cell-surface receptor-ligand engagement

A nonfouling interfacial interpenetrating polymer network (IPN) of poly(acrylamide-co-ethylene glycol/acrylic acid) [p(AAm-co-EG/AAc)] was grafted to polystyrene for use as a novel platform for the development of high-throughput assays for screening of specific bimolecular interactions (i.e., receptor-ligand engagement). For the development of the IPN, a water-soluble hydrogen-abstracting photoinitiator was investigated: (4-benzoylbenzyl)trimethylammonium chloride. IPN-modified polystyrene surfaces were characterized using XPS, contact angle goniometry, and protein adsorption analysis. These IPN surfaces minimized fibrinogen adsorption compared to tissue culture polystyrene (>96% reduction), prevented mammalian cell adhesion, and served as nonfouling surfaces to graft biological ligands. For bimolecular interaction studies, a model peptide ligand from bone sialoprotein (Ac-CGGNGEPRGDTYRAY-NH(2)) was grafted to p(AAm-co-EG/AAc) via a 3400 M(w) linear pEG spacer. Ligand density measurements, cell culture, and a centrifugal adhesion assay were used to study cell adhesion to peptide-modified IPNs (i.e., receptor-ligand engagement). Ligand density (Gamma) was controllable from approximately 1 to 20 pmol/cm(2) by modulating the peptide input concentration (0.02-20 microM). Cell adhesion was directly dependent on the ligand density. This technology creates a powerful high-throughput system to simultaneously probe a myriad of cell-surface receptor-ligand interactions.

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.

A finite element model of cell deformation during magnetic bead twisting

Magnetic twisting cytometry probes mechanical properties of an adherent cell by applying a torque to a magnetic bead that is tightly bound to the cell surface. Here we have used a three-dimensional finite element model of cell deformation to compute the relationships between the applied torque and resulting bead rotation and lateral bead translation. From the analysis, we computed two coefficients that allow the cell elastic modulus to be estimated from measurements of either bead rotation or lateral bead translation, respectively, if the degree of bead embedding and the cell height are known. Although computed strains in proximity of the bead can be large, the relationships between applied torque and bead rotation or translation remain virtually linear up to bead rotations of 15 degrees, above which geometrical nonlinearities become significant. This appreciable linear range stands in contrast to the intrinsically nonlinear force-displacement relationship that is observed when cells are indented during atomic force microscopy. Finally, these computations support the idea that adhesive forces are sufficient to keep the bead firmly attached to the cell surface throughout the range of working torques.

Effect of contact time and inhibitor concentration on the affinity mediated adsorption of cells to surfaces

Cell detachment by shear stress under conditions of laminar flow was used to investigate the effect of incubation time and soluble binding competitors on affinity mediated cell/surface interactions. Fractional attachment between yeast and a Concanavalin A (Con A) coated surface was studied as a function of adhesion time prior to exposure to shear in a parallel plate flow chamber. Two, four and sixteen hours adhesion times gave rise to significantly different fractional attachment profiles, with four hours giving greater cell retention.The effect of dextran as a competitive displacer of pre-attached cells was also examined using a number of exposure regimes. While the presence of dextran in the displacement buffer led to higher fractional displacement of pre-attached cells, this effect was magnified if an equilibration period between dextran solution and pre-attached cells was allowed before detachment was attempted. The decline in fractional attachment increased with incubation time up to 30 min, with longer periods resulting in a smaller effect. Pre-incubation of the Con A surface with dextran prior to the introduction of cells led to a 60% reduction in attachment.Attempts to determine critical shear values were complicated by the presence of a tightly bound cell fraction of approximately 15% that was not removed at the highest shear values used.

The effect of peptide surface density on mineralization of a matrix deposited by osteogenic cells

The density of Arg-Gly-Asp-containing peptides covalently grafted to solid materials has been shown to affect adhesion, spreading, and focal contact formation. The objective of this study was to examine the effect of ligand density on mineralization of the extracellular matrix deposited by osteoblasts. In particular, RGD-modified quartz surfaces with ligand densities varying over two orders (0.01-3.6 pmol/cm(2)) of magnitude were prepared to assess the long-term function of osteoblasts on peptide-derivatized surfaces. After 3 weeks in culture, surfaces modified with a 15 amino acid peptide (Ac-Cys-Gly-Gly-Asn-Gly-Glu-Pro-Arg-Gly-Asp-Thr-Tyr-Arg-Ala-Tyr-NH(2) ) at a density > or =0.62 pmol/cm(2) significantly (p<0.05) enhanced mineralization compared with a RGD surface density of 0.01 pmol/cm(2), RGE surfaces, or clean surfaces adsorbed with serum proteins. These results suggest that regulation of the surface density of adhesive ligands on biomaterial surfaces is a critical determinant in a strategy to alter the degree of extracellular matrix maturation in contact with solid surfaces (e.g., implants). Further studies are required to elucidate the intracellular signal transduction pathways that mediate long-term matrix mineralization through the initial engagement of these adhesive ligands.

The kinetics of affinity-mediated cell-surface attachment

Data and a semi-empirical model are presented that describe the affinity interaction of yeast cells with a Concanavalin A derivatised surface. The model uses 3 parameters to describe the time course of cell attachment from a flowing suspension of yeast cells, over a range of flow rates, and gives an effective global fit to the data obtained. Further modifications allow the effects of a soluble competitor (glucose) on binding to be quantified in terms of a saturation effect, and an effective global fit is obtained. A comparison was made between the relationship between steady-state attached fraction and applied shear with similar data reported earlier (Ming, F. et al, 1998) for the detachment of pre-adsorbed cells. This shows that there is an order of magnitude difference between the forces required to effect complete detachment in the two systems, and that the nature of the relationship between shear and attached fraction is profoundly different. The magnitude of this time-dependent stabilization might be explained in terms of a progressive reorientation of cell relative to the surface such that the number of bonds is maximized.

Type IV pili of pathogenic Neisseriae elicit cortical plaque formation in epithelial cells

The pathogenic Neisseriae Neisseria meningitidis and Neisseria gonorrhoeae, initiate colonization by attaching to host cells using type IV pili. Subsequent adhesive interactions are mediated through the binding of other bacterial adhesins, in particular the Opa family of outer membrane proteins. Here, we have shown that pilus-mediated adhesion to host cells by either meningococci or gonococci triggers the rapid, localized formation of dramatic cortical plaques in host epithelial cells. Cortical plaques are enriched in both components of the cortical cytoskeleton and a subset of integral membrane proteins. These include: CD44v3, a heparan sulphate proteoglycan that may serve as an Opa receptor; EGFR, a receptor tyrosine kinase; CD44 and ICAM-1, adhesion molecules known to mediate inflammatory responses; f-actin; and ezrin, a component that tethers membrane components to the actin cytoskeleton. Genetic analyses reveal that cortical plaque formation is highly adhesin specific. Both pilE and pilC null mutants fail to induce cortical plaques, indicating that neisserial type IV pili are required for cortical plaque induction. Mutations in pilT, a gene required for pilus-mediated twitching motility, confer a partial defect in cortical plaque formation. In contrast to type IV pili, many other neisserial surface structures are not involved in cortical plaque induction, including Opa, Opc, glycolipid GgO4-binding adhesins, polysialic acid capsule or a particular lipooligosaccharide variant. Furthermore, it is shown that type IV pili allow gonococci to overcome the inhibitory effect of heparin, a soluble receptor analogue, on gonococcal invasion of Chang and A431 epithelial cells. These and other observations strongly suggest that type IV pili play an active role in initiating neisserial infection of the mucosal surface in vivo. The functions of type IV pili and other neisserial adhesins are discussed in the specific context of the mucosal microenvironment, and a multistep model for neisserial colonization of mucosal epithelia is proposed.

Calreticulin affects focal contact-dependent but not close contact-dependent cell-substratum adhesion

We used two cell lines expressing fast (RPEfast) and slow (RPEslow) attachment kinetics to investigate mechanisms of cell-substratum adhesion. We show that the abundance of a cytoskeletal protein, vinculin, is dramatically decreased in RPEfast cells. This coincides with the diminished expression level of an endoplasmic reticulum chaperone, calreticulin. Both protein and mRNA levels for calreticulin and vinculin were decreased in RPEfast cells. After RPEfast cells were transfected with cDNA encoding calreticulin, both the expression of endoplasmic reticulum-resident calreticulin and cytoplasmic vinculin increased. The abundance of other adhesion-related proteins was not affected. RPEfast cells underexpressing calreticulin displayed a dramatic increase in the abundance of total cellular phosphotyrosine suggesting that the effects of calreticulin on cell adhesiveness may involve modulation of the activities of protein tyrosine kinases or phosphatases which may affect the stability of focal contacts. The calreticulin and vinculin underexpressing RPEfast cells lacked extensive focal contacts and adhered weakly but attached fast to the substratum. In contrast, the RPEslow cells that expressed calreticulin and vinculin abundantly developed numerous and prominent focal contacts slowly, but adhered strongly. Thus, while the calreticulin overexpressing RPEslow cells "grip" the substratum with focal contacts, calreticulin underexpressing RPEfast cells use close contacts to "stick" to it.

Effect of seeding duration on the strength of chondrocyte adhesion to articular cartilage

Chondrocyte adhesion to cartilage may play an important role in the repair of articular defects by maintaining cells in positions where their biosynthetic products can contribute to the repair process. The objective of this in vitro study was to determine the effect of the duration of seeding time on the ability of chondrocytes to resist detachment from cartilage when subjected to mechanical perturbation (fluid-induced shear stress). Suspensions of adult bovine articular chondrocytes were prepared from primary, high-density monolayer cultures and infused into a parallel-plate shear-flow chamber where they settled onto 50-microm-thick sections of bovine articular cartilage at a density of approximately 20,000 cells/cm2. The chondrocytes were seeded and allowed to attach to the cartilage surface for specific durations (5-40 minutes) in medium including 10% serum at 22 degrees C, after which the cells were exposed to fluid flow-induced shear stresses (6-90 Pa). The fraction of detached cells at each shear stress was calculated from microscopic images. Shear stress was applied for 1 minute because this length of time was sufficient to induce steady-state cell detachment. Increasing the duration of cell seeding led to a more firm attachment of chondrocytes to cartilage. After 9 minutes of seeding, 50% cell detachment was induced by gravitational force alone. After 40 minutes of seeding, 50% detachment required 26 Pa of shear stress. Extrapolation of the data to account for the effect of repeated applications of cell suspensions to an individual cartilage substrate indicated that for a freshly prepared cartilage section, 50% detachment was induced by gravity after 25 minutes of seeding and by 2.3 Pa of shear stress after 40 minutes of seeding. The increase in resistance to shear stress-induced cell detachment with increasing seeding duration suggests that it may be beneficial to allow chondrocytes to stabilize in the absence of applied load for some time after chondrocyte transplantation for cartilage repair in vivo.

A probabilistic approach to measure the strength of bone cell adhesion to chemically modified surfaces

Patterned surfaces with alternating regions of amino silanes [N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (EDS)] and alkyl silanes [dimethyldichlorosilane (DMS)] have been used to alter the kinetics of spatial distribution of cells in vitro. In particular, we have previously observed the preferential spatial distribution of bone cells on the EDS regions of EDS/ DMS patterned surfaces (10). In this study, we examined whether the mechanism of spatial distribution of cells on the EDS regions was adhesion mediated. Homogeneous layers of EDS and DMS were immobilized on quartz substrates and characterized by contact angle. X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The strength of bone cell attachment to the modified substrates was examined using a radial flow apparatus, within either 20 min or 2 hr of cell incubation in the presence of serum. A Weibull distribution was chosen to characterize the strength of cell-substratum adhesion. Within 20 min of cell exposure, the strength of adhesion was significantly larger on EDS and clean surfaces, compared with DMS surfaces (p < 0.001). Within 2 hr of cell incubation, there was no statistical difference between the strength of cell adhesion to EDS, DMS, and clean surfaces. The results of this study suggest that the surface chemistry mediates adhesion-based spatial cell arrangement through a layer of adsorbed serum proteins.

Effect of receptor-ligand affinity on the strength of endothelial cell adhesion

The objective of this study was to determine the effect of receptor-ligand affinity on the strength of endothelial cell adhesion. Linear and cyclic forms of the fibronectin (Fn) cell-binding domain peptide Arg-Gly-Asp (RGD) were covalently immobilized to glass, and Fn was adsorbed onto glass slides. Bovine aortic endothelial cells attached to the surfaces for 15 min. The critical wall shear stress at which 50% of the cells detached increased nonlinearly with ligand density and was greater with immobilized cyclic RGD than with immobilized linear RGD or adsorbed Fn. To directly compare results for the different ligand densities, the receptor-ligand dissociation constant and force per bond were estimated from data for the critical shear stress and contact area. Total internal reflection fluorescence microscopy was used to measure the contact area as a function of separation distance. Contact area increased with increasing ligand density. Contact areas were similar for the immobilized peptides but were greater on surfaces with adsorbed Fn. The dissociation constant was determined by nonlinear regression of the net force on the cells to models that assumed that bonds were either uniformly stressed or that only bonds on the periphery of the contact region were stressed (peeling model). Both models provided equally good fits for cells attached to immobilized peptides whereas the peeling model produced a better fit of data for cells attached to adsorbed Fn. Cyclic RGD and linear RGD both bind to the integrin alpha v beta 3, but immobilized cyclic RGD exhibited a greater affinity than did linear RGD. Receptor affinities of Fn adsorbed to glycophase glass and Fn adsorbed to glass were similar. The number of bonds was calculated assuming binding equilibrium. The peeling model produced good linear fits between bond force and number of bonds. Results of this study indicate that 1) bovine aortic endothelial cells are more adherent on immobilized cyclic RGD peptide than linear RGD or adsorbed Fn, 2) increased adhesion is due to a greater affinity between cyclic RGD and its receptor, and 3) the affinity of RGD peptides and adsorbed Fn for their receptors is increased after immobilization.