An equilibrium model of endothelial cell adhesion via integrin-dependent and integrin-independent ligands

A Review of Cell Adhesion Studies for Biomedical and Biological Applications

Cell adhesion is essential in cell communication and regulation, and is of fundamental importance in the development and maintenance of tissues. The mechanical interactions between a cell and its extracellular matrix (ECM) can influence and control cell behavior and function. The essential function of cell adhesion has created tremendous interests in developing methods for measuring and studying cell adhesion properties. The study of cell adhesion could be categorized into cell adhesion attachment and detachment events. The study of cell adhesion has been widely explored via both events for many important purposes in cellular biology, biomedical, and engineering fields. Cell adhesion attachment and detachment events could be further grouped into the cell population and single cell approach. Various techniques to measure cell adhesion have been applied to many fields of study in order to gain understanding of cell signaling pathways, biomaterial studies for implantable sensors, artificial bone and tooth replacement, the development of tissue-on-a-chip and organ-on-a-chip in tissue engineering, the effects of biochemical treatments and environmental stimuli to the cell adhesion, the potential of drug treatments, cancer metastasis study, and the determination of the adhesion properties of normal and cancerous cells. This review discussed the overview of the available methods to study cell adhesion through attachment and detachment events.

Experimental and computational analysis of a novel flow channel to assess the adhesion strength of sessile marine organisms

Bioadhesives produced by marine macroalgae represent a potential source of inspiration for the development of water-resistant adhesives. Assessing their adhesion strength, however, remains difficult owing to low volumes of adhesive material produced, low solubility and rapid curing time. These difficulties can be circumvented by testing the adhesion strength of macroalgae propagules attached to a substrate. In this paper, we present a simple, novel flow channel used to test the adhesion strength of the germlings of the fucalean alga Hormosira banksii to four substrates of biomedical relevance (PMMA, agar, gelatin and gelatin + lipid). The adhesion strength of H. banksii germlings was found to increase in a time-dependent manner, with minimal adhesion success after a settlement period of 6 h and maximum adhesion strength achieved 24 h after initial settlement. Adhesion success increased most dramatically between 6 and 12 h settlement time, while no additional increase in adhesion strength was recorded for settlement times over 24 h. No significant difference in adhesion strength to the various substrates was observed. Computational fluid dynamics (CFD) was used to estimate the influence of fluid velocity and germling density on drag force acting on the settled organisms. CFD modelling showed that, on average, the drag force decreased with increasing germling number, suggesting that germlings would benefit from gregarious settlement behaviour. Collectively, our results contribute to a better understanding of the mechanisms allowing benthic marine organisms to thrive in hydrodynamically stressful environments and provide useful insights for further investigations.

Finite element analysis of traction force microscopy: influence of cell mechanics, adhesion, and morphology

The interactions between adherent cells and their extracellular matrix (ECM) have been shown to play an important role in many biological processes, such as wound healing, morphogenesis, differentiation, and cell migration. Cells attach to the ECM at focal adhesion sites and transmit contractile forces to the substrate via cytoskeletal actin stress fibers. This contraction results in traction stresses within the substrate/ECM. Traction force microscopy (TFM) is an experimental technique used to quantify the contractile forces generated by adherent cells. In TFM, cells are seeded on a flexible substrate and displacements of the substrate caused by cell contraction are tracked and converted to a traction stress field. The magnitude of these traction stresses are normally used as a surrogate measure of internal cell contractile force or contractility. We hypothesize that in addition to contractile force, other biomechanical properties including cell stiffness, adhesion energy density, and cell morphology may affect the traction stresses measured by TFM. In this study, we developed finite element models of the 2D and 3D TFM techniques to investigate how changes in several biomechanical properties alter the traction stresses measured by TFM. We independently varied cell stiffness, cell-ECM adhesion energy density, cell aspect ratio, and contractility and performed a sensitivity analysis to determine which parameters significantly contribute to the measured maximum traction stress and net contractile moment. Results suggest that changes in cell stiffness and adhesion energy density can significantly alter measured tractions, independent of contractility. Based on a sensitivity analysis, we developed a correction factor to account for changes in cell stiffness and adhesion and successfully applied this correction factor algorithm to experimental TFM measurements in invasive and noninvasive cancer cells. Therefore, application of these types of corrections to TFM measurements can yield more accurate estimates of cell contractility.

Patterning microscale extracellular matrices to study endothelial and cancer cell interactions in vitro

The extracellular matrix (ECM) of the tumor niche provides support to residing and migrating cells and presents instructive cues that influence cellular behaviours. The ECM protein fibronectin (Fn) enables vascular network formation, while hyaluronic acid (HA) is known to facilitate breast tumor development. To recapitulate aspects of the tumor microenvironment, we developed systems of spatially defined Fn and HA for the co-culture of endothelial colony forming cells (ECFCs) and breast cancer cells (BCCs). A micropatterned system was developed using sequential microcontact printing of HA and Fn. This approach supported the preferential adhesion of ECFCs to Fn, but did not support the preferential adhesion of BCCs to HA. Thus, we developed a microstructured analog to spatially organize BCC-laden HA micromolded hydrogels adjacent to ECFCs in fibrin hydrogels. These novel, miniaturized systems allow the analysis of the spatial and temporal mechanisms regulating tumor angiogenesis, and can be applied to mimic other microenvironments of healthy and diseased tissues.

Size matters: influence of the size of nanoparticles on their interactions with ligands immobilized on the solid surface

The correlation between the size of biotinylated nanoparticles and their affinity in relation to interactions with the solid surface was investigated. The silica particles with a diameter of 50-200 nm containing amino groups on the surface were labeled with different quantities of biotin. The affinity properties of biotinylated nanoparticles were studied using a Biacore 3000 instrument equipped with a streptavidin-coated sensor chip (SA chip). It was shown that the increase in the particle size from 50 to 200 nm reduced the affinity (K(D)) of biotin-streptavidin interactions from 1.2 x 10(-12) to 1.2 x 10(-10) M. It was found that the particles with higher concentrations of immobilized biotin on particle surfaces demonstrated stronger binding with streptavidin.

Targeting microspheres and cells to polyethylene glycol-modified biological surfaces

It has previously been demonstrated that damaged arterial tissue can be acutely modified with protein-reactive polyethylene glycol (PEG) to block undesirable platelet deposition. This concept might be expanded by employing PEG-biotin and its strong interaction with avidin for site-specific targeted delivery. Toward this end, cultured endothelial cells (ECs) were surface modified with PEG-biotin and the available biotin was quantified with flow cytometry. NeutrAvidin-coated microspheres and PEG-biotin modified ECs with NeutrAvidin as a bridging molecule were delivered under arterial shear stress to PEG-biotin modified ECs on a coverslip as well as scrape-damaged bovine carotid arteries. After incubation with a 10 mM solution for 1 min, 8 x 10(7) PEG-biotin molecules/EC were found and persisted for up to 120 h. Perfused microspheres adhered to NHS-PEG-biotin treated bovine carotid arteries with 60 +/- 16 microspheres/mm(2) versus 11 +/- 4 microspheres/mm(2) for control arteries (p < 0.015). Similarly, 22 +/- 5 targeted ECs/mm(2) adhered to NHS-PEG-biotin treated bovine carotid arteries versus 6 +/- 2 ECs/mm(2) for control arteries (p < 0.01). The targeting strategy demonstrated here might ultimately find application for drug delivery, gene therapy, or cell therapy where localization to specific labeled vascular regions is desired following catheter-based or surgical procedures.

Flow and High Affinity Binding Affect the Elastic Modulus of the Nucleus, Cell Body and the Stress Fibers of Endothelial Cells

Cell mechanical properties are important in the adhesion of endothelial cells to synthetic vascular grafts exposed to shear flow. We hypothesized that the local apparent elastic modulus of the nucleus and the cell body would increase to a greater extent for cells adherent via the dual ligand (integrin-fibronectin/avidin-biotin) and exposed to flow, than for cells treated with either ligand alone. High affinity avidin-biotin bonds and in vitro flow exposure were used to improve adhesion to grafts thereby altering the mechanical properties of endothelial cells. Introduction of the dual ligand chemistry at the cell-substrate interface increased the apparent elastic modulus of the cells as compared to cells adherent with the fibronectin-integrin bonds only. Cells cultured on the dual ligand surface exhibited higher elastic moduli of the nucleus and cell body relative to cells cultured on fibronectin alone. Exposure of cells to flow increased the apparent elastic modulus of the cell body, nucleus, and stress fibers of cells adherent to the fibronectin surface. A similar effect was seen for cells adherent to the dual ligand surface, although there was little effect on the elastic modulus of the nucleus. While the dual ligand surface produces an increase in adhesion strength, focal contact area and elastic modulus, the change in elastic modulus after exposure to flow is due only to an increase in stress fibers and not an increase in contact area.

Fibronectin and culture temperature modulate the efficacy of an avidin–biotin binding system for chondrocyte adhesion and growth on biodegradable polymers

Cell adhesion to a scaffold is a prerequisite for tissue engineering. Many studies have been focused on enhancing cell adhesion to synthetic materials that are used for scaffold fabrication. Previously, we showed that immobilization of biotin molecules to chondrocyte surfaces enhanced cell adhesion to avidin-coated biodegradable polymers such as poly-L-lactic acid, poly-D,L-lactic acid and polycaprolactone. However, the endocytosis of cell membrane biotin molecules decreases binding strength between biotinylated-chondrocytes (B-chondrocytes) and avidin-coated substrata, and therefore decreases cell spreading and discourages long-term chondrocytes culture. In this study, we proposed two strategies to solve the shortcoming of the avidin-biotin binding system. First, the avidin-biotin binding system is combined with the intrinsic integrin-dependent adhesion systems in order to enhance long-term cell culture. Second, the incubation temperature is lowered in order to slow down the endocytosis process. We found that the avidin-biotin binding system in combination with FN-integrin binding system enhanced cell adhesion, cell spreading and cell growth. Decrease of cell culture temperature to 4 degrees C enhanced the adhesion of B-chondrocytes to the avidin-coated surfaces, but decreased cell viability and proliferation, compared to culture temperature of 37 degrees C. Whether there is an optimal seeding temperature between 4 and 37 degrees C for both adhesion and proliferation of B-chondrocytes needs further investigation. Our results indicated that modulation of the adhesion conditions could further enhance the efficacy of the avidin-biotin binding system in mediating cell adhesion, and subsequent tissue culture.

Gravity Spun Polycaprolactone Fibers for Applications in Vascular Tissue Engineering: Proliferation and Function of Human Vascular Endothelial Cells

Poly(epsilon-caprolactone) (PCL) fibers produced by wet spinning from solutions in acetone under lowshear (gravity-flow) conditions resulted in fiber strength of 8 MPa and stiffness of 0.08 Gpa. Cold drawing to an extension of 500% resulted in an increase in fiber strength to 43 MPa and stiffness to 0.3 GPa. The growth rate of human umbilical vein endothelial cells (HUVECs) (seeded at a density of 5 x 10(4) cells/mL) on as-spun fibers was consistently lower than that measured on tissue culture plastic (TCP) beyond day 2. Cell proliferation was similar on gelatin-coated fibers and TCP over 7 days and higher by a factor of 1.9 on 500% cold-drawn PCL fibers relative to TCP up to 4 days. Cell growth on PCL fibers exceeded that on Dacron monofilament by at least a factor of 3.7 at 9 days. Scanning electron microscopy revealed formation of a cell layer on samples of cold-drawn and gelatin-coated fibers after 24 hours in culture. Similar levels of ICAM-1 expression by HUVECs attached to PCL fibers and TCP were measured using RT-PCR and flow cytometry, indicative of low levels of immune activation. Retention of a specific function of HUVECs attached to PCL fibers was demonstrated by measuring their immune response to lipopolysaccharide. Levels of ICAM-1 expression increased by approximately 11% in cells attached to PCL fibers and TCP. The high fiber compliance, favorable endothelial cell proliferation rates, and retention of an important immune response of attached HUVECS support the use of gravity spun PCL fibers for three-dimensional scaffold production in vascular tissue engineering.

Mylar and Teflon-AF as cell culture substrates for studying endothelial cell adhesion

The textured and opaque nature of Dacron and ePTFE has prevented the use of these fabrics in conventional cell culture techniques normally employed to optimize cell attachment and retention. This lack of optimization has led, in part, to the poor performance of endothelialization strategies for improving vascular graft patency. Here we show that thin, transparent films of Mylar and Teflon-AF are viable in vitro cell culture mimics of Dacron and ePTFE vascular graft materials, particularly for the study of protein mediated endothelial cell (EC) attachment, spreading and adhesion. Glass substrates were used as controls. X-ray photoelectron spectroscopy (XPS) and contact angle analysis showed that Mylar and Teflon-AF have surface chemistries that closely match Dacron and ePTFE. (125)I radiolabeling was used to quantify fibronectin (FN) adsorption, and FN and biotinylated-BSA "dual ligand" co-adsorption onto glass, Mylar and Teflon-AF substrates. Native human umbilical vein endothelial cells (HUVEC) and streptavidin-incubated biotinylated-HUVEC (SA-b-HUVEC) spreading was measured using phase contrast microscopy. Cell retention and adhesion was determined using phase contrast microscopy under laminar flow. All surfaces lacking protein pre-treatment, regardless of surface type, showed the lowest degree of cell spreading and retention. Dual ligand treated Mylar films showed significantly greater SA-b-HUVEC spreading up to 2 h, but were similar to HUVEC on FN treated Mylar at longer times; whereas SA-b-HUVEC spreading on dual ligand treated Teflon-AF was never significantly different from HUVEC on FN treated Teflon-AF at any time point. SA-b-HUVEC retention was significantly greater on dual ligand treated Mylar compared to HUVEC on FN treated Mylar over the entire range of shear stresses tested (3.54-28.3 dynes/cm(2)); whereas SA-b-HUVEC retention to dual ligand and HUVEC retention to FN treated Teflon-AF gave similar results at each shear stress, with only the mid-range of stresses showing significant difference in cell retention to Teflon-AF.

Effect of an avidin-biotin binding system on chondrocyte adhesion, growth and gene expression

Cell adhesion to synthetic biomaterials is a prerequisite for anchorage cell culture and tissue engineering. The current study investigated utilization of an avidin-biotin binding system in enhancing chondrocyte adhesion to tissue culture polystyrene (TCPS). Biotinylated chondrocytes adhered to avidin-coated TCPS more quickly than untreated chondrocytes to bare TCPS. Also the avidin-biotin binding system enhanced cell initial spreading. However, the effects were only transient. The growth of biotinylated chondrocytes was first decreased during the first 3 days but increased afterwards. The progeny of biotinylated chondrocytes still maintained the ability in expressing cartilage extracellular matrix proteins such as type II collagen, type IX collagen and aggrecan. These results show potential for the application of the avidin-biotin binding system to cell culture and tissue engineering.

Effects of an Avidin-Biotin Binding System on Chondrocyte Adhesion and Growth on Biodegradable Polymers

Cell adhesion to a scaffold is a prerequisite for tissue engineering. Many studies have been focused on enhancing cell adhesion to synthetic materials that are used for scaffold fabrication. In this study, we applied an avidin-biotin binding system to enhance chondrocyte adhesion to biodegradable polymers. Biotin molecules were conjugated to the cell membrane of chondrocytes, and mediated cell adhesion to avidin-coated surfaces. We demonstrated that immobilization of biotin molecules to chondrocyte surfaces enhanced cell adhesion to avidin-coated biodegradable polymers such as poly(L-lactic acid), poly(D,L-lactic acid), and polycaprolactone, compared to the adhesion of normal chondrocytes to the same type of biodegradable polymer. The biotinylated chondrocytes still maintained their proliferation ability. This study showed the promise of applying the avidin-biotin system in cartilage tissue engineering. [diagram in text].

Effect of streptavidin RGD mutant on the adhesion of endothelial cells

Adhesion of endothelial cells (EC) to surfaces can be enhanced by supplementing the integrin-mediated adhesion with high-affinity streptavidin (SA) that links a biotinylated EC to a biotinylated surface. Biotin pullout from the EC membrane limits the effectiveness of this treatment, leading to a predominance of EC detachment by cohesive failure. In this study we investigated whether a RGD-SA mutant that links SA to EC integrin receptors, and eliminates EC biotinylation, improves EC adhesion. Suspended EC were incubated with the RGD-SA mutant prior to cell seeding, primarily via attachment to the RGD binding site on alpha(v)beta(3) integrin. RGD-SA-incubated EC were subsequently seeded onto a surface preadsorbed with a mixture of fibronectin (Fn) and biotinylated bovine serum albumin (b-BSA). Results showed EC adhesion supplemented with the RGD-SA-biotin system significantly increased cell retention under flow, critical shear stresses for detachment, focal contact area, and force per bond relative to SA used with biotinylated EC. These increases were accompanied by significant reductions in membrane fragments left behind following EC detachment, which suggested cohesive failure via cell membrane rupture was significantly reduced, and enhanced phosphorylation of focal adhesion kinase, which suggested activation and clustering of integrin receptors. Together, these results show that the integrin-independent augmentation of EC adhesion using SA-biotin can be further improved through use of an RGD-SA mutant.

Effect of streptavidin-biotin on endothelial vasoregulation and leukocyte adhesion

The current study examines whether the adhesion promoting arginine-glycine-aspartate-streptavidin mutant (RGD-SA) also affects two important endothelial cell (EC) functions in vitro: vasoregulation and leukocyte adhesion. EC adherent to surfaces via fibronectin (Fn) or Fn plus RGD-SA were subjected to laminar shear flow and media samples were collected over a period of 4h to measure the concentration of nitric oxide (NO), prostacyclin (PGI(2)), and endothelin-1 (ET-1). Western blot analysis was used to quantify the levels of endothelial-derived nitric oxide synthase (eNOS) and cyclooxygenase II (COX II). In a separate set of experiments, fluorescent polymorphonuclear leukocyte (PMN) adhesion to EC was quantified for EC with and without exposure to flow preconditioning. When cell adhesion was supplemented with the SA-biotin system, flow-induced production of NO and PGI(2) increased significantly relative to cells adherent on Fn alone. Previous exposure of EC to shear flow also significantly decreased PMN attachment to SA-biotin supplemented EC, but only after 2h of exposure to shear flow. The observed decrease in PMN-EC adhesion was negated by NG-nitro-L-arginine methyl ester (L-NAME), an antagonist of NO synthesis, but not by indomethacin, an inhibitor to PGI(2) synthesis, indicating the induced effect of PMN-EC interaction is primarily NO-dependent. Results from this study suggest that the use of SA-biotin to supplement EC adhesion encourages vasodilation and PMN adhesion in vitro under physiological shear-stress conditions. We postulate that the presence of SA-biotin more efficiently transmits the shear-stress signal and amplifies the downstream events including the NO and PGI(2) release and leukocyte-EC inhibition. These results may have ramifications for reducing thrombus-induced vascular graft failure.

Human microvascular endothelial cell growth and migration on biomimetic surfactant polymers

Successful engineering of a tissue-incorporated vascular prosthesis requires cells to proliferate and migrate on the scaffold. Here, we report on a series of "ECM-like" biomimetic surfactant polymers that exhibit quantitative control over the proliferation and migrational properties of human microvascular endothelial cells (HMVEC). The biomimetic polymers consist of a poly(vinyl amine) (PVAm) backbone with hexanal branches and varying ratios of cell binding peptide (RGD) to carbohydrate (maltose). Proliferation and migration behavior of HMVEC was investigated using polymers containing RGD: maltose ratios of 100:0, 75:25 and 50:50, and compared with fibronectin (FN) coated glass (1 microg/cm2). A radial Teflon fence migration assay was used to examine HMVEC migration at 12 h intervals over a 48 h period. Migration was quantified using an inverted optical microscope, and HMVEC were examined by confocal microscopy for actin and focal adhesion organization/ arrangement. Over the range of RGD ligand density studied (approximately 0.19-0.6 peptides/nm2), our results show HMVEC migration decreases with increasing RGD density in the polymer. HMVEC were least motile on the 100% RGD polymer (approximately 0.38-0.6 peptides/nm2) with an average migration of 0.20 mm2/h in area covered, whereas HMVEC showed the fastest migration of 0.48+/-0.06 mm2/h on the 50% RGD surface ( approximately 0.19-0.30 peptides/nm2). In contrast, cell proliferation increased with increasing surface peptide density; proliferation on the 50% RGD surface was 1.5%+/-0.06/h compared with 2.2%+/-0.07/h on the 100% RGD surface. Our results show that surface peptide density affects cellular functions such as growth and migration, with the highest peptide density supporting the most proliferation but the slowest migration.

Effect of substrate mechanics on chondrocyte adhesion to modified alginate surfaces

This study characterized the attachment of chondrocytes to RGD-functionalized alginate by examining the effect of substrate stiffness on cell attachment and morphology. Bovine chondrocytes were added to wells coated with 2% alginate or RGD-alginate. The alginate was crosslinked with divalent cations ranging from 1.25 to 62.5 mmol/g alginate. Attachment to RGD-alginate was 10-20 times higher than attachment to unmodified alginate and was significantly inhibited by antibodies to integrin subunits alpha3l and beta1, cytochalasin-D, and soluble RGD peptide. The equilibrium level and rate of attachment increased with crosslink density and substrate stiffness. Substrate stiffness also regulated chondrocyte morphology, which changed from a rounded shape with nebulous actin on weaker substrates to a predominantly flat morphology with actin stress fibers on stiffer substrates. The dependence of attachment on integrins and substrate stiffness suggests that chondrocyte integrins may play a role in sensing the mechanical properties of the matrices to which they are attached.

Synergistic effect of shear stress and streptavidin-biotin on the expression of endothelial vasodilator and cytoskeleton genes

Dual ligand treatment of streptavidin(SA)-biotin and fibronectin (Fn) enhances the adhesion of endothelial cells (EC) onto synthetic surfaces and promotes the quiescent phenotype of adherent EC. The current study investigates the effect of the dual ligand on the expression of endothelial genes in static culture and under shear stress (4 h at 10 dynes/cm2). Expression of 23 genes in the classes of signaling, cytoskeleton/ECM, vasoregulation, and shear-responsive were examined. Eight genes (argininosuccinate synthetase, K+ channel, TGFbeta, Mn-SOD, alpha-tubulin, t-PA, COX2, and eNOS) were significantly upregulated by shear stress. Two genes (caveolin-1 and ET-1) were downregulated by shear stress. Three genes (RhoA, elastin, alpha-actinin) were upregulated by the dual ligand treatment in static culture, and four genes (FAK, elastin, COX2, and eNOS) were upregulated when the dual ligand and shear stress were applied simultaneously. Northern blot analyses on FAK, RhoA, elastin, and alpha-actinin revealed similar results. The results suggest (1) the use of SA-biotin to supplement EC adhesion enhances the integrity of the EC cytoskeleton by upregulating the expression of cytoskeleton/ECM genes, and (2) a likely relationship between the expression of cytoskeleton/ECM genes and the downstream events, such as the shear-induced expression of eNOS and COX2 genes. Analyses presented in this study provide insights into the mechanism by which SA-biotin-supplemented EC mediate gene expression.

High-affinity augmentation of endothelial cell attachment: long-term effects on focal contact and actin filament formation

Coadsorption of high-affinity avidin with lower affinity cell adhesion protein fibronectin has been shown to significantly augment short-term (1 h) adhesion and spreading of endothelial cells; however, the longer term persistence of avidin binding and its effect on endothelial cell adhesion have not been addressed. In this study, the presence of avidin-biotin bonds 24 h after cell adhesion to the dual ligand surfaces was verified by laser confocal microscopy of a fluorescent avidin analog, streptavidin. Total internal reflection microscopy showed that the focal contact area, focal contact density, and cell spreading all increased significantly at 24 h compared to fibronectin-treated control surfaces. Focal contact area was identical when measured with cells that were labeled with either the fluorescent streptavidin or a carbocyanine dye incorporated in the cell membrane. Confocal images of stress fibers formed in cells adherent to dual ligand surfaces after 24 h were thicker and more numerous compared to cells adherent to fibronectin controls. The results indicate that 24 h after initial attachment avidin-biotin is localized to focal contacts on the basal surface and affects cell spreading, actin filament organization, and focal contact density.

Synergistic effect of high-affinity binding and flow preconditioning on endothelial cell adhesion

The current study examined whether the combined introduction of high-affinity avidin-biotin bonds and fibronectin-integrin bonds (i.e., dual ligand treatment) would further augment the adhesion of flow-preconditioned endothelial cells to model substrates via contributions to the actin cytoskeleton and the formation of focal contacts. Human umbilical vein endothelial cells (HUVEC) were grown under static conditions or exposed to a flow-preconditioning regimen for 24 h. Cell retention was determined by exposure to 75 dynes/cm(2). The combination of flow preconditioning and the dual ligand treatment yielded higher cell retention under flow compared to the cells adherent via fibronectin-integrin bonds only. This increase in adhesion strength correlated with a greater focal contact area. Elongation of the HUVEC occurred after exposure to flow preconditioning; however, orientation of dual ligand adherent cells was restricted due to the presence of the high-affinity ligand. Flow-preconditioned cells showed increased stress fiber formation compared to nonconditioned cells although the stress fibers per cell for flow-preconditioned cells were the same on both the ligand systems employed. The results indicate that enhanced adhesion strength is due to a combination of increased focal contact area, stress fiber formation, and cell alignment.

Local cell membrane deformations due to receptor-ligand bonding as seen by reflection microscopy

Understanding surface receptor clustering and redistribution processes at the cell-matrix contact zone requires detailed knowledge of the spatial integration of these molecules in the architecture of this complex interface. Here we present and discuss critically a procedure to extract such information combining reflection contrast microscopy (RCM) and reflection interference microscopy (RIM). As model system, we used living human umbilical vein endothelial cells (HUVEC) adhering to laminin-coated surfaces and investigated the distribution of the alpha2beta1 (CD29/CD49b) integrin at the contact zone of these cells. First, we applied freeze-fracture electron microscopy to gain information on microscopic details of the alpha2beta1 distribution at the contact zone. Next, we visualized and analyzed the overall lateral distribution of the integrins applying RCM using immunogold-labeling with 10 nm labels and a special silver enhancement technique. We found that RCM can be used to determine the lateral position of the marked receptor molecules to an accuracy of about 100-200 nm, instead of large morphological changes at the contact zone during silver enhancement. Finally, we combined RCM with RIM and analyzed the interference pattern of the contact zone around the label positions. Thus, we were able to detect changes of the average shape of the cell membrane due to receptor-ligand bonding of a size down to the resolution of the techniques.

Integrin-dependent interaction of human vascular endothelial cells on biomimetic peptide surfactant polymers

Biomimetic surfactant polymers designed by molecular grafting of pendant RGD peptides (Pep) and dextran oligosaccharides (Dex) in different ratios onto the backbone of poly(vinyl amine) (PVAm) were examined for their ability to promote endothelial cell (EC) growth. Adhesion, formation of focal contacts, and expression of integrin receptors were examined in EC seeded onto a series of novel surfactants containing 100% dextran (PVAm[Pep (0%)]) to 100% peptide (PVAm[Pep (100%)]) compared to fibronectin control. Interaction of EC on polymer was specific, as soluble GRGDSP, but not GRGESP, was able to inhibit both adhesion and spreading of EC. At three hours, EC attachment and spreading were rapid and comparable on fibronectin and PVAm[Pep (100%)], rounded on PVAm[Pep (0%)], and intermediate on PVAm[Pep (25%)], (PVAm[Pep (50%)], and PVAm[Pep (75%)], with increasing peptide ratio favoring more spreading, although all the substrates had similar hydrophilicity. Cells that spread well on fibronectin and PVAm[Pep (100%)] had sharp spikes of vinculin localized at the termination point of actin stress fibers. Formation of stress fibers and focal adhesions on other substrates were correlated with spreading pattern of EC and the peptide content. EC seeded on fibronectin expressed alpha5beta1 integrins all along the stress fibers and throughout the entire cytoskeleton, but this distribution pattern was less prominent on PVAm[Pep (100%)]. However, expression and distribution of vitronectin receptors (alpha(v)beta3) were similar on both fibronectin and PVAm[Pep (100%)], suggesting a strong cell adhesion on PVAm[Pep (100%)]. Viability of EC was also comparable on both fibronectin and PVAm[Pep (100%)] at 24 h. Substrates with high proportion of dextran limited cell adhesion, probably by decreasing protein adsorption. These results suggest that it may be possible to engineer substrates that promote cell adhesion in a receptor-dependent manner while blocking nonspecific protein adsorption, which may have potential as interface materials for prostheses used in cardiovascular system.