The dynamics and mechanics of endothelial cell spreading

Mechanisms of Cell Propulsion by Active Stresses

The mechanisms by which cytoskeletal flows and cell-substrate interactions interact to generate cell motion are explored using a simplified model of the cytoskeleton as a viscous gel containing active stresses. This model yields explicit general results relating cell speed and traction forces to the distributions of active stress and cell-substrate friction. It is found that 1) the cell velocity is given by a function that quantifies the asymmetry of the active-stress distribution, 2) gradients in cell-substrate friction can induce motion even when the active stresses are symmetrically distributed, 3) the traction-force dipole is enhanced by protrusive stresses near the cell edges or contractile stresses near the center of the cell, and 4) the cell velocity depends biphasically on the cell-substrate adhesion strength if active stress is enhanced by adhesion. Specific experimental tests of the calculated dependences are proposed.

Integrin-mediated adhesion and proliferation of human MSCs elicited by a hydroxyproline-lacking, collagen-like peptide

In this study, we evaluated the competence of a rationally designed collagen-like peptide (CLP-Cys) sequence - containing the minimal essential Glycine-Glutamic acid-Arginine (GER) triplet but lacking the hydroxyproline residue - for supporting human mesenchymal stem cell (hMSC) adhesion, spreading and proliferation. Cellular responses to the CLP-Cys sequence were analyzed by conjugating the peptide to two different substrates - a hard, planar glass surface and a soft hyaluronic acid (HA) particle-based hydrogel. Integrin-mediated cell spreading and adhesion were observed for hMSCs cultivated on the CLP-Cys functionalized surfaces, whereas on control surfaces lacking the peptide motif, cells either did not adhere or maintained a round morphology. On the glass surface, CLP-Cys-mediated spreading led to the formation of extended and well developed stress fibers composed of F-actin bundles and focal adhesion complexes while on the soft gel surface, less cytoskeletal reorganization organization was observed. The hMSCs proliferated significantly on the surfaces presenting CLP-Cys, compared to the control surfaces lacking CLP-Cys. Competitive binding assay employing soluble CLP-Cys revealed a dose-dependent inhibition of hMSC adhesion to the CLP-Cys-presenting surfaces. Blocking the α(2)β(1) receptor on hMSC also resulted in a reduction of cell adhesion on both types of CLP-Cys surfaces, confirming the affinity of CLP-Cys to α(2)β(1) receptors. These results established the competence of the hydroxyproline-free CLP-Cys for eliciting integrin-mediated cellular responses including adhesion, spreading and proliferation. Thus, CLP-Cys-modified HA hydrogels are attractive candidates as bioactive scaffolds for tissue engineering applications.

How matrix properties control the self-assembly and maintenance of tissues

The mechanism by which cells organize into tissues is fundamental to developmental biology and tissue engineering. Likewise, the disruption of cellular order within tissues is a hallmark of many diseases including cancer and atherosclerosis. Tissue formation is regulated, in part, by a balance between cell-cell cohesion and cell-extracellular matrix (ECM) adhesion. Here, experiments and approaches to alter this balance are discussed, and the nature of this balance in the formation of microvasculature is explored. Using matrices of tailored stiffness and matrix presentation, the role of the mechanical properties and ligand density in angiogenesis has been investigated. Decreasing cell-matrix adhesion by either reducing matrix stiffness or matrix ligand density induces the self-assembly of endothelial cells into network-like structures. These structures are stabilized by the polymerization of the extracellular matrix protein fibronectin. When fibronectin polymerization is inhibited, network formation does not occur. Interestingly, this interplay between substrate mechanics, ECM assembly, and tissue self-assembly is not limited to endothelial cells and has been observed in other cell types as well. These results suggest novel approaches to foster stable cell-cell adhesion and engineer tissues.

Stochastic modeling and identification of emergent behaviors of an Endothelial Cell population in angiogenic pattern formation

Despite a high level of stochasticity and heterogeneity, a population of biological cells can collectively construct a complex structure that emerges from individual cell behaviors. Endothelial Cells (ECs), for example, create a vascular network with a tubular structure through interactions with the surrounding scaffold and other cells. Individual cells make a series of discrete decisions whether to migrate, proliferate, or die, leading to network pattern formation. This paper presents a methodology for deriving agent behavior models from EC time lapse data in an in vitro micro-fluidic environment. Individual cells are modeled as stochastic agents that detect growth factors (chemical molecules) and the scaffold conditions, and that make stochastic phenotype state transitions. Based on observed behaviors, a model is obtained for predicting the behavior of a population of interacting cells, which will sprout out, form a tubular structure, and create a branch to generate a vascular network − the process referred to as angiogenesis. A Maximum Likelihood method for estimating model parameters from angiogenesis process time lapse data is then presented. The identified mechanism of emergent pattern formation, although investigated in the context of angiogenesis, provides useful insights for swarm and modular robotics.

The regulation of traction force in relation to cell shape and focal adhesions

Mechanical forces provide critical inputs for proper cellular functions. The interplay between the generation of, and response to, mechanical forces regulate such cellular processes as differentiation, proliferation, and migration. We postulate that adherent cells respond to a number of physical and topographical factors, including cell size and shape, by detecting the magnitude and/or distribution of traction forces under different conditions. To address this possibility we introduce a new simple method for precise micropatterning of hydrogels, and then apply the technique to systematically investigate the relationship between cell geometry, focal adhesions, and traction forces in cells with a series of spread areas and aspect ratios. Contrary to previous findings, we find that traction force is not determined primarily by the cell spreading area but by the distance from cell center to the perimeter. This distance in turn controls traction forces by regulating the size of focal adhesions, such that constraining the size of focal adhesions by micropatterning can override the effect of geometry. We propose that the responses of traction forces to center-periphery distance, possibly through a positive feedback mechanism that regulates focal adhesions, provide the cell with the information on its own shape and size. A similar positive feedback control may allow cells to respond to a variety of physical or topographical signals via a unified mechanism.

The role of the cytoskeleton in cellular force generation in 2D and 3D environments

To adhere and migrate, cells generate forces through the cytoskeleton that are transmitted to the surrounding matrix. While cellular force generation has been studied on 2D substrates, less is known about cytoskeletal-mediated traction forces of cells embedded in more in vivo-like 3D matrices. Recent studies have revealed important differences between the cytoskeletal structure, adhesion, and migration of cells in 2D and 3D. Because the cytoskeleton mediates force, we sought to directly compare the role of the cytoskeleton in modulating cell force in 2D and 3D. MDA-MB-231 cells were treated with agents that perturbed actin, microtubules, or myosin, and analyzed for changes in cytoskeletal organization and force generation in both 2D and 3D. To quantify traction stresses in 2D, traction force microscopy was used; in 3D, force was assessed based on single cell-mediated collagen fibril reorganization imaged using confocal reflectance microscopy. Interestingly, even though previous studies have observed differences in cell behaviors like migration in 2D and 3D, our data indicate that forces generated on 2D substrates correlate with forces within 3D matrices. Disruption of actin, myosin or microtubules in either 2D or 3D microenvironments disrupts cell-generated force. These data suggest that despite differences in cytoskeletal organization in 2D and 3D, actin, microtubules and myosin contribute to contractility and matrix reorganization similarly in both microenvironments.

A predictive model of cell traction forces based on cell geometry

Recent work has indicated that the shape and size of a cell can influence how a cell spreads, develops focal adhesions, and exerts forces on the substrate. However, it is unclear how cell shape regulates these events. Here we present a computational model that uses cell shape to predict the magnitude and direction of forces generated by cells. The predicted results are compared to experimentally measured traction forces, and show that the model can predict traction force direction, relative magnitude, and force distribution within the cell using only cell shape as an input. Analysis of the model shows that the magnitude and direction of the traction force at a given point is proportional to the first moment of area about that point in the cell, suggesting that contractile forces within the cell act on the entire cytoskeletal network as a single cohesive unit. Through this model, we demonstrate that intrinsic properties of cell shape can facilitate changes in traction force patterns, independently of heterogeneous mechanical properties or signaling events within the cell.

BCL2 interaction with actin in vitro may inhibit cell motility by enhancing actin polymerization

In addition to its well-defined role as an antagonist in apoptosis, we propose that BCL2 may act as an intracellular suppressor of cell motility and adhesion under certain conditions. Our evidence shows that, when over-expressed in both cancer and non-cancer cells, BCL2 can form a complex with actin and gelsolin that functions to decrease gelsolin-severing activity to increase actin polymerization, and, thus, suppress cell adhesive processes. The linkage between increased BCL2 and increased actin polymerization on the one hand, and suppression of cell adhesion, spreading, and motility on the other hand, is a novel observation that may provide a plausible explanation for why BCL2 over-expression in some tumors is correlated with improved patient survival. In addition, we have identified conditions in vitro in which F-actin polymerization can be increased while cell motility is reduced. These findings underscore the possibility that BCL2 may be involved in modulating cytoskeleton reorganization, and may provide an opportunity to explore signal transduction pathways important for cell adhesion and migration and to develop small molecule therapies for suppression of cancer metastasis.

Influence of substrate stiffness on cell-substrate interfacial adhesion and spreading: a mechano-chemical coupling model

Cell interactions with extracellular matrix, such as cell adhesion and spreading, are crucial for many biological functions and processes. Recent experimental progresses have demonstrated that substrate rigidities exert a remarkable influence on cell-substrate interfacial adhesion and spreading behaviors. The underlying biophysical mechanism, however, remains elusive. Based on the classical Bell-Dembo's theory, this paper develops a mechano-chemical coupling model to physically describe cell adhesion and spreading mediated by substrate stiffness. By investigating the competitive nature between cell-substrate specific attraction and non-specific repulsion, the kinetic relation of receptor-ligand interplay is established, in which the influences of receptor-ligand separation, substrate elasticity and non-specific repulsion on cell adhesions are especially addressed. According to mechanical equilibrium conditions between cell membranes and underlying elastic substrates, an analytical expression is then deduced to relate the cell-substrate interfacial adhesion strength to the substrate rigidity. Moreover, by means of the conventional wetting theory, the dependence of steady-state cell spreading on substrate stiffness is also quantitatively studied. Comparisons with the existing experimental data show that the proposed model can be used to explore cell-substrate interactions regulated by substrate rigidities.

Preparation of complaint matrices for quantifying cellular contraction

The regulation of cellular adhesion to the extracellular matrix (ECM) is essential for cell migration and ECM remodeling. Focal adhesions are macromolecular assemblies that couple the contractile F-actin cytoskeleton to the ECM. This connection allows for the transmission of intracellular mechanical forces across the cell membrane to the underlying substrate. Recent work has shown the mechanical properties of the ECM regulate focal adhesion and F-actin morphology as well as numerous physiological processes, including cell differentiation, division, proliferation and migration. Thus, the use of cell culture substrates has become an increasingly prevalent method to precisely control and modulate ECM mechanical properties. To quantify traction forces at focal adhesions in an adherent cell, compliant substrates are used in conjunction with high-resolution imaging and computational techniques in a method termed traction force microscopy (TFM). This technique relies on measurements of the local magnitude and direction of substrate deformations induced by cellular contraction. In combination with high-resolution fluorescence microscopy of fluorescently tagged proteins, it is possible to correlate cytoskeletal organization and remodeling with traction forces. Here we present a detailed experimental protocol for the preparation of two-dimensional, compliant matrices for the purpose of creating a cell culture substrate with a well-characterized, tunable mechanical stiffness, which is suitable for measuring cellular contraction. These protocols include the fabrication of polyacrylamide hydrogels, coating of ECM proteins on such gels, plating cells on gels, and high-resolution confocal microscopy using a perfusion chamber. Additionally, we provide a representative sample of data demonstrating location and magnitude of cellular forces using cited TFM protocols.

Integrins and extracellular matrix in mechanotransduction

Integrins bind extracellular matrix fibrils and associate with intracellular actin filaments through a variety of cytoskeletal linker proteins to mechanically connect intracellular and extracellular structures. Each component of the linkage from the cytoskeleton through the integrin-mediated adhesions to the extracellular matrix therefore transmits forces that may derive from both intracellular, myosin-generated contractile forces and forces from outside the cell. These forces activate a wide range of signaling pathways and genetic programs to control cell survival, fate, and behavior. Additionally, cells sense the physical properties of their surrounding environment through forces exerted on integrin-mediated adhesions. This article first summarizes current knowledge about regulation of cell function by mechanical forces acting through integrin-mediated adhesions and then discusses models for mechanotransduction and sensing of environmental forces.

Matrix density mediates polarization and lumen formation of endothelial sprouts in VEGF gradients

Endothelial cell (EC) sprouting morphogenesis is a critical step during angiogenesis, the formation of new blood vessels from existing conduits. Here, three-dimensional sprouting morphogenesis was examined using in vitro microfluidic devices that enabled the separate and simultaneous tuning of biomechanical and soluble biochemical stimuli. Quantitative analysis of endothelial sprout formation demonstrated that the ability of vascular endothelial growth factor (VEGF) to regulate stable sprout formation was mediated by the density of the surrounding collagen/fibronectin matrix. The coordinated migration and proliferation of multiple ECs to form stable sprouts were enhanced at intermediate matrix densities (1.2-1.9 mg ml(-1)), while lower densities resulted in uncoordinated migration (0.3-0.7 mg ml(-1)) and higher densities resulted in broad cell clusters that did not elongate (2.7 mg ml(-1)). Within the permissive range of matrix biomechanics, higher density matrices resulted in shorter, thicker, and slower-growing sprouts. The sprouts in higher density matrices also were more likely to polarize towards higher VEGF concentrations, included more cells per cross-sectional area, and demonstrated more stable lumen formation compared to sprouts in lower density matrices. These results quantitatively demonstrate that matrix density mediates VEGF-induced sprout polarization and lumen formation, potentially by regulating the balance between EC migration rate and proliferation rate.

Study of microscale hydraulic jump phenomenon for hydrodynamic trap-and-release of microparticles

Easy trap-and-release of microparticles is necessary to study biological cellular behavior. The hydraulic jump phenomenon inspired us to conceive a microfluidic device for the hydrodynamic trap-and-release of microparticles. A sudden height increase in a microfluidic channel leads to a dramatic decrease in flow velocity, allowing effective trapping of the microparticles by energy conversion. The trapped particles can be released by stronger inertial force based on simply increasing the flow velocity. We present a systematic, numerical study of trap-and-release of the microparticles using multiphase Navier-Stokes equations. Effect of geometry flow velocity, particle diameter, and adhesion force on trap-and-release was studied.

Effects of Morphology vs. Cell-Cell Interactions on Endothelial Cell Stiffness

Biological processes such as atherogenesis, wound healing, cancer cell metastasis, and immune cell transmigration rely on a delicate balance between Cell-Cell and cell-substrate adhesion. Cell mechanics have been shown to depend on substrate factors such as stiffness and ligand presentation, while the effects of Cell-Cell interactions on the mechanical properties of cells has received little attention. Here, we use atomic force microscopy to measure the Young's modulus of live human umbilical vein endothelial cells (HUVECs). In varying the degree of Cell-Cell contact in HUVECs (single cells, groups, and monolayers), we observe that increased cell stiffness correlates with an increase in cell area. Further, we observe that HUVECs stiffen as they spread onto a glass substrate. When we weaken Cell-Cell junctions (i.e., through a low dose of cytochalasin B or treatment with a VE-cadherin antibody), we observe that cell-substrate adhesion increases, as measured by focal adhesion size and density, and the stiffness of cells within the monolayer approaches that of single cells. Our results suggest that while morphology can roughly be used to predict cell stiffness, Cell-Cell interactions may play a significant role in determining the mechanical properties of individual cells in tissues by careful maintenance of cell tension homeostasis.

Substrate stiffening promotes endothelial monolayer disruption through enhanced physical forces

A hallmark of many, sometimes life-threatening, inflammatory diseases and disorders is vascular leakage. The extent and severity of vascular leakage is broadly mediated by the integrity of the endothelial cell (EC) monolayer, which is in turn governed by three major interactions: cell-cell and cell-substrate contacts, soluble mediators, and biomechanical forces. A potentially critical but essentially uninvestigated component mediating these interactions is the stiffness of the substrate to which the endothelial monolayer is adherent. Accordingly, we investigated the extent to which substrate stiffening influences endothelial monolayer disruption and the role of cell-cell and cell-substrate contacts, soluble mediators, and physical forces in that process. Traction force microscopy showed that forces between cell and cell and between cell and substrate were greater on stiffer substrates. On stiffer substrates, these forces were substantially enhanced by a hyperpermeability stimulus (thrombin, 1 U/ml), and gaps formed between cells. On softer substrates, by contrast, these forces were increased far less by thrombin, and gaps did not form between cells. This stiffness-dependent force enhancement was associated with increased Rho kinase activity, whereas inhibition of Rho kinase attenuated baseline forces and lessened thrombin-induced inter-EC gap formation. Our findings demonstrate a central role of physical forces in EC gap formation and highlight a novel physiological mechanism. Integrity of the endothelial monolayer is governed by its physical microenvironment, which in normal circumstances is compliant but during pathology becomes stiffer.

Effect of dynamic stiffness of the substrates on neurite outgrowth by using a DNA-crosslinked hydrogel

Central nervous system tissues, like other tissue types, undergo constant remodeling, which potentially leads to changes in their mechanical stiffness. Moreover, mechanical compliance of central nervous system tissues can also be modified under external load such as that experienced in traumatic brain or spinal cord injury, and during pathological processes. Thus, the neuronal responses to the dynamic stiffness of the microenvironment are of significance. In this study, we induced decrease in stiffness by using a DNA-crosslinked hydrogel, and subjected rat spinal cord neurons to such dynamic stiffness. The neurons respond to the dynamic cues as evidenced by the primary neurite structure, and the response from each neurite property (e.g., axonal length and primary dendrite number) is consistent with the behavior on static gels of same substrate rigidity, with one exception of mean primary dendrite length. The results on cell population distribution confirm the neuronal responses to the dynamic stiffness. Quantification on the focal adhesion kinase expression in the neuronal cell body on dynamic gels suggests that neurons also modify adhesion in coping with the dynamic stiffnesses. The results reported here extend the neuronal mechanosensing capability to dynamic stiffness of extracellular matrix, and give rise to a novel way of engineering neurite outgrowth in time dimension.

Preparation of hydrogel substrates with tunable mechanical properties

The modulus of elasticity of the extracellular matrix (ECM), often referred to in a biological context as "stiffness," naturally varies within the body, e.g., hard bones and soft tissue. Moreover, it has been found to have a profound effect on the behavior of anchorage-dependent cells. The fabrication of matrix substrates with a defined modulus of elasticity can be a useful technique to study the interactions of cells with their biophysical microenvironment. Matrix substrates composed of polyacrylamide hydrogels have an easily quantifiable elasticity that can be changed by adjusting the relative concentrations of its monomer, acrylamide, and cross-linker, bis-acrylamide. In this unit, we detail a protocol for the fabrication of statically compliant and radial-gradient polyacrylamide hydrogels, as well as the functionalization of these hydrogels with ECM proteins for cell culture. Included as well are suggestions to optimize this protocol to the choice of cell type or stiffness with a table of relative bis-acrylamide and acrylamide concentrations and expected elasticity after polymerization.

Cytoskeleton reorganization of spreading cells on micro-patterned islands: a functional model

Predictive numerical models of cellular response to biophysical cues have emerged as a useful quantitative tool for cell biology research. Cellular experiments in silico can augment in vitro and in vivo investigations by filling gaps in what is possible to achieve through 'wet work'. Biophysics-based numerical models can be used to verify the plausibility of mechanisms regulating tissue homeostasis derived from experiments. They can also be used to explore potential targets for therapeutic intervention. In this perspective article we introduce a single cell model developed towards the design of novel biomaterials to elicit a regenerative cellular response for the repair of diseased tissues. The model is governed by basic mechanisms of cell spreading (lamellipodial and filopodial extension, formation of cell-matrix adhesions, actin reinforcement) and is developed in the context of cellular interaction with functionalized substrates that present defined points of potential adhesion. To provide adequate context, we first review the biophysical underpinnings of the model as well as reviewing existing cell spreading models. We then present preliminary benchmarking of the model against published experiments of cell spreading on micro-patterned substrates. Initial results indicate that our mechanistic model may represent a potentially useful approach in a better understanding of cell interactions with the extracellular matrix.

Neutrophil adhesion and chemotaxis depend on substrate mechanics

Neutrophil adhesion to the vasculature and chemotaxis within tissues play critical roles in the inflammatory response to injury and pathogens. Unregulated neutrophil activity has been implicated in the progression of numerous chronic and acute diseases such as rheumatoid arthritis, asthma, and sepsis. Cell migration of anchorage-dependent cells is known to depend on both chemical and mechanical interactions. Although neutrophil responses to chemical cues have been well characterized, little is known about the effect of underlying tissue mechanics on neutrophil adhesion and migration. To address this question, we quantified neutrophil migration and traction stresses on compliant hydrogel substrates with varying elasticity in a micro-machined gradient chamber in which we could apply either a uniform concentration or a precise gradient of the bacterial chemoattractant fMLP. Neutrophils spread more extensively on substrates of greater stiffness. In addition, increasing the stiffness of the substrate leads to a significant increase in the chemotactic index for each fMLP gradient tested. As the substrate becomes stiffer, neutrophils generate higher traction forces without significant changes in cell speed. These forces are often displayed in pairs and focused in the uropod. Increases in the mean fMLP concentration beyond the K(D) of the receptor lead to a decrease in chemotactic index on all surfaces. Blocking with an antibody against beta(2)-integrins leads to a significant reduction but not an elimination of directed motility on stiff materials, but no change in motility on soft materials, suggesting neutrophils can display both integrin-dependent and integrin-independent motility. These findings are critical for understanding how neutrophil migration may change in different mechanical environments in vivo and can be used to guide the design of migration inhibitors that more efficiently target inflammation.

Fluctuations of cytoskeleton-bound microbeads--the effect of bead-receptor binding dynamics

The cytoskeleton (CSK) of living cells is a crosslinked fiber network, subject to ongoing biochemical remodeling processes that can be visualized by tracking the spontaneous motion of CSK-bound microbeads. The bead motion is characterized by anomalous diffusion with a power-law time evolution of the mean square displacement (MSD), and can be described as a stochastic transport process with apparent diffusivity D and power-law exponent β: MSD ∼ D (t/t(0))(β). Here we studied whether D and β change with the time that has passed after the initial bead-cell contact, and whether they are sensitive to bead coating (fibronectin, integrin antibodies, poly-L-lysine, albumin) and bead size (0.5-4.5 µm). The measurements are interpreted in the framework of a simple model that describes the bead as an overdamped particle coupled to the fluctuating CSK network by an elastic spring. The viscous damping coefficient characterizes the degree of bead internalization into the cell, and the spring constant characterizes the strength of the binding of the bead to the CSK. The model predicts distinctive signatures of the MSD that change with time as the bead couples more tightly to the CSK and becomes internalized. Experimental data show that the transition from the unbound to the tightly bound state occurs in an all-or-nothing manner. The time point of this transition shows considerable variability between individual cells (2-30 min) and depends on the bead size and bead coating. On average, this transition occurs later for smaller beads and beads coated with ligands that trigger the formation of adhesion complexes (fibronectin, integrin antibodies). Once the bead is linked to the CSK, however, the ligand type and bead size have little effect on the MSD. On longer timescales of several hours after bead addition, smaller beads are internalized into the cell more readily, leading to characteristic changes in the MSD that are consistent with increased viscous damping by the cytoplasm and reduced binding strength.

The effects of substrate elasticity on endothelial cell network formation and traction force generation

While the growth factors and cytokines known to influence angiogenesis and vasculogenesis have garnered widespread attention, less is known about how the mechanical environment affects blood vessel formation and cell assembly. In this study, we investigate the relationship between substrate elasticity, endothelial cell-cell connectivity and traction force generation. We find that on more compliant substrates, endothelial cells self-assemble into network-like structures independently of additional exogenous growth factors or cytokines. These networks form from the assembly of sub-confluent endothelial cells on compliant (E = 200-1000Pa) substrates, and results from both the proliferation and migration of endothelial cells. Interestingly, stabilization of these cell-cell connections and networks requires fibronectin polymerization. Traction Force Microscopy measurements indicate that individual endothelial cells on compliant substrates exert forces which create substrate stains that propagate from the cell edge. We speculate that these strains draw the cells together and initiate self-assembly. Notably, endothelial cell network formation on compliant substrates is dynamic and transient; as cell number and substrate strains increase, the networks fill in through collective cell movements from the network edges. Our results indicate that network formation is mediated in part by substrate mechanics and that cellular traction force may promote cell-cell assembly by directing cell migration.

Quantitative reflection interference contrast microscopy (RICM) in soft matter and cell adhesion

Adhesion can be quantified by measuring the distance between the interacting surfaces. Reflection interference contrast microscopy (RICM), with its ability to measure inter-surface distances under water with nanometric precision and milliseconds time resolution, is ideally suited to studying the dynamics of adhesion in soft systems. Recent technical developments, which include innovative image analysis and the use of multi-coloured illumination, have led to renewed interest in this technique. Unambiguous quantitative measurements have been achieved for colloidal beads and model membranes, thus revealing new insights and applications. Quantification of data from cells shows exciting prospects. Herein, we review the basic principles and recent developments of RICM applied to studies of dynamical adhesion processes in soft matter and cell biology and provide practical hints to potential users.

Dissecting the impact of matrix anchorage and elasticity in cell adhesion

Extracellular matrices determine cellular fate decisions through the regulation of intracellular force and stress. Previous studies suggest that matrix stiffness and ligand anchorage cause distinct signaling effects. We show herein how defined noncovalent anchorage of adhesion ligands to elastic substrates allows for dissection of intracellular adhesion signaling pathways related to matrix stiffness and receptor forces. Quantitative analysis of the mechanical balance in cell adhesion using traction force microscopy revealed distinct scalings of the strain energy imparted by the cells on the substrates dependent either on matrix stiffness or on receptor force. Those scalings suggested the applicability of a linear elastic theoretical framework for the description of cell adhesion in a certain parameter range, which is cell-type-dependent. Besides the deconvolution of biophysical adhesion signaling, site-specific phosphorylation of focal adhesion kinase, dependent either on matrix stiffness or on receptor force, also demonstrated the dissection of biochemical signaling events in our approach. Moreover, the net contractile moment of the adherent cells and their strain energy exerted on the elastic substrate was found to be a robust measure of cell adhesion with a unifying power-law scaling exponent of 1.5 independent of matrix stiffness.

BCL2 inhibits cell adhesion, spreading, and motility by enhancing actin polymerization

BCL2 is best known as a multifunctional anti-apoptotic protein. However, little is known about its role in cell-adhesive and motility events. Here, we show that BCL2 may play a role in the regulation of cell adhesion, spreading, and motility. When BCL2 was overexpressed in cultured murine and human cell lines, cell spreading, adhesion, and motility were impaired. Consistent with these results, the loss of Bcl2 resulted in higher motility observed in Bcl2-null mouse embryonic fibroblast (MEF) cells compared to wild type. The mechanism of BCL2 regulation of cell adhesion and motility may involve formation of a complex containing BCL2, actin, and gelsolin, which appears to functionally decrease the severing activity of gelsolin. We have observed that the lysate from MCF-7 and NIH3T3 cells that overexpressed BCL2 enhanced actin polymerization in cell-free in vitro assays. Confocal immunofluorescent localization of BCL2 and F-actin during spreading consistently showed that increased expression of BCL2 resulted in increased F-actin polymerization. Thus, the formation of BCL2 and gelsolin complexes (which possibly contain other proteins) appears to play a critical role in the regulation of cell adhesion and migration. Given the established correlation of cell motility with cancer metastasis, this result may explain why the expression of BCL2 in some tumor cell types reduces the potential for metastasis and is associated with improved patient prognosis.

Substrate Stiffness and Cell Area Predict Cellular Traction Stresses in Single Cells and Cells in Contact

Cells generate traction stresses against their substrate during adhesion and migration, and traction stresses are used in part by the cell to sense the substrate. While it is clear that traction stresses, substrate stiffness, and cell area are related, it is unclear whether or how area and substrate stiffness affect force generation in cells. Moreover, multiple studies have investigated traction stresses of single cells, but few have focused on forces exerted by cells in contact, which more closely mimics the in vivo environment. Here, cellular traction forces were measured where cell area was modulated by ligand density or substrate stiffness. We coupled these measurements with a multilinear regression model to show that both projected cell area and underlying substrate stiffness are significant predictors of traction forces in endothelial cells, and interestingly, substrate ligand density is not. We further explored the effect of cell-cell contact on the interplay between cell area, substrate stiffness, and force generation and found that again both area and stiffness play a significant role in cell force generation. These data indicate that cellular traction force cannot be determined by cell area alone and that underlying substrate stiffness is a significant contributor to traction force generation.

Effects of immobilized glycosaminoglycans on the proliferation and differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are adult stem cells with potential for multilineage differentiation. They represent an attractive cell source alternative to embryonic stem cells for therapeutic applications. Optimal utilization of MSCs for tissue engineering requires improved biomaterials that can enhance their growth and direct differentiation. The biological activity of glycosaminoglycans (GAGs) has been previously exploited for use in tissue engineering applications. In this study, MSC proliferation and differentiation was studied on GAG-derivatized chitosan membranes. The GAGs included heparin, heparan sulfate, dermatan sulfate, chondroitin 4-sulfate, chondroitin 6-sulfate, and hyaluronic acid. The covalent GAG immobilization method and amount of immobilized GAG were varied. It was found that MSC growth increased as much as fivefold on GAG-immobilized surfaces compared to tissue culture plastic and chitosan-only controls. The MSC growth rates increased significantly with increasing GAG density on the culture surfaces. The MSC proliferation rates on heparin, heparan sulfate, dermatan sulfate, and chondroitin 6-sulfate exhibited nonlinear increases with the level of fibronectin binding on these surfaces. In contrast, MSC proliferation on hyaluronic acid and chondroitin 4-sulfate was found to be independent of fibronectin or vitronectin binding on the surfaces, suggesting that these GAGs influenced MSC proliferation through different mechanisms. In conclusion, the results indicate that GAG immobilization on chitosan scaffolds provides an effective means of manipulating MSC proliferation and has promising potential for directing MSC differentiation in tissue engineering applications employing chitosan.

Tethering a laminin peptide to a crosslinked collagen scaffold for biofunctionality

Cell adhesion peptide regulates various cellular functions like proliferation, attachment, and spreading. The cellular response to laminin peptide (PPFLMLLKGSTR), a motif of laminin-5 alpha3 chain, tethered to type I collagen, crosslinked using microbial transglutaminase (mTGase) was investigated. mTGase is an enzyme that initiates crosslinking by reacting with the glutamine and lysine residues on the collagen fibers stabilizing the molecular structure. In this study that tethering of the laminin peptide in a mTGase crosslinked collagen scaffold enhanced cell proliferation and attachment. Laminin peptide tethered crosslinked scaffold showed unaltered cell morphology of 3T3 fibroblasts when compared with collagen and crosslinked scaffold. The triple helical structure of collagen remained unaltered by the addition of laminin peptide. In addition a dose-dependent affinity of the laminin peptide towards collagen was seen. The degree of crosslinking was measured by amino acid analysis, differential scanning calorimeter and fourier transform infrared spectroscopy. Increased crosslinking was observed in mTGase crosslinked group. mTGase crosslinking showed higher shrinkage temperature. There was alteration in the fibrillar architecture due to the crosslinking activity of mTGase. Hence, the use of enzyme-mediated linking shows promise in tethering cell adhesive peptides through biodegradable scaffolds.

Critical stresses for cancer cell detachment in microchannels

We present experiments involving cancer cells adhering to microchannels, subjected to increasing shear stresses (0.1-30 Pa). Morphological studies were carried out at different shear stresses. Cells exhibit spreading patterns similar to those observed under static conditions, as long as the shear stress is not too high. At critical wall shear stresses (around 2-5 Pa), cell-substrate contact area decreases until detachment at the larger stresses. Critical shear stresses are found to be lower for higher confinements (i.e. smaller cell height to channel height ratio). Fluorescent techniques were used to locate focal adhesions (typically 1 lm(2) in size) under various shearing conditions, showing that cells increase the number of focal contacts in the region facing the flow. To analyze such data, we propose a model to determine the critical stress, resulting from the competition between hydrodynamic forces and the adhesive cell resistance. With this model, typical adhesive stresses exerted at each focal contact can be determined and are in agreement with previous works.

Computational modeling for cell spreading on a substrate mediated by specific interactions, long-range recruiting interactions, and diffusion of binders

A continuum model was proposed to study cell spreading on a flat substrate mediated by specific interaction, long-range recruiting interaction, and the diffusion of binders. Specific interactions between the mobile receptors embedded in the cell membrane and ligands coated on the substrate surface result in cell adhesion to the substrate surface. This receptor-ligand interaction was described by a chemical reaction equation. Long-range recruiting interactions between the receptors and the substrate were simplified by a traction-separation law. The governing equations and boundary conditions were formulated for the entire process of cell spreading and solved using a finite element scheme. Parametric studies were conducted to investigate the effect of system parameters on the cell spreading kinetics. It is shown that kinetic factors play an important role in cell adhesion and three regimes, that is, the binder reaction limited regime, long-range recruiting force-driven binder recruitment limited regime, and the concentration gradient-driven diffusion limited regime, were identified.

Cell-Matrix De-Adhesion Dynamics Reflect Contractile Mechanics

Measurement of the mechanical properties of single cells is of increasing interest both from a fundamental cell biological perspective and in the context of disease diagnostics. In this study, we show that tracking cell shape dynamics during trypsin-induced de-adhesion can serve as a simple but extremely useful tool for probing the contractility of adherent cells. When treated with trypsin, both SW13(-/-) epithelial cells and U373 MG glioma cells exhibit a brief lag period followed by a concerted retraction to a rounded shape. The time-response of the normalized cell area can be fit to a sigmoidal curve with two characteristic time constants that rise and fall when cells are treated with blebbistatin and nocodazole, respectively. These differences can be attributed to actomyosin-based cytoskeletal remodeling, as evidenced by the prominent buildup of stress fibers in nocodazole-treated SW13(-/-) cells, which are also two-fold stiffer than untreated cells. Similar results observed in U373 MG cells highlights the direct association between cell stiffness and the de-adhesion response. Faster de-adhesion is obtained with higher trypsin concentration, with nocodazole treatment further expediting the process and blebbistatin treatment blunting the response. A simple finite element model confirms that faster contraction is achieved with increased stiffness.

Cell traction forces direct fibronectin matrix assembly

Interactions between cells and the surrounding matrix are critical to the development and engineering of tissues. We have investigated the role of cell-derived traction forces in the assembly of extracellular matrix using what we believe is a novel assay that allows for simultaneous measurement of traction forces and fibronectin fibril growth at discrete cell-matrix attachment sites. NIH3T3 cells were plated onto arrays of deformable cantilever posts for 2-24 h. Data indicate that developing fibril orientation is guided by the direction of the traction force applied to that fibril. In addition, cells initially establish a spatial distribution of traction forces that is largest at the cell edge and decreases toward the cell center. This distribution progressively shifts from a predominantly peripheral pattern to a more uniform pattern as compressive strain at the cell perimeter decreases with time. The impact of these changes on fibrillogenesis was tested by treating cells with blebbistatin or calyculin A to tonically block or augment, respectively, myosin II activity. Both treatments blocked the inward translation of traction forces, the dissipation of compressive strain, and fibronectin fibrillogenesis over time. These data indicate that dynamic spatial and temporal changes in traction force and local strain may contribute to successful matrix assembly.

Fibronectins in vascular morphogenesis

Fibronectin is an extracellular matrix protein found only in vertebrate organisms containing endothelium-lined vasculature and is required for cardiovascular development in fish and mice. Fibronectin and its splice variants containing EIIIA and EIIIB domains are highly upregulated around newly developing vasculature during embryogenesis and in pathological conditions including atherosclerosis, cardiac hypertrophy, and tumorigenesis. However, their molecular roles in these processes are not entirely understood. We review genetic studies examining functions of fibronectin and its splice variants during embryonic cardiovascular development, and discuss potential roles of fibronectin in vascular disease and tumor angiogenesis.

Mechanotransduction - a field pulling together?

Mechanical stresses are ever present in the cellular environment, whether through external forces that are applied to tissues or endogenous forces that are generated within the active cytoskeleton. Despite the wide array of studies demonstrating that such forces affect cellular signaling and function, it remains unclear whether mechanotransduction in different contexts shares common mechanisms. Here, I discuss possible mechanisms by which applied forces, cell-generated forces and changes in substrate mechanics could exert changes in cell function through common mechanotransduction machinery. I draw from examples that are primarily focused on the role of adhesions in transducing mechanical forces. Based on this discussion, emerging themes arise that connect these different areas of inquiry and suggest multiple avenues for future studies.

The role of actomyosin contractility in the formation and dynamics of actin bundles during fibroblast spreading

We studied the process of formation of stress fibres and involvement of phosphorylation of myosin-II during spreading of Swiss 3T3 fibroblasts. In cells that were allowed to spread for 1 h on a glass surface, circular bundles of actin and myosin-II filament were present. At 2-3 h after the plating, cells showed a polygonal and polarized shape. The proportion of the cells having circular bundles was decreased, whereas that of the cells with straight bundles of actin filaments was increased. At 4 h after the plating, cells were completely polarized and stress fibres were present at the periphery and the dorsal and ventral surfaces of the cells. Thus, spreading cells possessed different forms of actomyosin bundles corresponding to the cell shape. In circular bundles and stress fibres, myosin regulatory light chains were diphosphorylated. Formation of circular bundles and stress fibres was suppressed after the treatment of the cells with Y-27632, a Rho-kinase inhibitor, or blebbistatin, a myosin-II inhibitor. In digitonin-extracted cells, circular bundles as well as stress fibres contracted following the addition of Mg-ATP. These results suggest that circular bundles are contractile structures containing actin and phosphorylated myosin-II filaments, and the formation of circular bundles is regulated by Rho-kinase.

Continuum modeling of forces in growing viscoelastic cytoskeletal networks

Mechanical properties of the living cell are important in cell movement, cell division, cancer development and cell signaling. There is considerable interest in measuring local mechanical properties of living materials and the living cytoskeleton using micromechanical techniques. However, living materials are constantly undergoing internal dynamics such as growth and remodeling. A modeling framework that combines mechanical deformations with cytoskeletal growth dynamics is necessary to describe cellular shape changes. The present paper develops a general finite deformation modeling approach that can treat the viscoelastic cytoskeleton. Given the growth dynamics in the cytoskeletal network and the relationship between deformation and stress, the shape of the network is computed in an incremental fashion. The growth dynamics of the cytoskeleton can be modeled as stress dependent. The result is a consistent treatment of overall cell deformation. The framework is applied to a growing 1-d bundle of actin filaments against an elastic cantilever, and a 2-d cell undergoing wave-like protrusion dynamics. In the latter example, mechanical forces on the cell adhesion are examined as a function of the protrusion dynamics.

Living microlens arrays

Both individual cells and sheets of cells exert traction forces on the substrate and these forces have been investigated using a wide range of methods. Here we compare the mechanical properties of fibroblasts and epithelial cells using a novel surface geometry. Living cells are added to a thin film of polystyrene [PS] attached to a substrate of crosslinked poly(dimethyl siloxane) [PDMS] microwells. The contractile nature of the cells attached to the surface and the compliance of the PDMS surface geometry allows the PS thin film to buckle, forming arrays of convex microlenses. The resulting curvature of the microlenses allows us to determine the applied strain of growing cell sheets. We report that a monolayer of epithelial cells exerts more stress on the substrate than fibroblasts and attribute this to the collective behavior of the epithelium. By subsequently adding different chemical triggers to the system, the contractile nature of the cells changes, thus modifying the focal length of the microlenses. Together, these findings demonstrate the importance of studying the mechanics of cell sheets and also introduce a new design paradigm for advanced materials, offering great promise for a range of applications.

Cell-cell mechanical communication through compliant substrates

The role of matrix mechanics on cell behavior is under intense investigation. Cells exert contractile forces on their matrix and the matrix elasticity can alter these forces and cell migratory behavior. However, little is known about the contribution of matrix mechanics and cell-generated forces to stable cell-cell contact and tissue formation. Using matrices of varying stiffness and measurements of endothelial cell migration and traction stresses, we find that cells can detect and respond to substrate strains created by the traction stresses of a neighboring cell, and that this response is dependent on matrix stiffness. Specifically, pairs of endothelial cells display hindered migration on gels with elasticity below 5500 Pa in comparison to individual cells, suggesting these cells sense each other through the matrix. We believe that these results show for the first time that matrix mechanics can foster tissue formation by altering the relative motion between cells, promoting the formation of cell-cell contacts. Moreover, our data indicate that cells have the ability to communicate mechanically through their matrix. These findings are critical for the understanding of cell-cell adhesion during tissue formation and disease progression, and for the design of biomaterials intended to support both cell-matrix and cell-cell adhesion.

Measurement of two-dimensional binding constants between cell-bound major histocompatibility complex and immobilized antibodies with an acoustic biosensor

Gaining insights into the dynamic processes of molecular interactions that mediate cell-substrate and cell-cell adhesion is of great significance in the understanding of numerous physiological processes driven by intercellular communication. Here, an acoustic-wave biosensor is used to study and characterize specific interactions between cell-bound membrane proteins and surface-immobilized ligands, using as a model system the binding of major histocompatibility complex class I HLA-A2 proteins to anti-HLA-A2 monoclonal antibodies. The energy of the acoustic signal, measured as amplitude change, was found to depend directly on the number of HLA-A2/antibody complexes formed on the device surface. Real-time acoustic data were used to monitor the surface binding of cell suspensions at a range of 6.0 x 10(4) to 6.0 x 10(5) cells mL(-1). Membrane interactions are governed by two-dimensional chemistry because of the molecules' confinement to the lipid bilayer. The two-dimensional kinetics and affinity constant of the HLA-A2/antibody interaction were calculated (k(a) = 1.15 x 10(-5) mum(2) s(-1) per molecule, k(d) = 2.07 x 10(-5) s(-1), and K(A) = 0.556 mum(2) per molecule, at 25 degrees C), based on a detailed acoustic data analysis. Results indicate that acoustic biosensors can emerge as a significant tool for probing and characterizing cell-membrane interactions in the immune system, and for fast and label-free screening of membrane molecules using whole cells.

Systematic variation of microtopography, surface chemistry and elastic modulus and the state dependent effect on endothelial cell alignment

We examined how variations in elastic modulus, surface chemistry and the height and spacing of micro-ridges interact and effect endothelial cell (EC) alignment. Specifically, we employed independent control of the surface properties in order to elucidate the relative importance of each factor. Polydimethylsiloxane elastomer (PDMSe) was fabricated with 1.5 or 5 microm tall, 5 microm spaced and 5, 10, or 20 microm wide ridge microtopographies. Elastic modulus was varied from 0.3, 1.0, 1.4, and 2.3 MPa by controlling oligomeric additives and crosslink density. Surface chemistry was left untreated, argon plasma treated, coated with fibronectin (Fn) or patterned with Fn tracks on flat PDMSe or the tops of micro-ridges. Primary porcine vascular ECs were cultured on the PDMSe substrates and nuclear form factor (NFF) was used to determine cell orientation relative to surface microtopography. Experimental results showed that microtopographical variation strongly altered EC alignment on Fn coated surfaces, but not on plasma treated surfaces. Interestingly, similar alignment was achieved with different orientation cues, either micropatterned chemistry (2D) or microtopography (3D). In total, the effect of varying one of the experimental parameters depended strongly on the state of the others, highlighting the need for multi-factor analysis of surface properties for applications where cells and tissue will contact synthetic materials.