Adhesion dynamics and durotaxis in migrating cells

Cell-Substrate Interactions Feedback to Direct Cell Migration along or against Morphological Polarization

In response to external stimuli, cells polarize morphologically into teardrop shapes prior to moving in the direction of their blunt leading edge through lamellipodia extension and retraction of the rear tip. This textbook description of cell migration implies that the initial polarization sets the direction of cell migration. Using microfabrication techniques to control cell morphologies and the direction of migration without gradients, we demonstrate that after polarization, lamelipodia extension and attachment can feedback to change and even reverse the initial morphological polarization. Cells do indeed migrate faster in the direction of their morphologically polarization. However, feedback from subsequent lamellipodia extension and attachment can be so powerful as to induce cells to reverse and migrate against their initial polarization, albeit at a slower speed. Constitutively active mutants of RhoA show that RhoA stimulates cell motility when cells are guided either along or against their initial polarization. Cdc42 activation and inhibition, which results in loss of directional motility during chemotaxis, only reduces the speed of migration without altering the directionality of migration on the micropatterns. These results reveal significant differences between substrate directed cell migration and that induced by chemotactic gradients.

A computational study of stress fiber-focal adhesion dynamics governing cell contractility

We apply a recently developed model of cytoskeletal force generation to study a cell's intrinsic contractility, as well as its response to external loading. The model is based on a nonequilibrium thermodynamic treatment of the mechanochemistry governing force in the stress fiber-focal adhesion system. Our computational study suggests that the mechanical coupling between the stress fibers and focal adhesions leads to a complex, dynamic, mechanochemical response. We collect the results in response maps whose regimes are distinguished by the initial geometry of the stress fiber-focal adhesion system, and by the external load on the cell. The results from our model connect qualitatively with recent studies on the force response of smooth muscle cells on arrays of polymeric microposts.

Induction of focal adhesions and motility in Drosophila S2 cells

In this study, normally immotile S2 cells are engineered to induce the formation of focal adhesions and cell motility by the transfection of a single gene encoding an integrin subunit. It is demonstrated that the focal adhesions recruit expected components and exhibit mechanosensitive behavior on integrin-ligand substrates of different stiffnesses.

Focal adhesions are dynamic structures that interact with the extracellular matrix on the cell exterior and actin filaments on the cell interior, enabling cells to adhere and crawl along surfaces. We describe a system for inducing the formation of focal adhesions in normally non–ECM-adherent, nonmotile Drosophila S2 cells. These focal adhesions contain the expected molecular markers such as talin, vinculin, and p130Cas, and they require talin for their formation. The S2 cells with induced focal adhesions also display a nonpolarized form of motility on vitronectin-coated substrates. Consistent with findings in mammalian cells, the degree of motility can be tuned by changing the stiffness of the substrate and was increased after the depletion of PAK3, a p21-activated kinase. A subset of nonmotile, nonpolarized cells also exhibited focal adhesions that rapidly assembled and disassembled around the cell perimeter. Such cooperative and dynamic fluctuations of focal adhesions were decreased by RNA interference (RNAi) depletion of myosin II and focal adhesion kinase, suggesting that this behavior requires force and focal adhesion maturation. These results demonstrate that S2 cells, a cell line that is well studied for cytoskeletal dynamics and readily amenable to protein manipulation by RNAi, can be used to study the assembly and dynamics of focal adhesions and mechanosensitive cell motility.

Probing cellular mechanoadaptation using cell-substrate de-adhesion dynamics: experiments and model

Physical properties of the extracellular matrix (ECM) are known to regulate cellular processes ranging from spreading to differentiation, with alterations in cell phenotype closely associated with changes in physical properties of cells themselves. When plated on substrates of varying stiffness, fibroblasts have been shown to exhibit stiffness matching property, wherein cell cortical stiffness increases in proportion to substrate stiffness up to 5 kPa, and subsequently saturates. Similar mechanoadaptation responses have also been observed in other cell types. Trypsin de-adhesion represents a simple experimental framework for probing the contractile mechanics of adherent cells, with de-adhesion timescales shown to scale inversely with cortical stiffness values. In this study, we combine experiments and computation in deciphering the influence of substrate properties in regulating de-adhesion dynamics of adherent cells. We first show that NIH 3T3 fibroblasts cultured on collagen-coated polyacrylamide hydrogels de-adhere faster on stiffer substrates. Using a simple computational model, we qualitatively show how substrate stiffness and cell-substrate bond breakage rate collectively influence de-adhesion timescales, and also obtain analytical expressions of de-adhesion timescales in certain regimes of the parameter space. Finally, by comparing stiffness-dependent experimental and computational de-adhesion responses, we show that faster de-adhesion on stiffer substrates arises due to force-dependent breakage of cell-matrix adhesions. In addition to illustrating the utility of employing trypsin de-adhesion as a biophysical tool for probing mechanoadaptation, our computational results highlight the collective interplay of substrate properties and bond breakage rate in setting de-adhesion timescales.

Quantification of magnetically induced changes in ECM local apparent stiffness

The stiffness of the extracellular matrix (ECM) is known to influence cell behavior. The ability to manipulate the stiffness of ECM has important implications in understanding how cells interact mechanically with their microenvironment. This article describes an approach to manipulating the stiffness ECM, whereby magnetic beads are embedded in the ECM through bioconjugation between the streptavidin-coated beads and the collagen fibers and then manipulated by an external magnetic field. It also reports both analytical results (obtained by formal modeling and numerical simulation) and statistically meaningful experimental results (obtained by atomic force microscopy) that demonstrate the effectiveness of this approach. These results clearly suggest the possibility of creating desired stiffness gradients in ECM in vitro to influence cell behavior.

Differential nuclear expression of Yap in basal epithelial cells across the cornea and substrates of differing stiffness

Corneal epithelium is maintained throughout life by well-orchestrated proliferation of limbal epithelial stem cells, followed by migration and maturation centripetally across the ocular surface. The present study sets out to explore the role tissue stiffness (compliance) may have in directing both differentiation and centripetal migration of limbal epithelial stem cells during homeostasis. For that, we analysed the localization of the Yes-associated protein (Yap), a transcriptional co-activator previously shown to mediate cellular response and mechanical stimuli. Using both models of ocular surface compliance and normal bovine corneas we evaluated the nuclear/cytoplasmic expression ratio of Yap. Expression levels within corneal epithelial cells were compared in situ between the limbus and central cornea, and in vitro between limbal epithelial stem cells expanded upon biomimetic collagen gels of increasing stiffness. Nuclear expression of Yap was shown to increase within the expanded cells upon substrates of increasing stiffness. Subsequently, Yap was used as a novel molecular probe to investigate the mechanical microenvironment within a normal ocular surface. The in situ localization of Yap was predominantly cytoplasmic within basal limbal epithelial cells and nuclear within basal central corneal epithelial cells. Furthermore, nuclear p63 expression was not co-localized with Yap in basal limbal epithelial cells. In conclusion, the current investigation provides new insights into the relationship between Yap and distinct cell populations across the ocular surface indicating that cells experience a different mechanical environment between the limbus and central cornea. A new hypothesis is put forward, in which centripetal differences in substrate stiffness drives the migration and differentiation of limbal epithelial stem cells, thus controlling corneal epithelium homeostasis.

Nonlinear finite element simulations of injuries with free boundaries: application to surgical wounds

Wound healing is a process driven by biochemical and mechanical variables in which a new tissue is synthesised to recover original tissue functionality. Wound morphology plays a crucial role in this process, as the skin behaviour is not uniform along different directions. In this work, we simulate the contraction of surgical wounds, which can be characterised as elongated and deep wounds. Because of the regularity of this morphology, we approximate the evolution of the wound through its cross section, adopting a plane strain hypothesis. This simplification reduces the complexity of the computational problem; while allows for a thorough analysis of the role of wound depth in the healing process, an aspect of medical and computational relevance that has not yet been addressed. To reproduce wound contraction, we consider the role of fibroblasts, myofibroblasts, collagen and a generic growth factor. The contraction phenomenon is driven by cell-generated forces. We postulate that these forces are adjusted to the mechanical environment of the tissue where cells are embedded through a mechanosensing and mechanotransduction mechanism. To solve the nonlinear problem, we use the finite element method (FEM) and an updated Lagrangian approach to represent the change in the geometry. To elucidate the role of wound depth and width on the contraction pattern and evolution of the involved species, we analyse different wound geometries with the same wound area. We find that deeper wounds contract less and reach a maximum contraction rate earlier than superficial wounds.

The mechanochemistry of cytoskeletal force generation

In this communication, we propose a model to study the non-equilibrium process by which actin stress fibers develop force in contractile cells. The emphasis here is on the non-equilibrium thermodynamics, which is necessary to address the mechanics as well as the chemistry of dynamic cell contractility. In this setting, we are able to develop a framework that relates (a) the dynamics of force generation within the cell and (b) the cell's response to external stimuli to the chemical processes occurring within the cell, as well as to the mechanics of linkage between the stress fibers, focal adhesions and extracellular matrix.

A cell-regulatory mechanism involving feedback between contraction and tissue formation guides wound healing progression

Wound healing is a process driven by cells. The ability of cells to sense mechanical stimuli from the extracellular matrix that surrounds them is used to regulate the forces that cells exert on the tissue. Stresses exerted by cells play a central role in wound contraction and have been broadly modelled. Traditionally, these stresses are assumed to be dependent on variables such as the extracellular matrix and cell or collagen densities. However, we postulate that cells are able to regulate the healing process through a mechanosensing mechanism regulated by the contraction that they exert. We propose that cells adjust the contraction level to determine the tissue functions regulating all main activities, such as proliferation, differentiation and matrix production. Hence, a closed-regulatory feedback loop is proposed between contraction and tissue formation. The model consists of a system of partial differential equations that simulates the evolution of fibroblasts, myofibroblasts, collagen and a generic growth factor, as well as the deformation of the extracellular matrix. This model is able to predict the wound healing outcome without requiring the addition of phenomenological laws to describe the time-dependent contraction evolution. We have reproduced two in vivo experiments to evaluate the predictive capacity of the model, and we conclude that there is feedback between the level of cell contraction and the tissue regenerated in the wound.

Initiating a regenerative response; cellular and molecular features of wound healing in the cnidarian Nematostella vectensis

Background

Wound healing is the first stage of a series of cellular events that are necessary to initiate a regenerative response. Defective wound healing can block regeneration even in animals with a high regenerative capacity. Understanding how signals generated during wound healing promote regeneration of lost structures is highly important, considering that virtually all animals have the ability to heal but many lack the ability to regenerate missing structures. Cnidarians are the phylogenetic sister taxa to bilaterians and are highly regenerative animals. To gain a greater understanding of how early animals generate a regenerative response, we examined the cellular and molecular components involved during wound healing in the anthozoan cnidarian Nematostella vectensis.

Results

Pharmacological inhibition of extracellular signal-regulated kinases (ERK) signaling blocks regeneration and wound healing in Nematostella. We characterized early and late wound healing events through genome-wide microarray analysis, quantitative PCR, and in situ hybridization to identify potential wound healing targets. We identified a number of genes directly related to the wound healing response in other animals (metalloproteinases, growth factors, transcription factors) and suggest that glycoproteins (mucins and uromodulin) play a key role in early wound healing events. This study also identified a novel cnidarian-specific gene, for a thiamine biosynthesis enzyme (vitamin B synthesis), that may have been incorporated into the genome by lateral gene transfer from bacteria and now functions during wound healing. Lastly, we suggest that ERK signaling is a shared element of the early wound response for animals with a high regenerative capacity.

Conclusions

This research describes the temporal events involved during Nematostella wound healing, and provides a foundation for comparative analysis with other regenerative and non-regenerative species. We have shown that the same genes that heal puncture wounds are also activated after oral-aboral bisection, indicating a clear link with the initiation of regenerative healing. This study demonstrates the strength of using a forward approach (microarray) to characterize a developmental phenomenon (wound healing) at a phylogenetically important crossroad of animal evolution (cnidarian-bilaterian ancestor). Accumulation of data on the early wound healing events across numerous systems may provide clues as to why some animals have limited regenerative abilities.

Biased migration of confined neutrophil-like cells in asymmetric hydraulic environments

Cells integrate multiple measurement modalities to navigate their environment. Soluble and substrate-bound chemical gradients and physical cues have all been shown to influence cell orientation and migration. Here we investigate the role of asymmetric hydraulic pressure in directional sensing. Cells confined in microchannels identified and chose a path of lower hydraulic resistance in the absence of chemical cues. In a bifurcating channel with asymmetric hydraulic resistances, this choice was preceded by the elaboration of two leading edges with a faster extension rate along the lower resistance channel. Retraction of the "losing" edge appeared to precipitate a final choice of direction. The pressure differences altering leading edge protrusion rates were small, suggesting weak force generation by leading edges. The response to the physical asymmetry was able to override a dynamically generated chemical cue. Motile cells may use this bias as a result of hydraulic resistance, or "barotaxis," in concert with chemotaxis to navigate complex environments.

Characterization of uniaxial stiffness of extracellular matrix embedded with magnetic beads via bio-conjugation and under the influence of an external magnetic field

In this paper, we study the deformation, and experimentally quantify the change in stiffness, of an extracellular matrix (ECM) embedded with magnetic beads that are bio-conjugated with the collagen fibers and under the influence of an external magnetic field. We develop an analytical model of the viscoelastic behavior of this modified ECM, and design and implement a stretch test to quantify (based on statistically meaningful experiment data) the resulting changes in its stiffness induced by the external magnetic field. The analytical results are in close agreement with that obtained from the experiments. We discuss the implication of these results that point to the possibility of creating desired stiffness gradients in an ECM in vitro to influence cell behavior.

Impact of cell shape on cell migration behavior on elastic substrate

Cell shape is known to have profound effects on a number of cell behaviors. In this paper we have studied its role in cell migration through modeling the effect of cell shape on the cell traction force distribution, the traction force dependent stability of cell adhesion and the matrix rigidity dependent traction force formation. To quantify the driving force of cell migration, a new parameter called the motility factor, that takes account of the effect of cell shape, matrix rigidity and dynamic stability of cell adhesion, is proposed. We showed that the motility factor depends on the matrix rigidity in a biphasic manner, which is consistent with the experimental observations of the biphasic dependence of cell migration speed on the matrix rigidity. We showed that the cell shape plays a pivotal role in the cell migration behavior by regulating the traction force at the cell front and rear. The larger the cell polarity, the larger the motility factor is. The keratocyte-like shape has a larger motility factor than the fibroblast-like shape, which explains why keratocyte has a much higher migration speed. The motility factor might be an appropriate parameter for a quantitative description of the driving force of cell migration.

How linear tension converts to curvature: geometric control of bone tissue growth

This study investigated how substrate geometry influences in-vitro tissue formation at length scales much larger than a single cell. Two-millimetre thick hydroxyapatite plates containing circular pores and semi-circular channels of 0.5 mm radius, mimicking osteons and hemi-osteons respectively, were incubated with MC3T3-E1 cells for 4 weeks. The amount and shape of the tissue formed in the pores, as measured using phase contrast microscopy, depended on the substrate geometry. It was further demonstrated, using a simple geometric model, that the observed curvature-controlled growth can be derived from the assembly of tensile elements on a curved substrate. These tensile elements are cells anchored on distant points of the curved surface, thus creating an actin “chord” by generating tension between the adhesion sites. Such a chord model was used to link the shape of the substrate to cell organisation and tissue patterning. In a pore with a circular cross-section, tissue growth increases the average curvature of the surface, whereas a semi-circular channel tends to be flattened out. Thereby, a single mechanism could describe new tissue growth in both cortical and trabecular bone after resorption due to remodelling. These similarities between in-vitro and in-vivo patterns suggest geometry as an important signal for bone remodelling.

Mechanosensing can result from adhesion molecule dynamics

When plated onto a substrate, cells spread. Many cell types, including smooth muscle cells and fibroblasts, spread more effectively on stiffer substrates, and a simple empirical relationship has been determined that relates a cell's spread area to the substrate stiffness. In addition, some crawling cells when plated onto a stiff substrate will not migrate onto soft substrate, a process called "durotaxis". Here we show that the resistive force predicted previously for dynamic adhesion molecules can account for both of these mechanosensory effects.