Cell adhesion strength from cortical tension - an integration of concepts

Characterizing Inner Pressure and Stiffness of Trophoblast and Inner Cell Mass of Blastocysts

It has long been recognized that mechanical forces underlie mammalian embryonic shape changes. Before gastrulation, the blastocyst embryo undergoes significant shape changes, namely, the blastocyst cavity emerges and expands, and the inner cell mass (ICM) forms and changes in shape. The embryo's inner pressure has been hypothesized to be the driving mechanical input that causes the expansion of the blastocyst cavity and the shape changes of the ICM. However, how the inner pressure and the mechanics of the trophoblast and the ICM change during development is unknown because of the lack of a suitable tool for quantitative characterization. This work presents a laser-assisted magnetic tweezer technique for measuring the inner pressure and Young's modulus of the trophoblast and ICM of the blastocyst-stage mouse embryo. The results quantitatively showed that the inner pressure and Young's modulus of the trophoblast and ICM all increase during progression of mouse blastocysts, providing useful data for understanding how mechanical factors are physiologically integrated with other cues to direct embryo development.

Getting in shape and swimming: the role of cortical forces and membrane heterogeneity in eukaryotic cells

Recent research has shown that motile cells can adapt their mode of propulsion to the mechanical properties of the environment in which they find themselves-crawling in some environments while swimming in others. The latter can involve movement by blebbing or other cyclic shape changes, and both highly-simplified and more realistic models of these modes have been studied previously. Herein we study swimming that is driven by membrane tension gradients that arise from flows in the actin cortex underlying the membrane, and does not involve imposed cyclic shape changes. Such gradients can lead to a number of different characteristic cell shapes, and our first objective is to understand how different distributions of membrane tension influence the shape of cells in an inviscid quiescent fluid. We then analyze the effects of spatial variation in other membrane properties, and how they interact with tension gradients to determine the shape. We also study the effect of fluid-cell interactions and show how tension leads to cell movement, how the balance between tension gradients and a variable bending modulus determine the shape and direction of movement, and how the efficiency of movement depends on the properties of the fluid and the distribution of tension and bending modulus in the membrane.

Cell segregation and border sharpening by Eph receptor-ephrin-mediated heterotypic repulsion

Eph receptor and ephrin signalling has a major role in cell segregation and border formation, and may act through regulation of cell adhesion, repulsion or tension. To elucidate roles of cell repulsion and adhesion, we combined experiments in cell culture assays with quantitations of cell behaviour which are used in computer simulations. Cells expressing EphB2, or kinase-inactive EphB2 (kiEphB2), segregate and form a sharp border with ephrinB1-expressing cells, and this is disrupted by knockdown of N-cadherin. Measurements of contact inhibition of locomotion reveal that EphB2-, kiEphB2- and ephrinB1-expressing cells have strong heterotypic and weak homotypic repulsion. EphB2 cells have a transient increase in migration after heterotypic activation, which underlies a shift in the EphB2–ephrinB1 border but is not required for segregation or border sharpening. Simulations with the measured values of cell behaviour reveal that heterotypic repulsion can account for cell segregation and border sharpening, and is more efficient than decreased heterotypic adhesion. By suppressing homotypic repulsion, N-cadherin creates a sufficient difference between heterotypic and homotypic repulsion, and enables homotypic cohesion, both of which are required to sharpen borders.

A cold plasma jet accelerates wound healing in a murine model of full-thickness skin wounds

Cold plasma has been successfully applied in several fields of medicine that require, for example, pathogen inactivation, implant functionalization or alteration of cellular activity. Previous studies have provided evidence that plasma supports the healing of wounds owing to its beneficial mixtures of reactive species and modulation of inflammation in cells and tissues. To investigate the wound healing activity of an atmospheric pressure plasma jet in vivo, we examined the cold plasma's efficacy on dermal regeneration in a murine model of dermal full-thickness ear wound. Over 14 days, female mice received daily plasma treatment. Quantitative analysis by transmitted light microscopy demonstrated a significantly accelerated wound re-epithelialization at days 3-9 in comparison with untreated controls. In vitro, cold plasma altered keratinocyte and fibroblast migration, while both cell types showed significant stimulation resulting in accelerated closure of gaps in scratch assays. This plasma effect correlated with the downregulation of the gap junctional protein connexin 43 which is thought to be important in the regulation of wound healing. In addition, plasma induced profound changes in adherence junctions and cytoskeletal dynamics as shown by downregulation of E-cadherin and several integrins as well as actin reorganization. Our results theorize cold plasma to be a beneficial treatment option supplementing existing wound therapies.

Adhesion forces and cortical tension couple cell proliferation and differentiation to drive epidermal stratification

To establish and maintain organ structure and function, tissues need to balance stem cell proliferation and differentiation rates and coordinate cell fate with position. By quantifying and modelling tissue stress and deformation in the mammalian epidermis, we find that this balance is coordinated through local mechanical forces generated by cell division and delamination. Proliferation within the basal stem/progenitor layer, which displays features of a jammed, solid-like state, leads to crowding, thereby locally distorting cell shape and stress distribution. The resulting decrease in cortical tension and increased cell-cell adhesion trigger differentiation and subsequent delamination, reinstating basal cell layer density. After delamination, cells establish a high-tension state as they increase myosin II activity and convert to E-cadherin-dominated adhesion, thereby reinforcing the boundary between basal and suprabasal layers. Our results uncover how biomechanical signalling integrates single-cell behaviours to couple proliferation, cell fate and positioning to generate a multilayered tissue.

An asymmetric attraction model for the diversity and robustness of cell arrangement in nematodes

During early embryogenesis in animals, cells are arranged into a species-specific pattern in a robust manner. Diverse cell arrangement patterns are observed, even among close relatives. In the present study, we evaluated the mechanisms by which the diversity and robustness of cell arrangements are achieved in developing embryos. We successfully reproduced various patterns of cell arrangements observed in various nematode species in Caenorhabditis elegans embryos by altering the eggshell shapes. The findings suggest that the observed diversity of cell arrangements can be explained by differences in the eggshell shape. Additionally, we found that the cell arrangement was robust against eggshell deformation. Computational modeling revealed that, in addition to repulsive forces, attractive forces are sufficient to achieve such robustness. The present model is also capable of simulating the effect of changing cell division orientation. Genetic perturbation experiments demonstrated that attractive forces derived from cell adhesion are necessary for the robustness. The proposed model accounts for both diversity and robustness of cell arrangements, and contributes to our understanding of how the diversity and robustness of cell arrangements are achieved in developing embryos.

Summary: The cell arrangement pattern at the four-cell stage in nematode embryos is both diverse and robust. A numerical model that incorporates attractive, as well as repulsive, forces was constructed to explain this diversity and robustness.

Single-Cell Defects Cause a Long-Range Mechanical Response in a Confluent Epithelial Cell Layer

Epithelial cells are responsible for tissue homeostasis and form a barrier to maintain chemical gradients and mechanical integrity. Therefore, rapid wound closure is crucial for proper tissue function and restoring homeostasis. In this study, the mechanical properties of cells surrounding a single-cell wound are investigated during closure of the defect. The single-cell wound is induced in an intact layer using micropipette action and responses in neighboring cells are monitored with atomic force microscopy. Direct neighbors reveal a rise in the apparent pretension, which is dominated by cortical tension. The same effect was observed for a single-cell wound induced by laser ablation and during closure of a not fully confluent layer. Moreover, changes in the apparent pretension are far reaching and persist even in cells separated by three cell widths from the defect. This shows that epithelial cells respond to minimal wounds in a collective fashion by increased contractility with substantial reach.

Mechanics of Fluid-Filled Interstitial Gaps. I. Modeling Gaps in a Compact Tissue

Fluid-filled interstitial gaps are a common feature of compact tissues held together by cell-cell adhesion. Although such gaps can in principle be the result of weak, incomplete cell attachment, adhesion is usually too strong for this to occur. Using a mechanical model of tissue cohesion, we show that, instead, a combination of local prevention of cell adhesion at three-cell junctions by fluidlike extracellular material and a reduction of cortical tension at the gap surface are sufficient to generate stable gaps. The size and shape of these interstitial gaps depends on the mechanical tensions between cells and at gap surfaces, and on the difference between intracellular and interstitial pressures that is related to the volume of the interstitial fluid. As a consequence of the dependence on tension/tension ratios, the presence of gaps does not depend on the absolute strength of cell adhesion, and similar gaps are predicted to occur in tissues of widely differing cohesion. Tissue mechanical parameters can also vary within and between cells of a given tissue, generating asymmetrical gaps. Within limits, these can be approximated by symmetrical gaps.

Mechanics of Fluid-Filled Interstitial Gaps. II. Gap Characteristics in Xenopus Embryonic Ectoderm

The ectoderm of the Xenopus embryo is permeated by a network of channels that appear in histological sections as interstitial gaps. We characterized this interstitial space by measuring gap sizes, angles formed between adjacent cells, and curvatures of cell surfaces at gaps. From these parameters, and from surface-tension values measured previously, we estimated the values of critical mechanical variables that determine gap sizes and shapes in the ectoderm, using a general model of interstitial gap mechanics. We concluded that gaps of 1-4 μm side length can be formed by the insertion of extracellular matrix fluid at three-cell junctions such that cell adhesion is locally disrupted and a tension difference between cell-cell contacts and the free cell surface at gaps of 0.003 mJ/m² is generated. Furthermore, a cell hydrostatic pressure of 16.8 ± 1.7 Pa and an interstitial pressure of 3.9 ± 3.6 Pa, relative to the central blastocoel cavity of the embryo, was found to be consistent with the observed gap size and shape distribution. Reduction of cell adhesion by the knockdown of C-cadherin increased gap volume while leaving intracellular and interstitial pressures essentially unchanged. In both normal and adhesion-reduced ectoderm, cortical tension of the free cell surfaces at gaps does not return to the high values characteristic of the free surface of the whole tissue.

Ingression-type cell migration drives vegetal endoderm internalisation in the Xenopus gastrula

During amphibian gastrulation, presumptive endoderm is internalised as part of vegetal rotation, a large-scale movement that encompasses the whole vegetal half of the embryo. It has been considered a gastrulation process unique to amphibians, but we show that at the cell level, endoderm internalisation exhibits characteristics reminiscent of bottle cell formation and ingression, known mechanisms of germ layer internalisation. During ingression proper, cells leave a single-layered epithelium. In vegetal rotation, the process occurs in a multilayered cell mass; we refer to it as ingression-type cell migration. Endoderm cells move by amoeboid shape changes, but in contrast to other instances of amoeboid migration, trailing edge retraction involves ephrinB1-dependent macropinocytosis and trans-endocytosis. Moreover, although cells are separated by wide gaps, they are connected by filiform protrusions, and their migration depends on C-cadherin and the matrix protein fibronectin. Cells move in the same direction but at different velocities, to rearrange by differential migration.

Sorting at embryonic boundaries requires high heterotypic interfacial tension

The establishment of sharp boundaries is essential for segregation of embryonic tissues during development, but the underlying mechanism of cell sorting has remained unclear. Opposing hypotheses have been proposed, either based on global tissue adhesive or contractile properties or on local signalling through cell contact cues. Here we use ectoderm-mesoderm separation in Xenopus to directly evaluate the role of these various parameters. We find that ephrin-Eph-based repulsion is very effective at inducing and maintaining separation, whereas differences in adhesion or contractility have surprisingly little impact. Computer simulations support and generalise our experimental results, showing that a high heterotypic interfacial tension between tissues is key to their segregation. We propose a unifying model, in which conditions of sorting previously considered as driven by differential adhesion/tension should be viewed as suboptimal cases of heterotypic interfacial tension.The mechanisms that cause different cells to segregate into distinct tissues are unclear. Here the authors show in Xenopus that formation of a boundary between two tissues is driven by local tension along the interface rather than by global differences in adhesion or cortical contractility.

Cigarette smoke disrupts monolayer integrity by altering epithelial cell-cell adhesion and cortical tension

Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality. Cigarette smoke (CS) drives disease development and progression. The epithelial barrier is damaged by CS with increased monolayer permeability. However, the molecular changes that cause this barrier disruption and the interaction between adhesion proteins and the cytoskeleton are not well defined. We hypothesized that CS alters monolayer integrity by increasing cell contractility and decreasing cell adhesion in epithelia. Normal human airway epithelial cells and primary COPD epithelial cells were exposed to air or CS, and changes measured in protein levels. We measured the cortical tension of individual cells and the stiffness of cells in a monolayer. We confirmed that the changes in acute and subacute in vitro smoke exposure reflect protein changes seen in cell monolayers and tissue sections from COPD patients. Epithelial cells exposed to repetitive CS and those derived from COPD patients have increased monolayer permeability. E-cadherin and β-catenin were reduced in smoke exposed cells as well as in lung tissue sections from patients with COPD. Moreover, repetitive CS caused increased tension in individual cells and cells in a monolayer, which corresponded with increased polymerized actin without changes in myosin IIA and IIB total abundance. Repetitive CS exposure impacts the adhesive intercellular junctions and the tension of epithelial cells by increased actin polymer levels, to further destabilize cell adhesion. Similar changes are seen in epithelial cells from COPD patients indicating that these findings likely contribute to COPD pathology.

Patterned cortical tension mediated by N-cadherin controls cell geometric order in the Drosophila eye

Adhesion molecules hold cells together but also couple cell membranes to a contractile actomyosin network, which limits the expansion of cell contacts. Despite their fundamental role in tissue morphogenesis and tissue homeostasis, how adhesion molecules control cell shapes and cell patterns in tissues remains unclear. Here we address this question in vivo using the Drosophila eye. We show that cone cell shapes depend little on adhesion bonds and mostly on contractile forces. However, N-cadherin has an indirect control on cell shape. At homotypic contacts, junctional N-cadherin bonds downregulate Myosin-II contractility. At heterotypic contacts with E-cadherin, unbound N-cadherin induces an asymmetric accumulation of Myosin-II, which leads to a highly contractile cell interface. Such differential regulation of contractility is essential for morphogenesis as loss of N-cadherin disrupts cell rearrangements. Our results establish a quantitative link between adhesion and contractility and reveal an unprecedented role of N-cadherin on cell shapes and cell arrangements.

DOI:http://dx.doi.org/10.7554/eLife.22796.001

A new strategy to measure intercellular adhesion forces in mature cell-cell contacts

Intercellular adhesion plays a major role in tissue development and homeostasis. Yet, technologies to measure mature cell-cell contacts are not available. We introduce a methodology based on fluidic probe force microscopy to assess cell-cell adhesion forces after formation of mature intercellular contacts in cell monolayers. With this method we quantify that L929 fibroblasts exhibit negligible cell-cell adhesion in monolayers whereas human endothelial cells from the umbilical artery (HUAECs) exert strong intercellular adhesion forces per cell. We use a new in vitro model based on the overexpression of Muscle Segment Homeobox 1 (MSX1) to induce Endothelial-to-Mesenchymal Transition (EndMT), a process involved in cardiovascular development and disease. We reveal how intercellular adhesion forces in monolayer decrease significantly at an early stage of EndMT and we show that cells undergo stiffening and flattening at this stage. This new biomechanical insight complements and expands the established standard biomolecular analyses. Our study thus introduces a novel tool for the assessment of mature intercellular adhesion forces in a physiological setting that will be of relevance to biological processes in developmental biology, tissue regeneration and diseases like cancer and fibrosis.

Modeling the two-way feedback between contractility and matrix realignment reveals a nonlinear mode of cancer cell invasion

Cancer cell invasion from primary tumors is mediated by a complex interplay between cellular adhesions, actomyosin-driven contractility, and the physical characteristics of the extracellular matrix (ECM). Here, we incorporate a mechanochemical free-energy-based approach to elucidate how the two-way feedback loop between cell contractility (induced by the activity of chemomechanical interactions such as Ca2+ and Rho signaling pathways) and matrix fiber realignment and strain stiffening enables the cells to polarize and develop contractile forces to break free from the tumor spheroids and invade into the ECM. Interestingly, through this computational model, we are able to identify a critical stiffness that is required by the matrix to break intercellular adhesions and initiate cell invasion. Also, by considering the kinetics of the cell movement, our model predicts a biphasic invasiveness with respect to the stiffness of the matrix. These predictions are validated by analyzing the invasion of melanoma cells in collagen matrices of varying concentration. Our model also predicts a positive correlation between the elongated morphology of the invading cells and the alignment of fibers in the matrix, suggesting that cell polarization is directly proportional to the stiffness and alignment of the matrix. In contrast, cells in nonfibrous matrices are found to be rounded and not polarized, underscoring the key role played by the nonlinear mechanics of fibrous matrices. Importantly, our model shows that mechanical principles mediated by the contractility of the cells and the nonlinearity of the ECM behavior play a crucial role in determining the phenotype of the cell invasion.

The biomechanical properties of an epithelial tissue determine the location of its vasculature

An important question is how growing tissues establish a blood vessel network. Here we study vascular network formation in pancreatic islets, endocrine tissues derived from pancreatic epithelium. We find that depletion of integrin-linked kinase (ILK) in the pancreatic epithelial cells of mice results in glucose intolerance due to a loss of the intra-islet vasculature. In turn, blood vessels accumulate at the islet periphery. Neither alterations in endothelial cell proliferation, apoptosis, morphology, Vegfa expression and VEGF-A secretion nor ‘empty sleeves' of vascular basement membrane are found. Instead, biophysical experiments reveal that the biomechanical properties of pancreatic islet cells, such as their actomyosin-mediated cortex tension and adhesive forces to endothelial cells, are significantly changed. These results suggest that a sorting event is driving the segregation of endothelial and epithelial cells and indicate that the epithelial biomechanical properties determine whether the blood vasculature invades or envelops a growing epithelial tissue.

Vasculature is denser in soft than in stiff tissues. Kragl et al. suggest a mechanistic link between biomechanical tissue properties and vascularization by showing that integrin-linked kinase reduces the contractile forces of the cell cortex in endocrine pancreatic cells, facilitating their adhesion to blood vessels and enabling pancreatic islet vascularization.

The Expression of HMGB1 in Bone Marrow MSCs Is Upregulated by Hypoxia with Regulatory Effects on the Apoptosis and Adhesion

Background and Aims. Hypoxia regulates the survival of mesenchymal stem cells (MSCs) but the mechanism is unclear. In hypoxia, the level of high mobility group box 1 (HMGB1) was increased in many cells which may be involved in the regulation of cell biology. The aim is to determine whether hypoxia affects the expression of HMGB1 in bone marrow MSCs (BM-MSCs) and to investigate the role of HMGB1 in the apoptosis and adhesion. Methods. BM-MSCs were exposed to hypoxia (1% O₂) and normoxia (20% O₂) and the expression of HMGB1 was measured by RT-PCR and western blotting. The apoptosis and adhesion of BM-MSCs were evaluated after interfered by different concentrations of HMGB1. Results. Expression of HMGB1 in BM-MSCs showed a significant upregulation in hypoxia when compared to those in normoxia. The adhesion of BM-MSCs was increased by HMGB1 in a concentration-dependent manner; the apoptosis effect of HMGB1 depended on its concentrations: HMGB1 at low concentration (50 ng/mL) promoted the apoptosis of BM-MSCs while HMGB1 at high concentration (≥100 ng/mL) reduced this apoptosis. Conclusions. Hypoxia enhanced the expression of HMGB1 in BM-MSCs with influences on apoptosis and adhesion and this could have a significant effect on the regenerative potential of MSC-based strategies.

The mechanical properties of a cell-based numerical model of epithelium

In this work we use a computational cell-based model to study the influence of the mechanical properties of cells on the mechanics of epithelial tissues. We analyze the effect of the model parameters on the elasticity and the mechanical response of tissues subjected to stress loading application. We compare our numerical results with experimental measurements of epithelial cell monolayer mechanics. Unlike previous studies, we have been able to estimate in physical units the parameter values that match the experimental results. A key observation is that the model parameters must vary with the tissue strain. In particular, it was found that, while the perimeter contractility and the area elasticity of cells remain constant at lower strains (<20%), they must increase to respond to larger strains (>20%). However, above a threshold of 50% extension, the cells stop counteracting the tissue strain and reduce both their perimeter contractility and area elasticity.