Mechanics of epithelial closure over non-adherent environments

Wound Healing Coordinates Actin Architectures to Regulate Mechanical Work

How cells with diverse morphologies and cytoskeletal architectures modulate their mechanical behaviors to drive robust collective motion within tissues is poorly understood. During wound repair within epithelial monolayers in vitro, cells coordinate the assembly of branched and bundled actin networks to regulate the total mechanical work produced by collective cell motion. Using traction force microscopy, we show that the balance of actin network architectures optimizes the wound closure rate and the magnitude of the mechanical work. These values are constrained by the effective power exerted by the monolayer, which is conserved and independent of actin architectures. Using a cell-based physical model, we show that the rate at which mechanical work is done by the monolayer is limited by the transformation between actin network architectures and differential regulation of cell-substrate friction. These results and our proposed mechanisms provide a robust physical model for how cells collectively coordinate their non-equilibrium behaviors to dynamically regulate tissue-scale mechanical output.

Mechanical forces in cell monolayers

In various physiological processes, the cell collective is organized in a monolayer, such as seen in a simple epithelium. The advances in the understanding of mechanical behavior of the monolayer and its underlying cellular and molecular mechanisms will help to elucidate the properties of cell collectives. In this Review, we discuss recent in vitro studies on monolayer mechanics and their implications on collective dynamics, regulation of monolayer mechanics by physical confinement and geometrical cues and the effect of tissue mechanics on biological processes, such as cell division and extrusion. In particular, we focus on the active nematic property of cell monolayers and the emerging approach to view biological systems in the light of liquid crystal theory. We also highlight the mechanosensing and mechanotransduction mechanisms at the sub-cellular and molecular level that are mediated by the contractile actomyosin cytoskeleton and cell-cell adhesion proteins, such as E-cadherin and α-catenin. To conclude, we argue that, in order to have a holistic understanding of the cellular response to biophysical environments, interdisciplinary approaches and multiple techniques - from large-scale traction force measurements to molecular force protein sensors - must be employed.

Geometric Effect for Biological Reactors and Biological Fluids

As expressed "God made the bulk; the surface was invented by the devil" by W. Pauli, the surface has remarkable properties because broken symmetry in surface alters the material properties. In biological systems, the smallest functional and structural unit, which has a functional bulk space enclosed by a thin interface, is a cell. Cells contain inner cytosolic soup in which genetic information stored in DNA can be expressed through transcription (TX) and translation (TL). The exploration of cell-sized confinement has been recently investigated by using micron-scale droplets and microfluidic devices. In the first part of this review article, we describe recent developments of cell-free bioreactors where bacterial TX-TL machinery and DNA are encapsulated in these cell-sized compartments. Since synthetic biology and microfluidics meet toward the bottom-up assembly of cell-free bioreactors, the interplay between cellular geometry and TX-TL advances better control of biological structure and dynamics in vitro system. Furthermore, biological systems that show self-organization in confined space are not limited to a single cell, but are also involved in the collective behavior of motile cells, named active matter. In the second part, we describe recent studies where collectively ordered patterns of active matter, from bacterial suspensions to active cytoskeleton, are self-organized. Since geometry and topology are vital concepts to understand the ordered phase of active matter, a microfluidic device with designed compartments allows one to explore geometric principles behind self-organization across the molecular scale to cellular scale. Finally, we discuss the future perspectives of a microfluidic approach to explore the further understanding of biological systems from geometric and topological aspects.

Active superelasticity in three-dimensional epithelia of controlled shape

Fundamental biological processes are carried out by curved epithelial sheets that enclose a pressurized lumen. How these sheets develop and withstand three-dimensional deformations has remained unclear. Here we combine measurements of epithelial tension and shape with theoretical modelling to show that epithelial sheets are active superelastic materials. We produce arrays of epithelial domes with controlled geometry. Quantification of luminal pressure and epithelial tension reveals a tensional plateau over several-fold areal strains. These extreme strains in the tissue are accommodated by highly heterogeneous strains at a cellular level, in seeming contradiction to the measured tensional uniformity. This phenomenon is reminiscent of superelasticity, a behaviour that is generally attributed to microscopic material instabilities in metal alloys. We show that in epithelial cells this instability is triggered by a stretch-induced dilution of the actin cortex, and is rescued by the intermediate filament network. Our study reveals a type of mechanical behaviour-which we term active superelasticity-that enables epithelial sheets to sustain extreme stretching under constant tension.

Cooperation of dual modes of cell motility promotes epithelial stress relaxation to accelerate wound healing

Collective cell migration in cohesive units is vital for tissue morphogenesis, wound repair, and immune response. While the fundamental driving forces for collective cell motion stem from contractile and protrusive activities of individual cells, it remains unknown how their balance is optimized to maintain tissue cohesiveness and the fluidity for motion. Here we present a cell-based computational model for collective cell migration during wound healing that incorporates mechanochemical coupling of cell motion and adhesion kinetics with stochastic transformation of active motility forces. We show that a balance of protrusive motility and actomyosin contractility is optimized for accelerating the rate of wound repair, which is robust to variations in cell and substrate mechanical properties. This balance underlies rapid collective cell motion during wound healing, resulting from a tradeoff between tension mediated collective cell guidance and active stress relaxation in the tissue.

Pressure-driven collective growth mechanism of planar cell colonies

The growth of cell colonies is determined by the migration and proliferation of the individual cells. This is often modeled with the Fisher-Kolmogorov (FK) equation, which assumes that cells diffuse independently from each other, but stop to proliferate when their density reaches a critial limit. However, when using measured, cell-line specific parameters, we find that the FK equation drastically underestimates the experimentally observed increase of colony radius with time. Moreover, cells in real colonies migrate radially outward with superdiffusive trajectories, in contrast to the assumption of random diffusion. We demonstrate that both dicrepancies can be resolved by assuming that cells in dense colonies are driven apart by repulsive, pressure-like forces. Using this model of proliferating repelling particles (PRP), we find that colony growth exhibits different dynamical regimes, depending on the ratio between a pressure-related equilibrium cell density and the critial density of proliferation arrest.

Electric field-induced migration and intercellular stress alignment in a collective epithelial monolayer

During wound healing, cells migrate with electrotactic bias as a collective entity. Unlike the case of the electric field (EF)-induced single-cell migration, the sensitivity of electrotactic response of the monolayer depends primarily on the integrity of the cell–cell junctions. Although there exist biochemical clues on how cells sense the EF, a well-defined physical portrait to illustrate how collective cells respond to directional EF remains elusive. Here, we developed an EF stimulating system integrated with a hydrogel-based traction measurement platform to quantify the EF-induced changes in cellular tractions, from which the complete in-plane intercellular stress tensor can be calculated. We chose immortalized human keratinocytes, HaCaT, as our model cells to investigate the role of EF in epithelial migration during wound healing. Immediately after the onset of EF (0.5 V/cm), the HaCaT monolayer migrated toward anode with ordered directedness and enhanced speed as early as 15 min. Cellular traction and intercellular stresses were gradually aligned perpendicular to the direction of the EF until 50 min. The EF-­induced reorientation of physical stresses was then followed by the delayed cell-body reorientation in the direction perpendicular to the EF. Once the intercellular stresses were aligned, the reversal of the EF direction redirected the reversed migration of the cells without any apparent disruption of the intercellular stresses. The results suggest that the dislodging of the physical stress alignment along the adjacent cells should not be necessary for changing the direction of the monolayer migration.

Theoretical Analysis of Stress Distribution and Cell Polarization Surrounding a Model Wound

A growing amount of experimental evidence shows that the local elastic field acting on cells governs their spatial organization and polarity in a tissue. Interestingly, experiments on wound healing reveal a universal formation of thick actomyosin bundles around the margins of epithelial gaps. Although the forces involved in this process have been measured, the mechanisms governing cellular alignment and contractile ring formation are still not fully understood. To theoretically investigate this process, we have carried out a self-consistent calculation of the elastic field that is actively generated around a circular gap in a contractile cell monolayer that is adhered to an elastic substrate, taking into account the responsiveness of actomyosin activity to the locally generated stress. We model actomyosin contractility by a radial distribution of point force dipoles that may alter in magnitude and orientation in response to the local elastic stress. In addition, the model takes into account the forces exerted by leader cells on the margins of the cell monolayer. Our model suggests that the presence of a hole in the center of a contractile cell monolayer creates a mechanical tendency for actomyosin forces to polarize tangentially around the hole margin. In addition, it predicts that this tendency optimizes with substrate rigidity, thickness, and strength of cell adhesion to the substrate. Our calculations support the view that the universal formation of a peripheral contractile ring is a consequence of actomyosin contractility in the bulk and its inherent responsiveness to the local stress.

Role of the supracellular actomyosin cable during epithelial wound healing

The closure of wounds in epithelia is center to many physiological processes in both development and repair of multicellular organisms. Depending on the biochemical and mechanical environment as well as cell type, this collective cellular movement often involves coordinated cell crawling and the purse-string contraction of a supracellular actomyosin ring around the wound. However, it remains uncertain how these two mechanisms cooperatively contribute to the wound healing, and especially the role of the ring is not clear. To decipher this complex process, we develop a particle-based model that includes purse-string contraction, cell crawling and other properties incorporated with monolayers of Madin-Darby canine kidney (MDCK) cells. Our model captures the traction force patterns under several different conditions in experiments. In addition to traction force pointing away from the wound on the leading edge, we observed patterns of traction force pointing towards the wound. We show this inward pointing force pattern is induced by the purse-string contraction. Our model also explains the effects of the purse-string ring and which parameters affect the relative efficiency of these two mechanisms.

PDGF-A signaling is required for secondary alveolar septation and controls epithelial proliferation in the developing lung

Platelet-derived growth factor A (PDGF-A) signaling through PDGF receptor α is essential for alveogenesis. Previous studies have shown that Pdgfa−/− mouse lungs have enlarged alveolar airspace with absence of secondary septation, both distinctive features of bronchopulmonary dysplasia. To study how PDGF-A signaling is involved in alveogenesis, we generated lung-specific Pdgfa knockout mice (Pdgfafl/−; Spc-cre) and characterized their phenotype postnatally. Histological differences between mutant mice and littermate controls were visible after the onset of alveogenesis and maintained until adulthood. Additionally, we generated Pdgfafl/−; Spc-cre; PdgfraGFP/+ mice in which Pdgfra+ cells exhibit nuclear GFP expression. In the absence of PDGF-A, the number of PdgfraGFP+ cells was significantly decreased. In addition, proliferation of PdgfraGFP+ cells was reduced. During alveogenesis, PdgfraGFP+ myofibroblasts failed to form the α-smooth muscle actin rings necessary for alveolar secondary septation. These results indicate that PDGF-A signaling is involved in myofibroblast proliferation and migration. In addition, we show an increase in both the number and proliferation of alveolar type II cells in Pdgfafl/−; Spc-cre lungs, suggesting that the increased alveolar airspace is not caused solely by deficient myofibroblast function.

Hybrid cell-centred/vertex model for multicellular systems with equilibrium-preserving remodelling

We present a hybrid cell-centred/vertex model for mechanically simulating planar cellular monolayers undergoing cell reorganisation. Cell centres are represented by a triangular nodal network, while the cell boundaries are formed by an associated vertex network. The two networks are coupled through a kinematic constraint which we allow to relax progressively. Special attention is paid to the change of cell-cell connectivity due to cell reorganisation or remodelling events. We handle these situations by using a variable resting length and applying an Equilibrium-Preserving Mapping on the new connectivity, which computes a new set of resting lengths that preserve nodal and vertex equilibrium. We illustrate the properties of the model by simulating monolayers subjected to imposed extension and during a wound healing process. The evolution of forces and the Equilibrium-Preserving Mapping are analysed during the remodelling events. As a by-product, the proposed technique enables to recover fully vertex or fully cell-centred models in a seamless manner by modifying a numerical parameter of the model.

Theory of frequency response of mechanically driven cardiomyocytes

We theoretically predict and compare with experiments, transitions from spontaneous beating to dynamical entrainment of cardiomyocytes induced by an oscillating, external mechanical probe. In accord with recent experiments, we predict the dynamical behavior as a function of the probe amplitude and frequency. The theory is based on a phenomenological model for a non-linear oscillator, motivated by acto-myosin contractility. The generic behavior is independent of the detailed, molecular origins of the dynamics and, consistent with experiment, we find three regimes: spontaneous beating with the natural frequency of the cell, entrained beating with the frequency of the probe, and a “bursting” regime where the two frequencies alternate in time. We quantitatively predict the properties of the “bursting” regime as a function of the amplitude and frequency of the probe. Furthermore, we examine the pacing process in the presence of weak noise and explain how this might relate to cardiomyocyte physiology.

Injectable hyperbranched poly(β-amino ester) hydrogels with on-demand degradation profiles to match wound healing processes

1A series of hyperbranched poly(β-amino ester) polymers have been synthesized via a Michael addition approach for the fabrication of hydrogels for wound healing.

Adjusting biomaterial degradation profiles to match tissue regeneration is a challenging issue. Herein, biodegradable hyperbranched poly(β-amino ester)s (HP-PBAEs) were designed and synthesized via “A2 + B4” Michael addition polymerization, and displayed fast gelation with thiolated hyaluronic acid (HA-SH) via a “click” thiol–ene reaction. HP-PBAE/HA-SH hydrogels showed tunable degradation profiles both in vitro and in vivo using diamines with different alkyl chain lengths and poly(ethylene glycol) diacrylates with varied PEG spacers. The hydrogels with optimized degradation profiles encapsulating ADSCs were used as injectable hydrogels to treat two different types of humanized excisional wounds – acute wounds with faster healing rates and diabetic wounds with slower healing and neo-tissue formation. The fast-degrading hydrogel showed accelerated wound closure in acute wounds, while the slow-degrading hydrogel showed better wound healing for diabetic wounds. The results demonstrate that the new HP-PBAE-based hydrogel in combination with ADSCs can be used as a well-controlled biodegradable skin substitute, which demonstrates a promising approach in the treatment of various types of skin wounds.

In vivo imaging of epithelial wound healing in the cnidarian Clytia hemisphaerica demonstrates early evolution of purse string and cell crawling closure mechanisms

Background

All animals have mechanisms for healing damage to the epithelial sheets that cover the body and line internal cavities. Epithelial wounds heal either by cells crawling over the wound gap, by contraction of a super-cellular actin cable (“purse string”) that surrounds the wound, or some combination of the two mechanisms. Both cell crawling and purse string closure of epithelial wounds are widely observed across vertebrates and invertebrates, suggesting early evolution of these mechanisms. Cnidarians evolved ~600 million years ago and are considered a sister group to the Bilateria. They have been much studied for their tremendous regenerative potential, but epithelial wound healing has not been characterized in detail. Conserved elements of wound healing in bilaterians and cnidarians would suggest an evolutionary origin in a common ancestor. Here we test this idea by characterizing epithelial wound healing in live medusae of Clytia hemisphaerica.

Results

We identified cell crawling and purse string-mediated mechanisms of healing in Clytia epithelium that appear highly analogous of those seen in higher animals, suggesting that these mechanisms may have emerged in a common ancestor. Interestingly, we found that epithelial wound healing in Clytia is 75 to >600 times faster than in cultured cells or embryos of other animals previously studied, suggesting that Clytia may provide valuable clues about optimized healing efficiency. Finally, in Clytia, we show that damage to the basement membrane in a wound gap causes a rapid shift between the cell crawling and purse string mechanisms for wound closure. This is consistent with work in other systems showing that cells marginal to a wound choose between a super-cellular actin cable or lamellipodia formation to close wounds, and suggests a mechanism underlying this decision.

Conclusions

1. Cell crawling and purse string mechanisms of epithelial wound healing likely evolved before the divergence of Cnidaria from the bilaterian lineage ~ 600mya 2. In Clytia, the choice between cell crawling and purse string mechanisms of wound healing depends on interactions between the epithelial cells and the basement membrane.

Electronic supplementary material

The online version of this article (10.1186/s12861-017-0160-2) contains supplementary material, which is available to authorized users.

Dynamic cellular finite-element method for modelling large-scale cell migration and proliferation under the control of mechanical and biochemical cues: a study of re-epithelialization

Computational modelling of cells can reveal insight into the mechanisms of the important processes of tissue development. However, current cell models have limitations and are challenged to model detailed changes in cellular shapes and physical mechanics when thousands of migrating and interacting cells need to be modelled. Here we describe a novel dynamic cellular finite-element model (DyCelFEM), which accounts for changes in cellular shapes and mechanics. It also models the full range of cell motion, from movements of individual cells to collective cell migrations. The transmission of mechanical forces regulated by intercellular adhesions and their ruptures are also accounted for. Intra-cellular protein signalling networks controlling cell behaviours are embedded in individual cells. We employ DyCelFEM to examine specific effects of biochemical and mechanical cues in regulating cell migration and proliferation, and in controlling tissue patterning using a simplified re-epithelialization model of wound tissue. Our results suggest that biochemical cues are better at guiding cell migration with improved directionality and persistence, while mechanical cues are better at coordinating collective cell migration. Overall, DyCelFEM can be used to study developmental processes when a large population of migrating cells under mechanical and biochemical controls experience complex changes in cell shapes and mechanics.

Linking biological and physical aging: Dynamical scaling of multicellular regeneration

The fight against biological aging (bio-aging) is long-standing, with the focus of intense research aimed at maintaining high rates of tissue regeneration to promote health and longevity. Nevertheless, there are overwhelming complexities associated with the quantitative analysis of aging. In this study, we sought to quantify bio-aging based on physical aging, by mapping instances of multicellular regeneration to the relaxation of physical systems. An experiment of delayed wound healing assays was devised to obtain delay-dependent healing data. The experiment confirmed the slowdown of healing events, which fitted dynamical scaling just as relaxation events do in physical aging. The scaling exponent, which describes the aging rate in physics, is here similarly proposed as an indicator of the deterioration rate of tissue-regenerative power. Parallel equation-based and cell-based simulations also revealed that asymmetric cell cycle-regulatory mechanisms under strong growth-inhibitory conditions predominantly control the critical slowdown of healing analogous to physical criticality. By establishing a direct link between physical aging and biological aging, we are able to estimate the aging rate of tissues and to achieve an integrated understanding of bio-aging mechanism which may improve the modulation of regeneration for clinical use.

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.

Hypotonicity promotes epithelial gap closure by lamellipodial protrusion

The closure of gaps within epithelia is an essential part of many physiological and pathological processes, such as embryonic development, organ remodeling and wound healing. Emerging evidence proved that the physical microenvironment plays important roles in cell behaviors. However, the effect of osmolarity of extracellular medium on gap closure is least understood. Using a gap closure model of epithelial cells, we found that hypotonic condition significantly facilitated the process of gap closure. Moreover, instead of actomyosin ring, enhanced migration leading by lamellipodia primarily contributed to the rapid gap closure in hypotonic condition. These findings provide insights for understanding the physiology of epithelial gap closure.

Orientational Coupling Locally Orchestrates a Cell Migration Pattern for Re-Epithelialization

Re-epithelialization by collective migration of epithelial cells over a heterogeneous environment to restore tissue integrity and functions is critical for development and regeneration. Here, it is reported that the spatial organization of adjacent adherent paths within sparsely distributed extracellular matrix (ECM) has a significant impact on the orientational coupling between cell polarization and collective cell migration. This coupling effect determines the migration pattern for human keratinocytes to regain their cohesion, which impacts the occupancy of epithelial bridge and the migration velocity in wound repair. Statistical studies suggest the converging organization of ECM, in which adjacent paths become closer to each other and finally converge to a junctional point, facilitating collective cell migration mostly within variable ECM organization, as the polarization of the advancing cell sheet is remodeled to align along the direction of cell migration. The findings may help to design implantable ECM to optimize efficient skin regeneration.

Actomyosin contractility and collective migration: may the force be with you

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Collective migration relies on the ability of a multicellular co-ordinated unit to efficiently respond to physical changes in their surrounding matrix.

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Conversely, migrating cohorts physically alter their microenvironment using mechanical forces.

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During collective migration, actomyosin contractility acts as a central hub coordinating mechanosensing and mechanotransduction responses.

Collective cell migration is essential during physiological processes such as development or wound healing and in pathological conditions such as cancer dissemination. Cells migrating within multicellular tissues experiment different forces which play an intricate role during tissue formation, development and maintenance. How cells are able to respond to these forces depends largely on how they interact with the extracellular matrix. In this review, we focus on mechanics and mechanotransduction in collective migration. In particular, we discuss current knowledge on how cells integrate mechanical signals during collective migration and we highlight actomyosin contractility as a central hub coordinating mechanosensing and mechanotransduction responses.

Collective cell migration: a physics perspective

Cells have traditionally been viewed either as independently moving entities or as somewhat static parts of tissues. However, it is now clear that in many cases, multiple cells coordinate their motions and move as collective entities. Well-studied examples comprise development events, as well as physiological and pathological situations. Different ex vivo model systems have also been investigated. Several recent advances have taken place at the interface between biology and physics, and have benefitted from progress in imaging and microscopy, from the use of microfabrication techniques, as well as from the introduction of quantitative tools and models. We review these interesting developments in quantitative cell biology that also provide rich examples of collective out-of-equilibrium motion.

Ezrin enhances line tension along transcellular tunnel edges via NMIIa driven actomyosin cable formation

Transendothelial cell macroaperture (TEM) tunnels control endothelium barrier function and are triggered by several toxins from pathogenic bacteria that provoke vascular leakage. Cellular dewetting theory predicted that a line tension of uncharacterized origin works at TEM boundaries to limit their widening. Here, by conducting high-resolution microscopy approaches we unveil the presence of an actomyosin cable encircling TEMs. We develop a theoretical cellular dewetting framework to interpret TEM physical parameters that are quantitatively determined by laser ablation experiments. This establishes the critical role of ezrin and non-muscle myosin II (NMII) in the progressive implementation of line tension. Mechanistically, fluorescence-recovery-after-photobleaching experiments point for the upstream role of ezrin in stabilizing actin filaments at the edges of TEMs, thereby favouring their crosslinking by NMIIa. Collectively, our findings ascribe to ezrin and NMIIa a critical function of enhancing line tension at the cell boundary surrounding the TEMs by promoting the formation of an actomyosin ring.

Active Vertex Model for cell-resolution description of epithelial tissue mechanics

We introduce an Active Vertex Model (AVM) for cell-resolution studies of the mechanics of confluent epithelial tissues consisting of tens of thousands of cells, with a level of detail inaccessible to similar methods. The AVM combines the Vertex Model for confluent epithelial tissues with active matter dynamics. This introduces a natural description of the cell motion and accounts for motion patterns observed on multiple scales. Furthermore, cell contacts are generated dynamically from positions of cell centres. This not only enables efficient numerical implementation, but provides a natural description of the T1 transition events responsible for local tissue rearrangements. The AVM also includes cell alignment, cell-specific mechanical properties, cell growth, division and apoptosis. In addition, the AVM introduces a flexible, dynamically changing boundary of the epithelial sheet allowing for studies of phenomena such as the fingering instability or wound healing. We illustrate these capabilities with a number of case studies.

Contractile forces at tricellular contacts modulate epithelial organization and monolayer integrity

Monolayered epithelia are composed of tight cell assemblies that ensure polarized exchanges. EpCAM, an unconventional epithelial-specific cell adhesion molecule, is assumed to modulate epithelial morphogenesis in animal models, but little is known regarding its cellular functions. Inspired by the characterization of cellular defects in a rare EpCAM-related human intestinal disease, we find that the absence of EpCAM in enterocytes results in an aberrant apical domain. In the course of this pathological state, apical translocation towards tricellular contacts (TCs) occurs with striking tight junction belt displacement. These unusual cell organization and intestinal tissue defects are driven by the loss of actomyosin network homoeostasis and contractile activity clustering at TCs, yet is reversed by myosin-II inhibitor treatment. This study reveals that adequate distribution of cortical tension is crucial for individual cell organization, but also for epithelial monolayer maintenance. Our data suggest that EpCAM modulation protects against epithelial dysplasia and stabilizes human tissue architecture.

Tension (re)builds: Biophysical mechanisms of embryonic wound repair

Embryonic tissues display an outstanding ability to rapidly repair wounds. Epithelia, in particular, serve as protective layers that line internal organs and form the skin. Thus, maintenance of epithelial integrity is of utmost importance for animal survival, particularly at embryonic stages, when an immune system has not yet fully developed. Rapid embryonic repair of epithelial tissues is conserved across species, and involves the collective migration of the cells around the wound. The migratory cell behaviours associated with wound repair require the generation and transmission of mechanical forces, not only for the cells to move, but also to coordinate their movements. Here, we review the forces involved in embryonic wound repair. We discuss how different force-generating structures are assembled at the molecular level, and the mechanisms that maintain the balance between force-generating structures as wounds close. Finally, we describe the mechanisms that cells use to coordinate the generation of mechanical forces around the wound. Collective cell movements and their misregulation have been associated with defective tissue repair, developmental abnormalities and cancer metastasis. Thus, we propose that understanding the role of mechanical forces during embryonic wound closure will be crucial to develop therapeutic interventions that promote or prevent collective cell movements under pathological conditions.

Deletion of Calponin 2 in Mouse Fibroblasts Increases Myosin II-Dependent Cell Traction Force

Cell traction force (CTF) plays a critical role in controlling cell shape, permitting cell motility, and maintaining cellular homeostasis in many biological processes such as angiogenesis, development, wound healing, and cancer metastasis. Calponin is an actin filament-associated cytoskeletal protein in smooth muscles and multiple types of non-muscle cells. An established biochemical function of calponin is the inhibition of myosin ATPase in smooth muscle cells. Vertebrates have three calponin isoforms. Among them, calponin 2 is expressed in epithelial cells, endothelial cells, macrophages, myoblasts, and fibroblasts and plays a role in regulating cytoskeleton activities such as cell adhesion, migration, and cytokinesis. Knockout (KO) of the gene encoding calponin 2 (Cnn2) in mice increased cell motility, suggesting a function of calponin 2 in modulating CTF. In this study, we examined fibroblasts isolated from Cnn2 KO and wild-type (WT) mice using CTF microscopy. Primary mouse fibroblasts were cultured on polyacrylamide gel substrates embedded with fluorescent beads to measure root-mean-square traction, total strain energy, and net contractile movement. The results showed that calponin 2-null fibroblasts exhibit traction force greater than that of WT cells. Adherent calponin 2-null fibroblasts de-adhered faster than the WT control during mild trypsin treatment, consistent with an increased CTF. Blebbistatin, an inhibitor of myosin II ATPase, is more effective upon an alteration in cell morphology when calponin 2 is present in WT fibroblasts than that on Cnn2 KO cells, indicating their additive effects in inhibiting myosin motor activity. The novel finding that calponin 2 regulates myosin-dependent CTF in non-muscle cells demonstrates a mechanism for controlling cell motility-based functions.

Modeling cell shape and dynamics on micropatterns

Adhesive micropatterns have become a standard tool to study cells under defined conditions. Applications range from controlling the differentiation and fate of single cells to guiding the collective migration of cell sheets. In long-term experiments, single cell normalization is challenged by cell division. For all of these setups, mathematical models predicting cell shape and dynamics can guide pattern design. Here we review recent advances in predicting and explaining cell shape, traction forces and dynamics on micropatterns. Starting with contour models as the simplest approach to explain concave cell shapes, we move on to network and continuum descriptions as examples for static models. To describe dynamic processes, cellular Potts, vertex and phase field models can be used. Different types of model are appropriate to address different biological questions and together, they provide a versatile tool box to predict cell behavior on micropatterns.

Cast Tube Assay: A 3-D in vitro assay for visualization and quantification of horizontal chemotaxis and cellular invasion

Directed cell motility, as controlled by soluble factors, is crucial for many biological processes, including development, cancer progression, and wound healing. The use of directed cell motility also shows promise for applications in regenerative medicine such as therapeutic angiogenesis. Unfortunately, current in vitro 3-D migration and invasion models limit our understanding and application of these processes. Here, we present a novel and cost-effective 3-D chemotaxis assay for assessing the invasive response of cells to a chemoattractant extracellular matrix (ECM). Our system takes advantage of a custom-casting chamber to set two gels in contact with each other along a defined front, one containing a suitable chemoattractant and the other the cells. Rotation of the chamber allows easy visualization of invasion across the interface. The effectiveness of the assay was demonstrated by studying the invasion of both human dermal fibroblasts (FBs) and smooth muscle cells (SMCs) into a polyethylene glycol (PEG) hydrogel containing basic fibroblast growth factor (bFGF). Incorporation of bFGF resulted in significantly increased and directional invasion for both cell groups.

Mechanics of epithelial tissues during gap closure

The closure of gaps is crucial to maintaining epithelium integrity during developmental and repair processes such as dorsal closure and wound healing. Depending on biochemical as well as physical properties of the microenvironment, gap closure occurs through assembly of multicellular actin-based contractile cables and/or protrusive activity of cells lining the gap. This review discusses the relative contributions of 'purse-string' and cell crawling mechanisms regulated by cell-substrate and cell-cell interactions, cellular mechanics and physical constraints from the environment.

Dynamics of Cell Ensembles on Adhesive Micropatterns: Bridging the Gap between Single Cell Spreading and Collective Cell Migration

The collective dynamics of multicellular systems arise from the interplay of a few fundamental elements: growth, division and apoptosis of single cells; their mechanical and adhesive interactions with neighboring cells and the extracellular matrix; and the tendency of polarized cells to move. Micropatterned substrates are increasingly used to dissect the relative roles of these fundamental processes and to control the resulting dynamics. Here we show that a unifying computational framework based on the cellular Potts model can describe the experimentally observed cell dynamics over all relevant length scales. For single cells, the model correctly predicts the statistical distribution of the orientation of the cell division axis as well as the final organisation of the two daughters on a large range of micropatterns, including those situations in which a stable configuration is not achieved and rotation ensues. Large ensembles migrating in heterogeneous environments form non-adhesive regions of inward-curved arcs like in epithelial bridge formation. Collective migration leads to swirl formation with variations in cell area as observed experimentally. In each case, we also use our model to predict cell dynamics on patterns that have not been studied before.

Bio-Inspired Mechanotactic Hybrids for Orchestrating Traction-Mediated Epithelial Migration

A platform of mechanotactic hybrids is established by projecting lateral gradients of apparent interfacial stiffness onto the planar surface of a compliant hydrogel layer using an underlying rigid substrate with microstructures inherited from 3D printed molds. Using this platform, the mechanistic coupling of epithelial migration with the stiffness of the extracellular matrix (ECM) is found to be independent of the interfacial compositional and topographical cues.

Cellular forces and matrix assembly coordinate fibrous tissue repair

Planar in vitro models have been invaluable tools to identify the mechanical basis of wound closure. Although these models may recapitulate closure dynamics of epithelial cell sheets, they fail to capture how a wounded fibrous tissue rebuilds its 3D architecture. Here we develop a 3D biomimetic model for soft tissue repair and demonstrate that fibroblasts ensconced in a collagen matrix rapidly close microsurgically induced defects within 24 h. Traction force microscopy and time-lapse imaging reveal that closure of gaps begins with contractility-mediated whole-tissue deformations. Subsequently, tangentially migrating fibroblasts along the wound edge tow and assemble a progressively thickening fibronectin template inside the gap that provide the substrate for cells to complete closure. Unlike previously reported mechanisms based on lamellipodial protrusions and purse-string contraction, our data reveal a mode of stromal closure in which coordination of tissue-scale deformations, matrix assembly and cell migration act together to restore 3D tissue architecture.

Planar in vitro models for wound closure stress the role of lamellipodial protrusions and purse-string contraction. Here the authors develop a 3D biomimetic model for tissue repair and show a mode of stromal closure that relies on whole tissue deformations, cell migration and matrix deposition.

High-resolution micromechanical measurement in real time of forces exerted by living cells

The aim of this study was to compare uniaxial traction forces exerted by different cell types using a novel sensor design and to test the dependence of measured forces on cytoskeletal integrity. The sensor design detects forces generated between 2 contact points by cells spanning a gap. The magnitude of these forces varied according to cell type and were dependent on cytoskeletal integrity. The response time for drug-induced cytoskeletal disruption also varied between cell types: dermal fibroblasts exerted the greatest forces and had the slowest drug response times; EBV-transformed epithelial cells also had slow cytoskeletal depolymerisation times but exerted the lowest forces overall. Conversely, lung epithelial tumor cells exerted low forces but had the fastest depolymerisation drug response. These results provide proof of principle for a new design of force-measurement sensor based on optical interferometry, an approach that can be used to study cytoskeletal dynamics in real time.

Regulation of epithelial cell organization by tuning cell-substrate adhesion

Collective migration of cells is of fundamental importance for a number of biological functions such as tissue development and regeneration, wound healing and cancer metastasis. The movement of cell groups consisting of multiple cells connected by cell-cell junctions depends on both extracellular and intercellular contacts. Epithelial cell assemblies are thus regulated by a cross-talk between cell-substrate and cell-cell interactions. Here, we investigated the onset of collective migration in groups of cells as they expand from a few cells into large colonies as a function of extracellular matrix (ECM) protein coating. By varying the amount of ECM presented to the cells, we observe that the mode of colony expansion, as well as their overall geometry, is strongly dependent on substrate adhesiveness. On high ECM protein coated surfaces, cells at the edges of the colonies are well spread exhibiting large outward-pointing protrusive activity, whereas cellular colonies display more circular and convex shapes on less adhesive surfaces. Actin structures at the edge of the colonies also show different organizations with the formation of lamellipodial structures on highly adhesive surfaces and a pluricellular actin cable on less adhesive ones. The analysis of traction forces and cell velocities within the cellular assemblies confirm these results. By increasing ECM protein density, cells exert higher traction forces together with a higher outward motility at the edges. Furthermore, tuning cell-cell adhesion of epithelial cells modified the mode of expansion of the colonies. Finally, we used a recently developed computational model to recapitulate the emergent experimental behaviors of expanding cell colonies and extract that the main effect of the different cell-substrate interactions is on the ability of edge cells to form outward lamellipodia-driven motility. Overall, our data suggest that switching behaviors of epithelial cell assemblies result in a tug-of-war between friction forces at the cell-substrate interface and cell-cell interactions.

Tissue fusion over nonadhering surfaces

Tissue fusion eliminates physical voids in a tissue to form a continuous structure and is central to many processes in development and repair. Fusion events in vivo, particularly in embryonic development, often involve the purse-string contraction of a pluricellular actomyosin cable at the free edge. However, in vitro, adhesion of the cells to their substrate favors a closure mechanism mediated by lamellipodial protrusions, which has prevented a systematic study of the purse-string mechanism. Here, we show that monolayers can cover well-controlled mesoscopic nonadherent areas much larger than a cell size by purse-string closure and that active epithelial fluctuations are required for this process. We have formulated a simple stochastic model that includes purse-string contractility, tissue fluctuations, and effective friction to qualitatively and quantitatively account for the dynamics of closure. Our data suggest that, in vivo, tissue fusion adapts to the local environment by coordinating lamellipodial protrusions and purse-string contractions.

Gap geometry dictates epithelial closure efficiency

Closure of wounds and gaps in tissues is fundamental for the correct development and physiology of multicellular organisms and, when misregulated, may lead to inflammation and tumorigenesis. To re-establish tissue integrity, epithelial cells exhibit coordinated motion into the void by active crawling on the substrate and by constricting a supracellular actomyosin cable. Coexistence of these two mechanisms strongly depends on the environment. However, the nature of their coupling remains elusive because of the complexity of the overall process. Here we demonstrate that epithelial gap geometry in both in vitro and in vivo regulates these collective mechanisms. In addition, the mechanical coupling between actomyosin cable contraction and cell crawling acts as a large-scale regulator to control the dynamics of gap closure. Finally, our computational modelling clarifies the respective roles of the two mechanisms during this process, providing a robust and universal mechanism to explain how epithelial tissues restore their integrity.

The mechanotransduction machinery at work at adherens junctions

The shaping of a multicellular body, and the maintenance and repair of adult tissues require fine-tuning of cell adhesion responses and the transmission of mechanical load between the cell, its neighbors and the underlying extracellular matrix. A growing field of research is focused on how single cells sense mechanical properties of their micro-environment (extracellular matrix, other cells), and on how mechanotransduction pathways affect cell shape, migration, survival as well as differentiation. Within multicellular assemblies, the mechanical load imposed by the physical properties of the environment is transmitted to neighboring cells. Force imbalance at cell-cell contacts induces essential morphogenetic processes such as cell-cell junction remodeling, cell polarization and migration, cell extrusion and cell intercalation. However, how cells respond and adapt to the mechanical properties of neighboring cells, transmit forces, and transform mechanical signals into chemical signals remain open questions. A defining feature of compact tissues is adhesion between cells at the specialized adherens junction (AJ) involving the cadherin super-family of Ca(2+)-dependent cell-cell adhesion proteins (e.g., E-cadherin in epithelia). Cadherins bind to the cytoplasmic protein β-catenin, which in turn binds to the filamentous (F)-actin binding adaptor protein α-catenin, which can also recruit vinculin, making the mechanical connection between cell-cell adhesion proteins and the contractile actomyosin cytoskeleton. The cadherin-catenin adhesion complex is a key component of the AJ, and contributes to cell assembly stability and dynamic cell movements. It has also emerged as the main route of propagation of forces within epithelial and non-epithelial tissues. Here, we discuss recent molecular studies that point toward force-dependent conformational changes in α-catenin that regulate protein interactions in the cadherin-catenin adhesion complex, and show that α-catenin is the core mechanosensor that allows cells to locally sense, transduce and adapt to environmental mechanical constrains.

Live long and prosper: the enterprise of understanding diseased epithelium

The epithelium is a particularly complicated and dynamic tissue, and dysregulation of epithelial structure and function is a hallmark of several lung diseases. Motivated by the life and recent passing of Leonard Nimoy, we highlight several recent studies that explore the nuanced relationship between the epithelium and disease progression. Specifically, we focus on recent innovative and integrative approaches that shed new light on epithelial wounding, healing, and development.

Modelling wound closure in an epithelial cell sheet using the cellular Potts model

We use a two-dimensional cellular Potts model to represent the behavior of an epithelial cell layer and describe its dynamics in response to a microscopic wound.

Modeling the finger instability in an expanding cell monolayer

Curvature-controlled cellular forces at the edge of an expanding monolayer are sufficient for the initiation and growth of finger-like instability.