alpha-Catenin as a tension transducer that induces adherens junction development

Fusing VE-cadherin to α-catenin impairs fetal liver hematopoiesis and lymph but not blood vessel formation

We have recently shown that genetic replacement of VE-cadherin by a VE-cadherin-α-catenin fusion construct strongly impairs opening of endothelial cell contacts during leukocyte extravasation and induction of vascular permeability in adult mice. Here we show that this mutation leads to lethality at midgestation on a clean C57BL/6 background. Investigating the reasons for embryonic lethality, we observed a lack of fetal liver hematopoiesis and severe lymphedema but no detectable defects in blood vessel formation and remodeling. As for the hematopoiesis defect, VE-cadherin-α-catenin affected neither the generation of hematopoietic stem and progenitor cells (HSPCs) from hemogenic endothelium nor their differentiation into multiple hematopoietic lineages. Instead, HSPCs accumulated in the fetal circulation, suggesting that their entry into the fetal liver was blocked. Edema formation was caused by disturbed lymphatic vessel development. Lymphatic progenitor cells of VE-cadherin-α-catenin-expressing embryos were able to leave the cardinal vein and migrate to the site of the first lymphatic vessel formation, yet subsequently, these cells failed to form large lumenized lymphatic vessels. Thus, stabilizing endothelial cell contacts by a covalent link between VE-cadherin and α-catenin affects recruitment of hematopoietic progenitors into the fetal liver and the development of lymph but not blood vessels.

Mechanical control of the endothelial barrier

The integrity of the endothelial barrier is controlled by the combined action of chemical and mechanical signaling systems. Permeability-regulating factors signal through small GTPases to regulate the architecture of the cytoskeleton and this has a strong impact on the morphology and stability of VE-cadherin-based cell-cell junctions. The details of how structural and mechanical properties of the actin cytoskeleton influence cell-cell adhesion and how this impacts the dynamic regulation of the endothelial barrier, are beginning to be elucidated. In this review, we discuss the physical and regulatory interactions between the VE-cadherin complex and the actomysoin cytoskeleton, as they are the main determinants of cell-cell adhesion and the mechanical architecture of the cytoskeleton. We discuss, based on recent in vitro data, how a balance between Linear Adherens Junctions, paralleled by cortical actin bundles and Focal Adherens Junctions, connected to radial action bundles, determines endothelial barrier function. We discuss how small GTPases control this balance by regulating the spatial organization and mechanics of actomyosin. We propose a hypothetical model of how biochemical and mechanical signals cooperate locally, at the actomyosin-adhesion interface to open and re-seal the barrier in a rapid and controlled manner.

α-catenin cytomechanics--role in cadherin-dependent adhesion and mechanotransduction

The findings presented here demonstrate the role of α-catenin in cadherin-based adhesion and mechanotransduction in different mechanical contexts. Bead-twisting measurements in conjunction with imaging, and the use of different cell lines and α-catenin mutants reveal that the acute local mechanical manipulation of cadherin bonds triggers vinculin and actin recruitment to cadherin adhesions in an actin- and α-catenin-dependent manner. The modest effect of α-catenin on the two-dimensional binding affinities of cell surface cadherins further suggests that force-activated adhesion strengthening is due to enhanced cadherin-cytoskeletal interactions rather than to α-catenin-dependent affinity modulation. Complementary investigations of cadherin-based rigidity sensing also suggest that, although α-catenin alters traction force generation, it is not the sole regulator of cell contractility on compliant cadherin-coated substrata.

Cortical F-actin stabilization generates apical-lateral patterns of junctional contractility that integrate cells into epithelia

E-cadherin cell-cell junctions couple the contractile cortices of epithelial cells together, generating tension within junctions that influences tissue organization. Although junctional tension is commonly studied at the apical zonula adherens, we now report that E-cadherin adhesions induce the contractile actomyosin cortex throughout the apical-lateral axis of junctions. However, cells establish distinct regions of contractile activity even within individual contacts, producing high tension at the zonula adherens but substantially lower tension elsewhere. We demonstrate that N-WASP (also known as WASL) enhances apical junctional tension by stabilizing local F-actin networks, which otherwise undergo stress-induced turnover. Further, we find that cells are extruded from monolayers when this pattern of intra-junctional contractility is disturbed, either when N-WASP redistributes into lateral junctions in H-Ras(V12)-expressing cells or on mosaic redistribution of active N-WASP itself. We propose that local control of actin filament stability regulates the landscape of intra-junctional contractility to determine whether or not cells integrate into epithelial populations.

Epithelial adhesive junctions

Epithelial adhesive cell-to-cell contacts contain large, plasma membrane-spanning multiprotein aggregates that perform vital structural and signaling functions. Three prominent adhesive contacts are the tight junction, adherens junction, and the desmosome. Each junction type has unique cellular functions and a complex molecular composition. In this review, we comment on recent and exciting advances in our understanding of junction composition and function.

Epithelial bridges maintain tissue integrity during collective cell migration

The ability of skin to act as a barrier is primarily determined by the efficiency of skin cells to maintain and restore its continuity and integrity. In fact, during wound healing keratinocytes migrate collectively to maintain their cohesion despite heterogeneities in the extracellular matrix. Here, we show that monolayers of human keratinocytes migrating along functionalized micropatterned surfaces comprising alternating strips of extracellular matrix (fibronectin) and non-adherent polymer form suspended multicellular bridges over the non-adherent areas. The bridges are held together by intercellular adhesion and are subjected to considerable tension, as indicated by the presence of prominent actin bundles. We also show that a model based on force propagation through an elastic material reproduces the main features of bridge maintenance and tension distribution. Our findings suggest that multicellular bridges maintain tissue integrity during wound healing when cell-substrate interactions are weak and may prove helpful in the design of artificial scaffolds for skin regeneration.

A multicellular view of cytokinesis in epithelial tissue

The study of cytokinesis in single-cell systems provided a wealth of knowledge on the molecular and biophysical mechanisms controlling daughter cell separation. In this review, we outline recent advances in the understanding of cytokinesis in epithelial tissues. These findings provide evidence for how the cytokinetic machinery adapts to a multicellular context and how the cytokinetic machinery is itself exploited by the tissue for the preservation of tissue function and architecture during proliferation. We propose that cytokinesis in epithelia should be viewed as a multicellular process, whereby the biochemical and mechanical interactions between the dividing cell and its neighbors are essential for successful daughter cell separation while defining epithelial tissue organization and preserving tissue integrity.

Mechanobiology and developmental control

Morphogenesis is the remarkable process by which cells self-assemble into complex tissues and organs that exhibit specialized form and function during embryological development. Many of the genes and chemical cues that mediate tissue and organ formation have been identified; however, these signals alone are not sufficient to explain how tissues and organs are constructed that exhibit their unique material properties and three-dimensional forms. Here, we review work that has revealed the central role that physical forces and extracellular matrix mechanics play in the control of cell fate switching, pattern formation, and tissue development in the embryo and how these same mechanical signals contribute to tissue homeostasis and developmental control throughout adult life.

The interplay between cell signalling and mechanics in developmental processes

Force production and the propagation of stress and strain within embryos and organisms are crucial physical processes that direct morphogenesis. In addition, there is mounting evidence that biomechanical cues created by these processes guide cell behaviours and cell fates. In this Review we discuss key roles for biomechanics during development to directly shape tissues, to provide positional information for cell fate decisions and to enable robust programmes of development. Several recently identified molecular mechanisms suggest how cells and tissues might coordinate their responses to biomechanical cues. Finally, we outline long-term challenges in integrating biomechanics with genetic analysis of developing embryos.

The effects of artificial E-cadherin matrix-induced embryonic stem cell scattering on paxillin and RhoA activation via α-catenin

Mechanical forces have been shown to affect stem cell behavior in a large array of ways. However, our understanding of how these mechanical cues may regulate the behavior of embryonic stem cells (ESCs) remains in its infancy. Here, we aim to clarify the effect of cell scattering on the regulation of Rho family GTPases Rac1 and RhoA as well as paxillin. Allowing ESCs to spread and scatter on a synthetically designed E-cadherin substratum causes phosphorylation of paxillin on consensus phosphorylation sites leading to activation of Rac1 and inactivation of RhoA. By culturing cells in presence of RhoA activator or growing cells to a highly confluent state reverses the effect of cell scattering phenotype. Knockdown of E-cadherin-adapter protein α-catenin revealed that it negatively affects paxillin phosphorylation and up-regulates RhoA activity in compact cellular aggregates. Collectively these results indicate that cell scattering might cause a conformational change of α-catenin limiting its capacity to inhibit paxillin phosphorylation that causes an increase in Rac1 activation and RhoA deactivation. Understanding how synthetically designed extracellular matrix affect ESC signaling through mechanical cues brings a new aspect for stem cell engineers to develop technologies for controlling cell function.

Generating suspended cell monolayers for mechanobiological studies

Cell monolayers line most of the surfaces and cavities in the human body. During development and normal physiology, monolayers sustain, detect and generate mechanical stresses, yet little is known about their mechanical properties. We describe a cell culture and mechanical testing protocol for generating freely suspended cell monolayers and examining their mechanical and biological response to uniaxial stretch. Cells are cultured on temporary collagen scaffolds polymerized between two parallel glass capillaries. Once cells form a monolayer covering the collagen and the capillaries, the scaffold is removed with collagenase, leaving the monolayer suspended between the test rods. The suspended monolayers are subjected to stretching by prying the capillaries apart with a micromanipulator. The applied force can be measured for the characterization of monolayer mechanics. Monolayers can be imaged with standard optical microscopy to examine changes in cell morphology and subcellular organization concomitant with stretch. The entire preparation and testing protocol requires 3-4 d.

Control of creatine metabolism by HIF is an endogenous mechanism of barrier regulation in colitis

Mucosal surfaces of the lower gastrointestinal tract are subject to frequent, pronounced fluctuations in oxygen tension, particularly during inflammation. Adaptive responses to hypoxia are orchestrated largely by the hypoxia-inducible transcription factors (HIFs). As HIF-1α and HIF-2α are coexpressed in mucosal epithelia that constitute the barrier between the lumen and the underlying immune milieu, we sought to define the discrete contribution of HIF-1 and HIF-2 transactivation pathways to intestinal epithelial cell homeostasis. The present study identifies creatine kinases (CKs), key metabolic enzymes for rapid ATP generation via the phosphocreatine-creatine kinase (PCr/CK) system, as a unique gene family that is coordinately regulated by HIF. Cytosolic CKs are expressed in a HIF-2-dependent manner in vitro and localize to apical intestinal epithelial cell adherens junctions, where they are critical for junction assembly and epithelial integrity. Supplementation with dietary creatine markedly ameliorated both disease severity and inflammatory responses in colitis models. Further, enzymes of the PCr/CK metabolic shuttle demonstrate dysregulated mucosal expression in a subset of ulcerative colitis and Crohn disease patients. These findings establish a role for HIF-regulated CK in epithelial homeostasis and reveal a fundamental link between cellular bioenergetics and mucosal barrier.

Mechanotransduction

Physical forces are central players in development and morphogenesis, provide an ever-present backdrop influencing physiological functions, and contribute to a variety of pathologies. Mechanotransduction encompasses the rich variety of ways in which cells and tissues convert cues from their physical environment into biochemical signals. These cues include tensile, compressive and shear stresses, and the stiffness or elastic modulus of the tissues in which cells reside. This article focuses on the proximal events that lead directly from a change in physical state to a change in cell-signaling state. A large body of evidence demonstrates a prominent role for the extracellular matrix, the intracellular cytoskeleton, and the cell matrix adhesions that link these networks in transduction of the mechanical environment. Recent work emphasizes the important role of physical unfolding or conformational changes in proteins induced by mechanical loading, with examples identified both within the focal adhesion complex at the cell-matrix interface and in extracellular matrix proteins themselves. Beyond these adhesion and matrix-based mechanisms, classical and new mechanisms of mechanotransduction reside in stretch-activated ion channels, the coupling of physical forces to interstitial autocrine and paracrine signaling, force-induced activation of extracellular proteins, and physical effects directly transmitted to the cell's nucleus. Rapid progress is leading to detailed delineation of molecular mechanisms by which the physical environment shapes cellular signaling events, opening up avenues for exploring how mechanotransduction pathways are integrated into physiological and pathophysiological cellular and tissue processes.

Role of extracellular matrix and YAP/TAZ in cell fate determination

The mechanical signals transduced from cellular microenvironment can regulate cell shape and affect cell fate determination. However, how these mechanical signals are transduced to regulate biological processes of cells has remained elusive. Recent studies had elucidated a novel mechanism through which the interactions between mechanical signals from extracellular matrix and cell behavior regulation converged on the function of core components in Hippo signaling pathway, including YAP and TAZ in mammals. Moreover, several very recent studies have found a new crosstalk between Wnt and Hippo signaling in the regulation of cell fate determination. Such mechanism may explain how mechanical signals from microenvironment can regulate cell behavior and determine cell fate.

Signaling and mechanical roles of E-cadherin

The epithelium comprises an important tissue that lines the internal and external surfaces of metazoan organs. In order to organize sheets of epithelial cells into three-dimensional tissues, it requires the coordination of basic cellular processes such as polarity, adhesion, growth, and differentiation. Moreover, as a primary barrier to the external environment, epithelial tissues are often subjected to physical forces and damage. This critical barrier function dictates that these fundamental cellular processes are continually operational in order to maintain tissue homeostasis in the face of almost constant trauma and stress. A protein that is largely responsible for the organization and maintenance of epithelial tissues is the transmembrane protein, E-cadherin, found at the surface of epithelial cells. Though originally investigated for its essential role in mediating intercellular cohesion, its impact on a wide array of physiological processes underscores its fundamental contributions to tissue development and its perturbation in a variety of common diseases.

Patterns in space: coordinating adhesion and actomyosin contractility at E-cadherin junctions

Cadherin adhesion receptors are fundamental determinants of tissue organization in health and disease. Increasingly, we have come to appreciate that classical cadherins exert their biological actions through active cooperation with the contractile actin cytoskeleton. Rather than being passive resistors of detachment forces, cadherins can regulate the assembly and mechanics of the contractile apparatus itself. Moreover, coordinate spatial patterning of adhesion and contractility is emerging as a determinant of morphogenesis. Here we review recent developments in cadherins and actin cytoskeleton cooperativity, by focusing on E-cadherin adhesive patterning in the epithelia. Next, we discuss the underlying principles of cellular rearrangement during Drosophila germband extension and epithelial cell extrusion, as models of how planar and apical-lateral patterns of contractility organize tissue architecture.

Self-contact elimination by membrane fusion

Mutual, homophilic cell-cell adhesion between epithelial cells is required for proper maintenance of epithelial barrier function. Whereas opposing membranes from neighboring cells rapidly assemble junctional complexes, self-contacting membranes curiously do not, suggesting that cells have the ability to prevent the maturation of self-junctions. Using a self-contact-inducing microfabricated substrate, we show that self-contacts of normal epithelial cells are rapidly eliminated by membrane fusion between two opposing plasma membranes of a single cell. This membrane fusion is most frequently observed in E-cadherin-expressing epithelial cells, but not in fibroblasts. The efficiency of self-contact elimination depends on extracellular calcium concentration and the level of E-cadherin, suggesting that E-cadherin, although not required, enhances membrane fusion efficiency by bringing opposing membranes into close apposition to one another. Additionally, Rho-associated protein kinase inhibition decreases self-contact-induced membrane fusion of epithelial cells, suggesting that this fusion may be mechanically regulated through the actin-myosin network. This self-contact-induced membrane fusion is a key elimination mechanism for unwanted self-junctions and may be a feature of cell self-recognition.

Cadherin mechanotransduction in tissue remodeling

Mechanical forces are increasingly recognized as central factors in the regulation of tissue morphogenesis and homeostasis. Central to the transduction of mechanical information into biochemical signaling is the contractile actomyosin cytoskeleton. Fluctuations in actomyosin contraction are sensed by tension sensitive systems at the interface between actomyosin and cell adhesion complexes. We review the current knowledge about the mechanical coupling of cell-cell junctions to the cytoskeleton and highlight the central role of α-catenin in this linkage. We assemble current knowledge about α-catenin's regulation by tension and about its interactions with a diversity of proteins. We present a model in which α-catenin is a force-regulated platform for a machinery of proteins that orchestrates local cortical remodeling in response to force. Finally, we highlight recently described fundamental processes in tissue morphogenesis and argue where and how this α-catenin-dependent cadherin mechanotransduction may be involved.

Principles of E-cadherin supramolecular organization in vivo

BACKGROUND

E-cadherin plays a pivotal role in tissue morphogenesis by forming clusters that support intercellular adhesion and transmit tension. What controls E-cadherin mesoscopic organization in clusters is unclear.

RESULTS

We use 3D superresolution quantitative microscopy in Drosophila embryos to characterize the size distribution of E-cadherin nanometric clusters. The cluster size follows power-law distributions over three orders of magnitude with exponential decay at large cluster sizes. By exploring the predictions of a general theoretical framework including cluster fusion and fission events and recycling of E-cadherin, we identify two distinct active mechanisms setting the cluster-size distribution. Dynamin-dependent endocytosis targets large clusters only, thereby imposing a cutoff size. Moreover, interactions between E-cadherin clusters and actin filaments control the fission in a size-dependent manner.

CONCLUSIONS

E-cadherin clustering depends on key cortical regulators, which provide tunable and local control over E-cadherin organization. Our data provide the foundation for a quantitative understanding of how E-cadherin distribution affects adhesion and might regulate force transmission in vivo.

A molecular base for cell sorting at embryonic boundaries: contact inhibition of cadherin adhesion by ephrin/ Eph-dependent contractility

The mechanism responsible for subdividing the embryo into individual tissues is a fundamental, yet still poorly understood, question in developmental biology. Various general hypotheses have been proposed, involving differences in cell adhesion, contractility, or contact-mediated repulsion. However, the key parameter in tissue separation, i.e., the regulation of cadherin-based adhesion at the boundary, has not yet been investigated. We show that cadherin clustering is specifically inhibited at the vertebrate notochord-presomitic mesoderm boundary, preventing formation of adhesive bonds between cells of the two different types. This local regulation depends on differentially expressed ephrins and Eph receptors, which increase cell contractility and generate a membrane blebbing-like behavior along the boundary. Inhibiting myosin activity is sufficient to induce cadherin clustering and formation of stable contacts across the boundary, causing notochord and presomitic tissues to fuse. Local inhibition of cadherin adhesion explains how sharp separation can be achieved in response to cell-cell contact signals.

From tissue mechanics to transcription factors

Changes in tissue stiffness are frequently associated with diseases such as cancer, fibrosis, and atherosclerosis. Several recent studies suggest that, in addition to resulting from pathology, mechanical changes may play a role akin to soluble factors in causing the progression of disease, and similar mechanical control might be essential for normal tissue development and homeostasis. Many cell types alter their structure and function in response to exogenous forces or as a function of the mechanical properties of the materials to which they adhere. This review summarizes recent progress in identifying intracellular signaling pathways, and especially transcriptional programs, that are differentially activated when cells adhere to materials with different mechanical properties or when they are subject to tension arising from external forces. Several cytoplasmic or cytoskeletal signaling pathways involving small GTPases, focal adhesion kinase and transforming growth factor beta as well as the transcriptional regulators MRTF-A, NFκB, and Yap/Taz have emerged as important mediators of mechanical signaling.

Dynamics and regulation of epithelial adherens junctions: recent discoveries and controversies

Adherens junctions (AJs) are evolutionarily conserved plasma-membrane structures that mediate cell-cell adhesions in multicellular organisms. They are organized by several types of adhesive integral membrane proteins, most notably cadherins and nectins that are clustered and stabilized by a number of cytoplasmic scaffolds. AJs are key regulators of tissue architecture and dynamics via control of cell proliferation, polarity, shape, motility, and survival. They are absolutely critical for normal tissue morphogenesis and their disruption results in pathological abnormalities in different tissues. Although the field of adherens-junction research dramatically progressed in recent years, a number of important questions remain controversial and poorly understood. This review outlines basic principles that regulate organization of AJs in mammalian epithelia and discusses recent advances and standing controversies in the field. A special attention is paid to the regulation of AJs by vesicle trafficking and the intracellular cytoskeleton as well as roles and mechanisms of adherens-junction disruption during tumor progression and tissue inflammation.

Evolution and cell physiology. 3. Using Dictyostelium discoideum to investigate mechanisms of epithelial polarity

In Metazoa, a polarized epithelium forms a single-cell-layered barrier that separates the outside from the inside of the organism. In tubular epithelia, the apical side of the cell is constricted relative to the basal side, forming a wedge-shaped cell that can pack into a tube. Apical constriction is mediated by actomyosin activity. In higher animals, apical actomyosin is connected between cells by specialized cell-cell junctions that contain a classical cadherin, the Wnt signaling protein β-catenin, and the actin-binding protein α-catenin. The molecular mechanisms that lead to selective accumulation of myosin at the apical surface of cells are poorly understood. We found that the nonmetazoan Dictyostelium discoideum forms a polarized epithelium that surrounds the stalk tube at the tip of the multicellular fruiting body. Although D. discoideum lacks a cadherin homolog, it expresses homologs of β- and α-catenin. Both catenins are essential for formation of the tip epithelium, polarized protein secretion, and proper multicellular morphogenesis. Myosin localizes apically in tip epithelial cells, and it appears that constriction of this epithelial tube is required for proper morphogenesis. Localization of myosin II is controlled by the protein IQGAP1 and its binding partners cortexillins I and II, which function downstream of α- and β-catenin to exclude myosin from the basolateral cortex and promote apical accumulation of myosin. These studies show that the function of catenins in cell polarity predates the evolution of Wnt signaling and classical cadherins, and that apical localization of myosin is a morphogenetic mechanism conserved from nonmetazoans to vertebrates.

Contact inhibition of locomotion probabilities drive solitary versus collective cell migration

Contact inhibition of locomotion (CIL) is the process whereby cells collide, cease migrating in the direction of the collision, and repolarize their migration machinery away from the collision. Quantitative analysis of CIL has remained elusive because cell-to-cell collisions are infrequent in traditional cell culture. Moreover, whereas CIL predicts mutual cell repulsion and 'scattering' of cells, the same cells in vivo are observed to undergo CIL at some developmental times and collective cell migration at others. It remains unclear whether CIL is simply absent during collective cell migration, or if the two processes coexist and are perhaps even related. Here, we used micropatterned stripes of extracellular matrix to restrict cell migration to linear paths such that cells polarized in one of two directions and collisions between cells occurred frequently and consistently, permitting quantitative and unbiased analysis of CIL. Observing repolarization events in different contexts, including head-to-head collision, head-to-tail collision, collision with an inert barrier, or no collision, and describing polarization as a two-state transition indicated that CIL occurs probabilistically, and most strongly upon head-to-head collisions. In addition to strong CIL, we also observed 'trains' of cells moving collectively with high persistence that appeared to emerge from single cells. To reconcile these seemingly conflicting observations of CIL and collective cell migration, we constructed an agent-based model to simulate our experiments. Our model quantitatively predicted the emergence of collective migration, and demonstrated the sensitivity of such emergence to the probability of CIL. Thus CIL and collective migration can coexist, and in fact a shift in CIL probabilities may underlie transitions between solitary cell migration and collective cell migration. Taken together, our data demonstrate the emergence of persistently polarized, collective cell movement arising from CIL between colliding cells.

A mechanobiological perspective on cadherins and the actin-myosin cytoskeleton

Classical cadherin receptors mediate morphogenetic cell-cell interactions within many tissues of the body. Their biological impact often entails cooperation between cadherin adhesion and the actin cytoskeleton, but how this may occur and – even more urgently – how this leads to morphogenetic outcomes are questions that remain poorly understood. Here, we suggest that the emerging field of cadherin mechanobiology provides a useful new perspective from which to revisit these issues. We propose that the actin cytoskeleton can be considered as an active agent that mediates how cadherin junctions resist, sense and transduce forces between cells.

FRAP analysis reveals stabilization of adhesion structures in the epidermis compared to cultured keratinocytes

Proper development and tissue maintenance requires cell-cell adhesion structures, which serve diverse and crucial roles in tissue morphogenesis. Epithelial tissues have three main types of cell-cell junctions: tight junctions, which play a major role in barrier formation, and adherens junctions and desmosomes, which provide mechanical stability and organize the underlying cytoskeleton. Our current understanding of adhesion function is hindered by a lack of tools and methods to image junctions in mammals. To better understand the dynamics of adhesion in tissues we have created a knock-in ZO-1-GFP mouse and a BAC-transgenic mouse expressing desmoplakin I-GFP. We performed fluorescence recovery after photobleaching (FRAP) experiments to quantify the turnover rates of the tight junction protein ZO-1, the adherens junction protein E-cadherin, and the desmosomal protein desmoplakin in the epidermis. Proteins at each type of junction are remarkably stable in the epidermis, in contrast to the high observed mobility of E-cadherin and ZO-1 at adherens junctions and tight junctions, respectively, in cultured cells. Our data demonstrate that there are additional mechanisms for stabilizing junctions in tissues that are not modeled by cell culture.

Mechanotransduction: vinculin provides stability when tension rises

By beautiful imaging and state-of-the-art experiments, vinculin is established to be a central switch in mechanotransduction at integrin-based focal adhesions. Cycles of tension-regulated vinculin switching control focal adhesion dynamics and signaling to enable polarized cell migration and alignment.

Forces in tissue morphogenesis and patterning

During development, mechanical forces cause changes in size, shape, number, position, and gene expression of cells. They are therefore integral to any morphogenetic processes. Force generation by actin-myosin networks and force transmission through adhesive complexes are two self-organizing phenomena driving tissue morphogenesis. Coordination and integration of forces by long-range force transmission and mechanosensing of cells within tissues produce large-scale tissue shape changes. Extrinsic mechanical forces also control tissue patterning by modulating cell fate specification and differentiation. Thus, the interplay between tissue mechanics and biochemical signaling orchestrates tissue morphogenesis and patterning in development.

Vinculin, cadherin mechanotransduction and homeostasis of cell-cell junctions

Cell adhesion junctions characteristically arise from the cooperative integration of adhesion receptors, cell signalling pathways and the cytoskeleton. This is exemplified by cell-cell interactions mediated by classical cadherin adhesion receptors. These junctions are sites where cadherin adhesion systems functionally couple to the dynamic actin cytoskeleton, a process that entails physical interactions with many actin regulators and regulation by cell signalling pathways. Such integration implies a potential role for molecules that may stand at the interface between adhesion, signalling and the cytoskeleton. One such candidate is the cortical scaffolding protein, vinculin, which is a component of both cell-cell and cell-matrix adhesions. While its contribution to integrin-based adhesions has been extensively studied, less is known about how vinculin contributes to cell-cell adhesions. A major recent advance has come with the realisation that cadherin adhesions are active mechanical structures, where cadherin serves as part of a mechanotransduction pathway by which junctions sense and elicit cellular responses to mechanical stimuli. Vinculin has emerged as an important element in cadherin mechanotransduction, a perspective that illuminates its role in cell-cell interactions. We now review its role as a cortical scaffold and its role in cadherin mechanotransduction.

Oscillation and polarity of E-cadherin asymmetries control actomyosin flow patterns during morphogenesis

Actomyosin flows are involved in a variety of cellular processes, including cytokinesis, cell migration, polarization, and morphogenesis. In epithelia, flow polarization orients cell deformations. It is unclear, however, how flows are polarized and how global patterns of junction remodeling emerge from flow polarization locally. We address this question during intercalation-driving extension of the Drosophila germband. Intercalation is associated with polarized junction remodeling, whereby actomyosin pulses flow anisotropically toward dorsal-ventral junctions and shrink them. Here, we show that planar polarization of flows emerges from polarized fluctuations in the levels of E-cadherin clusters that produce transient and oscillating asymmetries of coupling. These fluctuations are triggered by polarized E-cadherin endocytosis and are amplified by flow itself. This work suggests that fluctuations and mechanical instability are not the consequences of limited control over the systems key parameters, but rather that they define the axis of symmetry breaking.

Myosin-1c regulates the dynamic stability of E-cadherin-based cell-cell contacts in polarized Madin-Darby canine kidney cells

Myo1c knockdown causes defects in E-cadherin localization, E-cadherin binding, and cell–cell contact of Madin–Darby canine kidney cells. Expression of wild-type Myo1c, but not motor-dead mutants or those unable to bind membrane, reverses the phenotype, evidence that Myo1c modulates the assembly/maintenance of adherens junctions.

Cooperation between cadherins and the actin cytoskeleton controls the formation and maintenance of cell–cell adhesions in epithelia. We find that the molecular motor protein myosin-1c (Myo1c) regulates the dynamic stability of E-cadherin–based cell–cell contacts. In Myo1c-depleted Madin–Darby canine kidney cells, E-cadherin localization was dis­organized and lateral membranes appeared less vertical with convoluted edges versus control cells. In polarized monolayers, Myo1c-knockdown (KD) cells were more sensitive to reduced calcium concentration. Myo1c separated in the same plasma membrane fractions as E-cadherin, and Myo1c KD caused a significant reduction in the amount of E-cadherin recovered in one peak fraction. Expression of green fluorescent protein (GFP)–Myo1c mutants revealed that the phosphatidylinositol-4,5-bisphosphate–binding site is necessary for its localization to cell–cell adhesions, and fluorescence recovery after photobleaching assays with GFP-Myo1c mutants revealed that motor function was important for Myo1c dynamics at these sites. At 18°C, which inhibits vesicle recycling, Myo1c-KD cells accumulated more E-cadherin–positive vesicles in their cytoplasm, suggesting that Myo1c affects E-cadherin endocytosis. Studies with photoactivatable GFP–E-cadherin showed that Myo1c KD reduced the stability of E-cadherin at cell–cell adhesions. We conclude that Myo1c stabilizes E-cadherin at adherens junctions in polarized epithelial cells and that the motor function and ability of Myo1c to bind membrane are critical.

beta-Catenin protects the epidermis from mechanical stresses

β-Catenin provides stability to epithelia exposed to mechanical stress in part by strengthening adherens junctions in response to tension.

Many tissues in our body experience mechanical stresses caused by both internal and external forces. The skin, for example, must tolerate diverse mechanical insults. In this paper, we report a role for β-catenin in providing stability to epithelia under stress. Loss of β-catenin during epidermal development caused perinatal lethality. Mutant embryos up-regulated stress responses at sites of active morphogenesis, which became more widespread after the stresses associated with birth. In addition, selective loss of tight junctions occurred in focal regions. This was recapitulated in cultured β-catenin–null cells exposed to externally applied forces. In addition, mutant cells were defective in tension-induced engagement of adherens junctions. We found that β-catenin was required to recruit vinculin to the cell cortex and to strengthen the junction’s association with the underlying cytoskeleton in response to tension. These data demonstrate that a complete understanding of the functions of cell adhesion proteins must take into account their roles in response to mechanical stresses.

ZO-1 recruitment to α-catenin--a novel mechanism for coupling the assembly of tight junctions to adherens junctions

The formation of a barrier between epithelial cells is a fundamental determinant of cellular homeostasis, protecting underlying cells against pathogens, dehydration and damage. Assembly of the tight junction barrier is dependent upon neighboring epithelial cells binding to one another and forming adherens junctions, but the mechanism for how these processes are linked is poorly understood. Using a knockdown and substitution system, we studied whether ZO-1 binding to α-catenin is required for coupling tight junction assembly to the formation of adherens junctions. We found that preventing ZO-1 binding to α-catenin did not appear to affect adherens junctions. Rather the assembly and maintenance of the epithelial barrier were disrupted. This disruption was accompanied by alterations in the mobility of ZO-1 and the organization of the actin cytoskeleton. Thus, our study identifies α-catenin binding to ZO-1 as a new mechanism for coupling the assembly of the epithelial barrier to cell-to-cell adhesion.

Rho activation is apically restricted by Arhgap1 in neural crest cells and drives epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transitions (EMTs) are crucial for morphogenesis and carcinoma metastasis, yet mechanisms controlling the underlying cell behaviors are poorly understood. RhoGTPase signaling has been implicated in EMT; however, previous studies have yielded conflicting results regarding Rho function, and its role in EMT remains poorly understood. Elucidation of precise Rho functions has been challenging because Rho signaling is highly context dependent and its activity is tightly regulated spatiotemporally within the cell. To date, few studies have examined how Rho affects cell motility in intact organisms, and the pattern of Rho activity during motile cell behaviors of EMT has not been determined in any system. Here, we image endogenous active Rho during EMT in vivo, and analyze effects of Rho and Rho-kinase (ROCK) manipulation on cell motility in vivo. We show that Rho is activated in a discrete apical region of premigratory neural crest cells during EMT, and Rho-ROCK signaling is essential for apical detachment and generation of motility within the neuroepithelium, a process that has been poorly understood. Furthermore, we find that Arhgap1 restricts Rho activation to apical areas, and this restriction is necessary for detachment. Our results provide new insight into mechanisms controlling local Rho activation and how it affects dynamic cell behaviors and actomyosin contraction during key steps of EMT in an intact living organism.

α-catenin, vinculin, and F-actin in strengthening E-cadherin cell-cell adhesions and mechanosensing

Classical cadherins play a crucial role in establishing intercellular adhesion, regulating cortical tension, and maintaining mechanical coupling between cells. The mechanosensitive regulation of intercellular adhesion strengthening depends on the recruitment of adhesion complexes at adhesion sites and their anchoring to the actin cytoskeleton. Thus, the molecular mechanisms coupling cadherin-associated complexes to the actin cytoskeleton are actively being studied, with a particular focus on α-catenin and vinculin. We have recently addressed the role of these proteins by analyzing the consequences of their depletion and the expression of α-catenin mutants in the formation and strengthening of cadherin-mediated adhesions. We have used the dual pipette assay to measure the forces required to separate cell doublets formed in suspension. In this commentary, we briefly summarize the current knowledge on the role of α-catenin and vinculin in cadherin-actin cytoskeletal interactions. These data shed light on the tension-dependent contribution of α-catenin and vinculin in a mechanoresponsive complex that promotes the connection between cadherin and the actin cytoskeleton and their requirement in the development of adhesion strengthening.

An autoinhibited structure of α-catenin and its implications for vinculin recruitment to adherens junctions

α-Catenin is an actin- and vinculin-binding protein that regulates cell-cell adhesion by interacting with cadherin adhesion receptors through β-catenin, but the mechanisms by which it anchors the cadherin-catenin complex to the actin cytoskeleton at adherens junctions remain unclear. Here we determined crystal structures of αE-catenin in the autoinhibited state and the actin-binding domain of αN-catenin. Together with the small-angle x-ray scattering analysis of full-length αN-catenin, we deduced an elongated multidomain assembly of monomeric α-catenin that structurally and functionally couples the vinculin- and actin-binding mechanisms. Cellular and biochemical studies of αE- and αN-catenins show that αE-catenin recruits vinculin to adherens junctions more effectively than αN-catenin, partly because of its higher affinity for actin filaments. We propose a molecular switch mechanism involving multistate conformational changes of α-catenin. This would be driven by actomyosin-generated tension to dynamically regulate the vinculin-assisted linkage between adherens junctions and the actin cytoskeleton.

Fluid shear stress on endothelial cells modulates mechanical tension across VE-cadherin and PECAM-1

Fluid shear stress (FSS) from blood flow acting on the endothelium critically regulates vascular morphogenesis, blood pressure, and atherosclerosis. FSS applied to endothelial cells (ECs) triggers signaling events including opening of ion channels, activation of signaling pathways, and changes in gene expression. Elucidating how ECs sense flow is important for understanding both normal vascular function and disease. EC responses to FSS are mediated in part by a junctional mechanosensory complex consisting of VE-cadherin, PECAM-1, and VEGFR2. Previous work suggested that flow increases force on PECAM-1, which initiates signaling. Deletion of PECAM-1 blocks responses to flow in vitro and flow-dependent vascular remodeling in vivo. To understand this process, we developed and validated FRET-based tension sensors for VE-cadherin and PECAM-1 using our previously developed FRET tension biosensor. FRET measurements showed that in static culture, VE-cadherin in cell-cell junctions bears significant myosin-dependent tension, whereas there was no detectable tension on VE-cadherin outside of junctions. Onset of shear stress triggered a rapid (<30 s) decrease in tension across VE-cadherin, which paralleled a decrease in total cell-cell junctional tension. Flow triggered a simultaneous increase in tension across junctional PECAM-1, while nonjunctional PECAM-1 was unaffected. Tension on PECAM-1 was mediated by flow-stimulated association with vimentin. These data confirm the prediction that shear increases force on PECAM-1. However, they also argue against the current model of passive transfer of force through the cytoskeleton to the junctions, showing instead that flow triggers cytoskeletal remodeling, which alters forces across the junctional receptors.

Distinct Rap1 activity states control the extent of epithelial invagination via α-catenin

Localized cell shape change initiates epithelial folding, while neighboring cell invagination determines the final depth of an epithelial fold. The mechanism that controls the extent of invagination remains unknown. During Drosophila gastrulation, a higher number of cells undergo invagination to form the deep posterior dorsal fold, whereas far fewer cells become incorporated into the initially very similar anterior dorsal fold. We find that a decrease in α-catenin activity causes the anterior fold to invaginate as extensively as the posterior fold. In contrast, constitutive activation of the small GTPase Rap1 restricts invagination of both dorsal folds in an α-catenin-dependent manner. Rap1 activity appears spatially modulated by Rapgap1, whose expression levels are high in the cells that flank the posterior fold but low in the anterior fold. We propose a model whereby distinct activity states of Rap1 modulate α-catenin-dependent coupling between junctions and actin to control the extent of epithelial invagination.

On fundamental cellular processes for emergence of collective epithelial movement

In all animals, collective cell movement is an essential process in many events, including wound healing and embryonic development. However, our understanding of what characterizes the emergence of multicellular collective behavior is still far from complete. In this article we showed the fundamental cellular processes that drive collective cell movement by means of integrated approaches, including precise quantification measurements and mathematical modeling of measured data. First, we observed the dependence of the collective behaviors of cultured human skin cells on Ca²⁺ concentrations. When the culturing area confined by a PDMS sheet was suddenly expanded by removing the sheet, the group of cells moved to the expanded area with higher collectivity at higher Ca²⁺ concentrations. Next, we quantitatively measured cellular responses to the Ca²⁺ treatments, such as cell growth, cell division, and the strength of intercellular adhesion. Using a femtosecond-laser-based assay, an original method for estimating intercellular adhesion, we found that the strength of intercellular adhesion has an approximately 13-fold range in our treatments. Incorporating the quantitative data into a mathematical model, we then confirmed that the model well reproduced the multicellular behaviors we observed, demonstrating that the strength of intercellular adhesion sufficiently determines the generation of collective cell movement. Finally, we performed extensive numerical experiments, and the results suggested that the emergence of collective cell movement is derived by an optimal balance between the strength of intercellular adhesion and the intensity of cell migration.

An α-catenin déjà vu

α-Catenin exists as part of the cadherin-catenin adhesion complex as well as in a cytoplasmic pool. However, which of these pools is responsible for its biological impact remains controversial. A structure-function analysis in Drosophila melanogaster illuminates how the molecular properties of α-catenin translate into functional outcomes in an intact organism.

The circumferential actomyosin belt in epithelial cells is regulated by the Lulu2-p114RhoGEF system

In epithelial cells, myosin-II-dependent forces regulate many aspects of animal morphogenesis, such as apical constriction, cell intercalation, cell sorting, and the formation and maintenance of the adherens junction. These forces are mainly generated by the circumferential actomyosin belt, which is composed of F-actin–myosin II bundles located along apical cell–cell junctions. Although several of the molecular pathways regulating the belt have been identified, the precise mechanisms underlying its function are largely unknown. Our recent studies identified Lulu proteins (Lulu1 and Lulu2), FERM-domain-containing molecules, as the regulators of the belt. Lulus activate the circumferential actomyosin belt and thereby induce apical constriction in epithelial cells; conversely, RNAi-mediated Lulu-knockdown results in the severe disorganization of the circumferential actomyosin belt. We also showed that p114RhoGEF is a downstream molecule of Lulu2 in its regulation of the belt; Lulu2 enhances the catalytic activity of p114RhoGEF through a direct interaction and thereby activates the circumferential actomyosin belt. We further identified aPKC and Patj as regulators of Lulu2-p114RhoGEF. In this commentary, we discuss current knowledge of the circumferential actomyosin belt's regulation, focusing on the Lulu2-p114RhoGEF system.

Mechanosensitive systems at the cadherin-F-actin interface

Cells integrate biochemical and mechanical information to function within multicellular tissue. Within developing and remodeling tissues, mechanical forces contain instructive information that governs important cellular processes that include stem cell maintenance, differentiation and growth. Although the principles of signal transduction (protein phosphorylation, allosteric regulation of enzymatic activity and binding sites) are the same for biochemical and mechanical-induced signaling, the first step of mechanosensing, in which protein complexes under tension transduce changes in physical force into cellular signaling, is very different, and the molecular mechanisms are only beginning to be elucidated. In this Commentary, we focus on mechanotransduction at cell-cell junctions, aiming to comprehend the molecular mechanisms involved. We describe how different junction structures are associated with the actomyosin cytoskeleton and how this relates to the magnitude and direction of forces at cell-cell junctions. We discuss which cell-cell adhesion receptors have been shown to take part in mechanotransduction. Then we outline the force-induced molecular events that might occur within a key mechanosensitive system at cell-cell junctions; the cadherin-F-actin interface, at which α-catenin and vinculin form a central module. Mechanotransduction at cell-cell junctions emerges as an important signaling mechanism, and we present examples of its potential relevance for tissue development and disease.

Monomeric α-catenin links cadherin to the actin cytoskeleton

The linkage of adherens junctions to the actin cytoskeleton is essential for cell adhesion. The contribution of the cadherin-catenin complex to the interaction between actin and the adherens junction remains an intensely investigated subject that centres on the function of α-catenin, which binds to cadherin through β-catenin and can bind F-actin directly or indirectly. Here, we delineate regions within Drosophila α-Catenin (α-Cat) that are important for adherens junction performance in static epithelia and dynamic morphogenetic processes. Moreover, we address whether persistent α-catenin-mediated physical linkage between cadherin and F-actin is crucial for cell adhesion and characterize the functions of α-catenin monomers and dimers at adherens junctions. Our data support the view that monomeric α-catenin acts as an essential physical linker between the cadherin-β-catenin complex and the actin cytoskeleton, whereas α-catenin dimers are cytoplasmic and form an equilibrium with monomeric junctional α-catenin.

α-Catenin and vinculin cooperate to promote high E-cadherin-based adhesion strength

Maintaining cell cohesiveness within tissues requires that intercellular adhesions develop sufficient strength to support traction forces applied by myosin motors and by neighboring cells. Cadherins are transmembrane receptors that mediate intercellular adhesion. The cadherin cytoplasmic domain recruits several partners, including catenins and vinculin, at sites of cell-cell adhesion. Our study used force measurements to address the role of αE-catenin and vinculin in the regulation of the strength of E-cadherin-based adhesion. αE-catenin-deficient cells display only weak aggregation and fail to strengthen intercellular adhesion over time, a process rescued by the expression of αE-catenin or chimeric E-cadherin·αE-catenins, including a chimera lacking the αE-catenin dimerization domain. Interestingly, an αE-catenin mutant lacking the modulation and actin-binding domains restores cadherin-dependent cell-cell contacts but cannot strengthen intercellular adhesion. The expression of αE-catenin mutated in its vinculin-binding site is defective in its ability to rescue cadherin-based adhesion strength in cells lacking αE-catenin. Vinculin depletion or the overexpression of the αE-catenin modulation domain strongly decreases E-cadherin-mediated adhesion strength. This supports the notion that both molecules are required for intercellular contact maturation. Furthermore, stretching of cell doublets increases vinculin recruitment and α18 anti-αE-catenin conformational epitope immunostaining at cell-cell contacts. Taken together, our results indicate that αE-catenin and vinculin cooperatively support intercellular adhesion strengthening, probably via a mechanoresponsive link between the E-cadherin·β-catenin complexes and the underlying actin cytoskeleton.

Fine tuning of tissues' viscosity and surface tension through contractility suggests a new role for α-catenin

What governs tissue organization and movement? If molecular and genetic approaches are able to give some answers on these issues, more and more works are now giving a real importance to mechanics as a key component eventually triggering further signaling events. We chose embryonic cell aggregates as model systems for tissue organization and movement in order to investigate the origin of some mechanical constraints arising from cells organization. Steinberg et al. proposed a long time ago an analogy between liquids and tissues and showed that indeed tissues possess a measurable tissue surface tension and viscosity. We question here the molecular origin of these parameters and give a quantitative measurement of adhesion versus contractility in the framework of the differential interfacial tension hypothesis. Accompanying surface tension measurements by angle measurements (at vertexes of cell-cell contacts) at the cell/medium interface, we are able to extract the full parameters of this model: cortical tensions and adhesion energy. We show that a tunable surface tension and viscosity can be achieved easily through the control of cell-cell contractility compared to cell-medium one. Moreover we show that -catenin is crucial for this regulation to occur: these molecules appear as a catalyser for the remodeling of the actin cytoskeleton underneath cell-cell contact, enabling a differential contractility between the cell-medium and cell-cell interface to take place.

N-cadherin dependent collective cell invasion of prostate cancer cells is regulated by the N-terminus of α-catenin

Cancer cell invasion is the critical first step of metastasis, yet, little is known about how cancer cells invade and initiate metastasis in a complex extracellular matrix. Using a cell line from bone metastasis of prostate cancer (PC3), we analyzed how prostate cancer cells migrate in a physiologically relevant 3D Matrigel. We found that PC3 cells migrated more efficiently as multi-cellular clusters than isolated single cells, suggesting that the presence of cell-cell adhesion improves 3D cell migration. Perturbation of N-cadherin function by transfection of either the N-cadherin cytoplasmic domain or shRNA specific to N-cadherin abolished collective cell migration. Interestingly, PC3 cells do not express α-catenin, an actin binding protein in the cadherin complex. When the full-length α-catenin was re-introduced, the phenotype of PC3 cells reverted back to a more epithelial phenotype with a decreased cell migration rate in 3D Matrigel. Interestingly, we found that the N-terminal half of α-catenin was sufficient to suppress invasive phenotype. Taken together, these data suggest that the formation of N-cadherin junctions promotes 3D cell migration of prostate cancer cells, and this is partly due to an aberrant regulation of the N-cadherin complex in the absence of α-catenin.