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

Cortical tension initiates the positive feedback loop between cadherin and F-actin

Adherens junctions physically link two cells at their contact interface via extracellular binding between cadherin molecules and intracellular interactions between cadherins and the actin cytoskeleton. Cadherin and actomyosin cytoskeletal dynamics are regulated reciprocally by mechanical and chemical signals, which subsequently determine the strength of cell-cell adhesions and the emergent organization and stiffness of the tissues they form. However, an understanding of the integrated system is lacking. We present a new mechanistic computational model of intercellular junction maturation in a cell doublet to investigate the mechanochemical cross talk that regulates adherens junction formation and homeostasis. The model couples a two-dimensional lattice-based simulation of cadherin dynamics with a reaction-diffusion representation of the reorganising actomyosin network through its regulation by Rho signalling at the intracellular junction. We demonstrate that local immobilization of cadherin induces cluster formation in a cis-less-dependent manner. We then recapitulate the process of cell-cell contact formation. Our model suggests that cortical tension applied on the contact rim can explain the ring distribution of cadherin and actin filaments (F-actin) on the cell-cell contact of the cell doublet. Furthermore, we propose and test the hypothesis that cadherin and F-actin interact like a positive feedback loop, which is necessary for formation of the ring structure. Different patterns of cadherin distribution were observed as an emergent property of disturbances of this positive feedback loop. We discuss these findings in light of available experimental observations on underlying mechanisms related to cadherin/F-actin binding and the mechanical environment.

Actin-Associated Proteins and Small Molecules Targeting the Actin Cytoskeleton

Actin-associated proteins (AAPs) act on monomeric globular actin (G-actin) and polymerized filamentous actin (F-actin) to regulate their dynamics and architectures which ultimately control cell movement, shape change, division; organelle localization and trafficking. Actin-binding proteins (ABPs) are a subset of AAPs. Since actin was discovered as a myosin-activating protein (hence named actin) in 1942, the protein has also been found to be expressed in non-muscle cells, and numerous AAPs continue to be discovered. This review article lists all of the AAPs discovered so far while also allowing readers to sort the list based on the names, sizes, functions, related human diseases, and the dates of discovery. The list also contains links to the UniProt and Protein Atlas databases for accessing further, related details such as protein structures, associated proteins, subcellular localization, the expression levels in cells and tissues, mutations, and pathology. Because the actin cytoskeleton is involved in many pathological processes such as tumorigenesis, invasion, and developmental diseases, small molecules that target actin and AAPs which hold potential to treat these diseases are also listed.

Plectin-mediated cytoskeletal crosstalk controls cell tension and cohesion in epithelial sheets

The coordinated interplay of cytoskeletal networks critically determines tissue biomechanics and structural integrity. Here, we show that plectin, a major intermediate filament-based cytolinker protein, orchestrates cortical cytoskeletal networks in epithelial sheets to support intercellular junctions. By combining CRISPR/Cas9-based gene editing and pharmacological inhibition, we demonstrate that in an F-actin-dependent context, plectin is essential for the formation of the circumferential keratin rim, organization of radial keratin spokes, and desmosomal patterning. In the absence of plectin-mediated cytoskeletal cross-linking, the aberrant keratin-desmosome (DSM)-network feeds back to the actin cytoskeleton, which results in elevated actomyosin contractility. Also, by complementing a predictive mechanical model with Förster resonance energy transfer-based tension sensors, we provide evidence that in the absence of cytoskeletal cross-linking, major intercellular junctions (adherens junctions and DSMs) are under intrinsically generated tensile stress. Defective cytoarchitecture and tensional disequilibrium result in reduced intercellular cohesion, associated with general destabilization of plectin-deficient sheets upon mechanical stress.

Viscoelasticity, Like Forces, Plays a Role in Mechanotransduction

Viscoelasticity and its alteration in time and space has turned out to act as a key element in fundamental biological processes in living systems, such as morphogenesis and motility. Based on experimental and theoretical findings it can be proposed that viscoelasticity of cells, spheroids and tissues seems to be a collective characteristic that demands macromolecular, intracellular component and intercellular interactions. A major challenge is to couple the alterations in the macroscopic structural or material characteristics of cells, spheroids and tissues, such as cell and tissue phase transitions, to the microscopic interferences of their elements. Therefore, the biophysical technologies need to be improved, advanced and connected to classical biological assays. In this review, the viscoelastic nature of cytoskeletal, extracellular and cellular networks is presented and discussed. Viscoelasticity is conceptualized as a major contributor to cell migration and invasion and it is discussed whether it can serve as a biomarker for the cells' migratory capacity in several biological contexts. It can be hypothesized that the statistical mechanics of intra- and extracellular networks may be applied in the future as a powerful tool to explore quantitatively the biomechanical foundation of viscoelasticity over a broad range of time and length scales. Finally, the importance of the cellular viscoelasticity is illustrated in identifying and characterizing multiple disorders, such as cancer, tissue injuries, acute or chronic inflammations or fibrotic diseases.

Biophysical Approaches for Applying and Measuring Biological Forces

Over the past decades, increasing evidence has indicated that mechanical loads can regulate the morphogenesis, proliferation, migration, and apoptosis of living cells. Investigations of how cells sense mechanical stimuli or the mechanotransduction mechanism is an active field of biomaterials and biophysics. Gaining a further understanding of mechanical regulation and depicting the mechanotransduction network inside cells require advanced experimental techniques and new theories. In this review, the fundamental principles of various experimental approaches that have been developed to characterize various types and magnitudes of forces experienced at the cellular and subcellular levels are summarized. The broad applications of these techniques are introduced with an emphasis on the difficulties in implementing these techniques in special biological systems. The advantages and disadvantages of each technique are discussed, which can guide readers to choose the most suitable technique for their questions. A perspective on future directions in this field is also provided. It is anticipated that technical advancement can be a driving force for the development of mechanobiology.

Active chemo-mechanical feedbacks dictate the collective migration of cells on patterned surfaces

Recent evidence has demonstrated that, when cultured on micro-patterned surfaces, living cells can move in a coordinated manner and form distinct migration patterns, including flowing chain, suspended propagating bridge, rotating vortex, etc. However, the fundamental question of exactly how and why cells migrate in these fashions remains elusive. Here, we present a theoretical investigation to show that the tight interplay between internal cellular activities, such as chemo-mechanical feedbacks and polarization, and external geometrical constraints are behind these intriguing experimental observations. In particular, on narrow strip patterns, strongly force-dependent cellular contractility and intercellular adhesion were found to be critical for reinforcing the leading edge of the migrating cell monolayer and eventually result in the formation of suspended cell bridges flying over nonadhesive regions. On the other hand, a weak force-contractility feedback led to the movement of cells like a flowing chain along the adhesive strip. Finally, we also showed that the random polarity forces generated in migrating cells are responsible for driving them into rotating vortices on strips with width above a threshold value (~10 times the size of the cell).

Local contractions regulate E-cadherin rigidity sensing

E-cadherin is a major cell-cell adhesion molecule involved in mechanotransduction at cell-cell contacts in tissues. Because epithelial cells respond to rigidity and tension in tissue through E-cadherin, there must be active processes that test and respond to the mechanical properties of these adhesive contacts. Using submicrometer, E-cadherin-coated polydimethylsiloxane pillars, we find that cells generate local contractions between E-cadherin adhesions and pull to a constant distance for a constant duration, irrespective of pillar rigidity. These cadherin contractions require nonmuscle myosin IIB, tropomyosin 2.1, α-catenin, and binding of vinculin to α-catenin. Cells spread to different areas on soft and rigid surfaces with contractions, but spread equally on soft and rigid without. We further observe that cadherin contractions enable cells to test myosin IIA-mediated tension of neighboring cells and sort out myosin IIA-depleted cells. Thus, we suggest that epithelial cells test and respond to the mechanical characteristics of neighboring cells through cadherin contractions.

Regulating life after death: how mechanical communication mediates the epithelial response to apoptosis

It is increasingly evident that cells in tissues and organs can communicate with one another using mechanical forces. Such mechanical signalling can serve as a basis for the assembly of cellular communities. For this to occur, there must be local instabilities in tissue mechanics that are the source of the signals, and mechanisms for changes in mechanical force to be transmitted and detected within tissues. In this review, we discuss these principles using the example of cell death by apoptosis, when it occurs in epithelia. This elicits the phenomenon of apical extrusion, which can rapidly eliminate apoptotic cells by expelling them from the epithelium. Apoptotic extrusion requires that epithelial cells detect the presence of nearby apoptotic cells, something which can be elicited by the mechanotransduction of tensile instabilities caused by the apoptotic cell. We discuss the central role that adherens junctions can play in the transmission and detection of mechanical signals from apoptotic cells.

Mechanical disruption of E-cadherin complexes with epidermal growth factor receptor actuates growth factor-dependent signaling

Increased intercellular tension is associated with enhanced cell proliferation and tissue growth. Here, we present evidence for a force-transduction mechanism that links mechanical perturbations of epithelial (E)-cadherin (CDH1) receptors to the force-dependent activation of epidermal growth factor receptor (EGFR, ERBB1)-a key regulator of cell proliferation. Here, coimmunoprecipitation studies first show that E-cadherin and EGFR form complexes at the plasma membrane that are disrupted by either epidermal growth factor (EGF) or increased tension on homophilic E-cadherin bonds. Although force on E-cadherin bonds disrupts the complex in the absence of EGF, soluble EGF is required to mechanically activate EGFR at cadherin adhesions. Fully quantified spectral imaging fluorescence resonance energy transfer further revealed that E-cadherin and EGFR directly associate to form a heterotrimeric complex of two cadherins and one EGFR protein. Together, these results support a model in which the tugging forces on homophilic E-cadherin bonds trigger force-activated signaling by releasing EGFR monomers to dimerize, bind EGF ligand, and signal. These findings reveal the initial steps in E-cadherin-mediated force transduction that directly link intercellular force fluctuations to the activation of growth regulatory signaling cascades.

The Drosophila anterior-posterior axis is polarized by asymmetric myosin activation

The Drosophila anterior-posterior axis is specified at mid-oogenesis when the Par-1 kinase is recruited to the posterior cortex of the oocyte, where it polarizes the microtubule cytoskeleton to define where the axis determinants, bicoid and oskar mRNAs, localize. This polarity is established in response to an unknown signal from the follicle cells, but how this occurs is unclear. Here we show that the myosin chaperone Unc-45 and non-muscle myosin II (MyoII) are required upstream of Par-1 in polarity establishment. Furthermore, the myosin regulatory light chain (MRLC) is di-phosphorylated at the oocyte posterior in response to the follicle cell signal, inducing longer pulses of myosin contractility at the posterior that may increase cortical tension. Overexpression of MRLC-T21A that cannot be di-phosphorylated or treatment with the myosin light-chain kinase inhibitor ML-7 abolishes Par-1 localization, indicating that the posterior of MRLC di-phosphorylation is essential for both polarity establishment and maintenance. Thus, asymmetric myosin activation polarizes the anterior-posterior axis by recruiting and maintaining Par-1 at the posterior cortex. This raises an intriguing parallel with anterior-posterior axis formation in C. elegans, where MyoII also acts upstream of the PAR proteins to establish polarity, but to localize the anterior PAR proteins rather than Par-1.

Eph/Ephrin-Based Protein Complexes: The Importance of cis Interactions in Guiding Cellular Processes

Although intracellular signal transduction is generally represented as a linear process that transmits stimuli from the exterior of a cell to the interior via a transmembrane receptor, interactions with additional membrane-associated proteins are often critical to its success. These molecules play a pivotal role in mediating signaling via the formation of complexes in cis (within the same membrane) with primary effectors, particularly in the context of tumorigenesis. Such secondary effectors may act to promote successful signaling by mediating receptor-ligand binding, recruitment of molecular partners for the formation of multiprotein complexes, or differential signaling outcomes. One signaling family whose contact-mediated activity is frequently modulated by lateral interactions at the cell surface is Eph/ephrin (EphA and EphB receptor tyrosine kinases and their ligands ephrin-As and ephrin-Bs). Through heterotypic interactions in cis, these molecules can promote a diverse range of cellular activities, including some that are mutually exclusive (cell proliferation and cell differentiation, or adhesion and migration). Due to their broad expression in most tissues and their promiscuous binding within and across classes, the cellular response to Eph:ephrin interaction is highly variable between cell types and is dependent on the cellular context in which binding occurs. In this review, we will discuss interactions between molecules in cis at the cell membrane, with emphasis on their role in modulating Eph/ephrin signaling.

Langerhans cells and SFRP2/Wnt/beta-catenin signalling control adaptation of skin epidermis to mechanical stretching

Skin can be mechanically stimulated to grow through a clinical procedure called tissue expansion (TE). Using a porcine TE model, we determined that expansion promptly activates transcription of SFRP2 in skin and we revealed that in the epidermis, this protein is secreted by Langerhans cells (LCs). Similar to well‐known mechanosensitive genes, the increase in SFRP2 expression was proportional to the magnitude of tension, showing a spike at the apex of the expanded skin. This implies that SFRP2 might be a newly discovered effector of mechanotransduction pathways. In addition, we found that acute stretching induces accumulation of b‐catenin in the nuclei of basal keratinocytes (KCs) and LCs, indicating Wnt signalling activation, followed by cell proliferation. Moreover, TE‐activated LCs proliferate and migrate into the suprabasal layer of skin, suggesting that LCs rebuild their steady network within the growing epidermis. We demonstrated that in vitro hrSFRP2 treatment on KCs inhibits Wnt/b‐catenin signalling and stimulates KC differentiation. In parallel, we observed an accumulation of KRT10 in vivo in the expanded skin, pointing to TE‐induced, SFRP2‐augmented KC maturation. Overall, our results reveal that a network of LCs delivers SFRP2 across the epidermis to fine‐tune Wnt/b‐catenin signalling to restore epidermal homeostasis disrupted by TE.

The Autophagic Route of E-Cadherin and Cell Adhesion Molecules in Cancer Progression

Simple Summary

A hallmark of carcinoma progression is the loss of epithelial integrity. In this context, the deregulation of adhesion molecules, such as E-cadherin, affects epithelial structures and associates with epithelial to mesenchymal transition (EMT). This, in turn, fosters cancer progression. Autophagy endows cancer cells with the ability to overcome intracellular and environmental stress stimuli, such as anoikis, nutrient deprivation, hypoxia, and drugs. Furthermore, it plays an important role in the degradation of cell adhesion proteins and in EMT. This review focuses on the interplay between the turnover of adhesion molecules, primarily E-cadherin, and autophagy in cancer progression.

Abstract

Cell-to-cell adhesion is a key element in epithelial tissue integrity and homeostasis during embryogenesis, response to damage, and differentiation. Loss of cell adhesion and gain of mesenchymal features, a phenomenon known as epithelial to mesenchymal transition (EMT), are essential steps in cancer progression. Interestingly, downregulation or degradation by endocytosis of epithelial adhesion molecules (e.g., E-cadherin) associates with EMT and promotes cell migration. Autophagy is a physiological intracellular degradation and recycling process. In cancer, it is thought to exert a tumor suppressive role in the early phases of cell transformation but, once cells have gained a fully transformed phenotype, autophagy may fuel malignant progression by promoting EMT and conferring drug resistance. In this review, we discuss the crosstalk between autophagy, EMT, and turnover of epithelial cell adhesion molecules, with particular attention to E-cadherin.

Force-induced changes of α-catenin conformation stabilize vascular junctions independently of vinculin

Cadherin-mediated cell adhesion requires anchoring via the β-catenin–α-catenin complex to the actin cytoskeleton, yet, α-catenin only binds F-actin weakly. A covalent fusion of VE-cadherin to α-catenin enhances actin anchorage in endothelial cells and strongly stabilizes endothelial junctions in vivo, blocking inflammatory responses. Here, we have analyzed the underlying mechanism. We found that VE-cadherin–α-catenin constitutively recruits the actin adaptor vinculin. However, removal of the vinculin-binding region of α-catenin did not impair the ability of VE-cadherin–α-catenin to enhance junction integrity. Searching for an alternative explanation for the junction-stabilizing mechanism, we found that an antibody-defined epitope, normally buried in a short α1-helix of the actin-binding domain (ABD) of α-catenin, is openly displayed in junctional VE-cadherin–α-catenin chimera. We found that this epitope became exposed in normal α-catenin upon triggering thrombin-induced tension across the VE-cadherin complex. These results suggest that the VE-cadherin–α-catenin chimera stabilizes endothelial junctions due to conformational changes in the ABD of α-catenin that support constitutive strong binding to actin.

Summary: There are novel antibody epitopes at the actin-binding domain of α-catenin that correlate with high affinity binding and are exposed in junction-stabilizing VE-cadherin–α-catenin fusion proteins.

Direction of epithelial folding defines impact of mechanical forces on epithelial state

Cell shape dynamics during development is tightly regulated and coordinated with cell fate determination. Triggered by an interplay between biochemical and mechanical signals, epithelia form complex tissues by undergoing coordinated cell shape changes, but how such spatiotemporal coordination is controlled remains an open question. To dissect biochemical signaling from purely mechanical cues, we developed a microfluidic system that experimentally triggers epithelial folding to recapitulate stereotypic deformations observed in vivo. Using this system, we observe that the apical or basal direction of folding results in strikingly different mechanical states at the fold boundary, where the balance between tissue tension and torque (arising from the imposed curvature) controls the spread of folding-induced calcium waves at a short timescale and induces spatial patterns of gene expression at longer timescales. Our work uncovers that folding-associated gradients of cell shape and their resulting mechanical stresses direct spatially distinct biochemical responses within the monolayer.

Multivalent interactions make adherens junction-cytoskeletal linkage robust during morphogenesis

Embryogenesis requires cells to change shape and move without disrupting epithelial integrity. This requires robust, responsive linkage between adherens junctions and the actomyosin cytoskeleton. Using Drosophila morphogenesis, we define molecular mechanisms mediating junction-cytoskeletal linkage and explore the role of mechanosensing. We focus on the junction-cytoskeletal linker Canoe, a multidomain protein. We engineered the canoe locus to define how its domains mediate its mechanism of action. To our surprise, the PDZ and FAB domains, which we thought connected junctions and F-actin, are not required for viability or mechanosensitive recruitment to junctions under tension. The FAB domain stabilizes junctions experiencing elevated force, but in its absence, most cells recover, suggesting redundant interactions. In contrast, the Rap1-binding RA domains are critical for all Cno functions and enrichment at junctions under tension. This supports a model in which junctional robustness derives from a large protein network assembled via multivalent interactions, with proteins at network nodes and some node connections more critical than others.

Anti-Malignant Effect of Tensile Loading to Adherens Junctions in Cutaneous Squamous Cell Carcinoma Cells

Actomyosin contractility regulates various cellular processes including proliferation and differentiation while dysregulation of actomyosin activity contributes to cancer development and progression. Previously, we have reported that actomyosin-generated tension at adherens junctions is required for cell density-dependent inhibition of proliferation of normal skin keratinocytes. However, it remains unclear how actomyosin contractility affects the hyperproliferation ability of cutaneous squamous cell carcinoma (cSCC) cells. In this study, we find that actomyosin activity is impaired in cSCC cells both in vitro and in vivo. External application of tensile loads to adherens junctions by sustained mechanical stretch attenuates the proliferation of cSCC cells, which depends on intact adherens junctions. Forced activation of actomyosin of cSCC cells also inhibits their proliferation in a cell-cell contact-dependent manner. Furthermore, the cell cycle arrest induced by tensile loading to adherens junctions is accompanied by epidermal differentiation in cSCC cells. Our results show that the degree of malignant properties of cSCC cells can be reduced by applying tensile loads to adherens junctions, which implies that the mechanical status of adherens junctions may serve as a novel therapeutic target for cSCC.

Conventional rigid 2D substrates cause complex contractile signals in monolayers of human induced pluripotent stem cell-derived cardiomyocytes

Abstract

Human induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CM) in monolayers interact mechanically via cell–cell and cell–substrate adhesion. Spatiotemporal features of contraction were analysed in hiPSC‐CM monolayers (1) attached to glass or plastic (Young's modulus (E) >1 GPa), (2) detached (substrate‐free) and (3) attached to a flexible collagen hydrogel (E = 22 kPa). The effects of isoprenaline on contraction were compared between rigid and flexible substrates. To clarify the underlying mechanisms, further gene expression and computational studies were performed. HiPSC‐CM monolayers exhibited multiphasic contractile profiles on rigid surfaces in contrast to hydrogels, substrate‐free cultures or single cells where only simple twitch‐like time‐courses were observed. Isoprenaline did not change the contraction profile on either surface, but its lusitropic and chronotropic effects were greater in hydrogel compared with glass. There was no significant difference between stiff and flexible substrates in regard to expression of the stress‐activated genes NPPA and NPPB. A computational model of cell clusters demonstrated similar complex contractile interactions on stiff substrates as a consequence of cell‐to‐cell functional heterogeneity. Rigid biomaterial surfaces give rise to unphysiological, multiphasic contractions in hiPSC‐CM monolayers. Flexible substrates are necessary for normal twitch‐like contractility kinetics and interpretation of inotropic interventions.

Key points

Spatiotemporal contractility analysis of human induced pluripotent stem cell‐derived cardiomyocyte (hiPSC‐CM) monolayers seeded on conventional, rigid surfaces (glass or plastic) revealed the presence of multiphasic contraction patterns across the monolayer with a high variability, despite action potentials recorded in the same areas being identical.

These multiphasic patterns are not present in single cells, in detached monolayers or in monolayers seeded on soft substrates such as a hydrogel, where only ‘twitch’‐like transients are observed.

HiPSC‐CM monolayers that display a high percentage of regions with multiphasic contraction have significantly increased contractile duration and a decreased lusotropic drug response.

There is no indication that the multiphasic contraction patterns are associated with significant activation of the stress‐activated NPPA or NPPB signalling pathways.

A computational model of cell clusters supports the biological findings that the rigid surface and the differential cell–substrate adhesion underly multiphasic contractile behaviour of hiPSC‐CMs.

Mechanotransduction at the Plasma Membrane-Cytoskeleton Interface

Mechanical cues are crucial for survival, adaptation, and normal homeostasis in virtually every cell type. The transduction of mechanical messages into intracellular biochemical messages is termed mechanotransduction. While significant advances in biochemical signaling have been made in the last few decades, the role of mechanotransduction in physiological and pathological processes has been largely overlooked until recently. In this review, the role of interactions between the cytoskeleton and cell-cell/cell-matrix adhesions in transducing mechanical signals is discussed. In addition, mechanosensors that reside in the cell membrane and the transduction of mechanical signals to the nucleus are discussed. Finally, we describe two examples in which mechanotransduction plays a significant role in normal physiology and disease development. The first example is the role of mechanotransduction in the proliferation and metastasis of cancerous cells. In this system, the role of mechanotransduction in cellular processes, including proliferation, differentiation, and motility, is described. In the second example, the role of mechanotransduction in a mechanically active organ, the gastrointestinal tract, is described. In the gut, mechanotransduction contributes to normal physiology and the development of motility disorders.

Holding it together: when cadherin meets cadherin

Intercellular adhesion is the key to multicellularity, and its malfunction plays an important role in various developmental and disease-related processes. Although it has been intensively studied by both biologists and physicists, a commonly accepted definition of cell-cell adhesion is still being debated. Cell-cell adhesion has been described at the molecular scale as a function of adhesion receptors controlling binding affinity, at the cellular scale as resistance to detachment forces or modulation of surface tension, and at the tissue scale as a regulator of cellular rearrangements and morphogenesis. In this review, we aim to summarize and discuss recent advances in the molecular, cellular, and theoretical description of cell-cell adhesion, ranging from biomimetic models to the complexity of cells and tissues in an organismal context. In particular, we will focus on cadherin-mediated cell-cell adhesion and the role of adhesion signaling and mechanosensation therein, two processes central for understanding the biological and physical basis of cell-cell adhesion.

Efficient fabrication of stretching hydrogels with programmable strain gradients as cell sheet delivery vehicles

Fabricating functional cell sheets with excellent mechanical strength for tissue regeneration remains challenging. Therefore, we devised a novel 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide/N-hydroxy-succinimide crosslinked hydrogel carrier composed of gelatin (Ge) and beta-cyclodextrin (β-CD) that promoted the adhesion and proliferation of keratinocytes (Kcs) compared with those cultured on a Ge hydrogel due to significantly higher pore size, porosity, and stiffness, as confirmed using field emission scanning electron microscopy (FE-SEM) and shear wave elastography (SWE). Upon exposure to a programmable gradient microenvironment, cells displayed a stress/strain-dependent spatial-temporal distribution of extended cellular phenotypes and cytoskeletons. The promoted proliferation of Kcs and the increased retention of the undifferentiated cell phenotype on Ge-β-CD composite hydrogels under a 15% strain led to the accelerated detachment of cell sheets with retained cell-cell junctions. Moreover, the stretch-triggered upregulated expression of phosphorylated yes-associated protein (YAP) 1 suggested that this effect might be associated with the mechanical stimulation-induced activation of the YAP pathway.

Angulin-1 seals tricellular contacts independently of tricellulin and claudins

Tricellular tight junctions (tTJs) are specialized tight junctions (TJs) that seal the intercellular space at tricellular contacts (TCs), where the vertices of three epithelial cells meet. Tricellulin and angulin family membrane proteins are known constituents of tTJs, but the molecular mechanism of tTJ formation remains elusive. Here, we investigated the roles of angulin-1 and tricellulin in tTJ formation in MDCK II cells by genome editing. Angulin-1-deficient cells lost the plasma membrane contact at TCs with impaired epithelial barrier function. The C terminus of angulin-1 bound to the TJ scaffold protein ZO-1, and disruption of their interaction influenced the localization of claudins at TCs, but not the tricellular sealing. Strikingly, the plasma membrane contact at TCs was formed in tricellulin- or claudin-deficient cells. These findings demonstrate that angulin-1 is responsible for the plasma membrane seal at TCs independently of tricellulin and claudins.

Enhanced RhoA signalling stabilizes E-cadherin in migrating epithelial monolayers

Epithelia migrate as physically coherent populations of cells. Previous studies have revealed that mechanical stress accumulates in these cellular layers as they move. These stresses are characteristically tensile in nature and have often been inferred to arise when moving cells pull upon the cell-cell adhesions that hold them together. We now report that epithelial tension at adherens junctions between migrating cells also increases due to an increase in RhoA-mediated junctional contractility. We found that active RhoA levels were stimulated by p114 RhoGEF (also known as ARHGEF18) at the junctions between migrating MCF-7 monolayers, and this was accompanied by increased levels of actomyosin and mechanical tension. Applying a strategy to restore active RhoA specifically at adherens junctions by manipulating its scaffold, anillin, we found that this junctional RhoA signal was necessary to stabilize junctional E-cadherin (CDH1) during epithelial migration and promoted orderly collective movement. We suggest that stabilization of E-cadherin by RhoA serves to increase cell-cell adhesion to protect against the mechanical stresses of migration. This article has an associated First Person interview with the first author of the paper.

Strain induced mechanoresponse depends on cell contractility and BAG3-mediated autophagy

Basically all mammalian tissues are constantly exposed to a variety of environmental mechanical signals. Depending on the signal strength, mechanics intervenes in a multitude of cellular processes and is thus capable to induce simple cellular adaptations but also complex differentiation processes and even apoptosis. The underlying recognition typically depends on mechanosensitive proteins, which most often sense the mechanical signal for the induction of a cellular signaling cascade by changing their protein conformation. However, the fate of mechanosensors after mechanical stress application is still poorly understood and it remains unclear whether protein degradation pathways affect the mechanosensitivity of cells. Here, we show that cyclic stretch induces autophagosome formation in a time-dependent manner. Formation depends on the cochaperone BAG3 and thus likely involves BAG3-mediated chaperone-assisted selective autophagy. Furthermore, we demonstrate that strain-induced cell reorientation is clearly delayed upon inhibition of autophagy, suggesting a bidirectional crosstalk between mechanotransduction and autophagic degradation. The strength of the observed delay depends on stable adhesion structures and stress fiber formation in a RhoA-dependent manner.

Maintaining proteostasis under mechanical stress

Cell survival, tissue integrity and organismal health depend on the ability to maintain functional protein networks even under conditions that threaten protein integrity. Protection against such stress conditions involves the adaptation of folding and degradation machineries, which help to preserve the protein network by facilitating the refolding or disposal of damaged proteins. In multicellular organisms, cells are permanently exposed to stress resulting from mechanical forces. Yet, for long time mechanical stress was not recognized as a primary stressor that perturbs protein structure and threatens proteome integrity. The identification and characterization of protein folding and degradation systems, which handle force-unfolded proteins, marks a turning point in this regard. It has become apparent that mechanical stress protection operates during cell differentiation, adhesion and migration and is essential for maintaining tissues such as skeletal muscle, heart and kidney as well as the immune system. Here, we provide an overview of recent advances in our understanding of mechanical stress protection.

Investigating the nature of active forces in tissues reveals how contractile cells can form extensile monolayers

Actomyosin machinery endows cells with contractility at a single-cell level. However, within a monolayer, cells can be contractile or extensile based on the direction of pushing or pulling forces exerted by their neighbours or on the substrate. It has been shown that a monolayer of fibroblasts behaves as a contractile system while epithelial or neural progentior monolayers behave as an extensile system. Through a combination of cell culture experiments and in silico modelling, we reveal the mechanism behind this switch in extensile to contractile as the weakening of intercellular contacts. This switch promotes the build-up of tension at the cell-substrate interface through an increase in actin stress fibres and traction forces. This is accompanied by mechanotransductive changes in vinculin and YAP activation. We further show that contractile and extensile differences in cell activity sort cells in mixtures, uncovering a generic mechanism for pattern formation during cell competition, and morphogenesis.

Inside-out regulation of E-cadherin conformation and adhesion

Cadherin cell-cell adhesion proteins play key roles in tissue morphogenesis and wound healing. Cadherin ectodomains bind in two conformations, X-dimers and strand-swap dimers, with different adhesive properties. However, the mechanisms by which cells regulate ectodomain conformation are unknown. Cadherin intracellular regions associate with several actin-binding proteins including vinculin, which are believed to tune cell-cell adhesion by remodeling the actin cytoskeleton. Here, we show at the single-molecule level, that vinculin association with the cadherin cytoplasmic region allosterically converts weak X-dimers into strong strand-swap dimers and that this process is mediated by myosin II-dependent changes in cytoskeletal tension. We also show that in epithelial cells, ∼70% of apical cadherins exist as strand-swap dimers while the remaining form X-dimers, providing two cadherin pools with different adhesive properties. Our results demonstrate the inside-out regulation of cadherin conformation and establish a mechanistic role for vinculin in this process.

Sorting of cadherin-catenin-associated proteins into individual clusters

The cytoplasmic tails of classical cadherins form a multiprotein cadherin-catenin complex (CCC) that constitutes the major structural unit of adherens junctions (AJs). The CCC in AJs forms junctional clusters, "E clusters," driven by cis and trans interactions in the cadherin ectodomain and stabilized by α-catenin-actin interactions. Additional proteins are known to bind to the cytoplasmic region of the CCC. Here, we analyze how these CCC-associated proteins (CAPs) integrate into cadherin clusters and how they affect the clustering process. Using a cross-linking approach coupled with mass spectrometry, we found that the majority of CAPs, including the force-sensing protein vinculin, interact with CCCs outside of AJs. Accordingly, structural modeling shows that there is not enough space for CAPs the size of vinculin to integrate into E clusters. Using two CAPs, scribble and erbin, as examples, we provide evidence that these proteins form separate clusters, which we term "C clusters." As proof of principle, we show, by using cadherin ectodomain monoclonal antibodies (mAbs), that mAb-bound E-cadherin forms separate clusters that undergo trans interactions. Taken together, our data suggest that, in addition to its role in cell-cell adhesion, CAP-driven CCC clustering serves to organize cytoplasmic proteins into distinct domains that may synchronize signaling networks of neighboring cells within tissues.

Direct measurement of near-nanoNewton forces developed by self-organizing actomyosin fibres bound α-catenin

Actin cytoskeleton contractility plays a critical role in morphogenetic processes by generating forces that are then transmitted to cell-cell and cell-ECM adhesion complexes. In turn, mechanical properties of the environment are sensed and transmitted to the cytoskeleton at cell adhesion sites, influencing cellular processes such as cell migration, differentiation and survival. Anchoring of the actomyosin cytoskeleton to adhesion sites is mediated by adaptor proteins such as talin or α-catenin that link F-actin to transmembrane cell adhesion receptors, thereby allowing mechanical coupling between the intracellular and extracellular compartments. Thus, a key issue is to be able to measure the forces generated by actomyosin and transmitted to the adhesion complexes. Approaches developed in cells and those probing single molecule mechanical properties of α-catenin molecules allowed to identify α-catenin, an F-actin binding protein which binds to the cadherin complexes as a major player in cadherin-based mechanotransduction. However, it is still very difficult to bridge intercellular forces measured at cellular levels and those measured at the single-molecule level. Here, we applied an intermediate approach allowing reconstruction of the actomyosin-α-catenin complex in acellular conditions to probe directly the transmitted forces. For this, we combined micropatterning of purified α-catenin and spontaneous actomyosin network assembly in the presence of G-actin and Myosin II with microforce sensor arrays used so far to measure cell-generated forces. Using this method, we show that self-organizing actomyosin bundles bound to micrometric α-catenin patches can apply near-nanoNewton forces, paving the way for future studies on molecular/cellular mechanotarnsduction and mechanosensing. This article is protected by copyright. All rights reserved.

The Epithelial Cell Leak Pathway

The epithelial cell tight junction structure is the site of the transepithelial movement of solutes and water between epithelial cells (paracellular permeability). Paracellular permeability can be divided into two distinct pathways, the Pore Pathway mediating the movement of small ions and solutes and the Leak Pathway mediating the movement of large solutes. Claudin proteins form the basic paracellular permeability barrier and mediate the movement of small ions and solutes via the Pore Pathway. The Leak Pathway remains less understood. Several proteins have been implicated in mediating the Leak Pathway, including occludin, ZO proteins, tricellulin, and actin filaments, but the proteins comprising the Leak Pathway remain unresolved. Many aspects of the Leak Pathway, such as its molecular mechanism, its properties, and its regulation, remain controversial. In this review, we provide a historical background to the evolution of the Leak Pathway concept from the initial examinations of paracellular permeability. We then discuss current information about the properties of the Leak Pathway and present current theories for the Leak Pathway. Finally, we discuss some recent research suggesting a possible molecular basis for the Leak Pathway.

Feeling the force: Multiscale force sensing and transduction at the cell-cell interface

A universal principle of all living cells is the ability to sense and respond to mechanical stimuli which is essential for many biological processes. Recent efforts have identified critical mechanosensitive molecules and response pathways involved in mechanotransduction during development and tissue homeostasis. Tissue-wide force transmission and local force sensing need to be spatiotemporally coordinated to precisely regulate essential processes during development such as tissue morphogenesis, patterning, cell migration and organogenesis. Understanding how cells identify and interpret extrinsic forces and integrate a specific response on cell and tissue level remains a major challenge. In this review we consider important cellular and physical factors in control of cell-cell mechanotransduction and discuss their significance for cell and developmental processes. We further highlight mechanosensitive macromolecules that are known to respond to external forces and present examples of how force responses can be integrated into cell and developmental programs.

Decoding mechanical cues by molecular mechanotransduction

Cells are exposed to a variety of mechanical cues, including forces from their local environment and physical properties of the tissue. These mechanical cues regulate a vast number of cellular processes, relying on a repertoire of mechanosensors that transduce forces into biochemical pathways through mechanotransduction. Forces can act on different parts of the cell, carry information regarding magnitude and direction, and have distinct temporal profiles. Thus, the specific cellular response to mechanical forces is dependent on the ability of cells to sense and transduce these physical parameters. In this review, we will highlight recent findings that provide insights into the mechanisms by which different mechanosensors decode mechanical cues and how their coordinated response determines the cellular outcomes.

The EMT transcription factor Snai1 maintains myocardial wall integrity by repressing intermediate filament gene expression

The transcription factor Snai1, a well-known regulator of epithelial-to-mesenchymal transition, has been implicated in early cardiac morphogenesis as well as in cardiac valve formation. However, a role for Snai1 in regulating other aspects of cardiac morphogenesis has not been reported. Using genetic, transcriptomic, and chimeric analyses in zebrafish, we find that Snai1b is required in cardiomyocytes for myocardial wall integrity. Loss of snai1b increases the frequency of cardiomyocyte extrusion away from the cardiac lumen. Extruding cardiomyocytes exhibit increased actomyosin contractility basally as revealed by enrichment of p-myosin and α-catenin epitope α-18, as well as disrupted intercellular junctions. Transcriptomic analysis of wild-type and snai1b mutant hearts revealed the dysregulation of intermediate filament genes, including desmin b (desmb) upregulation. Cardiomyocyte-specific desmb overexpression caused increased cardiomyocyte extrusion, recapitulating the snai1b mutant phenotype. Altogether, these results indicate that Snai1 maintains the integrity of the myocardial epithelium, at least in part by repressing desmb expression.

Junctional ER Organization Affects Mechanotransduction at Cadherin-Mediated Adhesions

Cadherin-mediated adhesions (also known as adherens junctions) are adhesive complexes that connect neighboring cells in a tissue. While the role of the actin cytoskeleton in withstanding tension at these sites of contact is well documented, little is known about the involvement of microtubules and the associated endoplasmic reticulum (ER) network in cadherin mechanotransduction. Therefore, we investigated how the organization of ER extensions in close proximity of cadherin-mediated adhesions can affect such complexes, and vice versa. Here, we show that the extension of the ER to cadherin-mediated adhesions is tension dependent and appears to be cadherin-type specific. Furthermore, the different structural organization of the ER/microtubule network seems to affect the localization of ER-bound PTP1B at cadherin-mediated adhesions. This phosphatase is involved in the modulation of vinculin, a molecular clutch which enables differential engagement of the cadherin-catenin layer with the actomyosin cytoskeleton in response to tension. This suggests a link between structural organization of the ER/microtubule network around cadherin-specific adhesions, to control the mechanotransduction of adherens junctions by modulation of vinculin conformational state.

Mechanical processes underlying precise and robust cell matching

During the development of complicated multicellular organisms, the robust formation of specific cell-cell connections (cell matching) is required for the generation of precise tissue structures. Mismatches or misconnections can lead to various diseases. Diverse mechanical cues, including differential adhesion and temporally varying cell contractility, are involved in regulating the process of cell-cell recognition and contact formation. Cells often start the process of cell matching through contact via filopodia protrusions, mediated by specific adhesion interactions at the cell surface. These adhesion interactions give rise to differential mechanical signals that can be further perceived by the cells. In conjunction with contractions generated by the actomyosin networks within the cells, this differentially coded adhesion information can be translated to reposition and sort cells. Here, we review the role of these different cell matching components and suggest how these mechanical factors cooperate with each other to facilitate specificity in cell-cell contact formation.

Cadherin puncta are interdigitated dynamic actin protrusions necessary for stable cadherin adhesion

Cadherins harness the actin cytoskeleton to build cohesive sheets of cells using paradoxically weak bonds, but the molecular mechanisms are poorly understood. In one popular model, actin organizes cadherins into large, micrometer-sized clusters known as puncta. Myosin is thought to pull on these puncta to generate strong adhesion. Here, however, we show that cadherin puncta are actually interdigitated actin microspikes generated by actin polymerization mediated by three factors (Arp2/3, EVL, and CRMP-1). The convoluted membranes in these regions give the impression of cadherin clustering by fluorescence microscopy, but the ratio of cadherin to membrane is constant. Nevertheless, these interlocking fingers of membrane are important for adhesion because perturbing their formation disrupts cell adhesion. In contrast, blocking myosin-dependent contractility does not disrupt either the interdigitated microspikes or lateral membrane adhesion. "Puncta" are zones of strong cell-cell adhesion not due to cadherin clustering but that occur because the interdigitated microspikes expand the surface area available for adhesive bond formation and increase the asperity of the cell surface to promote friction between cells.

Role of Intermediate Filaments in Blood-Brain Barrier in Health and Disease

The blood–brain barrier (BBB) is a highly selective cellular monolayer unique to the microvasculature of the central nervous system (CNS), and it mediates the communication of the CNS with the rest of the body by regulating the passage of molecules into the CNS microenvironment. Limitation of passage of substances through the BBB is mainly due to tight junctions (TJ) and adherens junctions (AJ) between brain microvascular endothelial cells. The importance of actin filaments and microtubules in establishing and maintaining TJs and AJs has been indicated; however, recent studies have shown that intermediate filaments are also important in the formation and function of cell–cell junctions. The most common intermediate filament protein in endothelial cells is vimentin. Vimentin plays a role in blood–brain barrier permeability in both cell–cell and cell–matrix interactions by affecting the actin and microtubule reorganization and by binding directly to VE-cadherin or integrin proteins. The BBB permeability increases due to the formation of stress fibers and the disruption of VE–cadherin interactions between two neighboring cells in various diseases, disrupting the fiber network of intermediate filament vimentin in different ways. Intermediate filaments may be long ignored key targets in regulation of BBB permeability in health and disease.

Calcium signaling mediates a biphasic mechanoadaptive response of endothelial cells to cyclic mechanical stretch

The vascular system is precisely regulated to adjust blood flow to organismal demand, thereby guaranteeing adequate perfusion under varying physiological conditions. Mechanical forces, such as cyclic circumferential stretch, are among the critical stimuli that dynamically adjust vessel distribution and diameter, but the precise mechanisms of adaptation to changing forces are unclear. We find that endothelial monolayers respond to cyclic stretch by transient remodeling of the vascular endothelial cadherin–based adherens junctions and the associated actomyosin cytoskeleton. Time-resolved proteomic profiling reveals that this remodeling is driven by calcium influx through the mechanosensitive Piezo1 channel, triggering Rho activation to increase actomyosin contraction. As the mechanical stimulus persists, calcium signaling is attenuated through transient down-regulation of Piezo1 protein. At the same time, filamins are phosphorylated to increase monolayer stiffness, allowing mechanoadaptation to restore junctional integrity despite continuing exposure to stretch. Collectively, this study identifies a biphasic response to cyclic stretch, consisting of an initial calcium-driven junctional mechanoresponse, followed by mechanoadaptation facilitated by monolayer stiffening.

Mechanical Ring Interfaces between Adherens Junction and Contractile Actomyosin to Coordinate Entotic Cell-in-Cell Formation

Entosis is a cell-in-cell (CIC)-mediated death program. Contractile actomyosin (CA) and the adherens junction (AJ) are two core elements essential for entotic CIC formation, but the molecular structures interfacing them remain poorly understood. Here, we report the characterization of a ring-like structure interfacing between the peripheries of invading and engulfing cells. The ring-like structure is a multi-molecular complex consisting of adhesive and cytoskeletal proteins, in which the mechanical sensor vinculin is highly enriched. The vinculin-enriched structure senses mechanical force imposed on cells, as indicated by fluorescence resonance energy transfer (FRET) analysis, and is thus termed the mechanical ring (MR). The MR actively interacts with CA and the AJ to help establish and maintain polarized actomyosin that drives cell internalization. Vinculin depletion leads to compromised MR formation, CA depolarization, and subsequent CIC failure. In summary, we suggest that the vinculin-enriched MR, in addition to CA and AJ, is another core element essential for entosis.

Actin-based force generation and cell adhesion in tissue morphogenesis

The generation of organismal form - morphogenesis - arises from forces produced at the cellular level. In animal cells, much of this force is produced by the actin cytoskeleton. Here, we review how mechanisms of actin-based force generation are deployed during animal morphogenesis to sculpt organs and organisms. Furthermore, we consider how cytoskeletal forces are coupled through cell adhesions to propagate across tissues, and discuss cases where cytoskeletal force or adhesion is patterned across a tissue to direct shape changes. Together, our review provides a conceptual framework that reflects our current understanding of animal morphogenesis and gives perspectives on future opportunities for study.

Adherens junction engagement regulates functional patterning of the cardiac pacemaker cell lineage

Cardiac pacemaker cells (CPCs) rhythmically initiate the electrical impulses that drive heart contraction. CPCs display the highest rate of spontaneous depolarization in the heart despite being subjected to inhibitory electrochemical conditions that should theoretically suppress their activity. While several models have been proposed to explain this apparent paradox, the actual molecular mechanisms that allow CPCs to overcome electrogenic barriers to their function remain poorly understood. Here, we have traced CPC development at single-cell resolution and uncovered a series of cytoarchitectural patterning events that are critical for proper pacemaking. Specifically, our data reveal that CPCs dynamically modulate adherens junction (AJ) engagement to control characteristics including surface area, volume, and gap junctional coupling. This allows CPCs to adopt a structural configuration that supports their overall excitability. Thus, our data have identified a direct role for local cellular mechanics in patterning critical morphological features that are necessary for CPC electrical activity.

Hindbrain neuropore tissue geometry determines asymmetric cell-mediated closure dynamics in mouse embryos

Gap closure is a common morphogenetic process. In mammals, failure to close the embryonic hindbrain neuropore (HNP) gap causes fatal anencephaly. We observed that surface ectoderm cells surrounding the mouse HNP assemble high-tension actomyosin purse strings at their leading edge and establish the initial contacts across the embryonic midline. Fibronectin and laminin are present, and tensin 1 accumulates in focal adhesion-like puncta at this leading edge. The HNP gap closes asymmetrically, faster from its rostral than caudal end, while maintaining an elongated aspect ratio. Cell-based physical modeling identifies two closure mechanisms sufficient to account for tissue-level HNP closure dynamics: purse-string contraction and directional cell motion implemented through active crawling. Combining both closure mechanisms hastens gap closure and produces a constant rate of gap shortening. Purse-string contraction reduces, whereas crawling increases gap aspect ratio, and their combination maintains it. Closure rate asymmetry can be explained by asymmetric embryo tissue geometry, namely a narrower rostral gap apex, whereas biomechanical tension inferred from laser ablation is equivalent at the gaps' rostral and caudal closure points. At the cellular level, the physical model predicts rearrangements of cells at the HNP rostral and caudal extremes as the gap shortens. These behaviors are reproducibly live imaged in mouse embryos. Thus, mammalian embryos coordinate cellular- and tissue-level mechanics to achieve this critical gap closure event.

An asymmetric junctional mechanoresponse coordinates mitotic rounding with epithelial integrity

Epithelia are continuously self-renewed, but how epithelial integrity is maintained during the morphological changes that cells undergo in mitosis is not well understood. Here, we show that as epithelial cells round up when they enter mitosis, they exert tensile forces on neighboring cells. We find that mitotic cell-cell junctions withstand these tensile forces through the mechanosensitive recruitment of the actin-binding protein vinculin to cadherin-based adhesions. Surprisingly, vinculin that is recruited to mitotic junctions originates selectively from the neighbors of mitotic cells, resulting in an asymmetric composition of cadherin junctions. Inhibition of junctional vinculin recruitment in neighbors of mitotic cells results in junctional breakage and weakened epithelial barrier. Conversely, the absence of vinculin from the cadherin complex in mitotic cells is necessary to successfully undergo mitotic rounding. Our data thus identify an asymmetric mechanoresponse at cadherin adhesions during mitosis, which is essential to maintain epithelial integrity while at the same time enable the shape changes of mitotic cells.

Surface decoration of development-inspired synthetic N-cadherin motif via Ac-BP promotes osseointegration of metal implants

Research works on the synergistic effect of surface modified bioactive molecules and bone metal implants have been highlighted. N-cadherin is regarded as a key factor in directing cell-cell interactions during the mesenchymal condensation preceding the osteogenesis in the musculoskeletal system. In this study, the N-cadherin mimetic peptide (Cad) was biofunctionalized on the titanium metal surface via the acryloyl bisphosphonate (Ac-BP). To learn the synergistic effect of N-cadherin mimetic peptide, when tethered with titanium substrates, on promoting osteogenic differentiation of the seeded human mesenchymal stem cells (hMSCs) and the osseointegration at the bone-implant interfaces. Results show that the conjugation of N-cadherin mimetic peptide with Ac-BP promoted the osteogenic gene markers expression in the hMSCs. The biofunctionalized biomaterial surfaces promote the expression of the Wnt/β-catenin downstream axis in the attached hMSCs, and then enhance the in-situ bone formation and osseointegration at the bone-implant interfaces. We conclude that this N-cadherin mimetic peptide tethered on Ti surface promote osteogenic differentiation of hMSCs and osseointegration of biomaterial implants in vitro and in vivo. These findings demonstrate the importance of the development-inspired surface bioactivation of metal implants and shed light on the possible cellular mechanisms of the enhanced osseointegration.

Fluid shear stress promotes embryonic stem cell pluripotency via interplay between β-catenin and vinculin in bioreactor culture

The expansion of pluripotent stem cells (PSCs) as aggregates in stirred suspension bioreactors is garnering attention as an alternative to adherent culture. However, the hydrodynamic environment in the bioreactor can modulate PSC behavior, pluripotency and differentiation potential in ways that need to be well understood. In this study, we investigated how murine embryonic stem cells (mESCs) sense fluid shear stress and modulate a noncanonical Wnt signaling response to promote pluripotency. mESCs showed higher expression of pluripotency marker genes, Oct4, Sox2, and Nanog in the absence of leukemia inhibitory factor (LIF) in stirred suspension bioreactors compared to adherent culture, a phenomenon we have termed mechanopluripotency. In bioreactor culture, fluid shear promoted the nuclear translocation of the less well-known pluripotency regulator β-catenin and concomitant increase of c-Myc expression, an upstream regulator of Oct4, Sox2, and Nanog. We also observed similar β-catenin nuclear translocation in LIF-free mESCs cultured on E-cadherin substrate under defined fluid shear stress conditions in flow chamber plates. mESCs showed lower shear-induced expression of pluripotency marker genes when β-catenin was inhibited, suggesting that β-catenin signaling is crucial to mESC mechanopluripotency. Key to this process is vinculin, which is known to rearrange and associate more strongly with adherens junctions in response to fluid shear. When the vinculin gene is disrupted, we observe that nuclear β-catenin translocation and mechanopluripotency are abrogated. Our results indicate that mechanotransduction through the adherens junction complex is important for mESC pluripotency maintenance.

Specific substrates composed of collagen and fibronectin support the formation of epithelial cell sheets by MDCK cells lacking α-catenin or classical cadherins

The effect of the extracellular matrix substrates on the formation of epithelial cell sheets was studied using MDCK cells in which the α-catenin gene was disrupted. Although the mutant cells did not form an epithelial cell sheet in conventional cell culture, the cells formed an epithelial cell sheet when they were cultured on or in a collagen gel; the same results were not observed when cells were cultured on collagen-coated cover glasses or culture dishes. Moreover, the cells cultured on the cell culture inserts coated with fibronectin, Matrigel, or vitronectin formed epithelial cell sheets, whereas the cells cultured on cover glasses coated with these proteins did not form the structure, implying that the physical and chemical features of the substrates exert a profound effect on the formation of epithelial cell sheets. MDCK cells lacking the expression of E- and K-cadherins displayed similar properties. When the mutant MDCK cells were cultured in the presence of blebbistatin, they formed epithelial cell sheets, suggesting that myosin II was involved in the formation of these sheets. These cell sheets showed intimate cell-cell adhesion, and electron microscopy confirmed the formation of cell junctions. We propose that specific ECM substrates organize the formation of basic epithelial cell sheets, whereas classical cadherins stabilize cell-cell contacts and promote the formation of structures.

Activated nanoscale actin-binding domain motion in the catenin-cadherin complex revealed by neutron spin echo spectroscopy

As the core component of the adherens junction in cell-cell adhesion, the cadherin-catenin complex transduces mechanical tension between neighboring cells. Structural studies have shown that the cadherin-catenin complex exists as an ensemble of flexible conformations, with the actin-binding domain (ABD) of α-catenin adopting a variety of configurations. Here, we have determined the nanoscale protein domain dynamics of the cadherin-catenin complex using neutron spin echo spectroscopy (NSE), selective deuteration, and theoretical physics analyses. NSE reveals that, in the cadherin-catenin complex, the motion of the entire ABD becomes activated on nanosecond to submicrosecond timescales. By contrast, in the α-catenin homodimer, only the smaller disordered C-terminal tail of ABD is moving. Molecular dynamics (MD) simulations also show increased mobility of ABD in the cadherin-catenin complex, compared to the α-catenin homodimer. Biased MD simulations further reveal that the applied external forces promote the transition of ABD in the cadherin-catenin complex from an ensemble of diverse conformational states to specific states that resemble the actin-bound structure. The activated motion and an ensemble of flexible configurations of the mechanosensory ABD suggest the formation of an entropic trap in the cadherin-catenin complex, serving as negative allosteric regulation that impedes the complex from binding to actin under zero force. Mechanical tension facilitates the reduction in dynamics and narrows the conformational ensemble of ABD to specific configurations that are well suited to bind F-actin. Our results provide a protein dynamics and entropic explanation for the observed force-sensitive binding behavior of a mechanosensitive protein complex.

Distinct intramolecular interactions regulate autoinhibition of vinculin binding in αT-catenin and αE-catenin

α-Catenin binds directly to β-catenin and connects the cadherin–catenin complex to the actin cytoskeleton. Tension regulates α-catenin conformation. Actomyosin-generated force stretches the middle (M)-region to relieve autoinhibition and reveal a binding site for the actin-binding protein vinculin. It is not known whether the intramolecular interactions that regulate epithelial (αE)-catenin binding are conserved across the α-catenin family. Here, we describe the biochemical properties of testes (αT)-catenin, an α-catenin isoform critical for cardiac function and how intramolecular interactions regulate vinculin-binding autoinhibition. Isothermal titration calorimetry showed that αT-catenin binds the β-catenin–N-cadherin complex with a similar low nanomolar affinity to that of αE-catenin. Limited proteolysis revealed that the αT-catenin M-region adopts a more open conformation than αE-catenin. The αT-catenin M-region binds the vinculin N-terminus with low nanomolar affinity, indicating that the isolated αT-catenin M-region is not autoinhibited and thereby distinct from αE-catenin. However, the αT-catenin head (N- and M-regions) binds vinculin 1000-fold more weakly (low micromolar affinity), indicating that the N-terminus regulates the M-region binding to vinculin. In cells, αT-catenin recruitment of vinculin to cell–cell contacts requires the actin-binding domain and actomyosin-generated tension, indicating that force regulates vinculin binding. Together, our results show that the αT-catenin N-terminus is required to maintain M-region autoinhibition and modulate vinculin binding. We postulate that the unique molecular properties of αT-catenin allow it to function as a scaffold for building specific adhesion complexes.

Mechanical instability of adherens junctions overrides intrinsic quiescence of hair follicle stem cells

Vinculin, a mechanotransducer associated with both adherens junctions (AJs) and focal adhesions (FAs), plays a central role in force transmission through cell-cell and cell-substratum contacts. We generated the conditional knockout (cKO) of vinculin in murine skin that results in the loss of bulge stem cell (BuSC) quiescence and promotes continual cycling of the hair follicles. Surprisingly, we find that the AJs in vinculin cKO cells are mechanically weak and impaired in force generation despite increased junctional expression of E-cadherin and α-catenin. Mechanistically, we demonstrate that vinculin functions by keeping α-catenin in a stretched/open conformation, which in turn regulates the retention of YAP1, another potent mechanotransducer and regulator of cell proliferation, at the AJs. Altogether, our data provide mechanistic insights into the hitherto-unexplored regulatory link between the mechanical stability of cell junctions and contact-inhibition-mediated maintenance of BuSC quiescence.

MAGIs regulate aPKC to enable balanced distribution of intercellular tension for epithelial sheet homeostasis

Constriction of the apical plasma membrane is a hallmark of epithelial cells that underlies cell shape changes in tissue morphogenesis and maintenance of tissue integrity in homeostasis. Contractile force is exerted by a cortical actomyosin network that is anchored to the plasma membrane by the apical junctional complexes (AJC). In this study, we present evidence that MAGI proteins, structural components of AJC whose function remained unclear, regulate apical constriction of epithelial cells through the Par polarity proteins. We reveal that MAGIs are required to uniformly distribute Partitioning defective-3 (Par-3) at AJC of cells throughout the epithelial monolayer. MAGIs recruit ankyrin-repeat-, SH3-domain- and proline-rich-region-containing protein 2 (ASPP2) to AJC, which modulates Par-3-aPKC to antagonize ROCK-driven contractility. By coupling the adhesion machinery to the polarity proteins to regulate cellular contractility, we propose that MAGIs play essential and central roles in maintaining steady state intercellular tension throughout the epithelial cell sheet.

Matsuzawa et al. show that adhesion-related molecules MAGI-1 and MAGI-3 localize partitioning defective-3 (Par-3) at apical junctional complexes of cells throughout the epithelial monolayer. This study provides insights into how tension distribution contributes to cellular contractility in epithelial tissue homeostasis.