Lamellipodium extension and cadherin adhesion: two cell responses to cadherin activation relying on distinct signalling pathways

Mechanotransductive N-cadherin binding induces differentiation in human neural stem cells

The neural stem cell niche is a complex microenvironment that includes cellular factors, secreted factors, and physical factors that impact stem cell behavior and development. Cellular interactions through cadherins, cell-cell binding proteins, have implications in embryonic development and mesenchymal stem cell differentiation. However, little is known about the influence of cadherins within the neural stem cell microenvironment and their effect on human stem cell maintenance and differentiation. Therefore, the purpose of this study was to develop synthetic substrates to examine the effect of cadherin mechanotransduction on human neural stem cells. Glass substrates were fabricated using silane, protein A, and recombinant N-cadherin; we used these substrates to examine the effect of N-cadherin binding on neural stem cell proliferation, cytoskeletal structure and morphology, Yes-associated protein-1 (YAP) translocation, and differentiation. Bound exogenous N-cadherin induced concentration-dependent increases in adherens junction formation, YAP translocation, and early expression of neurogenic differentiation markers. Strong F-actin ring structures were initiated by homophilic N-cadherin binding, eliciting neuronal differentiation of cells within 96 ​h without added soluble differentiation factors. Our findings show that active N-cadherin binding plays an important role for differentiation of human iPS-derived neural stem cells towards neurons, providing a new tool to differentiate cells in vitro.

Cell Architecture and Dynamics of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hiPSC-CMs) on Hydrogels with Spatially Patterned Laminin and N-Cadherin

Controlling cellular shape with micropatterning extracellular matrix (ECM) proteins on hydrogels has been shown to improve the reproducibility of the cell structure, enhancing our ability to collect statistics on single-cell behaviors. Patterning methods have advanced efforts in developing human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) as a promising human model for studies of the heart structure, function, and disease. Patterned single hiPSC-CMs have exhibited phenotypes closer to mature, primary CMs across several metrics, including sarcomere alignment and contractility, area and aspect ratio, and force production. Micropatterning of hiPSC-CM pairs has shown further improvement of hiPSC-CM contractility compared to patterning single cells, suggesting that CM-CM interactions improve hiPSC-CM function. However, whether patterning single hiPSC-CMs on a protein associated with CM-CM adhesion, like N-cadherin, can drive similar enhancement of the hiPSC-CM structure and function has not been tested. To address this, we developed a novel dual-protein patterning process featuring covalent binding of proteins at the hydrogel surface to ensure robust force transfer and force sensing. The patterns comprised rectangular laminin islands for attachment across the majority of the cell area, with N-cadherin "end caps" to imitate CM-CM adherens junctions. We used this method to geometrically control single-cell CMs on deformable hydrogels suitable for traction force microscopy (TFM) to observe cellular dynamics. We seeded α-actinin::GFP-tagged hiPSC-CMs on dual-protein patterned hydrogels and verified the interaction between hiPSC-CMs and N-cadherin end caps via immunofluorescent staining. We found that hiPSC-CMs on dual-protein patterns exhibited higher cell area and contractility in the direction of sarcomere organization than those on laminin-only patterns but no difference in sarcomere organization or total force production. This work demonstrates a method for covalent patterning of multiple proteins on polyacrylamide hydrogels for mechanobiological studies. However, we conclude that N-cadherin only modestly improves single-cell patterned hiPSC-CM models and is not sufficient to elicit increases in contractility observed in hiPSC-CM pairs.

Structure and function of the membrane microdomains in osteoclasts

The cell membrane structure is closely related to the occurrence and progression of many metabolic bone diseases observed in the clinic and is an important target to the development of therapeutic strategies for these diseases. Strong experimental evidence supports the existence of membrane microdomains in osteoclasts (OCs). However, the potential membrane microdomains and the crucial mechanisms underlying their roles in OCs have not been fully characterized. Membrane microdomain components, such as scaffolding proteins and the actin cytoskeleton, as well as the roles of individual membrane proteins, need to be elucidated. In this review, we discuss the compositions and critical functions of membrane microdomains that determine the biological behavior of OCs through the three main stages of the OC life cycle.

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.

Cell-Cell Adhesion-Driven Contact Guidance and Its Effect on Human Mesenchymal Stem Cell Differentiation

Contact guidance has been extensively explored using patterned adhesion functionalities that predominantly mimic cell-matrix interactions. Whether contact guidance can also be driven by other types of interactions, such as cell-cell adhesion, still remains a question. Herein, this query is addressed by engineering a set of microstrip patterns of (i) cell-cell adhesion ligands and (ii) segregated cell-cell and cell-matrix ligands as a simple yet versatile set of platforms for the guidance of spreading, adhesion, and differentiation of mesenchymal stem cells. It was unprecedently found that micropatterns of cell-cell adhesion ligands can induce contact guidance. Surprisingly, it was found that patterns of alternating cell-matrix and cell-cell strips also induce contact guidance despite providing a spatial continuum for cell adhesion. This guidance is believed to be due to the difference between the potencies of the two adhesions. Furthermore, patterns that combine the two segregated adhesion functionalities were shown to induce more human mesenchymal stem cell osteogenic differentiation than monofunctional patterns. This work provides new insight into the functional crosstalk between cell-cell and cell-matrix adhesions and, overall, further highlights the ubiquitous impact of the biochemical anisotropy of the extracellular environment on cell function.

Mitotic cell responses to substrate topological cues are independent of the molecular nature of adhesion

Correct selection of the cell division axis is important for cell differentiation, tissue and organ morphogenesis, and homeostasis. Both integrins, which mediate interactions with extracellular matrix (ECM) components such as fibronectin, and cadherins, which mediate interactions between cells, are implicated in the determination of spindle orientation. We found that both cadherin- and integrin-based adhesion resulted in cell divisions parallel to the attachment plane and elicited identical spindle responses to spatial adhesive cues. This suggests that adhesion topology provides purely mechanical spatial cues that are independent of the molecular nature of the interaction or signaling from adhesion complexes. We also demonstrated that cortical integrin activation was indispensable for correct spindle orientation on both cadherin and fibronectin substrates. These data suggest that spindle orientation responses to adhesion topology are primarily a result of force anisotropy on the cell cortex and show that integrins play a central role in this process that is distinct from their role in cell-ECM interactions.

Myosin II isoforms play distinct roles in adherens junction biogenesis

Adherens junction (AJ) assembly under force is essential for many biological processes like epithelial monolayer bending, collective cell migration, cell extrusion and wound healing. The acto-myosin cytoskeleton acts as a major force-generator during the de novo formation and remodeling of AJ. Here, we investigated the role of non-muscle myosin II isoforms (NMIIA and NMIIB) in epithelial junction assembly. NMIIA and NMIIB differentially regulate biogenesis of AJ through association with distinct actin networks. Analysis of junction dynamics, actin organization, and mechanical forces of control and knockdown cells for myosins revealed that NMIIA provides the mechanical tugging force necessary for cell-cell junction reinforcement and maintenance. NMIIB is involved in E-cadherin clustering, maintenance of a branched actin layer connecting E-cadherin complexes and perijunctional actin fibres leading to the building-up of anisotropic stress. These data reveal unanticipated complementary functions of NMIIA and NMIIB in the biogenesis and integrity of AJ.

Mechanical stability of αT-catenin and its activation by force for vinculin binding

αT (Testes)-catenin, a critical factor regulating cell-cell adhesion in the heart, directly couples the cadherin-catenin complex to the actin cytoskeleton at the intercalated disk (ICD), a unique cell-cell junction that couples cardiomyocytes. Loss of αT-catenin in mice reduces plakophilin2 and connexin 43 recruitment to the ICD. Since αT-catenin is subjected to mechanical stretch during actomyosin contraction in cardiomyocytes, its activity could be regulated by mechanical force. To provide insight in how force regulates αT-catenin function, we investigated the mechanical stability of the putative, force-sensing middle (M) domain of αT-catenin and determined how force impacts vinculin binding to αT-catenin. We show that 1) physiological levels of force, <15 pN, are sufficient to unfold the three M domains; 2) the M1 domain that harbors the vinculin-binding site is unfolded at ∼6 pN; and 3) unfolding of the M1 domain is necessary for high-affinity vinculin binding. In addition, we quantified the binding kinetics and affinity of vinculin to the mechanically exposed binding site in M1 and observed that αT-catenin binds vinculin with low nanomolar affinity. These results provide important new insights into the mechanosensing properties of αT-catenin and how αT-catenin regulates cell-cell adhesion at the cardiomyocyte ICD.

High-throughput Exploration of the Network Dependent on AKT1 in Mouse Ovarian Granulosa Cells

The PI3K/AKT signaling pathway is known to regulate a broad range of cellular processes, and it is often altered in several types of cancers. Recently, somatic AKT1 mutations leading to a strong activation of this kinase have been reported in juvenile granulosa cell tumors. However, the molecular role of AKT1 in the supporting cell lineage of the ovary is still poorly understood. To get insights into its function in such cells, we depleted Akt1 in murine primary granulosa cells and assessed the molecular consequences at both the transcript and protein levels. We were able to corroborate the involvement of AKT1 in the regulation of metabolism, apoptosis, cell cycle, or cytoskeleton dynamics in this ovarian cell type. Consistently, we showed in established granulosa cells that depletion of Akt1 provoked altered directional persistent migration and increased its velocity. This study also allowed us to put forward new direct and indirect targets of the kinase. Indeed, a series of proteins involved in intracellular transport and mitochondrial physiology were significantly affected by Akt1 depletion. Using in silico analyses, we also propose a set of kinases and transcription factors that can mediate the action of AKT1 on the deregulated transcripts and proteins. Taken altogether, our results provide a resource of direct and indirect AKT1 targets in granulosa cells and may help understand its roles in this ovarian cell type.

High-throughput Exploration of the Network Dependent on AKT1 in Mouse Ovarian Granulosa Cells

The PI3K/AKT signaling pathway is known to regulate a broad range of cellular processes, and it is often altered in several types of cancers. Recently, somatic AKT1 mutations leading to a strong activation of this kinase have been reported in juvenile granulosa cell tumors. However, the molecular role of AKT1 in the supporting cell lineage of the ovary is still poorly understood. To get insights into its function in such cells, we depleted Akt1 in murine primary granulosa cells and assessed the molecular consequences at both the transcript and protein levels. We were able to corroborate the involvement of AKT1 in the regulation of metabolism, apoptosis, cell cycle, or cytoskeleton dynamics in this ovarian cell type. Consistently, we showed in established granulosa cells that depletion of Akt1 provoked altered directional persistent migration and increased its velocity. This study also allowed us to put forward new direct and indirect targets of the kinase. Indeed, a series of proteins involved in intracellular transport and mitochondrial physiology were significantly affected by Akt1 depletion. Using in silico analyses, we also propose a set of kinases and transcription factors that can mediate the action of AKT1 on the deregulated transcripts and proteins. Taken altogether, our results provide a resource of direct and indirect AKT1 targets in granulosa cells and may help understand its roles in this ovarian cell type.

Cadherin mechanotransduction in leader-follower cell specification during collective migration

Collective invasion drives the spread of multicellular cancer groups, into the normal tissue surrounding several epithelial tumors. Collective invasion recapitulates various aspects of the multicellular organization and collective migration that take place during normal development and repair. Collective migration starts with the specification of leader cells in which a polarized, migratory phenotype is established. Leader cells initiate and organize the migration of follower cells, to allow the group of cells to move as a cohesive and polarized unit. Leader-follower specification is essential for coordinated and directional collective movement. Forces exerted by cohesive cells represent key signals that dictate multicellular coordination and directionality. Physical forces originate from the contraction of the actomyosin cytoskeleton, which is linked between cells via cadherin-based cell-cell junctions. The cadherin complex senses and transduces fluctuations in forces into biochemical signals that regulate processes like cell proliferation, motility and polarity. With cadherin junctions being maintained in most collective movements the cadherin complex is ideally positioned to integrate mechanical information into the organization of collective cell migration. Here we discuss the potential roles of cadherin mechanotransduction in the diverse aspects of leader versus follower cell specification during collective migration and neoplastic invasion.

A new role for Notch in the control of polarity and asymmetric cell division of developing T cells

A fundamental question in biology is how single cells can reliably produce progeny of different cell types. Notch signalling frequently facilitates fate determination. Asymmetric cell division (ACD) often controls segregation of Notch signalling by imposing unequal inheritance of regulators of Notch. Here, we assessed the functional relationship between Notch and ACD in mouse T cell development. To attain immunological specificity, developing T cells must pass through a pivotal stage termed β-selection, which involves Notch signalling and ACD. We assessed functional interactions between Notch1 and ACD during β-selection using direct presentation of Notch ligands, DL1 and DL4, and pharmacological inhibition of Notch signalling. Contrary to prevailing models, we demonstrate that Notch controls the distribution of Notch1 itself and cell fate determinants, α-Adaptin and Numb. Further, Notch and CXCR4 signalling cooperated to drive polarity during division. Thus, Notch signalling directly orchestrates ACD, and Notch1 is differentially inherited by sibling cells.

Conjugation of carboxymethyl cellulose and dopamine for cell sheet harvesting

This manuscript focuses on the cell sheet preparation methodology with the conjugation of carboxymethylcellulose (CMC) and dopamine (DA).

Nanoscale mechanobiology of cell adhesions

Proper physiological functions of cells and tissues depend upon their abilities to sense, transduce, integrate, and generate mechanical and biochemical signals. Although such mechanobiological phenomena are widely observed, the molecular mechanisms driving these outcomes are still not fully understood. Cell adhesions formed by integrins and cadherins receptors are key structures that process diverse sources of signals to elicit complex mechanobiological responses. Since the nanoscale is the length scale at which molecules interact to relay force and information, the understanding of cell adhesions at the nanoscale level is important for grasping the inner logics of cellular decision making. Until recently, the study of the biological nanoscale has been restricted by available molecular and imaging tools. Fortunately, rapid technological advances have increasingly opened up the nanoscale realm to systematic investigations. In this review, we discuss current insights and key open questions regarding the nanoscale structure and function relationship of cell adhesions, focusing on recent progresses in characterizing their composition, spatial organization, and cytomechanical operation.

Changes in E-cadherin rigidity sensing regulate cell adhesion

Mechanical cues are sensed and transduced by cell adhesion complexes to regulate diverse cell behaviors. Extracellular matrix (ECM) rigidity sensing by integrin adhesions has been well studied, but rigidity sensing by cadherins during cell adhesion is largely unexplored. Using mechanically tunable polyacrylamide (PA) gels functionalized with the extracellular domain of E-cadherin (Ecad-Fc), we showed that E-cadherin-dependent epithelial cell adhesion was sensitive to changes in PA gel elastic modulus that produced striking differences in cell morphology, actin organization, and membrane dynamics. Traction force microscopy (TFM) revealed that cells produced the greatest tractions at the cell periphery, where distinct types of actin-based membrane protrusions formed. Cells responded to substrate rigidity by reorganizing the distribution and size of high-traction-stress regions at the cell periphery. Differences in adhesion and protrusion dynamics were mediated by balancing the activities of specific signaling molecules. Cell adhesion to a 30-kPa Ecad-Fc PA gel required Cdc42- and formin-dependent filopodia formation, whereas adhesion to a 60-kPa Ecad-Fc PA gel induced Arp2/3-dependent lamellipodial protrusions. A quantitative 3D cell-cell adhesion assay and live cell imaging of cell-cell contact formation revealed that inhibition of Cdc42, formin, and Arp2/3 activities blocked the initiation, but not the maintenance of established cell-cell adhesions. These results indicate that the same signaling molecules activated by E-cadherin rigidity sensing on PA gels contribute to actin organization and membrane dynamics during cell-cell adhesion. We hypothesize that a transition in the stiffness of E-cadherin homotypic interactions regulates actin and membrane dynamics during initial stages of cell-cell adhesion.

Intercellular adhesion molecule-1 augments myoblast adhesion and fusion through homophilic trans-interactions

The overall objective of the study was to identify mechanisms through which intercellular adhesion molecule-1 (ICAM-1) augments the adhesive and fusogenic properties of myogenic cells. Hypotheses were tested using cultured myoblasts and fibroblasts, which do not constitutively express ICAM-1, and myoblasts and fibroblasts forced to express full length ICAM-1 or a truncated form lacking the cytoplasmic domain of ICAM-1. ICAM-1 mediated myoblast adhesion and fusion were quantified using novel assays and cell mixing experiments. We report that ICAM-1 augments myoblast adhesion to myoblasts and myotubes through homophilic trans-interactions. Such adhesive interactions enhanced levels of active Rac in adherent and fusing myoblasts, as well as triggered lamellipodia, spreading, and fusion of myoblasts through the signaling function of the cytoplasmic domain of ICAM-1. Rac inhibition negated ICAM-1 mediated lamellipodia, spreading, and fusion of myoblasts. The fusogenic property of ICAM-1-ICAM-1 interactions was restricted to myogenic cells, as forced expression of ICAM-1 by fibroblasts did not augment their fusion to ICAM-1+ myoblasts/myotubes. We conclude that ICAM-1 augments myoblast adhesion and fusion through its ability to self-associate and initiate Rac-mediated remodeling of the actin cytoskeleton.

Heterogeneous Cadherin Expression and Multicellular Aggregate Dynamics in Ovarian Cancer Dissemination

Epithelial ovarian carcinoma spreads via shedding of cells and multicellular aggregates (MCAs) from the primary tumor into peritoneal cavity, with subsequent intraperitoneal tumor cell:mesothelial cell adhesion as a key early event in metastatic seeding. Evaluation of human tumor extracts and tissues confirms that well-differentiated ovarian tumors express abundant E-cadherin (Ecad), whereas advanced lesions exhibit upregulated N-cadherin (Ncad). Two expression patterns are observed: “mixed cadherin,” in which distinct cells within the same tumor express either E- or Ncad, and “hybrid cadherin,” wherein single tumor cell(s) simultaneously expresses both cadherins. We demonstrate striking cadherin-dependent differences in cell-cell interactions, MCA formation, and aggregate ultrastructure. Mesenchymal-type Ncad+ cells formed stable, highly cohesive solid spheroids, whereas Ecad+ epithelial-type cells generated loosely adhesive cell clusters covered by uniform microvilli. Generation of “mixed cadherin” MCAs using fluorescently tagged cell populations revealed preferential sorting into cadherin-dependent clusters, whereas mixing of cell lines with common cadherin profiles generated homogeneous aggregates. Recapitulation of the “hybrid cadherin” Ecad+/Ncad+ phenotype, via insertion of the CDH2 gene into Ecad+ cells, resulted in the ability to form heterogeneous clusters with Ncad+ cells, significantly enhanced adhesion to organotypic mesomimetic cultures and peritoneal explants, and increased both migration and matrix invasion. Alternatively, insertion of CDH1 gene into Ncad+ cells greatly reduced cell-to-collagen, cell-to-mesothelium, and cell-to-peritoneum adhesion. Acquisition of the hybrid cadherin phenotype resulted in altered MCA surface morphology with increased surface projections and increased cell proliferation. Overall, these findings support the hypothesis that MCA cadherin composition impacts intraperitoneal cell and MCA dynamics and thereby affects ultimate metastatic success.

Integration of Cadherin Adhesion and Cytoskeleton at Adherens Junctions

The cadherin-catenin adhesion complex is the key component of the intercellular adherens junction (AJ) that contributes both to tissue stability and dynamic cell movements in epithelial and nonepithelial tissues. The cadherin adhesion complex bridges neighboring cells and the actin-myosin cytoskeleton, and thereby contributes to mechanical coupling between cells which drives many morphogenetic events and tissue repair. Mechanotransduction at cadherin adhesions enables cells to sense, signal, and respond to physical changes in their environment. Central to this process is the dynamic link of the complex to actin filaments (F-actin), themselves structurally dynamic and subject to tension generated by myosin II motors. We discuss in this review recent breakthroughs in understanding molecular and cellular aspects of the organization of the core cadherin-catenin complex in adherens junctions, its association to F-actin, its mechanosensitive regulation, and dynamics.

15-Deoxy-Δ12,14-prostaglandin J2 inhibits migration of human thyroid carcinoma cells by disrupting focal adhesion complex and adherens junction

Metastasis is frequently observed in human follicular thyroid carcinoma. The present study investigated the peroxisome proliferator-activated receptor γ agonist, 15-deoxy-Δ^12,14-prostaglandin J² (15d-PGJ₂), and its effect on the migration of CGTH W-2 human thyroid carcinoma cells. 15d-PGJ₂ decreased the survival rate of CGTH W-2 cells in a dose-dependent manner. The Transwell migration assay demonstrated that 15d-PGJ₂ reduced the migration rate of CGTH W-2 cells by 35% following treatment with 30 µM 15d-PGJ₂ compared with control cells. The cell adhesion assay indicated that, following 15d-PGJ₂ treatment for 24 h, cell adhesion decreased by 26% compared with the control group. The expression levels of focal adhesion proteins, including integrin β1, phospho-focal adhesion kinase and p-paxillin, were downregulated following treatment with 15d-PGJ₂. Immunostaining revealed that the puncta of vinculin were reduced and the actin stress fiber was disassembled following 15d-PGJ₂ treatment. By contrast, p120-catenin (p120-ctn) and β-catenin levels staining accumulated in the region of the lamellipodium following 15d-PGJ₂ treatment. Membrane fractionation revealed that p120-ctn and N-cadherin were decreased in the cell membrane, but increased in the cytoplasm of 15d-PGJ₂-treated cells. Therefore, 15d-PGJ₂ inhibited human thyroid carcinoma cell migration and this may be due to the impairment of focal adhesion complexes and the accumulation of p120-ctn in the cytoplasm in the region of the lamellipodium.

Early events in the assembly of E-cadherin adhesions

E-cadherin is a calcium dependent cell adhesion molecule that is key to the organization of cells in the epithelial tissue. It is a multidomain, trans-membrane protein in which the extracellular domain forms the homotypic, adhesive interaction while the intracellular domain interacts with the actin cytoskeleton through the catenin family of adaptor proteins. A number of recent studies have provided novel insights into the mechanism of adhesion formation by this class of adhesion proteins. Here, we describe an updated view of the process of E-cadherin adhesion formation with an emphasis on the role of molecular mobility, clustering, and active cellular processes.

Nanoscale architecture of cadherin-based cell adhesions

Multicellularity in animals requires dynamic maintenance of cell-cell contacts. Intercellularly ligated cadherins recruit numerous proteins to form supramolecular complexes that connect with the actin cytoskeleton and support force transmission. However, the molecular organization within such structures remains unknown. Here we mapped protein organization in cadherin-based adhesions by super-resolution microscopy, revealing a multi-compartment nanoscale architecture, with the plasma-membrane-proximal cadherin-catenin compartment segregated from the actin cytoskeletal compartment, bridged by an interface zone containing vinculin. Vinculin position is determined by α-catenin, and following activation, vinculin can extend ∼30 nm to bridge the cadherin-catenin and actin compartments, while modulating the nanoscale positions of the actin regulators zyxin and VASP. Vinculin conformational activation requires tension and tyrosine phosphorylation, regulated by Abl kinase and PTP1B phosphatase. Such modular architecture provides a structural framework for mechanical and biochemical signal integration by vinculin, which may differentially engage cadherin-catenin complexes with the actomyosin machinery to regulate cell adhesions.

A phenomenological model of cell-cell adhesion mediated by cadherins

We present a phenomenological model intended to describe at the protein population level the formation of cell-cell junctions by the local recruitment of homophilic cadherin adhesion receptors. This modeling may have a much wider implication in biological processes since many adhesion receptors, channel proteins and other membrane-born proteins associate in clusters or oligomers at the cell surface. Mathematically, it consists in a degenerate reaction-diffusion system of two partial differential equations modeling the time-space evolution of two cadherin populations over a surface: the first one represents the diffusing cadherins and the second one concerns the fixed ones. After discussing the stability of the solutions of the model, we perform numerical simulations and show relevant analogies with experimental results. In particular, we show patterns or aggregates formation for a certain set of parameters. Moreover, perturbing the stationary solution, both density populations converge in large times to some saturation level. Finally, an exponential rate of convergence is numerically obtained and is shown to be in agreement, for a suitable set of parameters, with the one obtained in some in vitro experiments.

A Mechanogenetic Toolkit for Interrogating Cell Signaling in Space and Time

Tools capable of imaging and perturbing mechanical signaling pathways with fine spatiotemporal resolution have been elusive, despite their importance in diverse cellular processes. The challenge in developing a mechanogenetic toolkit (i.e., selective and quantitative activation of genetically encoded mechanoreceptors) stems from the fact that many mechanically activated processes are localized in space and time yet additionally require mechanical loading to become activated. To address this challenge, we synthesized magnetoplasmonic nanoparticles that can image, localize, and mechanically load targeted proteins with high spatiotemporal resolution. We demonstrate their utility by investigating the cell-surface activation of two mechanoreceptors: Notch and E-cadherin. By measuring cellular responses to a spectrum of spatial, chemical, temporal, and mechanical inputs at the single-molecule and single-cell levels, we reveal how spatial segregation and mechanical force cooperate to direct receptor activation dynamics. This generalizable technique can be used to control and understand diverse mechanosensitive processes in cell signaling. VIDEO ABSTRACT.

The formation of ordered nanoclusters controls cadherin anchoring to actin and cell-cell contact fluidity

Visualization of single cadherins within cell membrane at nanometric resolution shows that E-cadherins arrange in ordered clusters and that these clusters control the anchoring of cadherin to actin and cell–cell contact fluidity.

Oligomerization of cadherins could provide the stability to ensure tissue cohesion. Cadherins mediate cell–cell adhesion by forming trans-interactions. They form cis-interactions whose role could be essential to stabilize intercellular junctions by shifting cadherin clusters from a fluid to an ordered phase. However, no evidence has been provided so far for cadherin oligomerization in cellulo and for its impact on cell–cell contact stability. Visualizing single cadherins within cell membrane at a nanometric resolution, we show that E-cadherins arrange in ordered clusters, providing the first demonstration of the existence of oligomeric cadherins at cell–cell contacts. Studying the consequences of the disruption of the cis-interface, we show that it is not essential for adherens junction formation. Its disruption, however, increased the mobility of junctional E-cadherin. This destabilization strongly affected E-cadherin anchoring to actin and cell–cell rearrangement during collective cell migration, indicating that the formation of oligomeric clusters controls the anchoring of cadherin to actin and cell–cell contact fluidity.

Single cell rigidity sensing: A complex relationship between focal adhesion dynamics and large-scale actin cytoskeleton remodeling

Many physiological and pathological processes involve tissue cells sensing the rigidity of their environment. In general, tissue cells have been shown to react to the stiffness of their environment by regulating their level of contractility, and in turn applying traction forces on their environment to probe it. This mechanosensitive process can direct early cell adhesion, cell migration and even cell differentiation. These processes require the integration of signals over time and multiple length scales. Multiple strategies have been developed to understand force- and rigidity-sensing mechanisms and much effort has been concentrated on the study of cell adhesion complexes, such as focal adhesions, and cell cytoskeletons. Here, we review the major biophysical methods used for measuring cell-traction forces as well as the mechanosensitive processes that drive cellular responses to matrix rigidity on 2-dimensional substrates.

Converging and Unique Mechanisms of Mechanotransduction at Adhesion Sites

The molecular mechanisms by which physical forces control tissue development are beginning to be elucidated. Sites of adhesion between both cells and the extracellular environment [extracellular matrix (ECM) or neighboring cells] contain protein complexes capable of sensing fluctuations in tensile forces. Tension-dependent changes in the dynamics and composition of these complexes mark the transformation of physical input into biochemical signals that defines mechanotransduction. It is becoming apparent that, although the core constituents of these different adhesions are distinct, principles and proteins involved in mechanotransduction are conserved. Here, we discuss the current knowledge of overlapping and distinct aspects of mechanotransduction between integrin and cadherin adhesion complexes.

A ligand-independent integrin β1 mechanosensory complex guides spindle orientation

Control of spindle orientation is a fundamental process for embryonic development, morphogenesis and tissue homeostasis, while defects are associated with tumorigenesis and other diseases. Force sensing is one of the mechanisms through which division orientation is determined. Here we show that integrin β1 plays a critical role in this process, becoming activated at the lateral regions of the cell cortex in a ligand-independent manner. This activation is force dependent and polar, correlating with the spindle capture sites. Inhibition of integrin β1 activation on the cortex and disruption of its asymmetric distribution leads to spindle misorientation, even when cell adhesion is β1 independent. Examining downstream targets reveals that a cortical mechanosensory complex forms on active β1, and regulates spindle orientation irrespective of cell context. We propose that ligand-independent integrin β1 activation is a conserved mechanism that allows cell responses to external stimuli.

Front-Rear Polarization by Mechanical Cues: From Single Cells to Tissues

Directed cell migration is a complex process that involves front-rear polarization, characterized by cell adhesion and cytoskeleton-based protrusion, retraction, and contraction of either a single cell or a cell collective. Single cell polarization depends on a variety of mechanochemical signals including external adhesive cues, substrate stiffness, and confinement. In cell ensembles, coordinated polarization of migrating tissues results not only from the application of traction forces on the extracellular matrix but also from the transmission of mechanical stress through intercellular junctions. We focus here on the impact of mechanical cues on the establishment and maintenance of front-rear polarization from single cell to collective cell behaviors through local or large-scale mechanisms.

Synaptopodin couples epithelial contractility to α-actinin-4-dependent junction maturation

A novel tension-sensitive junctional protein, synaptopodin, can relay biophysical input from cellular actomyosin contractility to induce biochemical changes at cell–cell contacts, resulting in structural reorganization of the junctional complex and epithelial barrier maturation.

The epithelial junction experiences mechanical force exerted by endogenous actomyosin activities and from interactions with neighboring cells. We hypothesize that tension generated at cell–cell adhesive contacts contributes to the maturation and assembly of the junctional complex. To test our hypothesis, we used a hydraulic apparatus that can apply mechanical force to intercellular junction in a confluent monolayer of cells. We found that mechanical force induces α-actinin-4 and actin accumulation at the cell junction in a time- and tension-dependent manner during junction development. Intercellular tension also induces α-actinin-4–dependent recruitment of vinculin to the cell junction. In addition, we have identified a tension-sensitive upstream regulator of α-actinin-4 as synaptopodin. Synaptopodin forms a complex containing α-actinin-4 and β-catenin and interacts with myosin II, indicating that it can physically link adhesion molecules to the cellular contractile apparatus. Synaptopodin depletion prevents junctional accumulation of α-actinin-4, vinculin, and actin. Knockdown of synaptopodin and α-actinin-4 decreases the strength of cell–cell adhesion, reduces the monolayer permeability barrier, and compromises cellular contractility. Our findings underscore the complexity of junction development and implicate a control process via tension-induced sequential incorporation of junctional components.

Constructing modular and universal single molecule tension sensor using protein G to study mechano-sensitive receptors

Recently a variety of molecular force sensors have been developed to study cellular forces acting through single mechano-sensitive receptors. A common strategy adopted is to attach ligand molecules on a surface through engineered molecular tethers which report cell-exerted tension on receptor-ligand bonds. This approach generally requires chemical conjugation of the ligand to the force reporting tether which can be time-consuming and labor-intensive. Moreover, ligand-tether conjugation can severely reduce the activity of protein ligands. To address this problem, we developed a Protein G (ProG)-based force sensor in which force-reporting tethers are conjugated to ProG instead of ligands. A recombinant ligand fused with IgG-Fc is conveniently assembled with the force sensor through ProG:Fc binding, therefore avoiding ligand conjugation and purification processes. Using this approach, we determined that molecular tension on E-cadherin is lower than dsDNA unzipping force (nominal value: 12 pN) during initial cadherin-mediated cell adhesion, followed by an escalation to forces higher than 43 pN (nominal value). This approach is highly modular and potentially universal as we demonstrate using two additional receptor-ligand interactions, P-selectin & PSGL-1 and Notch & DLL1.

E-cadherin junction formation involves an active kinetic nucleation process

Epithelial (E)-cadherin-mediated cell-cell junctions play important roles in the development and maintenance of tissue structure in multicellular organisms. E-cadherin adhesion is thus a key element of the cellular microenvironment that provides both mechanical and biochemical signaling inputs. Here, we report in vitro reconstitution of junction-like structures between native E-cadherin in living cells and the extracellular domain of E-cadherin (E-cad-ECD) in a supported membrane. Junction formation in this hybrid live cell-supported membrane configuration requires both active processes within the living cell and a supported membrane with low E-cad-ECD mobility. The hybrid junctions recruit α-catenin and exhibit remodeled cortical actin. Observations suggest that the initial stages of junction formation in this hybrid system depend on the trans but not the cis interactions between E-cadherin molecules, and proceed via a nucleation process in which protrusion and retraction of filopodia play a key role.

The interdependence of the Rho GTPases and apicobasal cell polarity

Signaling via the Rho GTPases provides crucial regulation of numerous cell polarization events, including apicobasal (AB) polarity, polarized cell migration, polarized cell division and neuronal polarity. Here we review the relationships between the Rho family GTPases and epithelial AB polarization events, focusing on the 3 best-characterized members: Rho, Rac and Cdc42. We discuss a multitude of processes that are important for AB polarization, including lumen formation, apical membrane specification, cell-cell junction assembly and maintenance, as well as tissue polarity. Our discussions aim to highlight the immensely complex regulatory mechanisms that encompass Rho GTPase signaling during AB polarization. More specifically, in this review we discuss several emerging common themes, that include: 1) the need for Rho GTPase activities to be carefully balanced in both a spatial and temporal manner through a multitude of mechanisms; 2) the existence of signaling feedback loops and crosstalk to create robust cellular responses; and 3) the frequent multifunctionality that exists among AB polarity regulators. Regarding this latter theme, we provide further discussion of the potential plasticity of the cell polarity machinery and as a result the possible implications for human disease.

The mechanotransduction machinery at work at adherens junctions

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

Intramembrane binding of VE-cadherin to VEGFR2 and VEGFR3 assembles the endothelial mechanosensory complex

VE-cadherin plays a critical role in endothelial shear stress mechanotransduction by interacting with VEGFRs through their transmembrane domains.

Endothelial responses to fluid shear stress are essential for vascular development and physiology, and determine the formation of atherosclerotic plaques at regions of disturbed flow. Previous work identified VE-cadherin as an essential component, along with PECAM-1 and VEGFR2, of a complex that mediates flow signaling. However, VE-cadherin’s precise role is poorly understood. We now show that the transmembrane domain of VE-cadherin mediates an essential adapter function by binding directly to the transmembrane domain of VEGFR2, as well as VEGFR3, which we now identify as another component of the junctional mechanosensory complex. VEGFR2 and VEGFR3 signal redundantly downstream of VE-cadherin. Furthermore, VEGFR3 expression is observed in the aortic endothelium, where it contributes to flow responses in vivo. In summary, this study identifies a novel adapter function for VE-cadherin mediated by transmembrane domain association with VEGFRs.

E-cadherin interactome complexity and robustness resolved by quantitative proteomics

E-cadherin-mediated cell-cell adhesion and signaling plays an essential role in development and maintenance of healthy epithelial tissues. Adhesiveness mediated by E-cadherin is conferred by its extracellular cadherin domains and is regulated by an assembly of intracellular adaptors and enzymes associated with its cytoplasmic tail. We used proximity biotinylation and quantitative proteomics to identify 561 proteins in the vicinity of the cytoplasmic tail of E-cadherin. In addition, we used proteomics to identify proteins associated with E-cadherin-containing adhesion plaques from a cell-glass interface, which enabled the assignment of cellular localization to putative E-cadherin-interacting proteins. Moreover, by tagging identified proteins with GFP (green fluorescent protein), we determined the subcellular localization of 83 putative E-cadherin-proximal proteins and identified 24 proteins that were previously uncharacterized as part of adherens junctions. We constructed and characterized a comprehensive E-cadherin interaction network of 79 published and 394 previously uncharacterized proteins using a structure-informed database of protein-protein interactions. Finally, we found that calcium chelation, which disrupts the interaction of the extracellular E-cadherin domains, did not disrupt most intracellular protein interactions with E-cadherin, suggesting that the E-cadherin intracellular interactome is predominantly independent of cell-cell adhesion.

Force-dependent conformational switch of α-catenin controls vinculin binding

Force sensing at cadherin-mediated adhesions is critical for their proper function. α-Catenin, which links cadherins to actomyosin, has a crucial role in this mechanosensing process. It has been hypothesized that force promotes vinculin binding, although this has never been demonstrated. X-ray structure further suggests that α-catenin adopts a stable auto-inhibitory conformation that makes the vinculin-binding site inaccessible. Here, by stretching single α-catenin molecules using magnetic tweezers, we show that the subdomains MI vinculin-binding domain (VBD) to MIII unfold in three characteristic steps: a reversible step at ~5 pN and two non-equilibrium steps at 10-15 pN. 5 pN unfolding forces trigger vinculin binding to the MI domain in a 1:1 ratio with nanomolar affinity, preventing MI domain refolding after force is released. Our findings demonstrate that physiologically relevant forces reversibly unfurl α-catenin, activating vinculin binding, which then stabilizes α-catenin in its open conformation, transforming force into a sustainable biochemical signal.

Chemical genomic-based pathway analyses for epidermal growth factor-mediated signaling in migrating cancer cells

To explore the diversity and consistency of the signaling pathways that regulate tumor cell migration, we chose three human tumor cell lines that migrated after treatment with EGF. We then quantified the effect of fifteen inhibitors on the levels of expression or the phosphorylation levels of nine proteins that were induced by EGF stimulation in each of these cell lines. Based on the data obtained in this study and chemical-biological assumptions, we deduced cell migration pathways in each tumor cell line, and then compared them. As a result, we found that both the MEK/ERK and JNK/c-Jun pathways were activated in all three migrating cell lines. Moreover, GSK-3 and p38 were found to regulate PI3K/Akt pathway in only EC109 cells, and JNK was found to crosstalk with p38 and Fos related pathway in only TT cells. Taken together, our analytical system could easily distinguish between the common and cell type-specific pathways responsible for tumor cell migration.

Adhesive interactions of N-cadherin limit the recruitment of microtubules to cell–cell contacts through organization of actomyosin

Adhesive interactions of cadherins induce crosstalk between adhesion complexes and the actin cytoskeleton, allowing strengthening of adhesions and cytoskeletal organization. The underlying mechanisms are not completely understood, and microtubules (MTs) might be involved, as for integrin-mediated cell–extracellular-matrix adhesions. Therefore, we investigated the relationship between N-cadherin and MTs by analyzing the influence of N-cadherin engagement on MT distribution and dynamics. MTs progressed less, with a lower elongation rate, towards cadherin adhesions than towards focal adhesions. Increased actin treadmilling and the presence of an actomyosin contractile belt, suggested that actin relays inhibitory signals from cadherin adhesions to MTs. The reduced rate of MT elongation, associated with reduced recruitment of end-binding (EB) proteins to plus ends, was alleviated by expression of truncated N-cadherin, but was only moderately affected when actomyosin was disrupted. By contrast, destabilizing actomyosin fibers allowed MTs to enter the adhesion area, suggesting that tangential actin bundles impede MT growth independently of MT dynamics. Blocking MT penetration into the adhesion area strengthened cadherin adhesions. Taken together, these results establish a crosstalk between N-cadherin, F-actin and MTs. The opposing effects of cadherin and integrin engagement on actin organization and MT distribution might induce bias of the MT network during cell polarization.

Covalent and density-controlled surface immobilization of E-cadherin for adhesion force spectroscopy

E-cadherin is a key cell-cell adhesion molecule but the impact of receptor density and the precise contribution of individual cadherin ectodomains in promoting cell adhesion are only incompletely understood. Investigating these mechanisms would benefit from artificial adhesion substrates carrying different cadherin ectodomains at defined surface density. We therefore developed a quantitative E-cadherin surface immobilization protocol based on the SNAP-tag technique. Extracellular (EC) fragments of E-cadherin fused to the SNAP-tag were covalently bound to self-assembled monolayers (SAM) of thiols carrying benzylguanine (BG) head groups. The adhesive functionality of the different E-cadherin surfaces was then assessed using cell spreading assays and single-cell (SCSF) and single-molecule (SMSF) force spectroscopy. We demonstrate that an E-cadherin construct containing only the first and second outmost EC domain (E1-2) is not sufficient for mediating cell adhesion and yields only low single cadherin-cadherin adhesion forces. In contrast, a construct containing all five EC domains (E1-5) efficiently promotes cell spreading and generates strong single cadherin and cell adhesion forces. By varying the concentration of BG head groups within the SAM we determined a lateral distance of 5–11 nm for optimal E-cadherin functionality. Integrating the results from SCMS and SMSF experiments furthermore demonstrated that the dissolution of E-cadherin adhesion contacts involves a sequential unbinding of individual cadherin receptors rather than the sudden rupture of larger cadherin receptor clusters. Our method of covalent, oriented and density-controlled E-cadherin immobilization thus provides a novel and versatile platform to study molecular mechanisms underlying cadherin-mediated cell adhesion under defined experimental conditions.

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.

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.

Janmey P, Kresh JY. α-Catenin localization and sarcomere self-organization on N-cadherin adhesive patterns are myocyte contractility driven

The N-cadherin (N-cad) complex plays a crucial role in cardiac cell structure and function. Cadherins are adhesion proteins linking adjacent cardiac cells and, like integrin adhesions, are sensitive to force transmission. Forces through these adhesions are capable of eliciting structural and functional changes in myocytes. Compared to integrins, the mechanisms of force transduction through cadherins are less explored. α-catenin is a major component of the cadherin-catenin complex, thought to provide a link to the cell actin cytoskeleton. Using N-cad micropatterned substrates in an adhesion constrainment model, the results from this study show that α-catenin localizes to regions of highest internal stress in myocytes. This localization suggests that α-catenin acts as an adaptor protein associated with the cadherin mechanosensory apparatus, which is distinct from mechanosensing through integrins. Myosin inhibition in cells bound by integrins to fibronectin-coated patterns disrupts myofibiril organization, whereas on N-cad coated patterns, myosin inhibition leads to better organized myofibrils. This result indicates that the two adhesion systems provide independent mechanisms for regulating myocyte structural organization.

The study of polarisation in single cells using model cell membranes

The apicobasal polarisation of epithelial cells within an epithelium is critical for its function as a selective barrier. Microenvironmental parameters, including cell-matrix and cell-cell interactions, contribute to the initiation and orientation of this polarity. However, it is often non-trivial to decipher the differential effects of these parameters in a controlled manner using traditional in vitro platforms. A reductionist platform, consisting of E-cadherin coupled onto laterally mobile supported lipid bilayers, was utilised to mimic E-cadherin presentation in the cell membrane. These functionalised bilayers were generated either on flat 2D surfaces or the interior surfaces of round microwells. This platform enabled the study of E-cadherin adhesion and the initiation of polarisation in a controlled environment, where the dimensionality of the microenvironment, type of protein coating and cell shape could be independently studied. A high proportion of single epithelial cells interacted with and clustered cellular E-cadherin in the presence of E-cadherin functionalised bilayers, which was reduced in the presence of integrin-mediated adhesion. The differential response in E-cadherin clustering correlated with the polarisation of E-cadherin and Na,K-ATPase, a reporter for the induction of basolateral polarity. Neither the three-dimensional presentation of E-cadherin nor the cell shape affected E-cadherin clustering or polarisation in single cells. Thus, the mobile presentation of E-cadherin was sufficient to mimic a cell-cell contact and induce basolateral polarisation in single cells.

Crossroads of integrins and cadherins in epithelia and stroma remodeling

Adhesion events mediated by cadherin and integrin adhesion receptors have fundamental roles in the maintenance of the physiological balance of epithelial tissues, and it is well established that perturbations in their normal functional activity and/or changes in their expression are associated with tumorigenesis. Over the last decades, increasing evidence of a dynamic collaborative interaction between these complexes through their shared interactions with cytoskeletal proteins and common signaling pathways has emerged not only as an important regulator of several aspects of epithelial cell behavior, but also as a coordinated adhesion module that senses and transmits signals from and to the epithelia surrounding microenvironment. The tight regulation of their crosstalk is particularly important during epithelial remodeling events that normally take place during morphogenesis and tissue repair, and when defective it leads to cell transformation and aggravated responses of the tumor microenvironment that contribute to tumorigenesis. In this review we highlight some of the interactions that regulate their crosstalk and how this could be implicated in regulating signals across epithelial tissues to sustain homeostasis.

Mtss1 promotes cell-cell junction assembly and stability through the small GTPase Rac1

Cell-cell junctions are an integral part of epithelia and are often disrupted in cancer cells during epithelial-to-mesenchymal transition (EMT), which is a main driver of metastatic spread. We show here that Metastasis suppressor-1 (Mtss1; Missing in Metastasis, MIM), a member of the IMD-family of proteins, inhibits cell-cell junction disassembly in wound healing or HGF-induced scatter assays by enhancing cell-cell junction strength. Mtss1 not only makes cells more resistant to cell-cell junction disassembly, but also accelerates the kinetics of adherens junction assembly. Mtss1 drives enhanced junction formation specifically by elevating Rac-GTP. Lastly, we show that Mtss1 depletion reduces recruitment of F-actin at cell-cell junctions. We thus propose that Mtss1 promotes Rac1 activation and actin recruitment driving junction maintenance. We suggest that the observed loss of Mtss1 in cancers may compromise junction stability and thus promote EMT and metastasis.

N-cadherin mediates neuronal cell survival through Bim down-regulation

N-cadherin is a major adhesion molecule involved in the development and plasticity of the nervous system. N-cadherin-mediated cell adhesion regulates neuroepithelial cell polarity, neuronal precursor migration, growth cone migration and synaptic plasticity. In vitro, it has been involved in signaling events regulating processes such as cell mobility, proliferation and differentiation. N-cadherin has also been implicated in adhesion-dependent protection against apoptosis in non-neuronal cells. In this study, we investigated if the engagement of N-cadherin participates to the control of neuronal cells survival/death balance. We observed that plating either primary mouse spinal cord neurons or primary rat hippocampal neurons on N-cadherin recombinant substrate greatly enhances their survival compared to non-specific adhesion on poly-L-lysine. We show that N-cadherin engagement, in the absence of other survival factors (cell-matrix interactions and serum), protects GT1-7 neuronal cells against apoptosis. Using this cell line, we then searched for the signaling pathways involved in the survival effect of N-cadherin engagement. The PI3-kinase/Akt survival pathway and its downstream effector Bad are not involved, as no phosphorylation of Akt or Bad proteins in response to N-cadherin engagement was observed. In contrast, N-cadherin engagement activated the Erk1/2 MAP kinase pathway. Moreover, N-cadherin ligation mediated a 2-fold decrease in the level of the pro-apoptotic protein Bim-EL whereas the level of the anti-apoptotic protein Bcl-2 was unchanged. Inhibition of Mek1/2 kinases with U0126, and the resulting inhibition of Erk1/2 phosphorylation, induced the increase of both the level of Bim-EL and apoptosis of cells seeded on the N-cadherin substrate, suggesting that Erk phosphorylation is necessary for cell survival. Finally, the overexpression of a phosphorylation defective form of Bim-EL prevented N-cadherin-engagement induced cell survival. In conclusion, our results show that N-cadherin engagement mediates neuronal cell survival by enhancing the MAP kinase pathway and down-regulating the pro-apoptotic protein Bim-EL.