E-cadherin junction formation involves an active kinetic nucleation process

BRET-based biosensors for SARS-CoV-2 oligonucleotide detection

The need for the early detection of emerging pathogenic viruses and their newer variants has driven the urgent demand for developing point-of-care diagnostic tools. Although nucleic acid-based methods such as reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and loop-mediated isothermal amplification (LAMP) have been developed, a more facile and robust platform is still required. To address this need, as a proof-of-principle study, we engineered a prototype-the versatile, sensitive, rapid, and cost-effective bioluminescence resonance energy transfer (BRET)-based biosensor for oligonucleotide detection (BioOD). Specifically, we designed BioODs against the SARS-CoV-2 parental (Wuhan strain) and B.1.617.2 Delta variant through the conjugation of specific, fluorescently modified molecular beacons (sensor module) through a complementary oligonucleotide handle DNA functionalized with the NanoLuc (NLuc) luciferase protein such that the dissolution of the molecular beacon loop upon the binding of the viral oligonucleotide will result in a decrease in BRET efficiency and, thus, a change in the bioluminescence spectra. Following the assembly of the BioODs, we determined their kinetics response, affinity for variant-specific oligonucleotides, and specificity, and found them to be rapid and highly specific. Furthermore, the decrease in BRET efficiency of the BioODs in the presence of viral oligonucleotides can be detected as a change in color in cell phone camera images. We envisage that the BioODs developed here will find application in detecting viral infections with variant specificity in a point-of-care-testing format, thus aiding in large-scale viral infection surveillance.

A slow but steady nanoLuc: R162A mutation results in a decreased, but stable, nanoLuc activity

NanoLuc (NLuc) luciferase has found extensive application in designing a range of biological assays, including gene expression analysis, protein-protein interaction, and protein conformational changes due to its enhanced brightness and small size. However, questions related to its mechanism of interaction with the substrate, furimazine, as well as bioluminescence activity remain elusive. Here, we combined molecular dynamics (MD) simulation and mutational analysis to show that the R162A mutation results in a decreased but stable bioluminescence activity of NLuc in living cells and in vitro. Specifically, we performed multiple, all-atom, explicit solvent MD simulations of the apo and furimazine-docked (holo) NLuc structures revealing differential dynamics of the protein in the absence and presence of the ligand. Further, analysis of trajectories for hydrogen bonds (H-bonds) formed between NLuc and furimazine revealed substantial H-bond interaction between R162 and Q32 residues. Mutation of the two residues in NLuc revealed a decreased but stable activity of the R162A, but not Q32A, mutant NLuc in live cell and in vitro assays performed using lysates prepared from cells expressing the proteins and with the furimazine substrate. In addition to highlighting the role of the R162 residue in NLuc activity, we believe that the mutant NLuc will find wide application in designing in vitro assays requiring extended monitoring of NLuc bioluminescence activity. SIGNIFICANCE: Bioluminescence has been extensively utilized in developing a variety of biological and biomedical assays. In this regard, engineering of brighter bioluminescent proteins, i.e. luciferases, has played a significant role. This is acutely exemplified by the engineering of the NLuc luciferase, which is small in size and displays much enhanced bioluminescence and thermal stability compared to previously available luciferases. While enhanced bioluminescent activity is desirable in a multitude of biological and biomedical assays, it would also be useful to develop variants of the protein that display a prolonged bioluminescence activity. This is specifically relevant in designing assays that require bioluminescence for extended periods, such as in the case of biosensors designed for monitoring slow enzymatic or cellular signaling reactions, without necessitating multiple rounds of luciferase substrate addition or any specialized reagents that result in increased assay costs. In the current manuscript, we report a mutant NLuc that possesses a stable and prolonged bioluminescence activity, albeit lower than the wild-type NLuc, and envisage a wider application of the mutant NLuc in designing biosensors for monitoring slower biological and biomedical events.

Characterization of early and late events of adherens junction assembly

Cadherins are transmembrane adhesion receptors. Cadherin ectodomains form adhesive 2D clusters through cooperative trans and cis interactions, whereas its intracellular region interacts with specific cytosolic proteins, termed catenins, to anchor the cadherin-catenin complex (CCC) to the actin cytoskeleton. How these two types of interactions are coordinated in the formation of specialized cell-cell adhesions, adherens junctions (AJ), remains unclear. We focus here on the role of the actin-binding domain of α-catenin (αABD) by showing that the interaction of αABD with actin generates actin-bound CCC oligomers (CCC/actin strands) incorporating up to six CCCs. The strands are primarily formed on the actin-rich cell protrusions. Once in cell-cell interface, the strands become involved in cadherin ectodomain clustering. Such combination of the extracellular and intracellular oligomerizations gives rise to the composite oligomers, trans CCC/actin clusters. To mature, these clusters then rearrange their actin filaments using several redundant pathways, two of which are characterized here: one depends on the α-catenin-associated protein, vinculin and the second one depends on the unstructured C-terminus of αABD. Thus, AJ assembly proceeds through spontaneous formation of trans CCC/actin clusters and their successive reorganization.

Computational and experimental approaches to quantify protein binding interactions under confinement

Crowded environments and confinement alter the interactions of adhesion proteins confined to membranes or narrow, crowded gaps at adhesive contacts. Experimental approaches and theoretical frameworks were developed to quantify protein binding constants in these environments. However, recent predictions and the complexity of some protein interactions proved challenging to address with prior experimental or theoretical approaches. This perspective highlights new methods developed by these authors that address these challenges. Specifically, single-molecule fluorescence resonance energy transfer and single-molecule tracking measurements were developed to directly image the binding/unbinding rates of membrane-tethered cadherins. Results identified predicted cis (lateral) interactions, which control cadherin clustering on membranes but were not detected in solution. Kinetic Monte Carlo simulations, based on a realistic model of cis cadherin interactions, were developed to extract binding/unbinding rate constants from heterogeneous single-molecule data. The extension of single-molecule fluorescence measurements to cis and trans (adhesive) cadherin interactions at membrane junctions identified unexpected cooperativity between cis and trans binding that appears to enhance intercellular binding kinetics. Comparisons of intercellular binding kinetics, kinetic Monte Carlo simulations, and single-molecule fluorescence data suggest a strategy to bridge protein binding kinetics across length scales. Although cadherin is the focus of these studies, the approaches can be extended to other intercellular adhesion proteins.

Adhesion-induced cortical flows pattern E-cadherin-mediated cell contacts

Metazoan development relies on the formation and remodeling of cell-cell contacts. Dynamic reorganization of adhesion receptors and the actomyosin cell cortex in space and time plays a central role in cell-cell contact formation and maturation. Nevertheless, how this process is mechanistically achieved when new contacts are formed remains unclear. Here, by building a biomimetic assay composed of progenitor cells adhering to supported lipid bilayers functionalized with E-cadherin ectodomains, we show that cortical F-actin flows, driven by the depletion of myosin-2 at the cell contact center, mediate the dynamic reorganization of adhesion receptors and cell cortex at the contact. E-cadherin-dependent downregulation of the small GTPase RhoA at the forming contact leads to both a depletion of myosin-2 and a decrease of F-actin at the contact center. At the contact rim, in contrast, myosin-2 becomes enriched by the retraction of bleb-like protrusions, resulting in a cortical tension gradient from the contact rim to its center. This tension gradient, in turn, triggers centrifugal F-actin flows, leading to further accumulation of F-actin at the contact rim and the progressive redistribution of E-cadherin from the contact center to the rim. Eventually, this combination of actomyosin downregulation and flows at the contact determines the characteristic molecular organization, with E-cadherin and F-actin accumulating at the contact rim, where they are needed to mechanically link the contractile cortices of the adhering cells.

Epithelial cell spreading assay on E-cadherin-coated glass or PDMS substrates for microscopy-based analysis of cadherin adhesions

Adherens junctions (AJs) are multi-protein adhesion structures that couple contractile actomyosin networks of epithelial cells within a tissue. Here, we present an epithelial cell spreading assay on E-cadherin-coated glass or polydimethylsiloxane (PDMS) substrates for detailed microscopy-based analysis of cadherin adhesions. We describe steps for preparation of glass coverslips and PDMS gels, E-cadherin coating, and epithelial cell spreading. Epithelial cells can be seeded on E-cadherin-coated surfaces, thereby mimicking AJ formation in X-Y dimension, making it suitable for microscopy analysis. For complete details on the use and execution of this protocol, please refer to Noordstra et al. (2023).1.

Push-pull mechanics of E-cadherin ectodomains in biomimetic adhesions

E-cadherin plays a central role in cell-cell adhesion. The ectodomains of wild-type cadherins form a crystalline-like two-dimensional lattice in cell-cell interfaces mediated by both trans (apposed cell) and cis (same cell) interactions. In addition to these extracellular forces, adhesive strength is further regulated by cytosolic phenomena involving α and β catenin-mediated interactions between cadherin and the actin cytoskeleton. Cell-cell adhesion can be further strengthened under tension through mechanisms that have not been definitively characterized in molecular detail. Here we quantitatively determine the role of the cadherin ectodomain in mechanosensing. To this end, we devise an E-cadherin-coated emulsion system, in which droplet surface tension is balanced by protein binding strength to give rise to stable areas of adhesion. To reach the honeycomb/cohesive limit, an initial emulsion compression by centrifugation facilitates E-cadherin trans binding, whereas a high protein surface concentration enables the cis-enhanced stabilization of the interface. We observe an abrupt concentration dependence on recruitment into adhesions of constant crystalline density, reminiscent of a first-order phase transition. Removing the lateral cis interaction with a "cis mutant" shifts this transition to higher surface densities leading to denser, yet weaker adhesions. In both proteins, the stabilization of progressively larger areas of deformation is consistent with single-molecule experiments that show a force-dependent lifetime enhancement in the cadherin ectodomain, which may be attributed to the "X-dimer" bond.

A PDA-Functionalized 3D Lung Scaffold Bioplatform to Construct Complicated Breast Tumor Microenvironment for Anticancer Drug Screening and Immunotherapy

2D cell culture occupies an important place in cancer progression and drug discovery research. However, it limitedly models the "true biology" of tumors in vivo. 3D tumor culture systems can better mimic tumor characteristics for anticancer drug discovery but still maintain great challenges. Herein, polydopamine (PDA)-modified decellularized lung scaffolds are designed and can serve as a functional biosystem to study tumor progression and anticancer drug screening, as well as mimic the tumor microenvironment. PDA-modified scaffolds with strong hydrophilicity and excellent cell compatibility can promote cell growth and proliferation. After 96 h treatment with 5-FU, cisplatin, and DOX, higher survival rates in PDA-modified scaffolds are observed compared to nonmodified scaffolds and 2D systems. The E-cadhesion formation, HIF-1α-mediated senescence decrease, and tumor stemness enhancement can drive drug resistance and antitumor drug screening of breast cancer cells. Moreover, there is a higher survival rate of CD45+ /CD3+ /CD4+ /CD8+ T cells in PDA-modified scaffolds for potential cancer immunotherapy drug screening. This PDA-modified tumor bioplatform will supply some promising information for studying tumor progression, overcoming tumor resistance, and screening tumor immunotherapy drugs.

Mechanics of cell-cell junctions

Tissue cells in epithelial or endothelial monolayers are connected through cell-cell junctions, which are stabilized by transmembrane E-cadherin bonds and intracellular actin filaments. These bonds and junctions play a crucial role in maintaining the barrier function of epithelia and endothelia and are believed to transmit forces between cells. Additionally, E-cadherin bonds can impact the shape of cell-cell junctions. In this study, we develop a continuum mechanical model of the cell-cell junction by explicitly incorporating the cell membrane, distributions of E-cadherin bonds, cytoplasmic fluid pressure, and F-actin dynamics. The static force-balanced version of the model is able to analyze the influences of cell cortical tension, actin dynamics, and cytoplasmic pressure on the junction shape and E-cadherin bonds. Furthermore, an extended model that incorporates fluid flow, across the cell boundary as well as around the cell, is also examined. This model can couple cell-shape changes with cell cortical tension and fluid flow, and predicts the additional effect of fluid motion on cell-cell junction mechanics. Taken together, our models serve as an intermediate link between molecular-scale models of cell-junction molecules and cell-scale models of tissue and epithelia.

An ensemble of cadherin-catenin-vinculin complex employs vinculin as the major F-actin binding mode

The cell-cell adhesion cadherin-catenin complexes recruit vinculin to the adherens junction (AJ) to modulate the mechanical couplings between neighboring cells. However, it is unclear how vinculin influences the AJ structure and function. Here, we identified two patches of salt bridges that lock vinculin in the head-tail autoinhibited conformation and reconstituted the full-length vinculin activation mimetics bound to the cadherin-catenin complex. The cadherin-catenin-vinculin complex contains multiple disordered linkers and is highly dynamic, which poses a challenge for structural studies. We determined the ensemble conformation of this complex using small-angle x-ray and selective deuteration/contrast variation small-angle neutron scattering. In the complex, both α-catenin and vinculin adopt an ensemble of flexible conformations, but vinculin has fully open conformations with the vinculin head and actin-binding tail domains well separated from each other. F-actin binding experiments show that the cadherin-catenin-vinculin complex binds and bundles F-actin. However, when the vinculin actin-binding domain is removed from the complex, only a minor fraction of the complex binds to F-actin. The results show that the dynamic cadherin-catenin-vinculin complex employs vinculin as the primary F-actin binding mode to strengthen AJ-cytoskeleton interactions.

Adherens junction: the ensemble of specialized cadherin clusters

The cell-cell connections in adherens junctions (AJs) are mediated by transmembrane receptors, type I cadherins (referred to here as cadherins). These cadherin-based connections (or trans bonds) are weak. To upregulate their strength, cadherins exploit avidity, the increased affinity of binding between cadherin clusters compared with isolated monomers. Formation of such clusters is a unique molecular process that is driven by a synergy of direct and indirect cis interactions between cadherins located at the same cell. In addition to their role in adhesion, cadherin clusters provide structural scaffolds for cytosolic proteins, which implicate cadherin into different cellular activities and signaling pathways. The cluster lifetime, which depends on the actin cytoskeleton, and on the mechanical forces it generates, determines the strength of AJs and their plasticity. The key aspects of cadherin adhesion, therefore, cannot be understood at the level of isolated cadherin molecules, but should be discussed in the context of cadherin clusters.

Static and Dynamic: Evolving Biomaterial Mechanical Properties to Control Cellular Mechanotransduction

The extracellular matrix (ECM) is a highly dynamic system that constantly offers physical, biological, and chemical signals to embraced cells. Increasing evidence suggests that mechanical signals derived from the dynamic cellular microenvironment are essential controllers of cell behaviors. Conventional cell culture biomaterials, with static mechanical properties such as chemistry, topography, and stiffness, have offered a fundamental understanding of various vital biochemical and biophysical processes, such as cell adhesion, spreading, migration, growth, and differentiation. At present, novel biomaterials that can spatiotemporally impart biophysical cues to manipulate cell fate are emerging. The dynamic properties and adaptive traits of new materials endow them with the ability to adapt to cell requirements and enhance cell functions. In this review, an introductory overview of the key players essential to mechanobiology is provided. A biophysical perspective on the state-of-the-art manipulation techniques and novel materials in designing static and dynamic ECM-mimicking biomaterials is taken. In particular, different static and dynamic mechanical cues in regulating cellular mechanosensing and functions are compared. This review to benefit the development of engineering biomechanical systems regulating cell functions is expected.

Observing Membrane and Cell Adhesion via Reflection Interference Contrast Microscopy

Reflection interference contrast microscopy (RICM) is an optical microscopy technique ideally suited for imaging adhesion. While RICM (and the closely related interference reflection microscopy (IRM)) has been extensively used qualitatively or semiquantitatively to image cells, including immune cells, it can also be used quantitatively to measure membrane to surface distance, especially for model membranes. Here, we present a protocol for RICM and IRM imaging and the details of semiquantitative and quantitative analysis.

Nucleation of cadherin clusters on cell-cell interfaces

Cadherins mediate cell-cell adhesion and help the cell determine its shape and function. Here we study collective cadherin organization and interactions within cell-cell contact areas, and find the cadherin density at which a 'gas-liquid' phase transition occurs, when cadherin monomers begin to aggregate into dense clusters. We use a 2D lattice model of a cell-cell contact area, and coarse-grain to the continuous number density of cadherin to map the model onto the Cahn-Hilliard coarsening theory. This predicts the density required for nucleation, the characteristic length scale of the process, and the number density of clusters. The analytical predictions of the model are in good agreement with experimental observations of cadherin clustering in epithelial tissues.

A genetically encoded BRET-based SARS-CoV-2 Mpro protease activity sensor

The main protease, Mpro, is critical for SARS-CoV-2 replication and an appealing target for designing anti-SARS-CoV-2 agents. Therefore, there is a demand for the development of improved sensors to monitor its activity. Here, we report a pair of genetically encoded, bioluminescence resonance energy transfer (BRET)-based sensors for detecting Mpro proteolytic activity in live cells as well as in vitro. The sensors were generated by sandwiching peptides containing the Mpro N-terminal autocleavage sites, either AVLQSGFR (short) or KTSAVLQSGFRKME (long), in between the mNeonGreen and NanoLuc proteins. Co-expression of the sensors with Mpro in live cells resulted in their cleavage while mutation of the critical C145 residue (C145A) in Mpro completely abrogated their cleavage. Additionally, the sensors recapitulated the inhibition of Mpro by the well-characterized pharmacological agent GC376. Further, in vitro assays with the BRET-based Mpro sensors revealed a molecular crowding-mediated increase in the rate of Mpro activity and a decrease in the inhibitory potential of GC376. The sensors developed here will find direct utility in studies related to drug discovery targeting the SARS-CoV-2 Mpro and functional genomics application to determine the effect of sequence variation in Mpro.

Endothelial cell spreading on lipid bilayers with combined integrin and cadherin binding ligands

Endothelial cells play a central role in the vascular system, where their function is tightly regulated by both cell-extracellular matrix (e.g., via integrins) and cell-cell interactions (e.g., via cadherins). In this study, we incorporated cholesterol-modified integrin and N-cadherin peptide binding ligands in fluid supported lipid bilayers. Human umbilical vein endothelial cell adhesion, spreading and vinculin localization in these cells were dependent on ligand density. One composition led to observe a higher extent of cell spreading, where cells exhibited extensive lamellipodia formation and a qualitatively more distinct N-cadherin localization at the cell periphery, which is indicative of N-cadherin clustering and a mimic of cell-cell contact formation. The results can be used to reconstitute the endothelial-pericyte interface on biomedical devices and materials.

Ligand functionalization of titanium nanopattern enables the analysis of cell-ligand interactions by super-resolution microscopy

The spatiotemporal aspects of early signaling events during interactions between cells and their environment dictate multiple downstream outcomes. While advances in nanopatterning techniques have allowed the isolation of these signaling events, a major limitation of conventional nanopatterning methods is its dependence on gold (Au) or related materials that plasmonically quench fluorescence and, thus, are incompatible with super-resolution fluorescence microscopy. Here we describe a novel method that integrates nanopatterning with single-molecule resolution fluorescence imaging, thus enabling mechanistic dissection of molecular-scale signaling events in conjunction with nanoscale geometry manipulation. Our method exploits nanofabricated titanium (Ti) whose oxide (TiO₂) is a dielectric material with no plasmonic effects. We describe the surface chemistry for decorating specific ligands such as cyclo-RGD (arginine, glycine and aspartate: a ligand for fibronectin-binding integrins) on TiO₂ nanoline and nanodot substrates, and demonstrate the ability to perform dual-color super-resolution imaging on these patterns. Ti nanofabrication is similar to other metallic materials like Au, while the functionalization of TiO₂ is relatively fast, safe, economical, easy to set up with commonly available reagents, and robust against environmental parameters such as humidity. Fabrication of nanopatterns takes ~2-3 d, preparation for functionalization ~1.5-2 d, and functionalization 3 h, after which cell culture and imaging experiments can be performed. We suggest that this method may facilitate the interrogation of nanoscale geometry and force at single-molecule resolution, and should find ready applications in early detection and interpretation of physiochemical signaling events at the cell membrane in the fields of cell biology, immunology, regenerative medicine, and related fields.

Peeling dynamics of fluid membranes bridged by molecular bonds: moving or breaking

Biological adhesion is a critical mechanical function of complex organisms. At the scale of cell–cell contacts, adhesion is remarkably tunable to enable both cohesion and malleability during development, homeostasis and disease. It is physically supported by transient and laterally mobile molecular bonds embedded in fluid membranes. Thus, unlike specific adhesion at solid–solid or solid–fluid interfaces, peeling at fluid–fluid interfaces can proceed by breaking bonds, by moving bonds or by a combination of both. How the additional degree of freedom provided by bond mobility changes the mechanics of peeling is not understood. To address this, we develop a theoretical model coupling diffusion, reactions and mechanics. Mobility and reaction rates determine distinct peeling regimes. In a diffusion-dominated Stefan-like regime, bond motion establishes self-stabilizing dynamics that increase the effective fracture energy. In a reaction-dominated regime, peeling proceeds by travelling fronts where marginal diffusion and unbinding control peeling speed. In a mixed reaction–diffusion regime, strengthening by bond motion competes with weakening by bond breaking in a force-dependent manner, defining the strength of the adhesion patch. In turn, patch strength depends on molecular properties such as bond stiffness, force sensitivity or crowding. We thus establish the physical rules enabling tunable cohesion in cellular tissues and in engineered biomimetic systems.

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.

Probing the effect of clustering on EphA2 receptor signaling efficiency by subcellular control of ligand-receptor mobility

Clustering of ligand:receptor complexes on the cell membrane is widely presumed to have functional consequences for subsequent signal transduction. However, it is experimentally challenging to selectively manipulate receptor clustering without altering other biochemical aspects of the cellular system. Here, we develop a microfabrication strategy to produce substrates displaying mobile and immobile ligands that are separated by roughly 1 µm, and thus experience an identical cytoplasmic signaling state, enabling precision comparison of downstream signaling reactions. Applying this approach to characterize the ephrinA1:EphA2 signaling system reveals that EphA2 clustering enhances both receptor phosphorylation and downstream signaling activity. Single-molecule imaging clearly resolves increased molecular binding dwell times at EphA2 clusters for both Grb2:SOS and NCK:N-WASP signaling modules. This type of intracellular comparison enables a substantially higher degree of quantitative analysis than is possible when comparisons must be made between different cells and essentially eliminates the effects of cellular response to ligand manipulation.

P120 catenin potentiates constitutive E-cadherin dimerization at the plasma membrane and regulates trans binding

Cadherins are essential adhesion proteins that regulate tissue cohesion and paracellular permeability by assembling dense adhesion plaques at cell-to-cell contacts. Adherens junctions are central to a wide range of tissue functions; identifying protein interactions that potentiate their assembly and regulation has been the focus of research for over 2 decades. Here, we present evidence for a new, unexpected mechanism of cadherin oligomerization on cells. Fully quantified spectral imaging fluorescence resonance energy transfer (FSI-FRET) and fluorescence intensity fluctuation (FIF) measurements directly demonstrate that E-cadherin forms constitutive lateral (cis) dimers at the plasma membrane. Results further show that binding of the cytosolic protein p120ctn binding to the intracellular region is required for constitutive E-cadherin dimerization. This finding differs from a model that attributes lateral (cis) cadherin oligomerization solely to extracellular domain interactions. The present, novel findings are further supported by studies of E-cadherin mutants that uncouple p120ctn binding or with cells in which p120ctn was knocked out using CRISPR-Cas9. Quantitative affinity measurements further demonstrate that uncoupling p120ctn binding reduces the cadherin trans binding affinity and cell adhesion. These findings transform the current model of cadherin assembly at cell surfaces and identify the core building blocks of cadherin-mediated intercellular adhesions. They also identify a new role for p120ctn and reconcile findings that implicate both the extracellular and intracellular cadherin domains in cadherin clustering and intercellular cohesion.

Molecular Biomechanics Controls Protein Mixing and Segregation in Adherent Membranes

Cells interact with their environment by forming complex structures involving a multitude of proteins within assemblies in the plasma membrane. Despite the omnipresence of these assemblies, a number of questions about the correlations between the organisation of domains and the biomechanical properties of the involved proteins, namely their length, flexibility and affinity, as well as about the coupling to the elastic, fluctuating membrane, remain open. Here we address these issues by developing an effective Kinetic Monte Carlo simulation to model membrane adhesion. We apply this model to a typical experiment in which a cell binds to a functionalized solid supported bilayer and use two ligand-receptor pairs to study these couplings. We find that differences in affinity and length of proteins forming adhesive contacts result in several characteristic features in the calculated phase diagrams. One such feature is mixed states occurring even with proteins with length differences of 10 nm. Another feature are stable nanodomains with segregated proteins appearing on time scales of cell experiments, and for biologically relevant parameters. Furthermore, we show that macroscopic ring-like patterns can spontaneously form as a consequence of emergent protein fluxes. The capacity to form domains is captured by an order parameter that is founded on the virial coefficients for the membrane mediated interactions between bonds, which allow us to collapse all the data. These findings show that taking into account the role of the membrane allows us to recover a number of experimentally observed patterns. This is an important perspective in the context of explicit biological systems, which can now be studied in significant detail.

Cadherin cis and trans interactions are mutually cooperative

Cadherin transmembrane proteins are responsible for intercellular adhesion in all biological tissues and modulate tissue morphogenesis, cell motility, force transduction, and macromolecular transport. The protein-mediated adhesions consist of adhesive trans interactions and lateral cis interactions. Although theory suggests cooperativity between cis and trans bonds, direct experimental evidence of such cooperativity has not been demonstrated. Here, the use of superresolution microscopy, in conjunction with intermolecular single-molecule Förster resonance energy transfer, demonstrated the mutual cooperativity of cis and trans interactions. Results further demonstrate the consequent assembly of large intermembrane junctions, using a biomimetic lipid bilayer cell adhesion model. Notably, the presence of cis interactions resulted in a nearly 30-fold increase in trans-binding lifetimes between epithelial-cadherin extracellular domains. In turn, the presence of trans interactions increased the lifetime of cis bonds. Importantly, comparison of trans-binding lifetimes of small and large cadherin clusters suggests that this cooperativity is primarily due to allostery. The direct quantitative demonstration of strong mutual cooperativity between cis and trans interactions at intermembrane adhesions provides insights into the long-standing controversy of how weak cis and trans interactions act in concert to create strong macroscopic cell adhesions.

Intracellular Ionic Strength Sensing Using NanoLuc

Intracellular ionic strength regulates myriad cellular processes that are fundamental to cellular survival and proliferation, including protein activity, aggregation, phase separation, and cell volume. It could be altered by changes in the activity of cellular signaling pathways, such as those that impact the activity of membrane-localized ion channels or by alterations in the microenvironmental osmolarity. Therefore, there is a demand for the development of sensitive tools for real-time monitoring of intracellular ionic strength. Here, we developed a bioluminescence-based intracellular ionic strength sensing strategy using the Nano Luciferase (NanoLuc) protein that has gained tremendous utility due to its high, long-lived bioluminescence output and thermal stability. Biochemical experiments using a recombinantly purified protein showed that NanoLuc bioluminescence is dependent on the ionic strength of the reaction buffer for a wide range of ionic strength conditions. Importantly, the decrease in the NanoLuc activity observed at higher ionic strengths could be reversed by decreasing the ionic strength of the reaction, thus making it suitable for sensing intracellular ionic strength alterations. Finally, we used an mNeonGreen–NanoLuc fusion protein to successfully monitor ionic strength alterations in a ratiometric manner through independent fluorescence and bioluminescence measurements in cell lysates and live cells. We envisage that the biosensing strategy developed here for detecting alterations in intracellular ionic strength will be applicable in a wide range of experiments, including high throughput cellular signaling, ion channel functional genomics, and drug discovery.

EphrinB2 clustering by Nipah virus G is required to activate and trap F intermediates at supported lipid bilayer-cell interfaces

Paramyxovirus membrane fusion requires an attachment protein that binds to a host cell receptor and a fusion protein that merges the viral and host membranes. For Nipah virus (NiV), the G attachment protein binds ephrinB2/B3 receptors and activates F-mediated fusion. To visualize dynamic events of these proteins at the membrane interface, we reconstituted NiV fusion activation by overlaying F- and G-expressing cells onto ephrinB2-functionalized supported lipid bilayers and used TIRF microscopy to follow F, G, and ephrinB2. We found that G and ephrinB2 form clusters and that oligomerization of ephrinB2 is necessary for F activation. Single-molecule tracking of F particles revealed accumulation of an immobilized intermediate upon activation. We found no evidence for stable F-G protein complexes before or after activation. These observations lead to a revised model for NiV fusion activation and provide a foundation for investigating other multicomponent viral fusion systems.

Cell-cell interfaces as specialized compartments directing cell function

Cell-cell interfaces are found throughout multicellular organisms, from transient interactions between motile immune cells to long-lived cell-cell contacts in epithelia. Studies of immune cell interactions, epithelial cell barriers, neuronal contacts and sites of cell-cell fusion have identified a core set of features shared by cell-cell interfaces that critically control their function. Data from diverse cell types also show that cells actively and passively regulate the localization, strength, duration and cytoskeletal coupling of receptor interactions governing cell-cell signalling and physical connections between cells, indicating that cell-cell interfaces have a unique membrane organization that emerges from local molecular and cellular mechanics. In this Review, we discuss recent findings that support the emerging view of cell-cell interfaces as specialized compartments that biophysically constrain the arrangement and activity of their protein, lipid and glycan components. We also review how these biophysical features of cell-cell interfaces allow cells to respond with high selectivity and sensitivity to multiple inputs, serving as the basis for wide-ranging cellular functions. Finally, we consider how the unique properties of cell-cell interfaces present opportunities for therapeutic intervention.

Cadherin clusters stabilized by a combination of specific and nonspecific cis-interactions

We demonstrate a combined experimental and computational approach for the quantitative characterization of lateral interactions between membrane-associated proteins. In particular, weak, lateral (cis) interactions between E-cadherin extracellular domains tethered to supported lipid bilayers, were studied using a combination of dynamic single-molecule Förster Resonance Energy Transfer (FRET) and kinetic Monte Carlo (kMC) simulations. Cadherins are intercellular adhesion proteins that assemble into clusters at cell-cell contacts through cis- and trans- (adhesive) interactions. A detailed and quantitative understanding of cis-clustering has been hindered by a lack of experimental approaches capable of detecting and quantifying lateral interactions between proteins on membranes. Here single-molecule intermolecular FRET measurements of wild-type E-cadherin and cis-interaction mutants combined with simulations demonstrate that both nonspecific and specific cis-interactions contribute to lateral clustering on lipid bilayers. Moreover, the intermolecular binding and dissociation rate constants are quantitatively and independently determined, demonstrating an approach that is generalizable for other interacting proteins.

Quantitative evaluation of the impact of artificial cell adhesion via DNA hybridization on E-cadherin-mediated cell adhesion

Programmable cell adhesion with DNA hybridization is a promising approach for fabricating various tissue architectures without sophisticated instrumentation. However, little is known about how this artificial interaction influences the binding of cell adhesion proteins, E-cadherin. In this work, we designed a planar and fluid lipid membrane displaying E-cadherin and/or single-strand DNA with well-defined densities. Visualization of cells on membranes by fluorescence and interference microscopy revealed cell adhesion to be a two-step process: artificial adhesion by DNA hybridization within a few minutes followed by biological adhesion via cadherin-cadherin binding within hours. Furthermore, we discovered that DNA hybridization can substantially facilitate E-cadherin-mediated cell adhesion. The promotive effect is probably due to the enforced binding between E-cadherin molecules in geometrical confinement between two membranes. Our in vitro model of cell adhesion can potentially be used to design functional synthetic molecules that can regulate cell adhesion via cell adhesion proteins for tissue engineering.

Actin protrusions push at apical junctions to maintain E-cadherin adhesion

Cadherin-mediated cell-cell adhesion is actin-dependent, but the precise role of actin in maintaining cell-cell adhesion is not fully understood. Actin polymerization-dependent protrusive activity is required to push distally separated cells close enough to initiate contact. Whether protrusive activity is required to maintain adhesion in confluent sheets of epithelial cells is not known. By electron microscopy as well as live cell imaging, we have identified a population of protruding actin microspikes that operate continuously near apical junctions of polarized Madin-Darby canine kidney (MDCK) cells. Live imaging shows that microspikes containing E-cadherin extend into gaps between E-cadherin clusters on neighboring cells, while reformation of cadherin clusters across the cell-cell boundary correlates with microspike withdrawal. We identify Arp2/3, EVL, and CRMP-1 as 3 actin assembly factors necessary for microspike formation. Depleting these factors from cells using RNA interference (RNAi) results in myosin II-dependent unzipping of cadherin adhesive bonds. Therefore, actin polymerization-dependent protrusive activity operates continuously at cadherin cell-cell junctions to keep them shut and to prevent myosin II-dependent contractility from tearing cadherin adhesive contacts apart.

Molecular Mobility-Mediated Regulation of E-Cadherin Adhesion

Cells in epithelial tissues utilize homotypic E-cadherin interaction-mediated adhesions to both physically adhere to each other and sense the physical properties of their microenvironment, such as the presence of other cells in close vicinity or an alteration in the mechanical tension of the tissue. These position E-cadherin centrally in organogenesis and other processes, and its function is therefore tightly regulated through a variety of means including endocytosis and gene expression. How does membrane molecular mobility of E-cadherin, and thus membrane physical properties and associated actin cytoskeleton, impinges on the assembly of adhesive clusters and signaling is discussed.

DNA mechanotechnology reveals that integrin receptors apply pN forces in podosomes on fluid substrates

Podosomes are ubiquitous cellular structures important to diverse processes including cell invasion, migration, bone resorption, and immune surveillance. Structurally, podosomes consist of a protrusive actin core surrounded by adhesion proteins. Although podosome protrusion forces have been quantified, the magnitude, spatial distribution, and orientation of the opposing tensile forces remain poorly characterized. Here we use DNA nanotechnology to create probes that measure and manipulate podosome tensile forces with molecular piconewton (pN) resolution. Specifically, Molecular Tension-Fluorescence Lifetime Imaging Microscopy (MT-FLIM) produces maps of the cellular adhesive landscape, revealing ring-like tensile forces surrounding podosome cores. Photocleavable adhesion ligands, breakable DNA force probes, and pharmacological inhibition demonstrate local mechanical coupling between integrin tension and actin protrusion. Thus, podosomes use pN integrin forces to sense and respond to substrate mechanics. This work deepens our understanding of podosome mechanotransduction and contributes tools that are widely applicable for studying receptor mechanics at dynamic interfaces.

An X-ray Vision for Phosphoantigen Recognition

Invariant Vγ9Vδ2 T cells respond to "phosphoantigen" metabolites through binding to the B30.2 domain of butyrophilin BTN3A. Yang et al. (2019) use molecular dynamic simulations based on X-ray structures of distinct B30.2 domain dimers to identify the asymmetric dimer as most active, which has implications for the inside-out signaling mechanism.

Cadherin Extracellular Domain Clustering in the Absence of Trans-Interactions

While both cis and trans (adhesive)-interactions cooperate in the assembly of intercellular adhesions, computational simulations have predicted that two-dimensional confinement may promote cis-oligomerization, in the absence of trans-interactions. Here, single-molecule tracking of cadherin extracellular domains on supported lipid bilayers revealed the density-dependent formation of oligomers and cis-clusters in the absence of trans-interactions. Lateral oligomers were virtually eliminated by mutating a putative cis (lateral) binding interface. At low cadherin surface coverage, wild-type and mutant cadherin diffused rapidly, consistent with the motion of a lipid molecule within a cadherin-free supported bilayer and with cadherins diffusing as monomers. Although the diffusion of mutant cadherin did not change appreciably with increasing surface coverage, the average short-time diffusion coefficient of wild-type cadherin slowed significantly above a fractional surface coverage of ∼0.01 (∼1100 molecules/μm2). A detailed analysis of molecular trajectories suggested the presence of a broad size distribution of cis-cadherin oligomers. These findings verify predictions that two-dimensional confinement promotes cis-oligomerization, in the absence of trans-interactions.

Hybrid Live Cell–Supported Membrane Interfaces for Signaling Studies

A wide range of cell–microenvironmental interactions are mediated by membrane-localized receptors that bind ligands present on another cell or the extracellular matrix. This situation introduces a number of physical effects including spatial organization of receptor–ligand complexes and development of mechanical forces in cells. Unlike traditional experimental approaches, hybrid live cell–supported lipid bilayer (SLB) systems, wherein a live cell interacts with a synthetic substrate supported membrane, allow interrogation of these aspects of receptor signaling. The SLB system directly offers facile control over the identity, density, and mobility of ligands used for engaging cellular receptors. Further, application of various nano- and micropatterning techniques allows for spatial patterning of ligands. In this review, we describe the hybrid live cell–SLB system and its application in uncovering a range of spatial and mechanical aspects of receptor signaling. We highlight the T cell immunological synapse, junctions formed between EphA2- and ephrinA1-expressing cells, and adhesions formed by cadherin and integrin receptors.

Cell-cell adhesion interface: orthogonal and parallel forces from contraction, protrusion, and retraction

The epithelial lateral membrane plays a central role in the integration of intercellular signals and, by doing so, is a principal determinant in the emerging properties of epithelial tissues. Mechanical force, when applied to the lateral cell-cell interface, can modulate the strength of adhesion and influence intercellular dynamics. Yet the relationship between mechanical force and epithelial cell behavior is complex and not completely understood. This commentary aims to provide an investigative look at the usage of cellular forces at the epithelial cell-cell adhesion interface.

Fabrication of Multicomponent, Spatially Segregated DNA and Protein-Functionalized Supported Membrane Microarray

Deoxyribonucleic acid (DNA) has been used as a material for a variety of applications, including surface functionalization for cell biological or in vitro reconstitution studies. Use of DNA-based surface functionalization eliminates limitations of multiplexing posed by traditionally used methods in applications requiring spatially segregated surface functionalization. Recently, we have reported a stochastic, membrane fusion-based strategy to fabricate multicomponent membrane array substrates displaying spatially segregated protein ligands using biotin-streptavidin and Ni-NTA-polyhistidine interactions. Here, we report the delivery of DNA oligonucleotide-conjugated lipid molecules to membrane corrals, allowing spatially segregated membrane corral functionalization in a membrane microarray. Incubation of microbeads coated with the supported membrane resulted in an exchange of lipid contents with planar membrane corrals present on a micropatterned substrate. Increases in the system temperature and membrane corral size resulted in alterations in the rate constant of lipid exchange, which are in agreement with our previously developed analytical model and further confirm that lipid exchange is a diffusion-based process that takes place after the formation of a long "fusion-stalk" between the two membranes. We take advantage of the physical dimensions of the fusion-stalk with a large aspect ratio to deliver DNA oligonucleotide-conjugated lipid molecules to membrane corrals. We believe that the ability to functionalize membrane corrals with DNA oligonucleotides significantly increases the utility of the stochastic fusion-mediated lipid delivery strategy in the functionalization of biomolecules such as DNA or DNA-conjugated protein ligands.

Spatiomechanical Modulation of EphB4-Ephrin-B2 Signaling in Neural Stem Cell Differentiation

Interactions between EphB4 receptor tyrosine kinases and their membrane-bound ephrin-B2 ligands on apposed cells play a regulatory role in neural stem cell differentiation. With both receptor and ligand constrained to move within the membranes of their respective cells, this signaling system inevitably experiences spatial confinement and mechanical forces in conjunction with receptor-ligand binding. In this study, we reconstitute the EphB4-ephrin-B2 juxtacrine signaling geometry using a supported-lipid-bilayer system presenting laterally mobile and monomeric ephrin-B2 ligands to live neural stem cells. This experimental platform successfully reconstitutes EphB4-ephrin-B2 binding, lateral clustering, downstream signaling activation, and neuronal differentiation, all in a configuration that preserves the spatiomechanical aspects of the natural juxtacrine signaling geometry. Additionally, the supported bilayer system allows control of lateral movement and clustering of the receptor-ligand complexes through patterns of physical barriers to lateral diffusion fabricated onto the underlying substrate. The results from this study reveal a distinct spatiomechanical effect on the ability of EphB4-ephrin-B2 signaling to induce neuronal differentiation. These observations parallel similar studies of the EphA2-ephrin-A1 system in a very different biological context, suggesting that such spatiomechanical regulation may be a common feature of Eph-ephrin signaling.

About Chemical Strategies to Fabricate Cell-Instructive Biointerfaces with Static and Dynamic Complexity

Properly functioning cell-instructive biointerfaces are critical for healthy integration of biomedical devices in the body and serve as decisive tools for the advancement of our understanding of fundamental cell biological phenomena. Studies are reviewed that use covalent chemistries to fabricate cell-instructive biointerfaces. These types of biointerfaces typically result in a static presentation of predefined cell-instructive cues. Chemically defined, but dynamic cell-instructive biointerfaces introduce spatiotemporal control over cell-instructive cues and present another type of biointerface, which promises a more biomimetic way to guide cell behavior. Therefore, strategies that offer control over the lateral sorting of ligands, the availability and molecular structure of bioactive ligands, and strategies that offer the ability to induce physical, chemical and mechanical changes in situ are reviewed. Specific attention is paid to state-of-the-art studies on dynamic, cell-instructive 3D materials. Future work is expected to further deepen our understanding of molecular and cellular biological processes investigating cell-type specific responses and the translational steps toward targeted in vivo applications.

Myosin-1c promotes E-cadherin tension and force-dependent recruitment of α-actinin to the epithelial cell junction

Actomyosin II contractility in epithelial cell plays an essential role in tension-dependent adhesion strengthening. One key unsettling question is how cellular contraction transmits force to the nascent cell-cell adhesion when there is no stable attachment between the nascent adhesion complex and actin filament. Here, we show that myosin-1c is localized to the lateral membrane of polarized epithelial cells and facilitates the coupling between actin and cell-cell adhesion. Knockdown of myosin-1c compromised the integrity of the lateral membrane, reduced the generation of tension at E-cadherin, decreased the strength of cell-cell cohesion in an epithelial cell monolayer and prevented force-dependent recruitment of junctional α-actinin. Application of exogenous force to cell-cell adhesions in a myosin-1c-knockdown cell monolayer fully rescued the localization defect of α-actinin, indicating that junction mechanoregulation remains intact in myosin-1c-depleted cells. Our study identifies a role of myosin-1c in force transmission at the lateral cell-cell interface and underscores a non-junctional contribution to tension-dependent junction regulation.

Tunable cell-surface mimetics as engineered cell substrates

Most recent breakthroughs in understanding cell adhesion, cell migration, and cellular mechanosensitivity have been made possible by the development of engineered cell substrates of well-defined surface properties. Traditionally, these substrates mimic the extracellular matrix (ECM) environment by the use of ligand-functionalized polymeric gels of adjustable stiffness. However, such ECM mimetics are limited in their ability to replicate the rich dynamics found at cell-cell contacts. This review focuses on the application of cell surface mimetics, which are better suited for the analysis of cell adhesion, cell migration, and cellular mechanosensitivity across cell-cell interfaces. Functionalized supported lipid bilayer systems were first introduced as biomembrane-mimicking substrates to study processes of adhesion maturation during adhesion of functionalized vesicles (cell-free assay) and plated cells. However, while able to capture adhesion processes, the fluid lipid bilayer of such a relatively simple planar model membrane prevents adhering cells from transducing contractile forces to the underlying solid, making studies of cell migration and cellular mechanosensitivity largely impractical. Therefore, the main focus of this review is on polymer-tethered lipid bilayer architectures as biomembrane-mimicking cell substrate. Unlike supported lipid bilayers, these polymer-lipid composite materials enable the free assembly of linkers into linker clusters at cellular contacts without hindering cell spreading and migration and allow the controlled regulation of mechanical properties, enabling studies of cellular mechanosensitivity. The various polymer-tethered lipid bilayer architectures and their complementary properties as cell substrates are discussed.

Clustering on Membranes: Fluctuations and More

Clustering of extracellular ligands and proteins on the plasma membrane is required to perform specific cellular functions, such as signaling and endocytosis. Attractive forces that originate in perturbations of the membrane's physical properties contribute to this clustering, in addition to direct protein-protein interactions. However, these membrane-mediated forces have not all been equally considered, despite their importance. In this review, we describe how line tension, lipid depletion, and membrane curvature contribute to membrane-mediated clustering. Additional attractive forces that arise from protein-induced perturbation of a membrane's fluctuations are also described. This review aims to provide a survey of the current understanding of membrane-mediated clustering and how this supports precise biological functions.

Secret handshakes: cell-cell interactions and cellular mimics

Cell-cell junctions, acting as 'secret handshakes', mediate cell-cell interactions and make multicellularity possible. Work over the previous century illuminated key players comprising these junctions including the cadherin superfamily, nectins, CAMs, connexins, notch/delta, lectins, and eph/Ephrins. Recent work has focused on elucidating how interactions between these complex and often contradictory cues can ultimately give rise to large-scale organization in tissues. This effort, in turn, has enabled bioengineering advances such as cell-mimetic interfaces that allow us to better probe junction biology and to develop new biomaterials. This review details exciting, recent developments in these areas as well as providing both historical context and a discussion of some topical challenges and opportunities for the future.

Platelet integrins exhibit anisotropic mechanosensing and harness piconewton forces to mediate platelet aggregation

Platelet aggregation at the site of vascular injury is essential in clotting. During this process, platelets are bridged by soluble fibrinogen that binds surface integrin receptors. One mystery in the mechanism of platelet aggregation pertains to how resting platelets ignore soluble fibrinogen, the third most abundant protein in the bloodstream, and yet avidly bind immobile fibrinogen on the surface of other platelets at the primary injury site. We speculate that platelet integrins are mechanosensors that test their ligands across the platelet-platelet synapse. To investigate this model, we interrogate human platelets using approaches that include the supported lipid bilayer platform as well as DNA tension sensor technologies. Experiments suggest that platelet integrins require lateral forces to mediate platelet-platelet interactions. Mechanically labile ligands dampen platelet activation, and the onset of piconewton integrin tension coincides with calcium flux. Activated platelets display immobilized fibrinogen on their surface, thus mediating further recruitment of resting platelets. The distribution of integrin tension was shown to be spatially regulated through two myosin-signaling pathways, myosin light chain kinase and Rho-associated kinase. Finally, we discovered that the termination of integrin tension is coupled with the exposure of phosphatidylserine. Our work reveals the highest spatial and temporal resolution maps of platelet integrin mechanics and its role in platelet aggregation, suggesting that platelets are physical substrates for one another that establish mechanical feedback loops of activation. The results are reminiscent of mechanical regulation of the T-cell receptor, E-cadherin, and Notch pathways, suggesting a common feature for signaling at cell junctions.

Trade-offs between structural integrity and acquisition time in stochastic super-resolution microscopy techniques

The applicability of widefield stochastic microscopy, such as PALM or STORM, is limited by their long acquisition times. Images are produced from the accumulation of a large number of frames that each contain a scarce number of super-resolved localizations. We show that the random and uneven distribution of localizations leads to a specific type of trade-off between the spatial and temporal resolutions. We derive analytical predictions for the minimal time required to obtain a reliable image at a given spatial resolution. We find that the image completion time scales logarithmically with the ratio of the image size to the spatial resolution volume, with second order corrections due to spurious localization within the background noise. We validate our predictions against experimental localization sequences of labeled microtubule filaments obtained by STORM. Our theoretical framework makes it possible to compare the efficiency of emitters, define optimal labeling strategies, and allow implementation of a stopping criterion for data acquisitions that can be performed using real-time monitoring algorithms.

Supported lipid bilayer platforms to probe cell mechanobiology

Mammalian and bacterial cells sense and exert mechanical forces through the process of mechanotransduction, which interconverts biochemical and physical signals. This is especially important in contact-dependent signaling, where ligand-receptor binding occurs at cell-cell or cell-ECM junctions. By virtue of occurring within these specialized junctions, receptors engaged in contact-dependent signaling undergo oligomerization and coupling with the cytoskeleton as part of their signaling mechanisms. While our ability to measure and map biochemical signaling within cell junctions has advanced over the past decades, physical cues remain difficult to map in space and time. Recently, supported lipid bilayer (SLB) technologies have emerged as a flexible platform to mimic and perturb cell-cell and cell-ECM junctions, allowing one to study membrane receptor mechanotransduction. Changing the lipid composition and underlying substrate tunes bilayer fluidity, and lipid and ligand micro- and nano-patterning spatially control positioning and clustering of receptors. Patterning metal gridlines within SLBs confines lipid mobility and introduces mechanical resistance. Here we review fundamental SLB mechanics and how SLBs can be engineered as tunable cell substrates for mechanotransduction studies. Finally, we highlight the impact of this work in understanding the biophysical mechanisms of cell adhesion. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

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.

Sustained α-catenin Activation at E-cadherin Junctions in the Absence of Mechanical Force

Mechanotransduction at E-cadherin junctions has been postulated to be mediated in part by a force-dependent conformational activation of α-catenin. Activation of α-catenin allows it to interact with vinculin in addition to F-actin, resulting in a strengthening of junctions. Here, using E-cadherin adhesions reconstituted on synthetic, nanopatterned membranes, we show that activation of α-catenin is dependent on E-cadherin clustering, and is sustained in the absence of mechanical force or association with F-actin or vinculin. Adhesions were formed by filopodia-mediated nucleation and micron-scale assembly of E-cadherin clusters, which could be distinguished as either peripheral or central assemblies depending on their relative location at the cell-bilayer adhesion. Whereas F-actin, vinculin, and phosphorylated myosin light chain associated only with the peripheral assemblies, activated α-catenin was present in both peripheral and central assemblies, and persisted in the central assemblies in the absence of actomyosin tension. Impeding filopodia-mediated nucleation and micron-scale assembly of E-cadherin adhesion complexes by confining the movement of bilayer-bound E-cadherin on nanopatterned substrates reduced the levels of activated α-catenin. Taken together, these results indicate that although the initial activation of α-catenin requires micron-scale clustering that may allow the development of mechanical forces, sustained force is not required for maintaining α-catenin in the active state.