Mechanism of the cadherin-catenin F-actin catch bond interaction

Catch Bonds in Immunology

Catch bonds are molecular bonds that last longer under force than slip bonds, which become shorter-lived under force. Although catch bonds were initially discovered in studies of leukocyte and bacterial adhesions two decades ago, they have since been found in many other contexts, including platelet binding to blood vessel walls during clotting, structural support within the cell and between cells, force transmission in the cell's machineries for motility and mechanotransduction, viral infection of host cells, and immunoreceptor mechanosensing. Catch bonds are strengthened by increasing force, which induces structural changes in one or both interacting molecules either locally or allosterically to enable additional contacts at their binding interface, thus lengthening bond lifetimes. They can be modeled by the kinetics of a system escaping from the energy well(s) of the bound state(s) over the energy barrier(s) to the free state by traversing along the dissociation path(s) across a hilly energy landscape modulated by force. Catch bond studies are important for understanding the mechanics of biological systems and developing treatment strategies for infectious diseases, immune disorders, cancer, and other ailments.

Fast Actin Disassembly and Fimbrin Mechanosensitivity Support Rapid Turnover in a Model of Clathrin-Mediated Endocytosis

The actin cytoskeleton is central to force production in numerous cellular processes in eukaryotic cells. During clathrin-mediated endocytosis (CME), a dynamic actin meshwork is required to deform the membrane against high membrane tension or turgor pressure. Previous experimental work from our lab showed that several endocytic proteins, including actin and actin-interacting proteins, turn over several times during the formation of a vesicle during CME in yeast, and their dwell time distributions were reminiscent of gamma distributions with a peak around 1 s. However, the distribution for the filament cross-linking protein fimbrin contains a second peak around 0.5 s. To better understand the nature of these dwell time distributions, we developed a stochastic model for the dynamics of actin and its binding partners. Our model demonstrates that very fast actin filament disassembly is necessary to reproduce experimental dwell time distributions. Our model also predicts that actin-binding proteins bind rapidly to nascent filaments and filaments are fully decorated. Last, our model predicts that fimbrin detachment from actin endocytic structures is mechanosensitive to explain the extra peak observed in the dwell time distribution.

α-Catenin force-sensitive binding and sequestration of LZTS2 leads to cytokinesis failure

Epithelial cells can become polyploid upon tissue injury, but mechanosensitive cues that trigger this state are poorly understood. Using an Madin Darby Canine Kidney (MDCK) cell knock-out/reconstitution system, we show that α-catenin mutants that alter force-sensitive binding to F-actin or middle (M)-domain promote cytokinesis failure and binucleation, particularly near epithelial wound-fronts. We identified Leucine Zipper Tumor Suppressor 2 (LZTS2), a factor previously implicated in abscission, as a conformation sensitive proximity partner of α-catenin. We show that LZTS2 enriches not only at midbody/intercellular bridges but also at apical adhering junctions. α-Catenin mutants with persistent M-domain opening show elevated junctional enrichment of LZTS2 compared with wild-type cells. LZTS2 knock-down leads to elevated rates of binucleation. These data implicate LZTS2 as a mechanosensitive effector of α-catenin that is critical for cytokinetic fidelity. This model rationalizes how persistent mechanoactivation of α-catenin may drive tension-induced polyploidization of epithelia after injury and suggests an underlying mechanism for how pathogenic α-catenin M-domain mutations drive macular dystrophy.

Actin-dependent α-catenin oligomerization contributes to adherens junction assembly

Classic cadherins, specifically E-cadherin in most epithelial cells, are transmembrane adhesion receptors, whose intracellular region interacts with proteins, termed catenins, forming the cadherin-catenin complex (CCC). The cadherin ectodomain generates 2D adhesive clusters (E-clusters) through cooperative trans and cis interactions, while catenins anchor the E-clusters 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. Here, we focus on the role of the actin-binding domain of α-catenin (αABD) by showing that the interaction of the αABD with actin generates actin-bound linear CCC oligomers (CCC/actin strands) incorporating up to six CCCs. This actin-driven CCC oligomerization, which is cadherin ectodomain independent, preferentially occurs along the actin cortex enriched with key basolateral proteins, myosin-1c, scribble, and DLG1. In cell-cell contacts, the CCC/actin strands integrate with the E-clusters giving rise to the composite oligomers, E/actin clusters. Targeted inactivation of strand formation by point mutations emphasizes the importance of this oligomerization process for blocking intercellular protrusive membrane activity and for coupling AJs with the actomyosin-derived tensional forces.

Proximity proteomics provides a new resource for exploring the function of Afadin and the complexity of cell-cell adherens junctions

The network of proteins at the interface between cell-cell adherens junctions and the actomyosin cytoskeleton provides robust yet dynamic connections that facilitate cell shape change and motility. While this was initially thought to be a simple linear connection via classic cadherins and their associated catenins, we now have come to appreciate that many more proteins are involved, providing robustness and mechanosensitivity. Defining the full set of proteins in this network remains a key objective in our field. Proximity proteomics provides a means to define these networks. Mammalian Afadin and its Drosophila homolog Canoe are key parts of this protein network, facilitating diverse cell shape changes during gastrulation and other events of embryonic morphogenesis. Here we report results of several proximity proteomics screens, defining proteins in the neighborhood of both the N- and C-termini of mammalian Afadin in the premier epithelial model, MDCK cells. We compare our results with previous screens done in other cell types, and with proximity proteomics efforts with other junctional proteins. These reveal the value of multiple screens in defining the full network of neighbors and offer interesting insights into the overlap in protein composition between different epithelial cell junctions.

Afadin mediates cadherin-catenin complex clustering on F-actin linked to cooperative binding and filament curvature

The E-cadherin-β-catenin-αE-catenin (cadherin-catenin) complex couples the cytoskeletons of neighboring cells at adherens junctions (AJs) to mediate force transmission across epithelia. Mechanical force and auxiliary binding partners converge to stabilize the cadherin-catenin complex's inherently weak binding to actin filaments (F-actin) through unclear mechanisms. Here, we show that afadin's coiled-coil (CC) domain and vinculin synergistically enhance the cadherin-catenin complex's F-actin engagement. The cryo-electron microscopy (cryo-EM) structure of an E-cadherin-β-catenin-αE-catenin-vinculin-afadin-CC supra-complex bound to F-actin reveals that afadin-CC bridges adjacent αE-catenin actin-binding domains along the filament, stabilizing flexible αE-catenin segments implicated in mechanical regulation. These cooperative binding contacts promote the formation of supra-complex clusters along F-actin. Additionally, cryo-EM variability analysis links supra-complex binding along individual F-actin strands to nanoscale filament curvature, a deformation mode associated with cytoskeletal forces. Collectively, this work elucidates a mechanistic framework by which vinculin and afadin tune cadherin-catenin complex-cytoskeleton coupling to support AJ function across varying mechanical regimes.

Biochemical and structural bases for talin ABSs-F-actin interactions

Focal adhesions (FAs) are large intracellular macromolecular assemblies that play a critical role in cell polarization and migration. Talin serves as a direct connection between integrin receptor and actomyosin cytoskeleton within FAs. Talin contains three actin-binding sites (ABS1-3) that engage discreetly during the development of FAs, thus acting as a critical player in FA initiation and maturation. However, the molecular basis of the ABS-F-actin interactions remains unknown. Here, we explore interactions of ABSs with F-actin to understand the multivalent behavior of talin. Particularly, the cryo-EM structure of the F-actin-ABS3 complex at 2.9 Å shows ABS3 spanning through two actin monomers along the filament axis, each occupied by the R13 rod subdomain and the DD domain. The dimerization of ABS3 occurs through the DD domain where both protomers interact on the actin surface, and the dimerization of talin to the actin surface is necessary for the engagement to F-actin. The R13 helical bundle is distorted upon binding to F-actin and releases the H1 helix from the rest of the bundle. This phenomenon has also been observed with other tension-sensing proteins like vinculin and α-catenin, highlighting that unfolding is relevant for its force sensing activity. On the contrary, ABS2 (R4R8 subdomains), which is thought to be critical for the maintenance of mature FAs, had multiple F-actin-binding regions within ABS2 and the binding likely occurred by these subdomains running through the surface of F-actin, thus strengthening the interactions upon the maturation of FAs.

Low-intensity pulsed ultrasound induces multifaced alterations in chromosome segregation, cytoskeletal filaments and cell junctions

Low-intensity pulsed ultrasound (LIPUS) is a widely used non-invasive approach with therapeutic purposes since it provides physical stimulation with minimal thermal effects. The skin epithelium is the first barrier of the human body that interfaces with LIPUS and is subjected to the highest intensity. Little is known about the impact of LIPUS on the skin surface. This work investigates the biological effects of one-hour exposure to 1 MHz LIPUS on human keratinocytes HaCaT and tumoral SK-MEL-28 skin cells. Specifically, we evaluated the cellular state immediately after LIPUS treatment by analyzing cytogenetic endpoints and the response of cytoskeleton and cell junction proteins. Herein we demonstrate that LIPUS induces genomic damage as shown by an increase of chromosome malsegregation and a consequent decrease of cellular proliferation. The mechanical stimulus produced by LIPUS is also transmitted to the cytoskeletal compartment, inducing the expression and re-organization of junction proteins (i.e., E-cadherin and Desmosomes) and intermediate filaments (i.e., F-actin and Cytokeratins) with impact on cell morphology and cell adhesion. These in vitro results highlight the different outcomes following the cytogenetic damage and the resilience response exerted by the cytoskeleton upon mechanical stress, laying the foundation for future in vivo investigations.

α-catenin phosphorylation is elevated during mitosis to resist apical rounding and epithelial barrier leak

Epithelial cell cohesion and barrier function critically depend on α-catenin, an actin-binding protein and essential constituent of cadherin-catenin-based adherens junctions. α-catenin undergoes actomyosin force-dependent unfolding of both actin-binding and middle domains to strongly engage actin filaments and its various effectors; this mechanosensitivity is critical for adherens junction function. We previously showed that α-catenin is highly phosphorylated in an unstructured region that links the mechanosensitive middle and actin-binding domains (known as the P-linker region), but the cellular processes that promote α-catenin phosphorylation have remained elusive. Here, we leverage a previously published phospho-proteomic data set to show that the α-catenin P-linker region is maximally phosphorylated during mitosis. By reconstituting α-catenin CRISPR knockout MDCK cells with wild-type, phospho-mutant and phospho-mimic forms of α-catenin, we show that full phosphorylation restrains mitotic cell rounding in the apical direction, strengthening the interactions between dividing and non-dividing neighbors to limit epithelial barrier leak. As the major scaffold components of adherens junctions, tight junctions and desmosomes are also differentially phosphorylated during mitosis, we reason that epithelial cell division may be a tractable system to understand how junction complexes are coordinately regulated to sustain barrier function under tension-generating morphogenetic processes.

PDZ Domains from the Junctional Proteins Afadin and ZO-1 Act as Mechanosensors

Intercellular adhesion complexes must withstand mechanical forces to maintain tissue cohesion while also retaining the capacity for dynamic remodeling during tissue morphogenesis and repair. Many cell-cell adhesion complexes contain at least one PSD95/Dlg/ZO-1 (PDZ) domain situated between the adhesion molecule and the actin cytoskeleton. However, PDZ-mediated interactions are characteristically nonspecific, weak, and transient, with multiple binding partners per PDZ domain, micromolar dissociation constants, and bond lifetimes of seconds or less. Here, we demonstrate that the bonds between the PDZ domain of the cytoskeletal adaptor protein afadin and the intracellular domains of the adhesion molecules nectin-1 and JAM-A form molecular catch bonds that reinforce in response to mechanical load. In contrast, the bond between the PDZ3-SH3-GUK (PSG) domain of the cytoskeletal adaptor ZO-1 and the JAM-A intracellular domain becomes dramatically weaker in response to ∼2 pN of load, the amount generated by single molecules of the cytoskeletal motor protein myosin II. Thus, physiologically relevant forces can exert dramatic and opposite effects on the stability of two of the major linkages between cell-cell adhesion proteins and the F-actin cytoskeleton. Our data demonstrate that that PDZ domains can serve as force-responsive mechanical anchors at cell-cell adhesion complexes. More broadly, our findings suggest that mechanical force may serve as a previously unsuspected regulator of the hundreds of PDZ-ligand interactions present in animal cells.

Effects of NaV1.5 and Rac1 on the Epithelial-Mesenchymal Transition in Breast Cancer

Breast cancer is a disease that seriously endangers the health of women. However, it is difficult to treat due to the emergence of metastasis and drug resistance. Exploring the metastasis mechanism of breast cancer is helpful to aim for the appropriate target. The epithelial-mesenchymal transition (EMT) is an important mechanism of breast cancer metastasis. Sodium channel 1.5(NaV1.5) and the GTPase Rac1 are factors related to the degree of malignancy of breast tumors. The expression of NaV1.5 and the activation of Rac1 are both involved in EMT. In addition, NaV1.5 can change the plasma membrane potential (Vm) by promoting the inflow of Na+ to depolarize the cell membrane, induce the activation of Rac1 and produce a cascade of reactions that lead to EMT in breast cancer cells; this sequence of events further induces the movement, migration and invasion of tumor cells and affects the prognosis of breast cancer patients. In this paper, the roles of NaV1.5 and Rac1 in EMT-mediated breast cancer progression were reviewed.

De novo DNA-based catch bonds

All primary chemical interactions weaken under mechanical stress, which imposes fundamental mechanical limits on the materials constructed from them. Biological materials combine plasticity with strength, for which nature has evolved a unique solution-catch bonds, supramolecular interactions that strengthen under tension. Biological catch bonds use force-gated conformational switches to convert weak bonds into strong ones. So far, catch bonds remain exclusive to nature, leaving their potential as mechanoadaptive elements in synthetic systems untapped. Here we report the design and realization of artificial catch bonds. Starting from a minimal set of thermodynamic design requirements, we created a molecular motif capable of catch bonding. It consists of a DNA duplex featuring a cryptic domain that unfolds under tension to strengthen the interaction. We show that these catch bonds recreate force-enhanced rolling adhesion, a hallmark feature of biological catch bonds in bacteria and leukocytes. This Article introduces catch bonds into the synthetic domain, and could lead to the creation of artificial catch-bonded materials.

Proximity proteomics provides a new resource for exploring the function of Afadin and the complexity of cell-cell adherens junctions

The network of proteins at the interface between cell-cell adherens junctions and the actomyosin cytoskeleton provides robust yet dynamic connections that facilitate cell shape change and motility. While this was initially thought to be a simple linear connection via classic cadherins and their associated catenins, we now have come to appreciate that many more proteins are involved, providing robustness and mechanosensitivity. Defining the full network of proteins in this network remains a key objective in our field. Proximity proteomics provides a means to define these networks. Mammalian Afadin and its Drosophila homolog Canoe are key parts of this protein network, facilitating diverse cell shape changes during gastrulation and other events of embryonic morphogenesis. Here we report results of several proximity proteomics screens, defining proteins in the neighborhood of both the N- and C-termini of mammalian Afadin in the premier epithelial model, MDCK cells. We compare our results with previous screens done in other cell types, and with proximity proteomics efforts with other junctional proteins. These reveal the value of multiple screens in defining the full network of neighbors and offer interesting insights into the overlap in protein composition between different epithelial cell junctions.

Engineering tunable catch bonds with DNA

Unlike most adhesive bonds, biological catch bonds strengthen with increased tension. This characteristic is essential to specific receptor-ligand interactions, underpinning biological adhesion dynamics, cell communication, and mechanosensing. While artificial catch bonds have been conceived, the tunability of their catch behaviour is limited. Here, we present the fish-hook, a rationally designed DNA catch bond that can be finely adjusted to a wide range of catch behaviours. We develop models to design these DNA structures and experimentally validate different catch behaviours by single-molecule force spectroscopy. The fish-hook architecture supports a vast sequence-dependent behaviour space, making it a valuable tool for reprogramming biological interactions and engineering force-strengthening materials.

Afadin mediates cadherin-catenin complex clustering on F-actin linked to cooperative binding and filament curvature

The E-cadherin-β-catenin-αE-catenin (cadherin-catenin) complex couples the cytoskeletons of neighboring cells at adherens junctions (AJs) to mediate force transmission across epithelia. Mechanical force and auxiliary binding partners converge to stabilize the cadherin-catenin complex's inherently weak binding to actin filaments (F-actin) through unclear mechanisms. Here we show that afadin's coiled-coil (CC) domain and vinculin synergistically enhance the cadherin-catenin complex's F-actin engagement. The cryo-EM structure of an E-cadherin-β-catenin-αE-catenin-vinculin-afadin-CC supra-complex bound to F-actin reveals that afadin-CC bridges adjacent αE-catenin actin-binding domains along the filament, stabilizing flexible αE-catenin segments implicated in mechanical regulation. These cooperative binding contacts promote the formation of supra-complex clusters along F-actin. Additionally, cryo-EM variability analysis links supra-complex binding along individual F-actin strands to nanoscale filament curvature, a deformation mode associated with cytoskeletal forces. Collectively, this work elucidates a mechanistic framework by which vinculin and afadin tune cadherin-catenin complex-cytoskeleton coupling to support AJ function across varying mechanical regimes.

Cross-regulations of two connected domains form a mechanical circuit for steady force transmission during clathrin-mediated endocytosis

Mechanical forces are transmitted from the actin cytoskeleton to the membrane during clathrin-mediated endocytosis (CME) in the fission yeast Schizosaccharomyces pombe. End4p directly transmits force in CME by binding to both the membrane (through the AP180 N-terminal homology [ANTH] domain) and F-actin (through the talin-HIP1/R/Sla2p actin-tethering C-terminal homology [THATCH] domain). We show that 7 pN force is required for stable binding between THATCH and F-actin. We also characterized a domain in End4p, Rend (rod domain in End4p), that resembles R12 of talin. Membrane localization of Rend primes the binding of THATCH to F-actin, and force-induced unfolding of Rend at 15 pN terminates the transmission of force. We show that the mechanical properties (mechanical stability, unfolding extension, hysteresis) of Rend and THATCH are tuned to form a circuit for the initiation, transmission, and termination of force between the actin cytoskeleton and membrane. The mechanical circuit by Rend and THATCH may be conserved and coopted evolutionarily in cell adhesion complexes.

Extracellular Matrix Components and Mechanosensing Pathways in Health and Disease

Glycosaminoglycans (GAGs) and proteoglycans (PGs) are essential components of the extracellular matrix (ECM) with pivotal roles in cellular mechanosensing pathways. GAGs, such as heparan sulfate (HS) and chondroitin sulfate (CS), interact with various cell surface receptors, including integrins and receptor tyrosine kinases, to modulate cellular responses to mechanical stimuli. PGs, comprising a core protein with covalently attached GAG chains, serve as dynamic regulators of tissue mechanics and cell behavior, thereby playing a crucial role in maintaining tissue homeostasis. Dysregulation of GAG/PG-mediated mechanosensing pathways is implicated in numerous pathological conditions, including cancer and inflammation. Understanding the intricate mechanisms by which GAGs and PGs modulate cellular responses to mechanical forces holds promise for developing novel therapeutic strategies targeting mechanotransduction pathways in disease. This comprehensive overview underscores the importance of GAGs and PGs as key mediators of mechanosensing in maintaining tissue homeostasis and their potential as therapeutic targets for mitigating mechano-driven pathologies, focusing on cancer and inflammation.

α -catenin phosphorylation is elevated during mitosis to resist apical rounding and epithelial barrier leak

Epithelial cell cohesion and barrier function critically depend on α -catenin, an actin-binding protein and essential constituent of cadherin-catenin-based adherens junctions. α -catenin undergoes actomyosin force-dependent unfolding of both actin-binding and middle domains to strongly engage actin filaments and its various effectors, where this mechanosensitivity is critical for adherens junction function. We previously showed that α -catenin is highly phosphorylated in an unstructured region that links mechanosensitive middle- and actin-binding domains (known as the P-linker region), but the cellular processes that promote α -catenin phosphorylation have remained elusive. Here, we leverage a previously published phosphor-proteomic data set to show that the α -catenin P-linker region is maximally phosphorylated during mitosis. By reconstituting α -catenin Crispr KO MDCK with wild-type, phospho- mutant and mimic forms of α -catenin, we show that full phosphorylation restrains mitotic cell rounding in the apical direction, strengthening interactions between dividing and non-dividing neighbors to limit epithelial barrier leak. Since major scaffold components of adherens junctions, tight junctions and desmosomes are also differentially phosphorylated during mitosis, we reason that epithelial cell division may be a tractable system to understand how junction complexes are coordinately regulated to sustain barrier function under tension-generating morphogenetic processes.

Molecular simulation approaches to probing the effects of mechanical forces in the actin cytoskeleton

In this article we give our perspective on the successes and promise of various molecular and coarse-grained simulation approaches to probing the effect of mechanical forces in the actin cytoskeleton.

Alternative molecular mechanisms for force transmission at adherens junctions via β-catenin-vinculin interaction

Force transmission through adherens junctions (AJs) is crucial for multicellular organization, wound healing and tissue regeneration. Recent studies shed light on the molecular mechanisms of mechanotransduction at the AJs. However, the canonical model fails to explain force transmission when essential proteins of the mechanotransduction module are mutated or missing. Here, we demonstrate that, in absence of α-catenin, β-catenin can directly and functionally interact with vinculin in its open conformation, bearing physiological forces. Furthermore, we found that β-catenin can prevent vinculin autoinhibition in the presence of α-catenin by occupying vinculin´s head-tail interaction site, thus preserving force transmission capability. Taken together, our findings suggest a multi-step force transmission process at AJs, where α-catenin and β-catenin can alternatively and cooperatively interact with vinculin. This can explain the graded responses needed to maintain tissue mechanical homeostasis and, importantly, unveils a force-bearing mechanism involving β-catenin and extended vinculin that can potentially explain the underlying process enabling collective invasion of metastatic cells lacking α-catenin.

Desmosomal Cadherin Tension Loss in Pemphigus Vulgaris Mediated by the Inhibition of Active RhoA at Cell-Cell Adhesions

Binding of autoantibodies to keratinocyte surface antigens, primarily desmoglein 3 (Dsg3) of the desmosomal complex, leads to the dissociation of cell-cell adhesion in the blistering disorder pemphigus vulgaris (PV). After the initial disassembly of desmosomes, cell-cell adhesions actively remodel in association with the cytoskeleton and focal adhesions. Growing evidence highlights the role of adhesion mechanics and mechanotransduction at cell-cell adhesions in this remodeling process, as their active participation may direct autoimmune pathogenicity. However, a large part of the biophysical transformations after antibody binding remains underexplored. Specifically, it is unclear how tension in desmosomes and cell-cell adhesions changes in response to antibodies, and how the altered tensional states translate to cellular responses. Here, we showed a tension loss at Dsg3 using fluorescence resonance energy transfer (FRET)-based tension sensors, a tension loss at the entire cell-cell adhesion, and a potentially compensatory increase in junctional traction force at cell-extracellular matrix adhesions after PV antibody binding. Further, our data indicate that this tension loss is mediated by the inhibition of RhoA at cell-cell contacts, and the extent of RhoA inhibition may be crucial in determining the severity of pathogenicity among different PV antibodies. More importantly, this tension loss can be partially restored by altering actomyosin based cell contractility. Collectively, these findings provide previously unattainable details in our understanding of the mechanisms that govern cell-cell interactions under physiological and autoimmune conditions, which may open the window to entirely new therapeutics aimed at restoring physiological balance to tension dynamics that regulates the maintenance of cell-cell adhesion.

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.

Vinculin is essential for sustaining normal levels of endogenous forces at cell-cell contacts

Transmission of cell-generated (i.e., endogenous) tension at cell-cell contacts is crucial for tissue shape changes during morphogenesis and adult tissue repair in tissues such as epithelia. E-cadherin-based adhesions at cell-cell contacts are the primary means by which endogenous tension is transmitted between cells. The E-cadherin-β-catenin-α-catenin complex mechanically couples to the actin cytoskeleton (and thereby the cell's contractile machinery) both directly and indirectly. However, the key adhesion constituents required for substantial endogenous force transmission at these adhesions in cell-cell contacts are unclear. Due to the role of α-catenin as a mechanotransducer that recruits vinculin at cell-cell contacts, we expected α-catenin to be essential for sustaining normal levels of force transmission. Instead, using the traction force imbalance method to determine the inter-cellular force at a single cell-cell contact between cell pairs, we found that it is vinculin that is essential for sustaining normal levels of endogenous force transmission, with absence of vinculin decreasing the inter-cellular tension by over 50%. Our results constrain the potential mechanical pathways of force transmission at cell-cell contacts and suggest that vinculin can transmit forces at E-cadherin adhesions independent of α-catenin, possibly through β-catenin. Furthermore, we tested the ability of lateral cell-cell contacts to withstand external stretch and found that both vinculin and α-catenin are essential to maintain cell-cell contact stability under external forces.

LY6/PLAUR domain containing 3 (LYPD3) maintains melanoma cell stemness and mediates an immunosuppressive microenvironment

BACKGROUND

Malignant melanoma is a highly heterogeneous skin cancer with the highest mortality rate among dermatological cancers. Catenins form functional networks in the nucleus to regulate gene expression and determine cell fate. Dysregulation of catenin expression correlates with the malignant characteristics of the tumor. We aimed to investigate the regulatory mechanisms of catenins in melanoma and to further define the function of catenin-related molecular signaling in the tumor microenvironment.

METHODS

In this study, a bioinformatics approach combined with experimental validation was used to explore the potential tumor biology mechanisms of catenin-related signaling.

RESULTS

Melanoma patients can be divided into two catenin clusters. Patients defined by high Junction Plakoglobin (JUP), Plakophilin 1 (PKP1), Plakophilin 3 (PKP3) levels (C2) had shorter survival time than other patients (C1). We demonstrated that JUP regulates Anterior Gradient 2 (AGR2)/LY6/PLAUR Domain Containing 3 (LYPD3) to maintain melanoma stemness and promotes glycolysis. We also found that LYPD3 was co-expressed with S100A9 and associated with immunosuppressive tumor microenvironment (TME).

CONCLUSION

The JUP/AGR2/LYPD3 signaling axis plays an important role in the malignant features of melanoma. Targeting the JUP/AGR2/LYPD3 signaling axis can help develop promising drugs.

The Interaction of Mechanics and the Hippo Pathway in Drosophila melanogaster

Drosophila melanogaster has emerged as an ideal system for studying the networks that control tissue development and homeostasis and, given the similarity of the pathways involved, controlled and uncontrolled growth in mammalian systems. The signaling pathways used in patterning the Drosophila wing disc are well known and result in the emergence of interaction of these pathways with the Hippo signaling pathway, which plays a central role in controlling cell proliferation and apoptosis. Mechanical effects are another major factor in the control of growth, but far less is known about how they exert their control. Herein, we develop a mathematical model that integrates the mechanical interactions between cells, which occur via adherens and tight junctions, with the intracellular actin network and the Hippo pathway so as to better understand cell-autonomous and non-autonomous control of growth in response to mechanical forces.

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

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.

Rap1 promotes epithelial integrity and cell viability in a growing tissue

Having intact epithelial tissues is critical for embryonic development and adult homeostasis. How epithelia respond to damaging insults or tissue growth while still maintaining intercellular connections and barrier integrity during development is poorly understood. The conserved small GTPase Rap1 is critical for establishing cell polarity and regulating cadherin-catenin cell junctions. Here, we identified a new role for Rap1 in maintaining epithelial integrity and tissue shape during Drosophila oogenesis. Loss of Rap1 activity disrupted the follicle cell epithelium and the shape of egg chambers during a period of major growth. Rap1 was required for proper E-Cadherin localization in the anterior epithelium and for epithelial cell survival. Both Myo-II and the adherens junction-cytoskeletal linker protein α-Catenin were required for normal egg chamber shape but did not strongly affect cell viability. Blocking the apoptotic cascade failed to rescue the cell shape defects caused by Rap1 inhibition. One consequence of increased cell death caused by Rap1 inhibition was the loss of polar cells and other follicle cells, which later in development led to fewer cells forming a migrating border cell cluster. Our results thus indicate dual roles for Rap1 in maintaining epithelia and cell survival in a growing tissue during development.

α-catenin mechanosensitivity as a route to cytokinesis failure through sequestration of LZTS2

Epithelial cells can become polyploid upon tissue injury, but mechanosensitive cues that trigger this state are poorly understood. Using α-catenin (α-cat) knock-out Madin Darby Canine Kidney (MDCK) cells reconstituted with wild-type and mutant forms of α-cat as a model system, we find that an established α-cat actin-binding domain unfolding mutant designed to reduce force-sensitive binding to F-actin (α-cat-H0-FABD+) can promote cytokinesis failure, particularly along epithelial wound-fronts. Enhanced α-cat coupling to cortical actin is neither sufficient nor mitotic cell-autonomous for cytokinesis failure, but critically requires the mechanosensitive Middle-domain (M1-M2-M3) and neighboring cells. Disease relevant α-cat M-domain missense mutations known to cause a form of retinal pattern dystrophy (α-cat E307K or L436P) are associated with elevated binucleation rates via cytokinesis failure. Similar binucleation rates are seen in cells expressing an α-cat salt-bridge destabilizing mutant (R551A) designed to promote M2-M3 domain unfurling at lower force thresholds. Since binucleation is strongly enhanced by removal of the M1 as opposed to M2-M3 domains, cytokinetic fidelity is most sensitive to α-cat M2-M3 domain opening. To identify α-cat conformation-dependent proximity partners that contribute to cytokinesis, we used a biotin-ligase approach to distinguished proximity partners that show enhanced recruitment upon α-cat M-domain unfurling (R551A). We identified Leucine Zipper Tumor Suppressor 2 (LZTS2), an abscission factor previously implicated in cytokinesis. We confirm that LZTS2 enriches at the midbody, but discover it also localizes to tight and tricellular junctions. LZTS2 knock-down promotes binucleation in both MDCK and Retinal Pigmented Epithelial (RPE) cells. α-cat mutants with persistent M2-M3 domain opening showed elevated junctional enrichment of LZTS2 from the cytosol compared α-cat wild-type cells. These data implicate LZTS2 as a mechanosensitive effector of α-cat that is critical for cytokinetic fidelity. This model rationalizes how persistent mechano-activation of α-cat may drive tension-induced polyploidization of epithelia post-injury and suggests an underlying mechanism for how pathogenic α-cat mutations drive macular dystrophy.

α-Catenin Dependent E-cadherin Adhesion Dynamics as Revealed by an Accelerated Force Ramp

Tissue remodeling and shape changes often rely on force-induced cell rearrangements occurring via cell-cell contact dynamics. Epithelial cell-cell contact shape changes are particularly dependent upon E-cadherin adhesion dynamics which are directly influenced by cell-generated and external forces. While both the mobility of E-cadherin adhesions and their adhesion strength have been reported before, it is not clear how these two aspects of E-cadherin adhesion dynamics are related. Here, using magnetic pulling cytometry, we applied an accelerated force ramp on the E-cadherin adhesion between an E-cadherin-coated magnetic microbead and an epithelial cell to ascertain this relationship. Our approach enables the determination of the adhesion strength and force-dependent mobility of individual adhesions, which revealed a direct correlation between these key characteristics. Since α-catenin has previously been reported to play a role in both E-cadherin mobility and adhesion strength when studied independently, we also probed epithelial cells in which α-catenin has been knocked out. We found that, in the absence of α-catenin, E-cadherin adhesions not only had lower adhesion strength, as expected, but were also more mobile. We observed that α-catenin was required for the recovery of strained cell-cell contacts and propose that the adhesion strength and force-dependent mobility of E-cadherin adhesions act in tandem to regulate cell-cell contact homeostasis. Our approach introduces a method which relates the force-dependent adhesion mobility to adhesion strength and highlights the morphological role played by α-catenin in E-cadherin adhesion dynamics.

What's the Catch? The Significance of Catch Bonds in T Cell Activation

One of the main goals in T cell biology has been to investigate how TCR recognition of peptide:MHC (pMHC) determines T cell phenotype and fate. Ag recognition is required to facilitate survival, expansion, and effector function of T cells. Historically, TCR affinity for pMHC has been used as a predictor for T cell fate and responsiveness, but there have now been several examples of nonfunctional high-affinity clones and low-affinity highly functional clones. Recently, more attention has been paid to the TCR being a mechanoreceptor where the key biophysical determinant is TCR bond lifetime under force. As outlined in this review, the fundamental parameters between the TCR and pMHC that control Ag recognition and T cell triggering are affinity, bond lifetime, and the amount of force at which the peak lifetime occurs.

Range and strength of mechanical interactions of force dipoles in elastic fiber networks

Mechanical forces generated by myosin II molecular motors drive diverse cellular processes, most notably shape change, division and locomotion. These forces may be transmitted over long range through the cytoskeletal medium - a disordered, viscoelastic network of biopolymers. The resulting cell size scale force chains can in principle mediate mechanical interactions between distant actomyosin units, leading to self-organized structural order in the cell cytoskeleton. Inspired by such force transmission through elastic structures in the cytoskeleton, we consider a percolated fiber lattice network, where fibers are represented as linear elastic elements that can both bend and stretch, and the contractile activity of myosin motors is represented by force dipoles. Then, by using a variety of metrics, we show how two such contractile force dipoles interact with each other through their mutual mechanical deformations of the elastic fiber network. As a prelude to two-dipole interactions, we quantify how forces propagate through the network from a single anisotropic force dipole by analyzing clusters of nodes connected by highly strained bonds, as well as through the decay rate of strain energy with distance from a force dipole. We show that predominant fiber bending screens out force propagation, resulting in reduced and strongly network configuration-dependent dipole interactions. On the other hand, stretching-dominated networks support longer-ranged inter-dipole interactions that recapitulate the predictions of linear elasticity theory. By characterizing the differences between tensile and compressive force propagation in the fiber network, we show how inter-dipole interaction depends on the dipoles' mutual separation and orientation. The resulting elastic interaction energy may mediate a force between multiple distant dipoles, leading to their self-organization into ordered configurations. This provides a potential pathway for active mechanical force-driven structural order in elastic biopolymer networks.

Evolution and developmental functions of the dystrophin-associated protein complex: beyond the idea of a muscle-specific cell adhesion complex

The Dystrophin-Associated Protein Complex (DAPC) is a well-defined and evolutionarily conserved complex in animals. DAPC interacts with the F-actin cytoskeleton via dystrophin, and with the extracellular matrix via the membrane protein dystroglycan. Probably for historical reasons that have linked its discovery to muscular dystrophies, DAPC function is often described as limited to muscle integrity maintenance by providing mechanical robustness, which implies strong cell-extracellular matrix adhesion properties. In this review, phylogenetic and functional data from different vertebrate and invertebrate models will be analyzed and compared to explore the molecular and cellular functions of DAPC, with a specific focus on dystrophin. These data reveals that the evolution paths of DAPC and muscle cells are not intrinsically linked and that many features of dystrophin protein domains have not been identified yet. DAPC adhesive properties also are discussed by reviewing the available evidence of common key features of adhesion complexes, such as complex clustering, force transmission, mechanosensitivity and mechanotransduction. Finally, the review highlights DAPC developmental roles in tissue morphogenesis and basement membrane (BM) assembly that may indicate adhesion-independent functions.

How cells sense and integrate information from different sources

Cell signaling is a fundamental cellular process that enables cells to sense and respond to information in their surroundings. At the molecular level, signaling is primarily carried out by transmembrane protein receptors that can initiate complex downstream signal transduction cascades to alter cellular behavior. In the human body, different cells can be exposed to a wide variety of environmental conditions, and cells express diverse classes of receptors capable of sensing and integrating different signals. Furthermore, different receptors and signaling pathways can crosstalk with each other to calibrate the cellular response. Crosstalk occurs through multiple mechanisms at different levels of signaling pathways. In this review, we discuss how cells sense and integrate different chemical, mechanical, and spatial signals as well as the mechanisms of crosstalk between pathways. To illustrate these concepts, we use a few well-studied signaling pathways, including receptor tyrosine kinases and integrin receptors. Finally, we discuss the implications of dysregulated cellular sensing on driving diseases such as cancer. This article is categorized under: Cancer > Molecular and Cellular Physiology Metabolic Diseases > Molecular and Cellular Physiology.

The α-Catenin mechanosensing M region is required for cell adhesion during tissue morphogenesis

α-Catenin couples the cadherin-catenin complex to the actin cytoskeleton. The mechanosensitive α-Catenin M region undergoes conformational changes upon application of force to recruit interaction partners. Here, we took advantage of the tension landscape in the Drosophila embryo to define three different states of α-Catenin mechanosensing in support of cell adhesion. Low-, medium-, and high-tension contacts showed a corresponding recruitment of Vinculin and Ajuba, which was dependent on the α-Catenin M region. In contrast, the Afadin homolog Canoe acts in parallel to α-Catenin at bicellular low- and medium-tension junctions but requires an interaction with α-Catenin for its tension-sensitive enrichment at high-tension tricellular junctions. Individual M region domains make complex contributions to cell adhesion through their impact on interaction partner recruitment, and redundancies with the function of Canoe. Our data argue that α-Catenin and its interaction partners are part of a cooperative and partially redundant mechanoresponsive network that supports AJs remodeling during morphogenesis.

The nematode α-catenin ortholog, HMP1, has an extended α-helix when bound to actin filaments

The regulation of cell-cell junctions during epidermal morphogenesis ensures tissue integrity, a process regulated by α-catenin. This cytoskeletal protein connects the cadherin complex to filamentous actin at cell-cell junctions. The cadherin-catenin complex plays key roles in cell physiology, organism development, and disease. While mutagenesis of Caenorhabditis elegans cadherin and catenin shows that these proteins are key for embryonic morphogenesis, we know surprisingly little about their structure and attachment to the cytoskeleton. In contrast to mammalian α-catenin that functions as a dimer or monomer, the α-catenin ortholog from C. elegans, HMP1 for humpback, is a monomer. Our cryogenic electron microscopy (cryoEM) structure of HMP1/α-catenin reveals that the amino- and carboxy-terminal domains of HMP1/α-catenin are disordered and not in contact with the remaining HMP1/α-catenin middle domain. Since the carboxy-terminal HMP1/α-catenin domain is the F-actin-binding domain (FABD), this interdomain constellation suggests that HMP1/α-catenin is constitutively active, which we confirm biochemically. Our perhaps most surprising finding, given the high sequence similarity between the mammalian and nematode proteins, is our cryoEM structure of HMP1/α-catenin bound to F-actin. Unlike the structure of mammalian α-catenin bound to F-actin, binding to F-actin seems to allosterically convert a loop region of the HMP1/α-catenin FABD to extend an HMP1/α-catenin FABD α-helix. We use cryoEM and bundling assays to show for the first time how the FABD of HMP1/α-catenin bundles actin in the absence of force. Collectively, our data advance our understanding of α-catenin regulation of cell-cell contacts and additionally aid our understanding of the evolution of multicellularity in metazoans.