Myosin 2 is a key Rho kinase target necessary for the local concentration of E-cadherin at cell-cell contacts

Human lung microvascular endothelial cell protein modification by 2-chlorohexadecanoic acid: RhoA mediates 2-chlorohexadecanoic acid-elicited endothelial activation

Chlorolipids are produced during the neutrophil respiratory burst as a result of myeloperoxidase (MPO)-generated hypochlorous acid (HOCl) targeting the vinyl ether bond of plasmalogen phospholipids. The initial products of this reaction are 2-chlorofatty aldehydes (2-ClFALDs), which are subsequently oxidized to 2-chlorofatty acids (2-ClFAs). 2-Chlorohexadecanoic acid (2-ClHA) is the 16-carbon 2-ClFA species, and previous studies have shown that increased levels of plasma 2-ClHA associate with acute respiratory distress syndrome (ARDS)-caused mortality in human sepsis. 2-ClHA causes endothelial barrier dysfunction and increases neutrophil and platelet adherence to the endothelium. In this study, click chemistry analogs of 2-ClHA and hexadecanoic acid (HA) were used to identify proteins covalently modified by 2-ClHA and HA in human lung microvascular endothelial cells (HLMVECs). Eleven proteins were specifically modified by 2-ClHA, and an additional one hundred and ninety-four proteins were modified by both 2-ClHA and HA. STRING analysis of 2-ClHA-modified proteins revealed a network of proteins with RhoA as a hub. RhoA is one of the proteins specifically modified by 2-ClHA and not HA. The RhoA inhibitors, Rhosin and C3, inhibited both 2-ClHA-elicited HLMVEC barrier dysfunction and angiopoietin-2 (Ang-2) release from HLMVEC. Further studies showed 2-ClHA activates HLMVEC RhoA activity. The specificity of the 2-ClHA-RhoA pathway for endothelial activation was further confirmed since HA did not cause HLMVEC barrier dysfunction, Ang-2 release and RhoA activation. Collectively, these studies have identified multiple proteins modified exclusively by 2-ClHA in HLMVECs, including RhoA. These proteomics studies led to the key finding that RhoA is an important mediator of 2-ClHA-caused endothelial barrier dysfunction.

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.

Regulation of epithelial cell jamming transition by cytoskeleton and cell-cell interactions

Multicellular systems, such as epithelial cell collectives, undergo transitions similar to those in inert physical systems like sand piles and foams. To remodel or maintain tissue organization during development or disease, these collectives transition between fluid-like and solid-like states, undergoing jamming or unjamming transitions. While these transitions share principles with physical systems, understanding their regulation and implications in cell biology is challenging. Although cell jamming and unjamming follow physics principles described by the jamming diagram, they are fundamentally biological processes. In this review, we explore how cellular processes and interactions regulate jamming and unjamming transitions. We begin with an overview of how these transitions control tissue remodeling in epithelial model systems and describe recent findings of the physical principles governing tissue solidification and fluidization. We then explore the mechanistic pathways that modulate the jamming phase diagram axes, focusing on the regulation of cell fluctuations and geometric compatibility. Drawing upon seminal works in cell biology, we discuss the roles of cytoskeleton and cell-cell adhesion in controlling cell motility and geometry. This comprehensive view illustrates the molecular control of cell jamming and unjamming, crucial for tissue remodeling in various biological contexts.

Myosin II mediates Shh signals to shape dental epithelia via control of cell adhesion and movement

The development of ectodermal organs begins with the formation of a stratified epithelial placode that progressively invaginates into the underlying mesenchyme as the organ takes its shape. Signaling by secreted molecules is critical for epithelial morphogenesis, but how that information leads to cell rearrangement and tissue shape changes remains an open question. Using the mouse dentition as a model, we first establish that non-muscle myosin II is essential for dental epithelial invagination and show that it functions by promoting cell-cell adhesion and persistent convergent cell movements in the suprabasal layer. Shh signaling controls these processes by inducing myosin II activation via AKT. Pharmacological induction of AKT and myosin II can also rescue defects caused by the inhibition of Shh. Together, our results support a model in which the Shh signal is transmitted through myosin II to power effective cellular rearrangement for proper dental epithelial invagination.

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.

SCRIB controls apical contractility during epithelial differentiation

Although mutations in the SCRIB gene lead to multiple morphological organ defects in vertebrates, the molecular pathway linking SCRIB to organ shape anomalies remains elusive. Here, we study the impact of SCRIB-targeted gene mutations during the formation of the gut epithelium in an organ-on-chip model. We show that SCRIB KO gut-like epithelia are flatter with reduced exposed surface area. Cell differentiation on filters further shows that SCRIB plays a critical role in the control of apical cell shape, as well as in the basoapical polarization of myosin light chain localization and activity. Finally, we show that SCRIB serves as a molecular scaffold for SHROOM2/4 and ROCK1 and identify an evolutionary conserved SHROOM binding site in the SCRIB carboxy-terminal that is required for SCRIB function in the control of apical cell shape. Our results demonstrate that SCRIB plays a key role in epithelial morphogenesis by controlling the epithelial apical contractility during cell differentiation.

Actin-dependent recruitment of AGO2 to the zonula adherens

Adherens junctions are cadherin-based structures critical for cellular architecture. E-cadherin junctions in mature epithelial cell monolayers tether to an apical actomyosin ring to form the zonula adherens (ZA). We have previously shown that the adherens junction protein PLEKHA7 associates with and regulates the function of the core RNA interference (RNAi) component AGO2 specifically at the ZA. However, the mechanism mediating AGO2 recruitment to the ZA remained unexplored. Here, we reveal that this ZA-specific recruitment of AGO2 depends on both the structural and tensile integrity of the actomyosin cytoskeleton. We found that depletion of not only PLEKHA7, but also either of the three PLEKHA7-interacting, LIM-domain family proteins, namely LMO7, LIMCH1, and PDLIM1, results in disruption of actomyosin organization and tension, as well as disruption of AGO2 junctional localization and of its miRNA-binding ability. We also show that AGO2 binds Myosin IIB and that PLEKHA7, LMO7, LIMCH1, and PDLIM1 all disrupt interaction of AGO2 with Myosin IIB at the ZA. These results demonstrate that recruitment of AGO2 to the ZA is sensitive to actomyosin perturbations, introducing the concept of mechanosensitive RNAi machinery, with potential implications in tissue remodeling and in disease.

Scribble, Lgl1, and myosin IIA interact with α-/β-catenin to maintain epithelial junction integrity

E-cadherin-catenin complex together with the cytoskeleton, builds the core of Adherens junctions (AJs). It has been reported that Scribble stabilizes the coupling of E-cadherin with catenins promoting epithelial cell adhesion, but the mechanism remains unknown. We show that Scribble, Lgl1, and NMII-A reside in a complex with E-cadherin-catenin complex. Depletion of either Scribble or Lgl1 disrupts the localization of E-cadherin-catenin complex to AJs. aPKCζ phosphorylation of Lgl1 regulates AJ localization of Lgl1 and E-cadherin-catenin complexes. Both Scribble and Lgl1 regulate the activation and recruitment of NMII-A at AJs. Finally, Scribble and Lgl1 are downregulated by TGFβ-induced EMT, and their re-expression during EMT impedes its progression. Our results provide insight into the mechanism regulating AJ integrity by Scribble, Lgl1, and NMII-A.

Dynamics and functions of E-cadherin complexes in epithelial cell and tissue morphogenesis

Cell-cell adhesion is at the center of structure and dynamics of epithelial tissue. E-cadherin-catenin complexes mediate Ca2+-dependent trans-homodimerization and constitute the kernel of adherens junctions. Beyond the basic function of cell-cell adhesion, recent progress sheds light the dynamics and interwind interactions of individual E-cadherin-catenin complex with E-cadherin superclusters, contractile actomyosin and mechanics of the cortex and adhesion. The nanoscale architecture of E-cadherin complexes together with cis-interactions and interactions with cortical actomyosin adjust to junctional tension and mechano-transduction by reinforcement or weakening of specific features of the interactions. Although post-translational modifications such as phosphorylation and glycosylation have been implicated, their role for specific aspects of in E-cadherin function has remained unclear. Here, we provide an overview of the E-cadherin complex in epithelial cell and tissue morphogenesis focusing on nanoscale architectures by super-resolution approaches and post-translational modifications from recent, in particular in vivo, studies. Furthermore, we review the computational modelling in E-cadherin complexes and highlight how computational modelling has contributed to a deeper understanding of the E-cadherin complexes.

Transición epitelio – mesenquima y cáncer

Cancer cell migration and invasion are critical components of metastatic disease, the leading cause of death in cancer patients. The epithe-lium-mesenchyme-transition (EMT) and mesenchyme-epithelium-transition (MET) are pathways involved in cancer metastasis. This process involves the degradation of cell-cell and cell-extracellular matrix junctions and the subse-quent loss of regulation of binding proteins such as E-cadherin. Cells undergo a reorganization of the cytoskeleton. These alterations are associated with a change in cell shape from epithelial to mesenchymal morphology. Understand-ing EMT and MET’s molecular and cellular basis provides fundamental insights into cancer etiology and may lead to new therapeutic strategies. In this review, we discuss some of the regulatory mechanisms and pathological role of epitheli-al-mesenchymal plasticity, focusing on the knowledge about the complexity and dynamics of this phenomenon in cancer

Cyclic stretching combined with cell-cell adhesion is sufficient for inducing cell intercalation

Although the important role of cell intercalation within a collective has long been recognized particularly for morphogenesis, the underlying mechanism remains poorly understood. Here we investigate the possibility that cellular responses to cyclic stretching play a major role in this process. By applying synchronized imaging and cyclic stretching to epithelial cells cultured on micropatterned polyacrylamide (PAA) substrates, we discovered that uniaxial cyclic stretching induces cell intercalation along with cell shape change and cell-cell interfacial remodeling. The process involved intermediate steps as previously reported for cell intercalation during embryonic morphogenesis, including the appearance of cell vertices, anisotropic vertex resolution, and directional expansion of cell-cell interface. Using mathematical modeling, we further found that cell shape change in conjunction with dynamic cell-cell adhesions was sufficient to account for the observations. Further investigation with small-molecule inhibitors indicated that disruption of myosin II activities suppressed cyclic stretching-induced intercalation while inhibiting the appearance of oriented vertices. Inhibition of Wnt signaling did not suppress stretch-induced cell shape change but disrupted cell intercalation and vertex resolution. Our results suggest that cyclic stretching, by inducing cell shape change and reorientation in the presence of dynamic cell-cell adhesions, can induce at least some aspects of cell intercalation and that this process is dependent in distinct ways on myosin II activities and Wnt signaling.

The surface ectoderm exhibits spatially heterogenous tension that correlates with YAP localisation during spinal neural tube closure in mouse embryos

The single cell layer of surface ectoderm (SE) which overlies the closing neural tube (NT) plays a crucial biomechanical role during mammalian NT closure (NTC), challenging previous assumptions that it is only passive to the force-generating neuroepithelium (NE). Failure of NTC leads to congenital malformations known as NT defects (NTDs), including spina bifida (SB) and anencephaly in the spine and brain respectively. In several mouse NTD models, SB is caused by misexpression of SE-specific genes and is associated with disrupted SE mechanics, including loss of rostrocaudal cell elongation believed to be important for successful closure. In this study, we asked how SE mechanics affect NT morphology, and whether the characteristic rostrocaudal cell elongation at the progressing closure site is a response to tension anisotropy in the SE. We show that blocking SE-specific E-cadherin in ex utero mouse embryo culture influences NT morphology, as well as the F-actin cable. Cell border ablation shows that cell shape is not due to tension anisotropy, but that there are regional differences in SE tension. We also find that YAP nuclear translocation reflects regional tension heterogeneity, and that its expression is sensitive to pharmacological reduction of tension. In conclusion, our results confirm that the SE is a biomechanically important tissue for spinal NT morphogenesis and suggest a possible role of spatial regulation of cellular tension which could regulate downstream gene expression via mechanically-sensitive YAP activity.

Actomyosin-mediated cellular tension promotes Yap nuclear translocation and myocardial proliferation through α5 integrin signaling

The cardiomyocyte phenotypic switch from a proliferative to terminally differentiated state results in the loss of regenerative potential of the mammalian heart shortly after birth. Nonmuscle myosin IIB (NM IIB)-mediated actomyosin contractility regulates cardiomyocyte cytokinesis in the embryonic heart, and NM IIB levels decline after birth, suggesting a role for cellular tension in the regulation of cardiomyocyte cell cycle activity in the postnatal heart. To investigate the role of actomyosin contractility in cardiomyocyte cell cycle arrest, we conditionally activated ROCK2 kinase domain (ROCK2:ER) in the murine postnatal heart. Here, we show that α5/β1 integrin and fibronectin matrix increase in response to actomyosin-mediated tension. Moreover, activation of ROCK2:ER promotes nuclear translocation of Yap, a mechanosensitive transcriptional co-activator, and enhances cardiomyocyte proliferation. Finally, we show that reduction of myocardial α5 integrin rescues the myocardial proliferation phenotype in ROCK2:ER hearts. These data demonstrate that cardiomyocytes respond to increased intracellular tension by altering their intercellular contacts in favor of cell-matrix interactions, leading to Yap nuclear translocation, thus uncovering a function for nonmuscle myosin contractility in promoting cardiomyocyte proliferation in the postnatal heart.

Cytokinesis machinery promotes cell dissociation from collectively migrating strands in confinement

Cells tune adherens junction dynamics to regulate epithelial integrity in diverse (patho)physiological processes, including cancer metastasis. We hypothesized that the spatially confining architecture of peritumor stroma promotes metastatic cell dissemination by remodeling cell-cell adhesive interactions. By combining microfluidics with live-cell imaging, FLIM/FRET biosensors, and optogenetic tools, we show that confinement induces leader cell dissociation from cohesive ensembles. Cell dissociation is triggered by myosin IIA (MIIA) dismantling of E-cadherin cell-cell junctions, as recapitulated by a mathematical model. Elevated MIIA contractility is controlled by RhoA/ROCK activation, which requires distinct guanine nucleotide exchange factors (GEFs). Confinement activates RhoA via nucleocytoplasmic shuttling of the cytokinesis-regulatory proteins RacGAP1 and Ect2 and increased microtubule dynamics, which results in the release of active GEF-H1. Thus, confining microenvironments are sufficient to induce cell dissemination from primary tumors by remodeling E-cadherin cell junctions via the interplay of microtubules, nuclear trafficking, and RhoA/ROCK/MIIA pathway and not by down-regulating E-cadherin expression.

Follicle cell contact maintains main body axis polarity in the Drosophila melanogaster oocyte

In Drosophila melanogaster, the anterior-posterior body axis is maternally established and governed by differential localization of partitioning defective (Par) proteins within the oocyte. At mid-oogenesis, Par-1 accumulates at the oocyte posterior end, while Par-3/Bazooka is excluded there but maintains its localization along the remaining oocyte cortex. Past studies have proposed the need for somatic cells at the posterior end to initiate oocyte polarization by providing a trigger signal. To date, neither the molecular identity nor the nature of the signal is known. Here, we provide evidence that mechanical contact of posterior follicle cells (PFCs) with the oocyte cortex causes the posterior exclusion of Bazooka and maintains oocyte polarity. We show that Bazooka prematurely accumulates exclusively where posterior follicle cells have been mechanically detached or ablated. Furthermore, we provide evidence that PFC contact maintains Par-1 and oskar mRNA localization and microtubule cytoskeleton polarity in the oocyte. Our observations suggest that cell-cell contact mechanics modulates Par protein binding sites at the oocyte cortex.

Sculpting an Embryo: The Interplay between Mechanical Force and Cell Division

The journey from a single fertilised cell to a multicellular organism is, at the most fundamental level, orchestrated by mitotic cell divisions. Both the rate and the orientation of cell divisions are important in ensuring the proper development of an embryo. Simultaneous with cell proliferation, embryonic cells constantly experience a wide range of mechanical forces from their surrounding tissue environment. Cells must be able to read and respond correctly to these forces since they are known to affect a multitude of biological functions, including cell divisions. The interplay between the mechanical environment and cell divisions is particularly crucial during embryogenesis when tissues undergo dynamic changes in their shape, architecture, and overall organisation to generate functional tissues and organs. Here we review our current understanding of the cellular mechanisms by which mechanical force regulates cell division and place this knowledge within the context of embryogenesis and tissue morphogenesis.

Unique and redundant functions of cytoplasmic actins and nonmuscle myosin II isoforms at epithelial junctions

The integrity and functions of epithelial barriers depend on the formation of adherens junctions (AJs) and tight junctions (TJs). A characteristic feature of AJs and TJs is their association with the cortical cytoskeleton composed of actin filaments and nonmuscle myosin II (NM-II) motors. Mechanical forces generated by the actomyosin cytoskeleton are essential for junctional assembly, stability, and remodeling. Epithelial cells express two different actin proteins and three NM-II isoforms, all known to be associated with AJs and TJs. Despite their structural similarity, different actin and NM-II isoforms have distinct biochemical properties, cellular distribution, and functions. The diversity of epithelial actins and myosin motors could be essential for the regulation of different steps of junctional formation, maturation, and disassembly. This review focuses on the roles of actin and NM-II isoforms in controlling the integrity and barrier properties of various epithelia. We discuss the effects of the depletion of individual actin isoforms and NM-II motors on the assembly and barrier function of AJs and TJs in model epithelial monolayers in vitro. We also describe the functional consequences of either total or tissue-specific gene knockout of different actins and NM-II motors, with a focus on the development and integrity of different epithelia in vivo.

Multifunctional role of GPCR signaling in epithelial tube formation

Epithelial tube formation requires Rho1-dependent actomyosin contractility to generate the cellular forces that drive cell shape changes and rearrangement. Rho1 signaling is activated by G-protein-coupled receptor (GPCR) signaling at the cell surface. During Drosophila embryonic salivary gland (SG) invagination, the GPCR ligand Folded gastrulation (Fog) activates Rho1 signaling to drive apical constriction. The SG receptor that transduces the Fog signal into Rho1-dependent myosin activation has not been identified. Here, we reveal that the Smog GPCR transduces Fog signal to regulate Rho kinase accumulation and myosin activation in the medioapical region of cells to control apical constriction during SG invagination. We also report on unexpected Fog-independent roles for Smog in maintaining epithelial integrity and organizing cortical actin. Our data support a model wherein Smog regulates distinct myosin pools and actin cytoskeleton in a ligand-dependent manner during epithelial tube formation.

Non-small cell lung carcinoma spheroid models in agarose microwells for drug response studies

There is a growing interest in developing personalized treatment strategies for each cancer patient, especially those with non-small cell lung carcinoma (NSCLC) which annually accounts for the majority of cancer related deaths in the US. Yet identifying the optimal NSCLC treatment strategy for each cancer patient is critical due to a multitude of mutations, some of which develop following initial therapy and can result in drug resistance. A key difficulty in developing personalized therapies in NSCLC is the lack of clinically relevant assay systems that are suitable to evaluate drug sensitivity using a minuscule amount of patient-derived material available following biopsies. Herein we leverage 3D printing to demonstrate a platform based on miniature microwells in agarose to culture cancer cell spheroids. The agarose wells were shaped by 3D printing molds with 1000 microwells with a U-shaped bottom. Three NSCLC cell lines (HCC4006, H1975 and A549) were used to demonstrate size uniformity, spheroid viability, biomarker expressions and drug response in 3D agarose microwells. Results show that our approach yielded spheroids of uniform size (coefficient of variation <22%) and high viability (>83% after 1 week-culture). Studies using epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKIs) drugs gefitinib and osimertinib showed clinically relevant responses. Based on the physical features, cell phenotypes, and responses to therapy of our spheroid models, we conclude that our platform is suitable for in vitro culture and drug evaluation, especially in cases when tumor sample is limited.

Epithelial Cell-Like Elasticity Modulates Actin-Dependent E-Cadherin Adhesion Organization

E-cadherin adhesions are essential for cell-to-cell cohesion and mechanical coupling between epithelial cells and reside in a microenvironment that comprises the adjoining epithelial cells. While E-cadherin has been shown to be a mechanosensor, it is unknown if E-cadherin adhesions can differentially sense stiffness within the range of that of epithelial cells. A survey of literature shows that epithelial cells' Young's moduli of elasticity lie predominantly in the sub-kPa to few-kPa range, with cancer cells often being softer than noncancerous ones. Here, we devised oriented E-cadherin-coated soft silicone substrates with sub-kPa or few-kPa elasticity but with similar viscous moduli and found that E-cadherin adhesions differentially organize depending on the magnitude of epithelial cell-like elasticity. Our results show that the actin cytoskeleton organizes E-cadherin adhesions in two ways─by supporting irregularly shaped adhesions at localized regions of high actin density and linear shaped adhesions at the end of linear actin bundles. Linearly shaped E-cadherin adhesions associated with radially oriented actin─but not irregularly shaped E-cadherin adhesions associated with circumferential actin foci─were much more numerous on 2.4 kPa E-cadherin substrates compared to 0.3 kPa E-cadherin substrates. However, the total amount of E-cadherin in both types of adhesions taken together was similar on the 0.3 and 2.4 kPa E-cadherin substrates across many cells. Our results show how the distribution of E-cadherin adhesions, supported by actin density and architecture, is modulated by epithelial cell-like elasticity and have significant implications for disease states like carcinomas characterized by altered epithelial cell elasticity.

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

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

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

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

Micron-scale supramolecular myosin arrays help mediate cytoskeletal assembly at mature adherens junctions

Epithelial cells assemble specialized actomyosin structures at E-Cadherin–based cell–cell junctions, and the force exerted drives cell shape change during morphogenesis. The mechanisms that build this supramolecular actomyosin structure remain unclear. We used ZO-knockdown MDCK cells, which assemble a robust, polarized, and highly organized actomyosin cytoskeleton at the zonula adherens, combining genetic and pharmacologic approaches with superresolution microscopy to define molecular machines required. To our surprise, inhibiting individual actin assembly pathways (Arp2/3, formins, or Ena/VASP) did not prevent or delay assembly of this polarized actomyosin structure. Instead, as junctions matured, micron-scale supramolecular myosin arrays assembled, with aligned stacks of myosin filaments adjacent to the apical membrane, overlying disorganized actin filaments. This suggested that myosin arrays might bundle actin at mature junctions. Consistent with this idea, inhibiting ROCK or myosin ATPase disrupted myosin localization/organization and prevented actin bundling and polarization. We obtained similar results in Caco-2 cells. These results suggest a novel role for myosin self-assembly, helping drive actin organization to facilitate cell shape change.

A picket fence function for adherens junctions in epithelial cell polarity

Adherens junctions are a defining feature of all epithelial cells, providing cell-cell adhesion and contractile ring formation that is essential for cell and tissue morphology. In Drosophila, adherens junctions are concentrated between the apical and basolateral plasma membrane domains, defined by aPKC-Par6-Baz and Lgl/Dlg/Scrib, respectively. Whether adherens junctions contribute to apical-basal polarization itself has been unclear because neuroblasts exhibit apical-basal polarization of aPKC-Par6-Baz and Lgl in the absence of adherens junctions. Here we show that, upon disruption of adherens junctions in epithelial cells, apical polarity determinants such as aPKC can still segregate from basolateral Lgl, but lose their sharp boundaries and also overlap with Dlg and Scrib - similar to neuroblasts. In addition, control of apical versus basolateral domain size is lost, along with control of cell shape, in the absence of adherens junctions. Manipulating the levels of apical Par3/Baz or basolateral Lgl polarity determinants in experiments and in computer simulations confirms that adherens junctions provide a 'picket fence' diffusion barrier that restricts the spread of polarity determinants along the membrane to enable precise domain size control. Movement of adherens junctions in response to mechanical forces during morphogenetic change thus enables spontaneous adjustment of apical versus basolateral domain size as an emergent property of the polarising system.

Mechanical processes underlying precise and robust cell matching

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

Quantitative and Multiplexed Fluorescence Lifetime Imaging of Intercellular Tensile Forces

Mechanical interactions between cells have been shown to play critical roles in regulating cell signaling and communications. However, the precise measurement of intercellular forces is still quite challenging, especially considering the complex environment at cell-cell junctions. In this study, we report a fluorescence lifetime-based approach to image and quantify intercellular molecular tensions. Using this method, tensile forces among multiple ligand-receptor pairs can be measured simultaneously. We first validated our approach and developed lifetime measurement-based DNA tension probes to image E-cadherin-mediated tension on epithelial cells. These probes were then further applied to quantify the correlations between E-cadherin and N-cadherin tensions during an epithelial-mesenchymal transition process. The modular design of these probes can potentially be used to study the mechanical features of various physiological and pathological processes.

Assembly of Peripheral Actomyosin Bundles in Epithelial Cells Is Dependent on the CaMKK2/AMPK Pathway

Defects in the maintenance of intercellular junctions are associated with loss of epithelial barrier function and consequent pathological conditions, including invasive cancers. Epithelial integrity is dependent on actomyosin bundles at adherens junctions, but the origin of these junctional bundles is incompletely understood. Here we show that peripheral actomyosin bundles can be generated from a specific actin stress fiber subtype, transverse arcs, through their lateral fusion at cell-cell contacts. Importantly, we find that assembly and maintenance of peripheral actomyosin bundles are dependent on the mechanosensitive CaMKK2/AMPK signaling pathway and that inhibition of this route leads to disruption of tension-maintaining actomyosin bundles and re-growth of stress fiber precursors. This results in redistribution of cellular forces, defects in monolayer integrity, and loss of epithelial identity. These data provide evidence that the mechanosensitive CaMKK2/AMPK pathway is critical for the maintenance of peripheral actomyosin bundles and thus dictates cell-cell junctions through cellular force distribution.

Non-muscle myosin II regulates aortic stiffness through effects on specific focal adhesion proteins and the non-muscle cortical cytoskeleton

Non‐muscle myosin II (NMII) plays a role in many fundamental cellular processes including cell adhesion, migration, and cytokinesis. However, its role in mammalian vascular function is not well understood. Here, we investigated the function of NMII in the biomechanical and signalling properties of mouse aorta. We found that blebbistatin, an inhibitor of NMII, decreases agonist‐induced aortic stress and stiffness in a dose‐dependent manner. We also specifically demonstrate that in freshly isolated, contractile, aortic smooth muscle cells, the non‐muscle myosin IIA (NMIIA) isoform is associated with contractile filaments in the core of the cell as well as those in the non‐muscle cell cortex. However, the non‐muscle myosin IIB (NMIIB) isoform is excluded from the cell cortex and colocalizes only with contractile filaments. Furthermore, both siRNA knockdown of NMIIA and NMIIB isoforms in the differentiated A7r5 smooth muscle cell line and blebbistatin‐mediated inhibition of NM myosin II suppress agonist‐activated increases in phosphorylation of the focal adhesion proteins FAK Y925 and paxillin Y118. Thus, we show in the present study, for the first time that NMII regulates aortic stiffness and stress and that this regulation is mediated through the tension‐dependent phosphorylation of the focal adhesion proteins FAK and paxillin.

FACEts of mechanical regulation in the morphogenesis of craniofacial structures

During embryonic development, organs undergo distinct and programmed morphological changes as they develop into their functional forms. While genetics and biochemical signals are well recognized regulators of morphogenesis, mechanical forces and the physical properties of tissues are now emerging as integral parts of this process as well. These physical factors drive coordinated cell movements and reorganizations, shape and size changes, proliferation and differentiation, as well as gene expression changes, and ultimately sculpt any developing structure by guiding correct cellular architectures and compositions. In this review we focus on several craniofacial structures, including the tooth, the mandible, the palate, and the cranium. We discuss the spatiotemporal regulation of different mechanical cues at both the cellular and tissue scales during craniofacial development and examine how tissue mechanics control various aspects of cell biology and signaling to shape a developing craniofacial organ.

PLEKHG4B enables actin cytoskeletal remodeling during epithelial cell-cell junction formation

Cell-cell junction formation requires actin cytoskeletal remodeling. Here, we show that PLEKHG4B, a Rho-guanine nucleotide exchange factor (Rho-GEF), plays a crucial role in epithelial cell-cell junction formation. Knockdown of PLEKHG4B decreased Cdc42 activity and tended to increase RhoA activity in A549 cells. A549 monolayer cells showed 'closed junctions' with closely packed actin bundles along the cell-cell contacts, but PLEKHG4B knockdown suppressed closed junction formation, and PLEKHG4B-knockdown cells exhibited 'open junctions' with split actin bundles located away from the cell-cell boundary. In Ca²⁺-switch assays, PLEKHG4B knockdown delayed the conversion of open junctions to closed junctions and β-catenin accumulation at cell-cell junctions. Furthermore, PLEKHG4B knockdown abrogated the reduction in myosin activity normally seen in the later stage of junction formation. The aberrant myosin activation and impairments in closed junction formation in PLEKHG4B-knockdown cells were reverted by ROCK inhibition or LARG/PDZ-RhoGEF knockdown. These results suggest that PLEKHG4B enables actin remodeling during epithelial cell-cell junction maturation, probably by reducing myosin activity in the later stage of junction formation, through suppressing LARG/PDZ-RhoGEF and RhoA-ROCK pathway activities. We also showed that annexin A2 participates in PLEKHG4B localization to cell-cell junctions.This article has an associated First Person interview with the first author of the paper.

Differential cell adhesion implemented by Drosophila Toll corrects local distortions of the anterior-posterior compartment boundary

Maintaining lineage restriction boundaries in proliferating tissues is vital to animal development. A long-standing thermodynamics theory, the differential adhesion hypothesis, attributes cell sorting phenomena to differentially expressed adhesion molecules. However, the contribution of the differential adhesion system during tissue morphogenesis has been unsubstantiated despite substantial theoretical support. Here, we report that Toll-1, a transmembrane receptor protein, acts as a differentially expressed adhesion molecule that straightens the fluctuating anteroposterior compartment boundary in the abdominal epidermal epithelium of the Drosophila pupa. Toll-1 is expressed across the entire posterior compartment under the control of the selector gene engrailed and displays a sharp expression boundary that coincides with the compartment boundary. Toll-1 corrects local distortions of the boundary in the absence of cable-like Myosin II enrichment along the boundary. The reinforced adhesion of homotypic cell contacts, together with pulsed cell contraction, achieves a biased vertex sliding action by resisting the separation of homotypic cell contacts in boundary cells. This work reveals a self-organizing system that integrates a differential adhesion system with pulsed contraction of cells to maintain lineage restriction boundaries.

Mutations in non-muscle myosin 2A disrupt the actomyosin cytoskeleton in Sertoli cells and cause male infertility

Mutations in non-muscle myosin 2A (NM2A) encompass a wide spectrum of anomalies collectively known as MYH9-Related Disease (MYH9-RD) in humans that can include macrothrombocytopenia, glomerulosclerosis, deafness, and cataracts. We previously created mouse models of the three mutations most frequently found in humans: R702C, D1424N, and E1841K. While homozygous R702C and D1424N mutations are embryonic lethal, we found homozygous mutant E1841K mice to be viable. However the homozygous male, but not female, mice were infertile. Here, we report that these mice have reduced testis size and defects in actin-associated junctions in Sertoli cells, resulting in inability to form the blood-testis barrier and premature germ cell loss. Moreover, compound double heterozygous (R702C/E1841K and D1424/E1841K) males show the same abnormalities in testes as E1841K homozygous males. Conditional ablation of either NM2A or NM2B alone in Sertoli cells has no effect on fertility and testis size, however deletion of both NM2A and NM2B in Sertoli cells results in infertility. Isolation of mutant E1841K Sertoli cells reveals decreased NM2A and F-actin colocalization and thicker NM2A filaments. Furthermore, A^E1841K/A^E1841K and double knockout Sertoli cells demonstrate microtubule disorganization and increased tubulin acetylation, suggesting defects in the microtubule cytoskeleton. Together, these results demonstrate that NM2A and 2B paralogs play redundant roles in Sertoli cells and are essential for testes development and normal fertility.

Quantifying tensile forces at cell-cell junctions with a DNA-based fluorescent probe

Cells are physically contacting with each other. Direct and precise quantification of forces at cell–cell junctions is still challenging. Herein, we have developed a DNA-based ratiometric fluorescent probe, termed DNAMeter, to quantify intercellular tensile forces. These lipid-modified DNAMeters can spontaneously anchor onto live cell membranes. The DNAMeter consists of two self-assembled DNA hairpins of different force tolerance. Once the intercellular tension exceeds the force tolerance to unfold a DNA hairpin, a specific fluorescence signal will be activated, which enables the real-time imaging and quantification of tensile forces. Using E-cadherin-modified DNAMeter as an example, we have demonstrated an approach to quantify, at the molecular level, the magnitude and distribution of E-cadherin tension among epithelial cells. Compatible with readily accessible fluorescence microscopes, these easy-to-use DNA tension probes can be broadly used to quantify mechanotransduction in collective cell behaviors.

A DNA-based fluorescent probe to quantify the magnitude and distribution of tensile forces at cell–cell junctions.

The multifarious regulation of the apical junctional complex

Epithelial cells form highly organized polarized sheets with characteristic cell morphologies and tissue architecture. Cell–cell adhesion and intercellular communication are prerequisites of such cohesive sheets of cells, and cell connectivity is mediated through several junctional assemblies, namely desmosomes, adherens, tight and gap junctions. These cell–cell junctions form signalling hubs that not only mediate cell–cell adhesion but impact on multiple aspects of cell behaviour, helping to coordinate epithelial cell shape, polarity and function. This review will focus on the tight and adherens junctions, constituents of the apical junctional complex, and aims to provide a comprehensive overview of the complex signalling that underlies junction assembly, integrity and plasticity.

Photocleavable Cadherin Inhibits Cell-to-Cell Mechanotransduction by Light

Precise integration of individual cell behaviors is indispensable for collective tissue morphogenesis and maintenance of tissue integrity. Organized multicellular behavior is achieved via mechanical coupling of individual cellular contractility, mediated by cell adhesion molecules at the cell-cell interface. Conventionally, gene depletion or laser microsurgery has been used for functional analysis of intercellular mechanotransduction. Nevertheless, these methods are insufficient to investigate either the spatiotemporal dynamics or the biomolecular contribution in cell-cell mechanical coupling within collective multicellular behaviors. Herein, we present our effort in adaption of PhoCl for attenuation of cell-to-cell tension transmission mediated by E-cadherin. To release intercellular contractile tension applied on E-cadherin molecules with external light, a genetically encoded photocleavable module called PhoCl was inserted into the intracellular domain of E-cadherin, thereby creating photocleavable cadherin (PC-cadherin). In response to light illumination, the PC-cadherin cleaved into two fragments inside cells, resulting in attenuating mechanotransduction at intercellular junctions in living epithelial cells. Light-induced perturbation of the intercellular tension balance with surrounding cells changed the cell shape in an epithelial cell sheet. The method is expected to enable optical manipulation of force-mediated cell-to-cell communications in various multicellular behaviors, which contributes to a deeper understanding of embryogenesis and oncogenesis.

Tensile Forces and Mechanotransduction at Cell-Cell Junctions

Cell-cell junctions are specializations of the plasma membrane responsible for physically integrating cells into tissues. We are now beginning to appreciate the diverse impacts that mechanical forces exert upon the integrity and function of these junctions. Currently, this is best understood for cadherin-based adherens junctions in epithelia and endothelia, where cell-cell adhesion couples the contractile cytoskeletons of cells together to generate tissue-scale tension. Junctional tension participates in morphogenesis and tissue homeostasis. Changes in tension can also be detected by mechanotransduction pathways that allow cells to communicate with each other. In this review, we discuss progress in characterising the forces present at junctions in physiological conditions; the cellular mechanisms that generate intrinsic tension and detect changes in tension; and, finally, we consider how tissue integrity is maintained in the face of junctional stresses.

Signaling pathways in cytoskeletal responses to plasma membrane depolarization in corneal endothelial cells

In previous work, we reported that plasma membrane potential depolarization (PMPD) provokes cortical F-actin remodeling in bovine corneal endothelial (BCE) cells in culture, which eventually leads to the appearance of intercellular gaps. In kidney epithelial cells it has been shown that PMPD determines an extracellular-signal-regulated kinase (ERK)/Rho-dependent increase in diphosphorylated myosin light chain (ppMLC). The present study investigated the signaling pathways involved in the response of BCE cells to PMPD. Differently to renal epithelial cells, we observed that PMPD leads to a decrease in monophosphorylated MLC (pMLC) without affecting diphosphorylated MLC. Also, that the pMLC reduction is a consequence of cyclic adenosine 3',5'-monophosphate (cAMP)/protein kinase A (PKA) activation. In addition, we found evidence that the cAMP increase mostly depends on soluble adenylyl cyclase activity. Inhibition of this enzyme reduces the effect of PMPD on the cAMP rise, F-actin remodeling, and pMLC decrease. No changes in phosho-ERK were observed, although we could determine that RhoA undergoes activation. Our results suggested that active RhoA is not involved in the intercellular gap formation. Overall, the findings of this study support the view that, differently to renal epithelial cells, in BCE cells PMPD determines cytoskeletal reorganization via activation of the cAMP/PKA pathway.

Myosin II isoforms play distinct roles in adherens junction biogenesis

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

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

Myosin IIA-mediated forces regulate multicellular integrity during vascular sprouting

Angiogenic sprouting is a critical process involved in vascular network formation within tissues. During sprouting, tip cells and ensuing stalk cells migrate collectively into the extracellular matrix while preserving cell–cell junctions, forming patent structures that support blood flow. Although several signaling pathways have been identified as controlling sprouting, it remains unclear to what extent this process is mechanoregulated. To address this question, we investigated the role of cellular contractility in sprout morphogenesis, using a biomimetic model of angiogenesis. Three-dimensional maps of mechanical deformations generated by sprouts revealed that mainly leader cells, not stalk cells, exert contractile forces on the surrounding matrix. Surprisingly, inhibiting cellular contractility with blebbistatin did not affect the extent of cellular invasion but resulted in cell–cell dissociation primarily between tip and stalk cells. Closer examination of cell–cell junctions revealed that blebbistatin impaired adherens-junction organization, particularly between tip and stalk cells. Using CRISPR/Cas9-mediated gene editing, we further identified NMIIA as the major isoform responsible for regulating multicellularity and cell contractility during sprouting. Together, these studies reveal a critical role for NMIIA-mediated contractile forces in maintaining multicellularity during sprouting and highlight the central role of forces in regulating cell–cell adhesions during collective motility.

Tensile Loads on Tethered Actin Filaments Induce Accumulation of Cell Adhesion-Associated Proteins in Vitro

Focal adhesions (FAs) and adherens junctions (AJs), which serve as a mechanical interface of cell-matrix and cell-cell interactions, respectively, experience tensile force either originating from the deformation of the surrounding tissues or generated by the actomyosin machinery in the cell. These mechanical inputs cause enlargement of FAs and AJs, while the detailed mechanism for the force-dependent development of FAs and AJs remain unclear. Both FAs and AJs provide sites for tethering of actin filaments and actin polymerization. Here, we develop a cell-free system, in which actin filaments are tethered to glass surfaces, and show that application of tensile force to the tethered filaments in the cell extract induces accumulation of several FA and AJ proteins, associated with further accumulation of actin filaments via de novo actin polymerization. Decline in the tensile force results in a decrease in the amount of the accumulated proteins. These results suggest that the tensile force acting on the tethered actin filaments plays a crucial role in the accumulation of FA and AJ proteins.

p90 ribosomal S6 kinase (RSK) phosphorylates myosin phosphatase and thereby controls edge dynamics during cell migration

Cell migration is essential to embryonic development, wound healing, and cancer cell dissemination. Cells move via leading-edge protrusion, substrate adhesion, and retraction of the cell's rear. The molecular mechanisms by which extracellular cues signal to the actomyosin cytoskeleton to control these motility mechanics are poorly understood. The growth factor-responsive and oncogenically activated protein extracellular signal-regulated kinase (ERK) promotes motility by signaling in actin polymerization-mediated edge protrusion. Using a combination of immunoblotting, co-immunoprecipitation, and myosin-binding experiments and cell migration assays, we show here that ERK also signals to the contractile machinery through its substrate, p90 ribosomal S6 kinase (RSK). We probed the signaling and migration dynamics of multiple mammalian cell lines and found that RSK phosphorylates myosin phosphatase–targeting subunit 1 (MYPT1) at Ser-507, which promotes an interaction of Rho kinase (ROCK) with MYPT1 and inhibits myosin targeting. We find that by inhibiting the myosin phosphatase, ERK and RSK promote myosin II–mediated tension for lamella expansion and optimal edge dynamics for cell migration. These findings suggest that ERK activity can coordinately amplify both protrusive and contractile forces for optimal cell motility.

Supracellular migration - beyond collective cell migration

Collective cell migration is a highly complex process in which groups of cells move together. A fundamental question is how cell ensembles can migrate efficiently. In some cases, the group is no more than a collection of individual cells. In others, the group behaves as a supracellular unit, whereby the cell group could be considered as a giant 'supracell', the concept of which was conceived over a century ago. The development of recent tools has provided considerable evidence that cell collectives are highly cooperative, and their migration can better be understood at the tissue level, rather than at the cell level. In this Review, we will define supracellular migration as a type of collective cell migration that operates at a scale higher than the individual cells. We will discuss key concepts of supracellular migration, review recent evidence of collectives exhibiting supracellular features and argue that many seemingly complex collective movements could be better explained by considering the participating cells as supracellular entities.

Mesenchymal actomyosin contractility is required for androgen-driven urethral masculinization in mice

The morphogenesis of mammalian embryonic external genitalia (eExG) shows dynamic differences between males and females. In genotypic males, eExG are masculinized in response to androgen signaling. Disruption of this process can give rise to multiple male reproductive organ defects. Currently, mechanisms of androgen-driven sexually dimorphic organogenesis are still unclear. We show here that mesenchymal-derived actomyosin contractility, by MYH10, is essential for the masculinization of mouse eExG. MYH10 is expressed prominently in the bilateral mesenchyme of male eExG. Androgen induces MYH10 protein expression and actomyosin contractility in the bilateral mesenchyme. Inhibition of actomyosin contractility through blebbistatin treatment and mesenchymal genetic deletion induced defective urethral masculinization with reduced mesenchymal condensation. We also suggest that actomyosin contractility regulates androgen-dependent mesenchymal directional cell migration to form the condensation in the bilateral mesenchyme leading to changes in urethral plate shape to accomplish urethral masculinization. Thus, mesenchymal-derived actomyosin contractility is indispensable for androgen-driven urethral masculinization.

The cerebral cavernous malformation disease causing gene KRIT1 participates in intestinal epithelial barrier maintenance and regulation

Epithelial barrier maintenance and regulation requires an intact perijunctional actomyosin ring underneath the cell-cell junctions. By searching for known factors affecting the actin cytoskeleton, we identified Krev interaction trapped protein 1 (KRIT1) as a major regulator for epithelial barrier function through multiple mechanisms. KRIT1 is expressed in both small intestinal and colonic epithelium, and KRIT1 knockdown in differentiated Caco-2 intestinal epithelium decreases epithelial barrier function and increases cation selectivity. KRIT1 knockdown abolished Rho-associated protein kinase-induced and myosin II motor inhibitor-induced barrier loss by limiting both small and large molecule permeability but did not affect myosin light chain kinase-induced increases in epithelial barrier function. These data suggest that KRIT1 participates in Rho-associated protein kinase- and myosin II motor-dependent (but not myosin light chain kinase-dependent) epithelial barrier regulation. KRIT1 knockdown exacerbated low-dose TNF-induced barrier loss, along with increased cleaved caspase-3 production. Both events are blocked by pan-caspase inhibition, indicating that KRIT1 regulates TNF-induced barrier loss through limiting epithelial apoptosis. These data indicate that KRIT1 controls epithelial barrier maintenance and regulation through multiple pathways, suggesting that KRIT1 mutation in cerebral cavernous malformation disease may alter epithelial function and affect human health.-Wang, Y., Li, Y., Zou, J., Polster, S. P., Lightle, R., Moore, T., Dimaano, M., He, T.-C., Weber, C. R., Awad, I. A., Shen, L. The cerebral cavernous malformation disease causing gene KRIT1 participates in intestinal epithelial barrier maintenance and regulation.

Putting VE-cadherin into JAIL for junction remodeling

Junction dynamics of endothelial cells are based on the integration of signal transduction, cytoskeletal remodeling and contraction, which are necessary for the formation and maintenance of monolayer integrity, but also enable repair and regeneration. The VE-cadherin–catenin complex forms the molecular basis of the adherence junctions and cooperates closely with actin filaments. Several groups have recently described small actin-driven protrusions at the cell junctions that are controlled by the Arp2/3 complex, contributing to cell junction regulation. We identified these protrusions as the driving force for VE-cadherin dynamics, as they directly induce new VE-cadherin-mediated adhesion sites, and have accordingly referred to these structures as junction-associated intermittent lamellipodia (JAIL). JAIL extend over only a few microns and thus provide the basis for a subcellular regulation of adhesion. The local (subcellular) VE-cadherin concentration and JAIL formation are directly interdependent, which enables autoregulation. Therefore, this mechanism can contribute a subcellularly regulated adaptation of cell contact dynamics, and is therefore of great importance for monolayer integrity and relative cell migration during wound healing and angiogenesis, as well as for inflammatory responses. In this Review, we discuss the mechanisms and functions underlying these actin-driven protrusions and consider their contribution to the dynamic regulation of endothelial cell junctions.

The force-sensitive protein Ajuba regulates cell adhesion during epithelial morphogenesis

The reorganization of cells in response to mechanical forces converts simple epithelial sheets into complex tissues of various shapes and dimensions. Epithelial integrity is maintained throughout tissue remodeling, but the mechanisms that regulate dynamic changes in cell adhesion under tension are not well understood. In Drosophila melanogaster, planar polarized actomyosin forces direct spatially organized cell rearrangements that elongate the body axis. We show that the LIM-domain protein Ajuba is recruited to adherens junctions in a tension-dependent fashion during axis elongation. Ajuba localizes to sites of myosin accumulation at adherens junctions within seconds, and the force-sensitive localization of Ajuba requires its N-terminal domain and two of its three LIM domains. We demonstrate that Ajuba stabilizes adherens junctions in regions of high tension during axis elongation, and that Ajuba activity is required to maintain cell adhesion during cell rearrangement and epithelial closure. These results demonstrate that Ajuba plays an essential role in regulating cell adhesion in response to mechanical forces generated by epithelial morphogenesis.

Nonmuscle myosin IIA is involved in recruitment of apical junction components through activation of α-catenin

MDCK dog kidney epithelial cells express two isoforms of nonmuscle myosin heavy chain II, IIA and IIB. Using the CRISPR/Cas9 system, we established cells in which the IIA gene was ablated. These cells were then transfected with a vector that expresses GFP-IIA chimeric molecule under the control of a tetracycline-responsible element. In the absence of Dox (doxycyclin), when GFP-IIA is expressed (GFP-IIA+), the cells exhibit epithelial cell morphology, but in the presence of Dox, when expression of GFP-IIA is repressed (GFP-IIA-), the cells lose epithelial morphology and strong cell-cell adhesion. Consistent with these observations, GFP-IIA- cells failed to assemble junction components such as E-cadherin, desmoplakin, and occludin at cell-cell contact sites. Therefore, IIA is required for assembly of junction complexes. MDCK cells with an ablation of the α-catenin gene also exhibited the same phenotype. However, when in GFP-IIA- cells expressed α-catenin lacking the inhibitory region or E-cadherin/α-catenin chimeras, the cells acquired the ability to establish the junction complex. These experiments reveal that IIA acts as an activator of α-catenin in junction assembly.