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

Altering Cell Junctional Tension in Spheroids through E-Cadherin Engagement Modulation

Cadherin-mediated tension at adherens junctions (AJs) is fundamental for cell-cell adhesion and maintaining epithelial integrity. Despite the importance of manipulating AJs to dissect cell-cell interactions, existing three-dimensional (3D) multicellular models have not adequately addressed the precise manipulation of these junctions. To fill this gap, we introduce E-cadherin-modified tension gauge tethers (TGTs) at the junctions within spheroids. The system enables both quantification and modulation of junctional tension with specific DNA triggers. Using rupture-induced fluorescence, we successfully measure mechanical forces in 3D spheroids. Furthermore, mechanically strong TGTs can maintain normal E-cadherin-mediated adhesion. Employing toehold-mediated strand displacement allowed us to disrupt E-cadherin-specific cell-cell adhesion, consequently altering intracellular tension within the spheroids. Our methodology offers a robust and precise way to manipulate cell-cell adhesion and intracellular mechanics in spheroid models.

Actin-templated Structures: Nature's Way to Hierarchical Surface Patterns (Gecko's Setae as Case Study)

The hierarchical design of the toe pad surface in geckos and its reversible adhesiveness have inspired material scientists for many years. Micro- and nano-patterned surfaces with impressive adhesive performance have been developed to mimic gecko's properties. While the adhesive performance achieved in some examples has surpassed living counterparts, the durability of the fabricated surfaces is limited and the capability to self-renew and restore function-inherent to biological systems-is unimaginable. Here the morphogenesis of gecko setae using skin samples from the Bibron´s gecko (Chondrodactylus bibronii) is studied. Gecko setae develop as specialized apical differentiation structures at a distinct cell-cell layer interface within the skin epidermis. A primary role for F-actin and microtubules as templating structural elements is necessary for the development of setae's hierarchical morphology, and a stabilization role of keratins and corneus beta proteins is identified. Setae grow from single cells in a bottom layer protruding into four neighboring cells in the upper layer. The resulting multicellular junction can play a role during shedding by facilitating fracture of the cell-cell interface and release of the high aspect ratio setae. The results contribute to the understanding of setae regeneration and may inspire future concepts to bioengineer self-renewable patterned adhesive surfaces.

Inactivating negative regulators of cortical branched actin enhances persistence of single cell migration

The Rac1-WAVE-Arp2/3 pathway pushes the plasma membrane by polymerizing branched actin, thereby powering membrane protrusions that mediate cell migration. Here, using knockdown (KD) or knockout (KO), we combine the inactivation of the Arp2/3 inhibitory protein arpin, the Arp2/3 subunit ARPC1A and the WAVE complex subunit CYFIP2, all of which enhance the polymerization of cortical branched actin. Inactivation of the three negative regulators of cortical branched actin increases migration persistence of human breast MCF10A cells and of endodermal cells in the zebrafish embryo, significantly more than any single or double inactivation. In the triple KO cells, but not in triple KD cells, the 'super-migrator' phenotype was associated with a heterogenous downregulation of vimentin (VIM) expression and a lack of coordination in collective behaviors, such as wound healing and acinus morphogenesis. Re-expression of vimentin in triple KO cells largely restored normal persistence of single cell migration, suggesting that vimentin downregulation contributes to the maintenance of the super-migrator phenotype in triple KO cells. Constant excessive production of branched actin at the cell cortex thus commits cells into a motile state through changes in gene expression.

Changes of Ex Vivo Cervical Epithelial Cells Due to Electroporation with JMY

The ionic environment within the nucleoplasm might diverge from the conditions found in the cytoplasm, potentially playing a role in the cellular stress response. As a result, it is conceivable that interactions of nuclear actin and actin-binding proteins (ABPs) with apoptosis factors may differ in the nucleoplasm and cytoplasm. The primary intracellular stress response is Ca2+ influx. The junctional mediating and regulating Y protein (JMY) is an actin-binding protein and has the capability to interact with the apoptosis factor p53 in a Ca2+-dependent manner, forming complexes that play a regulatory role in cytoskeletal remodelling and motility. JMY's presence is observed in both the cytoplasm and nucleoplasm. Here, we show that ex vivo ectocervical squamous cells subjected to electroporation with JMY protein exhibited varying morphological alterations. Specifically, the highly differentiated superficial and intermediate cells displayed reduced nuclear size. In inflamed samples, nuclear enlargement and simultaneous cytoplasmic reduction were observable and showed signs of apoptotic processes. In contrast, the less differentiated parabasal and metaplastic cells showed increased cytoplasmic activity and the formation of membrane protrusions. Surprisingly, in severe inflammation, vaginosis or ASC-US (Atypical Squamous Cells of Undetermined Significance), JMY appears to influence only the nuclear and perinuclear irregularities of differentiated cells, and cytoplasmic abnormalities still existed after the electroporation. Our observations can provide an appropriate basis for the exploration of the relationship between cytopathologically relevant morphological changes of epithelial cells and the function of ABPs. This is particularly important since ABPs are considered potential diagnostic and therapeutic biomarkers for both cancers and chronic inflammation.

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.

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.

MAP4K4 regulates forces at cell-cell and cell-matrix adhesions to promote collective cell migration

Collective cell migration is not only important for development and tissue homeostasis but can also promote cancer metastasis. To migrate collectively, cells need to coordinate cellular extensions and retractions, adhesion sites dynamics, and forces generation and transmission. Nevertheless, the regulatory mechanisms coordinating these processes remain elusive. Using A431 carcinoma cells, we identify the kinase MAP4K4 as a central regulator of collective migration. We show that MAP4K4 inactivation blocks the migration of clusters, whereas its overexpression decreases cluster cohesion. MAP4K4 regulates protrusion and retraction dynamics, remodels the actomyosin cytoskeleton, and controls the stability of both cell-cell and cell-substrate adhesion. MAP4K4 promotes focal adhesion disassembly through the phosphorylation of the actin and plasma membrane crosslinker moesin but disassembles adherens junctions through a moesin-independent mechanism. By analyzing traction and intercellular forces, we found that MAP4K4 loss of function leads to a tensional disequilibrium throughout the cell cluster, increasing the traction forces and the tension loading at the cell-cell adhesions. Together, our results indicate that MAP4K4 activity is a key regulator of biomechanical forces at adhesion sites, promoting collective migration.

A platform for dissecting force sensitivity and multivalency in actin networks

The physical structure and dynamics of cells are supported by micron-scale actin networks with diverse geometries, protein compositions, and mechanical properties. These networks are composed of actin filaments and numerous actin binding proteins (ABPs), many of which engage multiple filaments simultaneously to crosslink them into specific functional architectures. Mechanical force has been shown to modulate the interactions between several ABPs and individual actin filaments, but it is unclear how this phenomenon contributes to the emergent force-responsive functional dynamics of actin networks. Here, we engineer filament linker complexes and combine them with photo-micropatterning of myosin motor proteins to produce an in vitro reconstitution platform for examining how force impacts the behavior of ABPs within multi-filament assemblies. Our system enables the monitoring of dozens of actin networks with varying architectures simultaneously using total internal reflection fluorescence microscopy, facilitating detailed dissection of the interplay between force-modulated ABP binding and network geometry. We apply our system to study a dimeric form of the critical cell-cell adhesion protein α-catenin, a model force-sensitive ABP. We find that myosin forces increase α-catenin's engagement of small filament bundles embedded within networks. This activity is absent in a force-sensing deficient mutant, whose binding scales linearly with bundle size in both the presence and absence of force. These data are consistent with filaments in smaller bundles bearing greater per-filament loads that enhance α-catenin binding, a mechanism that could equalize α-catenin's distribution across actin-myosin networks of varying sizes in cells to regularize their stability and composition.

Engineering tools for quantifying and manipulating forces in epithelia

The integrity of epithelia is maintained within dynamic mechanical environments during tissue development and homeostasis. Understanding how epithelial cells mechanosignal and respond collectively or individually is critical to providing insight into developmental and (patho)physiological processes. Yet, inferring or mimicking mechanical forces and downstream mechanical signaling as they occur in epithelia presents unique challenges. A variety of in vitro approaches have been used to dissect the role of mechanics in regulating epithelia organization. Here, we review approaches and results from research into how epithelial cells communicate through mechanical cues to maintain tissue organization and integrity. We summarize the unique advantages and disadvantages of various reduced-order model systems to guide researchers in choosing appropriate experimental systems. These model systems include 3D, 2D, and 1D micromanipulation methods, single cell studies, and noninvasive force inference and measurement techniques. We also highlight a number of in silico biophysical models that are informed by in vitro and in vivo observations. Together, a combination of theoretical and experimental models will aid future experiment designs and provide predictive insight into mechanically driven behaviors of epithelial dynamics.

Junctional integrity and directional mobility of lymphatic endothelial cell monolayers are disrupted by saturated fatty acids

The lymphatic circulation regulates transfer of tissue fluid and immune cells toward the venous circulation. While obesity impairs lymphatic vessel function, the contribution of lymphatic endothelial cells (LEC) to metabolic disease phenotypes is poorly understood. LEC of lymphatic microvessels are in direct contact with the interstitial fluid, whose composition changes during the development of obesity, markedly by increases in saturated fatty acids. Palmitate, the most prevalent saturated fatty acid in lymph and blood, is detrimental to metabolism and function of diverse tissues, but its impact on LEC function is relatively unknown. Here, palmitate (but not its unsaturated counterpart palmitoleate) destabilized adherens junctions in human microvascular LEC in culture, visualized as changes in VE-cadherin, α-catenin, and β-catenin localization. Detachment of these proteins from cortical actin filaments was associated with abundant actomyosin stress fibers. The effects were Rho-associated protein kinase (ROCK)- and myosin-dependent, as inhibition with Y27632 or blebbistatin, respectively, prevented stress fiber accumulation and preserved junctions. Without functional junctions, palmitate-treated LEC failed to directionally migrate to close wounds in two dimensions and failed to form endothelial tubes in three dimensions. A reorganization of the lymphatic endothelial actin cytoskeleton may contribute to lymphatic dysfunction in obesity and could be considered as a therapeutic target.

Actin-rich lamellipodia-like protrusions contribute to the integrity of epithelial cell-cell junctions

Metastasis-suppressor 1 (MTSS1) is a membrane-interacting scaffolding protein that regulates the integrity of epithelial cell-cell junctions and functions as a tumor suppressor in a wide range of carcinomas. MTSS1 binds phosphoinositide-rich membranes through its I-BAR domain, and is capable of sensing and generating negative membrane curvature in vitro. However, the mechanisms by which MTSS1 localizes to intercellular junctions in epithelial cells, and contributes to their integrity and maintenance have remained elusive. By carrying out electron microscopy and live-cell imaging on cultured Madin-Darby canine kidney (MDCK) cell monolayers, we provide evidence that adherens junctions of epithelial cells harbor lamellipodia-like, dynamic actin-driven membrane folds, which exhibit high negative membrane curvature at their distal edges. BioID proteomics and imaging experiments demonstrated that MTSS1 associates with an Arp2/3 complex activator, the WAVE-2 complex, in dynamic actin-rich protrusions at cell-cell junctions. Inhibition of Arp2/3 or WAVE-2 suppressed actin filament assembly at adherens junctions, decreased the dynamics of junctional membrane protrusions, and led to defects in epithelial integrity. Together, these results support a model in which membrane-associated MTSS1, together with the WAVE-2 and Arp2/3 complexes, promotes the formation of dynamic lamellipodia-like actin protrusions that contribute to the integrity of cell-cell junctions in epithelial monolayers.

Cadherins and the cortex: A matter of time?

Cell adhesion systems commonly operate in close partnership with the cytoskeleton. Adhesion receptors bind to the cortex and regulate its dynamics, organization and mechanics; conversely, the cytoskeleton influences aspects of adhesion, including strength, stability and ductility. In this review we consider recent advances in elucidating such cooperation, focusing on interactions between classical cadherins and actomyosin. The evidence presents an apparent paradox. Molecular mechanisms of mechanosensation by the cadherin-actin apparatus imply that adhesion strengthens under tension. However, this does not always translate to the broader setting of confluent tissues, where increases in fluctuations of tension can promote intercalation due to the shrinkage of adherens junctions. Emerging evidence suggests that understanding of timescales may be important in resolving this issue, but that further work is needed to understand the role of adhesive strengthening across scales.

Scribble and E-cadherin cooperate to control symmetric daughter cell positioning by multiple mechanisms

The fate of the two daughter cells is intimately connected to their positioning, which is in turn regulated by cell junction remodelling and orientation of the mitotic spindle. How multiple cues are integrated to dictate the ultimate positioning of daughters is not clear. Here, we identify novel mechanisms of regulation of daughter positioning in single MCF10A cells. The polarity protein, Scribble cooperates with E-cadherin for sequential roles in daughter positioning. First Scribble stabilises E-cadherin at the mitotic cortex as well as the retraction fibres, to mediate spindle orientation. Second, Scribble re-locates to the junction between the two daughters to allow a new E-cadherin-based-interface to form between them, influencing the width of the nascent daughter-daughter junction and subsequent cell positioning. Thus, E-cadherin and Scribble dynamically relocate to different intracellular sites during cell division to orient the mitotic spindle and control placement of the daughter cells after cell division. This article has an associated First Person interview with the first author of the paper.

EVL Promotes Osteo-/Odontogenic Differentiation of Dental Pulp Stem Cells via Activating JNK Signaling Pathway

Objective

Human dental pulp stem cells (hDPSCs) were recognized as a suitable and promising source of stem cells in dental pulp regeneration. However, the mechanism by which hDPSCs differentiation into osteo-/odontogenic lineage remains unclear. Ena/VASP-like protein (EVL) has been found to be involved in diverse biological processes. In this study, we explored the role and underlying mechanism of EVL in osteo-/odontogenic differentiation of hDPSCs.

Methods

Expression of EVL was detected in hDPSCs by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and western blot (WB) analyses during osteo-/odontogenic differentiation. The function of EVL in osteo-/odontogenic differentiation and involvement of MAPK signaling pathways were evaluated by alkaline phosphatase (ALP) staining and activity, alizarin red staining (ARS), and qRT-PCR and western blot analyses.

Results

The expression of EVL was upregulated during osteo-/odontogenic differentiation of hDPSCs. Overexpression of EVL significantly increased osteo-/odontogenic capacity of hDPSCs, which was reflected in increased alkaline phosphatase (ALP) staining, ALP activity, mineralized nodule formation, and the expressions of genes related to osteo-/odontogenic differentiation, while downregulation of EVL inhibited it. In addition, EVL activated the JNK pathway and phosphorylation of p38 MAPK during differentiation procedure of hDPSCs. The EVL-enhanced differentiation of DPSCs was suppressed by blocking the JNK pathway, rather than the p38 MAPK pathway.

Conclusion

EVL promotes the osteo-/odontogenic differentiation of hDPSCs by activating the JNK pathway, providing a future target for osteo-/odontogenic differentiation and dental pulp regeneration.

Sensory axons induce epithelial lipid microdomain remodeling and determine the distribution of junctions in the epidermis

Epithelial cell properties are determined by the polarized distribution of membrane lipids, the cytoskeleton, and adhesive junctions. Epithelia are often profusely innervated, but little work has addressed how neurites affect epithelial organization. We previously found that basal keratinocytes in the zebrafish epidermis enclose axons in ensheathment channels sealed by autotypic junctions. Here we characterized how axons remodel cell membranes, the cytoskeleton, and junctions in basal keratinocytes. At the apical surface of basal keratinocytes, axons organized lipid microdomains quantitatively enriched in reporters for PI(4,5)P2 and liquid-ordered (Lo) membranes. Lipid microdomains supported the formation of cadherin-enriched, F-actin protrusions, which wrapped around axons, likely initiating ensheathment. In the absence of axons, cadherin-enriched microdomains formed on basal cells but did not organize into contiguous domains. Instead, these isolated domains formed heterotypic junctions with periderm cells, a distinct epithelial cell type. Thus, axon endings dramatically remodel polarized epithelial components and regulate epidermal adhesion.

SCAR/WAVE complex recruitment to a supracellular actomyosin cable by myosin activators and a junctional Arf-GEF during Drosophila dorsal closure

Expansive Arp2/3 actin networks and contractile actomyosin networks can be spatially and temporally segregated within the cell, but the networks also interact closely at various sites, including adherens junctions. However, molecular mechanisms coordinating these interactions remain unclear. We found that the SCAR/WAVE complex, an Arp2/3 activator, is enriched at adherens junctions of the leading edge actomyosin cable during Drosophila dorsal closure. Myosin activators were both necessary and sufficient for SCAR/WAVE accumulation at leading edge junctions. The same myosin activators were previously shown to recruit the cytohesin Arf-GEF Steppke to these sites, and mammalian studies have linked Arf small G protein signaling to SCAR/WAVE activation. During dorsal closure, we find that Steppke is required for SCAR/WAVE enrichment at the actomyosin-linked junctions. Arp2/3 also localizes to adherens junctions of the leading edge cable. We propose that junctional actomyosin activity acts through Steppke to recruit SCAR/WAVE and Arp2/3 for regulation of the leading edge supracellular actomyosin cable during dorsal closure.

A lateral protrusion latticework connects neuroepithelial cells and is regulated during neurogenesis

Dynamic contacts between cells within the developing neuroepithelium are poorly understood but play important roles in cell and tissue morphology and cell signalling. Here, using live-cell imaging and electron microscopy we reveal multiple protrusive structures in neuroepithelial apical endfeet of the chick embryonic spinal cord, including sub-apical protrusions that extend laterally within the tissue, and observe similar structures in human neuroepithelium. We characterise the dynamics, shape and cytoskeleton of these lateral protrusions and distinguish them from cytonemes, filopodia and tunnelling nanotubes. We demonstrate that lateral protrusions form a latticework of membrane contacts between non-adjacent cells, depend on actin but not microtubule dynamics, and provide a lamellipodial-like platform for further extending fine actin-dependent filipodia. We find that lateral protrusions depend on the actin-binding protein WAVE1 (also known as WASF1): misexpression of mutant WAVE1 attenuated protrusion and generated a round-ended apical endfoot morphology. However, this did not alter apico-basal cell polarity or tissue integrity. During normal neuronal delamination, lateral protrusions were withdrawn, but precocious protrusion loss induced by mutant WAVE1 was insufficient to trigger neurogenesis. This study uncovers a new form of cell-cell contact within the developing neuroepithelium, regulation of which prefigures neuronal delamination. This article has an associated First Person interview with the first author of the paper.

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.

Cadmium and Cadmium/BDE (47 or 209) Exposure Affect Mitochondrial Function, DNA Damage/Repair Mechanisms and Barrier Integrity in Airway Epithelial Cells

Heavy metals and Brominated diphenyl ether flame-retardants (BDEs) often coexist in the environment and are capable of inducing injury, cytotoxicity or genotoxicity in human epithelial cells of the lung. We studied the effects of single Cadmium chloride (CdCl2) or CdCl2/BDE (47 or 209) mixtures in airway epithelial cells, using A549 cell line cultured at submerged conditions and air–liquid interface (ALI) (an in vitro model described as physiologically relevant in vivo-like). We evaluated cell viability, oxidative stress, apoptosis, DNA damage/repair (Comet assay, γH2AX phosphorylation ser139), mitochondrial redox balance (NOX-4, Nrf2 and TFAM) and cell barrier integrity (TEER, ZO-1, Claudin-1, E-cadherin-1) in A549 cells exposed to CdCl2 (1 nM to 10 µM), or to CdCl2 (100 nM)/BDEs (47 or 209) (100 nM). CdCl2 (10 μM) reduced cell viability and increased apoptosis. CdCl2 (100 nM) significantly affected DNA-damage/repair (Olive Tail length production), γH2AX phosphorylation and oxidative stress (ROS/JC-1 production) in submerged cell cultures. CdCl2 (100 nM) decreased viability, TEER, ZO-1, Claudin-1 and E-cadherin-1 mRNA expression, and Nrf2 and TFAM while increased NOX-4, in ALI culture of cells. In both cell culture approaches, the cells stimulated with Cadmium/BDEs mixtures did not show a significant increase in the effects observed in the cells treated with CdCl2 alone. CdCl2 inhalation might exert cytotoxicity and genotoxicity, playing a pivotal role in the uncontrolled oxidative stress, damaging DNA and gene expression in airway epithelial cells. No additional or synergistic adverse effects of CdCl2/BDEs mixture were observed in comparison to CdCl2 alone in lung epithelium.

KRas-transformed epithelia cells invade and partially dedifferentiate by basal cell extrusion

Metastasis is the main cause of carcinoma-related death, yet we know little about how it initiates due to our inability to visualize stochastic invasion events. Classical models suggest that cells accumulate mutations that first drive formation of a primary mass, and then downregulate epithelia-specific genes to cause invasion and metastasis. Here, using transparent zebrafish epidermis to model simple epithelia, we can directly image invasion. We find that KRas-transformation, implicated in early carcinogenesis steps, directly drives cell invasion by hijacking a process epithelia normally use to promote death-cell extrusion. Cells invading by basal cell extrusion simultaneously pinch off their apical epithelial determinants, endowing new plasticity. Following invasion, cells divide, enter the bloodstream, and differentiate into stromal, neuronal-like, and other cell types. Yet, only invading KRas^V12 cells deficient in p53 survive and form internal masses. Together, we demonstrate that KRas-transformation alone causes cell invasion and partial dedifferentiation, independently of mass formation.

Nucleation, stabilization, and disassembly of branched actin networks

Arp2/3 complex is an actin filament nucleation and branching machinery conserved in all eukaryotes from yeast to human. Arp2/3 complex branched networks generate pushing forces that drive cellular processes ranging from membrane remodeling to cell and organelle motility. Several molecules regulate these processes by directly inhibiting or activating Arp2/3 complex and by stabilizing or disassembling branched networks. Here, we review recent advances in our understanding of Arp2/3 complex regulation, including high-resolution cryoelectron microscopy (cryo-EM) structures that illuminate the mechanisms of Arp2/3 complex activation and branch formation, and novel cellular pathways of branch formation, stabilization, and debranching. We also identify major gaps in our understanding of Arp2/3 complex inhibition and branch stabilization and disassembly.

Mitotic Centromere-Associated Kinesin (MCAK/KIF2C) Regulates Cell Migration and Invasion by Modulating Microtubule Dynamics and Focal Adhesion Turnover

The microtubule (MT) cytoskeleton is crucial for cell motility and migration by regulating multiple cellular activities such as transport and endocytosis of key components of focal adhesions (FA). The kinesin-13 family is important in the regulation of MT dynamics and the best characterized member of this family is the mitotic centromere-associated kinesin (MCAK/KIF2C). Interestingly, its overexpression has been reported to be related to increased metastasis in various tumor entities. Moreover, MCAK is involved in the migration and invasion behavior of various cell types. However, the precise molecular mechanisms were not completely clarified. To address these issues, we generated CRISPR/dCas9 HeLa and retinal pigment epithelium (RPE) cell lines overexpressing or downregulating MCAK. Both up- or downregulation of MCAK led to reduced cell motility and poor migration in malignant as well as benign cells. Specifically, it's up- or downregulation impaired FA protein composition and phosphorylation status, interfered with a proper spindle and chromosome segregation, disturbed the assembly and disassembly rate of FA, delayed cell adhesion, and compromised the plus-tip dynamics of MTs. In conclusion, our data suggest MCAK act as an important regulator for cell motility and migration by affecting the actin-MT cytoskeleton dynamics and the FA turnover, providing molecular mechanisms by which deregulated MCAK could promote malignant progression and metastasis of tumor cells.

Rac1 promotes kidney collecting duct integrity by limiting actomyosin activity

A polarized collecting duct (CD), formed from the branching ureteric bud (UB), is a prerequisite for an intact kidney. The small Rho GTPase Rac1 is critical for actin cytoskeletal regulation. We investigated the role of Rac1 in the kidney collecting system by selectively deleting it in mice at the initiation of UB development. The mice exhibited only a mild developmental phenotype; however, with aging, the CD developed a disruption of epithelial integrity and function. Despite intact integrin signaling, Rac1-null CD cells had profound adhesion and polarity abnormalities that were independent of the major downstream Rac1 effector, Pak1. These cells did however have a defect in the WAVE2–Arp2/3 actin nucleation and polymerization apparatus, resulting in actomyosin hyperactivity. The epithelial defects were reversible with direct myosin II inhibition. Furthermore, Rac1 controlled lateral membrane height and overall epithelial morphology by maintaining lateral F-actin and restricting actomyosin. Thus, Rac1 promotes CD epithelial integrity and morphology by restricting actomyosin via Arp2/3-dependent cytoskeletal branching.

Exploiting differential effects of actomyosin contractility to control cell sorting among breast cancer cells

In order to gain a greater understanding of the factors that drive spatial organization in multicellular aggregates of cancer cells, we investigate the segregation patterns of 6 breast cell lines of varying degree of mesenchymal character during formation of mixed aggregates. Cell sorting is considered in the context of available adhesion proteins and cellular contractility. It is found that the primary compaction mediator (cadherins or integrins) for a given cell type in isolation plays an important role in compaction speed, which in turn is the major factor dictating preference for interior or exterior position within mixed aggregates. In particular, cadherin-deficient, invasion-competent cells tend to position towards the outside of aggregates, facilitating access to extracellular matrix. Reducing actomyosin contractility is found to have a differential effect on spheroid formation depending on compaction mechanism. Inhibition of contractility has a significant stabilizing effect on cell-cell adhesions in integrin-driven aggregation and a mildly destabilizing effect in cadherin-based aggregation. This differential response is exploited to statically control aggregate organization and dynamically rearrange cells in pre-formed aggregates. Sequestration of invasive cells in the interior of spheroids provides a physical barrier that reduces invasion in three-dimensional culture, revealing a potential strategy for containment of invasive cell types.

The Wnt/PCP formin Daam1 drives cell-cell adhesion during nephron development

E-cadherin junctions facilitate assembly and disassembly of cell contacts that drive development and homeostasis of epithelial tissues. In this study, using Xenopus embryonic kidney and Madin-Darby canine kidney (MDCK) cells, we investigate the role of the Wnt/planar cell polarity (PCP) formin Daam1 (Dishevelled-associated activator of morphogenesis 1) in regulating E-cadherin-based intercellular adhesion. Using live imaging, we show that Daam1 localizes to newly formed cell contacts in the developing nephron. Furthermore, analyses of junctional filamentous actin (F-actin) upon Daam1 depletion indicate decreased microfilament localization and slowed turnover. We also show that Daam1 is necessary for efficient and timely localization of junctional E-cadherin, mediated by Daam1’s formin homology domain 2 (FH2). Finally, we establish that Daam1 signaling promotes organized movement of renal cells. This study demonstrates that Daam1 formin junctional activity is critical for epithelial tissue organization.

How cells remodel their adhesions through cell-surface proteins such as E-cadherin is a central question in epithelial tissue biology. Krneta-Stankic et al. show that the Wnt/PCP formin Daam1 regulates cytoskeletal membrane dynamics and E-cadherin localization within developing nephrons. These findings provide a new framework for studying cell-cell adhesion and nephron morphogenesis.

How adherens junctions move cells during collective migration

In this review, we consider how the association between adherens junctions and the actomyosin cytoskeleton influences collective cell movement. We focus on recent findings which reveal different ways for adherens junctions to promote the locomotion of cells within tissues: through lamellipodia and junctional contraction. These contributions reflect how classic cadherins establish sites of cortical actin assembly and how adherens junctions couple to contractile actomyosin, respectively. The diverse interplay between cadherin adhesion and the cytoskeleton thus provides different ways for adherens junctions to support epithelial locomotion.

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

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

E-Cadherin/HMR-1 Membrane Enrichment Is Polarized by WAVE-Dependent Branched Actin

Polarized epithelial cells adhere to each other at apical junctions that connect to the apical F-actin belt. Regulated remodeling of apical junctions supports morphogenesis, while dysregulated remodeling promotes diseases such as cancer. We have documented that branched actin regulator, WAVE, and apical junction protein, Cadherin, assemble together in developing C. elegans embryonic junctions. If WAVE is missing in embryonic epithelia, too much Cadherin assembles at apical membranes, and yet apical F-actin is reduced, suggesting the excess Cadherin is not fully functional. We proposed that WAVE supports apical junctions by regulating the dynamic accumulation of Cadherin at membranes. To test this model, here we examine if WAVE is required for Cadherin membrane enrichment and apical-basal polarity in a maturing epithelium, the post-embryonic C. elegans intestine. We find that larval and adult intestines have distinct apicobasal populations of Cadherin, each with distinct dependence on WAVE branched actin. In vivo imaging shows that loss of WAVE components alters post-embryonic E-cadherin membrane enrichment, especially at apicolateral regions, and alters the lateral membrane. Analysis of a biosensor for PI(4,5)P2 suggests loss of WAVE or Cadherin alters the polarity of the epithelial membrane. EM (electron microscopy) illustrates lateral membrane changes including separations. These findings have implications for understanding how mutations in WAVE and Cadherin may alter cell polarity.

Dynamic intracellular mechanical cues facilitate collective signaling responses

Collective behavior emerges in diverse life machineries, e.g., the immune responses to dynamic stimulations. The essential questions that arise here are that whether and how cells in vivo collectively respond to stimulation frequencies higher than their intrinsic natural values, e.g., the acute inflammation conditions. In this work, we systematically studied morphological and signaling responses of population fibroblasts in an interconnected cell monolayer and uncovered that, besides the natural NF-κB oscillation frequency of 1/90 min⁻¹, collective signaling response emerges in the cell monolayer at 1/20 min⁻¹ TNF-α input periodicity as well. Using a customized microfluidic device, we independently induced dynamic chemical stimulation and cytoskeleton reorganization on the stand-alone cells to exclude the effect of cell-cell communication. Our results reveal that, at this particular frequency, chemical stimulation is translated into dynamic intracellular mechanical cues through RAC1-medicated induction of dynamic cell-cell connections and cytoskeleton reorganizations, which synergize with chemical input to facilitate collective signaling responses.

•

Dynamic intracellular mechanical cues facilitate collective cellular responses

•

The dynamic chemical stimulations are translated into intracellular mechanical cues

•

The synergy between dynamic mechanical and chemical signal plays crucial roles

TRIP6 is required for tension at adherens junctions

Hippo signaling mediates influences of cytoskeletal tension on organ growth. TRIP6 and LIMD1 have each been identified as being required for tension-dependent inhibition of the Hippo pathway LATS kinases and their recruitment to adherens junctions, but the relationship between TRIP6 and LIMD1 was unknown. Using siRNA-mediated gene knockdown, we show that TRIP6 is required for LIMD1 localization to adherens junctions, whereas LIMD1 is not required for TRIP6 localization. TRIP6, but not LIMD1, is also required for the recruitment of vinculin and VASP to adherens junctions. Knockdown of TRIP6 or vinculin, but not of LIMD1, also influences the localization of myosin and F-actin. In TRIP6 knockdown cells, actin stress fibers are lost apically but increased basally, and there is a corresponding increase in the recruitment of vinculin and VASP to basal focal adhesions. Our observations identify a role for TRIP6 in organizing F-actin and maintaining tension at adherens junctions that could account for its influence on LIMD1 and LATS. They also suggest that focal adhesions and adherens junctions compete for key proteins needed to maintain attachments to contractile F-actin.

The cell pushes back: The Arp2/3 complex is a key orchestrator of cellular responses to environmental forces

The Arp2/3 complex orchestrates the formation of branched actin networks at the interface between the cytoplasm and membranes. Although it is widely appreciated that these networks are useful for scaffolding, creating pushing forces and delineating zones at the membrane interface, it has only recently come to light that branched actin networks are mechanosensitive, giving them special properties. Here, we discuss recent advances in our understanding of how Arp2/3-generated actin networks respond to load forces and thus allow cells to create pushing forces in responsive and tuneable ways to effect cellular processes such as migration, invasion, phagocytosis, adhesion and even nuclear and DNA damage repair.

Cell Junction Mechanics beyond the Bounds of Adhesion and Tension

Cell-cell junctions, in particular adherens junctions, are major determinants of tissue mechanics during morphogenesis and homeostasis. In attempts to link junctional mechanics to tissue mechanics, many have utilized explicitly or implicitly equilibrium approaches based on adhesion energy, surface energy, and contractility to determine the mechanical equilibrium at junctions. However, it is increasingly clear that they have significant limitations, such as that it remains challenging to link the dynamics of the molecular components to the resulting physical properties of the junction, to its remodeling ability, and to its adhesion strength. In this perspective, we discuss recent attempts to consider the aspect of energy dissipation at junctions to draw contact points with soft matter physics where energy loss plays a critical role in adhesion theories. We set the grounds for a theoretical framework of the junction mechanics that bridges the dynamics at the molecular scale to the mechanics at the tissue scale.

The Arf-GEF Steppke promotes F-actin accumulation, cell protrusions and tissue sealing during Drosophila dorsal closure

Cytohesin Arf-GEFs promote actin polymerization and protrusions of cultured cells, whereas the Drosophila cytohesin, Steppke, antagonizes actomyosin networks in several developmental contexts. To reconcile these findings, we analyzed epidermal leading edge actin networks during Drosophila embryo dorsal closure. Here, Steppke is required for F-actin of the actomyosin cable and for actin-based protrusions. steppke mutant defects in the leading edge actin networks are associated with improper sealing of the dorsal midline, but are distinguishable from effects of myosin mis-regulation. Steppke localizes to leading edge cell-cell junctions with accumulations of the F-actin regulator Enabled emanating from either side. Enabled requires Steppke for full leading edge recruitment, and genetic interaction shows the proteins cooperate for dorsal closure. Inversely, Steppke over-expression induces ectopic, actin-rich, lamellar cell protrusions, an effect dependent on the Arf-GEF activity and PH domain of Steppke, but independent of Steppke recruitment to myosin-rich AJs via its coiled-coil domain. Thus, Steppke promotes actin polymerization and cell protrusions, effects that occur in conjunction with Steppke's previously reported regulation of myosin contractility during dorsal closure.

β-catenin regulates muscle glucose transport via actin remodelling and M-cadherin binding

Objective

Skeletal muscle glucose disposal following a meal is mediated through insulin-stimulated movement of the GLUT4-containing vesicles to the cell surface. The highly conserved scaffold-protein β-catenin is an emerging regulator of vesicle trafficking in other tissues. Here, we investigated the involvement of β-catenin in skeletal muscle insulin-stimulated glucose transport.

Methods

Glucose homeostasis and transport was investigated in inducible muscle specific β-catenin knockout (BCAT-mKO) mice. The effect of β-catenin deletion and mutation of β-catenin serine 552 on signal transduction, glucose uptake and protein–protein interactions were determined in L6-G4-myc cells, and β-catenin insulin-responsive binding partners were identified via immunoprecipitation coupled to label-free proteomics.

Results

Skeletal muscle specific deletion of β-catenin impaired whole-body insulin sensitivity and insulin-stimulated glucose uptake into muscle independent of canonical Wnt signalling. In response to insulin, β-catenin was phosphorylated at serine 552 in an Akt-dependent manner, and in L6-G4-myc cells, mutation of β-catenin^S552 impaired insulin-induced actin-polymerisation, resulting in attenuated insulin-induced glucose transport and GLUT4 translocation. β-catenin was found to interact with M-cadherin in an insulin-dependent β-catenin^S552-phosphorylation dependent manner, and loss of M-cadherin in L6-G4-myc cells attenuated insulin-induced actin-polymerisation and glucose transport.

Conclusions

Our data suggest that β-catenin is a novel mediator of glucose transport in skeletal muscle and may contribute to insulin-induced actin-cytoskeleton remodelling to support GLUT4 translocation.

•

Deletion of β-catenin from the muscles of adult mice attenuates skeletal muscle glucose uptake.

•

Insulin stimulates phosphorylation of β-catenin^S552 by a mechanism involving Akt, and this is required for insulin's effects on both GLUT4 trafficking and actin remodelling.

•

Insulin promotes β-catenin/M-cadherin binding, to support cortical actin remodelling associated with GLUT4 translocation.

Actin cytoskeleton dynamics during mucosal inflammation: a view from broken epithelial barriers

Disruption of epithelial barriers is a key pathogenic event of mucosal inflammation: It ignites the exaggerated immune response and accelerates tissue damage. Loss of barrier function is attributed to the abnormal structure and permeability of epithelial adherens junctions and tight junctions, driven by inflammatory stimuli through a variety of cellular mechanisms. This review focuses on roles of the actin cytoskeleton in mediating disruption of epithelial junctions and creation of leaky barriers in inflamed tissues. We summarize recent advances in understanding the role of cytoskeletal remodeling driven by actin filament turnover and myosin II-dependent contractility in the homeostatic regulation of epithelial barriers and barrier disruption during mucosal inflammation. We also discuss how the altered biochemical and physical environment of the inflamed tissues could affect the dynamics of the junction-associated actomyosin cytoskeleton, leading to the disruption of epithelial barriers.

Meeting Report - Workshop 'Actin-based mechanosensation and force generation in health and disease'

International experts in the fields of cellular motility, force generation and mechanosensation met in Baeza, a UNESCO World Heritage city, from the 10th to the 13th of November, 2019. The meeting, part of the 'Current Trends in Biomedicine' series, took place at the 'Sede Antonio Machado', a beautiful 17th century building turned into a conference center of the Universidad Internacional de Andalucía (UNIA), which sponsored the event. The meeting was organized by Alexis Gautreau, Pekka Lappalainen and Miguel Vicente-Manzanares, with the support of the European Molecular Biology Organization (EMBO) and the Spanish-based company IMPETUX. Fifty scientists presented recent results during the talks, poster sessions and thematic discussions. As Baeza itself served as a crossroads of medieval Christian, Moorish and Jewish cultures, the meeting brought together cell biologists, biochemists, biophysicists and engineers from around the world that provided an integrated vision of the role of the actin cytoskeleton, force generation and mechanosensation in diverse physiological processes and pathologies.