Dynamic microtubules regulate the local concentration of E-cadherin at cell-cell contacts

How cytoskeletal crosstalk makes cells move: Bridging cell-free and cell studies

Cell migration is a fundamental process for life and is highly dependent on the dynamical and mechanical properties of the cytoskeleton. Intensive physical and biochemical crosstalk among actin, microtubules, and intermediate filaments ensures their coordination to facilitate and enable migration. In this review, we discuss the different mechanical aspects that govern cell migration and provide, for each mechanical aspect, a novel perspective by juxtaposing two complementary approaches to the biophysical study of cytoskeletal crosstalk: live-cell studies (often referred to as top-down studies) and cell-free studies (often referred to as bottom-up studies). We summarize the main findings from both experimental approaches, and we provide our perspective on bridging the two perspectives to address the open questions of how cytoskeletal crosstalk governs cell migration and makes cells move.

Adherens junctions as molecular regulators of emergent tissue mechanics

Tissue and organ development during embryogenesis relies on the collective and coordinated action of many cells. Recent studies have revealed that tissue material properties, including transitions between fluid and solid tissue states, are controlled in space and time to shape embryonic structures and regulate cell behaviours. Although the collective cellular flows that sculpt tissues are guided by tissue-level physical changes, these ultimately emerge from cellular-level and subcellular-level molecular mechanisms. Adherens junctions are key subcellular structures, built from clusters of classical cadherin receptors. They mediate physical interactions between cells and connect biochemical signalling to the physical characteristics of cell contacts, hence playing a fundamental role in tissue morphogenesis. In this Review, we take advantage of the results of recent, quantitative measurements of tissue mechanics to relate the molecular and cellular characteristics of adherens junctions, including adhesion strength, tension and dynamics, to the emergent physical state of embryonic tissues. We focus on systems in which cell-cell interactions are the primary contributor to morphogenesis, without significant contribution from cell-matrix interactions. We suggest that emergent tissue mechanics is an important direction for future research, bridging cell biology, developmental biology and mechanobiology to provide a holistic understanding of morphogenesis in health and disease.

Recent insights into gap junction biogenesis in the cochlea

In the cochlea, connexin 26 (Cx26) and connexin 30 (Cx30) co-assemble into two types of homomeric and heteromeric gap junctions between adjacent non-sensory epithelial cells. These channels provide a mechanical coupling between connected cells, and their activity is critical to maintain cochlear homeostasis. Many of the mutations in GJB2 or GJB6, which encode Cx26 and Cx30 in humans, impair the formation of membrane channels and cause autosomal syndromic and non-syndromic hearing loss. Thus, deciphering the connexin trafficking pathways in situ should represent a major step forward in understanding the pathogenic significance of many of these mutations. A growing body of evidence now suggests that Cx26/Cx30 heteromeric and Cx30 homomeric channels display distinct assembly mechanisms. Here, we review the most recent advances that have been made toward unraveling the biogenesis and stability of these gap junctions in the cochlea.

Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment

Across complex, multi-time and -length scale biological systems, redundancy confers robustness and resilience, enabling adaptation and increasing survival under dynamic environmental conditions; this review addresses ubiquitous effects of cytoskeletal remodelling, triggered by biomechanical, biophysical and biochemical cues, on stem cell mechanoadaptation and emergent lineage commitment. The cytoskeleton provides an adaptive structural scaffold to the cell, regulating the emergence of stem cell structure-function relationships during tissue neogenesis, both in prenatal development as well as postnatal healing. Identification and mapping of the mechanical cues conducive to cytoskeletal remodelling and cell adaptation may help to establish environmental contexts that can be used prospectively as translational design specifications to target tissue neogenesis for regenerative medicine. In this review, we summarize findings on cytoskeletal remodelling in the context of tissue neogenesis during early development and postnatal healing, and its relevance in guiding lineage commitment for targeted tissue regeneration. We highlight how cytoskeleton-targeting chemical agents modulate stem cell differentiation and govern responses to mechanical cues in stem cells' emerging form and function. We further review methods for spatiotemporal visualization and measurement of cytoskeletal remodelling, as well as its effects on the mechanical properties of cells, as a function of adaptation. Research in these areas may facilitate translation of stem cells' own healing potential and improve the design of materials, therapies, and devices for regenerative medicine.

Alleles of unc-33/CRMP exhibit defects during Caenorhabditis elegans epidermal morphogenesis

BACKGROUND

Microtubule-associated proteins regulate the dynamics, organization, and function of microtubules, impacting a number of vital cellular processes. CRMPs have been shown to control microtubule assembly and axon outgrowth during neuronal differentiation. While many microtubule-associated proteins have been linked to roles in cell division and neuronal development, it is still unclear the complement that control the formation of parallel microtubule arrays in epithelial cells.

RESULTS

Here we show through time-lapse DIC microscopy that Caenorhabditis elegans embryos homozygous for the weak loss-of-function allele unc-33(e204) progress more slowly through epidermal morphogenesis, while animals homozygous for strong loss-of-function alleles exhibit more embryonic lethality. Identification of two novel missense mutations in unc-33(e572), Val476Gly and Ser731Thr, lead to computational approaches to determine the potential effects of these changes on UNC-33/CRMP structure. Molecular dynamics simulations show that for Asp389Asn and Arg502His, two other known missense mutations, local changes in protein-protein hydrogen bonding affect the stability of the protein. However, the Val476Gly/Ser731Thr combination does not alter the structure or energetics of UNC-33 drastically when compared to the wild-type protein.

CONCLUSIONS

These results support a novel role for UNC-33/CRMP in C. elegans epidermal development and shed light on how individual amino acid changes cause a loss-of-function in UNC-33. This article is protected by copyright. All rights reserved.

Feeling the force: Multiscale force sensing and transduction at the cell-cell interface

A universal principle of all living cells is the ability to sense and respond to mechanical stimuli which is essential for many biological processes. Recent efforts have identified critical mechanosensitive molecules and response pathways involved in mechanotransduction during development and tissue homeostasis. Tissue-wide force transmission and local force sensing need to be spatiotemporally coordinated to precisely regulate essential processes during development such as tissue morphogenesis, patterning, cell migration and organogenesis. Understanding how cells identify and interpret extrinsic forces and integrate a specific response on cell and tissue level remains a major challenge. In this review we consider important cellular and physical factors in control of cell-cell mechanotransduction and discuss their significance for cell and developmental processes. We further highlight mechanosensitive macromolecules that are known to respond to external forces and present examples of how force responses can be integrated into cell and developmental programs.

Junctional ER Organization Affects Mechanotransduction at Cadherin-Mediated Adhesions

Cadherin-mediated adhesions (also known as adherens junctions) are adhesive complexes that connect neighboring cells in a tissue. While the role of the actin cytoskeleton in withstanding tension at these sites of contact is well documented, little is known about the involvement of microtubules and the associated endoplasmic reticulum (ER) network in cadherin mechanotransduction. Therefore, we investigated how the organization of ER extensions in close proximity of cadherin-mediated adhesions can affect such complexes, and vice versa. Here, we show that the extension of the ER to cadherin-mediated adhesions is tension dependent and appears to be cadherin-type specific. Furthermore, the different structural organization of the ER/microtubule network seems to affect the localization of ER-bound PTP1B at cadherin-mediated adhesions. This phosphatase is involved in the modulation of vinculin, a molecular clutch which enables differential engagement of the cadherin-catenin layer with the actomyosin cytoskeleton in response to tension. This suggests a link between structural organization of the ER/microtubule network around cadherin-specific adhesions, to control the mechanotransduction of adherens junctions by modulation of vinculin conformational state.

The transcription factor Rreb1 regulates epithelial architecture, invasiveness, and vasculogenesis in early mouse embryos

Ras-responsive element-binding protein 1 (Rreb1) is a zinc-finger transcription factor acting downstream of RAS signaling. Rreb1 has been implicated in cancer and Noonan-like RASopathies. However, little is known about its role in mammalian non-disease states. Here, we show that Rreb1 is essential for mouse embryonic development. Loss of Rreb1 led to a reduction in the expression of vasculogenic factors, cardiovascular defects, and embryonic lethality. During gastrulation, the absence of Rreb1 also resulted in the upregulation of cytoskeleton-associated genes, a change in the organization of F-ACTIN and adherens junctions within the pluripotent epiblast, and perturbed epithelial architecture. Moreover, Rreb1 mutant cells ectopically exited the epiblast epithelium through the underlying basement membrane, paralleling cell behaviors observed during metastasis. Thus, disentangling the function of Rreb1 in development should shed light on its role in cancer and other diseases involving loss of epithelial integrity.

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.

Cytoskeleton systems contribute differently to the functional intrinsic properties of chondrospheres

Cytoskeleton systems, actin microfilaments, microtubules (MTs) and intermediate filaments (IFs) provide the biomechanical stability and spatial organization in cells. To understand the specific contributions of each cytoskeleton systems to intrinsic properties of spheroids, we've scrutinized the effects of the cytoskeleton perturbants, cytochalasin D (Cyto D), nocodazole (Noc) and withaferin A (WFA) on fusion, spreading on adhesive surface, morphology and biomechanics of chondrospheres (CSs). We confirmed that treatment with Cyto D but not with Noc or WFA severely affected CSs fusion and spreading dynamics and significantly reduced biomechanical properties of cell aggregates. Noc treatment affected spheroids spreading but not the fusion and surprisingly enhanced their stiffness. Vimentin intermediate filaments (VIFs) reorganization affected CSs spreading only. The analysis of all three cytoskeleton systems contribution to spheroids intrinsic properties was performed for the first time.

Tricellular adherens junctions provide a cell surface delivery platform for connexin 26/30 oligomers in the cochlea

In the cochlea, connexins 26 (Cx26) and 30 (Cx30) largely co-assemble into heteromeric gap junctions, which connect adjacent non-sensory epithelial cells. These channels are believed to ensure the rapid removal of K⁺ away from the base of sensory hair cells, resulting in K⁺ recycling back to the endolymph to maintain cochlear homeostasis. Many of the mutations in GJB2 and GJB6, which encode CX26 and CX30, impair the formation of membrane channels and cause autosomal hearing loss in humans. Although recent advances have been made, several important questions remain about connexin trafficking and gap junction biogenesis. Here we show that tricellular adherens junctions present at the crossroad between adjacent gap junction plaques, provide an unexpected cell surface delivery platform for Cx26/Cx30 oligomers. Using an in situ proximity ligation assay, we detected the presence of non-junctional Cx26/Cx30 oligomers within lipid raft-enriched tricellular junction sites. In addition, we observed that cadherin homophilic interactions are critically involved in microtubule-mediated trafficking of Cx26/Cx30 oligomers to the cell surface. Overall, our results unveil an unexpected role for tricellular junctions in the trafficking and assembly of membrane channels.

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.

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.

Ascarids exposed: a method for in vitro drug exposure and gene expression analysis of anthelmintic naïve Parascaris spp

Ascarid parasites infect a variety of hosts and regular anthelmintic treatment is recommended for all species. Parascaris spp. is the only ascarid species with widespread anthelmintic resistance, which allows for the study of resistance mechanisms. The purpose of this study was to establish an in vitro drug exposure protocol for adult anthelmintic-naïve Parascaris spp. and report a preliminary transcriptomic analysis in response to drug exposure. Live worms were harvested from foal necropsies and maintained in RPMI-1640 at 37 °C. Serial dilutions of oxibendazole (OBZ) and ivermectin (IVM) were prepared for in vitro drug exposure, and worm viability was monitored over time. In a second drug trial, worms were used for transcriptomic analysis. The final drug concentrations employed were OBZ at 40.1 μm (10 μg mL-1) and IVM at 1.1 μm (1 μg mL-1) for 24 and 3 h, respectively. The RNA-seq analysis revealed numerous differentially expressed genes, with some being potentially related to drug detoxification and regulatory mechanisms. This report provides a method for in vitro drug exposure and the phenotypic responses for Parascaris spp., which could be extrapolated to other ascarid parasites. Finally, it also provides preliminary transcriptomic data following drug exposure as a reference point for future studies of Parascaris spp.

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.

Microtubules promote intercellular contractile force transmission during tissue folding

During development, forces transmitted between cells are critical for sculpting epithelial tissues. Actomyosin contractility in the middle of the cell apex (medioapical) can change cell shape (e.g., apical constriction) but can also result in force transmission between cells via attachments to adherens junctions. How actomyosin networks maintain attachments to adherens junctions under tension is poorly understood. Here, we discovered that microtubules promote actomyosin intercellular attachments in epithelia during Drosophila melanogaster mesoderm invagination. First, we used live imaging to show a novel arrangement of the microtubule cytoskeleton during apical constriction: medioapical Patronin (CAMSAP) foci formed by actomyosin contraction organized an apical noncentrosomal microtubule network. Microtubules were required for mesoderm invagination but were not necessary for initiating apical contractility or adherens junction assembly. Instead, microtubules promoted connections between medioapical actomyosin and adherens junctions. These results delineate a role for coordination between actin and microtubule cytoskeletal systems in intercellular force transmission during tissue morphogenesis.

Cell Adhesion Structures in Epithelial Cells Are Formed in Dynamic and Cooperative Ways

There are many morphologically distinct membrane structures with different functions at the surface of epithelial cells. Among these, adherens junctions (AJ) and tight junctions (TJ) are responsible for the mechanical linkage of epithelial cells and epithelial barrier function, respectively. In the process of new cell-cell adhesion formation between two epithelial cells, such as after wounding, AJ form first and then TJ form on the apical side of AJ. This process is very complicated because AJ formation triggers drastic changes in the organization of actin cytoskeleton, the activity of Rho family of small GTPases, and the lipid composition of the plasma membrane, all of which are required for subsequent TJ formation. In this review, the authors focus on the relationship between AJ and TJ as a representative example of specialization of plasma membrane regions and introduce recent findings on how AJ formation promotes the subsequent formation of TJ.

Mechanosensing at Cellular Interfaces

At the plasma membrane interface, cells use various adhesions to sense their extracellular environment. These adhesions facilitate the transmission of mechanical signals that dictate cell behavior. This review discusses the mechanisms by which these mechanical signals are transduced through cell-matrix and cell-cell adhesions and how this mechanotransduction influences cell processes. Cell-matrix adhesions require the activation of and communication between various transmembrane protein complexes such as integrins. These links at the plasma membrane affect how a cell senses and responds to its matrix environment. Cells also communicate with each other through cell-cell adhesions, which further regulate cell behavior on a single- and multicellular scale. Coordination and competition between cell-cell and cell-matrix adhesions in multicellular aggregates can, to a significant extent, be modeled by differential adhesion analyses between the different interfaces even without knowing the details of cellular signaling. In addition, cell-matrix and cell-cell adhesions are connected by an intracellular cytoskeletal network that allows for direct communication between these distinct adhesions and activation of specific signaling pathways. Other membrane-embedded protein complexes, such as growth factor receptors and ion channels, play additional roles in mechanotransduction. Overall, these mechanoactive elements show the dynamic interplay between the cell, its matrix, and neighboring cells and how these relationships affect cellular function.

Loss of Neogenin1 in human colorectal carcinoma cells causes a partial EMT and wound-healing response

Neogenin1 (NEO1) is a receptor of the Deleted in Colorectal Carcinoma (DCC)/Frazzled/UNC-40 family, which regulates axon guidance but can also stabilize epithelial adherens junctions. NEO1 and DCC are also tumor suppressors that can inhibit metastasis by acting as dependence receptors. Given the role of NEO1 in maintaining adherens junctions we tested whether loss of NEO1 also promoted metastasis via an epithelial mesenchymal transition (EMT). Loss of NEO1 disrupted zonula adherens but tight junctions were unaffected. Neo1-depleted epithelial cells exhibited a more migratory morphology, had reduced F-actin rich stress-fibres and more basal lamellipodia. Microtubule density was decreased while microtubule outgrowth was faster. Live imaging showed that Neo1-depleted epithelial islands had increased lateral movement. Western blots and immunostaining revealed increased expression of mesenchymal markers such as Fibronectin and MMP1. Furthermore, RNA-seq analysis showed a striking decrease in expression of genes associated with oxidative phosphorylation, and increased expression of genes associated with EMT, locomotion, and wound-healing. In summary, loss of NEO1 in intestinal epithelial cells produces a partial EMT response, based on gene expression, cellular morphology and behaviour and cytoskeletal distribution. These results suggest that loss of NEO1 in carcinomas may contribute to metastasis by promoting a partial EMT and increased motility.

The role of microtubules in the regulation of epithelial junctions

The cytoskeleton is crucially important for the assembly of cell-cell junctions and the homeostatic regulation of their functions. Junctional proteins act, in turn, as anchors for cytoskeletal filaments, and as regulators of cytoskeletal dynamics and signalling proteins. The cross-talk between junctions and the cytoskeleton is critical for the morphogenesis and physiology of epithelial and other tissues, but is not completely understood. Microtubules are implicated in the delivery of junctional proteins to cell-cell contact sites, in the differentiation and spatial organization of the cytoplasm, and in the stabilization of the barrier and adhesive functions of junctions. Here we focus on the relationships between microtubules and junctions of vertebrate epithelial cells. We highlight recent discoveries on the molecular underpinnings of microtubule-junction interactions, and report new data about the interaction of cingulin and paracingulin with microtubules. We also propose a possible new role of junctions as “molecular sinks” for microtubule-associated signalling proteins.

A Theoretical Model: Elastic Analysis of the Evolution of the Crypt Opening Between the Fundic Gland and the Pyloric Gland

In recent years, with the development of magnified endoscopic technology, the microstructure of the gastric mucosa surface has been widely studied. However, it is unclear why the crypt opening shape of the fundic gland is different from that of the pyloric gland. We attempted to explain the problem by means of physical concepts, mathematical tools and some pathological perspectives. We first constructed an “L” type tubular structure on the basis of the pathology of the gastric mucosa and some geometric principles and then analyzed the distortion of marginal crypt epithelia after we added cells in the model via the mechanism of continuous regeneration. Finally, we determined that the crypt opening shape of the pyloric gland is derived mathematically from that of the fundic gland with the help of the idea of the Riemann sum. According to the derivation of the Euler force, it is possible that the epithelial-mesenchymal transition (EMT) protects the integrity of the gastric mucosa. Our model suggests that the evolution of the fundic gland and the pyloric gland triggers the EMT via elastic deformation. The basic logic of our model is the principle of least action.

Microtubules regulate brush border formation

Most epithelial cells contain apical membrane structures associated to bundles of actin filaments, which constitute the brush border. Whereas microtubule participation in the maintenance of the brush border identity has been characterized, their contribution to de novo microvilli organization remained elusive. Hereby, using a cell model of individual enterocyte polarization, we found that nocodazole induced microtubule depolymerization prevented the de novo brush border formation. Microtubule participation in brush border actin organization was confirmed in polarized kidney tubule MDCK cells. We also found that centrosome, but not Golgi derived microtubules, were essential for the initial stages of brush border development. During this process, microtubule plus ends acquired an early asymmetric orientation toward the apical membrane, which clearly differs from their predominant basal orientation in mature epithelia. In addition, overexpression of the microtubule plus ends associated protein CLIP170, which regulate actin nucleation in different cell contexts, facilitated brush border formation. In combination, the present results support the participation of centrosomal microtubule plus ends in the activation of the polarized actin organization associated to brush border formation, unveiling a novel mechanism of microtubule regulation of epithelial polarity.

Narrower insight to SIRT1 role in cancer: A potential therapeutic target to control epithelial-mesenchymal transition in cancer cells

The epithelial-mesenchymal transition (EMT) is a highly networked cellular process which involves cell transition from the immotile epithelial to the motile mesenchymal phenotype, whereby cells lose their cell-cell adhesion and cell polarity. This important process is one of the underlying mechanisms for enabling invasion and metastasis of cancer cells which is considered as malignant phase of tumor progression. However, the molecular mechanisms of this process are not fully clarified. It is reported that Sirtuin1 (SIRT1), a NAD+ dependent class III histone deacetylase is associated with tumor metastasis through positive regulation of EMT in several types of cancers. Recent studies confirmed that up and down regulation of SIRT1 expression remarkably change the migration ability of different cancer cells in vitro and tumor metastasis in vivo. Also, according to this fact that carcinomas as the main human solid tumors, originate from different epithelial cell types, SIRT1 role in EMT has received a great attention due to its potential role in tumor development and metastasis. Therefore, SIRT1 has been proposed as a key regulator of cancer metastasis by promoting EMT, although little is known about the cleared effect of SIRT1 in this transition. Our aim in this review is to explain in more detail the role of SIRT1 in various signaling pathways related to carcinogenesis, with the focus on the promoting role of SIRT1 in EMT as a potential therapeutic target to control EMT and to prevent cancer progression.

Desacetylvinblastine Monohydrazide Disrupts Tumor Vessels by Promoting VE-cadherin Internalization

Vinca alkaloids, the well-known tubulin-binding agents, are widely used for the clinical treatment of malignant tumors. However, little attention has been paid to their vascular disrupting effects, and the underlying mechanisms remain largely unknown. This study aims to investigate the vascular disrupting effect and the underlying mechanisms of vinca alkaloids.

Methods: The capillary disruption assay and aortic ring assay were performed to evaluate the in vitro vascular disrupting effect of desacetylvinblastine monohydrazide (DAVLBH), a derivate of vinblastine, and the in vivo vascular disrupting effect was assessed on HepG2 xenograft model using magnetic resonance imaging, hematoxylin and eosin staining and immunohistochemistry. Tubulin polymerization, endothelial cell monolayer permeability, western blotting and immunofluorescence assays were performed to explore the underlying mechanisms of DAVLBH-mediated tumor vascular disruption.

Results: DAVLBH has potent vascular disrupting activity both in vitro and in vivo. DAVLBH disrupts tumor vessels in a different manner than classical tubulin-targeting VDAs; it inhibits microtubule polymerization, promotes the internalization of vascular endothelial cadherin (VE-cadherin) and inhibits the recycling of internalized VE-cadherin to the cell membrane, thus increasing endothelial cell permeability and ultimately resulting in vascular disruption. DAVLBH-mediated promotion of VE-cadherin internalization and inhibition of internalized VE-cadherin recycling back to the cell membrane are partly dependent on inhibition of microtubule polymerization, and Src activation is involved in DAVLBH-induced VE-cadherin internalization.

Conclusions: This study sheds light on the tumor vascular disrupting effect and underlying mechanisms of vinca alkaloids and provides new insight into the molecular mechanism of tubulin-targeting VDAs.

Functional role for stable microtubules in lens fiber cell elongation

The process of tissue morphogenesis, especially for tissues reliant on the establishment of a specific cytoarchitecture for their functionality, depends a balanced interplay between cytoskeletal elements and their interactions with cell adhesion molecules. The microtubule cytoskeleton, which has many roles in the cell, is a determinant of directional cell migration, a process that underlies many aspects of development. We investigated the role of microtubules in development of the lens, a tissue where cell elongation underlies morphogenesis. Our studies with the microtubule depolymerizing agent nocodazole revealed an essential function for the acetylated population of stable microtubules in the elongation of lens fiber cells, which was linked to their regulation of the activation state of myosin. Suppressing myosin activation with the inhibitor blebbistatin could attenuate the loss of acetylated microtubules by nocodazole and rescue the effect of this microtubule depolymerization agent on both fiber cell elongation and lens integrity. Our results also suggest that acetylated microtubules impact lens morphogenesis through their interaction with N-cadherin junctions, with which they specifically associate in the region where lens fiber cell elongate. Disruption of the stable microtubule network increased N-cadherin junctional organization along lateral borders of differentiating lens fiber cells, which was prevented by suppression of myosin activity. These results reveal a role for the stable microtubule population in lens fiber cell elongation, acting in tandem with N-cadherin cell-cell junctions and the actomyosin network, giving insight into the cooperative role these systems play in tissue morphogenesis.

Ninein is essential for apico-basal microtubule formation and CLIP-170 facilitates its redeployment to non-centrosomal microtubule organizing centres

Differentiation of columnar epithelial cells involves a dramatic reorganization of the microtubules (MTs) and centrosomal components into an apico-basal array no longer anchored at the centrosome. Instead, the minus-ends of the MTs become anchored at apical non-centrosomal microtubule organizing centres (n-MTOCs). Formation of n-MTOCs is critical as they determine the spatial organization of MTs, which in turn influences cell shape and function. However, how they are formed is poorly understood. We have previously shown that the centrosomal anchoring protein ninein is released from the centrosome, moves in a microtubule-dependent manner and accumulates at n-MTOCs during epithelial differentiation. Here, we report using depletion and knockout (KO) approaches that ninein expression is essential for apico-basal array formation and epithelial elongation and that CLIP-170 is required for its redeployment to n-MTOCs. Functional inhibition also revealed that IQGAP1 and active Rac1 coordinate with CLIP-170 to facilitate microtubule plus-end cortical targeting and ninein redeployment. Intestinal tissue and in vitro organoids from the Clip1/Clip2 double KO mouse with deletions in the genes encoding CLIP-170 and CLIP-115, respectively, confirmed requirement of CLIP-170 for ninein recruitment to n-MTOCs, with possible compensation by other anchoring factors such as p150^Glued and CAMSAP2 ensuring apico-basal microtubule formation despite loss of ninein at n-MTOCs.

Inter-dependent apical microtubule and actin dynamics orchestrate centrosome retention and neuronal delamination

Detachment of newborn neurons from the neuroepithelium is required for correct neuronal architecture and functional circuitry. This process, also known as delamination, involves adherens-junction disassembly and acto-myosin-mediated abscission, during which the centrosome is retained while apical/ciliary membranes are shed. Cell-biological mechanisms mediating delamination are, however, poorly understood. Using live-tissue and super-resolution imaging, we uncover a centrosome-nucleated wheel-like microtubule configuration, aligned with the apical actin cable and adherens-junctions within chick and mouse neuroepithelial cells. These microtubules maintain adherens-junctions while actin maintains microtubules, adherens-junctions and apical end-foot dimensions. During neuronal delamination, acto-myosin constriction generates a tunnel-like actin-microtubule configuration through which the centrosome translocates. This movement requires inter-dependent actin and microtubule activity, and we identify drebrin as a potential coordinator of these cytoskeletal dynamics. Furthermore, centrosome compromise revealed that this organelle is required for delamination. These findings identify new cytoskeletal configurations and regulatory relationships that orchestrate neuronal delamination and may inform mechanisms underlying pathological epithelial cell detachment.

N-Cadherin, a novel and rapidly remodelling site involved in vasoregulation of small cerebral arteries

KEY POINTS

N-cadherin formed punctate adherens junctions (AJ) along the borders between vascular smooth muscle cells (VSMCs) in the pressurized rat superior cerebellar artery. The formation of N-cadherin AJs in the vessel wall depends on the intraluminal pressure and was responsive to treatment with phenylephrine (PE) (10^-5  m) and ACh (10^-5  m). N-cadherin-coated beads were able to induce clustering of N-cadherin-enhanced green fluorescent protein (EGFP) on the plasma membrane of isolated VSMCs, whereas treatment with PE (10^-5  m) or sodium nitroprusside (10^-5  m) induced a significant increase or decrease in the N-cadherin-EGFP clustering, respectively. Application of pulling force (∼1 nN) to the N-cadherin-coated beads via an atomic force microscope induced a localized mechanical response from the VSMCs that opposed the pulling.

ABSTRACT

N-cadherin is the major cell-cell adhesion molecule in vascular smooth muscle cells (VSMCs). We tested the hypothesis that N-cadherin is part of a novel mechanosensory mechanism in VSMCs and plays an active role in both the arteriolar myogenic response and during changes in vascular tone induced by vasomotor agonists. Intact and pressurized rat superior cerebellar arteries were labelled for confocal immunofluorescence imaging. N-cadherin formed punctate adherens junctions (AJ) along the borders between VSMCs. When the lumen pressure was raised from 50 to 90 mmHg, both the density and the average size of N-cadherin AJs increased significantly. Similarly, arteriolar constriction with phenylephrine (PE) (10^-5  m) induced a significant increase of N-cadherin AJ density at 50 mmHg, whereas vasodilatation induced by ACh (10^-5  m) was accompanied by a significant decrease in density and size of N-cadherin AJs. An atomic force microscope (AFM) was employed to further examine the mechano-responsive properties of N-cadherin adhesion sites in isolated VSMCs. AFM probes with an attached N-cadherin-coated microbead (5 μm) induced a progressive clustering of N-cadherin-enhanced green fluorescent protein (EGFP) on the VSMC surface. Application of pulling force (∼1 nN) to the N-cadherin-coated-beads with the AFM induced a localized mechanical response from the VSMCs that opposed the pulling. Treatment with PE (10^-5  m) or sodium nitroprusside (10^-5  m) induced a significant increase or decrease of the N-cadherin-EGFP clustering, respectively. These observations provide compelling evidence that N-cadherin AJs are sensitive to pressure and vasomotor agonists in VSMCs and support a functional role of N-cadherin AJs in vasomotor regulation.

Tension (re)builds: Biophysical mechanisms of embryonic wound repair

Embryonic tissues display an outstanding ability to rapidly repair wounds. Epithelia, in particular, serve as protective layers that line internal organs and form the skin. Thus, maintenance of epithelial integrity is of utmost importance for animal survival, particularly at embryonic stages, when an immune system has not yet fully developed. Rapid embryonic repair of epithelial tissues is conserved across species, and involves the collective migration of the cells around the wound. The migratory cell behaviours associated with wound repair require the generation and transmission of mechanical forces, not only for the cells to move, but also to coordinate their movements. Here, we review the forces involved in embryonic wound repair. We discuss how different force-generating structures are assembled at the molecular level, and the mechanisms that maintain the balance between force-generating structures as wounds close. Finally, we describe the mechanisms that cells use to coordinate the generation of mechanical forces around the wound. Collective cell movements and their misregulation have been associated with defective tissue repair, developmental abnormalities and cancer metastasis. Thus, we propose that understanding the role of mechanical forces during embryonic wound closure will be crucial to develop therapeutic interventions that promote or prevent collective cell movements under pathological conditions.

Plasticity of tumor cell invasion: governance by growth factors and cytokines

Tumor cell migration, the basis for metastatic dissemination, is an adaptive process which depends upon coordinated cell interaction with the environment, influencing cell-matrix and cell-cell adhesion, cytoskeletal dynamics and extracellular matrix remodeling. Growth factors and cytokines, released within the reactive tumor microenvironment and their intracellular effector signals strongly impact mechanocoupling functions in tumor cells and thereby control the mode and extent of tumor invasion, including collective and single-cell migration and their interconversions. Besides their role in controlling tumor cell growth and survival, cytokines and growth factors thus provide complex orchestration of the metastatic cascade and tumor cell adaptation to environmental challenge. We here review the mechanisms by which growth factors and cytokines control the reciprocal interactions between tumor cells and their microenvironment, and the consequences for the efficacy and plasticity of invasion programs and metastasis.

Shot and Patronin polarise microtubules to direct membrane traffic and biogenesis of microvilli in epithelia

In epithelial tissues, polarisation of microtubules and actin microvilli occurs along the apical-basal axis of each cell, yet how these cytoskeletal polarisation events are coordinated remains unclear. Here, we examine the hierarchy of events during cytoskeletal polarisation in Drosophila melanogaster epithelia. Core apical-basal polarity determinants polarise the spectrin cytoskeleton to recruit the microtubule-binding proteins Patronin (CAMSAP1, CAMSAP2 and CAMSAP3 in humans) and Shortstop [Shot; MACF1 and BPAG1 (also known as DST) in humans] to the apical membrane domain. Patronin and Shot then act to polarise microtubules along the apical-basal axis to enable apical transport of Rab11 endosomes by the Nuf-Dynein microtubule motor complex. Finally, Rab11 endosomes are transferred to the MyoV (also known as Didum in Drosophila) actin motor to deliver the key microvillar determinant Cadherin 99C to the apical membrane to organise the biogenesis of actin microvilli.

Microtubule-dependent balanced cell contraction and luminal-matrix modification accelerate epithelial tube fusion

Connection of tubules into larger networks is the key process for the development of circulatory systems. In Drosophila development, tip cells of the tracheal system lead the migration of each branch and connect tubules by adhering to each other and simultaneously changing into a torus-shape. We show that as adhesion sites form between fusion cells, myosin and microtubules form polarized bundles that connect the new adhesion site to the cells' microtubule-organizing centres, and that E-cadherin and retrograde recycling endosomes are preferentially deposited at the new adhesion site. We demonstrate that microtubules help balancing tip cell contraction, which is driven by myosin, and is required for adhesion and tube fusion. We also show that retrograde recycling and directed secretion of a specific matrix protein into the fusion-cell interface promote fusion. We propose that microtubule bundles connecting these cell–cell interfaces coordinate cell contractility and apical secretion to facilitate tube fusion.

During tracheal tube fusion in Drosophila, a pair of tip cells form an adherens junction and then fuse their plasma membranes. Here the authors show that a balanced pulling force mediated by myosin and microtubules, as well as localized deposition of matrix, promotes plasma membrane fusion.

Microtubule plus end-associated CLIP-170 initiates HSV-1 retrograde transport in primary human cells

Herpes simplex virus particles that enter the cell do not randomly associate with microtubule filaments, but require plus end–binding proteins EB1, CLIP-170, and dynactin to initiate retrograde transport to the nucleus.

Dynamic microtubules (MTs) continuously explore the intracellular environment and, through specialized plus end–tracking proteins (+TIPs), engage a variety of targets. However, the nature of cargoes that require +TIP-mediated capture for their movement on MTs remains poorly understood. Using RNA interference and dominant-negative approaches, combined with live cell imaging, we show that herpes simplex virus particles that have entered primary human cells exploit a +TIP complex comprising end-binding protein 1 (EB1), cytoplasmic linker protein 170 (CLIP-170), and dynactin-1 (DCTN1) to initiate retrograde transport. Depletion of these +TIPs completely blocked post-entry long-range transport of virus particles and suppressed infection ∼5,000-fold, whereas transferrin uptake, early endosome organization, and dynein-dependent movement of lysosomes and mitochondria remained unaffected. These findings provide the first insights into the earliest stages of viral engagement of MTs through specific +TIPs, akin to receptors, with therapeutic implications, and identify herpesvirus particles as one of a very limited number of cargoes absolutely dependent on CLIP-170–mediated capture to initiate transport in primary human cells.

Microtubules Inhibit E-Cadherin Adhesive Activity by Maintaining Phosphorylated p120-Catenin in a Colon Carcinoma Cell Model

Tight regulation of cadherin-mediated intercellular adhesions is critical to both tissue morphogenesis during development and tissue homeostasis in adults. Cell surface expression of the cadherin-catenin complex is often directly correlated with the level of adhesion, however, examples exist where cadherin appears to be inactive and cells are completely non-adhesive. The state of p120-catenin phosphorylation has been implicated in regulating the adhesive activity of E-cadherin but the mechanism is currently unclear. We have found that destabilization of the microtubule cytoskeleton, independent of microtubule plus-end dynamics, dephosphorylates p120-catenin and activates E-cadherin adhesion in Colo 205 cells. Through chemical screening, we have also identified several kinases as potential regulators of E-cadherin adhesive activity. Analysis of several p120-catenin phosphomutants suggests that gross dephosphorylation of p120-catenin rather than that of specific amino acids may trigger E-cadherin adhesion. Uncoupling p120-catenin binding to E-cadherin at the membrane causes constitutive adhesion in Colo 205 cells, further supporting an inhibitory role of phosphorylated p120-catenin on E-cadherin activity.

KIF17 regulates RhoA-dependent actin remodeling at epithelial cell-cell adhesions

The kinesin KIF17 localizes at microtubule plus-ends where it contributes to regulation of microtubule stabilization and epithelial polarization. We now show that KIF17 localizes at cell-cell adhesions and that KIF17 depletion inhibits accumulation of actin at the apical pole of cells grown in 3D organotypic cultures and alters the distribution of actin and E-cadherin in cells cultured in 2D on solid supports. Overexpression of full-length KIF17 constructs or truncation mutants containing the N-terminal motor domain resulted in accumulation of newly incorporated GFP-actin into junctional actin foci, cleared E-cadherin from cytoplasmic vesicles and stabilized cell-cell adhesions to challenge with calcium depletion. Expression of these KIF17 constructs also increased cellular levels of active RhoA, whereas active RhoA was diminished in KIF17-depleted cells. Inhibition of RhoA or its effector ROCK, or expression of LIMK1 kinase-dead or activated cofilin(S3A) inhibited KIF17-induced junctional actin accumulation. Interestingly, KIF17 activity toward actin depends on the motor domain but is independent of microtubule binding. Together, these data show that KIF17 can modify RhoA-GTPase signaling to influence junctional actin and the stability of the apical junctional complex of epithelial cells.

Impaired cytoskeletal arrangements and failure of ventral body wall closure in chick embryos treated with rock inhibitor (Y-27632)

AIM

Rho-associated kinase (ROCK) signaling regulates numerous fundamental developmental processes during embryogenesis, primarily by controlling actin-cytoskeleton assembly and cell contractility. ROCK knockout mice exhibit a ventral body wall defect (VBWD) phenotype due to disorganization of actin filaments at the umbilical ring. However, the exact molecular mechanisms leading to VBWD still remain unclear. Improper somitogenesis has been hypothesized to contribute to failure of VBW closure. We designed this study to investigate the hypothesis that administration of ROCK inhibitor (Y-27632) disrupts cytoskeletal arrangements in morphology during early chick embryogenesis, which may contribute to the development of VBWD.

METHODS

At 60 h incubation, chick embryos were explanted into shell-less culture and treated with 50 µL of vehicle for controls (n = 33) or 50 µL of 500 µM of Y-27632 for the experimental group (Y-27, n = 56). At 8 h post-treatment, RT-PCR was performed to evaluate mRNA levels of N-cadherin, E-cadherin and connexin43. Immunofluorescence confocal microscopy was performed to analyze the expression and distribution of actin, vinculin and microtubules in the neural tube and somites. A further cohort of embryos was treated in ovo by dropping 50 µL of vehicle or 50 µL of different concentrations of Y-27632 onto the embryo and allowing development to 12 and 14 days for further assessment.

RESULTS

Gene expression levels of N-cadherin, E-cadherin and connexin43 were significantly decreased in treated embryos compared with controls (p < 0.05). Thickened actin filament bundles were recorded in the neural tube of Y-27 embryos. In somites, cells were dissociated with reduced actin distribution in affected embryos. Clumping of vinculin expression was found in the neural tube and somites, whereas reduced expression of microtubules was observed in Y-27 embryos compared with controls. At 12 and 14 days of development, affected embryos presented with an enlarged umbilical ring and herniation of abdominal contents through the defect.

CONCLUSION

ROCK inhibition alters cytoskeletal arrangement during early chick embryogenesis, which may contribute to failure of anterior body wall closure causing VBWD at later stages of development.

Non-centrosomal epidermal microtubules act in parallel to LET-502/ROCK to promote C. elegans elongation

C. elegans embryonic elongation is a morphogenetic event driven by actomyosin contractility and muscle-induced tension transmitted through hemidesmosomes. A role for the microtubule cytoskeleton has also been proposed, but its contribution remains poorly characterized. Here, we investigate the organization of the non-centrosomal microtubule arrays present in the epidermis and assess their function in elongation. We show that the microtubule regulators γ-tubulin and NOCA-1 are recruited to hemidesmosomes and adherens junctions early in elongation. Several parallel approaches suggest that microtubule nucleation occurs from these sites. Disrupting the epidermal microtubule array by overexpressing the microtubule-severing protein Spastin or by inhibiting the C. elegans ninein homolog NOCA-1 in the epidermis mildly affected elongation. However, microtubules were essential for elongation when hemidesmosomes or the activity of the Rho kinase LET-502/ROCK were partially compromised. Imaging of junctional components and genetic analyses suggest that epidermal microtubules function together with Rho kinase to promote the transport of E-cadherin to adherens junctions and myotactin to hemidesmosomes. Our results indicate that the role of LET-502 in junctional remodeling is likely to be independent of its established function as a myosin II activator, but requires a microtubule-dependent pathway involving the syntaxin SYX-5. Hence, we propose that non-centrosomal microtubules organized by epidermal junctions contribute to elongation by transporting junction remodeling factors, rather than having a mechanical role.

Summary: During C. elegans embryonic elongation, microtubules nucleate at adjerens junctions and hemidesmosomes, and are important for the transport of junctional proteins.

Stable knockdown of Kif5b in MDCK cells leads to epithelial-mesenchymal transition

Polarization of epithelial cells requires vectorial sorting and transport of polarity proteins to apical or basolateral domains. Kif5b is the mouse homologue of the human ubiquitous Kinesin Heavy Chain (uKHC). To investigate the function of Kif5b in epithelial cells, we examined the phenotypes of Kif5b-deficient MDCK cells. Stable knockdown of Kif5b in MDCK cells resulted in reduced cell proliferation rate, profound changes in cell morphology, loss of epithelial cell marker, and gain of mesenchymal marker, as well as increased cell migration, invasion, and tumorigenesis abilities. E-cadherin and NMMIIA could interact with Kif5b in polarized MDCK cells, and their expression levels were decreased in Kif5b-deficient MDCK cells. Overexpression of E-cadherin and NMMIIA in Kif5b depleted MDCK cells could decrease mesenchymal marker expression and cell migration ability. These results indicate that stable knockdown of Kif5b in MDCK cells can lead to epithelial-mesenchymal transition, which is mediated by defective E-cadherin and NMMIIA expression.

An ELMO2-RhoG-ILK network modulates microtubule dynamics

Complexes that contain ELMO2, RhoG, and integrin-linked kinase are required to maintain microtubule stability. Mechanistically, these complexes are involved in activation of Rac1, which in turn interferes with the destabilizing activity of stathmin. In addition, these complexes also mediate activation of GSK-3β, which promotes CRMP2-mediated microtubule stabilization.

ELMO2 belongs to a family of scaffold proteins involved in phagocytosis and cell motility. ELMO2 can simultaneously bind integrin-linked kinase (ILK) and RhoG, forming tripartite ERI complexes. These complexes are involved in promoting β1 integrin–dependent directional migration in undifferentiated epidermal keratinocytes. ELMO2 and ILK have also separately been implicated in microtubule regulation at integrin-containing focal adhesions. During differentiation, epidermal keratinocytes cease to express integrins, but ERI complexes persist. Here we show an integrin-independent role of ERI complexes in modulation of microtubule dynamics in differentiated keratinocytes. Depletion of ERI complexes by inactivating the Ilk gene in these cells reduces microtubule growth and increases the frequency of catastrophe. Reciprocally, exogenous expression of ELMO2 or RhoG stabilizes microtubules, but only if ILK is also present. Mechanistically, activation of Rac1 downstream from ERI complexes mediates their effects on microtubule stability. In this pathway, Rac1 serves as a hub to modulate microtubule dynamics through two different routes: 1) phosphorylation and inactivation of the microtubule-destabilizing protein stathmin and 2) phosphorylation and inactivation of GSK-3β, which leads to the activation of CRMP2, promoting microtubule growth. At the cellular level, the absence of ERI species impairs Ca²⁺-mediated formation of adherens junctions, critical to maintaining mechanical integrity in the epidermis. Our findings support a key role for ERI species in integrin-independent stabilization of the microtubule network in differentiated keratinocytes.

Alpha-catenin-dependent recruitment of the centrosomal protein CAP350 to adherens junctions allows epithelial cells to acquire a columnar shape

Epithelial morphogenesis involves a dramatic reorganisation of the microtubule cytoskeleton. How this complex process is controlled at the molecular level is still largely unknown. Here, we report that the centrosomal microtubule (MT)-binding protein CAP350 localises at adherens junctions in epithelial cells. By two-hybrid screening, we identified a direct interaction of CAP350 with the adhesion protein α-catenin that was further confirmed by co-immunoprecipitation experiments. Block of epithelial cadherin (E-cadherin)-mediated cell-cell adhesion or α-catenin depletion prevented CAP350 localisation at cell-cell junctions. Knocking down junction-located CAP350 inhibited the establishment of an apico-basal array of microtubules and impaired the acquisition of columnar shape in Madin-Darby canine kidney II (MDCKII) cells grown as polarised epithelia. Furthermore, MDCKII cystogenesis was also defective in junctional CAP350-depleted cells. CAP350-depleted MDCKII cysts were smaller and contained either multiple lumens or no lumen. Membrane polarity was not affected, but cortical microtubule bundles did not properly form. Our results indicate that CAP350 may act as an adaptor between adherens junctions and microtubules, thus regulating epithelial differentiation and contributing to the definition of cell architecture. We also uncover a central role of α-catenin in global cytoskeleton remodelling, in which it acts not only on actin but also on MT reorganisation during epithelial morphogenesis.

In epithelial cells, the normally centrosomal protein CAP350 binds to α-catenin at adherens junctions and helps to establish the cells' parallel apico-basal microtubule array and columnar shape.

Epithelia cover all the surfaces of and the cavities throughout the body and serve as barriers between the organism and its external environment. Epithelial differentiation requires the coordination in space and time of several mechanisms that ultimately lead to the acquisition of distinctive epithelial features, including apical-basal polarity, specialised cell-cell junctions, and columnar shape. Epithelial differentiation also induces the reorganisation of three cytoskeletal networks: actin filaments, intermediate filaments, and microtubules. In simple epithelia, cadherins and their cytoplasmic binding partners catenins play a crucial role in connecting cell-cell junctions to the actin cytoskeleton. The cadherin extracellular domain forms adhesive contacts between adjacent cells, and their cytoplasmic tail indirectly binds the actin-binding protein α-catenin, thus linking cell-cell junctions to the underlying actin cytoskeleton. We report here an additional role of α-catenin in remodelling microtubules during epithelial differentiation. In most epithelial cells, microtubules are organised as parallel bundles aligned along the apico-basal axis and as apical and basal plasma membrane-associated networks. We demonstrate that the microtubule-binding protein CAP350, which is only localised at the centrosome in most cells, is also recruited at cell–cell junctions in epithelial cells through its binding to α-catenin. In the absence of junctional CAP350, microtubules are unable to reorganise in bundles, and cells do not acquire columnar shape. Our results suggest that recruitment of centrosomal proteins to cell-cell junctions could be a general mechanism to control microtubule reorganisation in neighbour cells during epithelial differentiation.

Microtubule-dependent apical restriction of recycling endosomes sustains adherens junctions during morphogenesis of the Drosophila tracheal system

Epithelial remodelling is an essential mechanism for organogenesis, during which cells change shape and position while maintaining contact with each other. Adherens junctions (AJs) mediate stable intercellular cohesion but must be actively reorganised to allow morphogenesis. Vesicle trafficking and the microtubule (MT) cytoskeleton contribute to regulating AJs but their interrelationship remains elusive. We carried out a detailed analysis of the role of MTs in cell remodelling during formation of the tracheal system in the Drosophila embryo. Induction of MT depolymerisation specifically in tracheal cells shows that MTs are essential during a specific time frame of tracheal cell elongation while the branch extends. In the absence of MTs, one tracheal cell per branch overelongates, ultimately leading to branch break. Three-dimensional quantifications revealed that MTs are crucial to sustain E-Cadherin (Shotgun) and Par-3 (Bazooka) levels at AJs. Maintaining E-Cadherin/Par-3 levels at the apical domain requires de novo synthesis rather than internalisation and recycling from and to the apical plasma membrane. However, apical targeting of E-Cadherin and Par-3 requires functional recycling endosomes, suggesting an intermediate role for this compartment in targeting de novo synthesized E-Cadherin to the plasma membrane. We demonstrate that apical enrichment of recycling endosomes is dependent on the MT motor Dynein and essential for the function of this vesicular compartment. In addition, we establish that E-Cadherin dynamics and MT requirement differ in remodelling tracheal cells versus planar epithelial cells. Altogether, our results uncover an MT-Dynein-dependent apical restriction of recycling endosomes that controls adhesion by sustaining Par-3 and E-Cadherin levels at AJs during morphogenesis.

The tumour suppressor DLC2 ensures mitotic fidelity by coordinating spindle positioning and cell-cell adhesion

Dividing epithelial cells need to coordinate spindle positioning with shape changes to maintain cell–cell adhesion. Microtubule interactions with the cell cortex regulate mitotic spindle positioning within the plane of division. How the spindle crosstalks with the actin cytoskeleton to ensure faithful mitosis and spindle positioning is unclear. Here we demonstrate that the tumour suppressor DLC2, a negative regulator of Cdc42, and the interacting kinesin Kif1B coordinate cell junction maintenance and planar spindle positioning by regulating microtubule growth and crosstalk with the actin cytoskeleton. Loss of DLC2 induces the mislocalization of Kif1B, increased Cdc42 activity and cortical recruitment of the Cdc42 effector mDia3, a microtubule stabilizer and promoter of actin dynamics. Accordingly, DLC2 or Kif1B depletion promotes microtubule stabilization, defective spindle positioning, chromosome misalignment and aneuploidy. The tumour suppressor DLC2 and Kif1B are thus central components of a signalling network that guides spindle positioning, cell–cell adhesion and mitotic fidelity.

Epithelial cells must position their mitotic spindle correctly to maintain cell–cell adhesion. Here Vitiello et al. show that the tumour suppressor DLC2 and the mitotic kinesin Kif1b coordinate microtubule–actin interactions upstream of mDia3, guiding spindle positioning and mitotic fidelity.

Minus end-directed motor KIFC3 suppresses E-cadherin degradation by recruiting USP47 to adherens junctions

KIFC3, a minus end–directed kinesin motor, recruits the ubiquitin-specific protease USP47, a protease that removes ubiquitin chains from substrates, to epithelial adherens junctions. This process suppresses the ubiquitination and resultant degradation of E-cadherin, thereby maintaining stable cell–cell adhesion in epithelial sheets.

The adherens junction (AJ) plays a crucial role in maintaining cell–cell adhesion in epithelial tissues. Previous studies show that KIFC3, a minus end–directed kinesin motor, moves into AJs via microtubules that grow from clusters of CAMSAP3 (also known as Nezha), a protein that binds microtubule minus ends. The function of junction-associated KIFC3, however, remains to be elucidated. Here we find that KIFC3 binds the ubiquitin-specific protease USP47, a protease that removes ubiquitin chains from substrates and hence inhibits proteasome-mediated proteolysis, and recruits it to AJs. Depletion of KIFC3 or USP47 promotes cleavage of E-cadherin at a juxtamembrane region of the cytoplasmic domain, resulting in the production of a 90-kDa fragment and the internalization of E-cadherin. This cleavage depends on the E3 ubiquitin protein ligase Hakai and is inhibited by proteasome inhibitors. E-cadherin ubiquitination consistently increases after depletion of KIFC3 or USP47. These findings suggest that KIFC3 suppresses the ubiquitination and resultant degradation of E-cadherin by recruiting USP47 to AJs, a process that may be involved in maintaining stable cell–cell adhesion in epithelial sheets.

Role of the microtubule-targeting drug vinflunine on cell-cell adhesions in bladder epithelial tumour cells

Background

Vinflunine (VFL) is a microtubule-targeting drug that suppresses microtubule dynamics, showing anti-metastatic properties both in vitro and in living cancer cells. An increasing body of evidence underlines the influence of the microtubules dynamics on the cadherin-dependent cell-cell adhesions. E-cadherin is a marker of epithelial-to-mesenchymal transition (EMT) and a tumour suppressor; its reduced levels in carcinoma are associated with poor prognosis. In this report, we investigate the role of VFL on cell-cell adhesions in bladder epithelial tumour cells.

Methods

Human bladder epithelial tumour cell lines HT1376, 5637, SW780, T24 and UMUC3 were used to analyse cadherin-dependent cell-cell adhesions under VFL treatment. VFL effect on growth inhibition was measured by using a MTT colorimetric cell viability assay. Western blot, immunofluorescence and transmission electron microscopy analyses were performed to assess the roles of VFL effect on cell-cell adhesions, epithelial-to-mesenchymal markers and apoptosis. The role of the proteasome in controlling cell-cell adhesion was studied using the proteasome inhibitor MG132.

Results

We show that VFL induces cell death in bladder cancer cells and activates epithelial differentiation of the remaining living cells, leading to an increase of E-cadherin-dependent cell-cell adhesion and a reduction of mesenchymal markers, such as N-cadherin or vimentin. Moreover, while E-cadherin is increased, the levels of Hakai, an E3 ubiquitin-ligase for E-cadherin, were significantly reduced in presence of VFL. In 5637, this reduction on Hakai expression was blocked by MG132 proteasome inhibitor, indicating that the proteasome pathway could be one of the molecular mechanisms involved in its degradation.

Conclusions

Our findings underscore a critical function for VFL in cell-cell adhesions of epithelial bladder tumour cells, suggesting a novel molecular mechanism by which VFL may impact upon EMT and metastasis.