Apical cytoskeletons and junctional complexes as a combined system in epithelial cell sheets

A short guide to the tight junction

Tight junctions (TJs) are specialized regions of contact between cells of epithelial and endothelial tissues that form selective semipermeable paracellular barriers that establish and maintain body compartments with different fluid compositions. As such, the formation of TJs represents a critical step in metazoan evolution, allowing the formation of multicompartmental organisms and true, barrier-forming epithelia and endothelia. In the six decades that have passed since the first observations of TJs by transmission electron microscopy, much progress has been made in understanding the structure, function, molecular composition and regulation of TJs. The goal of this Perspective is to highlight the key concepts that have emerged through this research and the future challenges that lie ahead for the field.

Age-progressive interplay of HSP-proteostasis, ECM-cell junctions and biomechanics ensures C. elegans astroglial architecture

Tissue integrity is sensitive to temperature, tension, age, and is sustained throughout life by adaptive cell-autonomous or extrinsic mechanisms. Safeguarding the remarkably-complex architectures of neurons and glia ensures age-dependent integrity of functional circuits. Here, we report mechanisms sustaining the integrity of C. elegans CEPsh astrocyte-like glia. We combine large-scale genetics with manipulation of genes, cells, and their environment, quantitative imaging of cellular/ subcellular features, tissue material properties and extracellular matrix (ECM). We identify mutants with age-progressive, environment-dependent defects in glial architecture, consequent disruption of neuronal architecture, and abnormal aging. Functional loss of epithelial Hsp70/Hsc70-cochaperone BAG2 causes ECM disruption, altered tissue biomechanics, and hypersensitivity of glia to environmental temperature and mechanics. Glial-cell junctions ensure epithelia-ECM-CEPsh glia association. Modifying glial junctions or ECM mechanics safeguards glial integrity against disrupted BAG2-proteostasis. Overall, we present a finely-regulated interplay of proteostasis-ECM and cell junctions with conserved components that ensures age-progressive robustness of glial architecture.

Electroacupuncture Improving Intestinal Barrier Function in Rats with Irritable Bowel Syndrome Through Regulating Aquaporins

BACKGROUND

Intestinal mucosal barrier dysfunction plays a crucial role in the pathogenesis of irritable bowel syndrome with diarrhea (IBS-D). In order to explore the mechanism of electroacupuncture (EA) treatment on intestinal mucosal barrier, this study observed the effect of EA on aquaporins (AQPs), tight junctions (TJs), NF-κB pathway and the gut microbiota in IBS-D rats.

METHODS

The IBS-D model was established by acetic acid enema combined with chronic restraint method. The effects of EA on the treatment of IBS-D were examined by the abdominal withdrawal reflex score, Bristol's fecal character score, fecal water content, small intestine propulsion rate and HE staining. AQPs, TJs and inflammation-related molecular mechanisms were explored. The fecal samples were applied for 16S rRNA sequencing to assess the effect of EA intervention to the intestinal bacterial abundance.

RESULTS

EA reduced intestinal sensitization, restored intestinal motility and improved inflammatory cell infiltration. Furthermore, EA improved intestinal inflammation and flora environment significantly, inhibited NF-κB signaling and inflammatory factors (IL-1β and TNF-α). It can also increase the gene and protein expression of AQPs (AQP1, AQP3, and AQP8) and the gene levels of TJs (ZO-1 and Occludin).

CONCLUSION

EA has an inhibitory effect on the NF-κB signaling pathway, and regulates the proteins of AQP1, AQP3, AQP8, and TJs to restore the balance of water metabolism and intestinal permeability in IBS-D, which also restored the function of the intestinal mucosa by regulating the intestinal flora.

Paracingulin recruits CAMSAP3 to tight junctions and regulates microtubule and polarized epithelial cell organization

Paracingulin (CGNL1) is recruited to tight junctions (TJs) by ZO-1 and to adherens junctions (AJs) by PLEKHA7. PLEKHA7 has been reported to bind to the microtubule minus-end-binding protein CAMSAP3, to tether microtubules to the AJs. Here, we show that knockout (KO) of CGNL1, but not of PLEKHA7, results in the loss of junctional CAMSAP3 and its redistribution into a cytoplasmic pool both in cultured epithelial cells in vitro and mouse intestinal epithelium in vivo. In agreement, GST pulldown analyses show that CGNL1, but not PLEKHA7, interacts strongly with CAMSAP3, and the interaction is mediated by their respective coiled-coil regions. Ultrastructure expansion microscopy shows that CAMSAP3-capped microtubules are tethered to junctions by the ZO-1-associated pool of CGNL1. The KO of CGNL1 results in disorganized cytoplasmic microtubules and irregular nuclei alignment in mouse intestinal epithelial cells, altered cyst morphogenesis in cultured kidney epithelial cells, and disrupted planar apical microtubules in mammary epithelial cells. Together, these results uncover new functions of CGNL1 in recruiting CAMSAP3 to junctions and regulating microtubule cytoskeleton organization and epithelial cell architecture.

Dual-color live imaging unveils stepwise organization of multiple basal body arrays by cytoskeletons

For mucociliary clearance of pathogens, tracheal multiciliated epithelial cells (MCCs) organize coordinated beating of cilia, which originate from basal bodies (BBs) with basal feet (BFs) on one side. To clarify the self-organizing mechanism of coordinated intracellular BB-arrays composed of a well-ordered BB-alignment and unidirectional BB-orientation, determined by the direction of BB to BF, we generated double transgenic mice with GFP-centrin2-labeled BBs and mRuby3-Cep128-labeled BFs for long-term, high-resolution, dual-color live-cell imaging in primary-cultured tracheal MCCs. At early timepoints of MCC differentiation, BB-orientation and BB-local alignment antecedently coordinated in an apical microtubule-dependent manner. Later during MCC differentiation, fluctuations in BB-orientation were restricted, and locally aligned BB-arrays were further coordinated to align across the entire cell (BB-global alignment), mainly in an apical intermediate-sized filament-lattice-dependent manner. Thus, the high coordination of the BB-array was established for efficient mucociliary clearance as the primary defense against pathogen infection, identifying apical cytoskeletons as potential therapeutic targets.

Cell Adhesion at the Tight Junctions: New Aspects and New Functions

Tight junctions (TJ) are cell-cell adhesive structures that define the permeability of barrier-forming epithelia and endothelia. In contrast to this seemingly static function, TJs display a surprisingly high molecular complexity and unexpected dynamic regulation, which allows the TJs to maintain a barrier in the presence of physiological forces and in response to perturbations. Cell-cell adhesion receptors play key roles during the dynamic regulation of TJs. They connect individual cells within cellular sheets and link sites of cell-cell contacts to the underlying actin cytoskeleton. Recent findings support the roles of adhesion receptors in transmitting mechanical forces and promoting phase separation. In this review, we discuss the newly discovered functions of cell adhesion receptors localized at the TJs and their role in the regulation of the barrier function.

Role of syntaxin3 an apical polarity protein in poorly polarized keratinocytes: regulation of asymmetric barrier formations in the skin epidermis

The skin epidermis exhibits an asymmetric structure composed of multilayered keratinocytes and those in the outer layers form two-way physical barriers, cornified cell envelope (CCE), and tight junctions (TJs). While undifferentiated keratinocytes in the basal layer continuously deliver daughter cells outward, which undergo successive differentiation with losing their polarized characteristics, they retain the expression of several polarity proteins. In the present study, we revealed that the t-SNARE protein syntaxin3, a critical element for the formation of the apical compartment in simple epithelial cells, is required to confer the ability to organize the physical barriers on "poorly polarized" keratinocytes in epidermal outer layers. HaCaT keratinocytes with genetic ablation of syntaxin3 readily succumbed to hydrogen peroxide-induced cell death. Additionally, they lost the ability to organize TJ and CCE structures, accompanied by notable downregulation of transglutaminase1 and caspase14 (a cornification regulator) expression. These syntaxin3-knockout cells appeared to restore oxidative stress tolerance and functional TJ formation ability, in response to the inducible re-expression of exogenous syntaxin3. While plausible mechanisms underlying these phenomena remain unclear, syntaxin3, an apical polarity protein in the simple epithelia, has emerged as a potentially crucial element for barrier formation in poorly polarized keratinocytes in polarized epidermal tissue.

Multiple pathways promote microtubule stabilization in senescent intestinal epithelial cells

Intestinal epithelial cells are critical for gastrointestinal homeostasis. However, their function declines during aging. The aging-related loss of organ performance is largely driven by the increase in senescent cells. To date, the hallmarks and molecular mechanisms related to cellular senescence are not fully understood. Microtubules control epithelial functions, and we identified microtubule stabilization as a phenotypic marker of senescent intestinal epithelial cells. The senescence inducer determined the pathway to microtubule stabilization. Specifically, enhanced microtubule stability was associated with α-tubulin hyperacetylation or increased abundance of the microtubule-binding protein tau. We show further that overexpression of MAPT, which encodes tau, augmented microtubule stability in intestinal epithelial cells. Notably, pharmacological microtubule stabilization was sufficient to induce cellular senescence. Taken together, this study provides new insights into the molecular mechanisms that control epithelial cell homeostasis. Our results support the concept that microtubule stability serves as a critical cue to trigger intestinal epithelial cell senescence.

DAPLE orchestrates apical actomyosin assembly from junctional polarity complexes

Establishment of apicobasal polarity and the organization of the cytoskeleton must operate coordinately to ensure proper epithelial cell shape and function. However, the precise molecular mechanisms by which polarity complexes directly instruct the cytoskeletal machinery to determine cell shape are poorly understood. Here, we define a mechanism by which the PAR polarity complex (PAR3–PAR6–aPKC) at apical cell junctions leads to efficient assembly of the apical actomyosin network to maintain epithelial cell morphology. We found that the PAR polarity complex recruits the protein DAPLE to apical cell junctions, which in turn triggers a two-pronged mechanism that converges upon assembly of apical actomyosin. More specifically, DAPLE directly recruits the actin-stabilizing protein CD2AP to apical junctions and, concomitantly, activates heterotrimeric G protein signaling in a GPCR-independent manner to favor RhoA-myosin activation. These observations establish DAPLE as a direct molecular link between junctional polarity complexes and the formation of apical cytoskeletal assemblies that support epithelial cell shape.

Porcine Sapovirus-Induced Tight Junction Dissociation via Activation of RhoA/ROCK/MLC Signaling Pathway

Tight junctions (TJs) are a major barrier and also an important portal of entry for different pathogens. Porcine sapovirus (PSaV) induces early disruption of the TJ integrity of polarized LLC-PK cells, allowing it to bind to the buried occludin co-receptors hidden beneath the TJs on the basolateral surface. However, the signaling pathways involved in the PSaV-induced TJ dissociation are not yet known. Here, we found that the RhoA/ROCK/MLC signaling pathway was activated in polarized LLC-PK cells during the early infection of PSaV Cowden strain in the presence of bile acid. Specific inhibitors of RhoA, ROCK, and MLC restored PSaV-induced reduction of transepithelial resistance, increase of paracellular flux, intracellular translocation of occludin, and lateral membrane lipid diffusion. Moreover, each inhibitor significantly reduced PSaV replication, as evidenced by a reduction in viral protein synthesis, genome copy number, and progeny viruses. The PKC/MLCK and RhoA/ROCK/MYPT signaling pathways, known to dissociate TJs, were not activated during early PSaV infection. Among the above signaling pathways, the RhoA/ROCK/MLC signaling pathway was only activated by PSaV in the absence of bile acid, and specific inhibitors of this signaling pathway restored early TJ dissociation. Our findings demonstrate that PSaV binding to cell surface receptors activates the RhoA/ROCK/MLC signaling pathway, which in turn disrupts TJ integrity via the contraction of the actomyosin ring. Our study contributes to understanding how PSaV enters the cells and will aid in developing efficient and affordable therapies against PSaV and other calicivirus infections.IMPORTANCEPorcine sapovirus (PSaV), one of the most important enteric pathogens, is known to disrupt tight junction (TJ) integrity to expose its buried co-receptor occludin in polarized LLC-PK cells. However, the cellular signaling pathways that facilitate TJ dissociation are not yet completely understood. Here, we demonstrate that early infection of PSaV in polarized LLC-PK cells in either the presence or absence of bile acids activates the RhoA/ROCK/MLC signaling pathway, whose inhibitors reverse the early PSaV infection-induced early dissociation of TJs and reduce PSaV replication. However, early PSaV infection did not activate the PKC/MLCK and RhoA/ROCK/MYPT signaling pathways, which are also known to dissociate TJs. This study provides a better understanding of the mechanism involved in early PSaV infection-induced disruption of TJs, which is important for controlling or preventing PSaV and other calicivirus infections.

Planar cell polarity induces local microtubule bundling for coordinated ciliary beating

Multiciliated cells (MCCs) in tracheas generate mucociliary clearance through coordinated ciliary beating. Apical microtubules (MTs) play a crucial role in this process by organizing the planar cell polarity (PCP)-dependent orientation of ciliary basal bodies (BBs), for which the underlying molecular basis remains elusive. Herein, we found that the deficiency of Daple, a dishevelled-associating protein, in tracheal MCCs impaired the planar polarized apical MTs without affecting the core PCP proteins, causing significant defects in the BB orientation at the cell level but not the tissue level. Using live-cell imaging and ultra-high voltage electron microscope tomography, we found that the apical MTs accumulated and were stabilized by side-by-side association with one side of the apical junctional complex, to which Daple was localized. In vitro binding and single-molecule imaging revealed that Daple directly bound to, bundled, and stabilized MTs through its dimerization. These features convey a PCP-related molecular basis for the polarization of apical MTs, which coordinate ciliary beating in tracheal MCCs.

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

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

A microtubule-LUZP1 association around tight junction promotes epithelial cell apical constriction

Apical constriction is critical for epithelial morphogenesis, including neural tube formation. Vertebrate apical constriction is induced by di‐phosphorylated myosin light chain (ppMLC)‐driven contraction of actomyosin‐based circumferential rings (CRs), also known as perijunctional actomyosin rings, around apical junctional complexes (AJCs), mainly consisting of tight junctions (TJs) and adherens junctions (AJs). Here, we revealed a ppMLC‐triggered system at TJ‐associated CRs for vertebrate apical constriction involving microtubules, LUZP1, and myosin phosphatase. We first identified LUZP1 via unbiased screening of microtubule‐associated proteins in the AJC‐enriched fraction. In cultured epithelial cells, LUZP1 was found localized at TJ‐, but not at AJ‐, associated CRs, and LUZP1 knockout resulted in apical constriction defects with a significant reduction in ppMLC levels within CRs. A series of assays revealed that ppMLC promotes the recruitment of LUZP1 to TJ‐associated CRs, where LUZP1 spatiotemporally inhibits myosin phosphatase in a microtubule‐facilitated manner. Our results uncovered a hitherto unknown microtubule‐LUZP1 association at TJ‐associated CRs that inhibits myosin phosphatase, contributing significantly to the understanding of vertebrate apical constriction.

Ruffles and spikes: Control of tight junction morphology and permeability by claudins

Epithelial barrier function is regulated by a family of transmembrane proteins known as claudins. Functional tight junctions are formed when claudins interact with other transmembrane proteins, cytosolic scaffold proteins and the actin cytoskeleton. The predominant scaffold protein, zonula occludens-1 (ZO-1), directly binds to most claudin C-terminal domains, crosslinking them to the actin cytoskeleton. When imaged by immunofluorescence microscopy, tight junctions most frequently are linear structures that form between tricellular junctions. However, tight junctions also adapt non-linear architectures exhibiting either a ruffled or spiked morphology, which both are responses to changes in claudin engagement of actin filaments. Other terms for ruffled tight junctions include wavy, tortuous, undulating, serpentine or zig-zag junctions. Ruffling is under the control of hypoxia induced factor (HIF) and integrin-mediated signaling, as well as direct mechanical stimulation. Tight junction ruffling is specifically enhanced by claudin-2, antagonized by claudin-1 and requires claudin binding to ZO-1. Tight junction spikes are sites of active vesicle budding and fusion that appear as perpendicular projections oriented towards the nucleus. Spikes share molecular features with focal adherens junctions and tubulobulbar complexes found in Sertoli cells. Lung epithelial cells under stress form spikes due to an increase in claudin-5 expression that directly disrupts claudin-18/ZO-1 interactions. Together this suggests that claudins are not simply passive cargoes controlled by scaffold proteins. We propose a model where claudins specifically influence tight junction scaffold proteins to control interactions with the cytoskeleton as a mechanism that regulates tight junction assembly and function.

Disruption of tight junctions contributes to hyposalivation of salivary glands in a mouse model of type 2 diabetes

Tight junction (TJ) plays an important role in regulating paracellular fluid transport in salivary glands; however, little is known about the involvement of TJs in diabetes salivary glands. This study aimed to investigate the alterations of TJs and their possible contribution in diabetes-induced hyposalivation. Here, we observed that the morphologies of submandibular glands (SMGs) were impaired, characterized by enlarged acini accumulation with giant secretory granules, which were significantly reduced in atrophic ducts in SMGs of db/db mice, a spontaneous model of type-2 diabetes. However, the secretory granules were increased and scattered in the acini of diabetes parotid glands (PGs). Other ultrastructural damages including swollen mitochondria, expansive endoplasmic reticulum, and autophagosomes were observed in the diabetes group. The levels of TJ proteins including claudin-1 (Cldn1) and claudin-3 (Cldn3) were increased, whereas those of claudin-4 (Cldn4), occludin (Ocln), and zonula occludens-1 (ZO-1) were decreased in SMGs of db/db mice. Higher Cldn1 and Cldn3 and lower claudin-10 (Cldn10) and Ocln levels were observed in PGs of diabetes mice. Taken together, the structures of SMGs and PGs were impaired in diabetes mice, and the disruption of TJ integrity in both SMGs and PGs may contribute to diabetes-induced hyposalivation.

Insights into amebiasis using a human 3D-intestinal model

Entamoeba histolytica is the causative agent of amebiasis, an infectious disease targeting the intestine and the liver in humans. Two types of intestinal infection are caused by this parasite: silent infection, which occurs in the majority of cases, and invasive disease, which affects 10% of infected persons. To understand the intestinal pathogenic process, several in vitro models, such as cell cultures, human tissue explants or human intestine xenografts in mice, have been employed. Nevertheless, our knowledge on the early steps of amebic intestinal infection and the molecules involved during human-parasite interaction is scarce, in part due to limitations in the experimental settings. In the present work, we took advantage of tissue engineering approaches to build a three-dimensional (3D)-intestinal model that is able to replicate the general characteristics of the human colon. This system consists of an epithelial layer that develops tight and adherens junctions, a mucus layer and a lamina propria-like compartment made up of collagen containing macrophages and fibroblast. By means of microscopy imaging, omics assays and the evaluation of immune responses, we show a very dynamic interaction between E. histolytica and the 3D-intestinal model. Our data highlight the importance of several virulence markers occurring in patients or in experimental models, but they also demonstrate the involvement of under described molecules and regulatory factors in the amoebic invasive process.

Monoubiquitination of p120-catenin is essential for TGFβ-induced epithelial-mesenchymal transition and tumor metastasis

Disassembly of intercellular junctions is a hallmark of epithelial-mesenchymal transition (EMT). However, how the junctions disassemble remains largely unknown. Here, we report that E3 ubiquitin ligase Smurf1 targets p120-catenin, a core component of adherens junction (AJ) complex, for monoubiquitination during transforming growth factor β (TGFβ)-induced EMT, thereby leading to AJ dissociation. Upon TGFβ treatment, activated extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylates T900 of p120-catenin to promote its interaction with Smurf1 and subsequent monoubiquitination. Inhibition of T900 phosphorylation or ubiquitination of p120-catenin abrogates TGFβ-induced AJ dissociation and consequent tight junction (TJ) dissociation and cytoskeleton rearrangement, hence markedly blocking lung metastasis of murine breast cancer. Moreover, the T900 phosphorylation level of p120-catenin is positively correlated with malignancy of human breast cancer. Hence, our study reveals the underlying mechanism by which TGFβ induces dissociation of AJs during EMT and provides a potential strategy to block tumor metastasis.

Pharmacological activation of ERβ by arctigenin maintains the integrity of intestinal epithelial barrier in inflammatory bowel diseases

Intestinal epithelial barrier dysfunction is deeply involved in the pathogenesis of inflammatory bowel diseases (IBD). Arctigenin, the main active constituent in Fructus Arctii (a traditional Chinese medicine), has previously been found to attenuate colitis induced by dextran sulfate sodium (DSS) in mice. The present study investigated whether and how arctigenin protects against the disruption of the intestinal epithelial barrier in IBD. Arctigenin maintained the intestinal epithelial barrier function of mice with DSS- and TNBS-induced colitis. In Caco-2 and HT-29 cells, arctigenin lowered the monolayer permeability, increased TEER, reversed the abnormal expression of tight junction proteins, and restored the altered localization of F-actin induced by TNF-α and IL-1β. The specific antagonist PHTPP or shRNA of ERβ largely weakened the protective effect of arctigenin on the epithelial barrier function of Caco-2 and HT-29 cells. Molecular docking demonstrated that arctigenin had high affinity for ERβ mainly through hydrogen bonds as well as hydrophobic effects, and the protective effect of arctigenin on the intestinal barrier function was largely diminished in ERβ-mutated (ARG346 and/or GLU305) Caco-2 cells. Moreover, arctigenin-blocked TNF-α induced increase of the monolayer permeability in Caco-2 and HT-29 cells and the activation of myosin light chain kinase (MLCK)/myosin light chain (MLC) pathway in an ERβ-dependent manner. ERβ deletion in colons of mice with DSS-induced colitis resulted in a significant attenuation of the protective effect of arctigenin on the barrier integrity and colon inflammation. Arctigenin maintained the integrity of the intestinal epithelial barrier under IBD by upregulating the expression of tight junction proteins through the ERβ-MLCK/MLC pathway.

Reciprocal Association between the Apical Junctional Complex and AMPK: A Promising Therapeutic Target for Epithelial/Endothelial Barrier Function?

Epithelial/endothelial cells adhere to each other via cell–cell junctions including tight junctions (TJs) and adherens junctions (AJs). TJs and AJs are spatiotemporally and functionally integrated, and are thus often collectively defined as apical junctional complexes (AJCs), regulating a number of spatiotemporal events including paracellular barrier, selective permeability, apicobasal cell polarity, mechano-sensing, intracellular signaling cascades, and epithelial morphogenesis. Over the past 15 years, it has been acknowledged that adenosine monophosphate (AMP)-activated protein kinase (AMPK), a well-known central regulator of energy metabolism, has a reciprocal association with AJCs. Here, we review the current knowledge of this association and show the following evidences: (1) as an upstream regulator, AJs activate the liver kinase B1 (LKB1)–AMPK axis particularly in response to applied junctional tension, and (2) TJ function and apicobasal cell polarization are downstream targets of AMPK and are promoted by AMPK activation. Although molecular mechanisms underlying these phenomena have not yet been completely elucidated, identifications of novel AMPK effectors in AJCs and AMPK-driven epithelial transcription factors have enhanced our knowledge. More intensive studies along this line would eventually lead to the development of AMPK-based therapies, enabling us to manipulate epithelial/endothelial barrier function.

Measles Virus Ribonucleoprotein Complexes Rapidly Spread across Well-Differentiated Primary Human Airway Epithelial Cells along F-Actin Rings

Measles virus (MeV) is a highly contagious human pathogen that continues to be a worldwide health burden. One of the challenges for the study of MeV spread is the identification of model systems that accurately reflect how MeV behaves in humans. For our studies, we use unpassaged, well-differentiated primary cultures of airway epithelial cells from human donor lungs to examine MeV infection and spread. Here, we show that the main components of the MeV ribonucleoprotein complex (RNP), the nucleocapsid and phosphoprotein, colocalize with the apical and circumapical F-actin networks. To better understand how MeV infections spread across the airway epithelium, we generated a recombinant virus incorporating chimeric fluorescent proteins in its RNP complex. By live cell imaging, we observed rapid movement of RNPs along the circumapical F-actin rings of newly infected cells. This strikingly rapid mechanism of horizontal trafficking across epithelia is consistent with the opening of pores between columnar cells by the viral membrane fusion apparatus. Our work provides mechanistic insights into how MeV rapidly spreads through airway epithelial cells, contributing to its extremely contagious nature.IMPORTANCE The ability of viral particles to directly spread cell to cell within the airways without particle release is considered to be highly advantageous to many respiratory viruses. Our previous studies in well-differentiated, primary human airway epithelial cells suggest that measles virus (MeV) spreads cell to cell by eliciting the formation of intercellular membrane pores. Based on a newly generated ribonucleoprotein complex (RNP) "tracker" virus, we document by live-cell microscopy that MeV RNPs move along F-actin rings before entering a new cell. Thus, rather than diffusing through the cytoplasm of a newly infected columnar cell, RNPs take advantage of the cytoskeletal infrastructure to rapidly spread laterally across the human airway epithelium. This results in rapid horizontal spread through the epithelium that does not require particle release.

DLITE Uses Cell-Cell Interface Movement to Better Infer Cell-Cell Tensions

Cell shapes and connectivities evolve over time as the colony changes shape or embryos develop. Shapes of intercellular interfaces are closely coupled with the forces resulting from actomyosin interactions, membrane tension, or cell-cell adhesions. Although it is possible to computationally infer cell-cell forces from a mechanical model of collective cell behavior, doing so for temporally evolving forces in a manner robust to digitization difficulties is challenging. Here, we introduce a method for dynamic local intercellular tension estimation (DLITE) that infers such evolution in temporal force with less sensitivity to digitization ambiguities or errors. This method builds upon previous work on single time points (cellular force-inference toolkit). We validate our method using synthetic geometries. DLITE's inferred cell colony tension evolutions correlate better with ground truth for these synthetic geometries as compared to tension values inferred from methods that consider each time point in isolation. We introduce cell connectivity errors, angle estimate errors, connection mislocalization, and connection topological changes to synthetic data and show that DLITE has reduced sensitivity to these conditions. Finally, we apply DLITE to time series of human-induced pluripotent stem cell colonies with endogenously expressed GFP-tagged zonulae occludentes-1. We show that DLITE offers improved stability in the inference of cell-cell tensions and supports a correlation between the dynamics of cell-cell forces and colony rearrangement.

Basal-type lumenogenesis in extraembryonic endoderm stem cells models the early visceral endoderm

Cultured rat primitive extraembryonic endoderm (pXEN) cells easily form free-floating multicellular vesicles de novo, exemplifying a poorly studied type of morphogenesis. Here, we reveal the underlying mechanism and the identity of the vesicles. We resolve the morphogenesis into vacuolization, vesiculation and maturation, and define the molecular characteristics and requirements of each step. Vacuolization is fueled by macropinocytosis and occurs by default if not blocked by high cell density or matrix proteins. Fine-tuned cell-cell contact then forms nascent three-cell vesicles with vacuole-derived lumina. In maturation, the vesicles complete epithelialization, expand via mitosis and continued fluid uptake, and differentiate further. The mature vesicles consist of a simple squamous epithelium with an apical-outside/basal-inside polarity that we trace back to the single cell stage. The polarity and gene expression pattern of the vesicles are similar to those of the early visceral endoderm. pXEN cells provide a useful in vitro model for study of matrix-independent, basal-type lumenogenesis and the physiology of the visceral endoderm.This article has an associated First Person interview with the first author of the paper.

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

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

Claudin-2 suppresses GEF-H1, RHOA, and MRTF, thereby impacting proliferation and profibrotic phenotype of tubular cells

The tight junctional pore-forming protein claudin-2 (CLDN-2) mediates paracellular Na⁺ and water transport in leaky epithelia and alters cancer cell proliferation. Previously, we reported that tumor necrosis factor-α time-dependently alters CLDN-2 expression in tubular epithelial cells. Here, we found a similar expression pattern in a mouse kidney injury model (unilateral ureteral obstruction), consisting of an initial increase followed by a drop in CLDN-2 protein expression. CLDN-2 silencing in LLC-PK¹ tubular cells induced activation and phosphorylation of guanine nucleotide exchange factor H1 (GEF-H1), leading to Ras homolog family member A (RHOA) activation. Silencing of other claudins had no such effects, and re-expression of an siRNA-resistant CLDN-2 prevented RHOA activation, indicating specific effects of CLDN-2 on RHOA. Moreover, kidneys from CLDN-2 knockout mice had elevated levels of active RHOA. Of note, CLDN-2 silencing reduced LLC-PK1 cell proliferation and elevated expression of cyclin-dependent kinase inhibitor P27 (P27KIP1) in a GEF-H1/RHOA-dependent manner. P27KIP1 silencing abrogated the effects of CLDN-2 depletion on proliferation. CLDN-2 loss also activated myocardin-related transcription factor (MRTF), a fibrogenic RHOA effector, and elevated expression of connective tissue growth factor and smooth muscle actin. Finally, CLDN-2 down-regulation contributed to RHOA activation and smooth muscle actin expression induced by prolonged tumor necrosis factor-α treatment, because they were mitigated by re-expression of CLDN-2. Our results indicate that CLDN-2 suppresses GEF-H1/RHOA. CLDN-2 down-regulation, for example, by inflammation, can reduce proliferation and promote MRTF activation through RHOA. These findings suggest that the initial CLDN-2 elevation might aid epithelial regeneration, and CLDN-2 loss could contribute to fibrotic reprogramming.

DAPLE and MPDZ bind to each other and cooperate to promote apical cell constriction

Dishevelled-Associating Protein with a high frequency of LEucines (DAPLE) belongs to a group of unconventional activators of heterotrimeric G-proteins that are cytoplasmic factors rather than membrane proteins of the G-protein–coupled receptor superfamily. During neurulation, DAPLE localizes to apical junctions of neuroepithelial cells and promotes apical cell constriction via G-protein activation. While junctional localization of DAPLE is necessary for this function, the factors it associates with at apical junctions or how they contribute to DAPLE-mediated apical constriction are unknown. MPDZ is a multi-PDZ (PSD95/DLG1/ZO-1) domain scaffold present at apical cell junctions whose mutation in humans is linked to nonsyndromic congenital hydrocephalus (NSCH). DAPLE contains a PDZ-binding motif (PBM) and is also mutated in human NSCH, so we investigated the functional relationship between both proteins. DAPLE colocalized with MPDZ at apical cell junctions and bound directly to the PDZ3 domain of MPDZ via its PBM. Much like DAPLE, MPDZ is induced during neurulation in Xenopus and is required for apical constriction of neuroepithelial cells and subsequent neural plate bending. MPDZ depletion also blunted DAPLE-­mediated apical constriction of cultured cells. These results show that DAPLE and MPDZ, two factors genetically linked to NSCH, function as cooperative partners at apical junctions and are required for proper tissue remodeling during early stages of neurodevelopment.

Necl-4/Cadm4 recruits Par-3 to the Schwann cell adaxonal membrane

Interactions between axons and Schwann cells are essential for the acquisition of Schwann cell radial and longitudinal polarity and myelin sheath assembly. In the internode, the largest of these longitudinal domains, axon-Schwann cell interactions are mediated by the Nectin-like (Necl) cell adhesion proteins, also known as SynCAMs or Cadms. In particular, Necl-1/Cadm3 expressed on the axon surface binds to Necl-4/Cadm4 expressed along the adaxonal membrane of myelinating Schwann cells. Necl-4 promotes myelination in vitro and is required for the timely onset of myelination and the fidelity of the organization of the myelin sheath and the internode in vivo. A key question is the identity of the downstream effectors of Necl-4 that mediate its effects. The cytoplasmic terminal region (CTR) of Necl-4 contains a PDZ-domain binding motif. Accordingly, we used the CTR of Necl-4 in an unbiased proteomic screen of PDZ-domain proteins. We identify Par-3, a multi-PDZ domain containing protein of the Par-aPKC polarity complex previously implicated in myelination, as an interacting protein. Necl-4 and Par-3 are colocalized along the inner Schwann cell membrane and coprecipitate from Schwann cell lysates. The CTR of Necl-4 binds to the first PDZ domain of Par-3 thereby recruiting Par-3 to sites of Necl-4/Necl-1 interaction. Knockdown of Necl-4 perturbs Par-3 localization to the inner membrane of Schwann cells in myelinating co-cultures. These findings implicate interactions of Necl-1/Necl-4 in the recruitment of Par-3 to the Schwann cell adaxonal membrane and the establishment of Schwann cell radial polarity.

Permeability analyses and three dimensional imaging of interferon gamma-induced barrier disintegration in intestinal organoids

The aberrant regulation of the epithelial barrier integrity is involved in many diseases of the digestive tract, including inflammatory bowel diseases and colorectal cancer. Intestinal epithelial cell organoid cultures provide new perspectives for analyses of the intestinal barrier in vitro. However, established methods of barrier function analyses from two dimensional cultures have to be adjusted to the analysis of three dimensional organoid structures. Here we describe the methodology for analysis of epithelial barrier function and molecular regulation in intestinal organoids. Barrier responses to interferon-γ of intestinal organoids with and without epithelial cell-specific deletion of the interferon-γ-receptor 2 gene were used as a model system. The established method allowed monitoring of the kinetics of interferon-γ-induced permeability changes in living organoids. Proteolytic degradation and altered localization of the tight junction proteins claudin-2, -7, and - 15 was detected using confocal spinning disc microscopy with 3D reconstruction. Hessian analysis was used for quantification of re-localization of claudins. In summary, we provide a novel methodologic approach for quantitative analyses of intestinal epithelial barrier functions in the 3D organoid model.

Early Porcine Sapovirus Infection Disrupts Tight Junctions and Uses Occludin as a Coreceptor

The genus Sapovirus belongs to the family Caliciviridae, and its members are common causative agents of severe acute gastroenteritis in both humans and animals. Some caliciviruses are known to use either terminal sialic acids or histo-blood group antigens as attachment factors and/or cell surface proteins, such as CD300lf, CD300ld, and junctional adhesion molecule 1 of tight junctions (TJs), as receptors. However, the roles of TJs and their proteins in sapovirus entry have not been examined. In this study, we found that porcine sapovirus (PSaV) significantly decreased transepithelial electrical resistance and increased paracellular permeability early in infection of LLC-PK cells, suggesting that PSaV dissociates TJs of cells. This led to the interaction between PSaV particles and occludin, which traveled in a complex into late endosomes via Rab5- and Rab7-dependent trafficking. Inhibition of occludin using small interfering RNA (siRNA), a specific antibody, or a dominant-negative mutant significantly blocked the entry of PSaV. Transient expression of occludin in nonpermissive Chinese hamster ovary (CHO) cells conferred susceptibility to PSaV, but only for a limited time. Although claudin-1, another TJ protein, neither directly interacted nor was internalized with PSaV particles, it facilitated PSaV entry and replication in the LLC-PK cells. We conclude that PSaV particles enter LLC-PK cells by binding to occludin as a coreceptor in PSaV-dissociated TJs. PSaV and occludin then form a complex that moves to late endosomes via Rab5- and Rab7-dependent trafficking. In addition, claudin-1 in the TJs opened by PSaV infection facilitates PSaV entry and infection as an entry factor.IMPORTANCE Sapoviruses (SaVs) cause severe acute gastroenteritis in humans and animals. Although they replicate in intestinal epithelial cells, which are tightly sealed by apical-junctional complexes, such as tight junctions (TJs), the mechanisms by which SaVs hijack TJs and their proteins for successful entry and infection remain largely unknown. Here, we demonstrate that porcine SaVs (PSaVs) induce early dissociation of TJs, allowing them to bind to the TJ protein occludin as a functional coreceptor. PSaVs then travel in a complex with occludin into late endosomes through Rab5- and Rab7-dependent trafficking. Claudin-1, another TJ protein, does not directly interact with PSaV but facilitates the entry of PSaV into cells as an entry factor. This work contributes to our understanding of the entry of SaV and other caliciviruses into cells and may aid in the development of efficient and affordable drugs to treat SaV infections.

Ping-Pong-Tumor and Host in Pancreatic Cancer Progression

Metastasis is the main cause of high pancreatic cancer (PaCa) mortality and trials dampening PaCa mortality rates are not satisfying. Tumor progression is driven by the crosstalk between tumor cells, predominantly cancer-initiating cells (CIC), and surrounding cells and tissues as well as distant organs, where tumor-derived extracellular vesicles (TEX) are of major importance. A strong stroma reaction, recruitment of immunosuppressive leukocytes, perineural invasion, and early spread toward the peritoneal cavity, liver, and lung are shared with several epithelial cell-derived cancer, but are most prominent in PaCa. Here, we report on the state of knowledge on the PaCIC markers Tspan8, alpha6beta4, CD44v6, CXCR4, LRP5/6, LRG5, claudin7, EpCAM, and CD133, which all, but at different steps, are engaged in the metastatic cascade, frequently via PaCIC-TEX. This includes the contribution of PaCIC markers to TEX biogenesis, targeting, and uptake. We then discuss PaCa-selective features, where feedback loops between stromal elements and tumor cells, including distorted transcription, signal transduction, and metabolic shifts, establish vicious circles. For the latter particularly pancreatic stellate cells (PSC) are responsible, furnishing PaCa to cope with poor angiogenesis-promoted hypoxia by metabolic shifts and direct nutrient transfer via vesicles. Furthermore, nerves including Schwann cells deliver a large range of tumor cell attracting factors and Schwann cells additionally support PaCa cell survival by signaling receptor binding. PSC, tumor-associated macrophages, and components of the dysplastic stroma contribute to perineural invasion with signaling pathway activation including the cholinergic system. Last, PaCa aggressiveness is strongly assisted by the immune system. Although rich in immune cells, only immunosuppressive cells and factors are recovered in proximity to tumor cells and hamper effector immune cells entering the tumor stroma. Besides a paucity of immunostimulatory factors and receptors, immunosuppressive cytokines, myeloid-derived suppressor cells, regulatory T-cells, and M2 macrophages as well as PSC actively inhibit effector cell activation. This accounts for NK cells of the non-adaptive and cytotoxic T-cells of the adaptive immune system. We anticipate further deciphering the molecular background of these recently unraveled intermingled phenomena may turn most lethal PaCa into a curatively treatable disease.

Ankyrin-G regulated epithelial phenotype is required for mouse lens morphogenesis and growth

Epithelial cell polarity, adhesion, proliferation, differentiation and survival are essential for morphogenesis of various organs and tissues including the ocular lens. The molecular mechanisms regulating the lens epithelial phenotype however, are not well understood. Here we investigated the role of scaffolding protein ankyrin-G (AnkG) in mouse lens development by conditional suppression of AnkG expression using the Cre-LoxP recombination approach. AnkG, which serves to link integral membrane proteins to the spectrin/actin cytoskeleton, was found to distribute predominantly to the lateral membranes of lens epithelium with several isoforms of the protein being detected in the mouse lens. Conditional deficiency of AnkG impaired mouse lens morphogenesis starting from embryonic stage E15.5, with neonatal (P1) AnkG cKO lenses exhibiting overt abnormalities in shape, size, epithelial cell height, sheet length and lateral membrane assembly together with defective fiber cell orientation relative to lenses from littermate AnkG floxed or Cre expressing mice. Severe disruptions in E-cadherin/β-catenin-based adherens junctions, and the membrane organization of spectrin-actin cytoskeleton, ZO-1, connexin-50 and Na⁺-K⁺-ATPase were noted in AnkG deficient lenses, along with detection in lens epithelium of α-smooth muscle actin, a marker of epithelial to mesenchymal transition. Moreover, lens epithelial cell proliferation and survival were severely compromised while differentiation appears to be normal in AnkG deficient mouse lenses. Collectively, these results indicate that AnkG regulates establishment of the epithelial phenotype via lateral membrane assembly, stabilization of E-cadherin-based cell-cell junctions, polarity and membrane organization of transport and adhesion proteins and the spectrin-actin skeleton, and provide evidence for an obligatory role for AnkG in lens morphogenesis and growth.

AMPK-dependent phosphorylation of cingulin reversibly regulates its binding to actin filaments and microtubules

Cytoskeletal organization is essential for the precise morphogenesis of cells, tissues, and organs. Cytoskeletons, bound to scaffolding proteins, regulate the apical junction complex (AJC), which is composed of tight and adherens junctions, and located at the apical side of epithelial cell sheets. Cingulin is a tight junction-associated protein that binds to both actin filaments and microtubules. However, how cingulin binds to microtubules and whether cingulin can bind to actin and microtubules simultaneously are unclear. Here we examined the mechanisms behind cingulin’s cytoskeleton-binding properties. First, using total internal reflection fluorescence microscopy, we detected cingulin at microtubule cross points. We then found the interdomain interactions in cingulin molecules. Notably, we found that this interaction was regulated by AMPK-dependent phosphorylation and changed cingulin’s conformation and binding properties to actin filaments and microtubules. Finally, we found that the AMPK-regulated cingulin properties regulated the barrier functions of epithelial cell sheets. We propose that the cellular metabolic state, which involves AMPK, can contribute to the organization and maintenance of epithelial tissues through cingulin’s tight junction/cytoskeleton regulation.

Secreted Giardia intestinalis cysteine proteases disrupt intestinal epithelial cell junctional complexes and degrade chemokines

Giardiasis is a common diarrheal disease caused by the protozoan parasite Giardia intestinalis. Cysteine proteases (CPs) are acknowledged as virulence factors in Giardia but their specific role in the molecular pathogenesis of disease is not known. Herein, we aimed to characterize the three main secreted CPs (CP14019, CP16160 and CP16779), which were identified by mass spectrometry in the medium during interaction with intestinal epithelial cells (IECs) in vitro. First, the CPs were epitope-tagged and localized to the endoplasmic reticulum and cytoplasmic vesicle-like structures. Second, we showed that recombinant CPs, expressed in Pichia pastoris, are more active in acidic environment (pH 5.5-6) and we determined the kinetic parameters using fluorogenic substrates. Third, excretory-secretory proteins (ESPs) from Giardia trophozoites affect the localization of apical junctional complex (AJC) proteins and recombinant CPs cleave or re-localize the AJC proteins (claudin-1 and -4, occludin, JAM-1, β-catenin and E-cadherin) of IECs. Finally, we showed that the ESPs and recombinant CPs can degrade several chemokines, including CXCL1, CXCL2, CXCL3, IL-8, CCL2, and CCL20, which are up-regulated in IECs during Giardia-host cell interactions. This is the first study that characterizes the role of specific CPs secreted from Giardia and our results collectively indicate their roles in the disruption of the intestinal epithelial barrier and modulating immune responses during Giardia infections.

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

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

Modulation of Intestinal Immune and Barrier Functions by Vitamin A: Implications for Current Understanding of Malnutrition and Enteric Infections in Children

The micronutrient vitamin A refers to a group of compounds with pleiotropic effects on human health. These molecules can modulate biological functions, including development, vision, and regulation of the intestinal barrier. The consequences of vitamin A deficiency and supplementation in children from developing countries have been explored for several years. These children live in an environment that is highly contaminated by enteropathogens, which can, in turn, influence vitamin A status. Vitamin A has been described to modulate gene expression, differentiation and function of diverse immune cells; however, the underlying mechanisms are not fully elucidated. This review aims to summarize the most updated advances on elucidating the vitamin A effects targeting intestinal immune and barrier functions, which may help in further understanding the burdens of malnutrition and enteric infections in children. Specifically, by covering both clinical and in vivo/in vitro data, we describe the effects of vitamin A related to gut immune tolerance/homeostasis, intestinal barrier integrity, and responses to enteropathogens in the context of the environmental enteric dysfunction. Some of the gaps in the literature that require further research are also highlighted.

Occludin induces microvillus formation via phosphorylation of ezrin in a mouse hepatic cell line

Apical and basolateral cell membranes are separated by tight junctions (TJs). Microvilli are limited to the apical cell membrane. TJs and microvilli are the landmarks for epithelial cell polarity. However, the direct relationship between TJ proteins (TJPs) and the components of microvilli remains unclear. In this study, we investigated whether occludin, which is considered to be a functional TJP, is involved in microvillus formation. In occludin knockout mouse hepatic cells (OcKO cells), the microvillus density was less than that in wild-type (WT) cells and the length of microvilli was short. Immunoreactivity of ezrin was decreased in OcKO cells compared with that in WT cells. Although there was no change in the expression level of ezrin, phosphorylation of ezrin was decreased in OcKO cells. The microvillus density and the length of microvilli were increased in OcKO cells by transfection of full-length mouse occludin and COOH-terminal domains of occludin. These results suggested that occludin induced microvillus formation via phosphorylation of ezrin and that the COOH-terminal domain of occludin, which is localized in non-TJ areas, might be able to induce microvilli formation. Our results provide new insights into the function of occludin.

ELK3-GATA3 axis modulates MDA-MB-231 metastasis by regulating cell-cell adhesion-related genes

GATA3 is a master regulator that drives mammary epithelial cell differentiation, and the suppression of GATA3 expression is associated with the development of aggressive breast cancer. However, the mechanism through which GATA3 loss drives cancer development is poorly understood. Previously, we reported that ELK3 suppression in MDA-MB-231 (ELK3 KD) resulted in the reprogramming of these cells from a basal to luminal subtype, which was associated with the induction of GATA3 expression, and that the ELK3-GATA3 axis orchestrated the metastatic characteristics of MDA-MB-231. Here, we show that GATA3 suppression in ELK3 knockdown MDA-MB-231 cells (ELK3/GATA3 DKD) restores the metastatic ability comparably to that of control MDA-MB-231 cells, even though the epithelial cell morphology and TGF-β signaling of ELK3 KD are not recovered in ELK3/GATA3 DKD. The expression of E-cadherin and tight junctional proteins, including occludin, claudin and ZO-1, which is activated in ELK3 KD, is suppressed in ELK3/GATA3 DKD. These results reveal the possibility that the ELK3-GATA3 axis determines the metastatic characteristics of MDA-MB-231 by regulating the expression of cell-cell adhesion factors.

Cell confluence regulates claudin-2 expression: possible role for ZO-1 and Rac

Claudin-2 (Cldn-2) is a channel-forming tight junction (TJ) protein in the proximal tubules that mediates paracellular Na+ transport and has also emerged as a regulator of proliferation and migration. Expression of Cldn-2 is altered by numerous stimuli, but the underlying mechanisms remain incompletely understood. Here we show that Cldn-2 protein and mRNA expression were low in subconfluent tubular cells and increased during junction maturation. Cldn-1 or occludin did not exhibit similar confluence-dependence. Conversely, disruption of TJs by Ca2+ removal or silencing of zonula occludens-1 (ZO-1) or ZO-2 induced a large drop in Cldn-2 abundance. Immunofluorescent staining revealed a more uneven Cldn-2 staining in nascent, Cldn-1-positive TJs. Subconfluence and ZO-1 silencing augmented Cldn-2 degradation and reduced Cldn-2 promoter activity, suggesting that insertion into the TJs slows Cldn-2 turnover. Indeed, blocking endocytosis or lysosomal degradation increased Cldn-2 abundance. Cell confluence increased expression of the junctional adapters ZO-1 and -2, and the small GTPase Rac, and elevated Rac activity and p21-activated kinase (Pak) phosphorylation, suggesting that they might mediate confluence-dependent Cldn-2 regulation. Indeed, Rac silencing or Pak inhibition strongly reduced Cldn-2 protein abundance, which was likely the combined effect on turnover, as these interventions reduced Cldn-2 promoter activity and augmented Cldn-2 degradation. Taken together, our data suggest that TJ integrity and maturity, ZO-1 expression/TJ localization, and Rac/Pak control Cldn-2 degradation and synthesis. A feedback mechanism connecting Cldn-2 expression with junction remodeling, e.g., during wound healing, epithelial-mesenchymal transition, or tumor metastasis formation, may have important downstream effects on permeability, proliferation, and migration.

Calcium mediates cell shape change in human peritoneal mesothelial cells

Mast cells in the peritoneal membrane (PM) may degranulate to release preformed inflammatory mediators including histamine which is capable of diffusing into the surrounding interstitium, modulating cells in their vicinity including, human peritoneal mesothelial cells (hPMC). The present study aimed to investigate the quorum intracellular calcium ([Ca²⁺_i]) response to histamine compared to the membrane soluble ionophore, A23187, in adherent cultured hPMC. To examine [Ca²⁺_i] handling, Fura - 2 loaded cells were exposed to histamine and A23187. Agonist induced transient [Ca²⁺_i] event(s) (TCE) were defined and compared including, resting calcium, peak height, recovery and transient kinetics. Changes in cell shape were examined with immunocytochemistry of the cortical actin (CA) and microtubule (MT) cytoskeleton. To investigate whether histamine induced changes in cell shape were mediated by [Ca²⁺_i], mobilization of [Ca²⁺_i] was prevented with 20 μmol/l of the calcium chelator 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). Histamine produced a dose dependent increase of [Ca²⁺_i], maximal at 1.0 mmol/l which recovered to the pre-challenge resting value. Transient multiplicity with repeated challenge was evident below a histamine threshold of 10^-2 mmol/l. Morphometric analysis of MTs and CA showed significant cell elongation plus histamine and A23187. The histamine induced cell elongation was eliminated with [Ca²⁺_i] clamping. This data indicated that increased [Ca²⁺_i] was essential for cell elongation and the formation of stress fibres and therefore has a pivotal role in the regulation of the PM barrier.

Bridging the gap: Super-resolution microscopy of epithelial cell junctions

Cell junctions are critical for cell adhesion and communication in epithelial tissues. It is evident that the cellular distribution, size, and architecture of cell junctions play a vital role in regulating function. These details of junction architecture have been challenging to elucidate in part due to the complexity and size of cell junctions. A major challenge in understanding these features is attaining high resolution spatial information with molecular specificity. Fluorescence microscopy allows localization of specific proteins to junctions, but with a resolution on the same scale as junction size, rendering internal protein organization unobtainable. Super-resolution microscopy provides a bridge between fluorescence microscopy and nanoscale approaches, utilizing fluorescent tags to reveal protein organization below the resolution limit. Here we provide a brief introduction to super-resolution microscopy and discuss novel findings into the organization, structure and function of epithelial cell junctions.

Elevation of sensitivity to anticancer agents of human lung adenocarcinoma A549 cells by knockdown of claudin-2 expression in monolayer and spheroid culture models

Claudins, tight junctional proteins, regulate the paracellular permeability of ions and small molecules. Claudin-2 is highly expressed in human lung adenocarcinoma cells and is involved in the up-regulation of cell proliferation. However, the effect of claudin-2 on cellular sensitivity to anticancer agents has not been clarified. The cytotoxicity of anticancer agents such as cisplatin, gefitinib and doxorubicin (DXR) was increased by claudin-2 knockdown in A549 cells. Claudin-2 knockdown also significantly decreased the expression level of multidrug resistance-associated protein/ABCC2. The expression levels of other drug efflux transporters were unchanged. The intracellular accumulation of 5-chloromethylfluorescein diacetate (CMFDA) and DXR, substrates of ABCC2, was increased by claudin-2 knockdown, whereas the efflux was decreased. MK-571, an inhibitor of ABCC2, enhanced the cytotoxicity of anticancer agents. Claudin-2 knockdown decreased the levels of p-c-Jun and nuclear Sp1. SP600125, an inhibitor of c-Jun, and mithramycin, an inhibitor of Sp1, decreased the level of ABCC2. The promoter activity of ABCC2 was decreased by claudin-2 knockdown, SP600125 and mithramycin treatments, suggesting that claudin-2 is involved in the up-regulation of ABCC2 expression at the transcriptional level. Claudin-2 knockdown increased the paracellular permeability of DXR in a 2D monolayer culture model. In addition, the accumulation of DXR into spheroids was enhanced by claudin-2 knockdown, resulting in a reduction in cell viability. We suggest that claudin-2 may be a novel therapeutic target in lung adenocarcinoma, because claudin-2 knockdown increased the accumulation of anticancer agents in cancer cells and spheroids.