ZO-1 recruitment to α-catenin--a novel mechanism for coupling the assembly of tight junctions to adherens junctions

Afadin loss induces breast cancer metastasis through destabilisation of E-cadherin to F-actin linkage

Afadin is a multimodal scaffolding protein with essential functions in cell-cell adhesion. Although its loss of expression has been linked to breast cancer invasion and metastasis, the underlying mechanisms driving tumour progression upon mutational Afadin (AFDN) loss in breast cancers remains unclear. In the current study we identified a somatic frameshift AFDN mutation (p.Lys630fs) in an invasive breast cancer sample that coincides with loss of Afadin protein expression. Functional studies in E-cadherin-expressing breast cancer cells show that Afadin loss leads to immature and aberrant adherens junction (AJ) formation. The lack of AJ maturation results in a noncohesive cellular phenotype accompanied by Actomyosin-dependent anoikis resistance, which are classical progression hallmarks of single-cell breast cancer invasion. Reconstitution experiments using Afadin truncates show that proper F-actin organisation and epithelial cell-cell adhesion critically depend on the Coiled-Coil domain of Afadin but not on the designated C-terminal F-actin binding domain. Mouse xenograft experiments based on cell lines and primary patient-derived breast cancer organoids demonstrate that Afadin loss induces single-cell lobular-type invasion phenotypes and overt dissemination to the lungs and the peritoneum. In short, Afadin is a metastasis suppressor for breast cancer through stabilisation and maturation of a mechanical E-cadherin to F-actin outside-in link. © 2025 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Role of E-cadherin in epithelial barrier dysfunction: implications for bacterial infection, inflammation, and disease pathogenesis

Epithelial barriers serve as critical defense lines against microbial infiltration and maintain tissue homeostasis. E-cadherin, an essential component of adherens junctions, has emerged as a pivotal molecule that secures epithelial homeostasis. Lately, its pleiotropic role beyond barrier function, including its involvement in immune responses, has become more evident. Herein, we delve into the intricate relationship between (dys)regulation of epithelial homeostasis and the versatile functionality of E-cadherin, describing complex mechanisms that underlie barrier integrity and disruption in disease pathogenesis such as bacterial infection and inflammation, among others. Clinical implications of E-cadherin perturbations in host pathophysiology are emphasized; downregulation, proteolytic phenomena, abnormal localization/signaling and aberrant immune reactions are linked with a broad spectrum of pathology beyond infectious diseases. Finally, potential therapeutic interventions that may harness E-cadherin to mitigate barrier-associated tissue damage are explored. Overall, this review highlights the crucial role of E-cadherin in systemic health, offering insights that could pave the way for strategies to reinforce/restore barrier integrity and treat related diseases.

Gene expression of tight junctions in foreskin is not affected by HIV pre-exposure prophylaxis

Introduction

Tight junctions (TJs) serve as permeability filters between the internal and external cellular environment. A large number of proteins have been identified to be localized at the TJs. Due to limitations in tissue collection, TJs in the male genital tract have been understudied.

Methods

We analysed the transcriptomics of 132 TJ genes in foreskin tissue of men requesting voluntary medical male circumcision (VMMC) and enrolled in the Combined HIV Adolescent Prevention Study (CHAPS) trial conducted in South Africa and Uganda (NCT03986970). The trial evaluated the dose requirements for event-driven HIV pre-exposure prophylaxis (PrEP) with emtricitabine-tenofovir (FTC-TDF) or emtricitabine-tenofovir alafenamide (FTC-TAF) during insertive sex. A total of 144 participants were randomized to either control arm or one of 8 PrEP arms (n=16/arm), receiving oral FTC-TDF or FTC-TAF over one or two days. Following in vivo oral PrEP dosing and VMMC, the expression level of three important TJ proteins (CLDN-1, OCN and ZO-1) was measured ex vivo in foreskin tissue by Western blot. The expression of cytokine genes implicated in TJ regulation was determined. Non-parametric Kruskal-Wallis tests were used to compare TJ gene expression and protein levels by type of PrEP received, and Spearman's correlation coefficients were calculated to assess whether TJ gene expression levels were related to cytokine gene levels or to PrEP drug concentrations and their active intracellularly phosphorylated metabolites.

Results

A high level of expression in foreskin tissue was found for 118 (of 132) TJ genes analysed; this finding contributed to create a map of TJ components within the male genital tract. Importantly, PrEP regimens tested in the CHAPS trial did not affect the expression of TJ genes and the analysed proteins in the foreskin; thus, further supporting the safety of this prevention strategy against HIV-1 transmission during insertive sex. Additionally, we identified the level of several cytokines' genes to be correlated to TJ gene expression: among them, IL-18, IL-33 and VEGF.

Discussion

TJs can limit viral entry into target cells; to affect this biological function viruses can reduce the expression of TJ proteins. Our study, on the expression and regulation of TJs in the foreskin, contribute important knowledge for PrEP safety and further design of HIV-1 prophylaxis.

Restore Intestinal Barrier Integrity: An Approach for Inflammatory Bowel Disease Therapy

The intestinal barrier maintained by various types of columnar epithelial cells, plays a crucial role in regulating the interactions between the intestinal contents (such as the intestinal microbiota), the immune system, and other components. Dysfunction of the intestinal mucosa is a significant pathophysiological mechanism and clinical manifestation of inflammatory bowel disease (IBD). However, current therapies for IBD primarily focus on suppressing inflammation, and no disease-modifying treatments specifically target the epithelial barrier. Given the side effects associated with chronic immunotherapy, effective alternative therapies that promote mucosal healing are highly attractive. In this review, we examined the function of intestinal epithelial barrier function and the mechanisms of behind its disruption in IBD. We illustrated the complex process of intestinal mucosal healing and proposed therapeutic approaches to promote mucosal healing strategies in IBD. These included the application of stem cell transplantation and organ-like tissue engineering approaches to generate new intestinal tissue. Finally, we discussed potential strategies to restore the function of the intestinal barrier as a treatment for IBD.

ZO-1 regulates the migration of mesenchymal stem cells in cooperation with α-catenin in response to breast tumor cells

Mesenchymal stem cells are recruited from the bone marrow into breast tumors, contributing to the creation of a tumor microenvironment that fosters tropism for breast tumors. However, the intrinsic mechanisms underlying the recruitment of bone marrow-derived mesenchymal stem cells (MSCs) into the breast tumor microenvironment are still under investigation. Our discoveries identified zonula occludens-1 (ZO-1) as a specific intrinsic molecule that plays a vital role in mediating the collective migration of MSCs towards breast tumor cells and transforming growth factor beta (TGF-β), which is a crucial factor secreted by breast tumor cells. Upon migration in response to MDA-MB-231 cells and TGF-β, MSCs showed increased formation of adherens junction-like structures (AJs) expressing N-cadherin and α-catenin at their cell-cell contacts. ZO-1 was found to be recruited into the AJs at the cell-cell contacts between MSCs. Additionally, ZO-1 collaborated with α-catenin to regulate AJ formation, dependently on the SH3 and GUK domains of the ZO-1 protein. ZO-1 knockdown led to the impaired migration of MSCs in response to the stimuli and subsequent downregulation of AJs formation at the cell-cell contacts during MSCs migration. Overall, our study highlights the novel role of ZO-1 in guiding MSC migration towards breast tumor cells, suggesting its potential as a new strategy for controlling and re-engineering the breast tumor microenvironment.

Retinoic Acid-PPARα Mediates β-Carotene Resistance to Placental Dysfunction Induced by Deoxynivalenol

Deoxynivalenol (DON), one of the most polluted mycotoxins in the environment and food, has been proven to have strong embryonic and reproductive toxicities. However, the effects of DON on placental impairment and effective interventions are still unclear. This study investigated the effect of β-carotene on placental functional impairment and its underlying molecular mechanism under DON exposure. Adverse pregnancy outcomes were caused by intraperitoneal injection of DON from 13.5 to 15.5 days of gestation in mice, resulting in higher enrichment of DON in placenta than in other tissue samples. Interestingly, 0.1% β-carotene dietary supplementation could significantly alleviate DON-induced pregnancy outcomes. Additionally, in vivo and in vitro placental barrier models demonstrated the association of DON-induced placental function impairment with placental permeability barrier disruption, angiogenesis impairment, and oxidative stress induction. Moreover, β-carotene regulated DON-induced placental toxicity by activating the expressions of claudin 1, zonula occludens-1, and vascular endothelial growth factor-A through retinoic acid-peroxisome proliferator-activated receptor α signaling.

Circulating exosomes from Alzheimer's disease suppress VE-cadherin expression and induce barrier dysfunction in recipient brain microvascular endothelial cell

Background

Blood-brain barrier (BBB) breakdown is a component of the progression and pathology of Alzheimer's disease (AD). BBB dysfunction is primarily caused by reduced or disorganized tight junction or adherens junction proteins of brain microvascular endothelial cell (BMEC). While there is growing evidence of tight junction disruption in BMECs in AD, the functional role of adherens junctions during BBB dysfunction in AD remains unknown. Exosomes secreted from senescent cells have unique characteristics and contribute to modulating the phenotype of recipient cells. However, it remains unknown if and how these exosomes cause BMEC dysfunction in AD.

Objectives

This study aimed to investigate the potential roles of AD circulating exosomes and their RNA cargos in brain endothelial dysfunction in AD.

Methods

We isolated exosomes from sera of five cases of AD compared with age- and sex-matched cognitively normal controls using size-exclusion chromatography technology. We validated the qualities and particle sizes of isolated exosomes with nanoparticle tracking analysis and atomic force microscopy. We measured the biomechanical natures of the endothelial barrier of BMECs, the lateral binding forces between live BMECs, using fluidic force miscopy. We visualized the paracellular expressions of the key adherens junction protein VE-cadherin in BMEC cultures and a 3D BBB model that employs primary human BMECs and pericytes with immunostaining and evaluated them using confocal microscopy. We also examined the VE-cadherin signal in brain tissues from five cases of AD and five age- and sex-matched cognitively normal controls.

Results

We found that circulating exosomes from AD patients suppress the paracellular expression levels of VE-cadherin and impair the barrier function of recipient BMECs. Immunostaining analysis showed that AD circulating exosomes damage VE-cadherin integrity in a 3D model of microvascular tubule formation. We found that circulating exosomes in AD weaken the BBB depending on the RNA cargos. In parallel, we observed that microvascular VE-cadherin expression is diminished in AD brains compared to normal controls.

Conclusion

Using in vitro and ex vivo models, our study illustrates that circulating exosomes from AD patients play a significant role in mediating the damage effect on adherens junction of recipient BMEC of the BBB in an exosomal RNA-dependent manner. This suggests a novel mechanism of peripheral senescent exosomes for AD risk.

Surface Hydration Protects Cystic Fibrosis Airways from Infection by Restoring Junctional Networks

Defective hydration of airway surface mucosa is associated with recurrent lung infection in cystic fibrosis (CF), a disease caused by CF transmembrane conductance regulator (CFTR) gene mutations. Whether the composition and/or presence of an airway surface liquid (ASL) is sufficient to prevent infection remains unclear. The susceptibility to infection of polarized wild type and CFTR knockdown (CFTR-KD) airway epithelial cells was determined in the presence or absence of a healthy ASL or physiological saline. CFTR-KD epithelia exhibited strong ASL volume reduction, enhanced susceptibility to infection, and reduced junctional integrity. Interestingly, the presence of an apical physiological saline alleviated disruption of the airway epithelial barrier by stimulating essential junctional protein expression. Thus, rehydrated CFTR-KD cells were protected from infection despite normally intense bacterial growth. This study indicates that an epithelial integrity gatekeeper is modulated by the presence of an apical liquid volume, irrespective of the liquid's composition and of expression of a functional CFTR.

Cingulin binds to the ZU5 domain of scaffolding protein ZO-1 to promote its extended conformation, stabilization, and tight junction accumulation

Zonula occludens-1 (ZO-1), the major scaffolding protein of tight junctions (TJs), recruits the cytoskeleton-associated proteins cingulin (CGN) and paracingulin (CGNL1) to TJs by binding to their N-terminal ZO-1 interaction motif. The conformation of ZO-1 can be either folded or extended, depending on cytoskeletal tension and intramolecular and intermolecular interactions, and only ZO-1 in the extended conformation recruits the transcription factor DbpA to TJs. However, the sequences of ZO-1 that interact with CGN and CGNL1 and the role of TJ proteins in ZO-1 TJ assembly are not known. Here, we used glutathione-S-transferase pulldowns and immunofluorescence microscopy to show that CGN and CGNL1 bind to the C-terminal ZU5 domain of ZO-1 and that this domain is required for CGN and CGNL1 recruitment to TJs and to phase-separated ZO-1 condensates in cells. We show that KO of CGN, but not CGNL1, results in decreased accumulation of ZO-1 at TJs. Furthermore, ZO-1 lacking the ZU5 domain showed decreased accumulation at TJs, was detectable along lateral contacts, had a higher mobile fraction than full-length ZO-1 by fluorescence recovery after photobleaching analysis, and had a folded conformation, as determined by structured illumination microscopy of its N-terminal and C-terminal ends. The CGN–ZU5 interaction promotes the extended conformation of ZO-1, since binding of the CGN–ZO-1 interaction motif region to ZO-1 resulted in its interaction with DbpA in cells and in vitro. Together, these results show that binding of CGN to the ZU5 domain of ZO-1 promotes ZO-1 stabilization and accumulation at TJs by promoting its extended conformation.

Erythrocytes, a New Contributor to Age-Associated Loss of Blood-Brain Barrier Integrity

Blood exchanges between young and old partners demonstrate old blood has a detrimental effect on brain health of young animals. Previous studies primarily investigate soluble blood factors, such as transforming growth factor-beta, on the brain and the blood-brain barrier (BBB). However, the role of blood cellular components, particularly erythrocytes, has not been defined. Erythrocyte morphology and rigidity change as mammals age, altering their transport within the capillary bed. This impacts downstream biological events, such as the release of reactive oxygen species and hemoglobin, potentially compromising the BBB. Here, a micro electrical BBB (µE-BBB), with cocultured endothelial and astrocytic cells, and a built-in trans-endothelial electrical resistance (TEER) system is described to monitor the effect of capillary shear stress on erythrocytes derived from young and old mice and people and the subsequent effects of these cells on BBB integrity. This is monitored by the passage of fluorescein isothiocyanate-dextran and real-time profiling of TEER across the BBB after old and young erythrocyte exposure. Compared to young erythrocytes, old erythrocytes induce an increased permeability by 42% and diminished TEER by 2.9% of the µE-BBB. These results suggest that changes in circulating erythrocytes are a biomarker of aging in the context of BBB integrity.

The Cx43 Carboxyl-Terminal Mimetic Peptide αCT1 Protects Endothelial Barrier Function in a ZO1 Binding-Competent Manner

The Cx43 carboxyl-terminus (CT) mimetic peptide, αCT1, originally designed to bind to Zonula Occludens 1 (ZO1) and thereby inhibit Cx43/ZO1 interaction, was used as a tool to probe the role of Cx43/ZO1 association in regulation of epithelial/endothelial barrier function. Using both in vitro and ex vivo methods of barrier function measurement, including Electric Cell-Substrate Impedance Sensing (ECIS), a TRITC-dextran Transwell permeability assay, and a FITC-dextran cardiovascular leakage protocol involving Langendorff-perfused mouse hearts, αCT1 was found to protect the endothelium from thrombin-induced breakdown in cell-cell contacts. Barrier protection was accompanied by significant remodeling of the F-actin cytoskeleton, characterized by a redistribution of F-actin away from the cytoplasmic and nuclear regions of the cell, towards the endothelial cell periphery, in association with alterations in cellular chiral orientation distribution. In line with observations of increased cortical F-actin, αCT1 upregulated cell-cell border localization of endothelial VE-cadherin, the tight junction protein Zonula Occludens 1 (ZO1), and the Gap Junction Protein (GJ) Connexin43 (Cx43). A ZO1 binding-incompetent variant of αCT1, αCT1-I, indicated that these effects on barrier function and barrier-associated proteins, were likely associated with Cx43 CT sequences retaining ability to interact with ZO1. These results implicate the Cx43 CT and its interaction with ZO1, in the regulation of endothelial barrier function, while revealing the therapeutic potential of αCT1 in the treatment of vascular edema.

Tsc1 regulates tight junction independent of mTORC1

Tuberous sclerosis complex 1 (Tsc1) is a tumor suppressor that functions together with Tsc2 to negatively regulate the mechanistic target of rapamycin complex 1 (mTORC1) activity. Here, we show that Tsc1 has a critical role in the tight junction (TJ) formation of epithelium, independent of its role in Tsc2 and mTORC1 regulation. When an epithelial cell establishes contact with neighboring cells, Tsc1, but not Tsc2, migrates from the cytoplasm to junctional membranes, in which it binds myosin 6 to anchor the perijunctional actin cytoskeleton to β-catenin and ZO-1. In its absence, perijunctional actin cytoskeleton fails to form. In mice, intestine-specific or inducible, whole-body Tsc1 ablation disrupts adherens junction/TJ structures in intestine or skin epithelia, respectively, causing Crohn's disease-like symptoms in the intestine or psoriasis-like phenotypes on the skin. In patients with Crohn's disease or psoriasis, junctional Tsc1 levels in epithelial tissues are markedly reduced, concomitant with the TJ structure impairment, suggesting that Tsc1 deficiency may underlie TJ-related diseases. These findings establish an essential role of Tsc1 in the formation of cell junctions and underpin its association with TJ-related human diseases.

Ankyrin G organizes membrane components to promote coupling of cell mechanics and glucose uptake

The response of cells to forces is critical for their function and occurs via rearrangement of the actin cytoskeleton¹. Cytoskeletal remodelling is energetically costly^2,3, yet how cells signal for nutrient uptake remains undefined. Here we present evidence that force transmission increases glucose uptake by stimulating glucose transporter 1 (GLUT1). GLUT1 recruitment to and retention at sites of force transmission requires non-muscle myosin IIA-mediated contractility and ankyrin G. Ankyrin G forms a bridge between the force-transducing receptors and GLUT1. This bridge is critical for enabling cells under tension to tune glucose uptake to support remodelling of the actin cytoskeleton and formation of an epithelial barrier. Collectively, these data reveal an unexpected mechanism for how cells under tension take up nutrients and provide insight into how defects in glucose transport and mechanics might be linked.

Comparative proteomics analysis indicates that palmatine contributes to transepithelial migration by regulating cellular adhesion

CONTEXT

Palmatine, a biologically active isoquinoline alkaloid, possesses multiple pharmaceutical activities against mucosal infection and inflammation.

OBJECTIVE

There are no reports about the influence of palmatine on uterine mucosal epithelial cells.

MATERIALS AND METHODS

We used proteomics to analyse differentially expressed proteins (DEPs) in goat endometrial epithelial cells (EECs) stimulated by lipopolysaccharide (LPS, 5 μg/mL, the dosage can induce inflammatory response, according to our previous study) for 12 h and then treated with palmatine (80 μg/mL) for 8 h; the dosage was selected based on MTT assay. The EECs without any treatment were used as controls. Every group was treated in triplicate.

RESULTS

A total of 428 DEPs in LPS-stimulated group and 486 DEPs in the palmatine-treated group were identified. Functional annotation analysis showed that palmatine mainly regulated the protein expression of structural molecules involved in the response to stimuli. Pathway analysis showed that cell adhesion molecule (CaM) pathways were most significant enriched due to palmatine treatment. Junction adhesion molecule 1 (JAM1), nectin 1 (NECT1) and cadherin 5 (CDH5), which play important roles in the transepithelial migration (TEpM) of leukocytes, were significantly downregulated by palmatine. Meanwhile, other proteins essential to the maintenance of cell adhesion and those that facilitate leukocyte migration were upregulated after palmatine treatment. Discussion and conclusions: The results suggested that palmatine regulates the expression of CaMs to affect TEpM during uterine mucosal inflammation and provides novel insight to understanding and developing palmatine pharmacology. Palmatine is a promising drug for treatment of mucosal inflammation.

The tight junction and the epithelial barrier in coeliac disease

Epithelial barriers are essential to maintain multicellular organisms well compartmentalized and protected from external environment. In the intestine, the epithelial layer orchestrates a dynamic balance between nutrient absorption and prevention of microorganisms, and antigen intrusion. Intestinal barrier function has been shown to be altered in coeliac disease but whether it contributes to the pathogenesis development or if it is merely a phenomenon secondary to the aberrant immune response is still unknown. The tight junction complexes are multiprotein cell-cell adhesions that seal the epithelial intercellular space and regulate the paracellular permeability of ions and solutes. These structures have a fundamental role in epithelial barrier integrity as well as in signaling mechanisms that control epithelial-cell polarization, the formation of apical domains and cellular processes such as cell proliferation, migration, differentiation, and survival. In coeliac disease, the molecular structures and function of tight junctions appear disrupted and are not completely recovered after treatment with gluten-free diet. Moreover, zonulin, the only known physiological regulator of the tight junction permeability, appears augmented in autoimmune conditions associated with TJ dysfunction, including coeliac disease. This chapter will examine recent discoveries about the molecular architecture of tight junctions and their functions. We will discuss how different factors contribute to tight junction disruption and intestinal barrier impairment in coeliac disease. To conclude, new insights into zonulin-driven disruption of tight junction structures and barrier integrity in coeliac disease are presented together with the advancements in novel therapy to treat the barrier defect seen in pathogenesis.

An intact keratin network is crucial for mechanical integrity and barrier function in keratinocyte cell sheets

The isotype-specific composition of the keratin cytoskeleton is important for strong adhesion, force resilience, and barrier function of the epidermis. However, the mechanisms by which keratins regulate these functions are still incompletely understood. In this study, the role and significance of the keratin network for mechanical integrity, force transmission, and barrier formation were analyzed in murine keratinocytes. Following the time-course of single-cell wound closure, wild-type (WT) cells slowly closed the gap in a collective fashion involving tightly connected neighboring cells. In contrast, the mechanical response of neighboring cells was compromised in keratin-deficient cells, causing an increased wound area initially and an inefficient overall wound closure. Furthermore, the loss of the keratin network led to impaired, fragmented cell-cell junctions, and triggered a profound change in the overall cellular actomyosin architecture. Electric cell-substrate impedance sensing of cell junctions revealed a dysfunctional barrier in knockout (Kty-/-) cells compared to WT cells. These findings demonstrate that Kty-/- cells display a novel phenotype characterized by loss of mechanocoupling and failure to form a functional barrier. Re-expression of K5/K14 rescued the barrier defect to a significant extent and reestablished the mechanocoupling with remaining discrepancies likely due to the low abundance of keratins in that setting. Our study reveals the major role of the keratin network for mechanical homeostasis and barrier functionality in keratinocyte layers.

ZO-1 Regulates Intercalated Disc Composition and Atrioventricular Node Conduction

RATIONALE

ZO-1 (Zona occludens 1), encoded by the tight junction protein 1 (TJP1) gene, is a regulator of paracellular permeability in epithelia and endothelia. ZO-1 interacts with the actin cytoskeleton, gap, and adherens junction proteins and localizes to intercalated discs in cardiomyocytes. However, the contribution of ZO-1 to cardiac physiology remains poorly defined.

OBJECTIVE

We aim to determine the role of ZO-1 in cardiac function.

METHODS AND RESULTS

Inducible cardiomyocyte-specific Tjp1 deletion mice (Tjp1fl/fl; Myh6Cre/Esr1*) were generated by crossing the Tjp1 floxed mice and Myh6Cre/Esr1* transgenic mice. Tamoxifen-induced loss of ZO-1 led to atrioventricular (AV) block without changes in heart rate, as measured by ECG and ex vivo optical mapping. Mice with tamoxifen-induced conduction system-specific deletion of Tjp1 (Tjp1fl/fl; Hcn4CreERt2) developed AV block while tamoxifen-induced conduction system deletion of Tjp1 distal to the AV node (Tjp1fl/fl; Kcne1CreERt2) did not demonstrate conduction defects. Western blot and immunostaining analyses of AV nodes showed that ZO-1 loss decreased Cx (connexin) 40 expression and intercalated disc localization. Consistent with the mouse model study, immunohistochemical staining showed that ZO-1 is abundantly expressed in the human AV node and colocalizes with Cx40. Ventricular conduction was not altered despite decreased localization of ZO-1 and Cx43 at the ventricular intercalated disc and modestly decreased left ventricular ejection fraction, suggesting ZO-1 is differentially required for AV node and ventricular conduction.

CONCLUSIONS

ZO-1 is a key protein responsible for maintaining appropriate AV node conduction through maintaining gap junction protein localization.

Annexin A2 depletion exacerbates the intracerebral microhemorrhage induced by acute rickettsia and Ebola virus infections

Intracerebral microhemorrhages (CMHs) are small foci of hemorrhages in the cerebrum. Acute infections induced by some intracellular pathogens, including rickettsia, can result in CMHs. Annexin a2 (ANXA2) has been documented to play a functional role during intracellular bacterial adhesion. Here we report that ANXA2-knockout (KO) mice are more susceptible to CMHs in response to rickettsia and Ebola virus infections, suggesting an essential role of ANXA2 in protecting vascular integrity during these intracellular pathogen infections. Proteomic analysis via mass spectrometry of whole brain lysates and brain-derived endosomes from ANXA2-KO and wild-type (WT) mice post-infection with R. australis revealed that a variety of significant proteins were differentially expressed, and the follow-up function enrichment analysis had identified several relevant cell-cell junction functions. Immunohistology study confirmed that both infected WT and infected ANXA2-KO mice were subjected to adherens junctional protein (VE-cadherin) damages. However, key blood-brain barrier (BBB) components, tight junctional proteins ZO-1 and occludin, were disorganized in the brains from R. australis-infected ANXA2-KO mice, but not those of infected WT mice. Similar ANXA2-KO dependent CMHs and fragments of ZO-1 and occludin were also observed in Ebola virus-infected ANXA2-KO mice, but not found in infected WT mice. Overall, our study revealed a novel role of ANXA2 in the formation of CMHs during R. australis and Ebola virus infections; and the underlying mechanism is relevant to the role of ANXA2-regulated tight junctions and its role in stabilizing the BBB in these deadly infections.

Cell-cell junctions as sensors and transducers of mechanical forces

Epithelial and endothelial monolayers are multicellular sheets that form barriers between the 'outside' and 'inside' of tissues. Cell-cell junctions, made by adherens junctions, tight junctions and desmosomes, hold together these monolayers. They form intercellular contacts by binding their receptor counterparts on neighboring cells and anchoring these structures intracellularly to the cytoskeleton. During tissue development, maintenance and pathogenesis, monolayers encounter a range of mechanical forces from the cells themselves and from external systemic forces, such as blood pressure or tissue stiffness. The molecular landscape of cell-cell junctions is diverse, containing transmembrane proteins that form intercellular bonds and a variety of cytoplasmic proteins that remodel the junctional connection to the cytoskeleton. Many junction-associated proteins participate in mechanotransduction cascades to confer mechanical cues into cellular responses that allow monolayers to maintain their structural integrity. We will discuss force-dependent junctional molecular events and their role in cell-cell contact organization and remodeling.

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.

Piperazine Derivatives Enhance Epithelial Cell Monolayer Permeability by Increased Cell Force Generation and Loss of Cadherin Structures

A major obstacle for topical and enteral drug delivery is the poor transport of macromolecular drugs through the epithelium. One potential solution is the use of permeation enhancers that alter epithelial structures. Piperazine derivatives are known permeation enhancers that modulate epithelial structures, reduce transepithelial electrical resistance, and augment the absorption of macromolecular drugs. The mechanism by which piperazine derivatives disrupt the structures of epithelial monolayers is not well understood. Here, the effects of 1-phenylpiperazine and 1-methyl-4-phenylpiperazine are modeled in the epithelial cell line NRK-52E. Live-cell imaging reveals a dose-dependent gross reorganization of monolayers at high concentrations, but reorganization differs based on the piperazine molecule. Results show that low concentrations of piperazine derivatives increase myosin force generation within the cells and do not disrupt the cytoskeletal structure. Also, cytoskeletally attached cadherin junctions are disrupted before tight junctions. In summary, piperazines appear to increase myosin-mediated contraction followed by disruption of cell-cell contacts. These results provide new mechanistic insight into how transient epithelial permeation enhancers act and will inform of the development of future generations of transepithelial delivery systems.

Fusarium Mycotoxins Disrupt the Barrier and Induce IL-6 Release in a Human Placental Epithelium Cell Line

Deoxynivalenol, T-2 toxin, and zearalenone, major Fusarium mycotoxins, contaminate human food on a global level. Exposure to these mycotoxins during pregnancy can lead to abnormalities in neonatal development. Therefore, the aim of this study was to investigate the effects of Fusarium mycotoxins on human placental epithelial cells. As an in vitro model of placental barrier, BeWo cells were exposed to different concentrations of deoxynivalenol, zearalenone or T-2 toxin. Cytotoxicity, effects on barrier integrity, paracellular permeability along with mRNA and protein expression and localization of junctional proteins after exposure were evaluated. Induction of proinflammatory responses was determined by measuring cytokine production. Increasing mycotoxin concentrations affect BeWo cell viability, and T-2 toxin was more toxic compared to other mycotoxins. Deoxynivalenol and T-2 toxin caused significant barrier disruption, altered protein and mRNA expression of junctional proteins, and induced irregular cellular distribution. Although the effects of zearalenone on barrier integrity were less prominent, all tested mycotoxins were able to induce inflammation as measured by IL-6 release. Overall, Fusarium mycotoxins disrupt the barrier of BeWo cells by altering the expression and structure of junctional proteins and trigger proinflammatory responses. These changes in placental barrier may disturb the maternal–fetal interaction and adversely affect fetal development.

The mechanobiology of tight junctions

Tight junctions (TJ) play a central role in the homeostasis of epithelial and endothelial tissues, by providing a semipermeable barrier to ions and solutes, by contributing to the maintenance of cell polarity, and by functioning as signaling platforms. TJ are associated with the actomyosin and microtubule cytoskeletons, and the crosstalk with the cytoskeleton is fundamental for junction biogenesis and physiology. TJ are spatially and functionally connected to adherens junctions (AJ), which are essential for the maintenance of tissue integrity. Mechano-sensing and mechano-transduction properties of several AJ proteins have been characterized during the last decade. However, little is known about how mechanical forces act on TJ and their proteins, how TJ control the mechanical properties of cells and tissues, and what are the underlying molecular mechanisms. Here I review recent studies that have advanced our understanding of the relationships between mechanical force and TJ biology.

PAK2 links cell survival to mechanotransduction and metabolism

Campbell et al. show that force stimulates PAK2 activation at cell–cell junctions, where it protects cells under force from death and plays a key role in linking force-induced mechanotransduction, metabolism, and cell survival.

Too little or too much force can trigger cell death, yet factors that ensure the survival of cells remain largely unknown. Here, we demonstrate that E-cadherin responds to force by recruiting and activating p21-activated protein kinase 2 (PAK2) to allow cells to stiffen, metabolize, and survive. Interestingly, PAK2 activation and its control of the apoptotic response are specific for the amplitude of force applied. Specifically, under low amplitudes of physiological force, PAK2 is protected from proteolysis, thereby ensuring cell survival. In contrast, under higher amplitudes of physiological force, PAK2 is left unprotected and stimulates apoptosis, an effect that is prevented by cleavage-resistant forms of the protein. Finally, we demonstrate that PAK2 protection is conferred by direct binding of AMPK. Thus, PAK2 mediates the survival of cells under force. These findings reveal an unexpected paradigm for how mechanotransduction, metabolism, and cell survival are linked.

SHP-2 is activated in response to force on E-cadherin and dephosphorylates vinculin Y822

The response of cells to mechanical inputs is a key determinant of cell behavior. In response to external forces, E-cadherin initiates signal transduction cascades that allow the cell to modulate its contractility to withstand the force. Much attention has focused on identifying the E-cadherin signaling pathways that promote contractility, but the negative regulators remain undefined. In this study, we identify SHP-2 as a force-activated phosphatase that negatively regulates E-cadherin force transmission by dephosphorylating vinculin Y822. To specifically probe a role for SHP-2 in E-cadherin mechanotransduction, we mutated vinculin so that it retains its phosphorylation but cannot be dephosphorylated. Cells expressing the mutant vinculin have increased contractility. This work provides a mechanism for inactivating E-cadherin mechanotransduction and provides a new method for specifically targeting the action of phosphatases in cells.

Anisotropic organization of circumferential actomyosin characterizes hematopoietic stem cells emergence in the zebrafish

Hematopoiesis leads to the formation of blood and immune cells. Hematopoietic stem cells emerge during development, from vascular components, via a process called the endothelial-to-hematopoietic transition (EHT). Here, we reveal essential biomechanical features of the EHT, using the zebrafish embryo imaged at unprecedented spatio-temporal resolution and an algorithm to unwrap the aorta into 2D-cartography. We show that the transition involves anisotropic contraction along the antero-posterior axis, with heterogenous organization of contractile circumferential actomyosin. The biomechanics of the contraction is oscillatory, with unusually long periods in comparison to other apical constriction mechanisms described so far in morphogenesis, and is supported by the anisotropic reinforcement of junctional contacts. Finally, we show that abrogation of blood flow impairs the actin cytoskeleton, the morphodynamics of EHT cells, and the orientation of the emergence. Overall, our results underline the peculiarities of the EHT biomechanics and the influence of the mechanical forces exerted by blood flow.

As humans, we have two major types of blood cell: our red blood cells transport oxygen around the body, while our white blood cells fight disease. Both types of cell come from the same stem cells, which first appear early in embryonic development. These stem cells emerge from the walls of major blood vessels, including the aorta – which carries blood from the heart.

Stem cells have not yet decided which adult cell to become. Given the right signals, blood stem cells can form red blood cells or any of the different types of white blood cell. Understanding this process could allow scientists to recreate it in the laboratory, making blood stem cells that can give rise to all blood cells found in the body. But, this is not yet possible because we do not know all the conditions needed to make the cells and ensure they survive. One crucial gap in our understanding concerns the importance of blood flow. As the main blood vessel leaving the heart, the aorta experiences mechanical stress every time the heart beats. Lancino et al. wanted to find out whether this influences the development of the blood stem cells.

Zebrafish embryos are transparent, making it easy to see their bodies developing under a microscope. Like humans, they also produce both red blood cells and white blood cells meaning Lancino et al. could watch the birth of blood stem cells in these embryos from a part of the aorta called the aortic floor. A new software tool unwrapped pictures of the tube-shaped blood vessel into flat, two-dimensional maps, making it possible to see how the aorta changed over time. This revealed that, as blood stem cells leave the aortic floor, they bend and contract with the direction of the blood flow. Rings of actin and myosin proteins that formed around the stem cells as they are born helped the process along, while stopping the heartbeat changed the way the blood cells emerged. Without any blood flow, the actin proteins did not assemble properly; the stem cells also emerged in the wrong direction and some of them even died.

These findings show that physical forces play a role in the formation of blood stem cells. Understanding this process brings scientists a step closer to recreating the conditions for making different kinds of blood cells outside of the body.

Structural and Intermolecular Associations Between Connexin36 and Protein Components of the Adherens Junction-Neuronal Gap Junction Complex

Intimate structural and functional relationships between gap junctions and adherens junctions have been demonstrated in peripheral tissues, but have not been thoroughly examined in the central nervous system, where adherens junctions are often found in close proximity to neuronal gap junctions. Here, we used immunofluorescence approaches to document the localization of various protein components of adherens junctions in relation to those that we have previously reported to occur at electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36). The adherens junction constituents N-cadherin and nectin-1 were frequently found to localize near or overlap with Cx36-containing gap junctions in several brain regions examined. This was also true of the adherens junction-associated proteins α-catenin and β-catenin, as well as the proteins zonula occludens-1 and AF6 (aka, afadin) that were reported constituents of both adherens junctions and gap junctions. The deployment of the protein constituents of these junctions was especially striking at somatic contacts between primary afferent neurons in the mesencephalic trigeminal nucleus (MesV), where the structural components of adherens junctions appeared to be maintained in connexin36 null mice. These results support emerging views concerning the multi-molecular composition of electrical synapses and raise possibilities for various structural and functional protein-protein interactions at what now can be considered the adherens junction-neuronal gap junction complex. Further, the results point to intracellular signaling pathways that could potentially contribute to the assembly, maintenance and turnover of this complex, as well as to the dynamic nature of neuronal communication at electrical synapses.

Junctional adhesion molecule-A: functional diversity through molecular promiscuity

Cell adhesion molecules (CAMs) of the immunoglobulin superfamily (IgSF) regulate important processes such as cell proliferation, differentiation and morphogenesis. This activity is primarily due to their ability to initiate intracellular signaling cascades at cell-cell contact sites. Junctional adhesion molecule-A (JAM-A) is an IgSF-CAM with a short cytoplasmic tail that has no catalytic activity. Nevertheless, JAM-A is involved in a variety of biological processes. The functional diversity of JAM-A resides to a large part in a C-terminal PDZ domain binding motif which directly interacts with nine different PDZ domain-containing proteins. The molecular promiscuity of its PDZ domain motif allows JAM-A to recruit protein scaffolds to specific sites of cell-cell adhesion and to assemble signaling complexes at those sites. Here, we review the molecular characteristics of JAM-A, including its dimerization, its interaction with scaffolding proteins, and the phosphorylation of its cytoplasmic domain, and we describe how these characteristics translate into diverse biological activities.

NBPF7 promotes the proliferation of α-catenin-knockdown HaCaT cells via functional interaction with the NF-κB pathway

Loss of key components that form cell-cell adherens junctions, such as α-catenin, triggers severe epidermal hyperproliferation. However, the underlying molecular mechanisms remain largely unknown. We report here that neuroblastoma breakpoint family (NBPF) genes are upregulated and that NBPF7 specifically promotes cellular proliferation of α-catenin-silenced HaCaT cells through functional linkage with the NF-κB pathway. Genome-wide profiling of HaCaT cells shows that NBPF genes are upregulated following α-catenin knockdown. Data from western blot analyses are consistent with the activation of the NF-κB pathway as well as increased expression of NBPF7 by α-catenin knockdown. Co-immunoprecipitation assays indicate that NBPF7 could be detected in endogenous activated NF-κB immunoprecipitates. Immunoflurence analyses demonstrate that NBPF7 co-localizes with activated NF-κB in the nucleus after α-catenin silencing. Moreover, inhibition of NBPF7 decreases the proliferation of HaCaT cells and abolishes the enhanced proliferation associated with α-catenin knockdown in HaCaT cells. These results indicate that NBPF7 plays a key role in the α-catenin signaling pathway that regulates cell proliferation of keratinocytes. Our findings suggest that the classical NF-κB pathway plays a critical role in cellular proliferation and that NBPF7 is a functional mediator for α-catenin in the regulation of keratinocyte growth.

Cadherin-mediated cell-cell interactions in normal and cancer cells

Adherens junctions (AJs) are molecular complexes that mediate cell-cell adhesive interactions and play pivotal roles in maintenance of tissue organization in adult organisms and at various stages of development. AJs consist of cadherin adhesion receptors, providing homophilic ligation with cadherins on adjacent cells, and members of the catenin protein family: p120, β- and α-catenin. α-catenin's linkage with the actin cytoskeleton defines the linear or punctate organization of AJs in different cell types. Myosin II-dependent tension drives vinculin recruitment by α-catenin and stabilizes the linkage of the cadherin/catenin complex to F-actin. Neoplastic transformation leads to prominent changes in the organization, regulation and stability of AJs. Epithelial-mesenchymal transition (EMT) whereby epithelial cells lose stable cell-cell adhesion, and reorganize their cytoskeleton to acquire migratory activity, plays the central role in cancer cell invasion and metastasis. Recent data demonstrated that a partial EMT resulting in a hybrid epithelial/mesenchymal phenotype with retention of E-cadherin is essential for cancer cell dissemination. E-cadherin and E-cadherin-based AJs are required for collective invasion and migration, survival in circulation, and metastatic outgrowth.

The impact of aging on epithelial barriers

The epithelium has many critical roles in homeostasis, including an essential responsibility in establishing tissue barriers. In addition to the fundamental role in separating internal from external environment, epithelial barriers maintain nutrient, fluid, electrolyte and metabolic waste balance in multiple organs. While, by definition, barrier function is conserved, the structure of the epithelium varies across organs. For example, the skin barrier is a squamous layer of cells with distinct structural features, while the lung barrier is composed of a very thin single cell to minimize diffusion space. With the increased focus on age-dependent alterations in organ structure and function, there is an emerging interest in the impact of age on epithelial barriers. This review will focus on the impact of aging on the epithelial barrier of several organs, including the skin, lung, gastrointestinal tract and the kidney, at a structural and functional level.

Linking E-cadherin mechanotransduction to cell metabolism through force-mediated activation of AMPK

The response of cells to mechanical force is a major determinant of cell behaviour and is an energetically costly event. How cells derive energy to resist mechanical force is unknown. Here, we show that application of force to E-cadherin stimulates liver kinase B1 (LKB1) to activate AMP-activated protein kinase (AMPK), a master regulator of energy homeostasis. LKB1 recruits AMPK to the E-cadherin mechanotransduction complex, thereby stimulating actomyosin contractility, glucose uptake and ATP production. The increase in ATP provides energy to reinforce the adhesion complex and actin cytoskeleton so that the cell can resist physiological forces. Together, these findings reveal a paradigm for how mechanotransduction and metabolism are linked and provide a framework for understanding how diseases involving contractile and metabolic disturbances arise.

Integration of Cadherin Adhesion and Cytoskeleton at Adherens Junctions

The cadherin-catenin adhesion complex is the key component of the intercellular adherens junction (AJ) that contributes both to tissue stability and dynamic cell movements in epithelial and nonepithelial tissues. The cadherin adhesion complex bridges neighboring cells and the actin-myosin cytoskeleton, and thereby contributes to mechanical coupling between cells which drives many morphogenetic events and tissue repair. Mechanotransduction at cadherin adhesions enables cells to sense, signal, and respond to physical changes in their environment. Central to this process is the dynamic link of the complex to actin filaments (F-actin), themselves structurally dynamic and subject to tension generated by myosin II motors. We discuss in this review recent breakthroughs in understanding molecular and cellular aspects of the organization of the core cadherin-catenin complex in adherens junctions, its association to F-actin, its mechanosensitive regulation, and dynamics.

Interplay between tight junctions & adherens junctions

Cell-cell adhesions are critical for the development and maintenance of tissues. Present at sites of cell-cell contact are the adherens junctions and tight junctions. The adherens junctions mediate cell-cell adhesion via the actions of nectins and cadherins. The tight junctions regulate passage of ions and small molecules between cells and establish cell polarity. Historically, the adherens and tight junctions have been thought of as discrete complexes. However, it is now clear that a high level of interdependency exists between the two junctional complexes. The adherens junctions and tight junctions are physically linked, by the zonula occludens proteins, and linked via signaling molecules including several polarity complexes and actin cytoskeletal modifiers. This review will first describe the individual components of both the adherens and tight junctions and then discuss the coupling of the two complexes with an emphasis on the signaling links and physical interactions between the two junctional complexes.

The intestinal epithelial barrier: a therapeutic target?

A fundamental function of the intestinal epithelium is to act as a barrier that limits interactions between luminal contents such as the intestinal microbiota, the underlying immune system and the remainder of the body, while supporting vectorial transport of nutrients, water and waste products. Epithelial barrier function requires a contiguous layer of cells as well as the junctions that seal the paracellular space between epithelial cells. Compromised intestinal barrier function has been associated with a number of disease states, both intestinal and systemic. Unfortunately, most current clinical data are correlative, making it difficult to separate cause from effect in interpreting the importance of barrier loss. Some data from experimental animal models suggest that compromised epithelial integrity might have a pathogenic role in specific gastrointestinal diseases, but no FDA-approved agents that target the epithelial barrier are presently available. To develop such therapies, a deeper understanding of both disease pathogenesis and mechanisms of barrier regulation must be reached. Here, we review and discuss mechanisms of intestinal barrier loss and the role of intestinal epithelial barrier function in pathogenesis of both intestinal and systemic diseases. We conclude with a discussion of potential strategies to restore the epithelial barrier.

Regulatory mechanisms, prophylaxis and treatment of vascular leakage following severe trauma and shock

Vascular leakage, or increased vascular permeability, is a common but important pathological process for various critical diseases, including severe trauma, shock, sepsis, and multiple organ dysfunction syndrome (MODS), and has become one of the most important causes of death for intensive care units (ICU) patients. Currently, although there has been some progress in knowledge of the pathogenesis of these vascular disorders, the detailed mechanisms remain unclear, and effective prophylaxis and treatment are still lacking. In this study, we aimed to provide a review of the literature regarding the regulatory mechanisms and prophylaxis as well as the treatment of vascular leakage in critical diseases such as severe trauma and shock, which could be beneficial for the overall clinical treatment of vascular leakage disorders.

N-Butylphthalide Alleviates Blood-Brain Barrier Impairment in Rats Exposed to Carbon Monoxide

Carbon monoxide (CO) poisoning is one of the most important health concerns and may result in neuropathologic changes and neurologic sequelae. However, few studies have addressed the correlation between CO poisoning and blood–brain barrier (BBB) impairment. In this study, we investigated the effects of N-butylphthalide (NBP) on the expressions of zonula occludens-1 (ZO-1), claudin-5 and aquaporin-4 (AQP-4) proteins in a CO poisoning rat model. The results indicated that the brain water content was obviously increased, and the tight junctions between endothelial cells were disrupted, resulting in significant cerebral edema and BBB dysfunction in a rat model of CO poisoning. Meanwhile, the ultrastructure of endothelial cells and pericytes was seriously damaged, and the expressions of ZO-1 and claudin-5 were decreased at an early stage (<7 days). NBP treatment could efficiently maintain the ultrastructural and functional integrity of BBB, alleviate cerebral edema. Besides, NBP could also markedly increase the levels of both ZO-1 and claudin-5 proteins compared with those in rats exposed to CO (P < 0.05), whereas NBP had no apparent regulatory effect on AQP-4 expression. Taken together, this study highlights the importance of ZO-1 and claudin-5 proteins in maintaining BBB ultrastructure and function after CO poisoning. NBP, as a novel treatment approach, may effectively inhibit the down-regulation of ZO-1 and claudin-5 proteins (but not AQP-4), thereby preserving the barrier function and reducing cerebral edema after CO poisoning.

ZO-1 interactions with F-actin and occludin direct epithelial polarization and single lumen specification in 3D culture

Epithelia within tubular organs form and expand lumens. Failure of these processes can result in serious developmental anomalies. Although tight junction assembly is crucial to epithelial polarization, the contribution of specific tight junction proteins to lumenogenesis is undefined. Here, we show that ZO-1 (also known as TJP1) is necessary for the formation of single lumens. Epithelia lacking this tight junction scaffolding protein form cysts with multiple lumens and are defective in the earliest phases of polarization, both in two and three dimensions. Expression of ZO-1 domain-deletion mutants demonstrated that the actin-binding region and U5-GuK domain are crucial to single lumen development. For actin-binding region, but not U5-GuK domain, mutants, this could be overcome by strong polarization cues from the extracellular matrix. Analysis of the U5-GuK binding partners shroom2, α-catenin and occludin showed that only occludin deletion led to multi-lumen cysts. Like ZO-1-deficiency, occludin deletion led to mitotic spindle orientation defects. Single lumen formation required the occludin OCEL domain, which binds to ZO-1. We conclude that ZO-1-occludin interactions regulate multiple phases of epithelial polarization by providing cell-intrinsic signals that are required for single lumen formation.

Tight junctions: from simple barriers to multifunctional molecular gates

Epithelia and endothelia separate different tissue compartments and protect multicellular organisms from the outside world. This requires the formation of tight junctions, selective gates that control paracellular diffusion of ions and solutes. Tight junctions also form the border between the apical and basolateral plasma-membrane domains and are linked to the machinery that controls apicobasal polarization. Additionally, signalling networks that guide diverse cell behaviours and functions are connected to tight junctions, transmitting information to and from the cytoskeleton, nucleus and different cell adhesion complexes. Recent advances have broadened our understanding of the molecular architecture and cellular functions of tight junctions.

Synaptopodin couples epithelial contractility to α-actinin-4-dependent junction maturation

A novel tension-sensitive junctional protein, synaptopodin, can relay biophysical input from cellular actomyosin contractility to induce biochemical changes at cell–cell contacts, resulting in structural reorganization of the junctional complex and epithelial barrier maturation.

The epithelial junction experiences mechanical force exerted by endogenous actomyosin activities and from interactions with neighboring cells. We hypothesize that tension generated at cell–cell adhesive contacts contributes to the maturation and assembly of the junctional complex. To test our hypothesis, we used a hydraulic apparatus that can apply mechanical force to intercellular junction in a confluent monolayer of cells. We found that mechanical force induces α-actinin-4 and actin accumulation at the cell junction in a time- and tension-dependent manner during junction development. Intercellular tension also induces α-actinin-4–dependent recruitment of vinculin to the cell junction. In addition, we have identified a tension-sensitive upstream regulator of α-actinin-4 as synaptopodin. Synaptopodin forms a complex containing α-actinin-4 and β-catenin and interacts with myosin II, indicating that it can physically link adhesion molecules to the cellular contractile apparatus. Synaptopodin depletion prevents junctional accumulation of α-actinin-4, vinculin, and actin. Knockdown of synaptopodin and α-actinin-4 decreases the strength of cell–cell adhesion, reduces the monolayer permeability barrier, and compromises cellular contractility. Our findings underscore the complexity of junction development and implicate a control process via tension-induced sequential incorporation of junctional components.

TLR response pathways in NuLi-1 cells and primary human nasal epithelial cells

The present study describes and compares functional properties of Nuli-1 cells and primary human nasal epithelial cells (HNEC) including TLR expression and function. Differences in gene expression were identified for non-TLR genes that play a role in TLR response pathways. However, experiments comparing TLR gene expression for both Nuli-1 cells and HNECs indicated conserved expression in both cell types. Stimulation of the two cell types resulted in a conserved response to TLR3 agonists, but in differences in response to agonists for TLR5 and TLR6/2. HNECs were much more susceptible to infection with Staphylococcus aureus than NuLi-1 cells. Furthermore, when cultured at air-liquid interface (ALI), NuLi-1 cells possessed much lower trans-epithelial resistance than primary HNEC and did not exhibit maintenance of cell morphology or mucous production which was observed in HNECs. Nor did they produce the characteristic interconnecting pattern of tight junction complexes at the apicolateral margin of adjacent cells. Caution should therefore be exercised when selecting cell lines for immunological studies and a thorough screen of properties relevant to the study should always be carried out prior to commencement.

Epithelial junctions and Rho family GTPases: the zonular signalosome

The establishment and maintenance of epithelial cell-cell junctions is crucially important to regulate adhesion, apico-basal polarity and motility of epithelial cells, and ultimately controls the architecture and physiology of epithelial organs. Junctions are supported, shaped and regulated by cytoskeletal filaments, whose dynamic organization and contractility are finely tuned by GTPases of the Rho family, primarily RhoA, Rac1 and Cdc42. Recent research has identified new molecular mechanisms underlying the cross-talk between these GTPases and epithelial junctions. Here we briefly summarize the current knowledge about the organization, molecular evolution and cytoskeletal anchoring of cell-cell junctions, and we comment on the most recent advances in the characterization of the interactions between Rho GTPases and junctional proteins, and their consequences with regards to junction assembly and regulation of cell behavior in vertebrate model systems. The concept of “zonular signalosome” is proposed, which highlights the close functional relationship between proteins of zonular junctions (zonulae occludentes and adhaerentes) and the control of cytoskeletal organization and signaling through Rho GTPases, transcription factors, and their effectors.

α-Catenin-mediated cadherin clustering couples cadherin and actin dynamics

The function of the actin-binding domain of α-catenin, αABD, including its possible role in the direct anchorage of the cadherin-catenin complex to the actin cytoskeleton, has remained uncertain. We identified two point mutations on the αABD surface that interfere with αABD binding to actin and used them to probe the role of α-catenin-actin interactions in adherens junctions. We found that the junctions directly bound to actin via αABD were more dynamic than the junctions bound to actin indirectly through vinculin and that recombinant αABD interacted with cortical actin but not with actin bundles. This interaction resulted in the formation of numerous short-lived cortex-bound αABD clusters. Our data suggest that αABD clustering drives the continuous assembly of transient, actin-associated cadherin-catenin clusters whose disassembly is maintained by actin depolymerization. It appears then that such actin-dependent αABD clustering is a unique molecular mechanism mediating both integrity and reassembly of the cell-cell adhesive interface formed through weak cis- and trans-intercadherin interactions.

Endothelial cell-cell adhesion during zebrafish vascular development

The vertebrate vasculature is an essential organ network with major roles in health and disease. The establishment of balanced cell-cell adhesion in the endothelium is crucial for the functionality of the vascular system. Furthermore, the correct patterning and integration of vascular endothelial cell-cell adhesion drives the morphogenesis of new vessels, and is thought to couple physical forces with signaling outcomes during development. Here, we review insights into this process that have come from studies in zebrafish. First, we describe mutants in which endothelial adhesion is perturbed, second we describe recent progress using in vivo cell biological approaches that allow the visualization of endothelial cell-cell junctions. These studies underline the profound potential of this model system to dissect in great detail the function of both known and novel regulators of endothelial cell-cell adhesion.

Hydrocortisone enhances the barrier properties of HBMEC/ciβ, a brain microvascular endothelial cell line, through mesenchymal-to-endothelial transition-like effects

BACKGROUND

Because in vitro blood-brain barrier (BBB) models are important tools for studying brain diseases and drug development, we recently established a new line of conditionally immortalized human brain microvascular endothelial cells (HBMEC/ciβ) for use in such models. Since one of the most important functional features of the BBB is its strong intercellular adhesion, in this study, we aimed at improving HBMEC/ciβ barrier properties by means of culture media modifications, thus enhancing their use for future BBB studies. In addition, we simultaneously attempted to obtain insights on related mechanistic properties.

METHODS

Several types of culture media were prepared in an effort to identify the medium most suitable for culturing HBMEC/ciβ. The barrier properties of HBMEC/ciβ were examined by determining Na(+)-fluorescein permeability and transendothelial electric resistance (TEER). Endothelial marker mRNA expression levels were determined by quantitative real-time polymerase chain reaction. Adherens junction (AJ) formation was examined by immunocytochemistry. Cell migration ability was analyzed by scratch assay. Furthermore, cellular lipid composition was examined by liquid chromatography-time-of-flight mass spectrometry.

RESULTS

Our initial screening tests showed that addition of hydrocortisone (HC) to the basal medium significantly reduced the Na(+)-fluorescein permeability and increased the TEER of HBMEC/ciβ monolayers. It was also found that, while AJ proteins were diffused in the cytoplasm of HBMEC/ciβ cultured without HC, those expressed in cells cultured with HC were primarily localized at the cell border. Furthermore, this facilitation of AJ formation by HC was in concert with increased endothelial marker mRNA levels and increased ether-type phosphatidylethanolamine levels, while cell migration was retarded in the presence of HC.

CONCLUSIONS

Our results show that HC supplementation to the basal medium significantly enhances the barrier properties of HBMEC/ciβ. This was associated with a marked phenotypic alteration in HBMEC/ciβ through orchestration of various signaling pathways. Taken together, it appears that overall effects of HC on HBMEC/ciβ could be summarized as facilitating endothelial differentiation characteristics while concurrently retarding mesenchymal characteristics.

Alterations of the apical junctional complex and actin cytoskeleton and their role in colorectal cancer progression

Colorectal cancer represents the fourth highest mortality rate among cancer types worldwide. An understanding of the molecular mechanisms that regulate their progression can prevents or reduces mortality due to this disease. Epithelial cells present an apical junctional complex connected to the actin cytoskeleton, which maintains the dynamic properties of this complex, tissue architecture and cell homeostasis. Several studies have indicated that apical junctional complex alterations and actin cytoskeleton disorganization play a critical role in epithelial cancer progression. However, few studies have examined the existence of an interrelation between these 2 components, particularly in colorectal cancer. This review discusses the recent progress toward elucidating the role of alterations of apical junctional complex constituents and of modifications of actin cytoskeleton organization and discusses how these events are interlinked to modulate cellular responses related to colorectal cancer progression toward successful metastasis.

Mutation of EpCAM leads to intestinal barrier and ion transport dysfunction

Congenital tufting enteropathy (CTE) is a devastating diarrheal disease seen in infancy that is typically associated with villous changes and the appearance of epithelial tufts. We previously found mutations in epithelial cell adhesion molecule (EpCAM) to be causative in CTE. We developed a knock-down cell model of CTE through transfection of an EpCAM shRNA construct into T84 colonic epithelial cells to elucidate the in vitro role of EpCAM in barrier function and ion transport. Cells with EpCAM deficiency exhibited decreased electrical resistance, increased permeability, and decreased ion transport. Based on mutations in CTE patients, an in vivo mouse model was developed, with tamoxifen-inducible deletion of exon 4 in Epcam resulting in mutant protein with decreased expression. Tamoxifen treatment of Epcam (Δ4/Δ4) mice resulted in pathological features of villous atrophy and epithelial tufts, similar to those in human CTE patients, within 4 days post induction. Epcam (Δ4/Δ4) mice also showed decreased expression of tight junctional proteins, increased permeability, and decreased ion transport in the intestines. Taken together, these findings reveal mechanisms that may underlie disease in CTE.

KEY MESSAGES

Knock-down EpCAM cell model of congenital tufting enteropathy was developed. In vivo inducible mouse model was developed resulting in mutant EpCAM protein. Cells with EpCAM deficiency demonstrated barrier and ion transport dysfunction. Tamoxifen-treated Epcam (Δ4/Δ4) mice demonstrated pathological features. Epcam (Δ4/Δ4) mice showed improper barrier function and ion transport.