Differential roles for actin polymerization and a myosin II motor in assembly of the epithelial apical junctional complex

Vertex models capturing subcellular scales in epithelial tissues

Vertex models provide a robust theoretical framework for studying epithelial tissues as a network of cell boundaries. They have been pivotal in exploring properties such as cell packing geometry and rigidity transitions. Recently, extended vertex models have become instrumental in bridging the subcellular scales to the tissue scale. Here, we review extensions of the model aiming to capture experimentally observed subcellular features of epithelial tissues including heterogeneity in myosin activity across the tissue, non-uniform contractility structures, and mechanosensitive feedback loops. We discuss how these extensions change and challenge current perspectives on observables of macroscopic tissue properties. First, we find that extensions to the vertex model can change model properties significantly, impacting the critical threshold and in some cases even the existence of a rigidity transition. Second, we find that packing disorder can be explained by models employing different subcellular mechanisms, indicating a source of stochasticity and gradual local size changes as common mesoscopic motifs in the mechanics of tissue organization. We address complementary models and statistical inference, putting vertex models in a broader methodological context and we give a brief overview of software packages utilized in increasingly complex vertex model studies. Our review emphasizes the need for more comparative, systematic studies that identify specific classes of vertex models which share a set of well-defined properties, as well as a more in-depth discussion of modeling choices and their biological motivations.

Inflammatory bowel disease risk gene C1ORF106 regulates actin dynamics in intestinal epithelial cells

Background and aims

C1ORF106 has previously been associated with inflammatory bowel diseases (IBD) via large-scale genetic studies. Increased intestinal permeability is a hallmark of IBD and is observed in at-risk individuals prior to the appearance of clinical symptoms. C1ORF106 was previously shown to regulate intestinal barrier permeability through the regulation of adherens junction stability and through the formation of tight junctions, which impacted actin assembly. However, the downstream impact and molecular mechanisms involved in actin regulation by C1ORF106 haven't been explored. Our study aimed at identifying which pathways involved in intestinal epithelial barrier regulation and F-actin regulation are impacted by C1ORF106 and its IBD-associated variant.

Methods

We knocked down (KD) the expression of C1ORF106 in human colonic epithelial cells and characterized the function of the 333F variant in intestinal epithelial spheroid cultures obtained from patient-derived human induced pluripotent stem cell (hiPSC). We measured barrier permeability and characterized spheroid formation, actin regulation and cell migration though immunofluorescence, western blots and permeability assays.

Results

C1ORF106 KD leads to impaired cortical actin belt dynamics and regulation of stress fiber formation, resulting in increased cell constriction, impaired barrier permeability, cell polarity and cell migration. Moreover, we demonstrated that an inhibition of ROCK rescues the actin belt and cell polarity phenotypes in C1ORF106 KD cells, demonstrating that C1ORF106 regulates these phenotypes through a ROCK-dependent mechanism. We also observed an altered nmMYO2-P localization in C1ORF106 KD cells associated with the formation of Vacuolar Apical Compartments (VACs), which are important for 3D epithelial spheroid formation. We observed a similar impact on cell polarity in intestinal epithelial spheroids obtained from hiPSC carrying the 333F variant, providing additional support that this pathway is involved in disease development.

Conclusion

We provide insights into the molecular mechanisms by which C1ORF106 controls actin dynamics to regulate intestinal epithelial integrity.

Myosin Light Chain 12b and MASP1 as Novel Biomarker Candidates in Active Juvenile Idiopathic Arthritis─A Combined Proteomics/Bioinformatics Approach

Current diagnostic methods for JIA lack specificity, often failing to distinguish it from other childhood diseases of similar clinical presentations. The present study is a comparative cross-sectional study that identified potential biomarkers using label-free mass spectrometry and bioinformatics. Two differentially expressed proteins (DEPs), Myosin light chain 12b (Myl12b) and Mannose-binding lectin serine protease 1 (MASP1), showed increased expression in JIA patients. Receiver operating characteristic (ROC) analysis revealed strong predictive power (AUC: Myl12b = 0.757, MASP1 = 0.855), validated in a separate cohort via Western blot and ELISA. These findings suggest Myl12b and MASP1 as promising biomarkers for JIA diagnosis and treatment. Data: ProteomeXchange (PXD058863).

Actin-dependent α-catenin oligomerization contributes to adherens junction assembly

Classic cadherins, specifically E-cadherin in most epithelial cells, are transmembrane adhesion receptors, whose intracellular region interacts with proteins, termed catenins, forming the cadherin-catenin complex (CCC). The cadherin ectodomain generates 2D adhesive clusters (E-clusters) through cooperative trans and cis interactions, while catenins anchor the E-clusters to the actin cytoskeleton. How these two types of interactions are coordinated in the formation of specialized cell-cell adhesions, adherens junctions (AJ), remains unclear. Here, we focus on the role of the actin-binding domain of α-catenin (αABD) by showing that the interaction of the αABD with actin generates actin-bound linear CCC oligomers (CCC/actin strands) incorporating up to six CCCs. This actin-driven CCC oligomerization, which is cadherin ectodomain independent, preferentially occurs along the actin cortex enriched with key basolateral proteins, myosin-1c, scribble, and DLG1. In cell-cell contacts, the CCC/actin strands integrate with the E-clusters giving rise to the composite oligomers, E/actin clusters. Targeted inactivation of strand formation by point mutations emphasizes the importance of this oligomerization process for blocking intercellular protrusive membrane activity and for coupling AJs with the actomyosin-derived tensional forces.

Y-27632 enables long-term expansion of mouse submandibular gland epithelial cells via inactivation of TGF-β1/CTGF/p38 and ROCK2/JNK signaling pathway

OBJECTIVES

This study aimed to investigate the effects of Y-27632 on the long-term maintainence of mouse submandibular epithelial cells (SG-Epis) in vitro and to elucidate the underlying mechanisms.

METHODS

The role of the Rho-associated kinase (ROCK) inhibitor Y-27632 in maintaining SG-Epis and its underlying mechanisms were evaluated by examining the in vitro expansion of mouse SG-Epis. Changes in key cellular characteristics, such as proliferation, long-term expansion, and mRNA and protein expression, were assessed in the presence or absence of Y-27632.

RESULTS

Treatment with Y-27632 significantly enhanced the proliferative potential of SG-Epis, preserving Krt8 and Krt14 expression over 17 passages. In the absence of Y-27632, SG-Epis lost their epithelial morphology. However, Y-27632 treatment maintained the epithelial morphology and downregulated mRNA levels of Tgf-β1, Ctgf, and Rock2. Treatment with TGF-β1 indicated that TGF-β/CTGF/p38 signaling is responsible for the maintenance of SG-Epis, while RNA interference studies revealed that ROCK2/c-Jun N-terminal kinase (JNK) signaling is also crucial for SG-Epis proliferation and maintenance.

CONCLUSIONS

The TGF-β1/CTGF/p38 and ROCK2/JNK signaling pathways are responsible for SG-Epis proliferation, and Y-27632 treatment effectively inactivates these pathways, enabling long-term in vitro maintenance of SG-Epis. The culture method utilizing Y-27632 provides an effective approach for the in vitro expansion of SG-Epis.

Dragon's blood attenuates LPS-induced intestinal epithelial barrier dysfunction via upregulation of FAK-DOCK180-Rac1-WAVE2-Arp3 and downregulation of TLR4/NF-κB signaling pathways

Inflammation-induced intestinal epithelial barrier (IEB) dysfunction is one of the important reasons for the occurrence and development of intestinal inflammatory-related diseases, including ulcerative colitis (UC), Crohn's disease and necrotizing enterocolitis (NEC). Dragon's blood (DB) is a traditional Chinese medicine and has been clinically used to treat UC. However, the protective mechanism of DB on intestinal inflammatory-related diseases has still not been elucidated. The present study aimed to explore the protection mechanism of DB on IEB dysfunction in rat ileum and human colorectal adenocarcinoma cells (Caco-2)/human umbilical vein endothelial cells (HUVECs) coculture system induced by lipopolysaccharide (LPS). DB could ameliorate rat ileum mucosa morphological injury, reduce the accumulation of lipid-peroxidation products and increase the expression of junction proteins. DB also alleviated LPS-induced Caco-2 cells barrier integrity destruction in Caco-2/ HUVECs coculture system, leading to increased trans-endothelial electrical resistance (TEER), reduced cell permeability, and upregulation of expressions of F-actin and junction proteins. DB contributed to the assembly of actin cytoskeleton by upregulating the FAK-DOCK180-Rac1-WAVE2-Arp3 pathway and contributed to the formation of intercellular junctions by downregulating TLR4-MyD88-NF-κB pathway, thus reversing LPS-induced IEB dysfunction. These novel findings illustrated the potential protective mechanism of DB on intestinal inflammatory-related diseases and might be useful for further clinical application of DB.

Coactosin-like protein 1 regulates integrity and repair of model intestinal epithelial barriers via actin binding dependent and independent mechanisms

The actin cytoskeleton regulates the integrity and repair of epithelial barriers by mediating the assembly of tight junctions (TJs), and adherens junctions (AJs), and driving epithelial wound healing. Actin filaments undergo a constant turnover guided by numerous actin-binding proteins, however, the roles of actin filament dynamics in regulating intestinal epithelial barrier integrity and repair remain poorly understood. Coactosin-like protein 1 (COTL1) is a member of the ADF/cofilin homology domain protein superfamily that binds and stabilizes actin filaments. COTL1 is essential for neuronal and cancer cell migration, however, its functions in epithelia remain unknown. The goal of this study is to investigate the roles of COTL1 in regulating the structure, permeability, and repair of the epithelial barrier in human intestinal epithelial cells (IEC). COTL1 was found to be enriched at apical junctions in polarized IEC monolayers in vitro. The knockdown of COTL1 in IEC significantly increased paracellular permeability, impaired the steady state TJ and AJ integrity, and attenuated junctional reassembly in a calcium-switch model. Consistently, downregulation of COTL1 expression in Drosophila melanogaster increased gut permeability. Loss of COTL1 attenuated collective IEC migration and decreased cell-matrix attachment. The observed junctional abnormalities in COTL1-depleted IEC were accompanied by the impaired assembly of the cortical actomyosin cytoskeleton. Overexpression of either wild-type COTL1 or its actin-binding deficient mutant tightened the paracellular barrier and activated junction-associated myosin II. Furthermore, the actin-uncoupled COTL1 mutant inhibited epithelial migration and matrix attachment. These findings highlight COTL1 as a novel regulator of the intestinal epithelial barrier integrity and repair.

Buckling forces and the wavy folds between pleural epithelial cells

Cell shapes in tissues are affected by the biophysical interaction between cells. Tissue forces can influence specific cell features such as cell geometry and cell surface area. Here, we examined the 2-dimensional shape, size, and perimeter of pleural epithelial cells at various lung volumes. We demonstrated a 1.53-fold increase in 2-dimensional cell surface area and a 1.43-fold increase in cell perimeter at total lung capacity compared to residual lung volume. Consistent with previous results, close inspection of the pleura demonstrated wavy folds between pleural epithelial cells at all lung volumes. To investigate a potential explanation for the wavy folds, we developed a physical simulacrum suggested by D'Arcy Thompson in On Growth and Form. The simulacrum suggested that the wavy folds were the result of redundant cell membranes unable to contract. To test this hypothesis, we developed a numerical simulation to evaluate the impact of an increase in 2-dimensional cell surface area and cell perimeter on the shape of the cell-cell interface. Our simulation demonstrated that an increase in cell perimeter, rather than an increase in 2-dimensional cell surface area, had the most direct impact on the presence of wavy folds. We conclude that wavy folds between pleural epithelial cells reflects buckling forces arising from the excess cell perimeter necessary to accommodate visceral organ expansion.

Characterization of early and late events of adherens junction assembly

Cadherins are transmembrane adhesion receptors. Cadherin ectodomains form adhesive 2D clusters through cooperative trans and cis interactions, whereas its intracellular region interacts with specific cytosolic proteins, termed catenins, to anchor the cadherin-catenin complex (CCC) to the actin cytoskeleton. How these two types of interactions are coordinated in the formation of specialized cell-cell adhesions, adherens junctions (AJ), remains unclear. We focus here on the role of the actin-binding domain of α-catenin (αABD) by showing that the interaction of αABD with actin generates actin-bound CCC oligomers (CCC/actin strands) incorporating up to six CCCs. The strands are primarily formed on the actin-rich cell protrusions. Once in cell-cell interface, the strands become involved in cadherin ectodomain clustering. Such combination of the extracellular and intracellular oligomerizations gives rise to the composite oligomers, trans CCC/actin clusters. To mature, these clusters then rearrange their actin filaments using several redundant pathways, two of which are characterized here: one depends on the α-catenin-associated protein, vinculin and the second one depends on the unstructured C-terminus of αABD. Thus, AJ assembly proceeds through spontaneous formation of trans CCC/actin clusters and their successive reorganization.

PALS1 is a key regulator of the lateral distribution of tight junction proteins in renal epithelial cells

The evolutionarily conserved apical Crumbs (CRB) complex, consisting of the core components CRB3a (an isoform of CRB3), PALS1 and PATJ, plays a key role in epithelial cell-cell contact formation and cell polarization. Recently, we observed that deletion of one Pals1 allele in mice results in functional haploinsufficiency characterized by renal cysts. Here, to address the role of PALS1 at the cellular level, we generated CRISPR/Cas9-mediated PALS1-knockout MDCKII cell lines. The loss of PALS1 resulted in increased paracellular permeability, indicating an epithelial barrier defect. This defect was associated with a redistribution of several tight junction-associated proteins from bicellular to tricellular contacts. PALS1-dependent localization of tight junction proteins at bicellular junctions required its interaction with PATJ. Importantly, reestablishment of the tight junction belt upon transient F-actin depolymerization or upon Ca2+ removal was strongly delayed in PALS1-deficient cells. Additionally, the cytoskeleton regulator RhoA was redistributed from junctions into the cytosol under PALS1 knockout. Together, our data uncover a critical role of PALS1 in the coupling of tight junction proteins to the F-actin cytoskeleton, which ensures their correct distribution along bicellular junctions and the formation of tight epithelial barrier.

Adherens junction: the ensemble of specialized cadherin clusters

The cell-cell connections in adherens junctions (AJs) are mediated by transmembrane receptors, type I cadherins (referred to here as cadherins). These cadherin-based connections (or trans bonds) are weak. To upregulate their strength, cadherins exploit avidity, the increased affinity of binding between cadherin clusters compared with isolated monomers. Formation of such clusters is a unique molecular process that is driven by a synergy of direct and indirect cis interactions between cadherins located at the same cell. In addition to their role in adhesion, cadherin clusters provide structural scaffolds for cytosolic proteins, which implicate cadherin into different cellular activities and signaling pathways. The cluster lifetime, which depends on the actin cytoskeleton, and on the mechanical forces it generates, determines the strength of AJs and their plasticity. The key aspects of cadherin adhesion, therefore, cannot be understood at the level of isolated cadherin molecules, but should be discussed in the context of cadherin clusters.

Effect and Mechanism of Theaflavins on Fluoride Transport and Absorption in Caco-2 Cells

This paper investigated the effect and mechanism of theaflavins (TFs) on fluoride (F−) uptake and transport in the Caco-2 cell model through structural chemistry and transcriptome analysis. The results showed that the four major TFs (TF, TF3G, TF3′G and TFDG) at a 150 μg/mL concentration could all significantly decrease F− transport in Caco-2 cells after 2 h of treatment and, at 2 μg/mL F− concentration, the F− transport was more inclined to efflux. During transport, the F− retention in Caco-2 cells was significantly increased by TF3G while it was clearly decreased by TF. The interaction between TFs and F− was analyzed by Raman spectroscopy and isothermal titration calorimetry, and F− was shown to affect the π bond vibration on the benzene ring of TFs, thus influencing their stability. Additionally, F− showed weak binding to TF3G, TF3′G and TFDG, which may inhibit F− transport and absorption in the Caco-2 cell line. Transcriptome and RT-PCR analysis identified three key differentially expressed genes related to cell permeability, and TFs can be assumed to mediate F− transport by regulating the expression of permeability-related genes to change cell monolayer permeability and enhance cell barrier function; however, this needs to be further elucidated in future studies.

Acute low-dose phosphate disrupts glycerophospholipid metabolism and induces stress in juvenile turbot (Scophthalmus maximus)

Phosphate, as the main nutrient factor of lake eutrophication brought by pollutants discharged from agriculture and industry, is always considered to be a low-toxicity substance to aquatic animals. But the toxicity mechanism is unclear, and published information is limited. In this study, a 96 h acute stress experiment was conducted on juvenile turbot (Scophthalmus maximus) with 0, 10, and 60 mg/L phosphate solutions. Metabonomic analysis revealed that low-dose phosphate (10 mg/L) disrupted glycerophospholipid, purine, and glycolipid metabolism, as well as the tricarboxylic acid (TCA) cycle in juveniles, even at 96 h of stress, which may lead to cell structure damage and signal recognition disorder between cells. Upregulated key genes in the main glycerophospholipid metabolic pathways, which matched the results of the metabolomic study, were detected. Furthermore, low-dose phosphate (10 mg/L) induced oxidative stress and immunotoxicity in fish, resulting in the raising of relevant genes expression such as cat and sod in liver and kidney. In addition, all phosphate-treated groups had induced lesions on gill tissue, as evidenced by pathological observations. In this study on toxic effects on and mechanism of phosphate in aquatic animals using metabolomics, gene expression, and histopathology, we confirm that acute low-dose phosphate could disrupt glycerophospholipid metabolism and induce stress in juvenile turbot. This can provide advice on the amount of phosphate accumulation for marine fish farming and on protecting species diversity and marine ecosystem from the point of view of phosphate toxicity to marine animals.

Understanding disruption of the gut barrier during inflammation: Should we abandon traditional epithelial cell lines and switch to intestinal organoids?

Disruption of the intestinal epithelial barrier is a hallmark of mucosal inflammation. It increases exposure of the immune system to luminal microbes, triggering a perpetuating inflammatory response. For several decades, the inflammatory stimuli-induced breakdown of the human gut barrier was studied in vitro by using colon cancer derived epithelial cell lines. While providing a wealth of important data, these cell lines do not completely mimic the morphology and function of normal human intestinal epithelial cells (IEC) due to cancer-related chromosomal abnormalities and oncogenic mutations. The development of human intestinal organoids provided a physiologically-relevant experimental platform to study homeostatic regulation and disease-dependent dysfunctions of the intestinal epithelial barrier. There is need to align and integrate the emerging data obtained with intestinal organoids and classical studies that utilized colon cancer cell lines. This review discusses the utilization of human intestinal organoids to dissect the roles and mechanisms of gut barrier disruption during mucosal inflammation. We summarize available data generated with two major types of organoids derived from either intestinal crypts or induced pluripotent stem cells and compare them to the results of earlier studies with conventional cell lines. We identify research areas where the complementary use of colon cancer-derived cell lines and organoids advance our understanding of epithelial barrier dysfunctions in the inflamed gut and identify unique questions that could be addressed only by using the intestinal organoid platforms.

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

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

Respiratory syncytial virus disrupts the airway epithelial barrier by decreasing cortactin and destabilizing F-actin

Respiratory syncytial virus (RSV) infection is the leading cause of acute lower respiratory tract infection in young children worldwide. Our group recently revealed that RSV infection disrupts the airway epithelial barrier in vitro and in vivo. However, the underlying molecular pathways were still elusive. Here, we report the critical roles of the filamentous actin (F-actin) network and actin-binding protein cortactin in RSV infection. We found that RSV infection causes F-actin depolymerization in 16HBE cells, and that stabilizing the F-actin network in infected cells reverses the epithelial barrier disruption. RSV infection also leads to significantly decreased cortactin in vitro and in vivo. Cortactin-knockout 16HBE cells presented barrier dysfunction, whereas overexpression of cortactin protected the epithelial barrier against RSV. The activity of Rap1 (which has Rap1A and Rap1B forms), one downstream target of cortactin, declined after RSV infection as well as in cortactin-knockout cells. Moreover, activating Rap1 attenuated RSV-induced epithelial barrier disruption. Our study proposes a key mechanism in which RSV disrupts the airway epithelial barrier via attenuating cortactin expression and destabilizing the F-actin network. The identified pathways will provide new targets for therapeutic intervention toward RSV-related disease. This article has an associated First Person interview with the first author of the paper.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

A myosin chaperone, UNC-45A, is a novel regulator of intestinal epithelial barrier integrity and repair

The actomyosin cytoskeleton serves as a key regulator of the integrity and remodeling of epithelial barriers by controlling assembly and functions of intercellular junctions and cell-matrix adhesions. Although biochemical mechanisms that regulate the activity of non-muscle myosin II (NM-II) in epithelial cells have been extensively investigated, little is known about assembly of the contractile myosin structures at the epithelial adhesion sites. UNC-45A is a cytoskeletal chaperone that is essential for proper folding of NM-II heavy chains and myofilament assembly. We found abundant expression of UNC-45A in human intestinal epithelial cell (IEC) lines and in the epithelial layer of the normal human colon. Interestingly, protein level of UNC-45A was decreased in colonic epithelium of patients with ulcerative colitis. CRISPR/Cas9-mediated knock-out of UNC-45A in HT-29cf8 and SK-CO15 IEC disrupted epithelial barrier integrity, impaired assembly of epithelial adherence and tight junctions and attenuated cell migration. Consistently, decreased UNC-45 expression increased permeability of the Drosophila gut in vivo. The mechanisms underlying barrier disruptive and anti-migratory effects of UNC-45A depletion involved disorganization of the actomyosin bundles at epithelial junctions and the migrating cell edge. Loss of UNC-45A also decreased contractile forces at apical junctions and matrix adhesions. Expression of deletion mutants revealed roles for the myosin binding domain of UNC-45A in controlling IEC junctions and motility. Our findings uncover a novel mechanism that regulates integrity and restitution of the intestinal epithelial barrier, which may be impaired during mucosal inflammation.

Dimensionality-Dependent Mechanical Stretch Regulation of Cell Behavior

A variety of cells are subject to mechanical stretch in vivo, which plays a critical role in the function and homeostasis of cells, tissues, and organs. Deviations from the physiologically relevant mechanical stretch are often associated with organ dysfunction and various diseases. Although mechanical stretch is provided in some in vitro cell culture models, the effects of stretch dimensionality on cells are often overlooked and it remains unclear whether and how stretch dimensionality affects cell behavior. Here we develop cell culture platforms that provide 1-D uniaxial, 2-D circumferential, or 3-D radial mechanical stretches, which recapitulate the three major types of mechanical stretches that cells experience in vivo. We investigate the behavior of human microvascular endothelial cells and human alveolar epithelial cells cultured on these platforms, showing that the mechanical stretch influences cell morphology and cell-cell and cell-substrate interactions in a stretch dimensionality-dependent manner. Furthermore, the endothelial and epithelial cells are sensitive to the physiologically relevant 2-D and 3-D stretches, respectively, which could promote the formation of endothelium and epithelium. This study underscores the importance of recreating the physiologically relevant mechanical stretch in the development of in vitro tissue/organ models.

Differential responses of abomasal transcriptome to Haemonchus contortus infection between Haemonchus-selected and Trichostrongylus-selected merino sheep

Haemonchus contortus is the most prevalent and pathogenic gastrointestinal nematode infecting sheep and goats. The two CSIRO sheep resource flocks, the Haemonchus-selected flock (HSF) and Trichostrongylus-selected flock (TSF) were developed for research on host resistance or susceptibility to gastrointestinal nematode infection. A recent study focused on the gene expression differences between resistant and susceptible sheep within each flock, with lymphatic and gastrointestinal tissues. To identify features in the host transcriptome and understand the molecular differences underlying host resistance to H. contortus between flocks with different selective breeding and genetic backgrounds, we compared the abomasal transcriptomic responses of the resistant or susceptible animals between HSF and TSF flocks. A total of 11 and 903 differentially expressed genes were identified in the innate infection treatment in HSF and TSF flocks between resistant and susceptible sheep respectively, while 52 and 485 genes were identified to be differentially expressed in the acquired infection treatment, respectively. Among them, 294 genes had significantly different gene expression levels between HSF and TSF flock animals within the susceptible sheep by both the innate and acquired infections. Moreover, similar expression patterns of the 294 genes were observed, with 273 genes more highly expressed in HSF and 21 more highly expressed in the TSF within the abomasal transcriptome of the susceptible animals. Gene ontology enrichment of the differentially expressed genes identified in this study predicted the likely differing function between the two flock's susceptible lines in response to H. contortus infection. Nineteen pathways were significantly enriched in both the innate and adaptive immune responses in susceptible animals, which indicated that these pathways likely contribute to the host resistance development to H. contortus infection in susceptible sheep. Biological networks built for the set of genes differentially abundant in susceptible animals identified hub genes of PRKG1, PRKACB, PRKACA, and ITGB1 for the innate immune response, and CALM2, MYL1, COL1A1, ITGB1 and ITGB3 for the adaptive immune response, respectively. Our results offered a quantitative snapshot of host transcriptomic changes induced by H. contortus infection between flocks with different selective breeding and genetic backgrounds and provided novel insights into molecular mechanisms of host resistance.

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

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

Dual role of E-cadherin in cancer cells

E-cadherin is the main component of epithelial adherens junctions (AJs), which play a crucial role in the maintenance of stable cell-cell adhesion and overall tissue integrity. Down-regulation of E-cadherin expression has been found in many carcinomas, and loss of E-cadherin is generally associated with poor prognosis in patients. During the last decade, however, numerous studies have shown that E-cadherin is essential for several aspects of cancer cell biology that contribute to cancer progression, most importantly, active cell migration. In this review, we summarize the available data about the input of E-cadherin in cancer progression, focusing on the latest advances in the research of the various roles E-cadherin-based AJs play in cancer cell dissemination. The review also touches upon the "cadherin switching" in cancer cells where N- or P-cadherin replace or are co-expressed with E-cadherin and its influence on the migratory properties of cancer cells.

Association of the myosin heavy chain 9 gene single nucleotide polymorphism with inflammatory bowel disease

Background

To date, the cause of inflammatory bowel disease (IBD) remains a mystery. A balance between cell proliferation and apoptosis maintains intestinal tissue homeostasis. Dissociation-induced myosin-actin contraction results in stem cell apoptosis. This study aiming to evaluate the influence of the myosin heavy chain 9 (MYH9) gene single nucleotide polymorphisms (SNPs) on inflammatory bowel disease.

Subjects

and methods: The study carried on eighty patients with IBD and seventy controls. All participants subjected to history taking, thorough physical examination, colonoscopy and laboratory investigations. Genotyping performed for rs4821480 and rs3752462 by SNP assay real-time PCR methods.

Results

On analyzing rs3752462 CT and TT genotypes were significantly more frequent in IBD patients as compared to controls with 4.6 fold increase in the risk of IBD. While on analyzing rs4821480, The TG and GG genotypes have significant increased distribution among the IBD patients as compared to the controls with 5.3 fold increase in the risk of IBD and higher prevalence of GG genotype in patients with low hemoglobin level and higher BMI.

Conclusion

The rs3752462 T allele and rs4821480 G allele of MYH9 are associated with more susceptibility to IBD.

Obesity and its effects on the esophageal mucosal barrier

Obesity is associated with gastroesophageal reflux disease (GERD) and its complications including reflux esophagitis, Barrett's esophagus, and esophageal adenocarcinoma. Traditionally, these associations have been attributed to the mechanical effect of abdominal fat in increasing intra-abdominal pressure, thereby promoting gastroesophageal reflux and causing disruption of antireflux mechanisms at the esophagogastric junction. However, recent studies suggest that visceral adipose tissue (VAT) produces numerous cytokines that can cause esophageal inflammation and impair esophageal mucosal barrier integrity through reflux-independent mechanisms that render the esophageal mucosa especially susceptible to GERD-induced injury. In this report, we review mechanisms of esophageal mucosal defense, the genesis and remodeling of visceral adipose tissue during obesity, and the potential role of substances produced by VAT, especially the VAT that encircles the esophagogastric junction, in the impairment of esophageal mucosal barrier integrity that leads to the development of GERD complications.

Mechanosensitive myosin II but not cofilin primarily contributes to cyclic cell stretch-induced selective disassembly of actin stress fibers

Cells adapt to applied cyclic stretch (CS) to circumvent chronic activation of proinflammatory signaling. Currently, the molecular mechanism of the selective disassembly of actin stress fibers (SFs) in the stretch direction, which occurs at the early stage of the cellular response to CS, remains controversial. Here, we suggest that the mechanosensitive behavior of myosin II, a major cross-linker of SFs, primarily contributes to the directional disassembly of the actomyosin complex SFs in bovine vascular smooth muscle cells and human U2OS osteosarcoma cells. First, we identified that CS with a shortening phase that exceeds in speed the inherent contractile rate of individual SFs leads to the disassembly. To understand the biological basis, we investigated the effect of expressing myosin regulatory light-chain mutants and found that SFs with less actomyosin activities disassemble more promptly upon CS. We consequently created a minimal mathematical model that recapitulates the salient features of the direction-selective and threshold-triggered disassembly of SFs to show that disassembly or, more specifically, unbundling of the actomyosin bundle SFs is enhanced with sufficiently fast cell shortening. We further demonstrated that similar disassembly of SFs is inducible in the presence of an active LIM-kinase-1 mutant that deactivates cofilin, suggesting that cofilin is dispensable as opposed to a previously proposed mechanism.

Regulation of intestinal epithelial intercellular adhesion and barrier function by desmosomal cadherin desmocollin-2

The role of desmosomal cadherin desmocollin-2 (Dsc2) in regulating barrier function in intestinal epithelial cells (IECs) is not well understood. Here, we report the consequences of silencing Dsc2 on IEC barrier function in vivo using mice with inducible intestinal-epithelial-specific Dsc2 knockdown (KD) (Dsc2ERΔIEC). While the small intestinal gross architecture was maintained, loss of epithelial Dsc2 influenced desmosomal plaque structure, which was smaller in size and had increased intermembrane space between adjacent epithelial cells. Functional analysis revealed that loss of Dsc2 increased intestinal permeability in vivo, supporting a role for Dsc2 in the regulation of intestinal epithelial barrier function. These results were corroborated in model human IECs in which Dsc2 KD resulted in decreased cell-cell adhesion and impaired barrier function. It is noteworthy that Dsc2 KD cells exhibited delayed recruitment of desmoglein-2 (Dsg2) to the plasma membrane after calcium switch-induced intercellular junction reassembly, while E-cadherin accumulation was unaffected. Mechanistically, loss of Dsc2 increased desmoplakin (DP I/II) protein expression and promoted intermediate filament interaction with DP I/II and was associated with enhanced tension on desmosomes as measured by a Dsg2-tension sensor. In conclusion, we provide new insights on Dsc2 regulation of mechanical tension, adhesion, and barrier function in IECs.

Ca2+ homeostasis in brain microvascular endothelial cells

Blood brain barrier (BBB) is formed by the brain microvascular endothelial cells (BMVECs) lining the wall of brain capillaries. Its integrity is regulated by multiple mechanisms, including up/downregulation of tight junction proteins or adhesion molecules, altered Ca²⁺ homeostasis, remodeling of cytoskeleton, that are confined at the level of BMVECs. Beside the contribution of BMVECs to BBB permeability changes, other cells, such as pericytes, astrocytes, microglia, leukocytes or neurons, etc. are also exerting direct or indirect modulatory effects on BBB. Alterations in BBB integrity play a key role in multiple brain pathologies, including neurological (e.g. epilepsy) and neurodegenerative disorders (e.g. Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis etc.). In this review, the principal Ca²⁺ signaling pathways in brain microvascular endothelial cells are discussed and their contribution to BBB integrity is emphasized. Improving the knowledge of Ca²⁺ homeostasis alterations in BMVECa is fundamental to identify new possible drug targets that diminish/prevent BBB permeabilization in neurological and neurodegenerative disorders.

Phenotypic Plasticity of Cancer Cells Based on Remodeling of the Actin Cytoskeleton and Adhesive Structures

There is ample evidence that, instead of a binary switch, epithelial-mesenchymal transition (EMT) in cancer results in a flexible array of phenotypes, each one uniquely suited to a stage in the invasion-metastasis cascade. The phenotypic plasticity of epithelium-derived cancer cells gives them an edge in surviving and thriving in alien environments. This review describes in detail the actin cytoskeleton and E-cadherin-based adherens junction rearrangements that cancer cells need to implement in order to achieve the advantageous epithelial/mesenchymal phenotype and plasticity of migratory phenotypes that can arise from partial EMT.

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

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

Intestinal Inflammation as a Dysbiosis of Energy Procurement: New Insights into an Old Topic

Inflammatory bowel disease (IBD) coincides with profound shifts in microbiota and host metabolic energy supply and demand. The gastrointestinal epithelium is anatomically positioned to provide a selective barrier between the anaerobic luminal microbiota and host lamina propria, with the microbiota and epithelium participating in an intricate energy exchange necessary for homeostasis. Maintenance and restoration of the barrier requires high energy flux and places significant demands on available substrates to generate ATP. It is recently appreciated that components of the microbiota contribute significantly to a multitude of biochemical pathways within and outside of the mucosa. Decades-old studies have appreciated that byproducts of the microbiota provide essential sources of energy to the intestinal epithelium, especially the colon. More recent work has unveiled the existence of numerous microbial-derived metabolites that support energy procurement within the mucosa. It is now appreciated that disease-associated shifts in the microbiota, termed dysbiosis, places significant demands on energy acquisition within the mucosa. Here, we review the topic of host- and microbial-derived components that influence tissue energetics in health and during disease.

Thymosin β4 is essential for adherens junction stability and epidermal planar cell polarity

Planar cell polarity (PCP) is essential for tissue morphogenesis and homeostasis; however, the mechanisms that orchestrate the cell shape and packing dynamics required to establish PCP are poorly understood. Here, we identified a major role for the globular (G)-actin-binding protein thymosin-β4 (TMSB4X) in PCP establishment and cell adhesion in the developing epidermis. Depletion of Tmsb4x in mouse embryos hindered eyelid closure and hair-follicle angling owing to PCP defects. Tmsb4x depletion did not preclude epidermal cell adhesion in vivo or in vitro; however, it resulted in abnormal structural organization and stability of adherens junction (AJ) due to defects in filamentous (F)-actin and G-actin distribution. In cultured keratinocytes, TMSB4X depletion increased the perijunctional G/F-actin ratio and decreased G-actin incorporation into junctional actin networks, but it did not change the overall actin expression level or cellular F-actin content. A pharmacological treatment that increased the G/F-actin ratio and decreased actin polymerization mimicked the effects of Tmsb4x depletion on both AJs and PCP. Our results provide insights into the regulation of the actin pool and its involvement in AJ function and PCP establishment.

Highlighted Article: By regulating actin pool distribution and incorporation into junctional actin networks, thymosin β4 regulates cell–cell adhesion, planar cell polarity and epidermal morphogenesis.

Nonmuscle myosin-2 contractility-dependent actin turnover limits the length of epithelial microvilli

Brush border microvilli enable functions that are critical for epithelial homeostasis, including solute uptake and host defense. However, the mechanisms that regulate the assembly and morphology of these protrusions are poorly understood. The parallel actin bundles that support microvilli have their pointed-end rootlets anchored in a filamentous meshwork referred to as the "terminal web." Although classic electron microscopy studies revealed complex ultrastructure, the composition and function of the terminal web remain unclear. Here we identify nonmuscle myosin-2C (NM2C) as a component of the terminal web. NM2C is found in a dense, isotropic layer of puncta across the subapical domain, which transects the rootlets of microvillar actin bundles. Puncta are separated by ∼210 nm, the expected size of filaments formed by NM2C. In intestinal organoid cultures, the terminal web NM2C network is highly dynamic and exhibits continuous remodeling. Using pharmacological and genetic perturbations in cultured intestinal epithelial cells, we found that NM2C controls the length of growing microvilli by regulating actin turnover in a manner that requires a fully active motor domain. Our findings answer a decades-old question on the function of terminal web myosin and hold broad implications for understanding apical morphogenesis in diverse epithelial systems.

Loss of β-Cytoplasmic Actin in the Intestinal Epithelium Increases Gut Barrier Permeability in vivo and Exaggerates the Severity of Experimental Colitis

Intestinal epithelial barrier is critical for the maintenance of normal gut homeostasis and disruption of this barrier may trigger or exaggerate mucosal inflammation. The actin cytoskeleton is a key regulator of barrier structure and function, controlling the assembly and permeability of epithelial adherens and tight junctions. Epithelial cells express two actin isoforms: a β-cytoplasmic actin and γ-cytoplasmic actin. Our previous in vitro studies demonstrated that these actin isoforms play distinctive roles in establishing the intestinal epithelial barrier, by controlling the organization of different junctional complexes. It remains unknown, whether β-actin and γ-actin have unique or redundant functions in regulating the gut barrier in vivo. To address this question, we selectively knocked out β-actin expression in mouse intestinal epithelium. Mice with intestinal epithelial knockout of β-actin do not display gastrointestinal abnormalities or gross alterations of colonic mucosal architecture. This could be due to compensatory upregulation of γ-actin expression. Despite such compensation, β-actin knockout mice demonstrate increased intestinal permeability. Furthermore, these animals show more severe clinical symptoms during dextran sodium sulfate induced colitis, compared to control littermates. Such exaggerated colitis is associated with the higher expression of inflammatory cytokines, increased macrophage infiltration in the gut, and accelerated mucosal cell death. Consistently, intestinal organoids generated from β-actin knockout mice are more sensitive to tumor necrosis factor induced cell death, ex vivo. Overall, our data suggests that β-actin functions as an essential regulator of gut barrier integrity in vivo, and plays a tissue protective role during mucosal injury and inflammation.

The HIF target ATG9A is essential for epithelial barrier function and tight junction biogenesis

Intestinal epithelial cells (IECs) exist in a metabolic state of low oxygen tension termed "physiologic hypoxia." An important factor in maintaining intestinal homeostasis is the transcription factor hypoxia-inducible factor (HIF), which is stabilized under hypoxic conditions and mediates IEC homeostatic responses to low oxygen tension. To identify HIF transcriptional targets in IEC, chromatin immunoprecipitation (ChIP) was performed in Caco-2 IECs using HIF-1α- or HIF-2α-specific antibodies. ChIP-enriched DNA was hybridized to a custom promoter microarray (termed ChIP-chip). This unbiased approach identified autophagy as a major HIF-1-targeted pathway in IEC. Binding of HIF-1 to the ATG9A promoter, the only transmembrane component within the autophagy pathway, was particularly enriched by exposure of IEC to hypoxia. Validation of this ChIP-chip revealed prominent induction of ATG9A, and luciferase promoter assays identified a functional hypoxia response element upstream of the TSS. Hypoxia-mediated induction of ATG9A was lost in cells lacking HIF-1. Strikingly, we found that lentiviral-mediated knockdown (KD) of ATG9A in IECs prevents epithelial barrier formation by >95% and results in significant mislocalization of multiple tight junction (TJ) proteins. Extensions of these findings showed that ATG9A KD cells have intrinsic abnormalities in the actin cytoskeleton, including mislocalization of the TJ binding protein vasodilator-stimulated phosphoprotein. These results implicate ATG9A as essential for multiple steps of epithelial TJ biogenesis and actin cytoskeletal regulation. Our findings have novel applicability for disorders that involve a compromised epithelial barrier and suggest that targeting ATG9A may be a rational strategy for future therapeutic intervention.

Reproducible production and image-based quality evaluation of retinal pigment epithelium sheets from human induced pluripotent stem cells

Transplantation of retinal pigment epithelial (RPE) sheets derived from human induced pluripotent cells (hiPSC) is a promising cell therapy for RPE degeneration, such as in age-related macular degeneration. Current RPE replacement therapies, however, face major challenges. They require a tedious manual process of selecting differentiated RPE from hiPSC-derived cells, and despite wide variation in quality of RPE sheets, there exists no efficient process for distinguishing functional RPE sheets from those unsuitable for transplantation. To overcome these issues, we developed methods for the generation of RPE sheets from hiPSC, and image-based evaluation. We found that stepwise treatment with six signaling pathway inhibitors along with nicotinamide increased RPE differentiation efficiency (RPE6iN), enabling the RPE sheet generation at high purity without manual selection. Machine learning models were developed based on cellular morphological features of F-actin-labeled RPE images for predicting transepithelial electrical resistance values, an indicator of RPE sheet function. Our model was effective at identifying low-quality RPE sheets for elimination, even when using label-free images. The RPE6iN-based RPE sheet generation combined with the non-destructive image-based prediction offers a comprehensive new solution for the large-scale production of pure RPE sheets with lot-to-lot variations and should facilitate the further development of RPE replacement therapies.

The integrity of cochlear hair cells is established and maintained through the localization of Dia1 at apical junctional complexes and stereocilia

Dia1, which belongs to the diaphanous-related formin family, influences a variety of cellular processes through straight actin elongation activity. Recently, novel DIA1 mutants such as p.R1213X (p.R1204X) and p.A265S, have been reported to cause an autosomal dominant sensorineural hearing loss (DFNA1). Additionally, active DIA1 mutants induce progressive hearing loss in a gain-of-function manner. However, the subcellular localization and pathological function of DIA1(R1213X/R1204X) remains unknown. In the present study, we demonstrated the localization of endogenous Dia1 and the constitutively active DIA1 mutant in the cochlea, using transgenic mice expressing FLAG-tagged DIA1(R1204X) (DIA1-TG). Endogenous Dia1 and the DIA1 mutant were regionally expressed at the organ of Corti and the spiral ganglion from early life; alongside cochlear maturation, they became localized at the apical junctional complexes (AJCs) between hair cells (HCs) and supporting cells (SCs). To investigate HC vulnerability in the DIA1-TG mice, we exposed 4-week-old mice to moderate noise, which induced temporary threshold shifts with cochlear synaptopathy and ultrastructural changes in stereocilia 4 weeks post noise exposure. Furthermore, we established a knock-in (KI) mouse line expressing AcGFP-tagged DIA1(R1213X) (DIA1-KI) and confirmed mutant localization at AJCs and the tips of stereocilia in HCs. In MDCK^{AcGFP-DIA1(R1213X)} cells with stable expression of AcGFP-DIA1(R1213X), AcGFP-DIA1(R1213X) revealed marked localization at microvilli on the apical surface of cells and decreased localization at cell-cell junctions. The DIA1-TG mice demonstrated hazy and ruffled circumferential actin belts at AJCs and abnormal stereocilia accompanied with HC loss at 5 months of age. In conclusion, Dia1 plays a pivotal role in the development and maintenance of AJCs and stereocilia, ensuring cochlear and HC integrity. Subclinical/latent vulnerability of HCs may be the cause of progressive hearing loss in DFNA1 patients, thus suggesting new therapeutic targets for preventing HC degeneration and progressive hearing loss associated with DFNA1.

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

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

Molecular mechanisms of cell polarity in a range of model systems and in migrating neurons

Cell polarity is defined as the asymmetric distribution of cellular components along an axis. Most cells, from the simplest single-cell organisms to highly specialized mammalian cells, are polarized and use similar mechanisms to generate and maintain polarity. Cell polarity is important for cells to migrate, form tissues, and coordinate activities. During development of the mammalian cerebral cortex, cell polarity is essential for neurogenesis and for the migration of newborn but as-yet undifferentiated neurons. These oriented migrations include both the radial migration of excitatory projection neurons and the tangential migration of inhibitory interneurons. In this review, I will first describe the development of the cerebral cortex, as revealed at the cellular level. I will then define the core molecular mechanisms - the Par/Crb/Scrib polarity complexes, small GTPases, the actin and microtubule cytoskeletons, and phosphoinositides/PI3K signaling - that are required for asymmetric cell division, apico-basal and front-rear polarity in model systems, including C elegans zygote, Drosophila embryos and cultured mammalian cells. As I go through each core mechanism I will explain what is known about its importance in radial and tangential migration in the developing mammalian cerebral cortex.

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.

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.

PACSIN2-dependent apical endocytosis regulates the morphology of epithelial microvilli

Apical microvilli are critical for the homeostasis of transporting epithelia, yet mechanisms that control the assembly and morphology of these protrusions remain poorly understood. Previous studies in intestinal epithelial cell lines suggested a role for the F-BAR domain protein PACSIN2 in normal microvillar assembly. Here we report the phenotype of PACSIN2 KO mice and provide evidence that through its role in promoting apical endocytosis, this molecule plays a role in controlling microvillar morphology. PACSIN2 KO enterocytes exhibit reduced numbers of microvilli and defects in the microvillar ultrastructure, with membranes lifting away from rootlets of core bundles. Dynamin2, a PACSIN2 binding partner, and other endocytic factors were also lost from their normal localization near microvillar rootlets. To determine whether loss of endocytic machinery could explain defects in microvillar morphology, we examined the impact of PACSIN2 KD and endocytosis inhibition on live intestinal epithelial cells. These assays revealed that when endocytic vesicle scission fails, tubules are pulled into the cytoplasm and this, in turn, leads to a membrane-lifting phenomenon reminiscent of that observed at PACSIN2 KO brush borders. These findings lead to a new model where inward forces generated by endocytic machinery on the plasma membrane control the membrane wrapping of cell surface protrusions.

Gold nanoparticles regulate tight junctions and improve cetuximab effect in colon cancer cells

Aim: Colon cancer (CC) is the second cause of cancer death worldwide. The use of nanoparticles for drug delivery has been increasing in cancer clinical trials over recent years. Materials & methods: We evaluated cytotoxicity of citrate-capped gold nanoparticles (GNPs) and the role they play on cell-cell adhesion. We also used GNP for delivery of cetuximab into different CC cell lines. Results: CC cells with well-formed tight junctions impair GNP uptake. Noncytotoxic concentration of GNP increases paracellular permeability in Caco-2 cells in a reversible way, concomitantly to tight junctions proteins CLDN1 and ZO-1 redistribution. GNP functionalized with cetuximab increases death of invasive HCT-116 CC cells. Conclusion: GNP can be used for drug delivery and can improve efficiency of CC therapy.

CCAT1 lncRNA Promotes Inflammatory Bowel Disease Malignancy by Destroying Intestinal Barrier via Downregulating miR-185-3p

BACKGROUND

The long noncoding RNA (lncRNA) colon cancer-associated transcript-1 (CCAT1) has been reported to play a vital role in the development of cancer. Although the link between inflammation and cancer initiation is well established, whether CCAT1 is involved in inflammation and promotes inflammatory bowel disease (IBD) malignancy remains undetermined. We aimed to investigate the expression of CCAT1 in IBD and the effect of CCAT1 overexpression on intestinal epithelial barrier function.

METHODS

The relationship between CCAT1 and the inflammation-related pathway was analyzed in both colorectal cancer (CRC) and IBD patients. Gene expression was detected by real-time polymerase chain reaction and Western blot. Transepithelial electrical resistance (TEER) and FD-4 flux measurement were used to test the effect of CCAT1 and miR-185-3p on intestinal epithelial barrier function. Luciferase assay was performed to validate the target site of miR-185-3p on 3'-UTR of MLCK mRNA.

RESULTS

Gene set enrichment analysis revealed that several inflammation-related genes were enriched in the CCAT1 high-expressed group of CRC patients. The relationship between CCAT1 and inflammation activation in IBD patients was further confirmed. CCAT1 expression positively correlated with MLCK, which acts as a protein kinase to phosphorylate myosin light chain and induces tight junction protein distribution, whereas it was negatively correlated with miR-185-3p in IBD tissues. We also determined that CCAT1 overexpression increased Caco-2 monolayer permeability and upregulated MLCK. Furthermore, CCAT1-induced MLCK overexpression and IBD disease progression were significantly attenuated by miR-185-3p.

CONCLUSIONS

The CCAT1/miR-185-3p/MLCK signaling pathway is strongly activated to destroy barrier function and promotes the pathogenesis of IBD.

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.

Branched actin networks push against each other at adherens junctions to maintain cell-cell adhesion

Adherens junctions (AJs) are mechanosensitive cadherin-based intercellular adhesions that interact with the actin cytoskeleton and carry most of the mechanical load at cell-cell junctions. Both Arp2/3 complex-dependent actin polymerization generating pushing force and nonmuscle myosin II (NMII)-dependent contraction producing pulling force are necessary for AJ morphogenesis. Which actin system directly interacts with AJs is unknown. Using platinum replica electron microscopy of endothelial cells, we show that vascular endothelial (VE)-cadherin colocalizes with Arp2/3 complex-positive actin networks at different AJ types and is positioned at the interface between two oppositely oriented branched networks from adjacent cells. In contrast, actin-NMII bundles are located more distally from the VE-cadherin-rich zone. After Arp2/3 complex inhibition, linear AJs split, leaving gaps between cells with detergent-insoluble VE-cadherin transiently associated with the gap edges. After NMII inhibition, VE-cadherin is lost from gap edges. We propose that the actin cytoskeleton at AJs acts as a dynamic push-pull system, wherein pushing forces maintain extracellular VE-cadherin transinteraction and pulling forces stabilize intracellular adhesion complexes.

Attaching-and-Effacing Pathogens Exploit Junction Regulatory Activities of N-WASP and SNX9 to Disrupt the Intestinal Barrier

BACKGROUND & AIMS

Neural Wiskott-Aldrich Syndrome protein (N-WASP) is a key regulator of the actin cytoskeleton in epithelial tissues and is poised to mediate cytoskeletal-dependent aspects of apical junction complex (AJC) homeostasis. Attaching-and-effacing (AE) pathogens disrupt this homeostasis through translocation of the effector molecule early secreted antigenic target-6 (ESX)-1 secretion-associated protein F (EspF). Although the mechanisms underlying AJC disruption by EspF are unknown, EspF contains putative binding sites for N-WASP and the endocytic regulator sorting nexin 9 (SNX9). We hypothesized that N-WASP regulates AJC integrity and AE pathogens use EspF to induce junction disassembly through an N-WASP- and SNX9-dependent pathway.

METHODS

We analyzed mice with intestine-specific N-WASP deletion and generated cell lines with N-WASP and SNX9 depletion for dynamic functional assays. We generated EPEC and Citrobacter rodentium strains complemented with EspF bearing point mutations abolishing N-WASP and SNX9 binding to investigate the requirement for these interactions.

RESULTS

Mice lacking N-WASP in the intestinal epithelium showed spontaneously increased permeability, abnormal AJC morphology, and mislocalization of occludin. N-WASP depletion in epithelial cell lines led to impaired assembly and disassembly of tight junctions in response to changes in extracellular calcium. Cells lacking N-WASP or SNX9 supported actin pedestals and type III secretion, but were resistant to EPEC-induced AJC disassembly and loss of transepithelial resistance. We found that during in vivo infection with AE pathogens, EspF must bind both N-WASP and SNX9 to disrupt AJCs and induce intestinal barrier dysfunction.

CONCLUSIONS

Overall, these studies show that N-WASP critically regulates AJC homeostasis, and the AE pathogen effector EspF specifically exploits both N-WASP and SNX9 to disrupt intestinal barrier integrity during infection.

Enteropathogenic E. coli effectors EspF and Map independently disrupt tight junctions through distinct mechanisms involving transcriptional and post-transcriptional regulation

Enteropathogenic E. coli infection is characterized by rapid onset of diarrhea but the underlying mechanisms are not well defined. EPEC targets the tight junctions which selectively regulate the permeability of charged and uncharged molecules. Cooperative actions of the EPEC effectors EspF and Map have been reported to mediate tight junction disruption. To analyze the individual contributions of EspF and Map, we generated in vitro models where EspF and Map, derived from the EPEC strain E2348/69, were constitutively expressed in epithelial cells. Here we report that tight junction disruption by EspF and Map is caused by the inhibition of the junctional recruitment of proteins during tight junction assembly. Constitutive expression of EspF and Map depleted the levels of tight junction proteins. EspF down-regulated the transcript levels of claudin-1, occludin and ZO-1, while Map down-regulated only claudin-1 transcripts. Both effectors also caused lysosomal degradation of existing tight junction proteins. We also identified a novel interaction of Map with non-muscle myosin II. Consistent with earlier studies, EspF was found to interact with ZO-1 while actin was the common interacting partner for both effectors. Our data provides evidence for the distinct roles of Map and EspF in tight junction disruption through non-synergistic functions.

Myosins as fundamental components during tumorigenesis: diverse and indispensable

Myosin is a kind of actin-based motor protein. As the crucial functions of myosin during tumorigenesis have become increasingly apparent, the profile of myosin in the field of cancer research has also been growing. Eighteen distinct classes of myosins have been discovered in the past twenty years and constitute a diverse superfamily. Various myosins share similar structures. They all convert energy from ATP hydrolysis to exert mechanical stress upon interactions with microfilaments. Ongoing research is increasingly suggesting that at least seven kinds of myosins participate in the formation and development of cancer. Myosins play essential roles in cytokinesis failure, chromosomal and centrosomal amplification, multipolar spindle formation and DNA microsatellite instability. These are all prerequisites of tumor formation. Subsequently, myosins activate various processes of tumor invasion and metastasis development including cell migration, adhesion, protrusion formation, loss of cell polarity and suppression of apoptosis. In this review, we summarize the current understanding of the roles of myosins during tumorigenesis and discuss the factors and mechanisms which may regulate myosins in tumor progression. Furthermore, we put forward a completely new concept of “chromomyosin” to demonstrate the pivotal functions of myosins during karyokinesis and how this acts to optimize the functions of the members of the myosin superfamily.