The Claudins: From Tight Junctions to Biological Systems

Effect of Perfluorooctanoic Acid on the Epigenetic and Tight Junction Genes of the Mouse Intestine

Perfluorooctanoic acid (PFOA) has been implicated in various toxicities including neurotoxicity, genotoxicity, nephrotoxicity, epigenetic toxicity, immunotoxicity, reproductive toxicity, and hepatotoxicity. However, information on the accumulation of PFOA in the intestine and its toxic effects on intestinal epigenetics and tight junction (TJ) genes is sparse. CD1 mice were dosed with PFOA (1, 5, 10, or 20 mg/kg/day) for 10 days, and its accumulation and induced alterations in the expression of epigenetic and tight junction genes in the small intestine and colon were evaluated using LC-MS and qPCR techniques. PFOA reduced the expression levels of DNA methyltransferases (Dnmt1, Dnmt3a, Dnmt3b) primarily in the small intestine whereas, in the colon, a decrease was observed only at high concentrations. Moreover, ten-eleven translocation genes (Tet2 and Tet3) expression was dysregulated in the small intestine, whereas in the colon Tets remained unaffected. The tight junction genes Claudins (Cldn), Occludin (Ocln), and Tight Junction Protein (Tjp) were also heavily altered in the small intestine. TJs responded differently across the gut, in proportion to PFOA dosing. Our study reveals that PFOA triggers DNA methylation changes and alters the expression of genes essential for maintaining the physical barrier of intestine, with more profound effects in the small intestine compared to the colon.

Epigenetic effects of low-level sodium arsenite exposure on human liver HepaRG cells

Chronic exposure to inorganic arsenic is associated with a variety of adverse health effects, including lung, bladder, kidney, and liver cancer. Several mechanisms have been proposed for arsenic-induced tumorigenesis; however, insufficient knowledge and many unanswered questions remain to explain the integrated molecular pathogenesis of arsenic carcinogenicity. In the present study, using non-tumorigenic human liver HepaRG cells, we investigated epigenetic alterations upon prolonged exposure to a noncytotoxic concentration of sodium arsenite (NaAsO₂). We demonstrate that continuous exposure of HepaRG cells to 1 µM sodium arsenite (NaAsO₂) for 14 days resulted in substantial cytosine DNA demethylation and hypermethylation across the genome, among which the claudin 14 (CLDN14) gene was hypermethylated and the most down-regulated gene. Another important finding was a profound loss of histone H3 lysine 36 (H3K36) trimethylation, which was accompanied by increased damage to genomic DNA and an elevated de novo mutation frequency. These results demonstrate that continuous exposure of HepaRG cells to a noncytotoxic concentration of NaAsO₂ results in substantial epigenetic abnormalities accompanied by several carcinogenesis-related events, including induction of epithelial-to-mesenchymal transition, damage to DNA, inhibition of DNA repair genes, and induction of de novo mutations. Importantly, this study highlights the intimate mechanistic link and interplay between two fundamental cancer-associated events, epigenetic and genetic alterations, in arsenic-associated carcinogenesis.

Measuring Cell Growth and Junction Development in Epithelial Cells Using Electric Cell-Substrate Impedance Sensing (ECIS)

Electric Cell-substrate Impedance Sensing (ECIS) is an automated method that can be used to quantify processes such as cell attachment, growth, migration and barrier functions (i.e., the properties of tight junctions). The method provides simultaneous information on cell number and tight junction function by detecting electric parameters of cells grown on electrodes. Samples are probed with small alternating current (AC) over a range of frequencies, and changes in capacitance and impedance are measured over time. Capacitance reflects the degree of electrode coverage by cells, that correlates with cell number, and can be used to assess cell proliferation or migration. Impedance values inform about barrier function. Obtaining real-time simultaneous information on these parameters is unique to this system and is of great value for addressing fundamental questions such as the role of tight junction proteins in cell growth and migration. This protocol describes the use of ECIS to follow cell growth and tight junction-dependent barrier generation in tubular epithelial cells. We used this method to explore how depleting claudin-2, a tight junction protein affects tubular cell growth and barrier function. During the process, cells are transfected with control or claudin-2-specific siRNA, and 24h later plated on electrodes. ECIS automatically collects information on cell growth and barrier as the monolayer develops. The data are initially analyzed using the ECIS software and exported into a graph software for further processing.

Liver Cirrhosis and Sarcopenia from the Viewpoint of Dysbiosis

Sarcopenia in patients with liver cirrhosis (LC) has been attracting much attention these days because of the close linkage to adverse outcomes. LC can be related to secondary sarcopenia due to protein metabolic disorders and energy metabolic disorders. LC is associated with profound alterations in gut microbiota and injuries at the different levels of defensive mechanisms of the intestinal barrier. Dysbiosis refers to a state in which the diversity of gut microbiota is decreased by decreasing the bacterial species and the number of bacteria that compose the gut microbiota. The severe disturbance of intestinal barrier in LC can result in dysbiosis, several bacterial infections, LC-related complications, and sarcopenia. Here in this review, we will summarize the current knowledge of the relationship between sarcopenia and dysbiosis in patients with LC.

Immunofluorescent Staining of Claudin-2 in Cultured Kidney Tubular Cells

Members of the claudin family of tight junction proteins regulate paracellular permeability and modulate cell signaling. During junction remodeling, these proteins are selectively inserted into or retrieved from the tight junctions, but the control and coordination of these processes remain incompletely understood. Visualization of claudins allows the assessment of changes in their localization and abundance. We use the described protocol to stain claudin-2, but it can also be adapted to stain any tight junction protein. We found that using methanol for fixing allows the best preservation of claudin-2 both at the membrane and in cytoplasmic vesicles. Staining is done using a claudin-2 specific primary and a fluorescently labelled secondary antibody, along with DAPI to label nuclei. The samples are then imaged using confocal microscopy, and a z-stack is obtained allowing visualization of both junctional and intracellular claudin-2. Total claudin-2 signal can be quantified after 3D reconstruction of the images using the Imaris software.

Angulin-2/ILDR1, a tricellular tight junction protein, does not affect water transport in the mouse large intestine

Angulin-2/ILDR1 is a member of the angulin protein family, which is exclusively expressed at tricellular tight junctions in epithelia. Tricellular tight junctions are found where three cells meet and where three bicellular tight junction strands converge. Tricellular tight junctions are thought to be important for paracellular permeability of ions and water in epithelial tissues. It was recently reported that angulin-2/ILDR1 knockout mice have water transport abnormalities in the kidney. Since angulin-2/ILDR1 is the main tricellular tight junction protein in the large intestine, the goal of this research was to examine the effect of angulin-2/ILDR1 knockout on large intestinal paracellular water transport. We found that Ildr1 knockout mice showed no detectable phenotype other than deafness. In addition, paracellular transport as assessed by Ussing chamber was unchanged in Ildr1 knockout mice. However, we found that in the colon and the kidney of Ildr1 knockout mice, another tricellular tight junction protein, angulin-1/LSR, changes its expression pattern. We propose that with this replacement in tissue localization, angulin-1/LSR compensates for the loss of angulin-2/ILDR1 and maintains the barrier and function of the epithelia in the large intestine as well as the kidney.

Outside-in hypothesis revisited: The role of microbial, epithelial, and immune interactions

OBJECTIVE

Our understanding of the origin of allergic diseases has increased in recent years, highlighting the importance of microbial dysbiosis and epithelial barrier dysfunction in affected tissues. Exploring the microbial-epithelial-immune crosstalk underlying the mechanisms of allergic diseases will allow the development of novel prevention and treatment strategies for allergic diseases.

DATA SOURCES

This review summarizes the recent advances in microbial, epithelial, and immune interactions in atopic dermatitis, allergic rhinitis, chronic rhinosinusitis, and asthma.

STUDY SELECTIONS

We performed a literature search, identifying relevant recent primary articles and review articles.

RESULTS

Dynamic crosstalk between the environmental factors and microbial, epithelial, and immune cells in the development of atopic dermatitis, allergic rhinitis, chronic rhinosinusitis, and asthma underlies the pathogenesis of these diseases. There is substantial evidence in the literature suggesting that environmental factors directly affect barrier function of the epithelium. In addition, T-helper 2 (T_H2) cells, type 2 innate lymphoid cells, and their cytokine interleukin 13 (IL-13) damage skin and lung barriers. The effects of environmental factors may at least in part be mediated by epigenetic mechanisms. Histone deacetylase activation by type 2 immune response has a major effect on leaky barriers and blocking of histone deacetylase activity corrects the defective barrier in human air-liquid interface cultures and mouse models of allergic asthma with rhinitis. We also present and discuss a novel device to detect and monitor skin barrier dysfunction, which provides an opportunity to rapidly and robustly assess disease severity.

CONCLUSION

A complex interplay between environmental factors, epithelium, and the immune system is involved in the development of systemic allergic diseases.

Effect of long-term consumption of tea (Camellia sinensis L.) flower polysaccharides on maintaining intestinal health in BALB/c mice

Polysaccharides have various health-promoting functions. However, dietary polysaccharides cannot be digested by the human alimentary tract, thus the gut is the most important location where polysaccharides play their role. The effect of polysaccharides from tea (Camellia sinensis L.) flower (TFPS) on intestinal health was investigated in the present study. TFPS with the molecular weight of 1,316.29 kDa was prepared, and twenty 6-week-old BALB/c male mice were randomly allotted to a chow diet (normal control group, NC group) or with 200 mg/kg (body weight)/day of TFPS for 13 weeks (n = 10 each). Histomorphology observation of jejunum and colons showed that TFPS maintained the adequate gut barrier. qPCR analysis revealed that the expression of colonic tight junction proteins of claudin1 (1.29 ± 0.15 compared with 1.00 ± 0.13, P < 0.05) and claudin5 (2.91 ± 0.44 compared with 1.00 ± 0.27, P < 0.01) at mRNA level with a significant difference between TFPS supplement or not, while the expression of TLR4 and TNF-α mRNA was not changed statistically. 16S rDNA amplicons sequencing was applied to measure the compositions of gut microbiota from feces of mice. TFPS treatment exhibited similar relative abundances in Bacteroidetes and Firmicutes; however, it decreased the relative abundance of Akkermansia and increased that of Lactobacillus compared with the NC group. The contents of short-chain fatty acids after TFPS supplementation, both in cecal contents and feces, were significantly higher than those of the NC group. Besides, TFPS significantly increased IgA production. These results suggest that TFPS is beneficial to intestinal health and can improve intestinal adaptive immune tolerance. PRACTICAL APPLICATION: Dietary polysaccharides improve human intestinal health. Understanding the effect of TFPS, safe and healthy food components, on gut health increases the likelihood that TFPS will be developed as a functional food.

The Regulation of Intestinal Mucosal Barrier by Myosin Light Chain Kinase/Rho Kinases

The intestinal epithelial apical junctional complex, which includes tight and adherens junctions, contributes to the intestinal barrier function via their role in regulating paracellular permeability. Myosin light chain II (MLC-2), has been shown to be a critical regulatory protein in altering paracellular permeability during gastrointestinal disorders. Previous studies have demonstrated that phosphorylation of MLC-2 is a biochemical marker for perijunctional actomyosin ring contraction, which increases paracellular permeability by regulating the apical junctional complex. The phosphorylation of MLC-2 is dominantly regulated by myosin light chain kinase- (MLCK-) and Rho-associated coiled-coil containing protein kinase- (ROCK-) mediated pathways. In this review, we aim to summarize the current state of knowledge regarding the role of MLCK- and ROCK-mediated pathways in the regulation of the intestinal barrier during normal homeostasis and digestive diseases. Additionally, we will also suggest potential therapeutic targeting of MLCK- and ROCK-associated pathways in gastrointestinal disorders that compromise the intestinal barrier.

Post-translational modifications of tight junction transmembrane proteins and their direct effect on barrier function

Post-translational modifications (PTMs) such as phosphorylation, ubiquitination or glycosylation are processes affecting the conformation, stability, localization and function of proteins. There is clear evidence that PTMs can act upon tight junction (TJ) proteins, thus modulating epithelial barrier function. Compared to transcriptional or translational regulation, PTMs are rapid and more dynamic processes so in the context of barrier maintenance they might be essential for coping with changing environmental or external impacts. The aim of this review is to extract literature deciphering PTMs in TJ proteins directly contributing to epithelial barrier changes in permeability to ions and macromolecules. It is not intended to cover the entire scope of PTMs in TJ proteins and should rather be understood as a digest of TJ protein modifications directly resulting in the tightening or opening of the epithelial barrier.

The bicistronic gene würmchen encodes two essential components for epithelial development in Drosophila

Epithelial tissues are fundamental for the establishment and maintenance of different body compartments in multicellular animals. To achieve this specific task epithelial sheets secrete an apical extracellular matrix for tissue strength and protection and they organize a transepithelial barrier function, which is mediated by tight junctions in vertebrates or septate junctions in invertebrates. Here, we show that the bicistronic gene würmchen is functionally expressed in epithelial tissues. CRISPR/Cas9-mediated mutations in both coding sequences reveal two essential polypeptides, Würmchen1 and Würmchen2, which are both necessary for normal epithelial tissue development. Würmchen1 represents a genuine septate junction core component. It is required during embryogenesis for septate junction organization, the establishment of a transepithelial barrier function, distinct cellular transport processes and tracheal system morphogenesis. Würmchen2 is localized in the apical membrane region of epithelial tissues and in a central core of the tracheal lumen during embryogenesis. It is essential during the later larval development.

Tight Junctions as Targets and Effectors of Mucosal Immune Homeostasis

Defective epithelial barrier function is present in maladies including epidermal burn injury, environmental lung damage, renal tubular disease, and a range of immune-mediated and infectious intestinal disorders. When the epithelial surface is intact, the paracellular pathway between cells is sealed by the tight junction. However, permeability of tight junctions varies widely across tissues and can be markedly impacted by disease. For example, tight junctions within the skin and urinary bladder are largely impermeant and their permeability is not regulated. In contrast, tight junctions of the proximal renal tubule and intestine are selectively permeable to water and solutes on the basis of their biophysical characteristics and, in the gut, can be regulated by the immune system with remarkable specificity. Conversely, modulation of tight junction barrier conductance, especially within the gastrointestinal tract, can impact immune homeostasis and diverse pathologies. Thus, tight junctions are both effectors and targets of immune regulation. Using the gastrointestinal tract as an example, this review explores current understanding of this complex interplay between tight junctions and immunity.

Optimization of an oral mucosa in vitro model based on cell line TR146

During the last years, the popularity of saliva has been increasing for its applicability as a diagnostic fluid. Blood biomarker molecules have to cross the blood-saliva barrier (BSB) in order to appear in saliva. The BSB consists of all oral and salivary gland epithelial barriers. Within this context, the optimization of in vitro models for mechanistic studies about the transport of molecules across the oral mucosa is an important task. Here, we describe the optimization and comprehensive characterization of a Transwell model of the oral mucosa based on the epithelial cell line TR146. Through systematic media optimization investigating 12 different set-ups, a significant increase of barrier integrity upon airlift cultivation is described here for TR146 cell layers. The distinct improvement of the paracellular barrier was described by measurements of transepithelial electrical resistance (TEER) and carboxyfluorescein permeability assays. Histological characterization supported TEER data and showed a stratified, non-keratinized multilayer of the optimized TR146 model. High-Throughput qPCR using 96 selected markers for keratinization, cornification, epithelial-mesenchymal transition, aquaporins, mucins, tight junctions, receptors, and transporter proteins was applied to comprehensively characterize the systematic optimization of the cellular model and validate against human biopsy samples. Data revealed the expression of several genes in the oral mucosa epithelium for the first time and elucidated novel regulations dependent on culture conditions. Moreover, functional activity of ABC-transporters ABCB1 and ABCC4 was shown indicating the applicability of the model for drug transport studies. In conclusion, a Transwell model of the oral mucosa epithelium was optimized suitably for transport studies.

Deficiency in the anti-apoptotic protein DJ-1 promotes intestinal epithelial cell apoptosis and aggravates inflammatory bowel disease via p53

Parkinson disease autosomal recessive, early onset 7 (PARK7 or DJ-1) is involved in multiple physiological processes and exerts anti-apoptotic effects on multiple cell types. Increased intestinal epithelial cell (IEC) apoptosis and excessive activation of the p53 signaling pathway is a hallmark of inflammatory bowel disease (IBD), which includes ulcerative colitis (UC) and Crohn's disease (CD). However, whether DJ-1 plays a role in colitis is unclear. To determine whether DJ-1 deficiency is involved in the p53 activation that results in IEC apoptosis in colitis, here we performed immunostaining, real-time PCR, and immunoblotting analyses to assess DJ-1 expression in human UC and CD samples. In the inflamed intestines of individuals with IBD, DJ-1 expression was decreased and negatively correlated with p53 expression. DJ-1 deficiency significantly aggravated colitis, evidenced by increased intestinal inflammation and exacerbated IEC apoptosis. Moreover, DJ-1 directly interacted with p53, and reduced DJ-1 levels increased p53 levels both in vivo and in vitro and were associated with decreased p53 degradation via the lysosomal pathway. We also induced experimental colitis with dextran sulfate sodium in mice and found that compared with DJ-1-/- mice, DJ-1-/-p53-/- mice have reduced apoptosis and inflammation and increased epithelial barrier integrity. Furthermore, pharmacological inhibition of p53 relieved inflammation in the DJ-1-/- mice. In conclusion, reduced DJ-1 expression promotes inflammation and IEC apoptosis via p53 in colitis, suggesting that the modulation of DJ-1 expression may be a potential therapeutic strategy for managing colitis.

Claudins in Renal Physiology and Pathology

Claudins are integral proteins expressed at the tight junctions of epithelial and endothelial cells. In the mammalian kidney, every tubular segment express a specific set of claudins that give to that segment unique properties regarding permeability and selectivity of the paracellular pathway. So far, 3 claudins (10b, 16 and 19) have been causally traced to rare human syndromes: variants of CLDN10b cause HELIX syndrome and variants of CLDN16 or CLDN19 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis. The review summarizes our current knowledge on the physiology of mammalian tight junctions and paracellular ion transport, as well as on the role of the 3 above-mentioned claudins in health and disease. Claudin 14, although not having been causally linked to any rare renal disease, is also considered, because available evidence suggests that it may interact with claudin 16. Some single-nucleotide polymorphisms of CLDN14 are associated with urinary calcium excretion and/or kidney stones. For each claudin considered, the pattern of expression, the function and the human syndrome caused by pathogenic variants are described.

Components identification and nutritional value exploration of tea (Camellia sinensis L.) flower extract: Evidence for functional food

Camellia sinensis L., its fresh leaves and buds are used to make tea, is an important industrial crop with a long history. However, less attention has been paid to tea flowers. Indeed, tea flower extract (TFE) is a rich source of functional molecules, but its nutritional value remains unclear. This study, from the perspective of "whole food", aimed to investigate the composition of TFE and further explore its possible health-promoting effects on cyclophosphamide-induced mice. It was found that TFE was mainly composed of carbohydrates (34.02 ± 1.42%), phenolic compounds (11.57 ± 0.14%), crude proteins (27.72 ± 3.07%) and saponins (2.81 ± 0.00%). Supplementation of TFE at 200 mg/kg·BW/d regulated intestinal homeostasis by improving the intestinal barrier, alleviating dysbacteriosis (reverse 44 of 68 disordered genera), stimulated immunoreactions with significant enhancement of serum TNF-α, IFN-γ, IL-1β, IL-2 and IL-6. Furthermore, TFE could improve the liver function through decreasing the hepatic malondialdehyde and aminotransferase levels and increasing the levels of catalase, myeloperoxidase, superoxide dismutase and reduced glutathione. Notably, the ameliorating effects of TFE on cyclophosphamide-induced immunosuppression and the hepatic injury were associated with its modulation of gut microbiota. The results provide the evidence for the application of tea flower as potential functional food.

NCoR1 Protects Mice From Dextran Sodium Sulfate-Induced Colitis by Guarding Colonic Crypt Cells From Luminal Insult

BACKGROUND & AIMS

Colonic stem cells are essential for producing the mucosal lining, which in turn protects stem cells from insult by luminal factors. Discovery of genetic and biochemical events that control stem cell proliferation and differentiation can be leveraged to decipher the causal factors of ulcerative colitis and aid the development of more effective therapy.

METHODS

We performed in vivo and in vitro studies from control (nuclear receptor corepressor 1 [NCoR1^F/F]) and intestinal epithelial cell-specific NCoR1-deficient mice (NCoR1^ΔIEC). Mice were challenged with dextran sodium sulfate to induce experimental ulcerative colitis, followed by colitis examination, barrier permeability analysis, cell proliferation immunostaining assays, and RNA sequencing analysis. By using crypt cultures, the organoid-forming efficiency, cell proliferation, apoptosis, and histone acetylation were analyzed after butyrate and/or tumor necrosis factor α treatments.

RESULTS

NCoR1^ΔIEC mice showed a dramatic increase in disease severity in this colitis model, with suppression of proliferative cells at the crypt base as an early event and a concomitant increase in barrier permeability. Genome expression patterns showed an important role for NCoR1 in colonic stem cell proliferation and secretory cell differentiation. Colonic organoids cultured from NCoR1^ΔIEC mice were more sensitive to butyrate-induced cell growth inhibition and apoptosis, which were exaggerated further by tumor necrosis factor α co-treatment, which was accompanied by increased histone acetylation.

CONCLUSIONS

NCoR1 regulates colonic stem cell proliferation and secretory cell differentiation. When NCoR1 is disrupted, barrier protection is weakened, allowing luminal products such as butyrate to penetrate and synergistically damage the colonic crypt cells. Transcript profiling: RNA sequencing data have been deposited in the GEO database, accession number: GSE136153.

The Integral Role of Tight Junction Proteins in the Repair of Injured Intestinal Epithelium

The intestinal epithelial monolayer forms a transcellular and paracellular barrier that separates luminal contents from the interstitium. The paracellular barrier consists of a highly organized complex of intercellular junctions that is primarily regulated by apical tight junction proteins and tight junction-associated proteins. This homeostatic barrier can be lost through a multitude of injurious events that cause the disruption of the tight junction complex. Acute repair after injury leading to the reestablishment of the tight junction barrier is crucial for the return of both barrier function as well as other cellular functions, including water regulation and nutrient absorption. This review provides an overview of the tight junction complex components and how they link to other plasmalemmal proteins, such as ion channels and transporters, to induce tight junction closure during repair of acute injury. Understanding the components of interepithelial tight junctions and the mechanisms of tight junction regulation after injury is crucial for developing future therapeutic targets for patients experiencing dysregulated intestinal permeability.

Tight Junction Proteins and the Biology of Hepatobiliary Disease

Tight junctions (TJ) are intercellular adhesion complexes on epithelial cells and composed of integral membrane proteins as well as cytosolic adaptor proteins. Tight junction proteins have been recognized to play a key role in health and disease. In the liver, TJ proteins have several functions: they contribute as gatekeepers for paracellular diffusion between adherent hepatocytes or cholangiocytes to shape the blood-biliary barrier (BBIB) and maintain tissue homeostasis. At non-junctional localizations, TJ proteins are involved in key regulatory cell functions such as differentiation, proliferation, and migration by recruiting signaling proteins in response to extracellular stimuli. Moreover, TJ proteins are hepatocyte entry factors for the hepatitis C virus (HCV)—a major cause of liver disease and cancer worldwide. Perturbation of TJ protein expression has been reported in chronic HCV infection, cholestatic liver diseases as well as hepatobiliary carcinoma. Here we review the physiological function of TJ proteins in the liver and their implications in hepatobiliary diseases.

Cell junction proteins: Crossing the glomerular filtration barrier in diabetic nephropathy

Diabetic nephropathy as a deleterious complication of diabetes mellitus and an important cause of end-stage renal failure is characterized by changes in the molecular and cellular levels. Cell-cell communication via the gap and tight junctions are involved in the pathogenesis of diseases such as diabetes and kidney failure. Studying cell junctions including gap junctions, tight junctions, and anchoring junctions within the nephron can be used as an early sign of diabetic nephropathy. Furthermore, cell junctions may be an upcoming target by pharmacological methods to improve treatments of diabetic nephropathy and pave the way to introduce promising therapeutic strategies based on cell-cell communications effects and its translation into clinical studies for the treatment of diabetic nephropathy.

Enteropathogenic Escherichia coli (EPEC) Recruitment of PAR Polarity Protein Atypical PKCζ to Pedestals and Cell-Cell Contacts Precedes Disruption of Tight Junctions in Intestinal Epithelial Cells

Enteropathogenic Escherichia coli (EPEC) uses a type three secretion system to inject effector proteins into host intestinal epithelial cells, causing diarrhea. EPEC induces the formation of pedestals underlying attached bacteria, disrupts tight junction (TJ) structure and function, and alters apico-basal polarity by redistributing the polarity proteins Crb3 and Pals1, although the mechanisms are unknown. Here we investigate the temporal relationship of PAR polarity complex and TJ disruption following EPEC infection. EPEC recruits active aPKCζ, a PAR polarity protein, to actin within pedestals and at the plasma membrane prior to disrupting TJ. The EPEC effector EspF binds the endocytic protein sorting nexin 9 (SNX9). This interaction impacts actin pedestal organization, recruitment of active aPKCζ to actin at cell–cell borders, endocytosis of JAM-A S285 and occludin, and TJ barrier function. Collectively, data presented herein support the hypothesis that EPEC-induced perturbation of TJ is a downstream effect of disruption of the PAR complex and that EspF binding to SNX9 contributes to this phenotype. aPKCζ phosphorylates polarity and TJ proteins and participates in actin dynamics. Therefore, the early recruitment of aPKCζ to EPEC pedestals and increased interaction with actin at the membrane may destabilize polarity complexes ultimately resulting in perturbation of TJ.

Na+-Coupled Nutrient Cotransport Induced Luminal Negative Potential and Claudin-15 Play an Important Role in Paracellular Na+ Recycling in Mouse Small Intestine

Many nutrients are absorbed via Na⁺ cotransport systems, and therefore it is predicted that nutrient absorption mechanisms require a large amount of luminal Na⁺. It is thought that Na⁺ diffuses back into the lumen via paracellular pathways to support Na⁺ cotransport absorption. However, direct experimental evidence in support of this mechanism has not been shown. To elucidate this, we took advantage of claudin-15 deficient (cldn15^-/-) mice, which have been shown to have decreased paracellular Na⁺ permeability. We measured glucose-induced currents (ΔI_sc) under open- and short-circuit conditions and simultaneously measured changes in unidirectional ²²Na⁺ fluxes (ΔJ^Na) in Ussing chambers. Under short-circuit conditions, application of glucose resulted in an increase in ΔI_sc and unidirectional mucosal to serosal ²²Na⁺ (∆J^Na_MS) flux in both wild-type and cldn15^-/- mice. However, under open-circuit conditions, ΔI_sc was observed but ∆J^Na_MS was strongly inhibited in wild-type but not in cldn15^-/- mice. In addition, in the duodenum of mice treated with cholera toxin, paracellular Na⁺ conductance was decreased and glucose-induced ∆J^Na_MS increment was observed under open-circuit conditions. We concluded that the Na⁺ which is absorbed by Na⁺-dependent glucose cotransport is recycled back into the lumen via paracellular Na⁺ conductance through claudin-15, which is driven by Na⁺ cotransport induced luminal negativity.

Claudin-15 overexpression inhibits proliferation and promotes apoptosis of Schwann cells in vitro

Our previous experiments have discovered that Claudin-15 was up-regulated in Schwann cells of the distal nerve stumps of rat models of sciatic nerve injury. However, how Claudin-15 affects Schwann cell function is still unknown. This study aimed to identify the effects of Claudin-15 on proliferation and apoptosis of Schwann cells cultured in vitro and explore the underlying mechanisms. Primary Schwann cells were obtained from rats. Claudin-15 in Schwann cells was knocked down using siRNA (siRNA-1 group) compared with the negative control siRNA transfection group (negative control group). Claudin-15 in Schwann cells was overexpressed using pGV230-Claudin-15 plasmid (pGV230-Claudin-15 group). The pGV230 transfection group (pGV230 group) acted as the control of the pGV230-Claudin-15 group. Cell proliferation was analyzed with EdU assay. Cell apoptosis was analyzed with flow cytometric analysis. Cell migration was analyzed with Transwell inserts. The mRNA and protein expressions were analyzed with quantitative polymerase chain reaction assay and western blot assay. The results showed that compared with the negative control group, cell proliferation rate was up-regulated; p-AKT/AKT ratio, apoptotic rate, p-c-Jun/c-Jun ratio, mRNA expression of protein kinase C alpha, Bcl-2 and Bax were down-regulated; and mRNA expression of neurotrophins basic fibroblast growth factor and neurotrophin-3 were increased in the siRNA-1 group. No significant difference was found in cell migration between the negative control and siRNA-1 groups. Compared with the pGV230 group, the cell proliferation rate was down-regulated; apoptotic rate, p-c-Jun/c-Jun ratio and c-Fos protein expression increased; mRNA expression of protein kinase C alpha and Bax decreased; and mRNA expressions of neurotrophins basic fibroblast growth factor and neurotrophin-3 were up-regulated in the pGV230-Claudin-15 group. The above results demonstrated that overexpression of Claudin-15 inhibited Schwann cell proliferation and promoted Schwann cell apoptosis in vitro. Silencing of Claudin-15 had the reverse effect and provided neuroprotective effect. This study was approved by the Experimental Animal Ethics Committee of Jilin University of China (approval No. 2016-nsfc001) on March 5, 2016.

Single-Cell RNA Transcriptome Helps Define the Limbal/Corneal Epithelial Stem/Early Transit Amplifying Cells and How Autophagy Affects This Population

Purpose

Single-cell RNA-sequencing (scRNA-seq) was used to interrogate the relatively rare stem (SC) and early transit amplifying (TA) cell populations in limbal/corneal epithelia from wild-type and autophagy-compromised mice.

Methods

We conducted scRNA-seq on ocular anterior segmental tissue from wild-type and beclin 1–deficient (beclin1^+/−) mice, using a 10X Gemomics pipeline. Cell populations were distinguished by t-distributed stochastic neighbor embedding. Seurat analysis was conducted to compare gene expression profiles between these two groups of mice. Differential protein expression patterns were validated by immunofluorescence staining and immunoblotting.

Results

Unbiased clustering detected 10 distinct populations: three clusters of mesenchymal and seven clusters of epithelial cells, based on their unique molecular signatures. A discrete group of mesenchymal cells expressed genes associated with corneal stromal SCs. We identified three limbal/corneal epithelial cell subpopulations designated as stem/early TA, mature TA, and differentiated corneal epithelial cells. Thioredoxin-interacting protein and PDZ-binding kinase (PBK) were identified as novel regulators of stem/early TA cell quiescence. PBK arrested corneal epithelial cells in G2/M phase of the cell cycle. Beclin1^+/− mice displayed a decrease in proliferation-associated (Ki67, Lrig1) and stress-response (H2ax) genes. The most increased gene in beclin1^+/− mice was transcription factor ATF3, which negatively regulates limbal epithelial cell proliferation.

Conclusions

Establishment of a comprehensive atlas of genes expressed by stromal and epithelial cells from limbus and cornea forms the foundation for unraveling regulatory networks among these distinct tissues. Similarly, scRNA-seq profiling of the anterior segmental epithelia from wild-type and autophagy-deficient mice provides new insights into how autophagy influences proliferation in these tissues.

Cell adhesion signals regulate the nuclear receptor activity

Cell adhesion is essential for proper tissue architecture and function in multicellular organisms. Cell adhesion molecules not only maintain tissue integrity but also possess signaling properties that contribute to diverse cellular events such as cell growth, survival, differentiation, polarity, and migration; however, the underlying molecular basis remains poorly defined. Here we identify that the cell adhesion signal initiated by the tight-junction protein claudin-6 (CLDN6) regulates nuclear receptor activity. We show that CLDN6 recruits and activates Src-family kinases (SFKs) in second extracellular domain-dependent and Y196/200-dependent manners, and SFKs in turn phosphorylate CLDN6 at Y196/200. We demonstrate that the CLDN6/SFK/PI3K/AKT axis targets the AKT phosphorylation sites in the retinoic acid receptor γ (RARγ) and the estrogen receptor α (ERα) and stimulates their activities. Interestingly, these phosphorylation motifs are conserved in 14 of 48 members of human nuclear receptors. We propose that a similar link between diverse cell adhesion and nuclear receptor signalings coordinates a wide variety of physiological and pathological processes.

Structural Features of Tight-Junction Proteins

Tight junctions are complex supramolecular entities composed of integral membrane proteins, membrane-associated and soluble cytoplasmic proteins engaging in an intricate and dynamic system of protein-protein interactions. Three-dimensional structures of several tight-junction proteins or their isolated domains have been determined by X-ray crystallography, nuclear magnetic resonance spectroscopy, and cryo-electron microscopy. These structures provide direct insight into molecular interactions that contribute to the formation, integrity, or function of tight junctions. In addition, the known experimental structures have allowed the modeling of ligand-binding events involving tight-junction proteins. Here, we review the published structures of tight-junction proteins. We show that these proteins are composed of a limited set of structural motifs and highlight common types of interactions between tight-junction proteins and their ligands involving these motifs.

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

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

Claudin-2: Roles beyond Permeability Functions

Claudin-2 is expressed in the tight junctions of leaky epithelia, where it forms cation-selective and water permeable paracellular channels. Its abundance is under fine control by a complex signaling network that affects both its synthesis and turnover in response to various environmental inputs. Claudin-2 expression is dysregulated in many pathologies including cancer, inflammation, and fibrosis. Claudin-2 has a key role in energy-efficient ion and water transport in the proximal tubules of the kidneys and in the gut. Importantly, strong evidence now also supports a role for this protein as a modulator of vital cellular events relevant to diseases. Signaling pathways that are overactivated in diseases can alter claudin-2 expression, and a good correlation exists between disease stage and claudin-2 abundance. Further, loss- and gain-of-function studies showed that primary changes in claudin-2 expression impact vital cellular processes such as proliferation, migration, and cell fate determination. These effects appear to be mediated by alterations in key signaling pathways. The specific mechanisms linking claudin-2 to these changes remain poorly understood, but adapters binding to the intracellular portion of claudin-2 may play a key role. Thus, dysregulation of claudin-2 may contribute to the generation, maintenance, and/or progression of diseases through both permeability-dependent and -independent mechanisms. The aim of this review is to provide an overview of the properties, regulation, and functions of claudin-2, with a special emphasis on its signal-modulating effects and possible role in diseases.

The Role of the Islet Niche on Beta Cell Structure and Function

The islets of Langerhans or pancreatic islets are pivotal in the control of blood glucose and are complex microorgans embedded within the larger volume of the exocrine pancreas. Humans can have ~3.2 million islets [1] which, to our current knowledge, function in a similar manner to sense circulating blood glucose levels and respond with the secretion of a mix of different hormones that act to maintain glucose concentrations around a specific set point [2]. At a cellular level, the control of hormone secretion by glucose and other secretagogues is well-understood [3]. The key signal cascades have been identified and many details of the secretory process are known. However, if we shift focus from single cells and consider cells within intact islets, we do not have a comprehensive model as to how the islet environment influences cell function and how the islets work as a whole. This is important because there is overwhelming evidence that the structure and function of the individual endocrine cells are dramatically affected by the islet environment [4,5]. Uncovering the influence of this islet niche might drive future progress in treatments for Type 2 diabetes [6] and cell replacement therapies for Type 1 diabetes [7]. In this review, we focus on the insulin secreting beta cells and their interactions with the immediate environment that surrounds them including endocrine-endocrine interactions and contacts with capillaries.

Effects of cigarette smoke on barrier function and tight junction proteins in the bronchial epithelium: protective role of cathelicidin LL-37

Background

Airway epithelial barrier function is maintained by the formation of tight junctions (TJs) and adherens junctions (AJs). Inhalation of cigarette smoke causes airway epithelial barrier dysfunction and may contribute to the pathogenesis of chronic lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). We assessed the effects of cigarette smoke on barrier function and expression of multiple TJ and AJ proteins in the bronchial epithelium. We also examined whether treatment with glucocorticosteroids (GCSs), long-acting β₂-agonists (LABAs), and human cathelicidin LL-37 can protect against cigarette smoke extract (CSE)-induced barrier dysfunction.

Methods

Calu-3 cells cultured at the air-liquid interface were pretreated with or without GCSs, LABAs, GCSs plus LABAs, or LL-37, and subsequently exposed to CSE. Barrier function was assessed by transepithelial electronic resistance (TEER) measurements. Gene and protein expression levels of TJ and AJ proteins were analyzed by quantitative PCR and western blotting, respectively. Immunofluorescence staining of TJ and AJ proteins was performed.

Results

CSE decreased TEER and increased permeability in a concentration-dependent manner. CSE suppressed gene expression of claudin-1, claudin-3, claudin-4, claudin-7, claudin-15, occludin, E-cadherin, junctional adhesion molecule-A (JAM-A) and zonula occludens-1 (ZO-1) within 12 h post-CSE exposure, while suppressed protein expression levels of occludin at 12 h. CSE-treated cells exhibited discontinuous or attenuated immunostaining for claudin-1, claudin-3, claudin-4, occludin, ZO-1, and E-cadherin compared with untreated cells. GCS treatment partially restored CSE-induced TEER reduction, while LABA treatment had no effect. GCS and LABA combination treatment had no additive effect on CSE-induced TEER reduction and gene suppression of TJ and AJ proteins. Human cathelicidin LL-37 counteracted CSE-induced TEER reduction and prevented disruption of occludin and ZO-1. LL-37 also attenuated CSE-induced decreases in gene and protein expression levels of occludin.

Conclusions

CSE caused airway epithelial barrier dysfunction and simultaneously downregulated multiple TJ and AJ proteins. GCS and LABA combination treatment had no additive effect on CSE-induced TEER reduction. LL-37 counteracted CSE-induced TEER reduction and prevented disruption of occludin and ZO-1. Use of LL-37 to counteract airway epithelial barrier dysfunction may have significant benefits for respiratory diseases such as asthma and COPD.

Effects of Claudin-1 on the Action of Clostridium perfringens Enterotoxin in Caco-2 Cells

Clostridium perfringens enterotoxin (CPE) contributes to diarrhea and an often-lethal enterotoxemia. CPE action starts when it binds to claudin receptors, forming a small complex (90 kDa). Six small complexes then oligomerize to create prepores, followed by insertion of beta-hairpins from CPE to form beta-barrel pores named CH-1 or CH-2. Of the ~27 members of the human claudin protein family, only some bind CPE. However, both receptor claudins and the nonreceptor claudin-1 (CLDN-1) are associated with the small and CH-1/CH-2 CPE complexes. Therefore, this study evaluated whether claudin-1 affects CPE action by generating a CLDN-1 null mutant in Caco-2 cells using CRISPR-Cas9. Compared to wild-type Caco-2 cells, paracellular permeability of the CLDN-1 mutant was significantly enhanced, suggesting that claudin-1 may reduce CPE absorption during enterotoxemia. The CLDN-1 mutant was also markedly more sensitive than wild-type Caco-2 cells to apically-applied CPE. The mechanism behind this increased sensitivity involved higher CPE binding by the CLDN-1 mutant vs. wild-type Caco-2 cells, which led to more CH-1/CH-2 complex formation. However, the CH-1/CH-2 complexes formed by the CLDN-1 mutant were less stable or trypsin resistant than those of wild-type cells. These results indicate that, although a nonreceptor, CLDN-1 positively and negatively influences CPE action.

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.

Temporal Effects of Quercetin on Tight Junction Barrier Properties and Claudin Expression and Localization in MDCK II Cells

: Kidney stones affect 10% of the population. Yet, there is relatively little known about how they form or how to prevent and treat them. The claudin family of tight junction proteins has been linked to the formation of kidney stones. The flavonoid quercetin has been shown to prevent kidney stone formation and to modify claudin expression in different models. Here we investigate the effect of quercetin on claudin expression and localization in MDCK II cells, a cation-selective cell line, derived from the proximal tubule. For this study, we focused our analyses on claudin family members that confer different tight junction properties: barrier-sealing (Cldn1, -3, and -7), cation-selective (Cldn2) or anion-selective (Cldn4). Our data revealed that quercetin's effects on the expression and localization of different claudins over time corresponded with changes in transepithelial resistance, which was measured continuously throughout the treatment. In addition, these effects appear to be independent of PI3K/AKT signaling, one of the pathways that is known to act downstream of quercetin. In conclusion, our data suggest that quercetin's effects on claudins result in a tighter epithelial barrier, which may reduce the reabsorption of sodium, calcium and water, thereby preventing the formation of a kidney stone.

The Significance of Relative Claudin Expression in Odontogenic Tumors

Claudins are integral to the structure and function of tight junctions. Altered claudin expression has been shown to affect disease behavior and patient prognosis in various neoplasms. The objectives of this study were to analyze the claudin-1, -4 and -7 expression in odontogenic tumors and characterize their expression pattern in distinct tumor cell types in relation to the recurrence potential. Sixty-nine cases of odontogenic tumors, including 43 ameloblastomas (AM), 17 adenomatoid odontogenic tumors (AOT), 6 ameloblastic fibromas (AF) and 3 ameloblastic carcinomas (AC) were investigated for claudin-1, -4 and -7 expression immunohistochemically. The staining was analyzed semi-quantitatively and categorized into 4 levels, based on the percentage of positively stained neoplastic epithelial cells. Claudin-1 was expressed in all AOT and AF cases, whereas most AC (66.7%) showed no expression. The claudin-1 staining was moderate-to-intense in the odontogenic epithelium of AF. In contrast, its staining of ameloblast-like cells and stellate reticulum-like cells in AM was weak. Claudin-7 expression was noted in all tumor types studied, while the expression of claudin-4 was limited and mainly localized in the squamous differentiated cells of AM and AC. AM showed significantly higher claudin-4, but lower claudin-7 expression than AOT. In addition, AC showed diminished claudin-1 immunoreactivity, compared to AOT. Low claudin-1 expression in AM was significantly associated with the increased clinical recurrence. The loss of claudin-1 may underlie the locally invasive nature of AM.

R40.76 binds to the α domain of ZO-1: role of ZO-1 (α+) in epithelial differentiation and mechano-sensing

The barrier function of epithelia and endothelia depends on tight junctions, which are formed by the polymerization of claudins on a scaffold of ZO proteins. Two differentially spliced isoforms of ZO-1 have been described, depending on the presence of the α domain, but the function of this domain is unclear. ZO-1 also contains a C-terminal ZU5 domain, which is involved in a mechano-sensitive intramolecular interaction with the central (ZPSG) region of ZO-1. Here we use immunoblotting and immunofluorescence to map the binding sites for commercially available monoclonal and polyclonal antibodies against ZO-1, and for a new polyclonal antibody (R3) that we developed against the ZO-1 C-terminus. We demonstrate that antibody R40.76 binds to the α domain, and the R3 antibody binds to the ZU5 domain. The (α+) isoform of ZO-1 shows higher expression in epithelial versus endothelial cells, and in differentiated versus undifferentiated primary keratinocytes, suggesting a link to epithelial differentiation and a potential molecular adaptation to junctions subjected to stronger mechanical forces. These results provide new tools and hypotheses to investigate the role of the α and ZU5 domains in ZO-1 mechano-sensing and dynamic interactions with the cytoskeleton and junctional ligands.

Estrogen receptor β inhibits breast cancer cells migration and invasion through CLDN6-mediated autophagy

Background

Estrogen receptor β (ERβ) has been reported to play an anti-cancer role in breast cancer, but the regulatory mechanism by which ERβ exerts this effect is not clear. Claudin-6 (CLDN6), a tight junction protein, acts as a tumor suppressor gene in breast cancer. Our previous studies have found that 17β-estradiol (E2) induces CLDN6 expression and inhibits MCF-7 cell migration and invasion, but the underlying molecular mechanisms are still unclear. In this study, we aimed to investigate the role of ERβ in this process and the regulatory mechanisms involved.

Methods

Polymerase chain reaction (PCR) and western blot were used to characterize the effect of E2 on the expression of CLDN6 in breast cancer cells. Chromatin immunoprecipitation (ChIP) assays were carried out to confirm the interaction between ERβ and CLDN6. Dual luciferase reporter assays were used to detect the regulatory role of ERβ on the promoter activity of CLDN6. Wound healing and Transwell assays were used to examine the migration and invasion of breast cancer cells. Western blot, immunofluorescence and transmission electron microscopy (TEM) were performed to detect autophagy. Xenograft mouse models were used to explore the regulatory effect of the CLDN6-beclin1 axis on breast cancer metastasis. Immunohistochemistry (IHC) was used to detect ERβ/CLDN6/beclin1 expression in breast cancer patient samples.

Results

Here, E2 upregulated the expression of CLDN6, which was mediated by ERβ. ERβ regulated CLDN6 expression at the transcriptional level. ERβ inhibited the migration and invasion of breast cancer cells through CLDN6. Interestingly, this effect was associated with CLDN6-induced autophagy. CLDN6 positively regulated the expression of beclin1, which is a key regulator of autophagy. Beclin1 knockdown reversed CLDN6-induced autophagy and the inhibitory effect of CLDN6 on breast cancer metastasis. Moreover, ERβ and CLDN6 were positively correlated, and the expression of CLDN6 was positively correlated with beclin1 in breast cancer tissues.

Conclusion

Overall, this is the first study to demonstrate that the inhibitory effect of ERβ on the migration and invasion of breast cancer cells was mediated by CLDN6, which induced the beclin1-dependent autophagic cascade.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1359-9) contains supplementary material, which is available to authorized users.

Vinculin is critical for the robustness of the epithelial cell sheet paracellular barrier for ions

Vinculin in the apical junctional complex maintains the paracellular barrier function specifically for ions, but not for large solutes, by buffering mechanical fluctuations.

The paracellular barrier function of tight junctions (TJs) in epithelial cell sheets is robustly maintained against mechanical fluctuations, by molecular mechanisms that are poorly understood. Vinculin is an adaptor of a mechanosensory complex at the adherens junction. Here, we generated vinculin KO Eph4 epithelial cells and analyzed their confluent cell-sheet properties. We found that vinculin is dispensable for the basic TJ structural integrity and the paracellular barrier function for larger solutes. However, vinculin is indispensable for the paracellular barrier function for ions. In addition, TJs stochastically showed dynamically distorted patterns in vinculin KO cell sheets. These KO phenotypes were rescued by transfecting full-length vinculin and by relaxing the actomyosin tension with blebbistatin, a myosin II ATPase activity inhibitor. Our findings indicate that vinculin resists mechanical fluctuations to maintain the TJ paracellular barrier function for ions in epithelial cell sheets.

The peroxisomal SspA protein is redundant for purine utilization but essential for peroxisome localization in septal pores in Aspergillus nidulans

In an in silico search for correlated gene loss with fungal peroxisomal uric acid oxidase (UOX), we identified PMP22-like proteins, some of which function as promiscuous channels in organellar membranes. To investigate whether PMP22 channels have a role in peroxisomal uric acid transport and catabolism, we functionally analyzed the closest homologue in Aspergillus nidulans, named SspA. We confirmed that SspA is a peroxisomal membrane protein that co-localizes significantly with PTS1-tagged mRFP, UOX or HexA, the latter considered a protein of Woronin bodies (WB), organelles originating from peroxisomes that dynamically plug septal pores in ascomycetes. Our results suggest that in A. nidulans, unlike some other ascomycetes, there is no strict protein segregation of peroxisomal and WB-specific proteins. Importantly, genetic deletion of sspA, but not of hexA, led to lack of peroxisomal localization at septal pores, suggesting that SspA is a key factor for septal pore functioning. Additionally, ΔsspA resulted in increased sensitivity to oxidative stress, apparently as a consequence of not only the inability to plug septal pores, but also a recorded reduction in peroxisome biogenesis. However, deleting sspA had no effect on uric acid or purine utilization, as we hypothesized, a result also in line with the observation that expression of SspA was not affected by regulatory mutants and conditions known to control purine catabolic enzymes. Our results are discussed within the framework of previous studies of SspA homologues in other fungi, as well as, the observed gene losses of PMP22 and peroxisomal uric acid oxidase.

MiR-361-5p inhibits cell proliferation and induces cell apoptosis in retinoblastoma by negatively regulating CLDN8

PURPOSE

MiR-361-5p has been reported to act as tumor suppressor in several types of cancers. Retinoblastoma (RB) is the most common ocular tumor in childhood. The current study aimed to investigate the expression pattern and biological function of miR-361-5p in RB.

METHODS

Quantitative real time was utilized to determine and compare the expression of miR-361-5p in RB cells and normal retinal pigment epithelial cell line ARPE-19. CCK-8 and Edu assay were performed to assess cell proliferation. Cell apoptosis was evaluated using flow cytometry assay. Bioinformatics databases and luciferase reporter assay were applied to predict and confirm the target gene of miR-361-5p in RB cells.

RESULTS

Here, we found miR-361-5p was significantly downregulated in RB cells compared with normal retinal pigment epithelial cell line ARPE-19. MiR-361-5p overexpression significantly inhibited or silencing promoted cell proliferation in Y79 and SO-RB50 cells, respectively. Flow cytometry assay showed a significantly decreased cell apoptosis in miR-361-5p silencing Y79 cells and increased cell apoptosis in miR-361-5p overexpressing SO-RB50 cells. Moreover, miR-361-5p directly bound to the 3' untranslated region of claudin 8 (CLDN8) and inhibited the expression of CLDN8. Furthermore, we found knockdown of CLDN8 photocopied the effect of miR-361-5p on cell proliferation and apoptosis in RB cells.

CONCLUSION

These results indicated that overexpression of miR-361-5p might act as a suppressor in RB by targeting CLDN8 to inhibit the cellular function.

[Role and mechanism of muscarinic acetylcholine receptor in the regulation of submandibular gland secretion]

Muscarinic acetylcholine receptors (mAChRs), including M1-M5 subtypes, are classic receptors in regulating water, ion, and solute transport in salivary gland. Our work focuses on the studies on the expression pattern and function of mAChR in the submandibular gland (SMG), and the underlying mechanism involved in the mAChR-regulated secretion, together with the effect of parasympathectomy on the salivary secretion. Microvascular autotransplantation of SMG into the temporal fossa provides a continuous and endogenous source of fluids, and is currently an effective method for treating severe keratoconjunctivitis sicca. By using RT-PCR, Western blotting, and immunofluorescence, our data demonstrated that the expression of M1 and M3 subtypes were decreased in latent period in rabbit SMG autotransplantation model, whereas carbachol stimulation promoted the salivary secretion, as well as M1 and M3 expressions. By contrast, mAChRs were hypersensitive in epiphora SMGs, whereas atropine gel and botulinum toxin A application significantly inhibited the hypersecretion in both animal models and patients. Furthermore, the possible intracellular signal molecules involved in the mAChR-modulated salivary secretion were explored. Activation of mAChR upregulated the expression of aquaporin 5 (AQP5), the main transporter that mediated water secretion through transcellular pathway, and led to AQP5 trafficking from lipid rafts to non-lipid microdomain. Extracellular signal-regulated kinase 1/2 (ERK1/2) was involved in the mAChR-regulated AQP5 content. mAChR activation also modulated the expression, distribution, and function of tight junction proteins, and increased paracellular permeability. ERK1/2/β-arrestin2/clathrin/ubiquitin signaling pathway was responsible for the mAChR-regulated downregulation of tight junction molecule claudin-4. Cytoskeleton filamentous actin (F-actin) was also involved in the distribution and barrier function of epithelial tight junctions. Besides, endothelial tight junctions were opened by mAChR agonist-evoked salivation in the mice. Furthermore, parasympathetic denervation increased resting salivary secretion in the long terminrats and minipigs. Taken together, our work demonstrated that mAChR regulated saliva secretion via transcellular and paracellular pathways in SMG epithelium as well as tight junction opening in SMG endothelium. Modulation of mAChR might be a promising strategy to ameliorate SMG dysfunction.

Destination and consequences of Panx1 and mutant expression in polarized MDCK cells

The channel-forming membrane glycoprotein pannexin 1 (Panx1) is best characterized as an ATP release channel. To investigate the trafficking and sorting of Panx1, we used polarized MDCK cells and non-polarized BICR-M1R_k cells to track the fate of GFP-tagged Panx1. In non-polarized cells, Panx1 was found throughout the plasma membrane, including the lamellipodia of the tumor cells and the cell surface-targeting domain was mapped to residues 307-379. Panx1 was preferentially enriched at the apical membrane domain of polarized MDCK cells grown as monolayer sheets or as spheroids. Residual Panx1 localized within basolateral membranes of polarized MDCK cells was independent of a putative dileucine sorting motif LL365/6 found within the C-terminal of Panx1. Unexpectedly, stable expression of a Panx1 mutant, where a putative tyrosine-based basolateral sorting motif (YxxØ) was mutated (Y308F), or a truncated Δ379 Panx1 mutant, caused MDCK cells to lose cell-cell contacts and their ability to polarize as they underwent a switch to a more fibroblast-like phenotype. We conclude that Panx1 is preferentially delivered to the apical domain of polarized epithelial cells, and Panx1 mutants drive phenotypic changes to MDCK cells preventing their polarization.

Deficiency of Stomach-Type Claudin-18 in Mice Induces Gastric Tumor Formation Independent of H pylori Infection

BACKGROUND & AIMS

Epithelial cells are joined by tight junctions (TJs) to form a cell sheet. In the stomach, epithelial cell sheet forms an essential barrier against gastric material, including gastric acid. Although the decreased expression of stomach-type claudin-18 (stCldn18), a TJ protein, is generally observed in human gastritis and gastric cancer, its pathological roles are not fully understood. We previously reported that mice lacking stCldn18 (stCldn18-/-) exhibit gastric acid leakage through TJs, which induces active gastritis at a young age. Here, we examined the gastric pathologies in mice after long-term stCldn18 deficiency.

METHODS

The gastric pathologies in stCldn18-/- mice were sequentially analyzed from youth to old age, and compared to those in humans. To examine the relationship between stCldn18 deficiency-induced gastric pathologies and Wnt-dependent tumorigenesis, we generated Wnt1-overexpressing stCldn18-/- mice.

RESULTS

StCldn18-/- mice developed chronic active gastritis at middle age, with expression of the chemoattractant CCL28. At old age, 20-30% of these mice developed gastric tumors with CXCL5 expression, indicative of EMT. In this process, spasmolytic polypeptide-expressing metaplasia (SPEM) cells appeared. Increased expressions of CD44-variants, TLR2, and CXCL5 indicated age-dependent changes in cell characteristics. Some features of the stCldn18-/- mouse gastric tumorigenesis resembled H pylori-infection-related human carcinogenesis. The gastric tumorigenesis was accelerated in Wnt1-overexpressing stCldn18-/- mice, indicating that Wnt is involved in the stCldn18-/- mouse gastric tumorigenesis.

CONCLUSIONS

StCldn18 deficiency induced gastric tumorigenesis in mice without H pylori infection. Our findings revealed that several signaling networks, including the cytokine-, stemness-, and Wnt-signaling pathways, may be activated under the stCldn18-deficiency-induced chronic active gastritis to accelerate the gastric tumorigenesis.

Ping-Pong-Tumor and Host in Pancreatic Cancer Progression

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

Impact of deoxynivalenol and kaempferol on expression of tight junction proteins at different stages of Caco-2 cell proliferation and differentiation

The expression of tight junction proteins in human epithelial colorectal adenocarcinoma (Caco-2) cells was investigated after treatment by the mycotoxin of deoxynivalenol and phenolic compound of kaempferol in different stages of proliferation and differentiation.