Tight junctions and their regulation by non-coding RNAs.

Tight junction (TJ) is a "zippering up" junction structure located at the uppermost portion of adjacent epithelial/endothelial cells in organs and tissues. TJs maintain the relative stability of intracellular substances and functions by closing or opening intercellular pathways, coordinating the entry and exit of molecules of different sizes and charges, and regulating the permeability of paracellular barrier. TJs also prevent microbial invasion, maintain epithelial/endothelial cell polarity, and regulate cell proliferation. TJs are widely present in the skin and mucosal epithelial barriers, intestinal epithelial barrier, glomerular filtration barrier, bladder epithelial barrier, blood-brain barrier, brain-blood tumor barrier, and blood-testis barrier. TJ dysfunction in different organs can lead to a variety of diseases. In addition to signal pathways, transcription factors, DNA methylation, histone modification, TJ proteins can also be regulated by a variety of non-coding RNAs, such as micro-RNAs, long-noncoding RNAs, and circular RNAs, directly or indirectly. This review summarizes the structure of TJs and introduces the functions and regulatory mechanisms of TJs in different organs and tissues. The roles and mechanisms of non-coding RNAs in the regulation of TJs are also highlighted in this review.

Tandem mass tag (TMT) proteomic analysis of fetal lungs revealed differential expression of tight junction proteins in a rat model of congenital diaphragmatic hernia.

OBJECTIVE: Congenital diaphragmatic hernia (CDH) is a common and often lethal birth defect characterized by congenital lung malformation, which severely affects neonate prognosis and mortality. This study aimed to investigate differences in protein expression in order to elucidate the mechanism of CDH-associated pulmonary hypoplasia during the early stage of lung development using tandem mass tag (TMT) quantitative proteomics. METHODS: Nitrofen was administered orally to establish a rat CDH model, and pathological changes were evaluated through hematoxylin-eosin (H&E), PCNA, and Ki67 staining at the pseudoglandular stage. Fetal lungs were then collected, pooled before TMT labeling, and subjected to mass spectrometry. Immunohistochemistry (IHC), Western blotting, and Q-PCR were used to further validate the candidate proteins. RESULTS: A total of 79 differentially expressed proteins (DEPs) were identified when CDH and control lungs were compared, and further bioinformatics analysis showed that these proteins play important roles in tight-junctions, phospholipase D signaling, and the HIF-1 signaling pathway. Three differentially expressed proteins, Cldn3, Magi1, and Myh9 are involved in the tight-junction pathway (P < 0.05), and their differential expressions were confirmed by IHC, Western blotting, and Q-PCR. CONCLUSION: These findings indicate that alterations of tight-junction protein expression may play an important role in the pathogenesis of abnormal lung development in CDH. Further studies are warranted to verify the mechanism by which these tight-junction proteins influence the pathogenesis of CDH-associated pulmonary hypoplasia.

Role of Tricellular Tight Junction Protein Lipolysis-Stimulated Lipoprotein Receptor (LSR) in Cancer Cells.

Maintaining a robust epithelial barrier requires the accumulation of tight junction proteins, LSR/angulin-1 and tricellulin, at the tricellular contacts. Alterations in the localization of these proteins temporarily cause epithelial barrier dysfunction, which is closely associated with not only physiological differentiation but also cancer progression and metastasis. In normal human endometrial tissues, the endometrial cells undergo repeated proliferation and differentiation under physiological conditions. Recent observations have revealed that the localization and expression of LSR/angulin-1 and tricellulin are altered in a menstrual cycle-dependent manner. Moreover, it has been shown that endometrial cancer progression affects these alterations. This review highlights the differences in the localization and expression of tight junction proteins in normal endometrial cells and endometrial cancers and how they cause functional changes in cells.

Selective loss of cortical endothelial tight junction proteins during Alzheimer's disease progression.

While the accumulation and aggregation of amyloid-ß and tau are central events in the pathogenesis of Alzheimer's disease, there is increasing evidence that cerebrovascular pathology is also abundant in Alzheimer's disease brains. In brain capillaries, endothelial cells are connected closely with one another through transmembrane tight junction proteins forming the blood-brain barrier. Because the blood-brain barrier tightly regulates the exchange of molecules between brain and blood and maintains brain homeostasis, its impairment is increasingly recognized as a critical factor contributing to Alzheimer's disease pathogenesis. However, the pathological relationship between blood-brain barrier properties and Alzheimer's disease progression in the human brain is not fully understood. In this study, we show that the loss of cortical tight junction proteins is a common event in Alzheimer's disease, and is correlated with synaptic degeneration. By quantifying the amounts of major tight junction proteins, claudin-5 and occludin, in 12 brain regions dissected from post-mortem brains of normal ageing (n = 10), pathological ageing (n = 14) and Alzheimer's disease patients (n = 19), we found that they were selectively decreased in cortical areas in Alzheimer's disease. Cortical tight junction proteins were decreased in association with the Braak neurofibrillary tangle stage. There was also a negative correlation between the amount of tight junction proteins and the amounts of insoluble Alzheimer's disease-related proteins, in particular amyloid-ß40, in cortical areas. In addition, the amount of tight junction proteins in these areas correlated positively with those of synaptic markers. Thus, loss of cortical tight junction proteins in Alzheimer's disease is associated with insoluble amyloid-ß40 and loss of synaptic markers. Importantly, the positive correlation between claudin-5 and synaptic markers, in particular synaptophysin, was present independent of insoluble amyloid-ß40, amyloid-ß42 and tau values, suggesting that loss of cortical tight junction proteins and synaptic degeneration is present, at least in part, independent of insoluble Alzheimer's disease-related proteins. Collectively, these results indicate that loss of tight junction proteins occurs predominantly in the neocortex during Alzheimer's disease progression. Further, our findings provide a neuropathological clue as to how endothelial tight junction pathology may contribute to Alzheimer's disease pathogenesis in both synergistic and additive manners to typical amyloid-ß and tau pathologies.

Tight junction proteins in gastrointestinal and liver disease.

Over the past two decades a growing body of evidence has demonstrated an important role of tight junction (TJ) proteins in the physiology and disease biology of GI and liver disease. On one side, TJ proteins exert their functional role as integral proteins of TJs in forming barriers in the gut and the liver. Furthermore, TJ proteins can also be expressed outside TJs where they play important functional roles in signalling, trafficking and regulation of gene expression. A hallmark of TJ proteins in disease biology is their functional role in epithelial-to-mesenchymal transition. A causative role of TJ proteins has been established in the pathogenesis of colorectal cancer and gastric cancer. Among the best characterised roles of TJ proteins in liver disease biology is their function as cell entry receptors for HCV-one of the most common causes of hepatocellular carcinoma. At the same time TJ proteins are emerging as targets for novel therapeutic approaches for GI and liver disease. Here we review our current knowledge of the role of TJ proteins in the pathogenesis of GI and liver disease biology and discuss their potential as therapeutic targets.

Arsenic induces autophagy in developmental mouse cerebral cortex and hippocampus by inhibiting PI3K/Akt/mTOR signaling pathway: involvement of blood-brain barrier's tight junction proteins.

For the past decade, there has been an increased concern about the health risks from arsenic (As) exposure, because of its neurotoxic effects on the developing brain. The exact mechanism underlying As-induced neurotoxicity during sensitive periods of brain development remains unclear, especially the role of blood-brain barrier's (BBB) tight junction (TJ) proteins during As-induced neurotoxicity. Here, we highlight the involvement of TJ proteins in As-induced autophagy in cerebral cortex and hippocampus during developmental periods [postnatal day (PND) 21, 28, 35 and 42]. Here, the administration of arsenic trioxide (As2O3) at doses of 0.15 mg or 1.5 mg or 15 mg As2O3/L in drinking water from gestational to lactational and continued to the pups till PND42 resulted in a significant decrease in the mRNA expression levels of TJ proteins (Occludin, Claudin, ZO-1 and ZO-2) and Occludin protein expression level. In addition, As exposure significantly decreased PI3K, Akt, mTOR, and p62 with a concomitant increase in Beclin1, LC3I, LC3II, Atg5 and Atg12. Moreover, As exposure also significantly downregulated the protein expression levels of mTOR with a concomitant upregulation of Beclin 1, LC3 and Atg12 in all the developmental age points. However, no significant alterations were observed in low and medium dose-exposed groups of PND42. Histopathological analysis in As-exposed mice revealed decreased number of pyramidal neurons in hippocampus; and neurons with degenerating axons, shrinkage of cells, remarkable vacuolar degeneration in cytoplasm, karyolysis and pyknosis in cerebral cortex. Ultrastructural analysis by transmission electron microscopy revealed the occurrence of autophagosomes and vacuolated axons in the cerebral cortex and hippocampus of the mice exposed to high dose As at PND21 and 42. The severities of changes were found to more persist in the cerebral cortex than in the hippocampus of As-exposed mice. Finally, we conclude that the leaky BBB in cerebral cortex and hippocampus may facilitate the transfer of As and induces autophagy by inhibiting PI3K/Akt/mTOR signaling pathway in an age-dependent manner, i.e., among the four different developmental age points, PND21 animals were found to be more vulnerable to the As-induced neurotoxicity than the other three age points.

Hydrostatic pressure incubation affects barrier properties of mammary epithelial cell monolayers, in vitro.

During lactation, accumulation of milk in mammary glands (MG) causes hydrostatic pressure (HP) and concentration of bioactive compounds. Previously, a changed expression of tight junction (TJ) proteins was observed in mice MGs by accumulation of milk, in vivo. The TJ primarily determines the integrity of the MG epithelium. The present study questioned whether HP alone can affect the TJ in a mammary epithelial cell model, in vitro. Therefore, monolayers of HC11, a mammary epithelial cell line, were mounted into modified Ussing chambers and incubated with 10 kPa bilateral HP for 4 h. Short circuit current and transepithelial resistance were recorded and compared to controls, and TJ proteins were analyzed by Western blotting and immunofluorescent staining. In our first approach HC11 cells could withstand the pressure incubation and a downregulation of occludin was observed. In a second approach, using prolactin- and dexamethasone-induced cells, a decrease of short circuit current was observed, beginning after 2 h of incubation. With the addition of 1 mM barium chloride to the bathing solution the decrease could be blocked temporarily. On molecular level an upregulation of ZO-1 could be observed in hormone-induced cells, which was downregulated after the incubation with barium chloride. In conclusion, bilateral HP incubation affects mammary epithelial monolayers, in vitro. Both, the reduction of short circuit current and the change in TJ proteins may be interpreted as physiological requirements for lactation.

ß1-integrin-matrix interactions modulate cerebral microvessel endothelial cell tight junction expression and permeability.

Acutely following focal cerebral ischemia disruption of the microvessel blood-brain barrier allows transit of plasma proteins into the neuropil as edema formation that coincides with loss of microvessel endothelial ß1-integrins. We extend previous findings to show that interference with endothelial ß1-integrin-matrix adhesion by the monoclonal IgM Ha2/5 increases the permeability of primary cerebral microvascular endothelial cell monolayers through reorganization of claudin-5, occludin, and zonula occludens-1 (ZO-1) from inter-endothelial borders. Interference with ß1-integrin-matrix adhesion initiates F-actin conformational changes that coincide with claudin-5 redistribution. ß1-integrin-matrix interference simultaneously increases phosphorylation of myosin light chain (MLC), while inhibition of MLC kinase (MLCK) and Rho kinase (ROCK) abolishes the Ha2/5-dependent increased endothelial permeability by 6 h after ß1-integrin-matrix interference. These observations are supported by concordant observations in the cortex of a high-quality murine conditional ß1-integrin deletion construct. Together they support the hypothesis that detachment of ß1-integrins from abluminal matrix ligands increases vascular endothelial permeability through reorganization of tight junction (TJ) proteins via altered F-actin conformation, and indicate that the ß1-integrin-MLC signaling pathway is engaged when ß1-integrin detachment occurs. These findings provide a novel approach to the research and treatment of cerebral disorders where the breakdown of the blood-brain barrier accounts for their progression and complication.

The Endothelin ETB Receptor Antagonist BQ788 Protects against Brain Edema after Fluid Percussion Injury by Decreasing Vascular Endothelial Growth Factor-A Expression in Mice.

Brain edema is a severe morbid complication of brain injury, characterized by excessive fluid accumulation and an elevation of intracranial pressure. However, effective anti-brain edema drugs are lacking. One of the causes of brain edema is disruption of blood-brain barrier (BBB) function, which results in extravasation of intravascular fluid. After brain damage, astrocytes are activated, and astrocyte-derived vascular endothelial growth factor-A (VEGF-A) is known to induce BBB dysfunction. Therefore maintaining BBB integrity by regulating astrocyte function is a potentially effective strategy for treating brain edema. In this review, we focus on the endothelin ETB receptor and its role in regulation of astrocyte functions. In mice, brain damage was induced by fluid percussion injury (FPI), and the resulting BBB disruption and brain edema were observed in the mouse cerebrum. BQ788, a selective ETB receptor antagonist, attenuated the FPI-induced BBB disruption and brain edema. Levels of brain VEGF-A increased after FPI, mainly in reactive astrocytes. BQ788 suppressed the FPI-induced increase in VEGF-A expression in reactive astrocytes. Moreover, intraventricular administration of VEGF neutralizing antibody also attenuated FPI-induced BBB disruption and brain edema. Claudin-5 is an endothelial tight junction protein essential for normal BBB structure and function. Levels of claudin-5 protein were reduced by FPI. Furthermore, VEGF neutralizing antibody blocked FPI-induced decrease in claudin-5. These results suggest that the ETB receptor antagonist BQ788 protects against brain edema by inhibiting VEGF-A-mediated decrease in claudin-5.

Chronic type 2 diabetes reduces the integrity of the blood-brain barrier by reducing tight junction proteins in the hippocampus.

In the present study, we investigated the effects of type 2 diabetes-induced hyperglycemia on the integrity of the blood-brain barrier and tight junction markers in the rat hippocampus. Forty-week-old diabetic (Zucker diabetic fatty, ZDF) rats and littermate control (Zucker lean control, ZLC) rats were used in this study. We evaluated the integrity of the blood-brain barrier by measuring sodium fluorescein extravasation and blood vessel ultrastructure. In addition, tight junction markers, such as zona occludens-1, occludin and claudin-5, were quantified by western blot analysis. ZDF rats showed significantly increased sodium fluorescein leakage in the hippocampus. Tight junction markers, such as occludin and claudin-5, were significantly decreased in the hippocampi of ZDF rats compared to those of ZLC rats. In addition, ZDF rats showed ultrastructural changes with phagocytic findings in the blood vessels. These results suggest that chronic untreated diabetes impairs the permeability of the hippocampal blood-brain barrier by down-regulating occludin and claudin-5, indicating that chronic untreated diabetes may cause hippocampus-dependent dysfunction.

Diosmectite-zinc oxide composite improves intestinal barrier restoration and modulates TGF-ß1, ERK1/2, and Akt in piglets after acetic acid challenge.

The present study evaluated the beneficial effect of diosmectite-zinc oxide composite (DS-ZnO) on improving intestinal barrier restoration in piglets after acetic acid challenge and explored the underlying mechanisms. Twenty-four 35-d-old piglets (Duroc × Landrace × Yorkshire), with an average weight of 8.1 kg, were allocated to 4 treatment groups. On d 1 of the trial, colitis was induced via intrarectal injection of acetic acid (10 mL of 10% acetic acid [ACA] solution for ACA, DS-ZnO, and mixture of diosmectite [DS] and ZnO [DS+ZnO] groups) and the control group was infused with saline. Twenty-four hours after challenged, piglets were fed with the following diets: 1) control group (basal diet), 2) ACA group (basal diet), 3) DS-ZnO group (basal diet supplemented with DS-ZnO), and 4) DS+ZnO group (mixture of 1.5 g diosmectite [DS]/kg and 500 mg Zn/kg from ZnO [equal amount of DS and ZnO in the DS-ZnO treatment group]). On d 8 of the trial, piglets were sacrificed. The results showed that DS-ZnO supplementation improved (P < 0.05) ADG, ADFI, and transepithelial electrical resistance and decreased (P < 0.05) fecal scores, crypt depth, and fluorescein isothiocyanate-dextran 4 kDa (FD4) influx as compared with ACA group. Moreover, DS-ZnO increased (P < 0.05) occludin, claudin-1, and zonula occluden-1 expressions; reduced (P < 0.05) caspase-9 and caspase-3 activity and Bax expression; and improved (P < 0.05) Bcl2, XIAP, and PCNA expression. Diosmectite-zinc oxide composite supplementation also increased (P < 0.05) TGF-ß1 expression and ERK1/2 and Akt activation. These results suggest that DS-ZnO attenuates the acetic acid-induced colitis by improving mucosa barrier restoration, inhibiting apoptosis, and improving intestinal epithelial cells proliferation and modulation of TGF-ß1 and ERK1/2 and Akt signaling pathway.

Oral administration of putrescine and proline during the suckling period improves epithelial restitution after early weaning in piglets.

Polyamines are necessary for normal integrity and the restitution after injury of the gastrointestinal epithelium. The objective of this study was to investigate the effects of oral administration of putrescine and proline during the suckling period on epithelial restitution after early weaning in piglets. Eighteen neonatal piglets (Duroc × Landrace × Large Yorkshire) from 3 litters (6 piglets per litter) were assigned to 3 groups, representing oral administration with an equal volume of saline (control), putrescine (5 mg/kg BW), and proline (25 mg/kg BW) twice daily from d 1 to weaning at 14 d of age. Plasma and intestinal samples were obtained 3 d after weaning. The results showed that oral administration of putrescine or proline increased the final BW and ADG of piglets compared with the control (P < 0.05). Proline treatment decreased plasma D-lactate concentration but increased the villus height in the jejunum and ileum, as well as the percentage of proliferating cell nuclear antigen (PCNA) positive cells and alkaline phosphatase (AKP) activity in the jejunal mucosa (P < 0.05). The protein expressions for zonula occludens (ZO-1), occludin, and claudin-3 (P < 0.05) but not mRNA were increased in the jejunum of putrescine- and proline-treated piglets compared with those of control piglets. The voltage-gated K+ channel (Kv) 1.1 protein expression in the jejunum of piglets administrated with putrescine and the Kv1.5 mRNA and Kv1.1 protein levels in the ileum of piglets administrated with proline were greater than those in control piglets (P < 0.05). These findings indicate that polyamine or its precursor could improve mucosal proliferation, intestinal morphology, as well as tight junction and potassium channel protein expressions in early-weaned piglets, with implications for epithelial restitution and barrier function after stress injury.

Effects of cello-oligosaccharide on intestinal microbiota and epithelial barrier function of weanling pigs.

A total of 144 piglets (Duroc × Landrace × Yorkshire; average initial weight of 6.13 kg weaned at 21 ± 1 d age) were allotted to 4 treatments for 2 wk, each of which had 6 pens with 6 pigs per pen. After the feeding experiment, 6 pigs per treatment were slaughtered to investigate the effects of cello-oligosaccharide (COS) on intestinal microbiota and epithelial barrier function. The COS was added to the basal diet at 0, 1.5, 3.0, and 4.5 g/kg diet at the expense of corn, respectively. Plasma -lactate, diamine oxidase (DAO), and the Ussing chamber technique were used to determine the intestinal barrier function. 16S rRNA-based methods were used for intestinal microbiota analysis. The results showed that incremental levels of COS had no effect ( > 0.05) on growth performance. Incremental levels of COS increased lactobacilli in jejunal and colonic contents ( < 0.05); decreased in jejunal contents ( < 0.05) and and in colonic contents ( < 0.05); reduced plasma DAO (linear, = 0.013, and quadratic, = 0.037); increased jejunal mucosa DAO (linear, = 0.003, and quadratic, = 0.008); decreased fluorescein isothiocyanate dextran 4 kDa flux of jejunum and colon ( < 0.05); and increased transepithelial electrical resistance (TER) in colon ( < 0.05), claudin-1 protein expression in jejunal mucosa (linear, = 0.001, and quadratic, = 0.003), and protein expressions of claudin-1 and zonula occludens-1 (ZO-1) in colonic mucosa linearly ( = 0.001 and = 0.001, respectively) and quadratically ( = 0.001 and = 0.002, respectively). The results indicated that the improved microbial ecosystem in the presence of COS might contribute to improvement in intestinal barrier function and tight junction proteins. Results also showed that the appropriate dietary COS supplementation level was 3.0 g/kg in weaned pig diets under our trial conditions.

Tricellulin and its role in the epididymal epithelium of the rat.

Tricellulin is a tight-junction protein present at tricellular tight junctions. It has been suggested that basal cells are implicated in the blood-epididymis barrier. Basal cells express claudins, a component of tight junctions; however, there is no information regarding the potential architecture or regulation of basal cell-principal cell interactions. The present objectives were to determine the expression and localization of tricellulin in rat epididymis in relation to occludin, basal cell-principal cell interactions, and other junctional proteins. Tricellulin levels were similar in all segments of the adult epididymis, and the protein was localized to the apical region of the epithelium. Postnatal development showed that tricellulin levels increased with age and localization changed from cytoplasmic to membrane-bound as a function of age. Colocalization with occludin indicated that both proteins are in the region of the tight junction. In the initial segment, the proteins did not colocalize compared to the epididymis where they were both colocalized. Tricellulin did not colocalize with cytokeratin 5, a marker of basal cells, in any region of the epididymis, including the corpus and cauda epididymidis, where apical projections of basal cells were apparent. Tricellulin knockdown studies using small interfering RNA in rat caput epididymal principal cells resulted in decreased transepithelial resistance and was correlated with decreased levels of Cldn3, Cldn1, and occludin. Tight-junction protein1, also known as ZO-1, and cadherin1 levels were unchanged. This is the first report of tricellulin in the epididymis and on the interaction between tricellulin and other tight-junction proteins.

Emerging multifunctional roles of Claudin tight junction proteins in bone.

The imbalance between bone formation and resorption during bone remodeling has been documented to be a major factor in the pathogenesis of osteoporosis. Recent evidence suggests a significant role for the tight junction proteins, Claudins (Cldns), in the regulation of bone remodeling processes. In terms of function, whereas Cldns act "canonically" as key determinants of paracellular permeability, there is considerable recent evidence to suggest that Cldns also participate in cell signaling, ie, a "noncanonical function". To this end, Cldns have been shown to regulate cell proliferation, differentiation, and gene expression in a variety of cell types. The present review will discuss Cldns' structure, their expression profile, regulation of expression, and their canonical and non- canonical functions in general with special emphasis on bone cells. In order to shed light on the noncanonical functions of Cldns in bone, we will highlight the role of Cldn-18 in regulating bone resorption and osteoclast differentiation. Collectively, we hope to provide a framework for guiding future research on understanding how Cldns modulate osteoblast and osteoclast function and overall bone homeostasis. Such studies should provide valuable insights into the pathogenesis of osteoporosis, and may highlight Cldns as novel targets for the diagnosis and therapeutic management of osteoporosis.

[Relationship between tight junction proteins and Helicobacter pylori-associated gastric diseases].

Helicobacter pylori (Hp) infection is an important cause of chronic gastritis and peptic ulcer, but their pathogenesis is unclear. The role of gastric mucosal barrier dysfunction induced by impaired structure and function of tight junction in the pathogenesis of Hp-associated gastric diseases has received considerable attention in recent years. Tight junction is composed of a variety of proteins and molecules, including 3 integral membrane proteins (occludin, claudins, and junctional adhesion molecules) and a cytoplasmic protein (zonula occludens). This paper mainly describes the composition and function of various tight junction proteins, changes in tight junction protein function induced by Hp infection and their relationship with the incidence of gastric diseases, and the significance of enhancing the tight junction protein function in the prevention and treatment of Hp-associated gastric diseases.

Exercise regulation of intestinal tight junction proteins.

Gastrointestinal distress, such as diarrhoea, cramping, vomiting, nausea and gastric pain are common among athletes during training and competition. The mechanisms that cause these symptoms are not fully understood. The stress of heat and oxidative damage during exercise causes disruption to intestinal epithelial cell tight junction proteins resulting in increased permeability to luminal endotoxins. The endotoxin moves into the blood stream leading to a systemic immune response. Tight junction integrity is altered by the phosphoylation state of the proteins occludin and claudins, and may be regulated by the type of exercise performed. Prolonged exercise and high-intensity exercise lead to an increase in key phosphorylation enzymes that ultimately cause tight junction dysfunction, but the mechanisms are different. The purpose of this review is to (1) explain the function and physiology of tight junction regulation, (2) discuss the effects of prolonged and high-intensity exercise on tight junction permeability leading to gastrointestinal distress and (3) review agents that may increase or decrease tight junction integrity during exercise.

Tight junction proteins: from barrier to tumorigenesis.

The tight junction is a multi-protein complex and is the apical most junctional complex in certain epithelial and endothelial cells. A great deal of attention has been devoted to the understanding of these proteins in contributing to the barrier function - that is, regulating the paracellular flux or permeability between adjacent cells. However, tight junction proteins are now recognized as having functions beyond the barrier. The focus of this review is to discuss the barrier function of the tight junction and to summarize the literature with a focus on the role of tight junction proteins in proliferation, transformation, and metastasis.

NG2 regulates directional migration of oligodendrocyte precursor cells via Rho GTPases and polarity complex proteins.

The transmembrane proteoglycan NG2 is expressed by oligodendrocyte precursor cells (OPC), which migrate to axons during developmental myelination and remyelinate in the adult after migration to injured sites. Highly invasive glial tumors also express NG2. Despite the fact that NG2 has been implicated in control of OPC migration, its mode of action remains unknown. Here, we show in vitro and in vivo that NG2 controls migration of OPC through the regulation of cell polarity. In stab wounds in adult mice we show that NG2 controls orientation of OPC toward the wound. NG2 stimulates RhoA activity at the cell periphery via the MUPP1/Syx1 signaling pathway, which favors the bipolar shape of migrating OPC and thus directional migration. Upon phosphorylation of Thr-2256, downstream signaling of NG2 switches from RhoA to Rac stimulation. This triggers process outgrowth through regulators of front-rear polarity and we show using a phospho-mimetic form of NG2 that indeed NG2 recruits proteins of the CRB and the PAR polarity complexes to stimulate Rac activity via the GEF Tiam1. Our findings demonstrate that NG2 is a core organizer of Rho GTPase activity and localization in the cell, which controls OPC polarity and directional migration. This work also reveals CRB and PAR polarity complexes as new effectors of NG2 signaling in the establishment of front-rear polarity.

[Claudins as tight junction proteins: the molecular element of paracellular transport].

Epithelium is a key structure of tissue barriers ensuring a creation of electrochemical and osmotic gradients. There are transcellular and paracellular types of transepithelium transport of molecules and ions. Epithelial layer permeability for paracellular transport as well as restriction of lateral integrative protein diffusion in a plasma membrane is determined by apical intercellular complex including tight junctions. Integrative proteins of the claudin family are basic molecular components of tight junctions. Properties of single claudins and their complexes define the differences in a degree of epithelial permeability. The subfamily of claudins forming charge- and size-selective pores provides selective paracellular diffusion. The subfamily of claudins increasing epithelial impermeability strengthens epithelial barrier features.