Occludin 1B, a variant of the tight junction protein occludin

Occludin: a gatekeeper of brain Infection by HIV-1

Compromised structure and function of the blood-brain barrier (BBB) is one of the pathological hallmarks of brain infection by HIV-1. BBB damage during HIV-1 infection has been associated with modified expression of tight junction (TJ) proteins, including occludin. Recent evidence indicated occludin as a redox-sensitive, multifunctional protein that can act as both an NADH oxidase and influence cellular metabolism through AMPK kinase. One of the newly identified functions of occludin is its involvement in regulating HIV-1 infection. Studies suggest that occludin expression levels and the rate of HIV-1 infection share a reverse, bidirectional relationship; however, the mechanisms of this relationship are unclear. In this review, we describe the pathways involved in the regulation of HIV-1 infection by occludin. We propose that occludin may serve as a potential therapeutic target to control HIV-1 infection and to improve the lives of people living with HIV-1.

The effects of pectin on the gut microbiota and serum metabolites in mice fed with a high fat diet and exposed to low-dose antibiotics

A sedentary lifestyle, unhealthy diet, and antibiotic use among other environmental factors have been associated with an increased incidence of metabolic disorders and inflammation, as well as gut dysbiosis. Pectin is an edible polysaccharide that exists widely in the cell wall of plants. Our previous study has shown that pectin with various degrees of esterification displayed different effects on preventing acute colitis and regulating the gut microbiome and serum metabolome. This study aimed to further explore the differential effects of pectin with various degrees of esterification on mice simultaneously treated with a high-fat diet and low-dose antibiotics. The results showed that low-esterified pectin L102 improved the biomarkers of metabolic disorders including blood glucose and body weight. The high-esterified pectin H121 and the low-esterified pectin L13 ameliorated inflammatory markers such as superoxide dismutase (SOD). The enrichment of probiotic bacteria such as Lactobacillus by pectin L102, reduction of conditional pathogens such as Klebsiella by pectin L13, and changes in circulating metabolites like L-tryptophan and 3-indoleacrylate by all three types of pectins were detected. These data provide evidence for a differential effect of different types of pectin on the gut microbiota and metabolic health.

A critical role of AMP-activated protein kinase in regulating intestinal nutrient absorption, barrier function, and intestinal diseases

As one of the most important organs in animals, the intestine is responsible for nutrient absorption and acts as a barrier between the body and the environment. Intestinal physiology and function require the participation of energy. 5'-adenosine monophosphate-activated protein kinase (AMPK), a classical and highly expressed energy regulator in intestinal cells, regulates the process of nutrient absorption and barrier function and is also involved in the therapy of intestinal diseases. Studies have yielded findings that AMPK regulates the absorption of glucose, amino acids, and fatty acids in the intestine primarily by regulating transportation systems, as we detailed here. Moreover, AMPK is involved in the regulation of the intestinal mechanical barrier and immune barrier through manipulating the expression of tight junctions, antimicrobial peptides, and secretory immunoglobulins. In addition, AMPK also participates in the regulation of intestinal diseases, which indicates that AMPK is a promising therapeutic target for intestinal diseases and cancer. In this review, we summarized the current understanding regarding how AMPK regulates intestinal nutrient absorption, barrier function, and intestinal diseases.

Dysregulation of Intestinal Physiology by Aflatoxicosis in the Gilthead Seabream (Sparus aurata)

Aflatoxin B1 (AFB1) is a mycotoxin often present in food. This study aimed to understand the physiological effects of AFB1 on the seabream (Sparus aurata) gastrointestinal system. In a first in vitro approach, we investigated ion transport using the short-circuit current (Isc) technique in Ussing chambers in the anterior intestine (AI). Application of apical/luminal AFB1 concentrations of 8 and 16 μM to healthy tissues was without effect on tissue transepithelial electrical resistance (TER), and apparent tissue permeability (Papp) was measured using fluorescein FITC (4 kD). However, it resulted in dose-related effects on Isc. In a second approach, seabream juveniles fed with different AFB1 concentrations (1 and 2 mg AFB1 kg⁻¹ fish feed) for 85 days showed significantly reduced gill Na⁺/K⁺-ATPase (NKA) and H⁺-ATPase (HA) activities in the posterior intestine (PI). Moreover, dietary AFB1 modified Isc in the AI and PI, significantly affecting TER in the AI. To understand this effect on TER, we analyzed the expression of nine claudins and three occludins as markers of intestinal architecture and permeability using qPCR. Around 80% of the genes presented significantly different relative mRNA expression between AI and PI and had concomitant sensitivity to dietary AFB1. Based on the results of our in vitro, in vivo, and molecular approaches, we conclude that the effects of dietary AFB1 in the gastrointestinal system are at the base of the previously reported growth impairment caused by AFB1 in fish.

Single-cell transcriptomic profiling provides insights into retinal endothelial barrier properties

Purpose

To better characterize retinal endothelial barrier properties through analysis of individual transcriptomes of primary bovine retinal microvascular endothelial cells (RMECs).

Methods

Individual RMECs were captured on the Fluidigm C1 system, cDNA libraries were prepared using a Nextera XT kit, and sequencing was performed on a NextSeq system (Illumina). Data analysis was performed using R packages Scater, SC3, and Seurat, and the browser application Automated Single-cell Analysis Pipeline (ASAP). Alternative splicing events in single cells were quantified with Outrigger. Cytoscape was used for network analyses.

Results

Application of a single-cell RNA sequencing (scRNA-seq) analysis workflow showed that RMECs form a relatively homogeneous population in culture, with the main differences related to proliferation status. Expression of markers from along the arteriovenous tree suggested that most cells originated from capillaries. Average gene expression levels across all cells were used to develop an in silico model of the inner blood-retina barrier incorporating junctional proteins not previously reported within the retinal vasculature. Correlation of barrier gene expression among individual cells revealed a subgroup of genes highly correlated with PECAM-1 at the center of the correlation network. Numerous alternative splicing events involving exons within microvascular barrier genes were observed, and in many cases, individual cells expressed one isoform exclusively.

Conclusions

We optimized a workflow for single-cell transcriptomics in primary RMECs. The results provide fundamental insights into the genes involved in formation of the retinal-microvascular barrier.

Hepatitis C virus infection and tight junction proteins: The ties that bind

The hepatitis C virus (HCV) is a major cause of liver diseases ranging from liver inflammation to advanced liver diseases like cirrhosis and hepatocellular carcinoma (HCC). HCV infection is restricted to the liver, and more specifically to hepatocytes, which represent around 80% of liver cells. The mechanism of HCV entry in human hepatocytes has been extensively investigated since the discovery of the virus 30 years ago. The entry mechanism is a multi-step process relying on several host factors including heparan sulfate proteoglycan (HSPG), low density lipoprotein receptor (LDLR), tetraspanin CD81, Scavenger Receptor class B type I (SR-BI), Epidermal Growth Factor Receptor (EGFR) and Niemann-Pick C1-like 1 (NPC1L1). Moreover, in order to establish a persistent infection, HCV entry is dependent on the presence of tight junction (TJ) proteins Claudin-1 (CLDN1) and Occludin (OCLN). In the liver, tight junction proteins play a role in architecture and homeostasis including sealing the apical pole of adjacent cells to form bile canaliculi and separating the basolateral domain drained by sinusoidal blood flow. In this review, we will highlight the role of liver tight junction proteins in HCV infection, and we will discuss the potential targeted therapeutic approaches to improve virus eradication.

Oral Mucosal Epithelial Cells

Cellular Phenotype and Apoptosis: The function of epithelial tissues is the protection of the organism from chemical, microbial, and physical challenges which is indispensable for viability. To fulfill this task, oral epithelial cells follow a strongly regulated scheme of differentiation that results in the formation of structural proteins that manage the integrity of epithelial tissues and operate as a barrier. Oral epithelial cells are connected by various transmembrane proteins with specialized structures and functions. Keratin filaments adhere to the plasma membrane by desmosomes building a three-dimensional matrix.

Cell-Cell Contacts and Bacterial Influence: It is known that pathogenic oral bacteria are able to affect the expression and configuration of cell-cell junctions. Human keratinocytes up-regulate immune-modulatory receptors upon stimulation with bacterial components. Periodontal pathogens including P. gingivalis are able to inhibit oral epithelial innate immune responses through various mechanisms and to escape from host immune reaction, which supports the persistence of periodontitis and furthermore is able to affect the epithelial barrier function by altering expression and distribution of cell-cell interactions including tight junctions (TJs) and adherens junctions (AJs).

In the pathogenesis of periodontitis a highly organized biofilm community shifts from symbiosis to dysbiosis which results in destructive local inflammatory reactions.

Cellular Receptors: Cell-surface located toll like receptors (TLRs) and cytoplasmatic nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) belong to the pattern recognition receptors (PRRs). PRRs recognize microbial parts that represent pathogen-associated molecular patterns (PAMPs).

A multimeric complex of proteins known as inflammasome, which is a subset of NLRs, assembles after activation and proceeds to pro-inflammatory cytokine release.

Cytokine Production and Release: Cytokines and bacterial products may lead to host cell mediated tissue destruction. Keratinocytes are able to produce diverse pro-inflammatory cytokines and chemokines, including interleukin (IL)-1, IL-6, IL-8 and tumor necrosis factor (TNF)-α. Infection by pathogenic bacteria such as Porphyromonas gingivalis (P. gingivalis) and Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans) can induce a differentiated production of these cytokines.

Immuno-modulation, Bacterial Infection, and Cancer Cells: There is a known association between bacterial infection and cancer. Bacterial components are able to up-regulate immune-modulatory receptors on cancer cells. Interactions of bacteria with tumor cells could support malignant transformation an environment with deficient immune regulation.

The aim of this review is to present a set of molecular mechanisms of oral epithelial cells and their reactions to a number of toxic influences.

Implications of AMPK in the Formation of Epithelial Tight Junctions

Tight junctions (TJ) play an essential role in the epithelial barrier. By definition, TJ are located at the demarcation between the apical and baso-lateral domains of the plasma membrane in epithelial cells. TJ fulfill two major roles: (i) TJ prevent the mixing of membrane components; and (ii) TJ regulate the selective paracellular permeability. Disruption of TJ is regarded as one of the earliest hallmarks of epithelial injury, leading to the loss of cell polarity and tissue disorganization. Many factors have been identified as modulators of TJ assembly/disassembly. More specifically, in addition to its role as an energy sensor, adenosine monophosphate-activated protein kinase (AMPK) participates in TJ regulation. AMPK is a ubiquitous serine/threonine kinase composed of a catalytic α-subunit complexed with regulatory β-and γ-subunits. AMPK activation promotes the early stages of epithelial TJ assembly. AMPK phosphorylates the adherens junction protein afadin and regulates its interaction with the TJ-associated protein zonula occludens (ZO)-1, thereby facilitating ZO-1 distribution to the plasma membrane. In the present review, we detail the signaling pathways up-and down-stream of AMPK activation at the time of Ca2+-induced TJ assembly.

Epithelial barrier and oral bacterial infection

The oral epithelial barrier separates the host from the environment and provides the first line of defense against pathogens, exogenous substances and mechanical stress. It consists of underlying connective tissue and a stratified keratinized epithelium with a basement membrane, whose cells undergo terminal differentiation resulting in the formation of a mechanically resistant surface. Gingival keratinocytes are connected by various transmembrane proteins, such as tight junctions, adherens junctions and gap junctions, each of which has a specialized structure and specific functions. Periodontal pathogens are able to induce inflammatory responses that lead to attachment loss and periodontal destruction. A number of studies have demonstrated that the characteristics of pathogenic oral bacteria influence the expression and structural integrity of different cell-cell junctions. Tissue destruction can be mediated by host cells following stimulation with cytokines and bacterial products. Keratinocytes, the main cell type in gingival epithelial tissues, express a variety of proinflammatory cytokines and chemokines, including interleukin-1alpha, interleukin-1beta, interleukin-6, interleukin-8 and tumor necrosis factor-alpha. Furthermore, the inflammatory mediators that may be secreted by oral keratinocytes are vascular endothelial growth factor, prostaglandin E2 , interleukin-1 receptor antagonist and chemokine (C-C motif) ligand 2. The protein family of matrix metalloproteinases is able to degrade all types of extracellular matrix protein, and can process a number of bioactive molecules. Matrix metalloproteinase activities under inflammatory conditions are mostly deregulated and often increased, and those mainly relevant in periodontal disease are matrix metalloproteinases 1, 2, 3, 8, 9, 13 and 24. Viral infection may also influence the epithelial barrier. Studies show that the expression of HIV proteins in the mucosal epithelium is correlated with the disruption of epithelial tight junctions, suggesting a possible enhancement of human papilloma virus infection by HIV-associated disruption of tight junctions. Altered expression of matrix metalloproteinases was demonstrated in keratinocytes transformed with human papilloma virus-16 or papilloma virus-18,. To summarize, the oral epithelium is able to react to a variety of exogenous, possibly noxious influences.

Protective function of interleukin 27 in colitis-associated cancer via suppression of inflammatory cytokines in intestinal epithelial cells

Numerous studies have demonstrated that inflammation contributes to a variety of cancer formation, among them, colitis-associated cancer (CAC) represents a typical inflammation-related cancer. Interleukin 27 (IL-27) has been demonstrated to play an important role in inflammation-related disease. The effect of IL-27 in intestinal inflammation is controversial and its role in CAC is not elucidated yet. In our present study, we found that IL-27 has protective function in murine model of CAC through suppression of inflammatory cytokines in intestinal epithelial cells (IECs). IL-27Rα (WSX-1) deficiency promotes the CAC development in mice, which is driven by enhanced tumor cell proliferation, more intensive myeloid-derived suppressor cells (MDSC) accumulation in colon lamina propria and higher level of inflammatory cytokines and chemokines in IECs. The levels of IL-6, TNF-α, GM-CSF and CXCL1 triggered in vitro by toll-like receptor ligands are significantly upregulated in IECs from WSX-1 KO mice. Removal of commensal microorganism through antibiotic treatment in mice to eliminate TLR ligands deprives the protective function of IL-27 on CAC tumor growth. Thus, IL-27 suppresses CAC formation through an anti-inflammation mechanism targeting IECs and in turn resists the tumorigenesis. Hence, our study explained how IL-27 exerts its anti-inflammatory function on epithelial cells to fight against chronic-inflammation-associated cancer, which might provide new insights on the potential therapeutic strategies for cancer.

Lysophosphatidic acid regulates adhesion molecules and enhances migration of human oral keratinocytes

Oral keratinocytes are connected via cell-to-cell adhesions to protect underlying tissues from physical and bacterial damage. Lysophosphatidic acids (LPAs) are a family of phospholipid mediators that have the ability to regulate gene expression, cytoskeletal rearrangement, and cytokine/chemokine secretion, which mediate proliferation, migration, and differentiation. Several forms of LPA are found in saliva and gingival crevicular fluid, but it is unknown how they affect human oral keratinocytes (HOK). The aim of the present study was therefore to examine how different LPA forms affect the expression of adhesion molecules and the migration and proliferation of HOK. Keratinocytes were isolated from gingival biopsies obtained from healthy donors and challenged with different forms of LPA. Quantitative real-time RT-PCR, immunocytochemistry, and flow cytometry were used to analyze the expression of adhesion molecules. Migration and proliferation assays were performed. Lysophosphatidic acids strongly promoted expression of E-cadherin and occludin mRNAs and translocation of E-cadherin protein from the cytoplasm to the membrane. Occludin and claudin-1 proteins were up-regulated by LPA. Migration of HOK in culture was increased, but proliferation was reduced, by the addition of LPA. This indicates that LPA can have a role in the regulation of the oral epithelial barrier by increasing the expression of adhesion molecules of HOK, by promotion of migration and by inhibition of proliferation.

Influence of retinoic acid on human gingival epithelial barriers

BACKGROUND AND OBJECTIVES

The gingival epithelium plays an important role in the protection of oral tissues from microbial challenge. Oral keratinocytes form a barrier and show various cellular contacts, including tight junctions (TJ). To analyse the barrier function in vitro the transepithelial electrical resistance (TER) is commonly used. Retinoic acid (RA) is an important signalling molecule in most tissues, including epithelial differentiation. RA signalling is mediated through three RA receptors. The aim of the study was to investigate the influence of RA on human gingival barriers in vitro.

MATERIAL AND METHODS

Immortalized human gingival keratinocytes were seeded on culture plate inserts. The effect of RA with and without infection with Porphyromonas gingivalis W83 on the barrier was analysed by TER measurements. The expression of TJ proteins was investigated by western blot.

RESULTS

During differentiation, mean TER increased from 16 (1 h), 43 (4 h) to 62 (6 h) Ohm × cm² . Addition of 15 μm RA increased TER by +19 after 1 h, +25 after 4 h and +16 Ohm × cm² after 6 h. The pan-RA receptor inhibitor BMS 493 resulted in TER values comparable to the control. The mean established TER of the control was approximately 110 Ohm × cm² . Addition of 15 μm RA elevated TER to 127 Ohm × cm² after 1 h, 150 Ohm × cm² after 4 h and 189 Ohm × cm² after 6 h (p ≤ 0.01). RA plus infection with P. gingivalis W83 further increased the TER increasing effect but could not prevent the destruction of TER induced by bacterial infection. The protein expression of the TJ proteins claudin 4 and occludin was enhanced while ZO-1 was downregulated after 1 h of RA incubation.

CONCLUSION

RA provides barrier-positive elements to the gingival epithelial cell model that is accompanied by altered expression of TJ proteins.

Acetyl-L-Carnitine Prevents Methamphetamine-Induced Structural Damage on Endothelial Cells via ILK-Related MMP-9 Activity

Methamphetamine (METH) is a potent psychostimulant highly used worldwide. Recent studies evidenced the involvement of METH in the breakdown of the blood-brain-barrier (BBB) integrity leading to compromised function. The involvement of the matrix metalloproteinases (MMPs) in the degradation of the neurovascular matrix components and tight junctions (TJs) is one of the most recent findings in METH-induced toxicity. As BBB dysfunction is a pathological feature of many neurological conditions, unveiling new protective agents in this field is of major relevance. Acetyl-L-carnitine (ALC) has been described to protect the BBB function in different paradigms, but the mechanisms underling its action remain mostly unknown. Here, the immortalized bEnd.3 cell line was used to evaluate the neuroprotective features of ALC in METH-induced damage. Cells were exposed to ranging concentrations of METH, and the protective effect of ALC 1 mM was assessed 24 h after treatment. F-actin rearrangement, TJ expression and distribution, and MMPs activity were evaluated. Integrin-linked kinase (ILK) knockdown cells were used to assess role of ALC in ILK mediated METH-triggered MMPs' activity. Our results show that METH led to disruption of the actin filaments concomitant with claudin-5 translocation to the cytoplasm. These events were mediated by MMP-9 activation in association with ILK overexpression. Pretreatment with ALC prevented METH-induced activation of MMP-9, preserving claudin-5 location and the structural arrangement of the actin filaments. The present results support the potential of ALC in preserving BBB integrity, highlighting ILK as a new target for the ALC therapeutic use.

Effect of dietary isoleucine on the immunity, antioxidant status, tight junctions and microflora in the intestine of juvenile Jian carp (Cyprinus carpio var. Jian)

This study was conducted to investigate the effects of dietary isoleucine (Ile) on the immune response, antioxidant status, tight junctions, and microbial population in the intestine of juvenile Jian carp (Cyprinus carpio var. Jian). A total of 1200 juvenile Jian carp with average initial weight 6.9 ± 0.03 g were fed semi-purified isonitrogenous diets containing 4.2 (unsupplemented control group), 7.0, 9.5, 11.9, 13.9 and 16.9 g Ile kg(-1) diet for 60 days. Results indicated that Ile supplementation decreased malondialdehyde (MDA) and protein carbonyl content, and the amounts of Escherichia coli and Aeromonas in the intestine (P < 0.05), and increased the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR), glutathione content and the amounts of Lactobacillus and Bacillus in the intestine (P < 0.05). Furthermore, real time polymerase chain reaction revealed that relative mRNA expression of copper/zinc superoxide dismutase (Cu-ZnSOD), manganese superoxide dismutase (MnSOD), CAT, NF-E2-related factor 2 (Nrf2), p38 mitogen-activated protein kinases (p38MAPK) in the intestine were increased with increasing of dietary Ile up to a certain point (P < 0.05). Conversely, the relative mRNA expression of occludin, claudin-3, claudin-7, TNF-α, IL-10, Kelch-like-ECH- associated protein 1 (Keap1), extracellular signal-regulated kinase 1 (ERK1) in the intestine showed a downward trend (P < 0.05). In conclusion, dietary Ile improves intestinal immune function, antioxidant capacity and microbial population, and regulates gene expression of antioxidant enzyme, tight junctions, Nrf2, Keap1, p38 and ERK1 in the intestine of Jian carp.

ZEB2-transgene expression in the epidermis compromises the integrity of the epidermal barrier through the repression of different tight junction proteins

Epithelial homeostasis within the epidermis is maintained by means of multiple cell-cell adhesion complexes such as adherens junctions, tight junctions, gap junctions, and desmosomes. These complexes co-operate in the formation and the regulation of the epidermal barrier. Disruption of the epidermal barrier through the deregulation of the above complexes is the cause behind a number of skin disorders such as psoriasis, dermatitis, keratosis, and others. During epithelial-to-mesenchymal transition (EMT), epithelial cells lose their adhesive capacities and gain mesenchymal properties. ZEB transcription factors are key inducers of EMT. In order to gain a better understanding of the functional role of ZEB2 in epidermal homeostasis, we generated a mouse model with conditional overexpression of Zeb2 in the epidermis. Our analysis revealed that Zeb2 expression in the epidermis leads to hyperproliferation due to the combined downregulation of different tight junction proteins compromising the epidermal barrier. Using two epidermis-specific in vivo models and in vitro promoter assays, we identified occludin as a new Zeb2 target gene. Immunohistological analysis performed on human skin biopsies covering various pathogeneses revealed ZEB2 expression in the epidermis of pemphigus vulgaris. Collectively, our data support the notion for a potential role of ZEB2 in intracellular signaling of this disease.

The role of epithelial tight junctions involved in pathogen infections

Tight junctions (TJs) are sealing complexes between adjacent epithelial cells, functioning by controlling paracellular passage and maintaining cell polarity. These functions of TJs are primarily based on structural integrity as well as dynamic regulatory balance, indicating plasticity of TJ in response to external stimuli. An indispensable role of TJs involved in pathogen infection has been widely demonstrated since disruption of TJs leads to a distinct increase in paracellular permeability and polarity defects which facilitate viral or bacterial entry and spread. In addition to pathological changes in TJ integrity, TJ proteins such as occludin and claudins can either function as receptors for pathogen entry or interact with viral/bacterial effector molecules as an essential step for characterizing an infective stage. This suggests a more complicated role for TJ itself and especially specific TJ components. Thus, this review surveys the role of the epithelial TJs involved in various pathogen infections, and extends TJ targeted therapeutic and pharmacological application prospects.

In vitro models of the blood-brain barrier for the study of drug delivery to the brain

The most important obstacle to the drug delivery into the brain is the presence of the blood-brain barrier, which limits the traffic of substances between the blood and the nervous tissue. Therefore, adequate in vitro models need to be developed in order to characterize the penetration properties of drug candidates into the central nervous system. This review article summarizes the presently used and the most promising in vitro BBB models based on the culture of brain endothelial cells. Robust models can be obtained using primary porcine brain endothelial cells and rodent coculture models, which have low paracellular permeability and express functional efflux transporters, showing good correlation of drug penetration data with in vivo results. Models mimicking the in vivo anatomophysiological complexity of the BBB are also available, including triple coculture (culture of brain endothelial cells in the presence of pericytes and astrocytes), dynamic, and microfluidic models; however, these are not suitable for rapid, high throughput studies. Potent human cell lines would be needed for easily available and reproducible models which avoid interspecies differences.

Measuring collective cell movement and extracellular matrix interactions using magnetic resonance imaging

Collective cell behaviors in migration and force generation were studied at the mesoscopic-level using cells grown in a 3D extracellular matrix (ECM) simulating tissues. Magnetic resonance imaging (MRI) was applied to investigate dynamic cell mechanics at this level. MDCK, NBT2, and MEF cells were embedded in 3D ECM, forming clusters that then migrated and generated forces affecting the ECM. The cells demonstrated MRI contrast due to iron accumulation in the clusters. Timelapse-MRI enabled the measurement of dynamic stress fields generated by the cells, as well as simultaneous monitoring of the cell distribution and ECM deformation/remodeling. We found cell clusters embedded in the 3D ECM can exert translational forces to pull and push, as well as torque, their surroundings. We also observed that the sum of forces generated by multiple cell clusters may result in macroscopic deformation. In summary, MRI can be used to image cell-ECM interactions mesoscopically.

Impaired paracellular ion transport in the loop of Henle causes familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare tubular disorder caused by mutations in genes coding for tight junction (TJ) proteins. TJs define the paracellular path between adjacent cells and thereby play a pivotal role for the regulation of the paracellular ion permeability of epithelia. The family of TJ proteins comprise a variety of transmembrane proteins, including the claudins. Multiple distinct mutations in the genes for claudin-16 and -19 have been described to be responsible for FHHNC. Both encoded proteins are especially important for the paracellular reabsorption of Mg(2+) and Ca(2+) in the thick ascending limb of Henle's loop. Interestingly, in addition to ion disturbances, FHHNC leads to chronic renal failure and may be associated with extrarenal symptoms.

Enterocytes’ tight junctions: From molecules to diseases

Tight junctions (TJs) are structures between cells where cells appear in the closest possible contact. They are responsible for sealing compartments when epithelial sheets are generated. They regulate the permeability of ions, (macro) molecules and cells via the paracellular pathway. Their structure at the electron microscopic level has been well known since the 1970s; however, only recently has their macromolecular composition been revealed. This review first examines the major macromolecular components of the TJs (occludin, claudins, junctional adhesion molecule and tricellulin) and then the associated macromolecules at the intracellular plaque [zonula occludens (ZO)-1, ZO-2, ZO-3, AF-6, cingulin, 7H6]. Emphasis is given to their interactions in order to begin to understand the mode of assembly of TJs. The functional significance of TJs is detailed and several mechanisms and factors involved are discussed briefly. Emphasis is given to the role of intestinal TJs and the alterations observed or speculated in diverse disease states. Specifically, intestinal TJs may exert a pathogenetic role in intestinal (inflammatory bowel disease, celiac disease) and extraintestinal diseases (diabetes type 1, food allergies, autoimmune diseases). Additionally, intestinal TJs may be secondarily disrupted during the course of diverse diseases, subsequently allowing the bacterial translocation phenomenon and promoting the systemic inflammatory response, which is often associated with clinical deterioration. The major questions in the field are highlighted.

Possible involvement of tight junctions, extracellular matrix and nuclear receptors in epithelial differentiation

Tight junctions are intercellular junctions localized at the most apical end of the lateral plasma membrane. They consist of four kinds of transmembrane proteins (occludin, claudins, junctional adhesion molecules, and tricellulin) and huge numbers of scaffolding proteins and contribute to the paracellular barrier and fence function. The mutation and deletion of these proteins impair the functions of tight junctions and cause various human diseases. In this paper, we provide an overview of recent studies on transmembrane proteins of tight junctions and highlight the functional significance of tight junctions, extracellular matrix, and nuclear receptors in epithelial differentiation.

Occludin: one protein, many forms

Intercellular tight junctions (TJs) exhibit a complex molecular architecture involving the regulated cointeraction of cytoplasmic adaptor proteins (e.g., zonula occludens) and integral membrane linker proteins (e.g., occludin and claudins). They provide structural integrity to epithelial and endothelial tissues and create highly polarized barriers essential to homeostatic maintenance within vertebrate physiological systems, while their dysregulation is an established pathophysiological hallmark of many diseases (e.g., cancer, stroke, and inflammatory lung disease). The junctional complex itself is a highly dynamic signaling entity wherein participant proteins constantly undergo a blend of regulatory modifications in response to diverse physiological and pathological cues, ultimately diversifying the overall adhesive properties of the TJ. Occludin, a 65-kDa tetraspan integral membrane protein, contributes to TJ stabilization and optimal barrier function. This paper reviews our current knowledge of how tissue occludin is specifically modified at the posttranscriptional and posttranslational levels in diverse circumstances, with associated consequences for TJ dynamics and epithelial/endothelial homeostasis. Mechanistic concepts such as splice variance and alternate promoter usage, proteolysis, phosphorylation, dimerization, and ubiquitination are comprehensively examined, and possible avenues for future investigation highlighted.

Differential display RT-PCR reveals genes associated with lithium-induced neuritogenesis in SK-N-MC cells

Lithium is shown to be neurotrophic and protective against variety of environmental stresses both in vitro as well as in vivo. In view of the wider clinical applications, it is necessary to examine alterations in levels of expression of genes affected by lithium. Lithium induces neuritogenesis in human neuroblastoma cell line SK-N-MC. Our aim was to elucidate genes involved in lithium-induced neuritogenesis using SK-N-MC cells. The differential display reverse transcriptase polymerase chain reaction (DD-RT-PCR) technique was used to study gene expression profiles in SK-N-MC cells undergoing lithium-induced neuritogenesis. Differential expression of genes in control and lithium (2.5 mM, 24 h)-treated cells was compared by display of cDNAs generated by reverse transcription of mRNA followed by PCR using arbitrary primers. Expression of four genes was altered in lithium-treated cells. Real-time PCR was done to confirm the levels of expression of each of these genes using specific primers. Lithium significantly up-regulated NCAM, a molecule known to stimulate neuritogenesis, occludin, a molecule participating in tight junctions and PKD2, a molecule known to modulate calcium transport. ANP 32c, a gene whose function is not fully known yet, was found to be down-regulated by lithium. This is the first report demonstrating altered levels of expression of these genes in lithium-induced neuritogenesis and contributes four hitherto unreported candidates possibly involved in the process.

An update on necrotizing enterocolitis: pathogenesis and preventive strategies

Necrotizing enterocolitis (NEC) is one of the most critical morbidities in preterm infants. The incidence of NEC is 7% in very-low-birth-weight infants, and its mortality is 15 to 30%. Infants who survive NEC have various complications, such as nosocomial infection, malnutrition, growth failure, bronchopulmonary dysplasia, retinopathy of prematurity, and neurodevelopmental delays. The most important etiology in the pathogenesis of NEC is structural and immunological intestinal immaturity. In preterm infants with immature gastrointestinal tracts, development of NEC may be associated with a variety of factors, such as colonization with pathogenic bacteria, secondary ischemia, genetic polymorphisms conferring NEC susceptibility, anemia with red blood cell transfusion, and sensitization to cow milk proteins. To date, a variety of preventive strategies has been accepted or attempted in clinical practice with regard to the pathogenesis of NEC. These strategies include the use of breast feeding, various feeding strategies, probiotics, prebiotics, glutamine and arginine, and lactoferrin. There is substantial evidence for the efficacy of breast feeding and the use of probiotics in infants with birth weights above 1,000 g, and these strategies are commonly used in clinical practice. Other preventive strategies, however, require further research to establish their effect on NEC.

Occludin protein family: oxidative stress and reducing conditions

The occludin-like proteins belong to a family of tetraspan transmembrane proteins carrying a marvel domain. The intrinsic function of the occludin family is not yet clear. Occludin is a unique marker of any tight junction and is found in polarized endothelial and epithelial tissue barriers, at least in the adult vertebrate organism. Occludin is able to oligomerize and to form tight junction strands by homologous and heterologous interactions, but has no direct tightening function. Its oligomerization is affected by pro- and antioxidative agents or processes. Phosphorylation of occludin has been described at multiple sites and is proposed to play a regulatory role in tight junction assembly and maintenance and, hence, to influence tissue barrier characteristics. Redox-dependent signal transduction mechanisms are among the pathways modulating occludin phosphorylation and function. This review discusses the novel concept that occludin plays a key role in the redox regulation of tight junctions, which has a major impact in pathologies related to oxidative stress and corresponding pharmacologic interventions.

Necrotizing enterocolitis: an update

Necrotizing enterocolitis (NEC) is a leading cause of death among patients in the neonatal intensive care unit, carrying a mortality rate of 15-30%. Its pathogenesis is multifactorial and involves an overreactive response of the immune system to an insult. This leads to increased intestinal permeability, bacterial translocation, and sepsis. There are many inflammatory mediators involved in this process, but thus far none has been shown to be a suitable target for preventive or therapeutic measures. NEC usually occurs in the second week of life after the initiation of enteral feeds, and the diagnosis is made based on physical examination findings, laboratory studies, and abdominal radiographs. Neonates with NEC are followed with serial abdominal examinations and radiographs, and may require surgery or primary peritoneal drainage for perforation or necrosis. Many survivors are plagued with long term complications including short bowel syndrome, abnormal growth, and neurodevelopmental delay. Several evidence-based strategies exist that may decrease the incidence of NEC including promotion of human breast milk feeding, careful feeding advancement, and prophylactic probiotic administration in at-risk patients. Prevention is likely to have the greatest impact on decreasing mortality and morbidity related to NEC, as little progress has been made with regard to improving outcomes for neonates once the disease process is underway.

Expression of ZO-1 and occludin at mRNA and protein level during preimplantation development of the pig parthenogenetic diploids

Expression of mRNAs and proteins of ZO-1 and occludin was analyzed in pig oocytes and parthenogenetic diploid embryos during preimplantation development using real-time RT-PCR, western blotting and immunocytochemistry. All germinal vesicle (GV) and metaphase (M)II oocytes and preimplantation embryos expressed mRNAs and proteins of ZO-1 and occludin. mRNA levels of both ZO-1 and occludin decreased significantly from GV to MII, but increased at the 2-cell stage followed by temporal decrease during the early and late 4-cell stages. Then, both mRNAs increased after compaction. Relative concentration of zo1α− was highest in 2-cell embryos, while zo1α+ was expressed from the morula stage. Occludin expression greatly increased after the morula stage and was highest in expanded blastocysts. Western blotting analysis showed constant expression of ZO-1α− throughout preimplantation development and limited translation of ZO-1α+ from the blastocysts, and species-specific expression pattern of occludin. Immunocytochemistry analysis revealed homogeneous distribution of ZO-1 and occludin in the cytoplasm with moderately strong fluorescence in the vicinity of the contact region between blastomeres, around the nuclei in the 2-cell to late 4-cell embryos, and clear network localization along the cell-boundary region in embryos after the morula stage. Present results show that major TJ proteins, ZO-1 and occludin are expressed in oocytes and preimplantation embryos, and that ZO-1α+ is transcribed by zygotic gene activation and translated from early blastocysts with prominent increase of occludin at the blastocyst stage.

Recessive mutations in the gene encoding the tight junction protein occludin cause band-like calcification with simplified gyration and polymicrogyria

Band-like calcification with simplified gyration and polymicrogyria (BLC-PMG) is a rare autosomal-recessive neurological disorder showing highly characteristic clinical and neuroradiological features. Affected individuals demonstrate early-onset seizures, severe microcephaly, and developmental arrest with bilateral, symmetrical polymicrogyria (PMG) and a band of gray matter calcification on brain imaging; as such, the disorder can be considered as a "pseudo-TORCH" syndrome. By using autozygosity mapping and copy number analysis we identified intragenic deletions and mutations in OCLN in nine patients from six families with BLC-PMG. The OCLN gene encodes occludin, an integral component of tight junctions. Neuropathological analysis of an affected individual showed similarity to the mouse model of occludin deficiency with calcification predominantly associated with blood vessels. Both intracranial calcification and PMG are heterogeneous in etiology. Neuropathological and clinical studies of PMG have suggested that in utero ischemic or vascular insults may contribute to this common cortical abnormality. Tight junctions are functional in cerebral blood vessels early in fetal development and continue to play a vital role in maintenance of the blood-brain barrier during postnatal life. We provide evidence that the tight junction protein occludin (encoded by the OCLN gene) is involved in the pathogenesis of malformations of cortical development.

Splicing diversity of the human OCLN gene and its biological significance for hepatitis C virus entry

Persistent hepatitis C virus (HCV) infection is a primary etiological factor for the development of chronic liver disease, including cirrhosis and cancer. A recent study identified occludin (OCLN), an integral tight junction protein, as one of the key factors for HCV entry into cells. We explored the splicing diversity of OCLN in normal human liver and observed variable expression of alternative splice variants, including two known forms (WT-OCLN and OCLN-ex4del) and six novel forms (OCLN-ex7ext, OCLN-ex3pdel, OCLN-ex3del, OCLN-ex3-4del, OCLN-ex3p-9pdel, and OCLN-ex3p-7pdel). Recombinant protein isoforms WT-OCLN and OCLN-ex7ext, which retained the HCV-interacting MARVEL domain, were expressed on the cell membrane and were permissive for HCV infection in in vitro infectivity assays. All other forms lacked the MARVEL domain, were expressed in the cytoplasm, and were nonpermissive for HCV infection. Additionally, we observed variable expression of OCLN splicing forms across human tissues and cell lines. Our study suggests that the remarkable natural splicing diversity of OCLN might contribute to HCV tissue tropism and possibly modify the outcome of HCV infection in humans. Genetic factors crucial for regulation of OCLN expression and susceptibility to HCV infection remain to be elucidated.

Tight junctions in salivary epithelium

Epithelial cell tight junctions (TJs) consist of a narrow belt-like structure in the apical region of the lateral plasma membrane that circumferentially binds each cell to its neighbor. TJs are found in tissues that are involved in polarized secretions, absorption functions, and maintaining barriers between blood and interstitial fluids. The morphology, permeability, and ion selectivity of TJ vary among different types of tissues and species. TJs are very dynamic structures that assemble, grow, reorganize, and disassemble during physiological or pathological events. Several studies have indicated the active role of TJ in intestinal, renal, and airway epithelial function; however, the functional significance of TJ in salivary gland epithelium is poorly understood. Interactions between different combinations of the TJ family (each with their own unique regulatory proteins) define tissue specificity and functions during physiopathological processes; however, these interaction patterns have not been studied in salivary glands. The purpose of this review is to analyze some of the current data regarding the regulatory components of the TJ that could potentially affect cellular functions of the salivary epithelium.

Tight junction-associated MARVEL proteins marveld3, tricellulin, and occludin have distinct but overlapping functions

In vitro studies have demonstrated that occludin and tricellulin are important for tight junction barrier function, but in vivo data suggest that loss of these proteins can be overcome. The presence of a heretofore unknown, yet related, protein could explain these observations. Here, we report marvelD3, a novel tight junction protein that, like occludin and tricellulin, contains a conserved four-transmembrane MARVEL (MAL and related proteins for vesicle trafficking and membrane link) domain. Phylogenetic tree reconstruction; analysis of RNA and protein tissue distribution; immunofluorescent and electron microscopic examination of subcellular localization; characterization of intracellular trafficking, protein interactions, dynamic behavior, and siRNA knockdown effects; and description of remodeling after in vivo immune activation show that marvelD3, occludin, and tricellulin have distinct but overlapping functions at the tight junction. Although marvelD3 is able to partially compensate for occludin or tricellulin loss, it cannot fully restore function. We conclude that marvelD3, occludin, and tricellulin define the tight junction-associated MARVEL protein family. The data further suggest that these proteins are best considered as a group with both redundant and unique contributions to epithelial function and tight junction regulation.

Hepatic tight junctions: From viral entry to cancer metastasis

The tight junction (TJ) is a critical cellular component for maintenance of tissue integrity, cellular interactions and cell-cell communications, and physiologically functions as the “great wall” against external agents and the surrounding hostile environment. During the host-pathogen evolution, viruses somehow found the key to unlock the gate for their entry into cells and to exploit and exhaust the host cells. In the liver, an array of TJ molecules is localized along the bile canaliculi forming the blood-biliary barrier, where they play pivotal roles in paracellular permeability, bile secretion, and cell polarity. In pathology, certain hepatic TJ molecules mediate virus entry causing hepatitis infection; deregulation and functional abnormality of the TJ have also been implicated in triggering liver cancer development and metastasis. All these findings shed new insights on the understanding of hepatic TJs in the development of liver disease and provide new clues for potential intervention.

Identification of MarvelD3 as a tight junction-associated transmembrane protein of the occludin family

Background

Tight junctions are an intercellular adhesion complex of epithelial and endothelial cells, and form a paracellular barrier that restricts the diffusion of solutes on the basis of size and charge. Tight junctions are formed by multiprotein complexes containing cytosolic and transmembrane proteins. How these components work together to form functional tight junctions is still not well understood and will require a complete understanding of the molecular composition of the junction.

Results

Here we identify a new transmembrane component of tight junctions: MarvelD3, a four-span transmembrane protein. Its predicted transmembrane helices form a Marvel (MAL and related proteins for vesicle traffic and membrane link) domain, a structural motif originally discovered in proteins involved in membrane apposition and fusion events, such as the tight junction proteins occludin and tricellulin. In mammals, MarvelD3 is expressed as two alternatively spliced isoforms. Both isoforms exhibit a broad tissue distribution and are expressed by different types of epithelial as well as endothelial cells. MarvelD3 co-localises with occludin at tight junctions in intestinal and corneal epithelial cells. RNA interference experiments in Caco-2 cells indicate that normal MarvelD3 expression is not required for the formation of functional tight junctions but depletion results in monolayers with increased transepithelial electrical resistance.

Conclusions

Our data indicate that MarvelD3 is a third member of the tight junction-associated occludin family of transmembrane proteins. Similar to occludin, normal expression of MarvelD3 is not essential for the formation of functional tight junctions. However, MarvelD3 functions as a determinant of epithelial paracellular permeability properties.

Intestinal barrier function: molecular regulation and disease pathogenesis

The intestinal epithelium is a single-cell layer that constitutes the largest and most important barrier against the external environment. It acts as a selectively permeable barrier, permitting the absorption of nutrients, electrolytes, and water while maintaining an effective defense against intraluminal toxins, antigens, and enteric flora. The epithelium maintains its selective barrier function through the formation of complex protein-protein networks that mechanically link adjacent cells and seal the intercellular space. The protein networks connecting epithelial cells form 3 adhesive complexes: desmosomes, adherens junctions, and tight junctions. These complexes consist of transmembrane proteins that interact extracellularly with adjacent cells and intracellularly with adaptor proteins that link to the cytoskeleton. Over the past decade, there has been increasing recognition of an association between disrupted intestinal barrier function and the development of autoimmune and inflammatory diseases. In this review we summarize the evolving understanding of the molecular composition and regulation of intestinal barrier function. We discuss the interactions between innate and adaptive immunity and intestinal epithelial barrier function, as well as the effect of exogenous factors on intestinal barrier function. Finally, we summarize clinical and experimental evidence demonstrating intestinal epithelial barrier dysfunction as a major factor contributing to the predisposition to inflammatory diseases, including food allergy, inflammatory bowel diseases, and celiac disease.

Ovarian hormones control the changing expression of claudins and occludin in rat uterine epithelial cells during early pregnancy

Regulation of the uterine luminal environment is important for successful attachment and implantation of the blastocyst. The contents and volume of luminal fluid are regulated in part by the tight junctions. Using immunofluorescence microscopy, protein and RNA analysis, the cellular distributions of tight junction components claudins and occludin were observed during early pregnancy and under various hormonal regimens. Results indicate that occludin and claudin-4 distribution changed during early pregnancy and in response to ovarian hormones. At the time of implantation and in response to progesterone administration to ovariectomised rats, occludin and claudin-4 showed increased immunolabelling in luminal epithelium. Interestingly, occludin protein detection in uterine luminal epithelial cells at the time of implantation was statistically significantly decreased at the time of implantation compared to day 1 of pregnancy. This suggests that a cytoplasmic pool of occludin is present at day 1 of pregnancy and is redistributed to the tight junctions at the time of implantation. The presence of occludin and claudin-4 in the tight junctions at the time of implantation and in response to progesterone suggests that the paracellular pathway is impermeable to water and Na(+) at this time, and that the transport of such substances takes place via the transcellular pathway.

Effects of carprofen on the integrity and barrier function of canine colonic mucosa

OBJECTIVE

To measure effects of carprofen on conductance and permeability to mannitol and histologic appearance in canine colonic mucosa.

SAMPLE POPULATION

Colonic mucosa from 13 mature mixed-breed dogs. Procedures-Sections of mucosa from the transverse colon and proximal and distal portions of the descending colon were obtained immediately after dogs were euthanized. Sections were mounted in Ussing chambers. Carprofen (400 microg/mL) was added to the bathing solution for treated sections. Conductance was calculated at 15-minute intervals for 240 minutes. Flux of mannitol was calculated for three 1-hour periods. Histologic examination of sections was performed after experiments concluded. Conductance was graphed against time for each chamber, and area under each curve was calculated. Conductance X time, flux of mannitol, and frequency distribution of histologic findings were analyzed for an effect of region and carprofen.

RESULTS

Carprofen significantly increased mean conductance X time, compared with values for control (untreated) sections for all regions of colon. Carprofen significantly increased mean flux of mannitol from period 1 to period 2 and from period 2 to period 3 for all regions of colon. Carprofen caused a significant proportion of sections to have severe sloughing of cells and erosions involving >or= 10% of the epithelium, compared with control sections.

CONCLUSIONS AND CLINICAL RELEVANCE

Carprofen increased in vitro conductance and permeability to mannitol in canine colonic mucosa. Carprofen resulted in sloughing of cells and erosion of the colonic mucosa. These findings suggested that carprofen can compromise the integrity and barrier function of the colonic mucosa of dogs.

Understanding the susceptibility of the premature infant to necrotizing enterocolitis (NEC)

Necrotizing enterocolitis (NEC) is the most common life-threatening gastrointestinal emergency encountered in the neonatal intensive care unit. Despite advances in neonatal care, NEC remains a leading cause of morbidity and mortality among premature infants. Epidemiologic studies have identified multiple factors that increase an infant's risk for the development of NEC, although premature birth, bacterial colonization, and enteral feeding are thought to play central roles in disease pathogenesis. Appreciating factors that underlie the susceptibility of prematurely born infants to NEC is important for the development of new strategies aimed at the prevention and treatment of disease. In this review, we discuss defense mechanisms in the intestine and discuss how these systems may be insufficient in the prematurely born infant and thereby further contribute to initiation of NEC. In addition, based on a review of the literature, we suggest that, although numerous bacterial and viral pathogens have been associated with NEC, no individual organism is known to be responsible for disease. Finally, we comment on the possible role for probiotics in promoting maturation of intestinal defense mechanisms thereby attenuating or preventing the sequence of events that lead to NEC.

Occludin is overexpressed in Alzheimer's disease and vascular dementia

The tight junctions (TJs) are key players in the control of blood-brain barrier (BBB) properties, the most complex TJs in the vascular system being found in the endothelial cells of brain capillaries. One of the main TJs proteins is occludin, which anchors plasma membranes of neighbour cells and is present in large amounts in the brain endothelia. Previous studies demonstrated that disruption of BBB in various pathological situations associates with changes in occludin expression, and this change could be responsible for malfunction of BBB. Therefore in this study, applying an immunohistochemical approach, we decided to explore the occludin expression in frontal cortex (FC) and basal ganglia in ageing control, Alzheimer's disease (AD), and vascular dementia (VD) brains, as far as all these pathologies associate microangiopathy and disruption of BBB. Strikingly, we found selected neurons, astrocytes and oligodendrocytes expressing occludin, in all cases studied. To estimate the number of occludin-expressing neurons, we applied a stereological approach with random systematic sampling and the unbiased optical fractionator method. We report here a significant increase in ratio of occludin-expressing neurons in FC and basal ganglia regions in both AD and VD as compared to ageing controls. Within the cerebral cortex, occludin was selectively expressed by pyramidal neurons, which are the ones responsible for cognitive processes and affected by AD pathology. Our findings could be important in unravelling new pathogenic pathways in dementia disorders and new functions of occludin and TJs.

Roles of Rho/ROCK and MLCK in TNF-alpha-induced changes in endothelial morphology and permeability

Tumor necrosis factor-alpha (TNF-alpha) is known to induce changes in endothelial cell morphology and permeability, but the mechanisms have not been extensively characterized. TNF-alpha rapidly induced RhoA activation and myosin light chain phosphorylation, but caused only small changes to cortical F-actin, without significantly increasing paracellular permeability up to 30 min after stimulation. TNF-alpha subsequently caused a progressive increase in permeability and in stress fiber reorganization, cell elongation, and intercellular gap formation over 8-24 h. Consistent with the increased permeability, Occludin and JAM-A were removed from tight junctions and ZO-1 was partially redistributed. Rho/ROCK but not MLCK inhibition prevented the long-term TNF-alpha-induced changes in F-actin and cell morphology, but ROCK inhibition did not affect permeability. These results suggest that the gradual increase in permeability induced by TNF-alpha does not reflect contractile mechanisms mediated by Rho, ROCK, and MLCK, but involves long-term reorganization of tight junction proteins.

The amyloid-beta precursor protein is phosphorylated via distinct pathways during differentiation, mitosis, stress, and degeneration

Phosphorylation of amyloid-beta precursor protein (APP) at Thr(668) is a normal process linked to neurite extension and anterograde transport of vesicular cargo. By contrast, increased phosphorylation of APP is a pathological trait of Alzheimer's disease. APP is overexpressed in Down's syndrome, a condition that occasionally leads to increased APP phosphorylation, in cultured cells. Whether phosphorylation of APP in normal versus high APP conditions occurs by similar or distinct signaling pathways is not known. Here, we addressed this problem using brainstem-derived neurons (CAD cells). CAD cells that ectopically overexpress APP frequently show features of degenerating neurons. We found that, in degenerating cells, APP is hyperphosphorylated and colocalizes with early endosomes. By contrast, in normal CAD cells, phosphorylated APP (pAPP) is excluded from endosomes, and localizes to the Golgi apparatus and to transport vesicles within the neurites. Whereas the neuritic APP is phosphorylated by c-Jun NH(2)-terminal kinase through a pathway that is modulated by glycogen synthase kinase 3beta, the endosomal pAPP in degenerated CAD cells results from activation of cyclin-dependent kinase 5. Additional signaling pathways, leading to APP phosphorylation, become active during stress and mitosis. We conclude that distinct pathways of APP phosphorylation operate in proliferating, differentiating, stressed, and degenerating neurons.

Transmembrane proteins of tight junctions

Tight junctions contribute to the paracellular barrier, the fence dividing plasma membranes, and signal transduction, acting as a multifunctional complex in vertebrate epithelial and endothelial cells. The identification and characterization of the transmembrane proteins of tight junctions, claudins, junctional adhesion molecules (JAMs), occludin and tricellulin, have led to insights into the molecular nature of tight junctions. We provide an overview of recent progress in studies on these proteins and highlight their roles and regulation, as well as their functional significance in human diseases.

Adherens and tight junctions: structure, function and connections to the actin cytoskeleton

Adherens junctions and Tight junctions comprise two modes of cell-cell adhesion that provide different functions. Both junctional complexes are proposed to associate with the actin cytoskeleton, and formation and maturation of cell-cell contacts involves reorganization of the actin cytoskeleton. Adherens junctions initiate cell-cell contacts, and mediate the maturation and maintenance of the contact. Adherens junctions consist of the transmembrane protein E-cadherin, and intracellular components, p120-catenin, beta-catenin and alpha-catenin. Tight junctions regulate the paracellular pathway for the movement of ions and solutes in-between cells. Tight junctions consist of the transmembrane proteins occludin and claudin, and the cytoplasmic scaffolding proteins ZO-1, -2, and -3. This review discusses the binding interactions of the most studied proteins that occur within each of these two junctional complexes and possible modes of regulation of these interactions, and the different mechanisms that connect and regulate interactions with the actin cytoskeleton.

Altered permeability in inflammatory bowel disease: pathophysiology and clinical implications

PURPOSE OF REVIEW

To present the mechanisms behind barrier disturbance in inflammatory bowel disease and their functional consequences.

RECENT FINDINGS

A reduction in tight junction strands, strand breaks and alteration of tight junction protein content and composition characterize Crohn's disease. In ulcerative colitis, epithelial leaks appear early as a result of microerosions, upregulated epithelial apoptosis and tight junction protein changes with pronounced increases in claudin-2. T-helper type 1 cytokine effects by interferon-gamma and tumour necrosis factor alpha are important for epithelial damage in Crohn's disease. Interleukin-13 is the key effector cytokine in ulcerative colitis, stimulating epithelial cell apoptosis, and can upregulate claudin-2 expression. Together with interleukin-13-induced epithelial restitution arrest, this may explain why ulcer lesions occur in early stages of ulcerative colitis but are only observed in advanced inflammatory stages in Crohn's disease.

SUMMARY

Barrier dysfunction in inflammatory bowel disease contributes to diarrhea by a leak flux mechanism and can cause mucosal inflammation secondary to luminal antigen uptake. Barrier abnormalities, such as epithelial tight junction changes and apoptotic leaks, gross mucosal lesions, and epithelial restitution arrest are responsible for these abnormalities and are the result of immune dysregulation. Studying the underlying mechanisms is important in understanding the pathophysiology of inflammatory bowel disease and developing therapeutic strategies.