Specific modulation of airway epithelial tight junctions by apical application of an occludin peptide

Storming the gate: New approaches for targeting the dynamic tight junction for improved drug delivery

As biologics used in the clinic outpace the number of new small molecule drugs, an important challenge for their efficacy and widespread use has emerged, namely tissue penetrance. Macromolecular drugs - bulky, high-molecular weight, hydrophilic agents - exhibit low permeability across biological barriers. Epithelial and endothelial layers, for example within the gastrointestinal tract or at the blood-brain barrier, present the most significant obstacle to drug transport. Within epithelium, two subcellular structures are responsible for limiting absorption: cell membranes and intercellular tight junctions. Previously considered impenetrable to macromolecular drugs, tight junctions control paracellular flux and dictate drug transport between cells. Recent work, however, has shown tight junctions to be dynamic, anisotropic structures that can be targeted for delivery. This review aims to summarize new approaches for targeting tight junctions, both directly and indirectly, and to highlight how manipulation of tight junction interactions may help usher in a new era of precision drug delivery.

Abies holophylla Leaf Essential Oil Alleviates Allergic Rhinitis Based on Network Pharmacology

Abies holophylla is an evergreen coniferous species that has been widely used for treating pulmonary diseases and colds. Previous research has demonstrated the anti-inflammatory effect of Abies species and the anti-asthmatic activities of Abies holophylla leaf essential oil (AEO). As asthma and allergic rhinitis (AR) share pathophysiology and pharmacotherapeutic interventions, AEO inhalation can also ameliorate upper respiratory allergic diseases. This study explored the protective effects of AEO on AR with network pharmacological pathway prediction. The potential target pathways of AEO were analyzed by a network pharmacological approach. The BALB/c mice were sensitized by ovalbumin (OVA) and 10 μm particular matter (PM₁₀) to induce allergic rhinitis. Aerosolized AEO 0.0003% and 0.03% were delivered by nebulizer for 5 min a day, 3 times a week for 7 weeks. Nasal symptoms (sneezing and rubbing), histopathological changes in nasal tissues, serum IgE, and zonula occludens-1 (ZO-1) expressions on nasal tissues were analyzed. After AR induction with OVA+PM₁₀ and inhalation of AEO 0.0003% and 0.03% treatment, AEO significantly decreased allergic symptoms (sneezing and rubbing), hyperplasia of nasal epithelial thickness, goblet cell counts, and serum IgE level. The network analysis demonstrated that the possible molecular mechanism of AEO is highly associated with the IL-17 signaling pathway and tight junction. The target pathway of AEO was investigated in RPMI 2650 nasal epithelial cells. Treatment of AEO on PM₁₀-treated nasal epithelial cells significantly reduced the production of inflammatory mediators related to the IL-17 signaling pathway, NF-κB, and the MAPK signaling pathway and prevented the reduction in TJ-related factors. When taken together, AEO inhalation may be considered as a potential treatment for AR by alleviating nasal inflammation and recovering the tight junction.

Inflammation and the Intestinal Barrier: Leukocyte-Epithelial Cell Interactions, Cell Junction Remodeling, and Mucosal Repair

The intestinal tract is lined by a single layer of columnar epithelial cells that forms a dynamic, permeable barrier allowing for selective absorption of nutrients, while restricting access to pathogens and food-borne antigens. Precise regulation of epithelial barrier function is therefore required for maintaining mucosal homeostasis and depends, in part, on barrier-forming elements within the epithelium and a balance between pro- and anti-inflammatory factors in the mucosa. Pathologic states, such as inflammatory bowel disease, are associated with a leaky epithelial barrier, resulting in excessive exposure to microbial antigens, recruitment of leukocytes, release of soluble mediators, and ultimately mucosal damage. An inflammatory microenvironment affects epithelial barrier properties and mucosal homeostasis by altering the structure and function of epithelial intercellular junctions through direct and indirect mechanisms. We review our current understanding of complex interactions between the intestinal epithelium and immune cells, with a focus on pathologic mucosal inflammation and mechanisms of epithelial repair. We discuss leukocyte-epithelial interactions, as well as inflammatory mediators that affect the epithelial barrier and mucosal repair. Increased knowledge of communication networks between the epithelium and immune system will lead to tissue-specific strategies for treating pathologic intestinal inflammation.

Brain endothelial TAK1 and NEMO safeguard the neurovascular unit

Ridder et al. show that deletion of NEMO, a component of NF-kB signaling, in brain endothelial cells results in increased cerebral vascular permeability and endothelial cell death, and recapitulates the neurological symptoms observed in the genetic disease incontinentia pigmenti.

Inactivating mutations of the NF-κB essential modulator (NEMO), a key component of NF-κB signaling, cause the genetic disease incontinentia pigmenti (IP). This leads to severe neurological symptoms, but the mechanisms underlying brain involvement were unclear. Here, we show that selectively deleting Nemo or the upstream kinase Tak1 in brain endothelial cells resulted in death of endothelial cells, a rarefaction of brain microvessels, cerebral hypoperfusion, a disrupted blood–brain barrier (BBB), and epileptic seizures. TAK1 and NEMO protected the BBB by activating the transcription factor NF-κB and stabilizing the tight junction protein occludin. They also prevented brain endothelial cell death in a NF-κB–independent manner by reducing oxidative damage. Our data identify crucial functions of inflammatory TAK1–NEMO signaling in protecting the brain endothelium and maintaining normal brain function, thus explaining the neurological symptoms associated with IP.

Breaking barriers. New insights into airway epithelial barrier function in health and disease

Epithelial permeability is a hallmark of mucosal inflammation, but the molecular mechanisms involved remain poorly understood. A key component of the epithelial barrier is the apical junctional complex that forms between neighboring cells. Apical junctional complexes are made of tight junctions and adherens junctions and link to the cellular cytoskeleton via numerous adaptor proteins. Although the existence of tight and adherens junctions between epithelial cells has long been recognized, in recent years there have been significant advances in our understanding of the molecular regulation of junctional complex assembly and disassembly. Here we review the current thinking about the structure and function of the apical junctional complex in airway epithelial cells, emphasizing the translational aspects of relevance to cystic fibrosis and asthma. Most work to date has been conducted using cell culture models, but technical advancements in imaging techniques suggest that we are on the verge of important new breakthroughs in this area in physiological models of airway diseases.

Trends in drug delivery through tissue barriers containing tight junctions

A limitation in the uptake of many drugs is the restricted permeation through tissue barriers. There are two general ways to cross barriers formed by cell layers: by transcytosis or by diffusion through the intercellular space. In the latter, tight junctions (TJs) play the decisive role in the regulation of the barrier permeability. Thus, transient modulation of TJs is a potent strategy to improve drug delivery. There have been extensive studies on surfactant-like absorption enhancers. One of the most effective enhancers found is sodium caprate. However, this modulates TJs in an unspecific fashion. A novel approach would be the specific modulation of TJ-associated marvel proteins and claudins, which are the main structural components of the TJs. Recent studies have identified synthetic peptidomimetics and RNA interference techniques to downregulate the expression of targeted TJ proteins. This review summarizes current progress and discusses the impact on TJs' barrier function.

Tight junctions at the blood brain barrier: physiological architecture and disease-associated dysregulation

The Blood–brain barrier (BBB), present at the level of the endothelium of cerebral blood vessels, selectively restricts the blood-to-brain paracellular diffusion of compounds; it is mandatory for cerebral homeostasis and proper neuronal function. The barrier properties of these specialized endothelial cells notably depend on tight junctions (TJs) between adjacent cells: TJs are dynamic structures consisting of a number of transmembrane and membrane-associated cytoplasmic proteins, which are assembled in a multimolecular complex and acting as a platform for intracellular signaling. Although the structural composition of these complexes has been well described in the recent years, our knowledge about their functional regulation still remains fragmentary. Importantly, pericytes, embedded in the vascular basement membrane, and perivascular microglial cells, astrocytes and neurons contribute to the regulation of endothelial TJs and BBB function, altogether constituting the so-called neurovascular unit.

The present review summarizes our current understanding of the structure and functional regulation of endothelial TJs at the BBB. Accumulating evidence points to a correlation between BBB dysfunction, alteration of TJ complexes and progression of a variety of CNS diseases, such as stroke, multiple sclerosis and brain tumors, as well as neurodegenerative diseases like Parkinson’s and Alzheimer’s diseases. Understanding how TJ integrity is controlled may thus help improve drug delivery across the BBB and the design of therapeutic strategies for neurological disorders.

Claudin heterogeneity and control of lung tight junctions

Lung epithelial cells interconnected by tight junctions provide a barrier to the free diffusion of solutes into airspaces. Transmembrane tight junction proteins known as claudins are essential for epithelial barrier function. Claudins are regulated through interactions with each other that are coordinated with other transmembrane tight junction proteins and cytosolic scaffold proteins. Of the 14 claudins expressed by the alveolar epithelium, claudin-3, claudin-4, and claudin-18 are the most prominent; each confers unique properties to alveolar barrier function. In particular, a protective role for claudin-4 in preventing lung injury has emerged. By contrast, lung diseases that affect claudin expression and impair barrier function, including alcoholic lung syndrome and sepsis, prime the lung for pulmonary edema. Thus, approaches to restore and/or augment lung claudin expression provide potential targets for promoting healthy barrier function.

Thiol-redox antioxidants protect against lung vascular endothelial cytoskeletal alterations caused by pulmonary fibrosis inducer, bleomycin: comparison between classical thiol-protectant, N-acetyl-L-cysteine, and novel thiol antioxidant, N,N'-bis-2-mercaptoethyl isophthalamide

Lung vascular alterations and pulmonary hypertension associated with oxidative stress have been reported to be involved in idiopathic lung fibrosis (ILF). Therefore, here, we hypothesize that the widely used lung fibrosis inducer, bleomycin, would cause cytoskeletal rearrangement through thiol-redox alterations in the cultured lung vascular endothelial cell (EC) monolayers. We exposed the monolayers of primary bovine pulmonary artery ECs to bleomycin (10 µg) and studied the cytotoxicity, cytoskeletal rearrangements, and the macromolecule (fluorescein isothiocyanate-dextran, 70,000 mol. wt.) paracellular transport in the absence and presence of two thiol-redox protectants, the classic water-soluble N-acetyl-L-cysteine (NAC) and the novel hydrophobic N,N'-bis-2-mercaptoethyl isophthalamide (NBMI). Our results revealed that bleomycin induced cytotoxicity (lactate dehydrogenase leak), morphological alterations (rounding of cells and filipodia formation), and cytoskeletal rearrangement (actin stress fiber formation and alterations of tight junction proteins, ZO-1 and occludin) in a dose-dependent fashion. Furthermore, our study demonstrated the formation of reactive oxygen species, loss of thiols (glutathione, GSH), EC barrier dysfunction (decrease of transendothelial electrical resistance), and enhanced paracellular transport (leak) of macromolecules. The observed bleomycin-induced EC alterations were attenuated by both NAC and NBMI, revealing that the novel hydrophobic thiol-protectant, NBMI, was more effective at µM concentrations as compared to the water-soluble NAC that was effective at mM concentrations in offering protection against the bleomycin-induced EC alterations. Overall, the results of the current study suggested the central role of thiol-redox in vascular EC dysfunction associated with ILF.

Spiral progression in the development of absorption enhancers based on the biology of tight junctions

Epithelium covers the body and, therefore, separates the inner body from the outside environment. Passage across the epithelium is the first step in drug absorption. Tight junctions (TJs) seal the space between adjacent epithelial cells and prevent the free movement of solutes through the paracellular space. Modulation of the epithelial barrier is the most important strategy for enhancing drug absorption. Development of the strategy has accelerated with progress in understanding of the biology of the TJ seal. The first-generation absorption enhancers were screened on the basis of their absorption-enhancing activity in vivo. However, TJs were not well understood initially. The identification of TJ components, including those based on occludin and claudins, has led to the development of new strategies for drug absorption. Accumulation of knowledge of claudins has provided new insights into the paracellular transport of drugs. This review examines the relationship between advances in understanding of TJ biology and paracellular transport of drugs and discusses progress in the development of mucosal absorption enhancers.

Hepatitis C virus entry: beyond receptors

HCV is a blood-borne pathogen that affects approximately 3% of the global population and leads to progressive liver disease. Recent advances have identified an essential role for host cell molecules: tetraspanin CD81, scavenger receptor B1 and the tight junction proteins claudin-1 and occludin in HCV entry, suggesting a complex multi-step process. The conserved nature of this receptor-dependent step in the viral life cycle offers an attractive target for therapeutic intervention. Evidence is emerging that additional factors other than classical receptors, such as inflammatory mediators regulate the ability of hepatocytes to support HCV entry, and as such may provide potential avenues for drug design and development. In this review, we summarise the recent literature on HCV entry mechanisms with a view to realising the future potential of therapeutically targeting this process.

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.

Claudin and occludin expression and function in the seminiferous epithelium

Integral membrane proteins that contribute to function of the blood-testes barrier (BTB) in mice include claudins 3, 5 and 11 and occludin. Although claudin 11 is expressed throughout all stages of the seminiferous epithelial cycle, claudins 3 and 5 have specific expression at stage VIII. These differences in protein expression suggest that the interactions among, and functions of, these integral membrane proteins may shift over the course of the seminiferous epithelial cycle. Also, differences in expression among rodent species and men may make interpretation of studies across species challenging. This review will discuss the characteristics of claudins and occludin; the expression, regulation and function of these integral membrane proteins in the seminiferous epithelium; and how these properties relate to the unique features of BTB.

On-chip epithelial barrier function assays using electrical impedance spectroscopy

A bio-impedance chip has been developed for real-time monitoring of the kinetics of epithelial cell monolayers in vitro. The human bronchial epithelial cell line (16-HBE 14o-) was cultured in Transwells creating a sustainable and interactive model of the airway epithelium. Conducting polymer polypyrrole (PPy) doped with polystyrene sulfonate (PSS) was electrochemically deposited onto the surface of gold-plated electrodes to reduce the influence of the electrical double layer on the impedance measurements. Finite element and equivalent circuit models were used to model and determine the electrical properties of the epithelial cell monolayer from the impedance spectra. Electrically tight, confluent monolayers of 16 HBE 14o- cells were treated with increasing concentrations of either Triton X-100 to solubilize cell membranes or ethylene glycol-bis(2-aminoethyl-ether)-N,N,N'N'-tetraacetic acid (EGTA) to disrupt cell-cell adhesion. Experimental impedance data showed that disruption of epithelial barrier function in response to Triton X-100 and EGTA can be successfully measured by the bio-impedance chip. The results were consistent with the conventional hand-held trans-epithelial electrical resistance measurements. Immunofluorescent staining of the ZO-1 tight junction protein in the untreated and treated 16HBEs was performed to verify the disruption of the tight junctions by EGTA.

Extracellular matrix influences alveolar epithelial claudin expression and barrier function

The lung is dynamically remodeled in response to injury, which alters extracellular matrix composition, and can lead to either healthy or impaired lung regeneration. To determine how changes in extracellular matrix can influence alveolar epithelial barrier function, we examined the expression and function of tight junction proteins by rat alveolar epithelial type II cells cultured on one of three different matrix components: type I collagen or fibronectin, matrix glycoproteins which are highly expressed in injured lungs, or laminin, a basement membrane matrix component. Of note, alveolar epithelial cells cultured for 2 days on fibronectin formed high-resistance barriers and showed continuous claudin-3 and claudin-18 localization to the plasma membrane, as opposed to cells cultured on either type I collagen or laminin, which had low resistance monolayers and had areas of cell-cell contact that were claudin deficient. The barrier formed by cells cultured on fibronectin also had preferential permeability to chloride as compared with sodium. Regardless of the initial matrix composition, alveolar epithelial cells cultured for 5 days formed high-resistance barriers, which correlated with increased claudin-18 localization to the plasma membrane and an increase in zonula occludens-1. Day 5 cells on laminin had significantly higher resistance than cells on either fibronectin or type I collagen. Thus, although alveolar epithelial cells on fibronectin formed rapid barriers, it was at the expense of producing an optimized barrier.

Rhinovirus disrupts the barrier function of polarized airway epithelial cells

RATIONALE

Secondary bacterial infection following rhinovirus (RV) infection has been recognized in chronic obstructive pulmonary disease.

OBJECTIVES

We sought to understand mechanisms by which RV infection facilitates secondary bacterial infection.

METHODS

Primary human airway epithelial cells grown at air-liquid interface and human bronchial epithelial (16HBE14o-) cells grown as polarized monolayers were infected apically with RV. Transmigration of bacteria (nontypeable Haemophilus influenzae and others) was assessed by colony counting and transmission electron microscopy. Transepithelial resistance (R(T)) was measured by using a voltmeter. The distribution of zona occludins (ZO)-1 was determined by immunohistochemistry and immunoblotting.

MEASUREMENTS AND MAIN RESULTS

Epithelial cells infected with RV showed 2-log more bound bacteria than sham-infected cultures, and bacteria were recovered from the basolateral media of RV- but not sham-infected cells. Infection of polarized airway epithelial cell cultures with RV for 24 hours caused a significant decrease in R(T) without causing cell death or apoptosis. Ultraviolet-treated RV did not decrease R(T), suggesting a requirement for viral replication. Reduced R(T) was associated with increased paracellular permeability, as determined by flux of fluorescein isothiocyanate (FITC)-inulin. Neutralizing antibodies to tumor necrosis factor (TNF)-alpha, IFN-gamma and IL-1beta reversed corresponding cytokine-induced reductions in R(T) but not that induced by RV, indicating that the RV effect is independent of these proinflammatory cytokines. Confocal microscopy and immunoblotting revealed the loss of ZO-1 from tight junction complexes in RV-infected cells. Intranasal inoculation of mice with RV1B also caused the loss of ZO-1 from the bronchial epithelium tight junctions in vivo.

CONCLUSIONS

RV facilitates binding, translocation, and persistence of bacteria by disrupting airway epithelial barrier function.

Epithelial barrier modulation by a channel forming peptide

NC-1059 is a synthetic channel-forming peptide that provides for ion transport across, and transiently reduces the barrier integrity of, cultured epithelial monolayers derived from canine kidney (MDCK cells). Experiments were conducted to determine whether epithelial cells derived from other sources were similarly affected. Epithelial cells derived from human intestine (T-84), airway (Calu-3), porcine intestine (IPEC-J2) and reproductive duct (PVD9902) were grown on permeable supports. Basal short circuit current (Isc) was <3 microA cm(-2) for T-84, IPEC-J2 and PVD9902 cell monolayers and <8 microA cm(-2) for Calu-3 cells. Apical NC-1059 exposure caused, in all cell types, an increase in Isc to >15 microA cm(-2), indicative of net anion secretion or cation absorption, which was followed by an increase in transepithelial conductance (in mS cm(-2): T-84, 1.6 to 62; PVD9902, 0.2 to 51; IPEC-J2, 0.3 to 26; Calu-3, 2.3 to 13). These results are consistent with the peptide affecting transcellular ion movement, with a likely effect also on the paracellular route. NC-1059 exposure increased dextran permeation when compared to basal permeation, which documents an effect on the paracellular pathway. In order to evaluate membrane ion channels, experiments were conducted to study the dose dependence and stability of the NC-1059-induced membrane conductance in Xenopus laevis oocytes. NC-1059 induced a dose-dependent increase in oocyte membrane conductance that remained stable for greater than 2 h. The results demonstrate that NC-1059 increases transcellular conductance and paracellular permeation in a wide range of epithelia. These effects might be exploited to promote drug delivery across barrier epithelia.

Occludin oligomeric assembly at tight junctions of the blood-brain barrier is disrupted by peripheral inflammatory hyperalgesia

Tight junctions (TJs) at the blood-brain barrier (BBB) dynamically alter paracellular diffusion of blood-borne substances from the peripheral circulation to the CNS in response to external stressors, such as pain, inflammation, and hypoxia. In this study, we investigated the effect of lambda-carrageenan-induced peripheral inflammatory pain (i.e., hyperalgesia) on the oligomeric assembly of the key TJ transmembrane protein, occludin. Oligomerization of integral membrane proteins is a critical step in TJ complex assembly that enables the generation of tightly packed, large multiprotein complexes capable of physically obliterating the interendothelial space to inhibit paracellular diffusion. Intact microvessels isolated from rat brains were fractionated by detergent-free density gradient centrifugation, and gradient fractions were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis/ Western blot. Injection of lambda-carrageenan into the rat hind paw produced after 3 h a marked change in the relative amounts of oligomeric, dimeric, and monomeric occludin isoforms associated with different plasma membrane lipid raft domains and intracellular compartments in endothelial cells at the BBB. Our findings suggest that increased BBB permeability (i.e., leak) associated with lambda-carrageenan-induced peripheral inflammatory pain is promoted by the disruption of disulfide-bonded occludin oligomeric assemblies, which renders them incapable of forming an impermeant physical barrier to paracellular transport.

A key claudin extracellular loop domain is critical for epithelial barrier integrity

Intercellular tight junctions (TJs) regulate epithelial barrier properties. Claudins are major structural constituents of TJs and belong to a large family of tetra-spanning membrane proteins that have two predicted extracellular loops (ELs). Given that claudin-1 is widely expressed in epithelia, we further defined the role of its EL domains in determining TJ function. The effects of several claudin-1 EL mimetic peptides on epithelial barrier structure and function were examined. Incubation of model human intestinal epithelial cells with a 27-amino acid peptide corresponding to a portion of the first EL domain (Cldn-1(53-80)) reversibly interfered with epithelial barrier function by inducing the rearrangement of key TJ proteins: occludin, claudin-1, junctional adhesion molecule-A, and zonula occludens-1. Cldn-1(53-80) associated with both claudin-1 and occludin, suggesting both the direct interference with the ability of these proteins to assemble into functional TJs and their close interaction under physiological conditions. These effects were specific for Cldn-1(53-80), because peptides corresponding to other claudin-1 EL domains failed to influence TJ function. Furthermore, the oral administration of Cldn-1(53-80) to rats increased paracellular gastric permeability. Thus, the identification of a critical claudin-1 EL motif, Cldn-1(53-80), capable of regulating TJ structure and function, offers a useful adjunct to treatments that require drug delivery across an epithelial barrier.

Regulation of heterotypic claudin compatibility

Tissue barrier function is directly mediated by tight junction transmembrane proteins known as claudins. Cells that form tight junctions typically express multiple claudin isoforms which suggests that heterotypic (head-to-head) binding between different claudin isoforms may play a role in regulating paracellular permeability. However, little is known about motifs that control heterotypic claudin compatibility. We found that although claudin-3 and claudin-4 were heteromerically compatible when expressed in the same cell, they did not heterotypically interact despite having extracellular loop (EL) domains that are highly conserved at the amino acid level. Claudin-1 and -5, which were heterotypically compatible with claudin-3, did not heterotypically bind to claudin-4. In contrast, claudin-4 chimeras containing either the first EL domain or the second EL domain of claudin-3 were able to heterotypically bind to claudin-1, claudin-3, and claudin-5. Moreover, a single point mutation in the first extracellular loop domain of claudin-3 to convert Asn(44) to the corresponding amino acid in claudin-4 (Thr) produced a claudin capable of heterotypic binding to claudin-4 while still retaining the ability to bind to claudin-1 and -5. Thus, control of heterotypic claudin-claudin interactions is sensitive to small changes in the EL domains.

In vitro transdifferentiation of human fetal type II cells toward a type I-like cell

For alveolar type I cells, phenotype plasticity and physiology other than gas exchange await further clarification due to in vitro study difficulties in isolating and maintaining type I cells in primary culture. Using an established in vitro model of human fetal type II cells, in which the type II phenotype is induced and maintained by adding hormones, we assessed for transdifferentiation in culture toward a type I-like cell with hormone removal for up to 144 h, followed by electron microscopy, permeability studies, and RNA and protein analysis. Hormone withdrawal resulted in diminished type II cell characteristics, including decreased microvilli, lamellar bodies, and type II cell marker RNA and protein. There was a simultaneous increase in type I characteristics, including increased epithelial cell barrier function indicative of a tight monolayer and increased type I cell marker RNA and protein. Our results indicate that hormone removal from cultured human fetal type II cells results in transdifferentiation toward a type I-like cell. This model will be useful for continued in vitro studies of human fetal alveolar epithelial cell differentiation and phenotype plasticity.

Claudins--key pieces in the tight junction puzzle

Tight junctions restrict the flow of ions and aqueous molecules between cells by forming a selective barrier to the paracellular pathway. Permeability of the tight junction barrier is determined by a class of transmembrane proteins known as claudins. The relationship between claudins and paracellular permeability is complex and determined not only by the profile of claudin expression but also by the arrangement of claudins and other proteins into tight junction strands. This review summarizes progress in understanding how claudins are assembled into tight junctions and how they interact with other tight junction proteins.