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

Biophysics of claudin proteins in tight junction architecture: Three decades of progress

Tight junctions are cell-cell adhesion complexes that act as gatekeepers of the paracellular space. Formed by several transmembrane proteins, the claudin family performs the primary gate-keeping function. The claudin proteins form charge and size-selective diffusion barriers to maintain homeostasis across endothelial and epithelial tissue. Of the 27 known claudins in mammals, some are known to seal the paracellular space, while others provide selective permeability. The differences in permeability arise due to the varying expression levels of claudins in each tissue. The tight junctions are observed as strands in freeze-fracture electron monographs; however, at the molecular level, tight junction strands form when multiple claudin proteins assemble laterally (cis assembly) within a cell and head-on (trans assembly) with claudins of the adjacent cell in a zipper-like architecture, closing the gap between the neighboring cells. The disruption of tight junctions caused by changing claudin expression levels or mutations can lead to diseases. Therefore, knowledge of the molecular architecture of the tight junctions and how that is tied to tissue-specific function is critical for fighting diseases. Here, we review the current understanding of the tight junctions accrued over the last three decades from experimental and computational biophysics perspectives.

Intestinal absorption of bioactive oligopeptides: paracellular transport and tight junction modulation

Bioactive oligopeptides have gained increasing attention due to their diverse physiological functions, and these can be transported into the vasculature via transcellular and paracellular pathways. Among these, paracellular transport through the intercellular space is a passive diffusion process without energy consumption. It is currently the most frequently reported absorption route for food-derived bioactive oligopeptides. Previous work has demonstrated that paracellular pathways are mainly controlled by tight junctions, but the mechanism by which they regulate paracellular absorption of bioactive oligopeptides remains unclear. In this review, we summarized the composition of paracellular pathways across the intercellular space and elaborated on the paracellular transport mechanism of bioactive oligopeptides in terms of the interaction between oligopeptides and tight junction proteins, the protein expression level of tight junctions, the signaling pathways regulating intestinal permeability, and the properties of oligopeptides themselves. These findings contribute to a more profound understanding of the paracellular absorption of bioactive oligopeptides.

Systematic-Narrative Hybrid Literature Review: Crosstalk between Gastrointestinal Renin-Angiotensin and Dopaminergic Systems in the Regulation of Intestinal Permeability by Tight Junctions

In the first part of this article, the role of intestinal epithelial tight junctions (TJs), together with gastrointestinal dopaminergic and renin-angiotensin systems, are narratively reviewed to provide sufficient background. In the second part, the current experimental data on the interplay between gastrointestinal (GI) dopaminergic and renin-angiotensin systems in the regulation of intestinal epithelial permeability are reviewed in a systematic manner using the PRISMA methodology. Experimental data confirmed the copresence of DOPA decarboxylase (DDC) and angiotensin converting enzyme 2 (ACE2) in human and rodent enterocytes. The intestinal barrier structure and integrity can be altered by angiotensin (1-7) and dopamine (DA). Both renin-angiotensin and dopaminergic systems influence intestinal Na+/K+-ATPase activity, thus maintaining electrolyte and nutritional homeostasis. The colocalization of B0AT1 and ACE2 indicates the direct role of the renin-angiotensin system in amino acid absorption. Yet, more studies are needed to thoroughly define the structural and functional interaction between TJ-associated proteins and GI renin-angiotensin and dopaminergic systems.

Near-Infrared In Vivo Imaging of Claudin-1 Expression by Orthotopically Implanted Patient-Derived Colonic Adenoma Organoids

BACKGROUND

Claudin-1 becomes overexpressed during the transformation of normal colonic mucosa to colorectal cancer (CRC).

METHODS

Patient-derived organoids expressed clinically relevant target levels and genetic heterogeneity, and were established from human adenoma and normal colons. Colonoids were implanted orthotopically in the colon of immunocompromised mice. This pre-clinical model of CRC provides an intact microenvironment and representative vasculature. Colonoid growth was monitored using white light endoscopy. A peptide specific for claudin-1 was fluorescently labeled for intravenous administration. NIR fluorescence images were collected using endoscopy and endomicroscopy.

RESULTS

NIR fluorescence images collected using wide-field endoscopy showed a significantly greater target-to-background (T/B) ratio for adenoma versus normal (1.89 ± 0.35 and 1.26 ± 0.06) colonoids at 1 h post-injection. These results were confirmed by optical sections collected using endomicroscopy. Optical sections were collected in vivo with sub-cellular resolution in vertical and horizontal planes. Greater claudin-1 expression by individual epithelial cells in adenomatous versus normal crypts was visualized. A human-specific cytokeratin stain ex vivo verified the presence of human tissues implanted adjacent to normal mouse colonic mucosa.

CONCLUSIONS

Increased claudin-1 expression was observed from adenoma versus normal colonoids in vivo using imaging with wide field endoscopy and endomicrosopy.

The role and mechanism of tight junctions in the regulation of salivary gland secretion

Tight junctions (TJs) are cell-cell interactions that localize at the most apical portion of epithelial/endothelial cells. One of the predominant functions of TJs is to regulate material transport through paracellular pathway, which serves as a selective barrier. In recent years, the expression and function of TJs in salivary glands has attracted great interest. The characteristics of multiple salivary gland TJ proteins have been identified. During salivation, the activation of muscarinic acetylcholine receptor and transient receptor potential vanilloid subtype 1, as well as other stimuli, promote the opening of acinar TJs by inducing internalization of TJs, thereby contributing to increased paracellular permeability. Besides, endothelial TJs are also redistributed with leakage of blood vessels in cholinergic-stimulated submandibular glands. Furthermore, under pathological conditions, such as Sjögren's syndrome, diabetes mellitus, immunoglobulin G4-related sialadenitis, and autotransplantation, the integrity and barrier function of TJ complex are impaired and may contribute to hyposalivation. Moreover, in submandibular glands of Sjögren's syndrome mouse model and patients, the endothelial barrier is disrupted and involved in hyposecretion and lymphocytic infiltration. These findings enrich our understanding of the secretory mechanisms that link the importance of epithelial and endothelial TJ functions to salivation under both physiological and pathophysiological conditions.

Structure-function analysis of tight junction-directed permeation enhancer PIP250

The intestinal paracellular route of absorption is modulated via tight junction (TJ) structures located at the apical neck of polarized intestinal epithelial cells to restrict solute movement through the intercellular space between them. Tight junctions open or close in response to changes in the phosphorylation status of light chain (MLC) at position Ser-19. This phosphorylation event is primarily controlled by MLC kinase (MLCK) and MLC phosphatase (MLCP), the latter being a holoenzyme that involves interaction between protein phosphatase 1 (PP1) and myosin targeting protein 1 (MYPT1). An entirely D-amino acid Permeant Inhibitor of Phosphatase (PIP) peptide sequence designed to disrupt PP1-MYPT1 interactions at the cytoplasmic surface of TJs, PIP250 (rrfkvktkkrk) localized at intracellular TJ structures, altered expression levels of specific TJ proteins, increased cellular phosphorylated MLC (pMLC) levels, binding to PP1, decreased epithelial barrier function, and significantly increased systemic uptake of the poorly absorbed antibiotic gentamicin in vivo. A series of PIP250 peptide analogues showed that positions phe3 and val5 were critical to its functional properties, with some providing opportunities to tune the dynamic actions of its TJ modulation properties. These data confirm the activity of PIP250 as a rationally designed oral permeation enhancer and validated key amino acids involved in its interaction with PP1 that define its overall actions; the magnitude and duration of these enhancing properties were associated with the MYPT1-mimetic properties of the PIP250 peptide analogues described.

Cell spheroid creation by transcytotic intercellular gelation

Cell spheroids bridge the discontinuity between in vitro systems and in vivo animal models. However, inducing cell spheroids by nanomaterials remains an inefficient and poorly understood process. Here we use cryogenic electron microscopy to determine the atomic structure of helical nanofibres self-assembled from enzyme-responsive D-peptides and fluorescent imaging to show that the transcytosis of D-peptides induces intercellular nanofibres/gels that potentially interact with fibronectin to enable cell spheroid formation. Specifically, D-phosphopeptides, being protease resistant, undergo endocytosis and endosomal dephosphorylation to generate helical nanofibres. On secretion to the cell surface, these nanofibres form intercellular gels that act as artificial matrices and facilitate the fibrillogenesis of fibronectins to induce cell spheroids. No spheroid formation occurs without endo- or exocytosis, phosphate triggers or shape switching of the peptide assemblies. This study-coupling transcytosis and morphological transformation of peptide assemblies-demonstrates a potential approach for regenerative medicine and tissue engineering.

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.

Multiple plasma membrane reporters discern LHFPL5 region that blocks trafficking to the plasma membrane

The mechano-electrical transduction (MET) channel of the inner ear receptor cells, termed hair cells, is a protein complex that enables our senses of hearing and balance. Hair cell MET requires an elaborate interplay of multiple proteins that form the MET channel. One of the MET complex components is the transmembrane protein LHFPL5, which is required for hair cell MET and hearing. LHFPL5 is thought to form a multi-protein complex with other MET channel proteins, such as PCDH15, TMIE, and TMC1. Despite localizing to the plasma membrane of stereocilia, the mechanosensing organelles of hair cells, LHFPL5 requires its binding partner within the MET complex, PCDH15, to localize to the stereocilia tips in hair cells and to the plasma membrane in heterologous cells. Using the Aquaporin 3-tGFP reporter (AGR) for plasma membrane localization, we found that a region within extracellular loop 1, which interacts with PCDH15, precludes the trafficking of AGR reporter to the plasma membrane in heterologous cell lines. Our results suggest that the presence of protein partners may mask endoplasmic reticulum retention regions or enable the proper folding and trafficking of the MET complex components, to facilitate expression of the MET complex at the stereocilia membrane.

Effects of Houpo Mahuang Decoction on serum metabolism and TRPV1/Ca2+/TJs in asthma

ETHNOPHARMACOLOGICAL RELEVANCE

Houpo Mahuang Decoction (HPMHD is one of the classic traditional Chinese prescriptions that has been used in the treatment of asthma. The therapeutic effects and mechanism of HPMHD in aggravated asthma remain to be explored, especially from the perspective of metabolomics and Transient Receptor Potential Vanilloid-1 (TRPV1)/Ca²⁺/Tight junction (TJ) regulation.

AIM OF THE STUDY

To investigate the therapeutic and metabolic regulatory effects and the underlying mechanism of HPMHD in asthmatic rats.

MATERIALS AND METHODS

The asthmatic rats were administered with the corresponding HPMHD (at dosages of 5.54, 11.07, 22.14 mg/kg). Then inflammatory cells in peripheral blood and bronchoalveolar lavage fluid (BALF) were counted, the levels of interleukin (IL)-4 and IL-13 in BALF were measured, and the changes in enhanced pause (Penh) and pathological damage of lung tissues were also detected to evaluate the protective effects of HPMHD. The serum metabolic profile of HPMHD in asthmatic rats was explored using Ultra-High-Performance Liquid Chromatography-mass spectrometer (UHPLC-MS), and the regulatory effects on TRPV1 and TJs of HPMHD in asthmatic rats were detected by Western blotting analysis. In vitro, 16HBE cells were stimulated with IL-4 plus SO₂ derivatives and then administered HPMHD. The intracellular Ca²⁺ regulated by TRPV1, and the expression levels of TRPV1 and TJ proteins (TJs) were then detected by calcium imaging and Western blotting. The effects were verified by inhibition of TRPV1 and in short hairpin RNA (shRNA)-mediated TRPV1 silencing cells.

RESULTS

HPMHD significantly attenuated the airway inflammation of asthmatic rats, and reduced the levels of inflammatory cells in peripheral blood and BALF as well as the levels of IL-4 plus IL-13 in BALF. In addition, the airway hyperresponsiveness and lung pathological damage were alleviated. Serum metabolomic analysis showed that 31 metabolites were differentially expressed among the normal saline-, model-, and HPMHD-treated rats. Pathway enrichment analysis showed that the metabolites were involved in 45 pathways, among which, TJs regulation-relevant pathway was associated with the Ca²⁺ concentration change mediated by the TRP Vanilloid channel. In vivo and in vitro experiments indicated that HPMHD reduced the concentration of intracellular Ca²⁺ via suppressing the expression and activation of TRPV1, increased the expression of ZO-1, Occludin, and Claudin-3, and protected the integrity of TJs.

CONCLUSION

The current study indicates that HPMHD alleviates rat asthma and participates in the regulation of serum metabolism. The anti-asthma effects of HPMHD might be related to the protection of TJs by inhibiting the intracellular Ca²⁺ concentration via TRPV1.

Human Claudin-Derived Peptides Block the Membrane Fusion Process of Zika Virus and Are Broad Flavivirus Inhibitors

Zika virus (ZIKV) is a mosquito-borne flavivirus that emerged in the Pacific islands in 2007 and spread to the Americas in 2015. The infection remains asymptomatic in most cases but can be associated with severe neurological disorders. Despite massive efforts, no specific drug or vaccine against ZIKV infection is available to date. Claudins are tight-junction proteins that favor the entry of several flaviviruses, including ZIKV. In this study, we identified two peptides derived from the N-terminal sequences of claudin-7 and claudin-1, named CL7.1 and CL1.1, respectively, that inhibited ZIKV infection in a panel of human cell lines. Using cell-to-cell fusion assays, we demonstrated that these peptides blocked the ZIKV E-mediated membrane fusion. A comparison of the antiviral efficacy of CL1.1 and CL7.1 pointed to the importance of the peptide amphipathicity. Electron microscopic analysis revealed that CL1.1 altered the ultrastructure of the viral particles likely by binding the virus lipid envelope. However, amphipathicity could not fully explain the antiviral activity of CL1.1. In silico docking simulations suggested that CL1.1 may also interact with the E protein, near its stem region. Overall, our data suggested that claudin-derived peptides inhibition may be linked to simultaneous interaction with the E protein and the viral lipid envelope. Finally, we found that CL1.1 also blocked infection by yellow fever and Japanese encephalitis viruses but not by HIV-1 or SARS-CoV-2. Our results provide a basis for the future development of therapeutics against a wide range of endemic and emerging flaviviruses. IMPORTANCE Zika virus (ZIKV) is a flavivirus transmitted by mosquito bites that have spread to the Pacific Islands and the Americas over the past decade. The infection remains asymptomatic in most cases but can cause severe neurological disorders. ZIKV is a major public health threat in areas of endemicity, and there is currently no specific antiviral drug or vaccine available. We identified two antiviral peptides deriving from the N-terminal sequences of claudin-7 and claudin-1 with the latter being the most effective. These peptides block the envelope-mediated membrane fusion. Our data suggested that the inhibition was likely achieved by simultaneously interacting with the viral lipid envelope and the E protein. The peptides also inhibited other flaviviruses. These results could provide the basis for the development of therapies that might target a wide array of flaviviruses from current epidemics and possibly future emergences.

Modifying the blood-brain barrier by targeting claudin-5: Safety and risks

The blood-brain barrier is a major obstacle to the delivery of drugs to the central nervous system. In the blood-brain barrier, the spaces between adjacent brain microvascular endothelial cells are sealed by multiprotein complexes known as tight junctions. Among the many components of the tight junction, claudin-5 has received the most attention as a target for loosening the tight-junction seal and allowing drugs to be delivered to the brain. In mice, transient knockdown of claudin-5 and the use of claudin-5 binders have been shown to enhance the permeation of small molecules from the blood into the brain without apparent adverse effects. However, sustained knockdown of claudin-5 in mice is lethal within 40 days, and administration of an anti-claudin-5 antibody induced convulsions in a nonhuman primate. Here, we review the safety concerns of claudin-5-targeted technologies with respect to their clinical application.

Mitigative effects of Eleutheroside E against the mechanical barrier dysfunction induced by soybean agglutinin in IPEC-J2 cell line

Soybean agglutinin (SBA) is an anti-nutritional factor which decreases the mechanical barrier function in intestinal porcine jejunum epithelial cells (IPEC-J2). Eleutheroside E (EE) is a key part of Acanthopanax senticosus to exert pharmacological effects. This study aims to investigate the effects of EE on the barrier function in IPEC-J2 cells and to determine the ability of EE to enhance the protective effect of barrier function against SBA exposure. The IPEC-J2 cells were cultured in mediums with concentration of 0.1 mg/ml EE, 0.5 ml/ml SBA and 0.1 mg/ml EE pre-treated then treated with 0.5 mg/ml SBA. Then, the transepithelial electric resistance (TEER) value, inflammatory cytokines mRNA expression, tight junction mRNA and protein expression were tested by epithelial Voltohm meter, q-PCR and Western blot method respectively. The results showed that cells treated with 0.1 mg/ml EE had lower permeability (p < 0.05) while 0.5 mg/ml SBA treatment had higher permeability through tested TEER, and higher tight junction proteins (Claudin-3 and ZO-1) expressions and genes (Claudin-3, Occludin and ZO-1) expressions (p < 0.05) in 0.1 mg/ml EE group. IPEC-J2 cells pre-treated with 0.1 mg/ml EE could significantly improve the inflammatory response caused by 0.5 mg/ml SBA by up-regulation for IL-10, TGF-β, and down-regulation gene expression of IL-6, TNF-α and IFN-γ (p < 0.05). In conclusion, 0.1 mg/ml EE can improve the mechanical barrier function and could protect the effects while 0.5 mg/ml of SBA-induced barrier dysfunction in IPEC-J2.

Claudin-1-Targeted Nanoparticles for Delivery to Aging-Induced Alterations in the Blood-Brain Barrier

Aging-induced alterations to the blood-brain barrier (BBB) are increasingly being seen as a primary event in chronic progressive neurological disorders that lead to cognitive decline. With the goal of increasing delivery into the brain in hopes of effectively treating these diseases, a large focus has been placed on developing BBB permeable materials. However, these strategies have suffered from a lack of specificity toward regions of disease progression. Here, we report on the development of a nanoparticle (C1C2-NP) that targets regions of increased claudin-1 expression that reduces BBB integrity. Using dynamic contrast enhanced magnetic resonance imaging, we find that C1C2-NP accumulation and retention is significantly increased in brains from 12 month-old mice as compared to nontargeted NPs and brains from 2 month-old mice. Furthermore, we find C1C2-NP accumulation in brain endothelial cells with high claudin-1 expression, suggesting target-specific binding of the NPs, which was validated through fluorescence imaging, in vitro testing, and biophysical analyses. Our results further suggest a role of claudin-1 in reducing BBB integrity during aging and show altered expression of claudin-1 can be actively targeted with NPs. These findings could help develop strategies for longitudinal monitoring of tight junction protein expression changes during aging as well as be used as a delivery strategy for site-specific delivery of therapeutics at these early stages of disease development.

Modulating the Blood-Brain Barrier: A Comprehensive Review

The highly secure blood-brain barrier (BBB) restricts drug access to the brain, limiting the molecular toolkit for treating central nervous system (CNS) diseases to small, lipophilic drugs. Development of a safe and effective BBB modulator would revolutionise the treatment of CNS diseases and future drug development in the area. Naturally, the field has garnered a great deal of attention, leading to a vast and diverse range of BBB modulators. In this review, we summarise and compare the various classes of BBB modulators developed over the last five decades-their recent advancements, advantages and disadvantages, while providing some insight into their future as BBB modulators.

Claudin-5: A Pharmacological Target to Modify the Permeability of the Blood-Brain Barrier

Claudin-5 is the dominant tight junction protein in brain endothelial cells and exclusively limits the paracellular permeability of molecules larger than 400 Da across the blood-brain barrier (BBB). Its pathological impairment or sustained down-regulation has been shown to lead to the progression of psychiatric and neurological disorders, whereas its expression under physiological conditions prevents the passage of drugs across the BBB. While claudin-5 enhancers could potentially act as vascular stabilizers to treat neurological diseases, claudin-5 inhibitors could function as delivery systems to enhance the brain uptake of hydrophilic small-molecular-weight drugs. Therefore, the effects of claudin-5 manipulation on modulating the BBB in different neurological diseases requires further examination. To manipulate claudin-5 expression levels and function, several claudin-5 modulating molecules have been developed. In this review, we first describe the molecular, cellular and pathological aspects of claudin-5 to highlight the mechanisms of claudin-5 enhancers/inhibitors. We then discuss recently developed claudin-5 enhancers/inhibitors and new methods to discover these molecules.

Formulation strategies to improve the efficacy of intestinal permeation enhancers. Adv Drug Deliv Rev 2021;177:113925. [PMID: 34418495 DOI: 10.1016/j.addr.2021.113925]

The use of chemical permeation enhancers (PEs) is the most widely tested approach to improve oral absorption of low permeability active agents, as represented by peptides. Several hundred PEs increase intestinal permeability in preclinical bioassays, yet few have progressed to clinical testing and, of those, only incremental increases in oral bioavailability (BA) have been observed. Still, average BA values of ~1% were sufficient for two recent FDA approvals of semaglutide and octreotide oral formulations. PEs are typically screened in static in vitro and ex-vivo models where co-presentation of active agent and PE in high concentrations allows the PE to alter barrier integrity with sufficient contact time to promote flux across the intestinal epithelium. The capacity to maintain high concentrations of co-presented agents at the epithelium is not reached by standard oral dosage forms in the upper GI tract in vivo due to dilution, interference from luminal components, fast intestinal transit, and possible absorption of the PE per se. The PE-based formulations that have been assessed in clinical trials in either immediate-release or enteric-coated solid dosage forms produce low and variable oral BA due to these uncontrollable physiological factors. For PEs to appreciably increase intestinal permeability from oral dosage forms in vivo, strategies must facilitate co-presentation of PE and active agent at the epithelium for a sustained period at the required concentrations. Focusing on peptides as examples of a macromolecule class, we review physiological impediments to optimal luminal presentation, discuss the efficacy of current PE-based oral dosage forms, and suggest strategies that might be used to improve them.

Claudins as biomarkers of differential diagnosis and prognosis of tumors

Claudins are a superfamily of transmembrane proteins, the optimal expression and localization of which are important for the normal physiological function of the epithelium and any imbalance may have pathological consequences. Not only insufficient but also excessive production of claudins in cancer cells, as well as their aberrant localization, equally manifest the formation of a malignant phenotype. Many works are distinguished by contradictory data, which demonstrate the action of the same claudins both in the role of tumor-growth suppressors and promoters in the same cancers. The most important possible causes of significant discrepancies in the results of the works are a considerable variability of sampling and the absence of a consistent approach both to the assessment of the immune reactivity of claudins and to the differential analysis of their subcellular localization. Combined, these drawbacks hinder the histological assessment of the link between claudins and tumor progression. In particular, ambiguous expression of claudins in breast cancer subtypes, revealed by various authors in immunohistochemical analysis, not only fails to facilitate the identification of the claudin-low molecular subtype but rather complicates these efforts. Research into the role of claudins in carcinogenesis has undoubtedly confirmed the potential value of this class of proteins as significant biomarkers in some cancer types; however, the immunohistochemical approach to the assessment of claudins still has limitations, needs standardization, and, to date, has not reached a diagnostic or a prognostic value.

Role of microbiota and related metabolites in gastrointestinal tract barrier function in NAFLD

The Gastrointestinal (GI) tract is composed of four main barriers: microbiological, chemical, physical and immunological. These barriers play important roles in maintaining GI tract homeostasis. In the crosstalk between these barriers, microbiota and related metabolites have been shown to influence GI tract barrier integrity, and alterations of the gut microbiome might lead to an increase in intestinal permeability. As a consequence, translocation of bacteria and their products into the circulatory system increases, reaching proximal and distal tissues, such as the liver. One of the most prevalent chronic liver diseases, Nonalcoholic Fatty Liver Disease (NAFLD), has been associated with an altered gut microbiota and barrier integrity. However, the causal link between them has not been fully elucidated yet. In this review, we aim to highlight relevant bacterial taxa and their related metabolites affecting the GI tract barriers in the context of NAFLD, discussing their implications in gut homeostasis and in disease.

Chimeric Claudins: A New Tool to Study Tight Junction Structure and Function

The tight junction (TJ) is a structure composed of multiple proteins, both cytosolic and membranal, responsible for cell–cell adhesion in polarized endothelium and epithelium. The TJ is intimately connected to the cytoskeleton and plays a role in development and homeostasis. Among the TJ’s membrane proteins, claudins (CLDNs) are key to establishing blood–tissue barriers that protect organismal physiology. Recently, several crystal structures have been reported for detergent extracted recombinant CLDNs. These structural advances lack direct evidence to support quaternary structure of CLDNs. In this article, we have employed protein-engineering principles to create detergent-independent chimeric CLDNs, a combination of a 4-helix bundle soluble monomeric protein (PDB ID: 2jua) and the apical—50% of human CLDN1, the extracellular domain that is responsible for cell–cell adhesion. Maltose-binding protein-fused chimeric CLDNs (MBP-CCs) used in this study are soluble proteins that retain structural and functional aspects of native CLDNs. Here, we report the biophysical characterization of the structure and function of MBP-CCs. MBP-fused epithelial cadherin (MBP-eCAD) is used as a control and point of comparison of a well-characterized cell-adhesion molecule. Our synthetic strategy may benefit other families of 4-α-helix membrane proteins, including tetraspanins, connexins, pannexins, innexins, and more.

Target specific tight junction modulators

Intercellular tight junctions represent a formidable barrier against paracellular drug absorption at epithelia (e.g., nasal, intestinal) and the endothelium (e.g., blood-brain barrier). In order to enhance paracellular transport of drugs and increase their bioavailability and organ deposition, active excipients modulating tight junctions have been applied. First-generation of permeation enhancers (PEs) acted by unspecific interactions, while recently developed PEs address specific physiological mechanisms. Such target specific tight junction modulators (TJMs) have the advantage of a defined specific mechanism of action. To date, merely a few of these novel active excipients has entered into clinical trials, as their lack in safety and efficiency in vivo often impedes their commercialisation. A stronger focus on the development of such active excipients would result in an economic and therapeutic improvement of current and future drugs.

Why remission is not enough: underlying disease mechanisms in RA that prevent cure

Cure is the aspirational aim for the treatment of all diseases, including chronic inflammatory conditions such as rheumatoid arthritis (RA); however, it has only been during the twenty-first century that remission, let alone cure, has been a regularly achievable target in RA. Little research has been carried out on how to cure RA, and the term 'cure' still requires definition for this disease. Even now, achieving a cure seems to be a rare occurrence among individuals with RA. Therefore, this Review is aimed at addressing the obstacles to the achievement of cure in RA. The differences between remission and cure in RA are first defined, followed by a discussion of the underlying factors (referred to as drivers) that prevent the achievement of cure in RA by triggering sustained immune activation and effector cytokine production. Such drivers include adaptive immune system activation, mesenchymal tissue priming and so-called 'remote' (non-immune and non-articular) factors. Strategies to target these drivers are also presented, with an emphasis on the development of strategies that could complement currently used cytokine inhibition and thereby improve the likelihood of curing RA.

Mechanistic Approaches of Internalization, Subcellular Trafficking, and Cytotoxicity of Nanoparticles for Targeting the Small Intestine

Targeting the small intestine employing nanotechnology has proved to be a more effective way for site-specific drug delivery. The drug targeting to the small intestine can be achieved via nanoparticles for its optimum bioavailability within the systemic circulation. The small intestine is a remarkable candidate for localized drug delivery. The intestine has its unique properties. It has a less harsh environment than the stomach, provides comparatively more retention time, and possesses a greater surface area than other parts of the gastrointestinal tract. This review focuses on elaborating the intestinal barriers and approaches to overcome these barriers for internalizing nanoparticles and adopting different cellular trafficking pathways. We have discussed various factors that contribute to nanocarriers' cellular uptake, including their surface chemistry, surface morphology, and functionalization of nanoparticles. Furthermore, the fate of nanoparticles after their uptake at cellular and subcellular levels is also briefly explained. Finally, we have delineated the strategies that are adopted to determine the cytotoxicity of nanoparticles.

Intestinal permeation enhancers to improve oral bioavailability of macromolecules: reasons for low efficacy in humans

INTRODUCTION

Intestinal permeation enhancers (PEs) are substances that transiently alter the intestinal epithelial barrier to facilitate permeation of macromolecules with low oral bioavailability (BA). While a number of PEs have progressed to clinical testing in conventional formulations with macromolecules, there has been only low single digit increases in oral BA, irrespective of whether the drug met primary or secondary clinical endpoints.

AREAS COVERED

This article considers the causes of sub-optimal BA of macromolecules from PE dosage forms and suggests approaches that may improve performance in humans.

EXPERT OPINION

Permeation enhancement is most effective when the PE is co-localized with the macromolecule at the epithelial surface. Conditions in the GI tract impede optimal co-localization. Novel delivery systems that limit dilution and spreading of the PE and macromolecule in the small intestine have attempted to replicate promising enhancement efficacy observed in static drug delivery models.

Tight junction modulation at the blood-brain barrier: Current and future perspectives

The blood-brain barrier (BBB) is the one of the most robust physical barriers in the body, comprised of tight junction (TJ) proteins in brain microvascular endothelial cells. The need for drugs to treat central nervous systems diseases is ever increasing, however the presence of the BBB significantly hampers the uptake of drugs into the brain. To overcome or circumvent the barrier, many kinds of techniques are being developed. Modulating the paracellular route by disruption of the TJ complex has been proposed as a potential drug delivery system to treat brain diseases, however, it has several limitations and is still in a developmental stage. However, recent significant advance in medical equipment /tools such as targeted ultra-sound technologies may resolve these limitations. In this review, we introduce recent advances in site- or molecular size-selective BBB disruption/modulation technologies and we include details on pharmacological inhibitory molecules against intercellular TJ proteins to modulate the BBB.

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.

Molecular architecture and assembly of the tight junction backbone

The functional and structural concept of tight junctions has developed after discovery of claudin and TAMP proteins. Many of these proteins contribute to epi- and endothelial barrier but some, in contrast, form paracellular channels. Claudins form the backbone of tight junction (TJ) strands whereas other proteins regulate TJ dynamics. The current joined double-row model of TJ strands and channels is crucially based on the linear alignment of claudin-15 in the crystal. Molecular dynamics simulations, protein docking, mutagenesis, cellular TJ reconstitution, and electron microscopy studies largely support stability and functionality of the model. Here, we summarize in silico and in vitro data about TJ strand assembly including comparison of claudin crystal structures and alternative models. Sequence comparisons, experimental and structural data substantiate differentiation of classic and non-classic claudins differing in motifs related to strand assembly. Classic claudins seem to share a similar mechanism of strand formation. Interface variations likely contribute to TJ strand flexibility. Combined in vitro/in silico studies are expected to elucidate mechanistic keys determining TJ regulation.

Peptides Derived from the Tight Junction Protein CLDN1 Disrupt the Skin Barrier and Promote Responsiveness to an Epicutaneous Vaccine

Keratinocytes express many pattern recognition receptors that enhance the skin's adaptive immune response to epicutaneous antigens. We have shown that these pattern recognition receptors are expressed below tight junctions (TJ), strongly implicating TJ disruption as a critical step in antigen responsiveness. To disrupt TJs, we designed peptides inspired by the first extracellular loop of the TJ transmembrane protein CLDN1. These peptides transiently disrupted TJs in the human lung epithelial cell line 16HBE and delayed TJ formation in primary human keratinocytes. Building on these observations, we tested whether vaccinating mice with an epicutaneous influenza patch containing TJ-disrupting peptides was an effective strategy to elicit an immunogenic response. Application of a TJ-disrupting peptide patch resulted in barrier disruption as measured by increased transepithelial water loss. We observed a significant increase in antigen-specific antibodies when we applied patches with TJ-disrupting peptide plus antigen (influenza hemagglutinin) in either a patch-prime or a patch-boost model. Collectively, these observations demonstrate that our designed peptides perturb TJs in human lung as well as human and murine skin epithelium, enabling epicutaneous vaccine delivery. We anticipate that this approach could obviate currently used needle-based vaccination methods that require administration by health care workers and biohazard waste removal.

N-Acetyl-d-galactosamine prevents soya bean agglutinin-induced intestinal barrier dysfunction in intestinal porcine epithelial cells

Soya bean agglutinin (SBA) is a glycoprotein and the main anti-nutritional component in most soya bean feedstuffs. It is mainly a non-fibre carbohydrate-based protein and represents about 10% of soya bean-based anti-nutritional effects. In this study, we sought to determine the effects of N-Acetyl-D-galactosamine (GalNAc or D-GalNAc) on the damage induced by SBA on the membrane permeability and tight junction proteins of piglet intestinal epithelium (IPEC-J2) cells. The IPEC-J2 cells were pre-cultured with 0, 0.125 × 10-4 , 0.25 × 10-4 , 0.5 × 10-4 , 1.0 × 10-4 and 2.0 × 10-4  mmol/L GalNAc at different time period (1, 2, 4 and 8 hr) before being exposed to 0.5 mg/ml SBA for 24 hr. The results indicate that pre-incubation with GalNAc mitigates the mechanical barrier injury as reflected by a significant increase in trans-epithelial electric resistance (TEER) value and a decrease in alkaline phosphatase (ALP) activity in cell culture medium pre-treated with GalNAc before incubation with SBA as both indicate a reduction in cellular membrane permeability. In addition, mRNA levels of the tight junction proteins occludin and claudin-3 were lower in the SBA-treated groups without pre-treatment with GalNAc. The mRNA expression of occludin was reduced by 17.3% and claudin-3 by 42% (p < 0.01). Moreover, the corresponding protein expression levels were lowered by 17.8% and 43.5% (p < 0.05) respectively. However, in the GalNAc pre-treated groups, occludin and claudin-3 mRNAs were reduced by 1.6% (p > 0.05) and 2.7% (p < 0.01), respectively, while the corresponding proteins were reduced by 4.3% and 7.2% (p < 0.05). In conclusion, GalNAc may prevent the effect of SBA on membrane permeability and tight junction proteins on IPEC-J2s.

Physiologically Relevant, Humanized Intestinal Systems to Study Metabolism and Transport of Small Molecule Therapeutics

Intestinal disposition of small molecules involves interplay of drug metabolizing enzymes (DMEs), transporters, and host-microbiome interactions, which has spurred the development of in vitro intestinal models derived from primary tissue sources. Such models have been bioengineered from intestinal crypts, mucosal extracts, induced pluripotent stem cell (iPSC)-derived organoids, and human intestinal tissue. This minireview discusses the utility and limitations of these human-derived models in support of small molecule drug metabolism and disposition. Enteroids from human intestinal crypts, organoids derived from iPSCs using growth factors or small molecule compounds, and enterocytes extracted from mucosal scrapings show key absorptive cell morphology while are limited in quantitative applications due to the lack of accessibility to the apical compartment, the lack of monolayers, or low expression of key DMEs, transporters, and nuclear hormone receptors. Despite morphogenesis to epithelial cells, similar challenges have been reported by more advanced technologies that have explored the impact of flow and mechanical stretch on proliferation and differentiation of Caco-2 cells. Most recently, bioengineered human intestinal epithelial or ileal cells have overcome many of the challenges, as the DME and transporter expression pattern resembles that of native intestinal tissue. Engineering advances may improve such models to support longer-term applications and meet end-user needs. Biochemical characterization and transcriptomic, proteomic, and functional endpoints of emerging novel intestinal models, when referenced to native human tissue, can provide greater confidence and increased utility in drug discovery and development.

Cellular junctions in the epididymis, a critical parameter for understanding male reproductive toxicology

Epididymal sperm maturation is a critical aspect of male reproduction in which sperm acquire motility and the ability to fertilize an ovum. Sperm maturation is dependent on the creation of a specific environment that changes along the epididymis and which enables the maturation process. The blood-epididymis barrier creates a unique luminal micro-environment, different from blood, by limiting paracellular transport and forcing receptor-mediated transport of macromolecules across the epididymal epithelium. Direct cellular communication between cells allows coordinated function of the epithelium. A limited number of studies have directly examined the effects of toxicants on junctional proteins and barrier function in the epididymis. Effects on the integrity of the blood-epididymis barrier have resulted in decreased fertility and, in some cases, the development of sperm granulomas. Studies have shown that in addition to tight junctions, proteins implicated in the maintenance of adherens junctions and gap junctions alter epididymal functions. This review will provide an overview of the types and roles of cellular junctions in the epididymis, and how these are targeted by different toxicants.

Mechanisms regulating intestinal barrier integrity and its pathological implications

The gastrointestinal tract is a specialized organ in which dynamic interactions between host cells and the complex environment occur in addition to food digestion. Together with the chemical barrier of the mucosal layer and the cellular immune system, the epithelial cell layer performs a pivotal role as the first physical barrier against external factors and maintains a symbiotic relationship with commensal bacteria. The tight junction proteins, including occludin, claudins, and zonula occludens, are crucial for the maintenance of epithelial barrier integrity. To allow the transport of essential molecules and restrict harmful substances, the intracellular signaling transduction system and a number of extracellular stimuli such as cytokines, small GTPases, and post-translational modifications dynamically modulate the tight junction protein complexes. An imbalance in these regulations leads to compromised barrier integrity and is linked with pathological conditions. Despite the obscurity of the causal relationship, the loss of barrier integrity is considered to contribute to inflammatory bowel disease, obesity, and metabolic disorders. The elucidation of the role of diseases in barrier integrity and the underlying regulatory mechanisms have improved our understanding of the intestinal barrier to allow the development of novel and potent therapeutic approaches.

A better understanding of how the cells that line the inside of the intestines allow nutrients in, while keeping harmful substances and pathogens out could lead to new therapies for inflammatory bowel disease, obesity, and other conditions. A team from South Korea led by Sung Ho Ryu from Pohang University of Science and Technology review the regulatory mechanisms that help maintain the intestinal epithelial barrier. They discuss the role of tight junction proteins in forming a seal between adjacent cells and the various signaling pathways that loosen or tighten these junctions to enable limited transport. Loss of barrier integrity because of genetics, gut microbes, auto-immunity, diet, or other factors is often implicated in disease, and restoring barrier function with drugs or probiotics could help ameliorate many health problems.

Intestinal epithelial claudins: expression and regulation in homeostasis and inflammation

The intestinal epithelium forms a highly dynamic and selective barrier that controls absorption of fluid and solutes while restricting pathogen access to underlying tissues. Barrier properties are achieved by intercellular junctions that include an apical tight junction (TJ) and subjacent adherens junctions and desmosomes. The TJ tetraspan claudin proteins form pores between epithelial cells to control paracellular fluid and ion movement. In addition to regulation of barrier function, claudin family members control epithelial homeostasis and are expressed in a spatiotemporal manner in the intestinal crypt-luminal axis. This delicate balance of physiologic differential claudin protein expression is altered during mucosal inflammation. Inflammatory mediators influence transcriptional regulation, as well as endocytic trafficking, targeting, and retention of claudins in the TJ. Increased expression of intestinal epithelial claudin-1, -2, and -18 with downregulation of claudin-3, -4, -5, -7, -8, and -12 has been observed in intestinal inflammatory disorders. Such changes in claudin proteins modify the epithelial barrier function in addition to influencing epithelial and mucosal homeostasis. An improved understanding of the regulatory mechanisms that control epithelial claudin proteins will provide strategies to strengthen the epithelial barrier function and restore mucosal homeostasis in inflammatory disorders.

Intestinal permeation enhancers for oral peptide delivery

Intestinal permeation enhancers (PEs) are one of the most widely tested strategies to improve oral delivery of therapeutic peptides. This article assesses the intestinal permeation enhancement action of over 250 PEs that have been tested in intestinal delivery models. In depth analysis of pre-clinical data is presented for PEs as components of proprietary delivery systems that have progressed to clinical trials. Given the importance of co-presentation of sufficiently high concentrations of PE and peptide at the small intestinal epithelium, there is an emphasis on studies where PEs have been formulated with poorly permeable molecules in solid dosage forms and lipoidal dispersions.

Oral absorption of peptides and nanoparticles across the human intestine: Opportunities, limitations and studies in human tissues

In this contribution, we review the molecular and physiological barriers to oral delivery of peptides and nanoparticles. We discuss the opportunities and predictivity of various in vitro systems with special emphasis on human intestine in Ussing chambers. First, the molecular constraints to peptide absorption are discussed. Then the physiological barriers to peptide delivery are examined. These include the gastric and intestinal environment, the mucus barrier, tight junctions between epithelial cells, the enterocytes of the intestinal epithelium, and the subepithelial tissue. Recent data from human proteome studies are used to provide information about the protein expression profiles of the different physiological barriers to peptide and nanoparticle absorption. Strategies that have been employed to increase peptide absorption across each of the barriers are discussed. Special consideration is given to attempts at utilizing endogenous transcytotic pathways. To reliably translate in vitro data on peptide or nanoparticle permeability to the in vivo situation in a human subject, the in vitro experimental system needs to realistically capture the central aspects of the mentioned barriers. Therefore, characteristics of common in vitro cell culture systems are discussed and compared to those of human intestinal tissues. Attempts to use the cell and tissue models for in vitro-in vivo extrapolation are reviewed.

Using peptides to increase transport across the intestinal barrier

The oral route is the preferred for the administration of drugs; however, it has some serious limitations. One of the main disadvantages is poor permeability across the intestinal barrier. Various approaches are currently being adopted to overcome this issue. In this review, we describe the alternatives that use peptides to enhance intestinal absorption. First, we define the various sources of peptide enhancers followed by the analysis of the absorption mechanism used. We then comment on the possible toxic effects derived from their use as permeation enhancers, as well as potential formulation strategies. Finally, the advantages and drawbacks of peptides as intestinal enhancers are examined.

Regulation of claudin/zonula occludens-1 complexes by hetero-claudin interactions

Claudins are tetraspan transmembrane tight-junction proteins that regulate epithelial barriers. In the distal airspaces of the lung, alveolar epithelial tight junctions are crucial to regulate airspace fluid. Chronic alcohol abuse weakens alveolar tight junctions, priming the lung for acute respiratory distress syndrome, a frequently lethal condition caused by airspace flooding. Here we demonstrate that in response to alcohol, increased claudin-5 paradoxically accompanies an increase in paracellular leak and rearrangement of alveolar tight junctions. Claudin-5 is necessary and sufficient to diminish alveolar epithelial barrier function by impairing the ability of claudin-18 to interact with a scaffold protein, zonula occludens 1 (ZO-1), demonstrating that one claudin affects the ability of another claudin to interact with the tight-junction scaffold. Critically, a claudin-5 peptide mimetic reverses the deleterious effects of alcohol on alveolar barrier function. Thus, claudin controlled claudin-scaffold protein interactions are a novel target to regulate tight-junction permeability.

Identification of a Potent and Broad-Spectrum Hepatitis C Virus Fusion Inhibitory Peptide from the E2 Stem Domain

Hepatitis C virus (HCV) envelope proteins E1 and E2 play an essential role in virus entry. However, the fusion mechanisms of HCV remain largely unclear, hampering the development of efficient fusion inhibitors. Here, we developed two cell-based membrane fusion models that allow for screening a peptide library covering the full-length E1 and E2 amino acid sequences. A peptide from the E2 stem domain, named E27, was found to possess the ability to block E1E2-mediated cell-cell fusion and inhibit cell entry of HCV pseudoparticles and infection of cell culture-derived HCV at nanomolar concentrations. E27 demonstrated broad-spectrum inhibition of the major genotypes 1 to 6. A time-of-addition experiment revealed that E27 predominantly functions in the late steps during HCV entry, without influencing the expression and localization of HCV co-receptors. Moreover, we demonstrated that E27 interfered with hetero-dimerization of ectopically expressed E1E2 in cells, and mutational analysis suggested that E27 might target a conserved region in E1. Taken together, our findings provide a novel candidate as well as a strategy for developing potent and broad-spectrum HCV fusion inhibitors, which may complement the current direct-acting antiviral medications for chronic hepatitis C, and shed light on the mechanism of HCV membrane fusion.

Current trends in salivary gland tight junctions

Tight junctions form a continuous intercellular barrier between epithelial cells that is required to separate tissue spaces and regulate selective movement of solutes across the epithelium. They are composed of strands containing integral membrane proteins (e.g., claudins, occludin and tricellulin, junctional adhesion molecules and the coxsackie adenovirus receptor). These proteins are anchored to the cytoskeleton via scaffolding proteins such as ZO-1 and ZO-2. In salivary glands, tight junctions are involved in polarized saliva secretion and barrier maintenance between the extracellular environment and the glandular lumen. This review seeks to provide an overview of what is currently known, as well as the major questions and future research directions, regarding tight junction expression, organization and function within salivary glands.

Overexpressed Claudin-1 Can Be Visualized Endoscopically in Colonic Adenomas In Vivo

BACKGROUND & AIMS

Conventional white-light colonoscopy aims to reduce the incidence and mortality of colorectal cancer (CRC). CRC has been found to arise from missed polypoid and flat precancerous lesions. We aimed to establish proof-of-concept for real-time endoscopic imaging of colonic adenomas using a near-infrared peptide that is specific for claudin-1.

METHODS

We used gene expression profiles to identify claudin-1 as a promising early CRC target, and performed phage display against the extracellular loop of claudin-1 (amino acids 53-80) to identify the peptide RTSPSSR. With a Cy5.5 label, we characterized binding parameters and showed specific binding to human CRC cells. We collected in vivo near-infrared fluorescence images endoscopically in the CPC;Apc mouse, which develops colonic adenomas spontaneously. With immunofluorescence, we validated specific peptide binding to adenomas from the proximal human colon.

RESULTS

We found a 2.5-fold increase in gene expression for claudin-1 in human colonic adenomas compared with normal. We showed specific binding of RTSPSSR to claudin-1 in knockdown and competition studies, and measured an affinity of 42 nmol/L and a time constant of 1.2 minutes to SW620 cells. In the mouse, we found a significantly higher target-to-background ratio for both polypoid and flat adenomas compared with normal by in vivo images. On immunofluorescence, we found significantly greater intensity for human adenomas (mean ± SD, 25.5 ± 14.0) vs normal (mean ± SD, 9.1 ± 6.0) and hyperplastic polyps (mean ± SD, 3.1 ± 3.7; P = 10-5 and 8 × 10-12, respectively), and for sessile serrated adenomas (mean ± SD, 20.1 ± 13.3) vs normal and hyperplastic polyps (P = .02 and 3 × 10-7, respectively).

CONCLUSIONS

Claudin-1 is overexpressed in premalignant colonic lesions, and can be detected endoscopically in vivo with a near-infrared, labeled peptide.

Tight junction modulation of the blood brain barrier: CNS delivery of small molecules

The blood brain barrier (BBB) represents a major obstacle for targeted drug delivery to the brain for the treatment of central nervous system (CNS) disorders. Significant advances in barrier research over the past decade has led to the discovery of an increasing number of structural and regulatory proteins in tight junctions (TJ) and adherens junctions (AJ). These discoveries are providing the framework for the development of novel TJ modulators which can act specifically and temporarily to alter BBB function and regulate paracellular uptake of molecules. TJ modulators that have shown therapeutic potential in preclinical models include claudin-5 and occludin siRNAs, peptides derived from zonula occludens toxin as well as synthetic peptides targeting the extracellular loops of TJs. Adding to the array of modulating agents are novel mechanisms of BBB regulation such as focused ultrasound (FUS). This review will give a succinct overview of BBB biology and TJ modulation in general. Novel insights into BBB regulation in health and disease will also be summarized.

Claudin 1 in Breast Cancer: New Insights

Claudin 1 is a small transmembrane protein responsible for maintaining the barrier function that exists between epithelial cells. A tight junction protein that regulates the paracellular transport of small ions across adjacent cells, claudin 1 maintains cellular polarity and plays a major role in cell-cell communication and epithelial cell homeostasis. Long considered to be a putative tumor suppressor in human breast cancer, new studies suggest a role much more complex. While most invasive breast cancers exhibit a down regulation or absence of claudin 1, some aggressive subtypes that exhibit high claudin 1 levels have now been described. Furthermore, a causal role for claudin 1 in breast cancer progression has recently been demonstrated in some breast cancer cell lines. In this review we highlight new insights into the role of claudin 1 in breast cancer, including its involvement in collective migration and epithelial mesenchymal transition (EMT).

Claudin-1 enhances tumor proliferation and metastasis by regulating cell anoikis in gastric cancer

Claudin-1 (CLDN1) is overexpressed in gastric cancer and correlated with tumor invasion, metastasis and poor outcome. Here, we both down and up regulated CLDN1 expression in gastric cancer cells to elucidate its role in gastric carcinogenesis and tumor progression. We found that deficiency of CLDN1 inhibited cells migration, invasion, and colony formation in vitro and tumorigenicity, metastasis in vivo. Also, CLDN1 promoted cell aggregation and increased anoikis resistance. Down or up regulation of CLDN1 was accompanied with changes of membrane β-catenin expression as well as Akt and Src activities. When β-catenin was up-regulated in CLDN1-KD cells, cell aggregation and anoikis resistance were restored, and Akt and Src signal pathways were re-activated. Taken together, these findings suggest that CLDN1 is oncogenic in gastric cancer and its malignant potential may be attributed in part to regulation of anoikis, by mediating membrane β-catenin-regulated cell-cell adhesion and cell survival.