Role of the Cldn6 cytoplasmic tail domain in membrane targeting and epidermal differentiation in vivo

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.

Knockdown of CLAUDIN-6 Inhibited Apoptosis and Induced Proliferation of Bovine Cumulus Cells

This study aims to investigate the effects of CLAUDIN-6 (CLDN6) on cell apoptosis and proliferation of bovine cumulus cells (CCs). Immunofluorescence staining was used to localize CLDN6 protein in CCs. Three pairs of siRNA targeting CLDN6 and one pair of siRNA universal negative sequence as control were transfected into bovine CCs. Then, the effective siRNA was screened by real-time quantitative PCR (RT-qPCR) and Western blotting. The mRNA expression levels of apoptosis related genes (CASPASE-3, BAX and BCL-2) and proliferation related genes (PCNA, CDC42 and CCND2) were evaluated by RT-qPCR in CCs with CLDN6 knockdown. Cell proliferation, apoptosis and cell cycle were detected by flow cytometry with CCK-8 staining, Annexin V-FITC staining and propidium iodide staining, respectively. Results showed that the CLDN6 gene was expressed in bovine CCs and the protein was localized in cell membranes and cytoplasms. After CLDN6 was knocked down in CCs, the cell apoptosis rate significantly decreased and the pro-apoptotic genes BAX and CASPASE-3 were down-regulated significantly, whereas the anti-apoptotic gene BCL-2 was markedly up-regulated (p < 0.05). Additionally, CLDN6 knockdown significantly enhanced cell proliferation of CCs at 72 h after siRNA transfection. The mRNA levels of proliferation-related genes PCNA, CCND2 and CDC42 increased obviously in CCs with CLDN6 knockdown (p < 0.05). After CLDN6 was down-regulated, the percentage of CCs at S phase was significantly increased (p < 0.05). However, there was no remarkable difference in the percentages of cells at the G0/G1 phase and G2/M phase between CCs with or without CLDN6 knockdown (p > 0.05). Therefore, the expression of CLDN6 and its effects on cell proliferation, apoptosis and cell cycle of bovine CCs were first studied. CLDN6 low expression inhibited cell apoptosis, induced cell proliferation and cell cycle arrest of bovine CCs.

CLDN6: From Traditional Barrier Function to Emerging Roles in Cancers

Claudins (CLDNs) are the most important tight junction proteins, which are mainly expressed in endothelial cells or epithelial cells in a tissue-specific manner. As a member of the CLDNs family, CLDN6 is highly expressed in fetal tissues such as the stomach, pancreas, lung, and kidney, but is not expressed in corresponding adult tissues. The expression of CLDN6 is regulated by a variety of factors, including but not limited to stimuli and transcription factors, DNA methylation, and post-translational modifications. CLDN6 has been found to have a key role in the formation of barriers, especially the lung epithelial barrier and the epidermal permeability barrier (EPB). Importantly, the roles of CLDN6 in cancers have gained focus and are being investigated in recent years. Strong evidence indicates that the altered expression of CLDN6 is linked to the development of various cancers. Malignant phenotypes of tumors affected by CLDN6 include proliferation and apoptosis, migration and invasion, and drug resistance, which are regulated by CLDN6-mediated key signaling pathways. Given the important role in tumors and its low or no expression in normal tissues, CLDN6 is an ideal target for tumor therapy. This review aims to provide an overview of the structure and regulation of CLDN6, and its traditional barrier function, with a special emphasis on its emerging roles in cancers, including its impact on the malignant phenotypes, signal-modulating effects, the prognosis of tumor patients, and clinical applications in cancers.

The blood-brain and gut-vascular barriers: from the perspective of claudins

In some organs, such as the brain, endothelial cells form a robust and highly selective blood-to-tissue barrier. However, in other organs, such as the intestine, endothelial cells provide less stringent permeability, to allow rapid exchange of solutes and nutrients where needed. To maintain the structural and functional integrity of the highly dynamic blood–brain and gut–vascular barriers, endothelial cells form highly specialized cell-cell junctions, known as adherens junctions and tight junctions. Claudins are a family of four-membrane-spanning proteins at tight junctions and they have both barrier-forming and pore-forming properties. Tissue-specific expression of claudins has been linked to different diseases that are characterized by barrier impairment. In this review, we summarize the more recent progress in the field of the claudins, with particular attention to their expression and function in the blood–brain barrier and the recently described gut–vascular barrier, under physiological and pathological conditions.

Abbreviations: 22q11DS 22q11 deletion syndrome; ACKR1 atypical chemokine receptor 1; AD Alzheimer disease; AQP aquaporin; ATP adenosine triphosphate; Aβ amyloid β; BAC bacterial artificial chromosome; BBB blood-brain barrier; C/EBP-α CCAAT/enhancer-binding protein α; cAMP cyclic adenosine monophosphate (or 3ʹ,5ʹ-cyclic adenosine monophosphate); CD cluster of differentiation; CNS central nervous system; DSRED discosoma red; EAE experimental autoimmune encephalomyelitis; ECV304 immortalized endothelial cell line established from the vein of an apparently normal human umbilical cord; EGFP enhanced green fluorescent protein; ESAM endothelial cell-selective adhesion molecule; GLUT-1 glucose transporter 1; GVB gut-vascular barrier; H2B histone H2B; HAPP human amyloid precursor protein; HEK human embryonic kidney; JACOP junction-associated coiled coil protein; JAM junctional adhesion molecules; LYVE1 lymphatic vessel endothelial hyaluronan receptor 1; MADCAM1 mucosal vascular addressin cell adhesion molecule 1; MAPK mitogen-activated protein kinase; MCAO middle cerebral artery occlusion; MMP metalloprotease; MS multiple sclerosis; MUPP multi-PDZ domain protein; PATJ PALS-1-associated tight junction protein; PDGFR-α platelet-derived growth factor receptor α polypeptide; PDGFR-β platelet-derived growth factor receptor β polypeptide; RHO rho-associated protein kinase; ROCK rho-associated, coiled-coil-containing protein kinase; RT-qPCR real time quantitative polymerase chain reactions; PDGFR-β soluble platelet-derived growth factor receptor, β polypeptide; T24 human urinary bladder carcinoma cells; TG2576 transgenic mice expressing the human amyloid precursor protein; TNF-α tumor necrosis factor α; WTwild-type; ZO zonula occludens.

A SUMOylation-dependent HIF-1α/CLDN6 negative feedback mitigates hypoxia-induced breast cancer metastasis

Background

We have previously described CLDN6 as a tumor suppressor gene in breast cancer. Here, a new finding is that CLDN6 was upregulated under hypoxia, a commonly recognized factor that promotes tumor metastasis. In this study, we aim to explain this confusing finding and delineate the role of CLDN6 in the breast cancer metastasis induced by hypoxia.

Methods

RNAi and ChIP assays were used to confirm that CLDN6 is transcriptional regulated by HIF-1α. mRNA seq and KEGG analysis were performed to define the downstream pathways of CLDN6. The roles of the CLDN6/SENP1/HIF-1α signaling on tumor metastasis were evaluated by function experiments and clinical samples. Finally, the possible transcription factor of SENP1 was suspected and then validated by ChIP assay.

Results

We demonstrated a previously unrecognized negative feedback loop exists between CLDN6 and HIF-1α. CLDN6 was transcriptionally up-regulated by HIF-1α under hypoxia. On the other hand, in cytoplasm CLDN6 combines and retains β-catenin, a transcription factor of SENP1, causing β-catenin degradation and preventing its nuclear translocation. This process reduced SENP1 expression and prevented the deSUMOylation of HIF-1α, ultimately leading to HIF-1α degradation and breast cancer metastasis suppression.

Conclusions

Our data provide a molecular mechanistic insight indicating that CLDN6 loss may lead to elevated HIF-1α-driven breast cancer metastasis in a SUMOylation-dependent manner.

Self-organizing neuruloids model developmental aspects of Huntington's disease in the ectodermal compartment

Harnessing the potential of human embryonic stem cells to mimic normal and aberrant development with standardized models is a pressing challenge. Here we use micropattern technology to recapitulate early human neurulation in large numbers of nearly identical structures called neuruloids. Dual-SMAD inhibition followed by bone morphogenic protein 4 stimulation induced self-organization of neuruloids harboring neural progenitors, neural crest, sensory placode and epidermis. Single-cell transcriptomics unveiled the precise identities and timing of fate specification. Investigation of the molecular mechanism of neuruloid self-organization revealed a pulse of pSMAD1 at the edge that induced epidermis, whose juxtaposition to central neural fates specifies neural crest and placodes, modulated by fibroblast growth factor and Wnt. Neuruloids provide a unique opportunity to study the developmental aspects of human diseases. Using isogenic Huntington's disease human embryonic stem cells and deep neural network analysis, we show how specific phenotypic signatures arise in our model of early human development as a consequence of mutant huntingtin protein, outlining an approach for phenotypic drug screening.

Expression of CLDN6 in tissues of gastric cancer patients: Association with clinical pathology and prognosis

Expression of claudin-6 (CLDN6) in the tissues of gastric cancer patients and its association with clinical pathology and prognosis were investigated. A retrospective analysis was performed on 213 gastric cancer patients diagnosed and surgically treated in the Central Hospital of Zibo from January 2010 to January 2013. Cancer and normal adjacent tissues were obtained from the patients to detect the expression level of CLDN6 using reverse transcription-quantitative PCR (RT-qPCR). The association between the expression level of CLDN6 and the clinical and pathological features, as well as the prognosis of gastric cancer patients was analyzed. The expression level of CLDN6 was significantly lower in gastric cancer tissues than that in adjacent tissues (t=23.350, P<0.001). The expression level of CLDN6 was associated with age, lymph node metastasis, pathological staging, and distant metastasis (P<0.05). In this study, patients were separated into CLDN6 high-expression group (≥1.42) with 107 patients and CLDN6 low-expression group (<1.42) with 106 patients, with the median expression level of CLDN6 as the boundary. The 1-, 2- and 3-year survival rates of patients in the CLDN6 low-expression group were 80.19, 60.38 and 48.11%, respectively, and those in the CLDN6 high-expression group were 87.85, 73.83 and 66.36%, respectively. The survival rate was significantly better in the CLDN6 high-expression group than that in the CLDN6 low-expression group (P=0.009). In conclusion, the expression level of CLDN6 is low in the cancer tissues of gastric cancer patients, and associated with age, lymph node metastasis, pathological staging and distant metastasis. CLDN6 low expression has a certain negative impact on the prognosis of patients, and therefore, shows potential as an important indicator for the prognosis of gastric cancer patients.

Transcriptome Analysis of HepG2 Cells Expressing ORF3 from Swine Hepatitis E Virus to Determine the Effects of ORF3 on Host Cells

Hepatitis E virus- (HEV-) mediated hepatitis has become a global public health problem. An important regulatory protein of HEV, ORF3, influences multiple signal pathways in host cells. In this study, to investigate the function of ORF3 from the swine form of HEV (SHEV), high-throughput RNA-Seq-based screening was performed to identify the differentially expressed genes in ORF3-expressing HepG2 cells. The results were validated with quantitative real-time PCR and gene ontology was employed to assign differentially expressed genes to functional categories. The results indicated that, in the established ORF3-expressing HepG2 cells, the mRNA levels of CLDN6, YLPM1, APOC3, NLRP1, SCARA3, FGA, FGG, FGB, and FREM1 were upregulated, whereas the mRNA levels of SLC2A3, DKK1, BPIFB2, and PTGR1 were downregulated. The deregulated expression of CLDN6 and FREM1 might contribute to changes in integral membrane protein and basement membrane protein expression, expression changes for NLRP1 might affect the apoptosis of HepG2 cells, and the altered expression of APOC3, SCARA3, and DKK1 may affect lipid metabolism in HepG2 cells. In conclusion, ORF3 plays a functional role in virus-cell interactions by affecting the expression of integral membrane protein and basement membrane proteins and by altering the process of apoptosis and lipid metabolism in host cells. These findings provide important insight into the pathogenic mechanism of HEV.

Prognostic significance of claudin-1 and cyclin B1 protein expression in patients with hypopharyngeal squamous cell carcinoma

Claudin-l and cyclin B1 are abnormally expressed in certain malignancies, but their expression in hypopharyngeal squamous cell carcinoma (HSCC) has not been reported thus far. Studying the expression levels of claudin-1 and cylin B1 in HSCC tissues and their association with clinical stage, pathological grade and prognosis in patients with HSCC may provide a theoretical basis and guide future research on HSCC targeted therapy. The protein expression levels of the above two biomarkers was immunohistochemically detected in 97 HSCC cases and 90 matched adjacent tissue samples. The correlation between the expression levels of claudin-1 and cylin B1 and the patients' clinical parameters was analyzed via Pearson's χ² test, while survival analysis was performed using a log-rank test. The results of the current study revealed that claudin-1 and cyclin B1 were highly expressed in HSCC tissues, and the expression of claudin-1 was associated with tumor differentiation degree and lymph node metastasis, while cyclin B1 expression was associated with tumor differentiation degree. Furthermore, Kaplan-Meier analysis revealed that claudin-1 expression correlated with survival (P=0.003), and the expression levels of claudin-1 and cyclin B1 were observed to be positively correlated, in patients with HSCC. Cyclin B1 and claudin-1 exhibited an elevated expression in HSCC specimens, thus suggesting their use as tumor markers. Therefore, the joint detection of claudin-1 and cyclin B1 may aid to guide cancer therapy and to determine prognosis in HSCC. Furthermore, claudin-1 may be used as an HSCC-monitoring index, and may serve as a therapeutic target.

Systems Proteomics View of the Endogenous Human Claudin Protein Family

Claudins are the major transmembrane protein components of tight junctions in human endothelia and epithelia. Tissue-specific expression of claudin members suggests that this protein family is not only essential for sustaining the role of tight junctions in cell permeability control but also vital in organizing cell contact signaling by protein-protein interactions. How this protein family is collectively processed and regulated is key to understanding the role of junctional proteins in preserving cell identity and tissue integrity. The focus of this review is to first provide a brief overview of the functional context, on the basis of the extensive body of claudin biology research that has been thoroughly reviewed, for endogenous human claudin members and then ascertain existing and future proteomics techniques that may be applicable to systematically characterizing the chemical forms and interacting protein partners of this protein family in human. The ability to elucidate claudin-based signaling networks may provide new insight into cell development and differentiation programs that are crucial to tissue stability and manipulation.

Emerging multifunctional roles of Claudin tight junction proteins in bone

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

Developmental lung expression and transcriptional regulation of claudin-6 by TTF-1, Gata-6, and FoxA2

Background

Claudins are transmembrane proteins expressed in tight junctions that prevent paracellular transport of extracellular fluid and a variety of other substances. However, the expression profile of Claudin-6 (Cldn6) in the developing lung has not been characterized.

Methods and results

Cldn6 expression was determined during important periods of lung organogenesis by microarray analysis, qPCR and immunofluorescence. Expression patterns were confirmed to peak at E12.5 and diminish as lung development progressed. Immunofluorescence revealed that Cldn6 was detected in cells that also express TTF-1 and FoxA2, two critical transcriptional regulators of pulmonary branching morphogenesis. Cldn6 was also observed in cells that express Sox2 and Sox9, factors that influence cell differentiation in the proximal and distal lung, respectively. In order to assess transcriptional control of Cldn6, 0.5, 1.0, and 2.0-kb of the proximal murine Cldn6 promoter was ligated into a luciferase reporter and co-transfected with expression vectors for TTF-1 or two of its important transcriptional co-regulators, FoxA2 and Gata-6. In almost every instance, TTF-1, FoxA2, and Gata-6 activated gene transcription in cell lines characteristic of proximal airway epithelium (Beas2B) and distal alveolar epithelium (A-549).

Conclusions

These data revealed for the first time that Cldn6 might be an important tight junctional component expressed by pulmonary epithelium during lung organogenesis. Furthermore, Cldn6-mediated aspects of cell differentiation may describe mechanisms of lung perturbation coincident with impaired cell junctions and abnormal membrane permeability.

Claudins and other tight junction proteins

Epithelial transport relies on the proper function and regulation of the tight junction (TJ), other-wise uncontrolled paracellular leakage of solutes and water would occur. They also act as a fence against mixing of membrane proteins of the apical and basolateral side. The proteins determining paracellular transport consist of four transmembrane regions, intracellular N and C terminals, one intracellular and two extracellular loops (ECLs). The ECLs interact laterally and with counterparts of the neighboring cell and by this achieve a general sealing function. Two TJ protein families can be distinguished, claudins, comprising 27 members in mammals, and TJ-associated MARVEL proteins (TAMP), comprising occludin, tricellulin, and MarvelD3. They are linked to a multitude of TJ-associated regulatory and scaffolding proteins. The major TJ proteins are classified according to the physiological role they play in enabling or preventing paracellular transport. Many TJ proteins have sealing functions (claudins 1, 3, 5, 11, 14, 19, and tricellulin). In contrast, a significant number of claudins form channels across TJs which feature selectivity for cations (claudins 2, 10b, and 15), anions (claudin-10a and -17), or are permeable to water (claudin-2). For several TJ proteins, function is yet unclear as their effects on epithelial barriers are inconsistent (claudins 4, 7, 8, 16, and occludin). TJs undergo physiological and pathophysiological regulation by altering protein composition or abundance. Major pathophysiological conditions which involve changes in TJ protein composition are (1) effects of pathogens binding to TJ proteins, (2) altered TJ protein composition during inflammation and infection, and (3) altered TJ protein expression in cancers.

Mechanisms that determine the internal environment of the developing brain: a transcriptomic, functional and ultrastructural approach

We provide comprehensive identification of embryonic (E15) and adult rat lateral ventricular choroid plexus transcriptome, with focus on junction-associated proteins, ionic influx transporters and channels. Additionally, these data are related to new structural and previously published permeability studies. Results reveal that most genes associated with intercellular junctions are expressed at similar levels at both ages. In total, 32 molecules known to be associated with brain barrier interfaces were identified. Nine claudins showed unaltered expression, while two claudins (6 and 8) were expressed at higher levels in the embryo. Expression levels for most cytoplasmic/regulatory adaptors (10 of 12) were similar at the two ages. A few junctional genes displayed lower expression in embryos, including 5 claudins, occludin and one junctional adhesion molecule. Three gap junction genes were enriched in the embryo. The functional effectiveness of these junctions was assessed using blood-delivered water-soluble tracers at both the light and electron microscopic level: embryo and adult junctions halted movement of both 286Da and 3kDa molecules into the cerebrospinal fluid (CSF). The molecular identities of many ion channel and transporter genes previously reported as important for CSF formation and secretion in the adult were demonstrated in the embryonic choroid plexus (and validated with immunohistochemistry of protein products), but with some major age-related differences in expression. In addition, a large number of previously unidentified ion channel and transporter genes were identified for the first time in plexus epithelium. These results, in addition to data obtained from electron microscopical and physiological permeability experiments in immature brains, indicate that exchange between blood and CSF is mainly transcellular, as well-formed tight junctions restrict movement of small water-soluble molecules from early in development. These data strongly indicate the brain develops within a well-protected internal environment and the exchange between the blood, brain and CSF is transcellular and not through incomplete barriers.

Regulation of claudins in blood-tissue barriers under physiological and pathological states

Claudins are pivotal building blocks of tight junctions that form the paracellular barrier in epithelia and endothelia. In mammals, claudins are a 27-gene family that encodes tetraspan membrane proteins, playing a crucial role in the formation and integrity of tight junctions and regulate the barrier function. Claudin isoforms are expressed in a tissue- and/or developmental stage-dependent manner. A growing body of evidence indicates that pathological states characterized by neuroinflammation, such as Alzheimer disease, multiple sclerosis, diabetic retinopathy and retinopathy of prematurity share a common feature: the barrier breakdown. This review aims integrating and summarizing the most relevant and recent work developed in the field of claudins, with particular attention to their role in blood-brain and blood-retinal barriers, as well as describing their regulation in the aforementioned human diseases.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

Impaired tight junctions obstruct stratum corneum formation by altering polar lipid and profilaggrin processing

BACKGROUND

The stratum corneum (SC) is a well-known structure responsible for the cutaneous barrier. Tight junctions (TJs) function as a paracellular barrier beneath the SC and are involved in the cutaneous barrier. It remains unclear how TJs are involved in the cutaneous barrier.

OBJECTIVE

In order to clarify the role of TJs in the cutaneous barrier, we investigated skin equivalent models with disrupted TJ barriers focusing on the SC.

METHODS

Skin equivalents with disrupted TJ barriers were established using GST-C-CPE, a peptide with specific inhibitory action against specific claudins. The changes of the SC barrier in the skin equivalents with disrupted TJ barriers were investigated and compared with control skin equivalents.

RESULTS

An outside-to-inside skin barrier assay revealed a defective SC barrier in skin equivalents with disrupted TJ barriers. A detailed examination of the SC revealed an increase in the pH of the SC in the skin equivalent with disrupted TJ barriers. An electron microscopy showed the failure of lamellar structures to mature and the failure of keratohyalin granules to degrade in the skin equivalents with disrupted TJ barriers. A thin layer chromatography analysis showed an increase in polar lipids and a decrease in non-polar lipids. A western blot analysis showed an increase in filaggrin dimer and trimer and a decrease in filaggrin monomer.

CONCLUSION

We found that disrupted TJs obstructed the SC formation responsible for the cutaneous barrier. Our study indicates the possibility that impaired TJ barriers affect polar lipids and profilaggrin processing by disturbing the pH condition of the SC.

Transgene-mediated rescue of spermatogenesis in Cldn11-null mice

Claudins comprise a large family of tight junction (TJ) proteins that are often expressed broadly during development and in adult tissues and constitute the physical barriers that occlude the paracellular space in polarized epithelia. In mouse testis, the integrity of TJs is critical to normal spermatogenesis and is dependent on CLDN11 expression. In the current study, we have generated multiple transgenic mouse lines in which steady-state levels of transgene-derived Cldn11 mRNA are up to fourfold greater than endogenous gene expression. Spermatogenesis in all founder mice harboring two copies of the endogenous Cldn11 gene is normal. These animals breed well, indicating that transgene overexpression, at least at the level of mRNA, is well tolerated by Sertoli cells. In addition, we demonstrate that the promoter/enhancer of the transgene, comprising 5 kb of genomic sequence upstream of exon 1 of the mouse Cldn11 gene, is sufficient to rescue azoospermia in Cldn11-null mice. Finally, using transient transgenic mice, we narrow the location of Sertoli cell-specific cis regulatory elements to a 2-kb region upstream of the Cldn11 transcription start site. Together, these data provide essential information for further investigation of the biological regulation of CLDN11 TJs in the testis.

Deconstructing the skin: cytoarchitectural determinants of epidermal morphogenesis

To provide a stable environmental barrier, the epidermis requires an integrated network of cytoskeletal elements and cellular junctions. Nevertheless, the epidermis ranks among the body's most dynamic tissues, continually regenerating itself and responding to cutaneous insults. As keratinocytes journey from the basal compartment towards the cornified layers, they completely reorganize their adhesive junctions and cytoskeleton. These architectural components are more than just rivets and scaffolds - they are active participants in epidermal morphogenesis that regulate epidermal polarization, signalling and barrier formation.

Integration of tight junctions and claudins with the barrier functions of the retinal pigment epithelium

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by regulating the movement of solutes between the fenestrated capillaries of the choroid and the photoreceptor layer of the retina. Blood-tissue barriers use various mechanisms to accomplish their tasks including membrane pumps, transporters, and channels, transcytosis, metabolic alteration of solutes in transit, and passive but selective diffusion. The last category includes tight junctions, which regulate transepithelial diffusion through the spaces between neighboring cells of the monolayer. Tight junctions are extraordinarily complex structures that are dynamically regulated. Claudins are a family of tight junctional proteins that lend tissue specificity and selectivity to tight junctions. This review discusses how the claudins and tight junctions of the RPE differ from other epithelia and how its functions are modulated by the neural retina. Studies of RPE-retinal interactions during development lend insight into this modulation. Notably, the characteristics of RPE junctions, such as claudin composition, vary among species, which suggests the physiology of the outer retina may also vary. Comparative studies of barrier functions among species should deepen our understanding of how homeostasis is maintained in the outer retina. Stem cells provide a way to extend these studies of RPE-retinal interactions to human RPE.

The blood-retinal barrier: structure and functional significance

Formation and maintenance of the blood-retinal barrier is required for proper vision and loss of this barrier contributes to the pathology of a wide number of retinal diseases. The retina is responsible for converting visible light into the electrochemical signal interpreted by the brain as vision. Multiple cell types are required for this function, which are organized into eight distinct cell layers. These neural and glial cells gain metabolic support from a unique vascular structure that provides the necessary nutrients while minimizing interference with light sensing. In addition to the vascular contribution, the retina also possesses an epithelial barrier, the retinal pigment epithelium, which is located at the posterior of the eye and controls exchange of nutrients with the choroidal vessels. Together the vascular and epithelial components of the blood-retinal barrier maintain the specialized environment of the neural retina. Both the vascular endothelium and pigment epithelium possess a well-developed junctional complex that includes both adherens and tight junctions conferring a high degree of control of solute and fluid permeability. Understanding induction and regulation of the blood-retinal barrier will allow the development of therapies aimed at restoring the barrier when compromised in disease or allowing the specific transport of therapies across this barrier when needed. This chapter will highlight the anatomical structure of the blood-retinal barrier and explore the molecular structure of the tight junctions that provide the unique barrier properties of the blood--retinal barrier.

Involucrin–claudin-6 tail deletion mutant (CΔ206) transgenic mice: a model of delayed epidermal permeability barrier formation and repair

Preterm birth is a major global health problem that results in a large number of infant deaths, many of which are attributable to the complications of an immature epidermal permeability barrier (EPB), for which there is currently no effective therapeutic option. The mammalian EPB is formed during development and is essential for survival as it maintains thermoregulation and hydration, and provides a defense against infection. Using transgenic mouse technology, we have demonstrated the importance of claudin (Cldn)-containing tight junctions (TJs) in epidermal differentiation and, in particular, that epidermal suprabasal overexpression of Cldn6 results in an EPB-deficient phenotype that phenocopies the dysfunctional EPB of premature human infants. In this study, we used the same approach to target a Cldn6 tail deletion mutant to the epidermis of mice [involucrin (Inv)-Cldn6-CΔ206 transgenic mice]. The Inv-Cldn6-CΔ206 transgenic mice displayed a developmental delay in EPB formation, as shown by the expression of keratins and Cldns, and by X-Gal penetration assays. Trans-epidermal water loss measurements and immunolocalization studies indicated that the epidermal differentiation program was also perturbed in postnatal Inv-Cldn6-CΔ206 transgenic mice resulting in a delayed maturation. Notably, however, expression/localization of epidermal differentiation and maturation markers, including Cldns, indicated that the transgenic epidermis matured and normalized by postnatal day 10, which is 3 days after the wild-type epidermis. Our results suggest that activation of the extracellular signal-regulated kinase 1/2 (Erk1/2) pathway and Cldn1 phosphorylation are associated with the repair and maturation of the skin barrier processes. These studies provide additional support for the crucial role of Cldns in epidermal differentiation, maturation and the formation of the EPB, and describe a novel animal model for evaluating postnatal epidermal maturation and therapies that may accelerate the process.

Dermatitis and aging-related barrier dysfunction in transgenic mice overexpressing an epidermal-targeted claudin 6 tail deletion mutant

The barrier function of the skin protects the mammalian body against infection, dehydration, UV irradiation and temperature fluctuation. Barrier function is reduced with the skin's intrinsic aging process, however the molecular mechanisms involved are unknown. We previously demonstrated that Claudin (Cldn)-containing tight junctions (TJs) are essential in the development of the epidermis and that transgenic mice overexpressing Cldn6 in the suprabasal layers of the epidermis undergo a perturbed terminal differentiation program characterized in part by reduced barrier function. To dissect further the mechanisms by which Cldn6 acts during epithelial differentiation, we overexpressed a Cldn6 cytoplasmic tail deletion mutant in the suprabasal compartment of the transgenic mouse epidermis. Although there were no gross phenotypic abnormalities at birth, subtle epidermal anomalies were present that disappeared by one month of age, indicative of a robust injury response. However, with aging, epidermal changes with eventual chronic dermatitis appeared with a concomitant barrier dysfunction manifested in increased trans-epidermal water loss. Immunohistochemical analysis revealed aberrant suprabasal Cldn localization with marked down-regulation of Cldn1. Both the proliferative and terminal differentiation compartments were perturbed as evidenced by mislocalization of multiple epidermal markers. These results suggest that the normally robust injury response mechanism of the epidermis is lost in the aging Involucrin-Cldn6-CΔ196 transgenic epidermis, and provide a model for evaluation of aging-related skin changes.

Defective claudin-7 regulation by Tcf-4 and Sox-9 disrupts the polarity and increases the tumorigenicity of colorectal cancer cells

Tight junctions have recently emerged as essential signaling regulators of proliferation and differentiation in epithelial tissues. Here, we aimed to identify the factors regulating claudin-7 expression in the colon, and analyzed the consequences of claudin-7 overexpression in colorectal carcinoma (CRC). In healthy human colonic crypts, claudin-7 expression was found to be low in the stem/progenitor cell compartment, where Tcf-4 activity is high, but strong in differentiated and postmitotic cells, where Tcf-4 is inactive. In contrast, claudin-7 was overexpressed in areas with high Tcf-4 target gene levels in CRC samples. In vitro, Tcf-4 was able to repress claudin-7 expression, and the high mobility group-box transcription factor Sox-9 was identified as an essential mediator of this effect. Claudin-7 was strongly expressed in the intestine of Sox-9-deficient mice and in CRC cells with low Sox transcriptional activity. Sox-9 overexpression in these cells reinstated claudin-7 repression, and residual claudin-7 was no longer localized along the basolateral membrane, but was instead restricted to tight junctions. Using HT-29Cl.16E CRC cell spheroids, we found that Sox-9-induced polarization was completely reversed after virus-mediated claudin-7 overexpression. Claudin-7 overexpression in this context increased Tcf-4 target gene expression, proliferation, and tumorigenicity after injection in nude mice. Our results indicate that Tcf-4 maintains low levels of claudin-7 at the bottom of colonic crypts, acting via Sox-9. This negative regulation seems to be defective in CRC, possibly due to decreased Sox-9 activity, and the resulting claudin-7 overexpression promotes a loss of tumor cell polarization and contributes to tumorigenesis.

Genetic targeting of the endoderm with claudin-6CreER

A full description of the ontogeny of the beta cell would guide efforts to generate beta cells from embryonic stem cells (ESCs). The first step requires an understanding of definitive endoderm: the genes and signals responsible for its specification, proliferation, and patterning. This report describes a global marker of definitive endoderm, Claudin-6 (Cldn6). We report its expression in early development with particular attention to definitive endoderm derivatives. To create a genetic system to drive gene expression throughout the definitive endoderm with both spatial and temporal control, we target the endogenous locus with an inducible Cre recombinase (Cre-ER(T2)) cassette. Cldn6 null mice are viable and fertile with no obvious phenotypic abnormalities. We also report a lineage analysis of the fate of Cldn6-expressing embryonic cells, which is relevant to the development of the pancreas, lung, and liver.

Changes in the distribution pattern of Claudin tight junction proteins during the progression of mouse skin tumorigenesis

Background

Despite the fact that morphological and physiological observations suggest that the tight junction (TJ)-based permeability barrier is modified/disrupted in tumorigenesis, the role of members of the Claudin (Cldn) family of TJ proteins is not well-understood. Using a well-established two-stage chemical carcinogenesis model, we investigated the temporal and spatial changes in expression of those Cldns that we have previously demonstrated to be important in epidermal differentiation and the formation of the epidermal permeability barrier, i.e., Cldn1, Cldn6, Cldn11, Cldn12 and Cldn18.

Methods

The lower dorsal backskin of mice was treated topically with 7,12-dimethylbenz(a)anthracene (DMBA; 0.25 mg/ml in acetone) and following a 10-day incubation period, 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 25 μg/ml in acetone) was applied three times a week to the same area. Backskin samples were dissected 2, 4, 6, 8 and 12 weeks after the initiation of the experimental protocol and immunohistochemistry was performed on sections using antibodies against the following: Cldn1, Cldn6, Cldn11, Cldn12, Cldn18, Ki67 and CD3.

Results

Our data indicate that along with the changes in epidermal cell morphology and differentiation that occur during tumor formation, there is a dramatic change in Cldn distribution consistent with cell polarity and barrier selectivity changes. Specifically, in the early stages of DMBA/TPA treatment, the suprabasal-specific Cldns occupy an expanded zone of expression corresponding to an increased number of suprabasal epidermal cell layers. As tumorigenesis progressed, the number of suprabasal epidermal layers positive for Cldn6, Cldn11, Cldn12 and Cldn18 was reduced, especially in the lower strata of the expanded suprabasal zone. In addition, a variably reduced cell membrane association of those differentiation-specific Cldns was observed, especially within the infiltrating epidermal structures. In contrast, Cldn1 (which is normally expressed in all the living layers of the epidermis) remained restricted to the cell membrane throughout the tumorigenesis protocol. However commencing 2 weeks after treatment there was a marked decrease in the number of Cldn1-positive basal cells, and the zone of Cldn1-null epidermal cells was expanded up into the lower stratified epidermis throughout the progression of DMBA/TPA treatment. In addition, there was no Cldn1 localization in the infiltrating epidermal structures of the tumorigenic epidermis.

Conclusion

This is the first demonstration of the changes in Cldn expression in the progression of DMBA/TPA-induced skin tumors; however further investigation into the molecular mechanisms regulating the observed changes in barrier selectivity during tumorigenesis is required.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Claudin immunolocalization in neonatal mouse epithelial tissues

Emerging evidence supports the notion that claudins (Cldns) are dynamically regulated under normal conditions to respond to the selective permeability requirements of various tissues, and that their expression is developmentally controlled. We describe the localization of those Cldns that we have previously demonstrated to be functionally important in epidermal differentiation and the formation of the epidermal permeability barrier, e.g., Cldn1, Cldn6, Cldn11, and Cldn18, and the presence of Cldn3 and Cldn5 in various neonatal mouse epithelia including the epidermis, nail, oral mucosa, tongue, and stomach. Cldn1 is localized in the differentiated and/or undifferentiated compartments of the epidermis and nail and in the dorsal surface of the tongue and glandular compartment of the stomach but is absent from the oral mucosa and the keratinized compartment of the stomach. Cldn3 is present in the basal cells of the nail matrix and both compartments of the murine stomach but not in the epidermis, oral mucosa, or tongue. Cldn5 is found in the glandular compartment of the stomach but not in the epidermis, nail unit, oral mucosa, forestomach, and tongue. Cldn6, Cldn11, and Cldn18 occur in the differentiating suprabasal compartment of the epidermis, nail, and oral mucosa and in the dorsal and ventral surfaces of the tongue and the keratinized squamous epithelium of the stomach. The simple columnar epithelium of the glandular stomach stains for Cldn18 and reveals a non-membranous pattern for Cldn6 and Cldn11 expression. Our results demonstrate differential Cldn protein profiles in various epithelial tissues and their differentiation stages. Although the molecular mechanisms regulating Cldn expression are unknown, elucidation of their differential localization patterns in tissues with diverse permeability requirements should provide a better understanding of the role of tight junctions in tissue function.

Transmembrane proteins of tight junctions

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

The targeted overexpression of a Claudin mutant in the epidermis of transgenic mice elicits striking epidermal and hair follicle abnormalities

Skin is one of the largest organs of the body, and is formed during development through a highly orchestrated process involving mesenchymal-epithelial interactions, cell commitment, and terminal differentiation. It protects against microorganism invasion and UV irradiation, inhibits water loss, regulates body temperature, and is an important part of the immune system. Using transgenic mouse technology, we have demonstrated that Claudin (Cldn)-containing tight junctions (TJs) are intricately involved in cell signaling during epidermal differentiation and that an epidermal suprabasal overexpression of Cldn6 results in a perturbed epidermal terminal differentiation program with distinct phenotypic abnormalities. To delineate the role of the Cldn cytoplasmic tail domain in epidermal differentiation, we engineered transgenic mice targeting the overexpression of a Cldn6 cytoplasmic tail-truncation mutant in the epidermis. Transgenic mice were characterized by a lethal barrier dysfunction in addition to the existence of hyperproliferative squamous invaginations/cysts replacing hair follicles. Immunohistochemical analysis revealed an epidermal cytoplasmic accumulation of Cldn6, Cldn11, Cldn12, and Cldn18, downregulation of Cldn1 and aberrant expression of various classical markers of epidermal differentiation; namely the basal keratins as well as K1, involucrin, loricrin, and filaggrin. Collectively these studies suggest an important role for Cldns in epidermal/hair follicle differentiation programs likely involving cross talk to signaling pathways (e.g., Notch) directing cell fate selection and differentiation.

Dysregulation of claudin-7 leads to loss of E-cadherin expression and the increased invasion of esophageal squamous cell carcinoma cells

The claudins constitute a 24-member family of proteins that are critical for the function and formation of tight junctions. Here, we examine the expression of claudin-7 in squamous cell carcinoma (SCC) of the esophagus and its possible role in tumor progression. In the normal esophagus, expression of claudin-7 was confined to the cell membrane of differentiated keratinocytes. However, in the tumor samples, claudin-7 expression is often lost or localized to the cytoplasm. Assaying esophageal SCC lines revealed variable expression of claudin-7, with some lacking expression completely. Knockdown of claudin-7 in SCC cell lines using a small interfering RNA approach led to decreased E-cadherin expression, increased cell growth, and enhanced invasion into a three-dimensional matrix. The opposite was observed when claudin-7 was overexpressed in esophageal SCC cells lacking both claudin-7 and E-cadherin. In this context, the claudin-7-overexpressing cells became more adhesive and less invasive associated with increased E-cadherin expression. In summary, we demonstrate that claudin-7 is mislocalized during the malignant transformation of esophageal keratinocytes. We also demonstrate a critical role for claudin-7 expression in the regulation of E-cadherin in these cells, suggesting this may be one mechanism for the loss of epithelial architecture and invasion observed in esophageal SCC.

Proteomic and bioinformatic analysis of epithelial tight junction reveals an unexpected cluster of synaptic molecules

BACKGROUND

Zonula occludens, also known as the tight junction, is a specialized cell-cell interaction characterized by membrane "kisses" between epithelial cells. A cytoplasmic plaque of approximately 100 nm corresponding to a meshwork of densely packed proteins underlies the tight junction membrane domain. Due to its enormous size and difficulties in obtaining a biochemically pure fraction, the molecular composition of the tight junction remains largely unknown.

RESULTS

A novel biochemical purification protocol has been developed to isolate tight junction protein complexes from cultured human epithelial cells. After identification of proteins by mass spectroscopy and fingerprint analysis, candidate proteins are scored and assessed individually. A simple algorithm has been devised to incorporate transmembrane domains and protein modification sites for scoring membrane proteins. Using this new scoring system, a total of 912 proteins have been identified. These 912 hits are analyzed using a bioinformatics approach to bin the hits in 4 categories: configuration, molecular function, cellular function, and specialized process. Prominent clusters of proteins related to the cytoskeleton, cell adhesion, and vesicular traffic have been identified. Weaker clusters of proteins associated with cell growth, cell migration, translation, and transcription are also found. However, the strongest clusters belong to synaptic proteins and signaling molecules. Localization studies of key components of synaptic transmission have confirmed the presence of both presynaptic and postsynaptic proteins at the tight junction domain. To correlate proteomics data with structure, the tight junction has been examined using electron microscopy. This has revealed many novel structures including end-on cytoskeletal attachments, vesicles fusing/budding at the tight junction membrane domain, secreted substances encased between the tight junction kisses, endocytosis of tight junction double membranes, satellite Golgi apparatus and associated vesicular structures. A working model of the tight junction consisting of multiple functions and sub-domains has been generated using the proteomics and structural data.

CONCLUSION

This study provides an unbiased proteomics and bioinformatics approach to elucidate novel functions of the tight junction. The approach has revealed an unexpected cluster associating with synaptic function. This surprising finding suggests that the tight junction may be a novel epithelial synapse for cell-cell communication.

REVIEWERS

This article was reviewed by Gáspár Jékely, Etienne Joly and Neil Smalheiser.

The temporal and spatial expression of Claudins in epidermal development and the accelerated program of epidermal differentiation in K14-CaSR transgenic mice

The importance of the epidermal permeability barrier (EPB) in protecting the mammalian species against harmful UV irradiation, microorganism invasion and water loss is well recognized, as is the role of calcium (Ca(2+)) in keratinocyte differentiation, cell-cell contact and the EPB. In a previous study, we reported that the overexpression of the Ca(2+)-sensing receptor (CaSR) in the undifferentiated basal cells of the epidermis induced a modified epidermal differentiation program including an accelerated EPB formation in transgenic mice, suggesting a role for CaSR signaling in the differentiation of embryonic epidermal cells during development. We now describe the expression profile of claudins (Cldns) and keratin markers in the accelerated EPB formation of K14-CaSR transgenic mice during development as compared to the wild type from E12.5 to newborn stages. Our data show that the transgenic epidermis undergoes an advanced epidermal differentiation program as compared to the wild type as evidenced morphologically as well as by the expression of K14, K1, loricrin, Cldn6, Cldn18 and Cldn11. In addition, we report for the first time the sequential expression of Cldns in epidermal development and describe that the localization of some Cldns change within the epidermis as it matures. Furthermore, we demonstrate that Cldn6 is expressed very early in epidermal morphogenesis, followed by Cldn18, Cldn11 and Cldn1.