Damage control of epithelial barrier function in dynamic environments.

Epithelial tissues cover the surfaces and lumens of the internal organs of multicellular animals and crucially contribute to internal environment homeostasis by delineating distinct compartments within the body. This vital role is known as epithelial barrier function. Epithelial cells are arranged like cobblestones and intricately bind together to form an epithelial sheet that upholds this barrier function. Central to the restriction of solute and fluid diffusion through intercellular spaces are occluding junctions, tight junctions in vertebrates and septate junctions in invertebrates. As part of epithelial tissues, cells undergo constant renewal, with older cells being replaced by new ones. Simultaneously, the epithelial tissue undergoes relative rearrangement, elongating, and shifting directionally as a whole. The movement or shape changes within the epithelial sheet necessitate significant deformation and reconnection of occluding junctions. Recent advancements have shed light on the intricate mechanisms through which epithelial cells sustain their barrier function in dynamic environments. This review aims to introduce these noteworthy findings and discuss some of the questions that remain unanswered.

Bicellular Localization of Tricellular Junctional Protein Angulin-3/ILDR2 Allows Detection of Podocyte Injury.

Podocytes serve as part of the renal filtration unit with slit diaphragms. Although the structure of slit diaphragms between two cells is well characterized, how the tricellular contact of podocytes is organized and how it changes in injured podocytes remains unknown. This study focused on a tricellular junction protein, angulin-3, and its localization in healthy podocytes, in developmental stages, and in pathologic conditions, using a newly established monoclonal antibody. Angulin-3 was confined at tricellular junctions of primordial podocytes, then transiently localized at bicellular junctions as foot process interdigitation developed and the intercellular junctions rearranged into slit diaphragm, and eventually distributed in a sparse punctate pattern on the foot processes of adult podocytes. In the rodent podocyte injury models, angulin-3 showed bicellular localization between the foot processes, and the localization turned from punctate to dashed linear pattern along the effaced foot processes with the progression of podocyte injury. Angulin-3 also accumulated between foot processes in a linear pattern in kidney biopsy samples of human nephrotic syndrome. Additionally, the line length of angulin-3 staining signal correlated with risk of relapse under glucocorticoid therapy in patients with minimal change nephrotic syndrome. This study proposes an image program to score the linearity of the accumulation pattern of angulin-3 to evaluate the relapse risk of patients with minimal change nephrotic syndrome.

ZnUMBA - a live imaging method to detect local barrier breaches.

Epithelial barrier function is commonly analyzed using transepithelial electrical resistance, which measures ion flux across a monolayer, or by adding traceable macromolecules and monitoring their passage across the monolayer. Although these methods measure changes in global barrier function, they lack the sensitivity needed to detect local or transient barrier breaches, and they do not reveal the location of barrier leaks. Therefore, we previously developed a method that we named the zinc-based ultrasensitive microscopic barrier assay (ZnUMBA), which overcomes these limitations, allowing for detection of local tight junction leaks with high spatiotemporal resolution. Here, we present expanded applications for ZnUMBA. ZnUMBA can be used in Xenopus embryos to measure the dynamics of barrier restoration and actin accumulation following laser injury. ZnUMBA can also be effectively utilized in developing zebrafish embryos as well as cultured monolayers of Madin-Darby canine kidney (MDCK) II epithelial cells. ZnUMBA is a powerful and flexible method that, with minimal optimization, can be applied to multiple systems to measure dynamic changes in barrier function with spatiotemporal precision.

The Src-Family Kinases SRC and BLK Contribute to the CLDN6-Adhesion Signaling.

Cell adhesion molecules, including integrins, cadherins, and claudins (CLDNs), are known to activate Src-family kinases (SFKs) that organize a variety of physiological and pathological processes; however, the underlying molecular basis remains unclear. Here, we identify the SFK members that are coupled with the CLDN6-adhesion signaling. Among SFK subtypes, BLK, FGR, HCK, and SRC were highly expressed in F9 cells and concentrated with CLDN6 along cell borders during epithelial differentiation. Immunoprecipitation assay showed that BLK and SRC, but not FGR or HCK, form a complex with CLDN6 via the C-terminal cytoplasmic domain. We also demonstrated, by pull-down assay, that recombinant BLK and SRC proteins directly bind to the C-terminal cytoplasmic domain of CLDN6 (CLDN6C). Unexpectedly, both recombinant SFK proteins recognized the CLDN6C peptide in a phosphotyrosine-independent manner. Furthermore, by comparing phenotypes of F9:Cldn6:Blk-/- and F9:Cldn6:Src-/- cells with those of wild-type F9 and F9:Cldn6 cells, we revealed that BLK and SRC are essential for CLDN6-triggered cellular events, namely epithelial differentiation and the expression of retinoid acid receptor target genes. These results indicate that selective SFK members appear to participate in the CLDN-adhesion signaling.

Aberrant phosphorylation of human LRH1 at serine 510 is predictable of hepatocellular carcinoma recurrence.

We previously identified the AKT-phosphorylation sites in nuclear receptors and showed that phosphorylation of S379 in mouse retinoic acid γ and S518 in human estrogen receptor α regulate their activity independently of the ligands. Since this site is conserved at S510 in human liver receptor homolog 1 (hLRH1), we developed a monoclonal antibody (mAb) that recognized the phosphorylation form of hLRH1S510 (hLRH1pS510) and verified its clinicopathological significance in hepatocellular carcinoma (HCC). We generated the anti-hLRH1pS510 mAb and assessed its selectivity. We then evaluated the hLRH1pS510 signals in 157 cases of HCC tissues by immunohistochemistry because LRH1 contributes to the pathogenesis of diverse cancers. The developed mAb specifically recognized hLRH1pS510 and worked for immunohistochemistry of formalin-fixed paraffin-embedded tissues. hLRH1pS510 was exclusively localized in the nucleus of HCC cells, but the signal intensity and positive rates varied among the subjects. According to the semi-quantification, 45 cases (34.9%) showed hLRH1pS510-high, and the remaining 112 cases (65.1%) exhibited hLRH1pS510-low. There were significant differences in the recurrence-free survival (RFS) between the two groups, and the 5-year RFS rates in the hLRH1pS510-high and hLRH1pS510-low groups were 26.5% and 46.1%, respectively. In addition, high hLRH1pS510 was significantly correlated with portal vein invasion, hepatic vein invasion, and high levels of serum alpha-fetoprotein (AFP). Furthermore, multivariable analysis revealed that hLRH1pS510-high was an independent biomarker for HCC recurrence. We conclude that aberrant phosphorylation of hLRH1S510 is a predictor of poor prognosis for HCC. The anti-hLRH1pS510 mAb could provide a powerful tool to validate the relevance of hLRH1pS510 in pathological processes such as tumor development and progression.

SPON1 is an independent prognostic biomarker for ovarian cancer.

BACKGROUND: Ovarian cancer has the worst outcome among gynecological malignancies; therefore, biomarkers that could contribute to the early diagnosis and/or prognosis prediction are urgently required. In the present study, we focused on the secreted protein spondin-1 (SPON1) and clarified the prognostic relevance in ovarian cancer. METHODS: We developed a monoclonal antibody (mAb) that selectively recognizes SPON1. Using this specific mAb, we determined the expression of SPON1 protein in the normal ovary, serous tubal intraepithelial carcinoma (STIC), and ovarian cancer tissues, as well as in various normal adult tissues by immunohistochemistry, and verified its clinicopathological significance in ovarian cancer. RESULTS: The normal ovarian tissue was barely positive for SPON1, and no immunoreactive signals were detected in other healthy tissues examined, which was in good agreement with data obtained from gene expression databases. By contrast, upon semi-quantification, 22 of 242 ovarian cancer cases (9.1%) exhibited high SPON1 expression, whereas 64 (26.4%), 87 (36.0%), and 69 (28.5%) cases, which were designated as SPON1-low, possessed the moderate, weak, and negative SPON1 expression, respectively. The STIC tissues also possessed SPON1-positive signals. The 5-year recurrence-free survival (RFS) rate in the SPON1-high group (13.6%) was significantly lower than that in the SPON1-low group (51.2%). In addition, high SPON1 expression was significantly associated with several clinicopathological variables. Multivariable analysis revealed that high SPON1 was an independent prognostic factor for RFS of ovarian cancer. CONCLUSIONS: SPON1 represents a prognostic biomarker for ovarian cancer, and the anti-SPON1 mAb could be valuable as an outcome predictor.

Claudin-4-adhesion signaling drives breast cancer metabolism and progression via liver X receptor ß.

BACKGROUND: Cell adhesion is indispensable for appropriate tissue architecture and function in multicellular organisms. Besides maintaining tissue integrity, cell adhesion molecules, including tight-junction proteins claudins (CLDNs), exhibit the signaling abilities to control a variety of physiological and pathological processes. However, it is still fragmentary how cell adhesion signaling accesses the nucleus and regulates gene expression. METHODS: By generating a number of knockout and rescued human breast cell lines and comparing their phenotypes, we determined whether and how CLDN4 affected breast cancer progression in vitro and in vivo. We also identified by RNA sequencing downstream genes whose expression was altered by CLDN4-adhesion signaling. Additionally, we analyzed by RT-qPCR the CLDN4-regulating genes by using a series of knockout and add-back cell lines. Moreover, by immunohistochemistry and semi-quantification, we verified the clinicopathological significance of CLDN4 and the nuclear receptor LXRß (liver X receptor ß) expression in breast cancer tissues from 187 patients. RESULTS: We uncovered that the CLDN4-adhesion signaling accelerated breast cancer metabolism and progression via LXRß. The second extracellular domain and the carboxy-terminal Y197 of CLDN4 were required to activate Src-family kinases (SFKs) and the downstream AKT in breast cancer cells to promote their proliferation. Knockout and rescue experiments revealed that the CLDN4 signaling targets the AKT phosphorylation site S432 in LXRß, leading to enhanced cell proliferation, migration, and tumor growth, as well as cholesterol homeostasis and fatty acid metabolism, in breast cancer cells. In addition, RT-qPCR analysis showed the CLDN4-regulated genes are classified into at least six groups according to distinct LXRß- and LXRßS432-dependence. Furthermore, among triple-negative breast cancer subjects, the "CLDN4-high/LXRß-high" and "CLDN4-low and/or LXRß-low" groups appeared to exhibit poor outcomes and relatively favorable prognoses, respectively. CONCLUSIONS: The identification of this machinery highlights a link between cell adhesion and transcription factor signalings to promote metabolic and progressive processes of malignant tumors and possibly to coordinate diverse physiological and pathological events.

Tight-junction strand networks and tightness of the epithelial barrier.

Tight junctions (TJs) are cell-cell junction structures critical for controlling paracellular permeability. On freeze-fracture replica electron microscopy, they appear as a continuous network of fibrils (TJ strands). TJ strands function as zippers that create a physical barrier against paracellular diffusion of molecules. The morphology of the TJ strand network varies greatly between tissues, and in recent years, studies have highlighted the mechanisms regulating the morphology of TJ strand networks and on their relevance to barrier function. In this review, we discuss evidence regarding the components of the TJ strand and the mechanisms for creating the TJ strand network. Furthermore, we discuss and hypothesize how its morphology contributes to the establishment of the epithelial barrier.

EpCAM proteolysis and release of complexed claudin-7 repair and maintain the tight junction barrier.

TJs maintain the epithelial barrier by regulating paracellular permeability. Since TJs are under dynamically fluctuating intercellular tension, cells must continuously survey and repair any damage. However, the underlying mechanisms allowing cells to sense TJ damage and repair the barrier are not yet fully understood. Here, we showed that proteinases play an important role in the maintenance of the epithelial barrier. At TJ break sites, EpCAM-claudin-7 complexes on the basolateral membrane become accessible to apical membrane-anchored serine proteinases (MASPs) and the MASPs cleave EpCAM. Biochemical data and imaging analysis suggest that claudin-7 released from EpCAM contributes to the rapid repair of damaged TJs. Knockout (KO) of MASPs drastically reduced barrier function and live-imaging of TJ permeability showed that MASPs-KO cells exhibited increased size, duration, and frequency of leaks. Together, our results reveal a novel mechanism of TJ maintenance through the localized proteolysis of EpCAM at TJ leaks, and provide a better understanding of the dynamic regulation of epithelial permeability.

Effects of TAMP family on the tight junction strand network and barrier function in epithelial cells.

Occludin, tricellulin, and marvelD3 belong to the tight junction (TJ)-associated MARVEL protein family. Occludin and tricellulin jointly contribute to TJ strand branching point formation and epithelial barrier maintenance. However, whether marvelD3 has the same function remains unclear. Furthermore, the roles of the carboxy-terminal cytoplasmic tail, which is conserved in occludin and tricellulin, on the regulation of TJ strand morphology have not yet been explored in epithelial cells. We established tricellulin/occludin/marveld3 triple-gene knockout (tKO) MDCK II cells and evaluated the roles of marvelD3 in the TJ strand structure and barrier function using MDCK II cells and a mathematical model. The complexity of TJ strand networks and paracellular barrier did not change in tKO cells compared to that in tricellulin/occludin double-gene knockout (dKO) cells. Exogenous marvelD3 expression in dKO cells did not increase the complexity of TJ strand networks and epithelial barrier tightness. The expression of the carboxy-terminal truncation mutant of tricellulin restored the barrier function in the dKO cells, whereas occludin lacking the carboxy-terminal cytoplasmic tail was not expressed on the plasma membrane. These data suggest that marvelD3 does not affect the morphology of TJ strands and barrier function in MDCK II cells and that the carboxy-terminal cytoplasmic tail of tricellulin is dispensable for barrier improvement.

Claudin­9 is a novel prognostic biomarker for endometrial cancer.

The tight­junction protein claudin­9 (CLDN9) is barely distributed in normal adult tissues but is ectopically expressed in various cancer types. Although multiple databases indicated upregulation of CLDN9 in endometrial cancers at the mRNA level, its protein expression and biological roles remain obscure. In the present study, the prognostic significance of CLDN9 expression in endometrial cancer was evaluated by immunohistochemical staining and semi­quantification using formalin­fixed paraffin­embedded specimens obtained from 248 endometrial carcinoma cases. A total of 43 cases (17.3%) had high CLDN9 expression, whereas 205 cases (82.7%) exhibited low CLDN9 expression. The 5­year disease­specific survival rates in the high and low CLDN9 expression groups were 62.8 and 87.8% (P<0.001), respectively. In addition, multivariate analysis revealed that high CLDN9 expression was an independent prognostic factor (hazard ratio, 4.99; 95% CI, 1.96­12.70; P<0.001). Furthermore, CLDN9 expression was significantly correlated with the expression of CLDN6 (P<0.001), which is the closest CLDN member to CLDN9 and a poor prognostic factor for endometrial carcinoma. The 5­year disease­specific survival rate of cases with CLDN6­high/CLDN9­high, CLDN6­high/CLDN9­low and CLDN6­low/CLDN9­high status was 30.0, 37.5 and 72.7%, respectively, whereas that of CLDN6­low/CLDN9­low was 89.8% (P=0.004). In conclusion, aberrant CLDN9 expression is a predictor of poor prognosis for endometrial cancer and may be utilized in combination with CLDN6 to achieve higher sensitivity.

LXRß Activation Inhibits the Proliferation of Small-cell Lung Cancer Cells by Depleting Cellular Cholesterol.

BACKGROUND/AIM: Liver X receptors (LXRs) are nuclear receptors with various functions, including the regulation of cholesterol metabolism, glucose homeostasis, and inflammation. We previously reported that LXR activation inhibits the growth of oral cancer cells by inducing cellular cholesterol efflux and that LXRß is expressed mainly in small-cell lung cancer (SCLC) tissues. SCLC is one of the most aggressive cancers, and identifying an effective therapeutic target molecule is desirable. Therefore, we investigated whether LXRß could be an effective target molecule for SCLC treatment through in vitro experiments. MATERIALS AND METHODS: We evaluated the influence of treatment with the LXR agonist T0901317 on cell proliferation and apoptosis in SCLC cell lines using cell viability, BrdU-ELISA, FACS, and western blot analyses. Moreover, the mechanism by which T0901317 inhibits SCLC cell proliferation was elucidated using qRT-PCR, western blot, a cholesterol quantification assay, and a genome editing technique. RESULTS: We showed that cultivated SCLC cells expressed LXRß and that an LXR agonist inhibited the proliferation of SCLC cells without toxicity to normal cells. Furthermore, the antitumoral effect of an LXR agonist on SCLC cells was attributed to the induction of ABCA1 by LXRß activation, resulting in an increase in cellular cholesterol efflux via ABCA1. CONCLUSION: The activation of LXRß up-regulates ABCA1 expression, causing cholesterol depletion in cancer cells. This mechanism could be a novel target strategy for SCLC.

Aberrant expression of claudin-6 contributes to malignant potentials and drug resistance of cervical adenocarcinoma.

Recent studies have revealed that aberrant expression of tight junction (TJ) proteins is a hallmark of various solid tumors and it is recognized as a useful therapeutic target. Claudin-6 (CLDN6), a member of the family of TJ transmembrane proteins, is an ideal therapeutic target because it is not expressed in human adult normal tissues. In this study, we found that CLDN6 is highly expressed in uterine cervical adenocarcinoma (ADC) and that high CLDN6 expression was correlated with lymph node metastasis and lymphovascular infiltration and was an independent prognostic factor. Shotgun proteome analysis revealed that cell-cell adhesion-related proteins and drug metabolism-associated proteins (aldo-keto reductase [AKR] family proteins) were significantly increased in CLDN6-overexpressing cells. Furthermore, overexpression of CLDN6 enhanced cell-cell adhesion properties and attenuated sensitivity to anticancer drugs including doxorubicin, daunorubicin, and cisplatin. Taken together, the results indicate that aberrant expression of CLDN6 enhances malignant potentials and drug resistance of cervical ADC, possibly due to increased cell-cell adhesion properties and drug metabolism. Our findings provide an insight into a new therapeutic strategy, a CLDN6-targeting therapy, against cervical ADC.

Claudin-9 constitutes tight junctions of folliculo-stellate cells in the anterior pituitary gland.

The anterior pituitary gland regulates growth, metabolism, and reproduction by secreting hormones. Folliculo-stellate (FS) cells are non-endocrine cells located among hormone-producing cells in the anterior pituitary glands. They form follicular lumens, which are sealed by tight junctions (TJs). Although FS cells are hypothesized to contribute to fine-tuning of endocrine cells, little is known about the exact roles of FS cells. Here, we investigated the molecular composition of TJs in FS cells. We demonstrated that occludin is a good marker for TJs in the pituitary gland and examined the structure of the lumens surrounded by FS cells. We also found that claudin-9 is a major component of TJs in the FS cells. In immunoelectron microscopy, claudin-9 was specifically localized at TJs of the FS cells. The expression of claudin-9 was gradually increased in the pituitary gland after birth, suggesting that claudin-9 is developmentally regulated and performs some specific functions on the paracellular barrier of follicles in the pituitary gland. Furthermore, we found that angulin-1, angulin-2, and tricellulin are localized at the tricellular contacts of the FS cells. Our findings provide a first comprehensive molecular profile of TJs in the FS cells, and may lead us towards unveiling the FS cell functions.

CLDN15 is a novel diagnostic marker for malignant pleural mesothelioma.

Malignant mesothelioma is a cancer with a poor survival rate. It is difficult to diagnose mesotheliomas because they show a variety of histological patterns similar to those of various other cancers. However, since currently used positive markers for mesotheliomas may show false positives or false negatives, a novel mesothelial positive marker is required. In the present study, we screened 25 claudins and found that claudin-15 is expressed in the mesothelial cells. We made new rat anti-human claudin-15 (CLDN15) monoclonal antibodies that selectively recognize CLDN15, and investigated whether CLDN15 is a good positive marker for malignant pleural mesotheliomas (MPMs) using MPM tissue samples by immunohistochemistry and semi-quantification of the expression level using an immunoreactive score (IRS) method. Of 42 MPM samples, 83% were positive for CLDN15. The positive ratio was equal to or greater than other positive markers for MPMs including calretinin (81%), WT-1 (50%), and D2-40 (81%). In 50 lung adenocarcinoma sections, four cases were positive for CLDN15 and the specificity (92%) was comparable with other markers (90-100%). Notably, CLDN15 was rarely detected in 24 non-mesothelial tumors in the tissue microarray (12/327 cases). In conclusion, CLDN15 can be used in the clinical setting as a positive marker for MPM diagnosis.

Soluble JAM-C Ectodomain Serves as the Niche for Adipose-Derived Stromal/Stem Cells.

Junctional adhesion molecules (JAMs) are expressed in diverse types of stem and progenitor cells, but their physiological significance has yet to be established. Here, we report that JAMs exhibit a novel mode of interaction and biological activity in adipose-derived stromal/stem cells (ADSCs). Among the JAM family members, JAM-B and JAM-C were concentrated along the cell membranes of mouse ADSCs. JAM-C but not JAM-B was broadly distributed in the interstitial spaces of mouse adipose tissue. Interestingly, the JAM-C ectodomain was cleaved and secreted as a soluble form (sJAM-C) in vitro and in vivo, leading to deposition in the fat interstitial tissue. When ADSCs were grown in culture plates coated with sJAM-C, cell adhesion, cell proliferation and the expression of five mesenchymal stem cell markers, Cd44, Cd105, Cd140a, Cd166 and Sca-1, were significantly elevated. Moreover, immunoprecipitation assay showed that sJAM-C formed a complex with JAM-B. Using CRISPR/Cas9-based genome editing, we also demonstrated that sJAM-C was coupled with JAM-B to stimulate ADSC adhesion and maintenance. Together, these findings provide insight into the unique function of sJAM-C in ADSCs. We propose that JAMs contribute not only to cell-cell adhesion, but also to cell-matrix adhesion, by excising their ectodomain and functioning as a niche-like microenvironment for stem and progenitor cells.

Reduced Claudin-12 Expression Predicts Poor Prognosis in Cervical Cancer.

BACKGROUND: Within the claudin (CLDN) family, CLDN12 mRNA expression is altered in various types of cancer, but its clinicopathological relevance has yet to be established due to the absence of specific antibodies (Abs) with broad applications. METHODS: We generated a monoclonal Ab (mAb) against human/mouse CLDN12 and verified its specificity. By performing immunohistochemical staining and semiquantification, we evaluated the relationship between CLDN12 expression and clinicopathological parameters in tissues from 138 cases of cervical cancer. RESULTS: Western blot and immunohistochemical analyses revealed that the established mAb selectively recognized the CLDN12 protein. Twenty six of the 138 cases (18.8%) showed low CLDN12 expression, and the disease-specific survival (DSS) and recurrence-free survival rates were significantly decreased compared with those in the high CLDN12 expression group. We also demonstrated, via univariable and multivariable analyses, that the low CLDN12 expression represents a significant prognostic factor for the DSS of cervical cancer patients (HR 3.412, p = 0.002 and HR 2.615, p = 0.029, respectively). CONCLUSIONS: It can be concluded that a reduced CLDN12 expression predicts a poor outcome for cervical cancer. The novel anti-CLDN12 mAb could be a valuable tool to evaluate the biological relevance of the CLDN12 expression in diverse cancer types and other diseases.

The claudin-transcription factor signaling pathway.

Claudins (CLDNs) represent major transmembrane proteins of tight junctions and contribute to the barrier function. They also serve as anchors for several signaling proteins, but the underlying molecular basis has yet to be established. The present review covers the recent progress in our understanding of the CLDN signaling pathway in health and disease. We discuss the functional relevance of phosphotyrosine motifs in the C-terminal cytoplasmic domain of CLDNs and define mutual regulation between CLDNs and Src-family kinases (SFKs). In addition, we focus on the crosstalk between CLDN and transcription factor signaling. We also describe how aberrant CLDN-transcription factor signaling promotes or inhibits cancer progression. We propose that a link between various cell adhesion molecules and transcription factors coordinates a range of physiological and pathological events via activation or suppression of target genes.

The region-selective regulation of endothelial claudin-5 expression and signaling in brain health and disorders.

The neurovascular unit (NVU) consists of neurons, glial cells, microvascular cells, and extracellular matrix, and is involved in a variety of physiological and pathological processes in the central nervous system (CNS). Within the NVU, the microvascular endothelial cells and pericytes principally contribute to maintaining the integrity of the blood-brain barrier (BBB). Various types of cells are connected to each other in the NVU by diverse cell adhesion molecules, of which claudin-5 (CLDN5) is by far the most abundantly expressed tight-junction protein in brain microvascular endothelial cells and absolutely required for the maintenance of the BBB. This review highlights recent progress in understanding the region-specific regulation and dysregulation of CLDN5 expression in CNS health and disorders. We also discuss how CLDN5 expression is regionally disrupted within the NVU. In addition, we focus on the link between cell adhesion and transcription factor signalings and describe the possible involvement of CLDN5-adhesion signaling in brain health and disorders.