Altered Sweat Composition Due to Changes in Tight Junction Expression of Sweat Glands in Cholinergic Urticaria Patients.

In cholinergic urticaria (CholU), small, itchy wheals are induced by exercise or passive warming and reduced sweating has been reported. Despite the described reduced muscarinic receptor expression, sweat duct obstruction, or sweat allergy, the underlying pathomechanisms are not well understood. To gain further insights, we collected skin biopsies before and after pulse-controlled ergometry and sweat after sauna provocation from CholU patients as well as healthy controls. CholU patients displayed partially severely reduced local sweating, yet total sweat volume was unaltered. However, sweat electrolyte composition was altered, with increased K+ concentration in CholU patients. Formalin-fixed, paraffin-embedded biopsies were stained to explore sweat leakage and tight junction protein expression. Dermcidin staining was not found outside the sweat glands. In the secretory coils of sweat glands, the distribution of claudin-3 and -10b as well as occludin was altered, but the zonula occludens-1 location was unchanged. In all, dermcidin and tight junction protein staining suggests an intact barrier with reduced sweat production capability in CholU patients. For future studies, an ex vivo skin model for quantification of sweat secretion was established, in which sweat secretion could be pharmacologically stimulated or blocked. This ex vivo model will be used to further investigate sweat gland function in CholU patients and decipher the underlying pathomechanism(s).

Claudin-10 Expression and the Gene Expression Pattern of Thick Ascending Limb Cells.

Many genomic, anatomical and functional differences exist between the medullary (MTAL) and the cortical thick ascending limb of the loop of Henle (CTAL), including a higher expression of claudin-10 (CLDN10) in the MTAL than in the CTAL. Therefore, we assessed to what extent the Cldn10 gene expression is a determinant of differential gene expression between MTAL and CTAL. RNAs extracted from CTAL and MTAL microdissected from wild type (WT) and Cldn10 knock out mice (cKO) were analyzed by RNAseq. Differential and enrichment analyses (GSEA) were performed with interactive R Shiny software. Between WT and cKO MTAL, 637 genes were differentially expressed, whereas only 76 were differentially expressed between WT and cKO CTAL. Gene expression patterns and GSEA analyses in all replicates showed that WT MTAL did not cluster with the other replicates; no hierarchical clustering could be found between WT CTAL, cKO CTAL and cKO MTAL. Compared to WT replicates, cKO replicates were enriched in Cldn16, Cldn19, Pth1r, (parathyroid hormone receptor type 1), Casr (calcium sensing receptor) and Vdr (Vitamin D Receptor) mRNA in both the cortex and medulla. Cldn10 is associated with gene expression patterns, including genes specifically involved in divalent cations reabsorption in the TAL.

Claudin-10 in the Blood Brain Barrier Function of Cerebral Endothelial Cells and Transendothelial Invasion of Breast Cancer Cells.

BACKGROUND/AIM: Claudin-10 (CLDN10) is a membrane integral protein. It is one of the widely expressed tight junctional claudins with functions not well defined. In the present study, the expression profile and its role in cerebral endothelial cells and in the interaction between breast cancer and endothelial cells were investigated. MATERIALS AND METHODS: CLDN10 expression was examined in a wide range of cell types. Brain endothelial cell models with or without CLDN10 expression were generated using the hCMEC/D3 cell line and used to test the barrier and permeability functions. Transendothelial drug delivery and invasion were also evaluated. RESULTS: hCMEC/D3 cells express high levels of CLDN10, compared with peripheral endothelial cells, mesothelial cells, fibroblasts, and breast cancer cells, which were either negative or expressed low levels of CLDN10. Knockdown of CLDN10 in hCMEC/D3 cells resulted in impaired tight junctions as seen by reduced transendothelial electric resistance and paracellular permeability. It also accelerated invasion of breast cancer cells through the endothelial cell layer. CLDN10 knockdown in hCMEC/D3 cells led to an increase in transendothelial chemodrug delivery. Furthermore, the SRC kinase inhibitor (AZM475271) was able to decrease the impedance and increase the paracellular permeability in cerebral endothelial cells. CONCLUSION: Cerebral endothelial cells express high levels of CLDN10, a protein regulating barrier function and thereby drug permeability and cancer invasiveness in brain endothelial cells, suggesting that it is a novel therapeutic target for the treatment of brain metastasis-related diseases.

Claudin-10 is a new candidate prognostic marker in metastatic high-grade serous carcinoma.

The objective of this study was to analyze the expression and prognostic role of the tight junction protein claudin-10 in high-grade serous carcinoma (HGSC). Claudin-10 protein expression by immunohistochemistry was analyzed in 588 HGSC (414 effusions, 174 surgical specimens). Expression in mesotheliomas (n = 97; 47 effusions, 50 surgical specimens) was studied for comparative purposes. CLDN10 mRNA expression by quantitative RT-PCR (qRT-PCR) was analyzed in 40 HGSC effusions. Claudin-10 protein expression was found in 360/588 (61%) HGSC vs. 19/97 (20%) mesotheliomas (p < 0.001), and was higher in HGSC surgical specimens compared to effusions (p < 0.001). qRT-PCR confirmed the presence of CLDN10 mRNA in HGSC effusions. High (> 25%) claudin-10 expression in HGSC effusions was significantly associated with shorter overall survival (OS; p = 0.036) and progression-free survival (PFS; p = 0.045) in univariate analysis, and was an independent prognosticator of OS in multivariate analysis (p = 0.045). In conclusion, claudin-10 protein expression is higher in HGSC compared to mesothelioma, although the diagnostic power of this marker appear to be lesser than other claudin family members. Claudin-10 expression in HGSC effusions is marker of more aggressive disease.

Claudin-10 Expression Is Increased in Endometriosis and Adenomyosis and Mislocalized in Ectopic Endometriosis.

Claudins, as the major components of tight junctions, are crucial for epithelial cell-to-cell contacts. Recently, we showed that in endometriosis, the endometrial epithelial phenotype is highly conserved, with only minor alterations. For example, claudin-11 is strongly expressed; however, its localization in the endometriotic epithelial cells was impaired. In order to better understand the role of claudins in endometrial cell-to-cell contacts, we analyzed the tissue expression and localization of claudin-10 by immunohistochemistry analysis and two scoring systems. We used human tissue samples (n = 151) from the endometrium, endometriosis, and adenomyosis. We found a high abundance of claudin-10 in nearly all the endometrial (98%), endometriotic (98−99%), and adenomyotic (90−97%) glands, but no cycle-specific differences and no differences in the claudin-10 positive endometrial glands between cases with and without endometriosis. A significantly higher expression of claudin-10 was evident in the ectopic endometrium of deep-infiltrating (p < 0.01) and ovarian endometriosis (p < 0.001) and in adenomyosis in the cases with endometriosis (p ≤ 0.05). Interestingly, we observed a shift in claudin-10 from a predominant apical localization in the eutopic endometrium to a more pronounced basal/cytoplasmic localization in the ectopic endometria of all three endometriotic entities but not in adenomyosis. Significantly, despite the impaired endometriotic localization of claudin-10, the epithelial phenotype was retained. The significant differences in claudin-10 localization between the three endometriotic entities and adenomyosis, in conjunction with endometriosis, suggest that most of the aberrations occur after implantation and not before. The high similarity between the claudin-10 patterns in the eutopic endometrial and adenomyotic glands supports our recent conclusions that the endometrium is the main source of endometriosis and adenomyosis.

IL-22 regulates endometrial regeneration by enhancing tight junctions and orchestrating extracellular matrix.

The uterine endometrium uniquely regenerates after menses, postpartum, or after breaks in the uterine layer integrity throughout women's lives. Direct cell-cell contacts ensured by tight and adherens junctions play an important role in endometrial integrity. Any changes in these junctions can alter the endometrial permeability of the uterus and have an impact on the regeneration of uterine layers. Interleukin 22 (IL-22) is a cytokine that is recognized for its role in epithelial regeneration. Moreover, it is crucial in controlling the inflammatory response in mucosal tissues. Here, we studied the role of IL-22 in endometrial recovery after inflammation-triggered abortion. Fecundity of mice was studied in consecutive matings of the same animals after lipopolysaccharide (LPS) (10 µg per mouse)-triggered abortion. The fecundity rate after the second mating was substantially different between IL-22 knockout (IL-22-/-) (9.1%) and wild-type (WT) (71.4%) mice (p < 0.05), while there was no difference between the groups in the initial mating, suggesting that IL-22 deficiency might be associated with secondary infertility. A considerable difference was observed between IL-22-/- and WT mice in the uterine clearance following LPS-triggered abortion. Gross examination of the uteri of IL-22-/- mice revealed non-viable fetuses retained inside the horns (delayed clearance). In contrast, all WT mice had completed abortion with total clearance after LPS exposure. We also discovered that IL-22 deficiency is associated with a decreased expression of tight junctions (claudin-2 and claudin-10) and cell surface pathogen protectors (mucin-1). Moreover, IL-22 has a role in the remodeling of the uterine tissue in the inflammatory environment by regulating epithelial-mesenchymal transition markers called E- and N-cadherin. Therefore, IL-22 contributes to the proper regeneration of endometrial layers after inflammation-triggered abortion. Thus, it might have a practical significance to be utilized as a treatment option postpartum (enhanced regeneration function) and in secondary infertility caused by inflammation (enhanced barrier/protector function).

Unrecognized role of claudin-10b in basolateral membrane infoldings of the thick ascending limb.

Claudin-10b is an important component of the tight junction in the thick ascending limb (TAL) of Henle's loop and allows paracellular sodium transport. In immunofluorescence stainings, claudin-10b-positive cells exhibited extensive extra staining of basolateral, column-like structures. The precise localization and function have so far remained elusive. In isolated cortical TAL segments from C57BL/6J mice, kidney-specific claudin-10 knockout mice (cKO), and respective litter mates (WT), we investigated the localization and protein expression and function by fluorescence microscopy and electrophysiological measurements. Ultrastructural analysis of TAL in kidney sections was performed by electron microscopy. Claudin-10b colocalized with the basolateral Na+ -K+ ATPase and the Cl- channel subunit barttin, but the lack of claudin-10b did not influence the localization or abundance of these proteins. However, the accessibility of the basolateral infolded extracellular space to ouabain or fluorescein was increased by basolateral Ca2+ removal and in the absence of claudin-10b. Ultrastructural analysis by electron microscopy revealed a widening of basolateral membrane infoldings in cKO in comparison to WT. We hypothesize that claudin-10b shapes neighboring membrane invaginations by trans interaction to stabilize and facilitate high-flux salt transport in a water-tight epithelium.

Defective claudin-10 causes a novel variation of HELIX syndrome through compromised tight junction strand assembly.

Formation of claudin-10 based tight junctions (TJs) is paramount to paracellular Na+ transport in multiple epithelia. Sequence variants in CLDN10 have been linked to HELIX syndrome, a salt-losing tubulopathy with altered handling of divalent cations accompanied by dysfunctional salivary, sweat, and lacrimal glands. Here, we investigate molecular basis and phenotypic consequences of a newly identified homozygous CLDN10 variant that translates into a single amino acid substitution within the fourth transmembrane helix of claudin-10. In addition to hypohidrosis (H), electrolyte (E) imbalance with impaired urine concentrating ability, and hypolacrimia (L), phenotypic findings include altered salivary electrolyte composition and amelogenesis imperfecta but neither ichthyosis (I) nor xerostomia (X). Employing cellular TJ reconstitution assays, we demonstrate perturbation of cis- and trans-interactions between mutant claudin-10 proteins. Ultrastructures of reconstituted TJ strands show disturbed continuity and reduced abundance in the mutant case. Throughout, both major isoforms, claudin-10a and claudin-10b, are differentially affected with claudin-10b showing more severe molecular alterations. However, expression of the mutant in renal epithelial cells with endogenous TJs results in wild-type-like ion selectivity and conductivity, indicating that aberrant claudin-10 is generally capable of forming functional paracellular channels. Thus, mutant proteins prove pathogenic by compromising claudin-10 TJ strand assembly. Additional ex vivo investigations indicate their insertion into TJs to occur in a tissue-specific manner.

Claudin-10 overexpression suppresses human clear cell renal cell carcinoma growth and metastasis by regulating ATP5O and causing mitochondrial dysfunction.

Our previous study has proved that down-regulation of CLDN10 (Claudin-10) in ccRCC (clear cell renal cell carcinoma) was closely related to tumor metastasis and predicted an unfavorable prognosis by analyzing TCGA-KIRC data. However, the effects of CLDN10 on the progression of ccRCC and its mechanisms of action remain elusive. During the study, a large number of clinical samples were utilized to verify the reduced expression of CLDN10 in ccRCC and its association with tumor metastasis and poor prognosis, and our results confirmed that lower CLDN10 expression was an independent predictor of shorter OS (HR: 4.0860, 95%CI: 2.4737-6.7490, P<0.0001) and DFS (HR: 4.3680, 95%CI: 2.2800-8.3700, P<0.0001) in metastatic ccRCC patients. CLDN10 overexpression accelerated cell apoptosis and restrained cell proliferation, migration and invasion in vitro. Besides, CLDN10 overexpression suppressed ccRCC growth and lung metastasis and promoted apoptosis in orthotopic models. Mechanistically, we found that CLDN10 overexpression up-regulated the acetylation and expression levels of ATP5O (ATP synthase subunit O, mitochondrial), leading to the dysfunction of mitochondrial, thereby suppressing the growth and metastasis of ccRCC through increasing the levels of NDUFS2, ROS, Cleaved-Caspase 3, E-cadherin and SDHB and decreasing the levels of N-cadherin and mitochondrial membrane potential. Moreover, knockdown of ATP5O expression based on the overexpression of CLDN10 could reverse the increase in NDUFS2, ROS, Cleaved-Caspase 3, E-cadherin and SDHB levels, the decrease in N-cadherin and mitochondrial membrane potential levels and the inhibition of ccRCC phenotypes caused by CLDN10 overexpression. Taken together, these findings for the first time illuminate the mechanism by which CLDN10 overexpression suppresses the growth and metastasis of ccRCC.

Pathophysiological aspects of the thick ascending limb and novel genetic defects: HELIX syndrome and transient antenatal Bartter syndrome.

The thick ascending limb plays a central role in human kidney physiology, participating in sodium reabsorption, urine concentrating mechanisms, calcium and magnesium homeostasis, bicarbonate and ammonium homeostasis, and uromodulin synthesis. This review aims to illustrate the importance of these roles from a pathophysiological point of view by describing the interactions of the key proteins of this segment and by discussing how recently identified and long-known hereditary diseases affect this segment. The descriptions of two recently described salt-losing tubulopathies, transient antenatal Bartter syndrome and HELIX syndrome, which are caused by mutations in MAGED2 and CLDN10 genes, respectively, highlight the role of new players in the modulation of sodium reabsorption the thick ascending limb.

A novel claudin-10 mutation with a unique mechanism in two unrelated families with HELIX syndrome.

HELIX syndrome, characterized by hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia due to claudin-10 (CLDN10) mutations, was recognized in 2017. Here we describe two unrelated Saudi families with this syndrome due to a novel CLDN10 mutation with a unique mechanism of CLDN10 inactivation. The two consanguineous families include 12 affected individuals (three siblings in family 1 and nine members in family 2). They presented with hypokalemia and the above-mentioned features of HELIX syndrome. The underlying mutation was detected by whole exome sequencing, confirmed by Sanger sequencing and functionally indicated by RT-PCR, electrophysiological studies and immunohistochemical staining of transfected HEK293 and MDCK C7 cells, and skin and kidney biopsy tissues. A novel biallelic single nucleotide deletion was identified in exon 5 of CLDN10 (NM_182848.3: c.647delC, p.P216Lfs∗19 for CLDN10a or NM_006984.4: c.653delC, p.P218Lfs∗21 for CLDN10b). The mutation led to frameshift and extension of the original termination codon by nine amino acids with loss of the C-terminus pdz-binding motif. Functional studies showed mRNA degradation and protein retention in intracellular compartments and that the pdz-binding motif is crucial for proper localization of claudin-10 in tight junctions. In the kidney, claudin-10 was replaced by translocation of claudin-2 (proximal tubule) and claudin-19 (thick ascending limb), and in the sweat gland by claudin-3 and occludin. However, these claudins did not functionally compensate for loss of claudin-10. Thus, this novel CLDN10 mutation identified in these two families disrupted the C-terminus pdz-binding motif of claudin-10 causing HELIX syndrome.

Expression Patterns of Claudin Family Members During Tooth Development and the Role of Claudin-10 (Cldn10) in Cytodifferentiation of Stratum Intermedium.

There is growing evidence showing that tight junctions play an important role in developing enamel. Claudins are one of the main components of tight junctions and may have pivotal functions in modulating various cellular events, such as regulating cell differentiation and proliferation. Mutations in CLDN10 of humans are associated with HELIX syndrome and cause enamel defects. However, current knowledge regarding the expression patterns of claudins and the function of Cldn10 during tooth development remains fragmented. In this study, we aimed to analyze the expression patterns of claudin family members during tooth development and to investigate the role of Cldn10 in amelogenesis. Using cap analysis gene expression of developing mouse tooth germs compared with that of the whole body, we found that Cldn1 and Cldn10 were highly expressed in the tooth. Furthermore, single-cell RNA-sequence analysis using 7-day postnatal Krt14-RFP mouse incisors revealed Cldn1 and Cldn10 exhibited distinct expression patterns. Cldn10 has two isoforms, Cldn10a and Cldn10b, but only Cldn10b was expressed in the tooth. Immunostaining of developing tooth germs revealed claudin-10 was highly expressed in the inner enamel epithelium and stratum intermedium. We also found that overexpression of Cldn10 in the dental epithelial cell line, SF2, induced alkaline phosphatase (Alpl) expression, a marker of maturated stratum intermedium. Our findings suggest that Cldn10 may be a novel stratum intermedium marker and might play a role in cytodifferentiation of stratum intermedium.

Claudin 10 acts as a novel biomarker for the prognosis of patients with ovarian cancer.

Ovarian cancer (OC) is one of the most fatal gynecological malignancies in the world and confers a poor 5-year survival rate. The present study was designed to discover novel prognostic markers for patients with OC in order to estimate disease metastasis or recurrence. Based on the large cohorts of transcriptome data from multicenter sources, a comprehensive analysis was performed to explore potential prognostic markers. A total of 269 differentially expressed genes were identified, of which 32 were upregulated and 237 downregulated in OC tissues compared with the corresponding expression in normal tissues. Kaplan-Meier analysis, log-rank test and nomogram analysis were employed to demonstrate that low expression levels of claudin 10 (CLDN10) were associated with a less favorable disease prognosis. The most promising prognostic marker for OC was subsequently selected. Additionally, the prognostic nomogram was constructed in order to assess the 5-year survival rate using CLDN10 expression as a prognostic marker for OC. Furthermore, gene set enrichment analysis and analysis of the tumor-associated competing endogenous RNA network were performed to elucidate the potential biological processes associated with CLDN10 expression. The current results indicated that CLDN10 may influence OC progression via transforming growth factor-ß (TGF-ß)- or WNT/ß-catenin-induced epithelial-to-mesenchymal transition (EMT). The associations among CLDN10, microRNA-486-5p, TGF-ß, WNT/ß-catenin and EMT should be further investigated in future studies.

Claudins in Renal Physiology and Pathology.

Claudins are integral proteins expressed at the tight junctions of epithelial and endothelial cells. In the mammalian kidney, every tubular segment express a specific set of claudins that give to that segment unique properties regarding permeability and selectivity of the paracellular pathway. So far, 3 claudins (10b, 16 and 19) have been causally traced to rare human syndromes: variants of CLDN10b cause HELIX syndrome and variants of CLDN16 or CLDN19 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis. The review summarizes our current knowledge on the physiology of mammalian tight junctions and paracellular ion transport, as well as on the role of the 3 above-mentioned claudins in health and disease. Claudin 14, although not having been causally linked to any rare renal disease, is also considered, because available evidence suggests that it may interact with claudin 16. Some single-nucleotide polymorphisms of CLDN14 are associated with urinary calcium excretion and/or kidney stones. For each claudin considered, the pattern of expression, the function and the human syndrome caused by pathogenic variants are described.

Sox2 maintains epithelial cell proliferation in the successional dental lamina.

OBJECTIVES: The successional dental lamina is the distinctive structure on the lingual side of the vertebrate tooth germ. The aim of this study was to investigate the relationship among Sox2, Claudin10 and laminin5 and the role of Sox2 in successional dental lamina proliferation during vertebrate tooth development. MATERIALS AND METHODS: To understand the successional dental lamina, two types of successional tooth formation, that in geckos (with multiple rounds of tooth generation) and that in mice (with only one round of tooth generation), were analysed. RESULTS: Unique coexpression patterns of Sox2 and Claudin10 expression were compared in the successional dental lamina from the cap stage to the late bell stage in the mouse tooth germ and in juvenile gecko teeth to support continuous tooth replacement. Furthermore, Laminin5 expression was shown in the cap stage and decreased after the bell stage. Upon comparing the epithelial cell cycles and cell proliferation in successional dental lamina regions between mouse and gecko molars using BrdU and IdU staining and pulse-chase methods, distinctive patterns of continuous expression were revealed. Moreover, Sox2 overexpression with a lentiviral system resulted in hyperplastic dental epithelium in mouse molars. CONCLUSIONS: Our findings indicate that the regulation of Sox2 in dental lamina proliferation is fundamental to the successional dental lamina in both species.

CLDN10 promotes a malignant phenotype of osteosarcoma cells via JAK1/Stat1 signaling.

In our previous study, the expression profile of tight junction (TJ) protein claudins (CLDNs) in human osteosarcoma (OS) cells was examined, and the data found the CLDN10 was high expressed in OS cells versus fetal osteoblast cells. Hence, we aim to determine the impacts and the molecular mechanisms of CLDN10 in the metastatic phenotype of OS. The exact expression profiles of CLDN10 and phosphorylated Janus kinase 1 (JAK1) in noncancerous bone tissues and OS tissues were detected via a western blotting and immunohistochemistry method. The OS cells with CLDN10 or JAK1 silencing was established via an RNA interference (RNAi) method, and an osteoblast cell line stably expressing CLDN10 was established via cell transfection. Then, the transfection effects and activation states of JAK1/ signal transducer and activator of transcription1 (Stat1) pathway in OS and osteoblast cells were detected via a western blotting assay. Moreover, the metastatic ability of osteoblast cells and OS cells in vitro were evaluated by means of a cell counting kit-8 (CCK8) assay, colony formation assay in soft agar, transwell assay and wound-healing experiment. The present data revealed that CLDN10 and phospho-JAK1 were up-regulated in OS tissues compared with noncancerous bone tissues. Genetic loss of CLDN10 or JAK1 inhibited the activation of the Stat1 and the malignant phenotype in OS cells. To sum up, our study suggested the CLDN10 enhanced the metastatic phenotype of OS cells via the activation of the JAK1/Stat1 signaling pathway.

Hypokalemia Associated With a Claudin 10 Mutation: A Case Report.

Hypokalemia of renal origin can arise from genetic abnormalities in a variety of transporters or channel proteins that mediate tubular handling of potassium. Recently, mutations in claudin 10 have been documented in patients with hypokalemia in association with a range of other electrolyte abnormalities and skin and sweat gland manifestations. We report a 12-year-old Hispanic boy who presented with anhydrosis, aptyalism, alacrima, hypokalemia, and hypocalciuria, in whom we detected a homozygous mutation in the claudin 10 gene. During the 4-year follow-up period, he developed hypermagnesemia and a decline in estimated glomerular filtration rate to 59mL/min/1.73m2. His unaffected parents and siblings were heterozygous for the mutation. We summarize the clinical phenotype encountered in patients with claudin 10 mutations. It is characterized by significant heterogeneity in electrolyte and extrarenal abnormalities and is associated with a risk for progressive loss of kidney function in up to 33% of cases. Awareness of this association between claudin 10 mutations and electrolyte abnormalities, namely hypokalemia and hypermagnesemia, sheds new light on the physiology of potassium and magnesium handling along the nephron and increases the likelihood of identifying the underlying tubular mechanism in patients with newly diagnosed hypokalemia with or without concomitant hypermagnesemia.

Tight junction strand formation by claudin-10 isoforms and claudin-10a/-10b chimeras.

Claudins are integral components of tight junctions (TJs) in epithelia and endothelia. When expressed in cell lines devoid of TJs, claudins are able to form TJ-like strands at contacts between adjacent cells. According to a current model of TJ strand formation, claudin protomers assemble in an antiparallel double row within the plasma membrane of each cell (cis-interaction) while binding to corresponding double rows from the neighboring cells (trans-interaction). Cis-interaction was proposed to involve two interfaces of the protomers' first extracellular segment (extracellular loop (ECL)1). In the current study, three naturally occurring claudin-10 isoforms and two claudin-10 chimeras were used to investigate strand formation. All constructs were able to interact in cis (Förster/fluorescence resonance energy transfer (FRET)), to integrate into TJs of MDCK-C7 cells (confocal laser scanning microscopy), and to form TJ-like strands in HEK293 cells (freeze-fracture electron microscopy). Strand formation occurred despite the fact that isoform claudin-10a_i1 lacks both structural ECL1 elements reported to be crucial for cis-interaction. Furthermore, results from FRET experiments on claudin-10 chimeras indicated that identity of the first transmembrane region rather than ECL1 is decisive for claudin-10 cis-interaction. Therefore, in addition to the interaction interfaces suggested in the current model for TJ strand assembly, alternative interfaces must exist.

One gene, two paracellular ion channels-claudin-10 in the kidney.

Claudins are tight junction membrane proteins and regulate the paracellular passage of ions and water. They can seal the paracellular cleft against solute passage but also form paracellular channels. They are tetraspan proteins with two extracellular segments. Claudin-10 exists in at least two functional isoforms, claudin-10a and claudin-10b, that differ in their first transmembrane segment and first extracellular segment. Both isoforms act as selective paracellular ion channels, either for anions (claudin-10a) or for cations (claudin-10b). Their diverse functions are reflected in completely different expression patterns in the body, especially in the kidney. Their structural and functional similarities and differences make them ideal subjects to study determinants of claudin charge selectivity and pore formation. This review aims to summarise research on permeability properties of the claudin-10 channels and their role in physiology and pathophysiology of the kidney.

The role of club cell phenoconversion and migration in idiopathic pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) is an age-related multifactorial disease featuring non-uniform lung fibrosis. The decisive cellular events at early stages of IPF are poorly understood. While the involvement of club cells in IPF pathogenesis is unclear, their migration has been associated with lung fibrosis. In this study, we labeled club cells immunohistochemically in IPF lungs using a club cell marker Claudin-10 (Cldn10), a unique protein based on the recent report which demonstrated that the appearance of Cldn10 in developing and repairing lungs precedes other club cell markers including club cell secretory protein (CCSP). Cldn10-positive cells in IPF lungs displayed marked pleomorphism and were found in varied arrangements, suggesting their phenoconversion. These results were corroborated by immunogold labeling for Cldn10. Further, immunohistochemical double-labeling for Cldn10 and α-smooth muscle actin (α-SMA) demonstrated that aberrant α-SMA signals are frequently encountered near disorganized Cldn10-positive cells in hyperplastic bronchiolar epithelium and thickened interstitium of IPF lungs. Collectively, these data indicate that club cells actively participate in the initiation and progression of IPF through phenoconversion involving the acquisition of proliferative and migratory abilities. Thus, our new findings open the possibility for club cell-targeted therapy to become a strategic option for the treatment of IPF.