Permeability barrier dysfunction in transgenic mice overexpressing claudin 6

Ubiquitylated histone H2A: a molecular Jekyll and Hyde in the epidermis

The epidermis of the skin provides a barrier between the organism and the external environment. It is constantly subjected to physical and chemical insults, and thus susceptible to wounding and to neoplastic transformation. Long-lasting epigenetic modifications in epidermal stem cells are now shown to link responses to skin injuries with cell priming for carcinoma development, through regulation of histone H2A ubiquitylation.

Key Factors in the Complex and Coordinated Network of Skin Keratinization: Their Significance and Involvement in Common Skin Conditions

The epidermis serves many vital roles, including protecting the body from external influences and healing eventual injuries. It is maintained by an incredibly complex and perfectly coordinated keratinization process. In this process, desquamation is essential for the differentiation of epidermal basal progenitor cells into enucleated corneocytes, which subsequently desquamate through programmed death. Numerous factors control keratinocyte differentiation: epidermal growth factor, transforming growth factor-α, keratinocyte growth factor, interleukins IL-1-β and IL-6, elevated vitamin A levels, and changes in Ca2+ concentration. The backbone of the keratinocyte transformation process from mitotically active basal cells into fully differentiated, enucleated corneocytes is the expression of specific proteins and the creation of a Ca2+ and pH gradient at precise locations within the epidermis. Skin keratinization disorders (histologically characterized predominantly by dyskeratosis, parakeratosis, and hyperkeratosis) may be categorized into three groups: defects in the α-helical rod pattern, defects outside the α-helical rod domain, and disorders of keratin-associated proteins. Understanding the process of keratinization is essential for the pathogenesis of many dermatological diseases because improper desquamation and epidermopoiesis/keratinization (due to genetic mutations of factors or due to immune pathological processes) can lead to various conditions (ichthyoses, palmoplantar keratodermas, psoriasis, pityriasis rubra pilaris, epidermolytic hyperkeratosis, and others).

CLDN6 inhibits colorectal cancer proliferation dependent on restraining p53 ubiquitination via ZO-1/PTEN axis

Colorectal cancer (CRC) is one of the most common cancers in the world. Abnormal proliferation is a chief characteristic of cancer and is the initiation of CRC progression. As an important component of tight junctions, CLDN6 regulates the proliferation of multiple tumors. Our previous study showed that CLDN6 was low expressed in CRC, and CLDN6 overexpression inhibited CRC proliferation. However, the specific mechanism of how CLDN6 works remains unclear. This research aimed to reveal the relationship between CLDN6 and clinical features, as well as the molecular mechanism by which CLDN6 inhibited CRC proliferation. We found that low expression of CLDN6 was associated with pathological grade and prognosis of CRC patients, and confirmed that CLDN6 inhibited CRC proliferation dependent on p53. Mechanically, we elucidated that CLDN6 regulated ubiquitination to enhance p53 stability and nuclear import by PTEN/AKT/MDM2 pathway. Through the PDZ-binding motif (PBM), CLDN6 bound to ZO-1 to interact with PTEN, and regulate AKT/MDM2 pathway. Collectively, our data enriched the theoretical basis for CLDN6 as a potential biomarker for diagnosis, therapy and prognosis of CRC.

Transglutaminase-2 is critical for corneal epithelial barrier function via positive regulation of Claudin-1

PURPOSE

Transglutaminase (TG)-2 is a ubiquitous multi-functional protein expressed in all living cells. The purpose of the current study was to investigate the role of TG-2 in corneal barrier function and its potential regulation of epithelial junctional proteins and transcription factors.

METHODS

Corneal barrier function to ions in TG-2-/- and TG-2+/+ mice was assessed by Ussing chamber assay. Hypo-osmolar water or FITC-dextran was applied on top of mouse eyes to evaluate the corneal barrier function to water and macromolecules. Western blots, qPCR and immunofluorescent staining were used to investigate the expression of tight junction proteins in TG-2-/- and TG-2+/+ mouse corneas, and also in TG-2 knockdown human corneal epithelial cells.

RESULTS

Corneal explants from TG-2-/- mice had a lower trans-epithelial electrical resistance compared to TG-2+/+ mice. When challenged by hypo-osmolar water, the central corneal thickness of TG-2-/- mice increased faster, and these mice had a faster rise of fluorescence in the anterior chamber after ocular exposure to FITC-dextran, compared to TG-2+/+. Claudin-1 protein and transcript levels were reduced in the cornea of TG-2-/- mice and in TG-2 knockdown human corneal epithelial cells. Slug which previously reported suppressing Claudin-1 transcription, was increased at both protein and transcript level in TG-2 knockdown cells. TG-2 and Claudin-1 protein levels were unchanged in shRNA and shTG cells after MG132 treatment, while Slug accumulated in treated cells.

CONCLUSION

TG-2 may positively regulate Claudin-1 through repressing Slug at transcript level, and thus it is critical for normal corneal barrier function.

Double mutation of claudin‐1 and claudin‐3 causes alopecia in infant mice

Hair follicles (HFs) undergo cyclic phases of growth, regression, and rest in association with hair shafts to maintain the hair coat. Nonsense mutations in the tight junction protein claudin (CLDN)-1 cause hair loss in humans. Therefore, we evaluated the roles of CLDNs in hair retention. Among the 27 CLDN family members, CLDN1, CLDN3, CLDN4, CLDN6, and CLDN7 were expressed in the inner bulge layer, isthmus, and sebaceous gland of murine HFs. Hair phenotypes were observed in Cldn1 weaker knockdown and Cldn3-knockout (Cldn1^Δ/Δ Cldn3^-/- ) mice. Although hair growth was normal, Cldn1^Δ/Δ Cldn3^-/- mice showed striking hair loss in the first telogen. Simultaneous deficiencies in CLDN1 and CLDN3 caused abnormalities in telogen HFs, such as an aberrantly layered architecture of epithelial cell sheets in bulges with multiple cell layers, mislocalization of bulges adjacent to sebaceous glands, and dilated hair canals. Along with the telogen HF abnormalities, which shortened the hair retention period, there was an enhanced proliferation of the epithelium surrounding HFs in Cldn1^Δ/Δ Cldn3^-/- mice, causing accelerated hair regrowth in adults. Our findings suggested that CLDN1 and CLDN3 may regulate hair retention in infant mice by maintaining the appropriate layered architecture of HFs, a deficiency of which can lead to alopecia.

CLDN6 inhibits breast cancer metastasis through WIP-dependent actin cytoskeleton-mediated autophagy

BACKGROUND

As a breast cancer suppressor gene, CLDN6 overexpression was found to inhibit breast cancer metastasis in our previous studies, but the specific mechanism remains unclear. This study aimed to clarify the role and mechanism of CLDN6 in inhibiting breast cancer metastasis.

METHODS

Western blot, immunofluorescence and transmission electron microscopy were performed to detect autophagy. Wound healing, transwell assays and lung metastasis mouse models were used to examine breast cancer metastasis. Phalloidin staining and immunofluorescent staining were used to observe actin cytoskeleton. mRNA seq, RT-PCR, western blot, chromatin immunoprecipitation, dual luciferase reporter assay, co-immunoprecipitation and immunofluorescence were performed to define the molecular mechanism. The expression levels and clinical implication of CLDN6, WIP and LC3 in breast cancer tissues were evaluated using immunohistochemistry.

RESULTS

We demonstrated that CLDN6 inhibited breast cancer metastasis through autophagy in vitro and vivo. We unraveled a novel mechanism that CLDN6 regulated autophagy via WIP-dependent actin cytoskeleton assembly. Through its PDZ-binding motif, overexpressed CLDN6 interacted with JNK and upregulated JNK/c-Jun pathway. C-Jun promoted WIP expression at the transcriptional level. Notably, we observed c-Jun transcriptionally upregulated CLDN6 expression, and there was a positive feedback loop between CLDN6 and JNK/c-Jun. Finally, we found that CLDN6, WIP and LC3 expression correlated with each other, and WIP expression was significantly associated with lymph node metastasis of breast cancer patients.

CONCLUSIONS

The data provide a new insight into the inhibitory effects of CLDN6-mediated autophagy on breast cancer metastasis, and revealed the new mechanism of CLDN6 regulating autophagy through WIP-dependent actin cytoskeleton. Our findings enrich the theoretical basis for CLDN6 as a potential biomarker for breast cancer diagnosis and therapy.

CLDN6 Suppresses c-MYC-Mediated Aerobic Glycolysis to Inhibit Proliferation by TAZ in Breast Cancer

Claudin 6 (CLDN6) was found to be a breast cancer suppressor gene, which is lowly expressed in breast cancer and inhibits breast cancer cell proliferation upon overexpression. However, the mechanism by which CLDN6 inhibits breast cancer proliferation is unclear. Here, we investigated this issue and elucidated the molecular mechanisms by which CLDN6 inhibits breast cancer proliferation. First, we verified that CLDN6 was lowly expressed in breast cancer tissues and that patients with lower CLDN6 expression had a worse prognosis. Next, we confirmed that CLDN6 inhibited breast cancer proliferation through in vitro and in vivo experiments. As for the mechanism, we found that CLDN6 inhibited c-MYC-mediated aerobic glycolysis based on a metabolomic analysis of CLDN6 affecting cellular lactate levels. CLDN6 interacted with a transcriptional co-activator with PDZ-binding motif (TAZ) and reduced the level of TAZ, thereby suppressing c-MYC transcription, which led to a reduction in glucose uptake and lactate production. Considered together, our results suggested that CLDN6 suppressed c-MYC-mediated aerobic glycolysis to inhibit the proliferation of breast cancer by TAZ, which indicated that CLDN6 acted as a novel regulator of aerobic glycolysis and provided a theoretical basis for CLDN6 as a biomarker of progression in breast cancer.

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.

Rules of Engagement: Epithelial-Microbe Interactions and Inflammatory Bowel Disease

Inflammatory bowel diseases (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), are complex, multifactorial disorders that lead to chronic and relapsing intestinal inflammation. The exact etiology remains unknown, however multiple factors including the environment, genetic, dietary, mucosal immunity, and altered microbiome structure and function play important roles in disease onset and progression. Supporting this notion that the gut microbiota plays a pivotal role in IBD pathogenesis, studies in gnotobiotic mice have shown that mouse models of intestinal inflammation require a microbial community to develop colitis. Additionally, antimicrobial therapy in some IBD patients will temporarily induce remission further demonstrating an association between gut microbes and intestinal inflammation. Finally, a dysfunctional intestinal epithelial barrier is also recognized as a key pathogenic factor in IBD. The intestinal epithelium serves as a barrier between the luminal environment and the mucosal immune system and guards against harmful molecules and microorganisms while being permeable to essential nutrients and solutes. Beneficial (i.e., mutualists) bacteria promote mucosal health by strengthening barrier integrity, increasing local defenses (mucin and IgA production) and inhibiting pro-inflammatory immune responses and apoptosis to promote mucosal homeostasis. In contrast, pathogenic bacteria and pathobionts suppress expression and localization of tight junction proteins, cause dysregulation of apoptosis/proliferation and increase pro-inflammatory signaling that directly damages the intestinal mucosa. This review article will focus on the role of intestinal epithelial cells (IECs) and the luminal environment acting as mediators of barrier function in IBD. We will also share some of our translational observations of interactions between IECs, immune cells, and environmental factors contributing to maintenance of mucosal homeostasis, as it relates to GI inflammation and IBD in different animal models.

Visualizing the in-vivo application of zinc in sensitive skin using reflectance confocal microscopy

Findings obtained on objective assessments to evaluate sensitive skin do not correlate well with the symptomatology. We utilized reflectance confocal microscopy (RCM) to compare transepidermal application of zinc in sensitive and non-sensitive skin. Thirty-six subjects participated in this study. They were divided into groups based on lactic acid sting test (LAST):'stinger' and 'non-stinger'; transepidermal water loss (TEWL) measurements; and sensitivity self-assessments: 'sensitive' and 'non-sensitive'. RCM images were taken to visualize transepidermal application of topically-applied zinc. The intensity of zinc reflectance at different depths was measured by ImageJ software. Based on LAST scores, the 'stinger' group showed significantly higher reflectance of zinc at 8 µm (stratum corneum) [face (P < 0.001), forearm (P = 0.004)], and at 80-104 µm (dermo-epidermal junction layer) on the face. High-TEWL group showed increased zinc reflectance at 8-24 µm (tight junction layer, P < 0.001). There were no significant differences amongst subjects self-reporting 'sensitive' and 'non-sensitive' skin. RCM demonstrates that in sensitive skin, there is deeper and higher reflectance of zinc at multiple depths. Structural differences are also visualized. We suggest that RCM is a useful tool for evaluating skin barrier integrity.

Cell-cell junctions: structure and regulation in physiology and pathology

Epithelial and endothelial cell-cell contacts are established and maintained by several intercellular junctional complexes. These structurally and biochemically differentiated regions on the plasma membrane primarily include tight junctions (TJs), and anchoring junctions. While the adherens junctions (AJs) provide essential adhesive and mechanical properties, TJs hold the cells together and form a near leak-proof intercellular seal by the fusion of adjacent cell membranes. AJs and TJs play essential roles in vascular permeability. Considering their involvement in several key cellular functions such as barrier formation, proliferation, migration, survival, and differentiation, further research is warranted on the composition and signaling pathways regulating cell-cell junctions to develop novel therapeutics for diseases such as organ injuries. The current review article presents our current state of knowledge on various cell-cell junctions, their molecular composition, and mechanisms regulating their expression and function in endothelial and epithelial cells.

Claudin-6 is a single prognostic marker and functions as a tumor-promoting gene in a subgroup of intestinal type gastric cancer

BACKGROUND

We aimed to identify novel tumor-promoting drivers highly expressed in gastric cancer (GC) that contribute to worsened prognosis in affected patients.

METHODS

Genes whose expression was increased and correlated with worse prognosis in GC were screened using datasets from the Cancer Genome Atlas and Gene Expression Omnibus. We examined Claudin-6 (CLDN6) immunoreactivity in GC tissues and the effect of CLDN6 on cellular functions in GC cell lines. The mechanisms underlying GC-promoting function of CLDN6 were also investigated.

RESULTS

CLDN6 was identified as a gene overexpressed in GC tumors as compared with adjacent non-tumorous tissues and whose increased expression was positively correlated with worse overall survival of GC patients, particularly those with Lauren's intestinal type GC, in data from multiple publicly available datasets. Additionally, membranous CLDN6 immunoreactivity detected in intestinal type GC tumors was correlated with worse overall survival. In CLDN6-expressing GC cells, silencing of CLDN6 inhibited cell proliferation and migration/invasion abilities, possibly via suppressing transcription of YAP1 and its downstream transcriptional targets at least in part.

CONCLUSIONS

This study identified CLDN6 as a GC-promoting gene, suggesting that CLDN6 to be a possible single prognostic marker and promising therapeutic target for a subset of GC patients.

Dichotomous roles of claudins as tumor promoters or suppressors: lessons from knockout mice

Claudins are a family of integral tight junction proteins that regulate paracellular permeability in polarized epithelia. Overexpression or reduction of claudins can both promote and limit cancer progression, revealing complex dichotomous roles for claudins depending on cellular context. In contrast, recent studies demonstrating tumor formation in claudin knockout mouse models indicate a role for several claudin family members in suppressing tumor initiation. For example, intestine-specific claudin-7 knockout mice spontaneously develop atypical hyperplasia and intestinal adenomas, while claudin-18 knockout mice develop carcinomas in the lung and stomach. Claudin-4, -11, and -15 knockout mice show increased cell proliferation and/or hyperplasia in urothelium, Sertoli cells, and small intestinal crypts, respectively, possibly a precursor to cancer development. Pathways implicated in both cell proliferation and tumorigenesis include Yap/Taz and insulin-like growth factor-1 receptor (IGF-1R)/Akt pathways, among others. Consistent with the tumor suppressive role of claudins shown in mice, in humans, claudin-low breast cancer has been described as a distinct entity with a poor prognosis, and claudin-18-Rho GTPase activating protein 26 (CLDN18-ARHGAP26) fusion protein as a driver gene aberration in diffuse-type gastric cancer due to effects on RhoA. Paradoxically, claudins have also garnered interest as targets for therapy, as they are sometimes aberrantly expressed in cancer cells, which may or may not promote cancer progression. For example, a chimeric monoclonal antibody which targets cells expressing claudin-18.2 through antibody-dependent cell-mediated cytotoxicity has shown promise in multiple phase II studies. In this review, we focus on new findings supporting a tumor suppressive role for claudins during cancer initiation.

Holocrine Secretion Occurs outside the Tight Junction Barrier in Multicellular Glands: Lessons from Claudin-1-Deficient Mice

Holocrine secretion is a specific mode of secretion involving secretion of entire cytoplasmic materials with remnants of dead cells, as observed in multicellular exocrine glands of reptiles, birds, and mammals. Here, we found that sebaceous glands in mice, representative of multicellular exocrine glands of mammals, exhibit a form of polarized stratified epithelium equipped with tight junctions (TJs), and found that holocrine secretion occurred outside the TJ barriers. Sebaceous glands share characteristics of stratified epithelia with interfollicular epidermis, including basal-layer-restricted cell proliferation, TJ barrier formation at a specific single layer of cells with apico-basolateral plasma membrane polarity, and cell death outside the TJ barrier. Knockout of claudin-1, a transmembrane adhesive protein in TJs, in mice caused leakage of the TJ barrier in sebaceous glands and incomplete degradation of the plasma membrane and nuclei during holocrine secretion. Claudin-1 knockout resulted in the accumulation of incompletely degenerated sebocytes in sebaceous ducts, suggesting that the TJ barrier was necessary for differentiation of holocrine secretion. The redefinition of sebaceous glands as TJ-forming stratified epithelia provides an important framework to understand the molecular mechanism of holocrine secretion.

Bicellular Tight Junctions and Wound Healing

Bicellular tight junctions (TJs) are intercellular junctions comprised of a variety of transmembrane proteins including occludin, claudins, and junctional adhesion molecules (JAMs) as well as intracellular scaffold proteins such as zonula occludens (ZOs). TJs are functional, intercellular structures that form a barrier between adjacent cells, which constantly seals and unseals to control the paracellular passage of molecules. They are primarily present in the epithelial and endothelial cells of all tissues and organs. In addition to their well-recognized roles in maintaining cell polarity and barrier functions, TJs are important regulators of signal transduction, which modulates cell proliferation, migration, and differentiation, as well as some components of the immune response and homeostasis. A vast breadth of research data is available on TJs, but little has been done to decipher their specific roles in wound healing, despite their primary distribution in epithelial and endothelial cells, which are essential contributors to the wound healing process. Some data exists to indicate that a better understanding of the functions and significance of TJs in healing wounds may prove crucial for future improvements in wound healing research and therapy. Specifically, recent studies demonstrate that occludin and claudin-1, which are two TJ component proteins, are present in migrating epithelial cells at the wound edge but are absent in chronic wounds. This indicates that functional TJs may be critical for effective wound healing. A tremendous amount of work is needed to investigate their roles in barrier function, re-epithelialization, angiogenesis, scar formation, and in the interactions between epithelial cells, endothelial cells, and immune cells both in the acute wound healing process and in non-healing wounds. A more thorough understanding of TJs in wound healing may shed new light on potential research targets and reveal novel strategies to enhance tissue regeneration and improve wound repair.

Loss of ATP2A2 Allows Herpes Simplex Virus 1 Infection of a Human Epidermis Model by Disrupting Innate Immunity and Barrier Function

Destruction of epidermal barrier function associated with atopic dermatitis or Darier's disease often causes severe secondary skin infections. Patients with skin barrier disorders often repeatedly acquire Kaposi varicelliform eruption, which is caused by herpes simplex virus, but the underlying mechanisms and effective preventive methods have yet to be found. Viral infection through an impaired epidermal barrier can be prevented by enhancing innate immunity and/or inhibiting viral entry. In this study, we established a three-dimensional skin barrier dysfunction model by silencing ATP2A2, which is mutated in some Darier's disease patients. We confirmed the loss of desmosomes and presence of histopathological clefts in the suprabasal layer. Herpes simplex virus 1 applied to the stratum corneum infected the deep epidermis. An innate immune reaction was assessed by evaluating the expression of IFNB1 and related genes. Pretreatment with polyinosinic-polycytidylic acid alone or plus the antimicrobial peptide, LL37 enhanced IFN-β production and suppressed viral replication. Furthermore, topical application of a white petrolatum ointment containing heparin, which binds viral glycoproteins related to virus entry, strongly inhibited viral replication, probably by inhibiting invasion. Our human barrier-dysfunctional model will have future application for identifying the mechanism of Kaposi varicelliform eruption onset, preventive methods, and therapies.

The impact of aging on epithelial barriers

The epithelium has many critical roles in homeostasis, including an essential responsibility in establishing tissue barriers. In addition to the fundamental role in separating internal from external environment, epithelial barriers maintain nutrient, fluid, electrolyte and metabolic waste balance in multiple organs. While, by definition, barrier function is conserved, the structure of the epithelium varies across organs. For example, the skin barrier is a squamous layer of cells with distinct structural features, while the lung barrier is composed of a very thin single cell to minimize diffusion space. With the increased focus on age-dependent alterations in organ structure and function, there is an emerging interest in the impact of age on epithelial barriers. This review will focus on the impact of aging on the epithelial barrier of several organs, including the skin, lung, gastrointestinal tract and the kidney, at a structural and functional level.

Scaffolding proteins in the development and maintenance of the epidermal permeability barrier

The skin of mammals and other terrestrial vertebrates protects the organism against the external environment, preventing heat, water and electrolyte loss, as well as entry of chemicals and pathogens. Impairments in the epidermal permeability barrier function are associated with the genesis and/or progression of a variety of pathological conditions, including genetic inflammatory diseases, microbial and viral infections, and photodamage induced by UV radiation. In mammals, the outside-in epidermal permeability barrier is provided by the joint action of the outermost cornified layer, together with assembled tight junctions in granular keratinocytes found in the layers underneath. Tight junctions serve as both outside-in and inside-out barriers, and impede paracellular movements of ions, water, macromolecules and microorganisms. At the molecular level, tight junctions consist of integral membrane proteins that form an extracellular seal between adjacent cells, and associate with cytoplasmic scaffold proteins that serve as links with the actin cytoskeleton. In this review, we address the roles that scaffold proteins play specifically in the establishment and maintenance of the epidermal permeability barrier, and how various pathologies alter or impair their functions.

Time- and dose-dependent claudin contribution to biological functions: Lessons from claudin-1 in skin

There are many types of biologic compartments in the body. Tight junctions (TJs) function to create a dynamic paracellular barrier to separate these compartments and maintain homeostasis. The TJ component claudin-1 is the major transmembrane protein responsible for forming the paracellular barrier in the epidermis; other claudins in the epidermis include claudin-3, -4, -12, -23 and -25. Accumulating evidence points to a relationship between claudin-1 and skin diseases; for example, a decrease in claudin-1 is reported in human atopic dermatitis. However, how claudin-1 directly or indirectly contributes to disease in the context of the paracellular barrier is poorly understood. We recently established several mouse lines in which the claudin-1 expression was systematically regulated, and showed that claudin-1 time- and dose-dependently regulates epidermis function and disease in vivo. In this commentary, we will discuss recent progress on this topic, including our latest findings, and remaining or newly arisen issues.

Intestinal Epithelial Toll-Like Receptor 4 Signaling Affects Epithelial Function and Colonic Microbiota and Promotes a Risk for Transmissible Colitis

Evidence obtained from gene knockout studies supports the role of Toll-like receptor 4 (TLR4) in intestinal inflammation and microbiota recognition. Increased epithelial TLR4 expression is observed in patients with inflammatory bowel disease. However, little is known of the effect of increased TLR4 signaling on intestinal homeostasis. Here, we examined the effect of increased TLR4 signaling on epithelial function and microbiota by using transgenic villin-TLR4 mice that overexpress TLR4 in the intestinal epithelium. Our results revealed that villin-TLR4 mice are characterized by increases in the density of mucosa-associated bacteria and bacterial translocation. Furthermore, increased epithelial TLR4 signaling was associated with an impaired epithelial barrier, altered expression of antimicrobial peptide genes, and altered epithelial cell differentiation. The composition of the colonic luminal and mucosa-associated microbiota differed between villin-TLR4 and wild-type (WT) littermates. Interestingly, WT mice cohoused with villin-TLR4 mice displayed greater susceptibility to acute colitis than singly housed WT mice did. The results of this study suggest that epithelial TLR4 expression shapes the microbiota and affects the functional properties of the epithelium. The changes in the microbiota induced by increased epithelial TLR4 signaling are transmissible and exacerbate dextran sodium sulfate-induced colitis. Together, our findings imply that host innate immune signaling can modulate intestinal bacteria and ultimately the host's susceptibility to colitis.

Could tight junctions regulate the barrier function of the aged skin?

The skin is known to be the largest organ in human organism creating interface with outer environment. The skin provides protective barrier against pathogens, physical and chemical insults, and against uncontrolled loss of water. The barrier function was primarily attributed to the stratum corneum (SC) but recent studies confirmed that epidermal tight junctions (TJs) also play important role in maintaining barrier properties of the skin. Independent observations indicate that barrier function and its recovery is impaired in aged skin. However, trans-epidermal water loss (TEWL) values remains rather unchanged in elderly population. UV radiation as major factor of photoageing impairs TJ proteins, but TJs have great self-regenerative potential. Since it may be possible that TJs can compensate TEWL in elderly due to its regenerative and compensatory capabilities, important question remains to be answered: how are TJs regulated during skin ageing? This review provides an insight into TJs functioning as epidermal barrier and summarizes current knowledge about the impact of ageing on the barrier function of the skin and epidermal TJs.

Characterization of the first-in-class T-cell-engaging bispecific single-chain antibody for targeted immunotherapy of solid tumors expressing the oncofetal protein claudin 6

The fetal tight junction molecule claudin 6 (CLDN6) is virtually absent from any normal tissue, whereas it is aberrantly and frequently expressed in various cancers of high medical need. We engineered 6PHU3, a T-cell-engaging bispecific single chain molecule (bi-(scFv)₂) with anti-CD3/anti-CLDN6 specificities, and characterized its pharmacodynamic properties. Our data show that upon engagement by 6PHU3, T cells strongly upregulate cytotoxicity and activation markers, proliferate and acquire an effector phenotype. 6PHU3 exerts potent killing of cancer cells in vitro with EC₅₀ values in the pg/mL range. Subcutaneous xenograft tumors in NSG mice engrafted with human PBMCs are eradicated by 6PHU3 treatment and survival of mice is significantly prolonged. Tumors of 6PHU3-treated mice are strongly infiltrated with activated CD4⁺, CD8⁺ T cells and T_EM type cells but not T_regs and display a general activation of a mostly inflammatory phenotype. These effects are only observed upon bispecific but not monospecific engagement of 6PHU3. Together with the exceptionally cancer cell selective expression of the oncofetal tumor marker CLDN6, this provides a safeguard with regard to toxicity. In summary, our data shows that the concept of T-cell redirection combined with that of highly selective targeting of CLDN6-positive solid tumors is effective. Thus, exploring 6PHU3 for clinical therapy is warranted.

Placental claudin expression and its regulation by endogenous sex steroid hormones

Tight junctions (TJs) form continuous intercellular contacts controlling the paracellular transportation across the cell-to-cell junction. TJ components include the peripheral protein zonula occludens-1 (ZO-1), junctional adhesion molecules (JAMs), and integral proteins such as occludin and claudins. Among the junction proteins, claudins play a major role in regulation of paracellular electrolyte transportation. This study explores the expression and distribution of tight junctions and their regulation during pregnancy. To study the regulation of claudin family, we examined expression of mouse placental tight junction proteins, including claudin-1 to -24, with real-time PCR and Western blotting and distribution of tight junction proteins with immunohistochemistry. Pregnant C57/BL6 mice were used in this study. The pregnant mice were divided into three groups depending on pregnant day (on days 12, 16, and 20 of gestation). Regarding the transcription levels, claudin-1, claudin-2, claudin-4, and claudin-5 expression levels were relatively high compared to other claudin family in all periods of pregnancy. Claudin-4 and 5 expressions, which reduce ion permeability, were increased over a period of time. However, claudin-2 expression, that is the responsive protein for a decrease in paracellular conductance, was decreased. Following this modulation of expression during mid-term pregnancy, we identified endogenous hormonal modulation of claudin family using estrogen receptor antagonist ICI 182,780 and progesterone receptor antagonist RU-486. After administration of ICI and RU-486, expression of claudin-4 mRNA and protein was increased. In addition, immunohistochemistry was performed to identify their localization for inferring permeability in placenta. Due to the function of claudins as effectors of ion transport at the end of regulatory pathways, they must be transducing proteins that modulate the function of claudins and thus link the physiologic inputs to the final effectors. This study will provide the claudin expressions and their localization in the mouse placenta, and their regulation by endogenous hormones. Taken together, the results of this study may contribute to assuming the roles and regulatory mechanism of these tight junction genes regarding maternal-fetal ion transportation in the placenta.

A conditional transgenic mouse line for targeted expression of the stem cell marker LGR5

LGR5 is a known marker of embryonic and adult stem cells in several tissues. In a mouse model, Lgr5+ cells have shown tumour-initiating properties, while in human cancers, such as basal cell carcinoma and colon cancer, LGR5 expression levels are increased: however, the effect of increased LGR5 expression is not fully understood. To study the effects of elevated LGR5 expression levels we generated a novel tetracycline-responsive, conditional transgenic mouse line expressing human LGR5, designated TRELGR5. In this transgenic line, LGR5 expression can be induced in any tissue depending on the expression pattern of the chosen transcriptional regulator. For the current study, we used transgenic mice with a tetracycline-regulated transcriptional transactivator linked to the bovine keratin 5 promoter (K5tTA) to drive expression of LGR5 in the epidermis. As expected, expression of human LGR5 was induced in the skin of double transgenic mice (K5tTA;TRELGR5). Inducing LGR5 expression during embryogenesis and early development resulted in macroscopically and microscopically detectable phenotypic changes, including kink tail, sparse fur coat and enlarged sebaceous glands. The fur and sebaceous gland phenotypes were reversible upon discontinued expression of transgenic LGR5, but this was not observed for the kink tail phenotype. There were no apparent phenotypic changes if LGR5 expression was induced at three weeks of age. The results demonstrate that increased expression of LGR5 during embryogenesis and the neonatal period alter skin development and homeostasis.

Epidermal tight junctions in health and disease

The skin, the largest organ of the body, is an essential barrier that under homeostatic conditions efficiently protects and/or minimizes damage from both environmental (e.g. microorganisms, physical trauma, ultraviolet radiation) and endogenous (e.g., cancers, inflammation) factors. This formidable barrier function resides mainly in the epidermis, a dynamic, highly-stratified epithelium. The epidermis has 2 major barrier structures: stratum corneum, the outmost layer and tight junctions, intercellular junctions that seal adjacent keratinocytes in the stratum granulosum, found below the stratum corneum. In recent years there have been significant advances in our understanding of tight junction function, composition and regulation. Herein we review what is known about tight junctions in healthy skin and keratinocyte culture systems and highlight the dynamic crosstalk observed between tight junctions and the cutaneous immune system. Finally we discuss the preliminary observations suggesting that tight junction function or protein expression may be relevant for the pathogenesis of a number of common cutaneous inflammatory and neoplastic conditions.

Cell adhesion in epidermal development and barrier formation

Cell-cell adhesions are necessary for structural integrity and barrier formation of the epidermis. Here, we discuss insights from genetic and cell biological studies into the roles of individual cell-cell junctions and their composite proteins in regulating epidermal development and function. In addition to individual adhesive functions, we will discuss emerging ideas on mechanosensation/transduction of junctions in the epidermis, noncanonical roles for adhesion proteins, and crosstalk/interdependencies between the junctional systems. These studies have revealed that cell adhesion proteins are connected to many aspects of tissue physiology including growth control, differentiation, and inflammation.

Epidermal cell junctions and their regulation by p63 in health and disease

As the outermost tissue of the body, the epidermis is the first physical barrier for any pressure, stress or trauma. Several specialized cell-matrix and cell-cell adhesion structures, together with an intracellular network of dedicated intermediate filaments, are required to confer critical resilience to mechanical stress. The transcription factor p63 is a master regulator of gene expression in the epidermis and in other stratified epithelia. It has been extensively demonstrated that p63 positively controls a large number of tissue-specific genes, including those encoding a large fraction of tissue-restricted cell adhesion molecules. Consistent with p63 functions in cell adhesion and in epidermal differentiation, heterozygous mutations clustered mainly in the p63 C-terminus are causative of AEC syndrome, an autosomal dominant disorder characterized by cleft palate, ankyloblepharon and ectodermal dysplasia associated with severe skin erosions, bleeding and infections. The molecular basis of skin erosions in AEC patients is not fully understood, although defects in desmosomes and in other cell junctions are likely to be involved. Here, we provide an extensive review of the different epidermal cell junctions that cooperate to withstand mechanical stress and on the mechanisms by which p63 regulates gene expression of their components in healthy skin and in AEC syndrome. Collectively, advancement in understanding the molecular mechanisms by which epidermal cell junctions precisely exert their functions and how p63 orchestrates their coordinated expression, will ultimately lead to insight into developing future strategies for the treatment of AEC syndrome and more in generally for diseases that share an overlapping phenotype.

Epidermal tight junction barrier function is altered by skin inflammation, but not by filaggrin-deficient stratum corneum

BACKGROUND

The tight junction (TJ) barrier is located in the granular layer of the epidermis. Filaggrin deficiency predisposes patients to atopic dermatitis (AD) by impairing stratum corneum (SC) barrier function. Altered TJ barrier function has been observed in the skin of patients with AD; however, it remains unclear whether TJ function is influenced by filaggrin deficiency directly or secondarily via skin inflammation.

OBJECTIVE

To investigate the in vivo effects of filaggrin deficiency and skin inflammation on epidermal TJ function.

METHODS

Morphological changes in the TJ were investigated in filaggrin knockout mice and mice with hapten-induced dermatitis using en face visualization of epidermal sheets, and functional changes in the TJ were assessed with an in vivo permeation assay using tracers of various sizes.

RESULTS

In filaggrin knockout mice, there was no apparent change in the honeycomb morphology of the TJ, TJ component mRNA expression, or TJ barrier function in neonates and adults, indicating that filaggrin-deficiency had no direct effects on the TJ. By contrast, in mice with hapten-induced dermatitis, the mRNA expression of TJ components was decreased markedly and the TJ barrier function was size-dependently impaired: the TJ leaked small tracers (<5 kDa), but not large tracers (>30 kDa).

CONCLUSION

Filaggrin deficiency did not affect the epidermal TJ barrier directly, but once dermatitis occurred, the skin inflammation induced TJ dysfunction. Since TJ dysfunction induces the SC barrier impairment, skin inflammation will enhance skin permeability to external antigens and result in a vicious cycle of barrier dysfunction and skin inflammation.

Effects of in utero exposure of C57BL/6J mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin on epidermal permeability barrier development and function

BACKGROUND

Development of the epidermal permeability barrier (EPB) is essential for neonatal life. Defects in this barrier are found in many skin diseases such as atopic dermatitis.

OBJECTIVE

We investigated the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the development and function of the EPB.

METHODS

Timed-pregnant C57BL/6J mice were gavaged with corn oil or TCDD (10 μg/kg body weight) on gestation day 12. Embryos were harvested on embryonic day (E) 15, E16, E17, and postnatal day (PND) 1.

RESULTS

A skin permeability assay showed that TCDD accelerated the development of the EPB, beginning at E15. This was accompanied by a significant decrease in transepidermal water loss (TEWL), enhanced stratification, and formation of the stratum corneum (SC). The levels of several ceramides were significantly increased at E15 and E16. PND1 histology revealed TCDD-induced acanthosis and epidermal hyperkeratosis. This was accompanied by disrupted epidermal tight junction (TJ) function, with increased dye leakage at the terminal claudin-1-staining TJs of the stratum granulosum. Because the animals did not have enhanced rates of TEWL, a commonly observed phenotype in animals with TJ defects, we performed tape-stripping. Removal of most of the SC resulted in a significant increase in TEWL in TCDD-exposed PND1 pups compared with their control group.

CONCLUSIONS

These findings demonstrate that in utero exposure to TCDD accelerates the formation of an abnormal EPB with leaky TJs, warranting further study of environmental exposures, epithelial TJ integrity, and atopic disease.

The antimicrobial protein S100A7/psoriasin enhances the expression of keratinocyte differentiation markers and strengthens the skin's tight junction barrier

BACKGROUND

S100A7/psoriasin is a member of the S100 protein family and is encoded in the epidermal differentiation complex, which contains genes for markers of epidermal differentiation. S100A7/psoriasin is overexpressed in hyperproliferative skin diseases, where it is believed not only to exhibit antimicrobial functions, but also to induce immunomodulatory activities, including chemotaxis and cytokine/chemokine production.

OBJECTIVES

To evaluate the effect of S100A7/psoriasin on keratinocyte differentiation and regulation of the tight junction (TJ) barrier.

METHODS

Expression of differentiation markers and TJ proteins in human keratinocytes was determined by real-time polymerase chain reaction and Western blot. The changes in TJ barrier function were assessed by transepithelial electrical resistance and paracellular permeability assays. Glycogen synthase kinase-3 (GSK-3) and mitogen-activated protein kinase (MAPK) activation was analysed by Western blot, whereas β-catenin and E-cadherin activation was evaluated by Western blot and immunofluorescence.

RESULTS

S100A7/psoriasin enhanced the expression of several differentiation markers and selectively increased the expression of TJ proteins (e.g. claudins and occludin), which are known to strengthen the TJ barrier. Furthermore, S100A7/psoriasin increased β-catenin and E-cadherin accumulation at cell-cell contact, and enhanced transepithelial electrical resistance while reducing the paracellular permeability of keratinocyte layers. The data suggest that S100A7/psoriasin-mediated regulation of the TJ barrier was via both the GSK-3 and MAPK pathways, as evidenced by the inhibitory effects of inhibitors for GSK-3 and MAPKs.

CONCLUSIONS

Our finding that S100A7/psoriasin regulates differentiation and strengthens TJ barrier function provides novel evidence that, in addition to antimicrobial and immunoregulatory activities, S100A7/psoriasin is involved in skin innate immunity.

Cell-cell adhesions and cell contractility are upregulated upon desmosome disruption

Desmosomes are perturbed in a number of disease states – including genetic disorders, autoimmune and bacterial diseases. Here, we report unexpected changes in other cell-cell adhesion structures upon loss of desmosome function. We found that perturbation of desmosomes by either loss of the core desmosomal protein desmoplakin or treatment with pathogenic anti-desmoglein 3 (Dsg3) antibodies resulted in changes in adherens junctions consistent with increased tension. The total amount of myosin IIA was increased in desmoplakin-null epidermis, and myosin IIA became highly localized to cell contacts in both desmoplakin-null and anti-Dsg3-treated mouse keratinocytes. Inhibition of myosin II activity reversed the changes to adherens junctions seen upon desmosome disruption. The increased cortical myosin IIA promoted epithelial sheet fragility, as myosin IIA-null cells were less susceptible to disruption by anti-Dsg3 antibodies. In addition to the changes in adherens junctions, we found a significant increase in the expression of a number of claudin genes, which encode for transmembrane components of the tight junction that provide barrier function. These data demonstrate that desmosome disruption results in extensive transcriptional and posttranslational changes that alter the activity of other cell adhesion structures.

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.

Mice deficient for the epidermal dermokine β and γ isoforms display transient cornification defects

Expression of the human dermokine gene (DMKN) leads to the production of four dermokine isoform families. The secreted α, β and γ isoforms have an epidermis-restricted expression pattern, with Dmkn β and γ being specifically expressed by the granular keratinocytes. The δ isoforms are intracellular and ubiquitous. Here, we performed an in-depth characterization of Dmkn expression in mouse skin and found an expression pattern that was less complex than in humans. In particular, mRNA coding for the δ family were absent. Homozygous mice null for the Dmkn β and γ isoforms had no obvious phenotype but only a temporary scaly skin during the first week of life. The pups null for the Dmkn β and γ isoforms had smaller keratohyalin granules and their cornified envelopes were more sensitive to mechanical stress. At the molecular level, amounts of profilaggrin and filaggrin monomers were reduced whereas amino acid components of the natural moisturizing factor were increased. In addition, the electrophoretic mobility of involucrin was modified, suggesting post-translational modifications. Finally, the mice null for the Dmkn β and γ isoforms strongly overexpressed Dmkn α. These data are evocative of compensatory mechanisms relevant to the temporary phenotype. Overall, we improved the knowledge of Dmkn expression in mouse and highlighted a role for Dmkn β and γ in cornification.

Claudin-6, -10d and -10e contribute to seawater acclimation in the euryhaline puffer fish Tetraodon nigroviridis

Expression profiles of claudin-6, -10d and -10e in the euryhaline teleost fish Tetraodon nigroviridis revealed claudin-6 in brain, eye, gill and skin tissue, while claudin-10d and -10e were found in brain, gill and skin only. In fishes, the gill and skin are important tissue barriers that interface directly with surrounding water, but these organs generally function differently in osmoregulation. Therefore, roles for gill and skin claudin-6, -10d and -10e in the osmoregulatory strategies of T. nigroviridis were investigated. In the gill epithelium, claudin-6, -10d and -10e co-localized with Na(+)-K(+)-ATPase immunoreactive (NKA-ir) ionocytes, and differences in sub-cellular localization could be observed in hypoosmotic (freshwater, FW) versus hyperosmotic (seawater, SW) environments. Claudin-10d and -10e abundance increased in the gills of fish acclimated to SW versus FW, while claudin-6 abundance decreased in the gills of fish acclimated to SW. Taken together with our knowledge of claudin-6 and -10 function in other vertebrates, data support the idea that in SW-acclimated T. nigroviridis, these claudins are abundant in gill ionocytes, where they contribute to the formation of a Na(+) shunt and 'leaky' epithelium, both of which are characteristic of salt-secreting SW fish gills. Skin claudin-10d and -10e abundance also increased in fish acclimated to SW versus those in FW, but so did claudin-6. In skin, claudin-6 was found to co-localize with NKA-ir cells, but claudin-10d and -10e did not. This study provides direct evidence that the gill epithelium contains salinity-responsive tight junction proteins that are abundant primarily in ionocytes. These same proteins also appear to play a role in the osmoregulatory physiology of the epidermis.

Multiscale mathematical modeling and simulation of cellular dynamical process

Epidermal homeostasis is maintained by dynamic interactions among molecules and cells at different spatiotemporal scales. Mathematical modeling and simulation is expected to provide clear understanding and precise description of multiscaleness in tissue homeostasis under systems perspective. We introduce a stochastic process-based description of multiscale dynamics. Agent-based modeling as a framework of multiscale modeling to achieve consistent integration of definitive subsystems is proposed. A newly developed algorithm that particularly aims to perform stochastic simulations of cellular dynamical process is introduced. Finally we review applications of multiscale modeling and quantitative study to important aspects of epidermal and epithelial homeostasis.

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.

Cartilage-specific overexpression of ERRγ results in Chondrodysplasia and reduced chondrocyte proliferation

While the role of estrogen receptor-related receptor alpha (ERRα) in chondrogenesis has been investigated, the involvement of ERR gamma (ERRγ) has not been determined. To assess the effect of increased ERRγ activity on cartilage development in vivo, we generated two transgenic (Tg) lines overexpressing ERRγ2 via a chondrocyte-specific promoter; the two lines exhibited ∼3 and ∼5 fold increased ERRγ2 protein expression respectively in E14.5 Tg versus wild type (WT) limbs. On postnatal day seven (P7), we observed a 4–10% reduction in the size of the craniofacial, axial and appendicular skeletons in Tg versus WT mice. The reduction in bone length was already present at birth and did not appear to involve bones that are derived via intramembranous bone formation as the bones of the calvaria, clavicle, and the mandible developed normally. Histological analysis of P7 growth plates revealed a reduction in the length of the Tg versus WT growth plate, the majority of which was attributable to a reduced proliferative zone. The reduced proliferative zone paralleled a decrease in the number of Ki67-positive proliferating cells, with no significant change in apoptosis, and was accompanied by large cell-free swaths of cartilage matrix, which extended through multiple zones of the growth plate. Using a bioinformatics approach, we identified known chondrogenesis-associated genes with at least one predicted ERR binding site in their proximal promoters, as well as cell cycle regulators known to be regulated by ERRγ. Of the genes identified, Col2al, Agg, Pth1r, and Cdkn1b (p27) were significantly upregulated, suggesting that ERRγ2 negatively regulates chondrocyte proliferation and positively regulates matrix synthesis to coordinate growth plate height and organization.

Transcriptional profiling of epidermal barrier formation in vitro

BACKGROUND

Barrier function is integral to the health of epithelial tissues. Currently, there is a broad need to develop and improve our knowledge with regard to barrier function for reversal of mild skin irritation and dryness. However, there are few in vitro models that incorporate modulations of both lipids and epidermal differentiation programs for pre-clinical testing to aid in the understanding of barrier health.

OBJECTIVE

We have generated a reconstituted epidermis on a decellularized dermis (DED) and characterized its barrier properties relative to human epidermis in order to determine its utility for modeling barrier formation and repair.

METHODS

We followed the process of epidermal differentiation and barrier formation through immunocytochemistry and transcriptional profiling. We examined barrier functionality through measurements of surface pH, lipid composition, stratum corneum water content, and the ability to demonstrate topical dose-dependent exclusion of surfactant.

RESULTS

Transcriptional profiling of the epidermal model during its formation reveals temporal patterns of gene expression associated with processes regulating barrier function. The profiling is supported by gradual formation and maturation of a stratum corneum and expression of appropriate markers of epidermis development. The model displays a functional barrier and a water gradient between the stratum corneum and viable layers, as determined by confocal Raman spectroscopy. The stratum corneum layer displays a normal acidic pH and an appropriate composition of barrier lipids.

CONCLUSION

The epidermal model demonstrates its utility as an investigative tool for barrier health and provides a window into the transcriptional regulation of multiple aspects of barrier formation.

Papillomavirus E6 oncoprotein up-regulates occludin and ZO-2 expression in ovariectomized mice epidermis

We have studied the expression of the tight junction proteins (TJ) occludin, claudin-1 and ZO-2 in the epidermis of female mice. We observed a peak of expression of these proteins at postnatal day 7 and a decrease in 6 week-old mice to values similar to those found in newborn animals. We explored if the expression of the E6 oncoprotein from high-risk human papilloma virus type 16 (HPV16) in the skin of transgenic female mice (K14E6), altered TJ protein expression in a manner sensitive to ovarian hormones. We observed that in ovariectomized mice E6 up-regulates the expression of occludin and ZO-2 in the epidermis and that this effect was canceled by 17β-estradiol. Progesterone instead induced occludin and ZO-2 over-expression. However, the decreased expression of occludin and ZO-2 induced by 17β-estradiol in the epidermis was not overturned by E6 or progesterone. In addition, we employed MDCK cells transfected with E6, and observed that ZO-2 delocalizes from TJs and accumulates in the cell nuclei due to a decrease in the turnover rate of the protein. These results reinforce the view of 17β-estradiol and E6 as risk factors for the development of cancer through effects on expression and mislocalization of TJ proteins.

Rab3Gap1 mediates exocytosis of Claudin-1 and tight junction formation during epidermal barrier acquisition

Epidermal barrier acquisition during late murine gestation is accompanied by an increase in Akt kinase activity and cJun dephosphorlyation. The latter is directed by the Ppp2r2a regulatory subunit of the Pp2a phosphatase. This was accompanied by a change of Claudin-1 localisation to the cell surface and interaction between Occludin and Claudin-1 which are thought to be required for tight junction formation.

The aim of this study was to determine the nature of the barrier defect caused by the loss of AKT/Ppp2r2a function. There was a paracellular barrier defect in rat epidermal keratinocytes expressing a Ppp2r2a siRNA. In Ppp2r2a knockdown cells, Claudin-1 was located to the cytoplasm and its expression was increased. Inhibiting cJun phosphorylation restored barrier function and plasma membrane localisation of Claudin-1. Expression of the Rab3 GTPase activating protein, Rab3Gap1, was restored in Ppp2r2a siRNA cells when cJun phosphorylation was inhibited. During normal mouse epidermal development, Claudin-1 plasma membrane localisation and Rab3Gap1 cell surface expression were co-incident with Akt activation in mouse epidermis, strongly suggesting a role of Rab3Gap1 in epidermal barrier acquisition. Supporting this hypothesis, siRNA knockdown of Rab3Gap1 prevented plasma membrane Claudin-1 expression and the formation of a barrier competent epithelium. Replacing Rab3Gap1 in Ppp2r2a knockdown cells was sufficient to rescue Claudin-1 transport to the cell surface. Therefore these data suggest Rab3Gap1 mediated exocytosis of Claudin-1 is an important component of epidermal barrier acquisition during epidermal development.

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Barrier acquisition correlates with Ppp2r2a and cell surface Claudin-1 expression.

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Ppp2r2a knockdown results in a paracellular barrier defect.

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Ppp2r2a knockdown prevents cell-surface claudin-1 expression in a c-Jun dependent fashion.

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Barrier rescue by inhibition of c-Jun phosphorylation involves exocytosis and Rab3Gap1.

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Rab3Gap1 is induced during barrier acquisition and is necessary for cell surface claudin-1.

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.