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

Activation of the Ca2+ sensing receptor and the PKC/WNK4 downstream signaling cascade induces incorporation of ZO-2 to tight junctions and its separation from 14-3-3

Zonula occludens-2 (ZO-2) is a tight junction (TJ) cytoplasmic protein, whose localization varies according to cell density and Ca²⁺ in the media. In cells cultured in low calcium (LC), ZO-2 displays a diffuse cytoplasmic distribution, but activation of the Ca²⁺ sensing receptor (CaSR) with Gd³⁺ triggers the appearance of ZO-2 at the cell borders. CaSR downstream signaling involves activation of protein kinase C, which phosphorylates and activates with no lysine kinase-4 that phosphorylates ZO-2 inducing its concentration at TJs. In LC, ZO-2 is protected from degradation by association to 14-3-3 proteins. When monolayers are transferred to normal calcium, the complexes ZO-2/14-3-3ζ and ZO-2/14-3-3σ move to the cell borders and dissociate. The 14-3-3 proteins are then degraded in proteosomes, whereas ZO-2 integrates to TJs. From the plasma membrane residual ZO-2 is endocyted and degradaded in lysosomes. The unique region 2 of ZO-2, and S261 located within a nuclear localization signal, are critical for the interaction with 14-3-3 ζ and σ and for the efficient nuclear importation of ZO-2. These results explain the molecular mechanism through which extracellular Ca²⁺ triggers the appearance of ZO-2 at TJs in epithelial cells and reveal the novel interaction between ZO-2 and 14-3-3 proteins, which is critical for ZO-2 protection and intracellular traffic.

Relationship between G proteins coupled receptors and tight junctions

Tight junctions (TJs) are sites of cell-cell adhesion, constituted by a cytoplasmic plaque of molecules linked to integral proteins that form a network of strands around epithelial and endothelial cells at the uppermost portion of the lateral membrane. TJs maintain plasma membrane polarity and form channels and barriers that regulate the transit of ions and molecules through the paracellular pathway. This structure that regulates traffic between the external milieu and the organism is affected in numerous pathological conditions and constitutes an important target for therapeutic intervention. Here, we describe how a wide array of G protein-coupled receptors that are activated by diverse stimuli including light, ions, hormones, peptides, lipids, nucleotides and proteases, signal through heterotrimeric G proteins, arrestins and kinases to regulate TJs present in the blood-brain barrier, the blood-retinal barrier, renal tubular cells, keratinocytes, lung and colon, and the slit diaphragm of the glomerulus.

Activation of the Ca²+-sensing receptor induces deposition of tight junction components to the epithelial cell plasma membrane

The Ca(2+)-sensing receptor (CaSR) belongs to the G-protein-coupled receptor superfamily and plays essential roles in divalent ion homeostasis and cell differentiation. Because extracellular Ca(2+) is essential for the development of stable epithelial tight junctions (TJs), we hypothesized that the CaSR participates in regulating TJ assembly. We first assessed the expression of the CaSR in Madin-Darby canine kidney (MDCK) cells at steady state and following manipulations that modulate TJ assembly. Next, we examined the effects of CaSR agonists and antagonists on TJ assembly. Immunofluorescence studies indicate that endogenous CaSR is located at the basolateral pole of MDCK cells. Stable transfection of human CaSR in MDCK cells further reveals that this protein co-distributes with β-catenin on the basolateral membrane. Switching MDCK cells from low-Ca(2+) medium to medium containing a normal Ca(2+) concentration significantly increases CaSR expression at both the mRNA and protein levels. Exposure of MDCK cells maintained in low-Ca(2+) conditions to the CaSR agonists neomycin, Gd(3+) or R-568 causes the transient relocation of the tight junction components ZO-1 and occludin to sites of cell-cell contact, while inducing no significant changes in the expression of mRNAs encoding junction-associated proteins. Stimulation of CaSR also increases the interaction between ZO-1 and the F-actin-binding protein I-afadin. This effect does not involve activation of the AMP-activated protein kinase. By contrast, CaSR inhibition by NPS-2143 significantly decreases interaction of ZO-1 with I-afadin and reduces deposition of ZO-1 at the cell surface following a Ca(2+) switch from 5 µM to 200 µM [Ca(2+)]e. Pre-exposure of MDCK cells to the cell-permeant Ca(2+) chelator BAPTA-AM, similarly prevents TJ assembly caused by CaSR activation. Finally, stable transfection of MDCK cells with a cDNA encoding a human disease-associated gain-of-function mutant form of the CaSR increases the transepithelial electrical resistance of these cells in comparison to expression of the wild-type human CaSR. These observations suggest that the CaSR participates in regulating TJ assembly.

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.

Claudins reign: The claudin/EMP/PMP22/γ channel protein family in C. elegans

The claudin family of integral membrane proteins was identified as the major protein component of the tight junctions in all vertebrates. Since their identification, claudins, and their associated pfam00822 superfamily of proteins have been implicated in a wide variety of cellular processes. Claudin homologs have been identified in invertebrates as well, including Drosophila and C. elegans. Recent studies demonstrate that the C. elegans claudins, clc-1-clc- 5, and similar proteins in the greater PMP22/EMP/claudin/voltage-gated calcium channel γ subunit family, including nsy-4, and vab-9, while highly divergent at a sequence level from each other and from the vertebrate claudins, in many cases play roles similar to those traditionally assigned to their vertebrate homologs. These include regulating cell adhesion and passage of small molecules through the paracellular space, channel activity, protein aggregation, sensitivity to pore-forming toxins, intercellular signaling, cell fate specification and dynamic changes in cell morphology. Study of claudin superfamily proteins in C. elegans should continue to provide clues as to how claudin family protein function has been adapted to perform diverse functions at specialized cell-cell contacts in metazoans.

Tight junction properties change during epidermis development

In terrestrial animals, the epidermal barrier transitions from covering an organism suspended in a liquid environment in utero, to protecting a terrestrial animal postnatally from air and environmental exposure. Tight junctions (TJ) are essential for establishing the epidermal permeability barrier during embryonic development and modulate normal epidermal development and barrier functions postnatally. We now report that TJ function, as well as claudin-1 and occludin expression, change in parallel during late epidermal development. Specifically, TJ block the paracellular movement of Lanthanum (La(3+)) early in rat in vivo prenatal epidermal development, at gestational days 18-19, with concurrent upregulation of claudin-1 and occludin. TJ then become more permeable to ions and water as the fetus approaches parturition, concomitant with development of the lipid epidermal permeability barrier, at days 20-21. This sequence is recapitulated in cultured human epidermal equivalents (HEE), as assessed both by ultrastructural studies comparing permeation of large and small molecules and by the standard electrophysiologic parameter of resistance (R), suggesting further that this pattern of development is intrinsic to mammalian epidermal development. These findings demonstrate that the role of TJ changes during epidermal development, and further suggest that the TJ-based and lipid-based epidermal permeability barriers are interdependent.

Tight junctions and differentiation--a chicken or the egg question?

Skin barrier function is indispensable to prevent the uncontrolled loss of water and solutes and to protect the body from external assaults. To fulfil this function, keratinocytes undergo a complex pathway of differentiation that terminates in the formation of the stratum corneum. Additionally, tight junctions (TJs), which are cell-cell junctions localized in the stratum granulosum, are involved in the barrier function of the skin. Important biological and clinical roles of TJs are strongly suggested by altered TJ protein levels and distribution in skin diseases like psoriasis, ichthyosis and atopic dermatitis. Because these skin diseases show alterations in differentiation and TJs, it was suggested that changes in TJs might simply be a consequence of altered differentiation. However, in this viewpoint, we like to argue that the situation is not as simple and depends on the specific microenvironment. We discuss three hypotheses regarding the interplay between TJs/TJ proteins and differentiation: (1) TJs/TJ proteins are influenced by differentiation, (2) differentiation is influenced by TJs/TJ proteins, and (3) TJs/TJ proteins and differentiation are independent of each other. In addition, the concept is introduced that both processes are going on at the same time, which means that while one specific TJ protein/barrier component might be influenced by differentiation, the other may influence differentiation.

Re-assessing K15 as an epidermal stem cell marker

The intermediate filament keratin 15 (K15) is present in variable amounts in various stratified epithelia, but has also been reported to be a stem cell marker in the hair follicle. Using peptide specific antibodies, we evaluated the temporal and spatial distribution pattern of K15 expression/localization during normal epidermal development and initiation of hair follicle formation, and in the injured mature epidermis (e.g., during acute injury and repair and in tumorigenesis). During development, K15 expression is first localized to a subset of epidermal basal cells and the overlying periderm at E12.5, but its expression is seen throughout the basal layer by E15.5 and beyond. In hair follicle morphogenesis, initial peg formation occurs in a K15-null area at E14.5 and as peg elongation proceeds through to the mature hair follicle, K15 expression follows the leading edge with positive cells restricted to the outer root sheath. In an epidermal injury model, K15 is first up-regulated and associated with both the basal and suprabasal layers of the interfollicular epidermis then expression becomes sporadic and down-regulated before a basal layer-specific association is re-established in the repaired epidermis. During tumorigenesis, K15 is first mis-expressed, and is ultimately down-regulated. Our data suggest that K15 protein expression may reflect not only expression in a stem or progenitor cell subpopulation, but also reflects the activity and responsiveness of basal-like cells to loss of homeostasis of the epidermal differentiation program. Thus, the data suggest caution in using K15 alone to delineate epidermal stem cells, and underscore the need for further investigation of K15 and other markers in epidermal cell subpopulations.

CD44 regulates tight-junction assembly and barrier function

Upon barrier disturbance, adult CD44 knockout (KO) mice show delayed recovery of epidermal barrier function. This correlates with the loss of apical polarization of lamellar body (LB) secretion. As tight junctions (TJs) are crucial for barrier function and regulate polarized targeting of vesicles, we hypothesized that CD44 regulates TJs and associated cell polarity complexes, which in turn contributes to altered skin barrier function in CD44 KO mice. We show a delay in embryonic barrier formation associated with a loss of apical LB localization in CD44 KO mice, which correlates with alterations in TJ proteins and Par3. Simultaneously, the activity of Rac1, a major regulator of TJ barrier function, was reduced. Importantly, normalization of barrier function at E18.5 coincided with the recovery of these proteins. Tape-stripping experiments revealed that the loss of CD44 also affected TJ proteins upon induced disturbance of the barrier in adult mice. In CD44 KO keratinocytes, cell polarization and TJ barrier function were impaired. An alteration of differentiation markers was also observed, but was less pronounced than alterations of TJ proteins. Taken together, the results reveal an important function for CD44 in the assembly and function of TJs, suggesting their involvement in the skin barrier phenotype of CD44 KO mice.

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

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

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

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

Insights into the role of the calcium sensing receptor in epidermal differentiation in vivo

While the important role of calcium (Ca(++)) signaling is fundamental in epidermal cell physiology, a detailed knowledge of precisely how epidermal cells respond to Ca(++) levels is not clear. Using peptide-specific antibodies that we generated, we set out to evaluate the temporal and spatial distribution pattern of the Ca(++)-sensing receptor (CaSR) during epidermogenesis and to assess its involvement in the mature epidermis (e.g., in acute injury and tumorigenesis). Our data indicate a developmentally regulated expression of CaSR: up-regulation occurs in specific epidermal cells and cell layers in normal development or in response to injury when epidermal cells are induced to undergo commitment and early differentiation events, and down-regulation occurs in terminal differentiation stages. These results provide a new perspective on the role of the CaSR in these processes and describe a novel tool for evaluating Ca(++)-mediated epidermal differentiation.

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

Background

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

Methods

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

Results

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

Conclusion

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

Claudin immunolocalization in neonatal mouse epithelial tissues

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