Tight junctions in skin inflammation.

Inflammation of the skin is found after various external stimuli, e.g., UV radiation, allergen uptake, microbial challenge, or contact with irritants, as well as due to intrinsic, not always well-defined, stimuli, e.g., in autoimmune responses. Often, it is also triggered by a combination of both. The specific processes, which mean the kind of cytokines and immune cells involved and the extent of the reaction, depend not only on the trigger but also on the predisposition of the individual. Tight junctions (TJs) in the skin have been shown to form a barrier in the granular cell layer of the epidermis. Furthermore, TJ proteins were found in several additional epidermal layers. Besides barrier function, TJ proteins have been shown to be involved in proliferation, differentiation, cell-cell adhesion, and apoptosis in keratinocytes. In inflamed skin, TJ proteins are often affected. We summarize here the impact of skin inflammation on TJs, e.g., in various forms of dermatitis including atopic dermatitis, in skin infection, and in UV-irradiated skin, and discuss the role of TJs in these inflammatory processes.

Claudin-1 decrease impacts epidermal barrier function in atopic dermatitis lesions dose-dependently.

The transmembrane protein claudin-1 is a major component of epidermal tight junctions (TJs), which create a dynamic paracellular barrier in the epidermis. Claudin-1 downregulation has been linked to atopic dermatitis (AD) pathogenesis but variable levels of claudin-1 have also been observed in healthy skin. To elucidate the impact of different levels of claudin-1 in healthy and diseased skin we determined claudin-1 levels in AD patients and controls and correlated them to TJ and skin barrier function. We observed a strikingly broad range of claudin-1 levels with stable TJ and overall skin barrier function in healthy and non-lesional skin. However, a significant decrease in TJ barrier function was detected in lesional AD skin where claudin-1 levels were further reduced. Investigations on reconstructed human epidermis expressing different levels of claudin-1 revealed that claudin-1 levels correlated with inside-out and outside-in barrier function, with a higher coherence for smaller molecular tracers. Claudin-1 decrease induced keratinocyte-autonomous IL-1ß expression and fostered inflammatory epidermal responses to non-pathogenic Staphylococci. In conclusion, claudin-1 decrease beyond a threshold level results in TJ and epidermal barrier function impairment and induces inflammation in human epidermis. Increasing claudin-1 levels might improve barrier function and decrease inflammation and therefore be a target for AD treatment.

Biphasic influence of Staphylococcus aureus on human epidermal tight junctions.

Bacterial infections (e.g., with Staphylococcus aureus) are serious problems in skin with a compromised barrier, such as in patients with atopic dermatitis. Previously, it was shown that tight junction (TJ) proteins are influenced by staphylococcal infection, and TJ function is impaired after infection of the keratinocyte cell line HaCaT. However, functional studies in cells or models more similar to human skin are missing. Therefore, we investigated bacterial colonialization and infection with live S. aureus in primary human keratinocytes and reconstructed human epidermis (RHE). We show that short-term inoculation results in increased TJ barrier function-which could not be seen in HaCaT cells-hinting at an early protective effect. This is accompanied by occludin phosphorylation and sustained localization of occludin and claudin-4 at cell membranes. Long-term incubation resulted in decreased presence of claudin-1 and claudin-4 at cell membranes and decreased TJ barrier function. The agr regulon of S. aureus plays a role in the increasing but not in the decreasing effect. Proinflammatory cytokines, which are produced as a result of S. aureus inoculation, influence both phases. In summary, we show here that S. aureus can have short-term promoting effects on the TJ barrier, while in the long term it results in disturbance of TJs.

The role of tight junctions in skin barrier function and dermal absorption.

The skin protects our body from external assaults like pathogens, xenobiotics or UV irradiation. In addition, it prevents the loss of water and solutes. To fulfill these important tasks, a complex barrier system has developed which comprises the stratum corneum, tight junctions, the microbiome, the chemical barrier and the immunological barrier. These barriers do not act separately, but influence each other e.g. after external manipulation or in skin diseases. Especially the two mechanical barriers, i.e. stratum corneum and tight junctions, are of great interest for drug delivery, because they are the first interaction partners of drug delivery systems and play the major role in skin absorption. Tight junctions are of special interest, as they are centrally localized in this complex barrier system in the outermost viable layer - the stratum granulosum of the interfollicular epidermis and the companion cell layer of the hair follicle - and because they can react very quickly to stimuli. We summarize here our current knowledge about tight junction barrier function in mammalian interfollicular epidermis and hair follicles, and the interaction of tight junctions with other skin barrier components in health and disease. Furthermore, we discuss their relevance for drug delivery and provide examples for tight junction modulators.