The role of tight junctions in skin barrier function

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The epidermis of the skin is a multilayered epithelium which fulfills an indispensable barrier function for the protection of the body against external assaults, e.g. pathogens and environmental compounds, as well as against extensive water loss. On the other hand, this barrier has to be overcome for transdermal drug delivery. Thus, knowledge of the nature of skin barrier function is essential. For decades, this barrier was almost exclusively attributed to the lipid-protein structure of the stratum corneum (SC), the uppermost layer of the skin (SC barrier). However, in the last decade it became evident that tight junctions (TJs) are also present in the epidermis and form a second barrier (TJ barrier) which is localized in the stratum granulosum, the layer beneath the SC. In this project, we want to clarify the contribution of tight junctions to skin barrier function. To this end, we will (A) investigate the skin and TJ barrier to ions and agents of different size and polarity from outside to inside (substances applied to the skin surface) and inside to outside (substances applied from the basal side) in reconstructed human skin at different stages of skin/SC maturation and ex-vivo human and porcine skin. To reveal the contribution of individual TJ components, TJ protein knock-down (claudin-1, claudin-4, JAM-A) and opening of TJs by a claudin-1 peptidomimetic and a claudin-4-specific cCPE (C-terminal domain of Clostridium perfringens enterotoxin) mutant will be applied to these skin models. Most importantly, SC barrier and TJ barrier are by no means independent, but are likely to interact with each other. Thus, a pivotal part of the contribution of TJs to skin barrier function, in addition to being a barrier themselves, might be their influence on SC formation and maintenance. Therefore, the second aim of our project (B) is to elucidate the influence of TJs on the SC. We will test the role of claudin-1, claudin-4 and JAM-A in SC formation in the above-mentioned models and clarify the mechanisms involved. Finally, we want to shed light on the contribution of TJs to the impaired skin barrier in the pathologically altered skin of patients with atopic dermatitis (C). We will investigate lesional, non-lesional and healthy skin. Further, we will analyze lesional skin before, during and after treatment. Alterations of TJ protein amount and localization will be correlated to TJ and skin barrier function as well as disease parameters. Furthermore, the involvement of atopic dermatitis-relevant cytokines will be investigated in vitro.