The roles of organic osmolytes in the response of epidermal tight junctions to environmental stress

• Main Author: El Chami, Cecile
• Other Authors: O'Neill, Catherine
• Published: University of Manchester 2017
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.734240

The preservation of water is critical for terrestrial organisms and the epidermis of skin is a major permeability barrier to water loss from within the body. Epidermal barrier function is provided not only by the stratum corneum but also by the presence of tight junctions (TJs). However, cellular mechanisms of water conservation such as osmolyte accumulation are also important, in particular by helping to maintain cell volume during times of cellular stress. Cellular mechanisms of water homeostasis are largely unexplored in skin and additionally, nothing is known regarding how cellular and extracellular mechanisms may interact. The aim of this study was to investigate the role of organic osmolytes in the control of TJ structure and function in the presence or absence of ultraviolet irradiation (UVR), a major source of water loss in skin. Data obtained from cell culture experiments showed that irradiation of cultured keratinocytes with UVB reduced tight junction (TJ) barrier function, which appeared to be due to dislocalisation of TJ proteins claudin-1 and claudin-4 and phosphorylation of occludin. The presence of organic osmolytes, betaine, taurine or myo-inositol, negated these effects without any change in the gene expression of TJ proteins which suggests that osmolytes affect TJs via a post-translational mechanism which also appears to be independent of effects on cell volume and production of reactive oxygen species (ROS) at least to some extent. Data acquired from human studies showed that human skin expresses the betaine, taurine and myo-inositol transporters and these have transporter specific expression patterns. Moreover, the expression of these transporters is regulated by UVB. Treatment of human skin with osmolytes in an ex vivo organ culture model resulted in increased expression of claudins-1 and -4 but not claudin isoforms -7 and -12. However, these molecules where found to use different mechanisms to induce these effects depending on the osmolyte. Betaine appeared to stabilise existing TJ proteins whereas taurine induced the synthesis of new TJ proteins. This preliminary study shows for the first time that organic osmolytes are not solely important for maintaining intracellular osmolarity and volume homeostasis but also modulate TJ integrity and mitigate the damaging effect of UVB, which could contribute to the barrier property of the epidermis.