Dryness (xerosis) is a common characteristic of cutaneous ageing; however, the cellular mechanisms that regulate water homeostasis are yet to be explored in human skin. Cell volume regulation is vital for water homeostasis and one such control mechanism involves the movement of organic osmolytes to counteract effects of physiological stressors on cells. Therefore, we hypothesise that increasing age leads to epidermal keratinocyte shrinkage due to a loss of organic osmolyte-mediated cell volume regulation. Skin samples were collected from photoprotected and photoexposed sites of young and aged healthy volunteers. Morphological assessment revealed that epidermal keratinocyte size was significantly reduced with age and photoexposure (p<0.0001). Next, immunofluorescence was used to determine if this was due to dysregulation of osmolyte transporters. This demonstrated downregulation of the myoinositol transporter, SMIT, in photoexposed skin (young p<0.0001 and aged p=0.0002). Similarly, the taurine transporter, TAUT, was downregulated in aged skin (photoprotected p=0.0462 and photoexposed p<0.0001). To corroborate this, skin biopsies were taken from healthy volunteers at baseline and at 1-, 3- and 72-hours following 80mJ/cm2 solar simulated radiation (SSR). Immunofluorescence analysis showed a downregulation of SMIT (p=0.0104) and TAUT (p=0.0054) at 72-hours post-SSR. Next, single-cell live imaging was used to evaluate cell volume regulation of young and aged primary human keratinocytes (NHEKs). Hyperosmotic stress induced cell shrinkage in young and aged NHEKs; however, the presence of organic osmolytes improved the rate of cell recovery. Collectively, we demonstrate a role for organic osmolytes in mediating cell volume regulation of epidermal keratinocytes. Osmolyte transporter expression in skin is affected by extrinsic ageing and more specifically, acute UVR. Together, this suggests that the role of osmolytes in skin hydration could vitally improve our understanding of skin ageing pathophysiology.

Osmolyte-mediated cell volume regulation is dysregulated by ageing and UV exposure in human skin