The osmolyte strategy of ageing skin

Abstract

Skin presents the most visible signs of ageing which are often physically defined by characteristics such as wrinkling, dryness and hyperpigmentation. As the outermost organ of the human body, skin is continuously exposed to a harsh surrounding environment and this can accelerate the intrinsic ageing process. Although there has been much research into the structural changes of skin with age, the underlying molecular mechanisms of skin ageing remain somewhat unknown. Dryness (xerosis) is a common characteristic of cutaneous ageing. Water homeostasis is controlled by vital mechanisms such as cell volume regulation to maintain normal cell function. However, these mechanisms are yet to be explored in human skin. Organic osmolytes are naturally occurring compounds that can protect against the impact of physiological stress by counteracting changes to osmolarity of the cellular microenvironment. Therefore, the hypothesis is that with age, epidermal keratinocytes undergo cell shrinkage due to disrupted organic osmolyte-mediated cell volume regulation. To address the hypothesis, skin samples from healthy young and aged volunteers were utilised to investigate the impact of both intrinsic and extrinsic ageing on the organic osmolyte strategy of human skin. Primary normal human epidermal keratinocyte (NHEK) cell culture was also used to assess cell volume regulatory mechanisms in real time. Morphological assessment of human skin in vivo demonstrated that human epidermal keratinocytes undergo cell shrinkage with age and UV exposure. Immunofluorescence analysis revealed that the myoinositol transporter, SMIT, is significantly downregulated following chronic and acute UV exposure. Also, the taurine transporter, TAUT, is significantly downregulated with intrinsic and extrinsic ageing as well as following acute-UV exposure. This suggests that both ageing, and UV negatively impact the ability of epidermal keratinocytes to transport organic osmolytes. In addition, single-cell live imaging highlighted that cell volume regulation is slower and less effective in aged NHEKs compared to young NHEKs following hyperosmotic stress. However, the presence of taurine improved the rate of recovery of aged NHEKs. Therefore, the reduced cell volume observed in aged skin in vivo may be, in part, due to a reduced ability to recover cell volume in response to physiological stress. Next, the enzymes involved in betaine, myoinositol and taurine synthesis were characterised in human skin in vivo for the first time. It was demonstrated that the taurine biosynthetic pathway is impacted by UV exposure, demonstrating again, the importance of taurine in the skin and its potential role in maintaining skin health. Collectively, this data demonstrates that ageing and UV exposure significantly impact the organic osmolyte-mediated mechanisms in human skin in vivo. This suggests that increased knowledge the role of organic osmolytes in skin hydration could vitally improve our understanding of skin ageing pathophysiology. In conclusion, the findings of this thesis identify taurine as a novel candidate for anti-ageing strategies moving forward.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Rachel Watson (Supervisor) & Catherine O'Neill (Supervisor)