Chapter 29: Aquaporin

INTRODUCTION

Aquaporins (AQPs) are integral membrane proteins that form a water channel and facilitate water transport in various organs such as the skin, renal tubules, eyes, the digestive tract, and the brain. In 2003, Peter Agre received the Nobel Prize in chemistry for discovering AQPs. There are 13 isoforms of AQPs found in mammals classified as types 1 to 13. Functionally, they can be classified into two subtypes: AQPs 1, 2, 4, 5, and 8, which only transport water, and AQPs 3, 7, 9, and 10, which can conduct other substances such as glycerol (also known as glycerin) or urea in addition to water.¹ AQP-3 is the predominant water channel found in human epidermis, and is permeable to both water and glycerin. For years scientists have known that glycerin plays a superior role in hydrating skin,² but the reasons for this became more clear when AQP-3 was discovered. Studies have shown that defects in AQP-3 in mice models result in epidermal dryness as well as decreased stratum corneum hydration and glycerin content of the epidermis, followed by reduced elasticity and impaired skin barrier recovery.^3,4 Aquaporin facilitates the transport of water, glycerin, and solutes between keratinocytes.

AQPs are transmembrane structures arranged as homotetramers in the cell membrane. Each subunit of the tetramer consists of six α-helical domains and contains a distinct aqueous pore. The intricate shape and the transmembrane position make it impossible to exogenously add aquaporin to skin as the marketing claims of some cosmetic products imply. Ultraviolet light has been shown to diminish the expression of AQP-3, likely through MAP kinase pathways.⁵ Free radicals have also been demonstrated to decrease AQP-3 expression.⁵ This is one of the reasons that sun-exposed skin becomes dehydrated.⁵ In addition, aquaporin performance can be affected by topical ingredients that can increase the opening and closing of the aquaporin pore or upregulate AQP-3 expression. Ajuga turkestanica has been shown to increase AQP-3 function.⁶ Retinol and the antioxidant n-acetyl cysteine both have been shown to inhibit the downregulation of AQP-3 upon UV exposure.⁵ AQPs are intriguing water channels that allow water and glycerin to flow from cell to cell. In the next few years, many more facets of their dynamic mechanisms of action will be elucidated.

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