DECEMBER 2018 • Vol. 24 No. 10
New York Society of Cosmetic Chemists
A
www.nyscc.org
Aquaporins
quaporins are transmembrane protein structures located in the cell walls of animals, plants, and bacteria that transport water molecules into the cytoplasm of adjacent cells to maintain water homeostasis. Since water is essential for all known life forms, the importance of their discovery cannot be underestimated. Peter Agre’s discovery of aquaporins in the 1990s earned him a 2003 Nobel Prize in Chemistry. Professor Agre’s research proved that aquaporin channels bridge cell membranes and allow water, and sometimes structurally similar types of molecules, to enter the cytoplasm and actually be transported from cell to cell. Diffusion of water through phospholipid bilayers via osmosis occurs at a much slower rate. An aquaporin protein is comprised of six trans-membrane alpha helices tilted in right-hand bundles. They are not arranged as stand-alone monomers, but rather as tetramers (groups of four aquaporin monomers linked together) with a central channel running between them. At this time no one appears to be certain about the role of the central channel.1 There are many types of aquaporins. There are aquaporin-0, 1, 2, 4, 5, and 8, which are channels that permit only the transport of water. Aquaporins-3, 7, 9, and 10 are called aquaglyceroporins and they are water, glycerin (a.k.a. glycerol), and urea channels. Aquaporins-6, 11, and 12 have also been elucidated. Gonen and Walz measured the dimensions of the aquaporin-3 channel and determined it has a pore width of 8-10 ångströms. They also noted hydrophilic molecules ranging between 150-200 ångströms may pass through the channel, but it is completely impermeable to charged species, which protects the critically important electrochemical potential difference of the cell membrane.2
…by Joseph Albanese
Water molecules are actually transported through the aquaporin pores in single file. Brewster provides a good written description of how aquaporins function.3 One can also watch an entertaining and informative two-minute video online entitled “Aquaporins Claymation Project” to see how aquaporins work.4 While aquaporins appear in many types of cells in the human body, the primary one in skin is aquaporin-3. Specifically, aquaporin-3 is an aquaglyceroporin transporting not only water, but also glycerol and urea in epidermal cells. Nothing else can get through the channels. There is a wellaccepted direct correlation between the levels of aquaporin-3 and degree of skin hydration. One proof of this is that aquaporin-3 knockout mice have reduced glycerol levels in epidermis.5 Aquaporin-3 transports water, glycerol, and urea into the skin supporting skin elasticity and lipid synthesis. It is a key factor in maintaining adequate skin hydration. While the epidermis is normally 30% water, deeper dermal tissue is about 70% water. Reduced aquaporin-3 levels can cause xerosis and are also observed in aged and chronic sunexposed skin.6-8 Emollients (silicones, long-chain alcohols, oils, petroleum derivatives, etc.) and other skin conditioning agents may form a water-impermeable occlusive layer on the surface of skin to prevent trans-epidermal water loss (TEWLS) and help keep skin supple and smooth to the touch. Humectants—being hygroscopic—are capable of attracting ambient atmospheric water. Glycerol is an endogenous humectant that pulls water into the stratum corneum creating a reservoir important for elasticity and lipid synthesis. Natural moisturizing factor (NMF)—comprised of amino acids from the (Continued on page 4)
N Y S C C S U P P O R T S E D U C A T I O N N I G H T December 11th • Stage 48, New York, NY