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Aquaporins (AQPs) are integral membrane proteins that facilitate the transport of water across biological membranes along an osmotic gradient. There have been 13 AQP isoforms (AQP0-AQP12) identified in humans and rodents to date.
Aquaporins (AQPs) are integral membrane proteins that facilitate the transport of water across biological membranes along an osmotic gradient. AQPs were originally thought to be exclusively permeated by water; however, certain isoforms have been shown to also transport small solutes (glycerol, H2O2), ions (Na+, K+, Cl-) and gases (CO2, ammonia).
There have been 13 AQP isoforms (AQP0-AQP12) identified in humans and rodents to date. AQP0-2, 4-6 and 8 are generally considered to be exclusively water channels, whereas AQP3, 7, 9 and 10 are referred to as aquaglyceroporins, as they are also permeated by glycerol. The function and substrate specificity of AQP11 and 12 are yet to be determined. AQPs usually adopt a tetramer conformation within the cell membrane, with each monomer consisting of a single aqueous pore that is permeable to water. The movement of water and solutes through AQP channels is a passive process that is dependent on the concentration of the transported molecule.
AQPs are relatively ubiquitously expressed, although each isoform has a specific expression pattern that reflects their function. The AQPs are generally localized to the epithelia of tissues that require a high rate of water flux, where they function to regulate fluid homeostasis, secretion and reabsorption. These tissues include the kidney collecting ducts, lung capillaries, salivary glands and lens of the eye. Due to their role in fluid homeostasis, the AQPs have been associated with multiple disorders including kidney dysfunction, the onset of brain edema and loss of vision. For example AQP4 is highly expressed in mammalian astrocytes and has been implicated in the pathogenesis of cerebral edema, with AQP4 inhibition reducing ischemia-induced brain edema in mice.
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