Passive transport by Aquaporins

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Homo sapiens
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Aquaporins (AQP's) are six-pass transmembrane proteins that form channels in membranes. Each monomer contains a central channel formed in part by two asparagine-proline-alanine motifs (NPA boxes) that confer selectivity for water and/or solutes. The monomers assemble into tetramers. During passive transport by Aquaporins most aquaporins (i.e. AQP0/MIP, AQP1, AQP2, AQP3, AQP4, AQP5, AQP7, AQP8, AQP9, AQP10) transport water into and out of cells according to the osmotic gradient across the membrane. Four aquaporins (the aquaglyceroporins AQP3, AQP7, AQP9, AQP10) conduct glycerol, three aquaporins (AQP7, AQP9, AQP10) conduct urea, and one aquaporin (AQP6) conducts anions, especially nitrate. AQP8 also conducts ammonia in addition to water.
AQP11 and AQP12, classified as group III aquaporins, were identified as a result of the genome sequencing project and are characterized by having variations in the first NPA box when compared to more traditional aquaporins. Additionally, a conserved cysteine residue is present about 9 amino acids downstream from the second NPA box and this cysteine is considered indicative of group III aquaporins. Purified AQP11 incorporated into liposomes showed water transport. Knockout mice lacking AQP11 had fatal cyst formation in the proximal tubule of the kidney. Exogenously expressed AQP12 showed intracellular localization. AQP12 is expressed exclusively in pancreatic acinar cells.
Aquaporins are important in fluid and solute transport in various tissues. During Transport of glycerol from adipocytes to the liver by Aquaporins, glycerol generated by triglyceride hydrolysis is exported from adipocytes by AQP7 and is imported into liver cells via AQP9. AQP1 plays a role in forming cerebrospinal fluid and AQP1, AQP4, and AQP9 appear to be important in maintaining fluid balance in the brain. AQP0, AQP1, AQP3, AQP4, AQP8, AQP9, and AQP11 play roles in the physiology of the hepatobiliary tract.
In the kidney, water and solutes are passed out of the bloodstream and into the proximal tubule via the slit-like structure formed by nephrin in the glomerulus. Water is reabsorbed from the filtrate during its transit through the proximal tubule, the descending loop of Henle, the distal convoluted tubule, and the collecting duct. Aquaporin-1 (AQP1) in the proximal tubule and the descending thin limb of Henle is responsible for about 90% of reabsorption (as estimated from mouse knockouts of AQP1). AQP1 is located on both the apical and basolateral surface of epithelial cells and thus transports water through the epithelium and back into the bloodstream. In the collecting duct epithelial cells have AQP2 on their apical surfaces and AQP3 and AQP4 on their basolateral surfaces to transport water across the epithelium. Vasopressin regulates renal water homeostasis via Aquaporins by regulating the permeability of the epithelium through activation of a signaling cascade leading to the phosphorylation of AQP2 and its translocation from intracellular vesicles to the apical membrane of collecting duct cells.
Here, three views of aquaporin-mediated transport have been annotated: a generic view of transport mediated by the various families of aquaporins independent of tissue type (Passive transport by Aquaporins), a view of the role of specific aquaporins in maintenance of renal water balance (Vasopressin regulates renal water homeostasis via Aquaporins), and a view of the role of specific aquaporins in glycerol transport from adipocytes to the liver (Transport of glycerol from adipocytes to the liver by Aquaporins).

Literature References
PubMed ID Title Journal Year
19096781 Role of aquaporin-7 and aquaporin-9 in glycerol metabolism; involvement in obesity

Hibuse, T, Funahashi, T, Maeda, N

Handb Exp Pharmacol 2009
17513417 Molecular mechanisms of conduction and selectivity in aquaporin water channels

Wang, Y, Tajkhorshid, E

J Nutr 2007
17222168 Regulation and dysregulation of aquaporins in water balance disorders

Kwon, TH, Nielsen, S, Agre, P, Frøkiaer, J

J Intern Med 2007
15924268 The renal plumbing system: aquaporin water channels

Nejsum, LN

Cell Mol Life Sci 2005
18566824 Localization and trafficking of aquaporin 2 in the kidney

Ablimit, A, Takata, K, Hasegawa, T, Matsuzaki, T, Tajika, Y

Histochem Cell Biol 2008
19096775 Regulation of aquaporin-2 trafficking

Beulshausen, S, Klussmann, E, Tamma, G, Rosenthal, W, Valenti, G, Nedvetsky, PI

Handb Exp Pharmacol 2009
18173545 Aquaporins in the hepatobiliary tract. Which, where and what they do in health and disease

Svelto, M, Calamita, G, Portincasa, P, Palasciano, G

Eur J Clin Invest 2008
15901243 Aquaporins: highways for cells to recycle water with the outside world

Calamita, G

Biol Cell 2005
15340377 From structure to disease: the evolving tale of aquaporin biology

Kozono, D, Agre, P, King, LS

Nat Rev Mol Cell Biol 2004
11773613 Aquaporins in the kidney: from molecules to medicine

Marples, D, Knepper, MA, Kwon, TH, Nielsen, S, Agre, P, Frøkiaer, J

Physiol Rev 2002
19448080 Aquaporins: translating bench research to human disease

Verkman, AS

J Exp Biol 2009
19096770 Discovery of the aquaporins and development of the field

Carbrey, JM, Agre, P

Handb Exp Pharmacol 2009
16764946 Aquaporin-7 and glycerol permeability as novel obesity drug-target pathways

Frühbeck, G, Rodriguez, A, Gómez-Ambrosi, J, Catalán, V

Trends Pharmacol Sci 2006
17961083 A current view of the mammalian aquaglyceroporins

Praetorius, J, Rojek, A, Fenton, RA, Nielsen, S, Frøkiaer, J

Annu Rev Physiol 2008
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