Molecular cloning and expression of a member of the aquaporin family with permeability to glycerol and urea in addition to water expressed at the basolateral membrane of kidney collecting duct cells.

Abstract

Water transport in highly water-permeable membranes is conducted by water-selective pores-namely, water channels. The recent cloning of water channels revealed the water-selective characteristics of these proteins when expressed in Xenopus oocytes or reconstituted in liposomes. Currently, It is as d that the function of water ch ls is to transport only water. We now report the cloning of a member of the water channel that also transports nonionic small molecules such as urea and glycerol. We named this channel aquaporin 3 (AQP3) for its predominant water permeabilit. AQP3 has amino acid sequence Identity with major intrinsic protein (MIP) family proteins including AQPchannel-forming integral membrane protein, AQP-colecting duct, MIP, AQP-ytonoplast intrinsic protein, nodulin 26, and glycerol facilitator (33-42%). Thus, AQP3 is an additional member of the MEP family. Osmotic water permeability of Xenopus oocytes measured by videomicroscopy was 10-fold higher in oocytes injected with AQP3 transcript than with water-injected oocytes. The increase in osmotic water permeability was inhibited by HgC12, and this effect was reversed by a reducing agent, 2-mercaptoethanol. Although to a smaller degree, AQP3 also facilitated the transport of nonionic small solutes such as urea and glycerol, while the previously cloned water channels are permeable only to water when expressed in Xenopus oocytes. AQP3 mRNA was expressed abundantly in kidney medulla and colon. In kidney, it was exclusively immunokoalzed at the baolateral membrane of collecting duct cells. AQP3 may functon as a water and urea exit hanism in antidlure in ollecting duct cells.Water channels have been postulated for the pathway of selective water permeation in highly water-permeable membranes. Recent

Lack of Aquaporin 3 in bovine erythrocyte membranes correlates with low glycerol permeation

“…After the initial osmotic shrinkage due to glycerol gradients stimulated by fasting, AQP3 expressing erythrocytes would be enabled to rapidly recover their volume, a similar phenomenon hypothesised for urea when red blood cells cross the kidney medulla [33]. AQP3 can also moderately permeate urea although to a lesser extent [1] and this function might also play a role in volume regulation in hRBC. As for bRBC, besides a high expression level of the facilitated urea transporter [34] coexistent with urea permeability similar to the human specie [28], which itself may improve erythrocyte osmotic fragility, the physiological reasoning for the absence of a glycerol transporter remains ambiguous.…”

“…Aquaporins belong to a highly conserved group of membrane proteins called the major intrinsic proteins (MIPs) that are involved in the transport of water and small solutes such as glycerol, nitrate and urea [1,2].…”

“…Compared with other mammalian orthodox aquaporins (namely with AQP1), AQP3 is moderately permeable to water, but highly permeable to glycerol and possibly to urea [1]. AQP3 expression has been reported in several mammalian tissues including kidney, epidermis, urinary, respiratory and digestive tracts [11] and in human erythrocytes [13].…”

Defective water and glycerol transport in the proximal tubules of AQP7 knockout mice

“…At present, at least 13 AQPs have been identified and cloned in mammals (17). Some AQPs are classified as part of the aquaglyceroporin family, the members of which facilitate the transport of glycerol as well as water (7). AQP7 is an aquaglyceroporin that is abundantly expressed in kidney, testis, and adipocytes (6,9).…”

Gene expression profiling to identify markers associated with deregulated hTERT in HPV‐transformed keratinocytes and cervical cancer

“…Of these genes AQP3 was most significantly downregulated, which was confirmed by real time RT PCR in both hTERT positive hybrid cells and cervical SCC. AQP3 is an integral membrane protein that, in addition to water, also transports nonionic small molecules such as urea and glycerol 53 and of which expression has been reported in various normal human tissues. 54 To our knowledge this is the first study showing a downregulation of AQP3 expression in cervical carcinomas.…”