Cloning and functional expression of a new aquaporin (AQP9) abundantly expressed in the peripheral leukocytes permeable to water and urea, but not to glycerol

A new member (AQP9) of the Aquaporin family was identified from human leukocytes by homology cloning using PCR. A full length clone was obtained by screening human liver cDNA library. AQP9 encodes a 295-amino-acid protein with the amino acid sequence identity with AQP3 (48%), AQP7 (45%), and Other aquaporins (approximately 30%), suggesting that AQP3, AQP7, and AQP9 belong to a subfamily of the Aquaporin family. Injection of AQP9-cRNA into Xenopus oocytes stimulated the osmotic water permeability 7-folds with a low activation energy (4.2 kcal/mol) which was inhibited by 0.3 mM mercury chloride by 48%. AQP9 also facilitated urea transport 4-folds. However, in contrast to AQP3 and AQP7, AQP9 did not stimulate the glycerol permeability, suggesting a unique permeability character. Northern blot analysis revealed the high expression of 3.5-kb messages in peripheral leukocytes >> liver > lung = spleen, but not in thymus. The possible role of AQP9 in the immunological function of leukocytes is intriguing and the identification of AQP9 with unique permeability profile may expand our understanding of water and small solute transport in the body.