Functional and molecular characterization of the human neutral solute channel aquaporin-9

Abstract

In metabolically active cells, the coordinated transport of water and solutes is important for maintaining osmotic homeostasis. We recently identified a broad selective-neutral solute channel, AQP9, from rat liver that allows the passage of a wide variety of water and neutral solutes (H. Tsukaguchi, C. Shayakul, U. V. Berger, B. Mackenzie, S. Devidas, W. B. Guggino, A. N. van Hoek, and M. A. Hediger. J. Biol. Chem. 273: 24737-24743, 1998). A human homolog (hAQP9) with 76% amino acid sequence identity to rat AQP9 (rAQP9) was described, but its permeability was found to be restricted to water and urea (K. Ishibashi, M. Kuwahara, Y. Gu, Y. Tanaka, F. Marumo, and S. Sasaki. Biochem. Biophys. Res. Commun. 244: 268-274, 1998). Here we report a reevaluation of the functional characteristics of hAQP9, its tissue distribution, the structure of its gene, and its chromosomal localization. When expressed in Xenopus oocytes, hAQP9 allowed passage of a wide variety of noncharged solutes, including carbamides, polyols, purines, and pyrimidines in a phloretin- and mercurial-sensitive manner. These functional characteristics are similar to those of rAQP9. Based on Northern blot analysis, both rat and human AQP9 are abundantly expressed in liver, whereas, in contrast to rAQP9, hAQP9 is also expressed in peripheral leukocytes and in tissues that accumulate leukocytes, such as lung, spleen, and bone marrow. The human AQP9 gene is composed of 6 exons and 5 introns distributed over approximately approximately 25 kb. The gene organization is strikingly similar to that reported for human AQP3 and AQP7, suggesting their evolution from a common ancestral gene. The promoter region contains putative tonicity and glucocorticoid-responsive elements, suggesting that AQP9 may be regulated by osmolality and catabolism. Fluorescence in situ hybridization assigned its locus to chromosome 15 q22.1-22.2. Our data show that hAQP9 serves as a promiscuous solute channel expressed in both liver and peripheral leukocytes, where it is ideally suited to transport of metabolites and/or nutrients into and out of these cells