Publications

• Purification and functional characterization of aquaporin-8.

  Liu K., Nagase H., Huang C.G., Calamita G., Agre P.

  <h4>Background information</h4>Aquaporins (AQPs) are a family of channels permeable to water and some small solutes. In mammals, 13 members (AQP0-AQP12) have been found. AQP8 is widely distributed in many tissues and organs. Previous studies in frog oocytes suggested that AQP8 was permeable to wat ... >> More

  <h4>Background information</h4>Aquaporins (AQPs) are a family of channels permeable to water and some small solutes. In mammals, 13 members (AQP0-AQP12) have been found. AQP8 is widely distributed in many tissues and organs. Previous studies in frog oocytes suggested that AQP8 was permeable to water, urea and ammonium, but no direct characterization had yet been reported.<h4>Results</h4>We expressed recombinant rAQP8, hAQP8 and mAQP8 (rat, human and mouse AQP8 respectively) in yeast, purified the proteins to homogeneity and reconstituted them into proteoliposomes. Although showing high sequence similarity, AQP8 proteins from the three species had to be purified with different detergents prior to reconstitution. In stopped-flow studies, all three AQP8 proteoliposomes showed water permeability, which was inhibited by mercuric chloride and rescued by 2-mercaptoethanol. rAQP8 and hAQP8 proteoliposomes did not transport glycerol or urea but were permeable to formamide, which was also inhibited by mercuric chloride. In the oocyte transport assay, hAQP8-injected oocytes showed significantly higher [14C]methylammonium uptake than water-injected oocytes.<h4>Conclusions</h4>In the present study, we successfully purified rAQP8, hAQP8 and mAQP8 proteins and characterized their biochemical and biophysical properties. All three AQP8 proteins transport water. rAQP8 and hAQP8 are not permeable to urea or glycerol. Moreover, hAQP8 is permeable to ammonium analogues (formamide and methylammonium). Our results suggest that AQP8 may transport ammonium in vivo and physiologically contribute to the acid-base equilibrium. << Less

  Biol. Cell 98:153-161(2006) [PubMed] [EuropePMC]

  This publication is cited by 2 other entries.

• NH3 and NH4+ permeability in aquaporin-expressing Xenopus oocytes.

  Holm L.M., Jahn T.P., Moeller A.L., Schjoerring J.K., Ferri D., Klaerke D.A., Zeuthen T.

  We have shown recently, in a yeast expression system, that some aquaporins are permeable to ammonia. In the present study, we expressed the mammalian aquaporins AQP8, AQP9, AQP3, AQP1 and a plant aquaporin TIP2;1 in Xenopus oocytes to study the transport of ammonia (NH3) and ammonium (NH4+) under ... >> More

  We have shown recently, in a yeast expression system, that some aquaporins are permeable to ammonia. In the present study, we expressed the mammalian aquaporins AQP8, AQP9, AQP3, AQP1 and a plant aquaporin TIP2;1 in Xenopus oocytes to study the transport of ammonia (NH3) and ammonium (NH4+) under open-circuit and voltage-clamped conditions. TIP2;1 was tested as the wild-type and in a mutated version (tip2;1) in which the water permeability is intact. When AQP8-, AQP9-, AQP3- and TIP2;1-expressing oocytes were placed in a well-stirred bathing medium of low buffer capacity, NH3 permeability was evident from the acidification of the bathing medium; the effects observed with AQP1 and tip2;1 did not exceed that of native oocytes. AQP8, AQP9, AQP3, and TIP2;1 were permeable to larger amides, while AQP1 was not. Under voltage-clamp conditions, given sufficient NH3, AQP8, AQP9, AQP3, and TIP2;1 supported inwards currents carried by NH4+. This conductivity increased as a sigmoid function of external [NH3]: for AQP8 at a bath pH (pH(e)) of 6.5, the conductance was abolished, at pH(e) 7.4 it was half maximal and at pH(e) 7.8 it saturated. NH4+ influx was associated with oocyte swelling. In comparison, native oocytes as well as AQP1 and tip2;1-expressing oocytes showed small currents that were associated with small and even negative volume changes. We conclude that AQP8, AQP9, AQP3, and TIP2;1, apart from being water channels, also support significant fluxes of NH3. These aquaporins could support NH4+ transport and have physiological implications for liver and kidney function. << Less

  Pflugers Arch. 450:415-428(2005) [PubMed] [EuropePMC]

  This publication is cited by 2 other entries.