title: Aquaporin 4 as a NH3 Channel.
abstract: Ammonia is a biologically potent molecule, and the regulation of ammonia levels in the mammalian body is, therefore, strictly controlled. The molecular paths of ammonia permeation across plasma membranes remain ill-defined, but the structural similarity of water and NH3 has pointed to the aquaporins as putative NH3-permeable pores. Accordingly, a range of aquaporins from mammals, plants, fungi, and protozoans demonstrates ammonia permeability. Aquaporin 4 (AQP4) is highly expressed at perivascular glia end-feet in the mammalian brain and may, with this prominent localization at the blood-brain-interface, participate in the exchange of ammonia, which is required to sustain the glutamate-glutamine cycle. Here we observe that AQP4-expressing Xenopus oocytes display a reflection coefficient <1 for NH4Cl at pH 8.0, at which pH an increased amount of the ammonia occurs in the form of NH3 Taken together with an NH4Cl-mediated intracellular alkalization (or lesser acidification) of AQP4-expressing oocytes, these data suggest that NH3 is able to permeate the pore of AQP4. Exposure to NH4Cl increased the membrane currents to a similar extent in uninjected oocytes and in oocytes expressing AQP4, indicating that the ionic NH4 (+) did not permeate AQP4. Molecular dynamics simulations revealed partial pore permeation events of NH3 but not of NH4 (+) and a reduced energy barrier for NH3 permeation through AQP4 compared with that of a cholesterol-containing lipid bilayer, suggesting AQP4 as a favored transmembrane route for NH3 Our data propose that AQP4 belongs to the growing list of NH3-permeable water channels.
authors.affiliation: From the Department of Neuroscience and Pharmacology, University of Copenhagen, 2200 Copenhagen, Denmark.
authors.forename: Mette
authors.initials: M
authors.lastname: Assentoft
authors.affiliation: Computational Biomolecular Dynamics Group, Max Planck Institute for Biophysical Chemistry, 37077 Gottingen, Germany, and.
authors.forename: Shreyas
authors.initials: S
authors.lastname: Kaptan
authors.affiliation: Division of General Zoology, Department of Biology, University of Kaiserslautern, 67653 Kaiserslautern, Germany.
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Schneider
authors.affiliation: Division of General Zoology, Department of Biology, University of Kaiserslautern, 67653 Kaiserslautern, Germany.
authors.forename: Joachim W
authors.initials: JW
authors.lastname: Deitmer
authors.affiliation: Computational Biomolecular Dynamics Group, Max Planck Institute for Biophysical Chemistry, 37077 Gottingen, Germany, and.
authors.forename: Bert L
authors.initials: BL
authors.lastname: de Groot
authors.affiliation: From the Department of Neuroscience and Pharmacology, University of Copenhagen, 2200 Copenhagen, Denmark, macaulay@sund.ku.dk.
authors.forename: Nanna
authors.initials: N
authors.lastname: MacAulay
mesh_terms: D000641:Ammonia / Q000737:chemistry* / Q000378:metabolism*; D000643:Ammonium Chloride / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D051401:Aquaporin 4 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D020346:Aquaporins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D007473:Ion Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017136:Ion Transport / Q000502:physiology; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism; D056004:Molecular Dynamics Simulation; D009865:Oocytes; D051381:Rats; D014982:Xenopus laevis
chemical_list: C496032:Aqp4 protein, rat; D051401:Aquaporin 4; D020346:Aquaporins; D007473:Ion Channels; D008051:Lipid Bilayers; C108330:aquaporin 8; D000643:Ammonium Chloride; D000641:Ammonia
keywords: ammonia; aquaporin 4 (AQP4); membrane protein; molecular dynamics; permeability