TY - JOUR
T1 - Aquaporin 4 as a NH3 Channel
AU - Assentoft, Mette
AU - Kaptan, Shreyas
AU - Schneider, Hans-Peter
AU - Deitmer, Joachim W
AU - de Groot, Bert L
AU - MacAulay, Nanna
N1 - © 2016 by The American Society for Biochemistry and Molecular Biology, Inc.
PY - 2016/9/2
Y1 - 2016/9/2
N2 - 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 bloodbraininterface, 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.
AB - 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 bloodbraininterface, 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.
U2 - 10.1074/jbc.m116.740217
DO - 10.1074/jbc.m116.740217
M3 - Journal article
C2 - 27435677
SN - 0021-9258
VL - 291
SP - 19184
EP - 19195
JO - Journal of Biological Chemistry
JF - Journal of Biological Chemistry
IS - 36
ER -