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.jpg): Failed to open stream: No such file or directory in C:\Inetpub\vhosts\kidney.de\httpdocs\pget.php on line 117 J+Biol+Chem
2016 ; 291
(36
): 19184-95
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Aquaporin 4 as a NH3 Channel
#MMPMID27435677
Assentoft M
; Kaptan S
; Schneider HP
; Deitmer JW
; de Groot BL
; MacAulay N
J Biol Chem
2016[Sep]; 291
(36
): 19184-95
PMID27435677
show ga
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.
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