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 NH 3 has pointed to the aquaporins as putative NH 3 -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 bloodbrain-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 NH 4 Cl at pH 8.0, at which pH an increased amount of the ammonia occurs in the form of NH 3 . Taken together with an NH 4 Cl-mediated intracellular alkalization (or lesser acidification) of AQP4-expressing oocytes, these data suggest that NH 3 is able to permeate the pore of AQP4. Exposure to NH 4 Cl increased the membrane currents to a similar extent in uninjected oocytes and in oocytes expressing AQP4, indicating that the ionic NH 4 ؉ did not permeate AQP4.Molecular dynamics simulations revealed partial pore permeation events of NH 3 but not of NH 4 ؉ and a reduced energy barrier for NH 3 permeation through AQP4 compared with that of a cholesterol-containing lipid bilayer, suggesting AQP4 as a favored transmembrane route for NH 3 . Our data propose that AQP4 belongs to the growing list of NH 3 -permeable water channels.

Unraveling the substrate preference of an uncharacterized phylogenetic subgroup in Formate/Nitrite Transporter (FNT) family: Computational studies of anion transport inEscherichia coliFNT homolog

“…Hence, complete permeation of ammonia is less probable in EcYfdC with the neutral His as found for the formic acid substrate (Fig 8). However the energy barrier of 14 kJ/mol for the EcYfdC system with proptonated His is very similar to that observed for bacterial GlpF channel and human AQP4 [33,35], both are members of MIP superfamily. A slightly higher energy barrier is observed for human AQP1 and also for the ammonia gas permeating through POPE bilayer [35].…”

“…In this connection, two classes of membrane channels lend support to this hypothesis. The ammonium transporters AmtB transport ammonia and ammonium ion [21,22,[30][31][32] and several aquaporins have been shown to permeate neutral solutes including ammonia [30,33,34]…”

Organ Distribution of 13N Following Intravenous Injection of [13N]Ammonia into Portacaval-Shunted Rats

“…Although the uptake of [ 13 N]ammonia in several tissues may be similar in normal and PCS rats, the fact that the concentration of circulating ammonia in PCS rats is far greater than in normal rats ensures that long-term nitrogen homeostasis is disrupted in PCS rats, and many organs in PCS rats take up more unlabeled ammonia or its metabolites compared to normal rats. AQP4 may have a role in NH 3 uptake into astrocytes, kidney, and other organs/cells in both normal and PCS rats, since its expression can be influenced by ammonia exposure [26, 38, 39]. This is of special importance in the brain where increased ammonia ingress across the BBB is also reflected in an increase in ammonia ingress into the astrocytes due to the extraordinarily high permeability of the astrocyte cell membranes to NH 4 + [40] and possibly to increased NH 3 uptake across the AQP4 channel.…”

“…Ammonia ingress for many tissues is via transport of NH 4 + , whereas its entry into brain is largely by diffusion of NH 3 (and possibly some NH 4 + ) across the blood-brain barrier (BBB) [2, 10, 12, 14, 19–22]. Recently, the water channel aquaporin 4 (AQP4) that is highly expressed in astrocytic endfeet [23] and is found in many body organs, including kidney, lung, muscle, and testes [24, 25], was identified as an NH 3 -permeable channel [26]. In contrast, the ammonium ion (NH 4 + ) is not AQP4 permeable, and its plasma membrane passage may involve various K + transporters and channels ([26, 27] and references cited therein).…”

“…Recently, the water channel aquaporin 4 (AQP4) that is highly expressed in astrocytic endfeet [23] and is found in many body organs, including kidney, lung, muscle, and testes [24, 25], was identified as an NH 3 -permeable channel [26]. In contrast, the ammonium ion (NH 4 + ) is not AQP4 permeable, and its plasma membrane passage may involve various K + transporters and channels ([26, 27] and references cited therein).…”