Relative CO₂/NH₃ selectivities of mammalian aquaporins 0–9

Abstract

Geyer RR, Musa-Aziz R, Qin X, Boron WF. Relative CO2/NH3 selectivities of mammalian aquaporins 0 -9. Am J Physiol Cell Physiol 304: C985-C994, 2013. First published March 13, 2013; doi:10.1152/ajpcell.00033.2013.-Previous work showed that aquaporin 1 (AQP1), AQP4-M23, and AQP5 each has a characteristic CO2/NH3 and CO2/H2O permeability ratio. The goal of the present study is to characterize AQPs 0 -9, which traffic to the plasma membrane when heterologously expressed in Xenopus oocytes. We use video microscopy to compute osmotic water permeability (Pf) and microelectrodes to record transient changes in surface pH (⌬pHS) caused by CO2 or NH3 influx. Subtracting respective values for day-matched, H2O-injected control oocytes yields the channel-specific values Pf* and ⌬pHS*. We find that Pf* is significantly Ͼ0 for all AQPs tested except AQP6. (⌬pHS*)CO 2 is significantly Ͼ0 for AQP0, AQP1, AQP4-M23, AQP5, AQP6, and AQP9. (⌬pHS*)NH 3 is Ͼ0 for AQP1, AQP3, AQP6, AQP7, AQP8, and AQP9. The ratio (⌬pHS*)CO 2 /Pf* falls in the sequence AQP6 (ϱ)The ratio (⌬pHS*)CO 2 /(Ϫ⌬pHS*)NH 3 is indeterminate for both AQP2 and AQP4-M1. In summary, we find that mammalian AQPs exhibit a diverse range of selectivities for CO2 vs. NH3 vs. H2O. As a consequence, by expressing specific combinations of AQPs, cells could exert considerable control over the movements of each of these three substances.water transport; gas transport; gas channels; membrane protein; Xenopus oocytes THE FIRST GAS CHANNEL IDENTIFIED was the water channel aquaporin 1 (AQP1). The pioneering work of Agre and colleagues (51,52,74) showed that heterologously expressing AQP1 in Xenopus oocytes markedly increases osmotic water permeability (P f ). In later experiments in which they exposed oocytes to CO 2 , Nakhoul et al. (45) and Cooper and Boron (9) found that the expression of AQP1 also increases the rate at which intracellular pH (pH i ) declines due to the intracellular reactions:One stoppedflow study of AQP1 reconstituted into artificial lipid vesicles showed that AQP1 enhances CO 2 permeability (50), whereas another reached the opposite conclusion (71). However, Itel et al. (29) using an 18 O-exchange technique, recently showed that reconstituted AQP1 markedly increases CO 2 permeability. Moreover, several studies have shown that plant AQPs also function as CO 2 channels (31,32,66,67). Additional work has shown that AQP1 also conducts NH 3 (24,42,46) and nitric oxide (21).As an extension to the aforementioned pH i methodology for assessing the relative permeabilities of a cell membrane to dissolved gases, our laboratory exploited a principle revealed by earlier work in which investigators had monitored surface pH (pH S ) or tissue extracellular pH (pH o ) while exposing cells to CO 2 /HCO 3 Ϫ (11, 57) or NH 3 /NH 4 ϩ (8). The novelty of our approach was to push a relatively large pH-sensitive microelectrode up against the surface of an oocyte and to use transient changes in pH S to assess the membrane permeability of gases that alter pH (i.e., CO 2 and NH 3 ). Fo...

Aquaporin 4 as a NH3 Channel

“…This alkalization (or lesser acidification) suggests the ability of AQP4 to allow permeation of NH 3 . The observed AQP4-mediated NH 3 permeation is at odds with previous reports by Boron and co-workers (29,31) who were unable to demonstrate NH 3 permeability in AQP4-expressing oocytes. In those studies, NH 3 permeation was evaluated by monitoring the pH o at the external face of the oocyte plasma membrane with a blunt microelectrode pushed against the surface of an AQP-expressing oocyte.…”

“…29 and 31) versus 10 mM in this study. At the low NH 4 Cl concentration employed by Boron and co-workers (29,31), only 0.007 mM exists as NH 3 and in case AQP4 has a lower NH 3 permeability capacity than the other tested aquaporins, it may simply be experimentally undetectable at this concentration.…”

“…Permeability in AQP4 -To determine whether ammonia permeates AQP4, we monitored the ability of ammonia to drive osmotic water flux in Xenopus oocytes expressing AQP4 and, as a positive control, the ammonia-permeable AQP8 (22,31,45,46). An inherent advantage in this heterologous expression system is the low intrinsic water permeability of the native oocyte membrane; expression of AQP4 increases the osmotic water permeability of the oocyte membrane ϳ20-fold (47,48), thus providing a robust signal-to-noise ratio.…”

“…The 13 mammalian aquaporins are classified into three families based on their permeability profile and sequence homology: the aquaporins (AQP0, AQP1, AQP2, AQP4, 6 AQP5, AQP6, and AQP8), the aquaglyceroporins (AQP3, AQP7, AQP9, and AQP10), which in addition to water are permeable to small hydrophilic molecules such as urea and glycerol, and the unorthodox aquaporins (AQP11 and AQP12), which share little sequence homology with the other aquaporins (27,28). Several of both the aquaporins and aquaglyceroporins have been reported to be permeable to ammonia as well as water; AQP1, AQP3, AQP6, AQP7, AQP8, and AQP9 have been found to be permeable for NH 3 (21, 22, 29 -31), although the NH 3 permeability of AQP1 has been questioned (21,22,32), whereas AQP0, AQP2, AQP5, and AQP4 have been reported to lack NH 3 permeability (29,31). The sensitivity of the different experimental approaches employed to determine the NH 3 permeability is not defined, and it is possible that, as most K ϩ * The authors declare that they have no conflicts of interest with the contents of this article.…”

The water channel aquaporin-1a1 facilitates movement of CO2 and ammonia in zebrafish (Danio rerio) larvae

“…The findings of the present study suggest that AQP1a1 serves not only as a route for CO 2 excretion, but also as a pathway for ammonia movement. Although in vitro work supports the movement of ammonia through AQP1 (Musa-Aziz et al, 2009;Geyer et al, 2013a), the present study appears to be the first to have assessed the contribution of an AQP isoform to elimination of nitrogenous waste in vivo in a whole animal. Its location in the basolateral membrane suggests that AQP1a1 may provide a route for the entry of molecular CO 2 and NH 3 gases into ionocytes of the yolk sac epithelium, with exit to the external environment accomplished via apical Rhcg1 and/or diffusion through the lipid bilayer, depending on cell type.…”