Aquaporin-1, Nothing but a Water Channel

Abstract

Aquaporin-1 (AQP1) is a membrane channel that allows rapid water movement driven by a transmembrane osmotic gradient. It was claimed to have a secondary function as a cyclic nucleotide-gated ion channel. However, upon reconstitution into planar bilayers, the ion channel exhibited a 10-fold lower single channel conductance than in Xenopus oocytes and a 100-fold lower open probability (<10 ؊6 ) of doubtful physiological significance (Saparov, S. M., Kozono, D., Rothe, U., Agre, P., and Pohl, P. (2001) J. Biol. Chem. 276, 31515-31520). Investigating AQP1 expressed in human embryonic kidney cells, we now have shown that the discrepancy is not due to alterations of AQP1 properties upon reconstitution into bilayers but rather to regulatory processes of the oocyte expression system that may have been misinterpreted as AQP1 ion channel activity. As confirmed by laser scanning reflection microscopy, from 0.8 to 1.4 ؋ 10 6 AQP1 copies/cell contributed to osmotic cell swelling. The proper plasma membrane localization was confirmed by observing the fluorescence of the N-terminal yellow fluorescent protein tag. Whole-cell patch clamp experiments of wild type or tagged AQP1-expressing cells revealed that neither cGMP nor cAMP mediated ion channel activity. The lack of significant CNG ion channel activity rules out a secondary role of AQP1 water channels in cellular signal transduction.

Ion Channel Function of Aquaporin-1 Natively Expressed in Choroid Plexus

“…Opening probabilities for cGMP-gated AQP1 ion channels expressed in oocytes or reconstituted in bilayers are exceptionally low (Saparov et al, 2001;Yool and Weinstein, 2002) and undetectable in AQP1-transfected HEK cells (Tsunoda et al, 2004). Differences among these studies suggested that AQP1 ion channel activity might depend on the expression sys-tem, perhaps involving differentially expressed unidentified factors.…”

“…However, using the parallel water permeability of the channel, it is possible to estimate the total number of AQP1 channels in the oocyte, which outstrips the number of ion channels by Ͼ50,000-fold (Yool and Weinstein, 2002). When reconstituted in bilayers, the proportion of available ion channels is estimated at one per million (Saparov et al, 2001); ion channel activity was undetectable in AQP1-transfected HEK cells (Tsunoda et al, 2004). In lieu of explanation as an accident, these differences could suggest that AQP1 ion channel activity is sensitive to regulatory mechanisms or interactions that vary with the expression system.…”

Bumetanide Derivatives AqB007 and AqB011 Selectively Block the Aquaporin-1 Ion Channel Conductance and Slow Cancer Cell Migration

“…Loop D has been shown previously to be involved in cGMP-dependent gating of AQP1 ion channels (Yu et al, 2006). The low proportion of AQP1 water channels that are available to be gated as ion channels in reconstituted bilayers and heterologous expression systems has prompted uncertainty regarding the physiologic relevance of the dual water and ion channel function in AQP1 (Saparov et al, 2001;Tsunoda et al, 2004). Further work has indicated that the availability of AQP1 ion channels to be activated by cGMP depends in part on tyrosine phosphorylation at the carboxyl terminal domain .…”

Vasotocin and isotocin regulate aquaporin 1 function in the sea bream

“…This shows that AQP1 function is regulated by phosphorylation, which might constitute a rapid response mechanism to osmoregulatory challenges. This feature of sea bream AQP1 contrasts with human AQP1, which is insensitive to direct stimulation with cAMP, but instead resembles human AQP2 (Tsunoda et al, 2004). Strikingly, the fact that sea bream Aqp1a and Aqp1b are specifically stimulated via different intracellular signaling pathways further highlights sub-functionalization of these paralog genes.…”