pH and Calcium Regulate the Water Permeability of Aquaporin 0

Abstract

Aquaporins increase the water permeability in many cell types across many species. We investigated the effects of external pH and Ca 2؉ on water permeability of Xenopus oocytes injected with aquaporin cRNA by measuring the rate of swelling in hypotonic solutions. Lowering pH to 6.5 increased the water permeability of aquaporin (AQP0) 3.4 ؎ 0.4-fold. Diethylpyrocarbonate pretreatment increased water permeability 4.2 ؎ 0.5-fold and abolished pH sensitivity, suggesting that the pH regulation is mediated by an external histidine. Lowering Ca 2؉ increased water permeability 4.1 ؎ 0. acts at an internal site. Three different calmodulin inhibitors each increased AQP0 water permeability, suggesting that Ca 2؉ may act through calmodulin. None of the above altered the water permeability induced by AQP1 or AQP4. Because the greatest change in AQP0 water permeability is in the normal pH range found in the lens (7.2-6.5), this paper provides evidence for regulation of an aquaporin by pH under physiological conditions.The major intrinsic protein (MIP, now designated aquaporin 0 and abbreviated AQP0) 1 of the optical lens was the first sequenced member of the aquaporins, an ancient family of proteins found in bacteria, plants, and animals (1-4). Preston et al. (5) discovered that CHIP 28 (now called aquaporin 1 (AQP1)), a protein abundant in red blood cells, facilitates the diffusion of water across the plasma membrane when expressed in Xenopus oocytes. AQP4 (previously called MIWC for mercurial-insensitive water channel) is expressed strongly in the brain and kidney collecting duct (6, 7). Work in several laboratories subsequently demonstrated that many members of the aquaporin family facilitate the diffusion of water and other nonelectrolytes (3, 8 -12). Among the aquaporins, AQP0 forms a water channel with a relatively low water permeability (13,14); the water permeability per molecule is 40 times higher for AQP1 (13,15). The structural basis of this large difference in water permeability is unknown. AQP0 and AQP1 form tetrameric arrays in their native membranes and when reconstituted in lipid vesicles (6 -18). AQP0, AQP1, and AQP4 share ϳ40% sequence identity with each other. Attempts to increase AQP0 water permeability by exchanging parts of AQP0 for corresponding parts of AQP1 have been ineffective (19).Although low in water permeability per molecule, AQP0 comprises more than 60% of the membrane protein in the normal vertebrate lens and therefore provides the major permeability pathway for water movement across the membranes of lens fiber cells. If it is defective or missing from an otherwise normal lens, a cataract results (20, 21). In a chimeric mouse model, cataract can be prevented by the presence of 20% normal cells, which presumably supply the requisite AQP0 (22). The role of AQP0 in maintaining normal lens conditions is uncertain, but it likely facilitates the intrinsic circulation of fluid in the lens that maintains lens transparency and homeostasis in the absence of blood vessels (23). pH and Ca 2ϩ are likely ca...

Arabidopsis NIP2;1, a Major Intrinsic Protein Transporter of Lactic Acid Induced by Anoxic Stress

“…Interestingly, nodulin 26 showed an enhanced P f in response to lower pH (4.6 ϫ 10 Ϫ4 cm/s, pH 5; Fig. 6A), consistent with previous observations of pH-dependent gating stimulating the transport of some aquaporins (36). The water permeability of AtNIP2;1 remained low throughout the pH range, although a slight elevation of P f is observed at pH 4 ( Fig.…”

The channel architecture of aquaporin 0 at a 2.2-Å resolution

“…Measurements of water conductance using oocyte and proteoliposome swelling demonstrate that AQP0 water permeability is 15-to 45-fold less than AQP1 (11,(24)(25)(26)(27)(28)(29). Published waterpermeability data have varied from 0-to 43-fold over conduction through lipids alone or through the membranes of oocytes injected with water.…”

Functions of aquaporins in the eye

“…Unlike AQP1, AQP0 has widespread distribution throughout lens fibers, where it constitutes more than 50% of membrane protein (Bok et al, 1982), but is absent in lens epithelial cells. Another difference is that AQP0 (but not AQP1) water permeability is pH and Ca ++ regulated, with ~4 fold increase in AQP0 water permeability with reduced pH or [Ca ++ ] (Nemeth- Cahalan and Hall, 2000). AQP0 has at least 40 times lower water permeability than AQP1 Chandy et al, 1997).…”

“…Because of its low water permeability it has been proposed that AQP0 might be involved in regulating the resistance of the paracellular pathway, rather than in cell membrane water permeability (Nemeth-Cahalan and Hall, 2000;Zampighi et al, 2000). It has also been proposed that AQP0 acts as a scaffold for organizing gamma-crystallins in lens fibers (Fan et al, 2004).…”