The Aquaporin Zero Puzzle

Aquaporin zero (AQP0) is a water channel expressed almost exclusively in fiber cells of the lens of the eye. There is a long-standing hypothesis that AQP0 functions as both a water channel and a cell-to-cell adhesion protein in the lens. This hypothesis was supported in 2004 by the observation of AQP0 mediated cell-to-cell junctions in the lens (1) and in 2013 by observation of in vitro AQP0-mediated cell-to-cell adhesion (2). Absence of AQP0 or certain mutations in it lead to cataract and sometimes complete maldevelopment of the lens, so the essential nature of the protein to lens function is well established experimentally. In addition, the water permeability of AQP0 in model systems shows regulation by both calcium and pH, and phosphorylation alters the regulation by calcium (3). In normal lenses, protein kinase A (PKA) and AQP0 are associated with the A-kinase anchor protein AKAP2. In organ-cultured lenses, breaking the link between AKAP2 association with PKA and AQP0 results in a cortical cataract (4), suggesting a connection among phosphorylation, water permeability regulation, and cataract in vivo. Zebrafish lenses express two isoforms of AQP0: AQP0a is a water channel; AQP0b appears to provide functions other than water permeability, possibly the cell-to-cell adhesion function. Knockdown of AQP0a cannot be rescued by either a permanently phosphorylated form of AQP0 or a form that can never be phosphorylated; neither of these have calcium-mediated regulation of water permeability. Regulation of the water permeability of AQP0 is clearly essential for proper lens function. But this is only part of the story. A rising tide of data supports the view that the lens, which lacks blood vessels but is still too large to subsist on diffusion alone, generates its own intrinsic circulation of fluid (Fig. 1) (5–8). Compelling evidence argues that this circulation is powered by the Na/K ATPase in the equatorial epithelial cells, where sodium is extruded and a large transmembrane electrochemical potential for sodium entry is established. An inward current of sodium in lens extracellular spaces, combined with sodium leak channels in the lens fiber cells, generates an osmotic gradient to promote the flow of water through interstitial space and into the cells via osmotically driven flow through AQP0. The circulation carries nutrients and antioxidants into the lens, and it presumably also carries out waste products, although the evidence for this is circumstantial (9,10). The inward flow of water in the interior fiber cells of the lens leads to a buildup of fluid that generates a large intracellular hydrostatic pressure, of ∼1/3 of an atmosphere in the lens center, which drives fluid flow outward through gap junctions. This pressure may well be critical in increasing the index of refraction of the lens progressively toward the lens center and thus correcting for spherical aberration. Figure 1 The intrinsic lens circulation. (Left-hand panel) Path of currents generated by the action of the NaK ATPase. (External dotted lines) Path of measured currents; (dotted lines inside the lens) proposed current pathways based on modeling. (Right-hand panel ... What is the Puzzle? Because the fiber cell membrane water permeability is very large relative to the sodium permeability, altering water permeability of fiber cell membranes by a factor of 2–4 (similar to the regulatory changes in AQP0 water permeability) should have essentially no effect on fluid flow and produce only a very tiny change in the intracellular sodium concentration. (Note that we are interested only in the water permeability of the fiber cell membranes. There is plenty of aquaporin 1 in the epithelial cell surface layer, but this will have little effect on the internal lens circulation. In addition, aquaporin 5 has been found in fiber cells (2,11), but there is no experimental evidence on whether or not its water permeability is regulated or what effects its knockdown or knockout has on lens properties such as there is for AQP0.) This is because water flow entering the fiber cell membranes is essentially isotonic. To get a feeling for why the osmotic gradient is essentially zero, consider the limit of water permeability becoming infinite. If this is the case, even a vanishingly small osmotic gradient will produce very large fluid flow. This is called the isotonic limit (but it may help to think of it as the infinite water permeability limit). Now realize that if the water permeability is large enough compared to the sodium permeability, then if water permeability increases, the sodium generated osmotic gradient will decrease, but water flow will remain essentially constant nearly at its isotonic limit. This is apparently the case in the lens. Sodium leak permeability is quite small, so water permeability only needs to be small to allow fluid flow driven by very small sodium concentration gradients, essentially at the isotonic limit. Why then does water permeability, especially its regulation, matter? And what other functions might aquaporin zero supply? The possible answers to these questions constitute the subject of this review.