The water permeability of lens aquaporin-0 depends on its lipid bilayer environment

Abstract

Aquaporin-0 (AQP0), the primary water channel in lens fiber cells, is critical to lens development, organization, and function. In the avascular lens there is thought to be an internal microcirculation associated with fluid movement. Although AQP0 is known to be important in fluid fluxes across membranes, the water permeability of this channel has only been measured in Xenopus oocytes and in outer lens cortical membranes, but not in inner nuclear membranes, which have an increased cholesterol/phospholipid ratio. Here we measure the unit water permeability of AQP0 in different proteoliposomes with cholesterol/phospholipid ratios and external pHs similar to those found in the cortex and nucleus of the lens. Osmotic stress measurements were performed with proteoliposomes containing AQP0 and three different lipids mixtures: (1) phosphatidylcholine (PC) and phosphatidylglycerol (PG), (2) PC, PG, with 40 mol% cholesterol, and (3) sphingomyelin (SM), PG, with 40 mol% cholesterol. At pH 7.5 the unit permeabilities of AQP0 were 3.5 ± 0.5 ± 10−14 cm3/s (mean ± SEM), 1.1 ± 0.1 × 10−14 cm3/s, and 0.50 ± 0.04 × 10−14 cm3/s in PC:PG, PC:PG:cholesterol, and SM:PG:cholesterol, respectively. For lipid mixtures at pH 6.5, corresponding to conditions found in the lens nucleus, the AQP0 permeabilities were 1.5 ± 0.4 × 10−14 cm3/s and 0.76 ± 0.03 × 10−14 cm3/s in PC:PG:cholesterol and SM:PG:cholesterol, respectively. Thus, although AQP0 unit permeability can be modified by changes in pH, it is also sensitive to changes in bilayer lipid composition, and decreases with increasing cholesterol and SM content. These data imply that AQP0 water permeability is regulated by bilayer lipid composition, so that AQP0 permeability would be significantly less in the lens nucleus than in the lens cortex.

Proteomic Analysis of Lipid Raft-Like Detergent-Resistant Membranes of Lens Fiber Cells

“…A previous report showed that AQP0 water permeability was affected by membrane lipid composition and that increased cholesterol content decreased AQP0 water permeability. 51 This result also suggested a direct AQP0-cholesterol interaction.…”

High Cholesterol/Low Cholesterol: Effects in Biological Membranes: A Review

“…Such changes would be expected to alter the functioning of membrane integral proteins. However, this is not the case; presented data indicate that the saturating Chol content in these membranes (and the presence of CBDs) helps to maintain fiber cell membrane homeostasis during age-related changes of the PL composition without altering functions of integral proteins [11–14] and without compromising too much lens transparency. This constant saturating Chol content in the membranes of the fiber cells fulfills another important function as it makes the surfaces of the membrane smoother than otherwise.…”

“…Specific assemblies of membrane proteins allow membranes to perform the most complex functions such as signal transduction [5–7] and energy transformation [8]. Nevertheless, the lipid-bilayer portion of the biological membrane determines bulk membrane properties (including diffusion barriers [9, 10]) and can affect the functioning of membrane proteins [11–14]. …”

Highly permeable artificial water channels that can self-assemble into two-dimensional arrays

“…This result indicated that the contribution of the PAP channels to the overall water permeability of the vesicles was lower than that of the lipid membrane, and so the smaller exponential coefficient (k 2 ) was used to calculate the permeability of the channel. The same approach was recently used to measure the low water permeability of AQP0 (38). The permeability contributed by the PAP channels increased linearly when the mCLR was increased from 0 to 0.005 (Fig.…”