Role of Aquaporin 0 in lens biomechanics

Abstract

Maintenance of proper biomechanics of the eye lens is important for its structural integrity and for the process of accommodation to focus near and far objects. Several studies have shown specialized cytoskeletal systems such as the beaded filament (BF) and spectrin-actin networks contribute to mammalian lens biomechanics; mutations or deletion in these proteins alters lens biomechanics. Aquaporin 0 (AQP0), which constitutes ~45% of the total membrane proteins of lens fiber cells, has been shown to function as a water channel and a structural cell-to-cell adhesion (CTCA) protein. Our recent ex vivo study on AQP0 knockout (AQP0 KO) mouse lenses showed the CTCA function of AQP0 could be crucial for establishing the refractive index gradient. However, biomechanical studies on the role of AQP0 are lacking. The present investigation used wild type (WT), AQP5 KO (AQP5−/−), AQP0 KO (heterozygous KO: AQP0+/−; homozygous KO: AQP0−/−; all in C57BL/6J) and WT-FVB/N mouse lenses to learn more about the role of fiber cell AQPs in lens biomechanics. Electron microscopic images exhibited decreases in lens fiber cell compaction and increases in extracellular space due to deletion of even one copy of AQP0. Biomechanical assay revealed that loss of one or both copies of AQP0 caused a significant reduction in compressive load-bearing capacity of the lenses compared to WT lenses. Conversely, loss of AQP5 did not alter the lens load-bearing ability. Compressive load-bearing at the suture area of AQP0+/− lenses showed easy separation while WT lens remained intact. These data from KO mouse lenses in conjunction with previous studies on lens-specific BF proteins (CP49 and filensin) suggest that AQP0 and BF proteins could act co-operatively in establishing normal lens biomechanics. We hypothesize that AQP0, with its prolific expression at the fiber cell membrane, could provide anchorage for cytoskeletal structures like BFs and together they help to confer fiber cell shape, architecture and integrity. To our knowledge, this is the first report identifying the involvement of an aquaporin in lens biomechanics. Since accommodation is required in human lenses for proper focusing, alteration in the adhesion and/or water channel functions of AQP0 could contribute to presbyopia.

Sequential Application of Glass Coverslips to Assess the Compressive Stiffness of the Mouse Lens: Strain and Morphometric Analyses

“…These data also agree with previous observations made using a similar method 18 and by Brillouin optical microscopy 23 that mouse lenses increase in stiffness with age. Two other studies have used the described method to show that tropomodulin-1, an actin pointed-end capping protein, CP49, a beaded intermediate filament protein, and aquaporin 0 are needed to maintain lens stiffness 19,20 . With this method, the plethora of mouse models for lens pathologies and the accelerated aging of mice can be used to understand lens stiffness changes due to genetic variation and/or aging.…”

The molecular mechanisms underlying lens fiber elongation

“…Lens fiber cell elongation requires a large increase in membrane area (Bassnett, 2005; Borchman and Yappert, 2010; Piatigorsky, 1981), and the production/membrane insertion of fiber preferred membrane proteins such as aquaporin 0 (Bassnett et al, 2009; Chepelinsky, 2003; Sindhu Kumari et al, 2015). In neurons and other cell types, such membrane growth occurs via release of membranous vesicles from the endoplasmic reticulum/Golgi network, and trafficking of these vesicles to their site of membrane insertion.…”

Sub-nanometre mapping of the aquaporin–water interface using multifrequency atomic force microscopy

“…12,13 The AQP0 channel is remarkably efficient, allowing water flow rates that surpass the diffusion limit. [14][15][16][17] Deletion of the protein 8,18,19 can lead to several types of cataracts, the main cause of blindness in developing countries. 8 Given its importance, considerable research efforts have been dedicated to the investigation of AQP0 structure, 13,[20][21][22][23][24] function, 17,20,25,26 evolution 27 and the role it plays in the development of eye pathologies.…”