Zonular tension regulates the subcellular localization of AQP5 water channel in the rat lens at the influx and efflux zones of movement of water

Abstract

To maintain its transparency and refractive properties, the ocular lens operates a specialized microcirculatory system that utilises circulating fluxes of ions and water. These fluxes preferentially enter the lens at both anterior and posterior poles via an extracellular route associated with the lens sutures. The fluxes travel through the lens by an intercellular pathway before exiting the lens at the equator. In this thesis, I studied the water channels known as aquaporins (AQPs) that mediate the movement of water across cell membranes in the influx and efflux zones. The lens expresses three water channels (AQP1, AQP0 and AQP5) with different functional properties and distinct regional expression patterns. AQP1 is expressed only in epithelial cells and AQP0 only in fibre cells but AQP5 is present in both cells. AQP5 is cytoplasmic in the periphery of the lens but has been shown to change its localization from the cytoplasm to the cell membranes when the lens is organ cultured. To further characterise this shift in the membrane localization of AQP5, I used immunohistochemistry to study the expression and subcellular localization of all three water channels exclusively in the water influx and efflux regions of the lens. I investigated whether changes to zonular tension altered the subcellular localization of the lens AQPs, and whether this effect was transduced by the mechanosensitive ion channels TRPV1 and TRPV4. My results revealed that zonular tension plays a role in modulating the subcellular localization of AQP5 but had no effect on the subcellular location of AQP1 and AQP0. In the presence of zonular tension, AQP5 appeared to be predominantly membranous and was also found associated with the apical tips of the fibre cells that form the anterior suture but was not found in the basal tips that form the posterior suture. Mechanical and pharmacological reduction in zonular tension caused the removal of AQP5 from the membrane in the equatorial efflux and anterior influx zones, but this change in AQP5 distribution was not mediated by the activation of either TRPV1 or TRPV4. My results iii suggest that AQP5 acts as a regulated water channel in the rat lens and that changes in zonular tension can modulate the water permeability of fibre cell membranes in the efflux and anterior influx zones, by regulating AQP5 membrane trafficking. While further studies are required to characterise, the molecular pathways regulating the membrane trafficking of AQP5, my results raise the possibility of targeting AQP5 membrane trafficking as a means to upregulate the microcirculation system to enhance transparency and afford protection against age-related nuclear cataract (ARN).