Biodynamics of lens sterols and their role in membrane homeostasis

Abstract

Lifelong lens transparency is reliant on the physiological interactions between the lens proteome and the lens lipidome. The age-related collapse of α-crystallin chaperone activity, the increase of high molecular weight protein aggregates, and the progressive stiffening of lens fibre cell membranes due to cholesterol enrichment all contribute to the terminal manifestation of lens ageing which is cataract. The use of oxysterols as a pharmacotherapeutic agent for cataract has been proposed, though there is limited understanding behind the mechanisms underpinning the interactions of supplemental sterols with endogenous lens lipids and proteins. We have analysed the gross phenotypic effect of inhibition of cholesterol synthesis in the developing zebrafish and analysed the absorption and retention of deuterated sterols by the larval lens in the context of human syndromes presenting with congenital and juvenile cataract. The inhibition of cholesterol synthesis reduces the optomotor response in developing zebrafish by negatively affecting the visual acuity of the larvae, while changes in the lens epithelium affect the aspect ratio of the tissue. Lanosterol or cholesterol supplementation do not immediately rescue the physical phenotypes of synthesis inhibited zebrafish, but both sterols are absorbed, transported, and retained in the lens. In vitro analysis of the protein-lipid interactions of lens lysates revealed an α-crystallin dependent, temporal solubilisation of proteins including the cytoskeletal proteins vimentin and BFSP1, and the transmembrane water channel protein AQP0. This effect was not preserved in material derived from cataractous lenses and was inhibited using cysteine protease inhibitors E-64 and leupeptin. Finally, the supplementation of exogenous α-crystallin restored the temporal solubility of E-64-treated lens proteins and this effect was accelerated via the sensitisation of α-crystallin with oxysterols. These findings highlight the need for a better understanding of the pharmacokinetics of oxysterols in relation to their fate in the lens and support the concept of targeting the chaperone activity of α-crystallin as an intervention against the progression of cataract.