αB‐Crystallin Phosphorylation as a Precursor to Cataractogenesis

Abstract

PurposeThe molecular mechanism behind age‐related cataract formation is yet to be elucidated, although phosphorylation induced hyperactivity of αB‐crystallin (αB) and its subsequent association with fibre cell membranes has been implicated. This study evaluates the effect phosphorylation of αB at three well characterised sites (S19, S45 and S59) has on its chaperone activity and aims to characterise αB's membrane association with a focus on its role in cataractogenesis.MethodsThe chaperone activity of αB phosphomimics (S→D) were measured via aggregation assays using creatine phosphokinase (CPK) as a substrate. In vivo association of αB phosphoisotypes with two abundant membrane proteins, connexin 46 (cx46) and aquaporin 0 (AQP0) in aged human lenses was visualised using Duolink proximity assays and confocal microscopy.ResultsThe chaperone activity of αB increased with an increase in modification sites. Modification at the S19 and S59 residues activated αB to a greater extent than at S45. In the lens, phosphorylated αB was found to interact with Cx46 and AQP0 at cell membranes.ConclusionsThis study confirms that multiple phosphorylation events of αB cause a cumulative increase in activity, and that membrane association, at least in part, is mediated by interactions with membrane pore proteins. The proposed mechanism of cataractogenesis is that phosphorylation induced increases in substrate affinity of αB may lead to increased association with membrane pore proteins, leading to the obstruction of the diffusion of small molecules into inner regions of the lens, contributing to cataract formation.