Lipid-membrane interactions of the bovine eye lens

Abstract

The lens is a transparent, biconvex structure within the eye that has a very high refractive index. It focuses light onto the retina and enables vision. It is able to elastically deform to adjust the focal length of the eye for observing objects at varying distances, a process known as accommodation.
With age or damage, the lens becomes stiffer and more likely to develop opaque regions called cataracts. Levels of oxidised lipids such as 7-ketocholesterol increase with age and raised levels of these oxysterols are observed in cataractous lenses.
The goal of this project is to investigate the effect of raised levels of 7-ketocholesterol on the biophysical properties of lens membranes, which could in turn affect the structure of lens fibre cells and thereby impact on their individual function and the lens structure as a whole.
Nanomechanical measurements of the lens elasticity with Atomic Force Microscopy (AFM) show that the Young’s Modulus of model and extracted bovine lens lipid membranes are lower when incubated with biologically-relevant levels of 7-ketocholesterol, signalling decreased membrane stiffness. In addition to this, the force required to extract individual thiol-tagged cholesterols from the model membranes was observed to decrease with the presence of 7-ketocholesterol, indicating this is consistent with Fluorescence Recovery After Photobleaching observations of model membranes which support a more fluid membrane with 7-ketocholesterol present as the diffusion coefficient (recovery rate of the membrane) is higher.
Taken together, these findings show how the biophysical properties of the lipid membranes are altered by the presence of 7-ketocholesterol and provide insight for further investigations to examine how the eye lens is affected with age or oxidative damage. A novel model membrane for the lipids of the bovine eye lens was also created and tested, which could be used in further experiments.