Polarimetry in ex vivo retina from donors with Alzheimer’s disease

Abstract

Purpose: Alzheimer’s disease (AD) is characterized by the formation of insoluble fibrils (plaques) composed of amyloid β proteins. Polarization properties of stained amyloid β have been studied in brain tissue. Amyloid β has been found near the optic nerve fibre layer of the retina [1] in patients with AD using fluorescence staining [1,2]. We characterize the polarization properties of unstained amyloid β. The neural retina is optically accessible so these properties could be important to detection of amyloid β in the living eye.

Methods: Retinas were dissected from eyes obtained following informed consent under the auspices of the Eye Bank of Ontario and fixed in paraformaldehyde for those with a diagnosis of AD and age matched normals without AD or glaucoma. The retina was stained with 0.1% Thioflavin S. Firstly we tested, on one set of retinas, whether regions staining positively for Thioflavin S were also visible under crossed polarization. Retinal flat mounts without staining were prepared from subjects positive and negative for AD. Slides with pure amyloid β on glass was prepared by adding a Hepes buffer solution to ultra pure amyloid 1-42 and placed in a 37C incubator for 1, 6 and 23 hours. Polarized microscopy in red light was used to obtain the spatially resolved Mueller matrices of areas of the samples which showed increased brightness under crossed polarisers and negative regions of equal size. Spatial maps of diattenuation, polarizance, birefringence and depolarization were calculated. Histograms of these maps were compared using a Kolmogorov-Smirnov test.

Results: 82% of Thioflavin S positive areas showed increased brightness in crossed polarization. 15% of areas with increased brightness in crossed polarization were not stained. For unstained tissue, histograms of retardation from birefringence and depolarization were significantly different between presumed amyloid β deposits and surrounding retina as well as between deposited amyloid β and surrounding regions.

Conclusions: Unstained amyloid β deposits have polarization properties which can be quantified via Mueller matrix polarimetry. Amyloid β increases the amount of light scattered. In turn, these properties can make the deposits more visible. These studies of amyloid β in the retina may allow us to quantify progression of AD.

Keywords: 630 optical properties • 688 retina • 612 neuro-ophthalmology: diagnosis