Increased scattering by the RPE of Abca4^-/- mice relative to WT controls measured with OCT in vivo correlates with increased fundus autofluorescence measured with SLO

Abstract

Purpose : To understand the mechanism of increased scattering by the retinal pigment epithelium (RPE) of fifteen-month old agouti Abca4^-/- mice relative to age-matched agouti wild-type (WT) controls.

Methods : OCT and SLO imaging were repeatedly performed on both eyes of two agouti Abca4^-/- mice (129S4) and two agouti WT controls (129S1/SvlmJ). OCT imaging was performed with a custom-built swept source-based directional OCT system.The average A-scan profiles from OCT images were used to calculate the angular-dependent scattering from RPE. Fundus autofluorescence (AF) images were acquired using a custom-built SLO system with an excitation wavelength of 488 nm. The emission spectrum of the AF was acquired by an Ocean Optics spectrometer. Electron microscopy(EM) was performed to see the changes in melanosomes and other electron dense bodies in the RPE cells of Abca4^-/-and WT control.

Results : Comparison of the angular-dependent scattering profiles of WT controls and Abca4^-/- mice are shown in Figure 1 (a) and (b). The RPE of Abca4^-/- mice relative to its WT control exhibited an ~ 3-fold increase in scattering. Increased AF was also observed in the fundus images of Abca4^-/- mice as seen in Figure 1 (c) and (d), with an approximately 3-fold elevation above that of WT controls. Normalized spectra measured from Abca4^-/- and WT show a shift in the spectral band of the AF of Abca4-/- mice relative to controls (Figure 1(e) and 1(f)). EM images showed that number of melanosomes are less in Abca4^-/- compared to WT control.

Conclusions : Fundus AF generated by 488-nm excitation is attributable predominantly to bisretinoid byproducts that accumulate in the RPE as lipofuscin. We hypothesize that the increased back-scattering from the RPE arises from increased number of lipofuscin granules, as there appears to be fewer melanosomes in the Abca4^-/- mice in EM images.The changes in melanosomes are further supported by the changes in directional scattering from the RPE in Abca4^-/- mice. The shift in the emission spectra peak additionally suggest the variation of the fluorophores in RPE. We anticipate that the backscattered intensity along with AF from RPE may provide additional insight to the changes in AF granules associated with age and/or diseases.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

(a) & (b) Directional scattering from RPE. (c) & (d) AF Fundus images. (e) & (f) AF emission spectra.