Retinal dysfunction and progressive retinal cell death in SOD1-deficient mice

Abstract

The superoxide dismutase (SOD) family is a major antioxidant system, and deficiency of Cu,Zn-superoxide dismutase (SOD1) in mice leads to many different phenotypes that resemble accelerated aging. The purpose of this study was to examine the morphology and physiology of the sensory retina in Sod1(-/-) mice. The amplitudes of the a- and b-waves of electroretinograms elicited by stimuli of different intensity were reduced in senescent Sod1(-/-) mice, and this reduction in amplitude was more pronounced with increasing age. Retinal morphometric analyses showed a reduced number of nuclei in both the inner nuclear cell layer and outer nuclear cell layer. Electron microscopy revealed swollen cells and degenerated mitochondria in the inner nuclear cell and outer nuclear cell layer of senescent Sod1(-/-) mice indicating necrotic cell death. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling revealed no significant differences in the number of apoptotic cells between Sod1(-/-) and wild-type mice, and activated caspase-3 could not be detected in the retina of Sod1(-/-) mice. In addition to the age-related macular degeneration-like phenotypes previously reported, Sod1(-/-) mice also present progressive retinal degeneration. Our results indicate that Sod1(-/-) mice may be a good model system in which to study the mechanism of reactive oxygen species-mediated retinal degeneration.

Figure 1

ERGs recorded from age-matched WT and Sod1^−/− mice. Data of two representative cases of the WT and two Sod1^−/− mice are shown. ERGs were recorded under three different stimulus conditions (scotopic flash, photopic flash, photopic flicker). Stimulus intensities are shown at the left.

Figure 2

Mean amplitudes of a- and b-waves of the ERGs. ERGs were recorded from age-matched WT and Sod1^−/− mice >40 weeks of age (n = 5, each group). Asterisk indicates P < 0.05. White box, WT; black box, Sod1^−/−.

Figure 3

Age-related changes of ERGs. A: Age-related changes of the amplitudes of the scotopic a-wave. The amplitudes decrease with increasing age in Sod1^−/− mice (black squares), in contrast to those of the WT (white circles). B: Age-related changes of the scotopic b-wave. Stimulus intensity was 65.0 cd*sm⁻².

Figure 4

Retinal thinning of Sod1^−/− mice. Toluidine blue-stained retina Quetol 812 sections (2 μm) of a 15-month-old WT mouse and an age-matched Sod1^−/− mouse. Thinning of the INL and ONL is apparent. Scale bar = 50 μm.

Figure 5

Ultrastructure of sensory retina of the senescent WT and age-matched Sod1^−/− mice. A: INL of the WT of 15 months. B: Swollen nuclei in the INL of age-matched Sod1^−/− mice (asterisk). Vacuolization and destruction of plasma and nuclear membranes can be seen (arrow). C: ONL of a WT mouse at 15 months of age. D: ONL of an age-matched Sod1^−/− mouse. Cell layers are disorganized, and the cell density is decreased. The number of nuclei in the ONL is decreased in Sod1^−/− mice. Higher magnification of ONL. WT (E), and Sod1^−/− mice (F). Mitochondria in photoreceptors of ONL distinguished in Sod1^−/− mice, as compared with the WT (arrowheads in E). External limiting membrane in the WT (arrows in E) is not distinct in a 15-month-old Sod1^−/− mouse. Plasma membranes are not clear in a damaged cell of the Sod1^−/− mouse (arrow in F). Swelling of the inner segments of the photoreceptors can be seen (asterisk). Scale bars: 2 μm (B, F); 5 μm (D).