Decreased Proteasomal Activity Causes Retinal Degeneration in Mice

Abstract

Purpose: The ubiquitin proteasome system (UPS), one of the major proteolytic pathway, contributes to protein quality control in eukaryotic cells and the UPS dysfunction is known to trigger neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Retinitis pigmentosa (RP) is a group of inherited retinal neurodegenerative disorders characterized by progressive degeneration of photoreceptors, and the previous studies revealed that rhodopsin mutation results in the production and aggregation of unfolded rhodopsin protein, which causes the UPS dysfunction in vitro setting. However, it remains unclear whether impaired function of UPS, in particular, proteasome that is multiunit enzyme complex responsible for protein degradation in UPS pathway, causes photoreceptor degeneration in vivo. In this study, we investigated the retinal changes in β5t transgenic (β5t-Tg) mice, an animal model of senescence acceleration caused by decreased proteasomal activity.

Methods: For clinical evaluation of the retinal degeneration, fundus images of β5t-Tg and wild-type (WT) mice were taken using funduscopy. For histological examination, paraffin sections of eyes from β5t-Tg and WT mice were prepared and stained with hematoxylin-eosin. The thicknesses of the inner nuclear layer (INL) and outer nuclear layer (ONL) were measured in the posterior retina at 10 points, two on either side of the optic nerve that were 400, 800, 1200, 1600 and 2000μm apart, using the measurement software.

Results: Retinal degenerative changes with a number of white or yellowish spots and vessel attenuation were found in β5t-Tg, but not in age-matched WT mice. In histological examination, average INL thickness showed no significant changes between β5t-Tg (17.66±0.64μm) and WT mice (18.21±0.51μm, P=0.51) at 6 months of age. Similarly, there was no difference in INL thickness at 9 months of age (β5t-Tg 21.88±1.84μm vs. WT 21.69±0.60μm, P=0.90). By contrast, ONL thickness of β5t-Tg mice was significantly decreased at both 6 months (15.76±0.97μm) and 9 months (13.85±0.31μm) compared with those of WT mice (6 months, 29.89±0.71μm; 9 months, 31.74±0.60μm, P<0.001 each), indicating the progressive photoreceptor degeneration with age in β5t-Tg mice.

Conclusions: Decreased proteasomal activity causes photoreceptor degeneration in vivo. The current data suggest that impaired UPS function is intimately tied to the underlying cause of RP.

Keywords: 695 retinal degenerations: cell biology