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K. Sambamurti, C. Venugopal, A. Suram, M. Pappolla, B. Rohrer, P. Annamalai; Amyloid Precursor Protein Metabolism in Retinal Degeneration. Invest. Ophthalmol. Vis. Sci. 2007;48(13):26.
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© ARVO (1962-2015); The Authors (2016-present)
The Alzheimer's amyloid beta protein of 42 residues (AB42) aggregates to soluble oligomers and amyloid deposits in the aging brain and in Alzheimer's disease (AD). Mutations in the amyloid precursor protein (APP), presenilins (PS) 1 or 2 increase the yield of AB42 rather than the normally more abundant AB40. Since these mutations can cause the entire neurodegenerative cascade in AD at relatively young ages and AB42 aggregates to oligomers more rapidly than AB40, the current hypothesis is that AB42 cause all forms of AD. Deposits of AB42 are also seen in age-related macular degeneration, the most frequent cause of visual impairment in the elderly suggesting that mechanisms of neurodegeneration may be conserved in AD and AMD. The purpose of this study was to identify the expression and metabolism of APP in the eye regions and determine whether changes in AB42 production could explain retinal degeneration in model systems.
BALBC mice were exposed to continuous low or high intensity light for a range of time form 0 to 12 days and eyes were collected dissected to obtain the vitreous humor, the retina and retinal pigmented epithelial cell (RPE) layers. The layers were extracted and loaded on gels for analysis of APP and its metabolites by Western blotting using antibodies against the N- and C-terminal domains of APP as well as antibodies against components of gamma secretase (Nicastrin).
The data obtained show that APP is expressed at very high levels in the retina and RPE cells. The vitreous humor has very large quantities of secreted APP comparable to the cerebrospinal fluid. Light induced stress results in a large increase in the levels of APP C-terminal fragments. The accumulation of C-terminal fragments peak on day 8 of exposure to low intensity light and day 2 of high intensity light, when approximately 50% of the photoreceptors are lost. The increase in CTF is associated with a reduction in the levels of nicastrin, suggesting that gamma secretase, an important protease that turns over membrane-bound fragments of several key proteins, may be impaired.
Light damage may induce degeneration by inhibiting gamma secretase and accumulating C-terminal fragments of APP. Since this will reduce AB40 and AB42, it is likely that retinal degeneration (and by extension in AD) may be mediated by faulty protein processing and recycling of membranes in addition to AB42 toxicity, as recently proposed by our group (Curr. Alz. Res. 2006).
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