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K. Sambamurti, M. A. Pappolla, A.-C. Granholm, A. Prakasam; Alzheimer's Amyloid Protein Precursor and Neurotrophins in Age Related Retinal Degeneration. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3421.
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The amyloid β protein deposited in Alzheimer’s disease (AD) as well as in the drusen of Age Related Macular Degeneration (AMD) is processed by multiple complex pathways in the brain. Neurodegeneration is believed to be initiated by Aβ aggregates and facilitated by familial AD (FAD) mutations. Another characteristic of AD is the failure of NGF regulation with the accumulation of the pro-apoptotic form: pro-NGF. Our hypothesis has been that failure of normal protein processing by γ-secretase results in membrane-protein disruption leading to neurodegeneration in AD. The purpose of this study is to determine whether Amyloid protein precursor (APP) and Nerve Growth Factor (NGF) is processed in the eye as in the brain and and determine the effects of γ-secretase inhibitors on its processing.
Eyes of mice were carefully dissected to obtain the RPE, retina and vitreous humor fractions from control and γ-secretase inhibitor treated animals. These fractions were analyzed by Western blotting with either peptide or protein gels as appropriate to detect secreted or membrane-bound precursor proteins and cleavage products using commercial and in house antibodies against various domains of APP, Nicastrin, Presenilin, NGF and Pro-NGF.
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. The results also show that all the components necessary for the major APP proteolytic processing pathways (α- and β-secretase ) are present and follow the same relative levels in the eye as in the brain. Since the retina is primarily neuronal, the APP expressed in this tissue is primarily the shorter 695 aa protein unlike brain, which expresses large quantities of both APP695 and the longer APP 751. Animals treated with γ-secretase inhibitor accumulated C-terminal fragments of APP derived from α- and β-secretase pathways, as expected. In addition, these animals accumulated pro-NGF and yielded lower levels of mature NGF.
We find that APP and NGF are processed in the eye as in the brain and that inhibition of γ-secretase results in accumulation of the pro-apoptotic form of NGF that may play a role in retinal degeneration. These studies justify a careful examination of changes in γ-secretase with aging and retinal degeneration and analysis of methods to retain its function to prevent disease. They also caution against the use of γ-secretase inhibitors for treatment of AD and/or AMD.
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