Differential accumulation of secreted AbetaPP metabolites in ocular fluids.

Amyloid-beta (Abeta) accumulates in several types of retinal degeneration and in Alzheimer's disease (AD), but its source has been unclear. We detected the neuronal 695 amino acid form of amyloid-beta protein precursor (AbetaPP) in the normal retina and AbetaPP751 in the retinal pigment epithelium (RPE) and anterior eye tissues. Similar to the brain, alpha- and beta-secretases cleaved AbetaPP to soluble derivatives (sAbetaPP) alpha or beta and membrane-bound C-terminal fragments alpha or beta in the retina and RPE. Levels of sAbetaPP were particularly high in the vitreous and low in aqueous humor revealing a molecular barrier for AbetaPP. In contrast, Abeta40 and Abeta42 levels were only 50% lower in the aqueous than the vitreous humor, indicating relatively barrier-free movement of Abeta. These studies demonstrated a relatively high yield of AbetaPP and Abeta in the ocular fluids, which may serve as a trackable marker for AD. In addition, failure of free clearance from the eye may trigger retina degeneration in a manner similar to Abeta-related neurodegeneration in AD.