ORIGINAL ARTICLE: Inhibition of endocytosis activates alternative degradation pathway of [beta]APP in cultured cells

Alzheimer's disease associated [beta]APP is sequentially endoproteolyzed by alpha/[beta]-secretase and gamma-cleavage. In the process, extracellular shedding by alpha-secretase (ADAM 9/10/17) or [beta]-secretase (BACE 1/2) at position L17 or D1 (A[beta] numbering) are prerequisites for the generation of P3 or A[beta], respectively. In addition, several alternative extracellular cleavage sites in [beta]APP have been reported at position I-6, V-3, R5, E11, F20, and A21. Among these sites, position R5 is considered to be cleaved by alpha-secretase-like activity, whereas position E11, F20 and A21 are cleaved by [beta]-secretase. Therefore, extracellular shedding of [beta]APP is thought to be mediated exclusively by alpha/[beta]-secretase activities. However, so far the characteristics of cleavages at position V-3 and I-6 are not well understood. The aim of this study is to characterize these two cleavages of [beta]APP. We analyzed the conditioned media of [beta]APP wt or sw expressing cells with or without pharmacological agents. Here, we show that the cleavage at position I-6 of [beta]APP has characteristics distinct from that of alpha/[beta]-secretase, while the cleavage at V-3 seems to be mediated by [beta]-secretase. Although inhibition of endocytosis enhances the cleavages at both V-3 and I-6, PMA, an alpha-secretase stimulator, treatment enhances neither of these cleavages. Interestingly, a [beta]-secretase inhibitor, z-VLL-CHO, suppressed V-3 but not I-6 cleavage. The pathological [beta]APP Swedish mutant adjacent to the cleavage sites shows similar effects. Our data demonstrate that neither alpha nor [beta]-secretase undergoes extracellular shedding at I-6 of [beta]APP. Therefore, our data may indicate a novel alternative [beta]APP degradation pathway which is up-regulated upon low level of endocytosis.[PUBLICATION ABSTRACT]