X11 Proteins Regulate the Translocation of Amyloid β-Protein Precursor (APP) into Detergent-resistant Membrane and Suppress the Amyloidogenic Cleavage of APP by β-Site-cleaving Enzyme in Brain

Yuhki Saito,Sam Gandy,Tadashi Nakaya,Robert Vassar,Tohru Yamamoto,Yoshitake Sano,Toshiharu Suzuki

doi:10.1074/jbc.m801353200

Abstract

X11 and X11-like proteins (X11L) are neuronal adaptor proteins whose association to the cytoplasmic domain of amyloid beta-protein precursor (APP) suppresses the generation of amyloid beta-protein (Abeta) implicated in Alzheimer disease pathogenesis. The amyloidogenic, but not amyloidolytic, metabolism of APP was selectively increased in the brain of mutant mice lacking X11L (Sano, Y., Syuzo-Takabatake, A., Nakaya, T., Saito, Y., Tomita, S., Itohara, S., and Suzuki, T. (2006) J. Biol. Chem. 281, 37853-37860). To reveal the actual role of X11 proteins (X11s) in suppressing amyloidogenic cleavage of APP in vivo, we generated X11 and X11L double knock-out mice and analyzed the metabolism of APP. The mutant mice showed enhanced beta-site cleavage of APP along with increased accumulation of Abeta in brain and increased colocalization of APP with beta-site APP-cleaving enzyme (BACE). In the brains of mice deficient in both X11 and X11L, the apparent relative subcellular distributions of both mature APP and its beta-C-terminal fragment were shifted toward the detergent-resistant membrane (DRM) fraction, an organelle in which BACE is active and both X11s are not nearly found. These results indicate that X11s associate primarily with APP molecules that are outside of DRM, that the dissociation of APP-X11/X11L complexes leads to entry of APP into DRM, and that cleavage of uncomplexed APP by BACE within DRM is enhanced by X11s deficiency. Present results lead to an idea that the dysfunction of X11L in the interaction with APP may recruit more APP into DRM and increase the generation of Abeta even if BACE activity did not increase in brain.

Highlights

APP3 is a type I membrane protein and is the precursor of amyloid ␤-protein (A␤), which is the principal component of senile plaques, a pathological hallmark in the Alzheimer disease (AD) brain
In the brains of mice deficient in both X11 proteins (X11s) and X11-like proteins (X11L), the apparent relative subcellular distributions of both mature amyloid ␤-protein precursor (APP) and its ␤-C-terminal fragment were shifted toward the detergent-resistant membrane (DRM) fraction, an organelle in which BACE is active and both X11s are not nearly found. These results indicate that X11s associate primarily with APP molecules that are outside of DRM, that the dissociation of APP-X11/X11L complexes leads to entry of APP into DRM, and that cleavage of uncomplexed APP by BACE within DRM is enhanced by X11s deficiency
These results provide evidence that X11 proteins suppress translocation of APP into BACE- and ␥-secretase-rich DRM domains, raising the possibility that dysfunction of X11 proteins might play a role in the pathogenesis of AD even if the BACE activity did not increase in AD patients

Summary

Introduction

APP3 is a type I membrane protein and is the precursor of amyloid ␤-protein (A␤), which is the principal component of senile plaques, a pathological hallmark in the Alzheimer disease (AD) brain. We report here that the doubly deficient mice show increased levels of CTF␤ and A␤ in hippocampus and cerebral cortex, apparently because of the relatively increased colocalization of vesicles containing both APP and BACE1 in neurites of primary cultured neurons and that of APP and CTF␤ in DRM in brain when compared with what occurs in wild-type mice.

Results

Conclusion