Novel role of RanBP9 in BACE1 processing of amyloid precursor protein and amyloid beta peptide generation.

Accumulation of the amyloid beta (Abeta) peptide derived from the proteolytic processing of amyloid precursor protein (APP) is the defining pathological hallmark of Alzheimer disease. We previously demonstrated that the C-terminal 37 amino acids of lipoprotein receptor-related protein (LRP) robustly promoted Abeta generation independent of FE65 and specifically interacted with Ran-binding protein 9 (RanBP9). In this study we found that RanBP9 strongly increased BACE1 cleavage of APP and Abeta generation. This pro-amyloidogenic activity of RanBP9 did not depend on the KPI domain or the Swedish APP mutation. In cells expressing wild type APP, RanBP9 reduced cell surface APP and accelerated APP internalization, consistent with enhanced beta-secretase processing in the endocytic pathway. The N-terminal half of RanBP9 containing SPRY-LisH domains not only interacted with LRP but also with APP and BACE1. Overexpression of RanBP9 resulted in the enhancement of APP interactions with LRP and BACE1 and increased lipid raft association of APP. Importantly, knockdown of endogenous RanBP9 significantly reduced Abeta generation in Chinese hamster ovary cells and in primary neurons, demonstrating its physiological role in BACE1 cleavage of APP. These findings not only implicate RanBP9 as a novel and potent regulator of APP processing but also as a potential therapeutic target for Alzheimer disease.