Amyloid precursor protein–mediated endocytic pathway disruption induces axonal dysfunction and neurodegeneration
Wei Xu, … , William C. Mobley, Chengbiao Wu
Wei Xu, … , William C. Mobley, Chengbiao Wu
Published April 11, 2016
Citation Information: J Clin Invest. 2016;126(5):1815-1833. https://doi.org/10.1172/JCI82409.
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Research Article Neuroscience

Amyloid precursor protein–mediated endocytic pathway disruption induces axonal dysfunction and neurodegeneration

Abstract

The endosome/lysosome pathway is disrupted early in the course of both Alzheimer’s disease (AD) and Down syndrome (DS); however, it is not clear how dysfunction in this pathway influences the development of these diseases. Herein, we explored the cellular and molecular mechanisms by which endosomal dysfunction contributes to the pathogenesis of AD and DS. We determined that full-length amyloid precursor protein (APP) and its β-C-terminal fragment (β-CTF) act though increased activation of Rab5 to cause enlargement of early endosomes and to disrupt retrograde axonal trafficking of nerve growth factor (NGF) signals. The functional impacts of APP and its various products were investigated in PC12 cells, cultured rat basal forebrain cholinergic neurons (BFCNs), and BFCNs from a mouse model of DS. We found that the full-length wild-type APP (APPWT) and β-CTF both induced endosomal enlargement and disrupted NGF signaling and axonal trafficking. β-CTF alone induced atrophy of BFCNs that was rescued by the dominant-negative Rab5 mutant, Rab5S34N. Moreover, expression of a dominant-negative Rab5 construct markedly reduced APP-induced axonal blockage in Drosophila. Therefore, increased APP and/or β-CTF impact the endocytic pathway to disrupt NGF trafficking and signaling, resulting in trophic deficits in BFCNs. Our data strongly support the emerging concept that dysregulation of Rab5 activity contributes importantly to early pathogenesis of AD and DS.

Authors

Wei Xu, April M. Weissmiller, Joseph A. White II, Fang Fang, Xinyi Wang, Yiwen Wu, Matthew L. Pearn, Xiaobei Zhao, Mariko Sawa, Shengdi Chen, Shermali Gunawardena, Jianqing Ding, William C. Mobley, Chengbiao Wu

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Figure 2

The level of GTP-Rab5 correlated with App gene dose in mouse brain tissues.

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The level of GTP-Rab5 correlated with App gene dose in mouse brain tissu...
(A) Protein levels of APP, GAPDH, GTP-Rab5, and total Rab5 in brain homogenates from 12-month-old Ts65Dn and 2N littermates were assayed as described in Methods. (B) The levels of GTP-Rab5 in these samples were quantitated and normalized against internal control, showing a 73% increase in 12-month-old Ts65Dn samples relative to 2N littermates. The level of GTP-Rab5 in 2N littermates was set at 1. (C) The protein level of APP in the brain homogenates from E17 App+/+, App+/–, and App–/– embryos was detected using an antibody against the C-terminus of APP (top lane). The level of GTP-Rab5 in App+/+ mouse brains was set at 1, with the relative levels shown in D. (E) APP levels were detected in BFCNs of Ts65Dn mice that were treated with control or APP siRNA; β-tubulin was used as a loading control. (F) The size of Rab5+ puncta was significantly reduced by APP siRNA, compared with control siRNA (ctr siRNA). (G) Representative images showing Rab5 staining of BFCNs from Ts65Dn mice with APP knockdown (right) and control siRNA (Scramb., scrambled; left). Insets: Zoom-in (×2.5) images of the selected areas. Scale bars: 15 μm. All data represent mean ± SEM of >3 independent experiments, and P values were calculated using Student’s t test.