Amyloid β-protein oligomers upregulate the β-secretase, BACE1, through a post-translational mechanism involving its altered subcellular distribution in neurons

Naomi Mamada,Fuyuki Kametani,Wataru Araki,Daisuke Tanokashira,Ai Hosaka,Akira Tamaoka

doi:10.1186/s13041-015-0163-5

Abstract

Backgroundβ-Site amyloid precursor protein cleaving enzyme 1 (BACE1) is a membrane-bound aspartyl protease that initiates amyloid β-protein (Aβ) generation. Aberrant elevation of BACE1 levels in brains of Alzheimer’s disease (AD) patients may involve Aβ. In the present study, we used a neuron culture model system to investigate the effects of Aβ on BACE1 expression as well as the underlying mechanisms.ResultsRat primary cortical neurons were treated with relatively low concentrations (2.5 μM) of Aβ42 oligomers (Aβ-O) or fibrils (Aβ-F) for 2–3 days. Aβ-O induced a significant increase in protein levels of BACE1, while Aβ-F only had a marginal effect. Levels of amyloid precursor protein (APP) and the major α-secretase, ADAM10, remained unaltered upon treatment with both types of Aβ. Aβ-O treatment resulted in activation of eIF2α and caspase 3 in a time-dependent manner, with no changes in the endoplasmic reticulum (ER) stress marker, GRP78, indicating that a typical ER stress response is not induced under our experimental conditions. Furthermore, Aβ-O did not affect BACE1 mRNA expression but augmented the levels of exogenous BACE1 expressed via recombinant adenoviruses, indicating regulation of BACE1 protein expression, not at the transcriptional or translational but the post-translational level. Immunocytochemical analysis revealed that Aβ-O causes a significant increase in BACE1 immunoreactivity in neurites (both axons and dendrites), but not soma of neurons; this change appears relevant to the mechanism of Aβ-O-induced BACE1 elevation, which may involve impairment of BACE1 trafficking and degradation. In contrast, Aβ-O had no effect on APP immunoreactivity.ConclusionOur results collectively suggest that Aβ oligomers induce BACE1 elevation via a post-translational mechanism involving its altered subcellular distribution in neurons, which possibly triggers a vicious cycle of Aβ generation, thus contributing to the pathogenetic mechanism of AD.Electronic supplementary materialThe online version of this article (doi:10.1186/s13041-015-0163-5) contains supplementary material, which is available to authorized users.

Highlights

Alzheimer’s disease (AD) is the most common neurodegenerative dementia characterized by two major pathological hallmarks, extracellular amyloid plaques and intracellular neurofibrillary tangles composed of amyloid β-protein (Aβ) and phosphorylated tau protein, respectively [1]
Western blot analysis revealed that the Aβ42 oligomers (Aβ-O) preparation consists mainly of trimers and tetramers in addition to monomers and oligomers with molecular sizes of 30–40 kDa, while Aβ42 fibrils (Aβ-F) is composed of monomers and aggregates that do not enter the stacking gel (Fig. 1a)
We used a neuron model system to investigate the effects of Aβ-O and Aβ-F on BACE1 expression as well as the underlying mechanisms

Summary

Introduction

Alzheimer’s disease (AD) is the most common neurodegenerative dementia characterized by two major pathological hallmarks, extracellular amyloid plaques and intracellular neurofibrillary tangles composed of amyloid β-protein (Aβ) and phosphorylated tau protein, respectively [1]. The longer form of Aβ, Aβ42, is more aggregable and probably more pathogenic than Aβ40. Recent evidence further suggests that Aβ oligomers, soluble aggregated forms of Aβ, act as an initiator of AD by inducing the development of tau pathology and synaptic dysfunction [3, 4]. While the exact toxic forms of Aβ oligomers and mechanisms underlying their neurotoxicity remain elusive at present, Aβ oligomers are considered a viable therapeutic target for AD [3, 4].

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