Low molecular weight Aβ induces autophagosome accumulation via endoplasmic reticulum aggregation

Abstract

Alzheimer’s disease (AD) has been considered to be resulted from extracellular accumulation of β-amyloid protein (Aβ) and intracellular formation of neurofibrillary tangle composed of tau. Soluble Aβ was demonstrated to be synaptic toxic to neurons in most studies. Our previous report has shown that low molecular weight (LMW) Aβ induced aggregation of the endoplasmic reticulum (ER). However, the consequences of ER aggregation are not known. Recent report has shown that the ER membrane can contribute to the formation of autophagic vacuoles. Therefore, we hypothesize that collapse of the ER will trigger the autophagic-lysosomal degradation pathway in neurons, and the goal of our study is to prove whether autophagosome formation is derived from collapsed ER in the neurons exposed to LMW Aβ. By using primary cultures of hippocampal neurons, LMW Aβ triggered autophagy as indicated by elevated levels of LC3-II in Western-blot analysis and accumulation of autophagosomes in live cell imaging. Increased autophagosomes were accompanied by elevated numbers of omegosomes, which are PI3(P)-enriched compartment originally derived from the ER. These omegosomes colocalized with class III PI3 kinase Vps34-containing vesicles which are necessary for autophagosome initiation, suggesting that omegosome possibly participated in autophagosome formation. By using another ER-targeting PI3P probe GFP-ER-FYVE, we also found that autophagosome colocalized with GFP-ER-FYVE vesicles. Subsequently, we showed that some of the omegosomes accumulated at the site of ER aggregation in neurons upon LMW Aβ treatment. These results agree with our hypothesis by showing that Aβ-induced ER aggregation facilitates omegosome formation which will contribute to autophagosome production.