Aggregation of the endoplasmic reticulum triggered by oligomeric beta-amyloid peptides could initiate autophagy

Abstract

BACKGROUND: Autophagic vacuoles have been demonstrated from postmortem human Alzheimer’s brains. However, the underlying mechanisms of initiating autophagic vacuoles or autophagosomes remain unclear. We have shown that oligomeric Abeta peptides trigger aggregation of the endoplasmic reticulum (ER). Following this event, the number of autophagosomes is increased. The aim of this study is to investigate the mechanisms of forming autophagosomes from the aggregated ER. METHODS: Primary cultures of hippocampal neurons were treated with oligomeric Abeta. Neurons were then transfected with different DNA constructs to express GFP-DFCP1 for protein docking to Phosphatidylinositol phosphate (PI(3)K), DsRed-KDEL for protein expressing in the ER, GFP/AsRed-Atg14L, mRFPVps34 and mCherryAmbra-1 for proteins involving in the nucleation of autophagosomes. RESULTS: With the use of live cell imaging by multiphoton microscopy, we found that Atg14L was localized on the ER. Aggregation of the ER facilitated them to be clustered which can then attract Vps34 together with Beclin-1 to form a nucleation complex. Ambra-1 is not the protein to initiate the nucleation complex. Consequently, an intermediate form of vesicles called omegasomes was formed and will be furthered matured to be autophagosomes. All these initiation steps were not triggered by mTOR signaling pathway. CONCLUSIONS: Our results are among the first to demonstrate the initiation factors for formation of autophagosomes. Aggregation of the ER may be the starting point for forming autophagosomes. In addition, the initiation processes for formation of autophagosomes are different to other systems which are regulated by mTOR. The results can answer a long-term question of how autophagy is initiated.