Autophogy protects neuron from Aβ-induced cytotoxicity

Autophagy is a degradation pathway for the turnover of dysfunctional organelles or aggregated proteins in cells. Extracellular accumulation of P-amyloid peptide has been reported to be a major cause of Alzheimer disease (AD) and large numbers of autophagic vacuoles accumulate in the brain of AD patient. However, how autophagic process is involved in A beta-induced neurotoxicity and how A beta peptide is transported into the neuron and metabolized is still unknown. In order to study the role of autophagic process in A beta-induced neurotoxicity, EGFP-LC3 was overexpressed in SH-SY5Y cells (SH-SY5Y/pEGFP-LC3). It was found that treatment with A beta(25-35') A beta(1-42) or serum-starvation induced strong autophagy response in SH-SY5Y/pEGFP-LC3. Confocal double-staining image showed that exogenous application of A beta(1-42) in medium caused the colocalization of A beta(1-42) with LC3 in neuronal cells. Concomitant treatment of A beta with a selective alpha 7nAChR antagonist, alpha-bungarotoxin (alpha-BTX), enhanced A beta-induced neurotoxicity in SH-SY5Y cells. On the other hand, nicotine (nAChR agonist) enhanced the autophagic process and also inhibited cell death following A beta application. In addition, nicotine but not alpha-BTX increased primary hippocampal neuronal survival following A beta treatment. Furthermore, using Atg7 siRNA to inhibit autophagosome formation in an early step or alpha 7nAChR siRNA to knock down alpha 7nAChR significantly enhanced A beta-induced neurotoxicity. Confocal double-staining imaging shows that nicotine treatment in the presence of A beta enhanced the colocalization of alpha 7nAChR with autophagosomes. These results suggest that alpha 7nAChR may act as a carrier to bind with eA beta and internalize into cytoplasm and further inhibit A beta-induced neurotoxicity via autophagic degradation pathway. Our results suggest that autophagy process plays a neuroprotective role against A beta-induced neurotoxicity. Defect in autophagic regulation or A beta-alpha 7nAChR transport system may impair the clearance of A beta and enhance neuronal death.