아밀로이드 베타에 의한 mTORC1 활성이 cyclin/CDK 매개 신경세포 자멸사에 미치는 영향

Abstract

Alzheimer's disease (AD) is a neurodegenerative disorder induced by Amyloid β (Aβ) resulting in tau hyper-phosphorylation which is associated with neuronal apoptosis. Because no therapeutic targets for AD have been identified yet, I investigated relationship of cyclin-dependent kinases (CDKs) with tau phosphorylation in Aβ-induced neuronal apoptosis. In the present study, Aβ induced apoptosis in a time-dependent manner. Aβ increased the calcium influx which was dependent on the L-type calcium channel. I showed that the Aβ increased ROS (Reactive oxygen species) generation was blocked by EGTA (extracellular calcium chelator) but not by BAPTA-AM (intracellular calcium chelator) indicating that Aβ-induced calcium influx up-regulates ROS. The Aβ-induced ROS increased the HIF-1α expression which was blocked by N-acetyl-L-cysteine (NAC, ROS scavenger). Through HIF-1α siRNA transfection, I determined that Aβ stimulates mTORC1 phosphorylation through PI3K/AKT signaling. Moreover, my results show that Aβ-induced mTOR activation inhibits autophagy, while Aβ-activated mTOR increases cyclinD1/CDK4 and cyclinE/CDK2 expression. Interestingly, Aβ-inhibited autophagy up-regulated cyclinD1/CDK4 and cyclinE/CDK2 which were blocked by Trehalose (an autophagy inducer). These results suggest that Aβ-induced mTOR activation up-regulates the expression of cyclins and CDKs while the accumulation of cyclins and CDKs is attributed to Aβ-inhibited autophagy. CDKs were directly bound to tau, and leaded to phosphorylating tau which was blocked by Flavopiridol (CDK inhibitor). Phosphorylation of tau induced detachment from microtubule leading microtubule destabilization-mediated neuronal apoptosis which was blocked by Paclitaxel (microtubule stabilizer). In conclusion, Aβ-induced mTOR activation through HIF1α up-regulated the cyclinD1/CDK4 and cyclinE/CDK2 levels which were essential for tau hyper-phosphorylation leading to neuronal apoptosis.