Microcystin-Leucine-Arginine Induces Tau Pathology Through B alpha Degradation via Protein Phosphatase 2A Demethylation and Associated Glycogen Synthase Kinase-3 beta Phosphorylation

摘要

Microcystin-leucine-arginine (MC-LR) has been implicated as a potential environmental factor in Alzheimer's disease because of its potent inhibition of protein phosphatase 2A (PP2A) activity, but experimental evidence to support its detailed neurotoxic effects and their underlying mechanisms has been lacking. The present study investigated the role of PP2A catalytic subunit (PP2Ac) demethylation and its link with glycogen synthase kinase-3 beta (GSK)-3 beta in tau hyperphosphorylation induced by MC-LR. The results showed that MC-LR treatment significantly increased demethylation of PP2Ac, with a concomitant increase in GSK-3 beta phosphorylation at Ser9 resulting in elevated tau hyperphosphorylation at PP2A-favorable sites in SH-SY5Y cells and rat hippocampus. Coimmunoprecipitation experiments showed that MC-LR treatment dissociated PP2Ac from B alpha, making it incompetent in binding tau, thus causing tau hyperphosphorylation. Moreover, we found that inhibition of PP2A resulted in an increase in phosphorylation of GSK-3 beta at Ser9 and a decrease in GSK-3 beta activity, which further promoted demethylation of PP2Ac induced by MC-LR. These findings suggest a scenario in which MC-LR-mediated demethylation of PP2Ac is associated with GSK-3 beta phosphorylation at Ser9 and contributes to dissociation of B alpha from PP2Ac, which would result in B alpha degradation and disruption of PP2A/B alpha-tau interactions, thus promoting tau hyperphosphorylation and paired helical filaments-tau accumulation and, consequently, axonal degeneration and cell death.