Mechanisms Underlie Protein Phosphatase 2A-regulated APP Phosphorylation And AβProduction

Back ground:In Alzheimer's disease, amyloid plaque deposition is one of the major pathological changes. Cleavage of APP byβ-secretase generates an N-terminal soluble fragment (sAPP) and beta C-terminal fragment that is sequentially cleaved byγ-secretase to produce the Aβpeptides. Threonine 668 (T668) in the cytoplasmic domain of APP can be phosphorylated in vivo by a number of protein kinases including GSK-3β, CDK5, CDC2 and JNK etc., with increasing Aβgeneration. Previous work of our group found that down-regulation of PP-2A specific inhibition protein I2PP-2A by lentivirus increase the activity of PP-2A and decrease the phosphylation level of APP as well as Aβlevel in hAPP transgenic mouse. These results suggest that PP-2A, an important Ser/Thr phosphatase, may affect AP secretion via regulating APP phosphorylation at Thr668.Aim:To investigate whether PP-2A regulates the phosphorylation of APP at Thr668 and affects Aβgeneration.Methods:N2a/APP cells were treated with Okadaic acid or DES, or transiently transfected with PP-2A plasmids, or OA was injected into the hippocampus of SD rats, phospho-APP, total APP, CTFs and Aβpeptide were mearsured by immunoblotting. Activities of PP-2A and secretases were mearsured by enzyme activity assay kits.Results:1) Inhibition the activity of PP-2A by OA can increases the phosphorylation level of APP at Thr668.2) Activation of PP-2A by DES or overexpression of PP-2A decreases the phosphorylation level of APP.3) Inhibition of PP1 by Tautomycin decreases the phosphorylation level of APP.4) Phosphorylation level of APP also increased by OA injection into the hippocampus of SD rats.5) The activity ofβ- and y-secretase increased after OA treatment accompanied with elevated A(3 generation.