Phosphorylation of APP695 at Thr668 decreases γ-cleavage and extracellular Aβ

Phosphorylation of human APP695 at Thr668 seems to be specific to neuronal tissue and could affect Aβ production. Metabolism of APP mutated at Thr668 residue was analyzed in CHO cell line and primary cultures of rat cortical neurons. By site-directed mutagenesis, T668A or T668D substitutions were introduced in wild-type APP695. In CHO cells, wild-type APP695 was very slightly phosphorylated at Thr668 and produced similar levels of extracellular Aβ40 as compared to APPT668A. On the contrary, APPT668D was more efficiently cleaved by β-secretase. However, accumulated βCTF were less cleaved by γ-secretase and less extracellular Aβ40 was produced. Decreased susceptibility to cleavage by γ-secretase was confirmed upon expression of C99T668D. In neurons, part of APP695 was phosphorylated at Thr668. Following neuronal expression of APPT668A, extracellular Aβ40 production was increased. In conclusion, phosphorylation of human APP695 at Thr668 increases APP β-cleavage but decreases its γ-cleavage and extracellular Aβ40 production.