Tau reduction prevents Aβ-induced activation of GSK3β. (A–C) Phosphorylation of GSK3β at serine 9 (p-GSK3β), a modification that inhibits GSK3β activity (Sutherland et al., 1993), and total GSK3β (t-GSK3β) and GAPDH or actin levels in Tau^+/+ and Tau^−/− neurons were determined by Western blot analysis after treatment of neuronal cultures with vehicle (Veh), Aβ_1–42 oligomers (30 min), or the phosphoinositide 3-kinase inhibitor wortmannin (WM; 0.1 µM, 30 min). (A) Representative Western blot from a single gel that was scanned and digitally arranged. (B) Quantitation of the p-GSK3β/t-GSK3β ratio for each treatment. Aβ decreased the ratio (i.e., increased GSK3β activity) in Tau^+/+, but not Tau^−/−, neurons, whereas WM decreased the ratio in both types of neurons. (C) Quantitation of t-GSK3β levels revealed no significant difference between vehicle-treated Tau^+/+ and Tau^−/− neurons (t test). n = 7–18 wells per condition from three to six independent experiments at DIV 14. ***, P < 0.001 versus vehicle in the same genotype (Dunnett’s test). Data are means ± SEM.