Fig. 6

Induction of the unfolded protein response in CHO-Tau35 cells. a Western blot analysis of components of the PERK branch of the UPR. CHO-FL, CHO-Tau35 and CHO cell lysates were probed with antibodies to phosphorylated/total PERK, phosphorylated/total eIF2α, and GAPDH. Molecular weight markers (kDa) are shown on the left. b Graphs show the relative amounts of phosphorylated/total PERK, total PERK/GAPDH, and phosphorylated/total eIF2α. Data are displayed as percentage changes compared to untransfected CHO cells (100%). Values represent mean ± S.E.M., n = 4–6, one-way ANOVA, *P < 0.05, **P < 0.01. c Western blots of ATF6, IRE1 and CHOP components of the UPR. CHO-FL, CHO-Tau35 and CHO cell lysates were probed with antibodies to cleaved ATF6α (36 kDa), phosphorylated/total IRE1α, CHOP and β-actin. d Graphs show the relative amounts of cleaved ATF6α (36 kDa)/β-actin, total IRE1α/GAPDH, and total CHOP/β-actin. Data are displayed as percentage changes compared to untransfected CHO cells (100%). Values represent mean ± S.E.M., n = 4–6, one-way ANOVA, *P < 0.05, **P < 0.01, ***P < 0.001. e The major domains and amino acid numbering of human FL-tau and Tau35 are illustrated (upper panel) showing the projection (amino acids 1–198) and microtubule binding (amino acids 199–441) domains of FL-tau. N1 and N2 correspond to the alternatively spliced amino acid sequences encoded by exons 2 and 3, respectively. The proline-rich domain (PRD, amino acids 151–243) is followed by the microtubule binding repeat region (R1-R4, amino acids 244–368), including the alternatively spliced sequence encoded by exon 10 (R2). The scheme in the lower panel indicates potential mechanisms by which Tau35 may be involved in the development of tauopathy. Tau35 has a reduced ability to bind to and stabilize microtubules, which compromises microtubule organization (loss of function). Potential gains of toxic function by Tau35 include triggering the activation of multiple branches of the unfolded protein response (UPR) and an increase in CHOP, which may lead to the downstream suppression of Akt activity. The accumulation of unfolded Tau35 results in activation of GSK3β through perturbation of Akt signaling and increased tau phosphorylation. In parallel, Tau35 expression activates mTORC1-S6K1 signaling, resulting in inhibitory phosphorylation of IRS1 and suppression of Akt, rendering CHO-Tau35 cells less responsive to insulin