Aggregated forms of human tau progressively accumulate in the brain of mice injected with AAV-P301S. (a) Representative images of the hippocampus immunolabeled with the conformational MC1 antibody. Note the presence of MC1-positive neurons and neurites, 7 months after injection of either AAV-WT, AAV-P301S or AAV-2P. Scale bar: 100 μm. (b) Quantification of MC1-positive misfolded tau in forebrain protein homogenates using an HT7/MC1 AlphaLisa at 1.5 and 7 months post-injection. Statistical analysis: Kruskal-Wallis test, H(2, N = 38) = 28.25. (c) HT7/MC1 values are normalized for total human tau determined with a HT7/TAU-13 AlphaLisa. (d) The amount of human tau multimers is determined with a HT7/HT7 single-epitope AlphaLisa and normalized for total human tau. For c and d: data represent relative arbitrary units. Statistical analysis: two-way ANOVA followed by Tukey’s post-hoc test, group × time effect (c) F(2, 36) = 6.55, (d) F(2, 36) = 3.88, *p < 0.05; **p < 0.01; ***p < 0.001. (e) Significant correlation between the amount of MC1-immunoreactive human tau and human tau multimers (Pearson). (b–d) 1.5 and 3 months: AAV-maxFP: n = 2; AAV-WT and AAV-P301S: n = 6. 7 months: AAV-maxFP: n = 6; AAV-WT: n = 8; AAV-P301S: n = 10 mice. (f–h) Representative histological analysis, 7 months after AAV-P301S injection: (f) AT100 immunolabeling of phosphorylated tau in the hippocampus (CA1). Scale bar: 100 µm. (g) Thioflavine-S positive neurofibrillary deposits in cortical neurons. Scale bar: 25 µm. (h) Gallyas silver staining of argyrophilic filamentous inclusions in cortical neurons. Scale bar: 25 μm. (i) Detection of misfolded tau (HT7/MC1 AlphaLisa) in mouse forebrain homogenates, at 3 and 7 months after either AAV-WT or AAV-2P injection. Single mouse values were normalized for total human tau (HT7/TAU-13 AlphaLisa); n = 8 mice per group. (j) Detection of tau multimers (HT7/HT7 single-epitope AlphaLisa); n = 8 mice per group. Statistical analysis: two-tailed Student’s t test.