α-Syn load correlates with tau burden in the brainstem, but not cortical regions, of patients with PD in the absence of Aβ. (A–F) Representative images for mAb 81A p-α-syn and PHF1 p-tau pathology in tissue sections from the (A) substantia nigra (SN), (B) pons, (C) medulla, (D) thalamus, (E) hippocampus, and (F) entorhinal cortex (EC) in PD Braak stage 1–2 and non-PD Braak stage 1–2 patients. Scale bars, 100 µm. (G) A grouped bar graph shows a positive association between α-syn and tau percentage area stained. A two-tailed t test was performed to calculate the difference between groups. If the data were not normally distributed, a Mann–Whitney test was used. ****, P < 0.0001. (H and I) Correlation analysis between α-syn and tau in the brainstem (H; r = 0.87), hippocampus, and entorhinal cortex (I; r = 0.14). (J–M) IF double labeling was conducted for p-α-syn (red) and p-tau (green) using 81A and PHF1, respectively, in the medulla (J), entorhinal cortex (K), substantia nigra (L), and hippocampus (M) of PD patients. Scale bar, 40 µm. (N–Q) Percentage of p-tau–positive cells that were also positive for p-α-syn in the medulla (N), substantia nigra (O), hippocampus (P), and entorhinal cortex (Q) of PD patients. A two-tailed t test was performed to calculate the difference between groups; ****, P < 0.0001. Data are presented as mean ± SEM. All experimental data were quantified by two blinded scientists.