Fig. 2: Indirect evidence for trans-synaptic spread and synaptotoxicity of oligomeric tau.

a–d, A single 70 nm segmented section of PSP frontal cortex, immunostained for presynapses (SYO, cyan), postsynapses (PSD95, yellow) and oligomeric tau (T22, magenta). Scale bar = 10 µm. Oligomeric tau is seen in presynapses (b), postsynapses (c) and in both the presynapse and postsynapse of a synaptic pair (d). Boxes = 2 µm × 2 µm, 3D reconstructions made using IMARIS. e–j, Boxplots show the first quartile to third quartile (box) with the median marked, and whiskers show maximum and minimum values excluding outliers calculated from each image stack (two to three image stacks per case were taken as technical replicates). Data points show case means (n = 6 control, 7 PSP biological replicates). Data were Tukey transformed and analyzed with LMEMs (variable ~ diagnosis + 1 | case), and Tukey-corrected post hoc comparisons were performed. Quantification shows that in PSP frontal cortex there is an increase in the percentage of presynapses (e, estimate = 0.429, 95% CI (0.234, 0.624), t(10.7) = 4.85, P = 0.0005), postsynapses (f, estimate = 0.606, 95% CI (0.332, 0.88), t(10.6) = 4.89, P = 0.0005) and synaptic pairs colocalizing with oligomeric tau (g, estimate = 0.3, 95% CI (0.077, 0.522), t(8.9) = 3.052, P = 0.0139). The percentage of presynapses (h) and postsynapses (i) with oligomeric tau negatively correlates with the density of presynapses and postsynapses, respectively (Pearson correlations pre—t(27) = −2.348, 95% CI (−0.6763, −0.0533); post—t(27) = −2.857, 95% CI (−0.7210, −0.1400)). Postsynapses that are paired to a presynapse with oligomeric tau are around 86 times more likely to colocalize with oligomeric tau than all paired postsynapses, regardless of whether the adjacent presynapse also colocalizes with tau (j, estimate = 27.7, 95% CI (6.54, 48.8), t(11.8) = 2.856, P = 0.0146). Values shown in the schematic in j are means per group. Schematic created with biorender.com. k, Validation of oligomeric tau in synapses by immunogold electron microscopy. In postmortem PSP frontal cortex, anti-oligomeric tau (T22) immunogold staining is positive within the cytoplasm (arrow), presynaptic vesicles (arrowhead), oligomer-like structures in postsynapses (asterisk) and at the synaptic cleft (dotted arrow). Scale bar = 200 nm.