Fig. 5. S100B-Ca²⁺ inhibits tau surface-catalyzed secondary nucleation and colocalizes with tau fibrils and oligomers.

a Seeded fibril formation of 10 µM hTau441 (red) in 50 mM Tris pH 7.4, in the presence of 0.005% of preformed fibrils and 1.1 mM CaCl₂ at 37 °C under agitation, in the presence of 0.1 (orange), 0.2 (yellow), 0.4 (green), 0.6 (light blue), 0.8 (purple), 1.0 (blue), 2.0 (gray), and 4.0 (dark blue) molar equivalents of S100B. b ThT intensity in arbitrary units (arb. units) of the end-time point (same color code) and half-time reaction (inset) of the hTau441-seeded aggregation in the presence (light pink) or the absence (blue) of Ca²⁺, for different S100B molar equivalents. c hTau441-seeded aggregation in the same conditions with addition of equimolar S100B at 0 (gray), 5 (light blue, 1), 25 (dark blue, 2), and 50 (blue, 3) hours after aggregation start. Data are presented as mean values ± SD from n = 3 independent experiments except for samples with 0.0 and 0.1 equivalents of S100B for which n = 2. d–f TEM images of tau aggregates grown in the presence of S100B-Ca²⁺ and immunogold labeled against S100B (10 nm, arrow) and tau (15 nm, arrowhead). d Zoom from the black box in e. S100B bound to tau small aggregates and oligomers (boxes in f). Scale bar 100 nm (e and f). Independent experiments of immunogold labeling were performed three times.