Tetramerization of the S100B Chaperone Spawns a Ca2+ Independent Regulatory Surface that Enhances Anti-aggregation Activity and Client Specificity

S100B chaperone multimers suppress the formation of oligomers during Aβ42 aggregation

“…Our results revealed that dimeric and tetrameric S100B, the latter operating under sub-stoichiometric conditions and in the Ca 2+ free state, are able to drastically decrease the reactive influx toward oligomers and AβO total amounts-as inferred by reductions in the peak height (PH) and area under curve (AUC) of oligomerization rate distributions, in addition to X-34 positive Aβ42 oligomers (Figure 5). We demonstrate that such inhibitory behavior is a consequence of a preferential suppression of fibril catalyzed nucleation (k 2 ) of Aβ42 monomers into small aggregates (Figure 5A), in agreement with previous findings of an interaction between S100B and Aβ fibrils (Cristóvão et al, 2018;Figueira et al, 2022). With respect to fibril elongation, our results showed that although moderately, tetrameric apo-S100B is more competent than the dimer in suppressing the growth of aggregates (Figure 5B), a mechanism which constitute the main reactive sink for nucleated Figueira et al 10.3389/fnins.2023.1162741 oligomers (Staats et al, 2020).…”

“…Using an analogous procedure, we set out to investigate the mechanistic aspects driving the activity of tetrameric apo-S100B under sub-stoichiometric conditions. We tested tetrameric S100B in a Ca 2+ free state, as this allow us to selectively investigate the catalytic activity of the novel extended cleft present in tetrameric S100B ( Figueira et al, 2022 ). As reported, tetrameric apo-S100B displays an enhanced anti-aggregation activity with respect to dimer, being able to delay Aβ42 fibrillation even at sub-molar ratios ( Figures 1B, D ).…”

“…S100B concentrations were estimated as homodimer equivalents using by UV spectroscopy at 280 nm using the theoretical extinction coefficient value of ε 280 nm = 2,980 M –1 cm –1 . Biochemical characterization and oligomeric state validation of S100B dimer and tetramer preparations was performed by size-exclusion chromatography and electrophoresis analysis as described ( Supplementary Figure 5 ; Figueira et al, 2022 ).…”

“…Indeed, we recently established that S100B tetramerization spawns an extended hydrophobic surface which is formed by the lateral juxtaposition of homodimer C-terminal helices and whose solvent accessibility is independent of Ca 2+ binding. This novel regulatory cleft leads to a significant increase of Aβ42 anti-aggregation activity, also observable in the apo-form of tetrameric S100B (Figueira et al, 2022). Structural nuclear magnetic resonance (NMR), immunogold labeling electron microscopy and computational studies show that dimeric and tetrameric S100B target monomeric and fibrillar Aβ42 conformers (Cristóvão et al, 2018;Rodrigues et al, 2021), which indicates a modulation of Aβ42 primary nucleation, fibril elongation and, particularly, fibril catalyzed secondary nucleation of monomers into small aggregates (Arosio et al, 2016;Cristóvão et al, 2018).…”

“…Apart from its pro-inflammatory role, we have recently unveiled that the Ca 2+ -bound form of dimeric S100B inhibits the in vitro aggregation and toxicity of the Aβ forty two amino-acid variant (Aβ42) ( Cristóvão et al, 2018 , 2020 ) and the microtubule-associated protein tau ( Moreira et al, 2021 ), thus acting as a neuroprotective holdase-type chaperone. Not only, and although mainly present in the brain as a homodimer, S100B is also found as higher order functional oligomers such as octamers, hexamers and tetramers ( Ostendorp et al, 2007 ), the latter described as suppressors of Aβ42 aggregation ( Figueira et al, 2022 ). Indeed, we recently established that S100B tetramerization spawns an extended hydrophobic surface which is formed by the lateral juxtaposition of homodimer C-terminal helices and whose solvent accessibility is independent of Ca 2+ binding.…”