Cu2+-binding to S100B triggers polymerization of disulfide cross-linked tetramers with enhanced chaperone activity against amyloid-β aggregation

Secondary Modification of S100B Influences Anti Amyloid-β Aggregation Activity and Alzheimer's Disease Pathology

Proteinaceous aggregates accumulate in neurodegenerative diseases such as Alzheimer's Disease (AD), inducing cellular defense mechanisms and altering the redox status. S100 pro-inflammatory cytokines, particularly S100B, are activated during AD, but recent findings reveal an unconventional molecular chaperone role for S100B in hindering Aβ aggregation and toxicity. This suggests a potential protective role for S100B at the onset of Aβ proteotoxicity, occurring in a complex biochemical environment prone to oxidative damage. Herein, we report an investigation in which extracellular oxidative conditions are mimicked to test if the susceptibility of S100B to oxidation influences its protective activities. Resorting to mild oxidation of S100B, we observed methionine oxidation as inferred from mass spectrometry, but no cysteine-mediated crosslinking. Structural analysis showed that the folding, structure, and stability of oxidized S100B were not affected, and nor was its quaternary structure. However, studies on Aβ aggregation kinetics indicated that oxidized S100B was more effective in preventing aggregation, potentially linked to the oxidation of Met residues within the S100:Aβ binding cleft that favors interactions. Using a cell culture model to analyze the S100B functions in a highly oxidative milieu, as in AD, we observed that Aβ toxicity is rescued by the co-administration of oxidized S100B to a greater extent than by S100B. Additionally, results suggest a disrupted positive feedback loop involving S100B which is caused by its oxidation, leading to the downstream regulation of IL-17 and IFN-α2 expression as mediated by S100B.

M13 phage grafted with peptide motifs as a tool to detect amyloid-β oligomers in brain tissue

Oligomeric clusters of amyloid-β (Aβ) are one of the major biomarkers for Alzheimer's disease (AD). However, proficient methods to detect Aβ-oligomers in brain tissue are lacking. Here we show that synthetic M13 bacteriophages displaying Aβ-derived peptides on their surface preferentially interact with Aβ-oligomers. When exposed to brain tissue isolated from APP/PS1-transgenic mice, these bacteriophages detect small-sized Aβ-aggregates in hippocampus at an early age, prior to the occurrence of Aβ-plaques. Similarly, the bacteriophages reveal the presence of such small Aβ-aggregates in post-mortem hippocampus tissue of AD-patients. These results advocate bacteriophages displaying Aβ-peptides as a convenient and low-cost tool to identify Aβ-oligomers in post-mortem brain tissue of AD-model mice and AD-patients.

Tau liquid-liquid phase separation is modulated by the Ca2+ -switched chaperone activity of the S100B protein

Aggregation of the microtubule-associated protein tau is implicated in several neurodegenerative tauopathies including Alzheimer's disease (AD). Recent studies evidenced tau liquid-liquid phase separation (LLPS) into droplets as an early event in tau pathogenesis with the potential to enhance aggregation. Tauopathies like AD are accompanied by sustained neuroinflammation and the release of alarmins at early stages of inflammatory responses encompass protective functions. The Ca2+ -binding S100B protein is an alarmin augmented in AD that was recently implicated as a proteostasis regulator acting as a chaperone-type protein, inhibiting aggregation and toxicity through interactions of amyloidogenic clients with a regulatory surface exposed upon Ca2+ -binding. Here we expand the regulatory functions of S100B over protein condensation phenomena by reporting its Ca2+ -dependent activity as a modulator of tau LLPS induced by crowding agents (PEG) and metal ions (Zn2+ ). We observe that apo S100B has a negligible effect on PEG-induced tau demixing but that Ca2+ -bound S100B prevents demixing, resulting in a shift of the phase diagram boundary to higher crowding concentrations. Also, while incubation with apo S100B does not compromise tau LLPS, addition of Ca2+ results in a sharp decrease in turbidity, indicating that interactions with S100B-Ca2+ promote transition of tau to the mixed phase. Further, electrophoretic analysis and FLIM-FRET studies revealed that S100B incorporates into tau liquid droplets, suggesting an important stabilizing and chaperoning role contributing to minimize toxic tau aggregates. Resorting to Alexa488-labeled tau we observed that S100B-Ca2+ reduces the formation of tau fluorescent droplets, without compromising liquid-like behavior and droplet fusion events. The Zn2+ -binding properties of S100B also contribute to regulate Zn2+ -promoted tau LLPS as droplets are decreased by Zn2+ buffering by S100B, in addition to the Ca2+ -triggered interactions with tau. Altogether this work uncovers the versatility of S100B as a proteostasis regulator acting on protein condensation phenomena of relevance across the neurodegeneration continuum.

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

Alzheimer's disease (AD) hallmarks include the aggregation of amyloid-β (Aβ), tau and neuroinflammation promoted by several alarmins. Among these is S100B, a small astrocytic homodimeric protein, upregulated in AD, whose multiple biological activities depend on localization, concentration, and assembly state. S100B was reported to inhibit the aggregation and toxicity of Aβ42 and tau similarly to a holdase-type chaperone. This activity is dependent of Ca²⁺-binding, which triggers the exposure of a regulatory binding cleft at the S100B dimer interface with which amyloidogenic clients dynamically interact. Although the dimer prevails, a significant portion of secreted S100B in the human brain occurs as higher order multimers, whose protective functions remain uncharacterized and which we here investigate. Resorting to ThT-monitored aggregation kinetics, we determined that unlike the dimer, tetrameric S100B inhibits Aβ42 aggregation at sub/equimolar ratios, an effect that persists in the absence of Ca²⁺ binding. Structural analysis revealed that S100B tetramerization spawns a novel extended cleft accommodating an aggregation-prone surface that mediates interactions with monomeric Aβ client via hydrophobic interactions, as corroborated by Bis-ANS fluorescence and docking analysis. Correspondingly, at high ionic strength that reduces solvation and favours hydrophobic contacts, the inhibition of Aβ42 aggregation by tetrameric S100B is 3-fold increased. Interestingly, this extended Ca²⁺-independent surface favours Aβ42 as substrate, as tau K18 aggregation is not inhibited by the apo tetramer. Overall, results illustrate a mechanism through which oligomerization of the S100B chaperone fine-tunes anti-aggregation activity and client specificity, highlighting the potential functional relevance of S100B multimers in the regulation of AD proteotoxicity.

Dynamic interactions and Ca2+-binding modulate the holdase-type chaperone activity of S100B preventing tau aggregation and seeding

The microtubule-associated protein tau is implicated in the formation of oligomers and fibrillar aggregates that evade proteostasis control and spread from cell-to-cell. Tau pathology is accompanied by sustained neuroinflammation and, while the release of alarmin mediators aggravates disease at late stages, early inflammatory responses encompass protective functions. This is the case of the Ca²⁺-binding S100B protein, an astrocytic alarmin which is augmented in AD and which has been recently implicated as a proteostasis regulator, acting over amyloid β aggregation. Here we report the activity of S100B as a suppressor of tau aggregation and seeding, operating at sub-stoichiometric conditions. We show that S100B interacts with tau in living cells even in microtubule-destabilizing conditions. Structural analysis revealed that tau undergoes dynamic interactions with S100B, in a Ca²⁺-dependent manner, notably with the aggregation prone repeat segments at the microtubule binding regions. This interaction involves contacts of tau with a cleft formed at the interface of the S100B dimer. Kinetic and mechanistic analysis revealed that S100B inhibits the aggregation of both full-length tau and of the microtubule binding domain, and that this proceeds through effects over primary and secondary nucleation, as confirmed by seeding assays and direct observation of S100B binding to tau oligomers and fibrils. In agreement with a role as an extracellular chaperone and its accumulation near tau positive inclusions, we show that S100B blocks proteopathic tau seeding. Together, our findings establish tau as a client of the S100B chaperone, providing evidence for neuro-protective functions of this inflammatory mediator across different tauopathies.

The calcium binding protein S100B is an abundantly expressed protein in the brain and has neuro-protective functions by inhibiting Aβ aggregation and metal ion toxicity. Here, the authors combine cell biology and biochemical experiments with chemical kinetics and NMR measurements and show that S100B protein is an extracellular Tau chaperone and further characterize the interactions between S100B and Tau.

Computational Analysis of the Interactions between the S100B Extracellular Chaperone and Its Amyloid β Peptide Client

S100B is an astrocytic extracellular Ca²⁺-binding protein implicated in Alzheimer’s disease, whose role as a holdase-type chaperone delaying Aβ₄₂ aggregation and toxicity was recently uncovered. Here, we employ computational biology approaches to dissect the structural details and dynamics of the interaction between S100B and Aβ₄₂. Driven by previous structural data, we used the Aβ_25–35 segment, which recapitulates key aspects of S100B activity, as a starting guide for the analysis. We used Haddock to establish a preferred binding mode, which was studied with the full length Aβ using long (1 μs) molecular dynamics (MD) simulations to investigate the structural dynamics and obtain representative interaction complexes. From the analysis, Aβ-Lys28 emerged as a key candidate for stabilizing interactions with the S100B binding cleft, in particular involving a triad composed of Met79, Thr82 and Glu86. Binding constant calculations concluded that coulombic interactions, presumably implicating the Lys28(Aβ)/Glu86(S100B) pair, are very relevant for the holdase-type chaperone activity. To confirm this experimentally, we examined the inhibitory effect of S100B over Aβ aggregation at high ionic strength. In agreement with the computational predictions, we observed that electrostatic perturbation of the Aβ-S100B interaction decreases anti-aggregation activity. Altogether, these findings unveil features relevant in the definition of selectivity of the S100B chaperone, with implications in Alzheimer’s disease.