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

The "Jekyll Side" of the S100B Protein: Its Trophic Action in the Diet

The calcium-binding S100B protein is concentrated in glial cells (including enteroglial cells) in the nervous system. Its conformation and amino acid composition are significantly conserved in different species; this characteristic suggests conserved biological role(s) for the protein. The biological activity is concentration-dependent: low physiological concentrations exert a neurotrophic effect, while high concentrations exert a proinflammatory/toxic role. The proinflammatory/toxic role of S100B currently attracts the scientific community's primary attention, while the protein's physiological action remains unraveled-yet remarkably interesting. This is now a topical issue due to the recently consolidated notion that S100B is a natural trophic nutrient available in breast milk and/or other aliments, possibly interacting with other body districts through its impact on microbiota. These recent data may offer novel clues to understanding the role of this challenging protein.

Functional modulation of RAGE activation by multimeric S100B using single-domain antibodies

S100B is a multifunctional protein primarily found in the brain, where it plays crucial roles in cell proliferation, differentiation, and survival. It has intracellular and extracellular functions and, depending on S100B levels, can exhibit both neurotrophic and neurotoxic activities, both mediated by the receptor for advanced glycation end products (RAGEs). Here, we report the discovery and characterization of nanobodies (Nbs) targeting dimeric and tetrameric S100B, which are the two most abundant oligomeric functional forms of the protein, aiming to modulate S100B-mediated RAGE activation. Two Nbs were selected for detailed structural and functional studies and found to bind tetrameric S100B with high affinity, as determined by biolayer interferometry (BLI) analysis and size-exclusion chromatography-stable binary complex formation. Structural and docking analyses revealed preferential contact sites of Nbs with S100B regions implicated in interactions with RAGE, namely residues at the interfacial cleft on dimeric S100B and at hydrophobic cleft formed by the association of two homodimeric units in the tetramer. In accordance, assays in SH-SY5Y cells showed that Nbs modulate the RAGE-mediated neurotrophic activity of S100B by hindering its functional interactions with the receptor. BLI competition assays between tetrameric S100B and the RAGE-VC1 domain confirmed that Nbs selectively block S100B-mediated RAGE engagement, in agreement with cell activation experiments. These findings highlight Nbs as powerful tools for elucidating molecular and cellular mechanisms through the modulation of S100B and RAGE functions, inspiring potential therapeutic applications.

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.

ATP modulates self-perpetuating conformational conversion generating structurally distinct yeast prion amyloids that limit autocatalytic amplification

Prion-like self-perpetuating conformational conversion of proteins into amyloid aggregates is associated with both transmissible neurodegenerative diseases and non-Mendelian inheritance. The cellular energy currency ATP is known to indirectly regulate the formation, dissolution, or transmission of amyloid-like aggregates by providing energy to the molecular chaperones that maintain protein homeostasis. In this work, we demonstrate that ATP molecules, independent of any chaperones, modulate the formation and dissolution of amyloids from a yeast prion domain (NM domain of Saccharomyces cerevisiae Sup35) and restricts autocatalytic amplification by controlling the amount of fragmentable and seeding-competent aggregates. ATP, at (high) physiological concentrations in the presence of Mg²⁺, kinetically accelerates NM aggregation. Interestingly, ATP also promotes phase-separation-mediated aggregation of a human protein harboring a yeast prion-like domain. We also show that ATP disaggregates preformed NM fibrils in a dose-independent manner. Our results indicate that ATP-mediated disaggregation, unlike the disaggregation by the disaggregase Hsp104, yields no oligomers that are considered one of the critical species for amyloid transmission. Furthermore, high concentrations of ATP delimited the number of seeds by giving rise to compact, ATP-bound NM fibrils that exhibited nominal fragmentation by either free ATP or Hsp104 disaggregase to generate lower molecular weight amyloids. Additionally, (low) pathologically relevant ATP concentrations restricted autocatalytic amplification by forming structurally distinct amyloids which are found seeding-inefficient due to their reduced β-content. Our results provide key mechanistic underpinnings of concentration-dependent chemical chaperoning by ATP against prion-like transmissions of amyloids.

S100B chaperone multimers suppress the formation of oligomers during Aβ42 aggregation

Extracellular aggregation of the amyloid-β 1-42 (Aβ42) peptide is a major hallmark of Alzheimer's disease (AD), with recent data suggesting that Aβ intermediate oligomers (AβO) are more cytotoxic than mature amyloid fibrils. Understanding how chaperones harness such amyloid oligomers is critical toward establishing the mechanisms underlying regulation of proteostasis in the diseased brain. This includes S100B, an extracellular signaling Ca²⁺-binding protein which is increased in AD as a response to neuronal damage and whose holdase-type chaperone activity was recently unveiled. Driven by this evidence, we here investigate how different S100B chaperone multimers influence the formation of oligomers during Aβ42 fibrillation. Resorting to kinetic analysis coupled with simulation of AβO influx distributions, we establish that supra-stoichiometric ratios of dimeric S100B-Ca²⁺ drastically decrease Aβ42 oligomerization rate by 95% and AβO levels by 70% due to preferential inhibition of surface-catalyzed secondary nucleation, with a concomitant redirection of aggregation toward elongation. We also determined that sub-molar ratios of tetrameric apo-S100B decrease Aβ42 oligomerization influx down to 10%, while precluding both secondary nucleation and, more discreetly, fibril elongation. Coincidently, the mechanistic predictions comply with the independent screening of AβO using a combination of the thioflavin-T and X-34 fluorophores. Altogether, our findings illustrate that different S100B multimers act as complementary suppressors of Aβ42 oligomerization and aggregation, further underpinning their potential neuroprotective role in AD.