Molecular determinants of S100B oligomer formation

Time-Dependent Changes in the Biofluid Levels of Neural Injury Markers in Severe Traumatic Brain Injury Patients-Cerebrospinal Fluid and Cerebral Microdialysates: A Longitudinal Prospective Pilot Study

Monitoring protein biomarker levels in the cerebrospinal fluid (CSF) can help assess injury severity and outcome after traumatic brain injury (TBI). Determining injury-induced changes in the proteome of brain extracellular fluid (bECF) can more closely reflect changes in the brain parenchyma, but bECF is not routinely available. The aim of this pilot study was to compare time-dependent changes of S100 calcium-binding protein B (S100B), neuron-specific enolase (NSE), total Tau, and phosphorylated Tau (p-Tau) levels in matching CSF and bECF samples collected at 1, 3, and 5 days post-injury from severe TBI patients (n = 7; GCS 3-8) using microcapillary-based western analysis. We found that time-dependent changes in CSF and bECF levels were most pronounced for S100B and NSE, but there was substantial patient-to-patient variability. Importantly, the temporal pattern of biomarker changes in CSF and bECF samples showed similar trends. We also detected two different immunoreactive forms of S100B in both CSF and bECF samples, but the contribution of the different immunoreactive forms to total immunoreactivity varied from patient to patient and time point to time point. Our study is limited, but it illustrates the value of both quantitative and qualitative analysis of protein biomarkers and the importance of serial sampling for biofluid analysis after severe TBI.

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.

A model of full-length RAGE in complex with S100B

The receptor for advanced glycation end products (RAGE) is an immunoglobulin-type multiligand transmembrane protein expressed in numerous cell types, including the central nervous system cells. RAGE interaction with S100B, released during brain tissue damage, leads to RAGE upregulation and initialization of a spiral proinflammatory associated with different neural disorders. Here, we present the structural characterization of the hetero-oligomeric complex of the full-length RAGE with S100B, obtained by a combination of mass spectrometry-based methods and molecular modeling. We predict that RAGE functions as a tightly packed tetramer exposing a positively charged surface formed by V domains for S100B binding. Based on HDX results we demonstrate an allosteric coupling of the distal extracellular V domains and the transmembrane region, indicating a possible mechanism of signal transmission by RAGE across the membrane. Our model provides an insight into RAGE-ligand interactions, providing a basis for the rational design of the therapeutic modifiers of its activity.

Computational Design of Macrocyclic Binders of S100B(ββ): Novel Peptide Theranostics

S100B(ββ) proteins are a family of multifunctional proteins that are present in several tissues and regulate a wide variety of cellular processes. Their altered expression levels have been associated with several human diseases, such as cancer, inflammatory disorders and neurodegenerative conditions, and hence are of interest as a therapeutic target and a biomarker. Small molecule inhibitors of S100B(ββ) have achieved limited success. Guided by the wealth of available experimental structures of S100B(ββ) in complex with diverse peptides from various protein interacting partners, we combine comparative structural analysis and molecular dynamics simulations to design a series of peptides and their analogues (stapled) as S100B(ββ) binders. The stapled peptides were subject to in silico mutagenesis experiments, resulting in optimized analogues that are predicted to bind to S100B(ββ) with high affinity, and were also modified with imaging agents to serve as diagnostic tools. These stapled peptides can serve as theranostics, which can be used to not only diagnose the levels of S100B(ββ) but also to disrupt the interactions of S100B(ββ) with partner proteins which drive disease progression, thus serving as novel therapeutics.

Targeting S100B with Peptides Encoding Intrinsic Aggregation-Prone Sequence Segments

S100 proteins assume a diversity of oligomeric states including large order self-assemblies, with an impact on protein structure and function. Previous work has uncovered that S100 proteins, including S100B, are prone to undergo β-aggregation under destabilizing conditions. This propensity is encoded in aggregation-prone regions (APR) mainly located in segments at the homodimer interface, and which are therefore mostly shielded from the solvent and from deleterious interactions, under native conditions. As in other systems, this characteristic may be used to develop peptides with pharmacological potential that selectively induce the aggregation of S100B through homotypic interactions with its APRs, resulting in functional inhibition through a loss of function. Here we report initial studies towards this goal. We applied the TANGO algorithm to identify specific APR segments in S100B helix IV and used this information to design and synthesize S100B-derived APR peptides. We then combined fluorescence spectroscopy, transmission electron microscopy, biolayer interferometry, and aggregation kinetics and determined that the synthetic peptides have strong aggregation propensity, interact with S100B, and may promote co-aggregation reactions. In this framework, we discuss the considerable potential of such APR-derived peptides to act pharmacologically over S100B in numerous physiological and pathological conditions, for instance as modifiers of the S100B interactome or as promoters of S100B inactivation by selective aggregation.

Allosteric modulation of protein oligomerization: an emerging approach to drug design

Many disease-related proteins are in equilibrium between different oligomeric forms. The regulation of this equilibrium plays a central role in maintaining the activity of these proteins in vitro and in vivo. Modulation of the oligomerization equilibrium of proteins by molecules that bind preferentially to a specific oligomeric state is emerging as a potential therapeutic strategy that can be applied to many biological systems such as cancer and viral infections. The target proteins for such compounds are diverse in structure and sequence, and may require different approaches for shifting their oligomerization equilibrium. The discovery of such oligomerization-modulating compounds is thus achieved based on existing structural knowledge about the specific target proteins, as well as on their interactions with partner proteins or with ligands. In silico design and combinatorial tools such as peptide arrays and phage display are also used for discovering compounds that modulate protein oligomerization. The current review highlights some of the recent developments in the design of compounds aimed at modulating the oligomerization equilibrium of proteins, including the "shiftides" approach developed in our lab.

CD166/ALCAM mediates proinflammatory effects of S100B in delayed type hypersensitivity

Promiscuity of pattern recognition receptors, such as receptor for advanced glycation end products (RAGE), allows for a complex regulatory network controlling inflammation. Scavenging of RAGE ligands by soluble RAGE treatment is effective in reducing delayed-type hypersensitivity (DTH), even in RAGE(-/-) mice by 50% (p < 0.001). This has led to the hypothesis that molecules scavenged by soluble RAGE bind to receptors other than RAGE. This study identifies CD166/ALCAM (ALCAM) as a close structural and functional homolog of RAGE, and it shows that binding of S100B to CD166/ALCAM induces dose- and time-dependent expression of members of the NF-κB family in wild type (WT) and RAGE(-/-) mouse endothelial cells. Blocking CD166/ALCAM expression using small interfering RNA completely inhibited S100B-induced NF-κB activation in RAGE(-/-), but not in WT cells. The in vivo significance of these observations was demonstrated by attenuation of DTH in WT and RAGE(-/-) animals pretreated with CD166/ALCAM small interfering RNA by 50% and 40%, respectively (p < 0.001). Experiments in ALCAM(-/-) animals displayed an only slight reduction of 16% in DTH, explained by compensatory reciprocal upregulation of RAGE in animals devoid of CD166/ALCAM, and vice versa. Consistently, ALCAM(-/-) mice, but not WT mice treated with RAGE small interfering RNA show a 35% reduction in DTH, and ALCAM(-/-) RAGE(-/-) double-knockout mice show a 27% reduction in DTH reaction. Thus, S100B is a proinflammatory cytokine bridging RAGE and CD166/ALCAM downstream effector mechanisms, both being compensatory upregulated after genetic deletion of its counterpart.

The S100B protein in biological fluids: more than a lifelong biomarker of brain distress

S100B is a calcium-binding protein concentrated in glial cells, although it has also been detected in definite extra-neural cell types. Its biological role is still debated. When secreted, S100B is believed to have paracrine/autocrine trophic effects at physiological concentrations, but toxic effects at higher concentrations. Elevated S100B levels in biological fluids (CSF, blood, urine, saliva, amniotic fluid) are thus regarded as a biomarker of pathological conditions, including perinatal brain distress, acute brain injury, brain tumors, neuroinflammatory/neurodegenerative disorders, psychiatric disorders. In the majority of these conditions, high S100B levels offer an indicator of cell damage when standard diagnostic procedures are still silent. The key question remains as to whether S100B is merely leaked from injured cells or is released in concomitance with both physiological and pathological conditions, participating at high concentrations in the events leading to cell injury. In this respect, S100B levels in biological fluids have been shown to increase in physiological conditions characterized by stressful physical and mental activity, suggesting that it may be physiologically regulated and raised during conditions of stress, with a putatively active role. This possibility makes this protein a candidate not only for a biomarker but also for a potential therapeutic target.