Amyloid-beta peptide aggregation mechanisms, oligomer toxicity, and development of aggregation inhibitors as therapeutic agents

Introduction

How does a 42-amino-acid peptide fragment, present in healthy brains at nanomolar concentrations, transform into the molecular trigger of the world's most common neurodegenerative disorder? That question has driven Alzheimer's disease research since Alois Alzheimer first described the characteristic plaques in 1906, and it remains only partially answered [1] . The amyloid cascade hypothesis proposes that abnormal accumulation and aggregation of Aβ peptides initiates a pathological sequence leading to tau hyperphosphorylation, synaptic dysfunction, neuroinflammation, and neuronal death [2] . Aβ peptides are generated by sequential cleavage of amyloid precursor protein (APP) by βsecretase (BACE1) and γ-secretase. The predominant species are Aβ₄₀ and Aβ₄₂, with the latter more prone to aggregation due to two additional hydrophobic C-terminal residues [3] . Once released, Aβ₄₂ monomers can misfold and self-assemble through a nucleationdependent polymerization pathway: monomers form small oligomers, which grow into protofibrils and eventually mature into insoluble amyloid fibrils deposited as senile plaques [4] . A critical shift occurred when research groups demonstrated that soluble oligomers, not mature fibrils, are the primary toxic species. Oligomers disrupt membranes, dysregulate calcium, impair mitochondria, and damage synapses at concentrations far below fibril toxicity thresholds [5] . This redirected therapeutic efforts toward preventing oligomer formation, with several small molecules showing preclinical anti-aggregation activity [6,7] . However, direct comparisons of these inhibitors under identical experimental conditions are rare, making it difficult to rank their effectiveness or understand their distinct mechanisms [8] . This research compared three chemically diverse aggregation inhibitors curcumin, EGCG, and scyllo-inositol using standardized Aβ₄₂ preparations and a uniform panel of biophysical and cell-based assays conducted at a single laboratory in Cairo, Egypt [9,10] .

Theoretical background

Aβ aggregation follows sigmoidal kinetics: a lag phase (nucleation), growth phase (elongation), and plateau. During nucleation, monomers adopt β-sheet conformation through hydrophobic interactions at residues 17-21 and 29-42 [4] . The critical nucleus is estimated at 4-6 monomers. Nuclei recruit additional monomers, generating protofibrils that align into cross-β fibrils [3] . Inhibitors can stabilize monomeric conformation (scyllo-inositol), redirect aggregation toward non-toxic species (EGCG), or bind oligomeric interfaces (curcumin) [6,7] . Thioflavin T fluorescence serves as the standard kinetic reporter, binding selectively to cross-β structures [11] .

Material and Methods

Material

This research was conducted at the Neurobiology and Protein Misfolding Laboratory, Department of Biochemistry, Cairo Institute of Technology, Cairo, Egypt, between February 2023 and January 2024. Synthetic Aβ₄₂ peptide (>95% purity by HPLC) was purchased from AnaSpec Inc. (Fremont, CA). Curcumin (≥98%), EGCG (≥95%), and scyllo-inositol (≥99%) were obtained from Sigma-Aldrich. Thioflavin T, hexafluoroisopropanol (HFIP), dimethyl sulfoxide (DMSO), and MTT reagent were also from Sigma-Aldrich. SH-SY5Y human neuroblastoma cells were sourced from ATCC (Manassas, VA) and maintained in DMEM/F12 supplemented with 10% fetal bovine serum and 1% penicillin-streptomycin at 37 °C with 5% CO₂. All buffer solutions were prepared using ultrapure water and filtered through 0.22 μm membranes. The institutional ethics review board of Cairo Institute of Technology approved the research protocol (CIT-BIO-2022-0631, approved November 2022) [12] .

Methods

Aβ₄₂ monomer films were prepared by dissolving peptide in HFIP, evaporating to dryness, and resuspending in DMSO at 5 mM. Working solutions (25 μM) were prepared in PBS (pH 7.4) immediately before each experiment. For inhibitor testing, compounds were added at equimolar (25 μM) and double (50 μM) concentrations at time zero. Aggregation kinetics were tracked by ThT fluorescence (excitation 440 nm, emission 485 nm) in 96-well black plates (Corning) using a BioTek Synergy H1 reader at 37 °C with orbital shaking, recorded every 15 minutes for 48 hours [11] . Particle size distributions were measured by dynamic light scattering (Malvern Zetasizer Nano ZS) at 0, 6, 12, 24, and 48 hours. Morphology was assessed by TEM (JEOL JEM-1400) with negative staining using 2% uranyl acetate. For cytotoxicity, SH-SY5Y cells (10⁴/well, 96-well plates) were treated with Aβ₄₂ oligomeric preparations (0.5-50 μM) with or without inhibitors (50 μM) for 48 hours. Viability was measured by MTT assay at 570 nm [13] . All experiments were performed in triplicate across three independent preparations. Data were analyzed by one-way ANOVA with Tukey's HSD in GraphPad Prism v9.5 [14] .

Results

Table The scatter plot shows a clear concentration-dependent toxicity curve for untreated oligomers, with viability dropping below 50% at approximately 10 μM. All three inhibitors shifted the curve rightward, with scyllo-inositol providing the greatest protection at every concentration tested.

Comprehensive interpretation

Correlation between ThT fluorescence reduction and viability improvement was strong (r = 0.92, p<0.001), confirming anti-aggregation efficacy translates to cytoprotection. Scyllo-inositol's monomer-stabilizing mechanism appears more effective than curcumin's oligomer surface binding, as preventing nucleation eliminates toxic species entirely. EGCG's intermediate performance reflects its dual mechanism of preventing β-sheet formation while redirecting aggregation toward less toxic species [6,7] .

Discussion

The 68.4% oligomer reduction by scyllo-inositol exceeds previously reported values of 45-60%, likely because our standardized monomer preparation minimized pre-formed seeds that could bypass nucleation [7,9] . The 4.8-hour lag extension confirms action at the nucleation stage, consistent with binding to the hydrophobic core (residues 17-21) and stabilizing monomeric conformation [6] . EGCG's 54.7% oligomer reduction while redirecting Aβ toward amorphous aggregates is noteworthy. DLS showed EGCG-treated samples at 12.3 nm at 12 hours larger than monomers but smaller than oligomers suggesting offpathway species with reduced toxicity. This matches reports of EGCG remodeling Aβ into unstructured aggregates lacking membrane-disrupting β-sheet organization [8] . Curcumin showed weakest performance (41.2% reduction), likely limited by poor aqueous solubility and rapid pHdependent degradation [15] . Curcumin analogues with improved stability are under active investigation [16] . The strong ThT-viability correlation (r = 0.92) validates ThT-based screening for identifying cytoprotective inhibitors. However, translating in vitro results to clinical benefit remains challenging given blood-brain barrier penetration and metabolic stability hurdles [17] .

Limitations

This research used synthetic Aβ₄₂ rather than brain-derived peptide, which may differ in post-translational modifications. The SH-SY5Y cell model, while widely used, doesn't replicate the complexity of neuronal circuits or the inflammatory microenvironment present in Alzheimer's brain. Inhibitor concentrations tested (25-50 μM) may not be achievable in cerebrospinal fluid. Only a 48-hour treatment window was examined.

Conclusion

This research demonstrated that scyllo-inositol, EGCG, and curcumin all inhibit Aβ₄₂ oligomerization and reduce neurotoxicity with different efficacies. Scyllo-inositol achieved the greatest oligomer reduction (68.4%), longest nucleation delay (4.8 hours), and best cytoprotection (58.3% viability), outperforming EGCG and curcumin across all endpoints. The distinct mechanisms monomer stabilization, offpathway redirection, and surface binding suggest combination approaches may yield synergistic benefits. The ThT-viability correlation (r = 0.92) supports kinetic assays as reliable primary screens for inhibitor discovery. Future work should evaluate combination treatments, test brain-penetrant analogues in animal models, and examine whether these inhibitors can disaggregate pre-formed oligomers.

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