Insulin modulates Aβ_1-42 aggregation toward non-toxic forms: A comparative study with cis-[Ru(phen)₂(3,4Apy)₂]²

Intranasal insulin therapy for Alzheimer's disease (AD) has received increasing attention due to its potential to modulate amyloid-β (Aβ) aggregation, a hallmark of AD pathology. Molecular dynamics simulations suggest that insulin binds across U-shaped Aβ_1-42oligomers, disrupting β-strand alignment via its A-chain. In contrast, the Ru(II) complex cis-[Ru(phen)₂(3,4Apy)₂]²⁺(Ru3,4Apy; phen = 1,10-phenanthroline; 3,4Apy = 3,4-aminopyridine) primarily interacts with fibril surfaces. Although both target the central hydrophobic core motif, they bind to distinct regions of Aβ_1-42. Their co-aggregation with Aβ_1-42 may thus influence aggregation and cytotoxicity differently, providing insight into the molecular mechanisms underlying insulin's mitigation of Aβ-induced cytotoxicity. We monitored Aβ_1-42 aggregation using Tyr10 intrinsic fluorescence, circular dichroism, atomic force microscopy, and SH-SY5Y cell viability assays in the presence of insulin, Ru3,4Apy, or both. Ru3,4Apy reduced β-sheet formation but did not prevent Aβ_1-42-induced cytotoxicity. In contrast, insulin limited conformational flexibility, prevented Tyr10 burial, suppressed β-sheet formation, and promoted amorphous, non-toxic aggregates. Notable, in the Aβ_1-42/Ru3,4Apy/Insulin mixture, aggregation followed a non-toxic pathway bypassing the buried-core β-sheet typical of mature fibrils. These findings emphasize the specificity of Aβ_1-42/Insulin interactions and suggest targeting defined binding sites as a promising AD therapeutic strategy.