Sclerotiorin Stabilizes the Assembly of Nonfibrillar Abeta42 Oligomers with Low Toxicity, Seeding Activity, and Beta-sheet Content

The self-assembly of the 42-residue amyloid-beta peptide, A beta 42, into fibrillar aggregates is associated with neuronal dysfunction and toxicity in Alzheimer's disease (AD) patient brains, suggesting that small molecules acting on this process might interfere with pathogenesis. Here, we present experimental evidence that the small molecule sclerotiorin (SCL), a natural product belonging to the group of azaphilones, potently delays both seeded and nonseeded A beta 42 polymerization in cell-free assays. Mechanistic biochemical studies revealed that the inhibitory effect of SCL on fibrillogenesis is caused by its ability to kinetically stabilize small A beta 42 oligomers. These structures exhibit low beta-sheet content and do not possess seeding activity, indicating that SCL acts very early in the amyloid formation cascade before the assembly of seeding-competent, beta-sheet-rich fibrillar aggregates. Investigations with NMR WaterLOGSY experiments confirmed the association of A beta 42 assemblies with SCL in solution. Furthermore, using ion mobility-mass spectrometry, we observed that SCL directly interacts with a small fraction of A beta 42 monomers in the gas phase. In comparison to typical amyloid fibrils, small SCL-stabilized A beta 42 assemblies are inefficiently taken up into mammalian cells and have low toxicity in cell-based assays. Overall, these mechanistic studies support a pathological role of stable, beta-sheet-rich A beta 42 fibrils in AD, while structures with low beta-sheet content may be less relevant. (C) 2020 The Authors. Published by Elsevier Ltd.