Binding of Pro-inflammatory Proteins S100A8 or S100A9 to Amyloid-β Peptide Suppresses Its Fibrillation

Human serum albumin (HSA) is a natural depot of amyloid β-peptide (Aβ), a key player in the Alzheimer's disease (AD). HSA and pro-inflammatory Ca2+-binding proteins S100A8 and S100A9 are involved in Aβ metabolism and its deposition in the brain, serve as probable triggers and therapeutic targets in AD, but their interplay with regard to Aβ binding/fibrillation is unclear. To this end, here we explore in vitro binding of Ca2+-bound S100A8 or S100A9 to monomeric Aβ and influence of the S100 proteins on Aβ fibrillation. The equilibrium dissociation constants of the complexes of dimeric S100A8/S100A9 with Aβ40/42 estimated by biolayer interferometry are of 1-5 µM. S100A8 and S100A9 interfere with HSA binding to Aβ. Thioflavin T assay and electron microscopy data show that micromolar S100A8/S100A9 inhibit Aβ40 fibrillation, and the inhibitory effect of S100A8 exceeds that for HSA. The competition for Aβ between HSA and S100A8/S100A9 may contribute to the Aβ-HSA imbalance in the pro-inflammatory conditions in AD.