The stability and dynamics of the Aβ40/Aβ42 interlaced mixed fibrils

The accumulation of fibrillar amyloid-beta (A beta) aggregates in the brain, predominantly comprising 40- and 42-residue amyloid-beta (A beta 40 and A beta 42), is a major pathological hallmark of Alzheimer's disease (AD). A beta 40 and A beta 42 naturally coexist in the brain under normal physiological conditions, and their interplay is generally considered to be a critical factor in the progression of AD. In addition to forming homogeneous oligomers and fibrils, A beta 40 and A beta 42 are also reported to co-assemble into hetero-oligomers and interlaced mixed fibrils, as evidenced by solid-state nuclear magnetic resonance spectroscopy (NMR), high molecular weight mass spectrometry and cross-seeding experiments. However, the exact molecular mechanisms underlying these processes remain unclear. In this study, we have used a recently resolved structurally uniform 1:1 mixture of A beta 40/A beta 42 interlaced mixed fibril as a prototype to gain insights into the molecular-level interactions between A beta 40 and A beta 42. We employed fully atomistic molecular dynamics simulation and compared the results with a homogeneous U-shaped A beta 40 fibrillar model. Our simulations using two different force fields provide conclusive evidence that the A beta 40/A beta 42 interlaced mixed fibril is energetically more favorable than the homogeneous A beta 40 fibrillar model. Furthermore, we also show that the increase in stability observed in the mixed model stems primarily from the packing interfaces and the stacking interfaces between C-termini. Our simulation results provide valuable mechanistic insights that are not readily accessible in experiment and could have significant implications for both the pathogenesis of AD and the development of current therapeutic strategies.Communicated by Ramaswamy H. Sarma