(712g) The Transition State for the Growth of the Three-Fold Polymorph of Amyloid-? Fibrils Is Supported By Native Contacts

Alzheimer’s disease (AD) ranks among the top ten leading causes of death worldwide. AD is associated with the accumulation of amyloid plaques and neurofibrillary tangles throughout the brain, which are thought to cause the irreversible degradation and destruction of the neurons. Amyloid plaques are generated by the buildup of insoluble amyloid-beta (Aβ) fibrils that form by peptide self-assembly. Aβ fibrils attain numerous structures, called polymorphs, both in patients’ brains and in vitro. Prior observations reveal that in agitated solutions the two-fold polymorph of Aβ40 fibrils dominates, whereas a three-fold symmetric structure preferentially forms in the quiescent solutions; this molecular arrangement resembles the structures of the fibrils extracted from the brains of Alzheimer’s patients. Previous atomic force microscopy (AFM) studies have demonstrated that the two-fold symmetric Aβ40 polymorph grows steadily in a broad range of peptide concentrations. It was also established that the fibril growth rate is determined by a frustrated complex stationed at the tip of the fibril and composed of one or more peptide chains in non-native conformations; the rearrangement of the non-native configuration of the peptides at the fibril tip to the structure in the fibril bulk constitutes the transition state for growth [1]. We employ time-resolved in situ AFM to monitor the growth of fibrils nucleated in quiescent solutions. In contrast to the two-fold polymorph, which exhibits a linear correlation between the fibril growth rate and the peptide concentration, the growth rate of three-fold symmetric fibrils saturates at peptide concentrations above 5 μM. Kinetic analysis based on a Michaelis-Menten-type two-step sequence of reactions suggests that the transformation of the intermediate frustrated state of an incorporating peptide determines the overall growth rate. The addition of urea, known to destabilize contacts both in the fibril bulk and in the intermediate complex, increases the solubility and slows the growth of three-fold fibrils at all peptide concentrations. The urea-induced growth deceleration is in contrast to the response of two-fold fibrils, which grow substantially faster in the presence of urea. We interpret the saturating kinetic dependence and the response to urea as evidence that the intermediate complex in three-fold symmetric fibrils is supported only by native contacts. Our finding illuminates the complex interplay between fibril structures and the associated transition states during the growth of amyloid-β fibrils.

[1] Xu, Y. et al. Frustrated peptide chains at the fibril tip control the kinetics of growth of amyloid-β fibrils. Proceedings of the National Academy of Sciences 118, e2110995118, (2021).