The Threefold Polymorph of Amyloid Beta Fibrils Employ a Distinct Intermediate Frustrated State for Growth

Alzheimer’s disease ranks as the third leading cause of death amongst adults over the age of 60 behind heart disease and cancer. The accumulation of amyloid plaque and neurofibrillary tangles causes the irreversible degradation and destruction of neurons that drastically shrinks brain tissue. Amyloid plaques are caused by the buildup of insoluble amyloid beta (Aβ) fibrils formed from self-assembled propagation of peptides. Studies have shown that Aβ can take shape as soluble oligomers or insoluble fibrils with different morphologies. Aβ fibrils attain numerous structures, called polymorphs, both in patients’ brains and in vitro. Prior observations of fibrils grown in vitro, revealed that twofold polymorph of Aβ40 fibrils was dominantly expressed in agitated solutions; whereas the threefold polymorph of Aβ40 fibrils preferentially formed in the quiescent solutions, which also closely resemble the structures of Aβ fibrils extracted from Alzheimer’s patients’ brains.

Previous atomic force microscopy (AFM) studies have demonstrated that the twofold symmetric Aβ40 polymorph grew steadily at broad ranges of peptide solution concentrations. It was also established that the fibril growth rate was determined by an activated complex, comprised by a peptide chain partially bound to the fibril tip and folded in a conformation that is distinct from that of the fibril bulk; the rearrangement of the intermediate configuration of the incoming peptide to the structure in the bulk fibril constituted the transition state for growth [1]. Using time-resolved in situ AFM, the growth of seeds nucleated in quiescent solutions were monitored, and the data showed that the correlation between the growth rate with the peptide concentration in the solutions drastically diverged from the linear kinetic dependence shown by the twofold fibrils. For the threefold polymorph, the growth rate increased linearly with peptide concentration up to 5 mM, but at higher concentration it reached a saturation value. A model, analogous to the Michaelis-Menten law for enzyme kinetics, was developed to describe the kinetics of the frustrated complex at the fibril tip. Further analyses of the model have suggested that the distinct kinetic dependences of twofold and threefold fibrils show different properties of their respective frustrated complexes at the fibrils tips that were apparent as different kinetic parameters. Our findings illuminate the complex interplay between fibril structures and the associated transition states during the growth of amyloid β fibrils.