Origin of metastable oligomers and their effects on amyloid fibril self-assembly

Abstract

Simultaneous analysis of oligomer and fibril assembly kinetics reveals inhibitory effects of metastable oligomers on amyloid fibril formation.

Importance of micelle‐like multimers in the atypical aggregation kinetics of N‐terminal serum amyloid A peptides

“…This latter study specifically documented critical off‐pathway oligomers formed above a ‘critical oligomerization concentration’ competing with mature fibrils for recruitment of monomer, such that the lag times of mature fibril growth decrease with concentration below this concentration and increase above it. A similar situation is observed in our reactions, with the micelles serving the same inhibitory function as the ‘globular oligomers’ and ‘curvilinear fibrils’ observed in that study .…”

“…The noncanonical increase of lag times with increasing concentration of protein has also been observed in other proteins [9][10][11]21] and in each case was attributed to off-pathway species of varying type, for example, the 'inhibitory monomers' observed for calcitonin at high concentration, thought to compete with native monomers for addition to growing fibrils [9]. Off-pathway inhibitory oligomers, furthermore, have also been observed in amyloidogenic proteins and peptides such as glucagon [11], lysozyme [21], and an engineered Ab peptide dimer [21]. This latter study specifically documented critical off-pathway oligomers formed above a 'critical oligomerization concentration' competing with mature fibrils for recruitment of monomer, such that the lag times of mature fibril growth decrease with concentration below this concentration and increase above it.…”

“…In total, our data show that the SAA peptide fibrillization process displays at least three remarkable features: (a) a direct dependence of the t(50) on peptide concentration (i.e., slower fibrillization at higher concentration); (b) insensitivity to seeding by pre‐formed amyloid; and (c) formation of multimeric intermediates with some characteristics of micelles (ability to sequester pyrene), though these particles are larger than would be expected for typical micelles. Each of these features has been observed in other amyloidogenic systems , but to our knowledge this is the first instance of all three occurring in a single reaction.…”

“…For example, seeding‐independent fibrillization due to protein sequestration in off‐pathway oligomers was observed for κ‐casein as mentioned above . The noncanonical increase of lag times with increasing concentration of protein has also been observed in other proteins and in each case was attributed to off‐pathway species of varying type, for example, the ‘inhibitory monomers’ observed for calcitonin at high concentration, thought to compete with native monomers for addition to growing fibrils . Off‐pathway inhibitory oligomers, furthermore, have also been observed in amyloidogenic proteins and peptides such as glucagon , lysozyme , and an engineered Aβ peptide dimer .…”

“…Our model anticipates that below the CMC, the peptides should exhibit classical nucleation‐dependent kinetics, with an inverse dependence of lag phase on concentration and a seeding dependence, as seen below the critical oligomerization concentration by Hasecke et al . . Unfortunately, owing to the low ThT‐binding capacity of the SAA peptides, our assay was too insensitive to accurately monitor kinetics at peptide concentrations much below 50 μ m .…”