Critical aggregation concentration for the formation of early Amyloid-β (1–42) oligomers

Abstract

The oligomers formed during the early steps of amyloid aggregation are thought to be responsible for the neurotoxic damage associated with Alzheimer's disease. It is therefore of great interest to characterize this early aggregation process and the aggregates formed, especially for the most significant peptide in amyloid fibrils, Amyloid-β(1-42) (Aβ42). For this purpose, we directly monitored the changes in size and concentration of initially monomeric Aβ42 samples, using Fluorescence Correlation Spectroscopy. We found that Aβ42 undergoes aggregation only when the amount of amyloid monomers exceeds the critical aggregation concentration (cac) of about 90 nM. This spontaneous, cooperative process resembles surfactants self-assembly and yields stable micellelike oligomers whose size (≈50 monomers, R h ≈ 7-11 nm) and elongated shape are independent of incubation time and peptide concentration. These findings reveal essential features of in vitro amyloid aggregation, which may illuminate the complex in vivo process.Alzheimer's disease (AD) is a neurodegenerative disease characterized by the presence of Amyloid-β plaques in the brain. Although the causal relationship between these protein fibrillar aggregates and the neurodegenerative disease has not been established yet, the 'amyloid hypothesis' , that accumulation and aggregation of amyloid-β peptide initiates a cascade of neurodegenerative events, has been widely accepted [1][2][3] . Impairment of Amyloid-β clearance in AD patients seems to be the main cause for accumulation of the peptide 4,5 . It is thought that the neurotoxic species that trigger the amyloid cascade leading to neurodegeneration are early non-fibrillar aggregates, which may also be the precursors of the amyloid fibrils 2,6-8 . The dominant peptides in amyloid fibrils are Amyloid-β(1-42) (Aβ42) and Amyloid-β(1-40) (Aβ40), with Aβ42 being the more fibrillogenic of the two, with a much stronger tendency to aggregate [9][10][11] .There is ample literature on the mechanism underlying amyloid fibril formation 12,13 . Most kinetic studies agree on a complex nucleation-growth mechanism, where the differences in the microscopic rates and in the relevance of secondary nucleation processes determine the degree of aggregation and can account for the differences between Aβ40 and Aβ42 11,14 . For such nucleation-dependent processes, a critical aggregation concentration (cac) is predicted, above which aggregation takes place 10 . For Aβ40 the formation of micelle-like intermediates was reported, with a critical concentration in the micromolar range [15][16][17] , whereas recent studies have found nanomolar cac values for both Aβ40 and Aβ42 [18][19][20] . The latter values fit better with the reported physiological concentrations of Aβ in the picomolar to nanomolar range which may be locally higher due to accumulation or impairment of clearance 4,5,18,21,22 . A possible reason for the discrepancy in the cac values may be the strong adsorption of Amyloids Aβ40 and Aβ42 to interfaces 23 , which can lead to great ...

Solution‐Based Determination of Dissociation Constants for the Binding of Aβ42 to Antibodies

“…It should be mentioned that both AUC and MST report the information about the bound and unbound states of the labeled molecule. At a concentration lower than the critical aggregation concentration of about 90 nM, Aβ is unlikely to form aggregates within the time scale of both experiments, which definitely helps to reduce the incidence of multiple binding between Aβ oligomers and antibodies. The size distribution of all antibody‐containing FITC‐Aβ42 samples also revealed that complexes with only one antibody molecule were the dominant species in solution.…”

β‐Amyloid aggregation and heterogeneous nucleation

“…[44][45][46][47][48][49] The amphiphilicity of Aβ is itself a cause of its self-association. Several authors have noted the micelle-like nature of some Aβ soluble oligomers, [50][51][52][53][54] and in fibrils, the parallel, in-register nature of the β-sheets tends to minimize the exposure of the hydrophobic domains to the aqueous medium. In addition, it has been shown that a short internal fragment of Aβ, Aβ16-21, forms antiparallel β-sheets when the N-terminus is acetylated, but the orientation flips to parallel and in-register when the N-terminus is octanoylated-a change that also renders the peptide more amphiphilic in monolayers at the air-water interface.…”

Collagen hydrogel confinement of amyloid-βaccelerates aggregation and reduces cytotoxic effects

“…At time zero, unstructured protein globules of various sizes were identified (Figure 2a). The smallest structures have a diameter of ~3.6 nm, which correlates well to the size of Aβ monomer (hydrodynamic radius of 1.8 nm) [71]. Larger globular structures are 6 – 50 nm in diameter and lack any fibrillar structures.…”