Amyloid-β(1−42) Rapidly Forms Protofibrils and Oligomers by Distinct Pathways in Low Concentrations of Sodium Dodecylsulfate

Abstract

Alzheimer's disease (AD) is characterized by large numbers of senile plaques in the brain that consist of fibrillar aggregates of 40- and 42-residue amyloid-beta (Abeta) peptides. However, the degree of dementia in AD correlates better with the concentration of soluble Abeta species assayed biochemically than with histologically determined plaque counts, and several investigators now propose that soluble aggregates of Abeta are the neurotoxic agents that cause memory deficits and neuronal loss. These endogenous aggregates are minor components in brain extracts from AD patients and transgenic mice that express human Abeta, but several species have been detected by gel electrophoresis in sodium dodecylsulfate (SDS) and isolated by size exclusion chromatography (SEC). Endogenous Abeta aggregation is stimulated at cellular interfaces rich in lipid rafts, and anionic micelles that promote Abeta aggregation in vitro may be good models of these interfaces. We previously found that micelles formed in dilute SDS (2 mM) promote Abeta(1-40) fiber formation by supporting peptide interaction on the surface of a single micelle complex. In contrast, here we report that monomeric Abeta(1-42) undergoes an immediate conversion to a predominant beta-structured conformation in 2 mM SDS which does not proceed to amyloid fibrils. The conformational change is instead rapidly followed by the near quantitative conversion of the 4 kDa monomer SDS gel band to 8-14 kDa bands consistent with dimers through tetramers. Removal of SDS by dialysis gave a shift in the predominant SDS gel bands to 30-60 kDa. While these oligomers resemble the endogenous aggregates, they are less stable. In particular, they do not elute as discrete species on SEC, and they are completed disaggregated by boiling in 1% SDS. It appears that endogenous oligomeric Abeta aggregates are stabilized by undefined processes that have not yet been incorporated into in vitro Abeta aggregation procedures.

Specific Soluble Oligomers of Amyloid-β Peptide Undergo Replication and Form Non-fibrillar Aggregates in Interfacial Environments

“…Recently, a 12-16-mer oligomeric species of A␤42 called "globulomers" was reported to be formed independently of the fibril formation pathway (18), and a similar observation was reported for a 9 -15-mer species generated in the presence of SDS (19). Moreover, annular protofibrils (APFs) formed from prefibrillar oligomers (PFOs) failed to convert to fibrils even after an extended period of time, suggesting that these pathogenic oligomers are formed outside the nucleation-dependant fibril formation pathway (20).…”

“…This property has been recently demonstrated by Kayed et al (28), who reported that specific soluble oligomers called PFOs were able to seed their own replication upon interacting with monomeric A␤. The property of replication could be a manifestation of the unique structure of the oligomeric assembly and more importantly may complement our previously reported hypothesis that such oligomers are formed along a pathway different from fibril formation (1,19). To explore this, a 20 M sample of freshly purified, "seedfree" A␤42 was incubated with 0.4 M isolated LFAOs as seeds (2% molar ratio) at room temperature.…”

What Can the Kinetics of Amyloid Fibril Formation Tell about Off-pathway Aggregation?

“…The hyperbolic (concave) profiles exhibiting no inflection point or lag phase in the absence of seeding (Fig. 1a) are frequently reported in the literature during the aggregation of, for example, serum albumin (19,20), transthyretin (21,22), ␤ 2 -microglobulin (23), the four-repeat domain of Tau (24), apolipoprotein (25), and amyloid-␤ variants (26,27). This type of result is explained by the CLM as the result of predominant primary nucleation over the autocatalytic processes and is also well fitted by the "Ockham's razor" minimalistic model (18,28).…”

Distinct synthetic Aβ prion strains producing different amyloid deposits in bigenic mice

“…The anionic surface created by SDS has been found to promote fibril formation for a variety of proteins (33). In earlier studies by others, 0.2% SDS was reported to induce globular aggregates of Aβ; moreover, no fibrils were detected by atomic force microscopy (34).…”