Clusterin inhibits Aß42 aggregation through a "strawberry model" as detected by FRET-FCS.

Extracellular plaque deposits of ß-amyloid peptide (Aß) are one of the main pathological features of Alzheimer's disease (AD). The aggregation of Aß42 species, especially Aß42 oligomers, is still an active research field in AD pathogenesis. Secretory clusterin protein (sCLU), an extracellular chaperone, plays an important role in AD pathogenesis. Although sCLU interacts directly with Aß42 in vitro and in vivo, the mechanism is not clear. In this paper, His-tagged sCLU (sCLU-His) was cloned, expressed and purified, and we applied florescence resonance energy transfer-fluorescence correlation spectroscopy (FRET-FCS) to investigate the direct interaction of sCLU-His and Aß42 at the single-molecule fluorescence level in vitro. Here, we chose four different fluorescently labeled Aß42 oligomers to form two different groups of aggregation models, easy or difficult to aggregate. The results showed that sCLU-His could form complexes with both aggregation models, and sCLU-His inhibited the aggregation of Aß42/RB  ~ Aß42/Atto647 (easy to aggregate model). The complexes were produced as the Aß42/Label adhered to the sCLU-His, which is similar to a "strawberry model," as strawberry seeds are dotted on the outer surface of strawberries. This work provided additional insight into the interaction mechanism of sCLU and Aß42 .

The effects of fluorescent labels on Aß42 aggregation detected by fluorescence correlation spectroscopy.

Fluorescence-based methods are promising for measuring amyloid beta (Aß) oligomers, given their capacity to analyse a sample at the single-molecule level. As the attachment of fluorescent labels may influence the biochemical properties of the Aß oligomers, the effects of fluorescent labels on Aß oligomers must be evaluated. In this paper, we compared the impacts of five different fluorescent dyes on the aggregation of Aß42 oligomers using fluorescence correlation spectroscopy (FCS). We found that fluorescent labels of BODIPY® FL-C5 (BP), N-hydroxysuccinimide rhodamine B ester (RB) and rhodamine B isothiocyanate (RITC) increased the propensity of labelled Aß42 oligomers to aggregate, whereas 6-(fluorescein-5-carboxamido) hexanoic acid succinimidyl ester (5-SFX) and fluorescein 5(6)-isothiocyanate (5(6)-FITC) decreased the propensity of labelled Aß42 oligomers to aggregate. This difference originated from the different electric charges and hydrophobicity of the fluorescent dyes. These results provide valuable information for establishing different aggregation models for Aß42 oligomers in vitro using FCS.