Self-Assembly of Aβ40, Aβ42 and Aβ43 Peptides in Aqueous Mixtures of Fluorinated Alcohols

Abstract

Fluorinated alcohols such as hexafluoroisopropanol (HFIP) and trifluoroethanol (TFE) have the ability to promote α-helix and β-hairpin structure in proteins and peptides. HFIP has been used extensively to dissolve various amyloidogenic proteins and peptides including Aβ, in order to ensure their monomeric status. In this paper, we have investigated the self-assembly of Aβ40, Aβ42, and Aβ43 in aqueous mixtures of fluorinated alcohols from freshly dissolved stock solutions in HFIP. We have observed that formation of fibrillar and non-fibrillar structures are dependent on the solvent composition. Peptides form fibrils with ease when reconstituted in deionized water from freshly dissolved HFIP stocks. In aqueous mixtures of fluorinated alcohols, either predominant fibrillar structures or clustered aggregates were observed. Aqueous mixtures of 20% HFIP are more favourable for Aβ fibril formation as compared to 20% TFE. When Aβ40, Aβ42, and Aβ43 stocks in HFIP are diluted in 50% aqueous mixtures in phosphate buffer or deionized water followed by slow evaporation of HFIP, Aβ peptides form fibrils in phosphate buffer and deionized water. The clustered structures could be off-pathway aggregates. Aβ40, Aβ42, and Aβ43 showed significant α-helical content in freshly dissolved HFIP stocks. The α-helical conformational intermediate in Aβ40, Aβ42, and Aβ43 could favour the formation of both fibrillar and non-fibrillar aggregates depending on solvent conditions and rate of α-helical to β-sheet transition.

Fig 1. TEM images and ThT fluorescence of Aβ40, Aβ42, and Aβ43 from deionized water.

Images after 36 hours (Panels A, B, and C, respectively) and 72 hours (Panels D, E, and F, respectively) of incubation at 37°C. Scale bars correspond to 200 nm. ThT fluorescence intensities at 0 hour (immediately after dissolution), 36 hours and 72 hours of incubation at 37°C are shown in panel G. Values represent average values of ThT fluorescence intensity ± standard deviation. BL denotes basal fluorescence of ThT in absence of peptides.

Fig 2. TEM images and ThT fluorescence of Aβ40, Aβ42, and Aβ43 from PB.

HFIP stocks of Aβ40, Aβ42, and Aβ43 were diluted in 10 mM phosphate buffer, pH 7.4(PB) followed by 12 hours of incubation at 25°C. Aβ40, Aβ42, and Aβ43 were imaged from PB (Panels A, B, and C, respectively). Scale bars correspond to 200 nm. ThT fluorescence intensities observed over 12 hours of incubation in 2% HFIP at 25°C are shown in panel D. Values represent average values of ThT fluorescence intensity ± standard deviation. BL denotes basal fluorescence of ThT in absence of peptides.

Fig 3. TEM images of Aβ40, Aβ42, and Aβ43 from 20% HFIP in PB.

Aβ40 imaged after 2 hours of incubation at 25°C (Panel A), Aβ42 imaged after 12 hours of incubation at 25°C (Panel B), and Aβ43 imaged after 2 hours of incubation at 25°C (Panel C). Scale bars correspond to 200 nm. ThT fluorescence intensities observed over 12 hours of incubation at 25°C are shown in panel D. Values represent average values of ThT fluorescence intensity ± standard deviation. BL denotes basal fluorescence of ThT in absence of peptides.

Fig 4. TEM images of Aβ40, Aβ42, and Aβ43 from 20% TFE in PB.

Aβ40, Aβ42, and Aβ43 imaged after 12 hours of incubation at 25°C are shown in panels A, B, and C, respectively. Images from 37°C incubated stocks are shown in panels D, E, and F, respectively. Scale bars correspond to 200 nm. Fibrillar structures associated with clustered aggregates are indicated by arrows. ThT fluorescence intensities observed over 12 hours of incubation at 25°C are shown in panel G. Values represent average values of ThT fluorescence intensity ± standard deviation. BL denotes basal fluorescence of ThT in absence of peptides.

Fig 5. TEM images of Aβ40, Aβ42, and Aβ43 from 10% HFIP and 10% TFE in PB.

Aβ40, Aβ42, and Aβ43 imaged after 12 hours of incubation at 25°C from 10% HFIP (panels A, B, and C, respectively) and 10% TFE (Panels E, F, and G, respectively). Scale bars correspond to 200 nm. ThT fluorescence intensities observed over 12 hours of incubation at 25°C in 10% HFIP and 10% TFE are shown in panels D and H, respectively. Values represent average values of ThT fluorescence intensity ± standard deviation. BL denotes basal fluorescence of ThT in absence of peptides.

Fig 6. TEM images of Aβ40, Aβ42, and Aβ43 from 50% HFIP in PB.

Aβ40, Aβ42, and Aβ43 imaged after 10 days of incubation at 25°C (Panels A, B, and C, respectively). Scale bars correspond to 200 nm. ThT fluorescence intensities observed over 12 hours of incubation at 25°C are shown in panel D. Values represent average values of ThT fluorescence intensity ± standard deviation. BL denotes basal fluorescence of ThT in absence of peptides.

Fig 7. TEM images and ThT fluorescence of Aβ40, Aβ42, and Aβ43 from 50% aqueous mixtures of HFIP after complete evaporation of HFIP.

Images of Aβ40, Aβ42, and Aβ43 from PB (Panels A, B, and C, respectively) and deionized water (Panels E, F, and G, respectively) after evaporation of HFIP. Scale bars correspond to 200 nm. ThT fluorescence intensities at 482 nm recorded for the peptide solutions in PB (Panel D) and deionized water (Panel H). After evaporation of HFIP from PB and deionized water, peptides were diluted two fold in 10 mM and 20 mM PB, respectively prior to recording ThT spectra. Values of ThT fluorescence at 482 nm are presented as average ± standard deviation from three consecutive spectra. BL denotes basal fluorescence of ThT in absence of peptides.

Fig 8. TEM images of Aβ peptides from peptide solutions in deionized water HFIP mixtures.

Aβ40, Aβ42, and Aβ43 from 10% HFIP were imaged after 12 hours of incubation at 25°C (Panels A, B, and C, respectively). Aβ40, Aβ42, and Aβ43 from 20% HFIP were imaged after 2 hours of incubation at 25°C (Panels D, E, and F, respectively). Aβ40, Aβ42, and Aβ43 from 50% HFIP were imaged after 2 hours of incubation at 25°C (Panels G, H, and I, respectively). Scale bars correspond to 200 nm for panels A-C and 1 μm for panels D-I.

Fig 9. CD spectra recorded for Aβ40 (∙∙∙∙∙∙∙∙), Aβ42 (——-), and Aβ43 (—).

From freshly dissolved HFIP stocks at 20 μM peptide concentrations (Panel A), from deionized water stocks incubated for 72 hours at 100 μM peptide concentrations at 37°C, after diluting to 20 μM peptide concentrations (Panel B), and from freshly prepared solutions of 2% HFIP in PB (Panel C) at 20, 10, and 5 μM concentrations for Aβ40, Aβ42, and Aβ43, respectively. From freshly prepared solutions in 10% HFIP (Panel D), 20% HFIP (Panel E) and 50% HFIP (Panel F) in PB at 20, 10 and 5 μM concentrations for Aβ40, Aβ42, and Aβ43, respectively. From freshly prepared solutions in 10% HFIP (Panel G), 20% HFIP (Panel H) and 50% HFIP (Panel I) in deionized water at 20, 10, and 5 μM concentrations for Aβ40, Aβ42, and Aβ43, respectively.