Retardation of Abeta fibril formation by phospholipid vesicles depends on membrane phase behavior

Abstract

An increasing amount of evidence suggests that in several amyloid diseases, the fibril formation in vivo and the mechanism of toxicity both involve membrane interactions. We have studied Alzheimer's disease related amyloid beta peptide (Abeta). Recombinant Abeta(M1-40) and Abeta(M1-42) produced in Escherichia coli, allows us to carry out large scale kinetics assays with good statistics. The amyloid formation process is followed in means of thioflavin T fluorescence at relatively low (down to 380 nM) peptide concentration approaching the physiological range. The lipid membranes are introduced in the system as large and small unilamellar vesicles. The aggregation lagtime increases in the presence of lipid vesicles for all situations investigated and the phase behavior of the membrane in the vesicles has a large effect on the aggregation kinetics. By comparing vesicles with different membrane phase behavior we see that the solid gel phase dipalmitoylphosphatidylcholine bilayers cause the largest retardation of Abeta fibril formation. The membrane-induced retardation reaches saturation and is present when the vesicles are added during the lag time up to the nucleation point. No significant difference is detected in lag time when increasing amount of negative charge is incorporated into the membrane.

Figure 1

(A) Schematic illustration of the bilayer phases in the vesicles used in this study. By combining DOPC, DPPC, and cholesterol, three different phases were achieved with different degrees of translational diffusion and acyl chain order. (B) Chemical structure of used lipids. (C) Amino acid sequence of Aβ(M1–42) with hydrophobic residues in bold. Residues that are negatively (−) or positively (+) charged or titrating (t) at neutral pH are indicated under the sequence. The last two residues (IA) are missing in Aβ(M1–40).

Figure 2

ThT fluorescence as a function of time in the absence (dashed line) and presence (solid line) of DOPC large unilamellar vesicles. Each line represents one replicate in a separate well in a 96-well plate. (A) Aβ(M1–40) (4.8 μM) with or without 1.8 mM phospholipid. (B) Aβ(M1–42) (0.38 μM) with or without 1.1 mM phospholipid. All experiments were carried out in 20 mM Tris/HCl, 0.2 mM EDTA, 0.02% NaN₃, 20 μM ThT, pH 7.4. One replicate with peptide alone that never fibrillated was removed from A.

Figure 3

ThT fluorescence kinetic traces for 6.0 μM Aβ(M1–40) in the presence of (A) DOPC or (B) DPPC in small unilamellar vesicles with (solid lines) or without (dashed lines) membrane-incorporated cholesterol in 20 mM Tris/HCl, pH 7.4, 0.2 mM EDTA, 0.02% NaN₃ with 20 μM ThT. Cholesterol was added at a ratio of 2:1 PC/cholesterol and the total lipid concentration is in all cases 1.9 mM. Each line represents one replicate in a separate well.

Figure 4

Lagtime dependence on DPPC concentration. 6.8 μM Aβ(M1–40) was incubated with different concentrations of DPPC present as large unilamellar vesicles in 20 mM Tris/HCl, pH 7.4, 0.2 mM EDTA, 0.02% NaN₃ with 20 μM ThT. The half-time of completion, t_0.5, is defined as time at 50% of maximum intensity and the error bars represent the standard deviation from five wells.

Figure 5

Addition of DPPC vesicles at different time points during the amyloid formation process. The experiment was started with 50 μL 12 μM Aβ(M1–40) in 20 mM Tris/HCl, pH 7.4, 0.2 mM EDTA, 0.02% NaN₃ with 20 μM ThT. At different time points, 50 μL 2.7 mM DPPC large unilamellar vesicles or 50 μL buffer was added. (A) ThT fluorescence versus time. Vertical lines indicate the time of vesicle or buffer addition as also given as numbers in each panel. Solid lines represent data from wells to which vesicles were added and dashed lines data from wells to which buffer was added. (B) Lag time as a function of time of vesicle (solid circles) or buffer (open circles) addition. Each point represents an average of two replica and the lag time is defined as the time at which 10% of the maximum fluorescence is reached. Dashed lines indicate the lag time in the absence of vesicles.

Figure 6

Lagtime dependence on membrane charge. Aβ(M1–40) (6.7 μM) was incubated with large unilamellar vesicles of different DOPS/DOPC ratios in 20 mM Tris/HCl, pH 7.4, 0.2 mM EDTA, 0.02% NaN₃ with 20 μM ThT. The total phospholipid concentration was kept constant at 1.2 mM. Each point represents the mean incubation time until 50% of maximum intensity is reached ± standard deviation from five wells. Dashed lines represent standard deviation error limits for five samples of peptide in absence of lipids.

Figure 7

Langmuir monolayer experiment. 2 ml 30 μM Aβ(M1–40) was injected under a monolayer of DPPC or 2,3-dimercapto-1-propanesulfonic acid on a subphase of 20 mL 20 mM Tris/HCl, pH 7.4, 0.2 mM EDTA, 0.02% NaN_3. (A) Liquid expanded phase (π₀ = 6 mN/m) as a model for liquid disordered bilayer phase. (B) Liquid condensed phase (π₀ = 25 mN/m) as a model for gel bilayer phase. The small pressure drop during injection due to mechanical disturbance is corrected for and Δπ is thus defined as pressure increase after injection.