The interaction between amyloid-β peptides and anionic lipid membranes containing cholesterol and melatonin - PubMed (original) (raw)

The interaction between amyloid-β peptides and anionic lipid membranes containing cholesterol and melatonin

Hannah Dies et al. PLoS One. 2014.

Abstract

One of the hallmarks of Alzheimer's disease is the formation of senile plaques, primarily consisting of amyloid-β (Aβ) peptides. Peptide-membrane and peptide-lipid interactions are thought to be crucial in this process. We studied the interaction of Aβ₁₋₄₂ and Aβ₂₅₋₃₅ peptides with anionic lipid membranes made of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphoserine (DMPS) using X-ray diffraction. We compare the experimentally determined electron densities in the gel state of the membranes with density calculations from peptide structures reported in the Protein Data Bank in order to determine the position of the peptide in the bilayers. The full length peptide Aβ₁₋₄₂ was found to embed in the hydrocarbon core of the anionic lipid bilayers. Two populations were found for the Aβ₂₅₋₃₅ peptide: (1) membrane-bound states in the hydrophilic head group region of the bilayers, where the peptides align parallel to the membranes, and (2) an embedded state in the bilayer center. Aging plays an important role in the development of Alzheimer's, in particular with respect to changes in cholesterol and melatonin levels in the brain tissue. Immiscible cholesterol plaques were created by addition of 30 mol% cholesterol to the anionic membranes. The Aβ₂₅₋₃₅ peptides were found to strongly interact with the lipid bilayers, displacing further cholesterol molecules into the plaques, effectively lowering the cholesterol concentration in the membranes and increasing the total fraction of cholesterol plaques. Addition of 30 mol% melatonin molecules to the anionic membranes drastically reduced the population of the membrane-embedded Aβ state. These results present experimental evidence for an interaction between Aβ peptides, melatonin and cholesterol in lipid membranes.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. The materials and apparatus used for the experiment.

(a) Schematic representations of DMPC, DMPS, cholesterol, melatonin, amyloid-formula image and amyloid-formula image molecules. (b) Diagram of the experimental setup used for the X-ray diffraction measurements. Two-dimensional data sets were collected to study molecular structure perpendicular to the solid supported membranes (out-of-plane) and parallel to the membranes (in-plane).

Figure 2

Figure 2. Calculated electron distribution of A and A.

The molecular structures and calculations are based on the PDB structures 1QWP (Aformula image) and 1IYT (Aformula image). The peptides take helix-kink and helix-kink-helix configurations, respectively, in solution.

Figure 3

Figure 3. Two-dimensional data for (a) DMPC +3 DMPS membrane, (b) DMPC/DMPS+3 mol% A and (c) DMPC/DMPS+3 mol% A.

The area per lipid is obtained from the position of the lipid acyl chain correlation peak at formula image 1.5 Å−1.

Figure 4

Figure 4. Reflectivity measurements for (a) DMPC/DMPS membrane, (b) DMPC/DMPS+3 mol% A, and (c) DMPC/DMPS+3 mol% A.

Pronounced and equally spaced Bragg peaks were observed, which are indicative to a well ordered lamellar structure. Electron densities were calculated through Fourier transformation of the integrated peak intensities. The insets shows the formula image function, which was used to assess the phases of the corresponding Fourier components.

Figure 5

Figure 5. Electron density analyses obtained through Fourier transforms of the X-ray reflectivity data.

a) Electron densities for the DMPC/DMPS membrane (green), DMPC/DMPS+3 mol% Aformula image (blue), and DMPC/DMPS+3 mol% Aformula image (red). b) The location of the Aformula image peptide in the DMPC/DMPS membrane, as determined from the difference electron density profile peak positions. c) Gaussian fits to the difference electron density obtained by subtracting the electron density profile of the DMPC/DMPS sample from that of the DMPC/DMPS+3 mol% Aformula image sample. A total of 4 peaks in each half of the bilayer are needed to fit the experimental data. The formula image-axis encompasses the full bilayer.

Figure 6

Figure 6. Measured and calculated electron distribution of the membrane-embedded A a) and A b) peptides and their position in the membrane.

Good agreement between calculations and experiments is obtained for a position of Aformula image in the hydrocarbon membrane core. The peptide takes a slightly tilted orientation, in agreement with computer simulations . The full length Aformula image peptide was also found to embed in anionic lipid exclude a membrane-spanning formula image-sheet structure, as it was reported from molecular dynamics simulations –.

Figure 7

Figure 7. Two-dimensional X-ray scans for the a) DMPC +3 mol% DMPS +30 mol% cholesterol and b) DMPC +3 mol% DMPS 3 mol% A+30 mol% cholesterol membranes.

Data were collected under full hydration of the membranes (100% RH), in their physiologically relevant fluid state.

Figure 8

Figure 8. Reflectivity measurements for the a) DMPC/DMPS+30 mol% cholesterol membrane and the b) DMPC/DMPS+3 mol% A+30 mol% cholesterol membrane.

c) and d) show Lorentzian fits to corresponding in-plane data from Figures 7 c and d), scanned under 100formula image relative humidity. Note that Barrett et al. identified the blue peaks as corresponding to in-plane features of crystalline cholesterol plaques. e) Cartoon of the structure of the multi-lamellar membrane complexes with membranes and coexisting cholesterol plaques and corresponding formula image-spacing. The cholesterol molecules in the plaques form a monoclinic lattice.

Figure 9

Figure 9. Two-dimensional X-ray data for the a) DMPC/DMPS, b) DMPC/DMPS +30 mol% melatonin, and c) DMPC/DMPS +30 mol% melatonin +3 mol% A samples, with all scans done under full hydration.

Corresponding reflectivities are shown in d), e) and f). The phases can be determined from the formula image function (shown as insets to the plots) and were used to determine the electron densities plotted in Figure 10.

Figure 10

Figure 10. Electron densities for the a) DMPC/DMPS, b) DMPC/DMPS +30 mol% melatonin, and c) DMPC/DMPS +30 mol% melatonin +3 mol% A samples.

All samples were scanned under 100% RH. The difference in electron density in a) allows the determination of position of the melatonin molecules in the anionic lipid membranes. The location of the Aformula image peptide in the DMPC/DMPS+30 mol% melatonin membrane was determined from the difference electron density profile peak positions in b).

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This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), the National Research Council Canada (NRC), the Canada Foundation for Innovation (CFI) and the Ontario Ministry of Economic Development and Innovation. H.D. is the recipient of an NSERC Undergraduate Research Awards (USRA), L.T. is the recipient of an NSERC Graduate Scholarship (CGS), M.C.R. is the recipient of an Early Researcher Award of the Province of Ontario. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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