Small Angle X-ray Scattering Sensing Membrane Composition: The Role of Sphingolipids in Membrane-Amyloid β-Peptide Interaction

Abstract

The early impairments appearing in Alzheimer's disease are related to neuronal membrane damage. Both aberrant Aβ species and specific membrane components play a role in promoting aggregation, deposition, and signaling dysfunction. Ganglioside GM1, present with cholesterol and sphingomyelin in lipid rafts, preferentially interacts with the Aβ peptide. GM1 at physiological conditions clusters in the membrane, the assembly also involves phospholipids, sphingomyelin, and cholesterol. The structure of large unilamellar vesicles (LUV), made of a basic POPC:POPS matrix in a proportion of 9:1, and containing different amounts of GM1 (1%, 3%, and 4% mol/mol) in the presence of 5% mol/mol sphingomyelin and 15% mol/mol cholesterol, was studied using small angle X-ray scattering (SAXS). The effect of the membrane composition on the LUVs-Aβ-peptide interaction, both for Aβ_1-40 and Aβ_1-42 variants, was, thus, monitored. The presence of GM1 leads to a significant shift of the main peak, towards lower scattering angles, up to 6% of the initial value with SM and 8% without, accompanied by an opposite shift of the first minimum, up to 21% and 24% of the initial value, respectively. The analysis of the SAXS spectra, using a multi-Gaussian model for the electronic density profile, indicated differences in the bilayer of the various compositions. An increase in the membrane thickness, by 16% and 12% when 2% and 3% mol/mol GM1 was present, without and with SM, respectively, was obtained. Furthermore, in these cases, in the presence of Aβ₄₀, a very small decrease of the bilayer thickness, less than 4% and 1%, respectively, was derived, suggesting the inhibiting effect that the presence of sphingomyelin has on the GM1-Aβ interaction.

Keywords: Aβ; GM1; SAXS; large unilamellar vesicles; rafts; sphingomyelin.

Figure 1

(a) Gaussians electron distribution (dashed line); different electron density contributions (continuous lines): inner decoration (green), internal polar head (blue), tail region (red), external polar head (magenta), outer decoration (brown) and cholesterol perturbations, i.e. ch_in (cyan) and ch_out (violet). (b) Vesicle radius, R, definition as the distance from the center of the vesicle to δ_tail, i.e., the center of the bilayer.

Figure 2

SAXS spectra (symbol) and fit curves (line) (upper panels): B2 with GM1 0% (black), 1% (red), 2% (green), and 4% (blue) (a); B1 (black), B2 (maroon) and B3 (magenta) (b); B1 (black), B2 (maroon) and B1 with GM1 3% (yellow), B2 with GM1 2% (green) (c). Electron density obtained from the multi Gaussian analysis (lower panels): B2 without (black line) and with GM1 1% (red line), 2% (green line), and 4% (blue line) (a); B1 (black), B2 (maroon line) and B3 (magenta line) (b); B1 (black line), B2 (maroon line) and B1 with GM1 3% (yellow line), B2 with GM1 2% (green line) (c).

Figure 3

SAXS spectra and fit curves (upper panels) for B1 (a) and B1 with GM1 2% (b). LUV alone (black symbols for data and black line for fit curve), LUV with Aβ_1–40 (red symbols for data and orange line for fit curve), and LUV with Aβ_1–42 (green symbols for data and cyan line for fit curve). Electron density obtained from the multi Gaussian analysis (lower panels) for B1 (a) and B1 with GM1 2% (b). LUV alone (black line), LUV with Aβ_1–40 (orange line) and LUV with Aβ_1–42 (cyan line).

Figure 4

SAXS spectra and fit curves (upper panels) for B2 without (a) and with GM1 1% (b), GM1 3% (c) and GM1 4% (d). LUV alone (black symbols for data and black line for fit curve), LUV with Aβ_1–40 (red symbols for data and orange line for fit curve), and LUV with Aβ_1–42 (green symbols for data and cyan line for fit curve). Electron density obtained from the multi Gaussian analysis (lower panels) for B2 without (a) and with GM1 1% (b), GM1 3% (c) and GM1 4% (d). LUV alone (black line), LUV with Aβ_1–40 (orange line) and LUV with Aβ_1–42 (cyan line).