Association of amyloid-β peptide with membrane surfaces monitored by solid state NMR

Abstract

Amyloid-β peptide (Aβ), a key substance in Alzheimer’s disease (AD), is characterized by its abnormal folding into neurotoxic aggregates. Since Aβ comprises an extracellular and transmembrane domain, some of its neurotoxic actions might be exerted via interactions with neuronal membranes. Wideline and magic angle spinning ¹⁴N and ³¹P NMR have been used in combination with differential scanning calorimetry and circular dichroism spectroscopy to investigate the association between Aβ_1–40 peptide and membranes with different electrostatic surface potentials. Calorimetric measurements showed that all membrane systems were in the liquid crystalline state at 308 K. Binding of Aβ_1–40 at a 30 ∶ 1 lipid/peptide ratio to membranes composed of neutral dimyristoyl-phosphatidylcholine (DMPC) and negatively charged dimyristoylphosphatidylglycerol (DMPG) at a 4 ∶ 1 molar ratio is mainly driven electrostatically, reflected in characteristic changes of the isotropic ³¹P chemical shift values for both lipids. In addition, the average orientation of the choline headgroup of DMPC, with its electric P⁻–N⁺(CH₃)₃ dipole, changed directly in response to the reduced negative membrane surface potential. The deviation in tilt angle of the PN vector relative to the membrane surface is manifested in the observed ¹⁴N NMR quadrupole splitting and can therefore be described semiquantitatively. Adding Aβ_1–40 to membranes with nominal neutral surface charge, but composed of a ternary mixture of DMPC with DMPG and the cationic amphiphile didodecyldimethyl–ammonium bromide (DDAB) at a 3 ∶ 1 ∶ 1 molar ratio revealed surprisingly electrostatic interactions visible in the NMR spectra. Since Aβ_1–40 does not bind to neutral DMPC bilayers a model is proposed, in which on a molecular level the charged residues of Aβ_1–40 peptide can interact independently with lipid headgroups of various charges in these microscopically heterogeneous systems.