Structure determination of micelle-like intermediates in amyloid beta -protein fibril assembly by using small angle neutron scattering

Abstract

Increasing evidence supports the hypothesis that amyloid beta-protein (Abeta) assembly is a key pathogenic feature of Alzheimer's disease. Thus, understanding the assembly process offers opportunities for the development of strategies for treating this devastating disease. In prior studies, Abeta was found to form micelle-like aggregates under acidic conditions. These structures exhibited an average observed hydrodynamic radius of 7 nm. They were found to be in rapid equilibrium with Abeta monomers or low molecular weight oligomers, and were centers of fibril nucleation. Here the technique of small angle neutron scattering has been used to determine the structure of these Abeta micelles. The data reveal that the micellar assemblies comprise 30-50 Abeta monomers and have elongated geometries. The best fit of the data to a uniform spherocylinder yields a radius approximately 2.4 nm and cylinder length approximately 11 nm. These structure parameters remain constant over more than a decade in concentration range. The concentration independence of the length of the cylindrical aggregate indicates the presence of an internal nonrepetitive structure that spans the entire length of the Abeta assembly.

Figure 1

Time evolution of the apparent hydrodynamic radius of Aβ aggregates, obtained by QLS, at concentration 0.40 mg/ml. The solvents are 0.1 N HCl in H₂O at 25°C (□); 0.1 N DCl in D₂O at 25°C (○); 0.1 N DCl in D₂O at 4°C (▵).

Figure 2

Differential cross section per unit volume of neutron scattering I(Q) versus magnitude of the scattering vector Q. (A) c₀ = 5.0 mg/ml (top data points) and 2.2 mg/ml (bottom data points). The solid lines correspond to the theoretical fit for a spherocylinder. Data at 10.9°C (blue) and at 4°C (red) are fitted separately. (B) I(Q) for the second set of samples with lower concentrations: blue, c₀ = 0.60 mg/ml; red, c₀ = 0.40 mg/ml; green, c₀ = 0.31 mg/ml. The solid lines are the theoretical fits with the parameters listed in Table 1.

Figure 3

The same experimental data as in Fig. 2, shown in logarithmic scale. The solid lines are calculated for a spherocylinder with dimensions R = 2.4 nm and L = 11.0 nm and I listed in Table 1. All data shown were taken at 4°C. Two topmost curves correspond to samples of 5.0 mg/ml (▵) and 2.2 mg/ml (○) both at high and low Q range. The rest of the measurements, low Q range only, are: c₀ = 0.60 mg/ml (▿); c₀ = 0.40 mg/ml (◊, offset slightly to the right for clarity); and c₀ = 0.31 mg/ml (□).

Figure 4

SANS and QLS scattering intensities as functions of concentration at 4°C. (█, SANS forward cross section I; □, QLS intensity normalized so as to overlay with SANS data. The solid line represents the theoretical prediction for c* = 0.15 mg/ml and m₀ = 29.

Figure 5

Schematic model of Aβ assembly consistent with SANS data.